Differentially coherent code acquisition in the MIMO-aided multi-carrier DS-CDMA downlink

Both differentially coherent and non-coherent code acquisition schemes designed for the multiple-input multiple-output (MIMO)-aided multi-carrier (MC)-DS-CDMA downlink are analysed, when communicating over uncorrelated Rayleigh channels. The attainable mean acquisition time (MAT) performance is studied as a function of both the number of multiple transmit/multiple receive antennas and that of the number of subcarriers. It is demonstrated that in contrast to the expectations, when the number of multiple transmit antennas and/or that of the subcarriers is increased in both the differentially coherent and the non-coherent code acquisition scenarios, the achievable MAT deteriorates over the entire signal-to-interference plus noise ratio (SINR) per chip (E_c/I_o) range considered, except for the scenario of single-carrier (SC)-DS-CDMA using P = 2 transmit antennas and R = 1 receive antenna. As expected, the degree of performance degradation depends upon the specific scheme and the E_c/I_o ratio considered, although paradoxically, the correctly synchronised MIMO-aided system is capable of attaining its target bit error ratio performance at reduced SINR values.     

基于海面散射的莱斯MIMO信道容量研究

通过相关莱斯信道模型对海面散射水声多入多出(MIMO)信道容量进行了研究。从Helmholtz-Kirchhoff积分和Fresnel近似出发,得到海面散射信号的空间相关性,由于信道的协方差矩阵不可以表示成发射相关矩阵和接收相关矩阵的Kronecker积,通过Monte Carlo仿真研究了莱斯因子、接收信噪比和空间相关性对MIMO信道容量的影响。从对outage capacity和信道容量的CCDF(Complementary Cumulative Distribution Function)的仿真计算结果可以看出,莱斯衰落的MIMO信道容量并不总是小于瑞利衰落MIMO信道容量,当信道的空间相关性较大和接收信噪比较小时,由于信道的衰落作用将起主要作用,对于较小的收发阵元数,以非衰落的直达信号为主的莱斯MIMO信道容量将大于瑞利衰落的MIMO信道容量。     

Compact Rayleigh and Rician fading simulator based on random walk processes

This article describes a significantly improved sum-of-sinusoids-based model for the accurate simulation of time-correlated Rayleigh and Rician fading channels. The proposed model utilises random walk processes instead of random variables for some of the sinusoid parameters to more accurately reproduce the behaviour of wireless radio propagation. Every fading block generated using our model has accurate statistical properties on its own and hence, unlike previously proposed models, there is no need for time-consuming ensemble-averaging over multiple blocks. Using numerical simulation it is shown that the important statistical properties of the generated fading samples have excellent agreement with the theoretical reference functions. A fixed-point hardware implementation of the corresponding Rayleigh and Rician fading channel simulator on a field-programmable gate array (FPGA) is presented. By efficiently scheduling the operations, the reconfigurable fading channel simulator is compact enough that it can be efficiently used to simulate multipath scenarios and multiple-antenna systems (e.g. a 4 times 4 MIMO channel) using a single FPGA.     

Antenna subset selection in measured indoor channels

Antenna subset selection can greatly reduce the implementation complexity of multiple input multiple output (MIMO) systems while retaining most of their benefits. This paper investigates the diversity gain and capacity of such systems in wireless personal area networks. Considered scenarios include both the communication between access point to a laptop, and between two handheld devices. We analyse the performance of different antenna selection algorithms and signal combining methods in measured dual-polarised narrowband and wideband propagation channels. We find that line-of-sight and non-line-of-sight situations have fairly similar behaviour. Different polarisations result in similar signal-to-noise ratio gains when the multiple antennas are used for diversity, but result in noticeably different capacities in spatial-multiplexing systems. We also find that radiofrequency (RF) preprocessing of the signals is less effective for handheld handsets with non-uniform antenna arrangements than for uniform linear arrays. For communications between handheld devices, simple selection (of one out of four antennas) shows extremely high performance gains compared to no-selection. Finally, we compare bulk selection (same antenna subset is used for all frequency sub-channels) to per-tone selection (different antenna subsets can be used for each frequency sub-channel) for wideband channels. Bulk selection together with RF preprocessing performs almost as well as per-tone selection for some scenarios.     

