Abstract:
Systems and methods of information transmission are provided. An endpoint receiving packets with non-voice information transmits negative and positive acknowledgements over a dedicated channel in an uplink subframe. When the endpoint receives packets with voice information, the endpoint transmits negative acknowledgements over a shared channel, but does not transmit positive acknowledgements.

Description:
BACKGROUND OF THE INVENTION 
     Communication systems rely upon a wireless or wired transmission medium for exchanging communications between endpoints. Information transmitted across the transmission medium may be subject to impairments, and accordingly may not be correctly received or may not be received at all. Various techniques are employed to ensure that transmitted information is correctly received, including error correction encoding, power control, packet retransmission, and the like. 
     One technique that employs a combination of error correction encoding and packet retransmission is known as hybrid automatic retransmission request (HARQ). HARQ involves a combination of forward error correction encoding (FEC) and automatic retransmission requests (ARQ) such that an endpoint sends either an acknowledgement (ACK) or negative acknowledgment (NACK) for each received packet. When a NACK is transmitted, the original packet is FEC encoded and retransmitted to the endpoint. The endpoint attempts to recover the original packet information by using a combination of the original packet and the retransmitted packet. 
     SUMMARY OF THE INVENTION 
     HARQ is typically implemented to improve radio link reliability and cell-edge coverage. However, due to the need for very low latency, voice communications typically do not employ retransmission techniques. Instead, voice communications typically employ error protection techniques such as power control and/or modulation and coding schemes (MCS). 
     It has been recognized that some communication protocols (e.g., WiMAX) could implement HARQ and provide the low latency necessary for voice communications. HARQ, however, consumes a large amount of bandwidth when used in connection with the transmission of voice information. For example, in a WiMAX network a dedicated HARQ feedback channel is provided in the uplink subframe for each communication channel. Because the ACK/NACK packets are typically 6-8 bytes and voice packets are typically 20-30 bytes, implementing HARQ for voice communications introduces more than a 20% overhead to support that necessary feedback signaling. This additional overhead reduces the amount of network capacity available for the actual user data, and accordingly limits the number of simultaneous voice communication sessions that can be supported by an access point. 
     Exemplary embodiments of the present invention involve systems and methods for transmission of information for a call between a first and second endpoint. Specifically, exemplary embodiments of the present invention employ a shared channel to provide retransmission request feedback information to an access point for packets including voice information in the payload portion of the packet. An endpoint receiving the packet with voice information in the payload portion attempts to decode the packet. If the packet is not decodable, then the endpoint transmits a negative acknowledgement over a shared channel. If the packet is decodable then the endpoint does not transmit an acknowledgement to the access point. Accordingly, the access point will retransmit the packet when a negative acknowledgement is received over the shared channel and will transmit a next packet with voice information in the payload portion when a negative acknowledgement is not received. 
     In accordance with one aspect of the present invention, an access point transmits a first packet that includes voice information in a payload portion of the packet to a second endpoint. The access point determines that the first packet was not decodable when a negative acknowledgement is received over a shared channel. The access point transmits a second packet when a negative acknowledgement is received, wherein the second packet is a retransmission of the first packet. 
     In accordance with another aspect of the present invention a second endpoint receives a first packet that includes voice information in a payload portion of the first packet. The second endpoint determines whether the first packet is decodable and transmits a negative acknowledgement over a shared channel when it is determined that the first packet was not decodable. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a block diagram of an exemplary access point and endpoint in accordance with the present invention; 
         FIG. 2A  is a call flow diagram for transmission of packets including non-voice information in a payload portion of the packet in accordance with exemplary embodiments of the present invention; 
         FIG. 2B  is a block diagram of an exemplary frame in accordance with the present invention; 
         FIG. 3  is a call flow diagram for transmission of packets including voice information in a payload portion of the packet in accordance with exemplary embodiments of the present invention; 
         FIG. 4  is a flow diagram of an exemplary method for an access point in accordance with the present invention; and 
         FIG. 5  is a flow diagram of an exemplary method for an endpoint in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram of an exemplary access point and endpoint in accordance with the present invention. Access point  110  communicates with endpoint  150  using wireless signals over an air interface. Access point  110  includes memory  112 , processor  120 , one or more transceivers  130  and one or more antennas  140 . Access point  110  can be any type of access point, including, but not limited to, a base station. Endpoint  150  includes memory  152 , processor  160 , one or more transceivers  170  and one or more antennas  180 . Endpoint  150  can be any type of endpoint, including, but not limited to, a wireless telephone, a personal digital assistant (PDA), a laptop computer, a desktop computer and/or the like. As will be described in more detail below, exemplary embodiments of the present invention employ different types of HARQ processing for voice information and non-voice information. Accordingly, processors  120  and  160  include different types of logic for processing the different types of information. 
