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Cell-free massive multiple-input multiple-output (CF mMIMO) is known for its ability to provide ubiquitous connectivity. In this paper, we investigate the achievable spectral efficiency (SE) of a user-centric (UC) CF mMIMO system with both multi-antenna APs and users over joint-correlated Rayleigh fading channels. First, we provide a performance analytical framework for the system with the linear decorrelator and study the impact of hardware impairments (HIs) at transceivers on the uplink SE. Based on that, we discuss the local minimum mean-squared error (MMSE) and partial MMSE combining schemes and the partial large-scale fading decoding (LSFD) method from a scalable point of view. Besides, the exact closed-form SE expression is derived with maximum ratio combining (MRC). Then, we study the MMSE-based successive interference cancelation (MMSE-SIC) detector and give an approximate closed-form SE expression with MRC. In the simulations, we compare the linear decorrelator to the MMSE-SIC detector under different hardware-impaired scenarios. Numerical results correspond to the theoretical analyses and show that the impact of HIs can be mitigated by adding the number of receive antennas.

This letter investigates the secrecy performance of a non-orthogonal multiple access (NOMA) system assisted by simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS), where the imperfect successive interference cancelation (SIC) is considered. The cumulative distribution function of the signal-to-interference-noise ratio (SINR) of the legitimate users and the eavesdropping user are respectively deduced by using the central limit theorem. With this result, the theoretical secrecy rate (SR) and its approximate expressions are respectively derived by means of the Gauss-Chebyshev quadrature and Jensen’s inequality. Furthermore, the asymptotic SR analysis and corresponding expression are provided. The results indicate that the number of STAR-RIS elements and the imperfect SIC of NOMA users can affect the secrecy rate performance of the system. The simulations are offered to verify the effectiveness of theoretical analysis and the accuracy of approximate expressions.

The use of antenna arrays is pivotal in array signal processing, with the different antenna arrays having their own structural advantages. The uniform linear arrays (ULAs) are more robust against antenna failures, but their degrees of freedom (DOFs) and array aperture, which are crucial for the direction of arrival (DOA) estimation, are limited. On the other hand, the sparse arrays can offer more DOFs and a larger array aperture but may not necessarily be as robust as ULAs. Hence depending on the given requirement, the properties of certain antenna arrays need to be exploited for direction finding. However, most of the existing antenna array configurations are deprived of a universal design approach. This manuscript introduces a universal array designing framework (UADF), which can employ different antenna arrays and exhibit multiple desirable properties. The UADF is based on effectively cascading three identical levels that can be nested to another M-times, with only its initial level to be configured by the antenna array. Thus, it inherits the characteristics of configuration used in the initial level and simultaneously embeds a good balance between key features in terms of low cost, easier construction and extension, reliability, robustness, more DOFs, larger continuous virtual aperture, and higher spatial efficiency. Moreover, the UADF enjoys closed-form expressions for the array configuration and DOFs. We employed different antenna arrays to demonstrate the usefulness of UDAF, which is also confirmed via numerical experiments.