The cooperative communication is one of technologies which can explore the space diversity to resist fading channel. The spatial diversity is achieved by allowing various terminals behaving or a virtual antenna array and forwarding signal for a source terminal in cooperative manner. Under the existence of multiple sources, resource allocation to each source user is even more crucial to enhance the system performance and achieve higher diversity gain. In this work, we proposed a multiuser relaying strategy for a cooperative network with multiple sources sharing the radio resource provided by the cooperative relays simultaneously. Different from the existing work, the set of relays forwards signals of all source users over a common channel to raise spectral efficiency. With full channel information available at relays, the set of sub-optimal precoders and decoders was proposed in terms of maximal the average SNR over all users, subject to eliminating the multiple access interference (MAI) at each destination and satisfying total power constraint among all relays. It shows from the simulation results that, compared with the conventional cooperative strategy and direct transmission, the proposed scheme provides pronounced improvement on the outage capacity. Keywords: user cooperation, multiple access, resource allocation

We consider the uplink of a multicell multiuser single-input multiple-output system where the channel experiences both small and large-scale fading. The data detection is done by using the linear zero-forcing technique, assuming the base station (BS) has perfect channel state information of all users in its cell. We derive  new, exact analytical expressions for the uplink rate, symbol error rate, and outage probability per user, as well as alower bound on the achievable rate. This bound is very tight and becomes exact in the large-number-of-antennas limit. We further study the asymptotic system performance in the regimes of high signal-to-noise ratio (SNR), large number of antennas, and large number of users per cell. We show that at high SNRs, the system is interference-limited and hence, we cannot improve the system performance by increasing the transmit power of each user. Instead, by increasing the number of BS antennas, the effects of interference and noise can be reduced, thereby improving the system performance. We demonstrate that, with very large antenna arrays at the BS, the transmit power of each user can be made inversely proportional to the number of BS antennas while maintaining a desired quality-of-service. Numerical results are presented to verify our analysis.

In geometry we will consider n-dimensional generalizations of the Power of a Point Theorem and of Pascal's Hexagon Theorem. In generalizing the Power of a Point Theorem, we will consider collections of cones determined by the intersections of an (n-1)-sphere and a pair of hyperplanes. We will then use these constructions to produce an n-dimensional generalization of Pascal's Hexagon Theorem, a classical plane geometry result which states that "Given a hexagon inscribed in a conic section, the three pairs of continuations of opposite sides meet on a straight line." Our generalization of this theorem will consider a pair of n-simplices intersecting an (n-1)-sphere, and will conclude with the intersections of corresponding faces lying in a hyperplane. In graph theory we will explore the interaction between zero forcing and cut-sets. The color change rule which lies at the center of zero forcing says "Suppose that each of the vertices of a graph are colored either blue or white. If u is a blue vertex and v is its only white neighbor, then u can force v to change to blue." The concept of zero forcing was introduced by the AIM Minimum Rank - Special Graphs Work Group in 2007 as a way of determining bounds on the minimum rank of graphs. Later, Darren Row established results concerning the zero forcing numbers of graphs with a cut-vertex. We will extend his work by considering graphs with arbitrarily large cut-sets, and the collections of components they yield, to determine results for the zero forcing numbers of these graphs.

For a graph G we define S(G) to be the set of all real symmetric n by n matrices whose off-diagonal zero/nonzero pattern is described by G. We show how to compute the minimum rank of all matrices in S(G) for a class of graphs called outerplanar graphs. In addition, we obtain results on the possible eigenvalues and possible inertias of matrices in S(G) for certain classes of graph G. We also obtain results concerning the relationship between two graph parameters, the zero forcing number and the path cover number, related to the minimum rank problem.

No / We have studied the performance of multidimensional signaling techniques using wavelets based modulation within an orthogonally multiplexed communication system. The discrete wavelets transform and wavelet packet modulation techniques have been studied using Daubechies 2 and 8, Biothogonal1.5 and 3.1 and reverse Biorthognal 1.5 and 3.1 wavelets in the presence of Rayleigh multipath fading channels with AWGN. Results showed that DWT based systems outperform WPM systems both in terms of BER vs. SNR performance as well as processing. The performances of two different equalizations techniques, namely zero forcing (ZF) and minimum mean square error (MMSE), were also compared using DWT. When the channel is modeled using Rayleigh multipath fading, AWGN and ISI both techniques yield similar performance.

