Frequency-nonselective Channels Research Articles

Optimum receivers and low-dimensional spreaded modulation for multiuser space-time communications

The jointly optimum receiver is obtained for multi- user communications in a frequency nonselective Rayleigh-fading channel with N/sub T/ transmit antennas per user and N/sub R/ receive antennas. Based on a general analysis of quadratic receivers in zero-mean complex Gaussian vectors, asymptotically tight expressions (for high signal-to-noise ratio (SNR)) for the pairwise error probabilities are derived. Subsequently, it is shown that N/sub T/-dimensional single-user signaling suffices to provide full diversity order N=N/sub T/N/sub R/ for all the users. In other words, the presence of other users does not increase the minimum dimension required beyond what is needed for the single-user space-time channel. For the special case of low-rank "code-division multiple-access (CDMA)" signaling with N/sub T/=1 and provided the signatures of any two users are linearly independent, it is shown that the error probability of a K-user system asymptotically approaches single-user-like performance for every user. Remarkably, therefore, an increase in the number of users, and hence an increase in the aggregate spectral efficiency, does not require the users to transmit with more power to achieve single-user-like performance asymptotically. A signal design algorithm is proposed to illustrate this point and examples are given. These results are then generalized to the multiple transmit antenna case. A new (N/sub T/+1)-dimensional signaling strategy is proposed for the multiuser channel that leverages existing single-user space-time signal designs while ensuring full diversity order and single-user-like performance asymptotically for every user.

Spectrally efficient noncoherent communication

This paper considers noncoherent communication over a frequency-nonselective channel in which the time-varying channel gain is unknown a priori, but is approximately constant over a coherence interval. Unless the coherence interval is large, coherent communication, which requires explicit channel estimation and tracking prior to detection, incurs training overhead which may be excessive, especially for multiple-antenna communication. In contrast, noncoherent detection may be viewed as a generalized likelihood ratio test (GLRT) which jointly estimates the channel and the data, and hence does not require separate training. The main results in this paper are as follows. (1) We develop a space criterion for signal and code design for noncoherent communication, in terms of the distances of signal points from the decision boundaries. (2) The noncoherent metric thus obtained is used to guide the design of signals for noncoherent communication that are based on amplitude/phase constellations. These are significantly more efficient than conventional differential phase-shift keying (PSK), especially at high signal-to-noise ratio (SNR). Also, known results on the high-SNR performance of multiple-symbol demodulation of differential PSK are easily inferred from the noncoherent metric. (3) The GLRT interpretation is used to obtain near-optimal low-complexity implementations of noncoherent block demodulation. In particular, this gives an implementation of multiple symbol demodulation of differential PSK, which is of linear complexity (in the block length) and whose degradation from the exact, exponential complexity, implementation can be made as small as desired.

On the use of interference suppression to reduce intermodulation distortion in multicarrier CDMA systems

This paper presents a coded multicarrier direct-sequence code-division multiple-access (DS-CDMA) system that, by the use of a minimum mean-squared-error receiver, achieves frequency diversity (instead of path diversity as in a conventional single-carrier RAKE DS-CDMA) and has the ability to suppress the intermodulation distortion and partially compensate for the signal distortion introduced by a nonlinear amplifier at the transmitter. A frequency-selective Rayleigh fading channel is decomposed into M frequency-nonselective channels, based on the channel coherence bandwidth. A rate 1/M convolutional code, after being interleaved, is used to modulate M different DS-CDMA waveforms. The new system is shown to effectively combat intermodulation distortion in the presence of multiple-access interference.

Low-complexity iterative demodulation for noncoherent coded transmission over Ricean-fading channels

Power and bandwidth efficient noncoherent transmission over frequency nonselective Ricean-fading channels is studied. We propose a low-complexity receiver structure, which is very well suited to mobile communication scenarios with time-variant and nonstationary transmission channels. Applying bit-interleaved coded modulation with standard convolutional codes, substantial gains of several decibels in power efficiency compared to conventional differential detection are achieved. To obtain the novel noncoherent reception scheme, ideas of iterative decoding with hard-decision feedback and prediction-based branch metric calculation are combined and extended. Furthermore, the incorporation of combined phase and amplitude modulation for high bandwidth efficiency is focused on. The theoretical analysis of both the convergence and the achievable performance of iterative decoding are given by evaluating the corresponding prediction-error variance and the associated cutoff rate, respectively. The results from information theory are well confirmed by simulation results presented for different channel scenarios.

