Aufgaben:Exercise 1.09Z: Extension and/or Puncturing

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Extension and puncturing

Often you know a code that seems to be suitable for an application, but its code rate does not exactly match the specifications.

There are several possibilities for rate adaptation

Extension:
Starting from the  $(n, \, k)$ code whose parity-check matrix  $\mathbf{H}$  is given, one obtains a  $(n+1, \, k)$ code by extending the parity-check matrix by one row and one column and adding zeros and ones to the new matrix elements according to the upper graph. One adds a new parity bit

$$x_{n+1} = x_1 \oplus x_2 \oplus ... \hspace{0.05cm} \oplus x_n$$

and thus a new parity-check equation is added, which is considered in  $\mathbf{H}\hspace{0.05cm}'$  .

Puncturing:
According to the figure below, one arrives at a  $(n-1, \, k)$ code of larger rate by omitting a parity bit and a parity-check equation, which is equivalent to deleting one row and one column from the parity-check matrix  $\mathbf{H}$ .

Shortening:
If an information bit is omitted instead of a parity bit, the result is a  $(n-1, \, k-1)$ code of smaller rate.

In this exercise, starting from a  $(5, \, 2)$ block code

$$\mathcal{C} = \{ (0, 0, 0, 0, 0) \hspace{0.1cm}, (0, 1, 0, 1, 1) \hspace{0.1cm},(1, 0, 1, 1, 0) \hspace{0.1cm},(1, 1, 1, 0, 1) \}$$

the following codes are constructed and analyzed:

• one  $(6, \, 2)$ code by single extension,

• one  $(7, \, 2)$ code by extending it again,

• one  $(4, \, 2)$ code by puncturing.

The parity-check matrix and the generator matrix of the systematic  $(5, \, 2)$ code are:

$${ \boldsymbol{\rm H}}_{(5, 2)} = \begin{pmatrix} 1 &0 &1 &0 &0\\ 1 &1 &0 &1 &0\\ 0 &1 &0 &0 &1 \end{pmatrix} \hspace{0.3cm} \Leftrightarrow\hspace{0.3cm} { \boldsymbol{\rm G}}_{(5, 2)} = \begin{pmatrix} 1 &0 &1 &1 &0\\ 0 &1 &0 &1 &1 \end{pmatrix} \hspace{0.05cm}.$$

Hints :

• This exercise belongs to the chapter  General Description of Linear Block Codes.
• In the  Exercise 1.9  it is exemplified how the  $(7, \, 4, \, 3)$ Hamming code is turned into a  $(8, \, 4, \, 4)$-code by extension.

Questions

1

Specify the characteristics of the given  $(5, \, 2)$ code.

$R \ = \ $

$d_{\rm min} \ = \ $

2

What code words does the $(6, \, 2)$ code have after expansion?

	$(0 0 0 0 0 1), \ (0 1 0 1 1 0), \ (1 0 1 1 0 0), \ (1 1 1 0 1 1).$
	$(0 0 0 0 0 0), \ (0 1 0 1 1 1), \ (1 0 1 1 0 1), \ (1 1 1 0 1 0).$

3

Specify the characteristics of the extended  $(6, \, 2)$ code.

$R \ = \ $

$d_{\rm min} \ = \ $

4

What is the systematic generator matrix  $\boldsymbol{\rm G}$  of the  $(7, \, 2)$ code?

	Row 1 of  $\boldsymbol{\rm G} \text{:} \hspace{0.2cm} 1, \, 0, \, 1, \, 1, \, 0, \, 1, \, 0.$
	Row 2 of  $\boldsymbol{\rm G} \text{:} \hspace{0.2cm} 0, \, 1, \, 0, \, 1, \, 1, \, 1, \, 0.$

5

Specify the characteristics of the extended  $(7, \, 2)$ code.

$R \ = \ $

$d_{\rm min} \ = \ $

6

Which statements are true for the $(4, \, 2)$ code (puncturing the last parity bit)?

	The code rate is now  $R = 2/4 = 0.5$.
	$C_{(4, 2)} = \{(0, 0, 0, 0), \, (1, 0, 1, 1), \, (0, 1, 0, 1), \, (1, 1, 1, 0)\}$.
	The minimum distance remains unchanged from the  $(5, \, 2)$ code.

Solution

Solution

(1)  The rate of the $(5, \, 2)$ code is $R = 2/5 \ \underline{ = 0.4}$.

• From the given code, we further recognize the minimum distance $d_{\rm min} \ \underline{ = 3}$.

(2)  When extending from the $(5, \, 2)$ code to the $(6, \, 2)$ code, another parity bit is added.

• The codeword thus has the form

$$\underline{x} = ( x_1, x_2, x_3, x_4, x_5, x_6) = ( u_1, u_2, p_1, p_2, p_{3}, p_4) \hspace{0.05cm}.$$

