AT&T 3G MicroCell: A Comprehensive Exploration
by Brian Klug on April 1, 2010 1:55 AM EST- Posted in
- Smartphones
- AT&T
- Microcell Review
- Mobile
I mentioned earlier that voice and data both work fine, as long as you're inside the MicroCell's coverage radius. So what happens if you step outside? In theory and per advertising, your call is supposed to transition "seamlessly" to the nearest macrosite. In fact, the first time I tried walking out of location 1, it transitioned perfectly! I then proceeded to spend over a half hour trying to repeat my initial success...
The Theory of Handovers
But before I dive into the problems I experienced, let's briefly touch on some of the tech behind it. There are two types of handover that happen on a cellular network: soft and hard handovers. In a soft handover, the phone is continually talking to multiple macrocells at the same time - the transition happens seamlessly because you're already connected and communicating with towers you're going to handover to. This is a make-before-break transition. The transition is simply a matter of choosing which one has the best link quality, and this is generally how most transitions happen because it's robust.
The other case is a hard handover - this is less seamless break-before-make transition that's nearly instantaneous in practice, but still perceptible if you know what to look for. In a hard handover, the phone literally releases its active connection and transitions to another. Also of relevance are so-called vertical handoffs - these are transitions that happen across network technologies, in this case from UMTS to GSM, though other cross-technology handovers are possible as well. There's a huge amount of complexity to these systems; effectively handing off users while maintaining voice and data sessions is a challenging multidimensional problem. One that's often taken entirely for granted because of just how well we're used to it working.
So you can imagine my surprise when I discovered that the handover from femtocell back to public 3G wasn't just rough, it's downright crude. First of all, virtually every femtocell handover is of the hard variety, and in practice almost all of the ones I saw were also vertical, from UMTS (3G) to GSM (2G). During that long provisioning process, I mentioned that the femtocell is building a rough list of all the macrocells it can see. Hopefully, it can see a few of both 3G and 2G variety, which it passes onto the phone in the form of a neighbor list. The phone also builds out its own neighbor list, which you can see some of in the Field Trial (*3001#12345#*) dialer application. It's been gimped significantly since the iPhone 2G, but there's still enough here to get the point across:
Neighbor Cells - Just like advertised
Anyhow, when you leave the femtocell's radius, if there's a macrocell in range and your phone can hard handover to it, the call continues. Hopefully this is what happens, otherwise the call drops. Of course, the big caveat here is that if you're installing the femtocell in a place with little to no signal (why it's there in the first place) handover is going to be a difficult prospect. That's why I tried the AT&T MicroCell both in an area with good existing coverage, and one with poor coverage. The results at both were equally disappointing.
AT&T advertises an effective range of 5000 square feet. This seems about right, but it's signal dependent. Location one is just over of 2000 square feet, and continual femtocell coverage barely makes it across two rooms. Location two is effectively 5000 square feet, and femtocell coverage includes the whole house, guest house, and most of both long driveways. Again, this is largely in part because the femtocell is sitting atop existing 3G spectrum - there's no spectrum overlay just for microcells yet. The result is that if signal is bad where you are, your microcell range is larger. Conversely, if signal is already reasonable, you're not going to see more than a room or two of microcell coverage. It's a complex problem akin to the cell breathing phenomenon, and I think AT&T's 3G MicroCell product is doing a lot of ranging at setup.
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leexgx - Friday, April 2, 2010 - link
the UMA thing looks good idea, seems Way more piratical then these base stations ( http://en.wikipedia.org/wiki/Generic_Access_Networ... ) only ever seen it on blackberry phones thought, only issue i could see with them is if it doe snot work with the wireless router correctly or intermittently out of range of routerjulioromano - Friday, April 2, 2010 - link
Very nice and geeky review.Thanks for all the infos!
Simozene - Friday, April 2, 2010 - link
1. These units need to be very cheap or free for consumers.2. Any minutes or data usage that is routed over this instead of AT&Ts regular network should not be counted towards the limits on your data plan. It's not their network so you should not have to pay for using it.
If those two conditions are met I can see how this could be a very useful product.
sxr7171 - Friday, April 2, 2010 - link
But it's not. It can't even hand off properly.Chrisg331 - Friday, April 2, 2010 - link
1st off, great article. Good methodology as well. Will you be able to test different handsets (Primarily different chipsets/antenna designs to eliminate bias on the dropped calls), possibly test a repeater (as mentioned before) and possibly test data usage pattern(s) for those that may be bandwidth capped on their broadband? Great job. Could really be useful to those looking to ditch landlines.GregHH - Friday, April 2, 2010 - link
In your haste to slam AT&T you neglect to notice that the other cell carriers offer the same type of device. That implies their coverage must not be perfect and all encompassing. Everyone seems to think cell coverage should be ubiquitous whether in a metro area or in the wilderness. I feel good that my area finally got 3G coverage in December of 2010.JKflipflop98 - Saturday, April 3, 2010 - link
So, we're in the future looking back on the past then? Or we're in the past looking towards the current? Oh God, my head's going to explode.ivwshane - Friday, April 2, 2010 - link
There is no required monthly cost. Buy just the microcell and use your existing minutes and data plan. Buy a microcell with a $20 feature and you can have unlimited minutes while using it and you also get a $100 rebate.No one is forcing anyone to buy these, at&t is simply giving it's users a choice, if you want to improve your in home coverage then buy one, otherwise don't.
mikeshady - Saturday, April 3, 2010 - link
So if I understand it correctly the price,$20/month unlimited calling$10/month with AT&T DSL
$0 with AT&T landline.
Will i be able to use it for the unlimited free since i have att landline
mrSHEiK124 - Tuesday, April 6, 2010 - link
Those failed handover videos; that happens ALL OF THE TIME on at&t in the Tampa, FL area. If you're on a highway or main-road and venture off into the boonies, as you get booted off 3G all you can hear is distortion (the handover is successful and the call doesn't drop, but good luck continuing the phone call...) and weird audio artifacts. at&t...more bars in more places.