[pt] Modulação Espacial com Pré-codificação Generalizada (Generalized Precoding Aided Spatial Modulation, GPSM) é uma estratégia de comunicação em sistemas MIMO em que o transmissor codifica a informação em duas entidades: a transmissão paralela de símbolos pertencentes à constelação de uma modulação digital, e os índices das posições do vetor de informação que transportam estes símbolos, denominada information bearing positions (IBP), enquanto as demais posições transportam zero. Além disto, o transmissor, previamente à transmissão dos dados pré-codifica o vetor de informação, possibilitando a redução da complexidade do nó receptor. Entre as vantagens desta estratégia destaca-se a concentração da energia transmitida apenas nas posições do vetor de informação que efetivamente contém símbolos, favorecendo o desempenho do sistema. Esta tese considera um sistema MIMO multiusuário (MU-MIMO) que emprega GPSM no enlace direto. O modelo de sinais desenvolvido para descrever este sistema evidencia a influência das matrizes que codificam as IBP no desempenho do sistema. Com base neste modelo, o sistema GPSM MU-MIMO é apresentado para três pré-codificadores lineares: Zero-Forcing, Block Diagonalization e Block Diagonalization de Duas Fases. Para cada um destes pré-codificadores são propostas estratégias de seleção das matrizes de codificação IBP, de acordo com a matriz que descreve o canal MU-MIMO corrente, a serem empregadas pelo transmissor visando melhorar o desempenho de detecção do sistema. As curvas de desempenho de detecção são comparadas a limitantes semianalíticos desenvolvidos. Por fim, considera-se o cenário em que existem mais antenas disponíveis na estação rádio-base e/ou nos usuários do que o número de cadeias de radiofrequência que os equipam. Esta tese apresenta estratégias ótimas e de complexidade reduzida de se explorar o uso das antenas mais favoráveis à transmissão e/ou recepção, em adição à escolha das matrizes de codificação IBP, com o objetivo de prover melhorias ao desempenho do sistema. / [en] Generalized Pre-coding Aided Spatial Modulation (GPSM) is a MIMO system communication strategy in which the transmitter encodes the information in two entities: the parallel transmission of symbols belonging to a digital modulation constellation, and the choice of the indices of the information vector elements that carries the informaiton symbols, denoted information bearing positions (IBP), while the remaing positions are set to zero. Besides, the transmitter precodes, prior to transmission, the information vector, which lets the receiver node benefit from complexity reduction. Among the advantages of this strategies, the concentration of the transmitted energy only on the information vector positions that transports modulated symbols, resulting in system performance improvement. This thesis considers a multiuser MIMO (MU-MIMO) that employs GPSM in the donwlink transmission. The presented signal model to describe this system evidences the influence of the IBP coding matrices on the system performance. Based on this model, GPSM MU-MIMO system is presented considering three linear precoders: Zero-Forcing, Block Diagonalization and Double-Stage Block Diagonalization. For each precoder, strategies to select the IBP encoding matrix, acording to the matrix that describes the current MU-MIMO channel, is proposed. These matrices are to be employed by the user, aiming at system detection performance improvement. Detection performance curves are compared to semianalytic lower bounds. Finally, a scenario in which that are a number of available antennas at the BS and/or at the users that exceed the number of radiofrequency chains. this thesis porposes optimal and reduced complexity strategies that exploit the use of the most favorable antennas for transmission and/or reception, in addition to the choice of the IBP enconding matrices, aiming at system performance improvement.

Let G be an undirected graph on n vertices and let S(G) be the class of all real-valued symmetric n × n matrices whose nonzero off-diagonal entries occur in exactly the positions corresponding to the edges of G. Let V = {1, 2, . . . , n} be the vertex set of G. A scheme on G is a function f : V → {0, 1}. Given a scheme f on G, there is an associated class of matrices Sf (G) = {A ∈ S(G)|aii = 0 if and only if f(i) = 0}. A scheme f is said to be constructible if there exists a matrix A ∈ Sf (G) with rank A = min{rank M|M ∈ S(G)}. We explore properties of constructible schemes and give a complete classification of which schemes are constructible for paths and cycles. We also consider schemes on complete graphs and show the existence of a graph for which every possible scheme is constructible.

