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Shin, Wonjae,Lee, Namyoon,Yang, Heecheol,Lee, Jungwoo IEEE 2017 IEEE Transactions on Vehicular Technology VT Vol.66 No.1
<P>This paper proposes a novel relay-aided successive aligned interference cancellation (RaSAIC) method for a <TEX>$\text{2}\times K$</TEX> <TEX>$X$</TEX> channel with <TEX>$L$</TEX> full-duplex relays, each of which is fully connected to the channel. The core idea of the RaSAIC is to apply a cooperative relays' precoding technique that aligns interference signals at unintended receivers while creating an Alamouti structure for the desired signals. With the proposed cooperative relay transmission technique, it is shown that both the optimal sum degree of freedom (sum-DoF) gain of <TEX>$\text{2}K/(K+\text{1})$</TEX> and the diversity gain of two are achievable for both constant and time-varying channels when the numbers of relays' antennas are enough to meet the derived conditions, without requiring channel state information (CSI) at transmitters, but with global CSI at relays. Furthermore, it is demonstrated that the same sum-DoF and diversity gain are attainable, even in the presence of interrelay interference due to a full-duplex relaying operation if the memory at the relay nodes is exploited in the proposed manner.</P>
Relay-Aided NOMA in Uplink Cellular Networks
Wonjae Shin,Heecheol Yang,Vaezi, Mojtaba,Jungwoo Lee,Poor, H. Vincent IEEE Signal Processing Society 2017 IEEE signal processing letters Vol.24 No.12
<P>A new relay-aided non-orthogonal multiple access (NOMA) technique is proposed for multi-cell uplink cellular networks in which each cell supports K single-antenna users by its respective base station (BS) equipped with N (<< K) antennas. Cooperative relaying transmission is used to accommodate more than one user per orthogonal resource block in the context of interference-limited cellular networks. With the proposed relay-aided NOMA, an Alamouti structure of the desired symbol can further be generated at each BS free of interference, which gives rise to diversity gain of two. The proposed scheme does not require any channel state information (CSI) at users. Further, only limited CSI is required at relays and BSs, which can greatly reduce the control overhead.</P>
Non-Orthogonal Multiple Access in Multi-Cell Networks: Theory, Performance, and Practical Challenges
Shin, Wonjae,Vaezi, Mojtaba,Lee, Byungju,Love, David J.,Lee, Jungwoo,Poor, H. Vincent IEEE 2017 IEEE communications magazine Vol.55 No.10
<P>Non-orthogonal multiple access (NOMA) is a potential enabler for the development of 5G and beyond wireless networks. By allowing multiple users to share the same time and frequency, NOMA can scale up the number of served users, increase spectral efficiency, and improve user-fairness compared to existing orthogonal multiple access (OMA) techniques. While single-cell NOMA has drawn significant attention recently, much less attention has been given to multi-cell NOMA. This article discusses the opportunities and challenges of NOMA in a multi-cell environment. As the density of base stations and devices increases, inter-cell interference becomes a major obstacle in multi-cell networks. As such, identifying techniques that combine interference management approaches with NOMA is of great significance. After discussing the theory behind NOMA, this article provides an overview of the current literature and discusses key implementation and research challenges, with an emphasis on multi-cell NOMA.</P>
Coordinated Beamforming for Multi-Cell MIMO-NOMA
Shin, Wonjae,Vaezi, Mojtaba,Lee, Byungju,Love, David J.,Lee, Jungwoo,Poor, H. Vincent IEEE 2017 IEEE communications letters Vol.21 No.1
<P>In this letter, two novel coordinated beamforming techniques are developed to enhance the performance of non-orthogonal multiple access combined with multiple-input multiple-output communication in the presence of inter-cell interference. The proposed schemes successfully deal with intercell interference, and increase the cell-edge users' throughput, which in turn improves user fairness. In addition, they increase the number of served users, which makes them suitable for 5G networks where massive connectivity and higher spectral efficiency are required. Numerical results confirm the effectiveness of the proposed algorithms.</P>
Relay-Aided Space-Time Beamforming for Interference Networks With Partial Channel Knowledge
Shin, Wonjae,Lee, Namyoon,Lee, Jungwoo,Poor, H. Vincent Institute of Electrical and Electronics Engineers 2016 IEEE transactions on signal processing Vol.64 No.19
