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WO2002023791A2 - Retransmission for broadcast information - Google Patents

Retransmission for broadcast information Download PDF

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Publication number
WO2002023791A2
WO2002023791A2 PCT/EP2001/010126 EP0110126W WO0223791A2 WO 2002023791 A2 WO2002023791 A2 WO 2002023791A2 EP 0110126 W EP0110126 W EP 0110126W WO 0223791 A2 WO0223791 A2 WO 0223791A2
Authority
WO
WIPO (PCT)
Prior art keywords
return
broadcast
receiving units
unit
scheme
Prior art date
Application number
PCT/EP2001/010126
Other languages
French (fr)
Other versions
WO2002023791A3 (en
Inventor
Jacobus Haartsen
Original Assignee
Telefonaktiebolaget Lm Ericsson (Publ)
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Telefonaktiebolaget Lm Ericsson (Publ) filed Critical Telefonaktiebolaget Lm Ericsson (Publ)
Priority to AU2001295533A priority Critical patent/AU2001295533A1/en
Publication of WO2002023791A2 publication Critical patent/WO2002023791A2/en
Publication of WO2002023791A3 publication Critical patent/WO2002023791A3/en

Links

Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1809Selective-repeat protocols
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/1607Details of the supervisory signal
    • H04L1/1614Details of the supervisory signal using bitmaps
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1806Go-back-N protocols
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1829Arrangements specially adapted for the receiver end
    • H04L1/1858Transmission or retransmission of more than one copy of acknowledgement message
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L1/12Arrangements for detecting or preventing errors in the information received by using return channel
    • H04L1/16Arrangements for detecting or preventing errors in the information received by using return channel in which the return channel carries supervisory signals, e.g. repetition request signals
    • H04L1/18Automatic repetition systems, e.g. Van Duuren systems
    • H04L1/1867Arrangements specially adapted for the transmitter end
    • H04L1/1874Buffer management
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00Arrangements for detecting or preventing errors in the information received
    • H04L2001/0092Error control systems characterised by the topology of the transmission link
    • H04L2001/0093Point-to-multipoint

Definitions

  • the present invention relates to data communication systems.
  • the invention relates to a method and system for reliable data transmission in radio broadcast systems .
  • the aforementioned radio communications are based on peer communications and ad-hoc networking. This means that the system is not based on a hierarchical scheme with a fixed infrastructure of base stations and portable terminals that communicate with the base stations via radio signals.
  • peer communications all units have identical access rights to the network. There is no centralized control that can, for example, take care of resource or connection management, or provide other support services.
  • ad-hoc networks any unit can establish a connection to any other unit in range.
  • Ad-hoc networks are usually based on peer-to- peer communications. To support the cable replacement scenarios as mentioned above, the data traffic over the radio interface should be very flexible. The interface must support both symmetric and asymmetric (in arbitrary direction) data traffic flows.
  • BLUETOOTHTM provides a communications link having a very flexible slot structure without any multi-slot frames or the like.
  • the time axis in a BLUETOOTHTM communications link is divided into slots. The various units within communication range are free to allocate the slots for transmission or reception.
  • Radio communication systems for personal usage differ from radio systems like the public mobile phone network in that the former have to operate in an unlicensed band and have to deal with uncontrolled interference.
  • a suitable band is the Industrial, Scientific and Medical (ISM) band at 2.45 GHz, which is globally available.
  • ISM Industrial, Scientific and Medical
  • the band provides 83.5 MHz of radio spectrum. Since independent radio connections will share the same spectrum, mutual interference cannot be prevented.
  • communication systems apply retransmission schemes to retransmit data segments that have been received incorrectly by the receiving unit.
  • ARQ automatic retransmission query
  • the stop-and-wait scheme the next segment is only transmitted if the previous segment has been acknowledged.
  • the go-back-N scheme N segments can be sent before the first segment is checked to ensure correct reception. If not, all N segments are retransmitted irrespective of whether they were correctly received or not.
  • the selective-repeat ARQ scheme only the segment that failed is retransmitted and the receiver can request specific segments to be retransmitted. For practical reasons, the selective-repeat scheme is usually combined with a go-back-N scheme.
  • the go-back-N and selective-repeat schemes can be optimized if the round-trip delay over the link is known.
  • the delay is relatively constant and can be determined.
  • the delay is not known and can even vary. If the radio system can transmit packets with variable length arbitrarily in the forward and backward directions, the delay can vary considerably from packet to packet. Therefore, system delay cannot be used as a parameter for the optimization of the retransmission scheme.
  • a commercial radio concept calls for an efficient air protocol that requires minimum use of the radio spectrum. In particular, retransmission of segments that have already been received correctly should be prevented.
  • U.S. Provisional Application No. 60/180,095 entitled "Method and apparatus for retransmission,” by J.C. Haartsen a flexible retransmission protocol based on selective-repeat ARQ is presented which takes into account the buffer size at the receiving unit. This provisional application is hereby incorporated by reference herein in its entirety.
  • Broadcast systems are usually associated with familiar applications like TV broadcasting and AM/FM radio broadcasting. However, broadcast functionality can also occur on a smaller scale.
  • One application is to provide for wireless audio and video in and around the home.
  • Another application is information sharing between multiple laptop users in a meeting room.
  • Each laptop can be equipped with an inexpensive radio interface to communicate with other laptops (or other devices like cell phones, personal digital assistants (PDAs) , and the like) .
  • PDAs personal digital assistants
  • a retransmission scheme can be used to provide this robustness. With an automatic retransmission scheme, the reception of each bit of information is confirmed by each receiving unit .
  • a problem with ARQ protocols presented in the past is that they are intended for point-to-point applications.
  • a single receiving unit informs the transmitting unit whether the transmitted data was received correctly or not. If the same information has to be transmitted to multiple receiving units, as in a broadcast application, a problem arises with the acknowledgment messages.
  • the broadcast unit can of course establish multiple links, one link to each receiving unit, transmit the same data on the multiple links and carry out a retransmission protocol on each link separately. Transmitting the same information to each receiving unit separately and carrying out an ARQ scheme to each receiving unit separately is a possibility, but not an attractive one. Since all receiving units receive the same information, valuable resources in the sense of bandwidth and radio spectrum are wasted. In a broadcast situation, the transmitted data is only sent once and received by all receiving units simultaneously. To guarantee data integrity to all receiving units, each receiving unit should send a return message. Current ARQ protocols do not support this multi-acknowledgment feature.
  • the terms message, packet, and segment will be used to describe various aspects of the transmission from a broadcast unit.
  • the message is the largest unit transmitted and is typically fragmented into several segments which are contained in packets.
  • packets and segments are implicitly referenced.
  • One purpose of fragmenting the transmission is to facilitate both error correction/detection and data retransmission.
  • the current invention overcomes the prior art limitations by providing an automatic retransmission protocol between one broadcast unit and a plurality of receiving units and using a broadcast ARQ protocol in the broadcast unit and receiving units.
  • the broadcast unit transmits a broadcast message (using at least one packet) simultaneously to the plurality of receiving units .
  • the broadcast unit receives the return messages from the plurality of receiving units and forms a composite acknowledgment from the return messages. Then, the broadcast unit retransmits at least a portion of the packet, that is not acknowledged by the composite acknowledgment, simultaneously to all of the receiving units .
  • Fig. 1 shows a slotted time-division-duplex communication channel
  • Fig. 2 shows a system configuration of a broadcast application
  • Fig. 3 shows an example of return-slot allocation for fixed-sized packets of the invention
  • Fig. 4 shows an example of return-slot allocation for variable-sized packets of the invention
  • Fig. 5 shows an example of return-time allocation if no slotting is assumed
  • Fig. 6 shows an illustration of a broadcast retransmission scheme combined with the stop-and-wait ARQ protocol of the invention
  • Fig. 7 shows an illustration of a broadcast retransmission scheme combined with the go-back-N ARQ protocol of the invention
  • Fig. 8 shows an illustration of a broadcast retransmission scheme combined with the selective-repeat ARQ protocol of the invention
  • Fig. 9a shows an illustration of a broadcast retransmission scheme combined with an alternative selective-repeat ARQ protocol of the invention
  • Fig. 9b shows an illustration RX buffer contents of the alternative selective-repeat ARQ protocol presented in Fig. 9a; and Fig. 10 shows a flowchart illustrating a broadcast ARQ method of the invention.
