Wireless Communications & Mobile Telephony

Lecture Week 4

Last Lecture: 03 Telecommunications Systems

• Systems and System Specifics,

• Telecommunications Systems,

• Fundamental Blocks of Communication,

• Bandwidth in Analogue & Digital Systems,

• Transmission Media,

• Attenuation,

• The Decibel,

• Noise in Analogue & Digital Systems,

• Multiplexing Techniques (FDM, WDM & TDM),

• Modulation (AM & FM).

Today’s Lecture: 04Wireless Communications & Mobile Telephony

• Introduction to Wireless Communications.

• Radio Wave Frequency Bands.

• Modes of Propagation of Signals.

• Mobile Radio Systems.

• Offer good broadcasting ability and enable true mobile communications.

• Use unguided media as the transmission medium:

The transmission between transmitter and receiver is achieved via electromagnetic (E-M) wave propagation through free space.

Electromagnetic (E-M) waves in free space travel at the velocity of light (3 x 108 m/s).

• For terrestrial communications the unguided medium is the atmosphere and its layers, and for inter-satellite communications it is outer space.

Wireless (or Radio) Communications

• Works by:

modulating the input signal with a suitable carrier,

amplifying the modulated carrier wave,

radiating it in free space with the use of an antenna.

• A receiver tuned to the right carrier frequency can recover the transmitted information.

• An antenna can be used to both transmit and receive E-M waves.

A Basic Wireless Communications System

Antennas in Wireless Communications Systems

• Antennas are essential components of both the transmitter and receiver of a radio communications system.

When transmitting, the antenna radiates energy from a transmission line into free space in the desired direction.

When receiving, the antenna picks up the signals transmitted towards it.

• The signals received are then guided by a transmission line (usually coaxial cable) to the receiver circuit for demodulation.

Radio Wave Frequency Bands

• The usable radio frequencies span from 3 kHz to 100 GHz.

• The radio spectrum has been divided into a number of bands.

Classification Frequency Range

Very Low Frequency VLF 3 kHz - 30 kHz

Low Frequency LF 30 kHz - 300 kHz

Medium Frequency MF 300 kHz - 3 MHz

High Frequency HF 3 MHz - 30 MHz

Very High Frequency VHF 30 MHz - 300 MHz

Ultra High Frequency UHF 300 MHz - 3 GHz

Super High Frequency SHF 3 GHz - 30 GHz

Extra High Frequency EHF 30 GHz - 300 GHz

• Modes describe the way that E-M waves propagate in free space.

• Various physical characteristics affect the way by which E-M waves propagate in free space.

• Radio waves may be:

Reflected: From the Earth's surface.

Refracted: Using upper layers of the atmosphere, to increase range beyond optical horizon.

Diffracted: This enables long to medium wave bands to propagate around obstacles.

• The above physical effects can also be put to good use and help the propagation of radio waves in free space depending on the specific need.

Modes of Radio Wave Propagation

• Most popular Mobile systems include:

Radio Paging,

Cellular Radio,

Radio Data Service (RDS),

Mobile Satellite Communications,

Location & Navigation Systems.

Mobile Radio Systems

• Most mobile systems use VHF and UHF bands.

In VHF and UHF the E-M waves are very directional and diffraction is poor, therefore radio shadows can occur.

Even in flat terrain a shadow begins beyond the radio horizon (30% further than the visible horizon since upper atmospheric refraction is lower than at ground level).

• LF radio waves are able to diffract around buildings and natural obstacles but are not used for mobile systems.

Mobile Radio Systems Frequency Bands

• Transmission in mobile bands at distance appreciably beyond the horizon is greatly reduced.

This is used to advantage for “Frequency Re-use”.

• Coverage area of an antenna is approximately proportional to its height.

Although it may be tempting to reduce system costs by using tall antennas, the scope for frequency re-use diminishes and there is an optimum relationship between height, coverage area, frequency reuse and cost.

• Shadowing is greatly mitigated in built-up areas by reflections off hard-surfaced objects such as buildings.

Often no line-of-sight path exists in urban areas and reception relies upon such reflection.

Mobile Radio Systems Frequency Bands (…2)

• Multipath effects often come into play due to a number of reflections appearing at a receive antenna.

Multipath effects mean that the resultant signal is the sum of the individual rays.

May be additive or subtractive depending upon their respective phases. Sometimes subtraction may be so severe that the signal is lost in noise or at least is weaker than the receiver sensitivity (known as a 'fade‘). 

• Propagation within buildings is desirable, particularly for cellphones / mobiles and pagers. The mechanism is via doors, windows and thin non-metallic roofs.

Buildings increase loss.

• Other factors to consider are clothing and body orientation.

Mobile Radio Systems Frequency Bands (…3)

Co-Channel Interference

• Occurs when the various frequencies transmitted by the various transmitters interfere with each other.

• To avoid co-channel interference various techniques may be employed:

Frequency Re-Use: The solution favoured by cellular systems. Explained later on.

Sequential Transmission: While one transmitter is in use, all others are switched off.  At any one time, only one transmitter is operating.  Each transmitter operates in turn.

Simulcast: All transmitters within the network operate on the same frequency.  It is then arranged that they are all modulated in virtual synchronism in time.

