Advanced Wireless and Mobile Communications

Lecture 7

Spread Spectrum

Dr. Mohammed Usman

Recall

Bandwidth is a scarce and expensive resource

Primary design objective: Minimize the required transmission BW maximize BW efficiency

Spread spectrum systems: use transmission BW much greater than minimum required BW

BW inefficient for a single userBUT, with spread spectrum, several users can

share the same BW without significantly interfering with one another

With multiple users sharing the same BW, spread spectrum systems exhibit high spectral efficiency

Spread Spectrum

Frequency

Power

Spread Spectrum(Low Peak Power)

Narrowband(High Peak Power)

Spread spectrum advantagesResistance to narrowband interference &

jammingSecurity – signal can be hidden below the noise

floor making it difficult to be detected and intercepted

Spread spectrum receivers can be designed such that multipath can be used to advantage – reducing ISI

Transmitted power is spread over a wide frequency band – low PSD SS systems cause minimal interference to other narrowband systems

Allows multiple users to operate in the same frequency band with minimal interference

Spread spectrum - terminology

Signal of BW = Rs

Spreading operation

Spreading signal BW = Rc

>> Rs

Processing gain Gp = Rc/Rs

Also called as spreading factor or expansion factor

Gp represents the factor by which interference is

suppressed relative to the desired signal, when operating

in the same band

Gp is of the order of 100 to 1000 in commercial applications and

up to a million in military applications

Types of spread spectrum

Signal of BW = Rs

Spreading operation

Spreading signal BW = Rc >> Rs

Direct Sequence Spread

SpectrumFrequency

Hopping Spread Spectrum

Chirp Spread Spectrum

Time Hopping Spread

SpectrumHybrid system

CL

AS

SIF

IE

DC

OM

ME

RC

IA

L

Narrowband vs Spread spectrum

Narrowband transmission – easy to intercept and jam

Spread signal is embedded below the noise floor – hard to detect and intercept

Signal is spread over a wide frequency band – difficult to jam

Low PSD – does not interfere with other narrowband systems

Spread spectrum modulation

txb =dt x pnt

Spread Spectrum Demodulation

dr =rxb x pnr

For sync

pnr = pnt

Spreading and de-spreading

Spreading

For a receiver to decode the transmitted sequence, it must multiply the received sequence with the same pseudo-random code that was used at the transmitter pnr = pnt

If pnr ≠ pnt, there is no de-spreading action

A receiver that does not know pnt, cannot reproduce the transmitted data

ttb pndtx

rbr pnrxd

De-spreadingDespread dataAssuming error free channel rxb = txb

PN sequence selected such that auto-correlation is maximum

Therefore, dr = dt

If pnr ≠ pnt

PN sequence selected such that

Cross correlation is very small Orthogonal property of codes

rbr pnrxd

)()( ttttttr pnpndpnpndd

tallforpnpn tt 1

)()( rttrttrbr pnpndpnpndpnrxd

trallforzeroideallypnpn rt )(1

Spread spectrum in the presence of interference

Consider transmitting a spread spectrum signal txb

Received signal rxb is transmitted signal plus interference i.e.

To recover the original signal, multiply the received data with the same PN sequence used at the transmitter

Since pnt x pnt =1,

Multiplying the interfering signal i with pnt spreads the interfering signal, thereby lowering its power density

After de-spreading, data is narrowband (Rs) and interference is wideband (Rc)

LPF is used to extract the desired data and filter off most of the interference

Spread spectrum with narrowband interference

Spread spectrum with wideband interference

Source of wideband interference – Other SS users operating in the same frequency band (multiple access) &Gaussian Noise

PN SequencesChoice of proper PN sequences is important

for spread spectrum systemsAuto-correlation must be maximum for

perfect synchronization of two identical sequences

Autocorrelation is a measure of agreement between a sequence and time shifted versions of itself

PN SequencesCross-correlation must be zero (ideally) for all

shifts of two different codes orthogonalIn practice, codes are not perfectly orthogonalCross-correlation must therefore be as small

as possible to avoid performance degradation

Multiple users share the same frequency spectrum and are distinguished by their respective PN codes

For large number of users sharing the spectrum, PN codes must be chosen carefully to avoid interference between users

Cross-correlation is a measure of agreement between two different codes pni and pnj It defines the interference between codes pni and pnj

Code Division Multiple Access (CDMA)Multiplexing users by distinct (orthogonal

codes)

Each user has its own PN code and uses the same BW

All active users transmit simultaneously

CDMA

Suppose user 1’s data is required at the receiverMultiply received signal with pn1

Only user 1’s data is de-spread. All other users produce noise for user 1

Power control in CDMAEach user is a source of noise for other usersIf more power is received from an undesired user, it

contributes more noisePower control is necessary in CDMA to ensure

received signal (at the BS) from all users is of the same power

If all users transmit the same power, signal from a nearby receiver will be stronger than signal from far-away receiver – NEAR-FAR problem

Transmit power of all users is controlled such that received signal from all users is same (Prx)

If there are N simultaneous users, there are (N-1) interferers

As N increases, SNR decreases BER increases

1

1

)1(

NPN

PSNR

rx

rx

Multipath rejection in CDMA

τ – time delay between the two pathsReceiver PN code synchronized with PN code corresponding to direct pathReceiver PN code will be out of sync with the reflected path PN code by an amount τpn(t) x pn(t-τ) will be very small for τ greater than one chip interval Un-correlated paths do not contribute to multipath fadingMultipath signal not synchronized with receiver PN code are therefore rejected

SummarySS system advantages

Signal hiding under noise – security

Low PSD – does not interfere with other systems

Multiple access (CDMA)

Resistance to jamming, multipath fading and interference

Low probability of interception

SS system drawbacks

Does not do anything to deal with Gaussian noise

Increased BWIncreased

complexity and computation