 pilot and data sub-carriers. The number of used pilot and data sub-carriers depends on the MIMO mode, number of streams, and number of users, as well as the type of subframe. To better understand the difference between the localized and distributed allocations, consider the example shown in Figure 9-21. The frequency response of a multipath fading channel varies with time and frequency due to path loss, shadowing, and user mobility effects. Let's assume that user A and user B are two users with different channel conditions. The BS receives channel quality reports (typically in the form of CINR or SINR measurements) from the mobile stations in the cell. The BS scheduler may adopt either of the following resource allocation schemes depending on the channel condition reports and other considerations that will be discussed later in this chapter. One allocation strategy is to allocate user A and user B in the subchannels where the corresponding SINR is the best for that user. This is when a frequency-selective scheduling gain can be achieved through allocation of a group of physically adjacent sub-carriers in the subchannels with the relatively best SINR for this user. Another strategy is to permute the sub-carriers over the entire channel and form a logical group of distributed sub-carriers, and to assign them to a user. In this case, frequency diversity gain can be achieved through the use of distributed resource units for this user. The procedure for forming logical resource units from physical resource units can be described as follows. The PRUs are first subdivided into sub-bands (SB) and mini-bands (MB) where a sub-band 9.8 Subchannelization and permutation Signal Level for the Sub-channel Sub-carriers Allocated to User A Loading Gain Average Signal Level over the Entire Channel Sub-carriers Allocated to User A Frequency Frequency Sub-carriers Allocated to User B Sub-carriers Allocated to User B Frequency Frequency Combined OFDMA Signal Combined OFDMA Signal Frequency Frequency Channel Frequency Response Some S