d = 128;
\[Theta][t_] = 10000^(-2*t/d);
f[m_] = Sum[
    Norm[Sum[Exp[I*m*\[Theta][i]], {i, 0, j}]], {j, 0, d/2 - 1}]/(d/2);
Plot[f[m], {m, 0, 256}, AxesLabel -> {相对距离, 相对大小}]