Previously, i had already developed a code to encode complex constellation symbols into alamouti codes for two transmit antennas.

For the decoding process at the receiver (only one antenna currently) i am using the maximum likelihood receiver (mlr) algorithm for finding the original transmitted symbols. To find the advantage of using alamouti code i am comparing it with a siso transmission system. in this case i am using the QAM transreceiver example provided by NI. however besides the given AWGN, i am also including a Jake's model of rayleigh flat fading. this will indeed show the better noise immunity that the diversity can provide. when simulating alamouti code using mlr, an assumption is made that the channel path gain remains constant for a duration of two symbol time slots (i.e. one alamouti codeword).

the problem:
The given rayleigh fading vi however can neither be explicitly controlled to remain constant for two slots nor is it possible to directly extract the path gain values from it.

workaround (and not a solution, as such thing will be difficult to implement for other complex encoding process):
The path gain values are needed because the receiver needs to have information about the channel. as far as channel path gains are concerned they can be easily obtained by dividing the faded complex waveform with the original complex waveform. the requirement of the channel path gain remaining constant for two symbol time slots can be easily obtained by doing some programming.

the workaround proposed here is a bit clumsy. i would really like to know if it is possible to manipulate and modify vis such as the ones mentioned?? i would also like to know if there is any other better alternative of doing it.

added later:

 ohk i just had a chat with Rohit Khera online, as it turned out to be that the vi was not editable cause it was locked and not in edit mode. this can be done by simply pressing ctrl+m... 
it saved so much of labour for me 
thanks Rohit! :D