  • any such form of embodiment may be referred to herein as "logic configured to” perform a described action, or alternatively as “logic that” performs a described action.
  • the invention provides an automatic retransmission protocol between one broadcast unit and a plurality of receiving units.
  • the broadcast unit transmits a message containing information, which may consist of multiple packets and segments.
  • Each receiving unit is assigned a return window.
  • the return window position is related to the end of the transmitted information packet or some other reference. Thereby, each receiving unit can return acknowledgment of the packet.
  • Different receiving units use different return windows so that the return messages are received by the broadcast unit unambiguously (i.e., the broadcast unit knows which receiving unit sent which return message) .
  • the allocation of the return windows takes place when the broadcast link is established. Additional receiving units can optionally be added to an existing broadcast channel by simply assigning the new receiving unit a return window not yet used.
  • the receiving units each return their respective return message consecutively in time or in some other unambiguous manner, such as by assigning a different channel to each receiving unit .
  • channels can be defined by any number of strategies including, but not limited to, Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) , and Code Division Multiple Access (CDMA) .
  • FDMA Frequency Division Multiple Access
  • TDMA Time Division Multiple Access
  • CDMA Code Division Multiple Access
  • the scheme of multiple receiving units can be combined with the different basic ARQ schemes.
  • the herein-described broadcast ARQ protocol differs for the different basic ARQ schemes in the way the return information is combined.
  • For the stop-and-wait ARQ scheme a packet is retransmitted when at least one receiving unit has not acknowledged the reception. Retransmission can be overruled if the life time of the packet has expired, or the considered receiving unit has a lower quality of service.
  • the go-back-N ARQ protocol the lowest cumulative acknowledge is used to decide which sequence of segments to retransmit .
  • all retransmit requests are collected, identical requests from different receiving units are treated as one request, and for each request, a retransmission is carried out .
  • all of the specific ARQ schemes share the common features of a composite acknowledgment formed from the individual acknowledgments of the various receiving units.
  • the broadcasting unit creates the composite acknowledgment from the individual acknowledgments. Then, the broadcasting unit forms its retransmission response based on the composite acknowledgment as required by the particular ARQ scheme.
  • time-slotted channel 104 time is divided into equally sized time slots 100, as shown in Fig. ' l.
  • Time line 106 shows a transmission pattern from unit A
  • time line 108 shows a transmission pattern from Unit B.
  • Unit A transmits information in blocks 120, 122, 124, and 126.
  • Unit B transmits information in blocks 130, 132, 134 and 136.
  • Time slots 100 are generally applied in digital radio systems since they provide support for low-power modes and simplify synchronization procedures.
  • the radio transmissions sent over the air interface may cover multiple time slots. However, the transmission must always start at the time slot boundary. For example block 120 starts at time slot boundary 110 and continues to transmit over more than two time slots .
  • Fig. 2 is a block diagram of an embodiment of a radio broadcast system configuration.
  • One broadcast unit A broadcasts the information 55 to a number of other units B, C, and D.
  • Units A to D can, for example, be laptops where the user of laptop A wants to send a document to the users of laptops B, C, and D.
  • Each return link 65, 75, and 85 is to provide the broadcast unit with return messages .
  • each segment is equipped with parity bits (e.g., Cyclic Redundancy Check) to enable the receiving unit to check for errors .
  • parity bits e.g., Cyclic Redundancy Check
  • the broadcast unit A has a transceiver that transmits broadcast packets simultaneously to the receiving units B, C, and D and receives return messages from the receiving units B, C, and D.
  • the broadcast unit A also contains logic configured to implement an ARQ scheme, logic configured to assign return windows to corresponding receiving units B, C, and D, logic configured to form a composite acknowledgment from the return messages and logic configured to retransmit at least a portion of the packet, that is not acknowledged by the composite acknowledgment .
  • the receiving units B, C, and D each have a transceiver that receives the packets and transmits return messages during the return window to the broadcast unit A. Additionally, the receiving units B, C, and D each have logic configured to implement the ARQ scheme and logic configured to accept a return window assignment from the plurality of return windows assigned by broadcast unit A. Preferably each of the receiving units B, C, and D also has a circuit that places the receiving unit in a low power mode and a RX buffer. Both items are not shown in Fig. 2, but are well known in the art .
  • a broadcast ARQ protocol with multiple receiving units is shown wherein the broadcast message is sent in fixed-sized packets (covering a fixed number of slots) .
  • unit A is the broadcasting unit and units B, C, and D are the receiving units.
  • all units A, B, C, and D transmit and receive data.
  • Each receiving unit B, C, and D is allocated a return window in which it can place its return message 321, 322, and 323, respectively, based on the results of the error checking.
  • all receiving units B, C, D are informed not only of their own return window, but also of the return windows of the other receiving units. For example, unit B is informed of its return window in time slot 4.
  • unit B is informed of the return windows for units C and D, respectively. In this way, unit B can save additional power consumption by not listening to the return windows reserved for the return messages 322, 323 for the other receiving units.
  • the broadcast packet 320 is sent in a frame 310 of nine time slots 0-8.
  • the frame 310 consists of a broadcast packet that covers four broadcast time slots 0-3, three return windows in time slots 4-6, respectively, and two empty time slots 7 and 8.
  • receiving units B, C, and D are allocated return windows as time slots 4, 5 and 6, respectively, in order to return their return messages.
  • the broadcast packets 320, 330, 340, 350, and 360 require a data rate consistent with four out of nine time slots.
  • the empty time slots may be used to increase the broadcast rate, or to add two more receiving units that can use the remaining time slots 7, 8 as return windows.
  • broadcast packets 330, 340, 350, and 360 have corresponding return messages 331-333, 341-343, 351-353, and 361-363, from receiving units B, C, and D, respectively.
  • the broadcast packet size is not constant. This can be the case, for example, if the broadcast packet is data or compressed video. In this scenario, there are no fixed-sized frames, and therefore no fixed allocation of return windows in fixed time slots. Instead, the return window allocation depends on the end of the broadcast packet 410, 420, 430, 440, 450.
  • the offset can have an arbitrary integer value greater than or equal to zero.
  • the offset may provide a dead zone (overhead) which may be required in the system for processing time (i.e. to switch from transmit to receive or vice versa) . In Fig. 4, the offset is zero.
  • the location of the return windows is related to the end of the broadcast packet and therefore varies as the broadcast packet length varies. If no slotting is used, the same relative positioning concept can be used (for fixed or variable sized broadcast packets) .
  • the broadcast packet 410 is transmitted from unit A. Subsequently, return messages 411, 412 and 413 are sent by receiving units B, C and D respectively. The process is repeated wherein broadcast packets 420, 430, 440, and 450 have corresponding return messages 421-423, 431-433, 441-443, and 451-453, from receiving units B, C, and D, respectively. In each case the return messages begin in the time slot immediately after the end of the broadcast packet, because the offset is zero for this example. In Fig. 5, a system is shown with no fixed time slot.
  • unit B is allowed to transmit its return message 511 in a return window defined by the interval between tO+offset and tO+offset+T_return .
  • the offset is an arbitrary delay between the end of the broadcast packet 510 and the start of first return window 515, and T_return is the time allocated for each return window to transmit an return message 511 (e.g. acknowledging successful receipt of the packet) .
  • Unit C is allowed to return a return message 512 in the return window starting at time 516 defined by the interval between tO+offset+T_return and tO+offset+2*T_return .
  • unit D is allowed to return a return message 513 in the return window starting at time 517 defined by the interval between tO+offset+2*T_return and t0+offset+3*T_return .
  • broadcast packet 520 has corresponding return messages 521, 522 and 523, transmitted from receiving units B, C, and D, respectively.
  • the return messages begin after a time delay after the end of the broadcast packet, because the offset is not zero for this example.
  • the return messages 511-513 and 521-523 are transmitted during their respective return windows, but do not necessarily occupy the entire return window.
  • the applied scheme of multiple return windows for ACK (acknowledgment) or NAK (negative acknowledgment) messages can be applied to the various basic ARQ schemes.
  • ACK acknowledgenowledgment
  • NAK negative acknowledgment
  • the embodiment shown is a broadcast stop-and-wait ARQ scheme.
  • the broadcast unit A keeps repeating its packet until all the receiving units B, C, and D have at least once confirmed the reception of this segment .
  • a receiving unit may or may not confirm the retransmission (s) , but this has no impact on the procedure.
