Patent Application
20090327443
In a communication system, an access node may comprise a device or set of devices that serve as a gateway that client nodes may use for exchange of information with a network. This information may take the form of voice, data, or some other media. The client nodes may be wireline or wireless components.
In some types of communication systems, an access node explicitly acknowledges each packet of information that the access node receives from a client node. For example, if the access node receives ten voice packets from a client node, the access node may transmit ten acknowledgement messages, one for each voice packet, to the client node. These voice packets may include a representation of a voice signal, error detection coding, error correction coding, and possibly other information. By processing information in acknowledgment messages, the client node is able to determine if one or more of the voice packets it transmitted were not received by the access node. In response to such a determination, the client node may retransmit one or more voice packets to the access node.
There are at least two ways that a packet can be “lost” between a client node and the access node. The packet may be truly lost if it never arrives at the access node. Alternatively, the packet may arrive at the access node, but when the access node checks error detection coding in the packet, it finds that the packet has been corrupted. In this case, the access node may discard the packet, so the packet is effectively lost.
Such an acknowledgment-based transmission scheme is typically referred to as automatic repeat request (ARQ). For example, in one variation of ARQ referred to as stop-and-wait, a client node transmits a packet to an access node, then waits to receive an acknowledgement from the access node before transmitting another packet. If the client node does not receive an acknowledgement within a pre-determined time period or if the client node receives a negative acknowledgement, the client node may retransmit the packet.
Hybrid ARQ is a variation that can be applied to any type of ARQ. Using hybrid ARQ, a client node may divide a packet into multiple sub-packets, and transmit each sub-packet individually. These sub-packets may include redundant information for purposes of forward error correction. Thus, if the access node checks the error detection code of a sub-packet and finds that the sub-packet has been corrupted, the access node may be able to rebuild the correct packet from the redundant information. Nonetheless, some sub-packets may not include redundant information. For example, a given sub-packet may include only information and error detection codes, while subsequent sub-packets may include redundant information relating to the information in the given packet. Communication systems with lossy channels, such as wireless networks, may benefit from hybrid ARQ.
It is common for an access node to serve multiple client nodes, perhaps tens or hundreds of client nodes. Additionally, is it common for certain types of networks to support broadcast or multicast channels that can be used to transmit a packet such that the packet can be received by multiple destinations. Thus, it is possible for some access nodes to combine multiple acknowledgement messages destined for multiple clients into a single aggregated acknowledgement message (AAM). The access node then transmits this AAM to some or all of the client nodes using a broadcast or multicast channel. A client node that receives an AAM may parse the message to find the acknowledgement data pertaining to itself. The client node may discard acknowledgment data in the AAM that pertains to other client nodes.
By combining individual acknowledgment messages into a single AAM, network capacity is conserved, as the overhead associated with transmitting a large number of acknowledgment messages is eliminated. This technique is especially beneficial to an access node that is receiving packets from a potentially large number of client nodes.
Disclosed herein are methods and systems for improving aggregated acknowledgement schemes using ARQ-based or ARQ-like protocols. While these methods and systems are directed to high-speed wireless networks, they are applicable to any type of network that supports aggregating acknowledgements.
For purposes of simplicity, the terms “packet” and “sub-packet” will be used somewhat interchangeably. While the specific embodiment of hybrid ARQ divides packets into sub-packets and then transmits and acknowledges these individual sub-packets, the methods and systems described herein are not limited to operating on sub-packets, and may operate at the level of packets instead.
In order to improve the efficiency of encoding and processing AAMs, client nodes may be divided into groups based on an identifier assigned to each client that is unique for all client nodes served by a given access node or radio access network. The unique identifiers of clients within a group may be sequentially ordered and contiguous with respect to one another. When an access node receives a packet from each client in any given group during a predetermined time period, the access node may respond to all of the clients of the group with an AAM that contains acknowledgement data for each client in the group. If the access node does not receive a packet from all of the clients in the group within the predetermined time period, the access node may instead acknowledge each received packet individually.
