Link budget calculation
- 1. Training materials for wireless trainers Link Budget Calculation
- 2. 2 Goals ‣ To be able to calculate how far we can go with the equipment we have ‣ To understand why we need high masts for long links ‣ To learn about software that helps to automate the process of planning radio links
- 3. Free space loss ‣ Signal power is diminished by geometric spreading of the wavefront, commonly known as Free Space Loss. ‣ The power of the signal is spread over a wave front, the area of which increases as the distance from the transmitter increases. Therefore, the power density diminishes. Figure from http://en.wikipedia.org/wiki/Inverse_square 3
- 4. Free Space Loss (@2.45 GHz) ‣ Using decibels to express the loss and using 2.4 GHz as the signal frequency, the equation for the Free Space Loss is: Lfs = 100 + 20*log(D) ‣ ...where Lfs is expressed in dB and D is in kilometers. 4
- 5. Free Space Loss (any frequency) ‣ Using decibels to express the loss and using a generic frequency f, the equation for the Free Space Loss is: Lfs = 32,45 + 20*log(D) + 20*log(f) ‣ ...where Lfs is expressed in dB, D is in kilometers and f is in MHz. 5
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- 7. Power in a wireless system 7
- 8. Link budget ‣ The performance of any communication link depends on the quality of the equipment being used. ‣ Link budget is a way of quantifying the link performance. ‣ The received power in an 802.11 link is determined by three factors: transmit power, transmitting antenna gain, and receiving antenna gain. ‣ If that power, minus the free space loss of the link path, is greater than the minimum received signal level of the receiving radio, then a link is possible. ‣ The difference between the minimum received signal level and the actual received power is called the link margin. ‣ The link margin must be positive, and should be maximized (should be at least 10dB or more for reliable links). 8
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- 10. Example link budget calculation 10 Let’s estimate the feasibility of a 5 km link, with one access point and one client radio. The access point is connected to an antenna with 10 dBi gain, with a transmitting power of 20 dBm and a receive sensitivity of -89 dBm. The client is connected to an antenna with 14 dBi gain, with a transmitting power of 15 dBm and a receive sensitivity of -82 dBm. The cables in both systems are short, with a loss of 2dB at each side at the 2.4 GHz frequency of operation.
- 11. AP to Client link 11 +10 dBi -2 dB +20 dBm +14 dBi -2 dB -82 dBm ?? ? -114 dB @ 5 km
- 12. Link budget: AP to Client link 12 20 dBm (TX Power AP) + 10 dBi (Antenna Gain AP) - 2 dB (Cable Losses AP) + 14 dBi (Antenna Gain Client) - 2 dB (Cable Losses Client) 40 dB Total Gain -114 dB (free space loss @5 km) -73 dBm (expected received signal level) --82 dBm (sensitivity of Client) 8 dB (link margin)
- 13. Opposite direction: Client to AP 13 +14 dBi -2 dB +15 dBm +10 dBi -2 dB -89 dBm ?? ? -114 dB @ 5 km
- 14. Link budget: Client to AP link 14 15 dBm (TX Power Client) + 14 dBi (Antenna Gain Client) - 2 dB (Cable Losses Client) + 10 dBi (Antenna Gain AP) - 2 dB (Cable Losses AP) 35 dB Total Gain -114 dB (free space loss @5 km) -78 dBm (expected received signal level) --89 dBm (sensitivity of AP) 10 dB (link margin)
- 15. Fresnel Zone ‣ The First Fresnel Zone is an ellipsoid-shaped volume around the Line-of-Sight path between transmitter and receiver. ‣ The Fresnel Zone is important to the integrity of the RF link because it defines a volume around the LOS that must be clear of any obstacle for the the maximum power to reach the receiving antenna. ‣ Objects in the Fresnel Zone as trees, hilltops and buildings can considerably attenuate the received signal, even when there is an unobstructed line between the TX and RX. 15
- 16. 16 Line of Sight and Fresnel Zones a free line-of-sight IS NOT EQUAL TO a free Fresnel Zone r
- 17. Fresnel Zone ‣ The radius of the first Fresnel Zone at a given point between the transmitter and the receiver can be calculated as: ‣ ...where r is the radius of the zone in meters, d1 and d2 are distances from the obstacle to the link end points in meters, d is the total link distance in meters, and f is the frequency in MHz. ‣ Note that this gives you the radius of the zone, not the height above ground. To calculate the height above ground, you need to subtract the result from a line drawn directly between the tops of the two towers. r = 17.31 * sqrt((d1*d2)/(f*d)) 17
- 18. 18 Line of Sight and Fresnel Zones r = 17.31 * sqrt((d1 * d2) / (f * d)) rMAX d1 d2 r
- 19. Clearance of the Fresnel Zone and earth curvature Distance (km) 1st zone (m) 70% (m) Earth curvature (m) Required height (m) 1 5.5 3.9 0.0 3.9 5 12.4 8.7 0.4 9.1 10 17.5 12.2 1.5 13.7 15 21.4 15.0 3.3 18.3 20 24.7 17.3 5.9 23.2 25 27.7 19.4 9.2 28.6 30 30.3 21.2 13.3 34.5 This table shows the minimum height above flat ground required to clear 70% of the first Fresnel zone for various link distances at 2.4 GHz. Notice that earth curvature plays a small role at short distances, but becomes more important as the distance increases. 19
- 20. Fresnel Zone ‣ Considering the importance of the Fresnel Zone, it is important to quantify the degree to which it can be blocked. ‣ Typically, 20% - 40% Fresnel Zone blockage introduces little to no interference into the link. ‣ It is better to err to the conservative side allowing no more than 20% blockage of the Fresnel Zone. 20
- 21. Radio Mobile ‣ Radio Mobile is a free tool to aid in the design and simulation of wireless systems. ‣ It can automatically calculate the power budget of a radio link, calculating the Fresnel zone clearance. It can use digital maps, GIS (Geographical Information Systems), or any other digital map, including maps provided by yourself. ‣ Runs on Windows 95, 98, ME, NT, 2000 and XP. 21 http://www.cplus.org/rmw/english1.html
- 22. Radio Mobile 22 ‣ Uses Digital terrain Elevation Model for the calculation of coverage, indicating received signal strength at various point along the path. ‣ Radio Mobile automatically builds a profile between two points in the digital map showing the coverage area and 1st Fresnel zone. ‣ Different antenna heights can be tried to achieve optimum performance.
- 23. Radio Mobile 23
- 24. Win vs Web Radio Mobile 24 ‣ Web version ‣ Pros: runs on any machine (Linux, Mac, Tablet); does not require big downloads; saves sessions; user friendly ‣ Cons: requires connectivity; only certain frequencies ‣ Windows version ‣ Pros: runs offline; can use the GPS ‣ Cons: runs on Windows only; requires big downloads; hard to learn
- 25. Web Radio Mobile 25 Visit http://www.cplus.org/rmw/rmonline.html and create a new account!
- 26. Web Radio Mobile 26 Create a site (location)
- 30. Web Radio Mobile 30 Create a link
- 31. Web Radio Mobile 31 Use 2300 for 2.4 GHz and 5825 for 5.8 GHz
- 37. Chuuk link 37 TR's office: 7.452582 151.844061 Udot School: 7.384819 151.718185
- 38. Chuuk link 38 Questions to answer: 1) How high should the masts be? 2) How much output power should the radio give? 3) What antennas should we use? Please use the equipment in the lab to answer.
- 39. For more details about the topics presented in this lecture, please see the book Wireless Networking in the Developing World, available as free download in many languages at: http://wndw.net/ Thank you for your attention