comprehensive viva 405- cmc

8/7/2019 Comprehensive Viva 405- CMC

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-Anand Lahoti(07E31A0405)


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A mobile network is a radio network distributed over land areascalled cells, each served by at least one fixed-locationtransceiver known as a cell site or base station.

When joined together these cells provide radio coverage over awide geographic area.This enables a large number of portable transceivers (mobilephones, pagers, etc) to communicate with each other and withfixed transceivers and telephones anywhere in the network, viabase stations, even if some of the transceivers are movingthrough more than one cell during transmission.

Mobile networks offer a number of advantages over alternativesolutions:

increased capacity

reduced power usagelarger coverage area

reduced interference from other signals


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To distinguish signals from several different transmitters,frequency division multiple access (FDMA) and codedivision multiple access (CDMA) were developed.With FDMA, the transmitting and receiving frequenciesused in each cell are different from the frequencies usedin each neighbouring cell.In a simple taxi system, the taxi driver manually tuned to afrequency of a chosen cell to obtain a strong signal and toavoid interference from signals from other cells.The principle of CDMA is more complex, but achieves thesame result.Other available methods of multiplexing such aspolarization division multiple access (PDMA) and timedivision multiple access (TDMA) cannot be used to separate

signals from one cell to the next since the effects of bothvary with position and this would make signal separationpractically impossible.Time division multiple access, however, is used incombination with either FDMA or CDMA in a number ofsystems to give multiple channels within the coverage areaof a single cell.


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The key characteristic of a cellular networkis the ability to re-use frequencies toincrease both coverage and capacity.

As described above, adjacent cells mustutilise different frequencies, however thereis no problem with two cells sufficiently farapart operating on the same frequency.

The elements that determine frequencyreuse are the reuse distance and the reusefactor.


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The most common example of a cellular network is amobile phone (cell phone) network.

A mobile phone is a portable telephone which receives ormakes calls through a cell site (base station), ortransmitting tower.

Radio waves are used to transfer signals to and from thecell phone.

Modern mobile phone networks use cells because radiofrequencies are a limited, shared resource.

Cell-sites and handsets change frequency under computercontrol and use low power transmitters so that a limitednumber of radio frequencies can be simultaneously used bymany callers with less interference.


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A cellular network is used by the mobile phoneoperator to achieve both coverage and capacity fortheir subscribers.

Large geographic areas are split into smaller cells toavoid line-of-sight signal loss and to support a largenumber of active phones in that area.

All of the cell sites are connected to telephoneexchanges (or switches), which in turn connect to thepublic telephone network.

There are a number of different digital cellulartechnologies, including: Global System for Mobile

Communications (GSM), General Packet Radio Service(GPRS), Code Division Multiple Access (CDMA),Evolution-Data Optimized (EV-DO), Enhanced DataRates for GSM Evolution (EDGE), 3GSM, DigitalEnhanced Cordless Telecommunications (DECT),Digital AMPS (IS-136/TDMA), and Integrated DigitalEnhanced Network (iDEN).


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Structure of a 2G cellular networkA simple view of the cellular mobile-radio network consists ofthe following:A network of Radio base stations forming the Base stationsubsystem.

The core circuit switched network for handling voice calls andtextA packet switched network for handling mobile dataThe Public switched telephone network to connect subscribersto the wider telephony network

This network is the foundation of the GSM system network.There are many functions that are performed by this networkin order to make sure customers get the desired serviceincluding mobility management, registration, call set up, andhandover.


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As the phone user moves from one cell area to another cell whilsta call is in progress, the mobile station will search for a newchannel to attach to in order not to drop the call. Once a newchannel is found, the network will command the mobile unit toswitch to the new channel and at the same time switch the callonto the new channel.

As the user moves from one cell to another, the handset sets upradio links with multiple cell sites simultaneously.

This is known as "soft handoff" because, unlike with traditionalcellular technology, there is no one defined point where the

phone switches to the new cell.

If there is no ongoing communication or the communication canbe interrupted, it is possible for the mobile unit to spontaneouslymove from one cell to another and then notify the base stationwith the strongest signal.Cellular frequency choice in mobilephone networks


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This article discusses the mobile cellular networkaspect of telegraphic measurements.

Mobile radio networks have traffic issues that do not

arise in connection with the fixed line PSTN.

Important aspects of cellular traffic include: qualityof service targets, traffic capacity and cell size,spectral efficiency and sectorization, traffic capacityversus coverage, and channel holding time analysis.

Teletraffic engineering is a necessary field intelecommunications network planning to ensure thatnetwork costs are minimized without compromisingthe quality of service delivered to the user of thenetwork.


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The GSM architecture consists of threesubsystems

Base station subsystemNetwork Station subsystemOperating station Subsystem


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A cell site is a term used to describe a site where antennasand electronic communications equipment are placed on aradio mast or tower to create a cell in a cellular network.

