Frequency-selective Rayleigh Fading Channels Research Articles

Multi-Rate Access Schemes for MC-CDMA

In order to accommodate different types of traffic in future wireless communications, it is necessary to consider a system, which can operate satisfactorily at multiple transmission rates. Due to its capability to cope with the hostile frequency selective fading, that limits transmission rate, and its suitability to handle multi-rate data, multi-carrier CDMA (MC-CDMA) has recently drawn considerable attention as a suitable candidate for supporting multimedia services in wireless communications. Multi-rate access schemes where users are able to transmit at different data rates on MC-CDMA systems are presented in this paper. Four multi-rate access schemes: uncoded fixed spreading length (UFSL), coded fixed spreading length (CFSL), multi-code fixed spreading length (MFSL) and variable spreading length (VSL) schemes are proposed. With these schemes, different traffic such as voice, video and high rate data can be transmitted seamlessly through one MC-CDMA infrastructure. A chip-level minimum mean square error combining (MMSEC) technique is employed for joint energy combining and interference cancellation purpose. The performance of these schemes with MMSEC is compared by both theoretical derivations and simulation results under frequency selective Rayleigh fading channels.

Performance analysis of an improved multi‐carrier CDMA system under frequency‐selective Rayleigh fading channels

AbstractIn this article, an analytical method is proposed to study an improved orthogonal multi‐carrier DS/CDMA system, which uses QPSK for both spreading and carrier modulations without redundant sub‐carriers and time interleavers in each sub‐channel. The article concerns in particular such a system and its performance under frequency‐selective Rayleigh fading channels with various delay‐power profiles. The bit error probabilities under varying multipath‐diversity orders (using maximal ratio combing RAKE reception) and different numbers of resolvable multipaths are derived. It has been shown that the system, despite of its structural simplicity, performs well in mitigating frequency‐selective fading. The performance is also evaluated taking into account various other parameter sets, such as numbers of users and sub‐carriers, lengths of PN codes and the number of fingers in an RAKE receiver, etc. Copyright © 2003 John Wiley & Sons, Ltd.

Capacity of RAKE reception with energy randomization

We consider a RAKE receiver for coherent binary orthogonal signaling over a frequency selective multipath Rayleigh fading channel. The receiver uses maximal-ratio combining such that the weight estimation errors are not independent of the additive noise. We find the capacity-achieving energy randomization scheme with two energy levels for the cases of imperfect and perfect channel estimates. We observe that the capacity-achieving probability gets closer to 1/2 with increase in the number of paths. We also show that the capacity is higher if channel estimates are perfect and that the channel estimation errors have more pronounced effect on the capacity at low signal-to-noise ratios.

Differentially coherent combining for double-dwell code acquisition in ds-cdma systems

The use of differentially coherent combining is proposed to improve the performance of a double-dwell acquisition system by increasing the reliability of a decision in the verification stage. The detection and mean acquisition time performance of the acquisition scheme with the proposed combining scheme is analyzed in frequency-selective Rayleigh fading channels, and compared with that of two previously published double-dwell acquisition schemes based on long correlation intervals and noncoherent combining. It is shown that the proposed acquisition scheme outperforms the previous ones, and that the performance improvement increases as the frequency offset increases.

Design criteria and performance of space-frequency bit-interleaved coded modulations in frequency-selective Rayleigh fading channels

In this paper, we investigate design criteria and the performance of the space-frequency bit-interleaved coded modulation (SF-BICM) systems in frequency-selective Rayleigh fading channels. To determine the key parameters that affect the performance of SF-BICM, we derive the pairwise error probability (PEP) in terms of the determinant of the matrix corresponding to any two codewords. We prove that the bit-interleavers do the function of distributing the nonzero bits uniformly such that two or more nonzero bits are seldom distributed into the symbols that are transmitted in the same frequency bin. This implies that the bit-interleavers transform an SF-BICM system into an equivalent 1-antenna system. Based on this, we present design criteria of SF-BICM systems that maximizes the diversity order and the coding gain. Then, we analyze the performance of SF-BICM for the case of 2-transmit antennas and 2-multipaths by deriving a frame error rate (FER) bound. The derived bound is accurate and requires only the distance spectrum of the constituent codes of SF-BICM. Numerical results reveal that the bound is tight enough to estimate the performance of SF-BICM very accurately.

