Aufgaben:Exercise 5.4: Walsh Functions (PCCF, PACF)

• Return to book

Hadamard matrix  ${\mathbf{H}_{8}}$

The so-called  "Walsh functions",  which can be constructed by means of the Hadamard matrix,  are often used for band spreading and band compression.  Starting from the matrix

$${\mathbf{H}_{2}} = \left[ \begin{array}{ccc} +1 & +1 \\ +1 & -1 \end{array} \right] $$

the further Hadamard matrices  $ {\mathbf{H}_{4}}$,  $ {\mathbf{H}_{8}}$,  etc. can be derived by the following recursion:

$$ {\mathbf{H}_{2J}} = \left[ \begin{array}{ccc} \mathbf{H}_J & \mathbf{H}_J \\ \mathbf{H}_J & -\mathbf{H}_J \end{array} \right] \hspace{0.05cm}.$$

The diagram shows the matrix  $ {\mathbf{H}_{8}}$  for the spreading factor  $J = 8$.  From this we can derive the spreading sequences

$$ \langle w_\nu^{(1)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm}{+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm}{+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$ \langle w_\nu^{(2)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm}{+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm}{-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$...$$

$$\langle w_\nu^{(7)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm}{-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm}{+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm}$$

for seven CDMA subscribers.  The spreading sequence  $ \langle w_\nu^{(0)}\rangle$  corresponding to the first row in the Hadamard matrix is usually not assigned because it does not spread.

The questions mostly refer to the spreading factor  $J = 4$.  Thus,  correspondingly,  a maximum of three CDMA subscribers can be supplied with the spreading sequences  $ \langle w_\nu^{(1)}\rangle$,  $ \langle w_\nu^{(2)}\rangle$  and  $ \langle w_\nu^{(3)}\rangle$,  which result from the second, third and fourth rows of the matrix $ {\mathbf{H}_{4}}$.

Regarding the correlation functions, the following nomenclature shall apply in this exercise:

• The  periodic cross-correlation function  $\rm (PCCF)$  between the sequences  $ \langle w_\nu^{(i)}\rangle$  and  $ \langle w_\nu^{(j)}\rangle$  is denoted by  $φ_{ij}(λ)$.   Here:

$${\it \varphi}_{ij}(\lambda) = {\rm E}\left [ w_{\nu}^{(i)} \cdot w_{\nu+ \lambda}^{(j)} \right ] \hspace{0.05cm}.$$

• If  $φ_{ij} \equiv 0$  $($that is:  $φ_{ij}(λ) = 0$  for all values of  $λ)$,  the CDMA subscribers do not interfere with each other,  even if they have different propagation times.
• If at least  $φ_{ij}({\it λ} = 0) = 0$  applies,  then no interference occurs,  at least in synchronous CDMA operation  $($no or equal propagation times of all subscribers$).$ 
• The  periodic auto-correlation function  $\rm (PACF)$  of the Walsh function  $ \langle w_\nu^{(i)}\rangle$  is denoted by  $φ_{ii}(λ)$,  and it holds:

$${\it \varphi}_{ii}(\lambda) = {\rm E}\left [ w_{\nu}^{(i)} \cdot w_{\nu+ \lambda}^{(i)} \right ] \hspace{0.05cm}.$$

Notes:

• The exercise belongs to the chapter  Spreading Sequences for CDMA.
• Reference is made in particular to the section  Walsh functions  in the theory part.
• We would also like to draw your attention to the interactive applet  Generation of Walsh functions. 
• The abscissa is normalized to the chip duration  $T_c$.  This means that  $λ = 1$  actually describes a shift by the delay time  $τ = T_c$. 

Questions

1

What are the spreading sequences for  $J = 4$?

	$ \langle w_\nu^{(1)}\rangle = +\hspace{-0.05cm}1 -\hspace{-0.15cm}1 +\hspace{-0.15cm}1 -\hspace{-0.15cm}1$,
	$ \langle w_\nu^{(2)}\rangle = +\hspace{-0.05cm}1 +\hspace{-0.15cm}1 -\hspace{-0.15cm}1 -\hspace{-0.15cm}1$,
	$ \langle w_\nu^{(3)}\rangle = +\hspace{-0.05cm}1 -\hspace{-0.15cm}1 -\hspace{-0.15cm}1 +\hspace{-0.15cm}1$.

2

Which statements are true regarding the PCCF values  $φ_{ij}(λ = 0)$?

	For $J = 4$,   $φ_{12}(λ = 0) = 0$.
	For $J = 4$,   $φ_{13}(λ = 0) = 0$.
	For $J = 4$,   $φ_{23}(λ = 0) = 0$.
	For $J = 8$,   $φ_{ij}(λ = 0) ≠ 0$  may well hold  $(i ≠ j)$.
	In synchronous CDMA,  the subscribers do not interfere with each other.

3

Which statements are true for the PCCF values with  $λ ≠ 0$?

	For all values of  $λ$,  the PCCF is  $φ_{12}(λ) = 0$.
	For all values of  $λ$,  the PCCF is  $φ_{13}(λ) = 0$.
	For all values of  $λ$,  the PCCF is  $φ_{23}(λ) = 0$.
	In asynchronous CDMA,  the subscribers do not interfere with each other.

4

Which statements are true for the PACF curves?

	All   $φ_{ii}(λ)$  curves are periodic.
	$φ_{11}(λ = 0) = +\hspace{-0.05cm}1$  and  $φ_{11}(λ = 1) = -\hspace{-0.05cm}1$  hold.
	$φ_{22}(λ) = φ_{11}(λ)$  holds.
	$φ_{33}(λ) = φ_{22}(λ)$  holds.

