A multi-mode mobile phone is a phone which is designed to work on more than one mobile phone standard. Multimode phones have been valuable to enable roaming but are now becoming most important in allowing the introduction of WCDMA without customers having to give up the wide coverage of GSM. Almost every single true 3G phone sold is actually a WCDMA/GSM dual-mode mobile. This is also true of 2.75G phones such as those based on CDMA-2000 or Edge.
The special challenge involved in producing a multi-mode mobile is in finding ways to share the components between the different standards. Obviously, the phone keypad and display should be shared, otherwise it would be hard to treat as one phone. Beyond that, though, there are challenges at each level of integration. How difficult these challenges are depends on the differences between systems. The different variants of the GSM system have only different frequencies and so aren't even considered true multi-mode phones but rather are called multi-band phones. When talking about IS-95/GSM multi-mode phones, for example, or AMPS/IS-95 phones, the base band processing is very different from system to system. This leads to real difficulties in component integration and so to larger phones.
An interesting special case of multi-mode phones is the WCDMA/GSM phone. The radio interfaces are very different from each other, but mobile to core network messaging has some quite strong similarities, meaning that software sharing is quite easy. Probably more importantly, the WCDMA air interface has been designed with GSM compatibility in mind. It has a special mode of operation, known as punctured mode, in which, instead of transmitting continuously, the mobile is able to stop sending for a short period and try searching for GSM carriers in the area. This mode allows for safe inter-frequency handovers with channel measurements which can only be approximated using "pilot signals" in other CDMA based systems.
A final interesting case is that of mobiles covering DS-WCDMA and MC-CDMA the 3G variant of CDMA-2000. Initially, the chip rate of these phones was incompatible. As part of the negotiations related to patents, it was agreed to use compatible chip rates. This should mean that, despite the fact that the air and system interfaces are quite different, even on a philosophical level, much of the hardware for each system inside a phone should be common with differences being mostly contained to software.
Rrticle originally published on http://en.wikipedia.org
