针对相关Rayleigh信道，研究了使用发射天线选择(TAS)和正交空时分组码(OSTBC)的多输入多输出(MIMO)系统的平均符号误码率(ASEP)性能。首先采用了非常适合室内信道的Laplacian模型，得到了两个天线之间的相关系数，它是天线间隔，功率方位角谱(PAS)的标准差和平均到达角(AOA)三者的函数。然后引入了相关矩阵，建立了TAS/STBC系统的相关信道模型，基于等效的标量加性高斯白噪声(AWGN)信道的方法，得到了系统在接收端信噪比的表达式。最后对不同条件下的系统的平均符号误码率(ASEP)性能做了仿真，仿真结果表明：TAS/STBC系统的ASEP性能受到空间相关性的影响，尤其是天线间隔，功率方位角谱(PAS)的标准差和平均到达角(AOA)三者的影响。 The average symbol error probability (ASEP) of multiple-input multiple-output (MIMO) systems employing transmit antenna selection (TAS) and orthogonal space-time block code (OSTBC) over correlated Rayleigh fading channels is considered. We first derive the correlation coefficients between two antenna elements as a function of the antenna spacing, the standard deviation of the power azimuth spectrum (PAS) and the mean angle of arrival (AOA) for the truncated Laplacian model, which is appropriate for indoor channels. Then we introduce the correlation matrix, and establish a correlated channel model. Based on the equivalent scalar additive white Gaussian noise (AWGN) channel approach, the exact form expressions of the signal-to-noise ratio (SNR) at the receiving end are derived. The system performance under different conditions was evaluated through numerical simulations. Numerical results show that: the ASEP performance is affected by the spatial correlation, which varies according to the antenna spacing, PAS standard deviation and mean AOA.

−MIMO system recently emerged as a solution for the provision of wireless communications to improve capacity and decrease bit error rate. One of MIMO variant, which is used in this paper, is Space Time Block Code (STBC). STBC allows diversity gain using combination of spatial and time dimension without changing its bandwidth requirements. This paper presents an implementation of MIMO STBC 2x1 and 2x2 on HAPS channel with the assumption that the channel state condition is known at the receiver (perfect CSIR). HAPS channel characteristic is known to follow Ricean distribution in which it depends on its K factor. In case of HAPs, K factor also depends on its elevation angle. Using computer simulation, this paper analyzes HAPs channel performance using MIMO STBC 2x1 and 2x2 on the various of elevation angles. It is shown that MIMO STBC 2x1 and 2x2 are able to increase performance of HAPs channel including HAPs channel at low elevation angle. However from our simulation capacity improvement of MIMO STBC is obtained insignificant, therefore we propose MIMO spatial multiplexing, which is another variant of MIMO to obtain more capacity. Keywords− HAPs; MIMO; STBC; Ricean channel; K factor; Spatial Multiplexing.

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ii PhD Thesis by Gbenga Owojaiye University of Hertfordshire, Hatfield, Herts. AL10 9AB, United Kingdom Abstract In this thesis, space-time block codes originally developed for multiple antenna systems are extended to cooperative multi-hop networks. The designs are applicable to any wireless network setting especially cellular, adhoc and sensor networks where space limitations preclude the use of multiple antennas. The thesis first investigates the design of distributed orthogonal and quasi-orthogonal space time block codes in cooperative networks with single and multiple antennas at the destination. Numerical and simulation results show that by employing multiple receive antennas the diversity performance of the network is further improved at the expense of slight modification of the detection scheme. The thesis then focuses on designing distributed space time block codes for cooperative networks in which the source node participates in cooperation. Based on this, a source-assisting strategy is proposed for distributed orthogonal and quasi-orthogonal space time block codes. Numerical and simulation results show that the source-assisting strategy exhibits improved diversity performance compared to the conventional distributed orthogonal and quasi-orthogonal designs. Motivated by the problem of channel state information acquisition in practical wireless network environments, the design of differential distributed space time block codes is investigated. Specifically, a co-efficient vector-based differential encoding and decoding scheme is proposed for cooperative networks. The thesis then explores the concatenation of differential strategies with several distributed space time block coding schemes namely; the Alamouti code, square-real orthogonal codes, complex-orthogonal codes, and quasiorthogonal codes, using cooperative networks with different number of relay nodes. In order to cater for high data rate transmission in non-coherent cooperative networks, differential

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Wireless systems with multiple transmit and receive antennas (MIMO systems) provide high capacity due to the plurality of modes available in the channel. Previous code designs for MIMO systems have focused primarily on multiplexed signaling for high data rate or diversity signaling for high link reliability. Based on Ganesan and Stoica (2000) and Hassibi and Hochwald (2000), and using results from frame theory, we present a MIMO space-time code design which bridges the gap between multiplexing and diversity and performs well both in terms of ergodic capacity as well as error-probability. In particular, we demonstrate that designs performing well from an ergodic capacity point of view do not necessarily perform well from an error probability point of view. Simulations illustrate performance of the proposed codes in narrowband MIMO Rayleigh fading channels.

