Multiple input multiple output (MIMO) systems are now a proven area in current and future telecommunications research. MIMO wireless channels, in which both the transmitter and receiver have multiple antennas, have been shown to provide high bandwidth efficiency. In this thesis, we cover MIMO communications technology with a focus on cellular systems and the MIMO broadcast channel (MIMO-BC). Our development of techniques and analysis for the MIMO-BC starts with a study of single user MIMO systems. One such single user technique is that of antenna selection. In this thesis, we discuss various flavours of antenna selection, with the focus on powerful, yet straightforward, norm-based algorithms. These algorithms are analyzed and the results of this analysis produce a powerful and flexible power scaling factor. This power scaling factor can be used to model the gains of norm-based antenna selection via a single signal-to-noise ratio (SNR)-based parameter. This provides a powerful tool for engineers interested in quickly seeing the effects of antenna selection on their systems. A novel low complexity power allocation scheme follows on from the selection algorithms. Named “Poor Man’s Waterfilling” (PMWF), this scheme can provide significant gains in low SNR systems with very little extra complexity compared to selection alone. We then compare a variety of algorithms for the MIMO-BC, ranging from selection to beamforming, to the optimal, yet complex, iterative waterfilling (ITWF) solution. In this thesis we show that certain algorithms perform better in different scenarios, based on whether there is shadow fading or not. A power scaling factor analysis is also performed on these systems. In the cases where the user’s link gains are widely varying, such as when shadowing and distance effects are present, user fairness is impaired when optimal and near optimal throughput occurs. This leads to a key problem in the MIMO-BC, the balance between user fairness and throughput performance. In an attempt to find a suitable balance between these two factors, we modify the ITWF algorithm by both introducing extra constraints and also by using a novel utility function approach. Both these methods prove to increase user fairness with only minor loss in throughput over the optimal systems. The introduction of MIMO systems to the cellular domain has been hampered by the effects of interference between the cells. In this thesis we move MIMO to the cellular domain, addressing the interference using two different methods. We first use power control, where the transmit power of the base station is controlled to optimize the overall system throughput. This leads to promising results using low complexity methods. Our second method is a novel method of collaboration between base stations. This collaboration transforms neighbouring cell sectors into macro-cells and this results in substantial increases in performance.

Next generation of wireless communication mostly relies on multiple-input multipleoutput (MIMO) configuration and full-duplex relaying to improve data-rates, spectrale efficiency, spatial-multiplexing, quality-of-service and energy-efficiency etc. However, multiple radio frequency (RF) transceivers in MIMO system and multi-hops in relay networks, accumulate transceiver impairments, rendering an unacceptable system performance. Majority of the technical contributions either assume ideal hardware or inappropriately model hardware impairments which often induce misleading results especially for high data-rate communication systems. We propose statistical mathematical modeling of various hardware impairment (HWI) to characterize their deteriorating effects on the information signal. In addition, we model the aggregate HWI as improper Gaussian signaling (IGS), to fully characterize their asymmetric properties and the self-interfering signal attribute under I/Q imbalance. The proposed model encourages to adopt asymmetric transmission scheme, as opposed to traditional symmetric signaling. First, we present statistical baseband equivalent mathematical models for general MIMO system and two special scenarios of receive and transmit diversity systems under HWI. Then, we express their achievable rate under PGS and IGS transmit schemes. Moreover, we tune the IGS statistical characteristics to maximize the achievable rate. We also present optimal beam-forming/pre-coding and receive combiner vector for multiple-input single-output (MISO) and single-input multiple output (SIMO) systems, which lead to SDNR maximization. Moreover, we propose an adaptive scheme to switch between maximal IGS (MIGS) and PGS transmission based on the described conditions to reduce computational overhead. Subsequently, two case studies are presented. 1) Outage analysis has been carried out for SIMO, under transceiver distortion noise, for two diversity combining schemes 2) The benefits of employing IGS is investigated in full duplex relaying (FDR) suffering from two types of interference, the residual self-interference (RSI) and I/Q distortions. We further optimize the pseudo-variance to compensate the interference impact and improve end-to-end achievable rate. Finally, we validate the analytic expressions through simulation results, to quantify the performance degradation in the absence of ideal transceivers and the gain reaped from adopting IGS scheme compared with PGS scheme.

