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
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