Sung Ho Chae

Degrees of freedom of full-duplex multiantenna cellular networks

We study Please be advised that per instructions from the Communications Society this proof was formatted in Times Roman font and therefore some of the fonts will appear different from the fonts in your originally submitted manuscript. For instance, the math calligraphy font may appear different due to usage of the usepackage[mathcal]euscript. The Communications Society has decided not to use Computer Modern fonts in their publications. the degrees of freedom (DoF) of cellular networks in which a full duplex (FD) base station (BS) equipped with multiple transmit and receive antennas communicates with multiple mobile users. We consider two different scenarios. In the first scenario, we study the case when half duplex (HD) users, partitioned to either the uplink (UL) set or the downlink (DL) set, simultaneously communicate with the FD BS. In the second scenario, we study the case when FD users simultaneously communicate UL and DL data with the FD BS. Unlike conventional HD only systems, inter-user interference (within the cell) may severely limit the DoF, and must be carefully taken into account. With the goal of providing theoretical guidelines for designing such FD systems, we completely characterize the sum DoFs for both FD cellular networks. The key idea of the proposed scheme is to carefully allocate UL and DL streams using interference alignment and beam forming techniques. By comparing the DoFs of the FD systems with those of the conventional HD systems, we show that the DoF can approach the two-fold gain over the HD systems, when the number of users becomes large enough compared with the number of antennas at the BS.

Blind Interference Alignment for a Class of K-user Line-of-Sight Interference Channels

We consider a class of K-user line-of-sight interference channels without channel state information at transmitters (CSIT). All transmitters are fixed while receiver i has high mobility if i∈{1,2,⋯,K1} and has low mobility otherwise, where K-1≤ K1≤ K, and the channel coherence times for high- and low-mobility receivers are given by one and T, respectively. In addition, we assume that both transmitter i and receiver i use M≥ T antennas if i∈{1,2,⋯K1} and use only one antenna otherwise. In this paper, we propose an interference alignment (IA) scheme for these channels. The key idea is to group receivers with different mobility to have different coherence time. The results show that IA can improve the degrees of freedom even in the absence of CSIT under certain conditions.

On the degrees of freedom of rank deficient interference channels

We analyze the achievable degrees of freedom (DoF) of the time-varying K-user rank deficient (M, N, D) interference channel (IC) in which each transmitter and receiver uses M and N antennas, respectively, and the rank of each channel matrix is equal to D ≤ min (M, N). The proposed achievable schemes are based on zero forcing and interference alignment. We see that the total DoF of min equation is achievable, where equation and ⌊x⌋ denotes the integer part of x. The result shows that the achievable total DoF of the K-user (M, N, D) IC has similar tendency to that of the full rank IC.

Degrees of freedom of cellular networks: Gain from full-duplex operation at a base station

In this paper, we study the degrees of freedom (DoF) of cellular networks in which a base station with full-duplex operation simultaneously communicates to multiple half-duplex mobile stations. The half-duplex mobile stations are assumed to either communicate with the base station in the uplink or downlink. For such networks, inter-terminal interference needs to be taken into account to achieve the potential gain that doubles the throughput of conventional half-duplex cellular networks. We provide an achievable total DoF for such networks and show that having full-duplex operation at the base station only can indeed double the DoF when the number of mobile stations is sufficiently greater than the number of antennas at the base station. The key idea is to utilize the full DoF for the uplink transmission, while minimizing the inter-terminal interference via interference alignment. The downlink communication rate is then carefully chosen to occupy the remaining DoF at the receiving mobile stations.

Cooperative Relaying for the Rank-Deficient MIMO Relay Interference Channel

Rank-deficient multiple-input multiple-output (MIMO) relay interference channel (IC) is investigated in which K source-destination pairs communicate with the help of N relays. For the K-user rank-deficient MIMO IC with no relays, an achievable sum degrees of freedom (DoF) is upper bounded by Krds, where rds denotes the rank of each point-to-point channel matrix. However, if relays assist the communication, we show that there can be significant improvement on the DoF compared to the case where there is no relay. The proposed cooperative relaying uses relays not only for increasing ranks of desired channel matrices but also nullifying inter-user interference.

