SPREAD SPECTRUM

In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy; they spread the original spectrum needed for each station.

If the required bandwidth for each station is B, spread spectrum expands it to Bss such that Bss » B.

Spread spectrum achieves its goals through two principles:

1. The bandwidth allocated to each station needs to be, by far, larger than what is needed. This allows redundancy.
2. The expanding of the original bandwidth B to the bandwidth Bss must be done by a process that is independent of the original signal. In other words, the spreading process occurs after the signal is created by the source.

Spread spectrum

Topics discussed in this section:

• Frequency Hopping Spread Spectrum (FHSS)
• Direct Sequence Spread Spectrum Synchronous (DSSS)

Frequency Hopping Spread Spectrum(FHSS)

• 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 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.

Frequency hopping spread spectrum (FHSS)

FHSS Process

1. A pseudorandom code generator, called pseudorandom noise (PN), creates a k-bit pattern for every hopping period Th •
2. The frequency table uses the pattern to find the frequency to be used for this hopping period and passes it to the frequency synthesizer.
3. The frequency synthesizer creates a carrier signal of that frequency, and the source signal modulates the carrier signal.

FHSS Example

FHSS cycles

Fast FHSS Advantages

If there are many k-bit patterns and the hopping period is short, a sender and receiver can have privacy.

Intruder can only access a small piece of data because she does not know the spreading sequence.

The scheme has also an anti-jamming effect.

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 B ss 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 B ss if an appropriate modulation technique such as multiple FSK (MFSK) is used.

Bandwidth Sharing

Direct Sequence Spread Spectrum

• The direct sequence spread spectrum (DSSS) technique also expands the bandwidth of the original signal, but the process is different.
• In DSSS, we replace each data bit with n bits using a spreading code.
• In other words, each bit is assigned a code of 11 bits,called chips.

DSSS

DSSS Example

• Let us consider the sequence used in a wireless LAN, the famous
• Barker sequence where n is 11 .
• Let the spreading code is 11 chips having the pattern 10110111000.
• Let assume that the original signal and the chips in the chip generator use polar NRZ encoding.
• If the original signal rate is N, the rate of the spread signal is 11*N.

Advantages:

• The spread signal can provide privacy if the intruder does not know the code.
• It can also provide immunity against interference if each station uses a different code.

DSSS example