Fading rate and multipath delay effects on signal acquisition using sliding correlation in a multi-user environment

The ability of spread spectrum systems to combat interference can only be realised if the receiver is precisely synchronised with the incoming signal. The use of the sliding correlator for the acquisition of direct sequence spread spectrum signals is a popular method. Previous work considered either the single user in the fading and delay mobile environment or multi-users in the additive white Gaussian noise environment. In this paper the acquisition performance of the sliding correlator for direct sequence spread spectrum signals is investigated for the uplink case and in an environment where interference is caused both by multiple users and by multipath delays with different fading rates. The results show that a longer correlator length is required for optimum mean acquisition time in a slow fading environment. However in the multi-user, multipath environment when larger delays are present, the use of a longer correlation length is necessary in order to optimise the mean acquisition time.     

Error Probability Evaluation for Systems Employing Differential Detection in a Rician Fast Fading Environment and Gaussian Noise

A method is presented for determining the error probability of a receiver using differential detection in the presence of Gaussian noise and fast Rician fading. Equations for the covariances of the fading component are derived, which include the effect of IF filter distortion. It is shown that these equations may be readily evaluated numerically. A simple formula for the error probability is derived for systems using BPSK and a matched filter receiver. An example of the error probability is given using this receiver. Also given is an example of a system using MSK with a practical IF filter. Different spectral shapes and bandwidths for the fading process are investigated for this example and their effect on the error probability is determined.     

A comparison of long versus short spreading sequences in codedasynchronous DS-CDMA systems

The performance of turbo-coded asynchronous direct sequence code division multiple access (DS-CDMA) using long and short spreading sequences is compared by both analysis and simulation. For coded systems with a conventional matched filter (MF) receiver, three analytical methods with different complexity are compared: the standard Gaussian approximation, the improved Gaussian approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequence systems, the IGA gives an accurate estimate for the performance with much less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the flat Rayleigh fading channel and a MF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are not time-varying. However, the short-sequence systems are amenable to the use of an interference suppression technique designed to minimize the mean square error. Such a minimum mean square error (MMSE) receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel     

Combined Transmitter Diversity and Multi-Level Modulation Techniques

Orthogonal transmitter diversity such as frequency diversity and time diversity is quite simple to implement and, with optimum signal combining, can take full advantage of fading multipath channels. However, such a scheme has a bandwidth efficiency that decreases inversely with the number of diversity branches making it less attractive in wireless communications applications. This paper considers combined orthogonal transmitter diversity and multi-level linear modulation techniques. The idea is to view the signal constellations of the modulation scheme in an augmented signal space formed by the modulation signal dimension and the number of branches of the transmitter diversity scheme. This augmented signal space provides a good spread for the modulation signal points and can be quite efficient for high-level linear modulation techniques. The obtained results show that this combined scheme, not only improves the system performance on both additive white Gaussian noise and fading multipath channels, but also improves the bandwidth efficiency of orthogonal transmitter diversity.     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Decorrelating detectors for quasi-synchronous CDMA

To date, two forms of spread spectrum code division multiple access (CDMA) have been considered in the literature. Either synchronous CDMA, where all users are both chip and bit synchronized relative to each other, or asynchronous CDMA, where all users are neither chip nor bit synchronized relative to each other. Between these two extremes lies quasi-synchronous CDMA (QS-CDMA) where users are not chip synchronized but are approximately bit synchronized. Quasi-synchronous CDMA arises in microcell systems where the combined propagation time and delay spread produces a variation in the round trip delay time limited to a few chips.Detection of QS-CDMA at the base station is considered. It is shown that memoryless detection can be achieved by appending blank chips to the user's spreading codes.Two decorrelating detectors are proposed and analyzed. The first uses a matched filter bank to produce a discrete time signal, and the second uses a single oversampling integrate-and-dump filter to produce a discrete time signal. For the first detector, each matched filter is synchronized to its respective user, and for the second detector, the integrate-and-dump filter is synchronized to the base station transmissions. The performance of the two detectors is shown to be approximately equal provided the integrate-and-dump filter is sampled at least two times the chip rate.     

