The RF environment at stadiums - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: March 28, 2023, 01:37:09 am
content: Is wireless communication (ISM bands) viable in stadium type environments where there are perhaps 50,000 mobile phones, (some out of spec) filling the RF spectrum.How do you simulate this type of environment in the lab? Is there a 'stadium in a box' jammer device that simulates this type of RF environment?

author: Benta
author_rank: Super Contributor
timestamp: March 28, 2023, 11:18:54 am
content: Which ISM band? 305, 433, 868, 905, 2.4G?

author: mick_lee
author_rank: Newbie
timestamp: March 28, 2023, 12:46:56 pm
content: Wow, that's a really expansive question, and one that cannot be answered with a couple of quick lines.The reason it's not easy to answer is there are many factors that feed into how equipment works in a noisy or congested signal environment.The most fundamental, I suppose, is the Shannon Hartley channel capacity theorem - which places a fundamental limit on the amount of data that can be transmitted for a given bandwidth for a given signal to noise ratio.https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theoremThat's just the physical layer - on top of this we have the rest of the OSI 7 layer stackhttps://en.wikipedia.org/wiki/OSI_model- which will also play a big role in interoperability.Modern devices use adaptive modulation schemes such as COFDM - which adapt to use available spectrum. New to the market are systems that use electronically steerable antennas (MIMO) to allow what is referred to as 'spatial diversity'.So I'm sorry I can't give you a quick and simple answer- because there is one really!

author: tggzzz
author_rank: Super Contributor
timestamp: March 28, 2023, 01:15:56 pm
content: The OP's question is too vague to be answered. If he bothered to say thepurposeof the simulation, then it might be worth considering answering.He should also do some basic research on commercial equipment that is designed to be used in concerts halls, theatres etc. That should give some hints.As for having many cellphones, one of the more interesting specfications for a cellular system's capacity is "bits/s / Hz / m2". The "bits/s / Hz" is how close a call gets to Shannon's limit. The "/ m2" is how closely together you can squeeze the cells.

author: e100
author_rank: Frequent Contributor
timestamp: March 28, 2023, 02:57:12 pm
content: I shall rephrase the question.Does anyone have any practical experience of trying to use an ISM band (any ISM band) in a stadium environment?Was it a complete failure or did it sort of work?

author: ajb
author_rank: Super Contributor
timestamp: March 28, 2023, 07:11:19 pm
content: ISM bands are used successfully all the time in stadiums. Wireless microphones and IEMs, wireless lighting and camera control, wireless intercom for production and sports team staff (some of those may use licensed bands), those synchronized light up wrist bands that are handed out to the crowd at concerts (some use IR but some use RF), wireless pyrotechnic control.... All of those are used regularly in and around packed stadiums.The RF environment will vary significantly from stadium to stadium, and from event to event depending on the equipment in use. Looking at the background RF levels for the environment and selecting channels to avoid the most congested segments is part of the setup process for many events relying on RF systems, especially for things like microphones and monitors where dropouts would be especially objectionable.

author: e100
author_rank: Frequent Contributor
timestamp: March 29, 2023, 02:50:26 am
content: The Xyloband RF light up wrist bands seem the most interesting as they manage to cover the whole stadium using a small number of transmitters without any apparent dead spots. Pretty much all of them are are perfectly synchronized in time with no badly behaved devices doing their own thing.https://youtu.be/Qd_EOTziiSY?t=38

author: ejeffrey
author_rank: Super Contributor
timestamp: March 29, 2023, 03:17:18 am
content: Quote from: e100 on March 28, 2023, 01:37:09 amYou can mostly count on phones to not transmit on frequencies they are not assigned. So if you are using anything other than the 2.4 GHz and 5.6 GHz bands used by wifi radios you should be able to ignore that.If what you want is to run wifi for up to 50,000 clients that is extremely challenging but also common need and there are products and guides to doing this. Most of the options are going to be expensive of course.If you need to run some proprietary non wifi system, or just house wifi that isn't provided to the audience on the 2.4 GHz ISM band, then yeah you will have to deal with interference, but it strongly depends on what and where you need it