The AAM may be encoded efficiently so that it requires very few bits of overhead to indicate the group and/or the associated client nodes to which it pertains. Furthermore, the acknowledgement data may be organized according to the ordering of the unique identifiers of the client nodes in the group so that each client node may efficiently find its respective acknowledgement data in an AAM. Advantageously, in CDMA networks, the acknowledgement data may be transmitted without CDMA spreading, thus effectively broadcasting or multicasting the AAM to a plurality of client nodes using only a single message.
An AAM may also contain control bits transmitted from an access node to a group of client nodes. Alternatively, control bits for multiple client nodes can be combined into a separate aggregated control message (ACM) and either broadcast or multicast to all client nodes in the group independently from the AAM. In cases where each of the client nodes in a given group are to be transmitted distinct control bits, these bits may represented as a series of control bits in an ACM. In cases where all of the client nodes in a given group are to be transmitted identical control bits, these bits may represented as a single set of control bits in an ACM.
These and other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it should be understood that the foregoing overview is merely exemplary and is not intended to limit the scope of the invention as claimed.
While the methods and systems described herein are potentially applicable to any network that supports broadcasting or multicasting of acknowledgement messages, the following descriptions are directed to wireless networks for purposes of example and illustration.
BTSs 112, 114, 116 radiate to define the wireless coverage areas. Each wireless coverage area may provide air interface access to client nodes 106, 108, 110 and any other client nodes served by the wireless coverage area. A single BTS 112, 114, 116 may define one or more wireless coverage areas. The air interface may include forward links from a BTS to client nodes 106, 108, 110 and reverse links from client nodes 106, 108, 110 to a BTS. Client nodes 106, 108, 110 exchange signaling, voice, data, video, or other media with the BTS through the forward and reverse links. In this regard, client nodes 106, 108, 110 may use the wireless coverage areas defined by BTSs 112, 114, 116 to communicate with one or more endpoints, e.g., other client nodes, e-mail servers, world wide web servers, gaming servers, media servers, media gateways, or location-based services, via a packet-switched network (e.g., the Internet 124 or private IP network 126), and/or a circuit-switched network, such as the public switched telephone network (PSTN) 128. For example, BTSs 112, 114, 116 may be communicatively coupled to a base station controller (BSC) 120. BSC 120 may, in turn, be communicatively coupled to packet-switched networks 124, 126 via a packet data serving node (PDSN) 118. Alternatively or additionally, BSC 120 may be communicatively coupled to PSTN 128 via a mobile switching center (MSC) 122.
Although
Regardless of the composition of the RAN, at least one entity within the RAN serves as an access node by acknowledging packets and/or sub-packets transmitted by the client nodes in the RAN's coverage area. The acknowledging entity may be a BTS 112, 114, 116, a BSC 120, or some other device.
The entity or entities of the RAN preferably include at least one processor, data storage, and program instructions stored in the data storage and executable by the processor to carry RAN functions described herein. Similarly, a client node preferably includes at least one processor, data storage, and program instructions stored in the data storage and executable by the processor to carry out client node functions described herein. Furthermore, the client nodes and the RAN may operate in accordance to various types of wireless protocols, such as Code Division Multiple Access (CDMA), Worldwide Interoperability for Microwave Access (WIMAX), Universal Mobile Telecommunications System (UMTS), or other protocols now known or later developed.
CDMA wireless networks are exemplary types of communication systems in which the methods and systems herein can be implemented. Commercially deployed cellular CDMA systems are generally based on communication standards developed by the Third Generation Partnership Project 2 (3GPP2). 3GPP2 has supported the evolution and advancement of CDMA voice and data systems from low-speed systems to advanced integrated voice and data systems. One of the most recent commercially available CDMA systems is Evolution Data Only, Revision A (EVDO Rev. A), wherein a combination of CDMA and time division multiple access (TMDA) technologies can be employed to provide capacity on the order of megabits per second to client nodes. While the discussion herein is directed to EVDO Rev. A CDMA systems, the methods and procedures introduced may be applied to older or newer CDMA systems.