     Processors  120  and  160  can be any type of processor, including a field programmable gate array (FPGA), application specific integrated circuit (ASIC) and/or microprocessor. When either of the processors is a microprocessor, the associated logic can be processor executable code loaded from the associated memory. Memories  112  and  152  can be any type of memory, including random access memory (RAM), read only memory (ROM), flash memory, one or more hard disks, and/or the like. 
     It should be recognized that  FIG. 1  is a simplified block diagram and that access point  110  and endpoint  150  can include additional components beyond those illustrated. For example, endpoint  150  can include one or more display devices, one or more input devices, microphones, speakers and/or the like. Additionally, although  FIG. 1  illustrates only a single endpoint communicating with the access point, the access point can support more than one endpoint. Moreover, the system can include more than one access point. 
       FIG. 2A  is a call flow diagram for transmission of packets including non-voice information in accordance with the present invention. Specifically, each packet includes overhead information in a header portion of the packet and non-voice information in the payload portion of the packet. Access point  110  employs logic  124  to support this call flow and endpoint  150  employs logic  164  to support this call flow. The transmission of packets with non-voice information in the payload portion involves sending a NACK or ACK over a dedicated channel for each transmitted or retransmitted packet. Accordingly, logic  124  determines whether ACK or NACK feedback messages are received and controls the transceiver to retransmit the same packet or to transmit a next packet. Logic  164  determines whether the packets received by the endpoint are decodable and controls the associated transceiver to transmit ACK and NACK feedback messages. 
     The call flow of  FIG. 2A  assumes that the first packet is not correctly received (i.e., is not decodable) and that endpoint  150  transmits a NACK to access point  110  over a dedicated channel (step  205 ). Accordingly, access point  110  retransmits the first packet, endpoint  150  attempts to decode the information by using a combination of the first packet and the first retransmission of the packet, and sends a NACK to access point  110  over a dedicated channel indicating that the endpoint  150  still cannot decode the information in the packet (step  210 ). Access point  110  then sends a second retransmission of the first packet, endpoint  150  attempts to decode the information using the first packet and the first and second retransmissions of the packet, and sends an ACK over a dedicated channel to access point  110  indicating that the packet information was correctly decoded (step  215 ). Access point  110  then sends the next, second packet to endpoint  150  (step  220 ). If the information in the packet cannot be correctly decoded, a similar process to that described above will be performed. The retransmitted packets are not necessarily identical to the originally transmitted packet. For example, the retransmitted packets can be forward error correction (FEC) encoded, while the originally transmitted packet is not. 
       FIG. 2B  is a block diagram of an exemplary frame in accordance with the present invention. The exemplary frame is a WiMAX frame in which access point  110  uses the downlink (DL) subframe to transmit control and other information to endpoints, and the endpoints use the uplink (UL) subframe for transmitting control and other information to the access point. Those skilled in the art will recognize that WiMAX frames are transmitted using orthogonal frequency division multiplexing (OFDM). As illustrated in  FIG. 2B , the uplink subframe includes a portion for transmitting ACK and NACK messages as part of a HARQ procedure. Conventional HARQ techniques employed a dedicated channel in the uplink sub-frame assigned to each endpoint for transmitting ACK and NACK messages to the access point and the endpoint must transmit an ACK or a NACK to the access point in response to receipt of each packet in the assigned dedicated channel. As will be described in more detail below, exemplary embodiments of the present invention employ shared channels for endpoints to transmit HARQ feedback information for packets that include voice information in the payload portion of the packet and only transmit NACKs for these packets. Packets with voice information in the payload portion that are successfully decoded do not result in the transmission of an ACK message. 
