This thesis investigates the semi-blind channel estimation in uplink (UL) of Orthogonal Frequency Division Multiple Access (OFDMA) systems based on subspace decomposition. We exploit the orthogonality between signal subspace and noise subspace induced by virtual carriers (VCs) and cyclic prefix (CP) and the property of that the exclusive sub-carriers set is assigned to each user to estimate and identify the channels for each user individually. In OFDMA systems, when some users don¡¦t communicate with base station, the sub-carriers of non-active user provide extra redundancy for channel estimate to enhance the accuracy of channel estimation. Furthermore, the sufficient channel identifiability condition is developed. Furthermore, a novel scheme, called as virtual carriers recovery (VCR) scheme, is proposed to improve the performance of the subspace-based channel estimation method. It suppresses the noise interference by recovering the VCs to zeros at receiver. The simulation results illustrate that the enhancement of VCR scheme is particularly apparent for the partially loaded OFDMA system at low signal to noise ratio (SNR). In addition, the VCR scheme increases the convergence rate of the subspace-base semi-blind channel estimation.

H επιστήμη των τηλεπικοινωνιών τα τελευταία χρόνια έχει κάνει μεγάλα τεχνολογικά άλματα, σε σύγκριση με τις προηγούμενες δεκαετίες. Το ασύρματο κανάλι επικοινωνίας συχνά θεωρείται ο αδύναμος κρίκος της τηλεπικοινωνιακής αλυσίδας. Στην παρούσα διπλωματική εργασία εξετάζεται το πρόβλημα του διαμοιρασμού των διαθέσιμων πόρων ενός ασύρματου συστήματος επικοινωνίας στους χρήστες μέσα στην περιοχή μιας κυψέλης. Γίνεται η υπόθεση ότι το ασύρματο σύστημα κάνει χρήση της τεχνικής πολλαπλής πρόσβασης (multiple–access technique) OFDMA, δηλαδή ότι χρησιμοποιεί έναν αριθμό ορθογώνιων υποφορέων τον οποίο αναθέτει στους χρήστες για τη μετάδοση της πληροφορίας. Οι διαθέσιμοι πόροι του συστήματος είναι οι ορθογώνιοι υποφορείς και η συνολική διαθέσιμη ισχύς στο σταθμό βάσης. Θεωρούμε ότι ο σταθμός βάσης έχει στη διάθεσή του τέλεια γνώση του καναλιού που τον συνδέει με κάθε χρήστη (channel state information – CSI) μέσω καναλιών ανάδρασης. Η πληροφορία κατάστασης του καναλιού χρησιμοποιείται από τον σταθμό βάσης για την κατανομή των πόρων προς τους χρήστες με δυναμικό τρόπο με χρήση κατάλληλων αλγορίθμων. Στην παρούσα διπλωματική εργασία εξετάζονται διαφορετικά σενάρια κατανομής των πόρων που περιλαμβάνουν αλγορίθμους για την ανάθεση των υποφορέων και το διαμοιρασμό των κατάλληλων ποσοτήτων ισχύος στους υποφορείς. Ανάλογα με το στόχο και τις ανάγκες των χρηστών του συστήματος, καθώς και με το ποσό της διαθέσιμης πληροφορίας καναλιού στο σταθμό βάσης, χρησιμοποιούνται διαφορετικοί αλγόριθμοι. / The telecommunications science the last years has made big technological vaults, compared to the previous decades. Wireless communication channel is often considered as the weakest link of telecommunications' chain. In this diploma thesis, the downlink of a single-cell system is considered in the downlink transmission. The multiple–access technique that is used in this system is OFDMA and the objective is the allocation of the resources among users that access the system. The available resources to be distributed among the users of the OFDMA system comprise the subcarriers over which the signals of the users are transmitted and the available power that is allocated among subcarriers. It is assumed that users estimate and feedback perfectly the channel state information (CSI) to the base station of the cell, where subcarrier and power allocation are determined according to the CSI of the users and the resource–allocation algorithm. In this diploma thesis, different resource allocation strategies for the downlink of an OFDMA system are compared. Each algorithm has a different objective.

