DE20212017U1 - High speed downlink packet access (HSDPA) base station using an automatic physical layer retry request mechanism - Google Patents

Description

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InterDigital Technology Corporation 181243 GM

[0001 ] HIGH SPEED DOWNLINK PACKET ACCESS (HSDPA) BASE STATION USING AN AUTOMATIC PHYSICAL LAYER REPEAT REQUEST MECHANISM

[0002] BACKGROUND

[0003] The present invention relates to wireless

Communication systems. In particular, it relates to a modification of such systems through the use of a physical layer (PHY) automatic repeat request (ARQ) procedure.

[0004] Proposed communication systems with fixed wireless

Broadband fixed wireless access (BFWA) systems using either single carrier-frequency domain equalization (SC-FDE) or orthogonal frequency multiplexing (OFDM) plan to use a high speed downlink packet access (HSDPA) application. This application will send downlink packet data at high speeds. In BFWA, a building or group of buildings is connected either wirelessly or wired and operated as a single subscriber site. The data demand in such a system is quite high for the many end users of the single site, so large bandwidths are required.
[0005] The currently proposed system uses an automatic

Second layer repeat request (ARQ) system. Data blocks that are unsuccessfully sent to the subscribers are buffered and resent by the second layer. The data blocks stored in the second layer are typically large, are sent for reception with a high signal to noise ratio (SNR), are received with a low block error rate (BLER), and are rarely resent. In addition, ARQ signaling is

the second layer is typically slow and requires large buffers and long retransmission intervals.

[0006] It is therefore desirable, in addition to an ARQ system, to

second layer to have alternatives.
5

[0007] SUMMARY

[0008] A high-speed downlink packet

Access (HSDPA) base station has an automatic

Physical layer repeat request (ARQ) mechanism.

The base station has a transmitter. The transmitter has a physical layer transmitter for receiving data and formatting the received data into packets. Each packet has its own coding/data modulation. The own coding/data modulation uses an HSDPA modulation scheme and forward error correction (FEC) coding. Packets are transmitted, and in response to not receiving a corresponding acknowledgement for a particular packet, packets are retransmitted. An adaptive modulation and coding (AMC) controller collects retransmission statistics and adjusts the own coding/data modulation using the collected statistics.

[0009] BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. la and lb are simplified block diagrams of physical

Downlink and uplink ARQs.

[0011] Fig. 2 is a flow chart for using

Retransmission statistics for adaptive modulation and coding.
[0012] Figure 3 is a block diagram showing a multi-channel stop and wait architecture.

[0013] DETAILED DESCRIPTION OF THE PREFERRED

MODELS OF IMPLEMENTATION

[0014] Fig. 1a and 1b show a physical downlink ARQ 10 and 11, respectively.

physical uplink ARQ 20.

[0015] The physical downlink ARQ 10 comprises a base station 12

which receives packets from an ARQ transmitter 14a of a higher layer provided in the network 14. The packets from the transmitter 14a are forwarded to the ARQ transmitter 12a of the

physical layer in the base station 12. The ARQ transmitter 12a encodes the data with a forward error correction code (FEC), appends error check sequences (ECSs), modulates the data as directed by the adaptive modulation and coding (AMC) controller 12c, for example using binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or m-ary quadrature amplitude modulation (i.e., 16-QAM or 64-QAM). Additionally, in orthogonal frequency division multiple access (OFDMA), the AMC controller 12a may vary the subchannels used to carry the packets. The physical layer ARQ transmitter 12a sends packets to the subscriber unit 16 over an air interface 14 via a switch, circulator or duplexor 12d and an antenna 13. The transmitter 12a also temporarily stores the message in a buffer memory integrated in the transmitter 12a for retransmission if necessary.
[0016] An antenna 15 of the subscriber unit 16 receives the packet. The

Packet is input to the physical layer ARQ receiver 16a via a switch, circulator or duplexor 16b. At the receiver 16a, the packet is FEC decoded and checked for errors using the ECS. The receiver 16a then controls the acknowledge transmitter 16c to either acknowledge (ACK) receipt of a packet with an acceptable error rate or to request retransmission, preferably by withholding an acknowledgement signal or sending a negative acknowledgement (NAK).

[0017] The ACK is transmitted by the ACK transmitter 16c via the switch 16b and the

antenna 15 to base station 12. The ACK is transmitted over air interface 14 to antenna 13 of base station 12. The received ACK is processed by an acknowledgement receiver 12b in the base station. ACK receiver 12b provides the ACK/NAKs to adaptive modulation and coding (AMC) controller 12c and to transmitter 12a. AMC controller 12c analyzes the channel quality to subscriber unit 16 using statistics of the received ACKs and may vary the FEC coding and modulation methods of subsequent transmissions of the message, as will be described in more detail. When subscriber unit 16 acknowledges receipt of the packet, receipt of this ACK at base station 12 causes the original packet, which was temporarily stored in buffer memory, to be discarded in preparation for the next packet.

