Quality of service and the architecture of turbo codes in wireless communication

Abstract

Quality of Service (QoS) refers to the collective effect of service performance, which determines the degree of satisfaction of a user of the service. In wireless network the goal ofQoS is to provide quality as Bit Error Rate (BER), delay, delay variation and data rate. Among these , the major issue in the wireless communications is the improvement of BER. Error correcting codes are the main techniques to improve BER in wireless communication. Among all error-correcting codes, Turbo Codes shows the best performance in terrns of BER as it can reach near the Shannon's limit and require only 0.7 dB at BER of 10". So, we improve the architecture of Turbo codes and investigate its performance by changing the component of the architecture of the encoder and decoder. We simulate the classical Turbo codes, UMTS, cdmalOOOand CCSDS standards of Turbo codes and study their BER and Frame Error Rate (FER) performances for both the AWON and Rayleigh channels. The thesis investigates the performance variation of Turbo codes for varying decoding iterations, coding rate and frame size of the codes. Distance spectrum, which is one of the performance measures of Turbo codes at low BER, are measured and compared for classical and third generation Turbo codes.' The effect of frame size, code rate and number of decoding iterations on distance spectra of Turbo codes is shown. Then the influence of different interleavers on the performance of Turbo Codes is demonstrated. Among these interleavers, S -random interleaver shows the best performance. We suggest an improvement to the S -random in!erleaving algorithm. Simulation results to demonstrate the performance improvement due to modified structure of the S -random interleaver. Interleaver gains with improved S -random interleaver for different frame sizes, constraint lengths, code rates and number of decoding iterations are determined. Simulation time of Turbo Codes for different interleavers is determined. Then influence of encoder structure on the performance of Turbo codes' is investigated. It is shown that if the feedback polynomial of Turbo encoder is primitive polynomial, it performs better. Then we identify a Turbo encoder structure, which performs. better than the UMTS encoder. Following these results, improved Turbo codes can be constructed with better BER performance.