lec#01(basics of cellular commincation)

7/28/2019 Lec#01(Basics of Cellular Commincation)

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TELECOMMUNICATION SYSTEMS &

APPLICATIONS

CELLULAR COMMUNICATION

CELLULAR COMMUNICATION

FUNDAMENTALS


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Wireless Communications Wireless telecommunications can be divided into two broad categories:

mobile communications

fixed wireless communications. Mobile communications technology must be able to allow roaming

the ability to provide service to a mobile phone users while outsidetheir home system.

On the other hand, fixed wireless is simply an alternative to wiredcommunications.


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Cellular Communication-A new Technology

Cellular communication & networks are a relatively new form of communication,

and are rapidly evolving

as coverage is expanded both in terms of network area (reachability) &

number of users (capacity),

new services are added, and existing analog cellular systems are replaced

by digital ones.


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Cellular Communication- Technology Evolution

Development of handheld radio telephone technology from two-way radiosin vehicles to handheld cellular phones.

Initially, two-way radios were used in vehicles such as taxicabs, policecruisers, ambulances.

Originally, mobile phones were permanently installed in vehicles, butlater versions such as the so-called transportables or "bag phones" wereintroduced.


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Cellular Networks System Cellular networks got their name because of the way they divide service

areas into cells.

A cellular network is made up of radio cells each serve by a fixedtransmitter, known as cell site orbase station.

Cellular networks are comprised ofterminals and base stations.

Terminals are the end-user equipment, usually phones, and are oftencalled Mobile Stations.

Everything else in a cellular network is considered to be base stationequipment.


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Objectives of Cellular Network

Large Capacity

Efficient use of Resources (Frequency Spectrum) Availability

Adaptability to traffic density

Service to mobile terminals

Quality of Service

Affordability


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Cellular Network Organization Areas divided into cells.

Each cell is served by its own antenna, basestation consisting of transmitter, receiver, and

control unit. Each cell is allocated a Band of frequencies.

Cells are arrange in a way as the antennas ofall neighbors cells are at equidistant(hexagonal pattern).

Instead of covering the whole area with onetransmitter (base station) of high power we

deploy multiple base stations of moderate(lower) power.

Each base station (cite) covers some specificarea.

Each base station is assigned a portion of thetotal number of channels, while neighboringbase stations are assigned different groups of

channels so that the interference betweenbase stations (and mobile users) isminimized.

Figure: Cellular Network Organization


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Challenges of Cellular Communication Cellular networks must deal with all traditional challenges such as

call setup, switching, and so on.

However, cellular networks must face additional challenges due to

the wireless nature of the terminals.

Important one is Network Routing.

They also have to deal with many types ofsignal corruption caused

by sending the signals through the air.


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Netwo rk Rout ing

network routing issues can be address by use of multiple access methods.

All cellular networks use two or more multiple access methods to send multiplesignals over a single broadcast medium.

The reuse of given frequencies in different cells is a form of space divisionmultiple access, and requires the location of each terminal to be known.

This is provided through a service known as location management (ormobilitymanagement), and it is not simple since terminals may move from cell to cellduring a single call (this is called handover) and may even operate in an areacovered by a different service provider (this is called roaming).

Calls should not be dropped during handover, even when changing serviceproviders, and soft handovershould be used versus hard handover if possible(in soft handover, the terminal can continue to use the same channel whencommunicating with both cell sites rather than having to quickly switch to anentirely different communication channel).

All cellular networks also make use offrequency division multiple access.

Radio frequency is usually assigned to cellular communication in 50 MHz

blocks. These are divided into an equal number of uplink (terminal to base station) anddownlink (base station to terminal) bands.

There can be anywhere from 10 to 900 pairs of bands, and each band paircarries one or more bidirectional channels.


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Signal Corrupt ion

There are six major types of signal corruption that are individuallyaddressed by most cellular networks.

Multipath effects (signal reflections) cause two of them: Ricean fadingand Rayleigh fading.

Doppler effectsare caused by the mobility of terminals.

Blocking is caused by the fact that terminals may be at differentdistances from the cell sites.