Wireless systems are now popular worldwide to help people and machines to communicate with each other independently of their location. Hence, the demand for capacity in cellular and wireless local area networks has grown in a literally explosive manner during the last decade. But, data transmission over wireless communication suffers from several channel impairements such as fading and cochannel interference. An efficient technique to mitigate the fading and increase the communication link robustness is the spatial diversity. This thesis deals with the perfomance analysis of wireless communication systems exploiting spatial microand macro-diversity. The first part is concerned with the investigation of Multiple-Input Multiple-Output (MIMO) systems using Orthogonal Frequency division Multiplexing (OFDM), in the micro-diversity context. It is shown that taking into account the real channel delay profile can have an impact on the MIMO/OFDM system ergodic and outage capacities, when a parametrized cannel model is considered. Then, two space-time block coding schemes are proposed for a MIMO/OFDM system using two transmit antennas. These schemes, based on the Alamouti code, partially simulate the spatial diversity by the frequency one by taking advantages of the OFDM modulation, and present a rate of a half. We find that the diversity order of the proposed schemes converges to a half-rate scheme using 4 antennas instead of 2. The second part deals with the performance of wireless systems exploiting spatial macro-diversity. First, a realistic channel model based on a refinement of the Radio-Frequency Fornt-End (RFFE) is proposed. The impact of this realistic channel model on the MIMO system capacity is investigated, in both the microand macro-diversity contexts. It is shown that this refined channel model can describe the capacity behavior as observed in real implementations. Then, an optimal power allocation algorithm is proposed for the two-relay regenerative parallel transmission system. This optimization, subject to a cartain power budget to be allocated between the two relays, is done in order to minimize the BER at the destination. It is found that the relayed transmission can give better results than the direct transmission, and that the optimal power allocation is more useful when the relayed links are unbalanced.

Wireless communication systems always demand higher data rates and better quality of service (QoS). Transmission reliability in a wireless channel with high path loss, time-varying multipath fading and power and bandwidth limitations, is a testing issue. Multiple-input multiple-output (MIMO) systems can reduce the effect of these channel interferences and achieve reliable signal transmission at high data rate. Space-time block code (STBC) is a coding scheme for the use of multiple transmits antennas providing a simple transmit diversity scheme. In certain MIMO fading environments, the offered channel capacity can be very low, where despite rich local scattering and uncorrelated transmit and receive signals, the system has a single degree of freedom. This effect has been termed as keyhole or pinhole effect. This contribution analyzes the average symbol error rate (SER) performance of MIMO systems employing orthogonal STBC with M-PSK constellations over fading channels in the presence of the keyhole. The Nakagami-m distribution has been considered for MIMO channel modeling. We derive the probability density function (PDF) of instantaneous signal-to-noise ratio (SNR) and an integral equation to calculate the average SER after space-time block decoding in such channels. Numerical results show that the keyhole significantly degrades the SER performance of the STBC in MIMO channels. The performance of such channels is a function of fading figure, m and diversity.

Wennström, M., 2002. On MIMO Systems and Adaptive Arrays for Wireless Communications: Analysis and Practical Aspects, 291 pp. Uppsala. ISBN 91-5061619-6. This thesis is concerned with the use of multiple antenna elements in wireless communication over frequency non-selective radio channels. Both measurement results and theoretical analysis are presented. New transmit strategies are derived and compared to existing transmit strategies, such as beamforming and space time block coding (STBC). It is found that the best transmission algorithm is largely dependent on the channel characteristics, such as the number of transmit and receive antennas and the existence of a line of sight component. Rayleigh fading multiple input multiple output (MIMO) channels are studied using an eigenvalue analysis and exact expressions for the bit error rates and outage capacities for beamforming and STBC is found. In general are MIMO fading channels correlated and there exists a mutual coupling between antenna elements. These findings are supported by indoor MIMO measurements. It is found that the mutual coupling can, in some scenarios, increase the outage capacity. An adaptive antenna testbed is used to obtain measurement results for the SIMO channel. The results are analyzed and design guidelines are obtained for how a beamformer implemented in hardware shall be constructed. The effects of nonlinear transmit amplifiers in array antennas are also analyzed, and it is shown that an array reduces the effective intermodulation distortion (IMD) transmitted by the array antenna by a spatial filtering of the IMD. A novel frequency allocation algorithm is proposed that reduces IMD even further. The use of a low cost antenna with switchable directional properties, the switched parasitic antenna, is studied in a MIMO context and compared to array techniques. It is found that it has comparable performance, at a fraction of the cost for an array antenna.

