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Figure 1Encapsulation of the GSM handover messagein a "container" that is part of theWCDMA handover message.sages. The pre-defined WCDMA radiochannel configuration describes bit rates,data block sizes and other radio parametersof voice or video call service.Although the network solely communicateswith the mobile terminal using one accesstechnology at a time, the mobile terminalneeds to perform measurements onGSM while communicating in WCDMAand vice versa. Since WCDMA uses continuoustransmission and reception in activemode, a regular mobile terminal cannotmeasure GSM cells while communicating inWCDMA. To overcome this obstacle,Ericsson has introduced what it calls thecompressed mode method. As seen in Figure2, a short gap is created in transmissionand reception. To maintain a perceived constantbit rate, the actual transmission bitrate is increased just before and after the gap.A constant bit rate is required for servicessuch as voice, but for Web browsing andsimilar services, a constant bit rate is notnecessary. In the latter case, the transmissioncan thus be delayed to create a gap.When the mobile terminal is not in activemode it uses discontinuous transmissionand reception, and can therefore measureGSM cells.results are compared with a technologyspecificthreshold. Furthermore, additionalparameters, such as adjustable offsets, areprovided to control the selection betweenWCDMA and GSM cells. Where the GSMmeasurement of WCDMA cells is concerned,the main challenge is to fit the informationinto existing GSM messages.Figure 2Compressed mode creates gaps or idle spaces in time that WCDMA mobile terminals useto perform measurements on GSM cells.Since WCDMA and GSM are differenttechnologies, it is difficult to compare themeasurement results from these technologies.To overcome this challenge, the measuredEricsson Review No. 1, 2003 7