multipath-induced ISI, DPPM [18,19]

multipath-induced ISI, DPPM [18,19], dicode PPM [20], multiple PPM (MPPM) and overlapping PPM (OPPM) [21]. In order to gain improved bandwidth efficiency, a number of modified PPM schemes have been proposed, including (see Table 4.1) DPPM [18,19], dicode PPM [20], multiple PPM (MPPM) and overlapping PPM (OPPM) [21], EPM [29], PPM plus (PPM+) [4] and hybrid modulation. Tough PPM+, where a redundant low chip is inserted after each high chip, improves the reliability; it further decreases the bandwidth efficiency, whereas hybrid schemes, such as the differential amplitude pulse position modulation (DAPPM), offer the additional advantage of a peak-to-average power ratio [22]. In EPM, the time is divided into discrete slots greater than the rise time and jitter of a pulse; however, the pulse width can be wider than one time slot, thus allowing more information to be transmitted than for a comparable PPM scheme.