Constant Envelope Ofdm-key Technologies

OFDM (Orthogonal Frequency Division Multiplexing) is one of the most promising key technologies for future broadband wireless digital communication systems due to its advantages as follows:high bandwidth efficiency and strong immunity to multipath interference in wireless channel. The main drawback of OFDM is that it has a large peak to average power ratio (PAPR). This makes OFDM very sensitive to nonlinear distortion caused by the transmitter's power amplifier and requiring an extra input power back-off (IBO).This paper focuses mainly on the study of constant envelope OFDM (CE-OFDM). CE-OFDM modulates the OFDM signal on the phase of phase modulation, and solves the high PAPR problem of OFDM. The main contributions are as follows:a) The performance of CE-OFDM is investigated in additive white Gaussian noise (AWGN) and frequency-selective multipath fading channels. From the simulation, we obtain: the modulation index of CE-OFDM directly affects its spectrum efficiency and bit error rate (BER) performance; uncoded CE-OFDM can exploit frequency diversity in multipath fading channel.b) Base on the property of CE-OFDM signal, using time-domain pilot sequence based on circularly shifting Chu sequence, a Cauchy interpolation estimator (CIE) is proposed for CE-OFDM in slow time-variant channel. The CIE is implemented as follows:first, we obtain the channel impulse responses (CIR) corresponding to these pilot sequences by maximum likelihood, then we interpolate the CIRs of data symbols by Cauchy interpolation using those CIRs of pilot sequences. By simulation in typical urban channel, we find:CIE performs better than linear interpolation and second-order polynomial interpolation but worse than linear minimum square error in slow fading channels.c) We make an intensive investigation on spectrum efficiency, power amplifier (PA) efficiency, and BER performance of three block transmission techniques containing OFDM, CE-OFDM and single carrier frequency-domain equalization (SC-FDE). By simulation, we obtain:at low and middle signal-to-noise ratios (SNRs), their BER performance is in decreasing order: OFDM, SC-FDE and CE-OFDM; at high SNR, CE-OFDM performs better than OFDM and SC-FDE. However, their PAPRs have an increasing order as follows: CE-OFDM, SC-FDE, and OFDM.