Backscatter communication (BackCom) has gained increasing attention due to its low power consumption and low cost. However, its performance is severely degraded by the near-far effect due to the limited backscattering power at the tag side. In this paper, we consider two effective methods to solve the above problem by using the reconfigurable intelligent surface (RIS) and the relay. The objective is to maximize the received signal-to-noise ratio (SNR) subject to the transmit and circuit power control. To gain insight into both schemes, we derive a closed-form expression for the coverage radius of RIS-aided BackCom. Furthermore, considering the relay as a competing technology to the RIS, we compare their performance in terms of transmission performance, required transmit power, and quantify the number of reflecting elements needed for the RIS to outperform the relay. Simulation results demonstrate the RIS-aided BackCom with a sufficiently large number of elements achieves a superior performance, offers greater deployment flexibility, and is suitable for high self-interference levels.

As a core candidate technology of next-generation communication systems, reconfigurable intelligent surface (RIS) can effectively improve the channel quality, realizing the smart radio environment. It is recognized that the optimal deployment of RIS should be closer to the transmitter/receiver, naturally clarifying necessity of near-field channel for RIS. Due to the complexity of near-field channel as spherical waves, the channel state information (CSI) for RIS downlink is quite difficult to be estimated. To solve above issues, this work focuses on the RIS assisted nonorthogonal multiple access (RIS-NOMA) system based on the frequency division duplex (FDD), and proposes the bit-allocation schemes to solve that of the feedback bit limitations. The optimal deployment location of RIS is investigated for two-users case, which ensures the minimum channel fading effect and reduces the complexity of RIS beamforming. Specifically, we also derive the quantized channel gain as a function of feedback bits, when the feedback overhead is limited. Meanwhile, we explore the applicable CSI range for RIS-NOMA downlink designed in this article. According to the characteristics of base station-RIS cascade subchannel, three bit-allocation schemes are provided, which can be easily extended to multiuser scenarios. By applying the RIS deployment and bit-allocation schemes, the limited feedback issue of RIS-NOMA downlink can be solved easily. Numerical results show that the proposed RIS deployment and feedback schemes effectively improve the quantization sum-rate for overall system, mitigating the performance loss caused by limited feedback.

Reconfigurable intelligent surface (RIS) is a cost-effective component to enhance wireless energy harvest and spectral efficiency (SE) of wireless communications systems. In the paper, we investigate the performance of a RISs-aided cell-free massive multiple-input multiple-output (CF-mMIMO) system over Rician fading channels in conjunction with wireless power transfer (WPT). User equipments (UEs) harvest energy from the energy signal transmitted from access points (APs) with the help of RISs during the downlink and use it for the uplink pilot and data transmission. Specifically, we derive a closed-form expression for the uplink SE when using a two-layer decoding method and verify the accuracy of the derived results by Monte Carlo simulation. The results show that increasing the number of APs or the number of RIS elements is always beneficial for the system SE improvement. Furthermore, increasing the number of RIS elements can lead to a reduction in the required number of APs without a decrease in the average SE, which means that significant hardware and energy costs can be greatly reduced.

Non-terrestrial networks (NTNs) using flying objects such as satellites play key roles in the next-generation wireless system (6G). The NTN links with long propagation delay and random packet losses pose a great challenge to the performance of Transmission Control Protocol (TCP), which many Internet applications rely on. Performance enhancing proxy (PEP) is an easy-to-deploy approach for improving TCP's performance. In this paper, we design and implement a novel PEP calledPEPescwhich has two distinctive features. First, it featuresretransmission-freeloss recovery, using an adaptive packet-level forward erasure correction method called streaming coding (SC). Second, as packet losses are recovered by SC, the congestion control problem is simplified to rate control and local acknowledgement between entities based on bandwidth estimation. Based on a queueing theoretic analysis of the design, we carefully devise a protocol and implement PEPesc as an open-source application. Extensive evaluations show that PEPesc can achieve much higherandsmoother goodput than the canonical TCP variants and than other existing open-source PEPs in applications includingiperfand HTTP-based adaptive streaming, and achieves similar performance in web browsing. Finally, we also present a deployment case over a real-world geostationary satellite link.

Cell-free massive multiple-input multiple-output (mMIMO) has recently been proposed to improve cell edge performance. However, most prior works consider perfect hardware impairments (HIs), which are difficult to be achieved in practical systems. This paper studies the impact of HI in an uplink cell-free mMIMO system with both multi-antenna access points (APs) and multi-antenna user terminals (UTs) under the Weichselberger channel model.Firstly, we study a two-layer decoding scheme with local minimum mean-squared error or maximum ratio combining at the AP side and with optimal large-scale fading decoding in the central processing unit. We derive novel closed-form SE expressions and prove that the effect of HI can be mitigated in the case of UTs with multiple antennas. However, the achievable SE is constrained by the pilot contamination and pilot overhead. To this end, the superimposed pilot (SP) transmission method is considered in this paper, where all the coherence intervals are used for both pilot and data symbols transmission. Finally, numerical results verify our derived expressions and reveal the relationship between HI and the number of antennas per UT for different pilot schemes. Note that the advantages of SP over regular pilots disappear when the hardware quality decreases with multi-antenna UTs.