Why TX frequency is lower than RX one in mobile communication?

For mobile communication, TX frequency is always lower than RX frequency. Does anybody know this? and can give reasonable explaination. Thanks

That's not always the case. It depends on frequency allocations and protocols. For example, CDMA2000 has both situations, depending on the band class. This is probably determined by the services with adjacent allocations.

To comply with FCC frequency allocation:
If you are interested, check out at http://www.fcc.gov/oet/spectrum/table/

Hi,
for example in INMARSAT,Rx is lower than Tx.
Tx 1626.5 - 1660.5 MHz
Rx 1525.0 - 1559.0 MHz

For GSM, say EGSM, TX frequency is 880~915MHz and RX frequency is 925~960MHz. Are there some special considerations? Say, channel characteristics or RF issues.

There's no special design consideration that leads to commonly used frequency bands.

Don't forget that in Frequency Division Duplex (FDD) systems (like GSM) the Base-Station TX is Mobile-Station RX and vice-versa...

mogwai.

One thing that drives the frequencies is economics. There are more mobiles than bases. Therefore the system cost is lowest when the mobiles are lower cost. Generally, lower frequencies require hardware with lower costs. Also lower frequencies have more efficient power amplifiers which makes battery life longer.