• For the added parity bit must be valid:

$$p_4 = x_6 = x_1 \oplus x_2 \oplus x_3 \oplus x_4 \oplus x_5 \hspace{0.05cm}.$$

• That is, the new parity bit $p_{4}$ is chosen to result in an even number of ones in each codeword  ⇒  Answer 2.
• Solving this task with the parity-check matrix, we get

$${ \boldsymbol{\rm H}}_{(6,\hspace{0.05cm} 2)} = \begin{pmatrix} 1 &0 &1 &0 &0 &0\\ 1 &1 &0 &1 &0 &0\\ 0 &1 &0 &0 &1 &0\\ 1 &1 &1 &1 &1 &1 \end{pmatrix} \hspace{0.3cm} \Rightarrow\hspace{0.3cm} { \boldsymbol{\rm H}}_{{\rm (6,\hspace{0.05cm} 2)\hspace{0.05cm}sys}} = \begin{pmatrix} 1 &0 &1 &0 &0 &0\\ 1 &1 &0 &1 &0 &0\\ 0 &1 &0 &0 &1 &0\\ 1 &1 &0 &0 &0 &1 \end{pmatrix}\hspace{0.3cm} \Rightarrow\hspace{0.3cm} { \boldsymbol{\rm G}}_{{\rm (6,\hspace{0.05cm} 2)\hspace{0.05cm}sys}} = \begin{pmatrix} 1 &0 &1 &1 &0 &1\\ 0 &1 &0 &1 &1 &1 \end{pmatrix}\hspace{0.05cm}.$$

• The two rows of the generator matrix $\boldsymbol{\rm G}$ give two of the four codewords, the modulo $2$ sum gives the third, and finally the all zero word has to be considered.

(3)  After extension from the $(5, \, 2)$ code to the $(6, \, 2)$ code.

• decreases the rate from $R = 2/5$ to $R = 2/6 \ \underline{= 0.333}$,
• increases the minimum distance from $d_{\rm min} = 3$ to $d_{\rm min} \ \underline{= 4}$ .

In general:   Extending a code, the rate decreases and the minimum distance increases by $1$ if $d_{\rm min}$ was odd before.

(4)  Using the same procedure as in (3), we obtain

$${ \boldsymbol{\rm H}}_{(7,\hspace{0.05cm} 2)} \hspace{-0.05cm}=\hspace{-0.05cm} \begin{pmatrix} 1 &0 &1 &0 &0 &0 &0\\ 1 &1 &0 &1 &0 &0 &0\\ 0 &1 &0 &0 &1 &0 &0\\ 1 &1 &0 &0 &0 &1 &0\\ 1 &1 &1 &1 &1 &1 &1 \end{pmatrix} \hspace{0.15cm} \Rightarrow\hspace{0.15cm} { \boldsymbol{\rm H}}_{{\rm (7,\hspace{0.05cm} 2)\hspace{0.05cm}sys}} \hspace{-0.05cm}=\hspace{-0.05cm} \begin{pmatrix} 1 &0 &1 &0 &0 &0 &0\\ 1 &1 &0 &1 &0 &0 &0\\ 0 &1 &0 &0 &1 &0 &0\\ 1 &1 &0 &0 &0 &1 &0\\ 0 &0 &0 &0 &0 &0 &1 \end{pmatrix}\hspace{0.15cm} \Rightarrow\hspace{0.15cm} { \boldsymbol{\rm G}}_{{\rm (6,\hspace{0.05cm} 2)\hspace{0.05cm}sys}} \hspace{-0.05cm}=\hspace{-0.05cm} \begin{pmatrix} 1 &0 &1 &1 &0 &1 &0 \\ 0 &1 &0 &1 &1 &1 &0 \end{pmatrix}\hspace{0.05cm}.$$

⇒  Both answers are correct.

(5)  The rate is now $R = 2/7 = \underline{0.266}$.

• The minimum distance is still $d_{\rm min} \ \underline{= 4}$ , as can be seen from the $(7, \, 2)$ code words:

$$\mathcal{C} = \{ (0, 0, 0, 0, 0, 0, 0), \hspace{0.1cm}(0, 1, 0, 1, 1, 1, 0), \hspace{0.1cm}(1, 0, 1, 1, 0, 1, 0), \hspace{0.1cm}(1, 1, 1, 0, 1, 0, 0) \}\hspace{0.05cm}.$$

In general:   If the minimum distance of a code is even, it cannot be increased by extension.

(6)  Correct are the statements 1 and 2:

• By crossing out the last row and the last column, we obtain for parity-check matrix and generator matrix, respectively (each in systematic form):

$${ \boldsymbol{\rm H}}_{(4,\hspace{0.05cm} 2)} = \begin{pmatrix} 1 &0 &1 &0 \\ 1 &1 &0 &1 \end{pmatrix} \hspace{0.3cm} \Rightarrow\hspace{0.3cm} { \boldsymbol{\rm G}}_{{\rm (4,\hspace{0.05cm} 2)}} = \begin{pmatrix} 1 &0 &1 &1 \\ 0 &1 &0 &1 \end{pmatrix}\hspace{0.05cm}.$$

• From the generator matrix we get the mentioned codewords $(1, 0, 1, 1), \, (0, 1, 0, 1), \, (1, 1, 1, 0)$ as row sum as well as the null word $(0, 0, 0, 0)$. The minimum distance of this code is $d_{\rm min}= 2$, which is smaller than the minimum distance $d_{\rm min}= 3$ of the $(5, \, 2)$ code.

In general:   Puncturing makes $d_{\rm min}$ smaller by $1$ (if it was even before) or it stays the same. This can be illustrated by generating the $(3, \, 2)$ block code by another puncturing (of the parity bit $p_{2}$). This code

$$ \mathcal{C} = \{ (0, 0, 0), \hspace{0.1cm}(0, 1, 1), \hspace{0.1cm}(1, 0, 1), \hspace{0.1cm}(1, 1, 0) \}$$

has the same minimum distance $d_{\rm min}= 2$ as the $(4, \, 2)$ code.