Abstract:
A window error detector for a receiver capable of operating in a discontinuous transmit mode includes a soft decision decoder ( 210 ) producing soft output and generating window error signals. A detector ( 214 ) is coupled to the soft decision decoder for detecting a bad frame when the window error exceeds a bad frame threshold, wherein the threshold is altered based on the discontinuous transmit state. A turbo decoder ( 210 ) for a receiver includes a soft decision decoder and a window error detector ( 214 ) coupled to the soft decision decoder. The window error detector generates a bad frame indication. The turbo decoder stops iterative processing of the data associated with a window when the window error detector detects that the window does not result in a bad frame indication.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to bad frame detectors and turbo decoders. 
     BACKGROUND OF THE INVENTION 
     To maintain good audio quality in a radiotelephone, the radio receiver must pass to the speech decoder an indication of the quality of the speech frame. This binary indication must allow for the following considerations: a non-traffic channel input (noise or control channel) is always detected as bad; a high signal-to-noise ratio (SNR) traffic channel input is always detected as good; and a large majority of correctable traffic channel frames are detected as good. 
     By meeting these criteria, a high degree of muting and speech extrapolation by the speech decoder can be avoided. Additionally, a receiver meeting these requirements will operate to specification in a system such as the global system for mobile communication (GSM), IS-136, or IDEN, or other digital communication systems. However, it is recognized that it is difficult to meet these criteria. 
     One advantageous method of monitoring the speech frame quality uses window error detection in association with a soft decision decoder. For example, the soft decision output of a Viterbi equalizer can be used to detect a bad frame. U.S. Pat. No. 5,229,767, entitled DECODER FOR CONVOLUTIONALLY ENCODED INFORMATION, issued to Winter et al. on Jul. 20, 1993, discloses a decoder which detects a bad frame using the soft information from a decoder to determine whether or not to discard a frame. 
     One method of improving the performance of a receiver, is to provide so-called “turbo decoding”. A turbo decoder employs an iterative process which is repeated to provide improved decoding of a frame. A difficulty with these decoders is determining when to stop repeating the reiterative process. One technique that has been used to stop reiterative decoding is when the decoded values are no longer changing (i.e., when bits stop changing between a logic 1 and a logic 0). Another known method for determining when to stop the iterations of the turbo decoder uses a cyclical redundancy check (CRC). Bits are added to the frame to provide an error indication for turbo decoding. The reiterative process is finished when the error redundancy bits no longer indicate that there is an error. Yet another method uses a fixed number of repetitions (e.g., 10). Each of the above methods has a disadvantage. The use of turbo decoding CRC bits means that additional bits are added to the data frame for turbo decoding. This adds overhead to the data. The other two methods add overhead in the processing, in that many iterations are necessitated by the process itself. 
     Accordingly, there is need for improved decoding and error detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic in block diagram form illustrating a transceiver. 
     FIG. 2 is a circuit schematic in block diagram form illustrating a decoder. 
     FIG. 3 is circuit schematic in block diagram form illustrating a window error detector. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an improved window error detector and improved turbo decoder. The improved window error detector utilizes information relating to the discontinuous transmit state to alter a bad frame detector threshold. This allows for a tighter threshold in the discontinuous transmit mode and a looser threshold in a non-discontinuous transmit mode. 
     An improved turbo decoder takes advantage of the window error detector information to determine when to stop decoding iterations. In particular, the turbo decoder determines that it can stop iterative processing of the data associated with a window when the window error detector detects that the frame is not bad. This provides an indication of an acceptable decoded frame without the use of extra bits as would be required with CRC. Additionally, the iterative process is not repeated after it is no longer needed, as occurs with turbo decoders that stop only after a fixed number of iterations. The use of discontinuous transmit information also provides an advantage over those turbo decoders that continue iterations until the values no longer change, by providing an earlier cut off at a point when a frame can be decoded. 
     The turbo decoder is advantageous for a discontinuous transmit mode, when fewer frames are transmitted. The turbo decoder can work harder to insure that decodable frames are generated thereby. This insures that signals of improved quality are generated for the local speaker. 
