DS-CDMA RAKE Receiver with Time-Window Control Loop (TWCL) in Multipath Fading Environment

This paper presents a code synchronization scheme--the time-window control loop (TWCL)--for postdetection RAKE receiver systems that offers the path-diversity effect. Laboratory test results from a postdetection RAKE receiver system with TWCL under a frequency-selective fading environment are introduced. RAKE combining methods, a simple integral demodulation method within a time window, and a recursive least squares (RLS) demodulation method within the time window that uses the RLS algorithm to realize maximal ratio combining, are investigated. It is found that TWCL performance with delay paths (double-spike frequency-selective Rayleigh-fading model) surpasses the performance of both acquisition and tracking without delay paths (nonfrequency-selective Rayleigh-fading model) because of the path-diversity effect. The laboratory results show that TWCL provides good acquisition and tracking performance under the fading conditions experienced with maximum Doppler frequencies of 5 or 80 Hz. It is also shown that the signal energy per bit to noise-power spectral-density ratio ([E.sub.b]/[N.sub.0]) required to achieve the bit error rate (BER) of [10.sup.-3] under the double-spike frequency-selective Rayleigh-fading model is reduced by 10 dB compared to the nonfrequency-selective Rayleigh-fading model. The difference in path-diversity effect between the two RAKE combining methods is only 1 dB. From a structural viewpoint, the simple integral demodulation method supports miniaturization and high-speed data-transmission. Index Terms--CDMA, code synchronization, multipath fading, path diversity, RAKE receiver.