Detection of high order M-ARY QAM symbols under transmitter nonlinearities


Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2018

Student: ZİYA GÜLGÜN

Supervisor: ALİ ÖZGÜR YILMAZ

Abstract:

We will investigate the nonlinear effects of power amplifiers on large constellation Quadrature Amplitude Modulation (QAM) in this study. A more than potential feat to enhance transmission rates in next generation wireless networks is high order QAM along with mm-wave transmission. Meanwhile, different types of nonlinearities in the transmitter side may hamper the transmitter rate and decrease receiver performances. From literature, outermost constellation points of QAM schemes are usually more adversely affected by these deteriorations. To observe these effects, the Rapp and Saleh models that consider only amplitude deformation and amplitude in conjunction with phase deformation respectively are utilized. Error vector magnitude (EVM) of each QAM symbol is evaluated to discriminate error variances of each symbol. It is observed that in-phase and quadrature errors originating from the Rapp model can be assumed as independent. On the contrary, power amplifier with the Saleh model creates error such that in-phase and quadrature parts of the errors are not independent. According to our observations, the variances of in-phase and quadrature errors may not even be equal to each other. By accounting for all above issues, receivers that consider unequal EVM distribution for both models are proposed and their performances are compared with those of other receivers that exert average EVM for decoding. In addition to these receivers, a practical receiver is proposed that works on quantized observations based on a look-up table that keeps log-likelihood ratios to reduce computational complexity. Furthermore in this work, mismatched achievable rates of the equivalent nonlinear channels are presented to evaluate decoding thresholds of the receivers.