Propagation Measurements and Models for Wireless Communications Channels Brian Alexander

Introduction  A basic understanding of the channel is important for finding modulation and coding schemes.  It is also useful for deploying base station antennas.  Types of signals that occur in various environments and models of propagation parameters.

Concerns  Achieving reliable communications.  Energy should be sufficient, but not so strong as to cause interference.  Even with sufficient power, large errors may occur anyway.

Models 1. Parametric statistical models that on average describe behavior within a given error. 2. Deterministic models characterizing a specific street, building, etc.

Physics of Propagation Three basic propagation mechanisms: Three basic propagation mechanisms: 1. Reflection 2. Refraction 3. Scattering

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

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

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

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

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

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

Path Loss  Mobile radio systems are interference limited rather than noise limited.  Man-made noise effects are often insignificant compared to signal levels of co-channel users.  Path loss denotes the local average received signal power relative to transmit power.

Outdoor Propagation  Cell: an area in which path loss is at or below a given value.  Theoretically a honeycomb structure with hexagonally shaped cells.  In reality they have a much more irregular shape.

Types of Cells  Cells are classified roughly according to size as: 1.Macrocells 2.Microcells

Macrocells  Basis for the first generation systems meant for mobile users.  Generally have base stations at high points, with a coverage of several kilometers.

Microcells  Microcells can accomodate more subscribers per unit of area.  The smaller multipath delay spread and shallow fading makes possible broadband signal transmission without excessive countermeasures against multipath fading.

Street Microcells  Most of the signal power propagates along the street.  Corner refraction is a problem.  Due to the irregularity of buildings, excessive overlapping or non- overlapping microcells can occur.

Indoor Propagation  Important for extending voice and data communication services within the workplace.  Design issues linked to the propagation 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.

Obstructions  Hard partition – cannot be moved  Soft partition – movable obstruction  Propagation can be classified as: Line of sight (LOS) – visible path between transmitter and receiver.Line of sight (LOS) – visible path between transmitter and receiver. Obstructed (OBS) – objects in channel block a visible propagation path.Obstructed (OBS) – objects in channel block a visible propagation path.

Path Loss  Path loss is a measure of RF attenuation inside a building.  Measured by averaging received signal over several wavelengths at the receiver.  To avoid co-channel interference, frequencies must be reused on different floors.

Conclusions

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