Performance of type-II hybrid ARQ in a shadowed Rician fading LEOsatellite-based spread slotted ALOHA communication channel

Type-II hybrid ARQ is applied to a shadowed Rician fading low earth-orbiting (LEO) satellite-based spread slotted ALOHA communication channel. In particular it is found to be effective in combating heavy shadowing     

Spread-spectrum path diversity in a shadowed Rician fadingland-mobile satellite channel

The effects of two types of path diversity techniques, namely selection diversity and maximal ratio combining, on the bit error probability are investigated for direct-sequence spread-spectrum (DS/SS) transmission in a land mobile satellite channel using coherent binary phase-shift keying (BPSK) modulation. It is assumed that the channel consists of a log-normally shadowed line-of-sight signal plus Rayleigh distributed multipath signals. The bit error probability is evaluated for light, average, and heavy shadowing. The performance is also measured in terms of the outage probability     

Rician慢衰落信道下的正交循环码M元扩频系统性能分析

本文提出了一种基于正交循环码的M元扩频系统,在发端将一条原型扩频码循环移位构成M个相互正交的扩频码,实现M元扩频,而接收机利用一个时频变换域复数匹配滤波器就可实现正交循环码M元非相干解扩解调.与传统的M元扩频系统相比较,正交循环码M元扩频系统具有很低的复杂度.本文推导出该接收机在Rician慢衰落信道条件下的误比特率的计算公式,给出并分析了计算机仿真结果.     

Multitone spread spectrum multiple access communications system ina multipath Rician fading channel

The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated     

Trellis coded modulation schemes for shadowed Rician fading channels

The Canadian mobile satellite (MSAT) channel has been modelled as the sum of lognormal and Rayleigh components to represent foliage attenuation and multipath fading, respectively. The paper derives a Chernoff based error bound on the performance of trellis coded modulation (TCM) schemes operating on this channel.<>     

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     

Direct-sequence spread spectrum narrowband interference rejectionusing property restoration

A new method of rejecting narrowband interference in direct-sequence spread spectrum (DS/SS) communications is presented. A typical approach is to reject the interference using a filter with large attenuation at the interference frequencies before despreading. The interference rejection method presented incorporates vector space projection techniques to suppress the correlated interference. Several signal characteristics are formulated which lead to constraint surfaces in the vector space of possible solutions. These constraint surfaces describe interference rejection solutions which introduce minimal distortion to the spread spectrum signal and simultaneously remove the interference. The constraint surfaces essentially correspond to spread spectrum signal estimates which, after interference rejection, conform to known characteristics of the transmitted spread spectrum signal. The formulation of the surfaces relies on prior knowledge about the spread spectrum signal correlation and spectral properties     

Performance of direct sequence spread spectrum in a fading dispersive channel with jamming

The performance of a binary phase-shift-keyed direct-sequence spread-spectrum communication system in a fading dispersive channel with jamming is examined. The channel is modeled as a wide-sense-stationary uncorrelated scattering channel, dispersive only in time. The channel output consists of a strong stable component of the signal plus a continuum of dispersive components and is referred to as a frequency-selective Rician fading channel. To obtain specific results, the multipath intensity profile is modeled by triangular function. Both the uncoded and coded performances are analyzed. The multipath spread is chosen to limit intersymbol interference to K adjacent coded symbols in the coded case and to only adjacent data bits in the uncoded case. No specific interleaver design is considered. A closed-form expression for the probability of error is obtained, assuming random spreading sequences. Numerical results are presented for values of the system and channel parameters. It is shown that a jammer can optimize its strategy if the probability of error is not too high.<>     

Multicarrier spread spectrum watermarking for secure error concealment in fading channel

This paper proposes a novel multicarrier spread spectrum (SS) watermarking scheme for the application of image error concealment using multicarrier-code division multiple access (MC-CDMA) with binary phase shift keying (BPSK) transmission in Rayleigh fading channel. The goal is achieved by embedding important information (image digest) which is extracted from the original image itself, and is used to introduce sufficient redundancy in the transmitted image. Half-toning technique is applied to obtain image digest from its low-resolution version. At the decoder side, data demodulation as well as watermark decoding are done using minimum mean square error combining (MMSEC) strategy. The extracted image digest is used to correct the damaged regions. The integration of SS watermarking with the existing SS modulation not only simplifies the design but also offers significant performance improvement for error concealment in fading channel. Authorized users having the knowledge of code patterns for SS watermarking can only perform the error concealment operation and the method is secured. Experimental results duly support the effectiveness of the proposed scheme.     

