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[pt] Este trabalho é um estudo sobre o desempenho de um enlace típico do padrão UMTS/WCDMA para sistemas celulares de terceira geração, utilizando a técnica denominada modulação adaptativa. Esta técnica consiste em escolher, para cada quadro de bits transmitido, a modulação mais adequada às condições do canal. Os aspectos básicos do problema foram formulados analiticamente considerando a transmissão com modulações PSK e QAM coerentes em um canal com desvanecimento plano e efeito Doppler. Para considerar a aplicação a um modelo mais realista, foi desenvolvido um programa de simulação capaz de modelar esse mesmo problema considerando os principais componentes do sistema HSDPA (High Speed Data Packet Access), um sub-sistema do UMTS/WCDMA padronizado para transmissão de dados em alta velocidade no enlace de descida. O desempenho do enlace foi avaliado para diversas situações através da taxa de erro de bit, da taxa de transmissão média e da vazão obtida na transmissão, procurando-se investigar a influência de parâmetros do sistema nesse desempenho. / [en] This work is a study about the performance of a typical UMTS/WCDMA link for third generation mobile communications systems which uses the adaptive modulation technique. This technique consists of choosing, for each transmitted frame, the most efficient modulation according to the channel conditions. The basic aspects of the problem were analytically examined considering transmissions with PSK and QAM coherent techniques in a flat fading channel with Doppler effect. A simulator was developed in order to consider the application in a more realistic model. This simulator was capable of modeling this problem considering the main aspects of the HSDPA (High Speed Data Packet Access) system, which is a subsystem of UMTS/WCDMA that is standardized for high-speed data transmission in the downlink. The link level performance was evaluated for many different situations through average transmission bit error rate and throughput, investigating the influence of the system parameters in this performance.

Orthogonal frequency division multiplexing¡]OFDM¡^systems play an important role in modern wireless communications due to following advantages: bandwidth saving¡Bcombat with frequency selective fading channel and high throughput. The performance of wireless communications is often degraded by fading channel . adaptive modulation and subcarrier allocation are proposed to overcome the degration to meet the quality of servie¡]QoS¡^. Lagrange method and heuristics method, two of the subcarrier allocation technology under multi-user OFDM, can achieve the goal that maximizing bit rate with minimizing transmitted power. However, significantly high complexity of either Lagrange method or heuristics method makes the implementation difficult. Zhang and Letaief proposed a method of making subcarriers detected one by one to reduce the complexity. However, in piratical, an OFDM system accommodates hundred of , or even thousand of subcarriers, so the method can be improved. In this thesis, we propose a subcarrier allocation method. The users that are not satisfied with the QOS requirement are named demander, and the users satisfied with the QOS requirement are named supplier. In the proposed subcarrier allocation method, we evaluate the number of subcarriers that demanders need and remove the subcarriers from supplier to directly compensate demander. Then the system has lower complexity due to less iterations.

Recent research results have shown that the MIMO wireless communication architecture is a promising approach to achieve high bandwidth efficiencies. MIMO wireless channels can be simply defined as a link for which both the transmitting and receiving ends are equipped with multiple antenna elements. Adaptive modulation and power allocation could be used to further improve the performance of MIMO systems. This thesis focuses on developing a fast and high performance power allocation algorithm. Three power allocation algorithms are proposed in this thesis and their performances are compared in various system sizes and transceiver architectures. Among the three algorithms proposed in this thesis, the fast algorithm may be considered as the best power allocation algorithm since the performance of the fast algorithm is almost as good as the fullsearch (optimal)algorithm and the mean processing time is considerably less than the fullsearch algorithm. The fast algorithm achieves about 97.6% agreement with the optimal throughput on average. In addition, the time taken to find the power scaling factors using the fullsearch algorithm is about 2300 times longer than the processing time of the fast algorithm in a 6 x 6 system when the SNR is 20dB. As an extension to the power allocation process, excess power allocation methods are introduced. Excess power is the unused power during the power allocation process. The power allocation algorithm allocates power to each received SNR to maximize the throughput of the system whereas the excesspower allocation distributes the excess power to each SNR to improve both the instantaneous and temporal behavior of the system. Five different excess power allocation methods are proposed in this thesis. These methods were simulated in the Rayleigh fading channel with different Doppler frequencies, fD = 10Hz,50Hz and 100Hz, where the ACF of the channel coefficients are given by the Jakes' model. The equal BER improvement method showed a slightly better performance than the other methods. The equal BER improvement method enables the system to maintain the power scaling factors without sacrificing QoS for 19.6 ms on average when the maximum Doppler shift is 10Hz.

