This paper considers the use of the linear amplification with nonlinear components (LINC) technique for the power amplification of spectrally compact offset quadrature phase shift keying (OQPSK) signals allowing the use of highly efficient, low cost, and strongly nonlinear high power amplifiers (HPAs). However, the performance of the LINC signal separation and power combining procedures decreases with the rise of the signal's peak-to-average power ratio (PAPR). A new ring-type magnitude modulation (RMM) method is proposed for OQPSK signals that limits both its maximum and minimum complex envelope excursions avoiding zero crossings, without spreading the transmitted signal's spectrum. The performance results show that band-limited OQPSK signals whose envelope has low fluctuations produce LINC components with a narrower spectrum, with a considerable impact on the LINC transmitter regardless of the type of combiner chosen: when using a passive/matched combiner, the transmitter's power efficiency is significantly increased without spreading the combined signal's spectrum; for the highly efficient non-linear Chireix combiner, there is a reduction of the amount of spectral leakage produced by nonlinearly combining the LINC signal components. Finally, an iterative decoding scheme is also proposed, which employs estimates of the received symbols' RMM coefficients to compensate the RMM distortion.

The expectations for fifth generation (5G) systems involve handling huge throughputs together with power consumption. In order to address this issue, massive multiple-input and multiple-output (MIMO) schemes were developed exploring the fact that any M-ary constellation can be decomposed as several offset quadrature phase shift keying (OQPSK) signals, to be amplified by efficient saturated amplifiers and transmitted separately. Since nonlinear amplifiers should be employed with constant envelope signals to avoid nonlinear distortion, a ring-type magnitude modulation (RMM) technique was proposed for bandlimited OQPSK signals to mitigate its envelope fluctuations while maintaining its spectral characteristics. This technique was shown to improve the power amplification efficiency of linear amplification with nonlinear components (LINC) transmitters, although with some degradation of the of the detection performance. This paper proposes an iterative decoding scheme relying on the accuracy of the first iteration's log likelihood ratio (LLR) computation to estimate the RMM coefficients of the received symbols. Several LLR computation methods are compared with the ideal case where the receiver knows each symbol's RMM coefficients, and it is shown that a few iterations are enough to obtain a bit error rate (BER) performance close to the best case scenario.