Cognitive Cooperation for the Downlink of Frequency Reuse Small Cells

Hindawi Publishing Corporation EURASIP Journal on Advances in Signal Processing Volume 2011, Article ID 525271, 11 pages doi:10.1155/2011/525271 Research Article Cognitive Cooperation for the Downlink of Frequency Reuse Small Cells Salam Akoum,1 Marie Zwingelstein-Colin,2 Robert W. Heath Jr.,1 and Merouane Debbah3 1 Wireless Networking and Communications Group, Department of Electrical and Computer Engineering, The University of Texas at Austin, 1 University Station C0803, Austin, TX 78712-0240, USA 2 IEMN/DOAE, UMR 8520, University Lille Nord de France, 59000 Lille, France 3 Ecole Superieure d’Electrecite (SUPELEC), Alcatel-Lucent Chair on Flexible Radios, 3 rue Joliot-Curie, 91192 Gif sur Yvette Cedex, France Correspondence should be addressed to Marie Zwingelstein-Colin, Received 1 June 2010; Revised 28 September 2010; Accepted 16 November 2010 Academic Editor: Robert Schober Copyright © 2011 Salam Akoum et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We develop a cooperative diversity protocol coded over space, time, and frequency to achieve improved quality of service for mobile users in the downlink of small-cell frequency reuse networks. The proposed protocol, called cooperative frequency reuse (CFR), leverages the cellular frequency reuse concept to create space and frequency diversity among pairs of adjacent base stations. The CFR protocol is compatible with the half-duplex mode and is distributed in the sense that each base station acts in autonomy, without the need of a centralized entity. It is implemented in two phases. During the first phase, each base station independently serves its own users on its dedicated frequency band. It simultaneously listens to the symbols transmitted by neighboring base stations. Cognitive cooperation is introduced in the second phase, where each base station transmits on two frequency bands to the scheduled users in both base stations, by means of an appropriately chosen distributed space time code based on the Golden code. We analyze and discuss the performance of the proposed protocol in terms of bit error rate, probability of outage, and ergodic sum rate under different scenarios. Simulation results show that the proposed protocol yields considerable improvement over direct transmission frequency reuse strategies. 1. Introduction Small-cell wireless networks provide increased capacity and higher area spectral efficiency [1–3]. The benefits reaped from these networks come, however, at the expense of increased cochannel interference, especially at the cell edge. Conventional cellular networks manage the interference problem by requiring adjacent base stations (BSs) to transmit on different frequency bands. This mechanism is called frequency reuse (FR). It increases the reliability of the cellular networks while at the same time incurring a poor spatial reuse of the expensive frequency spectrum [1]. Fractional frequency reuse (FFR) achieves a higher spatial reuse of the spectrum and is suggested for next generation cellular systems [4, 5]. It divides the frequency bands into subchannels, to be shared orthogonally among BSs to serve users that are interference limited. It maintains, however, universal frequency reuse in the cell center. Small cell networks encounter conflicting requirements between providing an increased area spectral efficiency and maintaining quality of service for their mobile users. One way to resolve this tradeoff is through combining frequency reuse with cooperation between adjacent BSs. Implementing a cooperation algorithm that leverages the FFR concept of cellular systems achieves the dual benefit of higher reliability and higher spectral efficiency. Cooperation in cellular networks, depending on the level of data and channel state information (CSI) shared between BSs, can be implemented in several ways [6–9]. Cooperative space diversity [6, 10] is one such method. It exploits spatial diversity by implementing a virtual antenna array between adjacent BSs, and distributed space-time codes can be constructed over the formed virtual array to increase the reliability of the system [11–13]. In this paper, we design a 2 cooperation protocol based on space and frequency diversity, for cooperation between a pair of adjacent BSs. The proposed protocol, called Cooperative Frequency Reuse (CFR), leverages the frequency reuse concept of cellular systems, creating a virtual multiple-input single-output (MISO) system based on the sharing of OFDM frequency bands among adjacent BSs. It can be applied to cellular systems that use FFR, such as WiMAX and LTE, in a straightforward manner. It is cognitive in the sense that the BSs use, opportunistically in time, the frequency bands allocated to their adjacent BSs to transmit to the mobile users, hence creating cognitive diversity on the downlink of the cellular system. In contrast to the cooperative multicell transmission strategies available in the literature [7, 14, 15], where base stations jointly process the downlink signals of the mobile users, thereby creating a multiple-input multiple-output (MIMO) broadcast channel, the proposed CFR protocol implements cooperation through a distributed space time code. It is thus especially suitable for mobile flexible networks [16], where the BSs have limited or no wired backhaul communication. The proposed strategy is different from the cooperative transmit diversity in the multihop relay specification for WiMAX, the IEEE 802.16j standard [17]. In the latter, distributed space time codes are implemented across antennas of the deployed relays and the BS, in the same cell, over the same time and frequency resources. It is also different from the shared relay concept proposed in IEEE 802.16m [9], where a relay is placed at the intersection of two or more cells, and used to decode the signals from the intersecting BSs. The CFR protocol also differs from other cooperative protocols proposed in the literature such as in [10–12, 18]. The latter protocols are applied for cooperation among mobile nodes on the uplink of cellular systems and can be used for communication among terminals in adhoc networks. Assuming a half-duplex mode, whereby nodes cannot transmit and receive at the same time on the same frequency band, the CFR protocol is implemented, for a pair of adjacent BSs, in two phases. During the first phase, each BS serves its own users in a protected band, orthogonal to the frequency bands that the adjacent base stations transmit on. The BS listens, during the same phase, to the signal sent by the cooperating BS on another frequency band. The underlying assumption here is that the wireless link between the pair of adjacent BSs is reliable, which is generally the case in practice, when the BSs have a line of sight channel. In the second phase, the BSs divide their (...truncated)