Acquisition for DS/CDMA systems with multiple antennas in frequency-selective fading channels

A generalized code acquisition scheme for direct-sequence code-division multiple-access systems with multiple antennas is proposed over frequency-selective fading channels. The proposed scheme is developed on the framework of a generalized configuration of multiple antennas and correlators. The nonconsecutive search method is generalized and extended to multiple antenna systems to exploit multipath signals in improving acquisition performance over frequency-selective fading channels. The proposed scheme also adopts a hybrid decision strategy to make effective decisions on acquisition. The mean acquisition time performance of the proposed acquisition scheme is analyzed and evaluated in frequency-selective Rayleigh-fading channels with general multipath delay profiles and spatial-fading correlations. The effects of nonconsecutive search on mean acquisition time are investigated for various channel environments, and the optimal choice of decision strategy is discussed. Furthermore, effects of various configurations of multiple antennas and correlators, decision thresholds, and correlation interval on the performance are also investigated.

Use of multiple antennas for DS/CDMA code acquisition

A generalized acquisition scheme is proposed for direct sequence code-division multiple-access systems with multiple antennas. The proposed scheme employs grouping of multiple antennas as a means of a tradeoff between two important factors determining the mean acquisition time, combining gain and search time. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh-fading channels with consideration of spatial correlations. Numerical results show that the use of the largest number of antenna groups is preferable to reducing the mean acquisition time at low signal-to-interference ratio (SIR) values. At high SIR values, on the contrary, the mean acquisition time is found to increase in proportion to the number of antenna groups. In a typical environment, the presence of spatial correlation is shown to increase or decrease the mean acquisition time within 50% compared with the uncorrelated fading case.

Adaptive MBER decision feedback multiuser receivers in frequency selective fading channels

In this letter we investigate adaptive minimum bit error rate (BER) decision feedback multiuser receivers for DS-CDMA systems in fast frequency selective Rayleigh fading channels. We examine stochastic gradient adaptive algorithms and introduce fast algorithms for minimizing the BER cost function from training data.

Performance of multicode DS-CDMA using frequency domain equalisation in frequency selective fading channel

For the reception of multicode direct sequence (DS)-CDMA signals, the MMSE frequency domain equalisation is applied instead of RAKE combining. The achievable BER performance in a frequency selective Rayleigh fading channel is evaluated by computer simulation. It is shown by computer simulation that the DS-CDMA with MMSE frequency domain equalisation outperforms the DS-CDMA with RAKE combining and shows only slight performance degradation compared to the MC-CDMA with minimum mean square error combining (MMSEC).

Multicarrier cdma systems using time-domain and frequency-domain spreading codes

For wideband code-division multiple-access systems, the paper introduces a multicarrier modulation scheme that performs the spreading simultaneously in the time and frequency domains. This scheme attains higher flexibility and spectrum efficiency because system parameters can be selected at will. The performance is compared with that of a single carrier RAKE system by calculating the probability of error over a frequency-selective Rayleigh fading channel. The proposed scheme outperforms the single carrier RAKE system if the system parameters are selected properly for given conditions, such as bandwidth and delay spread.

A Performance Comparison of Code Acquisition Techniques in DS-CDMA System

This paper compares parallel acquisition with hybrid acquisition in terms of mean acquisition time in code acquisition system of forward link and reverse link of direct sequence code division multiple access (DS-CDMA) system. The mean acquisition time of the hybrid and parallel acquisitions is derived from the distribution of the multiple correct code-phase offsets. Especially, the mean acquisition time of the hybrid acquisition scheme is derived by considering both case 1 (the correct code-phase offsets are included in one subset) and case 2 (the correct code-phase offsets exist at the boundary of two subsets), which are caused by the distribution of the correct code-phase offsets in the subset. Results are provided for the performance comparison between hybrid acquisition and parallel acquisition under the same hardware complexity in the forward and reverse links. In our performance evaluation of code acquisition in DS-CDMA system, the hybrid acquisition shows a better performance than the parallel acquisition in terms of mean acquisition time of the both links in the frequency-selective Rayleigh fading channel.

Minimum BER power loading for OFDM in fading channel

A novel approach to power loading for orthogonal frequency-division multiplexing (OFDM) systems is proposed. The corresponding algorithm minimizes the aggregate bit-error rate, but appears to be rather complicated. Consequently, a much simpler, quasi-optimal loading algorithm is developed. The performance of various power loading strategies known in the literature are compared with the proposed quasi-optimal algorithm in the Rayleigh frequency-selective fading channel.

Interleaved TC 8DPSK/OFDM with decision-directed channel estimation on frequency-selective Rayleigh fading channels

Trellis-coded modulation (TCM) is a power and bandwidth efficient signaling scheme. In this letter, we propose interleaved trellis-coded (TC) 8DPSK/OFDM combined with decision-directed (DD) channel estimation on frequency-selective Rayleigh fading channels. We use a subchannel block interleaver in an OFDM symbol interval to randomize the burst errors due to the correlated dispersive fading channel. We also use DD channel estimation using the previous detected 8DPSK symbols in the temporal direction to improve the performance at fast fading. Computer simulations show that the proposed technique has good performance at fast fading.