Orientador: Prof. Dr. Ivan Roberto Santana Casella / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2017. / A busca por transmissoes de dados em altas taxas é um fator indispensavel para a evolução dos sistemas de comunicação Wireless. Dessa forma, se faz necessario o estudo e desenvolvimento de novos metodos que visam aprimorar o desempenho desses sistemas. Uma tecnica bastante utilizada para atingir este objetivo é a OFDM (Orthogonal Frequency Division Multiplexing), onde é possivel aumentar a robustez do sistema a canais com desvanecimento seletivo em frequencia. Outra tecnica bastante interessante e difundida para este fim é a alocaação de recursos, que possibilita a otimização do uso dos recursos do sistema de forma a maximizar a taxa de transmissão (ou outro parametro do sistema de interesse). Neste sentido, este trabalho visa estudar o problema de alocação de recursos em sistemas Wireless e analisar algumas estrategias especificas para sistemas OFDM com multiplos usuarios como os sistemas OFDMA (Orthogonal Frequency Division Multiple Access) empregados no downlink das redes LTE (Long Term Evolution). Os estudos apresentados irão focar nos metodos de alocaçao de subportadoras e potencia propostos por Shen e, posteriormente, por Wong e nos metodos baseados em Algoritmos Geneticos (GA). / The search for high-speed data transmissions is an indispensable factor for the evolution of Wireless communication systems. Thus, it is necessary to study and develop new methods that aim to improve the performance of these systems. A technique widely used to achieve this goal is OFDM (Orthogonal Frequency Division Multiplexing), where it is possible to increase the robustness of the system to channels with frequency selective fading. Another very interesting and widespread technique for this purpose is the allocation of resources, which makes it possible to optimize the use of system resources in order to maximize the transmission rate (or other parameter of the system of interest). In this sense, this work aims to study the problem of resource allocation in Wireless systems and to analyze some specic strategies for multi-user OFDM systems such as the Orthogonal Frequency Division Multiple Access (OFDMA) systems used in the downlink of LTE (Long Term Evolution) networks. The studies presented will focus on the methods of subcarrier allocation and power proposed by Shen and later by Wong and in the methods based on genetic algorithms (GA).

Single-carrier frequency division multiple access (SC-FDMA) has become a popular alternative to orthogonal frequency division multiple access (OFDM) in multi-user communication on LTE uplink systems. This is primarily due to the low peak-to-average power ratio (PAPR) of SC-FDMA compared to that of OFDM. Long-term evolution (LTE) uses SC-FDMA on the uplink to exploit this PAPR advantage to reduce transmit power amplifier (PA) back-off in mobile user terminals. However, the latest generation of communication systems requires high power efficiency and a large quantity of capacity in transmitting mobile data, bringing out some other critical problems: 1) the nonlinearity of Radio Frequency (RF) power amplifiers inevitably affects the power efficiency. Working beyond the saturation point is the main reason for power amplifiers having nonlinear properties; 2) In order to obtain adequate capacity, wide bandwidth is applied to the latest communication systems. Since in previous systems the pre-distorter would focus on memory-less distortion with relatively narrow bandwidth, this change makes memory distortion become a serious issue, degrading the transmission quality in the wireless communication systems. The intent of this thesis is to present nonlinear analysis of the SC-FDMA Spectrum with the RF power amplifier. Relevant mathematical models were considered and applied to the RF power amplifier in terms of intermodulation products and the third-order intercept point. The equivalent mathematical model is applied for the first time to SC-FDMA signals and with the previous formulation of the PA model, the derivation of the expressions for spectrum regrowth of amplified SC-FDMA signals was first established and finally simulated with Matlab software. The digital pre-distortion (DPD) technology was also applied to SC-FDMA signals for the first time in this thesis. An inverse intermodulation and Autoregressive Moving-Average (IM-ARMA) model was introduced to linearize the PA distortion with memory in the LTE uplink system. The DPD was finally implemented by Matlab R2010b. Conclusions are drawn that amplified power emission levels can be expressed by the form of third-order intercept point (IP_3). The expressions for spectrum regrowth of amplified SC-FDMA signals have been verified for the first time with the comparison of simulation, measurement and calculation results. The effects of third order intermodulation have a greater impact than higher order components with respect to out-of-band emission power levels. Furthermore, the DPD algorithm reduced the spectrum regrowth of SC-FDMA signals by 12 dB. The proposed pre-distorter can effectively solve the distortion problem caused by the memory effect in RF power amplifier.

