Normalization, windowing and quantization of soft-decision Viterbi decoder inputs in CDMA systems

In a code division multiple access system, a RAKE receiver is used to demodulate the received multipath signals. At the output of the RAKE receiver, a large dynamic range can occur depending on the channel condition and the number of multipaths received. After the RAKE receiver, the demodulated signal is sent into a soft-decision Viterbi (see Principles of Spread Spectrum Communication, Reading, MA, Addison-Wesley Publishing Company, 1995) decoder that requires only about 3 to 5 bits of input precision. We studied different ways of normalizing, windowing, and quantizing the received symbols at the RAKE receiver output before sending the quantized symbols into the Viterbi decoder. Using the IS-95 CDMA forward traffic channel system as an example, we analyzed and simulated the performance of these schemes. Overall, we found that if the received signal is properly normalized, windowed and quantized, a 4-bit input soft-decision Viterbi decoder loses around 0.2 dB in performance compared to that of an infinite input precision Viterbi decoder in Gaussian as well as Rayleigh fading channels with different Dopplers