In this paper, we investigate underlay device-to-device (D2D) communication in cellular networks where D2D transmitters transmit data to their receivers using cellular spectrum. We analyze the average sum throughput of D2D receivers in interference-limited channels. To enhance it, we propose a semi-distributed spectrum access scheme, which consists of two stages. In the first stage, the cellular base station divides whole D2D transmitters into multiple groups and assigns a different subchannel to each group. In the second stage, D2D transmitters in each group randomly access the subchannel assigned to it with predetermined access probability. We formulate an optimization problem to find groups and access probabilities, which maximize the average sum throughput. To overcome the prohibitive computational complexity to obtain its optimal solution, we decompose it into two subproblems: one to find groups and one to find access probabilities. A heuristic grouping algorithm is adopted to solve the former, and a branch-and-bound-based algorithm is proposed to solve the latter with achieving a target accuracy. The validity of the branch-and-bound-based algorithm is shown by performance comparison with an exhaustive search. It is shown that the heuristic grouping and the branch-and-bound-based algorithm achieve higher average sum throughput than conventional methods.

We propose a resource allocation scheme for the downlink of a two-tier wireless network in which femto-base stations are used as relays to macro- users. In the proposed scheme, subcarriers and relays are assigned to users assuming equal power allocation by all base stations to all subcarriers, and then, power is allocated to users. Simulation results show that the proposed scheme achieves higher sum rate of users than the random subcarrier assignment (RSA) scheme, more fair resource allocation to users than the greedy scheme, and lower total transmit power of base stations than the greedy and the RSA schemes.

In this paper, we analyze the outage probability and average throughput of opportunistic relaying with ARQ when the outdated channel state information (CSI) is available for relay selection. Its diversity order is analyzed to show that it is determined by the number of relays and the maximum number of ARQ rounds. Numerical results verify our analysis by comparison with computer simulation and show the effect of outdated CSI on its outage probability, diversity order, and average throughput.

Femtocell is one of promising key technologies to increase the efficiency and coverage of next generation broadband wireless networks. However, there exists interference problem between femtocells and macrocell due to co-channel operation. In this paper, we propose an efficient frequency resource allocation scheme for newly deployed femtocells in the legacy OFDMA networks in order to maximize the throughput of both macrocell/femtocell users without severe degradation to the existing macrocell users. Under the pre-allocated frequency bands for the macrocell users with the sectored-FFR is given, the proposed scheme allocates sub-bands to the femtocells efficiently to consider macrocell users having a priority over femtocell users. Simulation results show that the proposed one obtains higher total/edge throughputs than compared schemes.