[0018] If no ACK is received or a NAK is received,

the physical layer transmitter 12a repeats sending the original message or a selectively modified version of the original message to the subscriber unit 16. At the subscriber unit 16, the repeated transmission is combined with the original transmission if available. This method facilitates the reception of a correct message through the use of data redundancy or selective repeat combination. The packets with an acceptable error rate are transmitted to higher layers 16d for further processing. The acceptable received packets are delivered to the higher layers 16d in the same data order in which the data was delivered to the transmitter 12a in the base station (i.e., delivery in sequence). The maximum number of repeated transmissions is limited to an operator-defined integer value, such as in the range of 1 to 8. After the maximum number of repeated transmissions have been attempted, the buffer memory is cleared for use by the next packet. Decoding an acknowledgment using small packets at the physical layer reduces transmission delays and message processing times.

[0019] Since the PHY-ARQ takes place on the physical layer, the

Number of occurrences of repeated broadcasts for a particular channel, i.e.

Retransmission statistics, provide a good measure of the quality of that channel. Using the retransmission statistics, the AMC controller 12c can vary the modulation and coding schemes for that channel, as shown in Figure 2. In addition, the retransmission statistics can be combined with other link quality measurements, such as bit error rates (BERs) and block error rates (BLERs) from the AMC controller 12c to measure the channel quality and to determine if a change in the modulation and coding scheme is necessary.

[0020] To illustrate the case of SC-FDE, the occurrence

retransmissions for a particular channel to generate the retransmission statistics (60). A decision on whether to change the modulation scheme is made using the retransmission statistics (62). If retransmissions become excessive, a more robust coding and modulation scheme is used (64), usually with a reduced data transfer rate. The AMC controller

12c may increase the spreading factor and use more packets to transmit the packet data. Alternatively or additionally, the AMC controller may switch from a modulation scheme with a high data throughput to a lower one, such as from 64-QAM to 16-QAM or QPSK. If the rate of retransmissions is low, a switch is made to a modulation scheme of higher capacity, such as from QPSK to 16-QAM or 64-QAM (66). Preferably, both the retransmission rate and other link quality measurements coming from the receiver, such as BER or BLER (62) are used for the decision. The decision limits are preferably set by the system operator.

[0021] For OFDMA, the occurrence of retransmissions is

used to monitor the channel quality of each sub-channel. If the retransmission rate or the retransmission rate/

If the link quality for a particular subchannel indicates poor quality, that subchannel may be selectively cancelled from the OFDM frequency set (64) to exclude subchannels of such poor quality in the future. If the retransmission rate or the retransmission rate/link quality indicates high quality, a previously cancelled subchannel may be re-added to the OFDM frequency set (66).

[0022] The use of the occurrence of repeat broadcasts as a

The basis for AMC allows flexibility to tune the modulation and coding scheme to the average channel conditions for each user. In addition, the retransmission rate is insensitive to measurement error and reporting delay from the subscriber unit 16.

[0023] The uplink ARQ 20 is in its nature the

Downlink ARQ 10 is similar and consists of a subscriber unit 26 in which packets are transmitted from a higher layer ARQ transmitter 28a to a physical layer ARQ transmitter 26a. The message is sent to the base station antenna via a switch 26d, a subscriber antenna 25 and an air interface 24. The AMC controller can also vary the modulation and coding scheme using the retransmission statistics of a channel.

[0024] The ARQ receiver 22a of the physical layer determines similarly to

the receiver 16a of Fig. la, whether the message has an acceptable error rate and

must be resent. The acknowledge transmitter reports a status to the subscriber unit 26, causing the transmitter 26a to retransmit or, alternatively, to delete the original message temporarily stored at the transmitter 26a in preparation for receiving the next messages from the higher layers 28. Successfully received packets are sent to the network 24 for further processing.

[0025] Although this is not shown for reasons of clarity,

the system is preferably used for an HSDPA application in a BFWA system, although other implementations may be used. The BFWA system may use a frequency division duplex or time division duplex SC-FDE or OFDMA scheme. In such a system, the base station and all subscribers are at stationary locations. The system may comprise a base station and a large number of subscriber units. Each subscriber unit may serve a large number of users, for example in one building and in several neighboring buildings.

These applications typically require large bandwidth due to the large number of end users at a subscriber unit location.

