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DSSS versus FHSS-difference between DSSS and FHSS

This page on DSSS versus FHSS describes difference between DSSS and FHSS. Both are used in 802.11 WLAN and 802.15 WPAN networks.

DSSS-Direct Sequence Spread Spectrum

DSSS Transmitter Receiver Block Diagram

In DSSS, which stands for Direct Sequence Spread Spectrum, information bits are modulated by PN codes(chips). PN codes are Pseudonoise code symbols. This PN codes have short duration compare to information bits. Here transmitted information over the air occupies more bandwidth compare to user informaion bits. DSSS is the modulation technique adopted in IEEE 802.11 based WLAN compliant products. In DSSS systems entire system bandwidth is available for each user all the time.

Figure-1 depicts DSSS Transmitter and DSSS receiver Block Diagram. PRS stands for Pseudo-Random Sequence. Refer CCK vs DSSS vs OFDM>> which explains DSSS transmitter and receiver with signal waveforms.

FHSS-Frequency Hopping Spread Spectrum

FHSS Transmitter Receiver Block Diagram

In FHSS, which stands for Frequency Hopping Spread Spectrum, RF carrier frequency is changed according to the Pseudo-random sequence(PRS or PN sequence). This PN sequence is known to both transmitter and Receiver and hence help demodulate/decode the information. Within one chip duration, RF frequency does not vary. Based on this fact there are two types of FHSS, fast hopped FHSS and slow hopped FHSS.

In Fast hopped FHSS, hopping is done at the rate faster than message(information) bit rate. In slow hopped FHSS, hopping is done at the rate slower than information bit rate.

Figure-2 depicts FHSS Transmitter and DSSS receiver Block Diagram.

Difference between DSSS and FHSS

DSSS Spectrum vs FHSS Spectrum

Figure-3 depicts DSSS spectrum and FHSS spectrum.

• As FHSS systems rely on varying RF carrier frequency, it leads to bursty nature of errors due to frequency selective fading mainly. For more information on different types of channel models rayleigh, rician fading refer our page on channel model.

• In DSSS, information bits are spread across both frequency and time planes, hence minimizes effect of interference as well as fading. Hence DSSS system prone to errors but at low level compare to FHSS systems. FHSS produces strong bursty errors.

• DSSS delivers capacity upto 11 Mbps while FHSS supports upto 3 Mbps.

• DSSS is very sensitive technology while FHSS is very robust technology. This is observed in harsh environment comprising large coverage, noises, collocated cells, multi-path and presence of bluetooth frequency waves etc.

DSSS is ideal for point to point applications while FHSS can be used in point to multipoint deployment with excellent performance.

Useful Links

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CDMA tutorial, Read more

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