HK1068473B - Method for physical layer automatic repeat request for a base station - Google Patents

Description

Method for automatic retransmission request of base station physical layer

Technical Field

The present invention relates to a wireless communication system. More specifically, the present invention relates to a modification of the system implementation using a physical layer (PHY) automatic repeat request (ARQ) scheme.

Background

Wideband fixed wireless access (BFWA) communication systems using single carrier-frequency domain equalization (SC-FDE) or Orthogonal Frequency Division Multiplexing (OFDM) methods are presented when using a High Speed Downlink Packet Access (HSDPA) application. Such applications will transmit downlink packet data at high speed. In BFWA, a building or group of buildings are interconnected by wireless or wired connections and operate as a single subscriber station. The data size requirements of such systems are so great that a single station multiple user requires a large bandwidth.

The proposed system uses a layer 2 automatic repeat request (ARQ) system. Packets that cannot be successfully transmitted to the user will be buffered and retransmitted from layer 2. The data blocks stored in layer 2 are typically large, received with a high signal-to-noise ratio (SNR), received by a low block error rate (BLER), and retransmitted on a regular basis. Additionally, layer 2ARQ signaling is typically slow, requiring large buffers and long retransmission intervals.

Disclosure of Invention

Therefore, it is desirable to have variations other than layer 2ARQ systems.

A physical layer automatic repeat request system includes a transmitter and a receiver. A physical layer transmitter at the transmitter receives data and formats the received data into packets with a particular coding/data modulation. The physical layer transmitter includes n channels for transmitting the packets and retransmitting packets in response to not receiving a corresponding acknowledgment for a given packet. An adaptive modulation and coding controller in the transmitter collects retransmission statistics and uses the collected statistics to adjust the particular coding/data modulation. The receiver has a physical layer n-channel receiver for receiving the packets. The receiver includes an n-channel split ARQ combiner/decoder for combining packet transmissions, decoding packets, and detecting packet errors. The receiver includes an acknowledgment transmitter for transmitting an acknowledgment for a packet when the packet has an acceptable error rate. The receiver includes an in-sequence (in-sequence) delivery component to deliver acceptable packets to higher layers.

To further illustrate the above objects, structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

Drawings

Fig. 1A and 1B are simplified block diagrams of downlink and uplink actual ARQ.

Fig. 2 is a flow chart of adaptive modulation and coding using retransmission statistics.

Fig. 3 is a block diagram of a multi-channel stop-and-wait architecture.

Detailed Description

Fig. 1A and 1B show a downlink physical ARQ10 and an uplink physical ARQ20, respectively.

The downlink actual ARQ10 includes a base station 12 that receives packets from a higher layer transmitter 14a located in the network 14. The packet from transmitter 14a is applied to ARQ transmitter 12a in base station 12. The ARQ transmitter 12a modulates data by a Forward Error Correction (FEC) coded data, an additional Error Check Sequence (ECS), directed by an Adaptive Modulation and Coding (AMC) controller 12c and using Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-ary quadrature amplitude modulation (i.e., 16-QAM or 64-QAM). Additionally, for Orthogonal Frequency Division Multiplexing (OFDMA), the AMC controller 12a may change the branch channels used to carry the packet data. The physical layer ARQ transmitter 12a transmits packets to the subscriber unit 16 via the radio interface 14 through a switch, propagator (circulator) or duplexer 12d and an antenna 13. If necessary, the transmitter 12a also temporarily stores the information to be retransmitted in a buffer memory included in the transmitter 12 a.

The subscriber unit 16 receives the packet at antenna 15. The packet is input to the physical layer ARQ receiver 16a through a switch, propagator or duplexer 16 d. The packet will be FEC decoded in receiver 16a and checked for errors using ECS. The receiver 16a will then control the acknowledgment transmitter 16c to Acknowledge (ACK) receipt of a packet with an acceptable error rate or request retransmission by preferably suppressing an acknowledgment signal or transmitting a Negative Acknowledgment (NAK).

