How much spectrum is used in the US today for GSM/CDMA/UMTS today vs. in a typical country in Europe? Here's a couple of thoughts on this:
In the US there are mainly two bands used today:
850 MHz US (vs. 900 in Europe)
First, there's the 850 MHz band, used for GSM, CDMA and UMTS services. The bandwidth is 25 MHz according to this link. From my point of view that pretty much compares with the usage of the 900 MHz band in Europe which has a bandwidth of 35 MHz. Here, however, it is used for GSM only in the majority of cases and only few countries such as Finland and France also use it for UMTS in rural areas. In practice I guess it is quite tough for US carriers to squeeze both voice and broadband data services in a 25 MHz band.
1900 MHz Us (vs. 1800 in Europe)
The second major band used in the US is the 1900 MHz band with a bandwidth of 60 MHz. Again, this compares to the 1800 MHz band in Europe, which is 75 MHz wide. How much of these bands is used on each side of the Atlantic I am not quite sure but I would assume that in the US quite heavy use is made as based on it's width it must be the main band for data services. In Europe on the other hand, the 1800 MHz band is mostly used for voice services and quite a bit is still unused today. That might change though as a number of carriers have announced that they will start using the 1800 MHz resources for LTE.
And then there is this third band, in the 1700 MHz area for uplink transmissions and in the 2100 MHz area for downlink transmissions with a width of 45 MHz. Today, as far as I know, only T-Mobile uses it for UMTS services but other major carriers have significant (unused) holdings as well as discussed in a previous post. As only one network operator uses it, I don't think it can be compared today to the 2100 MHz band in Europe which has a width of 60 MHz and is used intensively by 3 to 4 network operators per country for UMTS services.
Other bands and the future
Yes, other bands should not be forgotten but from what I can tell they are not playing an important role just yet. There's around 30 MHz of spectrum in the 700 MHz range in the US that Verizon has just started to use 10 MHz for its LTE services. And then there is the 2500 MHz band used by Clearwire for WiMAX. The former compares to the 800 MHz digital dividend band in Europe that has just started to see intensive usage by a number of operators. A definite advantage on the European side is that the 30 MHz are all in one chunk while the 700 MHz resources in the US are cut into several chunks with uplink and downlink turned bottom up on one chunk, probably to protect TV transmissions. That probably complicates transceiver design for devices aiming to support more than just one chunk compared to the straight forward approach of the 800 MHz digital dividend band. And then on the European side of course, there's the 2600 MHz "LTE" band with a total of 70 MHz of bandwidth.
No doubt, this view is a bit "Europe biased" but when looking at it I can't help the impression that things are a bit smoother and broader outside the US!?