Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 33 / 4 / a

Let the $r$ distinct codewords be $x^{(1)},\dots ,x^{(r)}\in {\mathbb{F}}_2^N$. No linearity assumption is needed. Sum their Hamming distances over unordered distinct pairs:Each summand is at least the minimum Hamming distance $\delta$, so $S\ge \delta r(r-1)/2$.

For coordinate $a$, let $m_a$ be the number of codewords whose $a$th bit is $1$. Exactly $m_a(r-m_a)$ unordered pairs differ at that coordinate. Summing coordinate contributions givessince $m_a(r-m_a)=r^2/4-(m_a-r/2{)}^2$. Combining the two bounds and dividing by $r>0$ gives $2\delta (r-1)\le Nr$, hence $r(2\delta -N)\le 2\delta$. Under $2\delta >N$, division is legitimate, proving the Plotkin boundBecause $r$ is an integer, it also satisfies the floor of the right-hand side. The binary alphabet is essential to the coordinate count; this proof applies to arbitrary binary block codes, not just linear ones.