shot noise is lower with a low transmitted intensity. For input power higher than 13 dBm, significant EVM degradation occurs as the peak voltage amplitude of the input 802.11a signal reaches V of the EOM and therefore explores another nonlinear part of the EOM static characteristic. As expected by theoretical considerations about mixing with EOM [7], mixing power is higher when working at an LO power of 11 dBm instead of 7 dBm. Optimal LO power of 18.4 dBm would theoretically give even better EVM measurements, which shows a very efficient process for WLAN 802.11a frequency conversion. Figure 7 shows the lowest EVM measurements around 1.4% rms over all the 52 subcarriers of the up-converted 802.11a signal around 5.8 GHz for P IF ϭ 5 dBm and P LO ϭ 11 dBm.
5. CONCLUSION
In this paper, the performance of optical frequency conversion of a WLAN 802.11a signal centered at f IF is studied. Two optical frequency converters are compared: direct modulation of a LD and the use of a standard EOM. Both devices are biased in nonlinear regimes. Spectrum analysis has been investigated. It has been shown that a twice wider available conversion bandwidth of 2 f IF can be obtained with EOM compared to LD mixing technique as LO is rejected in the RF photodetected output spectrum.
Constellation diagrams for both mixing techniques have reported symmetrical constellation without any distortion caused by frequency up-conversion. EVM measurements of the up-converted 802.11a signal have shown low degradation caused by both optical mixing techniques compared to direct detection in the RoF link. The LD mixing technique offers cost effective 802.11a frequency conversion solution with high mixing power but with limited available bandwidth and small input power range from Ϫ5 dBm to 0 dBm, considering that bias current of the LD has to be set to a constant value.
Lowest values of EVM are obtained with EOM mixing technique at minimum of transmission as the shot noise is minimum with low transmitted intensity. For EOM, the range of optimal 802.11a input power is large, from Ϫ7 dBm up to 13 dBm. In this range, EVM measurements are lower than 3% rms, optimal input powers of P IF ϭ 5 dBm and P LO ϭ 11 dBm lead to a very low EVM value of 1.4% rms, which shows a degradation of only 0.1% compared to direct optical transmission without frequency conversion.