Research On Effects Of RF Impairments On System Performance Of802.11ac WLAN

EVM is widely used in communication standard to evaluate the in-channel modulationaccuracy at the transmitter end. In recent years, EVM measurements for receivers have beenemployed to get a rough estimation of their bit error rate (BER) performance. This is because EVMcontains all the non-idealities information which can be further extracted. Orthogonal frequencydivision multiplexing (OFDM) modulation, due to its high spectral efficiency and robustness tointerference, has been widely used in communication systems such as DVB, WLAN, Wimax andLTE. However, it is very sensitive to nonlinear distortions due to its high peak-to-average-powerratio (PAPR) and phase noise due to multi-carrier modulation. When OFDM modem and de-modemworks with its corresponding transceiver analog RF front-end circuits, the non-ideal circuits willintroduce extra distortions to the signal. Depending on the architecture of the transceiver circuits,different sources of impairments are concerned. Since low cost and low power devices are veryattractive in consumer electronics that are powered by batteries, direct conversion architecture hasbeen the first choice for RF transceiver designers. Despite its advantages, direct conversiontransceivers suffer from matching problems due to PVT variations. Now we can see that due to theinherent susceptibility of OFDM signal to non-linearity and phase noise, and the unavoidablematching problem in direct conversion architecture, it is very challenging for RF designers todesign direct conversion transceivers for OFDM system. The evolution of WLAN standard aims toprovide higher system throughput to make new applications possible. In this background, the newly802.11ac standard are proposed by the IEEE802.11WORKING GROUP. Compared to itspredecessors, the new standard employs higher bandwidth, denser constellation modulation, andmultiuser MIMO to improve the system data rate. However, this comes with the price of bothperformance improvement at both the digital baseband system and RF front-end transceivers. In theview of the RF transceiver design, wider bandwidth means more subcarriers in OFDM signals, andwhen combined with the denser constellation modulation, signals with higher peakto-average-power ratio (PAPR) are generated, which requires quite front-end circuits. In themeanwhile, the denser constellation will cause more stringent required on LO phase noise andsystem matching. In this paper, we first give a description of the802.11ac WLAN standard, afterthat theoretical analysis on the effects of various RF circuits impairments such as IQ mismatch, PAnon-linearity, phase noise, and noise are given. Then a system level simulation platform based on802.11ac standard is constructed, and effects of both individual RF impairments and their combined effects on EVM are simulated. At last, conclusions are made and some suggestions are given.