I have heard that EDGE transeiver is more difficult to design than GSM?
But in my team's design, they just use the same circuits.
What is the main difference between GSM and EDGE, in system level?
Many thanks!

The fundamental system difference that caused transceiver development issues is that EDGE modulation contains some amplitude modulation and GSM does not.

GSM only transmitters were developed to phase modulated a VCO. The VCO signal was buffered and sent to a PA without requiring a RF filter between the transmitter and PA.

Then EDGE came along and introduced amplitude modulation, so the dilemma became either how to get amplitude modulation from the existing system or scrap the existing system and go to a linear modulator like a I/Q quadrature modulator.

Either choice was a relatively difficult one to make for anyone trying to decide which way to go. It was really difficult to design an I/Q modulator based system that could meet all the GSM requirements and also not require an interstage filter between the transmitter and PA.

The savior of the original GSM phase modulated system was the golden era of "polar modulators." Polar modulation was/is the popular approach to adding  amplitude modulation to an existing GSM phase modulator. Only it turned out that matching the amplitude and phase modulation is tricky business that results in nonlinear distortion if the errors are not continually compensated.

Polar modulation has other system issues that makes it unattractive for amplitude modulation with wider bandwidths, so it turned out that the investment into GSM/EDGE polar modulation did not carry forward to 3G/4G transceiver development. So, in the end, some companies had to pay the development costs of both systems.

WCDMA, CDMA2000 EVDO, WiMAX, WLAN and LTE use linear I/Q modulation.

I hope this was informative.

That's good info - thanks!