Patent Grant
7564827
Related subject matter is disclosed in commonly owned U.S. patent application Ser. No. 09/982,317, entitled “A Hybrid Transmission Method for Wireless Communications”, which was filed on Oct. 18, 2001 and is incorporated by reference herein in its entirety.
The invention relates generally to wireless communication systems and, more particularly, to methods used for retransmitting information in such systems in the presence of errors and low quality communication channels.
In wireless communication systems, an air interface is used for the exchange of information between a mobile (e.g., cell phone) and a base station or other communication system equipment. The air interface typically comprises a plurality of communication channels. As is well known, CDMA-based systems employ unique codes for multiplexing separate, simultaneous transmissions over a communication channel. In a CDMA-based system, transmission duration is fixed while the users share the resources in the code domain. In the well-known High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, for example, the available resources are shared between circuit switched voice and data users within a standard five 5 MHz channel bandwidth. HSDPA employs 2 millisecond fixed frames comprising three time slots of equal duration, e.g., 0.67 milliseconds. Due to the real-time nature of the voice traffic, the resources (e.g., power, codes, etc.) are first allocated to voice. After satisfying the needs of real-time services, the remaining resources are then shared among the data users preferably in a time-multiplexed fashion on a frame by frame basis. HSDPA users can also be code multiplexed within a frame to fully utilize the resources in the frame, e.g., if a single user cannot fully utilize the resources in a frame.
The quality of any one of the communication channels can vary depending on factors such as user location, user speed, interference from other cells, and so on. For example, a particular channel between a base station and a mobile may have an acceptable throughput at one instant in time and unacceptable throughput at another instant in time. As such, the information transmitted through a relatively low quality communication channel can be adversely affected to such an extent that the information contains errors when received. In wireless communication systems, information is channel coded to compensate for errors arising during transmission. However, channel coding alone is not always sufficient to compensate for errors. Accordingly, systems employ various link layer recovery protocols, including retransmission methods, to compensate for these errors.
One widely used retransmission method is Automatic Repeat Request (ARQ). ARQ is a method of confirming that information transmitted through a communication channel has been received without any errors. Receiving equipment sends a message to transmitting equipment acknowledging that the transmitted information was received without errors. If the transmitted information was received with errors, then the receiving equipment sends a message to request a retransmission. The transmitter can retransmit all or part of the previously transmitted information using the same or different channel coding. As is well known, incremental redundancy (IR) transmission and soft combining (also referred to as Hybrid ARQ) are used to improve the efficiency of ARQ. For example, when a lost data frame is retransmitted, the receiving equipment may combine the multiple received copies of the frame to increase the likelihood of correct decoding. Alternately, the transmitter may transmit additional parity information instead of retransmitting another copy of the lost frame.
One method for incremental redundancy operation where the transmission rates can be different for different incremental redundancy transmissions was disclosed in U.S. patent application Ser. No. 09/725,438, entitled “Rate Adaptation in a Wireless Communication System” and filed Nov. 29, 2000, the subject matter of which is incorporated herein by reference in its entirety. As described, the different transmission rates are obtained by varying the transmission time while keeping the information block size unchanged. The aforementioned reference also described a scheme in which the retransmissions only provide the additional redundancy needed to successfully decode the frame based on the quality of the previous incremental redundancy transmissions. For example, if the first transmission is performed over four (4) time slots, the retransmission can be performed over two (2) slots if the quality of the previous transmission required only two (2) slot transmission for successful decoding of the information block.
Disadvantageously, the adaptive incremental redundancy scheme in the prior art requires variable length transmissions that lead to a more complex system in terms of implementation and control signaling transmission. Moreover, the granularity that is provided by the time slot duration for transmission of redundancy in most practical systems is coarse and therefore does not promote an efficient use of the bandwidth.
The shortcomings of the prior art are overcome according to the principles of the invention in an adaptive incremental redundancy (i.e., Hybrid ARQ) method that avoids variable length transmissions (in time) by keeping fixed length frames (in time) and varying the number of codes to accurately match the desired redundancy. This involves parallel transmissions (in time) via code division multiplexing. The operation of adaptive Hybrid ARQ in the code domain also provides a much finer granularity in which to transmit the redundancy more efficiently.
In one illustrative embodiment of the invention, a method for retransmitting information in a communication channel having a plurality of fixed length frames each divided into a plurality of time slots of equal duration includes the step of code multiplexing a retransmission of a previous transmission within one of the fixed length frames using one or more of a plurality of codes. The number of codes used for the retransmission is variable based on the condition of the communication channel. As such, the adaptive Hybrid ARQ scheme according to the principles of the invention can use a different modulation, different coding, and a different number of codes for a retransmission than is used in the original transmission, but still use fixed length frames in doing so. Consequently, the complexity and other problems associated with variable length transmissions are avoided. Keeping fixed length frames also ensures easy migration to future versions of third generation (3G) wireless systems.
