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.

In this dissertation, adaptive power control for single and multiuser opportunistic systems is investigated. First, a new adaptive power-controlled diversity combining scheme for single user systems is proposed, upon which is extended to the multiusers case. In the multiuser case, we first propose two new threshold based parallel multiuser scheduling schemes without power control. The first scheme is named on-off based scheduling (OOBS) scheme and the second scheme is named switched based scheduling (SBS) scheme. We then propose and study the performance of thresholdbased power allocation algorithms for the SBS scheme. Finally, we introduce a unified analytical framework to determine the joint statistics of partial sums of ordered RVs with i.i.d. and then the impact of interference on the performance of parallel multiuser scheduling is investigated based on our unified analytical framework.

In this thesis, we introduce a signal-to-noise ratio (SNR) estimator and analyze the performance degradation of the SNR estimator due to the synchronization error in the orthogonal frequency division multiplexing (OFDM) systems. This SNR estimator through the expectation-maximization (EM) algorithm is used in the adaptive modulation. When the synchronization is imperfectly done, the synchronization error reduces the performance of the OFDM systems and the accuracy of the SNR estimator. We investigate the estimation offset of the SNR estimator with the synchronization error. Simulation results demonstrate that the theoretical analyses are correct. In addition, the simulation results show that the more synchronization errors cause the more estimation errors of the SNR estimator. And the estimation errors are not decreased by the iteration of the EM algorithm.

In Earth-to-Space communications, well-known propagation effects such as path loss and atmospheric loss can lead to fluctuations in the strength of the communications link between a satellite and its ground station. Additionally, a less-often considered effect of shadowing due to the geometry of the satellite and its solar panels can also lead to link degradation. As a result of these anticipated channel impairments, NASA's communication links have been traditionally designed to handle the worst-case impact of these effects through high link margins and static, lower rate, modulation formats. This thesis first characterizes the propagation environment experienced by a software-defined radio on the NASA SCaN Testbed through a full link-budget analysis. Then, the following chapters propose, design, and model a link adaptation algorithm to provide an improved trade-off between data rate and link margin through varying the modulation format as the received signal-to-noise ratio fluctuates. / Master of Science

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 / Adaptive modulation is a communication technique capable of maximizing throughput while guaranteeing a fixed symbol error rate (SER). However, this technique requires instantaneous channel state information at the transmitter. This can be obtained by predicting channel states at the receiver and feeding them back to the transmitter. Existing algorithms used to predict single-input single-output (SISO) channels with recurrent neural networks (RNN) are extended to multiple-input multiple-output (MIMO) channels for use with adaptive modulation and their performance is demonstrated in several examples.

L’OFDM est une technique de modulation multiporteuses largement utilisée dans des applications de communications filaires et sans-fils comme le DVB-T/T2, le Wifi, et la 4G, grâce à sa robustesse contre les canaux sélectifs en fréquence en comparaison avec la modulation monoporteuse. Cependant, le signal OFDM souffre de grandes variations d’amplitude. Les fluctuations de l’enveloppe du signal OFDM génèrent des distorsions non-linéaires quand on introduit le signal dans un équipement non-linéaire comme l’amplificateur de puissance. Réduire les variations du signal améliore le rendement de l’amplificateur, réduit la consommation énergétique et diminue les émissions de CO2 des transmissions numériques.Le PAPR (rapport de la puissance crête sur la puissance moyenne) est une variable aléatoire qui a été introduite pour mesurer les variations du signal. Il existe plusieurs systèmes multiporteuses basés sur différentes bases de modulation et filtres de mise en forme. Nous prouvons d’abord dans ces travaux que le PAPR dépend de cette structure de modulation. Ensuite, nous étudions le comportement du PAPR vis-à-vis des formes d’ondes utilisées dans la modulation. Le problème de réduction du PAPR est ainsi formulé en un problème d’optimisation. Par ailleurs, une condition nécessaire pour construire des formes d’ondes avec un meilleur PAPR que l’OFDM est développée. Cette condition est notamment satisfaite par des bases en ondelettes. Enfin, une nouvelle forme d’onde en paquets d’ondelettes adaptative est proposée, permettant des gains significatifs en PAPR, tout en maintenant les avantages des modulations multiporteuses. / OFDM is a multicarrier modulation system widely used in wireline and wireless applications such as DVB-T/T2, Wifi, and 4G, due to its resilience against frequency selective channels compared with the single carrier modulation systems. However, the OFDM signal suffers from large amplitude variations. The fluctuations of the OFDM envelope generate non-linear distortions when we introduce the signal into a non-linear device like the power amplifier. Reducing the variations of the signal improves the power amplifier efficiency, reduces the energy consumption and decreases CO2 emissions.The peak-to-average power ratio (PAPR) has been introduced as a random variable that measures the power variations of the signal. There exist several multicarrier modulation systems based on different modulation basis and shaping filters. We first prove in this work that the PAPR depends on this modulation structure. Moreover, the behaviour of the PAPR regarding to the modulation waveforms is analysed and the PAPR reduction problem is formulated as an optimization problem. Furthermore, a necessary condition for designing waveforms with better PAPR than OFDM is developed. This necessary condition is particularly satisfied by wavelet basis. Finally, a new adaptive wavelet packet waveform is proposed, allowing significant gain in terms of PAPR, while keeping the advantages of multicarrier modulations.

This dissertation presents adaptation techniques that can achieve high spectral efficiency for single user multiple-input multiple-output (MIMO) systems. Two types of adaptation techniques, adaptive modulation and adaptive powe allocation, are employed to adapt the rate and the transmit power to fading channels. We start by investigating the adaptive modulation subject to a certain bit-error-ratio (BER) constraint, either instantaneous BER constraint or average BER constraint. The resulting average spectral efficiencies are obtained in closed-form expressions. It turns out that, by employing the average BER constraint, we can achieve the optimal average spectra efficiency at the cost of prohibitive computational complexity. On the other hand, instantaneous BER constraint leads to inferior performance with little computational complexity. In order to achieve comparable performance to the average BER constraint with limited complexity, a non-linear optimization method is proposed. To further enhance the average spectra efficiency, adaptive power allocation schemes are considered to adjust the transmit power across the temporal domain or the spatial domain, depending on the specific situation. Provided the closed-form expressions of the average spectral efficiency, the optimal MIMO coding scheme that offers the highest average spectral efficiency under the same circumstances can be identified. As we take into account the effect of imperfect channel estimation, the adaptation techniques are revised to tolerate interference introduced by the channel estimation errors. As a result, the degradation with respect to the average spectral efficiency is in proportion to signal-to-noise ratio (SNR). In order to facilitate fast development and verification of the adaptation schemes proposed for various MIMO systems, a reconfigurable Link Layer Simulator (LiLaS) which accommodates a variety of wireless/wireline applications is designed in the environment of MATLAB/OCTAVE. The idea of the simulator is originated from Software Defined Radio (SDR) and evolved to suit Cognitive Radio (CR) applications. For the convenience of modification and reconfiguration, LiLaS is functionally divided into generic blocks and all blocks are parameterized. / QC 20100812