Adaptive acquisition of PN code in multipath fading mobile channels

The adaptive acquisition processor (AAP), which controls false alarms and reduces miss detection of multipath signals by estimating the threshold level after censoring some highest samples in the sliding window, was introduced by the present author. In addition, an AAP was analysed in a single path situation. Here, the detection performance of an AAP in multipath situations, which may exist in the sliding window, is investigated. Results show that the AAP has good performance if the number of excision samples, Ne, is greater than or equal to the number of multipath signals in the window     

Decision-directed coherent detection in multicarrier systems onRayleigh fading channels

Novel coherent detectors for DQPSK and π/4-DQPSK on Rayleigh fading channels are presented. The detectors are based on decision feedback and minimum mean-square error (MMSE) channel estimation. The proposed detectors may be used in both single carrier and multicarrier systems. Analytical results in the form of expressions for channel estimation error variance and bit error probability, both with and without diversity, are derived. The MMSE-based detector has a significantly lower irreducible bit error probability floor compared to a conventional differential detector. For most practical applications, the error floor of the MMSE-based detector is negligible. Simulated results are presented to verify the analysis and justify the approximations made     

Utilization of multipaths for spread-spectrum code acquisition infrequency-selective Rayleigh fading channels

A novel acquisition scheme that utilizes multipaths to improve the acquisition performance is proposed for frequency-selective fading channels. The proposed acquisition scheme employs nonconsecutive search and joint triple-cell detection. The performance is analyzed in frequency-selective Rayleigh fading channels. Equations for the probabilities of detection and false alarm are derived, and an expression for the mean acquisition time is developed. The mean acquisition time performance of the proposed and conventional acquisition schemes is evaluated and compared. It is found that the proposed acquisition scheme significantly outperforms the conventional one. The effects of various channel parameters such as the number of resolvable paths, the shape of the multipath intensity profile (MIP) and the signal-to-interference ratio (SIR) on acquisition performance are also investigated     

Single-dwell and multidwell PN code acquisition in multipath Rayleigh fading channels

Traditional analysis on PN code acquisition was based on either AWGN or single-ray Rician/Rayleigh fading channels. In this paper, we analyze single-dwell and multidwell code acquisition schemes based on a two-ray Rayleigh fading channel model which is more appropriate to represent the real situations in mobile (personal) communication. Through the analytical and numerical results, we may figure out the performance of typical PN code acquisition schemes and the corresponding penalty in multipath fading channels when the second ray has much smaller power and the processing gain is not large. Otherwise, the second ray may be useful in communication and thus improves the acquisition performance as introducing a new correct acquisition state.     

Variable dwell-time code acquisition for direct-sequencespread-spectrum systems on time-variant Rayleigh fading channels

Variable dwell-time code acquisition based on multiple-dwell or sequential linear tests is investigated for direct-sequence spread-spectrum systems on time-variant Rayleigh fading channels. Unlike in the conventional additive white Gaussian noise channels, the channel memory incurred by fading renders the exact analysis of the acquisition systems extremely difficult, if not impossible. In this paper, a novel method is developed to evaluate the mean acquisition time of the acquisition systems very accurately. The effects of Rayleigh fading are evaluated, and comparisons are made between double-dwell and sequential linear tests. Numerical results show that Rayleigh fading may result in 1-4-dB loss in performance, and the sequential linear test can outperform double-dwell test by a margin of 1-2 dB. The analytical results are verified by computer simulations     

A universal lattice code decoder for fading channels

We present a maximum-likelihood decoding algorithm for an arbitrary lattice code when used over an independent fading channel with perfect channel state information at the receiver. The decoder is based on a bounded distance search among the lattice points falling inside a sphere centered at the received point. By judicious choice of the decoding radius we show that this decoder can be practically used to decode lattice codes of dimension up to 32 in a fading environment     

Performance of multiple-dwell pseudo-noise code acquisition with I-Q detector on frequency-nonselective multipath fading channels

Multiple-dwell pseudo-noise code acquisition with a noncoherent I-Q detector is analyzed for Rayleigh and Rician fading channels that takes into account the detection correlations resulting from multipath fading. Minimum mean acquisition times with optimized dwell times and thresholds are obtained, and the effects of multipath fading and frequency offsets are evaluated. In addition, a detailed comparison between I-Q and square-law detectors is conducted under various channel conditions.     

A comparison of slow-frequency-hop and direct-sequence spread-spectrum packet communications over doubly selective fading channels

The performance is compared for slow-frequency-hop and direct-sequence spread-spectrum packet communication systems of similar complexity with doubly selective fading channels characterized by several different multipath delay profiles and Doppler spreads. The delay profiles are chosen to reflect experimental measurements of mobile-radio channels, and the Doppler spreads are chosen to represent the range of carrier frequencies and vehicle speeds encountered in common mobile communication systems.     

