mobile computing

1. CDMA
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2. CONCEPT
• Based upon technology known as Spread
Spectrum.
• While CDMA focuses on multiple access,
Spread Spectrum offers a lot more.

3. STORY OF SPREAD SPECTRUM
INVENTION
• The frequency hopping technique was
invented by the actress Hedy Lamarr, bor
in Austria in 1914.
• She married Friedrich Mandl, who was in
the Defence Manufacturing business.
• During WW II, she attended many of her
husband’s business meetings.

4. STORY
• She learned about the problems of remote
controlled torpedoes used by the marines.
• Wirelessly controlled by English &
Americans to direct them against German
ships.
• Because the accuracy was not good,
manual fine tuning was needed.

5. STORY
• For the wireless control, a narrow band
signal was used.
• Once the German discovered the control
signal they could easily jam that signal.
• Hedy Lamarr came up with the idea of
multiple frequencies.

6. STORY
• She said that the comm should take place
on one freq at a time and then jump to
another channel fast enough so as not to
give enough time to tune in the new freq to
jam it.
• The change in freq should not be
predictable to the enemy.

7. STORY
• Lamarr’s problem was that this freq
hopping pattern had to be synchronized
between the sender on the boat and the
receiver in the torpedo.
• After some time, Lamarr’s friend George
Antheil came up with a solution.

8. STORY
• Antheil had a bar in Paris with automatic
pianos that were played by a piano roll.
• The roll had perforations in it which told
the piano which keys had to be played and
for how long.
• 88 perforations were possible in parallel.

9. STORY
• A tracker ball like thing was used to sense
the holes and give command to piano.
• The same mechanism was thought of for
the torpedo to keep synchronization.
• In 1962, spread spectrum was used by
military for the first time in Cuba crisis.

10. PRINCIPLE OF SPREAD SPECTRUM
• It gained popularity through the needs of
military communications.
• In contrast to narrow band comm, it is
more resistant to hostile jammers and
more difficult to intercept – fundamental
conditions for tactile comm.

11. PRINCIPLE OF SPREAD SPECTRUM
• For a comm system to be considered as Spread
Spectrum, the criteria to be satisfied are:
 BW of the transmitted spread signal has to be
greater than the information BW (this criterion is
also satisfied by FM, PCM and DM)
 The spread signal is composed of the
information signal and the spreading sequence.
The spreading sequence has to be independent
from the information in the statistical sense.

12. PRINCIPLE OF SPREAD SPECTRUM
• The ratio of the spreading BW Bs to info
BW Bi is denoted as the processing gain
Gspreading = Bs/Bi of a spread spectrum system.
• The processing gain does not combat
white noise with FM and PCM because
the spread signal is independent of
information signal.

13. PRINCIPLE OF SPREAD SPECTRUM
• However, spread spectrum signals offer
the following advantages for a wireless
comm system:
 spread spectrum modulation is capable of
dealing with the multi path signals
resulting from the radio channel.
These signals can be considered as
interference and therefore be suppressed.

14. PRINCIPLE OF SPREAD SPECTRUM
 the receiver of a spread spectrum system is
able to distinguish between different transmitted
signals using the spreading sequence.
Spreading sequence design is very important for
multiple access capability. The spreading
sequence is actually the identification for a
transmitter-receiver pair.
 Other advantages such as low probability Of
interception, privacy and anti-jamming capability
are more relevant for military needs.

15. SPREADING TECHNIQUES
• SPREAD SPECTRUM TECHNIQUES
DIRECT SEQUENCE TIME HOPPING FREQ HOPPING HYBRID
WIDEBAND NARROW BAND SLOW FAST
DS/FH DS/TH FH/TH DS/FH/TH

16. SPREADING TECHNIQUES
• In DSSS transmitter, the info signal is
directly modulated by a spreading
sequence.
• Main feature of DSSS is that multiple
access capability can be achieved without
synchronization between different
transmitters.

17. SPREADING TECHNIQUES
• FHSS systems periodically change the
carrier freq of the modulated information
signal in a random fashion.
• During a time interval thop the freq is
constant.
• Afterwards both the sender and receiver
hop to another frequency.