       FIG. 3  is a call flow diagram for transmission of packets including voice information in accordance with the present invention. Specifically, each packet includes overhead information in a header portion of the packet and voice information in the payload portion of the packet. The voice information can be formatted in accordance with, for example, Voice over IP (VoIP). Access point  110  employs logic  122  to support this call flow and endpoint  150  employs logic  162  to support this call flow. Accordingly, logic  122  determines whether NACK feedback messages are received and controls the transceiver to retransmit the same packet or to transmit a next packet. Logic  164  determines whether the packets received by the endpoint are decodable and controls the associated transceiver to transmit NACK feedback messages over a shared channel. 
     Access point  110  transmits the first packet to endpoint  150 , and endpoint  150  indicates that the packet could not be successfully decoded by transmitting a NACK over a shared channel in an uplink subframe of the same frame in which the packet was received (step  305 ). Access point  110  then retransmits the first packet to endpoint  150  which attempts to decode the information using both the first packet and the retransmission of the first packet, and when this cannot be successfully decoded, endpoint  150  transmits a NACK message back to access point  110  over a shared channel in an uplink subframe of the same frame in which the packet was received (step  310 ). Access point  110  then retransmits the first packet for a second time, and endpoint  150  attempts to decode the information using both the first packet and the first and second retransmissions of the first packet (step  315 ). Because the endpoint  150  can successfully decode the information, the endpoint does not transmit an ACK message. Accordingly, access point  110 , by not receiving a NACK message in the corresponding uplink subframe in which the second retransmission of the packet was transmitted, transmits the second packet to endpoint  150  (step  320 ). Endpoint  150  then attempts to decode the second packet and if it cannot, will repeat similar steps to those discussed above. The retransmitted packets are not necessarily identical to the originally transmitted packet. For example, the retransmitted packets can be forward error correction (FEC) encoded, while the originally transmitted packet is not. 
     As discussed above, exemplary embodiments of the present invention employ a shared channel for transmitting NACK messages for packets carrying voice information in the payload portion. The shared channel can be randomly selected from a group of shared channels that are used by other endpoints. Because the NACK messages are transmitted over randomly selected shared channels, NACK messages from two or more endpoints may collide when transmitted over the same shared channel at the same time. The collision ratio can be reduced to an acceptable level by selecting the correct number of shared NACK feedback channels based on the currently supported voice connections. The number of required shared feedback channels can be configured based on the number of active voice connections and voice packet error rate (PER) in the air interface. The optimum number of shared feedback channels is configured to balance HARQ feedback overhead and contention probability. Increasing the number of shared feedback channels reduces the probability of contention for transmitting NACK packets (improves customer experience), but increases the signaling overhead. 
     Assume, for example, that 200 voice calls are established, and each voice call transmits one packet every 20 msec. Also assume radio frame duration is 5 msec. Therefore, 200*5/20=50 voice packets are transmitted in each frame. Assuming that the PER is on average 10%, then on average only five endpoint would transmit a NACK message in each frame using shared feedback channels. In such a scenario, 7 to 10 shared feedback channels would be sufficient for HARQ packet transmission. 
       FIG. 4  is a flow diagram of an exemplary method for an access point in accordance with the present invention. Initially, access point  110  receives a packet for transmission in a downlink subframe (step  405 ), and determines whether a payload portion of the packet includes voice information (step  410 ). 
