- The spread spectrum technique:
- Spread-spectrum techniques
transmits a signal using
multiple frequencies
- The signal is transmitted on a
bandwidth
that
considerably larger
than the
frequency content
of the original information.
(That means it uses more frequencies from the bandwidth than necessary)
- Spread-spectrum techniques
transmits a signal using
multiple frequencies
- Some techniques to achieve "spread spectrum" transmission:
- Frequency hopping
- Direct sequence
(There are a few more, that I will not cover...)
- Frequency hopping
- The Frequency hopping technique:
- The sender and receiver first agree
on a list of frequences
f1,
f2, ..., fn
(This is usually done by selecting a seed number and use the same random number generator to generate n numbers)
- The sender sends the
first bit of the message on
frequence f1
Then sends the bit 2 of the message on frequence f2
...
And sends the bit n of the message on frequence fn
And repeat from the start...
- The receiver listens for the
first bit of the message on
frequence f1
Then listens for the bit 2 of the message on frequence f2
...
And listens for the bit n of the message on frequence fn
And repeat from the start...
NOTE:
- The message transmitted
using frequency hopping
can be
protected by
an error correcting code
- So when a few bits are corrupted, the message can be reconstructed.
- The sender and receiver first agree
on a list of frequences
f1,
f2, ..., fn
- Effect:
- Anti-jamming:
- A jam-signal will
corrupt transmission in
a certain range of frequencies
- By "spreading" the
transmission over
multiple frequencies,
many of the transmitted bit will be
received correctly
- A jam-signal will
corrupt transmission in
a certain range of frequencies
- Anti-eavesdropping:
- An eavesdropper tunes in
to the transmission in
a certain frequency
- By "spreading" the
transmission over
multiple frequencies,
most of the transmitted information will
not be eavesdropped
- An eavesdropper tunes 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
- Direct-sequence Spread Spectrum:
- The sender and the
receiver select a
sequence of N bits
This bit sequence is call a Psuedo Noise (PN) code
Each bit in the pattern is called a chip
The number of 0's and 1's in the PN code should be approximately the same (they are "randomly")
- When the sender wants to send
a "1" bit,
it transmits the
PN code
When the sender wants to send a "0" bit, it transmits the inverse of the PN code
Example:
- The receiver uses the following
decoding algorithm:
- Receive N bits
- If a majority of the
received N bits
matches the
PN code,
the receiver accepts it as a
"1" bit
- If a majority of the received N bits fails to matches the PN code, the receiver accepts it as a "0" bit
- Receive N bits
- The sender and the
receiver select a
sequence of N bits
- Example:
- Communication:
- A ⇒ B using chip = 0011011
- C ⇒ D using
chip = 1011010
All nodes are within range of each other....
Schematically:
- Effect of interference:
- Suppose we use
amplitude modulation:
- 1 = high amplitude wave
- 0 = low amplitude wave
-
Suppose
A &rArr B
and
C ⇒ D
simultaneously:
- A sends "1" to B ( A will send the PN code "0011011")
- C sends "1" to D ( C will send the PN code "1011010")
Graphically: (Amplitude Modulation)
- The interference
will cause the
waves to be
super-imposed
The receivers will "hear" (receive) the following pattern:
When both receiver try to decode the reception, they will receive the corrupted pattern:
- 1011011
- Suppose we use
amplitude modulation:
- Decoding a corrupted reception:
- Receiver B:
Received pattern: 1011011 PN code: 0011011 -------------- XOR 1000000The majority of the bits matches the PN code
B will receives "1"
- Receiver D:
Received pattern: 1011011 PN code: 1011010 -------------- XOR 0000001The majority of the bits matches the PN code
D will receives "1"
- Receiver B:
- Communication:
- Consider
all possible scenarios
that the transmissions
A ⇒ B and
C ⇒ D
can interact
with each other:
- At the bit level, the senders A and C can transmit either a "1" bit or a "0" bit
- There are then
4 possible
interference patterns:
Let's consider for each case whether the receivers B and D can decode the transmission correctly....
- The following figure consider these
interferences:
- A
sends "1"
and
C
sends "1"
- A sends "0" and C sends "0"
What the receivers B and D will decode:
Result:
- Both receivers B and D will decode correctly despite the interference
- A
sends "1"
and
C
sends "1"
- The following figure consider these
interferences:
- A
sends "1"
and
C
sends "0"
- A sends "1" and C sends "0"
What the receivers B and D will decode:
Result:
- Again, both receivers B and D will decode correctly despite the interference
- A
sends "1"
and
C
sends "0"
- Conclusion:
- Using these
specific chips,
each
sender/receiver pair
is able to
decode the transmitted bit correctly.
- Therefore,
the transmitting nodes
are able to
share
the transmission medium
(The direct-sequence spread spectrum has the "anti-jamming" property !!!).
- Using these
specific chips,
each
sender/receiver pair
is able to
decode the transmitted bit correctly.
- We did not get this
anti-jamming property
without
paying a price:
- In order to transmit one bit,
we transmit 7 bits !!!
I.e., we used 7 times the required bandwidth !!!
- In order to transmit one bit,
we transmit 7 bits !!!
- So we paid a
hefty price
in transmission efficiency
- Two PN codes
can be alike or
different
- The measure of
"likeness" is
correlation
defined as:
∞ 1 / Rc(τ) = --- | pi(t) pj(t+τ) dt (See: click here) T / 0The function pi(t) and pj(t) are the wave forms of the two PN codes.
(Don't worry, you don't need to know the details of correlation - it's there for completeness sake....)
- The correlation function
has these properties:
- It equals 1 if the two codes are identical
- It equals 0 of the two codes have nothing in common
Intermediate values (between 0 and 1) indicate how much the codes have in common.
- Property:
- The more they have in common, the harder it is for the receiver to extract the appropriate signal.
- The length
of the chip sequence is called
chip length
- Effect of the chip length:
- The longer
the chip length, the
more simultaneous transmissions
can be supported.
- But a longer chip length will also reduce the effective bandwidth (you need to send more signals per bit....)
- The longer
the chip length, the
more simultaneous transmissions
can be supported.
- Recall: Multiplexing
- The multiplexing technique
is used to shared
a transmission medium
- The two most common ways
to share the
transmission medium are:
- Time division multiplexing -
stations takes turn to
use the transmission medium
- Frequency division multiplexing - stations takes up different parts of the frequency spectrum of the transmission medium
- Time division multiplexing -
stations takes turn to
use the transmission medium
- The multiplexing technique
is used to shared
a transmission medium
- Now you have seen a third way
to share the
transmission medium
This form of multiplexing using PN codes is called:
- Code Division Multiplexing
(multiple concurrent transmissions can be received by their intended receivers correctly through encoding)
- Code Division Multiplexing