Space-time-frequency coded OFDM over frequency-selective fading channels

This paper proposes novel space-time-frequency (STF) coding for multi-antenna orthogonal frequency-division multiplexing (OFDM) transmissions over frequency-selective Rayleigh fading channels. Incorporating subchannel grouping and choosing appropriate system parameters, we first convert our system into a set of group STF (GSTF) systems. This enables simplification of STF coding within each GSTF system. We derive design criteria for STF coding and exploit existing ST coding techniques to construct both STF block and trellis codes. The resulting codes are shown to be capable of achieving maximum diversity and coding gains, while affording low-complexity decoding. The performance merits of our design are confirmed by corroborating simulations and compared with existing alternatives.

Effects of multipath diversity on adaptive QAM in frequency selective Rayleigh fading channels

We derive the average bit error rate (BER) and the average throughput for adaptive quadrature amplitude modulation (AQAM) with constant transmit power over frequency selective Rayleigh fading channels. Based on these derived BER and throughput expressions, we go on to investigate the effect of multipath diversity on AQAM using two BER constraints.

Multicarrier DS/SFH-CDMA systems

Multicarrier direct-sequence/slow-frequency-hopping (MC DS/SFH) code-division multiple-access (CDMA) systems are proposed, in which multiple carriers are modulated by the same DS waveform and hopped in frequency according to a random hopping pattern. The receiver dehops the received signal with the same pattern, provides RAKE receivers for each carrier, and combines the outputs with a maximal ratio combiner (MRC). The performance of the proposed system is investigated over a frequency-selective Rayleigh-fading channel and compared to that of the MC DS-CDMA systems. It is shown that for the same diversity order, the MC DS/SFH-CDMA systems are superior in reducing multiple-access interference (MAI) while preserving the good capability of narrowband interference suppression, when the system parameters are selected properly.

Performance of Reverse-Link Synchronous DS-CDMA System on a Frequency-Selective Multipath Fading Channel with Imperfect Power Control

We analyze the performance for reverse-link synchronous DS-CDMA system in a frequency-selective Rayleigh fading channel with an imperfect power control scheme. The performance degradation due to power control error (PCE), which is approximated by a log-normally distributed random variable, is estimated as a function of the standard deviation of the PCE. In addition, we investigate the impacts of the multipath intensity profile (MIP) shape and the number of resolvable paths on the performance. Finally, the coded bit error performance is evaluated in order to estimate the system capacity. Comparing with the conventional CDMA system, we show an achievable gain of from 59% to 23% for reverse-link synchronous transmission technique (RLSTT) in the presence of imperfect power control over asynchronous transmission for . As well, the effect of tradeoff between orthogonality and diversity can be seen according to the number of multipaths, and the tendency is kept even in the presence of PCE. We conclude that the capacity can be further improved via the RLSTT, because the DS-CDMA system is very sensitive to power control imperfections.

MC DS/SFH-CDMA systems for overlay systems

We introduce the slow frequency hopping (SFH) technique to the multicarrier (MC) code-division multiple-access (CDMA) systems for overlay situations. Using lower chip rate, which results in a narrower spectrum for each carrier and hopping the signal in frequency, the MC direct sequence (DS)/SFH system achieves better performance than the MC DS CDMA system in most cases, especially when the bandwidth of the narrowband interference (NBI) is narrower than one subchannel. It also exhibits a stable performance against the variations of the NBI location and bandwidth. When there is no NBI, the two systems perform approximately the same. The evaluation is performed over a frequency selective Rayleigh fading channel, with both Gaussian approximation and Monte Carlo simulation.

An adaptive asynchronous CDMA multiuser detector for frequency-selective Rayleigh fading channels

An adaptive asynchronous code-division multiple-access (CDMA) multiuser detector is proposed that uses a recently derived extended Kalman filter based algorithm (see Lim, T.J. and Rasmussen, L.K., IEEE Trans. Commun., vol.45, p.213-20, 1997) to perform joint data detection and parameter tracking in frequency-selective Rayleigh fading channels. A receiver structure based on this adaptive multiuser detector is presented and its performance in terms of parameter tracking and bit error rate (BER) is investigated. The receiver is a form of an adaptive RAKE that exploits multipaths to achieve performance gain.

Residue number system-based multicarrier CDMA system for high-speed broadband wireless access

This correspondence is concerned with a new method to enhance the bandwidth efficiency of a multicarrier CDMA system by using a residue number based representation for information symbols. The residues are mapped into a set of orthogonal sequences and are transmitted in parallel. A new modulation scheme is proposed to increase the data rate further by combining residue number representation, PSK/QAM modulation, orthogonal spreading and multicarrier modulation. The performance of the proposed system is evaluated in a frequency selective Rayleigh fading channel via extensive simulations. The present correspondence discusses the design issues associated with the system together with the performance results.