A Reliable OFDM-Based MMW Mobile Fronthaul with DSP-Aided Sub-Band Spreading and Time-Confined Windowing

Performance of digital signal processing (DSP) aided sub-band spreading orthogonal frequency division multiplexing (S-OFDM) implemented with the well-defined, time-located window is evaluated and compared with the cyclic prefix (CP) OFDM signal under severe interference circumstances. A four-quadrature amplitude modulation (QAM) mobile fronthaul over 20 km fiber link and 1 m millimeter wave 60-GHz wireless channel is demonstrated experimentally. The bit-error rate performance of applying time windowing on OFDM and S-OFDM are measured in the single channel scenario. In such simplified system, CP-OFDM is found to be a widely adopted waveform. While, as it suffers from the unexpected external interference, the received error vector magnitude (EVM) would be degraded dramatically for typical OFDM signals. By applying the Hanning window, the impact on the EVM performance can be alleviated as the interference is at the edge of the desired signal band and resulting a 2.5% EVM enhancement. However, an irrecoverable 20% data loss is obtained if the interference is totally immersed in the victim signal. In contrast, S-OFDM exhibits a frequency diversity property by grouping OFDM subcarriers into several sub-bands and spreading them into whole available bandwidth. Thus, as S-OFDM suffers from strong interference inside the signal band, about 12.61% and 43.53% EVM improvements of the average and the worst subcarrier performance with respect to the ordinary OFDM can be achieved via applying the spreading codes orthogonality restoring equalizer. All subcarriers in the S-OFDM are generated to meet the forward error correction threshold with reasonably flat performance. Therefore, the S-OFDM shows its superiority in an MMW mobile fronthaul for better reliability in data transmission.