RE: 2.4 GHz - A Broad Market Review

RE: 2.4 GHz - A Broad Market Review
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RE: 2.4 GHz - A Broad Market Review - 8/1/2007 2:26:47 PM

Flying Geezer

Posts: 1009
Joined: 4/12/2003
From: Bowling Green, KY, USA
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Correct me if I am wrong in assuming that you are an engineer with experiece in wave propagation.

I am an old broadcast engineer licensed for AM, FM, TV, and 2-WAY. I have not read up on the 2.4 band so I have a few questions.

I assume the output power on the 2.4 RC equipment much less than 1 watt because of the power consumption listed for the Extreme power system modules. Do you know what the power limit is?

Are point to point and point to multi-point units allowed on the same channels? Allowed the same power output? Raw power nor ERP.

How many different channels are there in the 2.4 gHz band? What is the bandwidth per channel?

Do you have a cell phone, if so, how does it work in your home? Do you have 2.4 gHz cordless phones? Do you have a wireless network within your home?

I live in a county of 100,000 people, most of whom have cell phones. Most of them seem to want to talk on them while they run the red light that is green for me. At the mall every little kid over 10 has a cell phone in their mouth instead of a lollipop. My home wireless network picks up 4 of my neighbors. I rarely get a tiny bit of noise on my cell phone and my network runs fine.

I need a more complete explanation of how these things work and I think you can help me.

Thanks in advance,

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Hughes RC

(in reply to Chaddt)

Post #: 76

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 3:12:52 PM

d_wheel

Posts: 516
Joined: 1/27/2003
From: Deep in the Heart Of, TX,
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quote:

ORIGINAL: Flying Geezer

I need a more complete explanation of how these things work and I think you can help me.

Thanks in advance,

I hope you don't mind if I but in. I'm sure Chaddt will come in with some good info but you can start here to learn a little about SS theory.

http://www.sss-mag.com/pdf/ds-v-fh.pdf

Later;

D.W.

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Post #: 77

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 3:23:54 PM

dirtybird

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Joined: 1/25/2002
From: Mesa, AZ, USA
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quote:

ORIGINAL: Chaddt

Yes you are only allowed 1watt or 30dBm of power from the radio itself but you are allowed another 6dBm for an antenna for a total system gain of 36dBm for point to mulitpoint "think access point here". If you increase the gain of the antenna you have to reduce the power output from the radio to keep the total system EIRP to 36dBm or under.

For point to point to point " connection point A to point B" You are allowed 42dBm of total system EIRP.

36dBm is 3.98 Watts
42dBm is 15.8 Watts

You do not increase the generated power with antemnna gain. You only increase the power concentration over what you would have with an omnidirectional antenna. You still have only one watt of generated power.
Also it only is true in the far field of the antenna - not the near field.

< Message edited by dirtybird -- 8/1/2007 3:26:12 PM >

(in reply to Chaddt)

Post #: 78

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 3:37:47 PM

Flying Geezer

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From: Bowling Green, KY, USA
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Thanks for the link D Wheel. This is interesting but is oriented towards computer networks which must carry much more data than a radio control transmitter.

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Hughes RC

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Post #: 79

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 4:01:14 PM

bruce88123

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From: Memphis, TN, USA
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Totally unrelated to anything but I thought an interesting way to wreck your warranty

http://arch.ced.berkeley.edu/kap2/php/wind/?p=34
This guy had a task and found a very creative way to meet it with 2.4 GHz.

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Post #: 80

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 6:10:42 PM

Chaddt

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Joined: 8/29/2006
From: Carlyle, IL, USA
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I am not an RF engineer but I have a degree in electronics and have learned what I have needed to own/run a wireless internet provider bussiness for the last four years. Most of what I know RF wise is related to wireless networking equipment. But the general RF theory is the same as anything else out there. I am by no means an expert but don't mind sharing the little that I know.

I dont know what the power output of the current 2.4 RC systems are. I haven't been able to find this information in any of the documentation online, if you have the FCC ID off one of the transmitters you may be able to find it on the FCC's website. Only other option would be to call them and ask. Also I don't know what the recieve sensitivity of the reciever is, or what the antenna gain is on each end or what type of modulation they are using.