  • broadcast packet 610 contains payload X and is transmitted by broadcast unit A.
  • Receiving units B, C, and D all transmit return messages 611, 612, and 613, respectively, acknowledging (ACK) successful receipt of payload X.
  • ACK acknowledging
  • a composite acknowledgment formed from the individual receiving units return messages would indicate no retransmission is necessary.
  • broadcast packet 620 containing payload Y is transmitted by broadcast unit A.
  • Receiving units B and D transmit return messages 621 and 623 acknowledging (ACK) successful receipt of payload Y by units B and D.
  • receiving unit C transmits a return message 622 containing a negative acknowledge (NAK) indicating payload Y was not successfully received.
  • NAK negative acknowledge
  • Broadcast unit A then retransmits payload Y in broadcast packet 630.
  • Receiving units C and D transmit return messages 632 and 633 acknowledging (ACK) successful receipt of payload Y by units C and D.
  • this time receiving unit B transmits return message 631 containing a negative acknowledge (NAK) indicating payload Y was not successfully received.
  • NAK negative acknowledge
  • the composite acknowledgment formed from this set of return messages would indicate no retransmission is necessary, since all receiving units B, C, and D have at least once ACKed receiving payload Y correctly. Therefore broadcast unit A continues with broadcast packet 640 containing payload Z. Subsequently, receiving units B, C, and D all transmit return messages 641, 642, 643 respectively, acknowledging (ACK) successful receipt of payload Z .
  • a broadcast ARQ protocol is shown for a go-back-N ARQ scheme with N equal to five.
  • the broadcast packets 710, 720, 730, 740 from broadcast unit A are each fragmented into segments and each segment is numbered sequentially (e.g. 0, 1, 2, ...) .
  • the receiving unit return message returns the number of the highest numbered segment that was correctly received in sequence.
  • the broadcast unit A collects the acknowledgment information from each receiving unit and generates a composite acknowledgment that contains the lowest number contained in the return messages and starts to retransmit segments based on that number.
  • the first transmission of broadcast packet 710 contains segments 0, 1, 2, 3, and 4.
  • Receiving units B and D have received segments in sequence up to number 3 and thus return messages 711 and 713 acknowledges segment 3. However receiving unit C only received in sequence segments up to number 2. Therefore, receiving unit C transmits return message 712 acknowledging segment 2.
  • the broadcast unit A takes the lowest cumulative segment number, namely 2 from receiving unit C, and transmits segments in sequence, beginning with segment 3. After the second transmission 720 of segments 3,4,5,6,7, receiving units B, C, and D transmit return messages 721, 722, and 723 acknowledging sequential receipt of segments. up to segments 7, 4, and 6 respectively.
  • the broadcast unit A takes the lowest cumulative segment number, 4 from receiving unit C, and transmits segments in sequence, beginning with segment 5.
  • the broadcast unit A takes the lowest cumulative segment number, 8 from receiving unit D, and transmits segments 9 and higher in sequence .
  • the fourth transmission 740 containing segments 9, 10, 11, 12, 13 receiving units B, C, and D transmit return messages 741, 742, and 743 acknowledging sequential receipt of segments up to segments 9, 9, and 9 respectively.
  • the process continues by broadcast unit A creating a composite acknowledgment that contains the lowest cumulative segment number and retransmitting the next and higher segments. If one of the receiving units does not update the cumulative segment number by a return message, its last received cumulative segment number is preferably used.
  • a broadcast ARQ protocol is shown for a general selective-repeat ARQ scheme with a transmit frame of ten segments.
  • Each broadcast packet 810, 820, 830 from broadcast unit A contains ten segments and each segment is numbered sequentially (e.g., 0-9) .
  • receiving units B, C, and D request particular segments to be retransmitted via return messages 811, 812, and 813, respectively.
  • unit B's retransmission request includes an identification of segment 3. Therefore, segment 3 will be among those that are retransmitted.
  • multiple requests for the same segment are treated as one request.
  • segment 4 is treated as one request .
  • Segment 4 will therefore be retransmitted only once.
  • the broadcast unit A retransmits segments 3 and 4 and the next sequence of segments beyond the first transmission 810, up to a total of ten segments.
  • the second transmission, packet 820 contains ten segments, specifically, segments 3, 4 and 10-17.
  • unit B requests segment 10, whereas both units C and D request segment 12.
  • Segments 10 and 12 are retransmitted in the third transmission, packet 830.
  • the next sequential data segments fill in the remainder of the ten segment transmission. Specifically, since segment 17 was the last segment to be transmitted in the second transmission, segments 18-25 fill in the remainder of the ten segment broadcast packet .
  • the modified ARQ protocol (hereafter REQ-BMS ARQ) presented in "Method and apparatus for retransmission," by J.C. Haartsen, U.S. Provisional Application No. 60/180,095 is applied to the broadcasting ARQ protocol .
  • the REQ-BMS ARQ method presented in the referenced disclosure operates as a function of the RX buffer size of the receiving unit.
  • the broadcasting unit A should only send that amount of new segments that the receiving unit RX buffer can contain.
  • the receiving units B, C, and D indicate with a request acknowledgment REQ_ACK which segments have been received in order, and also indicates with a bit map BMS, which segments after the REQ_ACK have failed.
  • REQ-BMS ARQ the modified ARQ protocol
  • the receiving units transmit REQ_ACKs which indicate that all segments with a sequence number lower than that indicated in the REQ_ACK have succeeded. Consequently, the segment indicated in the REQ_ACK has failed.
  • the receiving unit can indicate whether segments REQ_ACK+1, REQ_ACK+2 and REQ__ACK+3 were correctly received or not.
  • the different receiving units may have different RX buffers sizes. It is preferred that the modified ARQ scheme be based on the size of the shortest RX buffer among the receiving units.
  • the receiving units can indicate the failure or successful receipt of individual segments with REQ_ACK and BMS.
  • the broadcast unit combines all of this information and identifies which segment or segments require (s) retransmission. Multiple requests from several receiving units for retransmission of the same segment will result in only one retransmission of this segment.
  • the different receiving units may have different RX buffers sizes. It is preferred that the REQ-BMS ARQ scheme be based on the size of the shortest RX buffer among the receiving units.
  • receiving units B, C, and D have RX buffers 902, 904, and 906 having sizes of eight, seven, and seven segments, respectively.
  • the minimum RX buffer size of seven segments is used as the maximum ARQ window in this broadcast ARQ protocol .
  • the first transmission of broadcast packet 910 contains seven segments, namely segments 0-6.
  • a similar analysis can be given for receiving units C and D.
  • the buffer contents of the three receiving units B, C, and D are shown, in Fig. 9b, after reception of the broadcast packet. As soon as segments have correctly been received in order, they are removed from the RX buffer (after the first broadcast packet transmission, units B and D have received correctly all segments up to number 3, and therefore segments 0 to 3 have been removed from RX buffers 902 and 906; for unit C, segments 0 to 4 have been received correctly and removed from the RX buffer 904) .
  • segment 4 (requested by units B and D)
  • segment 5 (requested by units B and C)
  • segment 4 has not been acknowledged yet by all receiving units and the ARQ window is seven
  • the broadcast unit A cannot continue with segments 11 and higher.
  • Unit A must retransmit the negative acknowledged segments before it can continue. This results in the variation of the broadcast packet length.
  • the ARQ window remains constant at seven, as being the minimum RX buffer size (904 and 906) among the receiving units B, C, and D.
  • the RX buffers 902, 904, and 906 wrap around the end. Therefore, the position of the segments inside the RX buffer depends on the buffer size and how many segments were previously successfully transmitted.
  • broadcast unit A In the second transmission, broadcast unit A retransmits segments 4, 5, and transmits new segments 7 through 10. Consequently, broadcast packet 920 contains only 6 segments, segments 4, 5, 7, 8, 9, 10. Segment 6 is not retransmitted because all receiving units have successfully acknowledged receipt of segment 6.
  • the broadcast unit A In the third transmission, the broadcast unit A retransmits segments 7, 8, and 10 and transmits new segments 11 through 13. Consequently, broadcast packet 930 again contains only 6 segments, segments 7, 8, 10, 11, 12, and 13.
  • the broadcast unit A retransmits segments 7, 8, 10, and 12.
  • Broadcast packet 940 contains only 4 segments, segments 7, 8, 10, and 12.