A common practice for government defense agencies and commercial aeronautical companies is to use dual antennas on test flight air vehicles in order to overcome occlusion issues during high-speed telemetric maneuvers. The dual antennas, though never being masked at the same time, unfortunately lead to a drastic increase in nulls in the signal pattern. The result of this interference pattern can be compared to the effect of fading in a multiple-input multiple-output (MIMO) multi-path scattering environment. Confidence in this comparison leads to the use of unitary space-time MIMO codes to overcome the signal self-interference. The possibility and performance of several of these codes will be examined. Such criteria as training for channel estimation, use of shaped offset quadrature phase shift keying (SOQPSK), hardware facility, and data throughput will be compared for each code. A realistic telemetry channel will be derived to increase accuracy of simulated results and conclusions.

Employment of the multiple-antenna transmitters/receivers in communication systems is known as a promising solution to provide high-data-rate wireless links. In the multi-user environments, the problems of signaling and fairness for multi-antenna systems have emerged as challenging problems. This dissertation deals with these problems in several multi-antenna multi-user scenarios. In part one, a simple signaling method for the multi-antenna broadcast channels is proposed. This method reduces the MIMO broadcast system to a set of parallel channels. The proposed scheme has several desirable features in terms of: (i) accommodating users with different number of receive antennas, (ii) exploiting multi-user diversity, and (iii) requiring low feedback rate. The simulation results and analytical evaluations indicate that the achieved sum-rate is close to the sum-capacity of the underlying broadcast channel. In part two, for multiple-antenna systems with two transmitters and two receivers, a new non-cooperative scenario of data communication is studied in which each receiver receives data from both transmitters. For such a scenario, a signaling scheme is proposed which decomposes the system into two broadcast or two multi-access sub-channels. Using the decomposition scheme, it is shown that this signaling scenario outperforms the other known non-cooperative schemes in terms of the achievable multiplexing gain. In particular for some special cases, the achieved multiplexing gain is the same as the multiplexing gain of the system, where the full cooperation is provided between the transmitters and/or between the receivers. Part three investigates the problem of fairness for a class of systems for which a subset of the capacity region, which includes the sum-capacity facets, forms a polymatroid structure. The main purpose is to find a point on the sum-capacity facet which satisfies a notion of fairness among active users. This problem is addressed in the cases where the complexity of achieving interior points is not feasible, and where the complexity of achieving interior points is feasible. In part four, $K$-user memoryless interference channels are considered; where each receiver sequentially decodes the data of a subset of transmitters before it decodes the data of the designated transmitter. A greedy algorithm is developed to find the users which are decoded at each receiver and the corresponding decoding order such that the minimum rate of the users is maximized. It is proven that the proposed algorithm is optimal. The results of the parts three and four are presented for general channels which include the multiple-antenna systems as special cases.

Employment of the multiple-antenna transmitters/receivers in communication systems is known as a promising solution to provide high-data-rate wireless links. In the multi-user environments, the problems of signaling and fairness for multi-antenna systems have emerged as challenging problems. This dissertation deals with these problems in several multi-antenna multi-user scenarios. In part one, a simple signaling method for the multi-antenna broadcast channels is proposed. This method reduces the MIMO broadcast system to a set of parallel channels. The proposed scheme has several desirable features in terms of: (i) accommodating users with different number of receive antennas, (ii) exploiting multi-user diversity, and (iii) requiring low feedback rate. The simulation results and analytical evaluations indicate that the achieved sum-rate is close to the sum-capacity of the underlying broadcast channel. In part two, for multiple-antenna systems with two transmitters and two receivers, a new non-cooperative scenario of data communication is studied in which each receiver receives data from both transmitters. For such a scenario, a signaling scheme is proposed which decomposes the system into two broadcast or two multi-access sub-channels. Using the decomposition scheme, it is shown that this signaling scenario outperforms the other known non-cooperative schemes in terms of the achievable multiplexing gain. In particular for some special cases, the achieved multiplexing gain is the same as the multiplexing gain of the system, where the full cooperation is provided between the transmitters and/or between the receivers. Part three investigates the problem of fairness for a class of systems for which a subset of the capacity region, which includes the sum-capacity facets, forms a polymatroid structure. The main purpose is to find a point on the sum-capacity facet which satisfies a notion of fairness among active users. This problem is addressed in the cases where the complexity of achieving interior points is not feasible, and where the complexity of achieving interior points is feasible. In part four, $K$-user memoryless interference channels are considered; where each receiver sequentially decodes the data of a subset of transmitters before it decodes the data of the designated transmitter. A greedy algorithm is developed to find the users which are decoded at each receiver and the corresponding decoding order such that the minimum rate of the users is maximized. It is proven that the proposed algorithm is optimal. The results of the parts three and four are presented for general channels which include the multiple-antenna systems as special cases.

