Link Budget Analysis - Page 1
Forum section: electronics / rf-microwave

author: RobEE
author_rank: Contributor
timestamp: April 15, 2019, 05:08:31 pm
content: I have been tasked with doing an RF Link Budget analysis for a project I am working on. I have some RF experience, but I am not an expert, so I came here for some help. One of the variables in the link budget equation I am using is Miscellaneous Losses, for "fading margin, body loss, polarization mismatch, other losses..." Is there a good industry standard, rule-of-thumb, or even another equation, that I can use for the misc. losses?Also, I would appreciate it if you can provide a reference for your response. This is, at least in part, so I can follow up on this, educate myself, and understand this concept a little better. Thanks.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2019, 06:40:08 pm
content: Quote from: RobEE on April 15, 2019, 05:08:31 pmIt is strongly dependent on frequency and application. Satcomms are rather different to WLANs, 10MHz is rather different to 60GHz.Generic RF textbooks will give generic answers. Textbooks about specific applications may contain specific information. You may have to consult academic paper.I suggest you define your application domain, and then use google to search for that term plus "link budget analysis".And don't forget teh old saying: "the solution to all signal processing problems is to integrate for longer".

author: RobEE
author_rank: Contributor
timestamp: April 15, 2019, 07:18:41 pm
content: Quote from: tggzzz on April 15, 2019, 06:40:08 pmOK, I am working on something in the L and S bands, about 1-3 GHz; if that helps. But I will also try the other things you recommended.

author: coppercone2
author_rank: Super Contributor
timestamp: April 15, 2019, 10:23:08 pm
content: well all those parameters you mentioned should be right but their weight in the attenuation totalization will change depending on frequency.I think you would learn the most if you got a transmitter and receiver and did some field testing with battery equipment to detect gross errors.Fade margin will rely on transient phenomena like rain, temperature, humidity, power quality (if applicable) etc. Other factors should be static.SO for fade margin you would look at the resistance of free space in the signal transmission path. I don't know how easy it is to do (i.e. you can get something like a slice of fog in the way, or a full blown rain storm). Then the next factor might be the changing behavior of reflections based on moisture (no clue, how does multipath change when nature is wet or moisti wanna know). Then 'near field' effects of moisture and atmosphere, i.e. liquid film over antenna, dirt on antenna, dust storm on antenna, dust storm on wet antenna, construction worker leaving a bunch of grinder filings on top of antenna... and so on (also curious if this kind of stuff is a problem) ).Polarization mismatch is antenna dependent but so long the antennas don't move it should be static, so it comes down to what I assume is a mechanical specification of boresight rotation adjustment. Kind of sounds like you need to talk to the construction crew of the tower and a land surveyor to get some ideas on that parameter (i.e. if a tower starts to sink on one leg). It sounds like it will just kinda lump into other boresight errors and be one of those things that gets periodically reduced with maintenance when people just mess with the thing to get the best possible result without detailed analysis. If you had a full adjustment with motorized directional antennas then it might be feasible to actually distill information about adjustment error factors for speedy autocorrection (maybe even under wind conditions) but if its just a static thing its probobly going to get trouble shooted in the usual fashion (i.e. lets try some easy stuff first to see if we can increase the SNR). Unless some screws are loose I think you will end up getting a lumped boresight accuracy and boresight polarization error (i.e. flexing tower under heavy wind causing the bore to both rotate and 'miss the target', or the tower sinking and tilting in a particular direction because the foundation drops (like the leaning tower of piza). Or something in between like a foundation crack that changes the preload of various things in the tower and causes different behavior under wind load. Or a combination of all three.And you might have some weird funny business too, like ground resistance of a tower changing so the SMPSU powering stuff ends up throwing more noise to modulate the signal if its not built perfectly, or maybe a noise profile of a power supply changing with temperature (just thinkin of whatever here).

author: 1design
author_rank: Regular Contributor
timestamp: April 16, 2019, 01:55:59 am
content: You will get pretty close to a real world result with a simple budget:Transmit power-Feeder loss+TX antenna gain-Free space loss+RX antenna gain-Feeder loss= Received powerYour fade margin is the difference between your RX power and the threshold that you set as your limit.This might be the receive power to achieve 10E-6 BER, when the link goes down completely or whatever floats your boat.Tell us more about your application and we can propose the losses you should use. The one above is for point to point links for example.Polarization losses are not that common due a lot of factors, but might be more significant in your case. Systems that see a lot of such impairments are usually designed to have some sort of compensation mechanism to mitigate that.