Multiuser and cooperative orthogonal frequency division multiplexing(OFDM) systems are being actively researched and adopted in wireless standards, owing to their advantages of robustness to multipath fading, modularity, and ability to achieve high data rates. In OFDM based systems, perfect frequency and timing synchronization is essential to maintain orthogonality among the subcarriers at the receiver. In multiuser OFDM on the uplink, timing offsets (TOs) and/or carrier frequency offsets (CFOs) of different users, caused due to path delay differences between different users, Doppler and/or poor oscillator alignment, can destroy orthogonality among subcarriers at the receiver. This results in multiuser interference (MUI)and consequent performance degradation. In this thesis, we are concerned with the analysis and mitigation of the effect of large CFOs and TOs in multiuser OFDM systems, including uplink orthogonal frequency division multiple access (OFDMA),uplink single-carrier frequency division multiple access(SC-FDMA), and cooperative OFDM. Uplink OFDMA: In the first part of this thesis, we analytically quantify the effect of large CFOs and TOs on the signal-to-interference plus noise ratio(SINR) and uncoded bit error rate(BER) performance of uplink OFDMA on Rayleigh and Rician fading channels, and show analytical results to closely match with simulation results. Such an SINR/BER analysis for uplink OFDMA in the presence of both large CFOs as well as TOs has not been reported before. We also propose interference cancelling(IC) receivers to mitigate the performance degradation caused due to large CFOs and TOs of different users. SC-FDMA versus OFDMA: An issue with uplink OFDMA is its high peak-to-average power ratio(PAPR).Uplink SC-FDMA is proposed in the standards as a good low-PAPR alternative to uplink OFDMA; e.g., SC-FDMA has been adopted in the uplink of 3GPP LTE. A comparative investigation of uplink SC-FDMA and OFDMA from a sensitivity to large CFOs and TOs view point has not been reported in the literature. Consequently, in the second part of the thesis, we carry out a comparative study of the sensitivity of SC-FDMA and OFDMA schemes to large CFOs and TOs of different users on the uplink. Our results show that while SC-FDMA achieves better performance due to its inherent frequency diversity advantage compared to OFDMA in the case of perfect synchronization, its performance can get worse than that of OFDMA in the presence of large CFOs and TOs. We further show that use of low-complexity multistage IC techniques, with the knowledge of CFOs and TOs of different users at the receiver, can restore the performance advantage of SC-FDMA over OFDMA. Cooperative OFDM: Cooperative OFDM is becoming popular because of its ability to provide spatial diversity in systems where each node has only one antenna. In most studies on cooperative communications, perfect time synchronization among cooperating nodes is assumed. This implies that the transmissions from different cooperating nodes reach the destination receiver in orthogonal time slots. In practice, however, due to imperfect time synchronization, orthogonality among different nodes’ signals at the destination receiver can be lost, causing inter-symbol interference(ISI).In the third part of the thesis, we investigate cooperative OFDM communications using amplify-and-forward(AF) protocol at the relay, in the presence of imperfect timing synchronization. We derive analytical expressions for the ISI as function of timing offset for cooperative OFDM with AF protocol, and propose an IC receiver to mitigate the effects of timing offset induced ISI.

