Transmit Power Back-off (PBO)

for 2.5G and 5G BASE-T

Hossein Sedarat

Alireza Razavi

July 2015

Why Power Back-off is Needed

2

• Transmit power back-off (PBO) is a mechanism to

alleviate the effect of alien crosstalk

• Alien crosstalk is stronger when a remote aggressor is

on a shorter cable and closer to a victim

• PBO reduces the transmit power on short cables so that

the crosstalk to adjacent links is reduced

A1 A2

V1 V2

ANEXT AFEXT

𝐿𝑎

𝐿𝑣

Alien Crosstalk in BASE-T Systems

3

• Alien crosstalk is stronger when– Signaling bandwidth is wider

– Coupling between cables is stronger

• Alien crosstalk is considered negligible in 1000BASE-T due to low bandwidth and low SNR requirements– Annex 40A defines a limit on ANEXT for 1000BASE-T systems

• 10GBASE-T is very sensitive to alien crosstalk because of its wider bandwidth. Coping mechanisms:– Use of Cat6a with lower coupling factor

– PBO

• Alien crosstalk is an important noise source in 2.5G and more so in 5GBASE-T because– Cat5e/Cat6 cables have higher coupling factor

– Signaling bandwidth is fairly wide

Alien Crosstalk PSD in 10GBASE-T

4

• ANEXT power is mostly independent of cable length

• AFEXT varies with insertion loss and coupling length

0 50 100 150 200 250 300 350 400-148

-146

-144

-142

-140

-138

-136

-134

MHz

dB

m/H

z

ANEXT

AFEXT 100m

Total 100m

AFEXT 30m

Total 30m

PBO in 10GBASE-T

5

• Alien crosstalk may be equalized across cable lengths

with proper back-off of transmit power on shorter cables

• 10GBASE-T allows up to 14 dB of PBO in steps of 2 dB

0 100 200 300 400-148

-146

-144

-142

-140

-138

-136

-134Total Alien Crosstalk PSD: No PBO

dB

m/H

z

MHz

0 100 200 300 400-148

-146

-144

-142

-140

-138

-136

-134

MHz

Total Alien Crosstalk PSD: With PBO

30 m, PBO=10 dB

40 m, PBO= 8 dB

50 m, PBO= 6 dB

60 m, PBO= 4 dB

70 m, PBO= 2 dB

100 m, PBO= 0 dB

PBO Selection in 10G

6

• Method: reduce the transmit power on shorter cables

– so that the crosstalk on longer victim is minimized

– while the SNR on shorter cables is no sacrificed too much

• Criterion: SNR should remain above a target for any

combination of victim and aggressors cable lengths

40 60 80 10020

25

30

35

40

45

50Salz SNR: No PBO

dB

Aggressor length (m)

40 60 80 10020

25

30

35

40

45

50Salz SNR: With PBO

dB

Aggressor length (m)

Victim= 30m

Victim= 70m

Victim=100m

PBO Considerations in 5G/2.5G

7

• There is no limit-line or model for alien crosstalk of Cat5e

and Cat6 cables

• Usage model for 5G/2.5G allows mixture of victim and

aggressors with potentially different rates and PSDs

• Aggressors and victim can be any combination of the

following rates:

– 1000BASE-T: BW=62.5 MHz, no PBO

– 2.5GBASE-T: BW=100 MHz

– 5GBASE-T: BW=200 MHz

Alien Crosstalk Models for Cat6/5e

8

• For this analysis, Cat6a models are extended with an

additional constant offset

• 𝑋𝑓 and 𝑋𝑛 are alien crosstalk loss of Cat6a at 100 MHz

• 𝑋𝑜 is the offset of Cat6/5e from Cat6a

– An offset of 𝑋𝑜 ≈ 15 dB is used as the starting point

– Do we need different offset for ANEXT and AFEXT?

𝐴𝑁𝐸𝑋𝑇𝑃𝑆 = 𝑋𝑛 − 10 × 𝑙𝑜𝑔 𝑓 100 − 𝑿𝒐 𝑓 < 100 MHz

𝑋𝑛 − 15 × 𝑙𝑜𝑔 𝑓 100 − 𝑿𝒐 𝑓 ≥ 100 MHz

𝐴𝐸𝐿𝐹𝐸𝑋𝑇𝑃𝑆 = 𝑋𝑓 − 20 × 𝑙𝑜𝑔 𝑓 100 − 10 × 𝑙𝑜𝑔 𝐿 100 − 𝑿𝒐

Alien Crosstalk PSD for Various Rates

9

0 50 100 150 200-145

-140

-135

-130

-125

-120

1G over Cat5e (X0=15 dB)

dB

m/H

z

MHz

0 50 100 150 200-145

-140

-135

-130

-125

-120

2.5G over Cat5e (X0=15 dB)

dB

m/H

z

MHz

0 50 100 150 200-145

-140

-135

-130

-125

-120

5G over Cat5e (X0=15 dB)

dB

m/H

z

MHz

0 50 100 150 200-145

-140

-135

-130

-125

-12010G over Cat6a (for reference only)

dB

m/H

z

MHz

ANEXT

AFEXT 100m

Total 100m

AFEXT 30m

Total 30m

SNR Analysis

10

• Cable: Cat5e with alien crosstalk offset of 15 dB

• Transmit power:

– 5G/2.5G: 2 dBm

– 1G: 3.5 dBm (no spec, based on modeling and measurement)

• Bandwidth

– 5G: 200 MHz

– 2.5G: 100 MHz

– 1G: 62.5 MHz

• Background thermal noise: -150 dBm/Hz

– Implementation-depended noise sources are not included

• Mixed aggressor model: each of 6 aggressors

contributes 1/6 to the total crosstalk power

20 40 60 80 10020

25

30

35

Aggressor Length (m)

dB

Victim: 5G

20 40 60 80 10020

25

30

35

Aggressor Length (m)

dB

Victim: 2.5G

Aggressor: 5G

Aggressor: 2.5G

Aggressor: 1G

Salz SNR: 5G/2.5G over 100m – No PBO

11

SNR too low for 5G link over

100m Cat5e with 15 dB offsetA short 1G aggressor may be

a limiting factor for 5G/2.5G

Salz SNR: 1G over 100m

12

• 1000BASE-T is immune to 5G and 2.5G aggressors!

20 30 40 50 60 70 80 90 10035

36

37

38

39

40

41

42

43

Aggressor Length (m)

dB

Aggressor: 5G

Aggressor: 2.5G

Aggressor: 1G

SNR, PBO and Crosstalk Offset

13

• Increasing the transmit power of the victim increases the

SNR of the victim

• Increasing the transmit power of the aggressors reduces

the SNR of the victim

• Increasing the alien crosstalk offset reduces the SNR of

the victim

PBO Selection Algorithm

14

• Find the PBO and the alien crosstalk offset for each

length and rate such that:

– SNR of both victim and aggressors always remain above the

target level

– Crosstalk offset is maximized (i.e. widest support of cable plants

in the field)

– Maximum transmit power remains below a limit

• Other considerations:

– Transmit power in 1G is fixed

– Mixture of various aggressor rates

Optimal PBO and Crosstalk Offset

15

40 60 80 1000

1

2

3

4

5

6

7

8PBO

dB

Cable Length (m)

40 60 80 10010

12

14

16

18

20

22

24

26Alien Crosstalk Offset

dB

Cable Length (m)

5G

2.5G

5G

2.5G

30 40 50 60 70 80 90 10027.5

28

28.5

29

29.5

30

Aggressor Length (m)

dB

5G

2.5G

Minimum SNR

Victim: 100m Cat5e , Aggressors: Mixed

16

(6,0,0)

(3,3,0)

(5,0,1)

(3,2,1)

(0,5,1)

(0,5,1)(0,2,4)

number of aggressors from each rate

(5G, 2.5G, 1G)

in a 6-around-1 bundle

PBO: 2 dB Steps

17

40 60 80 1000

2

4

6

8PBO

dB

40 60 80 10010

15

20

25

30Alien Crosstalk Offset

dB

30 40 50 60 70 80 90 10027.5

28

28.5

29

29.5

30

Aggressor Length (m)

dB

Minimum SNR - 100m Cat5e, mixed rates

5G

2.5G

5G

2.5G

5G

2.5G

PBO Table

18

5G

Receive Power

(dBm)

Cable Length

(m)

PBO

(dB)

−5.8 ≤ 𝑃 𝐿 ≤ 35 8

−7.0 ≤ 𝑃 < −5.8 35 < 𝐿 ≤ 45 6

−9.2 ≤ 𝑃 < −7.0 45 < 𝐿 ≤ 65 4

−11 ≤ 𝑃 < −9.2 65 < 𝐿 ≤ 85 2

𝑃 < −11 85 < 𝐿 0

2.5G

Receive Power

(dBm)

Cable Length

(m)

PBO

(dB)

−4.3 ≤ 𝑃 𝐿 ≤ 45 2

𝑃 < −4.3 45 < 𝐿 0

Alien Crosstalk Offset

Cat6 vs Cat5e

19

30 40 50 60 70 80 90 10010

12

14

16

18

20

22

24

26

28

Aggressor Length (m)

dB

5G - Cat5e

2.5G - Cat5e

5G - Cat6

2.5G - Cat6

10G Victim with 5G/2.5G Aggressors

20

• Find the maximum offset that keeps the SNR above the

target for each length of 10G victim and all lengths of

10G aggressors

• Use these offsets in the case of mixed-rate aggressors

• SNR of 10G victim does not drop below the target with

mixed rate aggressors

30 40 50 60 70 80 90 10028

29

30

31

Aggressor Length (m)

dB

Victim:10G - Aggressors: 10G (solid) , mixed (dashed)

Victim = 30 m

Victim = 50 m

Victim = 100 m

Summary

21

• 5G:

– PBO is necessary

– A maximum of 8 dB back-off is sufficient

– Supported alien crosstalk offset from 11 dB on long cables to

more than 18 dB on short cables

• 2.5G

– PBO is beneficial particularly for 5G operation

– A maximum of 2 dB back-off is sufficient

– Supported range for alien crosstalk offset is from 18 to 26 dB

• 1G is insensitive to alien crosstalk from 2.5G and 5G

transceivers