The ACK is transmitted to the base station 12 by the ACK transmitter 16c via the switch 16b and the antenna 15. The ACK is sent to the antenna 13 of the base station 12 via the wireless interface 14. The received ACK is processed by an ACK receiver 12b in the base station. The ACK receiver 12b conveys the ACK/NAK to an Adaptive Modulation and Coding (AMC) controller 12c and to the transmitter 12 a. The AMC controller 12c will use the statistics of the received ACKs to analyze the channel quality connected to the subscriber unit 16 and may change the FEC coding and modulation techniques for subsequent information transmissions, as will be described in more detail below. If the subscriber unit 16 acknowledges the received packet, the original packet temporarily stored in a buffer memory is cleared upon receipt of the ACK at the base station 12 in preparation for the next packet to be provided for use.

If no ACK or a NAK is received, the physical layer transmitter 12a will again transmit the original information or a selectively modified version of the original information to the subscriber unit 16. If available, at the subscriber unit 16, the retransmission is combined with the original transmission. This technique will help receive a correct message by using data redundancy or selective retransmission combining. Packets with an acceptable error rate are sent to higher layers 16d for further processing. The acceptable received packets are delivered to the higher layer 16d in the same data order as the data provided to the transmitter 12a in the base station (i.e., in-sequence delivery). The maximum number of retransmissions is limited to a user-defined integer value, for example in the range of 1 to 8. When the maximum number of retransmissions has been attempted, the buffer memory is cleared for the next packet to be used. Decoding an acknowledgment using a small packet at the physical layer reduces transmission delay and message processing time.

Since PHYARQ occurs at the physical layer, the number of retransmissions, retransmission statistics, that occur for a particular channel can be used as a good metric for the channel quality. Using the retransmission statistics, the AMC controller 12c may change the modulation and coding scheme for the channel, as shown in fig. 2. Additionally, the retransmission statistics may combine other link quality metrics such as Block Error Rate (BER) and block error rate (BLER), with the AMC controller 12c determining the channel quality and determining whether a change in modulation and coding scheme is required.

To clarify the SC-FDE, retransmissions occurring on a particular channel may be measured to generate retransmission statistics (60). A decision as to whether to change the modulation scheme is implemented using retransmission statistics (62). If there is an excess of retransmissions, a more robust coding and modulation scheme is utilized (64) that typically reduces the data transmission rate. The AMC controller 12c may increase the spreading factor and use more codes to transmit the packet data. Alternatively or additionally, the AMC controller may switch from a high data throughput modulation scheme to a lower, such as 64-ary QAM to 16-ary QAM or QPSK. If the retransmission rate is lower, a higher capacity modulation scheme is switched, such as QPSK to 16-ary QAM or 64-ary QAM (66). The decision preferably utilizes both retransmission rates and other link quality metrics such as BER or BLER signaled by the receiver (62). The decision limits are set by the system operator.

For OFDMA, retransmission occurs to monitor the channel quality of each branch channel. If the retransmission rate or retransmission rate/link quality of a particular subchannel indicates poor quality, the subchannel may be selected to be deactivated (64) in the OFDM frequency set to preclude use of such poor quality subchannels for some future time period. If the retransmission rate or retransmission rate/link quality indicates high quality, a previously deactivated subchannel may be added back to the OFDM frequency set (66).

Using the occurrence of retransmissions as the basis for AMC, a per-user flexible matched modulation and coding scheme, and average channel conditions are provided. Additionally, the retransmission rate is insensitive to measurement errors and reporting delays from the subscriber unit 16.

The uplink ARQ20 is substantially similar to the downlink ARQ10 and includes a subscriber unit 26 in which packets from a higher layer ARQ transmitter 28a of the higher layer 28 are transmitted to the physical layer ARQ transmitter 26 a. Information is transmitted to the base station antenna 23 via the switch 26d, the user antenna 25 and the radio interface 24. AMC controller 26c may similarly change the modulation and coding scheme by using retransmission statistics for a channel.