  • broadcast unit A In the fifth transmission, the broadcast unit A retransmits segments 7 and 10. Consequently, broadcast packet 950 contains only 2 segments, segments 7 and 10. Receiving units C and D have received all segments up to 13 correctly and have emptied their buffers and request for segments 14 and higher. Since segment 7 has not been acknowledged by all receiving units and the ARQ window size is seven, the broadcast unit cannot continue with segments 14 and higher.
  • the broadcast unit may adapt the ARQ window depending on the receiving unit that requests retransmissions. Since the return slot is associated with the receiving unit, the broadcast unit knows exactly which receiving unit is requesting what segment . This may speed up the throughput . For example, in Fig. 9b, after the fourth transmission, it is clear that unit B with the longer RX buffer 902 is halting the progress of transmission. However, there is still one open position in unit B's RX buffer 902 since its RX buffer size is eight whereas the ARQ window is seven. In the fifth transmission, the broadcast unit could have increased the ARQ window size to eight without problems (i.e., segment 14 could be transmitted) .
  • a flowchart illustrating a broadcast ARQ method of the invention begins by a broadcast unit assigning return windows to a plurality of receiving units in step 1010.
  • the receiving units accept the return windows assigned to each in step 1015.
  • the broadcast unit then transmits its broadcast message (using at least one packet) in step 1020.
  • Each of the receiving units receives the packet and transmits a return message in its assigned return window in steps 1025 and 1035, respectively.
  • steps 1045 and 1055 optionally, if a low power mode is available and enabled in a receiving unit, that receiving unit will go into a low power mode during the return windows of the other receiving units to conserve energy.
  • the return messages will be formatted in accordance with an ARQ scheme such as described in the examples above.
  • the broadcast unit receives the return messages from the receiving units and forms a composite acknowledgment based on the ARQ scheme used. If all receiving units acknowledge successful receipt of the packet, no retransmission is necessary in step 1050. However, in step 1050, if all units have not successful received the packet , then based on the composite acknowledgment formed by the broadcast unit, the data segments not received will be included for retransmission with the subsequent packet in step 1060.
  • process may be modified in many ways without departing from the scope of the invention. For example, periodic checking of the local units may be used to identify when new receiving units are available and/or when existing receiving units are no longer available.
  • process could dynamically adjust the ARQ scheme used based on the amount of segments not received correctly.
  • Still another variation could include a further process for allowing the broadcast unit to become a receiving unit and one of the receiving units to become the broadcast unit. Many such variations are known for forming ad hoc networks and will be appreciated by those skilled in the art .
  • the broadcast unit may also differentiate between different receiving units.
  • the return channel indicates which receiving unit requests retransmissions. If the receiving unit is considered by the broadcast unit to be of lower importance, the broadcast unit may ignore the request, or restrict the number of retransmissions induced by this receiving unit .
  • the return channels shown in the previous description were based on slots in time. In other embodiments, these channels can equally well consist of another nature such as frequency or code division.
  • the return windows could be assigned cooperatively among the receiving units.
  • the broadcast unit could, for example, receive a return message containing an identifying portion, in addition to the acknowledgment, that identifies the receiving unit. In this system the broadcast unit would not be involved in assigning return windows.
  • the return message will always lag the broadcast message (or packets) , but the separation of the return messages need not include separation in time. As long as the channels are orthogonal, they do not mutually interfere.
  • the invention may be practiced by using any appropriate return message transmission method. For example, in using CDMA, it is contemplated that communication between broadcast unit and receiving units will be conducted using a spread-spectrum technique. By transmitting and receiving using different spreading codes, the broadcast unit can identify the return messages from each of the receiving units.

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Abstract

An automatic retransmission protocol between one broadcast unit and a plurality of receiving units is achieved by assigning an ARQ scheme to the broadcast unit and receiving units. A plurality of return windows are assigned for transmitting return messages from the corresponding plurality of receiving units to the broadcast unit. The broadcast unit broadcasts a packet simultaneously to the plurality of receiving units. The broadcast unit receives the return messages from the plurality of receiving units and forms a composite acknowledgement from the returnmessages. Then, the broadcast unit retransmits at least portion of the packet, that is not acknowledged by the composite acknowledgement, simultaneously to all of the receiving units.

Description

RETRANSMISSION FOR BROADCAST INFORMATION
BACKGROUND
The present invention relates to data communication systems. In particular, the invention relates to a method and system for reliable data transmission in radio broadcast systems .
In the last decade, progress in radio and VLSI technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, cellular phones, and the like, can now be produced having acceptable cost, size and power consumption for the consumer market .
Although wireless technology is today focused mainly on voice communications (e.g., with respect to handheld cell phones) , this field will likely expand in the near future to provide greater information flow to ' and from other types of portable devices and fixed devices. More specifically, it is likely that further advances in technology will provide very inexpensive radio equipment, which can easily be integrated into many devices. This will allow for further reduction of the number of cables currently used. For instance, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals. Recently, a new air interface named BLUETOOTH™ was introduced to replace all cables between mobile phones, laptop computers, headsets, PDAs, and the like. An introduction to the BLUETOOTH™ System can be found in "BLUETOOTH - The universal radio interface for ad hoc, wireless connectivity," by J.C. Haartsen, Ericsson Review No. 3, 1998.
The aforementioned radio communications are based on peer communications and ad-hoc networking. This means that the system is not based on a hierarchical scheme with a fixed infrastructure of base stations and portable terminals that communicate with the base stations via radio signals. In peer communications, all units have identical access rights to the network. There is no centralized control that can, for example, take care of resource or connection management, or provide other support services. In ad-hoc networks, any unit can establish a connection to any other unit in range. Ad-hoc networks are usually based on peer-to- peer communications. To support the cable replacement scenarios as mentioned above, the data traffic over the radio interface should be very flexible. The interface must support both symmetric and asymmetric (in arbitrary direction) data traffic flows. In addition, both synchronous traffic like voice as well as asynchronous traffic like internet access must be supported. BLUETOOTH™ provides a communications link having a very flexible slot structure without any multi-slot frames or the like. The time axis in a BLUETOOTH™ communications link is divided into slots. The various units within communication range are free to allocate the slots for transmission or reception.
Radio communication systems for personal usage differ from radio systems like the public mobile phone network in that the former have to operate in an unlicensed band and have to deal with uncontrolled interference. A suitable band is the Industrial, Scientific and Medical (ISM) band at 2.45 GHz, which is globally available. The band provides 83.5 MHz of radio spectrum. Since independent radio connections will share the same spectrum, mutual interference cannot be prevented. In order to obtain a link characterized by 100% data integrity, communication systems apply retransmission schemes to retransmit data segments that have been received incorrectly by the receiving unit. Numerous automatic retransmission query (ARQ) schemes have been studied in the past; see, for example, the book "Data Networks" by Bertsekas and Gallager, published by Prentice-Hall, Inc., 1992, ISBN 0-13-201674-5.
In principle, there are three types of ARQ schemes from which all the others are derived: the stop-and-wait, the go-back-N, and the selective-repeat ARQ schemes. In the stop-and-wait scheme the next segment is only transmitted if the previous segment has been acknowledged. In the go-back-N scheme, N segments can be sent before the first segment is checked to ensure correct reception. If not, all N segments are retransmitted irrespective of whether they were correctly received or not. In the selective-repeat ARQ scheme, only the segment that failed is retransmitted and the receiver can request specific segments to be retransmitted. For practical reasons, the selective-repeat scheme is usually combined with a go-back-N scheme. The go-back-N and selective-repeat schemes can be optimized if the round-trip delay over the link is known.
In most conventional communication systems, the delay is relatively constant and can be determined. However, in the flexible radio connections intended for cable replacement, the delay is not known and can even vary. If the radio system can transmit packets with variable length arbitrarily in the forward and backward directions, the delay can vary considerably from packet to packet. Therefore, system delay cannot be used as a parameter for the optimization of the retransmission scheme. Moreover, a commercial radio concept calls for an efficient air protocol that requires minimum use of the radio spectrum. In particular, retransmission of segments that have already been received correctly should be prevented. In U.S. Provisional Application No. 60/180,095 entitled "Method and apparatus for retransmission," by J.C. Haartsen, a flexible retransmission protocol based on selective-repeat ARQ is presented which takes into account the buffer size at the receiving unit. This provisional application is hereby incorporated by reference herein in its entirety.
