Advanced signal processing algorithms for 3rd generation wireless mobile systems

Motivated by the worldwide activity around third generation mobile communication systems, this thesis addresses the problem of improving the physical layer reliability of W-CDMA third generation transceivers by designing simple and low complexity algorithms suited to be implemented on a real system. The work was developed in a framework of a joint project between Eur´ ecom and EPFL whose objective is to design a real-time Software Defined Radio platform able to validate advanced signal processing algorithms. The research focuses on several aspects of the physical layer such as synchronization, multi-user detection, and design of the front-end signal processing. About the synchronization we consider initial timing acquisition in DS-CDMA when propagation is affected by multipath and fading and where the Base Station broadcasts a synchronization pilot signal in the form of bursts of modulated chips transmitted periodically and separated by long silent intervals. Subject to certain sim-plifying assumptions we derive the Maximum Likelihood (ML) estimator by solving a constrained maximization problem. Our ML timing estimator has constant complexity per observation sample. The relation to other estimation methods is addressed, and performance comparisons are provided by simulation. The proposed estimator yields good performance independently of the multipath-intensity profile of the channel, provided that the delay spread is not larger than a given maximum spread. Moreover, our estimator is fairly robust to the mismatch in the fading Doppler spectrum and provides good performance for both fast and slow fading. In order to increase the system capacity we propose a low complexity multiuser joint Parallel Interference Cancellation decoder and Turbo Decision Feedback Equal-izer for CDMA. In our scheme, an estimate of the interference signal (both MAI and ISI) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic per-formance analysis of random-spreading CDMA, we optimize the feedback weights at each iteration. Then, we consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. We show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and we propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of 3rd generation systems, contemplating a combination of TDMA and CDMA and including frequency-selective fading channels, user asynchronism, and power control. For such application, Soft-Input Soft-Output decoders (e.g.implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings. Moreover we present low-complexity algorithms for transmitter and receiver front-end suited to the implementation of Software Defined radio terminals. The proposed algorithms make the processing sampling frequency independent of the symbol rate of the digitally modulated signal and use the "IF-sampling" technique for D/A and A/D conversion. As a case-study, we consider a training-based joint multiuser channel estimation and we show that our front-end algorithms work nicely when coupled with an efficient FFT-based joint channel estimator. Eventually we describe a software radio architecture developed for providing real-time wide-band radio communication capabilities in a form attractive for advanced 3G system research. It is currently being used to implement signaling methods and protocol similar, but not limited to, evolving 3G radio standards (e.g. UMTS, CDMA2000). An overview of the hardware system is provided along with example software implementation on both high performance DSP systems and conventional microprocessors.