Now let's talk about how far we can go with wireless connections, introducing the link budget. A linked budget is pretty much like a household budget it tells me how much I start with what i'm spending or losing along the way and then what i have left at the end of the month or the end of the journey of the wireless signal from a to b that's in short link budget and in order to use it we'll need a little mathematical help we'll introduce a unit called the db the decibel and we're doing this just because it's practical what is the decibel it's a logarithmic way of expressing the relation between two powers you could say example you have an output power at an axis point and then you measure what arrives on the other side so you have two powers you put them in relation divide one by the other take the logarithmic that in short gives you the decibel why are we doing this because nature behaves like that when a regular wave gets attenuated gets weakened in material say it loses one tenth with every meter then you can calculate after one meter you have one tenth after the second meter you have a hundredth after the third meter you have just the thousandth part of it so it goes ten hundred thousand same by the way in animal populations if they multiply by ten every year ten the one year one hundred the next one thousand the next now this is best expressed if we're using this logarithmic form the decibel because then we can just add decibels instead of having to multiply all the way and this is just handy it's practical for making these calculations that's the reason you see the decibel the db all over the place. You don't have to memorize the mathematical formula what is good to memorize is things like 3 dbs means twice the power 10 db means an order of magnitude more and then 20 db is 2 orders of magnitude 30 db three orders of magnitude and so forth you also find dbs that are relative to something else for example the dbm is db relative to the milliwatt one milliwatt defines zero dbm and then the same rule 10 milliwatt would be 10 added that's 10 dbms. 100 milliwatt, a number you often find in wi-fi as the upper limit you can do that's then 20 dbm and so forth similarly in antenna descriptions you often see the dbi the dbi is db relative to an isotropic antenna which is just a theoretical concept an isotropic antenna would be one that radiates in all directions with the same strength you can't build such an antenna it's a idea rather than a reality and then you describe a directed antenna and saying it has so and so many dbis it's that much stronger in a certain direction than this antenna that radiates into all directions that's the dbi so you find often numbers like an access point is described as doing 20 dbms you can say a cable has a certain loss one meter of cable loses for example half a db you can say an amplifier amplifies by so and so many db's so you find them all over the place and they are our main tool for doing our main trick our main way of doing link budgets now what's a link budget it's just adding up gains and losses as i said earlier much as you do with the financial budget you have something you lose something you win something end of the day you have something left hopefully we have two kinds of contributions to such a wireless link budget there's things that we cannot change and there's things we can change the things we cannot change is just the loss because of the distance and we'll look at that in a moment and then there's things we can change that's antennas cables amplifiers if we have them the output power the receive sensitivity so how much we're sending and how good we are in receiving that that's things we can change the part we can't change is the propagation over the distance in space and at this point we need to introduce a term the line of sight the line of sight and that's where the word comes from is probably easiest to imagine when we're just looking our eyesight if we can see something we can see a mountain on the horizon it's within our line of sight if it's behind something if it's hidden by another hill among well then it's non line of sight now same concept for radio waves just with a little difference because of the bigger wavelength the radial line of sight is not just a thin line as we can imagine the optical light um it's more like a cigar shape or a straight banana shape maybe it's it's wide it has a certain thickness because those wavefronts that we looked on earlier they need to contribute to the signal arriving on the other side it's not enough to just have a line you need a certain space around it we could go into detail here and talk about fresnel zones that'll be a more detailed talk somewhere else but important to know the concept line of sight non-line of sight and be aware you need a little bit more than just the absolute straight line now if we have line of sight if there's no obstacle if there's no mount no trees no building then the loss of signal along the way is described by free space loss also find the term path loss and it's pure geometry it's it only its only reason is that you produce an output power at some point and this output power needs to spread and fill a larger and large larger area pure geometry so the further i'm going the less power per surface unit i will receive simply because power needed to spread out that's all that's behind the path loss the free space loss we can write this down in formulas and depends on the frequency and the distance obviously you don't need to memorize the exact formula when you need it find it in literature or this presentation what's useful to memorize is take one fixed frequency 2.4 gigahertz wi-fi for example and memorize one kilometer is a loss of minus 100 db you can check this put the values into this formula you get roughly this one kilometer minus 100 and then you can just add for each order of magnitude that's another 20 db loss and if you want to change frequency well if you roughly go to something which is 10 times the frequency you get that same factor 20 of molars so the same db logic here just with a factor 20. keep that in mind the factor 20 comes from now it's not only a distance that matters but actually a surface area so we get a factor 2 to the 10 that we already had in the definition of the decibel the parts we can change in a linked budget first of all the output power so this is what the radio outputs brings into the antenna on the sending side on the originating side of our signal and you will find typical values for wi-fi is 20 dbms 100 milliwatt that is largely due to the regulations i talked about ism transfer earlier and you have limits maximum power you're allowed to use and in most places for wi-fi these days that would be the 20 dbms then on the other side you have the device listening and we call that the receive sensitivity it means how good you are in hearing what's being said remember in a communication it's not enough for one person to scream really loud you also need the other person to listen very carefully and the better we can listen the longer the connection we can make so receive sensitivity is crucial and it typically for wi-fi equipment is in the range of somewhat in the high minus 90 dbms 95.98 remember here lower is better the less you need in order to still understand the better you are in listening the better for your connection so a lower value minus 98 is better than minus 92. mobile phones typically have radius in them that can do like -85 that's not as good as a professional access point can do. Lastly of course antennas that's something we can do something about and we'll be looking at those later here at this point the important question so how much do we need to still have a signal and this is quite easy when we're looking at what we just said we said receivers have a sensitivity i said mobile phones maybe -85 so the symbols we're often seeing on mobile phones on laptops whether it's bars or the symbols you're seeing here they indicate some dbms and roughly we can say in order to have a working connection to a mobile phone or a laptop something like minus 70 minus 75 perhaps db m is needed and the reason for that is the received sensitivity we still need a little bit of margin we can't go all the way to the minimum received sensitivity we need a little bit of space still whether that's 5 or 10 dbms that depends on how stable we want the connection but rule of thumb minus 70 75 dbm is the very very low limit minus 50 that's a very good signal some link budget examples you can list these things row by row here's the example we just talked about a hotspot output power 20 dbms uh medium size relatively weak antenna 5 dbi uh on the other side the receiver just a very small maybe inbuilt antenna with two dbi's and you get over a distance of one kilometer you get a connection that just about works it's just enough to get through not a lot of margin left but it would probably work here's another example for a point-to-point link now we're going long distance 100 kilometers that's a loss of minus 140 dbs that we have to make up for at some point we are making up for that by using strong antennas on both sides plus 25 dbis we'll be looking at such antennas that is strong directional antennas and we again get a working connection with the little comment that you should check whether you're allowed to do 20 dbms output plus a strong antenna are you allowed to do that in most countries no because the output power looks at the sum of the radio output and what the antenna does that together gives you the the upper limit and that in most places is the 20 dbms including the antenna.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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