A more complete understanding of the present invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, with like elements referenced with like reference numerals, in which:
In the exemplary embodiment shown in
According to one exemplary embodiment, code multiplexing is used for transmitting information within fixed length data frames 101-103 in an adaptive incremental redundancy (i.e., Hybrid ARQ) scheme. Related subject matter dealing with a hybrid code and time multiplexing scheme for transmitting information within fixed length time slots is described in U.S. patent application Ser. No. (Das 12-16), entitled “A Hybrid Transmission Method for Wireless Communications”, the subject matter of which is incorporated herein by reference in its entirety.
As shown in
As previously described, retransmissions may be requested for certain transmissions that were received with errors. In the example shown in
According to the principles of the invention, code multiplexing allows for adaptive incremental redundancy within the fixed length frames 101-103. In particular, the number of codes can be varied depending on how much redundancy is needed for a particular transmission. For example, there could be changes in the quality of the communications channel (e.g., poorer quality) that warrant more redundancy in the retransmission, such as the case with user B in which twenty-one (21) codes are allocated for retransmission 116 in frame 103 even though the original transmission 112 only used six (6) codes. Conversely, there could be changes in the quality of the communications channel (e.g., better quality) that warrant less redundancy in the retransmission, such as the case with user D in which three (3) codes are allocated for retransmission 117 in frame 103 instead of the six (6) codes that were used in the original transmission 114.
Accordingly, a different coding rate can be obtained for retransmissions by changing the number of codes on retransmission. For example, the coding rate for user D's retransmission 117 on three (3) codes in frame 103 will be 2R, where R is the coding rate for user D's first transmission 114 in frame 101 on six (6) codes. If user D's original transmission 114 used quadrature phase shift keying (QPSK) over six (6) codes, then retransmission over three (3) codes using 16-QAM will provide the same coding rate since 16-QAM carries two times more bits per modulation symbol.
According to the principles of the invention, the number of codes can be changed at the time of retransmission to provide the desired redundancy for successful decoding of the data transmission. For example, user B's retransmission 116 uses twenty-one (21) codes while the original transmission 112 used six (6) codes and user D used three (3) codes on retransmission 117 for an original transmission 114 over six (6) codes.
For sake of brevity, only the differences between the embodiments shown in
As previously mentioned in the case of adaptive Hybrid ARQ, multiple transmissions may need to be sent to the same user in the same frame. Multi-level acknowledgement/negative acknowledgments (ACK/NACKs) will therefore need to be sent to the transmitter separately for all the transmissions within a frame. In one exemplary embodiment, when a simultaneous transmission and retransmission is received in the same frame (e.g., as with user B in frame 203), the receiver can change the format of the typical ACK/NACK message to make it a multi-level ACK/NACK. In one exemplary embodiment, multi-level ACK/NACK can be achieved by having a different repetition factor for the ACK/NACK bits. For example, if ‘1’ indicates an ACK and a “0” indicates a NACK in the normal signaling and is repeated n times then, in the case of an m-bit ACK/NACK, the bits will be repeated only (n/m) times. Since the transmitter is aware of the number of transmissions it performed in a given frame, it will interpret the ACK/NACK accordingly. For example, when one transmission is ACK'd and other NACK'd, the ‘10’ can be repeated as ‘1010101010’. In another exemplary embodiment, multi-level ACK/NACKs can be achieved by transmitting multiple ACK/NACK bits over separate code channels. Additionally, coding can be used to provide ACK/NACK reliability. Other alternatives will also be apparent to those skilled in the art and are contemplated by the teachings herein.
Referring to
In general, the foregoing embodiments are merely illustrative of the principles of the invention. Those skilled in the art will be able to devise numerous arrangements, which, although not explicitly shown or described herein, nevertheless embody those principles that are within the scope of the invention. For example, although the invention was described in the context of code multiplexing within fixed length frames, other techniques may also be suitable, e.g., frequency division multiple access (FDMA) using a variable number of frequencies, space division multiple access (SDMA) using a variable number of antennas, and so on. Accordingly, the embodiments shown and described herein are only meant to be illustrative and not limiting in any manner. The scope of the invention is limited only by the claims appended hereto.
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| Number | Date | Country |
|---|---|---|
| WO 0137506 | May 2001 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20030076783 A1 | Apr 2003 | US |