In a CDMA network, multiple communication channels can utilize the same carrier frequency without interference. Each channel is assigned a unique code from a set of mathematically orthogonal codes. Each code is a series of bits that has a very small or negligible cross-correlation with all other codes in the set. A code is applied to messages transmitted on the channel at a “chip rate” of some multiple of the messages' bit rate. For example a CDMA system with a chip rate of four means that four code bits will be applied to each bit of a message. A message is transmitted on a CDMA channel by multiplying repeating patterns of a channel's code with the message. Thus, the message is “spread” according to the code and the system's chip rate. Messages on all channels are additively combined and then transmitted simultaneously on the same carrier frequency.
In process 200, a message M1 is depicted as three bits long for simplicity of illustration. In practice, a message transmitted using CDMA is typically much longer than three bits and represents information, such as voice or data, being transmitted to or received from a client node. Modified message M1′ is created by spreading message M1 according to the chip rate of four. In other words, modified message M1′ represents each bit of message M1 being replaced by four bits of the same value. Orthogonal code W1 is a repeating four bit sequence. Modified message M1′ is multiplied bitwise with code W1 to create spread code C1. Similar procedures are illustrated in process 210 for a message M2 and an orthogonal code W2, and in process 220 for a message M3 and an orthogonal code W3. It can readily be demonstrated that orthogonal codes W1, W2, and W3 are in fact orthogonal with one another, as the bitwise cross-correlation of any two of these codes is zero. In process 230, the three resulting spread codes, C1, C2, and C3, are added bitwise to create a signal S. Signal S is then transformed into a bit stream and modulated onto a carrier frequency for transmission to the intended receivers of messages M1, M2, and M3.
A receiver may decode a message from a particular CDMA channel by multiplying the received signal with the channel's code, then adding the resulting bits in each chip period.
In CDMA networks, voice or data packets transmitted on a reverse link traffic channel typically include error correcting codes. These codes insert redundant information into a voice or data bit stream so that the access node has a higher probability of being able to properly decode the packet. Popular types of error correcting codes include Reed-Solomon codes and turbo codes, and CDMA systems may use these or other types of error correcting codes.
Typically, a client node will apply an error correcting code to a voice or data packet, then divide the packet into a number of sub-packets. Preferably, the client node transmits each sub-packet independently to the access node according to ARQ procedures.
Due to the fact that redundant information is encoded in each sub-packet, it is possible that the access node may be able to decode the entire transmitted packet before it receives all of the sub-packets for that packet. For example, assuming that the number of sub-packets per packet is four, a client node may divide a packet into sub-packets 1 through 4 and then may transmit them according to ARQ procedures. After receiving each sub-packet, the access node will attempt to decode the entire original packet. It may be possible for the access node to do so after it receives sub-packets 1 and 2. In this case, the access node will instruct the client node to not transmit sub-packets 3 and 4, thus avoiding unnecessary utilization of network capacity. Such a benefit may be referred to as early termination gain.
Hybrid ARQ in CDMA networks facilitate this process. An access node uses three bits transmitted in the forward direction to indicate whether early termination gain can be applied to a packet. The H-ARQ bit (not to be confused with the overall process of hybrid ARQ) is used to acknowledge (ACK) or negatively acknowledge (NACK) the first three sub-packets of a packet. The L-ARQ bit is used to ACK or NACK the fourth sub-packet of a packet. The P-ARQ bit is used to ACK or NACK the full packet.
In first series 300, client node 304 transmits the first sub-packet, SP1(1) 310. Access node 306 responds with a H-ARQ ACK 315 to indicate that SP1(1) 310 was successfully received. Similarly, client node 304 transmits SP1(2) 320 and access node 306 responds with an H-ARQ ACK 325, and client node 304 transmits SP1(3) 330 and again, access node 306 responds with an H-ARQ ACK 335. Client node 304 transmits SP1(4) 340 and access node 306 responds with an L-ARQ ACK 345 to indicate that SP1(4) 340 is the final sub-packet of the packet. After transmitting L-ARQ ACK 345, the access node reassembles the four sub-packets into a packet. This process is successful, so access node 306 transmits a P-ARQ ACK 347 to indicate that access node 306 has successfully received the entire packet.