     A transceiver  100  (FIG. 1) includes an antenna  102  for receipt of signals from a communication link  101 . The illustrated device is a transceiver for wireless communication, but it will be recognized that the device may find application in other communication devices, such as satellite phones, landline phones and modems, or any other device that communicates digital information. A receiver  103  receives data from the antenna, processes it, and outputs information to the digital signal processor  104 . The digital signal processor converts the data sequence using a program stored in memory  106 . Received speech signals are converted into analog signals in digital to analog converter  107 , and amplified in amplifier  108  to drive speaker  109 . 
     Signals to be transmitted are detected by microphone  120 , amplified in amplifier  121 , and converted into digital signals in analog to digital converter  122 . The digital signals are input to digital signal processor  104 . A transmitter  124  generates signals for transmission over the air link  101 . 
     The transceiver  100  can operate in a continuous transmit mode or a discontinuous transmit mode. In the discontinuous transmit mode, the data capacity of a wireless transmission system in which transceiver  100  operates is increased by a remote site (e.g., a base station) transmitting less information, as is known in the art. Thus intervals between speech, for example, are not transmitted by the remote site. For example, in a time division multiple access (TDMA) system, information is communicated during each time slot dedicated to device  100  in a continuous mode, but only during those time slots that speech is present for discontinuous transmit mode. Digital signal processor  104  identifies the discontinuous transmit mode in a conventional manner. 
     To decode the data, information received from the transmission link  101  is de-interleaved by an optional deinterleaver  202  (FIG.  2 ). Those skilled in the art will recognize that the deinterleaver is not required where the data was not interleaved prior to transmission. Deinterleaved uncoded data is processed in a hard decision circuit  204 , and reordered in an optional reorderer  206 . 
     A soft decision decoder  210  is employed to decode the encoded data. The soft decision decoder may be provided by any soft decision decoder that produces soft outputs, such as a soft output Viterbi equalizer or a maximum a posterior decoder, suboptimal variations thereof, or the like. Additionally, the decoder can be used without the hard decision circuit  204 . The decoder may advantageously be a turbo decoder. The data may also be convolutionaly or block encoded, or the like. 
     The output of the soft decision decoder  210  is input to the bit orderer  206 . Information from the soft decision decoder  210  is also provided to a window error detector  214 . Window error detector  214  analyzes the information responsive to soft decision information in the decoder, and optionally responsive to a discontinuous transmit signal at input  222 , to detect a bad frame. The discontinuous transmit signal may be advantageously employed with or without a turbo decoder for soft decision decoder  210 . If the soft decision decoder is not a turbo decoder, the feedback from the window error detector  214  to the soft decision decoder  210  is not required. Likewise, a turbo decoder for the soft decision decoder  210  can employ a window error detector with or without the discontinuous transmit information. 
     A soft value detector  216  may also be employed to detect an unreliable frame. The bad frame indication from window error detector  214  and the unreliable frame indicator from soft value detector  216  are input to the speech decoder  208 . The speech signals from the decoder are provided to drive speaker  109  (FIG.  1 ). The decoder  210  and window error detector  214  of FIG. 2 are implemented in the digital signal processor  104 , which can be implemented using a microprocessor, a microcomputer, a digital signal processor, or the like. 
     With reference to FIG. 3, the window error detector  214  derives its input from the decoded bit likelihoods in a soft decision decoder  210 . In one embodiment, the differences between the path metric of the surviving metric (i.e., the path through the decoder that is selected) and the best deleted path is output as Li. A larger path metric difference corresponds to a more reliable decoder output. In other words, the greater the difference between the selected path and the next best path, the more reliable the path is determined to be. 
     Using the last 25 path metric values, the minimum amongst them is selected as the reliability factor in minimum detector  312 . A sum of the 12 previous selected minimum reliability factors is also generated using shift register  314  and summer  316 . The long term measurement interval is chosen to be 12 as it is considered to be the shortest interval of speech frames that is likely to contain one good speech frame. 