ITC/USA 2008 Conference Proceedings / The Forty-Fourth Annual International Telemetering Conference and Technical Exhibition / October 27-30, 2008 / Town and Country Resort & Convention Center, San Diego, California / In a multi-user cognitive radio network, there arises a need for coordination among the network users for efficient utilization of the available electromagnetic spectrum. While adaptive modulation alone helps cognitive radios actively determine the channel quality metric for the next transmission, Game theory combined with an adaptive modulation system helps them achieve mutual coordination among channel users and avoids any possible confusion about transmitting/receiving through a channel in the future. This paper highlights how the concepts of game theory and adaptive modulation can be incorporated in a cognitive radio framework to achieve better communication for telemetry applications.

ITC/USA 2007 Conference Proceedings / The Forty-Third Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2007 / Riviera Hotel & Convention Center, Las Vegas, Nevada / Cognitive radio is a reasonably new branch of research aimed at more fully utilizing the RF spectrum. This is accomplished by allowing wireless communication systems to dynamically choose a frequency band, and a modulation technique, based on the current state of the RF spectrum as perceived by the cognitive radio network. This paper will give a brief introduction of cognitive radio networks, and describe a hardware platform designed at the IFT/UMR Telemetry Learning Center. The test-bed will accommodate future research into cognitive networks, by allowing the user to dynamically change both its carrier frequency and modulation technique through software. A general description of the design of the platform is provided.

ITC/USA 2007 Conference Proceedings / The Forty-Third Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2007 / Riviera Hotel & Convention Center, Las Vegas, Nevada / While investigating methods for more efficiently allocating the available spectrum researchers noticed that in many geographical locations, there are broad bands of frequencies that are lightly utilized. Such inefficiencies are inevitable with fixed spectral allocation rules. Cognitive Radios actively measure the spectral utilization and adapt their modulation, frequencies, bandwidths, power, etc. to take advantage of these lightly used “spectral holes” or “white spaces”. Much of the research work in cognitive radios has not taken into account some of the challenges faced in the telemetry community-including multipaths and a guaranteed quality of service. This paper highlights how some mathematical models of adaptive modulation discussed extensively in many research papers and textbooks can be used in Cognitive Radios as well.

Multiple-input multiple-output (MIMO) systems have recently emerged as one of the most significant wireless techniques, as they can greatly improve the channel capacity and link reliability of wireless communications. These benefits have encouraged extensive research on a virtual MIMO system where the transmitter has multiple antennas and each of the receivers has a single antenna. Single-antenna receivers can work together to form a virtual antenna array and reap some performance benefits of MIMO systems. The idea of receiver-side local cooperation is attractive for wireless networks since a wireless receiver may not have multiple antennas due to size and cost limitations. In this thesis we investigate a virtual-MIMO wireless system using the receiver-side cooperation with the compress-and-forward (CF) protocol. Firstly, to perform CF at the relay, we propose to use standard source coding techniques, based on the analysis of its expected rate bound and the tightness of the bound. We state upper bounds on the system error probabilities over block fading channels. With sufficient source coding rates, the cooperation of the receivers enables the virtual-MIMO system to achieve almost ideal MIMO performance. A comparison of ideal and non-ideal conference links within the receiver group is also investigated. Considering the short-range communication and using a channel-aware adaptive CF scheme, the impact of the non-ideal cooperation link is too slight to impair the system performance significantly. It is also evident that the practicality of CF cooperation will be greatly enhanced if a efficient source coding technique can be used at the relay. It is even more desirable that CF cooperation should not be unduly sensitive to carrier frequency offsets (CFOs). Thus this thesis then presents a practical study of these two issues. Codebook designs of the Voronoi VQ and the tree-structure vector quantization (TSVQ) to enable CF cooperation at the relay are firstly described. A comparison in terms of the codebook design complexity and encoding complexity is presented. It is shown that the TSVQ is much simpler to design and operate, and can achieve a favourable performance-complexity tradeoff. We then demonstrate that CFO can lead to significant performance degradation for the virtual MIMO system. To overcome it, it is proposed to maintain clock synchronization and jointly estimate the CFO between the relay and the destination. This approach is shown to provide a significant performance improvement. Finally, we extend the study to the minimum mean square error (MMSE) detection, as it has a lower complexity compared to maximum likelihood (ML) detection. A closed-form upper bound for the system error probability is derived, based on which we prove that the smallest singular value of the cooperative channel matrix determines the system error performance. Accordingly, an adaptive modulation and cooperation scheme is proposed, which uses the smallest singular value as the threshold strategy. Depending on the instantaneous channel conditions, the system could therefore adapt to choose a suitable modulation type for transmission and an appropriate quantization rate to perform CF cooperation. The adaptive modulation and cooperation scheme not only enables the system to achieve comparable performance to the case with fixed quantization rates, but also eliminates unnecessary complexity for quantization operations and conference link communication.