A New Adaptive Code-Acquisition Algorithm Using Parallel Subfilter Structure

This paper presents a new adaptation algorithm using parallel subfilter structure, which enables fast-code acquisition for direct-sequence spread-spectrum systems. The proposed algorithm adapts the filter, based on a concept of dividing it into several subfilters, thus reducing the adaptation time. The detection and false-alarm probabilities are derived analytically. Simulation results show that the adaptive code-acquisition scheme, using the proposed algorithm, improves the mean-acquisition-time performance by 4-5 and 3-4 dB on additive white Gaussian noise and multipath Rayleigh fading channels for 64 taps, respectively     

Chip-delay locked matched filter for DS-CDMA systems using longsequence spreading

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small     

High level trellis-coded modulation with slow frequency hopping forland mobile communications

The use of high level modulation systems such as trellis-coded 8-PSK and trellis-coded 16-QAM, with slow frequency hopping, pilot symbol aided and small-sized interleaving techniques is proposed for land mobile communications. The bit error rate (BER) performance of those systems in the presence of additive white Gaussian noise (AWGN), cochannel interference (CCI), and slow Rayleigh fading are evaluated by computer simulation. Space diversity is applied to enhance the system performance. The authors show that by choosing optimum interleaving size and number of frequency hopped (FH) channels, the proposed systems give considerable performance improvement and much less delay, especially in a slow fading environment     

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     

Unquantized and uncoded channel state information feedback in multiple-antenna multiuser systems

We propose a channel state information (CSI) feedback scheme based on unquantized and uncoded (UQ-UC) transmission. We consider a system where a mobile terminal obtains the downlink CSI and feeds it back to the base station using an uplink feedback channel. If the downlink channel is an independent Rayleigh fading channel, then the CSI may be viewed as an output of a complex independent identically distributed Gaussian source. Further, if the uplink feedback channel is an additive white Gaussian noise channel, and the downlink CSI is perfectly known at the mobile terminal, it can be shown that UQ-UC CSI transmission (that incurs zero delay) is optimal in that it achieves the same minimum mean-squared error distortion as a scheme that optimally (in the Shannon sense) quantizes and encodes the CSI, while theoretically incurring infinite delay. Since the UQ-UC transmission is suboptimal on correlated wireless channels, we propose a simple linear CSI feedback receiver that can be used to improve the performance of UQ-UC transmission while still retaining the attractive zero-delay feature. We provide bounds on the performance of such UQ-UC CSI feedback and study its impact on the achievable information rates. Furthermore, we explore its application and performance in multiple-antenna multiuser wireless systems, and also propose a corresponding pilot-assisted channel-state estimation scheme.     

Application of Adaptive Filtering to Direct-Sequence Spread-Spectrum Code Acquisition

A novel hybrid scheme utilizing an adaptive FIR filter is proposed for acquisition of DS-SS signals. Timing information on the delay offset between the incoming DS-SS signal and the locally generated replica of the spreading code is extracted from the tap-weight vector of the acquisition adaptive filter. Expressions for the mean acquisition time, detection, and false alarm probabilities for a coherent, chip synchronous DS-SS system in AWGN are derived. The improvement in acquisition performance over serial search techniques is twice the length of the adaptive filter. This is similar to that gained by other hybrid schemes that search the same number of cells at a time. However, a significant reduction in hardware complexity is obtained. The proposed system is also compared to a system utilizing a partial matched filter structure. Moreover, the same hardware could be used for code tracking and, hence, eliminating the need for a separate tracking loop.     

Differential detection of Gaussian MSK in a mobile radio environment

Minimum shift keying with Gaussian shaped transmit pulses is a strong candidate for a modulation technique that satisfies the stringent out-of-band radiated power requirements of the mobil radio application. Numerous studies and field experiments have been conducted by the Japanese on urban and suburban mobile radio channels with systems employing Gaussian minimum-shift keying (GMSK) transmission and differentially coherent reception. A comprehensive analytical treatment is presented of the performance of such systems emphasizing the important trade-offs among the various system design parameters such as transmit and receiver filter bandwidths and detection threshold level. It is shown that two-bit differential detection of GMSK is capable of offering far superior performance to the more conventional one-bit detection method both in the presence of an additive Gaussian noise background and Rician fading.     

