A source of information emits a set of symbols denoted by
Two candidate modulation schemes, namely pulse-duration modulation (PDM) and pulse-position modulation (PPM), are considered for the electrical representation of this set of symbols. In PDM, the ith symbol is represented by a pulse of unit amplitude and duration (i/M)T.
On the other hand, in PPM, the ith symbol is represented by a short pulse of unit amplitude and fixed duration, which is transmitted at time t = (i/M)T.
Show that PPM is the only one of the two that can produce an orthogonal set of signals over the interval 0 £ t £ T.