The work reported in this thesis is divided in to two parts. In the first part, we report a closed-form bit error rate (BER) performance analysis of orthogonal frequency division multiple access (OFDMA) on the uplink in the presence of carrier frequency offsets (CFOs) and/or timing offsets (TOs) of other users with respect to a desired user. We derive BER expressions using probability density function (pdf) and characteristic function approaches, for a Rician faded multi-cluster multi-path channel model that is typical of indoor ultrawideband channels and underwater acoustic channels. Numerical and simulation results show that the BER expressions derived accurately quantify the performance degradation due to non-zero CFOs and TOs. Ultrawideband channels in indoor/industrial environment and underwater acoustic channels are severely delay-spread channels, where the number of multipath components can be of the order of tens to hundreds. In the second part of the thesis, we report low complexity equalization algorithms for cyclic-prefixed single carrier (CPSC)systems that operate on such inter-symbol interference(ISI) channels characterized by large delay spreads. Both single-input single-output and multiple-input multiple-output(MIMO) systems are considered. For these systems, we propose a low complexity graph based equalization carried out in frequency domain. Because of the noise whitening effect that happens for large frame sizes and delay spreads in the frequency domain processing, improved performance compared to time domain processing is shown to be achieved. Since the graph based equalizer is a soft-input soft-output equalizer, iterative techniques(turbo-equalization) between detection and decoding are shown to yield good coded BER performance at low complexities in convolutional and LDPC coded systems. We also study joint decoding of LDPC code and equalization of MIMO-ISI channels using a joint factor graph.

Στην παρούσα διπλωματική εργασία εξετάζεται η εκμετάλλευση της διαφορετικότητας πολλών χρηστών και της προσαρμοστικής διαμόρφωσης στα OFDMA συστήματα. Οι αλγόριθμοι που εκμεταλλεύονται αυτά τα κέρδη δεν προσδιορίζονται από το πρότυπο WiMAX, και έτσι όλοι οι κατασκευαστές WiMAX να είναι ελεύθεροι να αναπτύξουν τις δικές τους καινοτόμες διαδικασίες. Η ιδέα είναι η ανάπτυξη αλγορίθμων για την κατανομή των υποφορέων στους χρήστες, για τον προσδιορισμό των ποσοτήτων ισχύος σε αυτούς. Η μελέτη εστιάζεται στην κάτω ζεύξη (downlink) του συστήματος OFDMA, δηλαδή στη μετάδοση δεδομένων από το σταθμό βάσης της κυψέλης στους χρήστες – δέκτες. Οι διαθέσιμοι πόροι του συστήματος είναι οι ορθογώνιοι υποφορείς και η συνολική διαθέσιμη ισχύς στο σταθμό βάσης. Οι χρήστες κάνουν εκτίμηση και ανατροφοδότηση της πληροφορίας κατάστασης του καναλιού (CSI-channel state information) σε έναν κεντρικό σταθμό βάσης, όπου υποφορείς και κατανομή ισχύος προσδιορίζονται σύμφωνα με CSI των χρηστών και τη διαδικασία κατανομής των διαθέσιμων πόρων. Μόλις οι υποφορείς για κάθε χρήστη καθοριστούν, ο σταθμός βάσης πρέπει να ενημερώνει τον κάθε χρήστη για το ποιοι υποφορείς έχουν ανατεθεί στον καθένα. Συνήθως, η κατανομή των πόρων πρέπει να γίνεται σε χρονικά διαστήματα της τάξης του χρόνου συνοχής, αν και μπορεί να γίνει πιο συχνά, αν υπάρχουν πολλοί χρήστες που ανταγωνίζονται για τους πόρους που είναι