Bpsk implementation python

Question

I found this code in a book to generate polar RZ for a bpsk modulation, if someone can explain to me how the upfirdn function is used here to generate it:

Function to modulate an incoming binary stream using BPSK (baseband)

 def bpsk_mod(ak,L):
   
 #ak : input binary data stream (0's and 1's) to modulate
    
 #L : oversampling factor (Tb/Ts)
  
   from scipy.signal import upfirdn
   
   s_bb = upfirdn(h=[1]*L, x=2*ak-1, up 
                  = L) # NRZ encoder
  
   t=np.arange(start = 0,stop = 
           len(ak)*L) #discrete timebase
  
   return (s_bb,t)

Answer 1

First, the multiplication by 2 and subtraction of 1 transforms the 01-stream to +1 and -1. Then, as you can easily see in the function's documentation, an upsampling by factor L is performed with h being a sample-and-hold-filter. The output will be a stream of +1 and -1 with L samples per original bit and no zeros in it. Thus, an NRZ representation of the the signal.