There are a total of 11 channels in 2.4Ghz for 802.11b equipment starting at 2.401Ghz and ending at 2.473Ghz each channel is 20Mhz wide so there are only 3 non overlapping channles these are 1, 6, and 11. Here is a pretty good chart showing frequency and power information for wireless networking equipment, it will show you the center frequency for all the channels.

http://www.demarctech.com/techsupport/ism-info.html

Yes both point to point and point to multipoint use the same channels, so you have to be carefull on channel selection as to not interfere with yourself or others operating in the area.

Yes we have both 2.4Ghz phones and I have a 2.4Ghz network in the house and both work fine for two reasons.

1. I am pretty well directly under our tower so the RF generated from the tower is shooting over the top of our house. This would not be the case if I had an RC plane flying up there.

2. Within the house the furthest a cordless phone gets from its base station or a notebook PC gets from its access point is only a distance of 20-30 feet. So they have really strong signals to the base station and to the access point. Its all about your receive signal level vs your noise floor as long as your signal is better than your noise floor you should be ok. Say if your 2.4Ghz noise floor is -90 and your acctual signal is -70 "less negative is stronger signal" your equipment would operate fine. This is what happens with your neighbors wireless networks, even if you are on the same channel as they are as long as you are recieving a much stronger signal from and to your own access point you would be fine, you may see some service degration from the extra noise in the area but more than likely you would be ok.

Cell phone carriers operate on licenced frequencies. Each carrier has a certain amount of spectrum to operate on that no one else is allowed to use so they don't have to worry about interference issues like you do with 2.4Ghz. The way they are able to have so many users at the same time is based off of the modulation/media access method of the equipment they are using. Thier access point is able to concurrently handle so many calls at the same time and is constantly switching between each persons phone passing data back and fourth.

802.11 network equipment works in the same way I have a single access point with 35 users connected to it at the same time using CSMA/CA "Carrier Sense Multiple Access with Collision Avoidance" it allows all users on the system to have access to the internet at the same time. There are different modulation schemes out there some work better than others in different conditions, some allow more users, some have better overall bandwidth, some have better latency, some are better in noisy areas, there is always a trade off though.

Here is a link that explains CSMA/CA

Hope this helps. Sorry for taking this discussion off topic.

Thanks,
Chadd

quote:

ORIGINAL: Flying Geezer

I assume the output power on the 2.4 RC equipment much less than 1 watt because of the power consumption listed for the Extreme power system modules. Do you know what the power limit is?

Are point to point and point to multi-point units allowed on the same channels? Allowed the same power output? Raw power nor ERP.

How many different channels are there in the 2.4 gHz band? What is the bandwidth per channel?

Do you have a cell phone, if so, how does it work in your home? Do you have 2.4 gHz cordless phones? Do you have a wireless network within your home?

I live in a county of 100,000 people, most of whom have cell phones. Most of them seem to want to talk on them while they run the red light that is green for me. At the mall every little kid over 10 has a cell phone in their mouth instead of a lollipop. My home wireless network picks up 4 of my neighbors. I rarely get a tiny bit of noise on my cell phone and my network runs fine.

I need a more complete explanation of how these things work and I think you can help me.

Thanks in advance,

< Message edited by Chaddt -- 8/1/2007 6:55:59 PM >

(in reply to Flying Geezer)

Post #: 81

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 6:40:12 PM

sass25479

Posts: 77
Joined: 8/5/2005
From: Oak Hills, CA, USA
Status: offline

I'm also struggling with the choices at this time. It's time for me to replace one of my radios.

The way I see it my options are a 9303 with spectrum, or with the XPS module. Or wait for the Futuba 12FG. I'm concerned with spectrum because it is DSSS and has a long reboot time after a lock out. XPS uses FHSS which I believe is better but I have major concerns with only one antenna. So it seems that the 12FG seems to be the best choice, but I don't like the $1200 price tag.

And if that happens I'm going to be eating a lot of humble pie!

< Message edited by sass25479 -- 8/1/2007 6:42:38 PM >

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Post #: 82

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 6:54:18 PM

bruce88123

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I think it was mentioned elsewhere in this thread that XPS had a multi-receiver version coming out. Check into that if you are interested.

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Post #: 83

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 7:43:34 PM

pbunn

Posts: 124
Joined: 5/19/2003
From: Spartanburg, SC, USA
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A "gain " antenna does not increase power at the transmitter. It increases the effective radiated power. It does so by forcing the energy into a beam like a light with a reflector lens- sending energy in a specific direction and reduces it in other direction. With a high gain dish, most of the power is sent in a narrow beam with near zero radiation in all other directions. At 2.4 Ghz, it is relatively easy to concentrate Rf energy into a specific direction and use all the power for the job at hand. At 72 Mhz, this is not possible with reasonably sized antennas.

