Spread spectrum is a technique that spreads user data signals across a larger frequency band to provide resistance to narrowband interference. It works by converting a narrowband signal into a broadband signal at the sender, then reconverting it back to narrowband at the receiver while spreading any interference. Two common spread spectrum techniques are direct sequence spread spectrum (DSSS), which spreads signals using a chipping sequence, and frequency hopping spread spectrum (FHSS), which splits the bandwidth into smaller channels and hops between them.

1. Spread Spectrum

2. • It is a technique of spreading the bandwidth
needed to transmit data and provides
resistance to narrowband interference.
Narrowband signal from a sender of user data
Sender

3. • The sender now spreads the signal by
converting the narrowband signal into a
broadband signal.
• The energy needed to transmit the signal is
the same, but it is now spread over a larger
frequency range.

4. reciever
Reciever:
• During transmission, narrowband and broadband
interference add to the signal.
• The sum of interference and user signal is received.
• The receiver now knows how to despread the signal,
converting the spread user signal into a narrowband
signal again.
• while spreading the narrowband interference and
leaving the broadband interference.

5. • The receiver applies a bandpass filter to cut off
frequencies left and right of the narrowband
signal.
• Finally, the receiver can reconstruct the original
data because the power level of the user signal is
high enough, i.e., the signal is much stronger than
the remaining interference.

6. • spread spectrum helps to deal with narrowband
interference for a single channel, it can be used
for several channels.
• Consider the situation, six different channels use
FDM for multiplexing, which means that each
channel has its own narrow frequency band for
transmission.
• Between each frequency band a guard space is
needed to avoid adjacent channel interference.

7. Narrowband interference without
spread spectrum

8. • Depending on receiver characteristics, channels 1, 2, 5,
and 6 could be received while the quality of channels 3
and 4 is too bad to reconstruct transmitted data.
• Narrowband interference destroys the transmission of
channels 3 and 4.
• All narrowband signals are now spread into broadband
signals using the same frequency range.
• No more frequency planning is needed and all senders
use the same frequency band.
• But how can receivers recover their signal?

9. Spread spectrum to avoid narrowband
interference

10. • To separate different channels, CDM is now used
instead of FDM.
• Spreading of a narrowband signal is achieved
using a special code.
• Each channel is allotted its own code, which the
receivers have to apply to recover the signal.
• Without knowing the code, the signal cannot be
recovered and behaves like background noise.

11. Features
• CDM is very attractive for military
• Robustness against narrowband interference,
• high security
• background noise.
• Only the appropriate (secret) codes have to
be exchanged.

12. Disadvantage
• Increased complexity of receivers that have to
despread a signal.
• Despreading can be performed up to high
data rates can use digital signal processing.
• large frequency band that is needed due to
the spreading of the signal.
• spread signals appear more like noise.

13. Spread Spectrum Techniques
• Direct sequence spread spectrum (DSSS)-
take a user bit stream and perform an (XOR)
with a so-called chipping sequence
• Frequency hopping spread spectrum (FHSS)-
the total available bandwidth is split into
many channels of smaller bandwidth plus
guard spaces between the channels.

14. Direct sequence spread spectrum
• DSSS systems take a user bit stream and perform
an (XOR) with a so-called chipping sequence.
• The example shows that the result is either the
sequence 0110101 (if the user bit equals 0) or its
complement 1001010 (if the user bit equals 1).
• If the chipping sequence is generated properly
appears as random noise: this sequence is also
sometimes called pseudo-noise sequence.

15. Spreading with DSSS

16. DSSS sender

17. DSSS receiver

18. Frequency hopping spread spectrum
(FHSS)
• The total available bandwidth is split into many
channels of smaller bandwidth plus guard spaces
between the channels.
• Transmitter and receiver stay on one of these
channels for a certain time and then hop to
another channel.
• This system implements FDM and TDM. The
pattern of channel usage is called the hopping
sequence,and the time spend on a channel with
a certain frequency is called the dwell time.

19. FHSS

20. Slow Hopping means the transmitter uses the
frequency f2 for transmitting the first three
bits during the dwell time td.
Then, the transmitter hops to the next
frequency f3.
• Fast hopping: the transmitter changes the
frequency several times during the
transmission of a single bit.

21. Advantages
• In FHSS systems spreading is simpler.
• FHSS systems use a portion of the total band
at any time, while DSSS systems always use
the total bandwidth available.
• DSSS systems on the other hand are more
resistant to fading and multi-path effects.
DSSS signals are much harder to detect
without knowing the spreading code.