Tarik Akbudak

An LTE-femtocell dynamic system level simulator

A dynamic system level simulator for LTE networks was developed for investigating the interference behaviour of femtocells placed within macrocells. We simulate a multi-cell, multi-user and multi-carrier system in the downlink for Single-Input, Single-Output (SISO) and Multiple-Input, Multiple-Output (MIMO) antenna configurations.

Distributed power control and scheduling for decentralized OFDMA networks

In this paper, the downlink power control problem in decentralized orthogonal frequency division multiple access (OFDMA) networks is addressed. First, the optimization problem which minimizes the overall transmission power subject to individual rate and power constraints in a decentralized network is considered. An optimal solution requires global link information. However, obtaining such information is almost impossible. This paper introduces a power control and scheduling algorithm which can be implemented in a distributed manner. This algorithm delivers suboptimal solutions, since it is based on local information. However, simulation results show that the suboptimal solutions obtained through this algorithm are close to the global optimum, as long as it exists. Furthermore, it is shown that the obtained solutions are very similar to the ones obtained through a central controller.

MIMO capacity of class a impulsive noise channel for different levels of information availability at transmitter

In this paper, we analyze the capacity of a multiple input multiple output (MIMO) channel with impulsive noise interference. The Middleton class A model of electromagnetic interference has been employed to model the power line communication (PLC) channel. PLC can utilize MIMO to increase data rate and performance gain. However, an expression for the channel capacity of MIMO power line channel with Middleton class A noise has not yet been published. The expression for channel capacity of the MIMO power line channel is derived in two levels of information availability at transmitter. The channel state information is considered to be known at receiver. Numerical results are given for some parameters of the channel as well as a comparison of the capacity with and without channel state information at transmitter.

CoMP in heterogeneous networks: A joint cooperative resource allocation approach

Coordinated multipoint transmission (CoMP) is considered for a heterogeneous network where different kinds of base stations (BS) are located. We focus on linear transceiver design including user scheduling with the goal of maximizing the weighted sum rate in the downlink of a multiple-input multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) system, where each subchannel is allocated to a single user. Since the optimal solutions are extremely computationally complex to obtain, we propose a suboptimal joint resource allocation algorithm with a lower overall complexity. The proposed algorithm is able to take various linear power constraints into account, and simulation results indicate that using the proposed method achieves near-optimal solutions.

CoMP in heterogeneous networks: A low-complexity linear transceiver design

Coordinated multipoint transmission (CoMP) is considered for a heterogeneous network where different kinds of base stations (BS) are located. We focus on linear transceiver design with the goal of maximizing the weighted sum rate in the downlink of a multiple-input multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) system. The problem is shown to be non-convex, thus, the problem of finding the global optimum is an intrinsically intractable, NP hard problem. We propose an algorithm which divides the main problem into several convex subproblems, and, is shown to provide efficient suboptimal solutions.

CoMP in heterogeneous networks: Linear transceiver design using the minimum weighted geometric-MSE criterion

Coordinated multipoint transmission (CoMP) is considered for a heterogeneous network where different kinds of base stations (BSs) are located. We focus on linear transceiver design with the goal of maximizing the weighted sum rate in the downlink of a multiple-input multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) system, where each subchannel is allocated to a single user. We address this in the context of minimizing the weighted geometric mean of the mean-square-errors (MSEs), i.e., weighted geometric-MSE minimization. The problem is non-convex as in multi-user case, thus, the problem of finding the global optimum is an intrinsically intractable, NP hard problem. We propose an algorithm which divides the main problem into several convex subproblems. The proposed algorithm is shown to provide computationally efficient suboptimal solutions.

Symmetric capacity of multi-user MIMO downlink under per-base station power constraints

We consider a multiuser multiple-input multiple output (MIMO) Gaussian broadcast channel (BC). Assuming a sum-power constraint over all transmit antennas, the sum capacity of such a system can be achieved with dirty paper coding (DPC) which has been found as a very promising strategy. Rather than the usual sum-power constraint, in this paper we focus on the downlink of a cooperative multicellular network where each transmit antenna has an individual power constraint. This scenario is much more realistic than the case where a sum power constraint is considered, since in practical systems transmit antennas have their own power amplifiers. We extend this approach to the scenario where each base station (BS) is assumed to have an individual power constraint. We propose a capacity calculation algorithm which is able to take both per-BS and per-antenna power constraints into account.

A cross-layer resource allocation scheme for spatial multiplexing-based MIMO-OFDMA systems

We investigate the resource allocation problem for the downlink of a multiple-input multiple-output orthogonal frequency division multiple access (MIMO-OFDMA) system. The sum rate maximization itself cannot cope with fairness among users. Hence, we address this problem in the context of the utility-based resource allocation presented in earlier papers. This resource allocation method allows to enhance the efficiency and guarantee fairness among users by exploiting multiuser diversity, frequency diversity, as well as time diversity. In this paper, we treat the overall utility as the quality of service indicator and design utility functions with respect to the average transmission rate in order to simultaneously provide two services, real-time and best-effort. Since the optimal solutions are extremely computationally complex to obtain, we propose a suboptimal joint subchannel and power control algorithm that converges very fast and simplifies the MIMO resource allocation problem into a single-input single-output resource allocation problem. Simulation results indicate that using the proposed method achieves near-optimum solutions, and the available resources are distributed more fairly among users.

System level evaluation of cooperative MIMO-OFDMA-based heterogeneous networks

Coordinated multipoint transmission (CoMP) is considered for a heterogeneous network where different kinds of base stations (BSs) are located. We present a system level simulator for investigating the gains that can be achieved from the use of cooperative transmission (CoMP) in an urban heterogeneous cellular environment. It is shown that the spectral efficiency of the users located within a macro-cellular environment may significantly increase by employing low-power nodes (femtocells). On the other hand, the spectral efficiency of cell-edge users is decreased due to the additional inter-cell interference introduced by the use of such femtocells. Furthermore, it is shown that significant gain can be achieved by the cooperation of the BSs. From a users point of view, the inter-cell interference is reduced, and thus, the spectral efficiency per user can be significantly enhanced.

A low-complexity resource allocation scheme for single-user MIMO transmission

We investigate the resource allocation problem for the downlink of a spatial multiplexing (SM) based multiple input multiple output orthogonal frequency division multiple access (MIMO-OFDMA) based network. We address this problem in the context of the utility-based resource allocation presented in earlier papers in order to enhance the efficiency and guarantee fairness among users by exploiting multiuser diversity, frequency diversity, as well as time diversity. We design utility functions with respect to the average rate in order to simultaneously provide two services, real-time and best-effort. We propose a low-complexity joint subchannel and power control algorithm that converges very fast. Simulation results indicate that using the proposed method achieves near-optimum solutions and the available resources are distributed more fairly among users.