System and method for performing accurate demodulation of turbo-encoded signals via pilot assisted coherent demodulation

A system for providing an accurate log-likelihood value to improve receiver performance in a wireless communications system comprising:

• first means for extracting a pilot signal and a data signal from a received composite signal;
• second means for computing a carrier signal-to-interference ratio based on said pilot signal and said data signal and providing first signal-to-interference ratio and a second signal-to-interference ratio based in response thereto;
• third means for computing a log-likelihood ratio scaling factor based on said a first signal-to-interference ratio and said second signal-to-interference ratio;
• fourth means for calculating a log-likelihood ratio as a function of a channel estimate based on said pilot signal;
• and
• fifth means for scaling said log-likelihood ratio by said log-likelihood ratio scaling factor and providing said accurate log-likelihood value in response thereto.

A system for increasing the signal-to-noise ratio of a receiver employing reference symbol aided demodulation comprising:

• means for combining information from a reference signal transmitted over said channel with information from a corresponding known transmitted reference signal to obtain an estimate of a channel over which said reference symbol is transmitted by a transmitter and received by said receiver;
• means for calculating a log-likelihood ratio estimate for a data signal received by said receiver over said channel, said log-likelihood ratio estimate a function of said received data signal, said channel estimate, and a noise variance of said signal;
• means for applying a scaling factor to said log-likelihood ratio estimate to provide an accurate log-likelihood ratio, said scaling factor based on a noise variance of said channel estimate, said noise variance of said data signal, and an average of received energy per bit in said data signal;
• and
• means for employing said accurate log-likelihood value to demodulate said received data signal.

A communications system employing pilot assisted coherent demodulation comprising:

• a transmitter having a turbo encoder for encoding a data signal and transmitting said data signal with a pilot signal;
• a first receiver section having a turbo decoder and a priori knowledge of said pilot signal for receiving said data signal and said pilot signal and providing an estimate of said channel based on said received pilot signal;
• a log-likelihood calculation circuit in communication with said first receiver section for generating an accurate log-likelihood ratio that is a function of a noise variance of said data signal, a noise variance of said estimate of said channel, an average received energy per information bit included in said data signal, and said data signal;
• and
• a second receiver section for employing said log-likelihood ratio as a metric to demodulate said received data signal.