Coding for the multiple access binary channel

• Main Author: Khalid, Abbas
• Published: Lancaster University 2012
• Subjects: 621.3845
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659445

Transmitting the maximum amount of information in minimum possible bandwidth is always desired . Multiple access (MA) communication is often used to achieve this objective. However, the mutual interference among the users handicaps the performance considerably. Addition of redundancy bits for reliable transmission demands more bandwidth. Power line communication (PLC) is considered an attractive candidate to overcome the scarcity of the bandwidth and the associated huge cost. PLC uses power lines as a communication medium which were originally designed for power distribution rather than data transmission and are more harsh compared to other communication media. Frequency-selective fading and inter-symbol-interference (ISI) due to multipaths degrade the bit error rate (BER) substantially. Furthermore, devices connected to the grid introduce impulsive noise on the network. Robust coding and modulation schemes are therefore required to increase the communication reliability. Multi-fold turbo coding is a technique used to improve the error performance of conventional turbo codes. Multi-fold turbo coding increases the randomness of a turbo code by dividing the long information sequence in small subsequences and making use of multiple pseudo random interleavers. Multiple interleavers spread the error burst over several symbols making the errors appear in random. Orthogonal frequency-division multiplex (OFDM) distributes the overall transmitted data in parallel on several orthogonal subcarriers and transforms a frequency-selective fading channel to a group of many flat-fading channels. OFDM possesses a unique property which disperses impulsive noise burst across its several sub-carriers; hence Abstract is able to cope better in an impulsive noise environment. This thesis presents multi-fold turbo coding scheme for MA channels. Specifically, a member of multi-fold family called two-fold has been adopted for the simplest MA channel, the two-user binary adder channel (2-BAC). Each user uses a distinct code to encode the information and the decoder employed uses iterative decoding to decode the received signal. Making use of distinct codes not only allows the correction of errors due to noise but also the correction of errors due to interference between users. Performance of multi-fold turbo codes has been evaluated under Gaussian and power-line conditions. Depedance of performance on number of iterations and blocklength is also presented. A simplified multipath model approach is introduced for complex power-line channels in which the power-line network is divided into number of small segments and each segment is considered as an independent sub-channel. Transfer function of each sub-channel is determined. The transfer function of the whole network is taken as a product of all component transfer functions. The approach has been applied to model two PLC networks that are used as reference channels for the work presented in the thesis. Multi-fold turbo codes can be modified to provide unequal error protection (UEP) levels to the information having different ranks of importance where the most significant information is protected more than the information with least importance. To demonstrate the practicality of the UEP mechanism for the 2-BAC, two test images are decomposed into luminance (£), saturation (8) and hue (if) components. The L component of each image is protected twice than the other two components. The performance of the modified multi-fold turbo codes is compared with the multi-fold turbo codes and conventional turbo codes in terms of pixel error rate (PER.) in Gaussian and power-line environments. The visual effects of PER. for each image are also presented.