διαθέσιμοι. Στην παρούσα διπλωματική εργασία εξετάζονται διαφορετικά σενάρια κατανομής των πόρων που περιλαμβάνουν αλγορίθμους για την ανάθεση των υποφορέων και το διαμοιρασμό των κατάλληλων ποσοτήτων ισχύος στους υποφορείς. Ανάλογα με το στόχο και τις ανάγκες των χρηστών του συστήματος, καθώς και με το ποσό της διαθέσιμης πληροφορίας καναλιού στο σταθμό βάσης, χρησιμοποιούνται διαφορετικοί αλγόριθμοι. Η κατανομή των διαθέσιμων πόρων συνήθως διατυπώνεται ως πρόβλημα βελτιστοποίησης με περιορισμούς, είτε (1) ελαχιστοποίηση της συνολικής εκπεμπόμενης ισχύoς με περιορισμό στο ρυθμό μετάδοσης δεδομένων στο χρήστη είτε (2) μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων με περιορισμό στην συνολική εκπεμπόμενη ισχύ. Ο αλγόριθμος με περιορισμό αναλογίας των ρυθμών μετάδοσης (Proportional Rate Constraints algorithm - PRC) έχει ως στόχο τη μεγιστοποίηση της συνολικής διεκπεραίωσης με τον περιορισμό ότι ο ρυθμός μετάδοσης δεδομένων σε κάθε χρήστη είναι ανάλογος με ένα σύνολο από προκαθορισμένες παραμέτρους του συστήματος. Ενώ ο στόχος του αλγόριθμου μέγιστου ολικού ρυθμού μετάδοσης (MSR- maximum sum rate algorithm), είναι η μεγιστοποίηση του συνόλου των ρυθμών μετάδοσης όλων των χρηστών, λαμβάνοντας υπόψη έναν περιορισμό της συνολικής εκπεμπόμενης ισχύoς. / In this diploma thesis, multiuser diversity and adaptive modulation in OFDMA systems is considered. Algorithms that take advantage of these gains are not specified by the WiMAX standard, and all WiMAX developer are free to develop their own innovative procedures. The idea is to develop algorithms for determining which users to schedule, how to allocate subcarriers to them, and how to determine the appropriate power levels for each user on each subcarrier. The downlink of a single-cell system is considered in the downlink transmission. The available resources to be distributed among the users of the OFDMA system comprise the subcarriers over which the signals of the users are transmitted and the available power that is allocated among subcarriers. Users estimate and feedback the channel state information (CSI) to a centralized base station, where subcarrier and power allocation are determined according to users’ CSI and the resource-allocation procedure. Once the subcarriers for each user have been determined, the base station must inform each user which subcarriers have been allocated to it. Typically, the resource allocation must be performed on the order of the channel coherence time, although it may be performed more frequently if a lot of users are competing for resources. In this diploma thesis, different resource allocation strategies for the downlink of an OFDMA system are compared. Each algorithm has a different objective. The resource allocation is usually formulated as a constrained optimization problem, to either (1) minimize the total transmit power with a constraint on the user data rate or (2) maximize the total data rate with a constraint on total transmit power. The proportional rate constraints (PRC) algorithm is to maximize the sum throughput, with the additional constraint that each user’s data rate is proportional to a set of predetermined system parameters. While the objective of the maximum sum rate (MSR) algorithm, is to maximize the sum rate of all users, given a total transmit power constraint.

