CDMA(Code Division Multiple Access)

Evolution Of Cellular Systems• The advent of Cellular communications began in 1940s

known as MTS(Mobile Telephone Systems)• There has been a gradual advancement in the capacity and

signal strength.• The development can be broadly divided into 3 categories

namely first, second and third generations.

MULTIPLE ACCESS METHODS

• There are predominantly three types of multiple access methods.

1.FDMA(Frequency Division Multiple Access)2.TDMA(Time Division Multiple Access)3.CDMA(Code Division Multiple Access)

1.FDMA(Frequency Division Multiple Access)—• With FDMA, frequencies are separated in the

frequency domain, each user is allotted a different set of frequencies to operate upon.

• Subscribers are assigned a pair of voice channels (forward and reverse) for the duration of the call.

2.TDMA(Time Division Multiple Access)— In this system, each user is allocated a different

time slot. Forward link frequency and reverse link frequency is the same. A synchronous switch is responsible for the time switching.

3.CDMA(Code Division Multiple Access)—There is no restriction on time and frequency in this

scheme. All the users can transmit at all times and at all

frequencies. Because users are isolated by code, they can share

the same carrier frequency, eliminating the frequency reuse problem encountered in other technologies.

With CDMA, unlike other cellular telephone standards, subscriber data change in real time, depending on voice activity and requirements of the network and other users of the network.

• A comparative study between the above three access technologies with respect to time and frequency is as shown below.

Direct Sequence Spread Spectrum - DSSS CDMA

• In this method, the direct sequence(input data) which is spread over a limited bandwidth is multiplied with a code or spreading sequence (a pseudorandom sequence also known as PN sequence).

• This will spread the input data over the entire bandwidth of the communication channel.

• The power density is also reduced and is spread over the frequency spectrum and hence is known as spread spectrum method.

CDMA MODULATION

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.

Evolution of CDMA network• In 1984 Qualcomm Inc. proposed a cellular

telephone system and standard based on spread spectrum technology which marked the advent of CDMA.

• Since then the evolution has been rapid as shown:• IS-95 A ( 1995)• IS-95B • IS-95A as air-interface standard and IS-41 in core

network; the complete network known as CdmaOne.

• CDMA 2000 1x - • CDMA 2000 1x –EV-DO – data only 2.4 Mbps + RF

backward compatible.

Cellular Telephone System Topology• Base Station : The location of the transceivers are

called base stations. A base station serves as central control for all the users within the cell.

• Mobile Units: Such as automobiles and pedestrians carrying the mobile.

• MTSO(Mobile Telephone and Switching Office): The mobile units and the base station communicate directly with the MTSO.

• An MTSO controls channel assignment, call processing, call setup, and call termination which include signaling, switching, supervision and allocating r.f. channel.

CDMA Network and System Architecture

• Mobile Stations:• Mobile subscribers access the CDMA2000 1x data

network using a mobile station, such as a mobile phone, modem, a laptop with an embedded CDMA2000 chip, a broadband access wireless router, or PC Card on a laptop computer.

• The mobile station interacts with the access network (AN) to obtain radio resources in order to exchange data packets. The mobile station, in tethered mode, can also act as a modem for a computer.

• The mobile station automatically registers with the network upon power-up, and upon successful registration, it is ready for voice and data calls.

Access Network• There are two types of access networks: 1xRTT and 1xEV-DO. • The AN is the mobile station’s entry point into the mobile

network .• Maintains the communications link between the mobile station

and the core network. • The AN is composed of the following elements: • Base Transceiver Station• It is physically composed of antennas and towers. • The BTS manages radio resources including radio channel

assignment and transmit and receive power management and acts as the interface to mobile stations.

• Packet Control Function• It maintains the “connection state” between the access network

and mobile stations.• Radio Network Controller/Base Station Controller• These schedule packet transmission on the air interface and

manage handoffs between BTS`s .

