FHSS vs. DSSS
Presented by
Ali Alhajhouj
Presentation Outline
• Introduce the issues involved in the system
behaviors for FHSS and DSSS systems used in
WLAN (Wireless LAN) and BWA (Broadband
Wireless Access)
• Advantages of FHSS v. DSSS
• Disadvantages of FHSS v. DSSS
Systems Behaviors
• Some issues to study for FHSS and DSSS
systems:
– System collocation
– Noise and interference immunity
– Near/Far problem
– Multipath immunity
– Security
– Bluetooth interference
System collocation
• The issue: How many independent systems
may operate simultaneously without
interference ?
System collocation
• For installations requiring big coverage and
multiple collocated cells, it would be much easier
to used FHSS
• DSSS could be used but collocated cells should be
non-overlapping cells at the radio level, thus
requires using directional antennas:
– Directional antennas means limited coverage
– Requiring more systems to be installed
– Difficult to design because of the collocation issue
• This severe limitation of DSSS is in both 2Mbps
and 11Mbps
Noise and Interference Immunity
• The issue: What is the capability to operate
when other radio signals are present in the
same band ?
Noise and Interference Immunity
• FHSS systems operate with SNR(Signal to
Noise Ratio) of about 18 dB
• DSSS systems operate with SNR 12 dB because
it is using more efficient modulation
techniques, called Phase-shift keying (PSK)
Noise and Interference Immunity
• All band interference
– Noise
• Because DSSS systems operate with lower SNR, for
same level of transmitted energy, DSSS systems can
operate over longer distances than FHSS
– Interference
• Since whole spectrum used by the radio is 83.5 MHz in
FHSS, while it is 22 MHz in DSSS, there is a greater
chance of having interference in DSSS than FHSS
Near/Far problem
• The issue: The problems generated to a
receiver by other active transmitters located in
its proximity, also known as Near/Far problem
Near/Far problem
• If the receiver is using FHSS, the worst case
will be that “foreign” transmitter will block
some hops. Forcing the FHSS system to work
less than optimum conditions, but allowing it
to work
• In DSSS systems, the problem is more critical
and signals generated by the “foreign”
transmitter could not be heard at the receiver
Throughput
• The issue: What amount of data actually
carried by the system ?
Throughput
• Throughput is defined as the average amount
of data carried by the system
• DSSS systems provide greater throughput than
with non-synchronized FHSS
• Since synchronized FHSS systems never
collide, they provide more or less the same
throughput as DSSS systems
• With no synchronization, DSSS provides higher
throughput than FHSS
Multipath Immunity
• The issue: environments with reflective
surfaces generate multiple possible
propagation paths between transmitter and
receiver resulting in multiple copies of the
original (transmitted) signal, shifted in time
Multipath Immunity
• DSSS systems are extremely sensitive to the
multipath issue when compared to the FHSS
systems (especially when operated at 11 Mbps)
• To minimize multipath effects, point to multiple
topology systems have to be limited to small
geographical (such as offices)
• For long distance systems, the use of directional
antennas may help reducing the multipath
sensitivity in DSSS systems, however, FHSS
systems shows more resistance to multipath
immunity.
Security
• The issue: protecting the transmission against
eavesdropping
Security
• DSSS systems use spreading sequence of 11
chips, thus can modulate 14 channel defined
in the IEEE 802.11 standard.
• Since the carrier frequency is fixed for a given
DSSS system and possible frequencies is
limited, it would be easier for a listener to
“tune in” on the DSSS transmission
• This requires message encryption at the
application layer.
Security
• In FHSS, frequencies may be selected by the user
• In order for the bad guy to “tune in” on the FHSS
transmission, a bad guy should know:
1.
2.
3.
4.
# of frequencies selected in the system
The actual frequencies
The hopping sequence
The dwell time
• FHSS modulation acts as a layer 1 encryption
process, therefore, there is no need for
application level encryption.
Bluetooth Interference
• The issue: What is the system behavior in the
presence of Bluetooth/ IEEE 802.15 collocated
systems ?
Bluetooth Interference
• Bluetooth radio is a FHSS operating in the 2.4
GHz band ( with dwell time of 0.625ms)
• When DSSS executes a transmission, it is using
22 MHz for the duration of the transmitted
frame
• When FHSS executes a transmission, it is using
1 MHz for the duration of the transmitted
frame
Bluetooth Interference
• The chances of Bluetooth “hitting” the DSSS
systems is much higher than the chances of
hitting the FHSS systems
• Therefore, the DSSS is more sensitive to
Bluetooth interference rather than in the FHSS
systems
Conclusion
• DSSS provides higher (11Mbps) capacity than FHSS, but is
sensitive to many environmental factors such as (system
collocation, multipath effects, near/far problem, and
Bluetooth)
– For long distance applications, use point to point topologies
– For short distance applications, use point to multiple topologies
• FHSS provides only 3 Mbps, but it is very robust technology
( works better in harsh environment that is characterized by
large areas of coverage multiple collocated cells, noises &
interference, multipath, Bluetooth, and etc.)
– Best to use for big coverage areas with a big number of
collocated systems and where directional antennas are
impossible to implement. Therefore, it is more suitable for fixed
broadband wireless access (BWA)
End
Any Question ?