     A bad frame threshold is set in the bad frame detector  318  which is represented by the following criteria and illustrated by the graph in block  318  of FIG.  3 : 
     if the reliability factor is greater than a threshold T 1 , the input frame is indicated as good; 
     if the reliability factor is below a threshold K 1 , the input frame is reported as bad; 
     if the sum of 12 minima is below a threshold K 2 , the input is identified to be bad; 
     if the relationship between and the reliability factor and the sum of 12 minima is below a threshold T 2 , the input frame is reported as bad; and 
     in any other case, the frame is detected as good. 
     The thresholds T 1  and T 2  are varied depending upon the determination of whether the transceiver is in discontinuous transmit (DTX) mode. In the case of the discontinuous transmit state, tighter thresholds are used to ensure a lower percentage of bad frames reported as good frames passed to the speech decoder. In the illustrated embodiment, the bad frame indication occurs in the shaded area. The loose threshold (T 1 ) for no discontinuous transmit mode is set to 37. The tight threshold (T 1 ) for discontinuous transmit mode is set to 65. If the sum of Li is less than 100 (K 2 ), the frame is detected as bad. For T 2 , the slope is selected to be 4, such that the threshold for curve T 2  is selected to be ΣLi=168+4*Li. K 2  is 1. 
     Additionally, if an input speech frame is known to contain control information, the minimum is set to 0 in minimum detector  312  and the speech frame is detected to be bad (producing a bad frame indication at the output) by bad frame detector  318 . This prevents window error detector  214  from falsely indicating a good channel using the second criteria (i.e., the sum of the last 12 minimum). Then the speech decoder processes the frame as a non speech frame. 
     As can be seen, for discontinuous transmit (DTX), the threshold for detecting bad frames is moved to the right in block  318 , making it harder for frames to be pass as good frames. This is important, as during discontinuous transmit mode, some frames which would otherwise have been transmitted are not sent. Consequently, nothing is sent by the remote site in place of a signal that would otherwise be transmitted. This is done to reduce system interference so as to increase the capacity of the communication system. It is important that the frames associated with non-transmitted signals are not received and decoded as speech. By making the bad frame threshold in bad frame detector  318  greater, it is less likely that the frames associated with non transmitted signals will be detected as speech and decoded. 
     The use of turbo decoders for soft decision decoder  210  makes it possible to iteratively reprocess frames until a frame is detected as a good frame. The window error detector  214  provides a particularly advantageous method of determining when to stop the iterative process. Preferably, the number of iterations is set to a maximum, such as ten. The iterative process will be repeated until the window error detector no longer detects a bad frame, or the maximum number of iterations permitted is reached. If the communication system has a strong channel, then one decoding iteration by decoder  210  may be sufficient. In particularly problematic situation, the process will be repeated to the maximum number of permitted iterations. The maximum limits the resources required, but the ability to stop before the maximum significantly reduces the number of iterations required for a particular window. 
     A turbo decoder can be employed to help take advantage of the discontinuous transmit signal provided at input  222 . The bad frame threshold in  318  during discontinuous transmit is increased to insure that frames which were not transmitted do not result in decoder  208  producing a decoded frame. Because the bad frame threshold is higher, the number of iterations may be greater in the discontinuous transmit mode relative to the continuous transmit mode. In other words, it may take more iterations before the frame passes the bad frame detector  318  in the discontinuous mode. However, this allows the system to increase its capacity while the transceiver works to insure that only speech signals are decoded. 
     The soft value detector  216  may also be employed. The soft value detector includes a mean soft value generator  324  (FIG.  3 ). An unreliable frame detector  326  output an unreliable frame indication when the mean soft value is below a constant K 3  (illustrated to be  41 ) and the reliability factor Li is below a constant K 4  (illustrated to be  60 ). This additional information may be used by the speech decoder  208  in decoding the received data. 
     Thus it can be seen that an improved bad frame detector and turbo decoder are disclosed. While the inventions are illustrated in a speech decoder, those skilled in the art will recognize that the inventions can be used in non-speech systems.