tailieunhanh - Recent Advances in Wireless Communications and Networks Part 7

Tham khảo tài liệu 'recent advances in wireless communications and networks part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 170 Recent Advances in Wireless Communications and Networks r K Nc E W ZZLPk i i uwb w1 2 L J k-1 i-1 i-1 where z b is the signal transmitted over the Nc subcarriers and Wb k i of size Nfb X N represents the precoder of user k on subcarrier l at BS b . Centralized precoder vectors In this section we consider the SVD based precoding algorithm similar to the one proposed in Yu et al. 2004 . We assume that Nt Nr. Briefly we define Hk l as the following Nr - N X Nt matrix H k i H1 i .Hk-1 i Hk 1 i T 3 Tf 1 . 1 r T T T .1 .1 11 r T T 11 r -XT T -XT If we denote rank of Hk i as Lki then the null space of Hk i has dimension of Nt - Lki N . The SVD of H k is partitioned as follows H ũ D r Vi0 V 1 1 4 Hk i ũk iDk i _Vk i Vk i where Vk i holds the Nt - Lk i singular vectors in the null space of H k i. The columns of Vk0 are candidate for user k precoding matrix Wk i causing zero gain at the other users hence result in an effective SU-MIMO system. Since Vk0 potentially holds more precoders than the number of data streams user k can support an optimal linear combination of these vectors must be found to build matrix Wkl which can have at most N columns. To do this the following SVD is formed H V 0. ũ D rv 0 V 1 1 5 Hk ivk i ũk iDk i _vk i vk i where Dk i is Lki X Lki and v represents the Lk i singular vectors with non-zero singular values. The Lk i N columns of the product VkpV 1 represent precoders that further improve the performance subject to producing zero inter-user interference. The transmit precoder matrix will thus have the following form W rVYM i V P V .1 1P1 2 W P1 2 6 W-LV1 i V1 i - VK iVK i Pi WP The global precoder matrix with power allocation W1 - W1 . WK11 P11 2 as computed above block-diagonalizes the global equivalent channel H . H W - diag He 1 p. He Kl and the interference is completely removed considering perfect CSI. Let us define H Ht Wt - Ht Wt Ph 2 of size N X N as the equivalent enhanced e k 1 k 1 k 1 k 1 k 1 k 1 rk r T- channel for user k