900Mhz UMTS is not the great saviour everyone is expecting. The lower the frequency, the more the cell will breathe, and the more erratic the service will become. The networks like it because it has greater area coverage, which means it saves them money when they roll out because they can put the base stations further apart. Great in rural areas, but for city centers, my preference would be the 2100 band where power modulation is better.
Oops. It has been (not so subtely) pointed out to me that I may have been a bit harsh here. I don't want it to come across that I am anti 900mhz UMTS. Let me explain :
1. The 900Mhz spectrum that is assigned to the Mobile operators is currently mainly used for GSM. In order to use UMTS in this spectrum, they will have to rob Paul to pay Peter, so to speak. In other words, fewer time slots will be available for users of older phones that would not automatically hunt up to the 1800Mhz spectrum.
2. In an urban rollout, you would not get the coverage benefit because of noise pollution. The network engineers constantly "tune" the power output of the Base Stations so that there is not too much overlap. In terms of users supported, a 900Mhz UMTS allocation will essentially carry the same amount of users with similar coverage areas as the 2100mhz allocation because they will have to "shrink" the 900mhz coverage to reduce noise pollution with the GSM transmitters. In addition to this, the power modulation issue - the greater the area coverage of the cell, the more handsets will be in it. In an urban or city situation, you would quickly raise the noise floor to a point where the power will have to be adjusted to drop users out the cell (purely because of population density). It would boil down to a benefits analysis in urban situations : if the 900mhz spectrum is underutilised on the GSM network, it makes sense to use part of the spectrum for UMTS, and, in urban areas at least, it would push more GSM users onto the 1800mhz band. If you have capacity there, it makes sense. It would also depend on what the ratio is on the handset side between 3G capable phones with a 900Mhz radio and non-3G phones. Few 3G phones support the 900mhz spectrum at the moment...
3. In rural areas, where you do not have a lot of phones in the cell, and noise pollution and ever raising noise floors are not an issue, it makes sense to "part out" the 900mhz spectrum between UMTS and GSM. It is a lot easier (and cheaper!) to do it this way as you do not need to increase base station density to get effective 3G coverage (3G coverage that is more or less the same as existing 2G coverage). This would extend 3G coverage to people who would otherwise just have 2G coverage in these areas, and it would mean using the same high sites.
4. I am all for the gradual retiring of the GSM networks. UMTS is for more efficient with allocated spectrum, and soft handover is far better on UMTS than it is on (hard handover) GSM. On CDMA based technology, handsets talk to multiple base stations at once and the handset chooses the stream with the best quality, dynamically. You are seldom aware of handover. With TDMA (GSM) the base station decides when to hand you over to the next base station - your phone constantly reports its coverage to the base station and what the signal strength is to them, in order for the base station to make a decision. Handover is not as seemless - at best you have a blip in your conversation, at worst your call is dropped.
Hope this helps clarify my statement