Broadcast systems are usually associated with familiar applications like TV broadcasting and AM/FM radio broadcasting. However, broadcast functionality can also occur on a smaller scale. One application is to provide for wireless audio and video in and around the home. Another application is information sharing between multiple laptop users in a meeting room. Each laptop can be equipped with an inexpensive radio interface to communicate with other laptops (or other devices like cell phones, personal digital assistants (PDAs) , and the like) . In the laptop scenario, one user may send a file to each of the meeting participants. A retransmission scheme can be used to provide this robustness. With an automatic retransmission scheme, the reception of each bit of information is confirmed by each receiving unit .
A problem with ARQ protocols presented in the past is that they are intended for point-to-point applications. A single receiving unit informs the transmitting unit whether the transmitted data was received correctly or not. If the same information has to be transmitted to multiple receiving units, as in a broadcast application, a problem arises with the acknowledgment messages. The broadcast unit can of course establish multiple links, one link to each receiving unit, transmit the same data on the multiple links and carry out a retransmission protocol on each link separately. Transmitting the same information to each receiving unit separately and carrying out an ARQ scheme to each receiving unit separately is a possibility, but not an attractive one. Since all receiving units receive the same information, valuable resources in the sense of bandwidth and radio spectrum are wasted. In a broadcast situation, the transmitted data is only sent once and received by all receiving units simultaneously. To guarantee data integrity to all receiving units, each receiving unit should send a return message. Current ARQ protocols do not support this multi-acknowledgment feature.
SUMMARY
It should be emphasized that the terms "comprises" and "comprising", when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof .
In the following description of the invention, the terms message, packet, and segment will be used to describe various aspects of the transmission from a broadcast unit. In general the message is the largest unit transmitted and is typically fragmented into several segments which are contained in packets. Thus, when discussing, for instance a broadcast message, packets and segments are implicitly referenced. One purpose of fragmenting the transmission is to facilitate both error correction/detection and data retransmission.
The current invention overcomes the prior art limitations by providing an automatic retransmission protocol between one broadcast unit and a plurality of receiving units and using a broadcast ARQ protocol in the broadcast unit and receiving units. The broadcast unit transmits a broadcast message (using at least one packet) simultaneously to the plurality of receiving units . The broadcast unit receives the return messages from the plurality of receiving units and forms a composite acknowledgment from the return messages. Then, the broadcast unit retransmits at least a portion of the packet, that is not acknowledged by the composite acknowledgment, simultaneously to all of the receiving units .
The above features and advantages of the invention will be more apparent and additional features and advantages of the invention will be appreciated from the following detailed description of the invention made with reference to the drawings . BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the following figures, in which:
Fig. 1 shows a slotted time-division-duplex communication channel;
Fig. 2 shows a system configuration of a broadcast application;
Fig. 3 shows an example of return-slot allocation for fixed-sized packets of the invention;
Fig. 4 shows an example of return-slot allocation for variable-sized packets of the invention;
Fig. 5 shows an example of return-time allocation if no slotting is assumed;
Fig. 6 shows an illustration of a broadcast retransmission scheme combined with the stop-and-wait ARQ protocol of the invention;
Fig. 7 shows an illustration of a broadcast retransmission scheme combined with the go-back-N ARQ protocol of the invention;
Fig. 8 shows an illustration of a broadcast retransmission scheme combined with the selective-repeat ARQ protocol of the invention;
Fig. 9a shows an illustration of a broadcast retransmission scheme combined with an alternative selective-repeat ARQ protocol of the invention;
Fig. 9b shows an illustration RX buffer contents of the alternative selective-repeat ARQ protocol presented in Fig. 9a; and Fig. 10 shows a flowchart illustrating a broadcast ARQ method of the invention.
DETAILED DESCRIPTION OF INVENTION
Before addressing the specifics of the various embodiments of the invention a brief overview of the invention will be provided. To facilitate an understanding of the invention, many aspects of the invention are described in terms of sequences of actions to be performed by elements of a computer system. It will be recognized that in each of the embodiments, the various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function) , by program instructions being executed by one or more processors, or by a combination of both. Moreover, the invention can additionally be considered to be embodied entirely within any form of computer readable storage medium having stored therein an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein. Thus, the various aspects of the invention may be embodied in many different forms, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form of embodiment may be referred to herein as "logic configured to" perform a described action, or alternatively as "logic that" performs a described action.
The invention provides an automatic retransmission protocol between one broadcast unit and a plurality of receiving units. In exemplary embodiments, the broadcast unit transmits a message containing information, which may consist of multiple packets and segments. Each receiving unit is assigned a return window. The return window position is related to the end of the transmitted information packet or some other reference. Thereby, each receiving unit can return acknowledgment of the packet. Different receiving units use different return windows so that the return messages are received by the broadcast unit unambiguously (i.e., the broadcast unit knows which receiving unit sent which return message) . Preferably, the allocation of the return windows takes place when the broadcast link is established. Additional receiving units can optionally be added to an existing broadcast channel by simply assigning the new receiving unit a return window not yet used. The receiving units each return their respective return message consecutively in time or in some other unambiguous manner, such as by assigning a different channel to each receiving unit . As is well known in the art, channels can be defined by any number of strategies including, but not limited to, Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) , and Code Division Multiple Access (CDMA) .
The scheme of multiple receiving units can be combined with the different basic ARQ schemes. The herein-described broadcast ARQ protocol differs for the different basic ARQ schemes in the way the return information is combined. For the stop-and-wait ARQ scheme, a packet is retransmitted when at least one receiving unit has not acknowledged the reception. Retransmission can be overruled if the life time of the packet has expired, or the considered receiving unit has a lower quality of service. In the go-back-N ARQ protocol, the lowest cumulative acknowledge is used to decide which sequence of segments to retransmit . Finally, for the selective-repeat ARQ scheme, all retransmit requests are collected, identical requests from different receiving units are treated as one request, and for each request, a retransmission is carried out .
In the broadcast ARQ protocol presented, all of the specific ARQ schemes share the common features of a composite acknowledgment formed from the individual acknowledgments of the various receiving units. The broadcasting unit creates the composite acknowledgment from the individual acknowledgments. Then, the broadcasting unit forms its retransmission response based on the composite acknowledgment as required by the particular ARQ scheme.
In the various exemplary embodiments described herein, a time-slotted channel is assumed, although this is not absolutely necessary. In a time-slotted channel 104, time is divided into equally sized time slots 100, as shown in Fig.'l. Time line 106 shows a transmission pattern from unit A and time line 108 shows a transmission pattern from Unit B. Unit A transmits information in blocks 120, 122, 124, and 126. Unit B transmits information in blocks 130, 132, 134 and 136. Time slots 100 are generally applied in digital radio systems since they provide support for low-power modes and simplify synchronization procedures. The radio transmissions sent over the air interface may cover multiple time slots. However, the transmission must always start at the time slot boundary. For example block 120 starts at time slot boundary 110 and continues to transmit over more than two time slots .
Fig. 2 is a block diagram of an embodiment of a radio broadcast system configuration. One broadcast unit A broadcasts the information 55 to a number of other units B, C, and D. Units A to D can, for example, be laptops where the user of laptop A wants to send a document to the users of laptops B, C, and D. In contrast to well-known broadcast radio systems which are unidirectional (only from TV tower to TV sets) and are based on simplex transmission, in the current configuration, there is a return link 65, 75, and 85 from the receiving units B, C, and D, respectively, to the broadcast unit A. Each return link 65, 75, and 85 is to provide the broadcast unit with return messages . Preferably, each segment is equipped with parity bits (e.g., Cyclic Redundancy Check) to enable the receiving unit to check for errors .
The broadcast unit A has a transceiver that transmits broadcast packets simultaneously to the receiving units B, C, and D and receives return messages from the receiving units B, C, and D. The broadcast unit A also contains logic configured to implement an ARQ scheme, logic configured to assign return windows to corresponding receiving units B, C, and D, logic configured to form a composite acknowledgment from the return messages and logic configured to retransmit at least a portion of the packet, that is not acknowledged by the composite acknowledgment .
The receiving units B, C, and D each have a transceiver that receives the packets and transmits return messages during the return window to the broadcast unit A. Additionally, the receiving units B, C, and D each have logic configured to implement the ARQ scheme and logic configured to accept a return window assignment from the plurality of return windows assigned by broadcast unit A. Preferably each of the receiving units B, C, and D also has a circuit that places the receiving unit in a low power mode and a RX buffer. Both items are not shown in Fig. 2, but are well known in the art .
