Patent Publication Number: US-8116283-B2

Title: Reverse link automatic repeat request

Description:
CROSS-REFERENCES 
     Claim of Priority Under 35 U.S.C. §119 
     The present application is a continuation of U.S. application Ser. No. 10/305,338, filed Nov. 26, 2002, entitled “REVERSE LINK AUTOMATIC REPEAT REQUEST, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     The present invention relates generally to wireless communications, and more specifically to sending data in a wireless communication system. 
    
    
     BACKGROUND 
     The cdma2000 High Rate Packet Data Air Interface Specification published as TIA/EIA/IS-856, and more specifically the QUALCOMM CDMA2000 1xEV-DO version, is an air interface optimized for the delivery of high-speed wireless data to mobile and fixed terminals. 
     In 1xEV-DO, an access terminal (AT) is a device with a radio modem and a data interface that allows the user to access a packet data network through the 1xEV-DO access network. An AT is analogous to a Mobile Station in IS-41 cellular network. 
     An Access Network (AN) is network equipment providing data connectivity between a packet-switched data network, for example, the Internet, and the ATs. As referred to herein, the AN can comprise a modem pool transceiver alone, and, additionally, other network equipment, for example, a modem pool controller. 
     The forward link (FL) refers to communications from the AN to the AT. Correspondingly, the reverse link (RL) refers to communications from the AT to the AN. 
     Multiple ATs can be communicating within a sector covered by a single BTS. The AN is limited by a received threshold power. The received power is a function of the number of ATs in the sector and the data rate of each AT in the sector. An AT transmitting at high data rate transmits signals at a higher power than an AT transmitting at a low data rate. When an AN approaches its received threshold power, the AN can send a message on the reverse link activity channel to all ATs in the AN&#39;s sector to reduce transmit rate. The transmitted signals of an AT can cause channel interference for a particular AT. Multiple ATs transmitting signals can cause even greater channel interference for the particular AT. 
     A need therefore exists for a method and apparatus for reducing the channel interference in a sector serving multiple ATs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the transmission of signals between an AT and an AN; 
         FIG. 2  illustrates a method of data transmission by the AT; 
         FIG. 3  illustrates a method of data transmission by the AN; and 
         FIG. 4  illustrates a block diagram of an AT and an AN in a wireless communication system. 
     
    
    
