What does this code exactly do using spdiags

Hi what does the following code actually do? It states in the remarks that it is carrying out the hilbert transform but I am not so sure. I am not able to understand what is being acheieved by using the spdiags function also.a is a structural matrix, b consists of time domain data

 a.b=ifft(spdiags([1:length(a.b)]'<length(a.b)/2,0,length(a.b),length(a.b))*fft(a.b))

Answer 1

[Bhavesh]

http://www.mathworks.com/help/releases/R2016a/matlab/ref/spdiags.html