Effects of imperfections on the performance of OFDM-based UWB systems

The use of OFDM for UWB applications is a promising choice to simplify the receiver design and to maximize the bandwidth scalability, as well as to adapt to various spectrum allocations. However, most OFDM analyses assume idealized conditions when it comes to the waveform characteristics at the receiver. A cyclic prefix (CP) duration longer than the channel delay spread, and perfect time/frequency synchronization at the receiver makes the OFDM system appear as a set of N parallel orthogonal flat fading channels, where N is the number of subcarriers in the OFDM system. This work investigates how this subcarrier orthogonality is lost when we consider common imperfections such as a channel delay spread longer than the CP, and imperfect time/frequency synchronization at the receiver. We quantify the impact of these imperfections into the receiver bit error rate (BER) and we specialize the analysis to various UWB applications. From the analyses, we see graceful performance degradation with decreasing CP length and increasing residual frequency error. The performance degradation can be much more critical for timing synchronization errors.