me cases. The same drawback is experienced by linear precoding, which alleviates noise by boosting the transmit power. The Tomlinson-Harashima Precoding (THP) algorithm is a non-linear precoding technique orig- inally developed for multipath SISO channels. The THP algorithm is performed by moving the feedback part of the decision-feedback equalizer to the transmitter. It has also been applied for the pre- equalization of multi-user interference in MU-MIMO systems; hence, no error propagation occurs and the precoding can be performed for the interference-free channel. The MMSE precoding in combi- nation with THP is another approach to balance the multi-user interference in order to reduce the performance loss that occurs with zero interference techniques, while THP is used to reduce the multi- user interference and to improve the diversity. Space-Division Multiple Access (SDMA) is a special case of MU-MIMO where several users share the same time-frequency physical resource block due to their spatial separation. SDMA uses antenna beam direction or angle (using directional antennas) as another dimension in signal space which can be channelized and assigned to different users (see Figure 10-34). Orthogonal channels can be assigned only if the angular separation between users exceeds the angular resolution of the directional antenna. In practice, SDMA is often implemented using sectorized antenna arrays. The 360° angular space is divided into a number of sectors and in each sector a highly-directional antenna array is used to minimize the inter-user interference. The users within each sector can be served using multiple access schemes such as OFDMA, CDMA, etc. The SDMA system adapts to the mobility of the users by beam- steering or assigning the users to a new channel when the user changes its geographical location. 10.4 Multi-antenna transmission schemes User 3 User 2 Channel User 1 User k FIGURE 10-34 An illustration of the SDMA concept 10.4.6 Collaborative MIMO and Collaborative Spatial Multiplexing Collaborative s