Location-based spectrum allocation and partitioning scheme for cross-tier interference mitigation in macro-femtocell networks

Abstract

In this paper, we address the cross-tier interference problem in a two-tier network which consists of overlaid femtocells within a macrocell range. In the considered network, macro and femto base stations are assumed with the capabilities of power control and beamforming, differently from user terminals. Under such environments, we devise a decentralized solution where each femtocell determines whether to use shared spectrum or partitioned spectrum, considering users’ location information. Introducing two simple distance thresholds, we can optimally partition the spectrum and allocate them without detailed channel information. Simulation results demonstrate that our decentralized scheme achieves comparable performance to the centralized one with low overhead and feedback delay.

Introduction

Femtocells have received great attention as a cost-effective high-bandwidth solution for next-generation wireless services. Femtocell networks can enhance indoor coverage by providing low power short range access points towards a wired infrastructure network. They operate in the licensed spectrum owned by a mobile operator and currently provides mobile convergence services in 3GPP LTE networks [1]. It has been shown that the femtocell approach is beneficial to both the service provider and users in many aspects such as network capacity, cell coverage, and battery consumption at user handsets [2].

The coexistence of macrocell and femtocell networks at a same spectrum, however, incurs additional control challenges due to the cross-tier interference between macrocell and femtocells as well as the co-tier interference. Although the co-tier interference control has been extensively studied in the literature [3], [4], [5], [6], [7], [8], the cross-tier interference control still has been an open problem and remains as a key technical challenge [2], [9].

Studies on the two-tier network have often assumed operator planned deployment of the microcell network where the cross-tier interference is somewhat tractable. They mainly have focused on uplink capacity in an overlaid macro and microcell network under code division multiple access (CDMA) environments [10], [11]. However, these approaches suffer from high deployment cost and low spatial spectrum reuse. To overcome these weaknesses, a low power user deployed femtocell network architecture has emerged as an attractive alternative [9]. Since femtocell networks are commonly deployed by subscribers, femto base stations (HeNBs) can be arbitrarily placed, which makes the cross-tier interference problem more challenging.

When macro and femtocell networks coexist, it is desirable for the femtocells to generate minimal impact on the performance of the macrocell networks [1], [12]. To do so, an intuitive approach is assigning non-overlapped spectrums to the macrocell and femtocells, respectively [13]. However, this separate spectrum allocation limits spectrum bandwidth and accordingly degrades spectrum efficiency. The spectrum efficiency can be significantly improved by the cochannel deployment where macrocell and femtocells use the same spectrum [14].

With the goal of improving performance of both macrocell and femtocell users under the cochannel deployment, an MIMO beam subset selection strategy at microcells has been proposed in [15]. It maximizes the macrocell throughput by optimizing the trade-off relationship between multiplexing gain and multiuser interference, and also suppresses the cross-tier interference with a reduced number of beams. In [16], the authors have solved beamforming optimization problems that deal with total transmit power minimization, mean-square error balancing, and interference minimization. A centralized beamforming strategy that adaptively changes beam patterns and controls the transmit power at each cell is proposed in [17]. However, these cochannel deployments are still exposed to the cross-tier interference problem.

In [18], the authors have developed a way of hybrid spectrum use. This approach groups femtocells according to a distance threshold from the macro base station (MeNB) first. Differently from the macrocell, a group of femtocells use a partitioned spectrum, and the other group use the same spectrum with the macrocell. In [19], the cochannel operation is allowed for the femtocells that are distant from the MeNB. To classify such cochannel HeNBs, the interference-limited coverage area has been estimated. However, these solutions are limited to the case of static network settings, and operate in a centralized manner that incurs significant cross-tier feedback delay. Therefore they are not suited for dynamic network environments where users actively join and leave the network. To reflect such dynamics, the frequency spectrum should be allocated in a short time without using detailed channel information.

In this paper, we develop a dynamic spectrum allocation and partitioning scheme according to users’ location information to mitigate the cross-tier interference. Considering the beamforming gain, we design a novel decentralized scheme that partitions spectrum and decides which HeNBs to use the shared spectrum without requiring users’ detailed channel information. The main contributions of this paper are three fold.

• We analytically obtain the average gain of beamforming transmission in a macro-femtocell network and formulate spectrum partitioning as a network utility maximization problem.

• We develop a low-complexity location-based solution to the spectrum allocation problem that requires minimal cross-tier feedback information. Then we find an optimal spectrum partitioning ratio to maximize the total cell utility.

• We confirm that our proposed scheme shows comparable performance to the centralized scheme that is impractical due to long cross-tier feedback delay.

The rest of this paper is organized as follows. In Section 2, we describe the system model and formulate the problem of spectrum allocation and partitioning as a network utility maximization problem. In Section 3, we consider a centralized spectrum allocation and partitioning scheme as a reference scheme. It finds a best set of shared femtocells but it is impractical due to cross-tier feedback delay. Then we develop a decentralized location-based scheme that uses distance thresholds, and find an optimal shared spectrum ratio to maximize the cell utility. In Section 4, we evaluate our decentralized location-based schemes through extensive simulations, and conclude our paper in Section 5.