Mehdi Ashraphijuo

On the Capacity Region and the Generalized Degrees of Freedom Region for the MIMO Interference Channel With Feedback

In this paper, we study the effect of feedback on the two-user MIMO interference channel. The capacity region of the MIMO interference channel with feedback is characterized within a constant number of bits, where this constant is independent of the channel matrices. Further, it is shown that the capacity region of the MIMO interference channel with feedback and its reciprocal interference channel are within a constant number of bits. Finally, the generalized degrees of freedom region for the MIMO interference channel with feedback is characterized.

On the Capacity and Degrees of Freedom Regions of Two-User MIMO Interference Channels With Limited Receiver Cooperation

This paper gives the approximate capacity region of a two-user multiple-input multiple-output (MIMO) interference channel with limited receiver cooperation, where the gap between the inner and outer bounds is in terms of the total number of receive antennas at the two receivers and is independent of the actual channel values. The approximate capacity region is then used to find the degrees of freedom region. For the special case of symmetric interference channels, we also find the amount of receiver cooperation in terms of the backhaul capacity beyond, which the degrees of freedom do not improve. Further, the generalized degrees of freedom is found for MIMO interference channels with equal number of antennas at all nodes. It is shown that the generalized degrees of freedom improves gradually from a W curve to a V curve with increase in cooperation in terms of the backhaul capacity.

A Receiver-centric Approach to Interference Management: Fairness and Outage Optimization

Effective interference management in the multiuser interference channel necessitates that the users form their transmission and interference management decisions in coordination, and adapt them to the state of the channel. Establishing such coordination, often facilitated through information exchange, is prohibitive in fast-varying channels, especially when the network size grows. This paper focuses on the multiuser Gaussian interference channel and offers a receiver-centric approach to interference management. In this approach, the transmitters deploy rate-splitting and superposition coding to generate their messages according to independent Gaussian codebooks. The receivers can freely decode any arbitrary set of interfering messages along with their designated messages in any desired joint or ordered fashion, and treat the rest of the interferers as Gaussian noise. The proposed receiver-centric interference management approach is applied to two class of problems (outage optimization and fairness-constrained rate allocation), and constructive proofs are provided to establish the following properties for the proposed approach: 1) the optimal set of codebooks to be decoded by each receiver is a local decision made by each receiver based on its local channel state information (CSI); 2) the globally optimal transmission rates are related to locally optimal rates computed by the receivers based on their local information, which implies that the transmitters do not require explicit knowledge of the CSI and can determine their rates via limited feedback from the receivers; and 3) obtaining the optimal interference management strategy at each receiver has controlled complexity.

On the Symmetric

In this paper, we develop achievability schemes for symmetric K-user interference channels with a rate-limited feedback from each receiver to the corresponding transmitter. We study this problem under two different channel models: the linear deterministic model, and the Gaussian model. For the deterministic model, the proposed scheme achieves a symmetric rate that is the minimum of the symmetric capacity with infinite feedback, and the sum of the symmetric capacity without feedback and the symmetric amount of feedback. For the Gaussian interference channel, we use lattice codes to propose a transmission strategy that incorporates the techniques of Han-Kobayashi message splitting, interference decoding, and decode and forward. This strategy achieves a symmetric rate, which is within a constant number of bits to the minimum of the symmetric capacity with infinite feedback, and the sum of the symmetric capacity without feedback and the amount of symmetric feedback. This constant is obtained as a function of the number of users, K. We note that for the special case of Gaussian IC with K = 2, our proposed achievability scheme results in a symmetric rate that is within at most 21.085 bits/s/Hz of the outer bound, which is the first constant gap bound despite the constant gap claim in [1]. The symmetric achievable rate is used to characterize the achievable generalized degrees of freedom, which exhibits a gradual increase from no feedback to perfect feedback in the presence of feedback links with limited capacity.

K

In this paper, we study the capacity regions of two-way diamond channels. We show that for a linear deterministic model the capacity of the diamond channel in each direction can be simultaneously achieved for all values of channel parameters, where the forward and backward channel parameters are not necessarily the same. We divide the achievability scheme into three cases, depending on the forward and backward channel parameters. For the first case, we use a reverse amplify-and-forward strategy in the relays. For the second case, we use four relay strategies based on the reverse amplify-and-forward with some modifications in terms of replacement and repetition of some stream levels. For the third case, we use two relay strategies based on performing two rounds of repetitions in a relay. The proposed schemes for deterministic channels are used to find the capacity regions within constant gaps for two special cases of the Gaussian two-way diamond channel. First, for the general Gaussian two-way relay channel, the capacity within a constant gap is achieved with a simpler coding scheme as compared with the prior works. Then, a special symmetric Gaussian two-way diamond model is considered and the capacity region is achieved within four bits.