In this thesis, CMs of linear and non-linear multiple antenna receivers, in particular linear minimum mean squared error (LMMSE) and LMMSE with decision feedback (LMMSE-DF), are developed. To develop these CMs, first a simple analytical expression of the distribution of the post processing signal to interference and noise (SINR) of an LMMSE receiver is developed. This expression is then used to develop SINR- and ABER-based CMs. However, the analytical forms of these CMs are derived only for the following scenarios: (i) any number of receive antennas with three users having arbitrary received powers and (ii) two antenna receiver with arbitrary number of equal received power users. For all the other scenarios a semi-analytical CM is used. The PHY abstractions or CMs are next used in the evaluation of a random access cellular network and an ad hoc network. Analytical model of the random access cellular network is developed using the SINR- and ABER-based CM of the LMMSE receiver. The impact of receiver processing is measured in terms of throughput. In this case, the random access mechanism is modeled by a single channel S-Aloha channel access scheme. Another analytical model is developed for single and multi-packet reception in a multi-channel S-Aloha channel access. An emph{ideal} receiver is modeled in this case, i.e. the packet(s) are successfully received as long as the total number of colliding packets is not greater than the number of antennas. Throughput and delay are used as performance metrics to study the impact of different PHY designs. Finally, the SINR-based semi-analytical CMs of LMMSE and LMMSE-DF are used to evaluate the performance of multi-hop ad hoc networks. Throughput is used as the performance evaluation metric. A novel MAC, called S-MAC, is proposed and its performance is compared against another MAC for wireless networks, called CSMA/CA(k).

Now-a-days wireless networks such as cellular communication have deeply affected human lives and became an essential part of it. The demand to buy high capacity and better performance devices and cellular services has been rapidly increased. There are more than two hundred different countries and almost three billion users all over the world which are using cellular services provided by Global System for Mobile (GSM), Universal Mobile Telecommunication System (UMTS), Wireless Local Area Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX). In the past decade, one antenna is connected to only one communication radio device at the same time but currently this scenario has been completely changed. To increase the capacity of the channels and to improve the bit error performance between mobile station and service station, it is now possible to connect one antenna with more than one communication radio device at the same time. Multiple Input Multiple Output (MIMO) systems are designed to obtain this requirement. In MIMO systems, antennas are combined in the form of small frames like coupling in cellular devices. Diversity means to obtain successful transmission and reception of radio signals with accordance to polarization and correlation. Due to diversity the capacity of the channels and bit error rate are improved, so diversity is one of the main and important properties of MIMO systems. This thesis is emphasized to study WiMAX systems by implementing multiple antenna techniques, by observing the bit error rate performance and data rate in WiMAX systems using two important and currently widely applied multiple access communication techniques. The research will also elaborate these techniques and explain the basic parameters, operations, mathematical calculations and different relevant observations. The simulation tool used in this research thesis is MATLAB which is also used to illustrate the results with figures