Error Correction mechanisms over Correlated Channels

  • Wijting, Carl Simon, (Projektdeltager)



    The project started in 2001 and is continued in 2002. Recently there has been a considerable increase in the development and design of wireless networks. As a result, a wide range of mobile networks is being deployed, such as cellular networks (GSM and UMTS) and Wireless LANs. In this context, performance degradation of TCP/IP protocols over wireless links is an aspect worth of attention. Error control mechanisms such as ARQ and FEC may be introduced at Radio Link Layer in order to enhance the reliability of the channel 'seen' by the transport layer. Many applications have latency requirements, which cause the data to be useless if it is not delivered within a certain delay. This requirement imposes some limits on the design of the error control mechanisms. In FEC, redundancy is added at the transmitter and is used at the receiver to correctly recover the information even in the presence of some transmission errors. FEC schemes allow for the trade-off between data rate and residual errors. Due to the correlation in the channel, error coding only marginally improves the performance, the channel has to be made memoryless (uncorrelated), to achieve this, an interleaver can be deployed. However this introduces additional delay to the system. In ARQ a smaller percentage of redundancy than in FEC is added to the data but the retransmission mechanism and the randomness of the channel errors introduce a random delay and it is usually discarded in case of services with strict delay requirements. This project investigates the impact of the interleaver depth, both in terms of residual Frame Error Rate and transmission delay, on the performance of error control mechanisms in presence of a correlated Rayleigh fading and delay constraints. It is found that in bursty channels, even though increasing the interleaver depth helps in improving the BER, it not always improves the performance in terms of residual FER. In fact, depending from the interleaver depth and burstiness level of the channel, the effect of the interleaver is to impact more code words, even if a higher number of code words can be corrected. To get an improvement also in terms of FER the interleaver depth should be too high, thus impacting the delay. Therefore the introduction of retransmissions, which generally helps in improving the channel reliability, in case of bursty channel also helps in fulfilling the delay requirements. Carl Wijting, Eva Braia, Ernestina Cianca
    Effektiv start/slut dato31/12/200331/12/2003