STBC块传输系统中的一种新型分集合并算法

该文在二发一收的空时分组码(STBC)单载波块传输系统中,提出一种空时和多径分集合并接收算法。通过基于STBC的单载波频域均衡(STBC-SC-FDE)算法得到对发送信号的估计,以此和信道状态信息(CSI)分离接收信号中的各多径分量,对各多径分量分别采用STBC合并,最后再将各多径分量的合并输出结果按照最大比(MRC)的方式进行合并,从而实现空时二维RAKE接收。该算法在较低的计算复杂度情况下,可以同时获得发射分集和多径分集,Monte Carlo仿真验证了该算法的性能。     

Sum of Squared Shadowed-Rice Random Variables and its Application to Communication Systems Performance Prediction

The distribution of the sum of non-negative random variables plays an essential role in the performance analysis of diversity schemes for wireless communications over fading channels. While for common fading models such as the Rayleigh, Rice, and Nakagami, the performance of diversity systems is well understood, a minor attention has been devoted to the shadowed-Rice (SR) case, namely a Rice fading channel with fluctuating (e.g. random) Line of Sight (LOS) component. Indeed, the analytical performance evaluation of diversity systems on SR fading channels requires the availability of handy expressions for the distribution of the combined received power. To this end, the rationale of this paper is twofold: first, to evaluate the distribution of the sum of SR random variables, both for the case of independent as well as correlated LOS components, and then to carry out an extensive performance analysis of maximal ratio combining (MRC) detection scheme on SR fading channels.     

Spectral efficiency of dual diversity selection combining schemes under correlated Nakagami-0.5 fading with unequal average received SNR

The spectral efficiency results for different adaptive transmission schemes over correlated diversity branches with unequal average signal to noise ratio (SNR) obtained so far in literature are not applicable for Nakagami-0.5 fading channels. In this paper, we investigate the effect of fade correlation and level of imbalance in the branch average received SNR on the spectral efficiency of Nakagami-0.5 fading channels in conjunction with dual-branch selection combining (SC). This paper derived the expressions for the spectral efficiency over correlated Nakagami-0.5 fading channels with unequal average received SNR. This spectral efficiency is evaluated under different adaptive transmission schemes using dual-branch SC diversity scheme. The corresponding expressions for Nakagami-0.5 fading are considered to be the expressions under worst fading conditions. Finally, numerical results are provided to illustrate the spectral efficiency degradation due to channel correlation and unequal average received SNR between the different combined branches under different adaptive transmission schemes. It has been observed that optimal simultaneous power and rate adaptation (OPRA) scheme provides improved spectral efficiency as compared to truncated channel inversion with fixed rate (TIFR) and optimal rate adaptation with constant transmit power (ORA) schemes under worst case fading scenario. It is very interesting to observe that TIFR scheme is always a better choice over ORA scheme under correlated Nakagami-0.5 fading channels with unequal average received SNR.     

Mobile-to-mobile fading channels in amplify-and-forward relay systems under line-of-sight conditions: statistical modeling and analysis

This paper deals with the modeling and analysis of narrowband mobile-to-mobile (M2M) fading channels for amplify-and-forward relay links under line-of-sight (LOS) conditions. It is assumed that a LOS component exists in the direct link between the source mobile station (SMS) and the destination mobile station (DMS), as well as in the links via the mobile relay (MR). The proposed channel model is referred to as the multiple-LOS second-order scattering (MLSS) channel model. The MLSS channel model is derived from a second-order scattering process, where the received signal is modeled in the complex baseband as the sum of a single and a double scattered component. Analytical expressions are derived for the mean value, variance, probability density function (PDF), cumulative distribution function (CDF), level-crossing rate (LCR), and average duration of fades (ADF) of the received envelope of MLSS channels. The PDF of the channel phase is also investigated. It is observed that the LOS components and the relay gain have a significant influence on the statistics of MLSS channels. It is also shown that MLSS channels include various other channel models as special cases, e.g., double Rayleigh channels, double Rice channels, single-LOS double-scattering (SLDS) channels, non-line-of-sight (NLOS) second-order scattering (NLSS) channels, and single-LOS second-order scattering (SLSS) channels. The correctness of all analytical results is confirmed by simulations using a high performance channel simulator. Our novel MLSS channel model is of significant importance for the system level performance evaluation of M2M communication systems in different M2M propagation scenarios. Furthermore, our studies pertaining to the fading behavior of MLSS channels are useful for the design and development of relay-based cooperative wireless networks.     

