I'd like to know if basic GSM functionality will be affected by the phasing out of the 2G networks. I have several devices with modules like SIM900, old 2G Enfora, etc. and they use SMS and USSD for communications (not GPRS/EDGE or any IP protocol). Most answers from the Internet are about IoT devices (which do use IP), so they don't apply to my situation. My poor understanding of the GSM base protocol tells me that SMS and USSD use a control channel as a transport, and as I understand it those channels would never be touched (e.g. the base antennas would negotiate a better protocol from them, when possible), or will they? I've heard of countries where complete bands would be reassigned. My devices are quad-band. Thank you!
Over-design the capacitance required by pretending the power supply is too slow (but seriously, the pulse length of 1.8A is ~2.3ms, which your supply WILL be able to help with) and just look at an ideal capacitor and it's discharge curve, which is inverse log.
To speed the calculation process, find some handy little online calculator like this: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html
Given the requirement of 1.8A through the load, I am assuming you are running a 4V rail as given in the question, so I put in the parameters:
Vo = 4V,
R = 2.22 (so that I_load is 1.8A)
and after some estimation and a few values, I found 4700uF was perfect
A capacitance of 4.7mF means that without any help from the power supply, your rail will drop to ~3.3V after 2ms of the 1.8A load draw from a 4V charged cap.
To fill in the gaps, you can safely assume the power supply will help out a little by then to reduce the rate at which the charge over the cap drops. You circuit may malfunction if the voltage drops below 2.7V or so..
Here is an example component that may suit the task at hand: http://www.digikey.com.au/product-detail/en/ECA-0JM472/P5118-ND/244977
No, they cannot. Cell phones contain cell modems, like the green module pictured in the question. They are controlled using "AT" commands, patterned after (but greatly extending) the AT commands developed in the original Hayes modems thirty years ago.
There is a core set of AT commands that are common across all cell modems; these generally have a prefix of "AT+". Then there are a number of proprietary AT commands developed by each cell modem manufacturer, they have prefixes like "AT$" or "AT%" etc where the third character is unique to the manufacturer.
I have worked with cell modems from four different manufacturers. The documentation for these commands typically runs over 500 pages long.
These are all designed to work with a cell tower. There is a command to get the signal strength (AT+CSQ), which is used to update the bars on the screen. There are commands to check whether the cell modem has registered on the network (connected to a cell tower). Another command to get the list of carriers supported by the cell tower.
A command to dial a call (ADT, just like the dial up modems from the past). When a call answers, the response is CONNECT. When an incoming call comes in, the modem says RING ... RING etc (sound familiar?). More commands to send SMS messages, send and receive data (used for accessing the Internet), commands to access the phone book, etc.
If these were to work for point-to-point communication with another cell phone, that other cell phone would have to have all of the capability of a cell tower. But there is no provision for that. There are no commands in any cell modem AT set which allow a cell phone to behave that way.