瑞利信道下基于信号采样自相关的信号检测技术

信号检测技术是认知无线电中的首要及关键技术之一。为了提高低信噪比条件下的信号检测性能,采用基于接收信号采样自相关的检测方法来进行授权用户信号检测,并用捕获的数字电视信号以及随机产生的二相相移键控信号作为信号源,在MATLAB中进行信号检测仿真。仿真结果表明,在瑞利衰落信道以及低信噪比(特别是信噪比≤–14 dB)情况下,该检测方法与传统的能量检测方法相比,有较好的检测性能。此外,该检测方法中被检测信号相关性程度对检测概率也有一定影响。     

BER of OFDM in Rayleigh fading environments with selective diversity

This paper derives the analytical bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) systems employing selective diversity in Rayleigh fading environments. First, the probability density function (pdf) of inter‐carrier interference (ICI) power in Rayleigh fading environments is derived. Second, the pdf of the signal‐to‐interference ratio (SIR) is then derived. The cumulative distribution function (cdf) and hence pdf of the SIR with selective diversity are then given in the third and fourth steps. Using the BER expressions of binary phase shift keying (BPSK) and differential BPSK (DBPSK) modulation in an added white Gaussian noise (AWGN), the corresponding BER expressions of OFDM systems can then be derived. Detailed discussions are given. Possible future work is also outlined. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimum scheduling for smart antenna systems in Rayleigh fading channels

Optimum and suboptimum scheduling schemes are proposed in the uplink of array antenna systems. We assume that once a user is permitted to transmit the data, the user transmits the data using the maximum power, and adjusts the transmission bit rate so that the energy-per-bit-to-interference-plus-noise-density ratio should be equal or higher than a threshold. In this system model, we consider an optimization problem: How many and which users should be selected to transmit their data at a time in order to maximize the throughput? Based on the analysis on the complexity of the optimum scheme, we propose another optimum scheme having reduced complexity. To reduce the complexity further, we also propose three suboptimum scheduling schemes by limiting the number of users simultaneously transmitting, by choosing the users with higher channel gains, and by partitioning a cell into multiple parts. In circular and linear arrays, the performance results of all the proposed schemes are presented and compared.     

The performance of a -steering beamformer in correlated Rayleigh fading

We consider the performance of a -steering adaptive beamformer in a distributed Rayleigh fading environment. To determine the beamformer coefficients, the signal and interference array response vectors must be estimated. An estimator that makes use of the knowledge of the spatial distribution of the signal is proposed and compared with other estimators that do not use this information. As the angular spread of the signal increases, the value of this information is reduced, until, for the case of isotropic scattering, the estimate that uses the spatial distribution is equivalent to the estimate that does not. Performance results are presented that compare the performance of the different estimators. We derive closed-form expressions for the signal-to-interference-plus-noise ratio (SINRO) and probability of error for the case where the training data are orthogonal.     

Performance of an array receiver with a Kalman channel predictorfor fast Rayleigh flat fading environments

We develop an approach for using an antenna array for tracking fast Rayleigh flat fading channels and suppressing cochannel interference. The fast flat fading process is assumed to be a general autoregressive (AR) process in order to characterize temporal variation of channels and evaluate its effect on the receiver structure and performance. The optimal array receiver structure that minimizes the probability of error for BPSK signals is derived, which includes a Kalman filter to predict the fading channels. A simple integral expression for the probability of error is also derived for the optimal receiver. In particular, we analyze the case with identical shaping filters. An irreducible probability of error is shown to exist due to the prediction error of multiple channels. Another interesting observation from the study is that the diversity gain with m antenna elements in the presence of k interferences is usually greater than (m-k), even in the presence of channel prediction error. Simulations are carried out to verify the theoretical analysis     

Multicellular Alamouti scheme performance in Rayleigh and shadow fading

In this paper, we study the performance of two downlink multicellular systems: a multiple inputs single output (MISO) system using the Alamouti code and a multiple inputs multiple outputs (MIMO) system using the Alamouti code at the transmitter side and a maximum ratio combining (MRC) as a receiver, in terms of outage probability. The channel model includes path-loss, shadowing, and fast fading, and the system is considered interference-limited. Two cases are distinguished: constant shadowing and log-normally distributed shadowing. In the first case, closed form expressions of the outage probability are proposed. For a log-normally distributed shadowing, we derive easily computable expressions of the outage probability. The proposed expressions allow for fast and simple performance evaluation for the two multicellular wireless systems: MISO Alamouti and MIMO Alamouti with MRC receiver. We use a fluid model approach to provide simpler outage probability expressions depending only on the distance between the considered user and its serving base station.     

