It is hard to say when you give so little description of what you are trying to achieve. I presume what you are doing is based on the 802.11g WiFi standard.
Firstly, I suspect you have not looked at how the randi() function should be called. I suspect you may want something more like:
Code Matlab M - [expand]
1
BitTx = randi([0,1],N_Num,1);
There are probably other issues with your code. For example, it seems likely to me that you should be using Bit_Num in some of the places that you have hard-coded the number 4. I suggest you spend a minute to describe what your intentions are.