Namyoon Lee

Power Control for D2D Underlaid Cellular Networks: Modeling, Algorithms, and Analysis

This paper proposes a random network model for a device-to-device (D2D) underlaid cellular system using stochastic geometry and develops centralized and distributed power control algorithms. The goal of centralized power control is twofold: ensure that the cellular users have sufficient coverage probability by limiting the interference created by underlaid D2D users, while scheduling as many D2D links as possible. For the distributed power control method, the optimal on-off power control strategy is proposed, which maximizes the sum rate of the D2D links. Expressions are derived for the coverage probabilities of cellular, D2D links, and the sum rate of the D2D links in terms of the density of D2D links and the path-loss exponent. The analysis reveals the impact of key system parameters on the network performance. For example, the bottleneck of D2D underlaid cellular networks is the cross-tier interference between D2D links and the cellular user, not the D2D intratier interference when the density of D2D links is sparse. Simulation results verify the exactness of the derived coverage probabilities and the sum rate of D2D links.

On the Design of Interference Alignment Scheme for Two-Cell MIMO Interfering Broadcast Channels

The interference alignment (IA) is a promising technique to effectively mitigate interferences in wireless communication systems. To show the potential benefits of such an IA scheme, this letter focuses on a two-cell multiple-input multiple-output (MIMO) Gaussian interfering broadcast channels (MIMO-IFBC) with M transmit antennas and N receive antennas. It corresponds to a downlink scenario for cellular networks with two base stations (BSs) with M transmit antennas per BS, and two users with N receive antennas per user, on the cell-boundary of each BS. In this scenario, we propose a novel IA technique jointly designing transmit and receive beamforming vectors in a closed-form expression without iterative computation. It is also analytically shown that the proposed IA algorithm achieves the optimal degrees of freedom (DoF) of 2N in the case of [¾N] ≤ M <; 2N. The simulations demonstrate that not only the analytical results are valid, but the sum-rate of our proposed scheme also outperforms those of conventional techniques, especially in the high signal-to-noise ratio (SNR) regime.

Achievable Degrees of Freedom on K-user Y Channels

In this paper, we consider K-user Y channels where K users simultaneously exchange messages with each other via an intermediate relay. Degrees of freedom (DOF) of Y channels with multiple antennas is not known in general. Investigation of the feasibility conditions of signal space alignment for network coding is an initial step for addressing this open problem. We verify that when user i with M_i antennas sends K-1 independent messages to the other users through a relay with N antennas and each message achieves the DOF of d, the total DOF of dK (K-1) is attained if Mi ≥ d(K-1), N ≥ {dK(K-1)/2} and N<; min {Mi+Mj -d|∀ i ≠ j}. It is accomplished by adopting the signal space alignment for the network coding during both the multiple access phase and the broadcast phase. It is shown that the proposed scheme obtains not only a network coding gain but also an alignment gain in terms of the normalized DOF, as K → ∞. Also for Y channels where all nodes have a single antenna, we show that the DOF of 2 is achieved regardless of the number of users by using the rational dimension framework.

A novel signaling for communication on MIMO Y channel: Signal space alignment for network coding

In this paper, we study a new network information flow for a multiple-input multiple-output (MIMO) wireless network system with three users and a single intermediate relay which of each is equipped with multiple antennas. In this system, each user wants to convey independent messages for different two users via the intermediate relay while receiving two independent messages from the other two users by using only two time slots. This network information flow is a generalized version of the two-way relay channel for more than two users case. We will call this network information flow as a ‘MIMO Y channel.’ To achieve much higher multiplexing gain in the MIMO Y channel, we propose two novel signaling techniques, which are signal space alignment for network coding during the first time slot and network coding based interference nulling beamforming during the second time slot. By evaluating the multiplexing gain it is shown that the proposed signaling scheme significantly outperforms the conventional time division multiple access and multi-user MIMO schemes.

Spectral Efficiency of Dynamic Coordinated Beamforming: A Stochastic Geometry Approach

This paper characterizes the performance of coordinated beamforming with dynamic clustering. A downlink model based on stochastic geometry is put forth to analyze the performance of such a base station (BS) coordination strategy. Analytical expressions for the complementary cumulative distribution function (CCDF) of the instantaneous signal-to-interference ratio (SIR) are derived in terms of relevant system parameters, chiefly the number of BSs forming the coordination clusters, the number of antennas per BS, and the pathloss exponent. Utilizing this CCDF, with pilot overheads further incorporated into the analysis, we formulate the optimization of the BS coordination clusters for a given fading coherence. Our results indicate that: 1) coordinated beamforming is most beneficial to users that are in the outer part of their cells yet in the inner part of their coordination cluster and that 2) the optimal cluster cardinality for the typical user is small and it scales with the fading coherence. Simulation results verify the exactness of the SIR distributions derived for stochastic geometries, which are further compared with the corresponding distributions for deterministic grid networks.

