OFDM design considerations using the HF fading channel

Investigates the design parameters of an orthogonal frequency division multiplexed (OFDM) signal and relates these to the transmission characteristics of the HF ionospheric channel. OFDM is essentially a parallel modem technique where a number of carriers are transmitted and phase modulated each with a low symbol rate. Similar systems have been used at HF for some years. Carriers are orthogonally spaced thereby avoiding intersymbol interference. Using the IFFT/FFT pair for modulation and demodulation, orthogonality between the carriers is implicit, the frequency separation between them being 1/T_S where T_S is the symbol period. The digital audio broadcast (DAB) standard uses OFDM with differential quadrature phase shift keying (DQPSK). This provides a good compromise between spectral efficiency and a low symbol error rate with a low signal to noise ratio. The DAB system is designed to alleviate the undesirable effects of multipath encountered by mobile receivers operating at VHF and UHF frequencies