Why does 3G mobile cellular signal use more power than 2G?

2G (original GSM) was deliberately designed to be very power-efficient only for low-data rate voice services.   3G was designed to be more spectrally efficient and to deliver higher-rate data services, but at the expense of using more power.

Specifically:
- 2G uses 200kHz channel vs 5MHz for WCDMA (25x more). Power is roughly proportional to bandwidth
- GSM used a very power-efficient constant envelope modulation scheme (GMSK). 3G uses CDMA and higher order modulation (up to 16QAM or even 64QAM), which needs much better linearity and hence the radio draws far more power.  2.5G (GPRS) and 2.75G (EDGE) use QAM, which deliver more data and are more spectrally efficient (bps/Hz) so they coverge with 3G somewhat.

You can improve battery life significantly by turning of 3G and only using 2G if you stick to voice  but if you are going to web-surf then the better data-rate (bps) and data efficiency probably make 3G more efficient.

This trend continues: 4G offers far faster data rates and is more spectrally efficient, but draws even more power.


Note: the above discussion is for GSM to WCDMA; for IS95/cdma2000 the situation is similar but the specifics are different
Check out on out post about Wifi and 3G battery Usage Here

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