Improving fixed-point accuracy of FFT cores in O-OFDM systems

Optical OFDM communication systems operating at data rates in the 40Gb/s (and higher) range require high-throughput/highly parallel fast Fourier transform (FFT) implementations. These consume a significant amount of chip resources; we aim to reduce costs by improving the system's accuracy per chip-area. For OFDM signals, we characterize the growth of data within the FFT and explore several cost-conscious methods for improving the fixed-point format. Using ASIC synthesis and hardware accurate simulations, we evaluate the corresponding system error and stability of these methods. We introduce Directive Scaling, which provides an average increase in overall accuracy without additional runtime-adaptive mechanisms. ASIC synthesis results show minimal overhead, and we explicitly evaluate and explain the inherent tradeoffs. When applied to an 8-bit IFFT design, our technique improves precision by approximately two bits with just a 4% area overhead, as opposed to the additional 32% area overhead required using standard methods.