Lecture 4

Spread Spectrum

• In some applications, we have some In some applications, we have some concerns that outweigh bandwidth efficiency. concerns that outweigh bandwidth efficiency.

• In wireless applications, stations must be In wireless applications, stations must be able to share this medium without able to share this medium without interception by an eavesdropper and without interception by an eavesdropper and without being subject to jamming from a malicious being subject to jamming from a malicious intruder to achieve these goalsintruder to achieve these goals

• spread spectrum techniques add spread spectrum techniques add redundancyredundancy

6.2

Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have if its frequency were not varied.

This wireless technique is used in LAN & WAN that requires a bandwidth several times more than original bandwidth

In SS signals from different sources are also combined to fit into larger bandwidth

SS spread the original spectrum needed for each stationBandwidth required by station =BBandwidth spreded by SS is Bss

Bss >> B

The expanded bandwidth allows the source to wrap its message in a protective envelope for more secure transmission; like we send a delicate costly gift

Figure explains the technique:

6.3

Figure: Spread spectrum

Spreading code

How is the SS signal different from the normal signal?The signal is different from a normal signal, in the following aspects :1.This signal occupies a larger bandwidth than that of a normal signal. (Therefore the name Spread Spectrum).2.The spread spectrum signal invariably uses some kind of coding. The spectrum spreading at the transmitter and dispreading (opposite to spreading) at the receiver is obtained with the help of this code word. The code word associated with an SS signal is independent of the information carried by a signal.

3. The most important point is that the SS signal is “pseudorandom” in nature. This makes it appear like “random noise”. Therefore the normal receiver cannot demodulate the SS signal. Only a specially designed receiver can demodulate it to recover the information.

Application of Spread Spectrum Modulation The spread spectrum signals are used in the following application:1.In combating the intentional interference (jamming).2.In rejecting the unintentional interference from some other user: This is possible to achieve by assigning a different code for the signals from various users. This type of communication which allows multiple users to simultaneously a common channel for transmission of information is called as code division multiple access (CDMA).

3. To avoid the self interference due to multipath propagation : A signal can take multiple paths while travelling over a communication channel. This signal components following different path lengths will result in a dispersed signal at the receiver. This can be considered as the self-interference. This type of interference also can be suppressed by using the SS modulation.

4. In low probability of intercept (LPI) signals : A message can be hidden in the background noise by spreading its bandwidth using the code word and then transmitting the coded signal at a low power level. Due to these modification, the probability of being intercepted (detected) is reduced to a great extent. Hence such a spread and coded signal is called as the low probability-of-intercept (LPI) signal.

5. In obtaining the message privacy : The message privacy can be obtained by superimposing a pseudorandom pattern on the transmitted message.

Information sequence

Spread spectrum signal

Channel encoder

Modulator Channel Demodulat

or Channel decoder

Pseudorandom pattern

generator

Pseudorandom

pattern generator

Output data

Figure : model of spread spectrum digital communication system

Model of Spread Spectrum

Operation of The Model of spread spectrum System The information sequence at the input of the

system is a binary information sequence. The same signal is recovered at the output of the system as output data signals.

This model is also using channel encoder, channel decoder, modulator and demodulator.

In digital communication system, two additional blocks called “pseudo-random” are also used. One of them is connected to the modulator one the transmitter side whereas the other is connected to the demodulator on the receiving side. Both these generators are identical to each other.

These generators generate a pseudorandom or pseudonoise (PN) binary sequence. It is impressed on the transmitted signal at the modulator. Thus the modulator signal along with pseudorandom sequence travels over the communication channel. This sequence spreads the signals randomly over a wide frequency band. Thus the output of the modulated is a spread spectrum signals.

Pseudorandom sequence is removed from the received signal, by the other “Pseudorandom generator” operating at the receiver. Thus the pseudorandom pattern generators operate in synchronization with each other.

The synchronization between these generators is achieved before the beginning of the signal transmission. This is done by transmitting a fixed a fixed pseudorandom bit pattern which a receiver can recognize even in presence of interference. Once this synchronization is established, it is possible to begin the transmission.

Thus in the spread spectrum receiver can demodulate the transmitted signal if and only is a known pseudo-noise sequence has been transmitted along with the information signals.

Two types of interference are present in the S.S digital communication system namely the narrow band or broadband interference.

The modulation techniques used are :1. Phase shift keying (PSK)2. Frequency shift keying (FSK)

If PSK is used then the PN sequence generated at the modulator is used along with the PSK modulation to shift the phase of the PSK signal pseudorandomly. The resulting signal at the modulator output is called as a “Direct sequence” spread spectrum signal.

If binary or M-ary FSK is being used, then the frequency of the FSK signals is shifted pseudorandomly. The resulting signals at the output of the modulator is called as “Frequency Hopped” (FH) spread spectrum signal.

SS works on following 2 principles :1. The bandwidth allocated to each station

needs to be, by far, larger than what is needed. This allows redundancy.

