Sprint 4G Mobile usage Franklin U600

[techwrench]

I have several vehicles that are mobile support deployment vehicles. Until recently we were on 3G and using Wilson electronics 2db interior window mount antennas to help with reception. These vehicles are equipped to be in " stealth" mode due to the equipment value and likelihood of theft or carjacking. If anything is to be mounted externally it needs to be as inconspicuous as possible.

We started with 4G trying the hotspot but as you all know they have many problems especially in our hot environment. We have just started using the Franklin U600 instead of the Hotspot with a PHS 300 cradlepoint router. The problem is the upload speeds and overall reception have not been as good as the Hotspot even though the equipment is way more reliable. I have checked upload and download speeds without the router. They are pretty much the same as with the router so I would surmise that eliminates router interference.

The upload and download speeds are excellent when I have 100% signal (5.5 mbps down and 1.2 mbps up). The problem we are having is that when the signal is less than 100% the upload rate is worse than 3G. The upload speed starts to go away at about 85% and in the 50% range runs 4.3 mbps down and a dismal 0.15 mbps up.

My area of expertise is not RFwireless so if there is something you need please let me know.

The typical center frequency is 2630500 - 2660500 kHZ
When reception is good I get the following readings.

Transmit power -1 to -2 dbm
Signal strength -61 dbm
CINR db 26

At roughly 50% signal these numbers look like

Transmit power 23 dbm (it actually reaches this at 85% signal strength and stays there no matter how low the signal strength gets)
Signal strength - 87 dbm
CINR db 11

I have 2 questions that I need help with:
1. Is there a better choice than the U600 that has antenna ports from sprint?
2. What 3G and 4G antennas would you recommend for this application? I would like interior and exterior mount antenna options if possible as well as the pros/cons of each one. The vehicles are stock appearing caravans not panel vans. We run these vehicle in areas that have tall buildings, areas that are hilly and rural areas around Houston. In most cases these vehicles do not have the ability to be repositioned to acquire the best signal so we are looking for the best overall suggestion that would work in most cases. The 3G is used most often in the rural and hilly areas if it helps.

Thanks in advance for any help you can offer!!

[NickDBLG]

I have checked upload and download speeds without the router. They are pretty much the same as with the router so I would surmise that eliminates router interference.

You are right this is rarely an issue.

The problem we are having is that when the signal is less than 100% the upload rate is worse than 3G. The upload speed starts to go away at about 85% and in the 50% range runs 4.3 mbps down and a dismal 0.15 mbps up.

This is unfortunate but expected for any 3G/4G broadband protocol that uses adaptive modulation & coding (AMC). As the Received signal strength (RSS) degrades it goes to a lower coding mode (lower datarate) in order to improve the Bit-error-rate due to lower signal-to-noise-ratio.

Transmit power -1 to -2 dbm
Signal strength -61 dbm
CINR db 26

At roughly 50% signal these numbers look like

Transmit power 23 dbm (it actually reaches this at 85% signal strength and stays there no matter how low the signal strength gets)
Signal strength - 87 dbm
CINR db 11

You can see from comments above that when your RSS goes down to -87dBm (a degradation of 26dB which is quite significant) you lose your speed. This is because the receiver sensitivity for highest modulation mode is above -87dBm and you can no longer transmit at the fastest speed.

You CINR (Carrier to Interference plus Noise Ratio) also degrades by a lot. This could be because of noise contribution or because of interference. An antenna can somewhat solve the noise issue particularly for wide-band noise sources that could get shaped to be out of band. However little can be done if the major contributor to CINR is channel interference (while having a narrow beamwidth antenna can somewhat isolate interference arriving from different angles beyond the beamwidth interference is usually a channel related problem that cannot be solved at the user equipment without changing the CPE).

The big problem you have based on your first comment on a "low-profile stealth antenna" is that these antennas are typically low gain. This is a fundamental RF engineering/ physics constraint on a passive element antenna. The larger the radiation elements the larger the gain - so when you miniaturize the antennas you can't get more than a few dBi of gain.

Here are some examples of low profile antennas:

• Low profile cellular 3G & wimax 4G magnet mount antenna
• WiFi wimax magnet mount antenna 2.3-2.7GHz
• 750-1250 MHz, 1650-2700 MHz Cellular, WiFi, WiMax Wide-band Surface Mount Antenna

You can check out other antennas here: http://www.rfwel.com/4g-data/wimax-a...wimax_antennas including a 6dBi spring-mounted antenna ECOS6-2600 that is suited for vehicular applications but is by no means low-profile.

Is there a better choice than the U600 that has antenna ports from sprint?