莱斯信道下大规模MIMO系统混合模数转换器的随机向量量化方案

针对大规模MIMO系统用配置单精度模数转换器(ADC)来降低系统能耗会导致系统损失部分性能的问题,提出了一种莱斯信道下大规模MIMO系统的采用混合ADC的随机向量量化(RVQ)改进方案。该方案在频分双工模式下,首先在基站端采用高分辨率ADC和低分辨率ADC混合的接收方案处理信号,使接收的导频信号和有用信号具有较高的转换精度;系统对导频信号进行信道估计后,再对信道状态信息(CSI)进行RVQ处理,以此减小系统的反馈开销;最后运用最小均方误差(MMSE)信号检测算法减轻由量化误差引起的的用户间干扰,从而达到降低能耗并减小系统性能损失的目的。实验结果表明,这种改进的RVQ方案能在降低系统能耗的基础上有效减小系统容量损失,并使其和速率接近传统的高分辨率ADC接收方案。     

Semi-deterministic channel model for MIMO systems Part-II: results

This article presents the results of applying `single interaction scattering reflecting' (SISTER) model to different multiple-input multiple-output (MIMO) channels. The SISTER model is a semi-deterministic channel model which is described in details in Part-I of this article. MIMO performance is investigated for outdoor and indoor environments considering both line of sight (LOS) and non-line of sight (NLOS) cases. Different results are obtained by changing antenna array parameters such as spacing and number of elements and antenna half power beam width (HPBW), as well as environment parameters such as ground`s and walls` conductivity, and number and distribution of scatterers. In addition, space and angle diversity methods are compared for most of the studied cases. As the results show for most scenarios, the MIMO performance may be further improved by using multi-beams (angle diversity) rather than multi-antenna elements (space diversity).     

MIMO-OFDM radar for direction estimation

Multiple-input?multiple-output (MIMO) radar makes use of orthogonal signals to obtain the phase delay for each transmitting/receiving antenna pair, and thus increasing the accuracy of direction estimation. The previously proposed MIMO radar assumes narrowband signals that guarantee the waveform orthogonality during the signals? transmission, propagation and reception. However, a narrowband system is unstable in target localisation because of the fluctuation of the target?s radar cross section. An MIMO-OFDM radar is proposed for target localisation. It adopts the OFDM technique to simultaneously transmit and receive a set of multiple narrowband orthogonal signals at orthogonal frequencies. A practical model accommodating a physical target is presented to simulate the MIMO-OFDM radar. As an example, a composite target composed of five infinite dielectric cylinders is localised by a four-element uniform linear array. The performance of the MIMO-OFDM radar is investigated by examining the estimation error for different numbers of sub-bands, different signal-to-noise ratios and different target directions. It is demonstrated by simulation that the MIMO-OFDM radar gives more statistically stable estimation by spreading the signal power over a wider spectrum.     

Detailed characterisation of an indoor MIMO channel in the double directional spatial domain

The spatial domain characteristics of an indoor multiple-input multiple-output (MIMO) communication system are examined in detail. Using a high-resolution algorithm, the authors were able to effectively estimate multipath parameters on the joint azimuth of arrival/azimuth of departure (AoA/AoD) domain. The analysis showed dense clustering phenomena at the vast majority of considered locations and a strong correlation between the azimuth spreads of the two angular domains with the presence of line-of-sight (Los) specular components or when the distance between the transmitter (Tx) and the receiver (Rx) is low. The authors also noticed higher azimuth dispersions, compared with those reported in the literature, which can be attributed to low antenna heights and local scattering interactions in the vicinity of both the transmit and receive arrays. Finally, the impact of the spatial global parameters on the ergodic MIMO capacity was tested revealing a direct relation between them.     

Exploiting multiple-input multiple-output in the personal sphere

An analysis of the propagation characteristics of communication links in the personal sphere is presented. The results of two wideband measurement campaigns in the 5.2 GHz band involving on-body multiple-input multiple-output (MIMO) arrays are reported. In the first measurement, transmission takes place between on-body antennas and a uniform linear array, positioned at close range and with line-of-sight (LOS) propagation conditions. Despite the LOS operation, MIMO is shown to offer a significant increase in the information theoretic capacity of the system when compared with a conventional single-input single-output (SISO) antenna system. Although this appears counter-intuitive to the well-known fact that uncorrelated scattering results in high capacity, two previously less well-known mechanisms are highlighted to explain this result. This analysis reveals the potential use of MIMO in personal area networks. The second measurement campaign uses two on-body MIMO arrays to focus on the subject of body area network propagation. Analysis of the SISO characteristics of the links highlights the influence of the user on the channel. Body shadowing and user motion are shown to lead to multiple rapid changes in the channel characteristics. Again, MIMO is shown to be able to offer performance enhancement. In the measured channels, polarisation diversity is shown to outperform spatial diversity.     

Fast antenna subset selection algorithms for multiple-input multiple-output relay systems

The antenna subset selection technique balances the performance and hardware cost in the multipleinput multiple-output (MIMO) systems, and the problems on the antenna selection in MIMO relay systems have not been fully solved. This paper considers antenna selection on amplify-and-forward (AF) and decode-andforward (DF) MIMO relay systems to maximise capacity. Since the optimal antenna selection algorithm has high complexity, two fast algorithms are proposed. The selection criterion of the algorithm for AF relay is to maximise a lower bound of the capacity, but not the exact capacity. This criterion reduces algorithmic complexity. The algorithm for DF relay is an extension of an existing antenna subset selection algorithm for one-hop MIMO systems. The authors show the derivations of the algorithms in detail, and analyse their complexity in terms of numbers of complex multiplications. Simulation results show that the proposed algorithms for both cases achieve comparable performance to the optimal algorithm under various conditions, and have decreased complexity.     