Basic Structure of Cellular or Mobile Networks

• A cellular / mobile communications system uses a large number of low-power wireless transmitters to create “cells”.

• Cells are the basic geographic service area of a wireless communications system.

Cells can have variable power levels which allows them to be sized according to the subscriber density and demand within a particular region.

Mobile Communications System Structure

• Each mobile uses a separate, temporary radio channel to talk to the cell site.

The cell site talks to many mobiles at once, using one channel per mobile.

• Channels use a pair of frequencies for communication.

One frequency, the forward link, is used for transmitting from the cell site,

Another frequency, the reverse link, is used for the cell site to receive calls from the users.

Mobile Communications Channels

• Structured similar to television broadcasting:

Uses one very powerful transmitter.

The transmitter is located at the highest spot in an area.

The transmitter would broadcast in a radius of up to fifty kilometers.

Traditional Mobile Service

• Different from the Traditional Mobile Structure.

• Many low-power transmitters instead of a powerful one.

• Placed throughout a coverage area.

The “Cellular Concept”

• The cellular concept employs variable low-power levels which allow cells to be sized according to the subscriber density and demand of a given area.

Cells can be added on demand to accommodate population grow.

• Conversations can be handed off from cell to cell to maintain constant phone service as the user moves between cells.

The cellular radio equipment (base station) can communicate with mobiles as long as they are within range.

The “Cellular Concept” (…2)

• Provisioning for each region is planned according to an engineering plan that includes:

Cells,

Clusters,

Frequency Re-Use,

Handovers.

The “Cellular Concept” (…3)

Cells

• A cell is the basic geographic unit of a cellular system.

The term "cellular" comes from the honeycomb shape of the areas into which a coverage region is divided.

• Cells are base stations transmitting over small geographic areas that are represented as hexagons.

• Each cell size varies depending on the landscape.

• Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon.

• Cells are divided / classified based on their size.

Cell Size Common Use Coverage

Pico-cells Within buildings A few metres

Nano-cells Within buildings < 10 m

In-building Micro-cells Office floor < 100 m

Nodal Cells High capacity network node

< 300 m

Street Macro-cells Pedestrian mobiles 10 m - 400 m

Street Macro-cells Vehicular mobile 300 m - 2 km

Mini-cells Pedestrian mobiles 500 m - 3 km

Macro-cells Cities and suburbia 1 km - 5 km

Large Cells Suburbia and rural 5 km - 35 km

Mega Cells Cities to counties 20 km - 100 km

Satellite Cells Satellites > 300 km

Cell Classification

Clusters

• Clusters are groups of Cells.

• Cells are constructed as polygons and their shape (number of sides) is chosen so that they tessellate (overlap) in order to prevent gaps occurring in the service area offered by a cluster.

Tessellations may be arranged to produce a wide range of cluster sizes.

• Clusters may have the following number of cells: 3, 4, 7, 9, 12, etc.

• Within a particular system the number of cells in a cluster (cluster size) is usually fixed.

• No channels are reused within a cluster.

• In the UK hexagonal cells have been chosen.

• Such cells are attractive because they approximate to a circle.

Clusters (…2)

• The solution the industry adopted to reuse radio channels in order to carry more than one conversation at a time.

• WHY? Because only a small number of radio channel frequencies were available for mobile systems.

• Also called frequency planning.

Frequency Re-Use

• Frequency re-use was implemented by restructuring the mobile telephone system architecture into the “cellular concept”.

The “Cellular Concept”:

• The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area.

• Cells are assigned a group of channels that is completely different from neighbouring cells.

• The coverage area of cells is called the footprint.

• The footprint is limited bya boundary so that the samegroup of channels can beused in different cells thatare far enough away fromeach other so that theirfrequencies do not interfere.

Frequency Re-Use (…2)

Cell Splitting

• Cell splitting is the splitting of a single area into smaller ones.

• WHY? Because economic considerations made the concept of creating full systems with many small areas impractical.

• To overcome this difficulty, system operators developed the idea of "cell splitting".

• As a service area becomes full of users, this approach is used to split a single area into smaller ones.

Urban centers can besplit into as many areasas necessary in orderto provide acceptableservice levels inheavy-traffic regions.

Larger, less expensivecells can be used tocover remote ruralregions.

Cell Splitting (…2)

Handoff or Handover

• Handoff or Handover relates to the process of transferring a call from one cell to another when a mobile subscriber travels from one cell’s coverage area to another’s during a call.

• Handoff occurs when the mobile telephone network automatically transfers a call from radio channel to radio channel as a mobile crosses adjacent cells.

• WHY? Because dropping the call is unacceptable. Adjacent areas do not use the same radio channels so a call must either be dropped or transferred from one radio channel to another.

• During a call, two parties are on one voice channel.

• When a mobile unit moves out of the coverage area of a given cell site, the reception becomes weak.

At this point, the cell site in use requests a handoff.

The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user.

The call continues as long as the user is talking, and the user does not notice the handoff.

If handoff does not take place then the call is dropped.

• A call is dropped only if there is no channel available, i.e. full utilisation of the cell.

Handoff Process

Handoff Process (…2)

Summary

• Introduction to Wireless Communications.

• Radio Wave Frequency Bands.

• Modes of Propagation of Signals.

• Mobile Radio Systems.

Thank You