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The product of 2 independent sources $(S_A,P_A)$ and $(S_B,P_B)$ is defined as

$$ (S,P)\text{ s.t. }S = \{s_As_B|s_A\in S_A,s_B\in B\}\text{ and }\ P(s_As_B) = P_A(s_A)\cdot P_B(s_B)\,\forall s_A\in S_A,s_B\in S_B $$

and the MinAveCodeLen is the minimum average codeword length which is equal to the average codeword length of a Huffman coding.

The problem is to prove

$$ \mathrm{MinAveCodeLen}(P)\leq\mathrm{MinAveCodeLen}(P_A)+\mathrm{MinAveCodeLen}(P_B) $$

The original problem is quite confusing. I am asked to prove $H(P_A)+H(P_B)=H(P)$ in the previous part, so I was trying to relate the problem with $H$ at first with no progress.

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It's so wired that again, I'm answering my own question... I have to admit I am not confident at all when confronted with constructive proof problems.

An encoding scheme for ${P}$ could be described graphically as follows: Suppose $T_A$, $T_B$ are 2 Huffman trees for ${P}_A$ and ${P}_B$ respectively. For each leaf $s_{Ai}$ in $T_A$ replace it with $T_B$, i.e. enter image description here Letting $|S_A| = q,|S_B| = q$, the average code word length of this coding scheme is $$C = \frac{\sum l_{Ai}+l_{Bj}}{pq} = \frac{q\sum l_{Ai}+p\sum l_{Bj}}{pq} = \mathrm{MinAveCodeLen}({P}_A)+\mathrm{MinAveCodeLen}\mathbb{P}_B)$$ where $l_{Ai}$, $l_{Bj}$ are the codeword length of $s_{Ai}$ and $s_{Bj}$ in the Huffman code corresponding to $T_A$, $T_B$. Therefore, $$\mathrm{MinAveCodeLen}({P})\leq C\leq\mathrm{MinAveCodeLen}({P}_A)+\mathrm{MinAveCodeLen}({P}_B)$$

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