Fractional sampling and SIMO are not exactly similar, SIMO system uses multi anntenna at receiver but Fractional sampling can be used with one antenna at receiver. It's said that, Fractional sampling is often used to convert the SISO system into SIMO system without using Multi Antennas at receiver. How?
Suppose a SISO system is used, where you use single Tx antenna with single Rx antenna, Fractional sampling means you oversample the received signal by G, where G is Oversample factor, that means your sampling rate becomes Ts/G, and Ts is the sampling rate used at transmitter. So, that means you don't change the structure of transmitter, but you oversample the receiver. How?
Suppose the received signal is :
where y,u,h are received signal, transmitted signal and channel respectively.
If y(t) is sampled with a rate of Ts/G instead of Ts, where G is the oversampling ratio, and "Ts is the sampling rate which was used at transmitter: its polyphase components can be expressed as:
where g = 1, ..., G.
each polyphase component should be demodulated in a separated demodulator (This the one of disadvantage of using Fractional sampling). So instead of having one demodulator, we will have multi demodulators.
Coming back to the previous equation,
and the channel hg[n] is a matrix as below:
It seems similar, but I can't confirm or give more details, I am not sure, AlexTP maybe can explain it better than me.
thank you and sorry for the notations, because I don't know how I can write the mathematical equations here.