Secure Transmission in Cell-Free Massive MIMO With RF Impairments and Low-Resolution ADCs/DACs

This paper considers the secure transmission in a
cell-free massive MIMO system with imperfect radio frequency
(RF) chains and low-resolution analog-to-digital converters
(ADCs) and digital-to-analog converters (DACs) at both access
points (APs) and legitimate users, where an active eavesdropper
attempts to wiretap the confidential data. The Gaussian RF
impairment model (GRFIM) and additive quantization noise
model (AQNM) are used to evaluate the impacts of the RF
impairments and low resolution ADCs/DACs, respectively. The
analytical results of the linear minimum mean square error
(MMSE) channel estimation show that there is nonzero floor
on the estimation error with respect to the RF impairments,
ADC/DAC precision and the pilot power of the eavesdropper
which is different from the conventional case with perfect
transceiver. Then, a tractable closed-form expression for the
ergodic secrecy rate is obtained with respect to key system
parameters, such as the antenna number per AP, the AP number,
user number, quality factors of the ADC/DAC and the RF
chain, pilot signal power of the eavesdropper, etc. Moreover, a
compensation algorithm between the imperfect RF components
and the inexpensive coarse ADCs/DACs is also presented. Finally,
numerical results are provided to illustrate the efficiency of the
achieved expressions and the devised algorithm, and show the
effects of RF impairments and low resolution ADC/DAC on the
secrecy performance.