Code acquisition of DS/SS signals in fading channels using an LMSadaptive filter

The mean acquisition time performance for a direct-sequence spread-spectrum (DS/SS) code acquisition technique utilizing a finite impulse response (FIR) least-mean-square (LMS)-adaptive filter is investigated in fading conditions. The optimum tap-weight vector for the adaptive filter is derived and computer simulation results are presented to assess the acquisition performance for the system. It is shown that fading causes a slight degradation to the mean acquisition time (less than 3-dB signal-to-noise ratio (SNR) degradation) which is far less than that incurred by other systems utilizing matched filters (about 10 dB). In frequency selective fading channels, the adaptive nature of the proposed scheme is exploited to acquire the strongest received path and as the paths fade the new strongest available path is acquired     

A new analysis of direct-sequence pseudonoise code acquisition onRayleigh fading channels

Accurate performance evaluation of direct-sequence pseudonoise code acquisition on Rayleigh fading channels is investigated. For fading channels the homogeneous Markov chain model, used to characterize the acquisition process over additive white Gaussian noise channels, is no longer valid due to the correlations between cell detections incurred by fading. Hence, the traditional direct and flow-graph approaches for performance evaluation of the code acquisition are not applicable to fading channels. In this paper, a new analysis is proposed for accurate evaluation of the acquisition performance on Rayleigh fading channels. The analysis is quite general and can include various search strategies, types of correlators, and test methods with different performance measures: probability mass function and/or moments of acquisition time. Analytical and simulation results show that the new method predicts the acquisition performance very accurately     

Influence of fading on the Gaussian approximation for BPSK and QPSK with asynchronous cochannel interference

We investigate the performance of BPSK and QPSK with coherent detection and matched filtering in the presence of both time and phase asynchronous cochannel interfering signals. More precisely, we analyze the role played by different channel statistics on the distribution of the decision variable at the output of the matched filter. The results show that the Gaussian approximation is accurate not only in the (obvious) case of a large number of interferers, but also when the desired signal is subject to fading, whatever the number of interferers is. For example, when the desired signal is subject to Rayleigh fading, even in the presence of only one unfaded interferer the Kullback-Leibler distance between the exact distribution of the decision variable and that obtained with the Gaussian approximation on the interference is lower than 0.01 [nats] for all cases of practical interest.     

Signal detection using third-order moments

The noise suppression capability of higher-order moments and spectra has made them attractive when the goal is to extract or reconstruct a signal that is contaminated by multivariate Gaussian noise or certain types of non-Gaussian noise. Two new detectors, one centralized and one distributed, which are based on the third-order moment of the data are proposed. The asymptotic performance of the centralized detector and the asymptotic distribution of the components of the distributed detector are analyzed. Further, the performance of these detectors is simulated and compared to that of the matched filter for three different types of interference: Gaussian noise, Gaussian noise corrupted by a sinusoid with random phase, and Arctic under-ice noise.     

Spread-response precoding for communication over fading channels

Interleaving is an important technique for improving the effectiveness of traditional error-correcting codes in data transmission systems that exhibit multipath fading. Such channels often arise in mobile wireless communications. We present an alternative to interleaving for such systems, which we term “spread-response precoding”. From the perspective of the coded symbol stream, spread-response precoding effectively transforms an arbitrary Rayleigh fading channel into a nonfading, simple white marginally Gaussian noise channel. Furthermore, spread-response precoding requires no additional power or bandwidth, and is attractive in terms of computational complexity, robustness, and delay considerations     

Direct sequence spread spectrum matched filter acquisition infrequency-selective Rayleigh fading channels

This paper presents the performance of a matched filter acquisition scheme in nonselective and frequency-selective Rayleigh fading channels. This acquisition scheme employs a reference filter to estimate the variance of the interference at the output of the detecting matched filter. Expressions for probability of detection and probability of false alarm are derived and the mean acquisition time is evaluated for various channel and system parameters. The results presented show the relationship between channel and system parameters and the system performance     

Chaos-Coded Modulations Over Rician and Rayleigh Flat Fading Channels

In this brief, we analyze a kind of chaos-coded modulations over both Rician and Rayleigh frequency non-selective uncorrelated fading in the presence of additive white Gaussian noise. We provide bounds both for the case when perfect channel-state information (CSI) is available at the decoder and when there is no CSI. We show that the bounds proposed can be tight enough to give reason of the behavior of these systems in a flat fading channel. We compare the results with a related trellis-coded modulation and show that the degradation in performance can be at least as low as with conventional coded modulation systems.