The U600 is just as good as many of the other Sprint modems. We like the Sierra 250U since there are only two ports to deal with. I.e the 3G/4G port is multiplexed in the primary antenna port so you can hook that up to a wideband cellular/wimax antenna and the secondary port is 4G only. Here is a generic drawing of what i mean (for an in-building application).

What 3G and 4G antennas would you recommend for this application?

Based on the discussion above on the gain of these antennas, not quite sure you would benefit much from any of them. Now if the problem is because the vehicle is shielding the RF signal from getting to the modem then any of these low-profile low-gain antennas would help. However if the -87dBm is what you measure outside the vehicle then these antennas would get you to ~-84dBm at best which is hardly sufficient to bring back your speed. So a simple way to say this is that adding a low profile antenna would ONLY improve your signal to the extent that placing your modem outside the vehicle would (and no more). On the 3G side there are cellular band RF bidirectional amplifiers/repeaters you could add that would work with the low-profile window mount antennas to drastically improve your 3G signal. These RF BDA's unfortunately don't yet exist for WiMax.

Hope this helps and do let me know if i can clarify anything here.

[techwrench]

Thanks for the reply!! Sorry it took me a while to respond. It has been a busy week and I wanted to move the modem outside at several stops to see what effect it had. These are averages from several different tests.

CINR +3 db
Signal +3 db
Transmit power -2 dbm
Signal strength +9%

From these changes it looks to me like an antenna will help some but may not be the perfect solution. The interesting thing is the upload speeds change much more than the download speeds which makes me think this is the key to my solution.

I also suspect the windows may have some kind of UV coating as the modem is mounted in the window and I get this big of a change setting it on the roof just above the window. I have a window mounted 3g antenna on my other vehicle and I'm going to try it inside and out to see if the same thing occurs.

As far as the "stealth" requirement whips are absolutely out. This year vehicle has a sirius radio antenna on the roof whether it has one or not so I was thinking if something similar in size replaced it no one would give it a second look especially since the luggage rack rails make it hard to see except from the rear. Since you sell mobile Mark I took a look at the site at part number SMD-3500-3C3C which could replace the sirius antenna and be made to blend in quite nicely if it was black. It is a 4dbi dual antenna which I think would provdie enough benefit to make it worth the effort if it will work in this application. What do you think about it for this application and is that something you can sell?

Thanks again for your input!!

[NickDBLG]

The interesting thing is the upload speeds change much more than the download speeds

This is typical but unfortunate. FOr one MIMO diversity techniques are only used for the forward link (download). The reverse link (upload) only uses a single antenna. This means it's less resilient to multipath fading effects on the reverse link. Add on to that the fact that the reverse link typically has less bandwidth allocation and you can see how uploads are much worse.

I also suspect the windows may have some kind of UV coating as the modem is mounted in the window and I get this big of a change setting it on the roof just above the window

If the windows are polarized this could be an issue. More often though it's just the attenuation of the signal through the window as well as the pertubation of the radiation pattern due to proximity with the window. Typically the modem should be kept several inches away from the window at all times (definitely a few wavelengths away and the wavelength of a 2.6GHz signal is about 12cm so about half wavelength or ~ 1/4ft would be the minimum required).

Here is a link to that mobile mark 2x MIMO WiMax low profile antenna (SMD-3500-3C3C): http://www.rfwel.com/shop/WIMAX-2xMI...ace-Mount.html. I think these just come in white but let me doublecheck on that.

[techwrench]

Thanks again for your help. I didn't know aboout the antenna position. I positioned the modem for best reception in the vehicle based on signal and upload speeds. It is actually hanging on a cloth mesh privacy blind which is about 3 inched away from the window. If it only comes in white would it be possible to paint it without affecting the signal?

[NickDBLG]

It comes in white but you should be able to paint it just fine. Practically we have never seen any appreciable difference in performance with radome-enclosed antennas even as high as at 2.4GHz. If you look at the physics rigidly you might think that it would be a problem and while it theoretically distorts the radiation properties somewhat a thin coat of spray paint should be just fine. Needless to say it does void the warranty

Do test out the performance first before you paint it lest you convince yourself the paint messed you up

[techwrench]

Nick,

In you last post you stated:

Practically we have never seen any appreciable difference in performance with radome-enclosed antennas even as high as at 2.4GHz. If you look at the physics rigidly you might think that it would be a problem and while it theoretically distorts the radiation properties somewhat a thin coat of spray paint should be just fine.


It looks like you were talking about the paint but I just wanted to clarify before ordering that it was paint you were talking about and not the antenna itself!!

Thanks

[NickDBLG]

Yes i was talking about painting the antenna. You should be fine with a thin coat of spray paint. What i meant was that while this theoretically would change the radiation properties of the antenna, the practical effect is minimal for radome-enclosed antennas such as this.