Power allocation in SVD- and GMD-assisted MIMO systems

Singular-value decomposition (SVD)-based multiple-input multiple-output (MIMO) systems have attracted a lot of attention in the wireless community where the whole MIMO channel is decomposed into a number of unequally weighted independent single-input single-output (SISO) channels. The unequal weighting of the SISO channels has led to intensive research on bit- and power allocation even in MIMO channel situations with poor scattering conditions identified as the antenna correlation effect. In this situation, the unequal weighting of the SISO channels becomes even much stronger. In comparison to the SVD-assisted MIMO transmission, geometric mean decomposition (GMD)-based MIMO systems are able to compensate for the drawback of weighted SISO channels when using SVD at the cost of remaining interferences which can be easily removed by using dirty paper precoding. Together with different QAM constellation sizes per layer, bit loading and power allocation can be helpful to balance the bit-error probabilities in the activated number of MIMO layers. The novel contribution of this paper is that optimal and suboptimal power allocation solutions are investigated under the assumption of unequal SISO channels as well as different QAM constellation sizes per MIMO layer. Our results show that GMD-based MIMO transmission has the potential to significantly simplify the process of bit and power loading and outperforms the SVD-based MIMO transmission as long as the same QAM-constellation size is used on all equally-weighted SISO channels.     

Experimental investigation of directional characteristics for ionospheric clutter in HF surface wave radar

The unwanted radar echoes from the ionosphere are collectively called ionospheric clutter. It has proved to be the greatest impediment to achieve consistently good performance in long-range detection of surface vessels and sea-state monitoring for high-frequency surface wave radar (HFSWR). Field experimental data recorded by the HFSWR OSMAR2003 (Ocean State Monitor and Analysis Radar, manufactured in 2003) has been used in detailed investigations of the directional characteristics for this ionosphere clutter, leading to the development of effective mitigation techniques based on antenna design and adaptive signal processing. Particular attention is given to the amplitude and phase relationship among multiple spatial channels for two types of ionospheric clutter. Preliminary experimental results show that the random gain and phase variation of the antenna pattern overhead null destroyed the amplitude and phase consistency among channels. As a result, no significant measured directivity is observed in this type of specular clutter. For the incidence clutter from a lower elevation angle, it is observed that the spread clutter possesses high directivity. A nonlinear receiving array composed of multiple V-shaped antennas without deep and broad null at near-vertical incidence is proposed for adaptive ionospheric clutter suppression     

Performance analysis of multiple-input multipleoutput singular value decomposition transceivers during fading and other cell interference

A generalised method is derived to compute the error probabilities of singular value decomposition (SVD)-based receivers for a multiple-input multiple-output (MIMO) system with uncoded transmission. The method can be used for a wide class of flat fading environments, including independent and identically distributed (i.i.d.) and semi-correlated Rayleigh and i.i.d. Ricean channels. Although the method is applied to equal-power binary phase shift keying, it can easily be extended to higher-order M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) signal constellations and adaptive 'water-filling' schemes. The error probability curves derived from closed-form formulas and simulations demonstrate very close agreement. The error performances of channel inversion, minimum mean square error and zero forcing receivers are compared with the SVD receiver for a single-user system. The impact of multiple users is considered by studying the performance of an adaptive MIMO SVD transmission scheme operating in a cellular environment. In particular, the effect of inter-cell interference on the performance of the scheme is quantified, modelling the interference as increased Gaussian noise. A number of cellular layouts are examined and the impact of the resulting singal-to-interference and noise ratio on the constellation sizes that can be supported, the BER and so on is considered. The primary metric used for our performance analysis is the error-free transmission rate, which is derived for our adaptive system. For the cellular scenarios considered, it can be found that the effect of interference is considerable and the performance of the adaptive MIMO SVD scheme is only marginally better than that provided by conventional diversity methods.     

一种双频MIMO天线的T型枝节解耦设计

本文设计了一种T型枝节解耦的双频MIMO天线.两个工作频段分别覆盖WLAN频率2.45 GHz/5.2 GHz/5.8 GHz.低频谐振单元为倒F天线,通过在低频枝节上增加短截线,用以产生高频谐振,实现双频工作.将天线单元沿水平方向对称放置形成二单元的MIMO天线,并采用在两个天线单元之间添加T型枝节的方法进行解耦.对...     