Parallel acquisition of multicarrier direct-sequence spread-spectrum signals in fading and partial-band interference

The frequency diversity of multicarrier direct-sequence signaling can potentially offer robust performance in frequency-selective channels. The paper focuses on the acquisition of multicarrier signals in channels containing fading and partial-band interference. The maximum-likelihood decision rule for parallel acquisition in frequency-selective fading and partial-band interference is derived. Several simpler, near-optimal decision rules are also discussed. The performance of these decision rules is compared to that of equal-gain combining for multicarrier acquisition. Results show that the decision rules designed specifically for partial-band interference give significantly better performance. Methods of acquisition with a limited number of correlators are also discussed. Finally, the potential benefits of estimating the signal strength on each subcarrier prior to acquisition are examined.     

Bandwidth efficient coding for fading channels: code constructionand performance analysis

The authors apply a general method of bounding the event error probability of TCM (trellis-coded modulation) schemes to fading channels and use the effective length and the minimum-squared-product distance to replace the minimum-free-squared-Euclidean distance as code design parameters for Rayleigh and Rician fading channels with a substantial multipath component. They present 8-PSK (phase-shift-keying) trellis codes specifically constructed for fading channels that outperform equivalent codes designed for the AWGN (additive white Gaussian noise) channel when v⩾5. For quasiregular trellis codes there exists an efficient algorithm for evaluating event error probability, and numerical results which demonstrate the importance of the effective length as a code design parameter for fading channels with or without side information have been obtained. This is consistent with the case for binary signaling, where the Hamming distance remains the best code design parameter for fading channels. The authors show that the use of Reed-Solomon block codes with expanded signal sets becomes interesting only for large value of E_s/N₀, where they begin to outperform trellis codes     

On the detection of bandlimited direct-sequence spread-spectrumsignals transmitted via fading multipath channels

Recent investigations suggest bandlimited direct-sequence spread-spectrum transmission for use in CDMA mobile radio systems. In this paper, transmission systems with signal bandwidths not exceeding the chip rate are considered. First, the optimum noncoherent single-user receiver and various suboptimum approximations thereof are derived. These receivers utilize the fact that the received signals can be sampled at chip rate. This results in receiver concepts, which are well suited for fully digital implementations. Then, it is shown that the main result on the structure of the optimum single-user receiver is directly applicable to optimum noncoherent multiuser joint detection and interference cancellation. In this paper, noncoherent detection is considered. By this, we mean that only the channel statistics and not the channel itself is known to the receiver. It is shown that due to this less restrictive assumption, more powerful channel coding and interleaving schemes can be employed     

Noncoherent sequential PN code acquisition using slidingcorrelation for chip-asynchronous direct-sequence spread-spectrumcommunications

Noncoherent sequential pseudonoise (PN) code acquisition using sliding correlation is proposed in this paper. Noncoherent detection and chip asynchronization should be taken care of to handle a severely noisy environment, while frequency offset and data modulation effects can be simultaneously dealt with in the proposed technique. To realize sequential detection, the cross-correlation sequences at the output end of the integrate/dump (LID) filter under out-of-lock conditions have to be modeled as either a Gaussian random sequence for chip-asynchronous applications or as their upper bound for chip-synchronous applications in order to avoid significantly high probabilities of false alarm caused by the conventional zero-sequence model. Meanwhile, the in-lock sequence also has to be modified by taking the frequency offset and chip-asynchronization effects into account in order to avoid the occurrence of high probabilities of miss. Extensive computer simulation results indicate that the proposed technique can achieve low probabilities of false alarm and miss and can outperform its fixed-sample-size (FSS) counterparts by roughly 2~4 dB. This superiority, furthermore, increases with decreasing SNRs and/or decreasing desired error probabilities     

Performance analysis and code optimization of low densityparity-check codes on Rayleigh fading channels

A numerical method has been presented to determine the noise thresholds of low density parity-check (LDPC) codes that employ the message passing decoding algorithm on the additive white Gaussian noise (AWGN) channel. In this paper, we apply the technique to the uncorrelated flat Rayleigh fading channel. Using a nonlinear code optimization technique, we optimize irregular LDPC codes for such a channel. The thresholds of the optimized irregular LDPC codes are very close to the Shannon limit for this channel. For example, at rate one-half, the optimized irregular LDPC code has a threshold only 0.07 dB away from the capacity of the channel. Furthermore, we compare simulated performance of the optimized irregular LDPC codes and turbo codes on a land mobile channel, and the results indicate that at a block size of 3072, irregular LDPC codes can outperform turbo codes over a wide range of mobile speeds     

多径衰落信道下的扩频码设计与联合检测

在多径衰落信道CDMA系统中，扩频码设计与联合检测为提高系统容量的两个重要手段。在扩频码设计中，通过构造零相关窗码可以消除MAI和ISI，从而提高系统容量。当扩频码相关特性不理想时，通过联合检测也可以消除干扰，提高系统容量。但二者都有一定局限性，对于零相关窗码来说，它的码字资源十分有限，对于联合检测来说，当用户数很大，它的复杂度不能容忍。该文将扩频码设计与联合检测结合起来，提出了一种具有组间零相关窗特性的新型扩频码。在扩频码设计时考虑联合检测，可以增加码字资源，在联合检测中考虑扩频设计，又可以降低联合检测的复杂度。     

