e MIMO scheme and type of CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) resource unit. The subsets of the base codebooks for open-loop SU-MIMO are optimized for performance in diversity allocations for both spatially correlated and uncorrelated channels, with the additional constraints of small-sized sets with nested property among the codebooks of different ranks, and containment of matrices with elements of constant modulus, in order to reduce the implementation complexity at the MS and BS. The subsets of the base codebooks for open-loop MU-MIMO are selected to provide sub-band selection gain and are composed of matrices with elements of constant modulus. The choice of the precoder from subsets of the base codebook does impact on the perfor- mance, especially in correlated channels with distributed allocations and single-stream transmissions, thus optimization of the open-loop precoders is necessary. For adaptive precoding, the form and derivation of the precoding matrix Wk= specific to implementation and is not specified in the standard. The precoding vector Wjk at the kth sub- carrier and the jth stream is derived at the BS using the feedback received from the MS. Beamforming is enabled with this precoding mechanism. If the columns of the composite precoding matrix are orthogonal to each other, it is defined as unitary precoding. Otherwise, it is defined as non-unitary precoding. Non-unitary precoding is only allowed with closed-loop MU-MIMO. In the downlink closed-loop SU-MIMO and MU-MIMO, all demodulation pilots are precoded similar to the data sub-carriers, regardless of the number of transmit antennas, allocation type, and MIMO transmission mode. The precoding matrix is signaled to the MS via precoding of the demodu- lation pilots. The IEEE 802. 16m specifies six downlink MIMO transmission modes as listed in Table 10- 21. The associated MIMO mode parameters and usage are given in Table 10-22 and Table 10-23, respectively. All downlink MIMO modes support either distributed or localized resource per