Propagation Measurements Propagation Measurements and Models for Wireless and Models for Wireless

Communications ChannelsCommunications Channels

Brian AlexanderBrian Alexander

IntroductionIntroduction

A basic understanding of the channel A basic understanding of the channel is important for finding modulation is important for finding modulation and coding schemes.and coding schemes.

It is also useful for deploying base It is also useful for deploying base station antennas.station antennas.

Types of signals that occur in various Types of signals that occur in various environments and models of environments and models of propagation parameters.propagation parameters.

ConcernsConcerns

Achieving reliable communications.Achieving reliable communications.

Energy should be sufficient, but not Energy should be sufficient, but not so strong as to cause interference.so strong as to cause interference.

Even with sufficient power, large Even with sufficient power, large errors may occur anyway.errors may occur anyway.

ModelsModels

1.1. Parametric statistical models that Parametric statistical models that on averageon average describe behavior within describe behavior within a given error.a given error.

2.2. Deterministic models characterizing Deterministic models characterizing a specific street, building, etc.a specific street, building, etc.

Physics of PropagationPhysics of Propagation

Three basic propagation mechanisms:Three basic propagation mechanisms:

1.1. ReflectionReflection

2.2. RefractionRefraction

3.3. ScatteringScattering

ReflectionReflection

Reflections occurs when an Reflections occurs when an electromagnetic wave bounces off an electromagnetic wave bounces off an obstruction with dimensions very obstruction with dimensions very large compared to the wavelength of large compared to the wavelength of the radio wave.the radio wave.

Continued …Continued …

Reflections from the surface of the Reflections from the surface of the earth and from buildings produce earth and from buildings produce reflected waves that may interfere reflected waves that may interfere constructively or destructively at a constructively or destructively at a receiver.receiver.

RefractionRefraction

Refraction occurs when the radio Refraction occurs when the radio path between the transmitter and path between the transmitter and receiver is obstructed by an receiver is obstructed by an impenetrable body.impenetrable body.

Continued …Continued …

Waves are formed behind the Waves are formed behind the obstructing body even though there obstructing body even though there is no line-of-sight (LOS).is no line-of-sight (LOS).

ScatteringScattering

Scattering occurs when the radio Scattering occurs when the radio channel contains objects with channel contains objects with dimensions that are on the order of dimensions that are on the order of the wavelength or less.the wavelength or less.

Continued …Continued …

Scattering follows the same physical Scattering follows the same physical principle as refraction, causes energy principle as refraction, causes energy from a transmitter to be radiated in from a transmitter to be radiated in many different directions.many different directions.

Path LossPath Loss

Mobile radio systems are interference Mobile radio systems are interference limited rather than noise limited.limited rather than noise limited.

Man-made noise effects are often Man-made noise effects are often insignificant compared to signal levels of insignificant compared to signal levels of co-channel users.co-channel users.

Path loss denotes the local average Path loss denotes the local average received signal power relative to transmit received signal power relative to transmit power.power.

Outdoor PropagationOutdoor Propagation

Cell: an area in which path loss is at Cell: an area in which path loss is at or below a given value.or below a given value.

Theoretically a honeycomb structure Theoretically a honeycomb structure with hexagonally shaped cells.with hexagonally shaped cells.

In reality they have a much more In reality they have a much more irregular shape.irregular shape.

Types of CellsTypes of Cells

Cells are classified roughly Cells are classified roughly according to size as:according to size as:

1.1. MacrocellsMacrocells

2.2. MicrocellsMicrocells

MacrocellsMacrocells

Basis for the first generation systems Basis for the first generation systems meant for mobile users.meant for mobile users.

Generally have base stations at high Generally have base stations at high points, with a coverage of several points, with a coverage of several kilometers.kilometers.

MicrocellsMicrocells

Microcells can accomodate more Microcells can accomodate more subscribers per unit of area.subscribers per unit of area.

The smaller multipath delay spread The smaller multipath delay spread and shallow fading makes possible and shallow fading makes possible broadband signal transmission broadband signal transmission without excessive countermeasures without excessive countermeasures against multipath fading.against multipath fading.

Street MicrocellsStreet Microcells

Most of the signal power propagates Most of the signal power propagates along the street.along the street.

Corner refraction is a problem.Corner refraction is a problem.

Due to the irregularity of buildings, Due to the irregularity of buildings, excessive overlapping or non-excessive overlapping or non-overlapping microcells can occur.overlapping microcells can occur.

Indoor PropagationIndoor Propagation

Important for extending voice and data Important for extending voice and data communication services within the communication services within the workplace.workplace.

Design issues linked to the propagation Design issues linked to the propagation environment include:environment include:

• Distance between servers.Distance between servers.• Expected portable battery life.Expected portable battery life.• Customer performance expectations.Customer performance expectations.• Appropriate radio link budget.Appropriate radio link budget.

ObstructionsObstructions

Hard partition – cannot be movedHard partition – cannot be moved Soft partition – movable obstructionSoft partition – movable obstruction

Propagation can be classified as:Propagation can be classified as:

• Line of sight (LOS) – visible path Line of sight (LOS) – visible path between transmitter and receiver.between transmitter and receiver.

• Obstructed (OBS) – objects in channel Obstructed (OBS) – objects in channel block a visible propagation path.block a visible propagation path.

Path LossPath Loss

Path loss is a measure of RF attenuation Path loss is a measure of RF attenuation inside a building.inside a building.

Measured by averaging received signal Measured by averaging received signal over several wavelengths at the receiver.over several wavelengths at the receiver.

To avoid co-channel interference, To avoid co-channel interference, frequencies must be reused on different frequencies must be reused on different floors.floors.

ConclusionsConclusions

Thank YouThank You