Solution

Solution

(1)  All solutions  are correct:

• The matrix  $ {\mathbf{H}_{4}}$  is the upper left submatrix of  $ {\mathbf{H}_{8}}$.
• The spreading sequences result from the rows 2,  3  and 4  of  $ {\mathbf{H}_{4}}$,  and agree with the given sequences.

(2)  Solutions 1, 2 and 3  are correct:

• According to the equations in the data section,  the following holds:

$${\it \varphi}_{12}(\lambda = 0) = 1/4 \cdot \left [ (+1) \cdot (+1) + (-1) \cdot (+1) + (+1) \cdot (-1) + (-1) \cdot (-1) \right ] = 0\hspace{0.05cm},$$

$${\it \varphi}_{13}(\lambda = 0) = 1/4\cdot \left [ (+1) \cdot (+1) + (-1) \cdot (-1) + (+1) \cdot (-1) + (-1) \cdot (+1) \right ] = 0\hspace{0.05cm},$$

$${\it \varphi}_{23}(\lambda = 0) =1/4 \cdot \left [ (+1) \cdot (+1) + (+1) \cdot (-1) + (-1) \cdot (-1) + (-1) \cdot (+1) \right ] = 0\hspace{0.05cm}.$$

• Also,  for larger values of  $J$,  for  $i ≠ j$  the PCCF value is always  $φ_{ij}(λ = 0)= 0$.
• It follows:   In synchronous CDMA,  the subscribers do not interfere with each other.

(3)  Solutions 1 and 2  are correct:

• For all values of  $λ$,  the PCCF is  $φ_{12}(λ) = 0$,  as shown by the following lines:

$$\langle w_\nu^{(1)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm},$$ $$\langle w_\nu^{(2)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm}, \hspace{0.3cm}{\rm product\hspace{0.1cm} with \hspace{0.1cm}}\langle w_\nu^{(1)}\rangle: {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$\langle w_{\nu+1}^{(2)}\rangle = {-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm}, \hspace{0.3cm}{\rm product\hspace{0.1cm} with \hspace{0.1cm}}\langle w_\nu^{(1)}\rangle: {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$\langle w_{\nu+2}^{(2)}\rangle = {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.05cm},\hspace{0.3cm}{\rm product\hspace{0.1cm} with \hspace{0.1cm}}\langle w_\nu^{(1)}\rangle: {-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$\langle w_{\nu+3}^{(2)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.05cm},\hspace{0.3cm}{\rm product\hspace{0.1cm} with \hspace{0.1cm}}\langle w_\nu^{(1)}\rangle: {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} \hspace{0.05cm},$$

$$\langle w_{\nu+4}^{(2)}\rangle = {+\hspace{-0.05cm}1}\hspace{0.15cm} {+\hspace{-0.05cm}1} \hspace{0.15cm} {-\hspace{-0.05cm}1}\hspace{0.15cm} {-\hspace{-0.05cm}1} = \langle w_\nu^{(2)}\rangle \hspace{0.05cm}.$$

Some PCCF and PACF curves

• The same is true for the PCCF  $φ_{13}(λ)$.
• In contrast,  for the PCCF between the sequences  $ \langle w_\nu^{(2)}\rangle$  and  $ \langle w_\nu^{(3)}\rangle$  we obtain:

$${\it \varphi}_{23}(\lambda ) = \left\{ \begin{array}{c}0 \\+1\\ -1 \\ \end{array} \right. \begin{array}{*{10}c} {\rm{for}} \\ {\rm{for}} \\ {\rm{for}} \\ \end{array} \begin{array}{*{20}c} \lambda = 0, \pm 2, \pm 4,\pm 6, ... \hspace{0.05cm}, \\ \hspace{0.14cm} \lambda = ... \hspace{0.05cm} , -3, +1, +5, ... \hspace{0.05cm}, \\ \hspace{0.14cm} \lambda = ... \hspace{0.05cm} , -5, -1, +3, ... \hspace{0.05cm}. \\ \end{array}$$

• This means:   If the signal from subscriber  $3$  is delayed by one spreading chip with respect to subscriber  $2$  or vice versa,  the subscribers can no longer be separated and there is a significant increase in the error probability.
• In the diagram,  the PCCF curves are drawn in dashed lines  (violet and red).

(4)  Statements 1,  2  and 4  are correct:

• Since the Walsh function no.  $1$  is periodic with  $T_0 = 2T_c$,  the PACF is also periodic with  $λ = 2$.
• The second statement is correct,  as shown by the following calculation  (green curve):

$${\it \varphi}_{11}(\lambda = 0) = 1/4 \cdot \big [ (+1) \cdot (+1) + (-1) \cdot (-1) + (+1) \cdot (+1) + (-1) \cdot (-1) \big ] = +1\hspace{0.05cm},$$

$${\it \varphi}_{11}(\lambda = 1) = 1/4 \cdot \big [ (+1) \cdot (-1) + (+1) \cdot (-1) + (+1) \cdot (-1) + (+1) \cdot (-1) \big ] = -1\hspace{0.05cm}.$$

• Since the two Walsh functions no.  $2$  and  $3$  differ only by a shift around  $T_c$  and a phase in the PACF has no effect in principle,  in fact,  according to the last statement,  $φ_{33}(λ) = φ_{22}(λ)$.  These two PACF functions are plotted in blue.
• In contrast,  $φ_{22}(λ)$  differs from  $φ_{11}(λ)$  by a different periodicity:   $φ_{22}(λ) = φ_{33}(λ)$  is twice as wide as  $φ_{11}(λ)$.