Mobile users need more capacity and higher velocity to work with new data applications. In 

order to satisfy these requirements, mobile systems of third generation were created and they 

are established every day in more places. Consequently second and third generation of system 

have to coexist. 

In this paper is presented a study about the co-existence between GSM and UMTS, the most 

popular systems in Europe. 900MHz is the frequency chosen to work with both systems, due to 

this low frequency allows: less path loss and more coverage, than 2100MHz the usual 

frequency for UMTS system.  

The situation evaluated is focused on introducing a UMTS carrier in a GSM operator, with the 

aim to determine the advantages and cons produced in the mobiles with this change. Two 

cases are distinguished: coordinated operation, GSM and UMTS900 in the same operator and 

un-coordinated operation, UMTS900 respect to other GSM operator. Users under study belong 

to both modified operator and a next GSM operator. Power control was programmed in both 

systems. 

It was determined how affect the interferences in system capacity of the operators and which 

is the guard band necessary to protect the system. Spectral efficiency (user/cluster/MHz) in 

the case GSM and UMTS900 in same band is twice as much as in a GSM operator. The 

separation between operators is set in 300KHz.But is important to know that UMTS carriers do 

not cause damage in others operators because is located in the middle of GSM carriers. The 

guard band fixed to protect each mobile in operator with shared spectrum is not necessary. 

The distance between carriers GSM and UMTS necessary is 2.4MHz while the width of one 

UMTS carrier is 2.5MHz.