izable binary phase characteristic. A chip (or chips) is (are) a result of direct sequence spreading by biphase modulating an RF carrier. Being that each chip has a biphase modulated characteristic, we can identify each one with a binary 1 or binary 0. These chips derive from biphase (PSK) modulating a carrier where the modulation is controlled by a pseudorandom (PN) sequence. If the sequence is long enough, without repeats, it is considered pseudorandom. The sequence is controlled by a key which is unique to our transmitter and its companion far-end receiver. Of course the receiver must be time-aligned and synchronized with its companion transmitter. A block diagram of this operation is shown in Figure 18.11. It is an in-line correlator. Let's look at an information bit divided into seven chips and coded by a PN sequence + + and shown in Figure 18.12a. Now replace the in-line correlator with matched filter. In this case the matched filter is an electrical delay line tapped at delay intervals, which correspond to the chip time duration. Each tap in the delay line feeds into Received Double Recovered balanced carrier f(c) g(m) mixer Modulated signal Code reference Figure 18.11 In-line correlator. 18.6 NETWORK ACCESS TECHNIQUES BIT DURATION =180° DELAY LINE SIGNAL INPUT 1' PHASE SHIFTERS ADDER BANDPASS OUTPUT FILTER BIT LENGTH, T- CHIP = T/N RECEIVED SIGNAL MODULO2 NARROWBAND THRESHOLD ADDER FILTER REFERENCE +++-+-- SIGNAL N/T 1/T MOD 2 OUTPUT SPREAD SIGNAL SPECTRUM COLLAPSED SIGNAL SPECTRUM Figure 18.12 (a) An information element divided into chips coded by a PN sequence; (b) matched filter for 7-chip PN code; (c) the correlation process collapses the spread signal spectrum to that of the original bit spectrum. (From Ref. 15. Reprinted with permission.) an arithmetic operator matched in sign to each chip in the coded sequence. If each delay line tap has the same sign (phase shift) as the chips in the sequence, we have a match. This is illustrated in Figure 18.12b. As shown here, the short sequence of seven chips i