In second series 302, client node 304 transmits the first sub-packet, SP2(1) 350. Access node 306 responds with an H-ARQ ACK 355 to indicate that SP(1) 350 was successfully received. However, after client node 304 transmits the second sub-packet, SP2(2) 360, access node 306 responds with an H-ARQ ACK 365, followed by a P-ARQ ACK 370. The P-ARQ ACK 370 indicates to client node 304 that access node 306 was able to decode the packet based on a combination of SP2(1) 350 and SP2(2) 360 and that client node 304 does not need to transmit any further sub-packets.
In both first series 300 and second series 302, access node 306 may transmit ACKs and NACKs at predetermined offsets from when client node 304 transmits the associated sub-packet. Thus, client node 304 can use these offsets to match ACKs and NACKs to the sub-packets these ACKs and NACKs are intended to acknowledge or negatively acknowledge, respectively.
While the example in
In many CDMA systems, the orthogonal codes used in the forward direction are Walsh codes, while the orthogonal codes used in the reverse direction are pseudo noise (PN) codes. Client nodes are each assigned a unique medium access control identifier (MAC_ID), corresponding to their assigned Walsh code. In EVDO Rev. A, MAC_IDs comprise an integer between 0 and 127, with MAC_IDs 6-63 and 72-127 being available to be assigned to client nodes. A MAC_ID may serve as a dynamically assigned address for the client node. Thus, there typically is a unique mapping in each of the forward and reverse directions between an orthogonal code, a MAC_ID, and a client node. However some orthogonal codes and/or MAC_IDs may be shared by more than one client node.
CDMA standards allow a base station to be deployed to support multiple sectors. For example, instead of using an omni-directional, 360-degree antenna, a base station can use three antennae, each transmitting and receiving signals over sectors of 120 degrees. Each sector may support a number of traffic channels and a number of signaling channels. The traffic channels may support bearer traffic and the signaling channels may facilitate functions including paging, handoffs, power control, and synchronization.
One of the forward direction signaling channels in EVDO Rev. A is the MAC channel. Each client node is assigned a MAC channel and a MAC_ID associated with this channel. The MAC channel supports hybrid ARQ signaling, and further comprises the reverse power control (RPC) sub-channel and the data rate control lock (DRCLock) sub-channel. The RPC sub-channel may be used to control the power that the client node uses to transmit in the reverse direction. The DRCLock sub-channel may be used to indicate the quality of the reverse link DRC channel.
After the bit streams are converted in signal point mapping functions 410, 420, 450, and 465, they are passed through channel gain functions 415, 425, 455, and 470, respectively. Outputs from channel gain functions 415 and 425 are combined by time-division multiplexor 430, then sent to CDMA spreader 435, where the bitstream is spread using the Walsh code associated with the target client node's MAC_ID. Similarly, outputs from channel gain functions 455 and 470 are combined by time-division multiplexor 475, then sent to CDMA spreader 480, where the bitstream is spread using the Walsh code associated with the target client node's MAC_ID. The CDMA spreaders, 435 and 480, use 128-bit Walsh codes in order to support the 128 possible MAC_IDs in EVDO Rev. A. The 128 bits of output from the two CDMA spreading functions, 435 and 480 are combined in a Walsh chip level summer 485 and the resulting 256-bit output is modulated onto the in-phase (I) and quadrature-phase (Q) components of the carrier frequency.
While the methods and systems described above work reasonably well when a relatively small number of client nodes are served by an access node, as the number of client nodes per access node increases, the CDMA MAC channel hybrid ARQ scheme becomes less efficient. For example, suppose that 56 client nodes are transmitting packets on their respective reverse traffic channels. For each sub-packet of each packet from each client node, the access node has to transmit at least one hybrid ARQ ACK or NACK message. Under the scheme described above, each hybrid ARQ message requires 256 bits. Thus, if each client node transmits a sub-packet at approximately the same time, the access node must transmit 14,336 bits of hybrid ARQ information across 56 different ARQ messages. However, hybrid ARQ information can be more efficiently encoded.
