Direct Sequence Spread Spectrum vs.

Frequency Hopping Spread Spectrum

Prof./Dr. Gordon L. Stüber

Direct Sequence Spread Spectrum vs.

Frequency Hopping Spread Spectrum

Prof./Dr. Gordon L. Stüber

ContentsContents

• Introduction• Processing Gain• Electromagnetic Compatibility• Interference Rejection• Radiolocation• Power Control• Detection• Multipath and Multiple-access Interference• Diversity• Add-on Flexibility• Spectral Efficiency

PN sequenceGenerator

PN sequenceGenerator

synchronized

BPSK, QPSKmodulator

Correlatordetector

)t(i)t(n Noise + Interference

Datainput

Dataoutput

nx nx̂

Conventional Correlator Detector

Direct-sequence Spread Spectrum (DSSS)

Direct-sequence Spread Spectrum (DSSS)

Direct-sequence SpreadSpectrum (DSSS)

Direct-sequence SpreadSpectrum (DSSS)

PN sequenceGenerator

BPSK, QPSKmodulator

Multi-userdetector

)t(i)t(n

Noise + Interference

Datainput

Dataoutput

nx nx̂

Multi-user Detector

Decorrelator detectorMMSE detector

FSKmod

FSKdemod

mixer mixerDecisiondevice

Frequencysynthesizer

Frequencysynthesizer

PN sequencegenerator

PN sequencegenerator

synchronized

nx

Datainput

)t(i)t(n Noise + Interference

+

nx̂

Dataoutput

Frequency-Hop Spread Spectrum (FHSS)

Frequency-Hop Spread Spectrum (FHSS)

Some Commercial System Examples

Some Commercial System Examples

• DSSS: – LANs and PANs: IEEE802.11, IEEE802.11b,

Wi-LAN Hopper Plus– Cellular: EIA/TIA IS-95, W-CDMA– IEEE802.11b complementary code keying (CCK) is a form

of orthogonal multipulse signaling– orthogonal frequency division multiplexing is also a form of

orthogonal multipulse signaling.

• FHSS:– LANs and PANs: Bluetooth– Cellular: GSM – slow frequency hop add-on

Processing GainProcessing Gain

• DSSS: the spread bandwidth (and processing gain)is limited by the clock rate of the PN sequence generator. A 100 Mcps clock rate with root-raised cosine chip shaping requires a 100-200 MHz bandwidth.

• FHSS: the spread bandwidth is not limited by clock speed. The processing gain is limited by the available bandwidth.– Bandwidth does not have to be contiguous.– Hop rate (for fast frequency hopping) is limited by clock

speed.

Electromagnetic Compatibility

Electromagnetic Compatibility

• DSSS: spreads the signal energy throughout the entire bandwidth, thereby minimizing interference to other systems.

• FHSS: uses a small instantaneous bandwidth. When the signal hops into a bandwidth that is occupied by another narrowband signal it will cause interference.

• DSSS: Rejects interference by interference averaging.

• At input to the DSSS demodulator

)t(Itcos)t(a)t(xP2)t(r cs waveformdata)t(x

waveformspreading)t(a erferenceintnarrowband)t(I

• At input to the data detector

tcos)t(a)t(I2)t(xP)t(v cs

Narrow-band data Wideband interference

Narrowbandinterference

DSSS Interference AveragingDSSS Interference Averaging

Narrowband data

Widebandinterference

Narrowband interference

Wideband data

f f

Before despreading After despreading

DSSS Interference AveragingDSSS Interference Averaging

• Short Code: each data symbol is spread by a full period of the spreading sequence.

DSSS Short Code in Tone Interference

DSSS Short Code in Tone Interference

• FHSS: Rejects interference by interference avoidance.

N1

1 2 M

FH bins

N

MPH

Narrow-band interference

Hit Probability:

FHSS Interference Avoidance

FHSS Interference Avoidance

Ranging and RadiolocationRanging and Radiolocation

• DSSS can use the code acquisition and tracking loops for ranging and time-based radiolocation.

• For a 3.84Mcps chip rate (UTRA W-CDMA) and a 1/8 chip resolution, the range estimates are accurate to within 10 m.

• Not possible with FHSS.

1r1BS

2r

3r

2BS

3BS

MS

cr ii

Power ControlPower Control

• For DSSS with a conventional correlator detector, we must have equal received power from all MSs at the BS, i.e.,– Otherwise a CDMA multiuser

detector is required.

• Power control is not a requirement with FHSS due to interference avoidance.

1MS

2MS

3MS

BS1P

2P

3P321 PPP

Near-fareffect

DetectionDetection

• DSSS: coherent pilot-aided detection is used.– non-coherent detection is employed when there is no

pilot.

• FHSS: non-coherent detection is used, since the channel is uncorrelated at different hop frequencies.– Coherent detection can be used with very slow

frequency hopping, e.g., Bluetooth.

• Coherent detection provides a 1 to 3 dB improvement in receiver sensitivity over non-coherent detection.

Multipath andMultiple-access Interference

Multipath andMultiple-access Interference

• Both DSSS and FHSS can avoid multiple-access interference by using synchronous CDMA, e.g., forward channel operation in cellular CDMA.

• Multiple-access interference is generated by asynchronous CDMA, e.g., reverse channel operation in cellular CDMA

• DSSS: multipath accentuates multiple-access interference.

• FHSS: signals do not suffer from multipath because of their narrow instantaneous bandwidth.

DiversityDiversity

• DSSS: A high resolution RAKE receiver can be used to obtain multipath diversity by resolving and combining signal replicas that are received at different delays.– Signal replicas are independently faded but must be separated

in time by at least a chip duration to be resolved.

• FHSS: Fast frequency hopping (FFH) can be used to obtain frequency diversity on frequency selective channels.– With FFH the data symbols are transmitted on multiple hops.– Successive hops must be separated in frequency by at least

the channel coherence bandwidth to yield independently faded replicas.

Add-on FlexibilityAdd-on Flexibility

• Frequency hopping is easy to include as an add-on feature to F/TDMA narrowband systems for the purpose of interference averaging.– Example: GSM with optional slow frequency hopping.

• Direct sequence spreading is difficult to include as an add-on feature to F/TDMA narrowband systems.

Spectral EfficiencySpectral Efficiency

• Orthogonal frequency division multiplexing (OFDM) and direct sequence CDMA can be combined.– High spectral efficiency and robust performance.– Reduced complexity of equalization or RAKE receiver– Finer partition of time, frequency and code domains gives

greater flexibility in allocation of radio resources.

• Several types of OFDM-CDMA are possible– Multicarrier CDMA (MC-CDMA)– Multicarrier direct sequence (DS)-CDMA (MC-DS-CDMA)– Multitone (MT)-CDMA

Processing Gain

Electromagnetic Compatibility

Interference Rejection

Radiolocation

Power Control

Detection

Multipath and Multiple-access Interference

Diversity

Add-on Flexibility

Spectral Efficiency

DSSS FHSS

Summary—AdvantagesSummary—Advantages