ted using a binary, M-ary FSK, or OFDM (Orthogonal Frequency Division Multiplex) modulator, whose output signal occu- pies a bandwidth, W. This signal is then frequency-shifted to somewhere in the full band- width, LW, by mixing it with a local oscillator signal derived from a frequency synthesizer controlled by a PN generator. The receiver has a matching PN generator, frequency synthe- sizer, and mixer, which shift the frequency block down to baseband again, where it is demodulated by a conventional demodulator matching the modulator. An alternative approach, which has better resistance to some kinds of jamming but requires a more complex demodulator, is called independent tone hopping. In this, each digit value is combined with the output of the PN generator to control the frequency synthesizer directly. The resulting frequency separation of the tones corresponding to different digit values at any particular time can be up to the full channel bandwidth. The terms fast hopping and slow hopping traditionally described the relationship between the frequency hopping rate and the symbol rate. However, these terms are now used with great license. Fast hopping can be used to combat a follower jammer, which attempts to detect the signal tone(s) and immediately broadcasts other tones with adjacent frequencies. However, when fast hopping is used, the fact that phase coherence cannot be maintained across hops means that the energy from the successive hops in one symbol must be combined incoher- ently; this causes a performance loss. FHSS signals are primarily used in AJ and CDMA ("FH-CDMA") systems, including Bluetooth, and in some cordless phones. Advantages of FHSS over DSSS are that it has less severe timing requirements and is less sensitive to channel gain and phase fluctuations. Bluetooth also includes adaptive FHSS, which improves interference resistance by avoiding the use of crowded frequencies in the hopping sequence. In the United States, use of FHSS is permitted in unlicensed bands around 900 MHz and 2.4 GHz, 