Generalizing and optimizing fractional frequency reuse in broadband cellular radio access networks

For broadband cellular access based on orthogonal frequency division multiple access (OFDMA), fractional frequency reuse (FFR) is one of the key concepts for mitigating inter-cell interference and optimizing cell-edge performance. In standard FFR, the number of OFDMA sub-bands and the reuse factor are fixed. Whereas this works well for an idealized cell pattern, it is neither directly applicable nor adequate for real-life networks with irregular cell layouts. In this article, we consider a generalized FFR (GFFR) scheme to allow for flexibility in the total number of sub-bands as well as the number of sub-bands in each cell-edge zone, to enable network-adaptive FFR. In addition, the GFFR scheme takes power assignment in consideration. We formalize the complexity of the optimization problem, and develop an optimization algorithm based on local search to maximize the cell-edge throughput. Numerical results using networks with realistic radio propagation conditions demonstrate the applicability of the GFFR scheme in performance engineering of OFDMA networks. Introduction Orthogonal frequency division multiple access (OFDMA) is a current technology for broadband radio access. In OFDMA, the radio spectrum is split into a large number of channels, referred to as sub-carriers. Data transmission is performed simultaneously over multiple sub-carriers, each carrying a low-rate bit stream. OFDMA is very flexible in exploring multi-user diversity with high spectrum efficiency and scalability. The technique is part of the downlink air interface in the fourth generation cellular systems based on 3GPP long term evolution (LTE) standards (see [1]), and IEEE 802.16 WiMAX (see [2]). Orthogonal frequency division multiple access subcarriers are orthogonal to each other. As a result, intracell interference is not present. Inter-cell interference, on the other hand, becomes a performance-limiting factor. For this reason, interference mitigation has become an important topic in performance engineering of OFDMA networks. *Correspondence: [email protected] 1Department of Science and Technology, Campus Norrköping, Linköping University, SE-60174, Norrköping, Sweden Fractional frequency reuse One sub-carrier allocation scheme in multi-cell OFDMA networks is frequency reuse with factor one (reuse-1), in which the entire spectrum is made available in all cells. Here and throughout the article, a reuse factor of X means that the spectrum block in question is used in every group of X cells. Users that benefit from the reuse-1 scheme are those located close to a base station antenna, i.e., users in the cell center. Because the channel condition is good and the interference from other cells is relatively low, the throughput in cell-center zones grows by bandwidth. For users located at cell-edge areas, however, the performance typically suffers severely in the reuse-1 scheme, because of the high interference from the surrounding cells in relation to the signal of the home cell. In other words, celledge zones are much more sensitive to interference than bandwidth. Previous studies (e.g., [3]) indicate that, if the overall system throughput is the only performance target, then reuse-1 is the best choice. At the same time, the resulting performance in the cell-edge zones tends to be unacceptably low. To balance the performance of cell center against that of cell edge, interference avoidance and mitigation techniques have been investigated in recent years. One of the key concepts specifically addressing the performance of © 2012 Chen and Yuan; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Chen and Yuan EURASIP Journal onWireless Communications and Networking 2012, 2012:230 Page 2 of 15 http://jwcn.eurasipjournals.com/content/2012/1/230 cell edge is fractional frequency reuse (FFR), see, for example, [4-6]. In FFR, the service area of every cell is split into a center zone and an edge zone. The spectrum is correspondingly partitioned into two parts. One part is allocated with reuse-1 in all cell-center zones. The second part is further split into sub-bands. These sub-bands, to be used in the cell-edges zones, have a higher reuse factor. As a result, significant interference reduction is achieved in cell-edge zones. In standard FFR, derived for an ideal network layout with hexagonally shaped cells, the edge band is split into three sub-bands, each with a reuse factor of three (reuse-3); every edge zone is allocated one of the three sub-bands, see Figure 1 for an illustration. For the cell layout in Figure 1, the standard sub-band allocation pattern with reuse-3 is very intuitive. The allocation pattern ensures that the sub-band of a cell-edge zone is not reused in any of the neighboring cells. In reallife cellular networks, however, the amount of interference is very irregular over the service area. The cells differ greatly in the number of significant interferers as well as the respective amounts of interference, causing difficulties in applying standard FFR. As the number of surrounding cells varies from one cell to another, allocating the sub-bands optimally is not straightforward. For the same reason, a single reuse factor, if applicable at all, is no longer optimal. In addition, because the sensitivity to interference varies by cell, the allocation of one sub-band per edge zone may not be adequate. Finally, scalability becomes an issue, because it is not optimal to replicate the allocation pattern of one part of the network to another. A generalized FFR scheme In this article we present and evaluate a generalized FFR (GFFR) scheme, in order to overcome the limitations of standard FFR in dealing with real-life networks with irregular cell layout. As a result, the allocation pattern is adapted to the characteristics of each individual network. The GFFR scheme that we consider extends the standard one in three aspects. First, the frequency band used for cell-edge zones can be partitioned into any number of sub-bands. Doing so is potentially useful for interference avoidance in cells with many surrounding interfering