Exploiting signal and noise statistics for fixed point FFT design optimization in OFDM systems

Scaling the different stage outputs in an FFT appropriately is crucial for getting a good Signal to Quantization noise ratio (SQNR) in fixed point FFT design. Traditional designs have either handled this through Convergent block floating point technique (CBFP), which has memory, computation and latency penalties or through time consuming simulations. In this paper, we consider the special case of FFT design for OFDM transceivers. We exploit the Gaussian nature of OFDM signals to predict the bit-growth of the signal through the various stages of the FFT and propose a technique to scale the signal appropriately. Additionally, we investigate the quantization error profile and propose a technique to improve SQNR by exploiting the presence of null tones at the band edges. With the proposed techniques, the performance comes close to the CBFP design, with no increase in complexity compared to existing static designs. Simulation results illustrating the performance improvements of the proposed technique are presented.