采用两条支路分集接收的相关瑞利衰落信道容量

本文研究采用两条支路最大比合并(MRC)或选择合并(SC)分集接收的相关瑞利衰落信道理论容量推导恒定发射功率自适应M进制正交幅度调制(M-QAM)的频谱效率,并将它们与独立同分布瑞利信道理论容量进行比较,其结果对收发信机之间无视距分量路径、接收机上分集天线之间的距离小于半个波长的无线通信系统设计具有指导作用.     

An iterative algorithm for Doppler spread estimation in LOS environments

An iterative algorithm is proposed to estimate the maximum Doppler frequency in a line-of-sight (LOS) environment. While the autocorrelation function of a Rayleigh fading signal depends only on the maximum Doppler frequency, that of a Rician fading signal is also related to the K factor and the angle of arrival (AOA) of the LOS component. The performance of conventional Doppler spread estimators based on the Rayleigh fading assumption degrades in LOS environments due to the autocorrelation function mismatch. The proposed estimator estimates the Rician K factor first, then iteratively estimates the Doppler spread and the AOA of the LOS component. The performance degradation due to channel parameter variations is investigated by means of matching the autocorrelation functions. The simulation results indicate that the proposed estimator is not only superior to the conventional autocorrelation-based Doppler spread estimator but also robust to the channel parameter variation in both LOS and non-LOS environments.     

Effect of noise imbalance on dual-MRC over Rayleigh fading channels

The performance analysis of dual (two-branch) maximal ratio combining (MRC) under imperfect weight, named imperfect MRC, due to the noise imbalance is derived over independent but non-identical Rayleigh fading channels. Considering the system (or channel) conditions that the noise level of each branch is different, we present the accurate performance analysis of imperfect dual MRC in terms of average combined signal-to-noise ratio (SNR), outage probability and average symbol error rate for a large class of modulations in closed-form and compare them with the performance of the perfect MRC and the perfect selection combining (SC) over non-identical but independent Rayleigh fading channels based on the interesting statistical results on the combined SNR. From the performance results we provide the criterion in choosing the imperfect MRC, perfect MRC, or SC depending on the degree of the difference of the noise level between branches.     

Performance comparison of different selection combining algorithms in presence of co-channel interference

In this paper, we present a unified approach for the computation of the outage probability, the level crossing rate (LCR), and the average outage duration (AOD) of selection combining (SC) in the presence of multiple cochannel interferences and under both minimum signal-to-interference ratio (SIR) and desired signal power constraints. We consider three selection algorithms, namely: 1) the best signal power algorithm; 2) the best SIR algorithm; and 3) the best total power (desired plus interference) algorithm. As a specific application example, we analyze the three algorithms for a low-complexity dual-branch SC receiver subject to multiple interferers over Rayleigh fading channels. When applicable, the new results are compared to those previously reported in the literature dealing with the outage probability, AOD, and LCR of 1) interference-limited systems and 2) power-limited systems. Numerical examples show that the minimum desired signal power constraint induces a floor to the outage probability, AOD, and LCR performance measures. They also show that the best SIR algorithm provides the best outage probability and AOD performance for low average SIR. On the other hand, the best signal power algorithm and the best S+I algorithm outperform the best SIR algorithm for high average SIR. It is also shown that the best SIR algorithm tends to have more outage level crossings.     

VEPSD: a novel velocity estimation algorithm for next-generation wireless systems

A novel algorithm called velocity estimation using the power spectral density (VEPSD), which uses the Doppler spread in the received signal envelope to estimate the velocity of a mobile user (MU), is introduced in this paper. The Doppler spread is estimated using the slope of the power spectral density (PSD) of the received signal envelope. The performance of the VEPSD algorithm is evaluated in both Rayleigh and Rician fading environments. The sensitivity of the estimation error to additive white Gaussian noise (AWGN), Rice factor (K), and the angle of arrival of the line-of-sight (LOS) component is analyzed and compared with the level crossing rate (LCR) and covariance-based velocity estimators. Simulation results show that VEPSD estimates the velocity of MUs accurately. It is also shown that VEPSD can be used for velocity estimation under nonisotropic scattering and is well suited for next-generation wireless systems (NGWSs).     

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.     