and graphs. / Nu för tiden trådlösa nätverk såsom cellulär kommunikation har stor påverkan på människors liv och blev en viktig del av det. Kravet att köpa hög kapacitet och bättre utrustning prestanda och mobiltelefoni har snabbt ökat. Det finns mer än två hundra olika länder och nästan tre miljarder användare över hela världen som använder mobiltelefoni som Global System for Mobile (GSM), Universal Mobile Telecommunication System (UMTS), Wireless Local Area Network (WLAN) och Worldwide Interoperability för Microwave Access (WiMAX). Under det senaste decenniet har en antenn ansluten till en kommunikationsradio enhet vid samma tidpunkt, men för närvarande detta scenario har fullständigt förändrats. Att öka kapaciteten av kanalerna och för att förbättra prestandan lite fel mellan mobil station och bensinstationen, är det nu möjligt att ansluta en antenn med mer än en kommunikationsradio enhet samtidigt. Multiple Input Multiple Output (MIMO) system är utformade för att uppnå detta krav. I MIMO-system, antenner kombineras i form av små ramar som kopplingsanordningar i cellulär enheter. Mångfald innebär att få lyckad överföring och mottagning av radiosignaler i enlighet med polarisation och korrelation. På grund av mångfalden kapacitet av kanalerna och bit error rate förbättras, så att mångfald är en av de största och viktigaste egenskaper MIMO-system. Denna avhandling är betonas att studera WiMAX system genom att genomföra flera antenn teknik, genom att observera bitars prestanda felfrekvensen och datahastighet i WiMAX system med hjälp av två viktiga och som för närvarande tillämpas allmänt flera tekniker tillgång kommunikation. Forskningen kommer också att utveckla dessa metoder och förklara de grundläggande parametrarna, operationer, matematiska beräkningar och olika relevanta iakttagelser. Den simuleringsverktyg som används i denna forskning avhandling är MATLAB som också används för att illustrera resultaten med siffror och diagram.

Multiple-Input Multiple-Output (MIMO) communication using multiple antennas has received significant attention in recent years, both in the academia and industry, as they offer additional spatial dimensions for high-rate and reliable communication, without expending valuable bandwidth. However, exploiting these promised benefits of MIMO systems critically depends on fast and accurate acquisition of Channel State Information (CSI) at the Receiver (CSIR) and the Transmitter (CSIT). In Time Division Duplex (TDD) MIMO systems, where the forward channel and the reverse channel are the same, it is possible to exploit this reciprocity to reduce the overhead involved in acquiring CSI, both in terms of training duration and power. Further, many popular and efficient transmission schemes such as beam forming, spatial multiplexing over dominant channel modes, etc. do not require full CSI at the transmitter. In such cases, it is possible to reduce the Reverse Channel Training (RCT) overhead by only learning the part of the channel that is required for data transmission at the transmitter. In this thesis, we propose and analyze several novel channel-dependent RCT schemes for MIMO systems and analyze their performance in terms of (a) the mean-square error in the channel estimate, (b) lower bounds on the capacity, and (c) the diversity-multiplexing gain tradeoff. We show that the proposed training schemes offer significant performance improvement relative to conventional channel-agnostic RCT schemes. The main take-home messages from this thesis are as follows: • Exploiting CSI while designing the RCT sequence improves the performance. • The training sequence should be designed so as to convey only the part of the CSI required for data transmission by the transmitter. • Power-controlled RCT, when feasible, significantly outperforms fixed power RCT.

