CDMA

What is
CDMA?
The term “CDMA” (Code Division Multiple
Access) refers to both a spread spectrum
technique and a cellular standards.
Initially it was restricted to the armed
forces, this technology was commercially
launched in 1995.
Multiple access basically means that
information by several transmitters can be
sent simultaneously onto a single
communication channel.

There are multiple users which are provided or assigned variant CDMA
codes and thus the users can access the entire band of frequencies or
the whole bandwidth.
This method does not limit the frequency range of the user. Hence,
with the help of CDMA, multiple users can share a band of frequencies
without any kind of undue interference between them.
CDMA makes the use of spectrum technology along with analog to
digital conversion(ADC).
It is thus used by various radio communication technologies. Mainly, it
is used for mobile communication.

History of CDMA
• The CDMA technology was originated during World War II.
• It was developed by English allies for protection of their wireless
transmissions from jamming from enemies.
• This technology was patented by Qualcomm post war and was made
commercially available for use.
• The first CDMA system was launched in 1995 in Hongkong by Hutchison
Telephone Company.

How it Works?
• Code: It refers to the string of binary sequence that the
transmitter and the receiver share. This code encodes the
information into a low frequency signal before it is
transmitted over a channel. This same code is used by the
receiver to decode the information. The receiver gets the
code with the help of the nearest base station.
• Division: In CDMA a single channel is divided into
numerous slots which can be used by multiple users. This
is possible because of the use of unique code.
• Multiple Accesses: Due to code based communication,
multiple users can communicate and access the same
channel simultaneously without any undesirable
interference and loses.

Features of CDMA
Some of the key features of CDMA are:
• Used in Global Positioning System (GPS)
• Every channel uses full available spectrum
• Better capacity for voice and data communications
• CDMA does not require synchronization
• Allocation of code word to each user helps to reduce interference
• Used by several mobile companies (Qualcomm standard IS-2000 known as CDMA2000)
• Emits less radiation
• Faster data transfers
• CDMA supports high speed push to talk and push to email services
• UTMS 3G mobile phone standard uses W-CDMA
• OmniTRACS satellite system uses CDMA for transportation

Encoder for DS-SS CDMA

• The input provided to the CDMA encoder can be in the form of Pulse
code modulation (PCM) encoded voice band signal or can be a digital
signal from computer.
• It is multiplied with N bit, which is a unique chip code.
• The output of balanced modulator is the product code.
• In IF carrier it is used as an PSK modulation.
• The balanced modulator is sometimes referred to as multiplier.
• Further the modulated signal is then converted to RF band and is used
for transmission purpose.
• The high-power amplifier basically raises the level of power to a very
high level and then the antennas transmit this signal.
• The encoder is also called as multiplexer.

Decoder for DS-SS CDMA

• The decoder helps in reconverting the RF signal to IF.
• A coherent PSK carrier is been obtained from IF.
• The chip code is been used by the receiver and it helps in synchronizing the receiver
station’s code generator.
• The recovered chip is then multiplied with recovered PSK carrier to generate PSK
modulated signal which contains PSK carrier and the chip code.
• The IF signal which is received, contains chip code, PSK carrier and data. In correlator it
is compared with the received IF signal.
• The correlator helps in comparing this two signals and helps in recovering the original
data.
• The decoder is also called as demultiplexer.

Limitations of CDMA
• Some of the Limitations of CDMA are:
• A large code length could induce delay or interference
• Time synchronization is required
• Reduction in capacity due to gradual transfers
• Constant tight power control is required resulting in multiple handovers (Causing dropped calls)

Types of CDMA Technologies
There are two types of CDMA technologies:
• Synchronous CDMA (orthogonal codes)
• Asynchronous CDMA (pseudorandom codes)

Difference between Synchronous CDMA
(orthogonal codes) & Asynchronous CDMA
(pseudorandom codes)
CDMA
Technology
Definition Advantages Disadvantages Best suited for
Synchronous
CDMA
It exploits the mathematical
properties orthogonally
between vectors
representing the data
strings.
•Can accommodate large
number of subscribers
•Gives better performance
•Orthogonal codes are
used
•Digital modulation is
analogues
•Issue of dropped
calls
•Quality degrades
with increased user
base
Mainly used by mobile telephone
operators.
Asynchronous
CDMA
Makes use of code space .
Pseudo-random or pseudo-
noise sequences are used.
•Pseudo-random code
(Pseudo-random Noise)
or Gold code is used
•Flexible allocation of
resources
•Slow speeds
•Uses random or
irregular time
intervals
Ideally suited for mobile networks
having large number of transmitters
are producing small amount of
traffic at irregular intervals.

Advantages
• Increased user capacity is an advantage of the CDMA as it supports a lot more users in
comparison to TDMA or FDMA.
• CDMA is more secure as the information transmitted is below the noise floor making the
intrusion of the spectrum difficult.
• CDMA systems have comparatively fewer dropouts than GSM. Thus, it can also be used
in rural areas.
• The cost of the calls in CDMA is lower in comparison to the cost in GSM.
• CDMA provides a high quality of voice with almost no noise during the calls.
• Using CDMA problems like multipath and fading do not occur.
• CDMA has a very low power requirement.

Disadvantages
• CDMA lacks the facility of international roaming which is provided by GSM.
• Since there is no limit to the number of users the system performance degrades with an
increase in the number of users.
• Self-jamming problem occurs in CDMA systems because of loss of orthogonality.
• The problem of channel pollution occurs in CDMA systems which thus degrades the
quality of audio.
• Since most of the mobile companies use GSM thus there is a lack of handsets for CDMA
technology.