Theoretically, full-duplex (FD) communications can double the spectral-efficiency (SE) of a wireless link if the problem of self-interference (SI) is completely eliminated. Recent developments towards SI cancellation techniques have allowed to realize the FD communications on low-power transceivers, such as small-cell (SC) base stations. Consequently, the FD technology is being considered as a key enabler of 5G and beyond networks. In the context of 5G, FD communications have been initially investigated in a single SC and then into multiple SC environments. Due to FD operations, a single SC faces residual SI and intra-cell co-channel interference (CCI), whereas multiple SCs face additional inter-cell CCI, which grows with the number of neighboring cells. The surge of interference in the multi-cell environment poses the question of the feasibility of FD communications. In this article, we first review the FD communications in single and multiple SC environments and then provide the state-of-the-art for the CCI mitigation techniques, as well as FD feasibility studies in a multi-cell environment. Further, through numerical simulations, the SE performance gain of the FD communications in ultra-dense massive multiple input multiple-output enabled millimeter wave SCs is presented. Finally, potential open research challenges of multi-cell FD communications are highlighted.
Greater device affordability has been driving increased smartphone adoption, which results in a tremendous increase in the subscriptions to cellular and broadband services. Besides, billions of Internet-of-Things (IoT) devices are expected to rely on cellular wireless networks. Based on a recent Ericsson's mobility report published in June 2017, it is anticipated that by the end of 2022 there will be 29 billion connected devices, which translates into an immense volume of wireless traffic. To meet this growing demand for wireless access, the research community strains to include in the emerging fifth generation (5G) and beyond cellular networks the most advanced wireless communication technologies. Among them, the full-duplex (FD) technology has lately gained attention due to its potential of theoretically doubling the spectral efficiency (SE) when compared to half-duplex (HD) communications. In addition, the FD communications bring several other potential benefits, as they avoid the hidden terminal problem, enhance network secrecy, improve sensing in cognitive radio networks and reduce the end-to-end packet delay/latency. Providing lowlatency is one of the key features of the 5G New Radio (NR) interface. The FD communications can be considered as a realizable technology to offer low-latency for NR.
FD communications facilitate the simultaneous transmission and reception of data over the same frequency band. However, due to superimposition of leaked high-power transmit and lowpower received signals at the FD transceiver, FD suffers from self-interference (SI), which hampers its operation. Recently, several works have reported important developments in SI cancelation (SIC) techniques. Essentially, a combination of passive and active techniques has been adopted for SIC. Although the SI is usually compensated for in the digital domain, this cannot compensate for the noise resulting from the analog circuitry of the transmitter. Accordingly, SIC relies on both analog and digital techniques, also known as active SIC. Additionally, passive SIC techniques can be employed, which are applied at the antenna level, e.g., physical separation between transmit and receive antennas. The combined use of the available SIC techniques attenuates the SI power to the thermal noise level. Experimental results have shown that this level of attenuation is sufficient for FD operation and identified low-power transceivers, such as small-cell (SC) base stations (SBSs), as potential deployment scenario.
Cellular operators worldwide are increasingly introducing small-size cells, such as micro-, pico-and femto-cells, within the coverage area of macro-cells to improve the capacity and coverage of networks. In the context of 5G and beyond wireless networks, researchers advocate the deployment of even smaller size cells and their densification, which basically means a low user-SBS ratio. Furthermore, SCs are inexpensive and easy to deploy. In addition, the dense deployment of SCs improves the spatial frequency reuse, facilitates user offloading from macro BS to SBS, and reduces the transmit power.
Since the deployment of the FD transceiver on the lowpower SBSs is possible, several works investigated FD communications in SCs. Earlier studies have been performed to
Performance in Ultra-Dense Small-Cell Wireless Networks Animesh Yadav, Georgios I. Tsiropoulos, and Octavia A. Dobre verify the capacity gain offered by the FD communications in a single SC. The main challenge the single SC faces is cochannel interference (CCI) from the uplink (UL) to the downlink (DL) transmission besides the residual SI (RSI) after SIC. Moving from single-cell to realistic multi-cell scenario introduces a huge surge of CCIs including both intra-and intercell CCI. For comparison purpose, it is important to know how the number of CCI links increases when moving from the HD to the FD communications. Figure 1 graphically depicts the CCIs arisen in a multi-cell conventional HD communications, where the DL and UL transmissions occur on orthogonal frequency bands. On the other hand, Figure 2 (a) depicts the number of CCI links when each SBS operates in the FD mode, i.e., both DL and UL transmissions occur on the same frequency band simultaneously. Recently, a few works have appeared that investigate the feasibility of FD communications in the multi-cell environment over HD communications. The results urge for novel design of intelligent radio resource allocation schemes and schedulers in order to extract the FD benefits. In the rest of this article, firstly, the FD communications in both single and multiple SC networks are presented. Secondly, the state-of-the-art interference mitigation schemes in multiple SC networks are introduced. Additionally, the feasibility of FD communications with 5G key enabling technologies, such as massive MIMO (mMIMO) and millimeter wave (mmWave), is discussed in the context of improving the channel characteristics, reducing the interfe
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