Radio frequency range considerations

Written by Chris Pinter                                                                                    

February 9, 2010                                                                                             Bookmark and Share

What affects the range of a wireless device?

Many developers who build their first radio prototype often get stumped on the range.  Many people do not get the range they are expecting.   So what is expected in regards to range?   Why does a Wi-Fi access point not have the same range as a cellular telephone? Here are the key elements we need to consider.

Transmitter power (Pt)

The transmit power is the energy generated from the transmitter to the antenna.  It is important to note that phase noise, which is an unwanted element in the output power contributes to the transmit output power.  So it is important to eliminate the phase noise as much as possible.

Transmitter antenna gain (Gt) 

This is the gain of the transmitter antenna.  Antennas do not have the same type of gain as an amplifier.  There is no energy added to create gain.  An antenna has aperture gain, which can be compared to a window in a building.  As you look through the window you see the scenery outside.  The bigger the window the more the scenery you can see.  Just like a window the larger the antenna aperture the larger the gain of the antenna. You can also increase the gain by directing the radiation nodes in one direction creating directivity gain.

Receiver antenna gain (Gr)

This is the gain of the receiver antenna.  If you are using the same antenna for the transmitter and the receiver the two gains should be equal.  Just remember that the two antennas must be aligned in the same manner.  Horizontal polarized antennas should both be arranged horizontally and vertical antennas arranged vertically.  Otherwise you will see a drop in the overall gain of the system.

Wavelength (λ) 

Wavelength is the measure of the frequency as it propagates though space.  This is referenced to the speed of light.  The shorter the wavelength the shorter the distance it can travel before it dissipates.  Short waves are more easily absorbed by the molecules in the air and have a harder time diffracting around obstacles.

Gain of the receiver front end

This is the gain of the low noise amplifier.  The low noise amplifier is a critical circuit for receiving quality low power signals.  The noise figure is a measure of the amount of noise the amplifier adds to the signal, so the lower the noise figure the better.

Sensitivity of the receiver (Pr)

The sensitivity of the radio IC is documented in the datasheet.  However, the sensitivity can be adversely affected by using poor PCB layout techniques and the proximity of noisy components around the radio IC.  Even if you are using a system on a chip device you have to be careful what other device you have close to this part.

Multipath interference 

Multipath interference is very unique to radio communications. You will not see multipath interference inside a wire.   As a radio wave propagates along a vector through space it reflects off of obstacles and will influence other vectors of the same wave causing peaks and valleys of different signal strength and phase.  The wireless environment is truly hostile.  There are two models that are commonly used to estimate the affects of multipath.   The ‘inverse square law’ takes the inverse square root of the distance and the mobile model takes the inverse 4th root of the distance.  If you can fix the height of the antenna in your system then you can use the inverse square law as the higher the antenna the less affect ground reflection will have on the multipath interference.

Environmental noise and interfering signals

One of the most popular radio frequency bands used today is the 2.45 GHz ISM band, which is from 2400 MHz to 2480 MHz.  This band has many interfering signals including, microwave ovens, cordless telephones, WI-Fi, Bluetooth and Zigbee plus a host of custom radios used in industrial applications.

Radio bands will experience environmental noise from motors, cellular telephones and other devices using the same band.   It is important to identify the application need and consider these sources of interference before you start your design.

Data rate

Environmental conditions have an effect on the data rate.  Lowering the data rate will lower the errors.  This is because the receiver can sample each bit more times to reproduce the signal.   Lowering the data rate is also a very common way of reducing spurious emissions in order to pass certification.  Make sure you design your system for a higher data rate than required, which will provide you with a margin of error.

Bandwidth

The bandwidth of the channel has a direct influence in the amount of noise that is received.  The minimum detectable signal a receiver can detect is related to the bandwidth, noise figure of the receiver and the frequency.

Range is one of the most critical requirements of any radio and there are a lot of characteristic that affect range and overall performance.  Wi-Fi devices will not have the same range as a cellular telephone because of a variety of reasons.  Only some of these characteristics have been described here.  So, it is very important to consider the factors that affect the range performance such as the frequency of operation, environment in which the radio will operate and the throughput requirements before you start your design.

Reference

Kevin McClaning, Tom Vito, “Radio Receiver Design” Nobile Publications, Atlanta, GA, ISBN 1-884932-07-X

John D. Kraus, Daniel A. Fleisch, “Electromagnetics with Application” WCB McGraw-Hill Publication, NewYork, NY,   ISBN 0-07-289969-7

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