1. Direct Sequence Spread Spectrum vs
Direct Sequence Spread Spectrum vs. Frequency Hopping Spread Spectrum Prof./Dr. Gordon L. Stüber

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

3. Direct-sequence Spread Spectrum (DSSS)
Noise + Interference
BPSK, QPSK
modulator
Correlator
detector
Data
output
Data
input
PN sequence
Generator
PN sequence
Generator
synchronized
Conventional Correlator Detector

4. Direct-sequence Spread Spectrum (DSSS)
Noise + Interference
BPSK, QPSK
modulator
Multi-user
detector
Data
output
Data
input
PN sequence
Generator
Decorrelator detector
MMSE detector
Multi-user Detector

5. Frequency-Hop Spread Spectrum (FHSS)
Noise + Interference
FSK
mod +
FSK
demod
Decision
device
mixer
mixer
Data
input
Data
output
Frequency
synthesizer
Frequency
synthesizer
PN sequence
generator
PN sequence
generator
synchronized

6. 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

7. Processing 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 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.

8. 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.

9. DSSS Interference Averaging
DSSS: Rejects interference by interference averaging.
At input to the DSSS demodulator
Narrowband
interference
At input to the data detector
Narrow-band data
Wideband interference

10. DSSS Interference Averaging
Narrowband interference
Narrowband data
Wideband
interference
Wideband data f f
Before despreading
After despreading

11. DSSS Short Code in Tone Interference
Short Code: each data symbol is spread by a full period of the spreading sequence.

12. FHSS Interference Avoidance
FHSS: Rejects interference by interference avoidance.
Narrow-band interference 1 2 M N 1
FH bins
Hit Probability:

13. Ranging 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.

14. Power Control Near-far effect
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.
Near-far
effect

15. Detection 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.

16. Multipath and Multiple-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.

17. Diversity
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.

18. Add-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.

19. Spectral 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

20. Summary—Advantages DSSS FHSS Processing Gain
Electromagnetic Compatibility
Interference Rejection
Radiolocation
Power Control
Detection
Multipath and Multiple-access Interference
Diversity
Add-on Flexibility
Spectral Efficiency