Referring to Fig. 3, a broadcast ARQ protocol with multiple receiving units according to an exemplary embodiment of the invention is shown wherein the broadcast message is sent in fixed-sized packets (covering a fixed number of slots) . In this configuration, unit A is the broadcasting unit and units B, C, and D are the receiving units. However, it should be noted that all units A, B, C, and D transmit and receive data. Each receiving unit B, C, and D is allocated a return window in which it can place its return message 321, 322, and 323, respectively, based on the results of the error checking. Preferably, all receiving units B, C, D, are informed not only of their own return window, but also of the return windows of the other receiving units. For example, unit B is informed of its return window in time slot 4. Additionally, unit B is informed of the return windows for units C and D, respectively. In this way, unit B can save additional power consumption by not listening to the return windows reserved for the return messages 322, 323 for the other receiving units. In Fig. 3, the broadcast packet 320 is sent in a frame 310 of nine time slots 0-8. The frame 310 consists of a broadcast packet that covers four broadcast time slots 0-3, three return windows in time slots 4-6, respectively, and two empty time slots 7 and 8. At connection establishment, receiving units B, C, and D are allocated return windows as time slots 4, 5 and 6, respectively, in order to return their return messages. In this example, it is assumed, the broadcast packets 320, 330, 340, 350, and 360 require a data rate consistent with four out of nine time slots. The empty time slots may be used to increase the broadcast rate, or to add two more receiving units that can use the remaining time slots 7, 8 as return windows. In subsequent frames the process is repeated wherein broadcast packets 330, 340, 350, and 360 have corresponding return messages 331-333, 341-343, 351-353, and 361-363, from receiving units B, C, and D, respectively. Referring to Fig. 4, an embodiment of the invention is shown where the broadcast packet size is not constant. This can be the case, for example, if the broadcast packet is data or compressed video. In this scenario, there are no fixed-sized frames, and therefore no fixed allocation of return windows in fixed time slots. Instead, the return window allocation depends on the end of the broadcast packet 410, 420, 430, 440, 450. If the last time slot used by the broadcast packet is slot m, than the return window for unit B is time slot tsB where tsB=m+l+offset. The return window for unit C is time slot tsC where tsC=m+2+offset and the return window for unit D is time slot tsD where tsD=m+3+ offset . The offset can have an arbitrary integer value greater than or equal to zero. The offset may provide a dead zone (overhead) which may be required in the system for processing time (i.e. to switch from transmit to receive or vice versa) . In Fig. 4, the offset is zero. The location of the return windows is related to the end of the broadcast packet and therefore varies as the broadcast packet length varies. If no slotting is used, the same relative positioning concept can be used (for fixed or variable sized broadcast packets) .
Specifically, referring to Fig. 4, the broadcast packet 410 is transmitted from unit A. Subsequently, return messages 411, 412 and 413 are sent by receiving units B, C and D respectively. The process is repeated wherein broadcast packets 420, 430, 440, and 450 have corresponding return messages 421-423, 431-433, 441-443, and 451-453, from receiving units B, C, and D, respectively. In each case the return messages begin in the time slot immediately after the end of the broadcast packet, because the offset is zero for this example. In Fig. 5, a system is shown with no fixed time slot. If the broadcast packet 510 ends at time to, then unit B is allowed to transmit its return message 511 in a return window defined by the interval between tO+offset and tO+offset+T_return . The offset is an arbitrary delay between the end of the broadcast packet 510 and the start of first return window 515, and T_return is the time allocated for each return window to transmit an return message 511 (e.g. acknowledging successful receipt of the packet) . Unit C is allowed to return a return message 512 in the return window starting at time 516 defined by the interval between tO+offset+T_return and tO+offset+2*T_return . Similarly, unit D is allowed to return a return message 513 in the return window starting at time 517 defined by the interval between tO+offset+2*T_return and t0+offset+3*T_return . The process is repeated wherein broadcast packet 520 has corresponding return messages 521, 522 and 523, transmitted from receiving units B, C, and D, respectively. In each case the return messages begin after a time delay after the end of the broadcast packet, because the offset is not zero for this example. Additionally, the return messages 511-513 and 521-523 are transmitted during their respective return windows, but do not necessarily occupy the entire return window. The applied scheme of multiple return windows for ACK (acknowledgment) or NAK (negative acknowledgment) messages can be applied to the various basic ARQ schemes. Several examples are provided. In the figures, only the payload of the packets is shown; each packet may contain additional bits such as preamble, header, and the like, which is not shown in the illustrations .
In Fig. 6, the embodiment shown is a broadcast stop-and-wait ARQ scheme. In this case, the broadcast unit A keeps repeating its packet until all the receiving units B, C, and D have at least once confirmed the reception of this segment . Note that after a receiving unit has confirmed the reception of the packet, it may or may not confirm the retransmission (s) , but this has no impact on the procedure. For example, broadcast packet 610 contains payload X and is transmitted by broadcast unit A. Receiving units B, C, and D all transmit return messages 611, 612, and 613, respectively, acknowledging (ACK) successful receipt of payload X. A composite acknowledgment formed from the individual receiving units return messages would indicate no retransmission is necessary. Next, broadcast packet 620 containing payload Y is transmitted by broadcast unit A. Receiving units B and D transmit return messages 621 and 623 acknowledging (ACK) successful receipt of payload Y by units B and D. However, receiving unit C transmits a return message 622 containing a negative acknowledge (NAK) indicating payload Y was not successfully received. The composite acknowledgment formed from this set of return messages would indicate retransmission is necessary. Broadcast unit A then retransmits payload Y in broadcast packet 630. Receiving units C and D transmit return messages 632 and 633 acknowledging (ACK) successful receipt of payload Y by units C and D. However, this time receiving unit B transmits return message 631 containing a negative acknowledge (NAK) indicating payload Y was not successfully received. The composite acknowledgment formed from this set of return messages would indicate no retransmission is necessary, since all receiving units B, C, and D have at least once ACKed receiving payload Y correctly. Therefore broadcast unit A continues with broadcast packet 640 containing payload Z. Subsequently, receiving units B, C, and D all transmit return messages 641, 642, 643 respectively, acknowledging (ACK) successful receipt of payload Z .
Referring to Fig. 7, a broadcast ARQ protocol is shown for a go-back-N ARQ scheme with N equal to five. The broadcast packets 710, 720, 730, 740 from broadcast unit A are each fragmented into segments and each segment is numbered sequentially (e.g. 0, 1, 2, ...) . In the go-back-N case, the receiving unit return message returns the number of the highest numbered segment that was correctly received in sequence. The broadcast unit A collects the acknowledgment information from each receiving unit and generates a composite acknowledgment that contains the lowest number contained in the return messages and starts to retransmit segments based on that number. For example, in Fig. 7, the first transmission of broadcast packet 710 contains segments 0, 1, 2, 3, and 4. Receiving units B and D have received segments in sequence up to number 3 and thus return messages 711 and 713 acknowledges segment 3. However receiving unit C only received in sequence segments up to number 2. Therefore, receiving unit C transmits return message 712 acknowledging segment 2. The broadcast unit A takes the lowest cumulative segment number, namely 2 from receiving unit C, and transmits segments in sequence, beginning with segment 3. After the second transmission 720 of segments 3,4,5,6,7, receiving units B, C, and D transmit return messages 721, 722, and 723 acknowledging sequential receipt of segments. up to segments 7, 4, and 6 respectively. The broadcast unit A takes the lowest cumulative segment number, 4 from receiving unit C, and transmits segments in sequence, beginning with segment 5. After the third transmission 730 of segments 5,6,7,8,9 receiving units B, C, and D transmit return messages 731, 732, and 733 acknowledging sequential receipt of segments up to segments 9, 9, and 8 respectively. The broadcast unit A takes the lowest cumulative segment number, 8 from receiving unit D, and transmits segments 9 and higher in sequence . After the fourth transmission 740 containing segments 9, 10, 11, 12, 13 receiving units B, C, and D transmit return messages 741, 742, and 743 acknowledging sequential receipt of segments up to segments 9, 9, and 9 respectively. The process continues by broadcast unit A creating a composite acknowledgment that contains the lowest cumulative segment number and retransmitting the next and higher segments. If one of the receiving units does not update the cumulative segment number by a return message, its last received cumulative segment number is preferably used.