author: jwet
author_rank: Frequent Contributor
timestamp: March 29, 2023, 04:23:02 am
content: I'm not an expert but I guess what the OP was looking for was guidance on "simulating" this mess. This is a really tough simulation problem and there is no simple canned solution. The biggest problem is that in most RF interference scenarios, superposition doesn't strictly apply. The effects aren't linear as they create all kinds of intermod products.There are a few concepts and techniques that you might want to be aware of that apply here and have been studied and published about extensively.In the original analog cable TV system, there is a closely related problem. You have all these 6 Mhz wide video signals riding down a cable together- perhaps a 100 or more. They could span from 30 MHz to near a GHz in a superband system. The question which is similar to yours is what is the noise contribution of 999 fully operating channels that are adjacent to into your quiet channel. This was called "all hostile cross talk" in the analog cable world and was studied extensively. Digital modulation helps a lot but doesn't solve it. There are some wicked intermod specs like Composite Triple Beat and related types of distortions that require a lot of linearity. Studying how this stuff was studied and solved in the 80's and early 90's would be fruitful probably.Another concept that is key is just the idea of a "blockers". These can be in-band like another 868 ISM band app stepping on your signal but more perniciously these can be out of band blockers that create intermod and mixer products that contaminate your piece of spectrum.As a simulation problem, its probably worthy of a PhD thesis. As an engineering problem, its pretty hard and would probably be side-stepped. As others have said, it will vary by locale and what systems are used. Certainly avoid anything cellular including 868 Mhz- close to a very old AMPS cellular allocation in the US. The lower frequencies like 315, 492 might be better. Consider an agile transceiver like those from TI/Chipcon that can operate over a wide range of ISM frequencies. You would also want narrow preselection filters and narrow IF's.Good luck, challenging problem. Go catch some games with a "Flipper Zero" or a small spectrum analyzer and look at the environment.

author: boB
author_rank: Frequent Contributor
timestamp: March 29, 2023, 04:31:01 am
content: Probably just looks like a lot of noise in that band.

author: GLouie
author_rank: Regular Contributor
timestamp: March 29, 2023, 01:59:14 pm
content: I have also heard that the digital display signs that ring modern arenas/stadiums emit a horrid hash of RF noise.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 30, 2023, 12:10:10 am
content: If your looking for a regularly available testenvironment..Try a large church. Especially one with a cell site next door.I run spectrum management for my church.150-300 phones looking for WiFi at a time pushed us to 5.6 Ghz for show control. My SA is a regular visitor. We "bait" the phones with 2.4 Ghz nodes and then use 5.6 and some really oddball wireless mic frequencies.Our sound mixing console is an X-32 Rack and mixing is done on touch screens on Ipads, this allows the sound guys sit with their families. The X-32 has its own access point, set to private.One of the keys is not using default frequencies on the nodes.Our eight wireless mic receivers have ethernetfor adaptive allocation of channels. They also scan the spectrum and display it on a PC.Intermod and mixing products from the cell tower makes life interesting. I use WA5VJB log periodics to "look under" the towers pattern and diversity receive setups.if you are a tour or theatre in the US, you have options for licensed channels, if you need 60 or more frequencies.About four times a year, or whenever the church secretary notes a a truck at the cell site, I will check for interference on the SA.Generally if node powers are kept to the minimum needed, channel reuse works fairly well at 2.4 Ghz with ODFM etc.I am utterly amazed how often the cell site hardware gets updated. They also have adaptive down-tilt on the antenna arrays.not too long ago, reforming and selling the TV spectrum resulted in many low power entertainment channels going away. Losing that frequency space to cellular cost my church 750$ per channel to re-equip. We're told we could have 240 mics in the digital system at once. In 30 Mhz of Spectrum.In our case, we found the automatic frequency selection system in the mic receivers often chose to operate right at the tail end of an active ATSC Digital TV channel with no noticeable interference. Of course, I will manually move them off the TV channel.Covid forced us to come up with a zero budget camera array for live streaming. Ipods/Ipads on 5.6 Ghz saved the day. We've found our limit on emitters to be quite high in a 60x100 for space. Ten in our case.This change by the FCC resulted in many studies and new protocols for the entertainment industry.So recent data is out there if you look.Fortunately for us, we had a wealthy donor. We would haverather spent the money on charitable causes.At a concert, immense work will be done in spectral planning. For something like the Olympics, it starts years in advance, and typically the National Authority makes draconian rules for the region around the event, and re-allocates spectrum on a temporary basis.915 Mhz is often considered useless in the Midwest US, every household with natural gas has a gas meter. Every gas meter is chirping loudly in the 915 band multiple times per minute with billing information.Yes, there are simulators, for cellular and WIFI, but not so much for ISM. Most nations severely limit the ISM duty cycle.try Rhode and Schwartz, and Candela Technologies for simulators.My cheap Siglent SA has an option for measuring spectral occupancy and adjacent channel interference etc. So do most other brands.My friends in the entertainment industry use WIFI all the time. So does a friend who does RF Engineering for Disney parks. Most of them do not care about the tiny details. They all tend to have hardwired backups using specialized ethernet protocols such as ArtNet or OSC or Dante. I would find a touring consultant or talk to a hardware manufacturer.Remember, good WIFI hardware in that environment is going to be constantly adaptive and monitor spectral occupancy. But, no two stadiums or theatres will ever be standardized.⁸Steve

