a practical approach to upstream rf signal management in hfc networks

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SCTE Autumn Lecture Meeting

27 September 2005, IEE, London

Janhein van BrienenPrincipal Engineer Telewest Broadband

A Practical Approach to Upstream RF signalManagement in HFC Networks



Slide 227/09/2005

Historical Context

• Evolution of Telewest Broadband HFC networks

– Today's Telewest Broadband is the result of a series of mergers and acquisitions.

• Consequences of this process

– Some 10 distinctly different architectures can be identified inthe Telewest Broadband HFC networks.

– Most vendors equipment is represented somewhere withinthe Telewest Broadband network.

– The regionalised character of Telewest Broadband hasresulted in different practises and approaches in theoperational environment.



Slide 327/09/2005

Historical Context

• Pre 2000 upstream network usage and status

– RF IPPV - simple and robust BPSK modulation schemetransferring very small amounts of data.

– Upstream calibration and data collection process best

described as organised chaos.– Network monitoring - Upstream FSK data carrier with

transponders answering a poll request.

– First use of 2-way sweep and network alignment equipment.– Most networks reverse path capable, but some not

maintained or equipment not commissioned.


http://slidepdf.com/reader/full/a-practical-approach-to-upstream-rf-signal-management-in-hfc-networks 4/28Slide 427/09/2005

Historical Context

• Post 2000 upstream network

– Preparation for DTV and DOCSIS started in 1999– Complete rework of return path combining in Hub sites and

Headends.

– Testing and removal of “non digital-ready” passives from thenetwork. (mainly splitters and isolators)

– Assessment of return path network performance required to

ensure meeting DOCSIS demands.– Re-commissioning of return path and verification of

upstream network performance.



Historical Context

• RPM installation in Hubsites

S8

A

B

C

D

E

F

G

H

C8

1

2

3

4

5

6

7

8

S8

C8

8-way Node

Combiners

(Service "A")

8-way Narrowcast

Services Splitters

1

2

3

4

5

6

7

8

A

B

C

D

E

F

G

H

Narrowcast Service

Channel Splitters

Narrowcast

Service "A"

Receivers

T o N a r r o w c a s t S e r v i c e s B - H

RPM Output

Amplifiers

RPM-S

RPM Patch Field

Optical Receiver

Platform

Narrowcast Service

Receiver Platform

Cisco UBR

platformwith

MC-16

RPM Input

Amplifiers

Rx1

Rx2

Rx3

Rx4

f U1

f U2

F r o m

N e t w o r k

N o d e s 2 - 4

Rx5

Rx6

Optical

Receivers

f U3

f U1

f U2

f U3

I/P TP O/P TPI/P TP O/P TP

S8



Upstream Network and Equipment Performance

• Downstream Network Performance Assessment

O

E

Intermediate

Amplifier Fibre Node

Star TapsWith Drop Cables

Distribution

Amplifier

O

E

Hub site



Slide 727/09/2005

O

E

Intermediate

Amplifier Fibre Node

Star TapsWith Drop Cables

Distribution

Amplifier

O

E

Hub site


• Upstream Network Performance Assessment



Slide 827/09/2005


• Network Performance Assessment

– Amplifiers generally not a problem - wide dynamic range andnoise performance ensures trouble free operation.

– Legacy Fibre Optic equipment poorly specified or not at all

documented - undesirable interoperability issues noted.– Upstream HFC network set-up different in most networks,

even between identical equipment.

– Conclusion: not possible to implement a uniform DOCSISservice in such an environment.



Slide 927/09/2005


• Testing Requirements

– Common test standard required to assess performance of alloptical equipment combinations.

– Referencing all test results against a common standard

which meets DOCSIS requirements with acceptable margin.– Translation of test results into spectrum utilisation to future

proof against changes in service requirements.

– Common standard required for commissioning equipmentacross all regions to ensure uniform DOCSIS performance.



Slide 1027/09/2005

– Uses a 32 MHz wide noise signal with a notch at 22.5 MHz– Measures the DUT noise input power in dBmV/Hz vs. the

noise floor to noise envelope ∆ in dB at the DUT output.

– Is repeated at 5 different temperatures to assessperformance over the operational temperature range.