Performance limitation of closed-loop transmit antenna diversityover fast Rayleigh fading channels

Previously, transmit antenna diversity (TAD) has been considered as a key technique to improve the capacity of wireless systems over a fading channel. A type of TAD is closed-loop TAD, which utilizes channel conditions. Closed-loop TAD has a drawback caused by the feedback delay under a fast fading channel environment. If the channel information is outdated due to the feedback delay, the performance can be severely degraded. In this correspondence, we find the bit error rate (BER) of the closed-loop TAD with the feedback delay for a time-variant Rayleigh fading channel. For example, it is observed that there is about 2-dB degradation at the BER of 10-2 when four transmit antennas are used, and the feedback delay is 5 ms with the updating rate 400 Hz     

Error performance of multiple-symbol differential detection of PSKsignals transmitted over correlated Rayleigh fading channels

The error performance of multiple-symbol differential detection of uncoded QPSK signals transmitted over correlated Rayleigh fading channels is studied. The optimal detector is presented, along with an exact expression for the corresponding pairwise error event probability. It is shown that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor associated with a conventional differential detector. In all of the cases investigated, a detector with an observation interval as small as two symbols is sufficient for this purpose. It is also found that the error performance of a multiple-symbol differential detector is not sensitive to the mismatch between the decoding metric and the channel fading statistics     

New performance results for MDPSK with noncoherent diversity in fast Rayleigh fading

The error performance for M-ary differential phase-shift keying (MDPSK) with non-coherent diversity in fast fading was recently tackled, ending up with a very general formula applicable to various operational environments. The only drawback of this formula is its expression with a double integral which, alongside a singularity existing in the integrand, poses considerable burdens in numerical calculation. In this correspondence, by restricting to the case of independent fast Rayleigh fading channels with equal power, we show how to obtain a much simpler solution with only one integral.     

Performance analysis of optimum combining in antenna array systems with multiple interferers in flat Rayleigh fading

In this letter, we examine the statistical distribution of the signal-to-interference-plus-noise ratio (SINR) of the optimum combining (OC) technique in receive-antenna diversity systems. We decompose the SINR into two components by projection onto the interference subspace for systems with more antennas than interferers. For the case when the number of interferers is larger than the number of antennas, upper and lower bounds on performance are provided.     

Performance analysis of the closed-loop transmit antenna diversitysystem over Rayleigh fading channels

As a diversity technique, transmit antenna diversity (TAD) recently has been considered for third-generation (3G) code-division multiple-access systems. There are two different types of TAD techniques: closed loop and open loop. In this correspondence, the performance of the closed-loop TAD system equipped with multiple transmit and receive antennas is analyzed using bounds of the pairwise error probability (PEP) under a flat fading channel environment. In addition, the cutoff rate from the Chernoff bound has been considered to understand the overall performance, which may include channel coding     

Iterative diversity detection for correlated continuous-time Rayleigh fading channels

An antenna array is proposed as a means of achieving a space-diversity effect that partly overcomes the severity of continuous-time Rayleigh fading channels. The investigated channel is assumed to be frequency-nonselective with correlated diversity links, where the correlation is related to the array geometry and the spatial and Doppler dispersions. Further, the error performance is improved by bit interleaving and channel coding, where the encoders/channel is viewed as a serially concatenated system: a convolutional code constitutes the outer code, whereas a differential encoder and the fading channel (having truncated memory) form a joint inner code. In order to obtain a practical detector structure it is desirable to perform iterative decoding by applying some a posteriori probability (APP) algorithms. For this purpose, we propose a novel generalization of the well-known Bahl-Cocke-Jelinek-Raviv (1974) algorithm that calculates the APPs over channels having memory. Numerical results indicate that iterative decoding becomes more powerful when the exploited channel memory depth is extended. Also, the error performance is significantly improved by introducing multiple antennas. The interleaver gain is, however, seen to be quite moderate, in contrast to additive white Gaussian noise channels.     

