A robust channel estimator for DS-CDMA systems under multipath fading channels

This paper addresses the problem of channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. The multipath fading channels are modeled as autoregressive (AR) models. A method is first proposed to convert the time-varying regression model due to the time-varying nature of users' information symbols into a time-invariant one. Then, a polynomial approach is proposed to obtain the minimum mean square error (MMSE) estimator. The uncertainty of the channel model and decision errors of the DS-CDMA detector are taken into consideration in the design of the MMSE estimator. Compared with the Kalman estimator, the computational complexity of the proposed algorithm is much lower. The simulation results show that the proposed estimator provides a comparable estimation performance with the Kalman estimator and is robust for fast-fading channels.     

On the asymptotic performance of threshold-based acquisition systems in multipath fading channels

The asymptotic performance of timing acquisition systems having fixed dwell time in multipath fading channels is investigated. The detrimental effect of the multipath channel fading on the acquisition performance is isolated by considering the asymptotic performance as the average signal-to-noise ratio (SNR) increases. It is found that for any threshold such that the average probability of false alarm is less than a given tolerance, the channel fading results in a lower bound on the asymptotic average probability of miss which is nontrivial for a variety of fading scenarios. A threshold-based direct-sequence spread-spectrum signal acquisition system is considered and it is found that the detrimental effect of channel fading on asymptotic acquisition performance, albeit nontrivial, is not very significant. The asymptotic acquisition performance of two threshold-based acquisition schemes for ultra-wideband (UWB) signals with time-hopping (TH) spreading are also evaluated and compared. For both schemes, the detrimental effect of the channel fading on the asymptotic acquisition performance turns out to be significant.     

BER and outage probability for land mobile satellite channel withmaximal ratio combining

An exact analytical technique is presented for computing the average bit error rate (BER) and outage probability of differentially detected PSK in the land mobile satellite channel (LMSC) when L branch maximal ratio combining (MRC) is employed. Following a previous empirical study, the LMSC is modelled as a weighted sum of Rice and Suzuki distributions. Numerical results are provided     

Method to measure and estimate total outage probability for PMD-impaired systems

A programmable polarization-mode-dispersion (PMD) source is used to measure receiver performance in the presence of first- and second-order PMD. Combined with scalable joint probability-distribution functions of PMD statistics, total outage probability can be estimated.     

Service outage based power and rate allocation for parallel fading channels

The service outage based allocation problem explores variable-rate transmission schemes and combines the concepts of ergodic capacity and outage capacity for fading channels. A service outage occurs when the transmission rate is below a given basic rate r/sub o/. The allocation problem is to maximize the expected rate subject to the average power constraint and the constraint that the outage probability is less than /spl epsi/. A general class of probabilistic power allocation schemes is considered for an M-parallel fading channel model. The optimum power allocation scheme is derived and shown to be deterministic except at channel states of a boundary set. The resulting service outage achievable rate ranges from 1-/spl epsi/ of the outage capacity up to the ergodic capacity with increasing average power. Two near-optimum schemes are also derived by exploiting the fact that the outage probability is usually small. The second near-optimum scheme significantly reduces the computational complexity of the optimum solution; moreover, it has a simple structure for the implementation of transmission of mixed real-time and non-real-time services.     

Performance analysis of cognitive relay systems with spectrum-sharing interference under a primary outage probability constraint简

This paper investigates the performance of an underlay cognitive relay system where secondary users（SUs） suffer from a primary outage probability constraint and spectrum-sharing interference imposed by a primary user（PU）. In particular, we consider a secondary multi-relay network operating in the selection decode-and-forward（SDF） mode and propose a best-relay selection criterion which takes into account the spectrum-sharing constraint and interference. Based on these assumptions, the closed-form expression of the outage probability of secondary transmissions is derived. We find that a floor of the outage probability occurs in high signal-to-noise ratio（SNR） regions due to the joint effect of the constraint and the interference from the PU. In addition, we propose a generalized definition of the diversity gain for such systems and show that a full diversity order is achieved. Simulation results verify our theoretical solutions.     

Ergodic capacity and outage probability optimization for secondary user in cognitive radio networks under interference outage constraint

This paper considers a cognitive radio network where a secondary user (SU) coexists with a primary user (PU). The interference outage constraint is applied to protect the primary transmission. The power allocation problem to jointly maximize the ergodic capacity and minimize the outage probability of the SU, subject to the average transmit power constraint and the interference outage constraint, is studied. Suppose that the perfect knowledge of the instantaneous channel state information (CSI) of the interference link between the SU transmitter and the PU receiver is available at the SU, the optimal power allocation strategy is then proposed. Additionally, to manage more practical situations, we further assume only the interference link channel distribution is known and derive the corresponding optimal power allocation strategy. Extensive simulation results are given to verify the effectiveness of the proposed strategies. It is shown that the proposed strategies achieve high ergodic capacity and low outage probability simultaneously, whereas optimizing the ergodic capacity (or outage probability) only leads to much higher outage probability (or lower ergodic capacity). It is also shown that the SU performance is not degraded due to partial knowledge of the interference link CSI if tight transmit power constraint is applied.     

