FPGA Central

Hi,
When I read "Digital Communication" written by John R. Barry, E.A. Lee
and D.G. Messerschmitt, I cannot understand an example on page 371. It
said: "The equivalent channel response for the WMF is Sh, which is
non-negative real on the unit circle. Excluding poles at z=0 and
z=(unlimit biggest), if Sh has any additional poles at all, then it
will have poles outside the unit circle.

My question is:
a. In this book, Sh is the frequency response of the autocorrelation.
Then, the sentence "The equivalent channel response for the WMF is Sh"
is wrong. It should be "The equivalent channel channel response for the
MF is Sh", even though I don't find this on its errata.

b. Why, "if Sh has any additional poles at all, then it will have poles
outside the unit circle."
>From Example 8-3 of this book, I find the poles could be in and out
unit circle. For a autocorrelation function, it couldn't have poles in
the unit circle?

This is the folded spectrum of an autocorrelation function of example
8-3.
1 1
Sh(z)=Eh(1-alpha^2)*------------------------ * -------------------
(1-alpha*z^(-1)) (1-alpha*z)


Any suggestion is appreciated
Thank you