In this paper, we investigate the performance of an intelligent omni-surface (IOS) assisted downlink non-orthogonal multiple access (NOMA) network with phase quantization errors and channel estimation errors, where the channels related to the IOS are spatially correlated. First, upper bounds on the average achievable rates of the two users are derived. Then, channel hardening is shown to occur in the proposed system, based on which we derive approximations of the average achievable rates of the two users. The analytical results illustrate that the proposed upper bound and approximation on the average achievable rates are asymptotically equivalent in the number of elements. Furthermore, it is proved that the asymptotic equivalence also holds for the average achievable rates with correlated and uncorrelated channels. Additionally, we extend the analysis by evaluating the average achievable rates for IOS assisted orthogonal multiple access (OMA) and IOS assisted multi-user NOMA scenarios. Simulation results corroborate the theoretical analysis and demonstrate that: i) low-precision elements with only two-bit phase adjustment can achieve the performance close to the ideal continuous phase shifting scheme; ii) The average achievable rates with correlated channels and uncorrelated channels are asymptotically equivalent in the number of elements; iii) IOS-assisted NOMA does not always perform better than OMA due to the reconfigurability of IOS in different time slots.Index Terms-Intelligent omni-surface (IOS), NOMA, spatial correlation, average achievable rate
I. INTRODUCTIONThe future sixth-generation (6G) is expected to support billions of connected devices with significant demands in spectrum efficiency, reliability, latency, and connectivity [1]. To handle these challenging requirements, a wide range of technologies, including massive multiple-input multipleoutput (MIMO), network densification, and millimeter-wave (mm-wave) communications, have been investigated extensively in recent years [2]. Among various technologies, nonorthogonal multiple access (NOMA) has been regarded as a superior multiple access technology in the future wireless networks due to its promising advantages in terms of spectrum