Increasing the lock-in probability of blind symbol time offset estimators in OFDM systems

For orthogonal frequency division multiplexing (OFDM) receivers, the mutual orthogonality of the subcarriers is preserved only if the estimated symbol timing error lies in the lock-in region. Conventionally, a bias is added to the coarse symbol time offset (STO) estimate to move the symbol timing error into the lock-in region, thus increasing the lock-in probability. Upon review of the literature, no analysis has been provided on determining which bias values can improve the lock-in probability. This paper is the first to investigate adding a bias to blind coarse STO estimators, and analyze its performance in terms of the lock-in probability. It will be shown that the optimal value of the bias term is dependent on the channel, which is not ideal for blind estimators. Hence, an upper bound on the optimal value is derived that is valid for all channels, and then the bound is made tighter for realistic wireless communication channels. It is shown that these bounds are dependent solely on the length of the cyclic prefix, thus the coarse STO estimators remain blind.