– Is measured at the optical transmitter input


• “Noise in the Notch” test adopted

White Noise

Generator Bandpass and

22.5Mhz Notch Filter

Precision Step

Attenuator Bandpass

Filter

Spectrum

Analyser

System Under Test (SUT)

Opt. Transmitter, 20Km Fibre and Opt. Receiver

P(f)O

E

G

dB

E

O



Slide 1127/09/2005

Noise in the Notch Bandpass and Notch Measurement Filter

Mkr 1

- 3 . 0 d B

Mkr 2

0 . 2 d B Mkr 3

- 3 . 0 d B

Mkr 4

- 5 4 . 5 d B

Mkr 5

- 3 . 0 d B

Mkr 6

- 3 . 0 d B

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

1.0 7.0 13.0 19.0 25.0 31.0 37.0 43.0 49.0 55.0 61.0

Frequency (MHz)

I n s e r t i o

n l o s s ( d B )

Filter responseMarker 1, 4.0MhzMarker 2, 17.5MhzMarker 3, 20.2MhzMarker 4, 22.5MhzMarker 5, 25.3MhzMarker 6, 42.1Mhz




Slide 1227/09/2005

Noise in the Notch Spectum Analyser Filter

Mkr 1

- 5 . 5 d B

Mkr 2

- 0 . 3 d B

Mkr 3

- 0 . 3 d B

Mkr 4

- 5 . 0 d B

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

1.0 7.0 13.0 19.0 25.0 31.0 37.0 43.0 49.0 55.0 61.0

Frequency (MHz)

I n s e r t i o

n l o s s ( d B )

Filter response

Marker 1, 15.1Mhz

Marker 2, 17.5Mhz

Marker 3, 22.5Mhz

Marker 4, 25.3Mhz




Slide 1327/09/2005

Noise in the slot test set-up system response

Mkr 1

- 5 4 . 2 d B

Mkr 2

- 0 . 5 d B Mkr 3

- 3 . 6 d B

Mkr 4

- 5 4 . 8 d B

Mkr 5

- 7 . 7 d B

Mkr 6

- 4 1 . 1 d B

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

1.0 7.0 13.0 19.0 25.0 31.0 37.0 43.0 49.0 55.0 61.0

Frequency (MHz)

I n s e r t i o n l o s s ( d B )

Overall responseMarker 1, 15.5MhzMarker 2, 17.5MhzMarker 3, 20.2MhzMarker 4, 22.5MhzMarker 5, 25.3MhzMarker 6, 42.1Mhz




Slide 1427/09/2005

• Theoretical NPR curve Noise Power

Ratio (dB)

RF Laser Input

ower

(dBmV/Hz)

Dynamic

Range

Minimum

required noise

performance




Slide 1527/09/2005

Noise Power Ratio vs RF Input SA RT-1 TX - HL 2220 RX @ 8dB OLB

20.0

25.0

30.0

35.0

40.0

45.0

-51 -50 -49 -48 -47 -46 -45 -44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -34 -33 -32

TX Input level (dBmV/Hz)

N P R ( d B )

Temp -10°C/05°C

Temp 00°C/17°C

Temp 20°C/36°C

Temp 40°C/57°C

Temp 60°C/77°C

Temp 75°C/92°C


• Actual NPR curve



Slide 1627/09/2005


• Application of results

– Measurement results expressed in “Power per Hertz” enableeasy translation to signals of different bandwidths.

– Current 3.2MHz upstream DOCSIS carrier operating point inprevious example is found by considering the measuredresult of -45dBmV/Hz + 10*LOG(3200000)Hz = 20dBmV.

– To find the RF input level to the fibre node station ports therecommended gain or attenuation setting of the node mustbe considered. This is labelled Node Return Gain (NRG)



Slide 1727/09/2005


• Application of results

– NRG is calculated to achieve the most beneficial upstreamCPE transmit power when considering cable and passiveattenuation between the final distribution node and thehome in the HFC network.

– The upstream network gain (input of distribution node toinput of fibre node) is normally 0.

– NRG is documented for all nodes in use in Telewest in theRPM calculator.



Slide 1827/09/2005


• Example of Fibre Node alignment

-1dB

20dB

Test Point

25 dBmV

@ Station Port

17 dBmV 32 dBmV

PassiveCombining

Loss

-6.3dB-0.7dB

H

L

Amplifier Module

+19dB -3dB

SA 6920 RT-1

-3 dB

Test

Probe

40 dBmV

@ Injection

Point

For Alignment, Adjust O/P Pad to Achieve NRG of +7dB.

PAD

20dB

Test Point

-1dB

SA

Probe

Main O/P T.P Reverse O/P

T.P.



Slide 1927/09/2005


• Upstream alignment

– RPM calculator tool allows independent calculation of DOCSISdata carriers and CW alignment carriers - long establishedpractises may remain unchanged.

– RPM calculator input data can be selected from 6 simple dropdown menus.

– Undesirable equipment combinations will not calculateresults.

– Calculator tool is updated whenever new equipment isintroduced in the network.



