## 5 Results and Discussion
### OFDM Versus APSK
Nevertheless, for APSK band rates around 200 MBaud, the results show that laser sources exhibiting linewidths in the order of 10's kHz will be required for heterodyne operation, with a standard mm-wave receiver architecture. When the PNC receiving architecture is used, the SSB information is down-converted to the IF stage by the carrier\(\times\)carrier term which is isolated by the 'Carrier Filter' highlighted in Figure 1. As discussed, the effect of this is to enable the cancellation of any phase noise (and any frequency offsets within the bandwidth of the filter) arising from the interaction of the optical carriers, via the heterodyne process. In this case, Figure 4a shows performances at the system error floor (i.e., performances attained for linewidth values of 0 kHz) for both APSK (EVM = 3.5%) and OFDM (EVM = 5%) modulation, for all linewidth values. The difference in optimum performance EVMs for both formats is a consequence of the relative impact of system intensity noise (RIN, PD shot noise and thermal noise) on APSK and OFDM, whose PAPRs are 1.79 dB and 12.46 dB, respectively--highlighting one of the major drawbacks  
Figure 4: (**a**) Performance of OFDM and APSK with optical source linewidth variation, with LO based and PNC receivers, and (**b**) APSK and OFDM performance of a wider range of optical source linewidths, where the PNC receiver is engaged. of conventional OFDM. Corresponding phase noise free 32-QAM OFDM and 32-APSK constellations can be observed in Figure 5b,d, respectively. Figure 4b shows the performances of both OFDM and APSK, where the PNC receiver is enabled and optical linewidth values are swept over a broad range. EVMs corresponding to the optimum performance for both modulation formats are apparent for optical source linewidth values up to about 3 MHz, after which the received signal deteriorates. The performance limitation evident in Figure 4b stems from the fact that the PNC receiver does not enable perfect phase noise cancellation in cases where the mm-wave carrier spectral profile is significantly band-limited by the PNC's carrier filter. The phase noise contained in the mm-wave carrier term can be thought of as 'information' required for analog noise cancellation at the mixing stage. In cases where the carrier filter bandwidth is large relative to the spectral profile of the mm-wave carrier, almost all of the carrier's Lorentzian-shaped phase noise spectral component passes to the IF mixer and is used to cancel the equivalent component in the information signal. In cases where the filter bandwidth is small relative to the carrier's spectral width, only partial phase noise cancellation can be performed, as the mm-wave carrier has been stripped of some of its phase noise detail, due to band-limiting. It follows that the performance of the system is proportional to the level of phase noise cancellation which can be performed, and hence the ratio of filter bandwidth to generated mm-wave phase noise. So, intuitively, the SNR of the received signal can be maximised when full phase noise cancellation is permitted by appropriate filtering. This points toward a relationship between the received signal SNR, carrier filter bandwidth and carrier linewidth, which is explored in the following section.