A diversity scheme for a phase-coherent frequency-hoppingspread-spectrum system

Frequency diversity for a phase-coherent frequency-hopping spread-spectrum system is proposed. The frequency band of the communicator is partitioned into L disjoint subbands on which L replicas of the communicator signal are simultaneously transmitted; each signal replica hops independently in its subband. Therefore, both phase continuity and coherent optimal combining of the received replicas are possible. Optimum diversity is thus at least 6 dB better than the conventional noncoherent FH-SS system. We show that a smart jammer should be present in all subbands. We also give a sufficient condition that forces the jammer to spread its interference signal on the whole communicator frequency band     

Joint parameter estimation and symbol detection for linear ornonlinear unknown channels

We present an iterative method for joint channel parameter estimation and symbol selection via the Baum-Welch algorithm, or equivalently the Expectation-Maximization (EM) algorithm. Channel parameters, including noise variance, are estimated using a maximum likelihood criterion. The Markovian properties of the channel state sequence enable us to calculate the required likelihood using a forward-backward algorithm. The calculated likelihood functions can easily give optimum decisions on information symbols which minimize the symbol error probability. The proposed receiver can be used for both linear and nonlinear channels. It improves the system throughput by making saving in the transmission of known symbols, usually employed for channel identification. Simulation results which show fast convergence are presented     

Differentially coherent detection of binary partial responsecontinuous phase modulation with index 0.5

The performance of binary partial response continuous phase modulation (with index 0.5) using a differentially coherent receiver depends on the choice of the receiver filter. An optimum MMSE design method for this filter is presented. The receiver filter is equivalent to the cascade of a matched filter and an equalizer in order to reduce inherent intersymbol interference (ISI). It is shown that performance degradation with respect to that of the differential binary phase shift keying (BPSK) system is due to inherent ISI contained in the signal and also to noise enhancement and correlation caused by the receiver filter. The bit error probability on the Gaussian channel is calculated by assuming that ISI is Gaussian. The Gaussian minimum shift keying (MSK) signal is used for illustration     

Zero forcing and minimum mean-square-error equalization formultiuser detection in code-division multiple-access channels

In code-division multiple-access (CDMA) systems transmitting over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. The conventional suboptimum receiver consisting of a bank of matched filters is often inefficient because interference is treated as noise. The optimum multiuser detector is too complex to be implemented at present. Four suboptimum detection techniques based on zero forcing (ZF) and minimum mean-square-error (MMSE) equalization with and without decision feedback (DF) are presented and compared. They combat both ISI and MAI. The computational complexity of all four equalizers is essentially the same. All four equalizers are independent of the size of the data symbol alphabet. It is shown that the performance of the MMSE equalizers is better than that of the corresponding ZF equalizers. Furthermore, the performance of the equalizers with DF is better than that of the corresponding equalizers without DF. The impairing effect of error propagation on the equalizers with DF is reduced by channel sorting     

A new iterative soft-output Viterbi algorithm and its application to the “dicode” channel

We present an iterative soft-output decoding algorithm for serially concatenated coding systems. It has better performance than the conventional noniterative decoding algorithm. When applied together with an outer convolutional code to the dicode channel with partial response (1 — D), we obtain an additional coding gain of about 1 dB at a bit-error rate of 10^-4 after two iterations. This new algorithm can also be applied advantageously to satellite communication and fading channels.     

Iterative Decoding and Soft Interference Cancellation for the Gaussian Multiple Access Channel

For the Gaussian multiple access channel, we studied a joint channel decoderand multiple access interference canceller. For each user, the decoderoutput was used to generate a soft estimate of the user signal. Theestimate was improved by iterating decoding and cancellation untilconvergence was reached. A final, hard decoding was then performed.Simulation results are presented.     

Block and trellis codes for the binary (1-D) partial responsechannel with simple maximum likelihood decoders

We consider block and convolutional codes for improving the reliability of data transmission over the binary precoded noisy (1-D) partial response channel. We concentrate on a class of codes for which the maximum likelihood decoder, matched to the encoder, precoder, and the channel has the same trellis structure as the encoder. Thus, doubling the number of states due to the channel memory is avoided, We show that the necessary and sufficient condition to belong to this class is that all codewords be of the same parity. The Reed-Muller and Golay codes belong to this class     

Performance comparison of frequency-diversity and frequency-hoppingspread-spectrum systems

In the presence of partial-band Gaussian interference, the frequency-hopping spread-spectrum system, without and with optimum diversity, performs worse than the frequency-diversity spread-spectrum system proposed by Kaleh (see ibid., p.886-93, vol.44, no.7, 1996) The latter offers the advantage of coherent reception with coherent diversity combining     

Frequency-diversity spread-spectrum communication system to counterbandlimited Gaussian interference

A spread spectrum system to counter bandlimited Gaussian interference is proposed. The optimum receiver for this system is easy to build. Frequency diversity which allows the receiver to distinguish unjammed signal replicas from their jammed versions is used. The system can also resist bandlimited partial-time jamming. The only choice left to a smart jammer to maximize the error probability is to spread its signal like the communicator. The optimum receiver, which jointly performs symbol detection and interference rejection, is derived. Side information needed by this receiver can easily be estimated. However, if the interference bandwidth is narrow compared to the signal bandwidth, side information on noise and interference levels is not needed by a simpler and near-optimum receiver. Bit-error probability is evaluated for quaternary phase shift keying (QPSK) modulation and compared to that of direct sequence spread spectrum. We also propose the use of polyphase filters for simple system implementation     

Channel equalization for block transmission systems

In a block transmission system the information symbols are arranged in the form of blocks separated by known symbols. Such a system is suitable for communication over time-dispersive channels subject to fast time-variations, e,g., the HF channel. The known reliable receiver for this system is the nonlinear data-directed estimator (NDDE). This paper presents appropriate equalization methods for this system. A nonstationary innovations representation based on Cholesky factorization is used in order to define a noise whitener and a maximum-likelihood block detector. Also block linear equalizers and block decision-feedback equalizers are derived. For each type we give the zero-forcing and the minimum-mean-squared-error versions. Performance evaluations and comparisons are given. We show that they perform better than conventional equalizers. As compared to the NDDE, the derived block decision-feedback equalizers perform better and are much less complex. Whereas the NDDE uses the Levinson algorithm to solve M/2 Toeplitz systems of decreasing order (where M is the number of symbols per block), the derived equalizers need to process only one Toeplitz system. Moreover, the Schur algorithm, proposed for Cholesky factorization allows us to further reduce the complexity