Equalized-FEXT method for Upstream Power Backoff (UPBO)

Chandan Jha

Introduction

• The DSL environment can be considered as a multiuser environment because telephone lines from different customer premise sites are bundled together on the way to the central office, and different lines in the bundle create crosstalk interference into each other.

Introduction

• Crosstalk: The coupling of unwanted signals from one or more lines into another line is known as crosstalk.

• VDSL has two forms of crosstalk:– NEXT:– FEXT:

Motivation

• Distributed topology may illustrate a realistic DSL deployment scenario.• The near-far problem arises when two transmitters located in different

distances attempt to communicate with the same central office at the same time. When one transmitter is much closer to the central office than the other, the interference coming from the closer transmitter overwhelms the signal from the farther transmitter.

• When both transmitters at the CP-side transmit with the same PSD, due to the difference in line attenuation, the FEXT caused by the short line can overwhelm the data signal in the long line. The upstream performance of the long line is therefore severely affected by the upstream transmission of the short line.

Objective

• Hence to mitigate this near-far effect, the short line must reduce its upstream PSD so that it does not cause excessive interference into the long line. This reduction of upstream transmit PSD is known as upstream power backoff (UPBO).

Power backoff methods

• Constant power back-off method• Reference length method• Multiple reference lengths method• Equalized-FEXT method• Reference noise method

Problem formulation

• The effects of the near-far FEXT may be minimized to a significant extent by shaping the PSD of the signals transmitted by the remote units based upon the relative distances of the remote units.

• By shaping the PSD of the transmissions of the closer units the signals carried on the disparate lines have similar PSDs and therefore, generate approximately the same amount of cross-talk interference.

• The shaping of the transmissions of the remote units is determined in part upon the transfer function , H(f), of the channels, or lines. The shaping of the transmissions of closer remote units becomes a ratio of the transfer function of the longest line and the transfer function of the remote unit.

Problem formulation

• FEXT can be modeled, as used by ANSI TIE 1.4 working group as,

where is the received power spectral density of the FEXT noise caused by

the other lines and injected into the line into consideration is the total number of active lines is the number of lines contributing FEXT noise to the line under

consideration is a constant is the length over which FEXT coupling occurs is the transmit PSD of the disturbing lines is the magnitude of the channel transfer function of the line under

consideration.

226.0

49

1fHfPSDkl

nfPSD distfext

fPSD fext

n

fPSDdist

1n

k

2fH

l

Equalized-FEXT

• In order to equalize the amount of FEXT noise generated by a first line, line (i-1), to a second line (i), the PSD of the line under consideration is computed such that line (i) receives the same amount of FEXT noise line (i) would have received if line (i-1) was of equal length as line (i).

• The expanded equation is :

• The ‘i’ index indicates line (i) is longer than line (i-1). The equation is the simplified version considering one line disturbing rather than all (n-1) lines.

iifextiifext fPSDfPSD 1

226.0

2

112

1

6.0

49

1

49

1fHfPSDfkdfHfPSDfkd iiiiii

fPSDfHd

fHdfPSD i

ii

iii 12

11

2

1.

.

Equalized FEXT

• The above equation provides the relationship between a longer and a shorter line necessary to shape the PSD of the signal carried on the shorter line so that the FEXT received by the central site is approximately same for both the lines.

• General equation based upon the longest line,

• ‘j’ is the index for the PSD of the shorter line at issue. ‘n’ is the index for the longest line. is any optimized PSD for increased data rates is used.

• The above equation describes that in order to equalize the amount of FEXT noise caused by a shorter line, the PSD of the transmission of the closer remote unit should be reduced by the ratio of the distance and magnitude of the longest line (or insertion loss) to the distance and magnitude of the shorter line.

fPSDfHd

fHdfPSD i

ii

iii 12

11

2

1.

.

fPSDfHd

fHdfPSD n

jj

nnj 2

2

.

.

fPSDn