Instead of transmitting individual ARQ messages to each client node, the access node may aggregate the ARQ information for some number of client nodes into a single, common ARQ message that is then broadcast or multicast to these clients. An additional goal is to be able to format the aggregated acknowledgement message (AAM) in such a way that it uses a small number of bits in an arrangement that is simple to decode. As a result, network capacity is conserved while client nodes may spend relatively few CPU cycles processing each AAM. Note that while the AAM is referred to as an aggregated acknowledgement message, it may contain indications of negative acknowledgements as well.
The unique identifiers assigned to the client nodes are preferably CDMA MAC_IDs, but other types of unique identifiers may also be used. Preferably the unique identifiers are sequentially numbered and client nodes are assigned to groups based on the numbering of their unique identifiers. For example, as noted above, EVDO Rev. A client nodes may have MAC_IDs assigned from the ranges 6-63 and 72-127. These MAC_IDs may be dynamically assigned when a client node becomes associated with an access node, or may be assigned by some other means. The MAC_IDs can also serve as unique client node identifiers and as the basis for dividing the client nodes into groups. One such division can be based on assigning client nodes with contiguously-numbered MAC_IDs to groups of a particular size; for instance, an access node may be configured to assign 5 client nodes to a group. Thus, the access node may assign the client nodes with MAC_IDs 6-10 into a first group, the client nodes with MAC_IDs 11-15 into a second group, and so on.
At step 512, an access node detects that it has received, within a given time period, a packet from each of the client nodes in a given group. These packets may be sub-packets transmitted according to EVDO Rev. A hybrid ARQ procedures, or may be transmitted or formatted according to other procedures. At step 514, the access node transmits a single AAM that contains acknowledgement data associated with all of the client nodes in the group. This transmission may be broadcast to all client nodes or multicast to just the client nodes in the group. Preferably, the AAM includes (1) a preamble indicating that the AAM contains aggregated acknowledgements, (2) a group identifier indicating the group of client nodes to which the acknowledgements relate, and (3) a series of acknowledgements, each acknowledging a sub-packet transmitted by one of the client nodes within the given group. The group identifier is preferably a compact representation of a group of client nodes rather than a list of the unique client node identifiers of each client node in the group. The series of acknowledgements is preferably organized according to the sequential numbering of the client nodes' MAC_IDs.
At step 516, a client node receives the AAM and the client node uses its unique identifier to locate its respective acknowledgement data within the AAM. Preferably, the client node (1) parses the preamble to determine that the message contains aggregated acknowledgements, (2) parses the group identifier and determines that the AAM is directed to the group that the client node was assigned to, and (3) uses its MAC_ID to determine which bits in the AAM contains the client node's acknowledgment data.
At step 614, a plurality of packets from at least two client nodes assigned to a given group are received within a given time period. These packets may be sub-packets transmitted according to EVDO Rev. A hybrid ARQ procedures, or the packets may be transmitted or formatted according to other procedures. At step 616, a determination is made as to whether packets were received from all of the client nodes assigned to the given group. If this is not the case, then, at step 618, individual acknowledgement messages are transmitted to each client node from which a packet was received. If this is the case, then at step 620 a single AAM is transmitted to all client nodes assigned to the given group. This transmission may be broadcast to all client nodes or multicast to just the client nodes in the given group. As described above, the AAM preferably includes (1) a preamble indicating that the AAM contains aggregated acknowledgements, (2) a group identifier indicating the given group of client nodes to which the acknowledgements relate, and (3) a series of acknowledgements, each acknowledging a packet transmitted by one of the client nodes within the given group.
In accordance with method 500 and method 600, it may be advantageous to group client nodes based on a sequential ordering of their unique identifiers. Doing so allows AAMs to be efficiently encoded by access nodes and efficiently decoded by client nodes. By encoding AAMs using relatively few bits to indicate the group of client nodes that the AAMs are directed to and the acknowledgment data for these client nodes, the AAMs will be relatively simple for client nodes to parse. Thus, client node CPU utilization may be reduced. Furthermore, if a client node is a battery-powered wireless device, reducing the number of bits that the client node needs to receive and process may extend the client node's battery life.
