Direct-Sequence Spread Spectrum vs Frequency-Hopping Spread Spectrum

1. Direct Sequence Spread Spectrum vs. Frequency Hopping Spread SpectrumProf./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 SpreadSpectrum (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. 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 Some Commercial System Examples

7. 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 availablebandwidth. Bandwidth does not have to be contiguous. Hop rate (for fast frequency hopping) is limited by clock speed. Processing Gain

8. 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. Electromagnetic Compatibility

9. DSSS: Rejects interference by interference averaging. At input to the DSSS demodulator DSSS Interference Averaging 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. Short Code: each data symbol is spread by a full period of the spreading sequence. DSSS Short Code in Tone Interference

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

13. 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. Ranging and Radiolocation

14. 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. Power Control Near-far effect

15. 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. Detection

16. 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. Multipath andMultiple-access Interference

17. 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. Diversity

18. 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. Add-on Flexibility

19. 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 Spectral Efficiency

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