     DETAILED DESCRIPTION 
     In a code division multiple access (CDMA) system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” (IS-95) data packets can be retransmitted on the FL. A technique for FL transmission is described in U.S. patent application No., “Method and Apparatus for High Rate Packet Data Transmission,” Ser. No. 08/963,386, filed Nov. 3, 1997. For example, a data packet can comprise a predetermined number of data units, with each data unit identified by a sequence number. Upon incorrect reception of one or more data units by a mobile station, the mobile station can send a negative acknowledgment (NACK), on the RL ACK channel, indicating the sequence numbers of the missing data units for retransmission from the base station. The base station receives the NACK message and can retransmit the data units received in error. 
     No such acknowledgement or retransmission exists for data transmission in the RL in a CDMA system. 
     The transmission unit of the physical layer of 1x-EVDO is a physical layer packet. Data is contained in a physical layer packet. In the RL, physical layer packets are contained in frames. Frames can be of 26.66 milliseconds (ms) in duration. A frame can comprise of 16 slots, with each slot 1.66 ms in duration. 
     Data is modulated at varying data rates in the Reverse Traffic Channel. The data rates can be 9.6 kilobits per second (kbps), 19.2 kbps, 38.4 kbps, 76.8 kbps, and 153.6 kbps. At data rates less than 76.8 kbps, data can be repeated in a frame. For example, at 9.6 kbps, data can be sent in the first two slots of a frame, and the same data repeated 7 times in the next 14 slots of a 16-slot frame; at 19.2 kbps, the data can be sent in the first 4 slots of a frame, and then repeated 3 times in the next 12 slots of a 16-slot frame; at 38.4 kbps, the data can be sent in the first 8 slots of a frame, and then repeated once in the next 8 slots of a 16-slot frame. Although data is not repeated in a frame for a data rate of 76.8 kbps, redundancy is provided by encoding, such as Turbo encoding, as encoding provides redundancy for data at other data rates. 
     The repetition of data within in a frame, and the redundancy provided by encoding, can advantageously be used to reduce the transmission of redundant data by individual ATs. By reducing the amount of redundant data transmitted by an individual AT in a sector, the channel interference in a sector serving multiple ATs can be reduced. 
     For data rates less than 153.6 kbps, the 16-slot frame can be divided into 16/N sets of slots before transmission by the AT, where N can be 1, 2, 4, or 8. For example, when N equals 2, the AT can transmit the first half of the frame (8 slots) to the AN and hold the second half of the frame in a queue for possible re-transmission of the data depending upon the successful decoding of the data of the first half of the frame as received by the AN. 
     Automatic repeat request (ARQ) refers to a protocol in which the receiver asks the transmitter to resend data. Upon successful decoding of the first half of the frame, the AN can send an acknowledge message (ACK) to the AT to indicate that the AN has successfully decoded the data received in the first half of the frame. Upon unsuccessful decoding of the first half of the frame, the AN can send a negative acknowledge message (NAK) to the AT to indicate that the AN has not successfully decoded the data received in the first half of the frame. 
     ACK is a message transmitted to indicate that some data has been received correctly. Typically, if the sender does not receive the ACK message after some predetermined time, or receives a NAK, the original data will be sent again. 
     NAK is a message transmitted to indicate that some data has been received incorrectly, for example the data may have a checksum error. An alternative to sending a NAK is to use only ACK messages, in which case the non-receipt of an ACK after a certain time is counted as a NAK. As used herein, NAK refers to the receipt of a NAK message or the non-receipt of an ACK. 
     The ACK or NAK can be sent over a FL ACK channel. In 1xEV-DO, the FL ACK channel can be introduced as a new MAC subchannel. The existing FL Medium Access Control (MAC) channel of 1xEV-DO includes a subchannel referred to as a Reverse Link Power Control (RPC) channel. The RPC channel uses binary phase-shift keying (BPSK) modulation for sending an RPC bit that requests the ATs in the sector to increase or decrease their power. The FL ACK channel can use BPSK modulation on the phase orthogonal to the phase of the RPC channel. 
     Other techniques can be used to create the FL ACK channel. There currently are 64 MAC subchannels. Some of these subchannels can be used for the FL ACK channel. In this example, the modulation phase of the FL ACK channel do not have to be orthogonal to the RPC channels. 
     When the AT is in a soft handoff, the ACK bit can be transmitted from only a serving sector or from all sectors in the AT&#39;s Active Set. 
       FIG. 1  illustrates the transmission of signals between an AT  102  and an AN  104  using an ARQ. 
     The AT sends a first set of slots  106  of packet n. In this example, the first set of slots comprises 8 slots. The first set of slots  116  of packet n are received by the AN. After the first set of slots  106  of packet n are sent by the AT, the AT gates off at  108  for a predetermined period of time after sending the first set of slots  106  of the first packet and before sending a next set of slots. In this example, the AT gates of for a duration of 8 slots. 
     Before sending the next set of slots, the AT receives an acknowledgement signal, ACK or NAK, from the AN, that indicates whether the first set of slots  116  of packet n has been successfully decoded or unsuccessfully decoded. The acknowledgement signal informs the AT whether to resend the data, for example, by sending a second set of slots of packet n containing redundant data, or to send new data, for example, a first set of slots from a packet n+1. 
     In this example, an ACK  122  is sent by the AN to indicate that the first set of slots  116  of packet n has been successfully decoded. The AT receives the ACK, indicating that the first set of slots  110  of packet n+1 can be sent by the AT. AT  102  sends the first set of slots  110  of packet n+1 and gates off at  112  for 8 slots. The AN receives the first set of slots  118  of packet n+1. 
     Before sending a next set of slots  114 , the AT receives an acknowledgement signal, ACK or NAK, from the AN that indicates whether the first set of slots  118  of packet n+1 has been successfully decoded or unsuccessfully decoded. In this example, the AN sends a NAK  124 . Based upon receipt of the NAK, the AT resends the data by sending a second set of slots  114  of packet n+1 containing redundant data. The AN receives the second set of slots  120  of packet n+1. The AN can combine the data contained in the first set of slots  118  of packet n+1 with the redundant data contained in the second set of slots  120  of packet n+1 and attempt to decode the data contained in the combined sets of slots. With the addition of the redundant data, the likelihood of successfully decoding the data is increased. 