On the Multiplexing Gain of K-user Line-of-Sight Interference Channels

We analyze achievable multiplexing gains (MUXGs) of fully connected K-user line-of-sight (LOS) interference channels (ICs). An array of polarimetric antennas, each composed of three orthogonal electric dipoles and three orthogonal magnetic dipoles in which all six elements are co-located, is used throughout this paper. For a K-user LOS IC with single polarization, the maximum achievable MUXG is only K, regardless of the number of transmit and receive antennas at each node due to the key-hole effect. If polarimetric antennas are used at each node, a trivial upper bound on the MUXG is now 2K. In this paper, we consider zero-forcing (ZF) and interference alignment (IA) schemes to achieve this upper bound. We show the optimal MUXG of 2K is achievable if M ≥ ((K+1)/6) polarimetric antennas are used at each node for any K. In addition, using the proposed ZF scheme, we obtain minimal dipole configurations at each node that achieve this upper bound for K <; 5. Furthermore, we analyze achievable MUXGs using the distributed interference alignment (DIA) algorithm. Although IA can generally provide a higher MUXG than ZF for LOS ICs, we observe that the number of required dipole elements to achieve the optimal MUXG of 2K under a ZF scheme is the same as that required under IA for all cases we consider.

Fundamental Limits of Spectrum Sharing Full-Duplex Multicell Networks

This paper studies the degrees of freedom (DoFs) of full-duplex (FD) multicell networks that share the spectrum among multiple cells. In the considered network, we assume that FD base stations (BSs) with multiple transmit and receive antennas communicate with multiple single-antenna uplink (UL) and downlink (DL) mobile users. By spectrum sharing among multiple cells, and with FD radio, the network can utilize the spectrum more efficiently. However, since spectrum sharing and FD induce additional inter-cell and intra-cell interferences, interference management is crucial to take advantage of these features. In this paper, we propose novel interference management strategies which take into account the new sources of interferences caused by spectrum sharing and FD to establish a general achievability result on the sum DoFs. The key ideas in our proposed scheme are to minimize the dimension of UL inter-cell and user-to-user interferences using interference alignment at the UL users. On the BS side, we propose an interference management strategy to align or null out DL intra-cell and inter-cell interferences, and BS-to-BS interferences. We further establish an upper bound on the sum DoFs and show that our lower and upper bounds match under certain conditions. Several numerical evaluations are shown which demonstrate that spectrum sharing and FD can significantly improve the throughput over conventional cellular networks, especially for a network with large number of users and/or cells.

Degrees of Freedom of the Rank-Deficient Interference Channel With Feedback

We study the sum degrees of freedom (DoFs) of the K-user rank-deficient interference channel with feedback. For the two-user case, we characterize the sum DoF by developing an achievable scheme and deriving a matching upper bound. For the three-user case, we develop a new achievable scheme which employs interference alignment to efficiently utilize the dimension of the received signal space. In addition, we derive an upper bound for the general K-user case and show the tightness of the bound when the number of antennas at each node is sufficiently large. As a consequence of these results, we show that feedback can increase the DoF when the number of antennas at each node is large enough as compared with the ranks of channel matrices. This finding is in contrast to the full-rank interference channel where feedback provides no DoF gain. The gain comes from using feedback to provide alternative signal paths, thereby effectively increasing the ranks of desired channel matrices.

Optimal multiplexing gain of K-user line-of-sight interference channels with polarization

We consider the multiplexing gain (MUXG) of the fully connected íí-user line-of-sight (LOS) interference channels (ICs). A polarimetric antenna composed of 3 orthogonal electric dipoles and 3 orthogonal magnetic dipoles is considered where all 6 dipoles are co-located. In case of K-user IC with single polarization, the maximum achievable MUXG is K regardless of the number of transmit and receive antennas because of the key-hole effect. With polarization, a trivial upper bound on the MUXG is 2K. We propose a zero forcing (ZF) scheme for the íí-user LOS IC, where each user uses one or more polarimetric antennas. By using the proposed ZF scheme, we find minimal antenna configurations that achieve this bound for K ≤ 5. For K > 5, we show that the optimal MUXG of 2K is achieved with M = [K + 1 ÷ 6] polarimetric antennas at each user.

Feedback can increase the degrees of freedom of the rank-deficient interference channel

We characterize the total degrees of freedom (DoF) of the two-user rank-deficient interference channel with feedback, in which transmitter i and receiver j use Mi and Nj antennas, respectively, and the rank of the channel matrix between transmitter i and receiver j is given by Dji ≤ min(Mi, Nj) ∀i, j = 1,2. One consequence of this result is that feedback can increase the DoF when the number of antennas at each node is large enough as compared to the ranks of channel matrices. This finding is in contrast to the full-rank interference channel where feedback provides no DoF gain. The gain comes from using feedback to provide alternative signal paths, thereby effectively increasing the ranks of desired channel matrices.