Exemplary embodiments of AAM encodings are presented in
The mapping data depicted in
In
After the bit streams are converted in signal point mapping functions 910 and 925, they are passed through channel gain functions 915 and 930, respectively. Outputs from channel gain functions 915 and 930 are combined by time-division multiplexor 935, then transmitted to a CDMA spreader 940, where the signal is spread using the Walsh code associated with the target client node's MAC_ID. CDMA spreader 940 uses 128-bit Walsh codes in order to support the 128 possible MAC_IDs in EVDO Rev. A. The 128 bits of output from CDMA spreader 940 are modulated onto either the in-phase (I) or the quadrature-phase (Q) components of the carrier frequency. Preferably, the 128 bits of output are modulated onto the I component of the carrier frequency if the target client node's MAC_ID is even, and modulated onto the Q component of the carrier frequency if the target client node's MAC_ID is odd.
An AAM bit stream, preferably arranged according to AAM format 830 and comprising 128 bits, serves as input into ARQ channel gain function 945. The 128-bit output from ARQ channel gain function 945 is modulated onto either the I or Q components of the carrier frequency without being spread according to CDMA procedures. Preferably, the 128-bit output is modulated onto either the I or Q component of the carrier frequency based on which of these phase components is not being used by the output of CDMA spreader 940.
As described earlier, if each of 56 client nodes transmit a sub-packet at approximately the same time, the access node must transmit 14,336 bits of hybrid ARQ information on a MAC channel, across 56 different ARQ messages. Assume that, using method 900 to transmit AAM messages formatted according to AAM format 830, all 56 client nodes are assigned to the same group and all 56 client nodes transmit sub-packets at approximately the same time. Then, the 128-bit output of CDMA spreader 940 is transmitted on a MAC channel to each respective client node, but the 128-bit AAM message is transmitted on a MAC channel just once, and is preferably received by all 56 client nodes. Thus, the total number of bits required for transmitting acknowledgements is reduced to 7,296 bits through use of the AAM scheme introduced herein. However, significant reductions in the number of bits required for transmitting acknowledgements to client nodes can be achieved even if the client node group size is smaller than 56.
An AAM may also contain control bits transmitted from an access node to a group of client nodes. For example, the CDMA RPC bits and DRCLock bits for each client node in a given group of can be represented as a series of bits in binary AAM format 830. The control bits may appear before, after, or interleaved with ARQ bit encodings 870. Regardless of their exact position in an AAM, in an ACM that contains control bits, the control bits are preferably ordered according to the numbering of the client nodes' MAC_IDs. Thus, when receiving an AAM containing control bits, a client node will know where within the AAM to find its control bits.
Alternatively, control bits for multiple client nodes can be combined into a separate aggregated control message (ACM) and either broadcast or multicast to all client nodes in the group independently from the AAM. Similar to the methods and encodings described in
In cases where each of the client nodes in a given group are to be transmitted distinct control bits, these bits may represented as a series of control bits in an ACM. For example, the CDMA RPC bits and DRCLock bits for each client node in a given group of can be represented as a series of bits in a format analogous to binary AAM format 830. In an ACM that contains control bits, the control bits are preferably ordered according to the numbering of the client nodes' MAC_IDs. Thus, when receiving an ACM containing control bits, a client node will know where within the ACM to find its control bits.
In cases where all of the client nodes in a given group are to be transmitted identical control bits, these bits may represented as a single set of control bits in an ACM. All client nodes that receive such an ACM will preferably retrieve the same control bits from the ACM.
Furthermore, the AAM and ACM techniques described herein can operate independently of, or in conjunction with, one another. In other words, for a given group of client nodes, an access node may transmit (1) an AAM, (2) an ACM, (3) both an AAM and an ACM, or (4) neither an AAM nor an ACM.
Exemplary embodiments have been described above. Those skilled in the art will understand, however, that changes and modifications may be made to these embodiments without departing from the true scope and spirit of the invention, which is defined by the claims.
