Page 919 - 5G Basics - Core Network Aspects

Transport aspects 2

NOTE 1 – The SOC signal uses robust bit mapping, repetitions to operate in the presence of strong FEXT from active
lines since FEXT from active lines into joining lines is not yet cancelled. The IDS helps to mitigate crosstalk from other
joining lines. In some cases, effectively only low-frequency tones can be used for receiving O-SIGNATURE.
NOTE 2 – To align US transmission of a joining line with active lines, the FTU-O sends to the FTU-R the initial value of
the time gap Tg1' to be applied between upstream and downstream transmissions. The initial value of the time gap is
indicated in O-SIGNATURE message; the time gap is further updated by the FTU-O during later stages of initialization,
specifically O-TG-UPDATE message (see clause 12.3.3.2.2) and O-UPDATE message (see clause 12.3.3.2.4).
NOTE 3 – In O-SIGNATURE, the FTU-O also sends markers that indicate time position of downstream and upstream
probe sequences in active lines.

Upon decoding of O-SIGNATURE message, the FTU-R synchronizes to upstream and downstream probe
sequences, applies parameter settings obtained from O-SIGNATURE message, and transitions to R-VECTOR
1 stage.

12.3.3.1.4 R-VECTOR 1 stage
After detection of R-P-VECTOR 1, the FTU-O shall stop transmitting O-SIGNATURE message and start to
transmit O-IDLE.
The FTU-O shall estimate the correction to the initial value of the time gap Tg1' and communicate the
updated time gap Tg1' to the FTU-R in the O-TG-UPDATE message. The FTU-O transmits the O-TG-UPDATE
message in AR mode.
The FTU-O continues transmission of sync symbols modulated by probe sequence and transmission of SOC
symbols over the first sds symbol positions of each downstream logical frame. The SOC is active and
continues transmission of O-IDLE followed by the O-TG-UPDATE message until the end of this stage.

The FTU-R transmits upstream sync symbols only modulated by probe sequence. The content of probe
sequence, its time position and other transmission parameters shall be those received in O-SIGNATURE.
NOTE 1 – During this stage, before the upstream symbol alignment becomes sufficiently accurate, upstream FEXT
channel estimation might not be necessarily used by the FTU-O to estimate an accurate time gap Tg1'.

Upon receiving the O-TG-UPDATE message, the FTU-R shall transmit upstream sync symbols modulated by
probe sequence using the updated value of the time gap Tg1' received from the FTU-O in the O-TG-UPDATE
message.
NOTE 2 – During this stage, after the upstream symbol alignment becomes sufficiently accurate after its update via
O-TG-UPDATE message, all active lines estimate upstream crosstalk channels from the joining lines, and all joining
lines estimate the direct channel and crosstalk channels from both the active lines and other joining lines.

After transmission of one or more probe sequences, the VCE may compute the upstream post-canceller
coefficients for active lines and for joining lines (to cancel crosstalk between all active lines and joining
line(s)). The necessary FEQ gains for the joining lines and FEQ gain update for active lines are established.
The FTU-O signals the completion of this stage by sending to the FTU-R an O-P-SYNCHRO 1 signal. After
completion of this stage, joining lines can transmit upstream SOC symbols without disturbing transmission
over active lines and crosstalk into joining lines is cancelled in the upstream direction.
R-VECTOR 1 is a sub-stage of Channel Discovery 1 stage (see Figures 12-8 and 12-9).

12.3.3.1.5 CHANNEL DISCOVERY 2 stage
The FTU-O continues transmission of sync symbols modulated by a probe sequence and transmission of
SOC symbols over the first sds symbol positions of each downstream logical frame. The FTU-R transmits sync
symbols modulated by probe sequence and SOC symbols over the first sus symbol positions of each
upstream logical frame.

909