Quasi-maximum-likelihood multiple-symbol differential detection for time-varying Rayleigh fading channel

The maximum-likelihood multiple-symbol differential detector (ML-MSDD) has better bit-error-rate performance than many other detectors for differential modulation. Unfortunately, the computational complexity of ML-MSDD quickly becomes prohibitive as the observation window size grows. While low-complexity MSDD algorithms for the time-invariant Rayleigh fading channel have been considered before, there is a need for low-complexity MSDD algorithms for general time-varying Rayleigh fading channels. A polynomial-time complexity approach called semi-definite relaxation (SDR) is employed to achieve differential detection with near maximum-likelihood (ML) performance. The proposed SDR quasi-maximum-likelihood (QML) multiple-symbol differential detection (SDR-QML-MSDD) is efficient in that its complexity is polynomial in the observation window size, even in the worst case, while it exhibits almost the same performance as ML-MSDD does.     

PERFORMANCE OF MULTIUSER DETECTION FOR CDMA SYSTEM IN A FLAT RAYLEIGH FADING ENVIRONMENT

In a CDMA communication network using conventional signal detection, system capacity is limited and its performance is degraded by the multi-access interference (MAI). Multiuser detection, which makes use of cross-correlation information between spread spectrum codes, can reduce or eliminate the MAI in a Gaussian channel so as to mitigate the near-far effects and increase the system capacity. This paper extends the multiuser detector to a flat Rayleigh fading CDMA environment, and discusses the bit error rate of typical multiuser detection algorithms in such a environment by combining theoretical analysis and computer simulation. It is shown that multiuser detection is superior to conventional detections in the flat Rayleigh fading channels.     

Signal design for transmitter diversity wireless communicationsystems over Rayleigh fading channels

Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters     

Signal space diversity: a power- and bandwidth-efficient diversitytechnique for the Rayleigh fading channel

The increasing need for high data-rate transmissions over time- or frequency-selective fading channels has drawn attention to modulation schemes with high spectral efficiency such as QAM. With the aim of increasing the “diversity order” of the signal set we consider multidimensional rotated QAM constellations. Very high diversity orders can be achieved and this results in an almost Gaussian performance over the fading channel, This multidimensional modulation scheme is essentially uncoded and enables one to trade diversity for system complexity, at no power or bandwidth expense     

Baseband performance of a pilot diversity system with simulated Rayleigh fading signals and co-channel interference

Using simulated Rayleigh-fading FM signals, the baseband performance of a diversity system that uses a separate pilot is studied. The effect of various RF/audio bandwidth expansion ratios is included. These results show an approach to a constant asymptotic baseband SNR for large expansion ratios, with diversity increasing this asymptote by 5 dB in the case of moderate to large interference and/or RF noise levels (9 to 15 dB below the carrier), to over 10 dB in the case of lower impairment levels (21 to 27 dB below the signal level). Assuming the usual hex-cell radio coverage plan, and a propagation law of the form r^-n, n = 3 or 4, bandwidth expansion ratios of about three to six seem to yield the most efficient RF spectrum utilization in many practical situations. The number of required channel subsets (cells) is strongly influenced by the propagation law ranging from about 19 for n = 3, to about 7 for n = 4.     

Adjacent channel interference cancellation for MSK-type signals using antenna diversity in Rayleigh fading environment

This paper addresses the problem of adjacent channel interference between minimum-shift keying (MSK)-type signals transmitted through Rayleigh fading channels. A receiver, based on the multiple-input, multiple-output antenna diversity concept and the maximum-likelihood criterion, allows the transmission of MSK-type signals with very small channel spacings, greatly improving the bandwidth efficiency of these signals. The concept of antenna diversity is used to greatly reduce the effects of adjacent channel interference for channels with Rayleigh fading. The results show that it is possible to transmit bit-synchronized MSK-type signals with a very small normalized channel spacing (e.g., /spl Delta/fT/sub b/=/spl plusmn/0.4) between channels, under almost any power imbalance between signals, with very small degradation in performance.     

Probability of cochannel interference in the case of noncoherentphasor addition with unequal powers in Rayleigh fading environments

New, closed form expressions are derived for calculation of the probability of cochannel interference (PCI) in Rayleigh fading environments when interfering phasors with unequal mean powers add noncoherently in the receiver. Expressions enable PCI calculation at every point within a cell and/or cluster, both in base stations and mobile units     

Error probability performance of L-branch diversity reception ofMQAM in Rayleigh fading

New exact expressions involving hypergeometric functions are derived for the symbol-error rate (SER) of M-ary quadrature amplitude modulation (MQAM) for L branch diversity reception in Rayleigh fading and additive white Gaussian noise (AWGN). The diversity combining techniques considered are maximum ratio combining (MRC) and selection combining (SC). MRC with identical channels and dissimilar channels are analyzed