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.
