What are the important considerations in selecting a radio transceiver (and power amplifier) for the design?
Any radio chip must have minimal spurious outputs so that it can easily pass the testing procedures imposed by regulatory bodies. The receiver should also have as much sensitivity as possible because this greatly extends the handset's useful range and minimizes dropouts. Both direct-conversion (zero IF) and low-IF designs work well. Multimode designs tend to favor direct conversion.
What mixed-signal circuits are important in the design?
These include the analog-to-digital and digital-to-analog converters that link the RF transceiver and the baseband circuits, as well as other converters for AGC, RF power control, voice-band codecs, amplifiers, and battery monitoring and control. It is desirable to have all of these mixed-signal devices fully integrated.
What should I look for in a baseband chip?
Baseband chips are pretty well defined by the standard and usually include a DSP for the voice encoding/compression-decompression, equalization, modulation, and demodulation and a control processor for the protocol stack and user interface. Power consumption is critical, but there's a need for high speed as well. Look for flexible interfaces so features like cameras and other peripherals can be added easily.
Is it best to try to source all the chips from one supplier or mix and match?
There are obvious benefits to using a single supplier for all of the chips. The interfaces, control software, and other parts will have been tested, making it easier to get the chips to work together, and if bugs arise it is clear who to call for support. Time-to-market improvements far outweigh any small cost savings, because market windows in the cell-phone business are very narrow.