Exercise 2.11Z: Erasure Channel for Symbols

Erasure channel for symbols: "m–BEC"

The channel model "Binary Erasure Channel" $\rm (BEC)$ describes a bit-level erasure channel:

A binary symbol $(0$ or $1)$ is correctly transmitted with probability $1 - \lambda$ and marked as an erasure "$\rm E$" with probability $\lambda$.

In contrast to the "Binary Symmetric Channel" $\rm (BSC)$, falsifications $(0 → 1, \ 1 → 0)$ cannot occur here.

A Reed–Solomon code is based on a Galois field ${\rm GF}(2^m)$ with integer $m$. Thus, each code symbol $c$ can be represented by $m$ bits. If one wants to apply the BEC model here, one has to modify it to the "$m$–BEC" model as shown in the diagram below for $m = 2$:

All code symbols $($in binary representation $00, \ 01, \ 10, \ 11)$ are transmitted correctly with probability $1 - \lambda_2$ .

Thus, the probability of an erased symbol is $\lambda_2$.

Note that already a single erased bit leads to the erased received symbol $y = \rm E$ .

Hints:

This exercise belongs to the chapter "Reed–Solomon Decoding at the Erasure Channel".

For a code based on ${\rm GF}(2^m)$ the outlined $2$–BEC model is to be extended to $m$–BEC.

The erasure probability of this model is then denoted by $\lambda_m$.

For the first subtasks, use always $\lambda = 0.2$ for the erasure probability of the basic model according to the upper graph.

Questions

${\rm Pr}(y = 0) \ = \ $

$\ \%$

${\rm Pr}(y = {\rm E}) \ = \ $

$\ \%$

${\rm Pr}(y = {\rm 1}) \ = \ $

$\ \%$

$\lambda_2 \ = \ $

$\ \%$

$\lambda_m \ = \ $

$\ \%$

${\rm Max} \ \big[\lambda\big ] \ = \ $

$\ \%$

${\rm Pr}(y_{\rm bin} = 00000000) \ = \ $

$\ \%$

Solution

(1) Due to the symmetry of the given BEC model (bit-level erasure channel), the erasure probability is:

$$\ {\rm Pr}(y = {\rm E}) = \lambda \ \underline{= 20\%}.$$

Since code symbols $0$ and $1$ are equally likely, we obtain ${\rm Pr}(y = 0) \ \underline{= 40\%}$ and ${\rm Pr}(y = 1) \ \underline{= 40\%}$ for their probabilities.

(2) Without limiting generality, we assume the code symbol $c_{\rm binary} = $ "$00$" to solve this exercise.

According to the 2 BEC model, the receive symbol $y_{\rm binary}$ can then be either "$00$" or canceled $(\rm E)$ and it holds:

$${\rm Pr}(y_{\rm bin} = "00"\hspace{0.05cm} |\hspace{0.05cm} c_{\rm bin} = "00") \hspace{-0.15cm} \ = \ \hspace{-0.15cm} ( 1 - \lambda)^2 = 0.8^2 = 0.64 = 1 - \lambda_2\hspace{0.3cm} \Rightarrow \hspace{0.3cm} \lambda_2 \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 1 - ( 1 - \lambda)^2 \hspace{0.15cm}\underline{= 36\%}\hspace{0.05cm}. $$

This assumes that an erasure is avoided only if neither of the two bits has been erased.

(3) The $\rm RSC \, (255, \, 223, \, 33)_{256}$ is based on the Galois field $\rm GF(256) = GF(2^8) \ \Rightarrow \ \it m = \rm 8$. The result of subtask (2) must now be adapted to this case. For the $8$ BEC holds:

$$1 - \lambda_8 = ( 1 - \lambda)^8 = 0.8^8 \approx 0.168 \hspace{0.3cm} \Rightarrow \hspace{0.3cm} \lambda_m = \lambda_8 \hspace{0.15cm}\underline{\approx 83.2\%}\hspace{0.05cm}. $$

(4) From the condition $\lambda_m ≤ 0.2$ it follows directly $1 - \lambda_m ≥ 0.8$. From this follows further:

$$( 1 - \lambda)^8 \ge 0.8 \hspace{0.3cm} \Rightarrow \hspace{0.3cm} 1 - \lambda \ge 0.8^{0.125} \approx 0.9725 \hspace{0.3cm} \Rightarrow \hspace{0.3cm}\lambda \hspace{0.15cm} \underline{\le 2.75\%}\hspace{0.05cm}.$$

(5) Here one proceeds as follows:

With $\lambda = 0.0275 \Rightarrow \ \lambda_m = 0.2$, $20\%$ of the receiving symbols are erasures.
The $2^8 = 256$ receiving symbols ($00000000$ ... $11111111$) are all equally likely. It follows:

$${\rm Pr}(y_{\rm bin} = 00000000) = \hspace{0.1cm}\text{...} \hspace{0.1cm}= {\rm Pr}(y_{\rm bin} = 11111111)= \frac{0.8}{256} \hspace{0.15cm}\underline{= 0.3125\%}\hspace{0.05cm}.$$