Cell search in frequency division : duplex WCDMA networks.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2006.Wireless radio access technologies have been progressively evolving to meet the high data rate demands of consumers. The deployment and success of voice-based second generation networks were enabled through the use of the Global System for Mobile Communications (GSM) and the Interim Standard Code Division Multiple Access (lS-95 CDMA) networks. The rise of the high data rate third generation communication systems is realised by two potential wireless radio access networks, the Wideband Code Division Multiple Access (WCDMA) and the CDMA2000. These networks are based on the use of various types of codes to initiate, sustain and terminate the communication links. Moreover, different codes are used to separate the transmitting base stations. This dissertation focuses on base station identification aspects of the Frequency Division Duplex (FDD) WCDMA networks. Notwithstanding the ease of deployment of these networks, their asynchronous nature presents serious challenges to the designer of the receiver. One of the challenges is the identification of the base station identity by the receiver, a process called Cell Search. The receiver algorithms must therefore be robust to the hostile radio channel conditions, Doppler frequency shifts and the detrimental effects of carrier frequency offsets. The dissertation begins by discussing the structure and the generation of WCDMA base station data along with an examination of the effects of the carrier frequency offset. The various cell searching algorithms proposed in the literature are then discussed and a new algorithm that exploits the correlation length structure is proposed and the simulation results are presented. Another design challenge presented by WCDMA networks is the estimation of carrier frequency offset at the receiver. Carrier frequency offsets arise due to crystal oscillator inaccuracies at the receiver and their effect is realised when the voltage controlled oscillator at the receiver is not oscillating at the same carrier frequency as that of the transmitter. This leads to a decrease in the receiver acquisition performance. The carrier frequency offset has to be estimated and corrected before the decoding process can commence. There are different approaches in the literature to estimate and correct these offsets. The final part of the dissertation investigates the FFT based carrier frequency estimation techniques and presents a new method that reduces the estimation error

Cell search algorithms for WCDMA systems

Wideband Code Division Multiple Access (WCDMA) system uses orthogonal channelization codes to distinguish physical channels in a base station, while base stations are identified by different downlink scrambling codes. User equipments (UEs) must achieve synchronization to the downlink scrambling code before decoding any messages from base stations. The process of searching for a base station and synchronization to the downlink scrambling code is often referred to as cell search. The performance of cell search has a significant impact on a UE's switch-on delay, and thus it is very important to UE design. The goal of designing a cell search algorithm is to achieve a balance between speed, accuracy and complexity. A basic three-stage cell search procedure has been defined by 3GPP. It employs synchronization channels and the common pilot channel to facilitate a fast cell search. This cell search scheme only works well if there is no frequency offset between a base station's transmitter and a UE's receiver and if sampling timing is perfect on a UE. In practice, however, imperfection of oscillator in a UE may cause a big frequency error as well as clock error. It usually results in phase rotations and sampling timing drifts, which may degrade cell search performance significantly. Some advanced cell search algorithms have been proposed for mitigating impacts of frequency error or clock error. However, there is no much discussion on comprehensive solutions that can deal with the two negative impacts at the same time. In this thesis, we propose an algorithm that considers both frequency error and clock error. A fast and accurate cell search with a relatively low level of complexity is achieved. The algorithms are based on a combination of four existing enhanced cell search algorithms that are designed for a toleration of either frequency error or clock error. We first introduce the 3GPP-defined cell search algorithm as a basis. Then the four existing enhanced algorithms, PSD (partial symbol de-spreading), DDCC (differential detection with coherent combining), STS-1 (serial test in stage-1) and RSPT (random sampling per trial) are presented. Next, we propose four possible combinations of the existing algorithms: PSD+STS-1, PSD+RSPT, DDCC+STS-1 and DDCC+RSPT. Through extensive computer simulations, we find the DDCC+RSPT algorithm to be the best one. It is superior to other combinations and also outperforms any existing algorithm in terms of acquisition time, detection probability and complexity. Therefore, it is highly recommended for practical uses