Performance of soft iterative channel estimation in turbo equalization

To combat the effect of intersymbol interference (ISI) while transmitting data over an ISI channel in a coded data transmission system, the impulse response of the channel is required. As part of the turbo equalization approach, which facilitates iterative equalization and decoding, we introduce a method to iteratively improve the quality of the estimate of the channel characteristics. This is done by incorporating soft information fed back by the decoder to improve the initial estimate, obtained for example using a training sequence. Decision criteria based on the analytical calculation of the variance of the channel estimation error are derived to decide whether the soft information improves the quality of the estimate. The considered estimation algorithm is the well-known recursive-least-squares algorithm. It turns out that incorporating soft information for iterative channel estimation does not always improve the quality of the estimate. If it does, the bit-error-rate performance improves significantly over a system not using soft iterative channel estimation.