From: pprice@qualcomm.com (Phil Price) Subject: Roaming, CDMA, TDMA etc. (was Re: GSM Interference) Date: 29 Nov 1993 23:37:19 GMT Organization: Qualcomm Inc. In article , lchiu@crl.com (Laurence Chiu) wrote: > I don't pretend to understand all this technical stuff about TDMA, GSM > or CDMA but does this mean if various companies decided to implement > different standards for digital cellular, then is roaming, which is > pretty hard already, going to be that much harder as the poor user is > going to have to know if his phone is compatible in the area he is > roaming in? Roaming problems are really unrelated to whether the phone uses AMPS, TDMA, E-TDMA, GSM or CDMA (I'll explain these later). Roaming between systems requires a supporting infrastructure and intersystem protocols. These protocols have been standardised, but are still evolving. The N.American standard is IS-41, and the European standard is GSM (more precisely, the MAP protocols). These protocols define the messaging and procedures for exchanging information between systems to allow roaming. Most of the new phones will be dual-mode -- AMPS plus one of the others, so you should be able to use it just about anywhere (without having to know about the system that you are currently using). Roaming is already available in Europe, and is rapidly being implemented in N.America. On a related issue, you should start seeing information about 'roaming' systems fairly soon. One company has just released a system claiming coverage over the whole of N.America (MobiLink?), but I think that is a PCN system. There are also several projects under way to provide global (i.e. world-wide) roaming -- the front runners in this are Qualcomm's Globalstar, Motorolla's Iridium and TRW's Odessey systems. These systems are all satellite-based but have fundamentally different operating strategies. I know the most about Globalstar, and I'm probably biased in my views, but I'll try to summarize them: Globalstar - has 48 low earth orbiting satellites covering 70 degrees north and south of the Equator. The satellites act as a communications link to ground stations (called gateways) that connect the user to the land network. the satellites have a very large coverage area on the ground (roughly 2000 miles radius, I think) and so can be used as an 'extension' facility e.g. for rural or undeveloped areas. The user terminals will generally be dual mode (though single-mode phones are planned), where one mode is Globalstar and the other mode is that of the local system that you choose (AMPS, CDMA, GSM etc.). The idea here is that a user will use the local system when coverage is available, or Globalstar coverage when it is not (i.e. when roaming) -- since the satellite covers the temperate regions all around the globe, this means that global roaming is potentially possible (when you get into it, the problems become political or commercial in nature, rather than technical). Iridium -- has lots (76? whatever the element number of Iridium is) of low-earth orbit satellites in polar orbits (I don't know what happens if a few of them get close to each other at the poles!?). The funky thing about Iridium is that the satellites do the switching in space between themselves and then sends the signal down to earth at some point close to the destination of the call i.e. it bypasses the terrestrial systems (but apparently they are discussing an option to interface to the terrestrial systems also). The advantages of this is that it doesn't depend upon the terrestrial network (especially if there isn't one!). The disadvantages are complexity (imagine all of that hardware and software in space) and cost. Odessey -- this is a TDMA-based system utilising Medium-earth orbit satellites (i.e. you don't need as many satellites, but the user terminals have to be much more powerful and there is more delay). I don't know anything more about this system. Maybe someone from Motorolla or TRW could clarify this a little? > As an aside, are TDMA and CDMA implementations of a technology known > generically as digital AMPS? All of the cellular technologies (except N-AMPS) are basically digital AMPS in that they are digital, use the AMPS call setup model and are based upon the original AMPS model (the architecture anyway). Just FYI, the main systems that are being pushed as successors to AMPS are: N-AMPS -- This system is based upon FM waveform (as is AMPS), but splits up the bands into sub-bands (three, I think) and also carries signalling data overlayed on the voice signal. This provides a 3:1 capacity gain over AMPS, but still has all of the old fading and error-correction problems of the old system (this is probably a biased opinion, since I have worked on both E-TDMA and CDMA ;-) TDMA - Known in the standards as IS-54, this uses a TDMA approach where each frequency band is divided into six repeating timeslots (frames). A user will be assigned two of these slots for conversation (or data etc.) i.e. there is a capacity gain of 3:1. Call setup is achieved using AMPS and then 'handed-off' to TDMA. E-TDMA -- Enhanced TDMA, developed by Hughes Network Systems, is an extension of TDMA where a 'pool' of (TDMA) frequencies are used to support many mobiles. The technology uses half rate vocoders (i.e. doubles capacity) and digital speech interpolation (i.e. slots are only assigned when needed). GSM -- the European equivalent of TDMA, but with a much more strongly defined architecture (on the network side) and a more integrated system. The system is fairly similar to TDMA, but doesn't rely on AMPS for call setup and has lots of other features built into the standard (data, FAX, messaging etc.). CDMA -- Code Division Multiple Access -- the relevant standard is IS-95. CDMA is a spread-spectrum technology i.e. instead of dividing the data into timeslots, the data is 'spread' across a wide band of frequencies using some pretty fancy coding tricks. All users can transmit on the same frequencies at the same time -- the signals are distinguished from each other by the coding schemes. Using this scheme, the number of users is limited by the coding, rather than the number of timeslots. I think the capacity gain is about 10:1 or more (over AMPS), but there have been plenty of articles in the trade magazines comparing CDMA to TDMA (and I'm no expert). Hope this helps, Phil