t detection. Non-Coherent Frequency-Hopped Systems FH CARRIER Figure 2.2. Example of SFH/MFSK signal, with K = 2 or M = 4 symbols (1, 2, 3, 4) Re=Rs = = Rb/2 = 3Rh, and N equally spaced FH tone frequencies. rapidly that today's FFH system will invariably become tomorrow's slow fre- quency-hopped (SFH) system under this terminology. For example, in the recent past it was common for FH systems to operate at several hundred hops/sec, and 10-20 Khops/sec was considered to be state-of-the-art. Yet pro- totype synthetizer implementations have now been developed that deliver several hundred Khops/sec, and rates greater than 1 Mhop/sec should be realizable in the near term. To avoid technology-dependent terminology, we will define an FFH system to be one in which the hop rate Rh is an integer multiple of the MFSK symbol rate Rs, while the term SFH denotes the reverse condition. Another terminological ambiguity is the widespread use of the word "chip" to refer to an individual FH/MFSK tone of shortest duration, which should not be confused with the PN chips that drive the frequency synthe- sizer. In an FFH system where there are multiple hops per M-ary symbol, each hop is a chip, whereas, in an SFH system as shown in Figure 2.2, a chip denotes an M-ary symbol. The chip rate R = max(R Rs) is the highest FH system clock rate; the energy detectors in Figure 2.1 generate outputs at this rate. In the FFH mode, the M-ary symbol metrics are usually formed by lin- early combining the Rh/Rs detected chip energies for each symbol, resulting in a non-coherent combining loss that will be discussed in detail later. With non-coherent detection, the MFSK tones on a given hop must be sep- arated in frequency by an integer multiple of R to provide orthogonality. This implies that a transmitted symbol will not produce any crosstalk in the other M - 1 energy detectors, and, if the M-ary band contains additive white Gaussian noise (AWGN), the components of that noise in each detector out- put will be uncorrelated. Figure 2.3(a) depicts a com