A communication system is formed, at a minimum, by a transmitter and a receiver interconnected by a communication channel. The transmitter forms at least a portion of a sending station, and the receiver forms at least a portion of a receiving station. Communication signals transmitted by the sending station are transmitted upon the communication channel to be received by the receiving station. Information contained in the communication signals transmitted by the sending station is recovered once received at the receiving station. In a digital communication system, information to be communicated to the receiving station is digitized. The digitized information is then used to form the communication signal.
In some conventional, digital communication systems, the communication signals are transmitted in bursts. Blocks of digital information are communicated by a sending station to a receiving station during transmission of the bursts upon the communication channel.
In a non-ideal communication system, the communication signal is distorted during its transmission upon the communication channel. Because of such distortion, when the communication signal is received at the receiving station, the received signal differs somewhat from the communication signal transmitted from the sending station. If the communication channel is of poor quality, and the amount of distortion is significant, the informational content of the communication signal cannot be properly recovered at the receiving station.
In a digital, radio communication system, for instance, multi-path and Rayleigh distortion is sometimes introduced upon the communication signal as it is transmitted on the communication channel from the sending station to the receiving station.
Various schemes have been developed to better ensure that the informational content of the communication signal transmitted upon a non-ideal channel can be recovered at a receiving station. Several of such schemes utilize a feedback arrangement in which the receiving station reports back to the sending station whether the informational content of the received signal can be adequately recovered.
Several of such schemes have been implemented in a digital communication system in which blocks of information are communicated in bursts. The receiving station determines whether a burst of the block of information received at the receiving station is of an acceptable quality level. Indications of the determinations, forming feedback information, are returned to the sending station by way of a feedback channel. The indication may also be communicated back to the sending station implicitly. That is to say, lack of transmission of feedback information from the receiving to the sending station may be indicative of successful recovery of the informational content of the block of digital information. Or, the feedback information may be provided to the sending station in a manner other than by way of a direct feedback channel extending directly to the sending station. The indication may alternately be communicated back to the sending station indirectly. For example, the sending station might measure the channel conditions and decide that the probability of the receiving station successfully decoding the block of digital information to be so low that retransmission of the block of digital information would be justifiable.
Most simply, the receiving station simply detects whether the received signal burst is of an acceptable quality level. If the signal quality level of the received signal burst is not good enough to permit recovery of the informational content thereof, the receiving station merely requests the sending station to retransmit the block of digital information in a subsequent burst. Such a scheme is sometimes referred to as an ARQ (automatic request) scheme.
Retransmission of the block of digital information can be repeated in successive bursts until the block of digital information is received at the receiving station with at least a minimum quality level. The receiving station makes the determination of the quality of the received block of digital information responsive, for example, to detection of an error detecting code, knowledge of the communication channel upon which the block of information is transmitted, or some other suitable scheme.
The block of digital information can also be transmitted using an error protecting code, such as type I hybrid ARQ (Automatic Repeat Request) scheme, a general channel decoding technique. When the receiving station receives the block of digital information, the received block is decoded by a decoder to extract the informational content from the received signal. A determination is made if the informational content of the block of digital information can be recovered with at least an acceptable quality level. Because of the error protection, the block of digital information is better able to be recovered even if transmitted upon a communication channel of lowered quality.
Furthermore, if the decoder has the ability to exploit, not only the values of the received symbols, but also reliability information on the symbols, so-called soft information, the performance will increase substantially. Decoder circuitry at the receiving station capable of utilizing soft information is referred to as a soft input decoder.
In some other schemes, such as a type II and type III, hybrid ARQ (Automatic Repeat Request) scheme, blocks of digital information determined by the receiving station to be corrupt, i.e., to be of poor quality, are not merely discarded. Instead, the corrupt blocks are combined with subsequently-transmitted blocks of digital information. Information contained in previously-transmitted blocks is accumulated and forms accumulated knowledge. Such accumulated knowledge can be used to facilitate recovery of subsequently-transmitted blocks. Thereby, "accumulated knowledge" results, the number of times in which the blocks of digital information must otherwise be retransmitted decreases. The advantages of utilization of accumulated knowledge permitted by the combining of successive retransmissions of the blocks of digital information occur with increased amounts of information contained in the blocks of digital information.
This is advantageous as the probability of correct decoding generally increases if the accumulated knowledge is used during the decoding rather than if only the latest retransmitted block is utilized. Decoder circuitry at the receiving station capable of utilizing soft information is referred to as a soft input decoder.
In another scheme, a sequence formed of other symbols created from the same block of digital information is instead transmitted. For instance, if the first sequence is formed of an original set of parity symbols (possibly including uncoded information symbols of the block of digital information) and retransmission is requested, additional numbers of parity symbols are transmitted by the sending station in the retransmitted sequence. The receiving station accumulates the received symbols of the various transmissions and the symbols accumulated during the various transmissions are utilized together during a decoding process.
A conventional block or convolutional low rate coding process can be performed to form an encoded signal. Then, the encoded signal is "punctured." That is, selected symbols of the coded block are marked so as not to be transmitted. Only the "unpunctured" symbols, i.e., symbols which have not been marked, are transmitted. While the coded signal is weakened by transmitting only some of the encoded symbols, a higher effective coding rate is possible for a given encoder structure. If the informational content of the block of digital information can not be recovered with a desired level of quality from the symbols first received at the receiving station, additional coded portions of the signal are requested by the receiving station to be transmitted thereto. Some of the previously-punctured, and not-yet sent, symbols are thereafter transmitted by the sending to the receiving station. The receiving station utilizes both the previously-transmitted symbols and the newly-transmitted symbols. If additional retransmissions are required, yet additional symbols are subsequently transmitted to the receiving station. Such a scheme is referred to as a successive redundancy transmission scheme.
A successive redundancy transmission scheme, designed to be able to cope with adverse channel conditions, however, requires a complex decoding process to be performed to recover the informational content of the block of digital information. And, if the block of digital information is coded using a convolutional coding scheme, the decoding required at a receiving station to decode the information is complex irrespective of the amount of puncturing of the code.
When using a punctured convolutional code, a set of received redundant symbols that would, by themselves, cause a decoding error to occur still cooperate in the same way to counteract any error correcting attempts supported by additional redundant bits.
When the channel conditions of the communication channel extending between the sending and receiving stations are of poor quality, conventional codes of moderate complexity do not perform well. High-complexity codes, which perform well at such conditions, however, add unnecessary complexity when the channel conditions are of good quality. That is to say, while complex coding is necessitated when the channel conditions are of poor quality levels, such channel coding is not necessary when the channel conditions are of good quality levels.
A manner by which to adaptively select the complexity of the coding of a block of digital information to be transmitted between a sending and a receiving station would therefore be advantageous. When channel conditions are of good quality, only limited portions of a coded signal would have to be transmitted to a receiving station to permit recovery of the informational content thereof. And, when channel conditions are of poor quality, additional portions of the coded signal could be transmitted, better to ensure that the informational content of the block of digital information can be recovered at the receiving station.
It is in light of this background information related to digital communication systems that the improvements of the present invention have evolved.