One of the biggest obstacles has to do with the lengthy (months to well over a year: having to get in the "queue" to get your unit certified, it can be costly to have this certification performed - can be in the tens of thousands of dollars) and without a LARGE quantity (as in many tens of thousands of units) this can often derail a product/project like the one being proposed - having worked for a cellular carrier in the USA, I have been through the process - and it is beyond painful.
That said, there are already a couple of options that are off the shelf today:
* One option MIGHT be to take a look at the stacking cellular module that Microduino has developed as a stackable module (similar to the Tinyduino form factor, but the connector system is significantly different. Most likely, you would build the entire project using the Microduino system. NOTE: I have not worked with this device, and do not know what carriers it is certified with or the countries in which it was designed to operate. With the sunset of 2G GSM/GPRS in the USA, it will not a viable candidate for very long if the product must work in the USA since support for 2G GSM/GPRS - largely provided by AT&T is going away in the USA by 2017.
* A more simple approach that accommodates various antenna options and supports GPS on some of the units is the Eurotech ReliaCELL (a USB-connected cellular modem).
* It comes in various flavors: 2G-only (1xRTT) in the $90 range in qty., 2G/3G in the $130 range in qty., or LTE.
It is worth noting that some cellular carriers, such as Sprint, will continue to support CDMA 2G 1XRTT for a number of years - which means that a less expensive 2G-only unit could be used for a while). However, the 3G-capable and LTE-capable models will have a longer useful life.
* The antenna can mount directly to it and it can be mounted to the outside of the device enclosure or even be mounted remotely (as in outside of a vehicle or building since it is weatherproof) at distances as far away as you can "legally" run USB according to the standard (active USB repeater cables can extend this distance significantly).
* The ReliaCELL approach has several advantages since cellular coverage can vary in different locations/environments/buildings and the cellular receiver may need to be mounted remotely from where the embedded device needs to be located. When that is the case, the Reliacell can be treated almost like an intelligent antenna that is USB-connected. No long coax runs are needed (and so no RF signal loss from those long coax runs) since USB is used.
The biggest advantage, though is that the ReliaCELL devices are already carrier certified. That means you can begin deploying in production immediately and not go through the pain, cost, and delay of carrier certification.
This is discussed in further detail along with additional info regarding adding cellular connectivity as well as the data sheets for the ReliaCELL, Microduino 2G shield, and the AT&T 2G GSM/CDMA sunset in the following thread on this forum:
http://forum.tinycircuits.com/index.php?topic=1591.msg2937#msg2937Hope this was helpful...
John
