- Slides: 20
Direct Sequence Spread Spectrum vs. Frequency Hopping Spread Spectrum Prof. /Dr. Gordon L. Stüber
Contents • • • Introduction Processing Gain Electromagnetic Compatibility Interference Rejection Radiolocation Power Control Detection Multipath and Multiple-access Interference Diversity Add-on Flexibility Spectral Efficiency
Direct-sequence Spread Spectrum (DSSS) Noise + Interference BPSK, QPSK modulator Data input Correlator detector PN sequence Generator synchronized Conventional Correlator Detector Data output
Direct-sequence Spread Spectrum (DSSS) Noise + Interference BPSK, QPSK modulator Data input Multi-user detector PN sequence Generator Data output Decorrelator detector MMSE detector Multi-user Detector
Frequency-Hop Spread Spectrum (FHSS) Noise + Interference + Data input FSK mod mixer Frequency synthesizer PN sequence generator synchronized FSK demod Decision device Data output
Some Commercial System Examples • DSSS: – LANs and PANs: IEEE 802. 11, IEEE 802. 11 b, Wi-LAN Hopper Plus – Cellular: EIA/TIA IS-95, W-CDMA – IEEE 802. 11 b 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 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 • 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 Interference Averaging • DSSS: Rejects interference by interference Narrowband averaging. interference • At input to the DSSS demodulator • At input to the data detector Narrow-band data Wideband interference
DSSS Interference Averaging Narrowband interference Wideband data Narrowband data Wideband interference f Before despreading f After despreading
DSSS Short Code in Tone Interference • Short Code: each data symbol is spread by a full period of the spreading sequence.
FHSS Interference Avoidance • FHSS: Rejects interference by interference avoidance. Narrow-band interference 1 2 M N 1 FH bins Hit Probability:
Ranging and Radiolocation • DSSS can use the code acquisition and tracking loops for ranging and time-based radiolocation. • For a 3. 84 Mcps chip rate (UTRA W-CDMA) and a 1/8 chip resolution, the range estimates are accurate to within 10 m. • Not possible with FHSS.
Power 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. Near-far effect • Power control is not a requirement with FHSS due to interference avoidance.
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 d. B improvement in receiver sensitivity over non-coherent detection.
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.
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.
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.
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
Summary—Advantages DSSS Processing Gain Electromagnetic Compatibility Interference Rejection Radiolocation Power Control Detection Multipath and Multiple-access Interference Diversity Add-on Flexibility Spectral Efficiency FHSS