A physical layer ARQ receiver 22a, similar to the receiver 16a of fig. 1a, determines whether there is an acceptable error rate and needs to be retransmitted. The acknowledgment transmitter will report status to the subscriber unit 26 to cause the transmitter 26a to retransmit, or alternatively to clear the original information temporarily stored at the transmitter 26a in preparation for receiving the next information from the higher layers 28. Successfully received packets are sent to the network 24 for further processing.

Although not shown for simplicity, the system is preferably used for an HSDPA application in a BFWA system, although other implementations may be used. The BFWA system may use frequency division duplex or time division duplex SC-FDE or OFDMA. In such a system, the base station and all users are located in fixed locations. The system may include a base station and a plurality of subscriber units. Each subscriber unit may serve multiple users, such as in a building or in multiple adjacent buildings. These applications will typically require a large bandwidth due to the large number of end users of a subscriber unit station.

A PHY ARQ configured in such systems is transparent to higher layers such as the Medium Access Controller (MAC). And (6) obtaining the result. PHY ARQ may be used in conjunction with higher layer ARQ, such as layer 2. In this case, the PHY ARQ will reduce retransmissions above the higher layer ARQ.

Fig. 3 is a diagram of an N-channel stop-and-wait structure for a PHY ARQ 30. The physical layer ARQ transmission function 38 may be located in the base station, the subscriber unit, or both, depending on the PHY ARQ using downlink, uplink, or both. The data block 34a is from the network. The network block is placed in a queue 34 for transmission on a data channel 41 of a wireless interface 43. An N-channel sequence 36 sequentially transmits the data of the blocks to the N transmitters 40-1 through 40-N. Each transmitter 40-1 to 40-n is associated with a transmission sequence in the data channel 41. Each transmitter 40-1 to 40-n is FEC encoded and provides block data ECS to generate packets for AMC modulation and transmission in a data channel 41. The FEC encoded/ECS data is stored in a buffer at the transmitters 40-1 to 40-n for possible retransmission. Additionally, control data is sent by the PHY ARQ transmitter 38 to synchronize reception, demodulation, and decoding at the receivers 46-1 through 46-n.

Each of the N receivers 46-1 through 46-N receives packets in its associated time slot. The received packets are sent to a respective hybrid ARQ decoder 51-1 to 50-n (50). The hybrid ARQ decoder 50 determines the error rate, such as BER or BLER, of the received packet. If the packet has an acceptable error rate, the packet is released to higher layers for further processing and an ACK is sent by ACK transmitter 54. If the error rate is unacceptable or not acceptable to any packet, no ACK will be sent or a NAK will be sent. Packets with unacceptable error rates will be buffered at decoder 50 and may be subsequently combined with a retransmission of the packet.

One method of combining packets using turbo codes (turbo codes) is as follows. If a turbo encoded packet is received with an unacceptable error rate, the packet data is retransmitted to facilitate code combining. Packets containing the same data are encoded differently. To decode the packet data, two packets are processed by the turbo decoder to recover the original data. Since the second packet has a different code, its soft symbols (soft symbols) are mapped to different points in the decoding scheme. Using two packets with different codes will increase the code separation and the transmission separation to improve the overall BER. In another approach the same signal is transmitted. The two received packets are combined using a maximum ratio of symbol combinations. The combined signal is then decoded.

The ACK for each receiver 46-1 through 46-n is transmitted in a Fast Feedback Channel (FFC) 45. The fast feedback channel 45 is preferably a low latency channel. For a time division duplex system, the ACK may be sent in the idle period between upstream and downstream transmissions. The FFC45 is preferably a low speed, high bandwidth Code Division Multiple Access (CDMA) channel that covers other co-band transmissions. The FFC CDMA code and modulation are selected to minimize interference to other co-band transmissions. To increase the capacity of such FFC45, multiple codes may be used.

