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Compressed Mode
Before the handover, UE normally measure the cell power (signal quality) of the target cell and report it to the network, so that network can make a decision whether to allow UE to handover to the target cell or not. It is not a big issue to measure the signal quality of the target cell if the target cell is at the same frequency as the current cell (Intrafrequency measurement). But there would be an issue when the target cell is at a different frequency from the current cell (Interfrequency measurement). Just in terms of logical sense of view, the simplest solution for Interfrequency measurement, the simplest solution for this would be to implement two RF tranciever on UE. However, there are some practical problems with this kind of two tranciever solution. One of the problems is cost issue. It would require additional cost to implement the additional tranciever. The other problem would be the possible interference between the current frequency and target frequency especially when the current frequency and target frequency are close to each other.
So they come out with a special techique called "Compressed Mode". The idea of the compressed mode is to create a small gap during which no transmission and reception happens. since there is no signal transmission and reception during the gap, UE can switch to the target cell and perform the signal quality measurement and come back to the current cell. To make this work seamlessly, there should be a well established agreement between UE and Network about the gap definition (e.g, Starting Position of the Gap, Gap length, number of Gaps etc) and this agreement is established by a couple of RRC messages (e.g, Measurement Control, Physical Channel Reconfiguration etc). Followings are a couple of example configuration that was proved working in my test setp.
Measurement GAP setting for WCDMA to WCDMA Measurement
| | +-modeSpecificInfo ::= CHOICE [fdd] | | | +-fdd ::= SEQUENCE [011] | | | +-defaultDPCH-OffsetValue ::= INTEGER OPTIONAL:Omit | | | +-dpch-CompressedModeInfo ::= SEQUENCE OPTIONAL:Exist | | | | +-tgp-SequenceList ::= SEQUENCE OF SIZE(1..maxTGPS[6]) [1] | | | | +-TGP-Sequence ::= SEQUENCE [1] | | | | +-tgpsi ::= INTEGER (1..maxTGPS[6]) [1] | | | | +-tgps-Status ::= CHOICE [activate] | | | | | +-activate ::= SEQUENCE | | | | | +-tgcfn ::= INTEGER (0..255) [0] | | | | +-tgps-ConfigurationParams ::= SEQUENCE [000000] OPTIONAL:Exist | | | | +-tgmp ::= ENUMERATED [fdd-Measurement] | | | | +-tgprc ::= INTEGER (0..511) [0] | | | | +-tgsn ::= INTEGER (0..14) [4] | | | | +-tgl1 ::= INTEGER (1..14) [7] | | | | +-tgl2 ::= INTEGER OPTIONAL:Omit | | | | +-tgd ::= INTEGER (15..270) [270] | | | | +-tgpl1 ::= INTEGER (1..144) [12] | | | | +-dummy ::= INTEGER OPTIONAL:Omit | | | | +-rpp ::= ENUMERATED [mode0] | | | | +-itp ::= ENUMERATED [mode0] | | | | +-ul-DL-Mode ::= CHOICE [ul-and-dl] | | | | | +-ul-and-dl ::= SEQUENCE | | | | | +-ul ::= ENUMERATED [sf-2] | | | | | +-dl ::= ENUMERATED [higherLayerScheduling] | | | | +-dl-FrameType ::= ENUMERATED [dl-FrameTypeA] | | | | +-deltaSIR1 ::= INTEGER (0..30) [10] | | | | +-deltaSIRAfter1 ::= INTEGER (0..30) [5] | | | | +-deltaSIR2 ::= INTEGER OPTIONAL:Omit | | | | +-deltaSIRAfter2 ::= INTEGER OPTIONAL:Omit | | | | +-nidentifyAbort ::= INTEGER OPTIONAL:Omit | | | | +-treconfirmAbort ::= INTEGER OPTIONAL:Omit
Measurement GAP setting for WCDMA to LTE Measurement
| | +-modeSpecificInfo ::= CHOICE [fdd] | | | +-fdd ::= SEQUENCE [111] | | | +-defaultDPCH-OffsetValue ::= INTEGER (0..599) [0] OPTIONAL:Exist | | | +-dpch-CompressedModeInfo ::= SEQUENCE OPTIONAL:Exist | | | | +-tgp-SequenceList ::= SEQUENCE OF SIZE(1..maxTGPS[6]) [1] | | | | +-TGP-Sequence-r8 ::= SEQUENCE [1] | | | | +-tgpsi ::= INTEGER (1..maxTGPS[6]) [1] | | | | +-tgps-Status ::= CHOICE [activate] | | | | | +-activate ::= SEQUENCE | | | | | +-tgcfn ::= INTEGER (0..255) [0] | | | | +-tgps-ConfigurationParams ::= SEQUENCE [00000] OPTIONAL:Exist | | | | +-tgmp ::= ENUMERATED [e-UTRA] | | | | +-tgprc ::= INTEGER (0..511) [0] | | | | +-tgsn ::= INTEGER (0..14) [8] | | | | +-tgl1 ::= INTEGER (1..14) [10] | | | | +-tgl2 ::= INTEGER OPTIONAL:Omit // In 3GPP, TGD value can be '15-269'. // So, 270 specified here would be interpreted as 'Undefined' // 'Undefined' is also valid setting in 3GPP | | | | +-tgd ::= INTEGER (15..270) [270] | | | | +-tgpl1 ::= INTEGER (1..144) [12] | | | | +-rpp ::= ENUMERATED [mode0] | | | | +-itp ::= ENUMERATED [mode0] | | | | +-ul-DL-Mode ::= CHOICE [ul-and-dl] | | | | | +-ul-and-dl ::= SEQUENCE | | | | | +-ul ::= ENUMERATED [sf-2] | | | | | +-dl ::= ENUMERATED [sf-2] | | | | +-dl-FrameType ::= ENUMERATED [dl-FrameTypeB] | | | | +-deltaSIR1 ::= INTEGER (0..30) [20] | | | | +-deltaSIRAfter1 ::= INTEGER (0..30) [10] | | | | +-deltaSIR2 ::= INTEGER OPTIONAL:Omit | | | | +-deltaSIRAfter2 ::= INTEGER OPTIONAL:Omit | | | | +-nidentifyAbort ::= INTEGER OPTIONAL:Omit | | | | +-treconfirmAbort ::= INTEGER OPTIONAL:Omit
HEX (Physical Channel Reconfiguration) : FB 7A 64 2B 12 A7 82 00 00 00 C1 30 80 80 1D C0 01 00 00 60 04 4F F8 59 1D 14 08 04 80 00 04 D8 00
Let's look into the definition of each parameters used about Compressed Mode (Refer to 10.3.6.33 DPCH compressed mode info for the detailed description and valid value range) .
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