[0026] A PHY-ARQ used in such a system is for the higher

Layers transparent, such as the medium access controllers (MACs). Consequently, PHY-ARQ can be used together with ARQs of a higher layer, such as Layer 2. In such cases, PHY-ARQ reduces the additional retransmissions of the higher layer ARQs.
[0027] Fig. 3 is an illustration of an N-channel stop and wait

Architecture for a PHY ARQ 30. The physical layer ARQ transmit function 38 may be located at the base station, the subscriber unit, or both, depending on whether downlink, uplink PHY ARQs, or both are used. Data blocks 34a arrive from the network. The network blocks are queued 34 for transmission over the data channel 41 of the air interface 43. An N-channel sequencer 36 sends data from the blocks sequentially to the N transmitters 40-1 through 40-n. Each transmitter 40-1 through 40-n is associated with a transmit sequence on the data channel 41. Each transmitter 40-1 to 40-n performs FEC coding and provides an ECS for the block data to generate packets for AMC modulation and transmission in data channel 41. The FEC encoded ECS data is stored in a buffer of the transmitters 40-1 to 40-n for possible repeated transmission. In addition,

Control information is sent from the PHY-ARQ transmitter 38 to synchronize reception, demodulation and decoding at the receivers 46-1 to 46-n. [0028] Each of the N receivers 46-1 to 46-n receives the packet in its

assigned time slot. The received packet is sent to a corresponding hybrid ARQ decoder 50-1 to 50-n (50). The hybrid ARQ decoder 50 determines the error rate, such as BER or BLER, for the received packet. If the packet has an acceptable error rate, it is released to the higher layers for further processing and an ACK is sent by the ACK transmitter 54. If the error rate is unacceptable or no packet was received, no ACK is sent or a NAK is sent. Packets with unacceptable error rates are buffered at the decoder 50 for potential combination with a retransmitted packet. [0029] One approach to combining packets under the

Use of turbo codes is as follows. When a turbo coded packet is received with an unacceptable error rate, the packet is retransmitted to support a code combination. The packet containing the same data is encoded differently. To decode the packet data, both packets are processed by the turbo decoder to recover the original data. Since the second packet has a different encoding, its soft symbols are mapped to different points in the decoding process. Using two packets with different encodings brings coding diversity and transmission diversity to improve the overall BER. Another approach is to transmit the identical signal. The two received packets are combined using a maximum ratio combination of symbols. The combined signal is then subsequently decoded.

[0030] The ACK for each receiver 46-1 to 46-n is sent in a

Fast Feedback Channel (FFC) 45. Fast Feedback Channel 45 is preferably a low latency channel. For a time division duplex system, the ACKs may be sent in idle periods between uplink and downlink transmissions. FFC 45 is preferably a low speed, high bandwidth CDMA channel that overlays other in-band transmissions. The FFC CDMA codes and modulations are selected to minimize interference with other in-band transmissions. To increase the capacity of such an FFC 45, multiple codes may be used.

[0031] The ACK receiver 56 detects the ACKs and displays the corresponding

Transmitters 40-1 through 40-n indicate whether the ACK was received. If the ACK was not received, the packet is retransmitted. The retransmitted packet may use a different modulation and coding scheme as directed by the AMC controller 12c, 26c. When the ACK is received, transmitters 40-1 through 40-n clear the previous packet from the buffer and accept a following packet for transmission.
[0032] The number of transmitters and receivers N depends on various

Design considerations such as channel capacity and ACK response time. For the preferred system described above, a two-channel architecture with even and odd transmitters and receivers is preferably used.

[0033] The PHY-ARQ method of the preferred embodiment provides a

7db gain in signal-to-noise ratio (SNR) compared to a system using only a higher layer ARQ. This is achieved by operating at higher block error rates (BLER) (5-20% BLER) and using smaller block sizes for Layer 1 than would be practical with a higher layer ARQ alone. The requirement for a lower SNR enables: increased capacity by switching to higher order modulation using an adaptive modulation and coding (AMC) scheme; reduced customer premise equipment (CPE) cost by using lower order RF (radio frequency) components in the PHY ARQ which compensates for the reduced implementation power; increased downlink range which increases the cell radius; reduced downlink power in the base station (BS) to minimize cell-to-cell interference; and increased power amplifier degradation when using a multiple carrier technique.

Claims (1)

A high speed downlink packet access (HSDPA) base station with a physical layer automatic repeat request (ARQ) mechanism, the base station comprising: a transmitter with: - a physical layer transmitter for receiving data, for formatting the received data into packets, each packet having its own coding/data modulation, the own coding/data modulation using an HSDPA modulation method and forward error correction (FEC) coding; for transmitting the packets and for retransmitting packets in response to not receiving a corresponding acknowledgement for a particular packet; and - an Adaptive Modulation and Coding (AMC) controller for collecting retransmission statistics and adjusting its own coding/data modulation using the collected statistics.