Need argent help regarding delay estimation

I am going through Average Magnitude Difference
Function (AMDF), while exploring on this I found a good PDF 
Enhancing Echo Cancellation via Estimation of delay
By Youhong lu,Ronald Fowler,Wenshun Tian and Leland Thompson.

In which the delay estimation is done.
I am going through this paper to understand the mathematical steps used
for estimation of delay.

I have got struck with the normalized coss correlation function formule
given in this formula.

The normalised cross correlation function (n,k) at the time n can
be defined as 

R(n,k) = Pxy(n,k) / squareroot[Px(n-k) Py(n)]

where x(n) is frend signal and y(n) be recieved echo, where n is the time
index.

The denominator part I have understood. Px(n) and Py(n) is the power
ofx(n) and y(n)

I have doubt in this part of the formula. I am not understand how cross
correlation is calculated.
Pxy(n,k) is the cross correlation function between x(n) and y(n) at ime
n.


                 L - 1
Pxy(n,k) = 1/ L summation x*(n- l �k) y(n � l)
                 l = 0 

Can you please help me in this regard Sir.

_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?

                 L - 1
> Pxy(n,k) = 1/ L summation x*(n- l -k) y(n - l)
>                  l = 0
>
> Can you please help me in this regard Sir.
>
> _____________________________________
> Do you know a company who employs DSP engineers?
> Is it already listed athttp://dsprelated.com/employers.php?



Look up convolution. Think of it almost like a convolution (in which
you don't have to flip one of the signals over the t=0 axis).