Multiple antenna techniques are used to increase the robustness and performance of wireless networks. Multiple antenna techniques can achieve diversity and increase bandwidth efficiency when specially designed channel codes are used at the scheme’s transmitter. These channel codes can be designed in the space, time and frequency domain. These specially designed channel codes in the space and time domain are actually designed for flat fading channels and in frequency selective fading channel, their performance may be degraded. To counteract this possible performance degradation in frequency selective fading channel, two main approaches can be applied to mitigate the effect of the symbol interference due to the frequency selective fading channel. These approaches are multichannel equalisation and orthogonal frequency division multiplexing (OFDM). In this thesis, a multichannel equalisation technique and OFDM were applied to channel codes specially designed for multiple antenna systems. An optimum receiver was proposed for super-orthogonal space-time trellis codes in a multichannel equalised frequency selective environment. Although the proposed receiver had increased complexity, the diversity order is still the same as compared to the code in a flat fading channel. To take advantage of the multipath diversity possible in a frequency selective fading channel, super-orthogonal block codes were employed in an OFDM environment. A new kind of super-orthogonal block code was proposed in this thesis. Super-orthogonal space-frequency trellis-coded OFDM was proposed to take advantage of not only the possible multipath diversity but also the spatial diversity for coded OFDM schemes. Based on simulation results in this thesis, the proposed coded OFDM scheme performs better than all other coded OFDM schemes (i.e. space time trellis-coded OFDM, space-time block coded OFDM, space-frequency block coded OFDM and super-orthogonal space-time trellis-coded OFDM). A simplified channel estimation algorithm was proposed for two of the coded OFDM schemes, which form a broad-based classification of coded OFDM schemes, i.e. trelliscoded schemes and block-coded schemes. Finally in this thesis performance analysis using the Gauss Chebychev quadrature technique as a way of validating simulation results was done for super-orthogonal block coded OFDM schemes when channel state information is known and when it is estimated. The results obtained show that results obtained via simulation and analysis are asymptotic and therefore the proposed analysis technique can be use to obtain error rate values for different SNR region instead of time consuming simulation. / Thesis (PhD)--University of Pretoria, 2012. / Electrical, Electronic and Computer Engineering / unrestricted

[pt] Os sistemas de múltiplas antenas empregam diferentes técnicas de processamento de sinais em ambos extremos do sistema de comunicações para se beneficiar das múltiplas dimensões espaciais e transmitir para diversos usuarios usando os mesmos recursos de tempo e frequência. Desta forma, uma alta eficiência espectral pode ser atingida sem precisar de largura de banda extra. No entanto, o desempenho depende de uma estimativa do canal altamente precisa do lado do transmissor, a qual é denominada channel state information at the transmitter (CSIT). Se o valor estimado do canal for perfeito, o sistema consegue suprimir a interferência multiusuário (MUI), que é a principal responsável pela degradação do desempenho do sistema. Porém, supor uma estimativa perfeita é bastante otimista pois sistemas reais introduzem incerteza devido ao processo de estimação, a erros de quantização e a retardos próprios dos sistemas. Nesse contexto, a técnica conhecida como divisão de taxas ou rate splitting (RS) surge como uma ferramenta promissora para lidar com as imperfeições na estimativa do canal. RS divide os dados em um fluxo comum e vários fluxos privados e então sobrepõe o fluxo comum no topo dos fluxos privados. Esta tese propõe várias técnicas de processamento que aumentam ainda mais os benefícios dos sistemas RS. Neste trabalho, consideramos o downlink (DL) de um sistema de comunicações sem fio onde o transmissor envia mensagens independentes para cada usuário. A métrica usada para avaliar o desempenho do sistema é a soma das taxas ergódica (ESR). Diferente dos trabalhos convencionais em RS, consideramos que os terminais dos usuários estão equipados com múltiplas antenas. Isso nos permite implementar na recepção combinadores de fluxos que aumentem a taxa do fluxo comum. Aumentar esta taxa é um dos grandes problemas dos sistemas RS, uma vez que a taxa comum é limitada pelo pior usuário o que pode degradar fortemente o desempenho do sistema. Assim, três combinadores de fluxos diferentes são propostos e as expressões analíticas para calcular a soma das taxas são apresentadas. Os combinadores são derivados empregando-se os critérios Min-Max, MRC e MMSE. O critério Min-Max seleciona para cada usuário a melhor antena para decodificar o símbolo comum. O MRC visa maximizar o SNR ao decodificar o símbolo comum. Finalmente, o critério MMSE minimiza o quadrado da diferença entre o símbolo comum e o sinal recebido. Até o momento, RS foi considerado com precodificadores lineares. Devido a isto, neste trabalho investigamos o desempenho do RS com precodificadores não lineares. Para este fim, usamos diferentes tipos de precodificador Tomlinson-Harashima (THP) baseados nos precodificadores lineares ZF e MMSE. Em seguida, propomos um algoritmo multi-branch (MB) adequado para o RS-THP proposto. Este