Presentation on
CDMA
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MULTIPLE ACCESS

Simultaneous use of a transmission
medium by multiple users.

Advantages :

Increased capacity: serve more users.
Reduced capital requirements since fewer media
can carry the traffic.
Decreased per user expense.
Easier to manage and administer.



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MULTIPLE ACCESS METHODS
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MULTIPLE
ACCESS
METHOD
FDMA
TDMA
CDMA
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
A comparative study between the above
three access technologies with respect to
time and frequency is as shown below.
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CDMA Concept
$%&@#
CDMA can bring big
business
This is John Doe
• Cocktail party in same room (same wide band channel)
• Use different languages (different digital codes)
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BPSK Modulation
Why Choose BPSK?

Easy to implement

Well suited for CDMA
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Spread Spectrum Concept
1800
1850
Mobile TX
m
1910
1930
1990
Cell TX
User 1
Code 1
User 2
Code 2
User 3
Code 3
User n
2000MH
Code n
Spread spectrum uses much larger slice(1.25MHz) of the available bandwidth.
Same slice is used for all user with no time multiplexing but user is assigned
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Spread Spectrum Principles
Many code channels are
individually “spread” and
added together to create
a “composite” signal.
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CDMA SPREADING
(Time Domain)
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CDMA SPREADING
(Frequency Domain)
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TYPES OF SPREAD-SPECTRUM

Direct sequence spread-spectrum.

Frequency hopping spread-spectrum.

Multi-carrier CDMA system.
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DIRECT SEQUENCE SPREAD SPECTRUM
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Frequency hopping
Frequency occupying order- function of code sequence.
Rate of hopping from one carrier to another function of
information rate.
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PN(pseudo noise) codes

PN –CODE is a sequence of high data rate
bits(“chips”) ranging from –1 to + 1 (polar)
or 0 to 1 (non polar).

By “chips” we mean the no. of small data bits
in the PN-CODE per single bit of the original
signal.

They are either linear codes generated by
EX-OR gates used in shift register feedback
path or otherwise non linear codes.
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Spreading a Bit by the means of
many chips
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Steps evolved for signal
transmission




A pseudo random code is generated,
different for each channel& successive
connections.
The information data modulates the
pseudo random codes.
The resulting signal modulates the
carrier.
The modulated carrier is amplified and
broadcast.
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Steps evolved for receiving the
signals
The carrier is received and amplified.
 The received signal is now demodulated
by receiving the codes.
 The receiver acquires the received code
and phase locks its own code to it.
 The received signal is correlated with
the generated code, extracting the
information data.

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CDMA MODULATION
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CDMA DEMODULATION

The modulated signal is transmitted over the
channel and all users can receive it but only the
user which knows the correct code can decode
the message.
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CDMA System Block Diagram
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History of CDMA
A method for multiple access
(single channel , multiple users).
 The conventional methods were FDMA.

Then after developing synchronization
TDMA was developed in early 1980s it
was introduced.
 Its first use was done in military.
 in early 1990s its first use was done in
cellular communication system.

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Origination of CDMA
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FEATURES OF CDMA

FLEXIBLE NETWORK.

COMPATIBILITY.

NETWORK ACCESS.

LINE UTILIZATION.

ERROR-FREE TRANSMISSION.

VOICE AND CALL QUALITY.
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Multipaths
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Rake Receiver

Mobile station receives multiple attenuated replicas of
the original signal.

Two signals are resolvable only if their relative delay
exceeds the chip period Tc .

Amplitudes and phases of multipath components are
found by correlating the received waveform with
multiple delayed versions of the signal (delay = nTc).

Searcher performs the above task for up to 3 different
multipath signals.

RAKE fingers isolate the multipath components and
the RAKE receiver combines them.
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CDMA RAKE Receiver
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RAKE RECEIVER
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CDMA Coding Procedure
CDMA is based on Coding Theory.
 Each station is assigned a code which is
a sequence of numbers called CHIPS.
 In this example we have four stations
each has a sequence of chips which
designate as A,B,C, and D.

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Rules For Encoding
We Adopt the following Rules For Encoding.
 If a station needs to send a 0 bit it sends -1.
 If a station needs to send a 1 bit it sends +1.
 When a Station is idle, it sends no signal which
is represented by 0.
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CDMA Multiplexer
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CDMA Demultiplexer
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Codes in CDMA
CDMA Codes
Orthogonal Codes
Walsh Codes
Codes
Walsh
Pseudo-noise (PN) Codes
Long PN Code
Short PN
PN Codes
Codes
Short
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R F OVERVIEW

What is cell?
A cell is a small area of
service within a city, serviced
by its own antenna.

Frequency Reuse In CDMA
All users use same
frequency.

Universal frequency reuse
applies to the users in the
same cell as well as to those in
others.

