How to implement Polyphase filter?
I assume that the Polyphase filter shown in the paper is used as a decimator such that the output rate is an integer fraction the input rate (and this would make sense in a correlation operation since the output frequency required is less due to the proper filtering of the correlator. Making a polyphase filter implementation is quite easy; given the desired coefficients for a simple FIR filter, you distribute those same coefficients in "row to column" format into the separate polyphase FIR components as explained in the following example:

Assume a FIR filter with 8 taps for simplicity of explanation, with  coefficients as follows: 

h[0], h[1], h[2], h[3], h[4], h[5], h[6], h[7], ...

and we use that to create a 4 element polyphase each with 2 taps:

Poly FIR1: h[0],   h[4]

Poly FIR2: h[1],   h[5]

Poly FIR3: h[2],   h[6]

Poly FIR4: h[3],   h[7]

From the above it is hopefully clearer in what I meant by "row to column" format.

The Polyphase Decimator would be structured as in the figure below:



A Polyphase Interpolator works the same way, and since I already have the graphics that explain clearly why we do the row to column mapping, I will include that below (and the beauty of the polyphase implementation itself intuitively explained!). This shows an input signal of successive pulses at a higher rate going through an interpolation operation. Normally, without the use of polyphase implementations, we can interpolate a signal by simply inserting zeros, and then following that with a low pass filter to get rid of the higher frequency aliases that occur due to the zero insert. The best filter design would pass our original spectrum with no distortion and completely eliminate the higher frequency components. It is the coefficients of this FIR filter that are used in the polyphase implementation since it mathematically performs the same result (that said I suspect the filter chosen in the paper based on windowing method of filter design is not the best choice).