The Test loop mode feature in UMTS Layer 2 is classified into
Test loop mode 1
The test loop mode 1 can be applied on the PDCP module, if the radio bearer setup includes configuration of PDCP protocol layer.Then every PDCP SDU received by the UE on the downlink active radio bearer shall be taken from the output of the PDCP service access point (SAP) and be input to the corresponding PDCP SAP and transmitted uplink.Normally, when loopback is not configured it is sent to the higher layers.
If UE test mode 1 has been selected and radio bearer setup does not include configuration of PDCP protocol layer ,then the loop back will be for RLC SDUs. The RLC SDUs received in the downlink will not be sent to the upper layers,but instead data block received by the UE on the downlink active radio bearer shall be taken from the output of the RLC service access point (SAP) and be input to the corresponding RLC SAP and transmitted uplink.
Here four scenarios are possible
Use of Test loop mode 1
Measurement of Bit Error Rate(BER),a receiver characteristics can be carried out in test loop mode 1 in RLC TM. BER is calculated by the System Simulator (SS) in RLC Transparent mode by checking the downlink bits that is looped back in the uplink direction.Here in this case, UL and DL transport block size should be same.
Another receiver characteristic, Block Error Rate(BLER) can be measured using UE test loop mode 1, by having the DL RLC protocol operating in acknowledged mode (AM). The SS can calculate BLER from the ratio of number of UE retransmission requests and the total number of blocks sent by the SS.
Test loop mode 2
In test loop mode 2, transport block and CRC bits received in the downlink by the MAC layer will not be sent to RLC layer , but instead it will be sent in the uplink by selecting the corresponding uplink transport block.The way the downlink data being looped back over the MAC layer is implementation specific.
When the uplink transport block size is bigger than the sum of downlink transport block size and size of downlink CRC, the extra bits will be padded and sent.
When the uplink transport block size is smaller than the sum of downlink transport block size and size of downlink CRC,then the extra bits will be truncated and sent.
Use of Test loop mode 2
The System Simulator (SS) can calculate BLER in test loop mode 2 by checking returned data and CRC and count number of block errors and the total number of sent blocks.
Test loop mode 2 can be used for measurement of transmitter DTX characteristics. DTX bits are used in downlink rate matching. DTX indication bits only indicate when the transmission should be turned off, they are not transmitted.
The SS can use UE test loop mode 2 for testing UE blind transport format detection by taking the possible downlink transport formats into consideration when configuring the uplink transport formats. i.e. for every downlink transport format there should be an uplink transport format for which the transport block size is equal to the sum of the downlink transport block size and the number of downlink CRC bits. Thus the SS can check the TFI of the UE transmitted transport format to verify that the UE has detected the correct downlink transport format. When no TFCI is transmitted, the receiver side detects the transport format combination using some information, for example, received power ratio of DPDCH to DPCCH or CRC check results. This is called as Blind Transport Format Detection(BTFD).
More information on BTFD procedure can be found in the following URL http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_07/Docs/Pdfs/R1-99d38.pdf
3GPP TS 34.109 Terminal logical test interface V3.10.0 (2004-09)