Quasi-constant envelope OQPSK through nonlinear radio and AWGN channel
A quasi-constant envelope OQPSK (QCEOQPSK) signal is formed using a linear OQPSK modulator to overcome the nonlinearity caused by fully saturated RF power amplifiers. The performance of the QCEOQPSK signal in a AWGN channel is studied for a variety of filters in the transmitter and matched linear filters followed by hard decision in the demodulator. It is shown that at -43 dB the mainlobe of the power spectral density of the QCEOQPSK is only 77% of the bandwidth corresponding to the mainlobe of the MSK power spectrum density. At a bit error rate of 10/sup -5/, the SNR degradation caused by fully saturated power amplifiers is 0.1 dB in the AWGN channel compared with the ideal BER of BPSK using linear power amplifier. This is 2.3 dB better than the bit error rate of QPSK using linear power amplifier in the broadband satellite communications systems. Employing rate 1/2 convolutional code with constraint length K = 6 and the Viterbi (1966) decoder, the SNR degradation is only 0.25 dB at BER = 10/sup -5/. Therefore, QCEOQPSK can be employed in broadband wireless or broadband satellite communication systems to achieve higher spectral efficiency, near optimal demodulation performance and significantly reduce terminal cost.