Selected mapping (SLM) schemes are commonly employed to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It has been shown that the computational complexity of the traditional SLM scheme can be substantially reduced by adopting conversion vectors obtained by using the inverse fast Fourier transform (IFFT) of the phase rotation vectors in place of the conventional IFFT operations. To ensure that the elements of these phase rotation vectors have an equal magnitude, conversion vectors should have the form of a perfect sequence. This study firstly presents three novel classes of perfect sequence, each of which comprises certain base vectors and their cyclically shifted versions. Three novel low-complexity SLM schemes are then proposed based upon the unique structures of these perfect sequences. It is shown that while the PAPR reduction performances of the proposed schemes are marginally poorer than that of the traditional SLM scheme, the three schemes achieve a substantially lower computational complexity. Since the three proposed PAPR reduction schemes cannot be utilized in the orthogonal frequency division multiple access (OFDMA) system. A low-complexity scheme for PAPR reduction in OFDMA uplink systems using either an interleaved or a sub-band sub-carrier assignment strategy is also proposed in the second part of this study. The proposed scheme requires just one IFFT operation. The PAPR reduction performance of the proposed scheme is only marginally poorer than that of the traditional SLM scheme. However, the proposed schemes have significantly lower computational complexities. Besides, multiple-input multiple-output (MIMO) OFDM systems with space-frequency block coding (SFBC) are well-known for their robust performance in time selective fading channels. However, SFBC MIMO-OFDM systems have a high computational complexity since the number of IFFTs required scales in direct proportion to the number of antennas at the transmitter. Furthermore, SFBC MIMO-OFDM systems have a high PAPR. Accordingly, a low-complexity PAPR reduction scheme for SFBC MIMO OFDM systems with the Alamouti encoding scheme is proposed in this study. Extending this scheme obtains two low-complexity transmitter architectures for SFBC MIMO-OFDM systems with a general encoding matrix and an arbitrary number of transmitter antennas. The proposed schemes achieve a significant reduction in computational complexity by fully exploiting the time-domain signal properties of the transmitted signal. In addition, a PAPR reduction scheme is presented based on the proposed transmitter schemes. It is shown that the PAPR reduction performance of the proposed scheme is almost as good as that of the traditional SLM scheme, but is achieved with a substantially lower computational complexity.

Multiple users compete for a common resource like bandwidth to communicate data in interference networks. Existing approaches in dealing with interference limit the rate of communication due to paucity of shared resources. This limitation in the rate gets more glaring as the number of users in the network increases. For example, existing wireless systems either choose to orthogonalize the users (for example, Frequency Division Multiple Access (FDMA) systems or Code Division Multiple Access (CDMA) systems) or treat interference as Gaussian noise at the receivers. It is well known that these approaches are sub-optimal in general. Orthogonalization of users limit the number of available interference-free channels (known as degrees of freedom, abbreviated as DoF) and treating interference as noise means that the receiver cannot make use of the structure in the interfering signals. This motivates the need to analyze alternate transmit and decoding schemes in interference networks. This thesis mainly analyzes transmit schemes that use linear precoding for various configurations of interference networks with some practical constraints imposed by the use of finite input constellations, propagation delays, and channel state availability at the transmitters. The main contributions of this thesis are listed below. Achievable rates using precoding with finite constellation inputs in Gaussian Interference Channels (GIC) is analyzed. A metric for finding the approximate angle of rotation to maximally enlarge the Constellation Constrained (CC) capacity of two-user Gaussian Strong Interference Channel (GSIC) is proposed. Even as the Gaussian alphabet FDMA rate curve touches the capacity curve of the GSIC, with both the users using the same finite constellation, we show that the CC FDMA rate curve lies strictly inside the CC capacity curve at high powers. For a K-user MIMO GIC, a set of necessary and sufficient conditions on the precoders under which the mutual information between between relevant transmit-receive pairs saturate like in the single user case is derived. Gradient-ascent based algorithms to optimize the sum-rate achieved by precoding with finite constellation inputs and treating interference as noise are proposed. For a class of Gaussian interference networks with general message demands, identified as symmetrically connected interference networks, the expected sumspectral efficiency (in bits/sec/Hz) is shown to grow linearly with the number of transmitters at finite SNR, using a time-domain Interference Alignment (IA) scheme in the presence of line of sight (LOS) channels. For a 2×2 MIMO X-Network with M antennas at each node, we identify spacetime block codes that could be coupled with an appropriate precoding scheme to achieve the maximum possible sum-DoF of 4M 3 , for M = 3, 4. The proposed schemes are shown to achieve a diversity gain of M with SNR-independent finite constellation inputs. The proposed schemes have lower CSIT requirements compared to existing schemes. This thesis also makes an attempt to guarantee a minimum throughput when the zero-interference conditions cannot be satisfied in a wireline network with three unicast sessions with delays, using Precoding Based Network Alignment (PBNA). Three different PBNA schemes namely PBNA with time-varying local encoding coefficients (LECs), PBNA using transform approach and time-invariant LECs, and PBNA using transform approach and block time-varying LECs are proposed and their feasibility conditions analyzed.

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