tion of the data and the PN binary sequence, it is important that the data clock and the PN sequence clock are coincident [12]. That is, the data transition times must be at the transition time of a PN Coherent Direct-Sequence Systems c(i) d(1) MODULATOR Figure 1.1. Direct-sequence modulation. sequence binary symbol. Figure 1.2 shows examples of data multiplied by a PN sequence for N = 10 where in (a) we do not have coincidence of the two sequence clocks. The problem here is that it may be possible for anyone to read the data directly from a clean copy of the multiplied binary sequence even when the PN sequence is unknown. One can first estimate the PN sequence clock (see (c)) and then determine unscheduled transitions (solid arrows in Figure 1.2(a)) in the sequence which must be due to the data. In (b) we show coincident data and PN sequence clocks where it is impossible to read the data without knowledge of the PN sequence. In (c) we show the clock times for the PN sequence. In this book it is assumed that the data clock is divided down from the PN sequence clock SO that possible transition times in the data line up with transition times of the PN sequence and no unscheduled transitions occur. Systems which have coincident data and PN sequence clocks are often said to have a data "privacy" feature since the data is hidden by the PN sequence. The most common data modulation is coherent binary phase-shift-keying (BPSK) for direct-sequence spread systems. This coherent direct-sequence 10 Tc 10 Tc 10 Tc 10 TC Figure 1.2. Data times PN sequence with (a) no coincident, (b) coincident, and (c) PN clocks. Direct-Sequence Spread Coherent Binary Phase-Shift-Keying system is the one considered in this chapter. At the receiver ideal chip and phase synchronization is assumed. Acquisition and synchronization tech- niques are considered in Part 4. 1.1 DIRECT-SEQUENCE SPREAD COHERENT BINARY PHASE-SHIFT-KEYING The simplest form of direct-sequence spread-spectrum communication sys- tems uses coherent binary phase-shift-keying (B