Performance Comparison of OFDM Transmission Affected by Phase Noise with and without PLL

Abstract

Oscillator phase noise present in OFDM systems limits the performance by producing a common phase error and additional intercarrier interference. While phase noise is most often modelled as a Wiener process corresponding to free running oscillators, we use a recently developed approach [1] to describe the stochastical behaviour of phase locked loops (PLL) to compare the performance degradation of OFDM-transmission for a receiver operating with a free running oscillator to one using a first order charge-pump PLL having a VCO with the same linewidth. In addition to the relative linewidth of the oscillator with respect to subcarrier spacing, a characteristic quantity determining the performance is the time constant of the PLL with respect to the OFDM symbol duration. Comparing the performance for different QAM constellations it is found, that a PLL will improve bit and symbol error rate, if its gain can be made high enough to obtain time constants smaller than the OFDM symbol duration.