2. The expanding the original bandwidth to 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 signal is created by the sourceAfter the signal is created by source, the spreading

process uses a spreading code & spreads the bandwidth.

Spreading code is a series of numbers that look like pattern.

Techniques of Spread Spectrum1. Frequency Hopping Spread

Spectrum(FHSS)2. Direct Sequence Spread

Spectrum(DSSS)

6.17

Frequency Hopping Spread Spectrum (FHSS)Frequency Hopping Spread Spectrum (FHSS)

Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

6.19

Figure: Frequency hopping spread spectrum (FHSS)

FHSS uses M different carrier frequencies that are modulated by sources signal

Its modulates one frequency at one moment then other frequency at other moment

After 8 hoppings the pattern repeats starting again from 101

6.20

6.21

Figure: FHSS cycles

How the eavesdropping & jamming is avoided? If a intruder tries to intercept the

transmitted signal, it can only access a small piece of data because it doesn’t know the spreading sequence to quickly adapt for next hop

Anti-jamming : a malicious sender may be able to send noise to jam the signal for one hopping period(randomly), but not for the whole period

Applications of FHSS : Military uses it.

6.22

Bandwidth Sharing in FHSS FHSS is similar to FDM It can use Multiple FSK (MFSK) Let no of hopping frequencies be M we can multiplex M channels into one by

using the same Bss In FDM, each station uses 1/M of bandwidth

but allocation is fixed In FHSS, each station uses 1/M of bandwidth

but allocation changes at every hop

6.23

6.24

Figure: Bandwidth sharing

Case of 4 frequencies

6.25

Sr. No.

Slow frequency hopping Fast frequency hopping

1 More than one symbols are transmitted per frequency hop.

More than one frequency hops are required to transmit one symbol.

2 Chip rate is equal to the symbol rate. Chip rate is equal to the hop rate.

3 Symbol rate is higher than hop rate. Hop rate is higher than symbol rate.

4 Same carrier frequency is used to transmit one or more symbols.

One symbol is transmitted over multiple carriers in different hops.

5 A jammer can detect this signal if the carrier frequency in one hop is known.

A jammer can’t detect this signal because one symbol is transmitted using more than one carrier frequencies.

There are two types of FHSS:1. Slow Frequency & 2. Fast Frequency SS

6.26

Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)

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 n bits, called chips, where the chip rate is n times that of the data bit. Figure 6.32 shows the concept of DSSS.

6.27

Figure 6.32: DSSS

6.28

Figure DSSS example • This example uses famous Barker sequence (finite

sequence of n values) where n is 11.• Example also uses polar NRZ encoding• The spreading code is 11 chips having pattern

10110111000• The original signal rate is N & the rate of spreading signal

is 11N• This means that required BW for spread signal is 11 times

larger than the original signal.

Bandwidth sharing in DSSS The sharing of bandwidth is conditional:

Bandwidth can’t be shared if we use spreading code that spreads signals from different stations that can not be combined and is separated e.g. Some Wireless LAN s use DSSS & spread bandwidth which can’t be shared.

Bandwidth can be shared, if we use a special type of sequence code that allows combining & separating of spread signals e.g. a special code allow us to use DSSS in cellular telephony

6.29

Special spreading code allows us to use DSSS in digital cellular telephony

& share the bandwidth between several users

Comparison of DS-SS and FH-SS SystemsSr.No. Parameter Direct sequence

spread spectrumFrequency

hopping spread spectrum

1 Definition PN sequence of large bandwidth is multiplied with narrow band data signal.

Data bits are transmitted in different frequency slots which are changed by PN sequence.

2 Chip rate 1It is fixed Rc =---

Tc

Rc = max (Rh, Rs)

3 Modulation technique BPSK M-ary FSK

4 Acquistion time Long Short

5 Effect of distance This system is distance relative

Effect of distance is less

6.30

Advantages and Disadvantages of DS-SS systemsAdvantages This system has a very high degree of

discrimination against the multipath signals. Therefore the interference caused by the multipath reception is minimized successfully.

The performance of DS-SS system in presence of noise is superior to other system such as FH-SS system.

This system combats the intentional interference (jamming) most effectively.

Disadvantages With the serial search system, the acquisition

time is too large. This makes the DS-SS system slow.

The sequence generated at the PN code generator output must have a high rate. The length of such a sequence needs to be long enough to make the sequence truly random.

The channel bandwidth required, is very large. But this bandwidth is less than that of a FH-SS system.

The synchronization is affected by the variable distance between the transmitter and receiver.

Advantages and disadvantages of the FH-SS systemAdvantages The synchronization is not greatly dependent

on the distance. The serial search system with FH-SS needs

shorter time for acquisition. The processing gain PG is higher than that of

DS-SS system.

Disadvantages The bandwidth of FH-SS system is too large (in

GHz). Complex and expensive digital frequency

synthesizers are required to be used.