10 Repeated Downlink FACCH

3GPP44.006Data Link (DL) layer specificationMobile Station - Base Station System (MS - BSS) interfaceRelease 17TS

10.1 General

The Repeated Downlink FACCH functionality is applicable when sending LAPDm frames on the TCH/F or TCH/H channel (excluding O-TCH and E-TCH channels). An MS supporting the TCH/F or TCH/H, as described above, shall support the Repeated Downlink FACCH functionality. The BSS may use the Repeated Downlink FACCH functionality when it considers it to be appropriate.

10.2 The FACCH Repetition

A repeated FACCH block shall be sent in such a way that, if the first burst of the downlink FACCH block containing the first instance of a LAPDm frame is sent in TDMA frame M, the first burst of the downlink FACCH block containing the repeated instance of the LAPDm frame is sent in TDMA frame M+8 or M+9 (the latter corresponding to the case where the two FACCH blocks are separated by either a SACCH frame or an idle frame, see 3GPP TS 45.002).

NOTE: On a channel using Repeated Downlink FACCH the value of T200 should be increased to cope with the case where the receiving entity failed to decode the downlink FACCH block used to send the first instance of a repeated LAPDm frame. This applies to a BSS supporting Repeated Downlink FACCH and to an MS that has signalled the Repeated ACCH Capability bit as ‘1’ (see 3GPP TS 24.008).

10.3 BSS requirements

The BSS may, based on implementation-dependent criteria (e.g. downlink measurements), decide to repeat any downlink LAPDm command frame sent in a FACCH block over the radio interface. If the MS has signalled the Repeated ACCH Capability bit as ‘1’ (see 3GPP TS 24.008), the BSS may also, based on implementation-dependent criteria, decide to repeat any downlink LAPDm response frame sent in a FACCH block over the radio interface. In both cases, the repeated FACCH block shall be sent as specified in sub-clause 10.2.

NOTE: If an MS has not signalled a Repeated ACCH Capability bit as ‘1’ the BSS may only use Repeated Downlink FACCH to send command frames.

10.4 MS requirements

The MS shall, when receiving a downlink FACCH block, always attempt to decode it without combining with any previously received FACCH block.

If the current FACCH block is successfully decoded and an identical FACCH block was previously received (successfully decoded and spaced in time from the current FACCH block as specified in sub-clause 10.2), the MS shall not send the LAPDm frame of the current FACCH block to the LAPDm entity.

If the current FACCH block is successfully decoded and there was no such previously received identical FACCH block the LAPDm frame of the current FACCH block is sent to the LAPDm entity.

If the current FACCH block is unsuccessfully decoded and there was an unsuccessfully decoded FACCH block spaced in time from the current FACCH block as specified in sub-clause 10.2, a new decoding using the information from both these FACCH blocks shall be performed. If this decoding is successful the LAPDm frame produced by the new decoding is sent to the LAPDm entity.