## 1 Introduction  
Cellular systems are targeting aggressive spectrum reuse via universal frequency reuse to achieve high spectral efficiency and simplify frequency planning [1, 2, 3, 4, 5]. However, universal frequency reuse leads to unacceptable interference level at cell borders, i.e., the quality of service (QoS) remarkably depends on user location. To control this, cellular systems adopt several interference mitigation techniques to achieve uniform levels of QoS. Among these, network multiple-input multiple-output (MIMO) is used to mitigate the serious co-channel interference (CCI) and improve cell-edge performance. With network MIMO, multiple geographically separated base stations (BSs) collectively serve their cell-edge users (CEUs) using their antennas. These collaborative BSs can together serve either each individual CEU or a group of their CEUs during cooperative network MIMO transmission, which are called the single-user (SU) and multi-user (MU)-network MIMO, respectively. Network MIMO is referred to as coordinated multi-point (CoMP) in the Long-Term Evolution-Advanced (LTE-A) [6]. Another key technique to combat the serious interference is inter-cell interference coordination (ICIC) [7, 8]. Among a variety of ICIC strategies, the soft frequency reuse (SFR) and fractional frequency reuse (FFR) are widely adopted [9, 10, 11]. Both schemes assign a frequency reuse factor of one for cell-center users (CCUs) and a larger frequency reuse factor for CEUs. In LTE-A cellular systems, FFR has been used to keep the inter-cell interference at cell edges as low as possible. Combining both network MIMO and FFR-based ICIC can have the advantages of complexity reduction and throughput enhancement [12]. In addition, the studies of network MIMO on top of the FFR-based cellular system have drawn increasing interest [11; 12; 13; 14; 15]. It has been shown that using a 3-cell coordinated network MIMO with tri-sector frequency partition can outperform the 7-cell one with omni-directional antennas [12]. The 3-cell coordinated network MIMO architectures have been particularly adopted by the IEEE802.16m WiMAX standard body, which has specified that the default number of neighboring cells in the collaborative network MIMO transmission is three. The FFR-related research was generally associated with the orthogonal frequency-division multiple access (OFDMA) systems [11; 13]. In this paper, we consider two FFR-based frequency reuse methods for 3-cell network MIMO with tri-sector cells in OFDMA-based downlink cellular systems. They are regular and rearranged frequency partitions as shown in Figure 1. In the conventional regular one, the adjacent sectors of neighboring cells are with different frequency bands [13; 14; 16]. On the contrary, the rearranged frequency partition has the adjacent sectors of neighboring cells use the same frequency band. Thus, the rearranged frequency partition would yield serious CCI for network MIMO transmission. Regular and rearranged frequency partitions are associated with SU- and MU-network MIMO, respectively, to realize multi-cell cooperation. In this paper, we investigate the performance comparison between SU- and MU-network MIMO. In OFDMA systems, a collaborative BS can together serve its own users and the CEUs of other partner BSs under a transmitter power constraint. As a result, power management among three types of users (i.e., intra-cell CCUs, intra-cell CEUs, and inter-cell CEUs) becomes necessary. To the best of our knowledge, however, there has been no related literature dealing with this issue. Accordingly, we will develop methods of managing power to help raise the signal strength of inter-cell CEUs and in the meantime gratify the performance of intra-cell users. The rest of the paper is organized as follows. Tri-sector FFR-based frequency partitions (regular and rearrange) are introduced in Section 2. In Section 3, we present the signal model. Joint preprocessing techniques to eliminate serious CCI for MU-network MIMO are discussed in Section 4.