introduction to rf basics

7/30/2019 Introduction to RF Basics

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Introduction to RF Basics


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Characteristics of a Radio Signal

These are three characteristics ofan RF signal which are commonlyvaried to carry information:

Amplitude

Magnitude of the signal

Frequency

Number of cycles per unit of time

Phase

The offset of the Signal


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Dominant Mechanisms of Mobile Propagation

Most propagation in the mobile

environment is dominated bythese mechanisms:

Free Space (Line-of-sight) No reflections, no obstructions

Attenuation caused only by air

Obstruction When a signal propagates through anobstruction it is attenuated

Knife-Edge diffraction Direct path is blocked by obstruction,

however some signal can go around

Additional Loss is introduced

Reflection Signal can be reflected off certain

surfaces (i.e. Glass, Water)

This could degrade or improve the signal


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Cellular Systems

Single Cell Advantages:

Simple System

Disadvantages: Coverage constraints

Limited number of conversations

Multi-cell Advantages:

More cells allow the samechannels to be reused inmultiple areas, allowing moreconversations and morecapacity

Mobiles can be made smallerwith lower transmit power

Disadvantages: More complicated system:

handoffs required


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Whats a Sector? Why Sectorize

A base station may be set up in variousconfigurations:

Omni for simplicity and low cost

2-sector to cover just a highway or transportationcorridor, with little coverage off to the sides:

Two sectors give more traffic-handling capabilityand more antenna gain than an omni station

3 sector to cover an area with population in alldirections:

Three sectors give almost three times the traffic-handling capability of an omni base station

Sector antennas also give better coverage since

they have more gain than omni antennas The sectors do not have equal widths and can be

grown , shrunk, and aimed to balance the amountsof traffic they get


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Frequency Reuse


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Structure of CDMA Signal

CDMA Carries are spaced 1.25MHz

apart

In the BTS downlink signal, different

codes belong to different users a

mobile only listens to its particular

code The mobile on its uplink uses the

same code to communicate with the

BTS

This allows many simultaneous calls

to be processed during the same

time


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References

Baxter, Scott. Physical Principals of Propagation, 2004

Baxter, Scott. Introduction to Wireless Technologies for Telematics

Applications, 2004

Baxter, Scott. How Wireless Developed, 2005

Baxter, Scott. CDMA Core Principals, 2007

Baxter, Scott. Introduction to Fixed Broadband Wireless Access, 2005 http://www.cellular-expert.com/_Rainbow/images/default/imgl4.jpg

http://static.commentcamarche.net/en.kioskea.net/pictures/telephonie-mobile-images-reseau-

cellulaire.png


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Any Questions?

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