Novel semi-blind channel estimation schemes for Rician fading MIMO channel

In this paper, we propose two novel semi-blind channel estimation techniques based on QR decomposition for Rician fading Multiple Input Multiple Output (MIMO) channel. In the first technique, the MIMO channel matrix H can be decomposed as an upper triangular matrix R and unitary rotation matrix Q as H = RQ. The matrix R is estimated blindly from only received data by using the orthogonal matrix triangularization based Householder QR decomposition, while the optimum rotation matrix Q is estimated exclusively from the algorithm of Orthogonal Pilot Maximum Likelihood Estimator (OPML) based on pilot information. In the second technique, the joint semi-blind channel and data estimation is performed by using the Least Square (LS) algorithm based on QR decomposition. The simulation results obtained for 4-PSK data modulation scheme using two transmitters and six receiver antennas for different Rice factor (K) have shown that the BER performance increases with an increase in the Rice factor. Finally, we compare these two new techniques with the conventional semi-blind channel estimation technique based on Whitening Rotation (WR), and the results show that the first proposed technique outperforms and the second technique achieves a very nearby performance as compared to the technique based on whitening rotation.     

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。     

Analysis of microdiversity and dual channel macrodiversity in shadowed fading channels using a compound fading model

Wireless channels are affected by short-term fading and long-term fading (shadowing). A compound fading model was proposed for the modeling of shadowed fading channels which resulted in a closed form solution for the probability density function (pdf) of the signal-to-noise ratio (SNR). This model is applied to a case where both micro- and macro-diversity schemes are implemented to mitigate short-term fading and shadowing, respectively. Using the compound fading model, it is shown that the pdf of the signal-to-noise ratio after the implementation of maximal ratio combining (MRC) at the micro level and selection combining (SC) at the macro level can be expressed in analytical form. Even when branch correlation exists, the pdf still can be expressed in analytical form. Thus, the compound pdf model offers significant improvement over approaches which use lognormal pdf for shadowing. The performance of a coherent binary phase shift keying (BPSK) modem is evaluated using this approach. The results demonstrate the simplicity and usefulness of the compound pdf in the performance analyses of shadowed fading channels even when branch correlation exists at the base station or correlation exists between base stations.     

Double differential MPSK on the fast Rician fading channel

An analysis of adjacent and nonadjacent double differential M -ary PSK (M-ary D²PSK) on the fast Rician fading channel is presented. The probability density function (PDF) of the first-order phase error (FOPE) and the probability density functions of the second-order phase error (SOPE) for both adjacent and nonadjacent D²PSK on a fading channel have been derived. Asymptotic approximations for these PDFs are proposed in the case of channels with weak fading (including the additive white Gaussian noise (AWGN) channel) and high signal-to-noise ratio. For nonadjacent D²PSK with independent FOPEs a simplified PDF of SOPE has been obtained. The derived PDFs allow calculation of error performance of D²PSK, as well as of DPSK, on the Rician channel including the AWGN channel and the Rayleigh channel and verification of some results reported in the literature earlier. It is shown that nonadjacent D²PSK can be optimized by choosing a proper separation between the first-order phase differences     

Multiple-frequency slotted ALOHA in a shadowed and Rician faded radio environment

A multiple-frequency channel is proposed to enhance the capacity of slotted ALOHA radio networks. The throughput is derived considering each packet being frequency assigned randomly in an uncorrelated shadowing and fading channel. Poisson and binomial distributions are assumed for the packet arrival models for an infinite and a finite number of users, respectively.<>     

Performance of asynchronous DS-CDMA using a maximal ratio combiningdiversity over a Rician fading channel

The final closed-form expression for bit error probability (BEP) is presented for a DS-CDMA system using a maximal ratio combining (MRC) diversity over a Rician fading channel. The accuracy of the BEP estimate evaluated by this expression is verified by comparison with a semi-analytic simulation result. The effect that diversity order has on the BEP is also considered for typical multipath delay profiles with different Rician ratios     

Diversity with distributed decisions combining for direct-sequenceCDMA in a shadowed Rician-fading land-mobile satellite channel

