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After the signal is created by the source, the spreading process uses a spreading code and spreads the bandwidth The figure shows the original bandwidth B and the spreaded bandwidth B ss The spreading code is a series of numbers that look random, but are actually a pattern There are two techniques to spread the bandwidth: frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS)
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The frequency hopping spread spectrum (FHSS) technique uses M different carrier frequencies that are modulated by the source signal At one moment, the signal modulates one carrier frequency; at the next moment, the signal modulates another carrier
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BANDWIDTH UTILIZATION: MULTIPLEXING AND SPREADING
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frequency Although the modulation is done using one carrier frequency at a time, M frequencies are used in the long run The bandwidth occupied by a source after spreading is B pHSS B Figure 628 shows the general layout for FHSS A pseudorandom code generator, called pseudorandom noise (PN), creates a k-bit pattern for every hopping period Th The frequency table uses the pattern to find the frequency to be used for this hopping period and passes it to the frequency synthesizer The frequency synthesizer creates a carrier signal of that frequency, and the source signal modulates the carrier signal Figure 628 Frequency hopping spread spectrum (FHSS)
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Modulator Original --I----------'l~ signal
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signal
Frequency table
Suppose we have decided to have eight hopping frequencies This is extremely low for real applications and is just for illustration In this case, Mis 8 and k is 3 The pseudorandom code generator will create eight different 3-bit patterns These are mapped to eight different frequencies in the frequency table (see Figure 629) Figure 629 Frequency selection in FHSS
First-hop frequency
k-bit k-bil patterns
Frequency
101 111 001 000 010 110 011 100
000 001
First selection
011 100 101
III Frequency table
200kHz 300kHz 400kHz 500kHz 600kHz 700kHz800kHz 900kHz
SECTION 62
SPREAD SPECTRUM
The pattern for this station is 101, 111, 001, 000, 010, all, 100 Note that the pattern is pseudorandom it is repeated after eight hoppings This means that at hopping period 1, the pattern is 101 The frequency selected is 700 kHz; the source signal modulates this carrier frequency The second k-bit pattern selected is 111, which selects the 900-kHz carrier; the eighth pattern is 100, the frequency is 600 kHz After eight hoppings, the pattern repeats, starting from 101 again Figure 630 shows how the signal hops around from carrier to carrier We assume the required bandwidth of the original signal is 100 kHz
Figure 630 FHSS cycles
Carrier frequencies
(kHz)
900 800 700 600 500 400 300 200
Cycle 1
Cycle 2
7 8 9 10 11 12 13 14 l5 16
Hop periods
It can be shown that this scheme can accomplish the previously mentioned goals
If there are many k-bit patterns and the hopping period is short, a sender and receiver can have privacy If an intruder tries to intercept the transmitted signal, she can only access a small piece of data because she does not know the spreading sequence to quickly adapt herself to the next hop The scheme has also an antijamming effect A malicious sender may be able to send noise to jam the signal for one hopping period (randomly), but not for the whole period
Bandwidth Sharing
If the number of hopping frequencies is M, we can multiplex M channels into one by using the same Bss bandwidth This is possible because a station uses just one frequency in each hopping period; M - 1 other frequencies can be used by other M - 1 stations In other words, M different stations can use the same Bss if an appropriate modulation technique such as multiple FSK (MFSK) is used FHSS is similar to FDM, as shown in Figure 631 Figure 631 shows an example of four channels using FDM and four channels using FHSS In FDM, each station uses 11M of the bandwidth, but the allocation is fixed; in FHSS, each station uses 11M of the bandwidth, but the allocation changes hop to hop
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