A cell site is composed of a tower or other elevatedstructure for mounting antennas, and one or more sets of

transmitter/receivers transceivers, digital signalprocessors, control electronics, a GPS receiver for timing(for CDMA2000 or IS-95 systems), regular and backupelectrical power sources, and sheltering.

A synonym for "cell site" is "cell tower", although many cellsite antennas are mounted on buildings rather than astowers. In GSM networks, the technically correct term isBase Transceiver Station (BTS), and colloquial BritishEnglish synonyms are "mobile phone mast" or "basestation".


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The effect of frequency on cell coverage means that differentfrequencies serve better for different uses. Low frequencies, such as450 MHz NMT, serve very well for countryside coverage.

Higher frequencies are a disadvantage when it comes to coverage,but it is a decided advantage when it comes to capacity.


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The recently released 4th generation, also known as Beyond 3G,aims to provide broadband wireless access with nominal data ratesof 100 Mbit/s to fast moving devices, and 1 Gbit/s to stationarydevices defined by the ITU-R, 4G systems may be based on the 3GPPLTE (Long Term Evolution) cellular standard, offering peak bit ratesof 326.4 Mbit/s.

A 4G system should be a complete replacement for current networkinfrastructure and is expected to be able to provide acomprehensive and secure IP solution where voice, data, andstreamed multimedia can be given to users on a "Anytime,Anywhere" basis, and at much higher data rates than previousgenerations.

Sprint in the US has claimed its WiMax network to be "4G network"which most cellular telecoms standardization experts disputerepeatedly around the world.Sprint's 4G is seen as a marketing gimmick as WiMax itself is part ofthe 3G air interface. The officially accepted, ITU ratified standards-

based 4G networks are not expected to be commercially launcheduntil 2011.


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Car phoneA type of telephone permanently mounted in a vehicle,these often have more powerful transmitters.

Cordless telephone (portable phone)Cordless phones are telephones which use one or moreradio handsets in place of a wired handset. The handsetsconnect wirelessly to a base station, which in turnconnects to a conventional land line for calling.

P rofessional Mobile RadioAdvanced professional mobile radio systems can be verysimilar to mobile phone systems. Notably, the IDENstandard has been used as both a private trunked radiosystem as well as the technology for several large publicproviders.


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Radio phone

This is a term which covers radios which couldconnect into the telephone network. Thesephones may not be mobile; for example, theymay require a mains power supply, or they mayrequire the assistance of a human operator toset up a PSTN phone call.

Satellite phoneThis type of phone communicates directly withan artificial satellite, which in turn relays callsto a base station or another satellite phone.


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IP P honeThis type of phone delivers or receives calls overinternet, LAN or WAN networks using VoIP asopposed to traditional CDMA and GSM networks.In business, the majority of these IP Phones tend

to be connected via wired Ethernet, howeverwireless varieties do exist. Several vendors havedeveloped standalone WiFi phones.Additionally, some cellular mobile phones

include the ability to place VoIP calls overcellular high speed data networks and/orwireless internet.


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Channel reuse-The concept of "maximum" range is misleading, however, in acellular network. Cellular networks are designed to create amass communication solution from a limited amount ofchannels (slices of radio frequency spectrum necessary tomake one conversation) that are licensed to an operator of acellular service. To overcome this limitation, it is necessary torepeat and reuse the same channels at different locations.Just as a car radio changes from one local station to acompletely different local station with the same frequencywhen you travel to another city, the same radio channel getsreused on a cell mast only a few miles away.

Signal strength-In telecommunications, particularly in radio, signal strengthrefers to the magnitude of the electric field at a referencepoint that is a significant distance from the transmittingantenna. It may also be referred to as received signal level orfield strength. Typically, it is expressed in voltage per lengthor signal power received by a reference antenna.


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Code division multiple access (CDMA) is a channelaccess method utilized by various radiocommunication technologies. It should not beconfused with the mobile phone standards calledcdmaOne and CDMA2000 (which are often referred toas simply CDMA), which use CDMA as an underlyingchannel access method.

Synchronous CDMASynchronous CDMA exploits mathematical properties

of orthogonality between vectors representing thedata strings. For example, binary string 1011 isrepresented by the vector (1, 0, 1, 1). Each user insynchronous CDMA uses a code orthogonal to theothers' codes to modulate their signal.


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Rural areas-In many rural areas the housing density is too low to make construction of a newbase station commercially viable.

Building construction material-Some construction materials very rapidly attenuate cell phone signal strength.

Older buildings, such as churches, which use lead in their roofing material will veryeffectively block any signal.

Building size-Large buildings, such as warehouses, hospitals and factories, often have no cellular

reception further than a few meters from the outside wall.

Multipath interference-Even in urban areas which usually have strong cellular signals throughout, there are

often dead zones caused by destructive interference of waves which have takendifferent paths (caused by the signal bouncing off buildings etc.).

Diffraction and general attenuation-The longer wavelengths have the advantage of being able to diffract to a greater

degree so are less reliant on line of sight to obtain a good signal, but still attenuatesignificantly.

comprehensive viva 405- cmc

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