e Power (dBm) Co-MIMO Performance Gain over SU-MIMO Overall Interference FIGURE 10-36 CDF of interference power of the cell-edge users as a function of cell-edge length [26] CHAPTER 10 The IEEE 802.16m Physical Layer (Part II) If the interference from cells outside the collaborative zone is taken into consideration, the reduction in overall interference level is less noticeable, and can be further improved by extending the collaborative zone to contain more coordinated base stations. The cumulative distribution function of the cell-edge users within the cell-edge area as a function of cell-edge length for SU-MIMO and Co-MIMO are compared and shown in Figure 10-36. It is shown that the interference power increases with increasing cell-edge length (see Figure 10-35). There is a considerable reduction in the inter- ference power from cells within the Co-MIMO zone when using Co-MIMO techniques compared to the conventional SU-MIMO schemes. The performance gain decreases when comparing the overall interference [26]. 10.5 IEEE 802.16M DOWNLINK MIMO SCHEMES The IEEE 802.16m standard supports several advanced multi-antenna techniques including single and multi-user MIMO (spatial multiplexing and beamforming), as well as a number of transmit diversity schemes. The classification of the IEEE 802.16m downlink-MIMO schemes is depicted in Figure 10-37. In this section, a detailed description of these schemes and their advantages over the legacy schemes will be provided. As mentioned in earlier sections, in a single-user MIMO scheme only one user can be scheduled over one (time, frequency, space) resource unit. In a multi-user MIMO, on the other hand, multiple users can be scheduled in one resource unit. Vertical encoding (or single codeword) utilizes one encoder block (or layer), whereas horizontal encoding (or multi-codeword) uses multiple encoders (or multiple layers). For SU-MIMO vertical encoding is utilized, whereas for MU-MIMO horizontal or multi-layer encoding is employed at the BS, and up to two streams can be transmit