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2019, 05:56:56 am
content: Fade margin is considerably more complex for cellular and WLAN systems where multipath is important and necessary. Then academic papers use two very different models: slow fading and fast fading.Even when those are both taken into account, models for choosing antenna sites is big business. Even then site surveys are necessary. And even then some cells "misbehave' for unknown reasons.

author: coppercone2
author_rank: Super Contributor
timestamp: April 17, 2019, 12:54:15 am
content: does a wet environment have an effect on multipath? or will it only effect LOS attenuation? I just mean the wet surface reflections, as in the case of uniform distribution of a fog/mist. I.e. all paths are equally lossy,but will the reflectivity change?do you get r eflections off of fogs and rain too, that result in complex multipath, or do you just get what can be modeled by a near perfect resistor? how does it look like across the high RF - MMW spectrum?

author: tggzzz
author_rank: Super Contributor
timestamp: April 17, 2019, 08:58:19 am
content: Quote from: coppercone2 on April 17, 2019, 12:54:15 amYes to all of that!Don't forget foliage. Leaves act as antennas, different size leavea at different frequencies. And branches wave around and leaves fall off.Some people moan about multipath reception causing dropouts. Others note that without multipath there would be no cellular phones.

author: 1design
author_rank: Regular Contributor
timestamp: April 17, 2019, 09:31:42 pm
content: As someone that designed and manages 100s of microwave and milimeter links, I would say that the above description is a bit dramatic.Sure, someone will argue that in cities, where cell is deployed, this is a major factor, but that is just one use case. When you deploy between structures, like in a city, where everything is made of concrete and steel, you will get a lot of multipath and MIMO will help you in regards to bandwidth and availability. Out in the fields where cell towers are 10 miles apart on farm ground or you can only see 1 tower on top of a mountain, good luck with MIMO. And that case is a lot more common.Again, on point to point links, the only major source of multipath is water or large structures within certain Fresnel zones. Fog, leaves etc. will only cause attenuation and if planned correctly, can actually reduce multipath by acting as an RF absorber to everything except LOS.Once RobEE shares a bit more about his use case...could be satellite comms as far as I know, we can help him with his specific case.

author: tggzzz
author_rank: Super Contributor
timestamp: April 17, 2019, 09:58:31 pm
content: Quote from: 1design on April 17, 2019, 09:31:42 pmThe latter is the key point.All of the effects mentioned previously are real and significant. How significant or insignificant depends on the site and frequency.

author: coppercone2
author_rank: Super Contributor
timestamp: April 17, 2019, 10:34:21 pm
content: Quote from: tggzzz on April 17, 2019, 08:58:19 amIfthe leaf has a radiation resistance and acts as an antenna, can whatever it is attached to act as a tank circuit and cause a re-radiation of the absorbed radiation at a random direction? So a wet leaf turns from reflector/attenuator to antenna? Or am I misunderstanding?Can you think about a reflector like a antenna some how?Like what would happen with a single photon hitting a leaf? When you say the leaf is an antenna, I imagine it has some kind of lobe pattern etc.So say you have a antenna connected to a passive load with a VSWR mismatch. And you illuminate the antenna with a far-field signal within the raidation resistance frequency of the antenna (say from a far away horn or parabola). The reflections from the load get re-emitted according to the radiation pattern of the antenna, regardless of the direction from the horn (this would only effect the power level of emissions from the antenna i.e. how much energy is available to be reflected and re-radiated). Is this the correct way to think about it, that the leaf-structure/tree is an antenna and mismatched load? So unlike a pure reflector, which would follow the law of optics we know (with incident angles), it would act to scatter the signal according to its radiation pattern? And the moisture effects the 'vswr' of the antenna (leaf/branch/tree) and its radiation pattern/frequency? So the incident radiation only serves as a power source and the radiation pattern only depends on the antenna parameters.But, can the leaf be both a 'classic reflector' (if there even is such a thing??) and a antenna, so the same structure both reflects some energy in the expected direction but also reflects some according to its antenna plot?.. I find that hard to imagine for some reason. Like the exclusion principle comes to mind, only one circuit in a particular region of space ...... how does that work with the region a photon occupies in space? I mean I dunno how to describe it, like some thin rectangle with a vector moving through space can only go at one particular path, or can some kind of dicotomy occur where it might decide to reflect at a incidence angle or absorb? into a antenna circuit? It would need two paths right, like a physical filter it falls through, where there are 'holes' in the reflector so it can go into antenna?