Topic: Telecommunication Engineering
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Maximum Ratio Combining for a WCDMA Rake Receiver
Wideband CDMA (WCDMA), a widely accepted third- generation interface, is based on direct-sequence (DS) CDMA technology. To minimize distortion of the signals in a DS- CDMA system, a rake receiver is used. A signal transmitted through the wireless channel may be severely distorted due to co-channel interference, adjacent channel interference (or multiple access interference), thermal noise, and multipath fading.
The most severe distortion comes from fading, which changes the bit error rate (BER) curve from an exponential to a linear curve. One technique the rake receiver employs to combat these distortions is diversity. There are different diversity techniques, including frequency, time, and space. This application note explains how the DS-CDMA rake receiver employs multipath diversity techniques to minimize distortion.
The first term on the right-hand side (R.H.S.) is the transmitted data weighted by the strength of the fading channel (β). The second term is inter-path interference (IPI). The last two terms (second and term term) can be combined and modeled as a single AWGN noise. As the noise from last two terms increases, the probability of error increases. If the fading channel is a Rayleigh distribution, β becomes an exponential distribution. If β is less than 1, the distance between the symbols in signal constellation shrinks, and the error in decoding becomes more likely than in a normal AWGN channel. In other words, fading shortens the distance in the signal constellation, changing the BER curve to linearly decrease as SNR goes up.
The performance of both combining schemes is the same under perfect channel estimation and path searching, assuming that the fading channel is constant over a symbol period. Table 1 shows the estimation of the computational loads of both combining schemes for one channel. The descrambling and despreading are combined and occur in one step. It is assumed that the scrambling code and the spreading code do not change during the transmission.
Source: www.freescale.com
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