We take a capacity view of a number of different space-time coding (STC) schemes. While the Shannon capacity of multiple-input multiple-output (MIMO) channels has been known for a number of years now, the attainment of these capacities remains a challenging issue in many cases. The introduction of space-time coding schemes in the last 2–3 years has, however, begun paving the way towards the attainment of the promised capacities. In this work we attempt to describe what are the attainable information rates of certain STC schemes, by quantifying their inherent capacity penalties. The obtained results, which are validated for a number of typical cases, cast some interesting light on the merits and tradeoffs of different techniques. Further, they point to future work needed in bridging the gap between the theoretically expected capacities and the performance of practical systems.

We study the performance of space-time block coding in terms of different transmission matrices over Nakagami-m fading channels in multiple-input multiple-output (MIMO) environments. A closed-form expression for the exact symbol error rate (SER) of orthogonal space-time block coding (OSTBC) over independent identically distributed Nakagami-m fading channels can be derived by means of the equivalent single-input single-output (SISO) model. The exact SERs of OSTBC with M-PSK and M-QAM modulations are expressed in terms of the moment generating function (MGF) of instantaneous SNR as well as the higher transcendental functions. In this paper, we provide performance evaluation results of several codes of OSTBC with the closed-form expressions; also, Monte Carlo simulation results are given, which demonstrate the accuracy of closed-form solutions. In addition, some new observations on the performance results of different code matrices are made.

We study space-time block coding for single-carrier block transmissions over frequency-selective multipath fading channels. We propose novel transmission schemes that achieve a maximum diversity of order N/sub t/N/sub r/(L+1) in rich scattering environments, where N/sub t/ (N/sub r/) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. We show that linear receiver processing collects full antenna diversity, while the overall complexity remains comparable to that of single-antenna transmissions over frequency-selective channels. We develop transmissions enabling maximum-likelihood optimal decoding based on Viterbi's ( 1998) algorithm, as well as turbo decoding. With single receive and two transmit antennas, the proposed transmission format is capacity achieving. Simulation results demonstrate that joint exploitation of space-multipath diversity leads to significantly improved performance in the presence of frequency-selective fading channels.

We show that an L-branch maximal ratio combining (MRC) receiver with an average signal-to-noise ratio (SNR) γ̅ in Nakagami-m fading channels is equivalent to an mL-branch MRC receiver with average SNR γ̅/m in Rayleigh fading channels in terms of symbol error probability (SEP) when mL is an integer. Moreover, the orthogonal space-time block code (OSTBC) employing M_t transmit antennas and M_r receive antennas with the average SNR γ̅ in Nakagami-m fading channels is shown to be equivalent to an mM_tM_r-branch MRC single-input-multiple-output (SIMO) receiver with the average SNR γ̅/mM_t in Rayleigh fading channels.

We propose an efficient space-frequency coded orthogonal frequency-division multiplexing (OFDM) system for high-speed transmission over wireless links. The analytical expression for the pairwise probability of the proposed space-frequency coded OFDM system is derived in slow, space- and frequency-selective fading channels. The design criteria of trellis codes used in the proposed system are then developed and discussed. It is shown that the proposed space-frequency coded OFDM can efficiently achieve the full diversity provided by the fading channel with low trellis complexity, while for traditional space-frequency coded OFDM systems, we need to design space-time trellis codes with high trellis complexity to exploit the maximum achievable diversity order. The capacity properties of space-frequency coded OFDM over multipath fading channels are also studied. Numerical results are provided to demonstrate the significant performance improvement obtained by the proposed space-frequency coded OFDM scheme, as well as the excellent outage capacity properties.

We propose a space-time block coding framework based on precoding. The code design is aimed to the tradeoff between rate, diversity and complexity. Specifically, for P transmit antennas, our block codes are based on L distinct precoding matrices, where L/spl les/P. We further propose a simple structure for the precoding matrices that are product of distinct permutation matrices and a diagonal unitary matrix, whose diagonal elements are design parameters. In particular, when L=P, the codewords are constructed as circulant matrices. And we explicitly give the expression of the eigenvalues of the error codeword matrix, derive the bounds for the coding gain. If the design parameters are chosen according to algebraic number theory, diversity advantage L can be guaranteed for the resulting codes. Our framework is also appealing on that simple decoding strategies can be applied.

