A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 2001

As demand for wireless communications continues to grow, third-generation cellular communications systems are being standardized to provide flexible voice and data services. Third-generation Wideband Code Division Multiple Access (WCDMA) systems promise significantly higher data rates (up to 2 Mbits/s) and a new array of mobile communication services, such as video teleconferencing and web browsing, which especially require uplink and downlink performance improvement. In the near future, the downlink capacity is expected to be more crucial than the capacity of the uplink due to the asymmetric capacity requirements, i.e., the downlink direction should offer higher capacity than the uplink. Therefore the employment of efficient downlink receivers is important. One promising approach is to use chip level channel equalizer type downlink receiver. In the thesis, performance of the adaptive chip level Linear Minimum Mean Square Error Equalizer receivers for downlink of WCDMA systems employing long scrambling code is studied over a correlated Rayleigh multipath fading channel through simulation. The adaptive chip level Linear Minimum Mean Square Error Equalizer receivers and conventional Rake receiver are implemented in this simulation based on the parameters closely matched to UMTS/IMT 2000 requirement. Two adaptive LMMSE equalization techniques for downlink WCDMA with QPSK modulation have been studied: estimating the inverse matrix and using Griffiths' algorithms. Performance of both adaptive equalization techniques has been investigated and compared with Conventional Rake receivers in a correlated fading environment. The adaptive chip level LMMSE Equalizer receivers essentially estimate the multiuser chip and thus, restore the orthogonality between different own cell users' signal. Simulation results show that while the conventional Rake receiver is impaired very much by the interfering user's signal, LMMSE equalizer receiver can sustain the effects of MAL The adaptive chip level LMMSE Equalizer receivers give superior performance to the conventional Rake since they are able to suppress intracell MAI based on equalizing the effects of the multipath channel. Based on the simulation results, it is shown that the method of estimating inverse matrix gives slightly better performance than the Griffiths' algorithms.