Date of Award
Doctor of Philosophy in Electrical Engineering - (Ph.D.)
Electrical and Computer Engineering
Ali N. Akansu
Richard A. Haddad
Matthijs A. Visser
Growing interest in cellular phones, wireless modems, instant messaging, pagers with the increasing use of the Internet in daily life indicate that wireless high speed Internet access and wireless multimedia applications are in great demand. The barrier to satisfy this demand is the provision of sufficiently high bit rates in severe wireless mobile channel conditions, for a multitude of services, each requiring different and perhaps even time-varying quality of services (QoS) that can be more demanding than the QoS required by today's voice service. New generation systems will enable provision of a multitude of services each with different QoS and data rate requirements. To resolve problems lying in the integration of new generation systems with today's wireless voice services, several contributions are made as listed below. Signal processing techniques that help analyze the spectral properties and models representing the mobile wireless channel are increasingly important in new generation wireless systems for applications such as multipath mitigation, geolocation, line of sight detection. Models representing the mobile wireless channel and methods to trace the energy concentration in its spectrum are tested and effects of the channel environment are examined. After irregular subbands, equal bin subbands in frequency spectrum which corresponds to Discrete Multitone or Orthogonal Frequency Division Multiplexing methods are analyzed for multicarrier option of next generation wireless services. Noise levels in the multicarriers, environmental effects and availability of the channel are related to achievable bit rates using generic adaptive loading. Advantages/disadvantages ages of using multicarrier systems versus single carrier systems are analyzed with decision feedback equalization. In CDMA systems, for a fixed bandwidth allocation, the transmission rate can be altered by varying one or more of the coding, interleaving, modulation and spreading block parameters. In the downlink of DS-CDMA systems, once the parameters of the above mentioned blocks are set, the base station will transmit at the lowest power level that can provide the QoS requirements of the service provided. Different QoS and data rate requirements of communication channels can be satisfied by varying one or more of the coding, digital modulation and spreading blocks. By varying interblock data rates, bit error rate performance simulations are done with the constraint of keeping transmission bandwidth fixed for all configurations as in IS-95. Various coding, digital modulation and spreading rate combinations yield different performance characteristics in AWGN, flat fading and multipath fading environments allowing trade-off decisions on choices of rate, QoS, and implementation. In addition, performance characteristics of multicode CDMA downlink with systematic convolutional (Turbo) coding are obtained under shadowed multipath fading mobile channel. Two different iterative decoding methods are compared in two different synchronous downlink schemes operating under given channel conditions with the same transmission bandwidth as in IS-95. Multicode CDMA with LogMAP decoding is observed to provide better QoS with lower bit and frame error rates, reasonable complexity and shorter decoding delay compared to DS-CDMA downlink with single code pair assigned to each user. With this scheme, higher rate services can be overlayed on existing voice services without any modifications to existing active system architecture.
Bircan, Anil, "Resource allocation for CDMA downlink to achieve maximum throughput" (2001). Dissertations. 444.