main where n = 4 and m=5. The above criteria can be generalized to the case of multiple transmit antennas where each transmit antenna (alternatively known as antenna port) is associated with a group of pilot sub-carriers. In the case of irregular pilot patterns, the pilot symbols can be irregularly placed in time, in frequency, or in both. The irregular patterns can be chosen according to certain criteria, each yielding 9.9 Pilot structure and channel estimation Pilot Sub-carrier Time (OFDM Symbols) FIGURE 9-32 An example of a regular pilot pattern for one spatial stream with n = 4 and m = 5 a specific pattern. Other important considerations in the design of pilot structures for support of multi- antenna OFDMA systems include the following: Identical pilot pattern for each physical resource block: pilot spacing in time and frequency must be in proportion to the resource block size. If the location of the pilot tones within a resource block is not maintained the same across all resource blocks within the system bandwidth, or alternatively the pilot sub-carriers have different positions within each resource block, the filtering and interpolation operations during channel estimation become excessively complex. Pilot density: proper pilot overhead must be considered as a trade-off between accurate channel estimation under various mobility conditions and higher throughput. Types of pilots: pilots are typically classified as common and dedicated. Each MS in the cell can estimate the channel over the entire bandwidth using the common pilots, while dedicated pilots in the MS-assigned resource block can be used for channel estimation over a fraction of the total bandwidth. In conventional MIMO schemes, the dedicated pilots are typically adaptively precoded. The dedicated pilots can be used in conjunction with common pilots or replace the common pilots. Pilot power boosting: in general, the power of pilot sub-carriers is boosted relative to data sub-carriers in order to enhance the channel estimation with pilot hopping or 