The performance of two diversity schemes with distributed decisions combining is analyzed and compared with that of the selection and maximal ratio-combining schemes for binary-phase shift keying (BPSK) direct-sequence code-division multiple access (DS-CDMA) in a shadowed Rician-fading land-mobile satellite channel. In the first scheme of distributed decisions combining, the combiner collects only the binary branch decisions to make the data decision. In the second scheme, the combiner makes the data decision using the branch decisions and the channel states. Both symmetrical and asymmetrical diversity branches are considered. Based on optimal likelihood ratio testing, a new data decision rule is devised for the detection of the spread-spectrum signals in the DS-CDMA system. Upper bounds are employed for the performance evaluation of the diversity combining schemes when the bit error probability is difficult to compute. Numerical results are presented for the satellite channel under light, average, and heavy shadowing     

Effect of channel estimation errors on MRC diversity in Rician fading channels

We study the effect of imperfect channel estimation (ICE) on the performance of M-ary phase shift keying (M-PSK) with maximum ratio combining (MRC) in generalized Rician fading channels. First, we derive the error probability formulas for M-PSK with MRC and ICE in arbitrary Rician fading channels. Furthermore, we derive the effective receiver output signal-to-noise ratio (SNR) statistics and the outage probability, and analytically quantify the average SNR loss of M-PSK caused by ICE, assuming independent diversity branches. Finally, we point out a major approximation in a popular approach used in the literature to evaluate the adverse effect of ICE.     

Analysis and measurement of FM click rate in fast Rician fading channel

For the prediction of the error probability performance of a digital frequency modulation (FM) receiver that uses a limiter-discriminator-integrator (LDI) detector, expressions for the rates of "clicks" are needed. Recently, formulas for FM click rates valid for the slow Rician fading channels have been derived by N.P. Secord et al. (see ibid., vol.42, p.197-204, 1993) by taking expectation of the classical click-rate formulas of Rice, which is valid for the additive white Gaussian noise (AWGN) channel, over the Rician distribution. We derive and present new closed-form expressions for average FM click rates for the fast Rician fading channels, taking into account the effect of Doppler spread or fading bandwidth. We exploit the fact that the multipath component of the Rician process can be combined with the AWGN into another Gaussian process, so that the classical click-rate analysis of Rice can be applied. An experiment using modern equipment is conducted, and the measured click rates match the theory closely for both the fast Rician and Rayleigh fading channels.     

Probability-of-error bounds for binary transmission on the slowly fading Rician channel

Chernoff bounds and tilted distribution arguments are applied to obtain error probability bounds for binary signaling on the slowly-fading Rician channel with L diversity. For the maximum likelihood receiver, the CB-optimum [optimum in the sense of minimizing the Chernoff (upper) bound on error probability] signal correlation is determined and plotted; it is found that antipodal signals should be used ifa > b^{2}(1 + b), where a is the signal-to-noise ratio of the specular components andbis that of the fading components. The CB-optimum number of diversity paths is then obtained. Ifa/b > 0.2, antipodal signaling with unlimited diversity is CB-optimum; whereas, ifa/b < 0.2, orthogonal signaling with properly chosen diversity is very nearly CB-optimum. If restricted to orthogonal signaling, unlimited diversity is CB-optimum whenevera/b > 1.0. Similar results are obtained for the generally nonoptimum square-law-combining receiver. In this case, orthogonal signaling with finite diversity is always CB-optimum.     

Symbol-aided plus decision-directed reception for PSK/TCMmodulation on shadowed mobile satellite fading channels

The symbol-aided (SA) synchronization concept developed by Moher and Lodge (1989) is applied to the MSAT channel modeled with a shadowed Rician process. Simulation data demonstrate that it can track the severe phase jitter encountered on the fading channel free of the false lock which plagues conventional techniques. The algorithm multiplexes known symbols into the data stream, establishing an absolute reference free of decision errors that is used to estimate the fading phase. An improvement to the SA algorithm which extracts phase information from the data-bearing symbols is proposed. It is found that the new technique is more effective for larger K. The improved algorithm is referred to as symbol-aided plus decision-directed (SADD) phase estimation. A system employing SADD phase estimation, trellis-coded modulation, interleaving, and amplitude weighting within the Viterbi decoder yielded the best BER performance on the shadowed MSAT channel considered