Frequency Coordination Between UMTS and GSM Systems at 900 MHZ

UTRA-FDD requirements for operation in the 900 MHz band have been recently standardized in 3 GPP paving the way for refarming of GSM spectrum to UMTS. UMTS900 networks will have to co-exist with GSM900 networks for some time. 3GPP (2005) has studied co-existence issues and has concluded that GSM900 and UMTS900 can co-exist in both coordinated and un-coordinated deployments in which the UMTS900 carrier is allocated 5 MHz in addition to a guard-band equal to 1 GSM channel (200 KHz) on each side. However, in some deployments the operator may not have sufficient available GSM spectrum to allow the GSM traffic to be offloaded from the spectrum allocated to UMTS900 to the remaining GSM spectrum. Hence of interest is to assess system performance under much more stringent spectrum clearing assumptions. Here we characterize transmitter and receiver performance based on lab tests conducted on commercial equipment, both UMTS900 and GSM900 terminals and base stations. From such measurements we assess the impact of mutual interference, GSM MS with UMTS NodeB and UMTS UE with GSM BTS, on receiver performance as a function of frequency offset and coupling loss between interfering transmitter and offended receiver. We show that the limiting factor is the interference caused by the GSM MS to the UMTS Node B, and that as little as 4.2 MHz of GSM spectrum can be cleared and allocated to one UMTS carrier with satisfactory system performance.