The relationship between fading and bandwidth for multipath channels

A signal transmitted over a multipath channel experiences fading, the variability of which is a function of: 1) signal bandwidth; and 2) power delay profiles of the channel's specular and diffuse components. We analyze this general relationship and, for several important classes of multipath channel, we derive simple, closed-form approximations for what we call the stability bandwidth, W/sub 0/. For signal bandwidths greater than W/sub 0/, the local area variation in the received signal power is acceptably small, e.g., less than 1 dB standard deviation of decibel power. We demonstrate the accuracy of our W/sub 0/ approximations for some representative cases. Moreover, we show that the root mean square delay spread-a statistic commonly used to characterize multipath channels-has limited utility in estimating W/sub 0/.     

Evaluation of outage probability of microwave radio relay systems carrying both analogue (FM) and digital (FSK) RF channels

In the letter, a new formula is derived for the (multipath) outage probability of (both analogue and digital) signals present in an existing analogue radio relay link operating below 11 GHz when some analogue channels are replaced by digital ones.     

On the average outage rate and average outage duration of wireless communication systems with multiple cochannel interferers

This paper studies the average outage rate [or average level crossing rate (LCR)] and average outage duration (AOD) of wireless communication systems subject to cochannel interference. In particular, it presents closed-form expressions for the LCR and AOD when a minimum desired signal power requirement is specified for satisfactory reception. The results are quite general and account for systems operating over independent identically distributed Rician and/or Nakagami fading environments. When applicable, these new expressions are compared to those previously reported in the literature dealing with the LCR and AOD of 1) interference-limited systems when both the desired and interfering signals are subject to Rayleigh type of fading and 2) power-limited systems operating over Rician or Nakagami fading channels. Corresponding numerical examples that illustrate applications of the results are also provided and discussed. These results show that specifying a certain minimum desired signal power requirement induces a floor on the AOD. They also show that the AOD is essentially affected by the the maximum Doppler frequencies (or equivalently the speed) of the desired users.     

On the secrecy outage probability and performance trade-off of the multi-hop cognitive relay networks

Telecommunication Systems - We study the physical-layer security of multi-hop secondary network under spectrum sharing constraint caused by the primary, which consists of many simultaneously...     

MIMO-DCSK通信系统在瑞利衰落信道下的误码性能分析

对多输入多输出差分混沌相移键控(MIMO-DCSK)通信系统在瑞利衰落信道下的性能进行了分析,给出了基于中心极限定理的高斯近似误码率表达公式,并与计算机仿真数据进行比较。仿真结果表明:随着扩频因子的增大,高斯近似误码率曲线与实际仿真误码率曲线的一致性也随之增强;相比DCSK系统,随着发送天线数量和接收天线数量的增加,MIMO-DCSK系统的误码性能有较大提升,且增加接收天线的数量可以比增加发送天线的数量获得更大的增益;随着发送天线数量的增多,不同天线间信号的非正交性导致理论误码率曲线与仿真曲线出现差别,在扩频因子增大后趋于一致。     

On the probability density functions of outage and inter-outage durations of the capacity of Rayleigh fading channels

Approximations for the probability density functions (PDFs) of the capacity outage and inter-outage durations over Rayleigh fading channels are studied. Exact closed form expressions for the Rice probability functions of the level-crossing intervals of the capacity process are derived, assuming a symmetrical Doppler power spectral density (PSD). These probability functions, which are obtained by applying the classical level-crossing theory, are known to describe the PDFs of outage and inter-outage durations only over their initial behavior. Additionally, the derived quantities are used to calculate approximate solutions for the considered PDFs based on the assumption of statistical independence between the level-crossing intervals. Numerical examples, considering Rayleigh mobile-tomobile fading channels, are presented together with simulation results to illustrate the analysis and examine the validity of the derived expressions. Particularly, it is shown that the theoretical results obtained provide accurate approximations for the PDFs of outage and inter-outage durations at low and high outage levels, respectively.     