Signal loss can be caused by a variety of things, usually physical

barriers. All other types of signal corruption can be classified as noise


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Ricean Fading

Ricean fading is the multipath effect, results because transmitted signal reachseveral time to the receiver because of echo.

After the direct transmission is received, echoes of the signal reach the receiver.

These can cause transmitted symbols to interfere with future symbols.

Ricean Fading (Multi path Effect)


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Rayleigh Fading

Rayleigh fading is a very similar multipath effect, except it results when obstaclesblock the direct path from the transmitter to the receiver.

Since the direct transmission is blocked, the reflected signals are not echoes, butthe first signals received.

Rayleigh Fading (Multipath Effect)


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Dopp ler Effects Doppler effects are caused by mobile receivers or transmitters, and affect the

frequency of the received signal.

If the receiver and transmitter are moving towards each other, the frequency of

the received signal is shifted up. If they are moving away, it is shifted down.

Doppler Effects: Wavefront as seen by stationary & Mobile Receiver


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Block ing

Blockingresults when a nearby high-power transmitter blocks out whole rangeof channels.

It is similar to driving by a radio station which can block out other stations onnearby frequencies.

If a terminal is too close to the nearby (overloaded) cell, it may lose the signalfrom the cell with which it is communicating.

Blocking


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Loss o f Signal

There are several possible causes for loss of signal, including over-

aggressive power management, although the most likely cause is

simply due to obstacles. If the direct transmission blocked and there are no reflected signals to

receive, the signal is lost and must be renegotiated.

Loss of Signal


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Noise

There are many type of noises.

most common noise is general background noise, which is

additive white Gaussian noise that is dependent only ontemperature and cannot be avoided.

The cellular network also introduces its own noise.

For example, the blockingeffect can be felt on a smaller scale, when acell that has greater power (i.e., is closer) than the cell with which aterminal is communicating is using a nearby frequency, only causing

noise rather then blanking out the other signal. This is called cell to-cell adjacent channelnoise.

Co-channelnoise is also caused by the reuse of frequencies.

There can be additional noise problems when two terminals are usingthe same cell to communicate with each other (a type of crosstalkproblem).


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cellular communication- Operational Domains

There are two major domains of cellular communication processing

Radio Frequency (RF) Base band.


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Radio Frequency Domain The RF processing includes modulation, demodulation, frequency synthesis,

amplification, and analog filtering .

Analog cellular phone systems typically stop here and do not do base band

processing (they also typically use more power and consume more radiobandwidth per channel).

Modulation and demodulation deals with putting base band signals on carrierwaves.

Digital cellular systems use different forms of frequency or phase-shift keying intheir modulation techniques.

CDMA systems also use a PN sequence before determining the actual chips tosend.

Frequency synthesis is the generation of carrier frequencies.

Cellular frequencies are allotted by the governing regulatory body (the FCC inthe U.S.).

They are typically allotted in 50MHz blocks (25 MHz for the base-to-terminaldownlink or forward channels and 25 MHz for the terminal-to-base uplink or

reverse channels). Typically, they are allotted at around 900 MHz. Other frequencies that are commonly used are 450, 1800, and 1900 MHz, with

PCS systems using the latter two.

In all cases, channels operate in pairs (one uplink, one downlink), which can betreated as a single bidirectional channel.


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Base band Domain

Base band processing in digital cellular systems consists of voice coding& decoding , channel coding & decoding, equalization, timing & control,

digital conversion, and encryption. Voice coding is simply compressing speech.

Channel coding or Forward Error Correction (FEC), builds redundancyand structure into messages so digital faults may be corrected, andhelps correct for Raleigh fading and noise.

Equalization corrects for analog corruption (including noise, multipath,and Doppler effects) by attempting to reconstruct parts of the messageand guess what phenomena affected them .

Timing and control coordinates and synchronizes between the RF andbase band processing and deals with high-level control issues such asprotocol handling .

Digital conversion is from digital to analog and analog to digital.Encryption is not always supported, but might be used to sendsubscriber information to the base station, for example.

Voice Coding

Channel Coding

Equalization

lec#01(basics of cellular commincation)

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