Performance limits of M-FSK with Reed-Solomon coding and diversity combining

This paper examines the asymptotic (M/spl rarr//spl infin/) performance of M-ary frequency-shift keying (M-FSK) in multi-channels, or multiple frequency-nonselective, slowly fading channels, with coding, side information, and diversity reception. In particular, Reed-Solomon (RS) coding is considered in conjunction with the ratio-threshold test (RTT), which generates side information regarding the reliability of received symbols. The asymptotic performance of orthogonal signaling in multichannels with maximal ratio combining (MRC), postdetection equal gain combining (EGC), hybrid selection combining (H-SC), and selection combining (SC) is derived for an arbitrary statistical fading model and diversity order. The derivations reveal that coherent and noncoherent implementations of diversity combining schemes yield the same performance asymptotically. In addition, the asymptotic results are evaluated assuming a Nakagami-m fading model, and the effect of fading severity, diversity order, code rate, and side information upon the performance of the various diversity combiners is investigated. The minimum signal-to-noise ratio (SNR) required to achieve arbitrarily reliable or error-free communication, as well as the associated optimal RS code rate, are determined for various cases.     

On the performance of underlay cognitive radio system with random mobility under imperfect channel state information

In this paper, the performance of an underlay cognitive radio system with random mobility and imperfect channel state information (CSI) is investigated. The mobile user (MU) utilises maximum ratio combining (MRC) and selection combining (SC) diversity techniques as signal reception to improve the quality of received signal‐to‐noise ratio (SNR). Under the Rayleigh fading, random waypoint mobility model is employed to characterised the effect of the MU random mobility on the system performance. Thus, novel probability density function (PDF) and cumulative distribution function (CDF) for the two considered diversity techniques are derived. Through these, the outage probability and average bit error rate (ABER) closed‐form analytical expressions are then obtained to quantify the system performance under the MRC and SC schemes. The results illustrate the effect of imperfect CSI, user mobility which is characterised by pathloss and the network topology on the system performance. Also, the results depict that MRC offers the system better performance compared with SC under the same system conditions. The accuracy of the derived analytical expressions is verified through Monte‐Carlo simulations.     

Mobile speed estimation for broadband wireless communications over Rician fading channels

In this paper, a new algorithm is proposed to estimate mobile speed for broadband wireless communications, which often encounter large number of fading channel taps causing severe intersymbol interference. Different from existing algorithms, which commonly assume that the fading channel coefficients are available for the speed estimators, the proposed algorithm is based on the received signals which contain unknown transmitted data, unknown frequency selective fading channel coefficients possibly including line-of-sight (LOS) components, and random receiver noise. Theoretical analysis is first carried out from the received signals, and a practical algorithm is proposed based on the analytical results. The algorithm employs a modified normalized auto-covariance of received signal power to estimate the speed of mobiles. The algorithm works well for frequency selective Rayleigh and Rician channels. The algorithm is very resistant to noise, it provides accurate speed estimation even if the signal-to-noise ratio (SNR) is as low as 0 dB. Simulation results indicate that the new algorithm is very reliable and effective to estimate mobile speed corresponding to a maximum Doppler up to 500 Hz. The algorithm has high computational efficiency and low estimation latency, with results being available within one second after communication is established.     

SER of M-ary NCFSK with S + N Selection Combining in Nakagami Fading for Integer m

The performance of M-ary orthogonal noncoherent frequency-shift keying (NCFSK) with N branch signal-plus-noise (S + N) selection combining (SC) in Nakagami-m fading (m, integer) is studied. Both independent, identically distributed (i.i.d) and independent, nonidentically distributed (i.n.d) diversity branches are considered and two S + N SC receiver structures are examined. The performances of the S + N SC receivers are compared to those of classical SC and square-law combining (SLC) receivers. The effects of modulation order, fading parameter and the number of diversity branches on the performance of S + N SC are compared to the effects on the performances of classical SC and SLC. For example, it is shown that in an i.n.d fading channel, the value of signal-to-noise ratio (SNR) at which the error rate curves of classical SC and S + N SC cross, decreases as the modulation order, M, increases. Our results indicate that in i.n.d fading channels classical SC outperforms S + N SC for small ranges of SNR, while for moderate to large SNR values S + N SC has superior performance over classical SC. It is also shown that increasing the diversity order will increase the performance gap of S 4N SC over classical SC and over SLC in both i.i.d and i.n.d Nakagami-m fading channels     