Advances in Multimedia communications have shown the need for high data rate wireless links over short distances. This is to enhance flexibility, accessibility, portability and mobility of devices in home and enterprise environment thereby making users more productive. In 2004, the WiMedia group proposed the Multiband Orthogonal Frequency Division Multiplex Ultra Wideband (MB-OFDM UWB) system with a target of delivering data rate of 480Mbps over 3 metres. However, by now no existing commercial UWB product can meet this proposed specification. The project aims to investigate the reason why UWB technology has failed to realise its potential by carrying out detailed analysis and to seek ways of solving the technical problems. Detailed system analyses were carried out on the UWB technology using a commercial UWB product and a MB-OFDM UWB Evaluation kit. UWB channel measurements of different scenarios were carried out in order to characterise both time varying and time invariant channels. The scenarios are the realistic environments where UWB devices are operating with human subjects in various movement patterns. It gives insight into the effects of human object blocking on the MB-OFDM system performance and estimates an acceptable feedback rate in a UWB time varying channel when implementing an adaptive modulation. The adaptive modulation was proposed and implemented in the MB-OFDM system model to demonstrate the improved Bit Error Rate (BER) performance. Modulating bits are varied across the sub-channels depending on the signal to noise ratio (SNR). Sub-channels experiencing severe fading employ lower or no bit-loading while sub-channels with little or no fading utilise higher bit-loading to maintain a constant system data rate. Spatial diversity was employed to exploit different properties of the radio channel to improve performance. Good diversity gain of two receiving diversity systems using maximal ratio combining and antenna selection techniques is demonstrated in the measurements with the different antenna orientations. An antenna selection circuit is designed and implemented working together with AT90CAP9 UWB Evaluation kit, verifying an improved performance of the UWB system in an indoor environment. The maximal ratio combining technique is also implemented and demonstrated to give a better system performance on a test bed after post-processing.

This dissertation deals with the design of bandwidth-efficient coding schemes with Low-Density Parity-Check (LDPC) for reliable wireless communications. Code design for wireless channels roughly falls into three categories: (1) when channel state information (CSI) is known only to the receiver (2) more practical case of partial CSI at the receiver when the channel has to be estimated (3) when CSI is known to the receiver as well as the transmitter. We consider coding schemes for all the above categories. For the first scenario, we describe a bandwidth efficient scheme which uses highorder constellations such as QAM over both AWGN as well as fading channels. We propose a simple design with LDPC codes which combines the good properties of Multi-level Coding (MLC) and bit-interleaved coded-modulation (BICM) schemes. Through simulations, we show that the proposed scheme performs better than MLC for short-medium lengths on AWGN and block-fading channels. For the first case, we also characterize the rate-diversity tradeoff of MIMO-OFDM and SISO-OFDM systems. We design optimal coding schemes which achieve this tradeoff when transmission is from a constrained constellation. Through simulations, we show that with a sub-optimal iterative decoder, the performance of this coding scheme is very close to the optimal limit for MIMO (flat quasi-static fading), MIMO-OFDM and SISO OFDM systems. For the second case, we design non-systematic Irregular Repeat Accumulate (IRA) codes, which are a special class of LDPC codes, for Inter-Symbol Interference (ISI) fading channels when CSI is estimated at the receiver. We use Orthogonal Frequency Division Multiplexing (OFDM) to convert the ISI fading channel into parallel flat fading subchannels. We use a simple receiver structure that performs iterative channel estimation and decoding and use non-systematic IRA codes that are optimized for this receiver. This combination is shown to perform very close to a receiver with perfect CSI and is also shown to be robust to change in the number of channel taps and Doppler. For the third case, we look at bandwidth efficient schemes for fading channels that perform close to capacity when the channel state information is known at the transmitter as well as the receiver. Schemes that achieve capacity with a Gaussian codebook for the above system are already known but not for constrained constellations. We derive the near-optimum scheme to achieve capacity with constrained constellations and then propose coding schemes which perform close to capacity. Through linear transformations, a MIMO system can be converted into non-interfering parallel subchannels and we further extend the proposed coding schemes to the MIMO case too.