ult to detect and jam. As discussed in the following sections, there are three basic types of spread spectrum techniques: Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Orthogonal Frequency Division Multiplexing (OFDM). FHSS With FHSS, which is the oldest of the spread spectrum techniques, the fre- quency hopping varies in a known pattern, and separate error correction must be included. The concept involves a system that hops between frequencies in a ran- dom pattern, known only to the transmitter and receiver (see Figure 13.8). The transmitter sends on a channel for only a short period of time and then hops to a different channel. Slow frequency hopping is used to combat fading. Along with providing resistance to jamming, an FHSS system also addresses the problem of frequency-selective fading. Because the transmitter does not stay on any one frequency for a long period of time, fading at a particular part of the frequency spectrum is minimized. The disadvantage of FHSS is that the more hopping chan- nels needed, the greater the bandwidth required, although a set of hopping channels can be shared by a number of users. Spectrum Utilization V(t,f) Figure 13.8 FHSS FHSS is used by the original Bluetooth standard (IEEE 802.15.1) for PANs, which is discussed in detail in Chapter 15. DSSS DSSS is used in most contemporary systems, including 3G cellular net- works, CDMA-based networks, and 802.11b WLANs. DSSS requires greater band- width than does FHSS, but that is exactly what makes its performance so good. With DSSS, each transmitted bit is first converted into a series of narrower pulses, referred to as chips. DSSS multiplies the data bits by a very fast pseudoran- dom bit pattern (PN code) that spreads the data into a large coded stream that takes the full bandwidth of the channel. The chips are then transmitted through a digital modulator. The bandwidth expansion factor refers to the number of bits in the spreading code. There are many types of spreading codes in use; for exam