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Last week Singtel/Optus announced it would be expanding UMTS (the dominant 3G standard worldwide) into regional/rural Australia, to compete against the “incumbent” (as they call Telstra). Let’s delve into some technical details on this topic.

Optus are investigating the possibility of deploying UMTS in the recently defined UMTS Band VIII (NodeB Tx: 925-960MHz, UE Tx: 880-915MHz). This is the exact same spectrum as EGSM900 (and deliberately so, to allow owners of GSM900 spectrum to transition to UMTS). For Optus, this means in regional/rural areas, Optus can re-allocate some of the EGSM900 spectrum to UMTS with no additional spectrum purchase required.

As it stands now, UMTS is offered in its original Band I (1900MHz up / 2100MHz down) by the Hutchison-Telstra alliance and the Optus-Vodafone alliance in metropolitan Australia. Additionally, Telstra’s heavily promoted ‘Next G’ operates on UMTS band V (850MHz), which operates on the exact same frequencies as used by GSM850 (again, for reasons discussed earlier). UMTS850 is not a commonly-used – yet - UMTS band - the only way Telstra can feasibly use this is because Cingular Wireless-AT&T in the United States (with over 60 million subscribers) has chosen to use this band, as it owns existing GSM850 spectrum.

With a 3rd UMTS band in Australia, 2 of which will be predominantly used in rural Australia, to be able to simply “change SIMs” to a new operator, as many do already with GSM phones, the handset will need to offer both the UMTS850 and UMTS900 bands (and preferably the UMTS2100 band as well).

Some believe adding a 3rd UMTS band in Australia may be a bad move. Whilst I’m not a big fan of expanding the number of bands used either (UMTS has no less than 10 bands defined in the most recent version of the standard - admittedly 2 of them are strict subsets of other bands), because the proposed final set of bands used in Australia will be 3 of the most popular in the world, I am confident the handsets covered will cover at least these 3 in much the same way as we have tri and quad band GSM handsets today.

The USA has a similar situation, where operators are using different UMTS bands based on spectrum availability. Like us, UMTS will be offered on 3 bands – UMTS850, UMTS1900 and UMTS Band X. The last is a special case of UMTS at 2100MHz, where the uplink from the UE to the Node B occurs on 1700MHz, rather than the 1900MHz for Band I. This is being done by operator T-Mobile due to constraints on the spectrum it owns.

How will UMTS900 affect GSM900 performance?

This is another question that I have been asked. Optus currently own about 41 contiguous physical channels, or about 1/3rd (the middle third) of the entire GSM900 band, which is at 200kHz/channel just over 8MHz per direction (uplink/downlink). UMTS operates on much wider 5MHz channels per direction, but multiple base stations in range of each other can share the same physical channel, due to the CDMA nature of the system. So a single 5MHz channel can be used – indeed that is all that will fit.

Optus would likely use a ‘coordinated’ scheme of roll-out in rural areas, where there is a 1:1 mapping between GSM900 and UMTS900 base stations (this significantly reduces inter-system interference considerations). However, a guard band is still needed between UMTS and GSM channels, which is recommended to be 2.8MHz (UMTS carrier – GSM carrier). So the actual practical bandwidth occupied is about 5.6MHz, leaving room for perhaps 2.4MHz = 12 channels of operation. In a rural environment few channels are typically used as capacity is not the major design factor but this is certainly a tight fit for GSM.

For an in-depth look at this general scenario see here and here.