RF Link Budget Calculator Manual

Transcription

1 RF Link Budget Calulator Manual Author Ivo van Ling for Software Release RF Link Distane Calulator, Version 1.00, Dated 4 January 2010 Manual Version 1.00 Date Page: 1(8)

2 Contents 1 Introdution Calulate Maximum Distane The Mathematis behind Maximum Distane Calulation... 5 Dislaimer The speifiations and information regarding the produts in this manual and the desribed online RF Link Budget Calulator tool are subjet to hange without notie. All statements, information, and reommendations in this manual are believed to be aurate but are presented without warranty of any kind, express or implied. Users must take full responsibility for their appliation of any produts. Notwithstanding any other warranty herein, all doument files and software are provided as is with all faults. The author dislaims all warranties, expressed or implied, inluding, without limitation, those of merhantability, fitness for a partiular purpose and noninfringement or arising from a ourse of dealing, usage, or trade pratie. In no event shall the author or his suppliers be liable for any indiret, speial, onsequential, or inidental damages, inluding, without limitation, lost profits or loss or damage to data arising out of the use or inability to use this manual and online tools, even if the author or his suppliers have been advised of the possibility of suh damages RF Link Budget Calulator Manual Page: 2(8)

3 1 Introdution This doument desribes the use of the RF Link Distane Calulator, Version 1.00, and Dated 4 January This is an online tool and an be used to alulate the Line-Of-Sight (LOS) distane between two radio systems dependent on radio type, antenna type and frequeny. Furthermore, it also alulates the required antenna height on eah side of the link to allow for the Fresnel zone and the urvature of the earth over the alulated link distane. This alulator tool an be used in two modes: Normal and Expert mode. The Normal mode is meant to alulate the maximum wireless link distane between two wireless nodes in ombination with a ertain set of antennas. In this mode, the tool an be used to quikly verify the validity of a proposed tehnial solution. The Expert mode an be used manually hange all parameters effeting the link distane. You an for example selet a radio and antenna type that loses mathes the required setup and then manually tweak the parameters that differ from the values that are stored in the database of the tool. The atual alulator an be found at the following URL: Please note that the distanes alulated by this tool are best ase values under ideal onditions assuming lear line of sight and should be treated as suh. In real life the onditions are far from ideal, so the radio waves transmitted from our wireless nodes tend to bump into things (buildings, leaves, and people) and either get absorbed or boune off again in some random diretion. Therefore, no rights an be obtained from the information displayed. This tool and the information ontained therein are provided as is and may be hanged without notie. 1.1 Calulate Maximum Distane The RF Link Distane alulator an be used to alulate the maximum link distane between two wireless network nodes. The distane overed between the nodes is dependent on the type of radio, antenna equipment and frequenies used on eah side of the link. One side of the link is alled the near side and the other side is alled the far side. In order to alulate the maximum distane between two nodes you have to selet the type of radio and type of antenna at eah side (Near and Far side) of the link. Consider the following example network setup with two Freedom nodes (Near Side and Far Side) eah equipped with a 3.5GHz 12dBi base station antenna. Figure 1: Example Network setup for maximum distane alulation Page: 3(8)

4 In the start-up sreen of the RF Link Distane Calulator you find a number of list boxes to selet the Near Side and Far Side equipment. In order to alulate the maximum distane between the two nodes in the example above make the following seletions for the near side radio equipment and antenna: Figure 2: Selet Equipment on the near side for maximum distane alulation Then selet the radio equipment and antenna for the far side: Figure 3: Selet Equipment on the far side for maximum distane alulation Upon pressing the button the RF Link Distane Calulator alulates the maximum distane between the two nodes dependent on the types of radio, antenna and usable frequenies to establish a working radio link. In this example the maximum distane between the Near Side and Far side is given as 2.88 kilometers. Figure 4: Calulated Maximum Distane between two nodes At the same time the maximum distane going from the Far to Near node is also alulated. In this example the distane is exatly the same, however if you use two different types of radios on eah side of the link these distanes might be different. Page: 4(8)

5 1.2 The Mathematis behind Maximum Distane Calulation If you only want to use the tool to determine the distane given a set of antennas and radio equipment then read no further. Otherwise this hapter desribes in more depth how the RF Link Distane Calulator alulates the maximum distane between two nodes. The maximum distane that you an over between those two wireless nodes is determined by a number of fators: 1. The radio output power of the near node. 2. The antenna able and onnetor loss at the near node. 3. The antenna gain of the near node. 4. The Free Spae Loss, or the attenuation of the radio signal, as it travels from the near node to the far node. 5. The antenna gain of the far node. 6. The antenna able and onnetor loss at the near node. 7. The reeive sensitivity of the far node. Of ourse the same holds true for the maximum distane going from the far node to the near node. For sake of simpliity we only onsider one way traffi, i.e. going from the near node to the far node. However, the maximum distane going from near to far might be different than going the other way. In the tool we alulate both paths simultaneously. In order to determine the maximum distane between the two nodes the Link Distane Calulator tool needs to alulate the seven fators above and then present you with the maximum distane in meters. To give you an example how the tool alulates the maximum distane we will use the same network example as shown in the previous hapter. Near Side Far Side 1: Radio output power 18dBm 4: Free Spae Loss (???db) 7: Reeive Sensitivity -73dBm 2: Cable/Connetor Loss -1dB 3: Antenna Gain 12dBi 5: Antenna Gain 12dBi 6: Cable/Connetor Loss -1dB Figure 5: Radio output power, able losses and reeive sensitivity are known values Here we have two nodes plaed at an unknown (???) distane apart. The Near Side node is transmitting radio signals with a ertain radio output power (+18dBm) that are being amplified (+12dBi) by the Near Side antenna. The radio signal travels a distane until they hit the antenna of the far node. There they amplified by the antenna (+12dBi) and then presented to the reeiver of the Far Side radio. The losses of the onnetors, pigtails and antenna onnetors at eah end of the link are set at a fixed value of -1dB. All values for onnetor and able losses an be altered and further speified when swithing to expert mode. Page: 5(8)

