Sau đó, hình thức một nhóm mã trên chòm sao điều chếTrong ví dụ trên, nó được giả định rằng. Nhìn chung, chiều dài từ mãgian có thể được lớn hơn hoặc bằng nT. Sự khác biệt giữa mã hóa / giải mã nguyên tắc đối với đề án điều chế đơn nhất không-thời gian thảo luận trong phần trước có thể được áp dụng cho không gian | Unitary Group Codes 243 Example For nT L 2 let D -1 1 1 1 ơ jr1 Mi 0 Ml j Mi0 j n 17 871 Q lL 0 1 Ju -1 0 J L 0 -j Ju j 0 J Then DQ forms a group code over the QPSK modulation constellation A 1 j -1 -j . In the above examples it is assumed that L nT .In general the space-time codeword length L can be greater than or equal to nT. The differential encoding decoding principles for unitary space-time modulation schemes discussed in the previous section can be applied to the space-time unitary group codes. The differential transmission scheme for a space-time unitary group code is illustrated in Fig. . At the t-th encoding block log2 G bits are mapped into the group code Q and they select a unitary matrix Gzt where zt e 0 1 2 . IQ - 1 . To initialize the differential transmission X0 D is sent from nT transmit antennas over L symbol periods. The differential encoding rule is given by 9 Xt Xt-1 Gzt The group structure ensures that Xt e AnTxL if Xt-1 e AnTxL. The received signals for the t-th transmission block are represented by an nR X L matrix Rt. The differential space-time decoding based on the current and previous received signal information Cl c2 c3 space-time modulation VZ1 VZ2 Vz3 differential modulation xo Xi VZ1 X2 VZ1VZ2 x3 VZ1VZ2VZ3 Figure A differential space-time group code Figure A differential space-time receiver 244 Differential Space-Time Block Codes matrices is given by 9 it arg max ReTr RZ-1GzR l Q argmax ReTr Gz Rf Rt-1 leQ where ReTr denotes the real part of the trace. The receiver with maximum-likelihood differential decoding for a space-time unitary group code is shown in Fig. 9 . Bibliography 1 S. M. Alamouti A simple transmit diversity technique for wireless communications IEEE Journal Select. Areas Commun. vol. 16 no. 8 pp. 1451-1458 Oct. 1998. 2 A. Wittneben A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation in Proc. IEEE ICC93 pp. 1630-1634 1993. 3 V. Tarokh H.
