Predrag B. Rapajic

Information capacity of a random signature multiple-input multiple-output channel

A closed-form expression for information capacity of the random signature multiple input multiple-output channel is given. A direct calculation of the capacity is provided additional to the proof involving the capacity of minimum mean-square-error systems provided by Verdu and Shamai (see IEEE Trans. Inform. Theory, vol.45, p.622-40, 1999).

Minimum mean-squared error multiuser decision-feedback detectors for DS-CDMA

Multiuser decision-feedback detectors (DFDs) for direct-sequence code-division multiple access, based on the minimum mean-squared error (MMSE) performance criterion, are described. Both successive and parallel feedback (interference cancellation) with hard decisions are considered. An iterative DFD is presented, which consists of cascaded DFDs, each performing successive cancellation. The two-stage DFD achieves the single-user bound in the absence of error propagation, and performs significantly better than an MMSE DFD with parallel feedback. The filter structures are generalized to include finite impulse response feedforward and feedback matrix filters, which account for asynchronous users and intersymbol interference. The effect of error propagation is illustrated through simulation. Both uncoded and coded performance results are presented. Although error propagation can significantly degrade performance, the DFDs still offer a significant performance gain relative to linear MMSE detection.

Adaptive MMSE maximum likelihood CDMA multiuser detection

The well-known code division multiple access maximum likelihood receiver (MF-ML) uses a bank of matched filters as a generator of sufficient statistics for maximum likelihood detection of users transmitted symbols. In this paper, the bank of matched filters is replaced by a bank of adaptive minimum mean squared error (MMSE) filters as the generator of sufficient statistics. This formal replacement of the MF bank by the adaptive MMSE filter bank has significant conceptual consequences and provides improvement by several performance measures. The adaptive MMSE-ML receivers digital implementation is significantly computationally simplified. The advantages of the proposed adaptive MMSE-ML receiver over the MF-ML receiver are: (1) ability to perform joint synchronization, channel parameter estimation, and signal detection where the signal is sent over an unknown, slowly time-varying, frequency-selective multipath fading channel; (2) increased information capacity in a multicellular environment; and (3) significantly improved bit error rate (BER) performance in a multicellular mobile communications environment. The information capacity and the BER of the proposed MMSE-ML receiver are analyzed. Numerical results showing the BER performance of the MMSE-ML receiver in a multipath channel environment are presented.

Merit factor based comparison of new polyphase sequences

New polyphase sequences, named Px sequences, of a square integer length are found. Standard merit factors of polyphase Frank (1963), P1, P3, P4, Golomb and Chu (1972) sequences are compared with merit factors of Px sequences. It is found that Px sequences have better sidelobe-energy merit factor and not worse maximum-sidelobe merit factor in comparison to all mentioned analytically constructed sequences.

Capacity analysis for finite-state Markov mapping of flat-fading channels

In this paper, time-varying flat-fading channels are modeled as first-order finite-state Markov channels (FSMC). The effect of this modeling on the channel information capacity is addressed. The approximation accuracy of the first-order memory assumption in the Markov model is validated by comparing the FSMC capacity with the channel capacity assuming perfect state information at the receiver side. The results indicate that the first-order Markovian assumption is accurate for normalized Doppler frequencies f/sub d/T /spl lsim/ 0.01, in amplitude-only quantization of the channel gain for noncoherent binary signaling. In phase-only and joint phase and amplitude quantization of the channel gain for coherent binary signaling, the first-order Markovian assumption is accurate for f/sub d/T /spl lsim/ 0.001. Furthermore, the effect of channel quantization thresholds on the FSMC capacity is studied. In high signal-to-noise ratio (SNR) conditions, nonuniform two-level amplitude quantization scheme outperforms equiprobable quantization method by 0.8-1.5 dB.

Multiuser decision-feedback detection: performance bounds and adaptive algorithms

Multiuser decision-feedback detection has been previously considered for DS-CDMA. Here we evaluate asymptotic MSE and capacity (averaged over the users) of the multiuser decision-feedback detector (DFD) in the absence of error propagation. Asymptotic means that the number of users K/spl rarr//spl infin/ and the processing gain N/spl rarr//spl infin/ with K/N fixed. The resulting capacity is the same as that achieved with optimal detection. (This is also true for finite K and N.) We also present and compare adaptive stochastic gradient and least squares algorithms for updating the DFD coefficients without explicit knowledge of channel state information.

Antenna selection for MIMO-OFDM spatial multiplexing system

Transmit antenna selection is a promising closed-loop approach to introduce transmit-diversity for MIMO system. It offers performance improvement compared to the open loop schemes with equivalent complexity by only using low rate feedback link. Two categories of selection criteria were reported, which are capacity-oriented criteria (R. Blum, et al., 2002)and performance-oriented criteria (R.W. Heath, et al., 2001). In this paper, we focus on the later case. We present an analytical bound of transmit-diversity gain provided by antenna selection in flat fading channels. An antenna selection criterion is proposed for MIMO-OFDM in frequency selective fading channels.

Capacity and coverage increase with repeaters in UMTS urban cellular mobile communication environment

In this letter, we propose to use repeaters to increase system information capacity in urban areas, where path loss exponents are higher (as much as 3.4 or more). There is at least a 10% increase in coverage area when repeaters are placed in every cell within the network to increase system capacity. The overall system information capacity is doubled at propagation exponent 3.7-3.9.

Information Capacity of the Space Division Multiple Access Mobile Communication System

An expression for the Shannon capacity lower bound of the space division multiple access (SDMA) mobile communication channel with interference is shown. The bound is tightly approaching the Shannon capacity of the orthogonal SDMA system over Gaussian channel with no interference. The information capacity of the SDMA system with adaptive MMSE receivers is presented as well. The SDMA system with MMSE receivers achieves at least 50% of the orthogonal SDMA system capacity for the input signal to noise ratio of practical interest in cellular mobile communications (10–30dB).

Turbo DFE algorithm with imperfect decision feedback

In this letter, we propose a new turbo decision feedback equalizer (DFE) algorithm that takes into account decision feedback error propagation. We derive the exact mathematical expression for the probability density function (pdf) of soft decision feedback symbols, assuming that soft outputs from channel decoder are independent identically distributed Gaussian random variables with known mean and variance. We find a new set of imperfect turbo minimum mean-square error DFE coefficients based on the derived pdf. The proposed turbo detector outperforms other turbo equalization algorithms of similar computational complexity in terms of bit-error rate. The achieved improvement is up to 3.8 dB for severe frequency-selective channels.