MIMO-assisted space-code-division multiple-access: linear detectors and performance over multipath fading channels

In this contribution, we propose and investigate a multiple-input-multiple-output space-division, code-division multiple-access (MIMO SCDMA) scheme. The main objective is to improve the capacity of the existing direct-sequence (DS)-CDMA systems, for example, for supporting an increased number of users, by deploying multiple transmit and receive antennas in the corresponding systems and by using some advanced transmission and detection algorithms. In the proposed MIMO SCDMA system, each user can be distinguished jointly by its spreading code signature and its unique channel impulse response (CIR) transfer function referred to as spatial signature. Hence, the number of users might be supported by the MIMO SCDMA system and the corresponding achievable performance are determined by the degrees of freedom provided by both the code signatures and the spatial signatures, as well as by how efficiently the degrees of freedom are exploited. Specifically, the number of users supported by the proposed MIMO SCDMA can be significantly higher than the number of chips per bit, owing to the employment of space-division. In this contribution, space-time spreading is employed for configuring the transmitted signals. Three types of low-complexity linear detectors, namely, correlation, decorrelating, and minimum mean-square error (MMSE) are considered for detecting the MIMO SCDMA signals. The bit-error rate performance of the MIMO SCDMA system associated with these linear detectors are evaluated by simulations, when assuming that the MIMO SCDMA signals are transmitted over multipath Rayleigh-fading channels. Our study and simulation results show that MIMO SCDMA assisted by multiuser detection is capable of facilitating joint space-time despreading, multipath combining, and receiver diversity combining, while simultaneously suppressing the multiuser interfering signals.     

Error probability performance prediction for multichannel reception of linearly modulated coherent systems on fading channels

This paper provides a framework for the analysis of error rate performance of maximal ratio combining multichannel reception of coherent linearly modulated systems over frequency-selective fading channels. Expressions for the error probability are developed, and the special cases for which closed-form or acceptable efficient numerical solutions are possible are delineated. A new efficient and accurate recursive method for the Ricean fading main resolved path case is presented for coherent quadrature amplitude modulation constellations. The effect of automatic gain control error on analysis of modulation schemes is demonstrated. Finally, numerical examples based on the maritime high-data-rate channel model are provided to demonstrate the usefulness of the framework.     

Effects of multipath propagation delay on uplink performance of synchronous DS-CDMA systems communicating in dispersive Rayleigh fading channels

The effect of the maximum asynchronous arrival time difference (AATD) of the delayed multipath components on the uplink performance of synchronous DS-CDMA systems is investigated, when communicating in a frequency-selective Rayleigh fading channel. Numerical results demonstrate that the achievable user capacity gain may be as high as 64% at a bit error rate (BER) of 10/sup -2/, when the maximum AATD is sufficiently low.     

Iterative receiver for joint detection and channel estimation in OFDM systems under mobile radio channels

In this paper, an iterative receiver for a joint data-detection and channel-estimation scheme is presented for orthogonal frequency-division multiplexing systems, which incorporates iterative decoding in the receiver. In the proposed scheme, a maximum a posteriori-based decoder and a channel estimator provide more reliable information on the coded bits for each other in an iterative manner. We first consider a practical implementation issue for the optimal minimum mean squared error two-dimensional (2-D) channel estimator as an essential element in the iterative receiver. To reduce the complexity of the 2-D estimator as suited to the iterative receiver, we focus on rigorously investigating how a separable estimator must be designed so that its structure may become asymptotically equivalent to that of the optimal 2-D estimator. Furthermore, we derive an analytical expression of the iterative process to evaluate a convergence performance as a function of the number of iterations and discuss its convergence property. Our simulation results demonstrate that the proposed iterative receiver achieves a near-ideal performance with only a few iterations under time-variant multipath fading channels.     

Choosing from redundant designs of power systems using system outage rate and cost

The push for high availability is on. As customers keep telling computer manufacturers, any outage to their operations is unacceptable. For those involved at the design stage, this usually means that functions within a computer's subsystems, like power, must be made fault-tolerant, or redundant. In its simplest form, redundancy replaces one component with two, so that the first failure will have no impact on the customer. More generally, a K-out-of-N design calls for N components to do the job of N − K + 1, so that the first K − 1 failures can be tolerated but the K-th failure causes a system outage. In this paper, we relate our own experience with power subsystems: how we chose from among different shades of redundancy by concentrating on outage rate and cost. To further simulate real life, we also considered the service strategy, i.e. whether (and when) to preventively replace a component whose failure did not cause the system to go down. A key result of the paper shows that a redundant design coupled with a well-chosen service strategy yields advantages beyond what either one alone would provide. The combination not only decreases the design's already low system outage rate, it makes the outage rate essentially constant—so that ease of predictability is ensured. Three power subsystem designs are outlined, as are the reliability and outage rate functions associated with them. Cost and service strategy are then considered, to help decide which is the optimal design based on total program cost projections and reliability objectives.