Ordered statistics decoding of linear block codes on frequency nonselective multipath channels

This paper investigates the performance of ordered statistics decoding (OSD) of linear block codes with binary differential phase-shift-keying (2DPSK) transmission on frequency nonselective multipath channels. First, the wireless channel is modeled as a frequency nonselective, slow fading environment without intersymbol interference. Second, block interleaving is embedded to decorrelate the successive received symbols so that the performance of OSD, designed essentially for memoryless channels, is retained. Third, we derive the log-likelihood ratio of received symbols based on 2DPSK modulation. Fourth, we derive the statistics of received symbols and the statistics of received symbols after ordering. Further, we analyze the system error performance for sufficient degree of interleaving. Specifically, we derive the asymptotic bit error rate (BER) of OSD at high SNRs using the first-order approximation, finally, we conduct experiments at various channel characteristics and interleaving degrees for (128, 61, 22) extended BCH code and (24, 12, 8) extended Golay code. Results show that the error performance improves tremendously with increased interleaving degree.

Decision-feedback differential detection based on linear prediction for MDPSK signals transmitted over Ricean fading channels

In this paper, linear prediction-based decision-feedback differential detection (DF-DD) for M-ary differential phase-shift keying (MDPSK) signals transmitted over Ricean fading channels is proposed. This scheme can improve conventional DD significantly for a multitude of frequency-nonselective channels, as shown analytically and by computer simulations. Prediction-based DF-DD is particularly well suited for application in mobile communications since the predictor coefficients may be updated regularly using the recursive least squares (RLS) algorithm. Here, adaptation can start blind, i.e., no training sequence and no a prior knowledge about the channel statistics are required. A further important characteristic of the proposed detection scheme is that no degradation occurs under frequency offset. The bit error rate (BER) performance of QDPSK with genie-aided prediction-based DF-DD is analyzed, and it is shown under which conditions the irreducible error floor of conventional DD can be removed entirely. In addition, the influence of Doppler shift is discussed. Last, the proposed scheme is compared with a second DF-DD scheme, which is based on multiple-symbol detection.

Bit error probability of OFDM system over frequency nonselective fast Rayleigh fading channels

The variation in the channel impulse response during one OFDM symbol, which occurs in fast fading channel environments, destroys the orthogonality between the subcarriers. As a result, a complicated multiple distortion and an extra additive distortion component are introduced. A theoretical derivation of the BER formula is presented, which involves a proposed method for calculating the probability density function (PDF) of the sum of N (N ≥ 3) correlated Rayleigh random variables (RVs). The results obtained using the derived BER formula agree well with the simulation results.

Bias terms in the optimal quadratic receiver

The bias terms in the likelihood functions of the optimal quadratic receiver are compared. In a Rician-fading frequency-nonselective channel with additive white Gaussian noise (AWGN) the terms are identical, and can be neglected if the transmitted signals have identical envelopes. In diversity systems including the RAKE the channels may have correlation and different average powers but no crosstalk between the channels or intersymbol interference (ISI) is allowed.

Simulation of frequency nonselective mobile channel with LOS component: A deterministic approach

A deterministic digital simulation model with finite sum of properly weighted sinusoids is used to simulate a frequency nonselective mobile channel with LOS component. Measurement data are used to optimise the parameters, which are then used in the deterministic simulation model. The probability distribution and level cross rate (LCR) obtained from the measured data agree well with the analytical and simulation results.

Error Probablity for Optimal and Suboptimal Quadratic Receivers in Rapid Rayleigh Fading Channels

The exact performance of optimal and suboptimal quadratic receivers in a binary hypothesis test between jointly distributed zero mean complex Gaussian variates is derived. The probability of error is given as a function of the characteristic values of a generalized eigenvalue problem set up in terms of the covariance matrix of the received signal-plus-noise and in the matrix of the quadratic form of the receiver. The results include the exact performance of various types of suboptimal receivers including those previously derived for the envelope matched filter for MFSK and the noncoherent DBPSK receiver in rapid Rayleigh fading, nonfrequency-selective channels. Also, the performance of near-optimal stationary process-long observation time [SPLOT] receivers, energy detectors, and other approximately optimal receivers may be calculated for noncoherent signaling in the same channel.