Introduction to Wireless Networks

Below we cover some of the common issues surrounding setting up a wireless network.  Matters microwave can be extremely complex and mathematical.  Here we try and simplify things to get across the main concepts.  There are three main sections:

 

1.0 Designing a Wireless Link

1.1 Introduction

A single wireless link between two points is really quite simple.  As shown in figure 1 below, the basic components are two network devices attached to radios and antennas at each end.

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Figure 1: Simple Wireless Link

 

1.2 Link Budget

For a wireless network to work reliably, sufficient radio power needs to get across the link in both directions.  As most networks utilise unlicensed spectrum (900MHz, 2.4GHz or 5.8GHz), the maximum radiated power is limited to certain levels set by the ACA.  These limits allow the unlicensed spectrum to be re-used many times over.  The best way to design a wireless link is to start by doing a link budget to understand how the link can be optimised.

Figure 2 shows the main components that should be considered in doing a link budget. Each of these elements is considered in turn below.

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Figure 2: Main Components in Link Budget

 

1.3 Radio Power

A radio produces power, typically expressed in mW. However as the output power from a radio gets severely attenuated over distance, mW is not a good measurement standard.  As an example the signal level reaching the receive antenna is often considerably less than one/millionth of a mW!

So instead a logarithmic scale is normally used instead – dBm.  Figure 3 shows some common mW levels expressed in dBm.

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Figure 3: Conversation of mW to dBm

 

1.4 Cable Attenuation

As the signal from the radio travels through a cable it is attenuated (reduced in strength).  The higher the frequency and the smaller the cable, the bigger the losses.  Typical losses for common cable types are shown in Figure 4. 

Losses can be significant.  Ten metres of LMR400 cable carrying a 5.8GHz signal will create a loss of -3.5dB (-0.35 x 10) which in absolute terms is more than half the signal!

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Figure 4: Attenuation of Common Cable Types

 

1.5 Antenna Gain

Section 2 discusses selection of antennas in depth.  For the purposes of link budget it is important to know the gain of an antenna.  Typically this is expressed in dBi.  It is a measure how much the antenna amplifies the section in a particular direction.

The combination of:

radio output power  - cable losses + antenna gain = EIRP
           
EIRP stands for Equivalent Isotropically Radiated Power.  The reason it is important is that this is the figure that has limits set by the ACA.  For 2.4GHz and 5.8GHz it is typically 4 watts or 36dBm.

 

1.6 Radio Path Losses

As the signal travels through the air it is attenuated.  Increasing frequency and distance both increase attenuation, as shown in figure 5. 

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Figure 5: Radio Path Loss Vs Distance

 

1.7 Choosing An Antenna

Your choice of antenna is important - if your antenna doesn’t work properly, your network won’t work properly either.  The following is a brief summary of the main factors to consider in choosing an antenna. 

 

1.8 The Receive End

At the Receive Antenna the process is repeated in reverse.  The receive antenna amplifies the signal by its gain and the resulting signal is attenuated by any cable losses.  Any signal left at the end of this journey then hits the receive radio.  A radio receiver is designed to receive very weak signals and amplify them so the data they contain can be extracted.  Most radios give a figure for receiver sensitivity (usually in dBm) – this is the minimum amount of power the radio receiver requires to operate.  So in terms of our link budget, receiver sensitivity is counted as a positive number.

   

1.9 The Total Link Budget

So referring back to Figure 2, our total link budget is:

  Radio Transmitter Power
- Transmit Cable Losses
+  Transmit Antenna gain
- Radio Path Losses
+ Receive Antenna gain
- Receive Cable Losses
+ Receiver Sensitivity
= Link Gain

Other things being equal, if the Link Gain is positive the link will work. 

In practice, for most link budget calculations a Fade Margin is also allowed to allow for a margin of error due to real world conditions.  The amount allowed depends upon a number of factors including distance and environmental conditions but -10, -15 or -20 dB is often used in practice.

--> go to "2.0 Choosing an Antenna"
--> go to "3.0 Implementing a Wireless Link"

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