Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-08-31
2002-11-26
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S283000, C375S325000, C375S341000, C375S348000, C714S794000
Reexamination Certificate
active
06487255
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to radiotelephones and more particularly to the coherent demodulation of differentially encoded quadrature phase shift keying (DQPSK) signals.
BACKGROUND OF THE INVENTION
The performance of receivers in wireless radio communications systems may degrade severely due to multipath fading. Although anti-fading techniques, like antenna diversity, equalization, and adaptive array processing, may be very effective in improving the performance of the receiver, forward error correction (FEC) techniques may be necessary to achieve acceptable voice and data transmission in wireless communication systems. FEC techniques provide redundancy by adding extra bits to the actual information bits, which allows the decoder to detect and correct errors. In the receiver, the decoding process can be performed by either using hard information values or soft information values, which are provided by the demodulator. Decoding using soft information values improves the receiver performance over decoding using hard information values. Therefore, to improve decoder performance, it may be important to provide accurate soft information from the demodulation process.
The U.S. digital cellular system (IS-136) uses &pgr;/4 shifted-DQPSK as a modulation scheme. Differential encoding of the transmitted signals allows both coherent and differential demodulation of the received signal. Although differential demodulators may not be complex to implement, it is widely accepted that the performance of differential detectors degrades rapidly in the presence of Inter-Symbol Interference (ISI) which may be caused by multi-path propagation. Therefore, coherent demodulators with equalizers are commonly used in many receivers. Such a receiver is described in U.S. Pat. No. 5,285,480 to Chennakeshu et al., entitled ADAPTIVE MSLE-VA RECEIVER FOR DIGITAL CELLULAR RADIO.
FIG. 1
depicts a block diagram of a &pgr;/4 shifted-DQPSK system with a conventional differential demodulator receiver. The transmitter
105
includes encoder
101
and differential modulator
102
. Information bits are encoded in encoder
101
to produce coded bits. The coded bits are differentially modulated in differential modulator
102
to produce a differentially modulated signal, which is provided to antenna
104
for transmission. The transmitted signal reaches the radio receiver after passing through a propagation medium (e.g., a mobile radio channel). The transmitted signal plus any noise are received at the receiver antenna
106
and the received signal provided to receiver
114
. The received signal is processed by the radio processor
108
which amplifies, mixes, filters, samples and quantizes the received signal to produce a baseband signal. A differential demodulator
110
demodulates the received signal and provides symbol or bit values to the decoder
112
which decodes the encoded bits and which may detect and correct possible errors in the received signal. As discussed above, the output of the demodulator
110
is preferably soft values which may provide higher performance in decoding.
Differential encoding of the transmitted signals allows both coherent and differential demodulation of the received signal.
FIG. 2
shows a block diagram of a known apparatus for differential demodulation of the DQPSK modulated signals. The differential detector uses received samples to get hard and/or soft decision values. The present received sample is coupled to the multiplier
203
. The present received sample is also fed into a delay
201
. The delay
201
is coupled to a conjugate operator
202
, and the output of the conjugate operator
202
is coupled to the multiplier
203
.
In operation, the present received sample and the delayed and conjugated received sample are multiplied to undo the effect of the differential encoder at the transmitter. The real
204
and imaginary
205
part of the output of the multiplier provide the soft bit values corresponding to the two bits sent in one, di-bit symbol. Also, the hard values can be obtained by taking the sign
206
and
207
of the soft values as desired.
FIG. 3
shows a block diagram of a known apparatus for coherent demodulation of the DQPSK modulated signals. The coherent receiver utilizes channel estimation unit
302
which estimates the amplitude and phase of the mobile radio channel. These channel estimates are passed to the coherent QPSK demodulator
301
where the estimates of the QPSK symbols are generated. The channel parameters can be estimated using the known data sequences which are periodically inserted into the transmitted information sequences. In systems where the channel parameters change over the transmission of two consecutive known data sequences, like the U.S. digital cellular system (IS-136), it is desirable to adapt the channel parameters during the transmission of unknown data sequences. Such an adaptive coherent receiver is described in U.S. Pat. No. 5,285,480 to Chennakeshu et al. entitled ADAPTIVE MLSE-VA RECEIVER FOR DIGITAL CELLULAR RADIO.
The output values of the coherent QPSK demodulator
301
, which are the hard coherent symbols, are passed through a differential detector
303
to undo the effect of the differential encoder in the transmitter. The outputs of the differential detector are the hard decision values corresponding to the transmitted information bits.
A semi-coherent demodulation of the DQPSK modulated signals is described in U.S. Pat. No. 5,706,313 to Blasiak et al. entitled SOFT DECISION DIGITAL COMMUNICATION AND METHOD AND APPARATUS in which only the phase and frequency offset are estimated using the received signal. After compensating the phase and frequency offset, the likelihood of each possible QPSK symbol value for each sample is calculated. Therefore, a likelihood vector for each sample is obtained and this likelihood vector is provided to the decoder. The decoder uses the likelihood vectors to estimate the transmitted symbol values.
Another conventional method for the soft decoding of differentially encoded QPSK signal is described in U.S. Pat. No. 5,754,600 to Rahnema entitled METHOD AND APPARATUS FOR OPTIMUM SOFT-DECISION VITERBI DECODING OF CONVOLUTIONAL DIFFERENTIAL ENCODED QPSK DATA IN COHERENT DETECTION. In this apparatus the differential and Viterbi decoders are integrated, i.e., differential decoding is part of the convolutional decoding process.
Soft information for maximum likelihood sequence estimation (MLSE) for frequency selective fading channels has been extensively studied, for example as described in J. Hagenauer and P. Hoeher, “A Viterbi algorithm with soft-decision outputs and its applications”,
Proceeding of IEEE Globecom Conference
, pp. 47.1.1-47.1.7, Dallas, Tex., USA, November 1989. These techniques have been extended to &pgr;/4 shifted-DQPSK systems for example as described in Jong Park, Stephan B. Wicker and Henry L. Owen, “Soft Output Equalization Techniques for &pgr;/4 DQPSK Mobile Radio”,
IEEE International Conference on Communications
, pp. 1503-1507, Dallas, Tex., USA, 1997. However, relatively little work has been directed towards soft information generation for coherent detection of &pgr;/4 shifted-DQPSK in non-ISI channels, i.e. channels without significant inter-symbol interference (ISI). A suboptimal approach is given that requires exponentiation and logarithm operation in Yow-Jong Liu, Mark Wallace and John W. Ketchum, “A Soft-Output Bidirectional Decision Feedback Equalization Technique for TDMA Cellular Radio”,
IEEE Journal on selected areas in commun
., Vol. 11, No. 7, September 1993. This approach does not consider all possible symbol values in determining a soft value and, further, does not result in a unique solution.
In light of the above discussion, a need exists for improved performance in soft value determination for coherent demodulation of differentially encoded signals for non-ISI channels.
SUMMARY OF THE INVENTION
In view of the above discussion, it is an object of the present invention to provide accurate soft values for differentially en
Arslan Hüseyin
Bottomley Gregory E.
Ramesh Rajaram
Chin Stephen
Ericsson Inc.
Ha Dac V.
Myers Bigel & Sibley & Sajovec
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