The ACK receiver 56 detects the ACK and indicates whether the corresponding transmitter 40-1 to 40-n has received the ACK. If no ACK is received, the packet is transmitted again. The retransmitted packet may have a different modulation and coding scheme as directed by the AMC controllers 12c, 26 c. If an ACK has been received, the transmitters 40-1 through 40-n will clear the previous packet from the buffer and accept a subsequent packet for transmission.

The number of transmitters and receivers N is based on various design considerations such as channel capacity and ACK response time. For the preferred system described above, a 2-channel structure is preferably utilized with even and odd numbers of transmitters and receivers.

The PHY ARQ technique of the preferred embodiment provides a signal to noise ratio (SNR) gain of 7db compared to a system using only higher layer ARQ. This is because it operates at a higher block error rate (BLER) (5% to 20% BLER) and uses a smaller layer 1 block size than if the higher layer AQR were implemented alone. The reduced SNR requirement allows: capacity is increased by switching to higher level modulation using an Adaptive Modulation and Coding (AMC) technique; reducing Customer Premises Equipment (CPE) costs by using lower order RF (radio frequency) components and compensating for reduced implementation performance by PHY ARQ; increasing downlink range to enlarge the cell radius; reducing downlink power in a Base Station (BS) to minimize cell-to-cell interference; and increasing a Power Amplifier (PA) back-off when a multiple carrier technique is employed.

Although the present invention has been described with reference to the present specific embodiments, it will be recognized by those skilled in the art that the above embodiments are illustrative only, and various equivalent changes and modifications may be made without departing from the spirit of the present invention, and therefore, it is intended that all changes and modifications to the above embodiments be included within the scope of the present invention as defined in the appended claims.

Claims (9)

1. A method of adjusting data modulation at a base station, comprising:

receiving, at a transmitter, data from a higher layer automatic repeat request device for transmission;

formatting the received data into a plurality of packets for transmission, wherein each of the packets has a particular type of coding/data modulation;

by a physical layer automatic repeat request device:

transmitting the packet;

monitoring a return channel for receipt of an acknowledgment indicating that one of the packets has been received;

if an acknowledgment for a packet is not received, retransmitting the packet at the transmitter;

collecting statistics of retransmissions; and

each of the particular coding/data modulations are adjusted using the collected retransmission statistics.

2. The method of claim 1 wherein the special type of coding/data modulation is forward error correction.

3. The method of claim 2 wherein the packets are transmitted using an orthogonal frequency division multiplexing wireless interface and the special FEC coding/data modulation adjustments are performed in an OFDM set except for selected null subchannel.

4. The method of claim 1 wherein the packets are transmitted using a single carrier radio interface with a frequency domain equalization.

5. The method of claim 1 wherein the return channel is a fast feedback channel when the packet is transmitted using a code division multiple access wireless interface.

6. The method of claim 1, further comprising

In response to receiving a negative acknowledgment, a packet is identified as having an unacceptable error rate.

7. A method of adjusting data modulation at a base station, comprising:

receiving data from a higher layer automatic repeat request device;

formatting the received data into a plurality of packets for transmission over a wireless radio interface;

transmitting the formatted packet by using a first physical layer automatic retransmission request device; and

by a second physical layer automatic repeat request device:

receiving said data packets over said wireless interface, wherein each of said packets has a particular code/data modulation; and

for each of the received packets, a positive acknowledgment is generated and transmitted at the physical layer of the wireless interface when the received packet has an acceptable error rate.

8. The method of claim 7 wherein the positive acknowledgement message is transmitted on a fast feedback channel when the wireless interface is a code division multiple access.

9. The method of claim 7 further comprising transmitting a negative acknowledgement if the packet has an unacceptable error rate.