Performance evaluation of COFDM for digital audio broadcasting. II. Effects of HPA nonlinearities

For pt. I see ibid. vol.43, no.1, p.64-75, 1997. The effects of the high power amplifier (HPA) nonlinearities on the performance of the Eureka 147 DAB system are studied by computer simulation. The performance is determined for three types of HPA: a travelling wave tube amplifier (TWTA), a solid state power amplifier (SSPA) and a perfectly linearized amplifier (PLA). Two related performance criteria are used: (a) the degradation, resulting from HPA nonlinearities, in the E_b /N₀ ratio required at the receiver to maintain a bit error rate of 10^-4 and (b) the total power degradation. These degradations are measured as a function of the HPA output backoff (OBO). The effect, on the E_b/N₀ degradation, of linearizing only the phase or only the amplitude transfer characteristic of the TWTA and the SSPA is also assessed. Correcting the phase distortion alone in both HPAs is found to reduce the E_b/N₀ degradation by less than 0.5 dB. Linearization of the amplitude characteristic alone, on the other hand, can reduce the E_b/N₀ degradation by several dBs at small OBO values (<2 dB). The optimum output backoff which minimizes the total power degradation is between 2 and 3 dB for both the TWTA and the SSPA in a terrestrial mobile channel and between 1 and 2 dB in an AWGN channel. The optimum output backoff for the PLA is 2 dB in the terrestrial channel and between 1 and 2 dB in the AWGN channel. At the optimal operation point, the power saved by linearizing the amplitude and phase characteristics of the TWTA or the SSPA is about 0.6 dB for the terrestrial mobile channel and 0.4 dB for the AWGN channel