Impact of Frequency-Modulated Harmonic Noises From PCs on OFDM-Based WLAN Systems

Theoretical and numerical analyses are conducted on the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM)-based wireless LAN (WLAN) systems that are subjected to electromagnetic interference (EMI) caused by radiated noises from personal computers (PCs). The aim of the analysis is to investigate the impact of frequency- modulated harmonic noises generated by spread-spectrum clock (SSC) systems in PCs. SSC techniques are commonly employed in order to reduce the spectral amplitude of clock harmonics measured in EMI compliance tests. The results of the analysis revealed that: (1) BER can be seriously affected in an actual situation where a WLAN antenna installed in or placed very close to a PC receives noise generated by the PC itself; (2) the forward error correction (FEC) in IEEE802.11 a/g becomes less effective as the frequency variation rate of the harmonic increases; and (3) frequency-modulated clock harmonics have a greater negative effect on BER than unmodulated harmonics do. Thus, SSC does not necessarily reduce the impact of clock harmonics on OFDM-based WLAN systems, even if the spectral amplitude of the harmonics is reduced. The impact of clock harmonics on WLAN is compared with that of other wideband noises. Discussion is further extended to cover how to mitigate the interference.