Pulse or digital communications – Receivers – Interference or noise reduction
Patent
1996-10-28
1999-05-04
Pham, Chi H.
Pulse or digital communications
Receivers
Interference or noise reduction
375365, 371 375, 371 377, H03D 104, H03K 501, H03K 604, H04B 110
Patent
active
059011866
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
The present invention relates to an apparatus and a method for bad frame detection in a digital communication system.
Methods for coding/decoding of speech signals in a radio communication system are well known and even standardized (for example IS-54 in the U.S. and GSM in Europe). Furthermore, methods for improving encoding/decoding of background sounds, primarily intended for digital cellular telephone systems, have been described in Swedish Patent Application 93 00290-5. Both these types of methods are primarily designed to handle a situation where the connection between the speech encoder and the speech decoder is close to ideal, in the sense that only a small amount of bit or transmission errors remain after channel decoding. However, since the connection is a radio channel the received signal may contain some bit or transmission errors. In such cases it may be necessary to modify the methods mentioned above.
Thus, it is essential to be able to reliably detect bad received frames in order to perform these modified methods. Usually a cyclic redundancy check (CRC) is used at the receiver as a quality measure. If the CRC fails this is an indication that the received frame may be bad. Another quality measure is so called soft information from the detector. This soft information essentially indicates the probability of the received speech frame (or parts of it) to be correct. A problem with the first type of measure (CRC) is that a speech frame is interleaved over a number of time slots. Since the CRC requires the bits from all these time slots before the check can be made, this implies a delay until the decision can be made and error concealment measures can be taken. A problem with soft information is that it is difficult to set thresholds correctly. If the thresholds are set too low a high false alarm rate will occur (acceptable frames treated as bad frames).
An object of the present invention is an apparatus and a method in which bad frames are more reliably detected to enable bad frame concealment in accordance with the above mentioned modified methods.
SUMMARY
In accordance with the invention this object is solved by a method and apparatus for detecting bad received frames in a receiver in a digital communication system in which information is transmitted and received in signal bursts. Some bits of each frame have a first interleaving depth, and other bits in each frame have a second interleaving depth, the second interleaving depth being lower than the first interleaving depth. Possible bit errors are detected in a predetermined redundancy-containing bit sequence having the second interleaving depth. A bad received frame is indicated if at least one bit error has been detected in the bit sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawing, in which
FIG. 1 is a schematic block diagram of the relevant parts of a receiver in a radio communication system containing an apparatus in accordance with the present invention;
FIG. 2 is a time diagram of a received signal strength and corresponding time slots; and
FIG. 3 is a flow chart of the method in accordance with the present invention.
DETAILED DESCRIPTION
To understand the operation of the invention it is useful to briefly review the operation of a typical digital cellular radio connection and typical bad frame concealment techniques.
In a communication link of a digital cellular telephone system the audio signal is first digitized and then a speech coding algorithm is applied (see for example "Applications of voice processing to telecommunications", Lawrence R. Rabiner, Proc. IEEE, Vol. 82, No 2, pp 199-228). This algorithm compresses the speech signal and transforms it to a number of quantized parameters (usually in a frame based manner). The resulting bits are thereafter protected by addition of coding redundancy, using channel encoding techniques (see fo
REFERENCES:
patent: 4967413 (1990-10-01), Otani
patent: 5113400 (1992-05-01), Gould et al.
Jamal Karim
Jansson Fredrik
Pham Chi H.
Telefonaktieboalget LM Ericsson
Webster Bryan
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