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CDMA stands for Code-Division Multiple Access,a technology which is also called digital cellular technology which uses technique of Spread Spectrum. CDMA has a special feature that it don't assign each user with specific frequency instead full available spectrum utilization is done by every existing channel.Coding is done following pseudo-random digital sequence. Better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time.
CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built.
One of the terms you'll hear in conjunction with CDMA is "Soft Handoff". A handoff occurs in any cellular system when your call switches from one cell site to another as you travel. In all other technologies, this handoff occurs when the network informs your phone of the new channel to which it must switch. The phone then stops receiving and transmitting on the old channel, and commences transmitting and receiving on the new channel. It goes without saying that this is known as a "Hard Handoff".
IMPORTANCE OF CDMA
CDMA changes the nature of the subscriber station from a predominately analog device to a predominately digital device. In CDMA technology multiple users can occupy the same frequency band ensuring frequency reusability. This is the reason why CDMA is the technology of choice for next generation digital wireless communications products and services. Some of the major importances of CDMA technologies are:
It is very useful for military purpose. Since, these signals are transmitted at different frequencies at that too in a very large bandwidth, so it becomes hard to either interpret or jam or cause interference to the transmission.
CDMA is more secured than other wireless communication service such as GSM. This is because CDMA uses a pseudo random code, which is integrated with the data and then removed at the receiver end to recollect the original data.
CDMA signals provide greatest coverage in the wireless industry. This requires to built fewer cell sites reducing expenses both on providers and customer side.
Because of CDMA's soft handoff policy, there are fewer call drops. This is because at any instant of time two or three cell are monitoring a call, and passing calls between cells is sharply in case of CDMA.
Here's how the three most popular technologies establish channels:
FDMA (Frequency Division Multiplex Access)
each user on a different frequency
a channel is a frequency
TDMA (Time Division Multiplex Access)
each user on a different window period in time ("time slot")
a channel is a specific time slot on a specific frequency
CDMA (Code Division Multiplex Access)
each user uses the same frequency all the time, but mixed with different distinguishing code patterns
a channel is a unique set of code patterns
Introduction to Spread Spectrum Communications
Three Types of Spread Spectrum Communications
There are three ways to spread the bandwidth of the signal:
The standard chosen is CDMA and it is a form of Direct Sequence Spread Spectrum communications.
The technology which CDMA uses is Direct Sequence Spread Spectrum.
Direct sequence Spread Spectrum:In this coding of digital data is done at much hgher frequency.Since the code generated is pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.
Direct Sequence Spread Spectrum
Since CDMA is a follower of Direct Sequence Spread Spectrum system. The whple System works on 64 kbit/sec digital signals. The signals can be easily present in the digitised voice, ISDN channels, modem data, etc.
Direct Sequence Spread Spectrum System
Explanation of Transmission:
Steps of Signal transmission
1. A code is generatedwhich is called pseudo-random,which is uncommon for all channels and for each next coming connection.
2. The pseudo-random code is modulated by Information data.
3. A carrier is resulted by a signal modulation.
4. The amplification and broadcasting is done of modulated carrier.
Steps of Signal reception:
1. Receiving and amplification of The carrier .
2. For recovering of the spread digital signal mixing of the received signal with a local carrier id done.
3. Generation of pseudo-random code .
4.Aquiring of the received code by the receiver and after the locking its own code to the recived code is done by phase..
5. By extracting the Information data the correalation of the received signal is done with the generated code,.
The technique followed by Direct Sequence Spread Spectrum is
QPSK (Quadrature Phase Shift Keying) Phase modulation algorithm.
Frequency modulation is having one version of it is Phase modulation in which modulation of carrier wave is done in terms of the phase for the purpose of encoding digital information of every bits after every phase change.
"PSK" stands for Phased Shift Keying .
It is aform of phase modulation which is achieved by using states of discrete number.
QPSK stands for PSK with 4 states.
The "Quad" in QPSK refers to four phases. Since a carrier is sent in QPSK: 45, 135, 225, and 315 degrees.
Since QPSK is having 4 possible states,so coding can be done using 2 bits only .
. QPSK uses four points on the constellation diagram, equispaced around a circle which is shown with grey coding.This shows that while maintaining bandwith QPSK can provide double the data rate comompared to a BPSK system .
Since there is an ambiguity problem with at the receiver side thus QPSK is more widely used
Conceptual transmitter structure for QPSK
Receiver structure for QPSK
This figure shows an overview of a CDMA Technology.
T1 or E1s
Twisted pair - copper
Fiber optic cable
Air interface (radio signals)
Mobile Telephone Exchange (MTX) provides call processing functions for AMPS/TDMA/CDPD/CDMA cellular systems
Base Station Manager (BSM) provides a Graphical User Interface (GUI) for operations, administration and maintenance of the BSC, BTS and itself
Base Station Controller (BSC) provides data routing, voice coding
and some hand-off functions
Base Station Transceiver Subsystem (BTS) provides the RF link
to the subscriber
MTX, BSC and BSM are identical for 800 and 1900 MHz products
Since CDMA Sysem is wireless communication channel and data is transferred through wireless channels so there are a lot of chances of eavesdropping .It is generally said that CDMA is safe from eavesdropping but the ground reality is it is possible..So for that purpose
Since today's service providers are realising the importance of security so it is possible that in near future switch can be used for the purpose of security.
