Performance Evaluation and Analysis of Power Limited Cognitive Radios

Abstract

Due to ever increasing demand of higher data rate, improved spectral efficiency, lower latency rate and reduced bit error rate, there is a need to develop an efficient communication system that skilfully utilizing the scarce spectrum resources and is capable of meeting above requirements. By taking this into consideration, the formulated objectives of the thesis lead to an improved communication system which incorporates underlay spectrum sharing cognitive radios, an improved diversity combining technique i.e. Generalized Selection Combining and transmit diversity, obtained through Transmit Antenna Selection as well as Space Time Block Coding in multiuser environment. First of all, this thesis discusses an underlay cognitive radio where the cognitive user and the primary user share the same spectrum bandwidth. The proposed system is analyzed by employing transmit antenna selection and generalized selection combining in multiuser scenario in context of average post processing signal-to-noise-ratio (SNR) and ergodic capacity. In scheduled multiuser environment, the ′𝐾′active users are competing for the spectrum resources. The results attained thus shows that all the performance metrics improve as the number of best combined receiver branches increases. Also enhanced performance is achieved when one best transmit antenna and the one best cognitive user is chosen on the basis of highest post processed SNR at the output of CR receiver. Likewise, another very effective transmit diversity scheme is applied i.e. space time coding with generalized selection combining in underlay cognitive radios. This system is investigated with average post processed SNR, ergodic capacity and the average bit error rate, when the transmit power of cognitive transmitter is constrained by the outage probability of the primary network. The improved results are obtained for all the performance metrics when the number of best combined branches increases from 1 (Selection combining) to 4 (Maximal ratio combining), respectively. This section concludes with the performance comparison of iv shared spectrum cognitive radios with transmit antenna selection and space time block coding with generalized selection combining in multiuser multi-input-multi-output (MIMO) system. The average post processed SNR, ergodic capacity and the average bit error rate is analyzed for both the proposed systems. The achieved outcomes reveals that the system employing transmit antenna selection provides better and improved performance as compared to the MIMO counterpart. Also it is shown that performance gains become even more when one best cognitive user is chosen out of all competing users. This thesis also focuses on the interference environment which is studied and analyzed with the optimal combining scheme i.e. Optimum Combining. The underlay cognitive radios is studied and analyzed with the average post processed SNR, ergodic capacity, outage probability and the average bit error rate, when the proposed system is constrained of interference temperature limit as defined by the primary network. The proposed work is also analyzed with the varying number of primary interferers and the number of receiver branches. All the achieved simulation results are well corroborated with their analytical counterpart.