-User Interference Channels With Limited Feedback

We consider a generic problem of multicast beamforming design in multicell networks where each base station (BS) has multiple independent messages to multicast and each user intends to decode an arbitrary subset of messages sent from all BSs using successive group decoding (SGD). We first formulate the total transmit power minimization problem subject to the constraints that a target rate vector is achievable by the SGDs at all receivers. This problem is a non-convex quadratically constrained quadratic program and NP-hard. We propose a new method based on solving a sequence of linearly regularized semi-definite programming (SDP) relaxation of the original problem that yields feasible and near-optimal solutions with high probability. Moreover, we propose a decentralized algorithm based on the alternating direction method of multipliers to solve each linearly regularized SDP, which consists of solving a quadratic program at the central controller, and closed-form analytic computations at each BS. Finally, we propose an iterative procedure for joint beamformer and rate optimization under the SGD framework. Numerical results confirm the superiority of the proposed beamformer design in both performance and complexity. It is also demonstrated that, compared with the traditional linear receivers, the SGD receivers achieve both significant rate improvement and energy savings.

On the Capacity Regions of Two-Way Diamond Channels

We consider content delivery to users in a system consisting of a macro base station (BS), several energy-harvesting small cells (SCs), and many users. Each SC has a large cache and stores a copy of all contents in the BS. A user’s content request can be either handled by the SC for free if it has enough energy, or by the BS that has a cost. Each user has a finite cache and can store some most popular contents. We propose an efficient coded content caching schemes and an optimal transmission schemes for this system to maximally off-load the data traffic from the macro BS to the energy-harvesting SCs, and therefore minimize the power consumption from the grid. Specifically, the proposed coded caching scheme stores fractions of some most popular contents, such that contents requested from multiple users can be simultaneously delivered by the BS or SC. Moreover, the optimal transmission policy is formulated and solved as a Markov decision process. Extensive simulation results are provided to demonstrate that the proposed coded caching and transmission schemes can provide significantly higher traffic off-loading capability compared with systems with no caching or with uncoded caching, as well as systems that employ heuristic-based transmission schemes.

Multicast Beamforming Design in Multicell Networks With Successive Group Decoding

We consider an energy harvesting point-to-point communication system where the transmitter is powered by an energy arrival process and is equipped with a finite battery of size Bmax, which has limited efficiencies for storing energy into the battery and withdrawing energy from the battery. We assume a discrete i.i.d. energy arrival process where at each time step, energy of size Ai is harvested with probability pi, ∀i ∈ {1, 2;..., K}, independent of the other time steps. We provide upper and lower bounds on the information-theoretic capacity of this channel. These bounds are within a constant gap for K = 2 and a special case (with perfect battery efficiencies) for K = 3.

Traffic Off-Loading With Energy-Harvesting Small Cells and Coded Content Caching

We consider an energy harvesting point-to-point communication system where the transmitter is powered by an energy arrival process and is equipped with a battery of finite capacity Bmax, which could be used for saving energy for future use. We assume a discrete i.i.d. energy arrival process where at each time step, energy of amount Ai is harvested with probability pi Vi ∈ {1, 2, .. ., K} independent of the other time steps. We provide upper and lower bounds on the capacity of this channel. These bounds are shown to be within a constant gap for K ≤ 3 for all parameters, and for K > 3 when the battery capacity Bmax is small or large enough, where this constant does not depend on any energy or battery parameters.

Energy harvesting communication using limited battery with efficiency

Two-way relay is potentially an effective approach to spectrum sharing and aggregation by allowing simultaneous bidirectional transmissions between source–destinations pairs. In this study, the two-way 2 × 2 × 2 relay network, a class of four-unicast networks, where there are four source/destination nodes and two relay nodes, with each source sending a message to its destination, is studied. They show that without relay caching the total degrees of freedom (DoF) is bounded from above by 8 / 3 , indicating that bidirectional links do not double the DoF (it is known that the total DoF of one-way 2 × 2 × 2 relay network is 2). Further, they show that the DoF of 8 / 3 is achievable for the two-way 2 × 2 × 2 relay network with relay caching. Finally, even though the DoF of this network is no more than 8 / 3 for generic channel gains, DoF of 4 can be achieved for a symmetric configuration of channel gains.