Space-time code design for CPFSK modulation over frequency-nonselective fading channels

We derive a novel space-time code (STC) design criterion for continuous-phase frequency-shift keying (CPFSK) over frequency-nonselective fading channels. Our derivation is based on a specific matrix that is related to the input symbols of the CPFSK modulators. With this code-design criterion, we propose a simple interleaved space-time encoding scheme for CPFSK modulation over frequency-nonselective correlated fading channels to exploit potential temporal and spatial diversity advantages. Such an encoding scheme consists of a ring convolutional encoder and a spatial encoder, between which a convolutional interleaver is placed. A decoding algorithm that generates symbol metrics for the Viterbi decoder of convolutional codes from the spatial modulation trellis is examined. Simulation results confirm that the advantages of the combination of the interleaved convolutional encoding (for temporal diversity) and the spatial encoding (for spatial diversity) are promising for various system parameters.     

M进制直扩系统在平坦衰落下的性能分析

分析了采用RS(n，k)编码的一种在MSK调制方式下基于循环移位的M进制直接序列扩频通信系统在Rayleigh衰落信道中的性能，给出了准确的误码率公式。并通过SPW软件仿真给出了该系统在Rayleigh衰落环境下的性能曲线，仿真得到的误码率与公式推导的结果完全吻合。     

Direct-sequence spread-spectrum parallel acquisition innonselective and frequency-selective Rician fading channels

This work presents the performance of the direct-sequence spread-spectrum (DS-SS) parallel acquisition system, previously proposed by the authors (1989, 1990), for nonselective and frequency-selective Rician (i.e. specular plus Rayleigh) fading channels. The acquisition system utilizes a bank of parallel I-Q noncoherent matched filters for the search mode, and a coincidence detector for the verification mode. The probabilities of detection and false alarm are derived and the mean and variance of the acquisition time are evaluated as a measure of the system performance. The nonselective channel is a Rayleigh fast fading channel, while the frequency-selective channel model is the so-called wide sense stationary uncorrelated scattering (WSSUS), selective only on frequency. These channels are typical for aircraft-satellite and line-of-sight (LOS) communications     

Code timing acquisition for DS-CDMA in fading channels bydifferential correlations

We propose simple and efficient algorithms for the code timing acquisition in the direct-sequence code-division multiple-access communication system. The essential assumption is that a preamble or an unmodulated pilot channel is available for the desired user. Then the correlation matrix R(τ) of the sampled data, where τ is suitably chosen time lag, contains the timing information only of desired user, while the contributions of uncorrelated interferers and noise are suppressed out. Hence, compared to the conventional approach, more interference suppression is achieved. Coarse delay estimates are then obtained by a matched filter (MF) or multiple signal classification-type approaches. In the latter case, only L eigenvectors are computed, where L is the number of resolvable paths. If only one path exists, an additional procedure is proposed to both approaches, by which the estimation accuracy is greatly improved with negligible increase in computation. More precisely, the chip timing offset due to chip-asynchronous sampling can be determined by solving a system of two second-order polynomials for each chip interval. Therefore, only at most 2C hypotheses are needed, where C is the processing gain. All the proposed methods are computationally quite simple, containing mainly MF-operations, or at most computation of only few eigenvectors. Mean acquisition time analysis is carried out semi-analytically. Numerical experiments speaks for the possibility of achieving significant performance gains compared to conventional acquisition, especially in the presence of strong multiple-access interference, making them attractive options to be attached for the next generation mobile receivers     

Filterbank-based blind code synchronization for DS-CDMA systems in multipath fading channels

We present a filterbank approach to blind code synchronization for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems. The key idea of the proposed scheme is to first pass the received signal through a bank of filters, which are designed to enhance signals of interest and suppress interference/noise, and then to derive the code timing from the filtered data. The only required knowledge by the proposed filterbank scheme is the spreading code of the desired user. It can be used in various environments, including frequency-nonselective and frequency-selective, time-invariant, and time-varying fading channels. It can deal with colored channel noise and unmodeled interference, such as inter-cell interference (ICI) and narrowband interference. It has relatively low complexity and can be readily implemented using standard adaptive algorithms. We show that under mild conditions, the proposed scheme yields statistically consistent [in signal-to-noise ratio (SNR)] code timing estimates, irrespective of the strength of the interference and with only a finite number of data samples. We also derive an unconditional Cramer-Rao bound (UCRB), which serves as a lower bound for all unbiased blind code synchronization schemes. Numerical results indicate that the proposed scheme compares favorably with a popular subspace-based method in terms of user capacity, near-far resistance, and robustness to time-varying fading and unmodeled interference.