Code Division Multiple Access (CDMA)

Code Division Multiple
Access (CDMA)
Presented by
EduTechLearners
www.edutechlearners.com

Multiple Access Techniques
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)

CDMA
In CDMA, a number of users occupy all of the
channel bandwidth all of the time. Their signals
are encoded so that information from an individual
transmitter can be detected and recovered only by
a properly synchronized receiving station that
knows the code being used.
Each receiving station has its own code, called its
“address” and a transmitting station simply
modulates its transmission with the address of the
intended receiver whenever it wishes to send a
message to that receiver.

Features of CDMA
CDMA is Full Duplex
Eliminates need to engineer reuse pattern
Increases Capacity by about 6 times
Multipath reception improves voice clarity
Security or Interference rejection : Cross-
correlating the code signal with a narrowband
signal spreads the power of the narrowband
signal, thereby reducing the interfering
information bandwidth.

Rake Receiver
Both the mobile and the BTS use rake receivers.

Power Control: Avoids near-far effect
and increases mobile unit battery life
POWER CONTROL

Forward link Power Control
The BTS independently adjusts the power
based on the information it receives from
the mobile.

Reverse Link Power Control
Open Loop: Prior to any transmission, the
mobile monitors the total power received by the
base station.
Closed Loop: The base station provides
continuous feedback to each mobile so that the
mobile varies its power accordingly.

Block Diagram of CDMA

CDMA Architecture
Consists of the following components
 Mobile user/subscriber (MS)
 Base Trans/Receive Station (BTS) & Antennae
 Mobile Switching Center (MSC)
 Authentication Center (AC)
 Operation, Administration & Maintenance
(OAM)

CDMA Architecture
IOS: Interoperability Specification IWF: Interworking Function

CDMA Channels
Forward Link Channels

CDMA Channels
Reverse Link Channels

CDMA Call Processing
There are four stages or modes in CDMA call
processing

CDMA Call Execution
Land to Mobile Call

CDMA Call Execution
Mobile to Land Call

CDMA Call Execution
Mobile to Mobile Call within same network

CDMA Call Execution
Mobile to Mobile Call in different network

CDMA Applications
In Bluetooth
 Bluetooth uses Fast FH-CDMA with a hopping
rate of 1,600 hops per second.
 Each piconet has its own spreading sequence.
 FH/CDMA in Bluetooth allows tens of piconets
to overlap in the same area providing an
effective throughput of more than 1 Mbps.

CDMA Applications
In Global Positioning System (GPS):
 In GPS there are two satellite signals or carriers.
 A PN-sequence of freq. 1023 MHz is used to
modulate one of these carriers to generate coded
signals that are needed for acquisition of the satellite
by the receiver.
 Different satellites use different sequences, which lets
them all broadcast on the same frequencies while still
allowing receivers to distinguish between satellites.

Limitations of CDMA
Synchronization Requirement
• Unavailability of large contiguous bandwidth
• Near-far Effect
• Self-Jamming

Conclusion
There are now different variants of CDMA, but
the original CDMA is now known as cdmaOne.
We now have cdma2000 and its variants like 1X
EV, 1XEV-DO, and MC 3X. These refer to
variants of usage of a 1.25Mhz channel. For
example, 3X uses a 5 MHz channel.
Over 35 countries have either commercial or trial
activity ongoing regarding CDMA. There are
already 43 Wireless Local Loop (WLL) systems in
22 countries using cdmaOne technology.

Code Division Multiple Access (CDMA)

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Code Division Multiple Access (CDMA)