author: pdenisowski
author_rank: Super Contributor
timestamp: March 30, 2023, 02:06:01 pm
content: I spent over a dozen years doing field interference hunting all over the United States and internationally. This included some large venues, including indoor and outdoor stadiums, etc.The short answer is that it's impossible to say for sure what the RF environment will be like on a given day without measuring it: I've seen environments go from quiet to chaos in a matter of hours (but usually not vice versa). Even in "licensed" spectrum like cellular bands, things can go very wrong in a hurry, and ISM bands are sometimes the "Wild West" of spectrum.Quote from: LaserSteve on March 30, 2023, 12:10:10 amThe easier way to simulate a crowded spectral environment is using AWGN (additive white Gaussian noise) with a defined bandwidth and SNR or EB/No. This isn't a perfect simulation as some residents of the ISM bands may be bursty / hopping or CW (vs. wider bandwidth) type signals, but it's the best practical approximation. R&S vector signal generators (like the SMW, etc.) can generate AWGN and/or add AWGN to a defined signal.Quote from: LaserSteve on March 30, 2023, 12:10:10 amI may or may not have once been on the roof of the Contemporary Resort in Orlando looking for RF interference ...

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2023, 02:34:31 pm
content: Quote from: pdenisowski on March 30, 2023, 02:06:01 pmJust so.The "rusty bolt" problem is unpredictable and can be severe. Apparently warships are particularly vulnerable.Certainly HMS Sheffield was sunk partly because it had turned off its primary radar in order to use comms equipment. Whether that wasbecauseof "rusty bolts" is unknown to me.

author: pdenisowski
author_rank: Super Contributor
timestamp: March 30, 2023, 03:44:28 pm
content: Quote from: tggzzz on March 30, 2023, 02:34:31 pmPassive intermodulation (PIM, a.k.a. the rusty bolt effect) is a serious enough issue that many major test and measurement instrument manufacturers offer a PIM tester that transmits two high power CW signals (tones) and then looks for intermod products at the expected (calculated) frequency. In my field experience, almost all PIM issues are (unsurprisingly) associated with high power transmitters, usually either broadcasters or cellular base stations.My understanding is that the Sheffield's radar was turned off due to interference caused by (co-channel) SATCOM, not PIM. Whatever the cause, the sinking of the Sheffield created a lot of interest in EMC MIL-STD testing that continues to this day: I can't count how many EMC presentations I've listened to that started with the story of the Sheffield.

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2023, 05:05:57 pm
content: Quote from: pdenisowski on March 30, 2023, 03:44:28 pmI initially heard SATCOMs, but the relatively recenthttps://www.theguardian.com/uk-news/2017/oct/15/revealed-full-story-behind-sinking-of-falklands-warship-hms-sheffieldmentions the more ambiguous "The radar on board the ship that could have detected incoming Super Étendard fighter aircraft had been blanked out by a transmission being made to another vessel.". Hence my weaker statement.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 30, 2023, 10:24:49 pm
content: We do need to be warned when coldplay might untintentionally heard.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 31, 2023, 01:04:37 pm
content: One of the additional hazards is front end overload.A security guard standing next to your gear with a 5 watt handietalkie can wreak havoc. I have a friend who lost a laser show projector from a HT keyed right next to the galvo scanners. At the time the scanners used RFat a few Mhz in capacitive position sensors.Steve