6 The equation for alulation of the reeived signal strength on the far node is as follows: FAR Re Sig = NEAR Pout - NEAR able loss + NEAR ant gain - FSPL - FAR able loss + FAR ant gain Where: FAR Re Sig NEAR Pout NEAR able loss NEAR ant gain FSPL FAR able loss FAR ant gain = The Reeived Signal on the far node. = The radio output power of the near node. = The able and onnetor losses at the near node. = The antenna gain of the near node = Free Spae Loss. = The able and onnetor losses at the far node. = The antenna gain of the far node. In our equation we know everything but the value of the Free Spae Loss. This is beause the Free Spae Loss is dependent on the distane between the nodes and the frequeny on whih the link between the nodes is established. As we are trying to work out what the maximum distane between the nodes an be we need to alulate the maximum Free Spae Loss allowed to establish a working link between the two nodes. We need to make one assumption, namely: As long as the radio signal that is being presented to the reeiver has higher signal strength than the reeive sensitivity of the reeiver (-73dBm) your are able to establish a working radio link. Thus the minimum signal level that need to be present at the reeiver is the same as the reeive sensitivity of the far node. We an rewrite the equation to read: FSPL = NEAR Pout + NEAR ant gain + FAR ant gain - FAR Re Sens - NEAR able loss - FAR able loss Where: FSPL NEAR Pout NEAR ant gain FAR ant gain FAR Re Sens NEAR able loss FAR able loss = Free Spae Power Loss. = The radio output power of the near node. = The antenna gain of the near node. = The antenna gain of the far node. = The Reeived Signal on the far node. = The able and onnetor losses at the near node = The able and onnetor losses at the far node When we fill in all the know variables from our example we get: FSPL = NEAR Pout + NEAR ant gain + FAR ant gain - FAR Re Sens - NEAR able loss - FAR able loss FSPL = 18dBm + 12dBi + 12dBi -(- 73dBm) 1dB FSPL = 18dBm + 12dBi + 12dBi + 73dBm - 1dB - 1dB - 1dB FSPL = 113dB The Free Spae Loss in this example an thus be 113dB before the Far Side node is unable to reeive the broadast oming from the near node. The further the distane between the two nodes the more the signal traveling from the near node to the far node will be losing power (attenuated). Free-spae power loss is proportional to the square of the distane between the transmitter and reeiver, and also proportional to the square of the frequeny of the radio signal. Page: 6(8)

7 The equation for free-spae power loss (FSPL) is: Where: FSPL = (4*π*d*f) FSPL = Free Spae Power Loss. f = the signal frequeny (in Hertz). d = the distane from the transmitter (in meters). = the speed of light in a vauum, meters per seond. π = taken as A onvenient way to express free-spae loss is in terms of db (sine we have alulated it as 113dB in the example above) is: FSPL(dB) = 20*log 10 (4*π*d*f) FSPL(dB) = 20*log 10 (d) + 20*log 10 (f) + 20*log 10 (4*π) --- In order to alulate the distane (d) from the transmitter we an rewrite the equation to read: 20*log 10 (d) = FSPL(dB) - 20*log 10 (f) - 20*log 10 (4*π) --- The RF Link Distane Calulator uses the formula above to alulate the maximum distane between the two nodes. The frequeny used for this alulation is the highest frequeny that an be used to make a working link between the two nodes. Sine the highest frequeny gives the highest attenuation of the radio signal the maximum distane is alulated based on worst ase onditions. The RF Link Distane Calulator determines the highest usable frequeny of a radio link based on the used radios and antennas at eah side of the link. For the example above the RF Link Distane Calulator uses the frequeny of 3700MHz, as this is the maximum frequeny of the antenna used. Figure 6: Automati seletion of highest usable frequeny based on radios and antennas Page: 7(8)

8 Filling in all the know values we get: 20*log 10 (d) = *log 10 (3700*10 6 ) - 20*log 10 (4*π) *log 10 (d) = *log 10 (d) = 69.2 log 10 (d) = 3.46 d = 2884 meter The RF Link Distane Calulator uses the method above to alulate both the maximum distane from the near node to the far node as vie versa. Beause in this example we use exatly the same radio and antenna at eah side of the link both distanes are exatly the same. Page: 8(8)