In Fig. 8, a broadcast ARQ protocol is shown for a general selective-repeat ARQ scheme with a transmit frame of ten segments. Each broadcast packet 810, 820, 830 from broadcast unit A contains ten segments and each segment is numbered sequentially (e.g., 0-9) . After the first transmission, in which packet 810 contains segments 0-9, receiving units B, C, and D request particular segments to be retransmitted via return messages 811, 812, and 813, respectively. In this example, unit B's retransmission request includes an identification of segment 3. Therefore, segment 3 will be among those that are retransmitted. In forming the composite acknowledgment, multiple requests for the same segment are treated as one request. Therefore, the request for segment 4 from all receiving units is treated as one request . Segment 4 will therefore be retransmitted only once. In the second transmission 820, the broadcast unit A retransmits segments 3 and 4 and the next sequence of segments beyond the first transmission 810, up to a total of ten segments. The second transmission, packet 820, contains ten segments, specifically, segments 3, 4 and 10-17. Now, unit B requests segment 10, whereas both units C and D request segment 12. Segments 10 and 12 are retransmitted in the third transmission, packet 830. The next sequential data segments fill in the remainder of the ten segment transmission. Specifically, since segment 17 was the last segment to be transmitted in the second transmission, segments 18-25 fill in the remainder of the ten segment broadcast packet .
In Figs. 9a and 9b, the modified ARQ protocol (hereafter REQ-BMS ARQ) presented in "Method and apparatus for retransmission," by J.C. Haartsen, U.S. Provisional Application No. 60/180,095 is applied to the broadcasting ARQ protocol . The REQ-BMS ARQ method presented in the referenced disclosure operates as a function of the RX buffer size of the receiving unit. The broadcasting unit A should only send that amount of new segments that the receiving unit RX buffer can contain. The receiving units B, C, and D indicate with a request acknowledgment REQ_ACK which segments have been received in order, and also indicates with a bit map BMS, which segments after the REQ_ACK have failed. In Fig. 9a and 9b, in general for the transmission of a broadcast packet, the receiving units transmit REQ_ACKs which indicate that all segments with a sequence number lower than that indicated in the REQ_ACK have succeeded. Consequently, the segment indicated in the REQ_ACK has failed. With the bit map BMS which is only three bits in this example (the particular size of the BMS is application-specific) , the receiving unit can indicate whether segments REQ_ACK+1, REQ_ACK+2 and REQ__ACK+3 were correctly received or not. In the broadcast ARQ case, the different receiving units may have different RX buffers sizes. It is preferred that the modified ARQ scheme be based on the size of the shortest RX buffer among the receiving units. The receiving units can indicate the failure or successful receipt of individual segments with REQ_ACK and BMS. The broadcast unit combines all of this information and identifies which segment or segments require (s) retransmission. Multiple requests from several receiving units for retransmission of the same segment will result in only one retransmission of this segment. In the broadcast ARQ case, the different receiving units may have different RX buffers sizes. It is preferred that the REQ-BMS ARQ scheme be based on the size of the shortest RX buffer among the receiving units.
In the example of Figs. 9a and 9b, receiving units B, C, and D have RX buffers 902, 904, and 906 having sizes of eight, seven, and seven segments, respectively. In the exemplary embodiment of the invention, the minimum RX buffer size of seven segments is used as the maximum ARQ window in this broadcast ARQ protocol . The first transmission of broadcast packet 910, contains seven segments, namely segments 0-6.
Receiving unit B fails to properly receive segments 4 and 5. Therefore, return message 911 from Unit B contains REQ_ACK=4 and BMS=101 since, segment 5 is in error, segment 6 is correct, and segment 7 has not been received. A similar analysis can be given for receiving units C and D. Receiving unit C fails to properly receive segment 5. Therefore, return message 912 from Unit C contains REQ_ACK=5 and BMS=011 since, segment 6 is correct, and segments 7 and 8 have not been received. In this embodiment a bit value of 1 indicates failure to receive a segment and a bit value of 0 indicates successful receipt of a segment. Receiving unit D fails to properly receive segment 4. Therefore, return message 913 from Unit D contains REQ_ACK=4 and BMS=001 since, segments 5 and 6 are correct, and segment 7 has not been received.
The buffer contents of the three receiving units B, C, and D are shown, in Fig. 9b, after reception of the broadcast packet. As soon as segments have correctly been received in order, they are removed from the RX buffer (after the first broadcast packet transmission, units B and D have received correctly all segments up to number 3, and therefore segments 0 to 3 have been removed from RX buffers 902 and 906; for unit C, segments 0 to 4 have been received correctly and removed from the RX buffer 904) .
Combining the information from the return messages 911, 912, 913, to form a composite acknowledgment, it can be derived that the following segments have to be retransmitted: segment 4 (requested by units B and D) , and segment 5 (requested by units B and C) . Since at minimum four segments have been received in order (segments 0 to 3) , four new segments can be sent before the RX buffers 902, 904, and 906 become full. However since segment 4 has not been acknowledged yet by all receiving units and the ARQ window is seven, the broadcast unit A cannot continue with segments 11 and higher. Unit A must retransmit the negative acknowledged segments before it can continue. This results in the variation of the broadcast packet length. For simplicity, the ARQ window remains constant at seven, as being the minimum RX buffer size (904 and 906) among the receiving units B, C, and D. The RX buffers 902, 904, and 906 wrap around the end. Therefore, the position of the segments inside the RX buffer depends on the buffer size and how many segments were previously successfully transmitted.
In the second transmission, broadcast unit A retransmits segments 4, 5, and transmits new segments 7 through 10. Consequently, broadcast packet 920 contains only 6 segments, segments 4, 5, 7, 8, 9, 10. Segment 6 is not retransmitted because all receiving units have successfully acknowledged receipt of segment 6. After the second transmission, unit B failed to receive segments 7 and 10 correctly. Therefore, return message 921 from Unit B contains REQ_ACK=7 and BMS=001 since, segments 8 and 9 are correct and segment 10 is in error. Receiving unit C fails to properly receive segment 10. Therefore, return message 922 from Unit C contains REQ_ACK=10 and BMS=111 since, segments 11, 12 and 13 have not been transmitted. Receiving unit D fails to properly receive segment 8. Therefore, return message 923 from Unit D contains REQ_ACK=8 and BMS=001 since, segments 9 and 10 are correct, and segment 11 has not been transmitted.
In the third transmission, the broadcast unit A retransmits segments 7, 8, and 10 and transmits new segments 11 through 13. Consequently, broadcast packet 930 again contains only 6 segments, segments 7, 8, 10, 11, 12, and 13. After the third transmission, unit B fails to receive segments 7 and 10 correctly. Therefore, return message 931 from Unit B contains REQ_ACK=7 and BMS=001 since, segments 8 and 9 are correct and segment 10 is in error. Receiving unit C fails to properly receive segment 12. Therefore, return message 932 from Unit C contains REQ_ACK=12 and BMS=011 since, segments 13 is correct and segments 14 and 15 have not been received. Receiving unit D fails to properly receive segment 8. Therefore, return message 933 from Unit D contains REQ_ACK=8 and BMS=000 since, segments 9, 10, and 11 are correct.
In the fourth transmission, the broadcast unit A retransmits segments 7, 8, 10, and 12. Broadcast packet 940 contains only 4 segments, segments 7, 8, 10, and 12. After the fourth transmission, unit B again fails to receive segments 7 and 10 correctly. Therefore, return message 941 from Unit B contains REQ_ACK=7 and BMS=001 since, segments 8 and 9 are correct and segment 10 is in error. Receiving units C and D properly receives all segments up to segment 13. Therefore, return messages 942 and 943 from units C and D, respectively, each contain REQ_ACK=14 and BMS=111 since, segments 15, 16, and 17 have not been received.
In the fifth transmission, the broadcast unit A retransmits segments 7 and 10. Consequently, broadcast packet 950 contains only 2 segments, segments 7 and 10. Receiving units C and D have received all segments up to 13 correctly and have emptied their buffers and request for segments 14 and higher. Since segment 7 has not been acknowledged by all receiving units and the ARQ window size is seven, the broadcast unit cannot continue with segments 14 and higher.
