UMTS Overlaid on GSM

Some network operators will deploy a green-field UMTS network. For
many, however, UMTS will be deployed alongside an existing GSM network.
Those operators will wish to reuse the components of the GSM network to
the greatest extent possible. There is a desire to reuse everything from cell
site locations to MSCs, SGSNs, and home location registers (HLRs). Because
of the fact that the core network of UMTS is essentially the same core network
as is used for GSM/GPRS, there is a significant opportunity to reuse
existing equipment. For example, a GSM MSC can be upgraded to simultaneously
support both GSM and UMTS. Similarly, SGSNs and Gateway
GPRS Support Node (GGSNs) can be upgraded to simultaneously support
both UMTS and General Packet Radio Service (GPRS).

In the radio access network, there is also some opportunity for reuse. For
most vendors, it will not be possible to upgrade a BSC to simultaneously
function as a BSC and an RNC. For base stations, however, several vendors
support both GSM and UMTS within the same base station cabinet. In such
a situation, it is possible for the GSM and UMTS transceivers to use the
same antennas. Even if a given vendor does not support both UMTS and
GSM transceivers within the same cabinet, or if the UMTS and GSM systems
are provided by different vendors, there may still be the opportunity
to co-locate a UMTS base station cabinet with a GSM base station cabinet.
This can reduce site acquisition costs and some construction costs.
For GSM systems operating at 1800 MHz or 1900 MHz, the footprint of
a UMTS cell and a GSM cell are very similar. In fact, for a cell loading factor
of up to approximately 65 percent, the footprint of a UMTS cell for voice
service is slightly greater than the equivalent footprint of a GSM cell. For
GSM900, however, the difference in frequencies is such that the GSM signal
propagates a great deal further, which means that the coverage of a
UMTS cell will be less than that of the GSM cell. Thus, when deploying
UMTS over an existing GSM900 network, extra cell sites will be required
for UMTS. In urban areas, the number of extra UMTS cell sites is likely to
be quite limited as the GSM sites will have been deployed in a more dense
arrangement for capacity reasons rather than just for coverage reasons. In
rural/highway areas, however, there will need to be many more UMTS sites
than GSM900 sites, simply because of less attenuation for the lower frequency
GSM900 signal.

In the case where a GSM base station and UMTS base station are colocated,
or even share the same cabinet, then they can also share the transmission
facilities back towards the BSC and RNC. Figure 12-11 shows an example of how this can be done. In that example, a UMTS cabinet is colocated
with an existing GSM cabinet. The GSM cabinet already has a T1
connection back to the BSC. Given that a GSM base station requires
between two and three DS0s per transceiver, it is quite possible that the T1
is not fully used. In fact, less than half of the T1 might be used, as would be
the case for, say, a three transceiver GSM BTS. Provided that the expected
Iub bandwidth requirement will consume less than the remaining bandwidth,
then we can use that fractional T1 capacity for the Iub interface. In
other words, we carry ATM on a fractional T1. Back at the BSC/RNC location,
we need to have a cross connect that can perform DS0-level grooming.
That cross-connect strips out that part of the T1 that is used by the GSM
BTS and sends it to the BSC. The part of the T1 that carries the Iub interface
is sent to the RNC. Of course, we also need a mini cross-connect at the
base station site. For many GSM base stations, such one-card devices are
available as it is not uncommon in many countries to daisy-chain GSM base
stations in order to reduce transmission cost.

Of course, we are likely to find that this type of transport sharing will be
possible only for those sites that expect relatively low demand. If we are cositing
a UMTS base station with a GSM base station in an urban area, for
example, we may find that the GSM base station is already consuming
more than half of a T1 (as would be the case for a six-transceiver base station).
We are likely to find that the UMTS base station will also require
more than half of a T1, particularly when we consider the overhead that the Iub interface needs to include. In that case, we have little choice but to
increase the transport bandwidth to the site.