CDMA allows multiple signals to occupy the same frequency channel by using unique codes to distinguish each signal. It works by converting analog voice signals to digital, encoding and channelizing the signals using codes, and transmitting the coded digital signals simultaneously over the same frequency. The receiving device uses the same codes to extract the intended signal from the multiple signals in the channel. CDMA provides advantages like increased capacity, better call quality, and lower radiation compared to other technologies.

1. By : Nur Azlin Binti Razali
Nur Fadzilah Binti Johar
Nur Fattahiah Binti Haslauhuddin
Siti Norhidayah Binti Kamarudin

2. Introduction to CDMA
 CDMA (Code-Division Multiple Access) refers to any of several protocols
used in so-called second-generation (2G) and third-generation (3G) wireless
communications.
 As the term implies, CDMA is a form of multiplexing, which allows numerous
signals to occupy a single transmission channel, optimizing the use of
available bandwidth. The technology is used in ultra-high-frequency (UHF)
cellular telephone systems in the 800-MHz and 1.9-GHz bands.
 All users can use same carrier frequency and may transmit simultaneously.
Each user in a CDMA system uses a different code to modulate their signal.
 The best performance will occur when there is good separation between the
signal of a desired user and the signals of other users.
 The separation of the signals is made by correlating the received signal with
the locally generated code of the desired user. If the signal matches the
desired user's code then the correlation function will be high and the system
can extract that signal.

3. How CDMA works?
 The words "code" and "division" are important parts of
how CDMA works.
 CDMA uses codes to convert between analog voice
signals and digital signals and also uses codes to
separate (or divide) voice and control data into data
streams called "channels."
 CDMA converts speech into digital information, which is
transmitted as a radio signal over wireless network.
 The unique about CDMA is that it uses a unique code to
distinguish each different call, thus enabling many
people to share the airwaves simultaneously; it thereby
eliminates cross talk and interference.
 CDMA is also used in data technology apart from voice
technology.

4.  There are five steps in generating a CDMA signal :
i) analog to digital conversion
ii) vocoding
iii) encoding and interleaving
iv) channelizing the signals
v) conversion of the digital signals to a Radio Frequency
(RF) signals

5. Analog to digital conversion
 The first step of CDMA signal generation
is analog to digital conversion,
sometimes called A/D conversion.
CDMA uses a technique called Pulse
Code Modulation (PCM) to accomplish
A/D conversion.

6. Vocoding
 The second step of CDMA signal generation is voice compression
 Vocoder : to accomplish voice compression.
 The term "vocoder" is a contraction of the words "voice" and "code.
 Vocoders are located at the BSC and in the phone. People pause between
syllables and words when they talk. CDMA takes advantage of these
pauses in speech activity by using a variable rate vocoder. A CDMA vocoder
varies compression of the voice signal into one of four data rates based on
the rate of the user's speech activity.

7. Encoding and interleaving
 encoding and interleaving that is built into the
BTS and the phones
 The purpose of the encoding and interleaving is
to build redundancy into the signal so that
information lost in transmission can be recovered

8.  The type of encoding done at this stage is
called "convolutional encoding.
 A digital message consists of four bits (A, B, C,
D) of vocoded data. Each bit is repeated three
times. These encoded bits are called symbols.
 The decoder at the receiver uses a majority
logic rule. Thus, if an error occurs, the
redundancy can help recover the lost
information.
 A burst error is a type of error in received
digital telephone signals. Burst errors occur in
clumps of adjacent symbols. These errors are
caused by fading and interference. Encoding
and interleaving reduce the effects of burst
errors.

9.  Interleaving is a simple but powerful method of reducing the effects of burst
errors and recovering lost bits. In the example shown here the symbols from
each group are interleaved (or scrambled) in a pattern that the receiver knows.
De-interleaving at the receiver unscrambles the bits, spreading any burst errors
that occur during transmission.

10.  The encoded voice data is further encoded to separate
it from other encoded voice data. The encoded symbols
are then spread over the entire bandwidth of the CDMA
channel. This process is called channelization. The
receiver knows the code and uses it to recover the voice
data.

11. Channelizing
CDMA uses two important types of
codes to channelize users.
 Walsh codes channelize users on the
forward link (BTS to mobile).
 Pseudorandom Noise (PN) codes
channelize users on the reverse link
(mobile to BTS)

12. Conversion of the digital signals to a
Radio Frequency (RF) signals
 The BTS combines channelized data from all
calls into one signal. It then converts the
digital signal to a Radio Frequency (RF)
signal for transmission.