18. SPREADING TECHNIQUES
• The whole BW B is divided into freq slices. The
set of available frequencies is called a hop set
with N freq slices.
• FHSS systems differ radically from DSSS
systems in terms of freq occupancy.
• While DSSS systems occupy the whole BW all
the time, FHSS systems use only one freq slice
at any point in time.

19. SPREADING TECHNIQUES
• If one info bit is transmitted over several freq
slices the freq hopper is referred to as fast.
• If several bits are transmitted over one freq slice
it is referred to as a slow hopper.
• The relation of hop duration to info bit duration
depends upon the number of hops per info bits.

20. SPREADING TECHNIQUES
• The adv of FHSS systems over DSSS systems is the
less strict synchronization requirement.
• Within THSS, the time axis is divided into frames.
• A single wireless terminal (WT) will use only one slot out
of k possible slots within one frame. Within this slot, the
WT sends with a k times higher data rate in contrast to
the situation in which the WT would transmit within the
whole frame.

21. SPREADING TECHNIQUES
• Interference among simultaneous wireless terminals can
be minimized if coordination between terminals can be
achieved.
• This also avoids the near-far effect.
• In the absence of coordination, situations occur in which
more than one terminal uses a time slot.
• The receiver will not be able to detect either of the
signals correctly.

22. SPREADING TECHNIQUES
• Both terminals collide on the wireless link.
For such cases, error correction schemes
are required.
• Time hoppers have nearly the same
acquisition time as that of discrete
sequence systems, but their
implementation is much simpler than that
of a freq hopper.

23. SPREADING TECHNIQUES
• HSSS consist of combination of two or
more basic SS systems.
• Complexity of Tx and Rx increases but
offers a combination of their advantages.

24. SPREADING TECHNIQUES
• DSSS:
• Advantages:
o Best behaviour for multi-path rejection
o Best anti-jam rejection
o Best interference rejection
o No synchronization among terminals
o Simple implementation
o Most difficult to detect

25. SPREADING TECHNIQUES
• DSSS:
• Disadvantages:
 Near-far problem
 Require coherent bandwidth
 Long acquisition time
 Synchronization of code signal within
fraction of chip time.

26. SPREADING TECHNIQUES
• FH:
• Advantages:
 great amount of spreading
No need of coherent BW
Short acquisition time
Inherent security
Less effected by the near-far effect

27. SPREADING TECHNIQUES
• FH:
• Disadvantages:
 Complex hardware
 Error correction is needed

28. SPREADING TECHNIQUES
• TH:
• Advantages:
High BW efficiency
Less complex hardware
Less affected by the near-far effect

29. SPREADING TECHNIQUES
• TH:
• Disadvantages:
 Error correction is needed
 Long acquisition time

30. CDMA BASICS
• In FDMA the users are separated in the
frequency domain and in TDMA the users
are separated in time domain.
• Basically, FDMA and TDMA assign
particular frequency or time slices to
different wireless terminals.

31. CDMA BASICS
• When all slices are occupied in the
system, no additional wireless terminal
can be accommodated.
• Multiple access capability is also provided
by CDMA.
• The most common technique of CDMA
are FH-CDMA or DS-CDMA.

32. CDMA BASICS
• CDMA allows multiple users to
simultaneously use a common channel
for transmission of information.
• A CDMA transmitter will code its
information with a code or spreading
sequence particularly allocated for that
user.

33. CDMA BASICS
• Afterwards, the transmitter sends the
coded signal to the receiver. Using the
same code sequence, the receiver
decodes the received signal.
• The bandwidth of the coded signal is
much larger than the information
bandwidth.

34. CDMA BASICS
• One may say that the information signal
was spread, where spread refers to the
broadening of the signal spectrum in the
frequency domain.
• The coding process is therefore also
called spread spectrum modulation while
the coded signal is called a spread
spectrum signal.

35. CDMA BASICS
• The spreading of the information signal
gives CDMA its multiple access capability.
• The main feature of CDMA system using
pseudo noise sequences is that it can
tolerate overload in terms of number of
simultaneously supported users.
• In CDMA, system capacity is not fixed.

36. CDMA BASICS