Security as well As Authentication in CDMA
In IS-41 standard authentication for CDMA access has been defined .
, For that purpose,Generation of intermediate keys called "Shared Secret Data (SSD)" is done.
In IS-41There are two guidelines
1.SSD-A for the purpose of authentication signature.(64 bits)
2. SSD-B for the purpose of cryptographic key generation.(64 bits)
Keys generated from SSD-B:
ï‚•ï€ The CMEA key (64 bits)
ï‚•ï€ The Voice Privacy Mask (520 bits)
ï‚•ï€ The DataKey (32 bits)
3.4 Authentication Signatures
Allowing a mobile phone to access the network is possible only when the phone had faced a challenge or master key with the help of key present in it's own , based on the SSD-A (IS-41) , and it also contains the special algorithm called CAVE for the purpose of calculating signature.
Security Concerns in CDMA
Algorithms for the purpose of encryption and authentication are used by CDMA
algorithm for authentication: CAVE
algorithm for encryption: CMEA
algorithm for privacy and integrity of data: ORYX
Core Network Security
Type of locality and the proximity of the nearest site decides usage of type of Technology.
More important than the type of building however is the proximity of the nearest site
CDMA shows its importance when we are out in the countryside and only few sites are there covering large masses of land. Here CDMA magic is shown ,by providing of extremely stable audio with few frame errors with less mess up or you can say negligible. This is because Channel Pollution is almost non-existent in these situations.
This CDMA technology is also used in locality of sparsely populated areas.
CDMA systems can survive open regions just outside densely populated areas, where signals from different sites are also coming from the nearby densely populated areas, but there should be no dominant signal strength.
CDMA is used at high grounds where TDMA fails .
Malls with glass roofs will generally provide better service than fully enclosed ones. More important than the type of building however is the proximity of the nearest site. When a site is located just outside of a building it can penetrate just about any building material. When a site is much further away however, the signals have a much harder time of getting past the walls of a structure
Utilities like electricity, oil, gas and water
Railroads and transportation
.Banks, hospitals, universities and corporations
for ranging and radar.
Advantages of CDMA:
One of the main advantages of CDMA is that dropouts occur only when the phone
is at least twice as far from the base station. Thus it is used in the rural areas where GSM
cannot cover. Another advantage is its capacity; it has a very high spectral capacity that it
can accommodate more users per MHz of bandwidth. It uses a vocoder EVRC for noise
reduction where the background noise is reduced. This is exclusively available in CDMA
Disadvantages of CDMA:
One major problem in CDMA technology is channel pollution, where signals
from too many cell sites are present in the subscriber's phone but none of them is
dominant. When this situation arises the quality of the audio degrades. Another
disadvantage in this technology when compared to GSM is the lack of international
roaming capabilities. The ability to upgrade or change to another handset is not easy with
this technology because the network service information for the phone is put in the actual
phone unlike GSM which uses SIM card for this. One another disadvantage is the limited
variety of the handset, because at present the major mobile companies use GSM
Problems faces while implementation of CDMA technology
In Code Division Multiple Access (CDMA), each user is assigned a different
pseudorandom binary sequence that modulates the carrier, spreading the spectrum of the
waveform and giving each user a unique code pattern.
This technology is used in ultra-high-frequency (UHF) cellular telephone systems in the
800-MHz and 1.9-GHz bands.
Just because of this we can cover only 30% of the world network.
The CDMA technology is generally known as
powerful security during communication. However,
the communication data might be eavesdropped and
forged, because the mobile communication sends
data through wireless communication channel. So it
is essentially necessary to setup the entire system
securely. This paper analyzes the security hole and
proves a weak point of CDMA system. Based on the
results, we emphasize the necessity of security in
CDMA system. For the sake of this, we analyze the
IS-95 protocol and propose the monitor mechanism
of forward channel in call processing procedure. The
IS-95 and GSM only define the security features
between the mobile station and the base station.
The monitor equipment is divided into two parts
that are a logic circuit part and RF circuit part. The
logic circuits are MSM(Mobile Station Modem), Audio
PCM Codec, speaker, memory, PC(Personal
Computer), and UART. RF circuit, which transforms
CDMA signal into BASE BAND signal, are BBA, PLL,
AMP, and filter. We propose only the methodology of
the forward channel monitoring by modifying the
terminal S/W partially in CDMA terminal. We propose
S/W and H/W architecture for the call process of the
forward channel monitor and analyze the call process
procedure. The REX Operating System in equipment
terminal was used to monitor the CDMA system. If
we know the ESN and the MIN, we can eavesdrop
the CDMA data. We can easily know the ESN and
MIN value by HP8924C instrument.
We suggest the FCM protocol to monitor CDMA
system. This paper proves a weakness of CDMA
system using the FCM protocol. We also prove that
the BS and the MS are the same data in
communicating each other. We implement the test
instruments including the FCM protocol. In the
experiment of monitoring system based on the FCM
protocol, we can monitor an user communication
message by the monitoring environments. In the
future, based on the result, we will suggest a new
system architecture for the secure CDMA system