<P>This paper proposes a novel multiantenna relay-aided interference management technique that requires limited channel knowledge for interference relay channels, which is referred to as <I>relay-aided space-time beamforming (r-STBF) </I>. Using the proposed method, it is shown that <TEX>$\frac{KM}{K+M-1}$</TEX> multiplexing gain is achievable in a <TEX>$K$</TEX>-user multiple-input single-output (MISO) interference relay channel when the relay has <TEX>$M$</TEX> antennas with a certain type of limited channel knowledge. By leveraging this result, it is demonstrated that the <I>interference-free</I> multiplexing gain of <TEX>$K$</TEX> is asymptotically achievable as <TEX>$M$</TEX> approaches infinity. Furthermore, the proposed r-STBF method is shown to achieve the <I>optimal</I> multiplexing gain of <TEX>$\frac{KL}{K+L-1}$</TEX> for a <TEX>$K\times L$</TEX> single-input single-output (SISO) <TEX>$X$</TEX> channel with an <TEX>$M$</TEX>-antenna relay. One major implication of these results is that the use of a relay is beneficial in obtaining the <I>optimal</I> multiplexing gains for relay-aided interference networks even with limited channel knowledge, which bridges the theory-practice gap by implementing advanced interference management methods with less overhead for practical wireless systems.</P>
Retrospective Interference Alignment for the Two-Cell MIMO Interfering Multiple Access Channel
Wonjae Shin,Jungwoo Lee IEEE 2015 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.14 No.7
<P>In this paper, we propose a new retrospective interference alignment for two-cell multiple-input multiple-output (MIMO) interfering multiple access channels (IMAC) with M receive antennas per base station (BS) and K users per cell each with N transmit antennas. The key idea of the retrospective IA lies in interference-purification during phase 1 and 2, and interference-repetition during phase 3. On the basis of the proposed method, we show that the achievable sum degrees of freedom (sum-DoF) is equal to 2M ([2M/N]+ 1) / (2 [2M/N] + 1) if K M <; NK and min{2KN, M} if NK ≤ M with completely delayed channel state information at the transmitters (CSIT). It is shown that having delayed CSIT can strictly increase the sum-DoF compared to the case with no CSIT for cellular networks, especially for uplink scenarios.</P>
Cooperative Wireless Powered Communication Networks With Interference Harvesting
Shin, Wonjae,Vaezi, Mojtaba,Lee, Jungwoo,Poor, H. Vincent IEEE 2018 IEEE Transactions on Vehicular Technology VT Vol.67 No.4
<P>A cooperative wireless-powered communication network protocol is proposed, in which user far away from a hybrid access point (H-AP) is capable of harvesting wireless powers by overhearing the uplink signals of users that are relatively nearer to the H-AP, in addition to downlink signal broadcast by the H-AP. In this setting, time allocations for the downlink transmission of the H-AP and the uplink transmission for the users are jointly optimized for two different purposes. First, the <I>weighted sum-rate maximization</I> problem is solved for a given total transmission time. Then, the <I>total transmission time</I> is minimized under a minimum rate constraint, and an efficient algorithm is also presented to solve the optimal solution by a line search strategy. Numerical results demonstrate that the proposed schemes significantly increase the far user's rate, which in turn improves user fairness and transmission delay.</P>
Cyclic Interference Alignment for Full-Duplex Multi-Antenna Cellular Networks
Wonjae Shin,Jong-Bu Lim,Hyun-Ho Choi,Jungwoo Lee,Vincent Poor, H. Institute of Electrical and Electronics Engineers 2017 IEEE Transactions on Communications Vol. No.
<P>This paper studies full-duplex (FD) cellular networks in which a base station (BS) operated in FD mode with multiple antennas supports multiple uplink and downlink users simultaneously in the same wireless channel. Two typical FD cellular scenarios are considered, one with half-duplex (HD) users and the other with FD users along with the FD BS. For both the cases, a novel constructive method is developed for finding a closed-form interference alignment (IA) solution, named cyclic IA. The core idea behind this approach is to construct a set of loop-equations enabling IA in a cyclic manner, so that beamforming vectors are sequentially determined by solving an eigenvalue problem. It is shown analytically that the proposed cyclic IA can achieve the optimal sum degrees-of-freedom (DoF) when the number of user antennas is large enough to meet the derived conditions. In particular, it is shown that the proposed scheme achieves a twofold DoF gain compared with conventional HD cellular networks even in the presence of inter-link interference, provided the number of users becomes large enough compared with the ratio of the number of BSs and user antennas. Simulation results demonstrate that not only are the analytical DoF results valid, but under a practical multi-cell scenario, the proposed cyclic IA offers significant throughput gains depending on the cell radius.</P>