Complicated reuse pattern is
not necessary.
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Frequency reuse
AMPS
CDMA
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CDMA Different Systems
 IS-95.
 CDMA-2000 (which is also called WCDMA
in Europe).
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CDMA Switch Overview
CDMA Network Architecture
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CDMA
1xEV/DO
2400
W-CDMA
stationary
W-CDMA
moving
2000
384
CDMA
1xRTT
144
GPRS
114
CDMA
64
PDC-P
28.8
GSM/PDC
Theoretical
data
transmission
speed
kbps
9.6
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CDMA channels
FORWARD CHANNELS:
Pilot channel.
Sync channel.
Paging channel.
Forward traffic channel.
REVERSE CHANNELS:
Access channels.
Reverse traffic channels.
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TECHNICAL DATA
FREQUENCY OF OPERATION
:800MHZTO1900MHZ
TRANSMISSION RATE : 9600 KBITS/SEC
OPERATION BAND : C-BAND
RF SPACING : 1.25 MHZ
COVERAGE : 20 KMS. WITH FIXED UNITS
GRADE OF SCIENCE : 1%
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Forward Link Channels in CDMA
On the forward link there are 4 channels used to transmit control and voice data
to the mobile. These code channels are:
Pilot Channel
 Synchronize Channel
 Paging Channel
 Traffic Channel

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Reverse Link Channels in CDMA
On the reverse link there are 2 types of channels used to transmit control and
voice data to the mobile. These channels are:
Access Channel
 Traffic Channel

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Call Processing in CDMA
There are 4 Call Processing modes in CDMA:

Initializing mode.

Idle mode.

Access mode.

Traffic mode.
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ADVANTAGES
Increased capacity.
Improved voice quality.
Enhanced privacy and security
Reduced interference to other electronic devices.
Co existence with the other technologies.
Reduction in the number of calls dropped due to
the handoff failures.
Improved battery life due to reduced average
transmitted power.
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CDMA SECURITY
Security becomes key to delivering
solutions that meet today’s demand for
mobility. Some of today’s top security
issues and concerns are:
 Unauthorized systems and network
access
 Auditability and compliance
 Customer data breaches
 Internal and external sabotage
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Handoff in CDMA System

In GSM hard handoff occurs at the cell boundary

Soft Handoff
– Mobile commences Communication with a new BS without
interrupting communication with old BS.
– same frequency assignment between old and new BS.
– provides different site selection diversity .

Softer Handoff
– Handoff between sectors in a cell.

CDMA to CDMA hard handoff
– Mobile transmits between two base stations with different
frequency assignment.
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Soft Handoff- A unique feature of
CDMA Mobile
Advantages

Contact with new base station is made before the call
is switched.

Diversity combining is used between multiple cell
sites

If the new cell is loaded to capacity, handoff can still
be performed for a small increase in BER

Neither the mobile nor the base station is required to
change frequency
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Handoff: act of transferring a call from one
cell to the other
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Disadvantages of CDMA
Technology

Multi-user interference or multipleaccess interference(MAI).

Multi-path fading.

Near-far problem.

M-commerce difficult.
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THE NEAR FAR PROBLEM

Users may be received with very different
powers:
 Users near the base station are received
with high power.
 Users far from the base station are
received with low power.
 Nearby users will completele swamp far
away users.
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Processing gain

It is the increase in S/N ratio for
successful data communication

Processing gain (Gp)
Gp=(band width of spectrum/message
signalbandwidth)
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Power Control in CDMA

CDMA goal is to maximize the number of
simultaneous users.

Power transmitted by mobile station must be
therefore controlled.
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Two factors important for power
control

Propagation loss
• due to propagation loss, power variations up to
80 dB
• a high dynamic range of power control required

Channel Fading
• average rate of fade is one fade per second per
mile hour of mobile speed
• power attenuated by more than 30 dB
• power control must track the fade
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Power control
 Reverse
link open
loop power control.
 Reverse
link closed
loop power control.
 Forward
link power
control.
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Services in CDMA







Voice: In the form of calls made.
SMS: It allows subscribers to send and receive
short text messages between mobile stations .
MMS: It allows for the transmission of images,
audio, video, and rich text using WAP
Pocket data ( 144 kbps / 2.4 Mbps ) –NIC & EVDO cards.
Prepaid and Postpaid Services
VPN(Virtual Private Network)
Bank ATM / Branch ATM connectivity
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DEPARTMENTS OF CDMA
RF
MSC
POI
CDMA
NLD/ID
TRANSPORT
OSP
FIXED ACCESS
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Current Market
480 million CDMA
subscribers world over.
 110 million CDMA
subscribers in India.
 25% of cellular
subscribers in India are
CDMA users.
 ARPU Rs 99 for
CDMA in India.

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CDMA COMPARED TO GSM
Technique Used-CDMA uses code
division multiple access whereas GSM
uses time division multiple access.
 Power Consumption and Adjacent
Channel Interference.
 Data Transfer Speed-CDMA has been
traditionally faster than GSM.
 Subscriber Identity Module (SIM) cards
and RUIM.

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In Technology CDMA Has Plus
Point Over GSM, Why?
CDMA is faster.
 CDMA is more secure.
 Connection on a CDMA network will
never get dropped when moving from
cell to cell.
 CDMA base stations cover a large area.

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Billing and Customer Care System
Prepaid Services: The customer is
charged for usage in the prior month.
 Postpaid Services: It asks customers to
add a certain amount to their accounts
prior to usage.
 Customer service-a series of activities to
enhance the level of customer satisfaction.
 Many organizations have implemented
feedback loops.

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Future Of CDMA

CDMA has overcome most cynicism to dominate the
worldwide wireless voice market.

GSM & CDMA both are moving towards improved technology
based on CDMA technique

Ongoing research on CDMA

Applying CDMA to other applications: optical CDMA, adhoc
networks, dense wireless LANs etc.

“MultiCDMA”: multiple antenna CDMA, multicarrier CDMA,
multicode CDMA etc.
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Conclusion
CDMA provides more number of users
per cell site.
 A very secure mode of communication
encouraging its use in military.
 It is power efficient and provides
extended battery life.
 Provide better call quality due to soft
handoff reducing the number of call
drops.

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