13. Architecture of CDMA
A CDMA network consists of the following
components :
 Mobile Station (MS )
 Base Station ( BS )
 Base Station Controller ( BSC )
 Mobile Switching Center ( MSC )
 Home Location Register ( HLR )
 Visitor Location Register ( VLR )
 Authentication Center (AC)
 Operation and Administration (OAM)

14. cCOMPONENT FUNCTION
Mobile station The CDMA mobile station (or mobile phone) communicates with other parts of the system through
the base station system.
Base station (BS) The base station (BS) handles the radio interface to the mobile station. The base station is the radio
equipment (transceivers and antennas).
Base station Controller (BSC) The BSC provides the control functions and physical links between the MSC and BTS. It provides
functions such as hand cover, cell configuration data and control of RF power levels in as transceiver
stations.
Mobile switching center The MSC performs the telephony switching functions of the system. Its also performs such functions
(MSC) as toll ticketing, network interfacing ,common channel signaling, and others.
Home location register (HLR) The HLR database is used for storage and management of subscriptions. The home location register
stores permanent data about subscribers, including a subscriber’s service profile, location
information, and activity status.
Visitor location register (VLR) The VLR database contains temporary information about subscribers that is needed by the mobile
services switching center (MSC) in order to service visiting subscribers. When a mobile station
roams into a new mobile services switching center (MSC)
Area, the visitor location register (VLR) connected to that MSC will request data about the mobile
station from the HLR, reducing the need for interrogation of the home location register (HLR).
Authentication (AC) The AC provides authentication and encryption parameters that verify the user’s identity and ensure
the confidentiality of each call. The authentication center (AUC) also protects network operators
from fraud.
Operation and administration The OAM is the functional entity from which the network operator monitors and control the system.
(OAM) The purpose of operation and support system is to offer support for centralized, regional and local
operational and maintenance activities that are required for a CDMA network.

15. Architecture of CDMA
Example CDMA 2nd Generation:

16. Architecture of CDMA
Example CDMA 3rd Generation:

17. CDMA Applications :
 One of the early applications for code division multiplexing
is in GPS. This predates and is distinct from its use
in mobile phones.
 The Qualcomm standard IS-95, marketed as cdma One.
 The Qualcomm standard IS-2000, known as CDMA2000.
This standard is used by several mobile phone
companies, including the Global star satellite
phone network.
 The UMTS 3G mobile phone standard, which uses W-
CDMA.
 CDMA has been used in the Omni TRACS satellite
system for transportation logistic


18. Advantages of CDMa Disadvantages of CDMA
 more advanced digital interface
technology  one major problem in CDMA
technology is channel
 the transmission power is
extremely small, so conversation polution,where signals from too
is not only less radiation, many cell sites are present in
the subscriber’s phone but
 also longer battery life mobile
phones. none of them is dominant.(the
quality of the audio degrades)
 better call quality, almost no noise
when making a call,  limited to North America and a
 there is no health adverse effects few other places, GSM is
because its radiation is negligible worldwide
 faster download speeds  limited variety of the handset
 best privacy (spread spectrum), because at present the major
mobile companies use GSM
 hardest to crack (the AIR
technologies.
INTERFACE)
 cost of calls is cheaper than in
GSM.

20. ADVANTAGES DISADVANTAGES
CDMA •Increased cellular communications •Due to its proprietary
security. nature, all of cdma’s
•Simultaneous conversations flaws are not known
•Increased efficiency •CDMA is relatively new,
•Smaller phones and all the network is not
•Low power requirements and little cell- as mature as GSM.
to-cell coordination needed by •Cannot offer
operators. international roaming
•Extended reach
GSM •Already used worldwide •Lack of access to
•Permits international roaming burgeoning American
•More stable network market.
•Smart cards that provide security data
encryption

21. Frequency hopping CDMA
There are two kinds of frequency hopping ;
 Slow Frequency Hopping (SFH)
In this case one or more data bits are
transmitted within one hop. An advantage is
that coherent data detection is possible. Often,
systems using slow hopping also employ
(burst) error control coding to restore loss of
(multiple) bits in one hop.
 Fast Frequency Hopping (FFH)
One data bit is divided over multiple hops. In
fast hopping, coherent signal detection is
difficult, and seldom used. Mostly, FSK or
MFSK modulation is used.

22. Direct sequence CDMA
 In Direct Sequence spread spectrum transmission, the
user data signal is multiplied by a code sequence. Mostly,
binary sequences are used. The duration of an element in
the code is called the "chip time". The ratio between the
user symbol time and the chip time is called the spread
factor. The transmit signal occupies a bandwidth that
equals the spread factor times the bandwidth of the user
data.
 As despreading is the same operation as spreading a
possible narrowband jammer signal in the radio channel
is spread before the detector. Thus, the jam signal is
attenuated by the spread factor ("spreading gain").