Performance of Cooperative Spectrum Sensing Under Rician and Nakagami Fading

In a dynamic spectrum access (DSA) network, a secondary user (SU) that deploys a simple energy detector could access the unoccupied spectrum upon sensing the absence of primary signal. However, sensing accuracy could be compromised due to signal fading. For improved accuracy, cooperative spectrum sensing (CSS) has been proposed, where spectrum occupancy is decided based on the combined results from multiple SUs. Two combining techniques that are commonly used are equal gain combining (EGC) and selection combining (SC). In this paper, we investigate the performance of CSS that implements (i) EGC, and (ii) SC technique, over independent and identically distributed (i) Rician, and (ii) Nakagami fading channels, which can model fading conditions that are less or more severe than the commonly used Rayleigh distribution. Novel expressions for the probability of detecting primary signal, P _d , have been derived and numerically evaluated.     

Performance of partially coherent selection combining receiver in α-μ fading channel

In this paper the impact of the imperfect reference signal extraction is investigated, the bit error rate (BER) performance of multibranch selection combining (SC) receiver for binary and quaternary phase-shift keying (BPSK and QPSK) signals in a generalized α-μ fading channel are shown. The combined effects of imperfect phase estimation of the received signal, diversity order, fading severity and average signal-to-noise ratio (SNR) per bit on BER values are examined. The analytical results are verified by Monte Carlo simulations.     

Serial acquisition performance of single-carrier and multicarrier DS-CDMA over Nakagami-m fading channels

We investigate the issue of pseudo noise (PN) code acquisition in single-carrier and multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems, when the channel is modeled by frequency-selective Nakagami-m (1960) fading. The PN code acquisition performance of single-carrier and MC DS-CDMA systems is analyzed and compared when communicating over Nakagami-m fading channels under the hypothesis of multiple synchronous states (H/sub 1/ cells) in the uncertainty region of the PN code. In the context of MC DS-CDMA, the code acquisition performance is evaluated, when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined by using equal gain combining (EGC) or selection combining (SC) schemes. The performance comparison of the above mentioned schemes shows that the code acquisition performance of the MC DS-CDMA scheme, especially when using the EGC scheme, is more robust, than that of single-carrier DS-CDMA schemes communicating over the multipath Nakagami-m fading channels encountered. However, our code acquisition performance comparison also shows that if the detection threshold was set inappropriately, the performance might be degraded, even if the channel fading becomes less severe.     

Performance of coherent receivers with hybrid SC/MRC overNakagami-m fading channels

A performance analysis of two hybrid selective combining/maximal ratio combining (SC/MRC) diversity receivers over Nakagami-m (1960) fading channels with a flat multipath intensity profile is presented and numerically compared with that of the conventional SC and MRC schemes. Numerical results for particular cases of interest show that the bit error rate (BER) degradation arising from the use of hybrid SC/MRC instead of MRC is independent of the average signal-to-noise ratio (SNR) regardless of the severity of the fading and that MRC provides a higher rate of improvement than the hybrid SC/MRC as the severity of fading decreases     

时变衰落信道中同步时间扩展信号的分集接收技术

扩频通信可以区分多径信号进行RAKE分集接收,是一种有效处理多径效应的手段。与此对应,时间扩展通信也可以在频域上进行类似RAKE的分集接收,有效处理时变衰落带来的影响。时间扩展会带来严重的符号间干扰,同步时间扩展可以有效控制和处理符号间干扰。基于信道分解的分集接收技术保证了同步时间扩展在时变衰落信道下的性能。获得10-4误码率,本文分集算法(扩展长度1024)在时变衰落信道中所需的信噪比与AWGN信道不分集处理所需的信噪比只相差1.4dB。     

一种基于STBC的SC-FDE系统双天线联合检测算法

针对双发双收的基于空时分组编码的单载波频域均衡(STBC-SC-FDE)系统,研究了一种双天线联合检测算法,通过信道估计、导频干扰消除、加权联合均衡等过程重构发送的信息。并根据信噪比对频域信号与均衡矩阵进行加权处理,使联合检测算法适用于2路接收信号信噪比不同的情况。仿真结果表明,所提出的双天线联合检测算法在SUI-3信道下最大可获得约8dB的接收分集增益。与时域最大比合并(MRC)算法相比,由于采用联合检测,该算法能有效改善系统在多径衰落信道下的误码率。