1. ‫م‬‫ي‬‫ح‬ّ‫الر‬‫من‬‫ح‬ّ‫الر‬‫هللا‬‫م‬‫س‬‫ا‬‫ب‬
‫ل‬
ُ‫ق‬ َ‫و‬ِّ‫ب‬َّ‫ر‬‫د‬ ِ‫ز‬ِ‫ي‬‫ن‬َ‫م‬‫ل‬ِ‫ع‬‫ا‬

2. INSTITUT NATIONAL DES TELECOMMUNICATIONS ET DES TIC
CDMA Concepts
Done By:
Ouchabane Raouf
Driouche Abderrachid
Promotion:
IGE 36
Jury:
Mr O.Kaid Omar
Supervisor:
Mr A.Tienti
Academic year: 2014 - 2015

3. Plan
 Introduction.
 Multiple Access Techniques.
 Spread Spectrum.
 Forward and reverse Link Channels.
 Handoff & Power control.
 Advantages and disadvantages.
 Conclusion.

4. Introduction
Today, we depend on mobile services not only for
communication, but also for education,
entertainment, healthcare and commerce…
This evolution of service requires a very high speed
data with high quality.
Pbs:
1. Limitted Spectrum.
2. Multiple users.

5. Multiple Access ??
• Goal: Multiple use of a shared
medium.
• There are three widely-used
policies:
- FDMA (Frequency Division Multiple Access)
- TDMA (Time Division Multiple Access)
- CDMA (Code Division Multiple Access

6. FDMA
▧ Different users are
assigned different
frequency channels.
A channel is a
frequency.
▧ 1 G (analog)
AMPS,TACS and
NMT.

7. TDMA
▧ The use of the same
frequency spectrum but
allows more users on the
same band of frequencies
by dividing the time into
“slots” and shares the
channel between users by
assigning them different
time slots.
▧ 2G (digital) :
D-AMPS;GSM.

8. ▧ Every communicator will
be allocated the entire
spectrum all of the time.
CDMA uses codes to
identify connections.
▧ 2G : IS-95 (CdmaOne)
▧ 3G : cdma2000,
W-CDMA, and TD-SCDMA
CDMA

9. CDMA
▧ In CDMA, the narrowband message signal is multiplied
by a very large bandwidth signal called spreading signal
(code) before modulation and transmission over the air.
This is called spreading

10. CDMA
▧ At the reception the large bandwidth received signal will
be multiplied by the same spreading signal to restore the
original message, This is called dispreading

11. Spread spectum
Most common types of spread-spectrum
systems:
1. Direct Sequence Spread Spectrum (DS-SS)
2. Frequency Hopped Spread Spectrum (FH-SS)
CDMA uses the DS-SS

12. Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSSS) is a
spread spectrum technique whereby the original
data signal is multiplied with a pseudo random
noise spreading code.
This spreading code has a higher chip rate (this
the bitrate of the code), which results in a
wideband signal.

13. Direct Sequence Spread Spectrum

14. Direct Sequence Spread Spectrum
∑=S1×C1+ S2×C2

15. Orthogonality
▧Two codes are orthogonal if there
correlation==0,
▧If not (Correlation==1)they are called identical
codes,

16. Direct Sequence Spread Spectrum
C1×∑= C1×(S1×C1+ S2×C2)
=
C1×S1×C1+ C1×S2×C2
=
S1

17. Example:

18. Scrambling and channelization
▧ In CDMA system the spreading consists of 2
steps:
1. Channelization.
2. Scrambling.

19. Channelization
▧ It is simply the spreading proccess where we are
going to increase the bandwidth of the signal by
transforming every data symbol into a number of
chips.
▧The number of chips per data symbol is called
the Spreading Factor (SF).
▧used to distinguish different physical channels of
one transmitter

20. Channelization
▧In the Downlink, Channelization Codes are used to
distinguish between data (and control) channels coming from
the same BS.(SF:from 4 to 256).
▧In the Uplink, Channelization Codes are used to distinguish
between data (and control) channels from the same UE
.(SF:from 4 to 512).

21. OVSF
▧To generate those codes we uses OVSF code
(Orthogonal Variable Spreading Factor) called also
WALSH codes,

22. SF
Spreading Factor (SF)
=
chip rate / symbol rate
▧ High data rates → low SF code.
▧ Low data rates → high SF code

23. Scrambling
▧ After the Channelization Codes, the data stream is multiplied by
a scrambling code to distinguish between different transmitters.
▧ In the Downlink, they are used to distinguish each cell
▧ In the Uplink, the are used to distinguish each UE

24. Forward and reverse
Link Channels

25. Forward Link Channels

26. Reverse Link Channels:

27. EXAMPLE of WCDMA frame
structure:

28. Handoff in CDMA

29. Handoff
transferring a call from one cell (or
sector) to another, as the phone moves
between them.

30. Hard Handoff
▧ Old connection is
released before a new
one is activated.
▧Short interruption time.
▧ Break-Before-Make.

31. Soft Handoff
▧ Activation of the new
connection is prior to breaking
the old one.
▧ Seamless handoff.
▧Make-Before-Break.

32. Softer Handoff
▧ Procedure similar to the soft handoff but in
this case the handover is between two sectors.

33. Power control

34. Why power control ??
▧ CDMA is interference limited multiple access system.
▧ Reduction in interference increase in capacity.
▧ If the received mobile’s signal has a too low level of
received power, a high level of interference is
experienced by this mobile and its performance will be
degraded .
▧ If the received power level is too high the performance
of this mobile is acceptable, but increases interference
to all other mobiles that are using the same channel .
▧This is called “Near Far problem” .

35. Why power control ??
▧ To solve the near far problem we do what we
call power control.
▧ Controlling the transmitted power so that it
should be enough to maintain the required
SNR for a satisfactory call quality.

36. Advantages and disadvantages
++++ - - - -

37. Advantages
▧ Frequency reuse factor is 1, network design
becomes much easier, and minimal frequency
planning is required.
▧ High capacity.
▧ Improvement in handoff.
▧ Longer mobile battery life and longer life
span of BTS power amplifiers thanks to the use
of power control techniques.
▧Smooth migration to 3G .

38. Disadvantages
▧ DSSS is more complex than techniques used
in TDMA/FDMA.
▧ As the number of users increases, the overall
quality of service decreases.
▧ Near-far problem.

39. “
Conclusion

40.  Uses a wider bandwidth increased immunity to
interference or jamming, and multiple user access.
 The CDMA will allow many signals to be transmitted at the
same channel at the same time. This is done by giving each
user a code sequence .
 These codes should be orthogonal between them.
 In order to receive the data, the receiver must have a
knowledge of the spreading code, without this it is not
possible to decipher the transmitted data, and this gives a
measure of security.

41. CDMA & 5G
5G
OFDMA + CDMA
=
SCMA

42. Thanks!
Any Questions? ?