và lặp đi lặp lại với các ước tính cập nhật au trong lặp đi lặp lại tiếp theo. Trong lần lặp thứ μ, ~ biểu tượng u thứ trở nên auChúng tôi xem xét các hệ thống ăng-ten nhiều với một số giống hệt nhau của ăng-ten nhận và truyền. Hơn nữa, không tương quan căn hộ Rayleigh fading kênh giữa các cặp khác nhau của ăng-ten truyền | MULTIPLE ANTENNA SYSTEMS 279 independent of NR and is equally distributed onto the diversity paths so that only the NRth part of Eb can be exploited at each receive antenna. In this scenario the gain obtained solely by diversity can be observed. On the contrary Figure b depicts the error rate versus the average Es N0 at each receive antenna. Therefore the total transmit power increases linearly with NR and the entire SNR after maximum ratio combining becomes NR times larger indicating the additional array gain. Comparing the difference between adjacent curves in both the plots we recognize a difference of 3 dB that exactly represents the gain obtained by doubling the number of receive antennas. We can conclude that receive diversity is an efficient and simple possibility to increase the link reliability. However its applicability becomes immediately limited if the size of the receiving terminal is very small. Cell phones for mobile radio communications have become smaller and smaller in recent years so that it is a difficult task to place several antennas on such small devices. Even if we succeed it is questionable whether the spacing would be large enough to guarantee uncorrelated channels. Although different polarizations represent a further dimension to obtain diversity the decoupling is generally imperfect leading to cross talk. In this situation the question arises whether diversity can also be exploited with multiple antennas at the transmitter. Performance Analysis of Space-Time Codes In this subsection the general concept of space-time transmit diversity is addressed that is using multiple antennas at the transmitter. A straightforward implementation where a signal x is transmitted simultaneously over several antennas will not provide the desired diversity gain. Looking at the received signal 1 NNt y f N X ÝEhv n we see that an incoherent superposition is obtained resulting in a new Rayleigh-distributed Hence the equivalent SISO channel
