Abstract
In a mobile environment, communication links are subject to a wide range of channel variations due to mobility, multipath fading, shadowing, interference, etc. These variations combined with limited allocated bandwidth often constrain the design of such links and result in degradation of the quality of service and lowered capacity. Techniques such as error correction code are often used to mitigate channel errors at the expense of bandwidth. Other techniques such as the use of smaller cells with extensive frequency re-use are used to enhance system capacity. However, this operation approaches the theoretical channel capacity and increases the likelihood of errors on the channel due to co-channel interference. Although marginal channel conditions are quite common in mobile communication links, they are intermittent. These conditions are encountered for example just prior to handover; during deep shadowing such as is experienced when a communicating mobile suddenly goes behind a building or hill; during the power ramp-on phase of new mobiles joining the system etc. Therefore, the design of a mobile communication link that can adapt to such conditions when it is necessary would be ideal. A multi-rate source and channel coding (MRC) is one form of link adaptation which represents a new approach to achieving consistent high quality speech combined with efficient spectrum usage. In MRC, link adaptation is achieved by trading-off the gross bit rate of each traffic channel between the speech and channel codecs according to the prevailing channel conditions. When channel conditions degrade excessively, the system allocates more bits for error protection thereby operating with a lower bit rate speech codec. In this case, the powerful error protection might well correct all the errors and thus speech synthesis proceeds with uncorrupted parameters. This thesis presents a design of a CDMA-based MRC system. A quad-rate speech coder is designed based on Pulse Residual Excited LPC (PRELP), an algebraic CELP coder. Unequal error protection is used in the design of a quad-rate convolutional channel coder. In order to evaluate the output speech quality and capacity over realistic channel conditions, a CDMA-based comprehensive dynamic link simulator (DLS) is developed and used as a testbed. The simulation results of the MRC show that it is capable of maximising the output speech quality during good channel conditions and maintaining a satisfactory quality in marginal channel conditions. Whilst the lower rate speech codec is expected to produce reduced quality speech, the perceived degradation is shown to be less than that of a fixed rate system in which the error protection would quickly fail, possibly resulting in cessation of the link. Errors on the rate switching commands show no impact on the output speech quality. Indeed, the MRC system keeps its integrity with severe channel conditions and occasional command errors. This is assisted, however, by the careful design of the switching algorithm in which switching can only occur in consecutive modes. Since the multi-rate system permits the use of minimum resources of speech and channel coding bits, capacity of the mobile system is obviously increased. However, in this work it is shown that the multi-rate system can be particularly useful in crowded cells where it can increase the robustness of the system and enhances the capacity.