We present an overview of multiple-input multiple-output (MIMO) wireless communication systems followed by some of the new techniques proposed by us to enhance performance. Firstly, some of the popular transmit and receive diversity schemes available in the literature are discussed and issues like diversity, rate, power distribution, decoding complexity, decoding delay, amount of feedback and feedback delay and their tradeoffs to achieve reliable and cost effective communications are described. We then analyze the merits and demerits of some of the open-loop (no feedback) and closed-loop (with feedback) schemes, and discuss various methods proposed by us recently to improve the performance of such schemes. The symbol error rate (SER) performance of the various schemes are compared through computer simulations.

We present an exact analysis of STBCs over uncorrelated memoryless block-fading channels with unequal and generalized fading statistics. First, the exact BER of an ideal (rate-one-full-diversity) linear STBCs with PSK or QAM modulation in the aforementioned channel is given. The formulas derive from a careful review and aggregation of results scattered in the literature, and can be applied to STBCs of various constructions. The exactness and generality of these analytical BER expressions are illustrated by comparison against corresponding simulation results presented in classic papers on STBCs, e.g. (Alamouti et al., 1998). The exact BER formulas are then utilized to analyze the effect of unequal fading statistics and uneven average transmit power distribution across diversity branches, onto the performance of a hypothetical STBC of unitary rate and full-diversity, constructible to any number of transmit antennas. The study reveals the little impact of uneven diversity branch average power distribution on the performance of such a hypothetical STBC. Finally, the formulas are employed to derive the mutual information of linear STBCs over realistic PSK and QAM modulations with block-wise hard-decision, which are then applied to reveal the concrete throughput potential of linear STBCs, in comparison to the capacity of the equivalent SIMO AWGN channel with the same transmit power.

We present a unified approach to designing space-time (ST) block codes using linear constellation precoding (LCP). Our designs are based either on parameterizations of unitary matrices, or on algebraic number-theoretic constructions. With an arbitrary number of N/sub t/ transmit- and N/sub r/ receive-antennas, ST-LCP achieves rate 1 symbol/s/Hz and enjoys diversity gain as high as N/sub t/N/sub r/ over (possibly correlated) quasi-static and fast fading channels. As figures of merit, we use diversity and coding gains, as well as mutual information of the underlying multiple-input-multiple-output system. We show that over quadrature-amplitude modulation and pulse-amplitude modulation, our LCP achieves the upper bound on the coding gain of all linear precoders for certain values of N/sub t/ and comes close to this upper bound for other values of N/sub t/, in both correlated and independent fading channels. Compared with existing ST block codes adhering to an orthogonal design (ST-OD), ST-LCP offers not only better performance, but also higher mutual information for N/sub t/>2. For decoding ST-LCP, we adopt the near-optimum sphere-decoding algorithm, as well as reduced-complexity suboptimum alternatives. Although ST-OD codes afford simpler decoding, the tradeoff between performance and rate versus complexity favors the ST-LCP codes when N/sub t/, N/sub r/, or the spectral efficiency of the system increase. Simulations corroborate our theoretical findings.

We investigate the rate maximizing power allocation strategy for a system where the source and the decode-and-forward (DF) relays cooperate by utilizing a distributed space-time block coding strategy. Under the assumption that all transmitters have perfect knowledge of the link gains, we analyze two limit cases - the case of no-collaboration among relays and the case of ideal collaboration among relays - and we provide a polynomial-time algorithm that computes the maximum achievable rate for the distributed space-time coding system despite the fact that the optimization problem is not convex and it belongs to a class of problems with variational inequality constraints. Due to the orthogonal structure of the space-time block code designs, the proposed algorithm can also be used, with minor modifications, in the computation of the optimum power allocation for the case of DF relays with orthogonal transmissions

We investigate the physical layer security of synchronous multiple access transmissions using the Alamouti space-time block code in fading channels where multiple users communicate with a single intended receiver in the presence of an eavesdropper. We propose an artificial-noise-aided technique to secure the transmissions by having the Alamouti users collaborate with each other, without exchanging information, to degrade the eavesdropper’s channel. Unlike previous work, which assumes that the transmitters have complete knowledge of the legitimate as well as the eavesdropper’s channels, our proposed technique requires no communications between the users, minimal knowledge of the legitimate channel, and no channel knowledge regarding the eavesdropper.