MIMO水声通信中的空时分组扩展编码方案探讨

采用多输入多输出（MIMO）结构和空时编码可以使水声通信系统获得分集增益,改善系统性能。针对水声信道中的频率选择性衰落,研究了一种用于MIMO水声通信系统的空时分组扩展编码方案（STBSC）,利用正交扩频编码克服了水声信道中多径干扰对空时分组编码（STBC）信号正交性的破坏。给出了方案的系统模型和实现框图,采用MIMO水声模型对方案的性能进行了仿真。仿真结果表明,STBSC方案在多径水声信道中可以得到完全发射分集。最后介绍了方案的水库试验,试验结果表明了方案的有效性。     

Performance study of linear and nonlinear diversity-combining techniques in synchronous FFH/MA communication systems over fading channels

Based on the Fourier-Bessel series and exponential characteristic function approach, the paper presents analytical bit-error-rate expressions for synchronous fast frequency-hopped spread-spectrum multiple-access systems over Rician fading channels with M-ary frequency-shift-keying modulation. Linear-combining, product-combining and clipper receivers are studied. Numerical results show that both the clipper and product-combining receivers perform significantly better than the linear-combining receiver. The clipper receiver with an optimum threshold level slightly outperforms the product-combining receiver. However, the product-combining receiver is the preferred diversity-combining method because it is completely nonparametric. In addition, the optimum diversity levels for various diversity-combining receivers over Rayleigh fading channels are higher than that of the Rician fading channels     

BER analysis of space-time diversity in CDMA systems over frequency-selective fading channels

The performance of direct-sequence code division multiple access (DS-CDMA) using space-time spreading system, over frequency-selective fading channels, is investigated. The underlying transmit diversity scheme, previously introduced in the literature, is based on two transmit and one receive antenna. It was shown that when employed in flat fast-fading channels, the received signal quality can be improved by utilising the spatial and temporal diversities at the receiver side. We study the problem of multiuser interference in asynchronous CDMA systems that employ transmit/receive diversity using space-time spreading. To overcome the effects of interference, a decorrelator detector is used at the base station. Considering binary phase-shift keying transmission, we analyse the system performance in terms of its probability of bit error. In particular, we derive the probability of error over frequency-selective Rayleigh fading channels for both fast and slow-fading channels. For the fast-fading channel, both simulations and analytical results show that the full system diversity is achieved. On the other hand, when considering a slow-fading channel, we show that the scheme reduces to conventional space-time spreading schemes where the diversity order is half of that of fast-fading.     

Pulse position modulated space?time trellis coding for ultra-wideband impulse radio multiple-input multiple-output communication systems

The design of pulse position modulated (PPM) space-time trellis codes (STTC) for ultra-wideband impulse radio (UWB-IR) multiple-input multiple-output (MIMO) communication systems over slow and fast fading multipath channels is considered. First, A probability of error analysis is carried out to derive upper bounds on pairwise symbol error probability at high and low signal-to-noise ratios (SNRs). From the upper bounds, A new distance notion is introduced and novel design criteria for optimal (in error rate performance) M-ary PPM STTC are deduced for UWB. An optimal binary-PPM STTC is designed for two transmit antennas. Finally, simulation results of the UWB-IR MIMO system, using the optimal STTC, confirm significant improvement in bit-error-rate performance over the uncoded UWB-IR single-input single-output system and also over previously proposed space-time coding scheme for UWB, at higher SNR.     

Second-order statistics of dual-hop transmission with an amplify-and-forward relay over Rician and Weibull fading channels

The level-crossing rate (LCR) and average fade duration (AFD) are the second-order statistics that are useful for the design of adaptive transmission and coding systems over wireless fading channels. These second-order statistics are analyzed for dual-hop transmissions with a fixed-gain amplify-and-forward relay, where the Rician and Weibull fading channels are considered. To evaluate the LCR and AFD, we also derived the lower and upper bounds on the LCR and AFD which are useful when the exact closed-forms of the LCR and AFD are difficult to obtain. To illustrate the performance of the dual-hop relay channel, numerical results of the outage probability, the LCR and AFD are presented.     

Channel capacity evaluation for a multiple-inputmultiple-output terminal in the presence of user?s hand

The impact of the user's hand holding a multiple-input-multiple-output (MIMO) terminal on system performance and specifically on channel capacity is investigated. A 4 times 4 MIMO system is considered with a personal digital assistant terminal equipped with a compact array of 4 patch elements. The proposed methodology is based on the use of radiation patterns of the antenna elements in the presence of the other elements and in the presence of a user's hand. Radiation patterns are obtained by means of simulations as well as of measurements conducted in an anechoic chamber. Two MIMO channel models are considered, namely a correlation-based model and a geometry-based model, where the antenna patterns are incorporated. The evaluation of MIMO channel capacity demonstrates the impact of the specific terminal antenna array. Moreover, significant degradation of channel capacity is proved because of the presence of the user's hand.