A New Design of Polar-Cap Differential Codebook for Temporally/Spatially Correlated MISO Channels

Accurate channel direction information is essential to achieve considerable capacity gains in multiple-input multiple-output (MIMO) wireless communication systems. Limited feedback using a polar-cap differential codebook which utilizes the temporal correlation in multiple-input single-output (MISO) channels is presented in this paper. We first describe the general properties of the polar-cap differential codebook and then explain the design methodology of the size of the polar-cap given the temporal correlation coefficient. We also propose an enhancement of the polar-cap differential codebook which is suitable for a spatially correlated channel. We compare the polar-cap differential codebook with a rotation-based differential codebook in terms of the chordal distance to demonstrate the superiority of the polar-cap differential codebook. Monte Carlo simulation results show that the polar-cap differential codebook facilitates a significant performance gain in both temporally and spatially correlated channels.

Not too delayed CSIT achieves the optimal degrees of freedom

Channel state information at the transmitter (CSIT) aids interference management in many communication systems. Due to channel state information (CSI) feedback delay and time-variation in the wireless channel, perfect CSIT is not realistic. In this paper, the CSI feedback delay-DoF gain trade-off is characterized for the multi-user vector broadcast channel. A major insight is that it is possible to achieve the optimal degrees of freedom (DoF) gain if the delay is less than a certain fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligeable. To show this, a new transmission method called space-time interference alignment is proposed, which actively exploits both the current and past CSI.

Adaptive Feedback Scheme on K-Cell MISO Interfering Broadcast Channel with Limited Feedback

In this paper, we study a K-cell multiple input single-output interfering broadcast channel (MISO-IFBC) with finite rate feedback. In this channel, we first derive the rate loss due to the quantization error by considering both a coordinated zero-forcing beamforming and random vector quantization method. Using this result, feedback bits allocation methods are proposed to minimize the performance degradation in K-cell MISO-IFBC. Furthermore, we investigate how many feedback bits per user are necessary to maintain the optimal multiplexing gain in K-cell MISO-IFBC. Through numerical evaluations, we show that our proposed feedback bits allocation strategy provides significant gain compared to a trivial bits allocation scheme.

Degrees of Freedom on the K-User MIMO Interference Channel with Constant Channel Coefficients for Downlink Communications

In this paper, we study degrees of freedom for the K-User multiple-input multiple-output (MIMO)-interference channel (IFC) with constant channel coefficients. In this channel, we investigate how many total number of transmit antennas, M1 + M2 + . . . + MK, are required in minimum to achieve di = 1, ∀i degrees of freedom when all receivers have N = 2 antennas, which is a downlink communication scenario. To answer this question, we propose a new interference alignment scheme based on intersection subspace property of the vector space. The proposed interference alignment scheme can be easily generalized regardless of the number of users. In addition, we investigate degrees of freedom for the partially connected MIMOIFC where some arbitrary interfering links are disconnected due to the large path loss or deep fades. In this channel model, we examine how these disconnected links are considered on designing the beamforming vectors for interference alignment.

Space-Time Interference Alignment and Degree-of-Freedom Regions for the MISO Broadcast Channel With Periodic CSI Feedback

This paper characterizes the degree-of-freedom (DoF) regions for the multiuser vector broadcast channel with periodic channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-time interference alignment is proposed, which exploits both the current and past CSI jointly. Using the proposed alignment technique, an inner bound of the sum-DoF region is characterized as a function of a normalized CSI feedback frequency, which measures CSI feedback speed compared to the speed of users channel variations. One consequence of the result is that the achievable sum-DoF gain is improved significantly when a user sends back both current and outdated CSI compared to the case where the user sends back current CSI only. Then, a tradeoff between CSI feedback delay and the sum-DoF gain is characterized for the multiuser vector broadcast channel in terms of a normalized CSI feedback delay that measures CSI obsoleteness compared to channel coherence time. A crucial insight is that it is possible to achieve the optimal DoF gain if the feedback delay is less than a derived fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligible.