Combined (3G & 2G) Compressed Mode Patterns

Key functionality in WCDMA is the use of compressed mode which creates measurement gaps that allow the UE to measure either 2G or 3G inter-frequency neighbours. Typically most UTRAN implementations restrict the usage of compressed mode to either 2G measurements or 3G inter-frequency measurements.

As almost all 3G network deployments today utilise multiple 3G FDD carriers, the operator has to make a choice. When radio conditions deteriorate, should traffic be handed over to another 3G FDD carrier or to 2G? Handing over to 2G is often seen as the safe choice to make, but why re-direct traffic to 2G when potentially other 3G carriers of good radio quality exist?

This problem can be solved by the use of combined (3G & 2G) compressed mode patterns, which is what I recently observed on a live network. With combined compressed mode patterns the UE is asked to measure both 2G and 3G inter-frequency neighbours at the same time. Based on the measurement reports received the RNC can then decide to which layer to handover the UE to (with an obvious preference to 3G if possible). The RRC signalling flow can be seen below (click to enlarge).

The procedure is initiated by the UE sending a  MEASUREMENT REPORT for event 2d (estimated quality of used frequency is below a certain threshold). The RNC responds by sending a PHYSICAL CHANNEL RECONFIGURATION message which configures the compressed mode patterns. The UE accepts these and then two MEASUREMENT CONTROL messages are sent. The first provides a 3G neighbour list for the UE to measure (and activates the compressed mode patterns) and the second provides the 2G neighbour list for the UE to measure. Following this the UE starts sending periodic MEASUREMENT REPORTS (separate ones for 2G & 3G) indicating which (if any) neighbours it has detected.

An extract of the PHYSICAL CHANNEL RECONFIGURATION message is provided below.

dpch-CompressedModeInfo
                            {
                              tgp-SequenceList
                              {
                                {
                                  tgpsi 2,
                                  tgps-Status deactivate : NULL,
                                  tgps-ConfigurationParams
                                  {
                                    tgmp gsm-CarrierRSSIMeasurement,
                                    tgprc 0,
                                    tgsn 4,
                                    tgl1 7,
                                    tgd 270,
                                    tgpl1 8,
                                    rpp mode1,
                                    itp mode0,
                                    ul-DL-Mode ul-and-dl :
                                      {
                                        ul sf-2,
                                        dl sf-2
                                      },
                                    dl-FrameType dl-FrameTypeA,
                                    deltaSIR1 12,
                                    deltaSIRAfter1 6
                                  }
                                },
                                {
                                  tgpsi 3,
                                  tgps-Status deactivate : NULL,
                                  tgps-ConfigurationParams
                                  {
                                    tgmp gsm-initialBSICIdentification,
                                    tgprc 0,
                                    tgsn 4,
                                    tgl1 7,
                                    tgd 270,
                                    tgpl1 8,
                                    rpp mode1,
                                    itp mode0,
                                    ul-DL-Mode ul-and-dl :
                                      {
                                        ul sf-2,
                                        dl sf-2
                                      },
                                    dl-FrameType dl-FrameTypeA,
                                    deltaSIR1 12,
                                    deltaSIRAfter1 6,
                                    nidentifyAbort 66
                                  }
                                },
                                {
                                  tgpsi 1,
                                  tgps-Status deactivate : NULL,
                                  tgps-ConfigurationParams
                                  {
                                    tgmp fdd-Measurement,
                                    tgprc 0,
                                    tgsn 4,
                                    tgl1 7,
                                    tgd 270,
                                    tgpl1 8,
                                    rpp mode1,
                                    itp mode0,
                                    ul-DL-Mode ul-and-dl :
                                      {
                                        ul sf-2,
                                        dl sf-2
                                      },
                                    dl-FrameType dl-FrameTypeA,
                                    deltaSIR1 12,
                                    deltaSIRAfter1 6
                                  }
                                }
                              }

As can be seen 3 TGPSI (Transmission Gap Pattern Sequence Identities) are configured. TGPSI 1 is for FDD measurements (i.e. other 3G frequencies), TGPSI 2 is for 2G RSSI measurements and TGPSI 3 is for initial BSIC identification of the 2G cells.

The compressed mode IEs for each TGPSI, result in a measurement gap as shown at the top of the post (for TGPSI 1). This essentially consists of a single measurement gap, 7 slots in duration (4.67ms). The other TGPSI are configured in an identical fashion. The overall repeating pattern is shown below (click to enlarge).