dulation pulse shape) with bandwidth Wss SS* Because of practical considerations (it is difficult to synchronize the PN gen- erator in the receiver to sub-nanosecond accuracy), current DS systems typ- ically operate with PN rates of 100 Mchips/sec or less, implying that Wss is limited to several hundred MHz. We will now analyze the other principal category of SS systems, namely, frequency hopping (FH) with non-coherent M-ary frequency-shift-keyed (MFSK) modulation. This is essentially a conventional MFSK scheme in which the carrier frequency is pseudorandomly hopped over Wss under the control of a PN sequence. Specifically, as illustrated in Figure 2.1, successive (not necessarily disjoint) k-chip segments of the PN sequence drive a fre- quency synthesizer which hops the carrier over 2k frequencies. On a given hop, the signal bandwidth is identical to conventional MFSK, which is typ- ically much smaller than W; however, averaged over many hops, the FH/MFSK spectrum occupies the entire SS bandwidth. Current technology permits FH bandwidths of the order of several GHz, which is an order of magnitude larger than implementable DS bandwidths. Because they must operate over such wide bandwidths, FH synthesizers generally do not main- tain phase coherence over successive hops: consequently, coherent data demodulation techniques are possible only within each hop. As mentioned above and shown in Figure 2.1, this chapter will consider only non-coherent energy detection. In other publications, the term "fast frequency hopping" (FFH) has occa- sionally been used to denote systems with relatively high hop rates, inde- pendent of the data rate. However, synthesizer technology is progressing SO BINARY BINARY BINARY K-TUPLE A TUPLE CODER BINARY M ENERGY A-TUPLE DETECTORS DEMOD JAMMER Figure 2.1. Functional block diagram of FH/MFSK system perturbed by jamming. Transmit one of M = 2K tones; carrier is hopped to one of 2k frequencies determined by k-chip segments of PN code; dehopping requires derived PN reference (PN); non-coheren