algoritmo cria vários padrões de transmissão e seleciona o melhor padrão para efetuar a transmissão. Esta técnica de préprocessamento aumentam ainda mais a soma das taxas obtida, uma vez que o desempenho do THP depende da ordem dos símbolos, porém também aumenta a complexidade computacional. Expressões analíticas para calcular a soma das taxas das técnicas propostas são derivadas por meio de análises estatísticas dos principais parâmetros. Finalmente, propomos quatro técnicas adaptativas diferentes de alocação de potência, as quais se caracterizam por sua baixa complexidade computacional. Duas destas técnicas são projetadas para sistemas SDMA convencionais, enquanto as outras duas são projetadas para sistemas RS. Um dos principais objetivos dos algoritmos propostos é realizar uma alocação de potência robusta capaz de lidar com os efeitos prejudicias das imperfeições no CSIT. É importante mencionar que a alocação de potência em sistemas RS é uma das tarefas mais importantes e deve ser realizada com extremo cuidado. Se a potência não for alocada corretamente, o desempenho do sistema RS será bastante degradado e as arquiteturas convencionais, como SDMA e NOMA, poderão ter um desempenho melhor. No entanto, a alocação de potência em sistemas RS precisa da solução de problemas complexos de otimização, o que aumenta o tempo gasto no processamento do sinal. Os algoritmos adaptativos propostos reduzem a complexidade computacional e são uma solução atrativa para aplicações práticas em sistemas de grande porte. / [en] Multiple-antenna systems employ different signal processing techniques at both ends of the communication to exploit the spatial dimensions and serve multiple users simultaneously in the same time-frequency domain. In this way, high spectral efficiency can be reached without the need of extra bandwidth. However, such gain depends on a highly accurate channel state information at the transmitter (CSIT). Perfect CSIT allows the system to suppress the multi user interference (MUI), which is the main responsible of the performance degradation. Nonetheless, assuming perfect CSIT is rather optimistic since the estimation procedure, quantization errors and delays of real system lead to CSIT uncertainties. In this context, rate splitting (RS) has arisen as a promising technique to deal with CSIT imperfections. Basically, RS splits the data into a common stream and private streams and then superimposes the common stream on top of the private streams. This thesis proposes several processing techniques which further enhance the benefits of RS systems. We consider the downlink (DL) of a wireless communications system, where the transmitter sends independent messages to each receiver. The ergodic sum rate (ESR) is adopted as the main metric to evaluate the performance of the system. Different from conventional RS works, we consider that the users are equipped with multiple antennas. This allows us to implement stream combiners for the common stream at the receivers. The implementations of the stream combiners improves the common rate performance, which is a major problem of RS systems since the common rate is limited by the performance of the worst user and can be heavily degraded. In this work, three different stream combiners are proposed along with analytical expressions to compute their sum rate performance. Specifically, the combiners are derived employing the min-max, maximum ratio combining (MRC), and minimum mean square error (MMSE) criteria. The min-max criterion selects at each user the best receive antenna to decode the common symbol. The MRC criterion aims at maximizing the SNR when decoding the common symbol. Finally, the MMSE criterion minimizes the squared difference between the common symbol and the received signal. So far, RS has been predominantly considered with channel inversiontype linear precoders. Therefore, this motivates us to investigate the performance of RS with non-linear precoders. For this purpose, we employ different architectures of the Tomlinson-Harashima precoder (THP) which are based on the zero-forcing (ZF) and MMSE precoders. We then propose a multi-branch (MB) algorithm for the proposed RS-THP, which creates several transmit patterns and selects the best for transmission. This pre-processing techniques further enhance the sum rate obtained since the performance of THP is dependent on the symbol ordering but also increases the computational complexity. Analytical expressions to calculate the sum rate of the proposed techniques are derived through statistical evaluation of key parameters. Finally, we propose four different adaptive power allocation techniques, which are characterized by their low computational complexity. Two of them are designed for conventional SDMA systems whereas the other two are intended for RS systems. One major objective of the proposed algorithms is to perform robust power allocation capable of dealing with the detrimental effects of imperfect CSIT. It is important to mention that power allocation in RS systems is one of the critical tasks that should be carefully performed. If the power is not properly allocated the performance of RS systems is heavily degraded and conventional architectures such as SDMA and NOMA could perform better. However, RS rely on solving complex optimization problems to perform power allocation, increasing the time and effort dedicated to signal processing. The proposed adaptive power allocation algorithms reduce the computational complexity and are an attractive solution for practical applications with large-scale systems.