PERFORMANCE OF PRE-RAKE DIVERSITY TECHNIQUE IN DS- CDMA

Abstract

A new method of multipath diversity combination has been proposed for Time Division Duplex —CDMA systems for portable communications. In this method, the transmitted signal from the base station (BS) is the sum of a number of same spread signal, each one delayed and scaled according to the delay and the strength of the multipaths of the transmission channel. As a result the received signal at the mobile unit (MU) will already be a Rake combination of the multipath signals. This new method is called Pre-Rake diversity combination because Rake diversity combination process is performed before transmission. By this method the size and complexity of the MU can be minimized and the MU is made as simple as a no combining single path receiver (conventional receiver). This Pre-rake method is used with TDD. Since the Up and down links are time slotted on the same carrier frequency, they have the same channel impulse response during short period of time. Instead of building rake receiver in MU, the BS can pre-rake the signal before transmission in the down-link using the channel impulse response estimated from the up-link. When pre-rake signal is convolved with channel impulse response, the function of rake receiver is automatically performed. A theoretical analysis of Signal to Noise Ratio (SNR) and Bit Error Rate (BER) results for the traditional Rake and Pre-Rake combiner as well as computer simulations (using Monte—Carlo (MC) and importance sampling (IS) techniques) for both orthogonal and random codes show that the performance of Pre-Rake and Rake for orthogonal codes are early same, while in case of random codes Pre-Rake over performed the Rake.