Electrical and Electronic Engineering - Theses

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    Energy efficient wireless system design
    Kudavithana, Dinuka ( 2015)
    The demand for telecommunication networks is increasing rapidly. Wireless access is a major contributor to this trend. On the other hand, wireless is considered as a least energy efficient transmission medium mainly due to its unguided nature. The general focus of increasing wireless system energy efficiency is on reduction of the transmit power. However, this strategy may not save energy in short distance communication systems as the processing energy in hardware becomes more significant compared to the transmit radio energy. This thesis focuses on looking at the energy consumption of wireless systems by modeling the energy consumption as a function of several parameters such as receiver SNR, RF bandwidth, information rate, modulation scheme and code rate. We propose energy models for synchronization systems and other digital signal processing modules by considering the computational complexity of the algorithm and the required circuitry. Initially we focus on the synchronization aspects of wireless receivers. We study various algorithms on symbol timing recovery, carrier frequency recovery and carrier phase recovery and compare the performance in order to identify the suitable algorithms to operate at different SNR regions. We then develop energy models for those synchronization sub-systems by analyzing the computational complexity of circuitries based on a number of arithmetic, logic and memory operations. We define a new metric - energy consumption to achieve a given performance as a function of SNR - in order to compare the energy efficiency of different estimation algorithms. Next, we investigate the energy-efficiency trade-offs of a point-to-point wireless system by developing energy models of both the transmitter and receiver that include practical aspects such as error control coding, synchronization and channel equalization. In our system, a multipath Rayleigh-fading channel model and a low-density parity check (LDPC) coding scheme are chosen. We then develop a closed-form approximation for the total energy consumption as a function of receiver SNR and use it to find a minimum-energy transmission configuration. The results reveal that low SNR operation (i.e. low transmit power) is not always the most energy efficient strategy, especially in short distance communication. We present an optimal-SNR concept which can save a significant amount of energy mainly in short-range transmission systems. We then focus on cooperative relay systems. We investigate the energy efficiency trade-offs of single--relay networks by developing energy models for two relay strategies: amplify-and-forward (AF) and detect-and-forward (DF). We then optimize the location and power allocation of the relay to minimize the total energy consumption. The optimum location is found in two-dimensional space for constrained and unconstrained scenarios. We then optimize the total energy consumption over the spectral efficiency and derive expressions for the optimal spectral efficiency values. We use numerical simulations to verify our results. Finally, we focus on energy efficiency of multi-relay systems by considering a dual-relay cooperative system using DF protocol with full diversity. We propose a location-and-power-optimization approach for the relays to minimize the transmit radio energy. We then minimize the total system energy from spectral efficiency perspective for two scenarios: throughput-constrained and bandwidth-constrained configurations. Our proposed approach reduces the transmit energy consumption compared to an equal-power allocated and equidistant-located relay system. Finally, we present an optimal transmission scheme as a function of distance by considering single-hop and multi-hop schemes. The overall results imply that more relays are required as the transmission distance increases in order to maintain a higher energy efficiency.
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    Resource allocation in cognitive radio networks
    LIMMANEE, ATHIPAT ( 2012)
    This thesis focuses on optimal power allocation problems for various types of spectrum-sharing based cognitive radio networks in the presence of delay-sensitive primary links. To guarantee the quality of service in the delay-sensitive primary network, primary user’s outage probability constraint (POC) is imposed such that the transmission outage probability of each primary user is confined under the predefined threshold. We first consider a cognitive radio network consisting of a secondary user (SU) equipped with orthogonal frequency-division multiplexing (OFDM) technology able to access N randomly fading frequency bands for transmitting delay-insensitive as well as delay-sensitive traffic. Each band is licensed to an individual single-antenna and delay-sensitive primary user (PU) whose quality of service is assured by a POC. Assuming full channel state information (CSI) is available at the secondary network, we solve the SU’s ergodic capacity maximization problem subject to SU’s average transmit power, SU’s outage probability constraints (SOC) and all POCs by using a rigorous probabilistic power allocation technique. A suboptimal power control policy is also proposed to reduce the high computational complexity when N is large. Next, we study cognitive broadcast channels with a single-antenna secondary base station (SBS) and M single-antenna secondary receivers (SRs) sharing