Slide 2027/09/2005


• RPM calculatorEnter optical node

Type

Alignment Level

(dBmV) on laser IP Enter Rx Type

Enter Normalised

O/P on RX (dBmV)

Enter Bandwidth

of Carrier (MHz)

input Amplifier

gain =

output Amplifier

gain =

Node Return Gain = 7 dB 6 dB 14 dB

Node IP Alignment CW(dBmV)

Level presented to laserTP* (dBmV)

Alignment CW(dBmV)

Alignment CW(dBmV)

Alignment CW(dBmV)

Alignment CW(dBmV)

25 32 27 33 27 17

Digital signal Level

(dBmV)


(dBmV)


(dBmV)


(dBmV)


(dBmV)


(dBmV)

13 20 15 21 15 5

Optical node type: Optical laser type:Optical receiver model

and type

SA 6920 SA RT-1 (-7dBm) HLW 2220 or 21105.0 dBmV for a bandwidth of 3.2

Enter return laser make and type

RX

Note: Set Expected Ubr level to:

Rev TX IPNode RF mother board

SA RT-1 (-7dBm)

HLW 2220 or 211032 27 3.2

RPM

Patchfield

MC-16

8-way

input splitter

UBr

SA 6920



Slide 2127/09/2005


• NPR test only part of fibre node tests.

– Temperature stability further tested by sweeping the fullupstream path at temperature points with a networkanalyser.

– Optical transmitter modules monitored for optical poweroutput, temperature and laser bias voltage test point.

– Test data forms part of full downstream testing and powersupply testing, which is performed on every fibre node.



Slide 2227/09/2005

1.6MHz to 3.2MHz upstream carrier upgrade

• 3.2 MHz DOCSIS carriers included in original design.

– Upgrade required an extra 3dB of Upstream transmit powerform all CPE devices.

– In practise larger transmit power swings may be seen due tothe default adjustment threshold of 1dB on the Cisco UBR

– RPM calculator showed that 1 fibre node type hadinsufficient Node Return Gain to accommodate this.

– Optical transmitters in these nodes were upgraded to highergain types enabling upgrade rollout without furtherproblems.



Slide 2327/09/2005

3.2 QPSK to 16QAM upgrade

• Problems with DOCSIS specification.

– CPE upstream transmit power

• +8 to +58 dBmV (QPSK) but +8 to +55 dBmV (16QAM).

– Most legacy Cable Modems follow DOCSIS specification,DHT’s generally don’t.

– RPM calculator shows that 1 fibre node type (ANTEC 1FTD)has insufficient Node Return Gain to compensate for thisdrop by providing an extra 5 or 6dB Gain.

– Therefore Carrier to Ingress will decrease by 3dB, as willnoise performance over the optical system.

2 QPSK 16QAM d



Slide 2427/09/2005


• Theoretical 1FTD area CPE TX power calculation

– With subscriber tap values and max number of CPE per dropcable, min and maximum attenuation levels can be calculated.

– The 1FTD requires 21dBmV for 3.2MHz carriers on eachstation port. The maximum passive attenuation is 39dB, theminimum is 12dB. Therefore the CPE TX power willtheoretically range between 33 and 60dBmV.

– Theoretical average upstream level therefore is 46.5dBmV.

– The Cisco UBR default “continuation” threshold of ± 2dBenables Cable Modems transmitting at 58dBmV to register.

3 2 QPSK 16QAM d



Slide 2527/09/2005


• 1FTD area UBR pollCPE Upstream Transmit levels UBR01 Edinburgh 12th of July

0

50

100

150

200

250

300

350

400

450

20 25 30 35 40 45 50 55 60

Upstream Transmit level (dBmV)

N u m b e r o f d e v i c e s

Edinburgh totals

47.5

3 2 QPSK t 16QAM d



Slide 2627/09/2005


• 1FTD area UBR poll

– Theoretical calculation matches the UBR poll almost exactly.

– CPE upstream TX curve sits too high in the power range of the CPE equipment.

– If no action was taken and the QPSK to 16QAM switch wasmade some 7% of Cable modems would no longer be able toregister on this UBR

– Further analysis by CPE type reveals that the upturn in thecurve at 58dBmV is due to devices at their maximum TXlevel, but not achieving requested UBR receive level.

3 2 QPSK t 16QAM d



Slide 2727/09/2005


• 1FTD upgrade solution

– A replacement fibre optic transmitter with a minimum of 6dBextra gain has been developed by one of Telewest’s currentsuppliers and is currently under evaluation as a replacementof the original RDL-2 transmitter.

– When approved this should remove one of the mainequipment obstacles preventing an upgrade to 16QAM

Questions



Slide 2827/09/2005

Questions

• Thank you for your attention

a practical approach to upstream rf signal management in hfc networks

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