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Figure 3Interoperability of mobile terminals andnetwork equipment from different vendorsis crucial for handover between WCDMAand GSM. Ericsson network equipment, incommercial operation in Japan since 2002(here represented by the Indoor MacroRBS 3202), has been publicly shown (at3GSM World Congress in Cannes, February18-21) to interoperate in WCDMA withall major mobile terminal vendors—hererepresented by two Sony Ericsson Z1010terminals.Figure 4Overview of the GSM and WCDMA nodesand interfaces involved in the cell reselectionand handover procedures.As mentioned above, a mobile terminal inWCDMA makes use of compressed mode tomeasure GSM. That is, if the mobile terminalhas a single radio receiver, it requirescompressed mode. If, on the other hand, themobile terminal contains separateWCDMA and GSM radio receivers, it canuse each receiver in parallel, performingGSM measurements without compressedmode in the downlink. Notwithstanding,each solution—compressed mode and dualreceivers—reduces talk time due to higherpower consumption in the terminal.In idle mode, standby time of the mobileterminal is mainly affected by how often itneeds to wake up to monitor radio channelsand perform measurements for cell reselection.Since a dual-mode terminal mustmeasure WCDMA and GSM cells, this hasa negative effect on standby time comparedto GSM-only mobile terminals. To improvestandby time, the mobile terminal is allowedto inhibit measurements on the otheraccess technology (for example, WCDMAwhen in GSM) when the quality of the currentaccess technology is adequate for thenetwork settings. Furthermore, comparedto re-selection between GSM cells, the measurementrequirements in the standard aremore relaxed for re-selection betweenWCDMA and GSM cells.Mobility procedures for interworkingbetween WCDMA and GSMThere are two basic modes of operation forhandling mobility:• the mobile terminal-controlled mode;and• the network-controlled mode.In the mobile terminal-controlled mode, themobile terminal selects the cell to which itwill connect. However, the network canbroadcast various parameters to influencethis process.In the network-controlled mode, the networkexplicitly orders the mobile terminalto connect to a specific cell. Ordinarily, thenetwork bases its decisions on measurementinformation provided by the mobile terminal.For either mode of operation, the networkshould consider cells that use each accesstechnology. Besides radio link quality,the network might also consider other aspectswhen selecting the cell, for example,the current load of the established service.Two procedures have been defined bywhich the network can order the mobile terminalto connect to a cell using another technology,namely the handover and cellchange order procedures. These are employedwhen the mobile terminal uses a dedicatedchannel. The handover procedureprovides a higher level of service, since it involvesa preparation phase in which resourcesin the target cell are reserved priorto the actual handover. Accordingly, thehandover procedure is employed when themobile terminal is providing circuitswitchedservice—for instance, voice. Thecell change order procedure applies whenthe mobile terminal is providing packetswitchedservice, such as Web browsing.Cell re-selection betweenWCDMA and GSMWhile in WCDMA, the mobile terminalperforms cell re-selection• in idle mode; and• in connected mode when common channelsare used for packet-switched service.The dual-mode mobile terminal re-selects aGSM cell when that cell is ranked higherthan the current WCDMA cell or any otherWCDMA cell. WCDMA and GSM cells areranked together according to signalstrength. This same type of ranking appliesin GSM.When performing cell re-selection inWCDMA, the mobile terminal either measuresGSM cells continuously or when thequality of the serving WCDMA cell fallsbelow a given threshold. The mobile terminalis solely allowed to select a newWCDMA or GSM cell when the average receivedquality and average signal strengthexceed a minimum threshold. Theminimum-quality threshold (signal-tonoiseratio) ensures that the mobile terminalcan receive the information transmittedEricsson Review No. 1, 2003