Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-11-30
2002-04-02
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S262000, C375S265000, C375S267000, C375S342000, C714S792000, C714S794000, C714S795000, C714S798000
Reexamination Certificate
active
06366624
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communication systems and more particularly to systems and methods for receiving modulated signals.
BACKGROUND OF THE INVENTION
Public wireless radiotelephone systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in
The Mobile Communications Handbook
, edited by Gibson and published by CRC Press (1996).
Wireless communications systems such as cellular radiotelephone systems typically include a plurality of communication channels which may be established between a first transceiver (such as a base station) and a second transceiver (such as a mobile terminal). The communication channels typically are subject to performance-degrading environmental effects such as multi-path fading and interference (noise). Fading effects include flat fading which may arise from the interaction of a transmitted signal (the main ray) with reflected versions of the transmitted signal that arrive concurrently at a receiver. Time dispersion, another type of fading, may arise from interaction of the main ray with time-delayed reflections of the main ray. Interference effects may be caused by interaction of non-orthogonal signals generated in the signal medium by sources other than the source of the desired transmitted signal. Equalization techniques such as maximum likelihood sequence estimation (MLSE) may be used to compensate for time dispersion. Interference may be reduced by using antenna beam steering to reduce reception of undesired signals.
Fading is typically a major detriment to the performance of demodulators in communication systems. The receiver of a mobile terminal typically includes a demodulator which may be a coherent demodulator such as a maximum likelihood sequence estimator (MLSE) demodulator (or equalizer). To provide for reliable demodulation of a received signal, an associated channel tracker is typically provided for the demodulator. After acquisition of a communicated signal by the receiver, the channel tracker maintains a channel estimate to provide a coherent reference between the demodulator and the received signal.
Combining demodulation and decoding via feedback from the decoder to the demodulator is a way to improve receiver performance. This may be accomplished by multi-pass demodulation. According to information theory, an optimal receiver jointly performs the operations of demodulation and decoding.
The complexity of such an operation is generally exorbitant especially when interleaving is used in the system. However, it is possible to bridge part of the gap between disjoint and joint demodulation and decoding by the use of feedback from the decoder to the demodulator. This is the idea behind multi-pass demodulation.
An example of such a multi-pass demodulator is described in U.S. Pat. No. 5,673,291 to Dent which is incorporated herein by reference in its entirety. The '291 patent discusses demodulating a received signal first, then decoding coded symbols, then feeding information obtained by re-encoding the decoder output back to the demodulator to re-demodulate the un-coded symbols with improved performance. The re-encoded symbols are exploited as known symbols with improved performance. The re-encoded symbols are exploited as known symbols by the demodulator, in the same way that it exploits sync symbols, which are true known symbols that have been inserted in the data prior to transmission. The methods and systems of the '291 patent generally are based in part on knowledge by the receiver of the order of placement of symbols in the transmitted stream and on the placement of any known sync symbols. In another approach, Garr et al. proposed a multi-pass demodulator for fully encoded bit streams with soft feedback to the demodulator. D. Garr et al., “Iterative Decoding of GSM Signals,” Conference on Information Sciences and Systems, Princeton University, March 1988. Yet another proposed approach as described in, for example, Berrou et al., “Near Shannon Limit Error-correcting Coding and Decoding: Turbo-codes (1),” Proceedings of the IEEE International Communication Conference, pages 1064-1070, 1993, proposes the use of turbo codes in which parallel concatenation of two recursive convolutional codes are used. Likewise, serial concatenation of two recursive convolutional codes was proposed in Benedetto et al., “Serial Concatenation of Interleaved Codes: performance Analysis, Design and Iterative Decoding,” TDA Progress Report 42-126, Politechnico Di Torino, Italy, Aug. 15, 1996.
A problem is encountered with methods such as that proposed in the '291 patent and for communications systems having unencoded bit classes. Examples of such codes associated with various telecommunication standards currently proposed are shown in
FIGS. 1A and 1B
.
FIG. 1A
illustrates a voice coding system such as that described for the IS-136 specification.
FIG. 1B
shows a similar format for the IS-641 specification. As shown in
FIG. 1A
coding system
10
includes a Vector-Sum Excited Linear Prediction (VSELP) vocoder outputting 159 bits as a data frame. The bits are designated into 3 coding classes referred to as Class 1A, Class 1B and Class 2. Twelve bits designated as Class 1A are first passed to CRC error detection coder
14
which appends a Cyclical Redundancy Check (CRC) error detection code to the twelve Class 1A bits before passing them to convolutional encoder
16
. An additional 65 bits, classified as Class 1B, are passed directly to convolutional encoder
16
without error detection coding. Finally, 82 bits, classified as Class 2 bits, are passed directly to interleaver
18
without error detection or correction encoding. The output of convolutional encoder
16
and the unprotected Class 2 bits are passed to 2-slot interleaver
18
.
Interleaver
18
breaks up the original data frame into two frames, each containing half the original information and each of which is placed in one of two adjacent slots (i.e. sequential transmission windows) by slot formatter
19
for transmission by a modulator (not shown).
Referring now to
FIG. 1B
, the structure of coding under the IS-641 standard will now be described. Adaptive Code Excited Linear Prediction (ACELP) vocoder
22
of coding system
20
provides a data frame of 148 bits. 48 of the bits are classified as Class 1A and pass to CRC error detection coder
24
where an error detection code is appended to the bits. An additional 48 of the bits from the 148 bit data frame are treated as Class 1B bits and provided to convolutional coder
26
without error detection coding. The remaining 52 bits are treated as Class 2 bits and provided directly to interleaver
28
without coding. The Class 1A and 1B bits are passed through convolutional encoder
26
and, in turn, the code is punctured by circuit
27
to provide a total of 260 bits to two-slot interleaver
28
when combined with the 52 Class 2 bits. As described above with respect to
FIG. 1A
, interleaver
28
and slot formatter
29
implement interleaving by dividing the 148 bits from source
22
into two separate slots which are provided to a modulator for transmission.
While these various approaches provide the potential for improved signal reception, there continues to be a need for improvements in performance of receivers for modulate
Balachandran Kumar
Khayrallah Ali
Ericsson Inc.
Pham Chi
Tran Khanh
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