Recent information theoretical results indicate that dirty-paper coding (DPC) achieves the entire capacity region of the Gaussian multiple-input multiple-output (MIMO) broadcast channel (BC). This thesis presents practical code designs for Gaussian BCs based on DPC. To simplify our designs, we assume constraints on the individual rates for each user instead of the customary constraint on transmitter power. The objective therefore is to minimize the transmitter power such that the practical decoders of all users are able to operate at the given rate constraints. The enabling element of our code designs is a practical DPC scheme based on nested turbo codes. We start with Cover's simplest two-user Gaussian BC as a toy example and present a code design that operates 1.44 dB away from the capacity region boundary at the transmission rate of 1 bit per sample per dimension for each user. Then we consider the case of the multiple-input multiple-output BC and develop a practical limit-approaching code design under the assumption that the channel state information is available perfectly at the receivers as well as at the transmitter. The optimal precoding strategy in this case can be derived by invoking duality between the MIMO BC and MIMO multiple access channel (MAC). However, this approach requires transformation of the optimal MAC covariances to their corresponding counterparts in the BC domain. To avoid these computationally complex transformations, we derive a closed-form expression for the optimal precoding matrix for the two-user case and use it to determine the optimal precoding strategy. For more than two users we propose a low-complexity suboptimal strategy, which, for three transmit antennas at the base station and three users (each with a single receive antenna), performs only 0.2 dB worse than the optimal scheme. Our obtained results are only 1.5 dB away from the capacity limit. Moreover simulations indicate that our practical DPC based scheme significantly outperforms the prevalent suboptimal strategies such as time division multiplexing and zero forcing beamforming. The drawback of DPC based designs is the requirement of channel state information at the transmitter. However, if the channel state information can be communicated back to the transmitter effectively, DPC does indeed have a promising future in code designs for MIMO BCs.

In this dissertation the aim was to investigate the usage of algorithms found in computer science and apply suitable algorithms to the problem of decoding multiple-input multipleoutput (MIMO) space-time-frequency block coded signals. It was found that the sphere decoder is a specific implementation of the A* tree search algorithm that is well known in computer science. Based on this knowledge, the sphere decoder was extended to include a priori information in the maximum a posteriori probability (MAP) joint decoding of the STFC block coded MIMO signals. The added complexity the addition of a priori information has on the sphere decoder was investigated and compared to the sphere decoder without a priori information. To mitigate the potential additional complexity several algorithms that determine the order in which the symbols are decoded were investigated. Three new algorithms incorporating a priori information were developed and compared with two existing algorithms. The existing algorithms compared against are sorting based on the norms of the channel matrix columns and the sorted QR decomposition. Additionally, the zero forcing (ZF) and minimum mean squared error (MMSE) decoderswith and without decision feedback (DF) were also extended to include a priori information. The developed method of incorporating a priori information was compared to an existing algorithm based on receive vector translation (RVT). The limitation of RVT to quadrature phase shift keying (QPSK) and binary shift keying (BPSK) constellations was also shown in its derivation. The impact of the various symbol sorting algorithms initially developed for the sphere decoder on these decoders was also investigated. The developed a priori decoders operate in the log domain and as such accept a priori information in log-likelihood ratios (LLRs). In order to output LLRs to the forward error correcting (FEC) code, use of the max-log approximation, occasionally referred to as hard-to-soft decoding, was made. In order to test the developed decoders, an iterative turbo decoder structure was used together with an LDPC decoder to decode threaded algebraic space-time (TAST) codes in a Rayleigh faded MIMO channel. Two variables that have the greatest impact on the performance of the turbo decoder were identified: the hard limit value of the LLRs to the LDPC decoder and the number of independently faded bits in the LDPC code. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Electrical, Electronic and Computer Engineering / unrestricted

Free-space optics (FSO) is an unlicensed communication technology that uses the free space as a propagation medium to connect two communicating terminal wire- lessly [1]. It is an attractive solution to the last-mile connectivity problems in commu- nication networks, mainly when installing optical fibers is expensive or unavailable. A possible idea to increase the throughput of wireless optical links in free space is to use spatial multiplexing (SMM) [2]. Optical beam distortion due to propagation through a turbulent channel is one of the main factors limiting performance of such a system. Therefore, overcoming the effect of turbulence is a major problem for structured light optical communication in free space. Usually, this problem is approached by using adaptive optics systems and various methods of digital signal processing (DSP) on the receiver side [3–5]. Recently, an idea of optical channel pre-compensation to mit- igate inter-modal crosstalk was proposed [6] and experimentally validated [7]. Such a method, if implemented, will allow the use of entirely passive receivers or, in the case of full-duplex transmission, increase throughput. Here, the performance of a zero-forcing precoding technique to mitigate the effects of an optical turbulence in a Laguerre Gaussian mode based SMM FSO is investigated. Equally, details on a close to reality simulation of the atmospheric turbulence and beam propagation are provided.