On Design and Simulation of FFT-based RAKE Receiver Architectures for Wideband CDMA Systems
|Keywords:||犁耙式接收機;多用戶偵測器;智慧型天線;RAKE Receiver;Multiuser Detector;Smart antenna|
Wireless cellular communications around the world are moving toward wideband code division multiple access (WCDMA) systems in almost all third generation approaches. A CDMA RAKE receiver plays an import role to combat the effect of multipath fading and improve the system performance. In this dissertation, we design three FFT-based RAKE receiver architectures for WCDMA systems and evaluate the system performance by computer simulations. We first proposed a downlink FFT-based RAKE receiver, which computes a data detection in the frequency domain. A spreading code matched filter, a channel matched filter, and a sounding receiver are all implemented by FFT-based matched filters and integrated in a unified architecture. Furthermore, the strong downlink pilot signal does not degrade the receiver BER performance because the receiver estimates the pilot signal and subtracts the estimated pilot signal from the received signal before data detection. Since the signal processing of the FFT-based matched filter is done on symbol-by-symbol basis, the RAKE receiver is robust to the changing of the channel. Our simulation results show that average bit error probability of the system is close to the theoretic optimal performance in a static channel. In a two-path fading channel, we found the system performance degrades as the maximum Doppler frequency increases. Advance technologies, such as multiuser detectors and smart antennas, have been suggested to increase the system capacity of CDMA systems. In this dissertation, we propose a multiuser receiver and a 2 dimensional (2D) RAKE receiver for WCDMA systems, in which FFT-based matched filters are used to reduce the computation complexity. An FFT-based multiuser receiver adopts parallel interference cancellation (PIC), a suboptimal multiuser detection, with the FFT-based RAKE receiver architecture for synchronous CDMA systems. At each stage of the multiuser receiver, we make data detections using FFT-base RAKE receivers and reconstruct the interference signals in the frequency domain. Then all the multiple access interferences (i.e. the replicas of the signals) are removed from the received signal in parallel before next stage data detection. Because the interference can be more and more reduced in this stage-by-stage PIC processes, the data decision will become much more accurate. Here, a soft decision is used in the receiver to account for the reliability of data dectection. For practical implementation, we modify a transmitted symbol by adding a cyclicly extended guard interval to loose the requirement for perfect multiuser synchronization. Through computer simulations, we found the FFT-based multiuser receiver significantly increases the system capacity. An FFT-based 2D RAKE receiver, which combine an antenna array with RAKE receivers, is implemented by a spatial-temporal matched filter in the frequency domain. To form a beam pattern, we calculate the spatial frequency spectra of received signals on the antenna array using fast Fourier transform (FFT). After FFT-beamforming, a bank of FFT-based matched filters is first used to perform code matching. Then, the code matched signals are combined with maximal-ratio combining technique by a spatial-temporal channel matched filter implemented in the frequency domain. This 2D RAKE receiver also includes a channel sounder, which estimates the spatial and temporal channel impulse response parameters such as delays, directions of arrivals, and complex gains of multipath components. Computer simulations show that the 2D RAKE receiver performs well in both a static 2D channel and a mobile radio channel environments.
|Appears in Collections:||Thesis|