Spread Spectrum Concept Frequency Hopping Spread Spectrum Direct - - PowerPoint PPT Presentation

CMPE 477

Lecture 6:

Spread Spectrum

 Concept Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum

CMPE 477 – Wireless and Mobile Networks

Spread Spectrum

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration

• f the interference, i.e., narrowband interference

Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference

detection at receiver interference spread signal signal spread interference f f power power

Spreading and Despreading

dP/df f i) dP/df f ii) sender dP/df f iii) dP/df f iv) receiver f v) user signal broadband interference narrowband interference dP/df

Spreading and frequency selective fading

frequency channel quality 1 2 3 4 5 6 narrow band signal guard space 2 2 2 2 2 frequency channel quality 1 spread spectrum

narrowband channels spread spectrum channels

Spread Spectrum Input is fed into a channel encoder

 Produces analog signal with narrow bandwidth

Signal is further modulated using sequence of digits

 Spreading code or spreading sequence  Generated by pseudonoise, or pseudo-random number

generator

Effect of modulation is to increase bandwidth of signal to be transmitted On receiving end, digit sequence is used to demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data

Spread Spectrum What can be gained from apparent waste of spectrum?

Immunity from various kinds of noise and

multipath distortion

Can be used for hiding and encrypting signals Several users can independently use the

same higher bandwidth with very little interference

Direct Sequence Spread Spectrum (DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band

Spread is in direct proportion to number of bits used

One technique combines digital information stream with the spreading code bit stream (chipping sequence) using exclusive-OR

An example

user data chipping sequence resulting signal 1 1 1 0 1 0 1 0 1 1 1 1 XOR 1 1 0 1 0 1 1 1 1 = tb tc

tb: bit period tc: chip period

Spreading factor: s = tb/tc ws= s.w (w: original bandwidth) Advantages reduces frequency selective fading in cellular networks base stations can use the same frequency range Disadvantages precise power control necessary

DSSS Transmitter and Receiver

X user data chipping sequence modulator radio carrier spread spectrum signal transmit signal transmitter demodulator received signal radio carrier X chipping sequence lowpass filtered signal receiver integrator products decision data sampled sums correlator

Integrator: Addition of products Decision: Whether it is 1 or 0

• acc. to the product

What if the multipath propagation? Rake receiver

Another Example

Frequency Hoping Spread Spectrum (FHSS) Signal is broadcast over seemingly random series

• f radio frequencies

A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of

input signal

Signal hops from frequency to frequency at fixed intervals

Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency

is selected

Frequency Hoping Spread Spectrum Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages

Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency

succeed only at knocking out a few bits

Frequency Hoping Spread Spectrum

FHSS Using MFSK

MFSK signal is translated to a new frequency every Tc (dwell-time) seconds by modulating the MFSK signal with the FHSS carrier signal For data rate of R:

duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds (L:

number of signal elements) Tc Ts - slow-frequency-hop spread spectrum Tc< Ts - fast-frequency-hop spread spectrum (Better at

• vercoming frequency selective fading)

FHSS Using MFSK

FHSS Performance Considerations

Advantages

 frequency selective fading and interference limited to short

period

 Results in a system that is quite resistant to jamming,

Jammer must jam all frequencies

 simple implementation  uses only small portion of spectrum at any time

Disadvantages

 not as robust as DSSS (DSSS is more robust to fading and

multi-path effects)

 simpler to detect  Large number of frequencies used

Where are they used?

FHSS

• Bluetooth

DSSS

• 802.11b
• The United States GPS and European Galileo satellite

navigation systems

• Cordless phones operating in the 900 MHz, 2.4 GHz and

5.8 GHz bands

• IEEE 802.15.4