On frequency offset estimation errors in coherent detection over equalized ISI channels

This paper applies a novel analysis technique to examine the impact of frequency offset estimation errors on a single-carrier system equipped with a pilot (or preamble) sequence to estimate the frequency offset and the channel and operating over an ISI channel. The analysis shows that when certain conditions are achieved, the equalizer output is a heterodyned version of the equalizer output when no frequency offset is present. This observation motivated the inclusion of a PLL to track out the frequency offset at the equalizer output. As a byproduct of the analysis, a criterion defining “frequency offset optimized” pilot sequences was developed. Simulation results using QPSK show that a 1 dB loss in BER performance occurs when the frequency offset error is 0:002 cycles/symbol. This was contrasted with a purely feedforward approach that required a more complex frequency offset estimator whose error is three orders of magnitude smaller. The example demonstrates how the addition of a relatively simple PLL at the equalizer output can relax the strict performance requirements of the frequency offset estimator.