Core Network• The core network acts as the gateway between the access

network and the Internet or enterprise private networks. • It provides authentication, authorization, and accounting

(AAA) services, provides access to network services, IP mobility, and manages IP addresses.

• The core network comprises the following elements:• PDSN/Foreign Agent• The PDSN is the gateway between the access network

and the core network. • AAA/Home Agent• The AAA and the home agent (HA) are used for

authentication, authorization, and accounting for data services. The AAA/HA stores and records usage and access information for billing and invoicing purposes.

CDMA Channels IS-95 FORWARD LINK CHANNELS• Pilot channel – For initial system acquisition & to

uniquely identify cell / sectors . Every sector of every cell site has a unique pilot channel.

• Sync channel - Subscriber unit with network information related to cell site identification, pilot transmit power & cell site PN offset.

• Paging channels – Used by base station to page the subs unit call set-up and traffic channel assignment information.

• Traffic Channels - The traffic channel carries the actual call, i.e, the voice and control information between the subs unit & base station.

CDMA CHANNELS IS-95 Reverse Link Channels• Access Channel -Provides communication from

Mobile to base station when mobile is not using traffic Channel.

• Traffic channels - The traffic channel carries the actual call traffic in reverse channel also.

• In CDMA 2000 1x , in addition to above channels, Fwd. Quick paging channel ,Fwd. Supplemental channel , Rev. pilot channel , Rev. supplement channels are available.

CDMA SPECIFICATIONS

1. Frequency of operation: 824-849Mhz & 869-894 Mhz2. Duplexing Method : Frequency Division Duplexing 3. Access Channel per carrier: Maximum 61 Channels4. RF Spacing: - 1.25 Mhzs 5. Coverage: - 5 Km with hand held

telephones and approx. 20 Km with fixed units. • CDMA channel in the trans and receive direction is a FDD

(Frequency Division Duplexing) channel. • Frequency-division duplexing means that the transmitter and

receiver operates at different carrier frequencies.

CDMA SECURITY• As wireless data networks become increasingly

prevalent, new possibilities and challenges continue to emerge.

• 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

CDMA SECURITY BENEFITS• The key inherent security benefits of CDMA technology can

be summarized as:• CDMA codes inherently spread the signals across the full

channel bandwidth of 1.25 MHz. • Soft handoff (multiple cells simultaneously supporting the

call) typical for the CDMA operation make it very difficult to “follow” the CDMA cellular call.

• Long code mask (LCM) provides “built-in” security at the physical layer.

• CDMA signals are very difficult to intercept.• CDMA attacks require sophisticated and expensive

equipment.• Access is only provided to authenticated mobile

stations/subscribers.

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 (wireless application protocol) technology and an MMS-capable mobile station.

• Pocket data ( 144 kbps / 2.4 Mbps ) –NIC & EV-DO cards.

• Prepaid and Postpaid Services• VPN(Virtual Private Network)• Bank ATM / Branch ATM connectivity

Billing and Customer Care System• Prepaid Services: The customer is charged for usage in the

prior month (hence 'post' usage), and hence does not feature any limitations on volume of service used.

• Postpaid Services: It asks customers to add a certain amount to their accounts prior to usage and their maximum usage is limited to the amount that is added to the account.

• Customer service is a series of activities designed to enhance the level of customer satisfaction – that is, the feeling that a product or service has met the customer expectation.

• Many organizations have implemented feedback loops that allow them to capture feedback at the point of experience.

• The operators provide the user with one dial customer care services and supplement them by having offices in cities.

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, though both technologies continue to rapidly leapfrog along this path. Both boast "3G" standards, or 3rd generation technologies.

• Subscriber Identity Module (SIM) cards and RUIM

CONCLUSION• In the age of growing connectivity CDMA plays

a crucial role in bringing us closer.• According to a survey CDMA networks support

over 270 million subscribers worldwide.• The techniques involved in it enable us to

achieve the data transfer speeds of almost 3.1 mbps(EVDV cards).

• Future advancement in technology is going to take us to new heights in terms of capacities and facilities offered to customers.