the same spectrum band with one single-antenna and delay-sensitive PU. The SBS aims to maximize the ergodic sum downlink throughput to all M SRs subject to a POC and a transmit power constraint at the SBS. With full CSI available at the secondary network, the optimal solution reveals that at each timeslot SBS will choose the SR with the highest direct channel power gain and allocate the timeslot to that user. The opportunistic scheduling aspect from the optimality condition allows us to further analyze the downlink throughput scaling behavior in Rayleigh fading channel as M grows large. We then examine a cognitive multiple-access channels with a single-antenna SBS and M single-antenna secondary transmitters sharing the same spectrum band with a single-antenna and delay-sensitive PU. Under an average transmit power constraint in each secondary transmitters and a POC at the primary link, we characterize the ergodic capacity region and two outage capacity regions, i.e. common outage capacity region and individual outage capacity region, in the secondary uplink network by exploiting the polymatroid structure of the problems. Also, the derivation of the associated optimal power allocation schemes are provided. The optimal solutions for the problems demonstrate that successive decoding is optimal and the decoding order can be solved explicitly as a function of joint channel state. Finally, we investigate a transmit power allocation problem for minimizing outage probability of a single-antenna SU subject to a POC at a delay-sensitive and single-antenna PU and an average transmit power constraint at the SU, providing that the SU has quantized channel side information via B-bit feedback from the band manager. By using nearest neighbourhood condition, we can derive the optimal channel partition structure for the vector channel space, making Karush-Kuhn-Tucker condition applicable as a necessary condition for finding a locally optimal solution. We also propose another low-complexity suboptimal algorithm. Numerical results show that the SU’s outage probability performance from the suboptimal algorithm approaches the SU’s outage probability performance in the locally-optimal algorithm as the number of feedback bits, B, increases. Besides, we include the asymptotic analysis on the SU’s outage probability when B is large.
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    Topics in resource optimization in wireless networks with limited feedback
    He, Yuan Yuan ( 2011)
    This thesis focuses on the design of various optimal resource allocation algorithms for wireless communication networks with imperfect channel state information (CSI) available at the transmitter obtained via a finite-rate feedback channel from the receiver (the so-called limited feedback technique). We first look at an M parallel block-Nakagami-fading channels where we seek to design an optimal power allocation scheme that minimize the outage probability under a long term average transmit power (ATP) constraint with quantized CSI. A simultaneous perturbation stochastic approximation algorithm (SPSA) based simulation-optimization method is applied to obtain a locally optimal power codebook. As this method is computationally intensive and time-consuming, we then derive a number of reduced-complexitysuboptimal finite-rate power codebook design algorithms. For the large number of parallel channels case, a Gaussian approximation based low-complexity power allocation strategy is provided. We then consider a secondary user (SU) transmit power control problem in a spectrum sharing cognitive radio networks scenario with quantized channel feedback for optimizing relevant performance measures such as secondary ergodic capacity or outage probability under interference power constraints (which can be restricted either by an average (AIP) or a peak (PIP) constraint) at primary user (PU) receivers to protect the PU, and an average transmit power (ATP) constraint on SU. Firstly, we study the problem of ergodic capacity maximization over M parallel channels (each of which is licensed to a distinct PU) of SU subject to an ATP constraint at SU and M individual AIP constraints on each PU with quantized feedback of the joint channel space of SU transmitter to SU receiver and SU transmitter to PU receivers. We develop a “modified generalized Lloyds-type algorithm (GLA)” for finding a locally optimal quantized power codebook. An approximate but computationally efficient quantized power allocation algorithm is then derived for the case of large number of feedback bits. It is seen that only 3-4 bits of feedback per channel band achieves SU ergodic capacity close to the full CSI based performance. We also extend these results to the noisy limited feedback case. We then consider the problem of throughput maximization of SU with a finite rate power codebook under an ATP constraint at SU and N individual PIP constraints on each PU receiver. With quantized channel (from the SU-TX to each PU-RX) knowledge, three different quantized power allocation schemes are proposed corresponding to three distinct forms of CSI obtained regarding the channel from SU-TX to SU-RX link at SU-TX : full CSI, noisy estimated CSI and quantized CSI. Finally, we consider the joint optimization of the quantization regions and the transmission power codebook such that the outage probability of the SU is minimized while an ATP constraint at the SU and an AIP constraint on the PU are met. Explicit expressions for asymptotic behavior of the SU outage probability at high rate quantization are also developed.