 Electrical and Electronic Engineering  Theses
Electrical and Electronic Engineering  Theses
Permanent URI for this collection
Search Results
Now showing
1  3 of 3

ItemEnergy efficient wireless system designKudavithana, 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 subsystems 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 energyefficiency tradeoffs of a pointtopoint 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 Rayleighfading channel model and a lowdensity parity check (LDPC) coding scheme are chosen. We then develop a closedform approximation for the total energy consumption as a function of receiver SNR and use it to find a minimumenergy 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 optimalSNR concept which can save a significant amount of energy mainly in shortrange transmission systems. We then focus on cooperative relay systems. We investigate the energy efficiency tradeoffs of singlerelay networks by developing energy models for two relay strategies: amplifyandforward (AF) and detectandforward (DF). We then optimize the location and power allocation of the relay to minimize the total energy consumption. The optimum location is found in twodimensional 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 multirelay systems by considering a dualrelay cooperative system using DF protocol with full diversity. We propose a locationandpoweroptimization approach for the relays to minimize the transmit radio energy. We then minimize the total system energy from spectral efficiency perspective for two scenarios: throughputconstrained and bandwidthconstrained configurations. Our proposed approach reduces the transmit energy consumption compared to an equalpower allocated and equidistantlocated relay system. Finally, we present an optimal transmission scheme as a function of distance by considering singlehop and multihop schemes. The overall results imply that more relays are required as the transmission distance increases in order to maintain a higher energy efficiency.

ItemResource allocation in cognitive radio networksLIMMANEE, ATHIPAT ( 2012)This thesis focuses on optimal power allocation problems for various types of spectrumsharing based cognitive radio networks in the presence of delaysensitive primary links. To guarantee the quality of service in the delaysensitive 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 frequencydivision multiplexing (OFDM) technology able to access N randomly fading frequency bands for transmitting delayinsensitive as well as delaysensitive traffic. Each band is licensed to an individual singleantenna and delaysensitive 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 singleantenna secondary base station (SBS) and M singleantenna secondary receivers (SRs) sharing the same spectrum band with one singleantenna and delaysensitive 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 multipleaccess channels with a singleantenna SBS and M singleantenna secondary transmitters sharing the same spectrum band with a singleantenna and delaysensitive 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 singleantenna SU subject to a POC at a delaysensitive and singleantenna PU and an average transmit power constraint at the SU, providing that the SU has quantized channel side information via Bbit feedback from the band manager. By using nearest neighbourhood condition, we can derive the optimal channel partition structure for the vector channel space, making KarushKuhnTucker condition applicable as a necessary condition for finding a locally optimal solution. We also propose another lowcomplexity 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 locallyoptimal 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.

ItemTopics in resource optimization in wireless networks with limited feedbackHe, 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 finiterate feedback channel from the receiver (the socalled limited feedback technique). We first look at an M parallel blockNakagamifading 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 simulationoptimization method is applied to obtain a locally optimal power codebook. As this method is computationally intensive and timeconsuming, we then derive a number of reducedcomplexitysuboptimal finiterate power codebook design algorithms. For the large number of parallel channels case, a Gaussian approximation based lowcomplexity 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 Lloydstype 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 34 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 SUTX to each PURX) knowledge, three different quantized power allocation schemes are proposed corresponding to three distinct forms of CSI obtained regarding the channel from SUTX to SURX link at SUTX : 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.