In comparing GSM to WCDMA/UMTS there are several aspects to consider. The first is that GSM is a combination
FDMA/TDMA system, where UMTS is a CDMA only system with no requirement for FDMA – this means that
any one for one MHz spectrum comparison is not possible.
WCDMA/UMTS = occupies 5MHz / channel – note however that the adjacent cell/base station occupies the same 5MHz – so there is no Frequency Division required. Every cell operates on the same frequency. Each cell has a voice only capacity around the 60-80 circuit mark for CS voice with all CS64, PS and HS bearers turned off – although this varies with distance due to one of the limiting resources being RF Ampplifier Power – so the general assumption is that traffic is distributed radially from the site fairly uniformly. (real world conditions can temper this).
GSM = occupies 200Khz/channel – but cant have an adjacent cell on the same channel without rendering C/I inoperable. So when first setting up a comparison we need to consider how much of an equivalent 5MHz bandwidth would be available to a signle GSM cell in a network. If we consider say a 4/12 re-use plan, each cell can have 3 frequencies reused three other times for three adjacent cells. 3 Frequencies gives GSM 24 circuits per cell. Effectively GSM implies that only 600KHz of the available 5000KHz can be used by one cell in any given location because of the need for FDMA in the GSM system.
Comparing capacity then, you can see that for the same 5MHz on a single cell in a multi-cell network 70 / 24 ~ 3 times more capacity available on a WCDMA network on the same or similar frequency band as GSM. (cf 850MHz UMTS vs 900MHz GSM).
As a bonus, when you compare similar band UMTS vs GSM, you get between 5-7dB improved depth of coverage because the UMTS system’s Ec/No requirements are about half those of GSM. If you wanted to trade that for capacity per sqkm then you can increase your capacity over GSM even further.