     When the payload portion of the packet does not contain voice information (“Non-Voice Information” path out of decision step  410 ), then the access point transmits the packet in the downlink subframe (step  415 ) and sets a timer (step  420 ). The timer is used by the access point in case uplink channel impairments prevent an ACK or NACK message from being received by the access point. Accordingly, while the timer is running the access point determines whether a NACK message has been received (step  425 ), an ACK message has been received (step  435 ), or whether the timer has expired (step  445 ). When a NACK message has been received (“Yes” path out of decision step  425 ), access point  110  selects the same packet for retransmission in the downlink subframe (step  430 ) and transmits the packet in the downlink subframe (step  415 ). When an ACK message has been received (“Yes” path out of decision step  435 ), the access point selects the next packet for transmission in the downlink subframe (step  440 ) and transmits the packet in the downlink subframe (step  415 ). When an ACK or NACK message has not been received (“No” paths out of decision steps  425  and  435 ) and the timer has expired (“Yes” path out of decision step  445 ), the access point selects the same packet for retransmission in the downlink subframe (step  450 ) and transmits the packet in the downlink subframe (step  415 ). In this latter case, the failure to receive an ACK or NACK message is considered as the endpoint not being able to correctly decode the transmitted packet. 
     Returning again to step  410 , when the received packet includes voice information in the payload portion (“Voice Information” path out of decision step  410 ), access point  110  transmits the packet in the downlink subframe (step  455 ) and determines whether a NACK message has been received in the uplink subframe of the same frame in which the packet was transmitted to the endpoint (step  460 ). When the access point does not receive a NACK message in the uplink subframe of the same frame in which the packet was transmitted to the endpoint (“No” path out of decision step  460 ), the access point assumes that the packet was correctly decoded and selects the next packet with voice information in the payload for transmission in the downlink subframe (step  465 ) and transmits the packet in the downlink subframe (step  455 ). When a NACK message has been received in the uplink subframe (“Yes” path out of decision step  460 ), access point  110  retransmits the packet in the downlink subframe (step  470 ) and waits to determine whether the uplink subframe includes a corresponding NACK message (step  460 ). 
       FIG. 5  is a flow diagram of an exemplary method for an endpoint in accordance with the present invention. When the endpoint receives a packet in the downlink subframe (step  505 ), the endpoint determines whether the packet includes voice information in the payload portion (step  510 ). When the packet does not include voice information in the payload portion (“Non-Voice Information” path out of decision step  510 ), the endpoint determines whether the packet has been received correctly and can be decoded (step  515 ). When the packet cannot be correctly decoded (“No” path out of decision step  515 ), the endpoint transmits a NACK message in a dedicated channel in the uplink subframe (step  520 ). If, however, a packet was received correctly (“Yes” path out of decision step  515 ), the endpoint transmits an ACK message in the dedicated channel in the uplink subframe (step  525 ) and processes and decodes the received packet (step  530 ). When the packet is a retransmitted packet the decoding can employ both the original packet and the retransmitted packet. 
     Returning again to step  510 , when the received packet includes voice information in the payload portion (“Voice Information” path out of decision step  510 ), the endpoint determines whether the packet was received and correctly decoded (step  535 ). When the packet can be correctly decoded (“Yes” path out of decision step  535 ), the endpoint processes the received packet (step  540 ) and waits to receive the next packet in the downlink subframe. When the packet is a retransmitted packet the decoding can employ both the original packet and the retransmitted packet. If, however, the packet was not correctly decoded (“No” path out of decision step  535 ), the endpoint selects a shared channel in the uplink subframe in the same frame in which the packet was received (step  545 ) and transmits a NACK message in the selected shared channel in the uplink subframe (step  550 ). 
     By implementing the HARQ techniques of the present invention an initial packet error rate (PER) can be reduced from 10% to between 1 and 2%. Furthermore, the exemplary HARQ techniques of the present invention introduce 5-10 times less overhead to support packet retransmissions compared to conventional HARQ techniques, while still providing similar performance to these conventional techniques. For example, assuming a downlink PER of 10% and 50 concurrent voice calls, the present invention would require, for example, only 7 HARQ feedback channels to support the voice calls, whereas conventional techniques that employ dedicated channels would require 50 HARQ feedback channels. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.