TAILIEUCHUNG - Wireless Communications over MIMO Channels phần 7

Xem xét trường hợp hai người dùng, cả hai tín hiệu truyền xu [k] được cho là giống hệt nhau, quyền hạn Es / Ts. Các ứng dụng đồng thời (5,44) cho tất cả các biểu tượng au, 1 ≤ u ≤ Nu, cũng gọi là thuật toán Jacobi và được gọi là | CODE DIVISION MULTIPLE ACCESS 203 user u are obtained by r Z CT Z HH Z Hu Z Cu Z a Z n Z . Although this approach maximizes the SNR and perfectly exploits diversity it does not consider MUI which dramatically limits the system performance Dekorsy 2000 Kaiser 1998 . The diagonal matrix HH Z Hu Z between cu Z and Cu Z in destroys the orthogonality of the spreading codes because the chips of the spreading codes are weighted with different magnitudes. The performance degradation is the same as in single-carrier CDMA systems. Orthogonal restoring combining ORC The influence of MUI can be easily overcome in OFDM-CDMA systems. Restoring the orthogonality is possible by perfectly equalizing the channel also known as ZF solution Fazel and Kaiser 2003 . In OFDM-based systems this is easily implemented by dividing each symbol in y Z with the corresponding channel coefficient. With H-1 Z diag H 1 Z 0 H 1 l Nc - 1ỊỊ we obtain ru Z eOrc Z y Z cu Z H-1 Z h Z C Z a Z n Z CT Z C Z a Z CT Z H-1 Z n Z . If the partial spreading codes cu Z Ẫ. of different users are mutually orthogonal CT Z C Z Onj x u-1 Nb lNb Nbx Nu-u Nb holds. Hence the multiplication with CT Z suppresses all users except user u and becomes ru Z au Z CT Z H-1 Z n Z . We see that the desired symbols au Z have been perfectly extracted and only the modified background noise disturbs a decision. However this same background noise is often significantly amplified by dividing through small channel coefficients leading to high error probabilities especially at low SNRs. This effect is well-known from ZF equalization Kammeyer 2004 and linear multiuser detection Moshavi 1996 . A comparison with the linear ZF detector in Subsection on page 234 shows the following equivalence. For a fully loaded system with Ns Nu C Z is an orthogonal Nu x Nu matrix. Neglecting time indices the ZF criterion delivers with S HC E SH S 1 SH C-1H-1H-h C-h Ch Hh CT H-1. Obviously coincides with EOrc Z in

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