In a more complex scheme, the broadcast unit may adapt the ARQ window depending on the receiving unit that requests retransmissions. Since the return slot is associated with the receiving unit, the broadcast unit knows exactly which receiving unit is requesting what segment . This may speed up the throughput . For example, in Fig. 9b, after the fourth transmission, it is clear that unit B with the longer RX buffer 902 is halting the progress of transmission. However, there is still one open position in unit B's RX buffer 902 since its RX buffer size is eight whereas the ARQ window is seven. In the fifth transmission, the broadcast unit could have increased the ARQ window size to eight without problems (i.e., segment 14 could be transmitted) .
Referring to Fig. 10, a flowchart illustrating a broadcast ARQ method of the invention is shown. The process begins by a broadcast unit assigning return windows to a plurality of receiving units in step 1010. The receiving units accept the return windows assigned to each in step 1015. The broadcast unit then transmits its broadcast message (using at least one packet) in step 1020. Each of the receiving units receives the packet and transmits a return message in its assigned return window in steps 1025 and 1035, respectively. In steps 1045 and 1055, optionally, if a low power mode is available and enabled in a receiving unit, that receiving unit will go into a low power mode during the return windows of the other receiving units to conserve energy. The return messages will be formatted in accordance with an ARQ scheme such as described in the examples above. In steps 1030 and 1040, the broadcast unit receives the return messages from the receiving units and forms a composite acknowledgment based on the ARQ scheme used. If all receiving units acknowledge successful receipt of the packet, no retransmission is necessary in step 1050. However, in step 1050, if all units have not successful received the packet , then based on the composite acknowledgment formed by the broadcast unit, the data segments not received will be included for retransmission with the subsequent packet in step 1060.
One skilled in the art will appreciate that the process may be modified in many ways without departing from the scope of the invention. For example, periodic checking of the local units may be used to identify when new receiving units are available and/or when existing receiving units are no longer available. In another example, the process could dynamically adjust the ARQ scheme used based on the amount of segments not received correctly. Still another variation could include a further process for allowing the broadcast unit to become a receiving unit and one of the receiving units to become the broadcast unit. Many such variations are known for forming ad hoc networks and will be appreciated by those skilled in the art .
In each of the examples provided, there may be a maximum to the number of retransmissions applied. If the lifetime of the data segment expires (e.g., it has to be sent within a certain time window) , the segment is discarded and the broadcast unit continues to make the next segments arrive in time. Examples are found in isochronous channels for transmitting audio and video. In such environments, a data segment can be retransmitted up to a certain point in time; thereafter, the next data segment has to be transmitted, which has an identical time interval. The broadcast unit may also differentiate between different receiving units. The return channel indicates which receiving unit requests retransmissions. If the receiving unit is considered by the broadcast unit to be of lower importance, the broadcast unit may ignore the request, or restrict the number of retransmissions induced by this receiving unit .
The foregoing has described the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. For example, the return channels shown in the previous description were based on slots in time. In other embodiments, these channels can equally well consist of another nature such as frequency or code division. In another example, the return windows could be assigned cooperatively among the receiving units. The broadcast unit could, for example, receive a return message containing an identifying portion, in addition to the acknowledgment, that identifies the receiving unit. In this system the broadcast unit would not be involved in assigning return windows.
The return message will always lag the broadcast message (or packets) , but the separation of the return messages need not include separation in time. As long as the channels are orthogonal, they do not mutually interfere. One skilled in the art will appreciate that the invention may be practiced by using any appropriate return message transmission method. For example, in using CDMA, it is contemplated that communication between broadcast unit and receiving units will be conducted using a spread-spectrum technique. By transmitting and receiving using different spreading codes, the broadcast unit can identify the return messages from each of the receiving units.
Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the invention as defined by the following claims.

Claims

CLAIMSWhat is claimed is:
1. A method for transmitting broadcast information from a broadcast unit to a plurality of receiving units comprising: broadcasting a packet simultaneously to the plurality of receiving units; receiving return messages from the plurality of receiving units, wherein each of the return messages is an acknowledgment message from a corresponding one of the receiving units; forming a composite acknowledgment from the return messages; and retransmitting at least a portion of the packet, that is not acknowledged by the composite acknowledgment, simultaneously to all of the receiving units .
2. The method of claim 1 further comprising: assigning a plurality of return windows for transmitting return messages from the corresponding plurality of receiving units to the broadcast unit.
3. The method of claim 2 wherein each of the plurality of return windows is defined by a separate frequency.
4. The method of claim 2 wherein each of the plurality of return windows is defined by a unique spreading code .
5. The method of claim 2 wherein each of the plurality of return windows has defined by a unique time slot .
6. The method of claim 5 wherein a first return window of the plurality of return windows is offset from the end of the packet by an offset time and each subsequent return window of the plurality of return windows begins at the end of a previous return window.
7. The method of claim 2 further comprising: defining a time frame containing a fixed number of time slots; wherein the step of transmitting the packet starts at a first time slot and occupies at least one time slot ; wherein each return window occupies at least one time slot; and wherein a first return window of the plurality of return windows starts at a time slot subsequent to a last time slot occupied by the broadcast packet and each subsequent return window of the plurality of return windows starts at a time slot subsequent to a last time slot occupied by a previous return window.
8. The method of claim 7 wherein the first return window starts in a time slot immediately following the last time slot occupied by the packet.
9. The method of claim 1 wherein the step of transmitting the packet has a fixed time duration.
10. The method of claim 1 wherein the step of transmitting the packet has a variable time duration.
11. The method of claim 1 wherein the return messages are formatted in accordance with a broadcast stop-and-wait ARQ scheme.
12. The method of claim 1 wherein the return message is formatted in accordance with a broadcast go-back-N ARQ scheme.
13. The method of claim 1 wherein the return message is formatted in accordance with a broadcast selective-repeat ARQ scheme.
14. The method of claim 1 wherein the return message is formatted in accordance with a broadcast REQ- BMS scheme .
15. A method for receiving broadcast information in a plurality of receiving units, wherein the broadcast information is transmitted by a broadcast unit, the method comprising: each receiving unit performing the steps of: receiving the packet; and transmitting to the broadcast unit a return message that is formatted in accordance with an ARQ scheme.
16. The method of claim 15 further comprising: receiving a plurality of return window allocations for transmitting return messages from the corresponding plurality of receiving units to the broadcast unit .
17. The method of claim 16 wherein each of the plurality of return windows is defined by a fixed time slot .
18. The method of claim 16 wherein each of the plurality of return windows is defined by a separate frequency.
19. The method of claim 16 wherein each of the plurality of return windows is defined by a unique spreading code .
20. The method of claim 16 further comprising: placing each receiving unit in a low power mode during the return windows of all other receiving units .
21. The method of claim 15 wherein the ARQ scheme is a broadcast stop-and-wait scheme.
22. The method of claim 15 wherein the ARQ scheme is a broadcast go-back-N scheme.
23. The method of claim 15 wherein the ARQ scheme is a broadcast selective-repeat scheme.
24. The method of claim 15 wherein the ARQ scheme is a broadcast REQ-BMS scheme.
25. A system for transmitting broadcast information comprising: a broadcast unit comprising: a transceiver that transmits packets simultaneously to a- lurality of receiving units and receives return messages from the plurality of receiving units; logic configured to implement an ARQ scheme; logic configured to form a composite acknowledgment from the return messages; and logic configured to retransmit at least a portion of the packet, that is not acknowledged by the composite acknowledgment; and the plurality of receiving units each comprising: logic configured to implement the ARQ scheme; and a transceiver that receives the packets and transmits return messages during a return window to the broadcast unit .
26. The system of claim 25 wherein: the broadcast unit further comprises : logic configured to assign a plurality return windows to a corresponding plurality of receiving units; and each receiving unit further comprises: logic configured to accept a return window assignment from the plurality of return windows.
27. The system of claim 25 wherein each receiving unit further comprises: a circuit that places the receiving unit in a low power mode .
28. The system of claim 25 wherein the ARQ scheme is a broadcast stop-and-wait scheme.
29. The system of claim 25 wherein the ARQ scheme is a broadcast go-back-N scheme.
30. The system of claim 25 wherein the ARQ scheme is a broadcast selective-repeat scheme.
31. The system of claim 25 wherein each receiving unit further comprises: a RX buffer.
32. The system of claim 31 wherein the ARQ scheme is a broadcast REQ-BMS scheme.
PCT/EP2001/010126 2000-09-12 2001-09-03 Retransmission for broadcast information WO2002023791A2 (en)

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