- The frequency hopping
spread spectrum (FHSS)
transmission technique:
- The sender
and receiver
first agree
on a list of frequences:
f1 f2 f3 f4 ... fN
- The sender
and receiver
then
pick (agree) on
a random sequence:
r1 r2 r3 r4 .... (indefinitely long)This can be achieve by using the same seed in a common random number generator.
- Sender:
- Send the
first bit of the message on
frequence
fr1
- Send the
bit 2 of the message on
frequence
fr2
...
- Send the
first bit of the message on
frequence
fr1
- Receiver:
- Receive the
first bit of the message on
frequence
fr1
- Receive the
bit 2 of the message on
frequence
fr2
...
- Receive the
first bit of the message on
frequence
fr1
- The sender
and receiver
first agree
on a list of frequences:
- Graphically:
Explanation:
- The picture above shows
4 different times
that FHSS transmits
- Each transmission uses a different frequency
NOTE:
- Some data
can still
be lost
due to jamming
- Data transmitted on frequencies that is being jammed will be lost
But:
- The data transmitted on the unjammed frequencies will be received !!!
- The picture above shows
4 different times
that FHSS transmits
- Fact:
- voice communication is
highly "redundant"
I.e.:
- It is possible to lose part of the conversation and the rest is still comprehensible !!!
- voice communication is
highly "redundant"
- Consequently:
- Frequency hopping Spread spectrum can effectively protect audio messages from being jammed
- Applications:
- Anti-jamming:
- Most of the
audio conversation will
be unjammed
- Audio with
some clipping is
highly understandable
- Most of the
audio conversation will
be unjammed
- Anti-eavesdropping:
- An eavesdropper
tunes (= listens) in
to the transmission in
a certain frequency
- By "spreading" the
transmission over
multiple randomized frequencies,
most
of the transmitted audio signal
will
not be eavesdropped
- An eavesdropper
tunes (= listens) in
to the transmission in
a certain frequency
- Anti-jamming:
- Note:
- This technique is
high effective in
protecting
audio transmission
- Fact:
- Audio is highly redundant
(you don't need to hear everything to understand what is communicated)
- So losing a few bits
in the audio stream
due to jamming
will not
prevent the communication
- Having a few bits "stolen" will not allowed the thief to understand the entire conversation
- This technique is
high effective in
protecting
audio transmission
- Frequency-hopping spread spectrum - Wikipedia: click here