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Post #: 84

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 7:51:47 PM

Flying Geezer

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Joined: 4/12/2003
From: Bowling Green, KY, USA
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Thanks for the info Chaddt. From what you have told me I am confident that the new systems are safe. I'm sure the engineers and marketers that have put their reputations on the line for this advance and have engineered some form of collision avoidance into the system.

Of course no radio system is bullet proof. No receiver can seperate and normal signal from an overpowering EFI loaded into it's antenna. It's like trying to shoot a guy with a squirt gun when you are both underwater. Proper range checking should take care of most of this, and anything else at the flying field will consistently cause problems in a certain area or direction.

Thanks again for the info, I feel better about the new technology.

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Flying Geezer
Hughes RC

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Post #: 85

RE: 2.4 GHz - A Broad Market Review - 8/1/2007 7:57:58 PM

Flying Geezer

Posts: 1009
Joined: 4/12/2003
From: Bowling Green, KY, USA
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I feel the same way you do about the 12 channel Futaba, it looks like a great radio. But I may be too old to wait. Futaba has really missed the boat on their late start, and I understand that there are some issues with a 12 channel TX and a 6 or 7 channel RX.

Yes, $1100 is a little heavy for a radio, making the 9303 attractive. I really don't want to go with a module. I have flown Futaba for a lot of years, and the older you are the less you like to change, but I guess i'm gonna'.

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Flying Geezer
Hughes RC

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Post #: 86

RE: 2.4 GHz - A Broad Market Review - 8/2/2007 4:00:43 AM

leyland384

Posts: 37
Joined: 2/5/2005
From: Depere, WI, USA
Status: offline

How spread spectrum for RC works:

There are no "Channels" in the 2.4 Ghz Band. The 2.4 band is just an allocation where manufacturers can put their equipment. The number of channels available for use by equipment is determined by the bandwidth needed for the amount of data and the speed of the transmission.

Since RC is relatively slow with much less data to transfer than PC based networks, less bandwidth is needed, and thus more "channels" can be created. This is why Spektrum says that their equipment has 80 "channels" available for their ground equipment. (40 channels for air equipment because they use 2 "channels"

Since the ISM band is from 2.4–2.4835 GHz, we could reasonably estimate that each Spektrum "channel" is around 1MHz wide. (2.4835Ghz - 2.4Ghz = .0835GHz = 83.5Mhz, now divide by the 80 "channels" available). The actual update speed of the system is about 45 Hz, with 7 channels, plus some overhead. This requires very little bandwidth to transmit. The part that makes this SPREAD spectrum is the fact that the modulation scheme takes this small bandwidth signal and stretches is way out using a mathematical process and the unique digital spreading code (Digital Spectrum Modulation, sounds kinda like DSM eh?)

FYI, the 45Hz packet transmission rate is the reason why the DX7 speaker buzzes ever so softly (It's barely noticeable, most people don't notice it until after 2 weeks of flying, then freak out and ask Danny what the heck is going on!) Everytime the transmitter sends a data packet, it pulses the voltage on the battery down just enough to cause that soft little hum. It's normal... and it will be okay.

FHSS on the other hand spreads the signal using a different method. The bandwidth of the data remains narrow and relatively unchanged, but the transmitter keeps hopping around onto different "channels" every few milliseconds. These channels might only be a few tens of Khz wide, so there could be several hundred tiny channels available for the FHSS systems. Every time the transmitter lands on another channel, it spreads the narrow data signal out over a much broader RF bandwidth.

Both methods are spread spectrum, just different approaches. I won't say which one is better, but I will say that each has qualities that make it suitable for different types of applications. The technique that works most suitably for RC will be decided by lots of empirical testing (in other words, lots and lots of flying time).

Most of this has been said before here on RCU and elsewhere, and I don't claim to be an "expert", just another hobbyist with a little bit of insight... so please hold the flames and enjoy!

Adam

(in reply to Flying Geezer)

Post #: 87

RE: 2.4 GHz - A Broad Market Review - 8/2/2007 4:24:29 AM

starzak

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That was nice Adam. Very understandable.

Thank you very much.

> Jim

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Post #: 88