Space Time Block Codes

Well i had talked abt us using coded signals. That brings us to Space Time codes. These are used at the transmitter to exploit the spatial domain, which means while the time, frequency remains the same the code alone differs for the different systems. This is analogous to CDMA technology but the difference lies in that while CDMA employs single channel in this multiple channels are realized.So here i give a gist on Space Time Block codes.
STBC is an extension of transmit diversty. Here at one instant of time the data is sent via the separate antennas by coding them using orthogonal codes. STBC is a generalization of Alamouti's code discovered by Alamouti for transmission using two transmitting antennas. The scheme can be described as below:
The information bits are first modulated using an M-ary modulation scheme. The encoder then takes a block of two modulated symbols s1 and s2 in each encoding operation and gives it to the transmit antennas according to the code matrix,
S=[s1 -s2* ; s2 s1*]
The first column represents the first transmission period and the second column the second transmission period. The first row corresponds to the symbols transmitted from the first antenna and the second row corresponds to the symbols transmitted from the second antenna. This implies that we are transmitting both in space (across two antennas) and time (two transmission intervals). This is space-time coding. By extending this to more than 2 antennas the theory behind STBC was formulated.
At the receiver side these various versions are combined and estimated. Then this value is sent to the maximum likelihood detector that minimizes the decision metric and decodes the symbols s1 and s2.