     One of ordinary skill in the art will appreciate that the channel interference of the system can been reduced by gating off after transmission of data, and gating on after an acknowledgement signal has been received. In this example of two sets of 8 slots, the channel interference can potentially be halved. 
       FIG. 2  illustrates a method of data transmission by the AT. At  202 , the AT sends the first set of slots of a first packet to the AN. At  204 , the AT gates off for a predetermined period of time before sending a next set of slots. One of ordinary skill in the art will appreciate that when the packet is divided into more than two sets of slots, for example, four sets of slots of four slots each, additional transmissions of first sets of packets can be performed before proceeding to  204 . 
     At  206 , the AT determines whether an ACK or a NAK from the AN has been received by the AT. In this example, the ACK indicates that the first set of slots of the first packet has been successfully decoded by the AN, and the NAK indicates that the first set of slots of the first packet has not been successfully decoded by the AN. 
     When the ACK has been received, the AT sends the first set of slots of a second packet at  210 . When the NAK has been received, the AT sends a second set of slots of the first packet at  208 . 
     The preceding example is for a packet that is divided into two sets of eight slots. One of ordinary skill in the art will appreciate that a packet can be divided into more than two halves. 
     For example, if the packet is divided into four sets of slots of four slots each, the AT can send a first set of slots of a first packet at  202 , and then the AT can send a first set of slots of a second packet at  202 . When an ACK is received by the AT corresponding to successful decoding of the first set of slots of the first packet by the AN at  206 , the AT can send a first set of slots of a third packet at  210 . When a NAK is received by the AT corresponding to unsuccessful decoding of the first set of slots of the first packet by the AN at  206 , the AT can send a second set of slots of the first packet at  208 . 
     Furthermore, when an ACK, corresponding to successful decoding of the first set of slots of the second packet by the AN (not shown), is received by the AT, the AT can either send (a) a first set of slots of a fourth packet provided that the first set of slots of the first packet was also successfully decoded, or (b) the first set of slots of the third packet provided that the first set of slots of the first packet was unsuccessfully decoded. The AT can then gate off. 
     When a NAK is received by the AT corresponding to unsuccessful decoding of the first set of slots of the second packet by the AN (not shown), the AT can send the second set of slots of the second packet. 
     The process can be continued similarly for other sets of slots. 
     Additionally, the AT can sequentially send the first set of slots of the first packet, the first set of slots of the second packet, the first set of slots of the third packet (not shown), and the first set of slots of the fourth packet (not shown), at  202 , before gating off. The AN can receive the first three set of slots before sending an acknowledgement signal for the first set of slots of the first packet. When an ACK is received by the AT corresponding to successful decoding of the first set of slots of the first packet by the AN at  206 , the AT can send a first set of slots of a fifth packet at  210 . When a NAK is received by the AT corresponding to unsuccessful decoding of the first set of slots of the first packet by the AN at  206 , the AT can send the second set of slots of the first packet at  208 . 
     Furthermore, when an ACK is received by the AT corresponding to successful decoding of the first set of slots of the second packet by the AN, the AT can send a first set of slots of a sixth packet (not shown). When a NAK is received by the AT corresponding to unsuccessful decoding of the first set of slots of the second packet by the AN, the AT can send the second set of slots of the second packet (not shown). 
     The process can be continued similarly for other sets of slots. 
     Moreover, one of ordinary skill in the art will appreciate that other combinations of sending slots by the AT and sending acknowledgment signals by the AN can be made and fall within the scope of the appended claims. 
       FIG. 3  illustrates a method of data transmission by the AN corresponding to the method of data transmission by the AT shown in  FIG. 2 . 
     At  302 , the AN receives the first set of slots of the first packet. At  306 , the AN attempts to decode the first set of slots of the first packet. Although the attempt to decode  306  is shown after the receipt of the first set of slots of the second packet, the AN can attempt to decode the first set of slots of the first packet before or after the receipt of the first set of slots of the second packet, or in parallel with the receipt of the first set of slots of the second packet. 
     At  308 , the AN determines whether the first set of slots of the first packet has been successfully decoded. When the AN is successful in decoding the first set of slots of the first packet, the AN sends an ACK to the AT at  310 , and receives the first set of slots of the second packet at  312 . When the AN is unsuccessful in decoding the first set of slots of the first packet, the AN sends a NAK to the AT at  314 , receives the second set of slots of the first packet at  316 , and attempts to decode the combination of the first set of slots of the first packet and the second set of slots of the first packet at  318 . 
     The preceding example is for a packet that is divided into two sets of eight slots. One of ordinary skill in the art will appreciate that a packet can be divided into more than two halves as described with reference to  FIG. 2 . 
     The timing of the sending of the ACK or the NAK can be independent of the number of sets of slots received. The AN can send the ACK or the NAK at a time after decoding a corresponding received set or sets of slots, for example, at  308 , and before the AT needs the acknowledgement signal to make the determination to send a particular set of slots at  206 . The timing of the sending of the ACK or the NAK can be independent of the interleaving of the sets of slots. 
       FIG. 4  illustrates a block diagram of an AT  102  and an AN  104  in wireless communication system. Conventional hardware and software techniques can be employed by the AT and the AN. Both the AT and the AN can comprise a logic and processing unit  402  or  404 , respectively, and a radio frequency unit  406  and  408 , respectively, for carrying out the above-described functionality. For example, one of ordinary skill in the art will appreciate that the radio frequency units can send and receive signals between the AN and AT, and that the logic and processing units can perform the logical operations and processing of signals. 
     Although the foregoing description was made with specific reference to the 1xEV-DO communication system, the reverse link automatic repeat request can also be utilized in other CDMA systems. 
     The previous description of the disclosed embodiments is provided to enable a person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.