Response to Mostafa : Actually depends on the equipment you are using. Regulatory bodies in the country may also restrict the maximum EIRP and hence the output power after the PA.
Response to Sachin : The term syncronization is not correct in the context. The line of thought you were following is correct.
The higher the frequency, the high the path loss. The power-limiting link when calculating the link budgets for GSM is in most cases (where the BTS has output power far larger than MS)the uplink hence in an attempt to favour the weaker link, the lower frequencies have been assigned to the uplink.
Note that this is applicable to almost all wireless communications systems.