Multicarrier, Quadrature-Frequency/Phase modulation for frequency-selective fading channels

Abstract

A Multicarrier system using Quadrature Frequency-Modulation, combined with Phase-Modulation (MCQFPM) is proposed for high bit rate communications in frequency-selective multipath fading channels. MCQFPM employs biorthogonal multi-dimensional signal points with dimensions in frequency and phase and loads, on the average, 44% of the subcarriers with a bandwidth efficiency of 1.5 bit/s/Hz. Smaller number of loaded subcarriers in each symbol interval result in 3 times lower Peak-to-Average Power ratio with reduced transmit power and lower amount of noise caused by Inter-Channel-Interference. Although the proposed scheme is more sensitive to ISI, this drawback is overcome by using guard time. For frequency-selective channels, the frequency shifting property of the loaded subcarriers provides an intrinsic diversity against deep fades and also randomizes the consistent errors. The performance of MCQFPM system is examined through simulations for multipath channels with exponential power-delay-profile and the results are compared with those of the OFDM system using QPSK modulation. With guard time, the sacrifice in bandwidth efficiency of MCQFPM is compensated by a similar increase in the transmission rate with improved average BER performance for low delay spreads, where BER< 10(-3), together with power efficiency capability.