64 qam upstream · pdf file64 qam upstream deployment why 64-qam? higher upstream data...

64 QAM

UPSTREAM

DEPLOYMENT

64 QAM UPSTREAM DEPLOYMENT

Why 64-QAM?

Higher upstream data throughput required for:

Voice, growing SMB’s using DOCSIS , Gamers, Peer to Peer.

Up to 120 Mbs for 4 bonded channels for DOCSIS 3 in the upstream.

Competition.

Business Services

Upstream 64-QAM: What Does it Take?

Cable 101 practices

A comprehensive and effective preventive maintenance

program

Aggressive leakage and ingress control

Tracking down high-transmit level modems

Manageable node sizes

Placing of 64-QAM carrier signal between 20 and 30 MHz

center frequency


Upstream 64 QAM Challenges

Once interference occurs in voice the data cannot be

retransmitted.

Measurements are more difficult because the signals are bursty.

64 QAM looses 3 dB of headroom because the maximum modem output is 52 dBmV as opposed to 58 dBmV for QPSK


More Upstream Challenges with 64 QAM

64 QAM is less robust than 16 QAM

Requires better BER and MER

QAM means that the carrier is amplitude modulated and therefore more susceptible to amplitude based impairments such as:

Ingress

Micro-reflections

Compression



One question that comes up is whether or not return

path Fabry Perot (FP) lasers will work with 64-QAM.

The short answer is yes, but a little “it depends” is

applicable here.

• In particular, can upstream optical links support the

additional channel loading of multiple bonded channels?



Properly set up FP links will support one or two

64-QAM channels in the real world, but more than that really requires DFB or digital return links.

FP lasers simply don’t have the bit error rate (BER)

dynamic range to handle a bunch of channels.

Pay particular attention to upstream fiber link

alignment—this is critical, and can significantly

affect CNR and MER (“upstream SNR”)


Standard DFB & FP TX

Noise Power Ratio (NPR) Performance

with 7 dB Optical Link

20

25

30

35

40

45

50

55

-70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15

Input Power per Hz (dBmV/Hz)

NP

R (

dB

)

Room Temp Std DFB

Room Temp Std FP

Carrier-

to-

Noise*

To

tal R

F

Inp

ut

Po

we

r

38 dB CNR

Linear Response

Non-Linear Response

(Clipping)


Transit time and velocity of propagation

A signal takes a certain amount of time to pass through a filter

• The transit time through the filter is a function of the filter’s velocity of propagation (also called velocity factor)

• Velocity of propagation is the speed that an electromagnetic signal travels through some medium, usually expressed as a percentage of the speed of light in a vacuum


Bandpass filter equivalent


Bandpass filter equivalent

Consider the 6 MHz spectrum occupied by an

analog TV channel or digitally modulated signal, the 5-42 MHz upstream spectrum, or any specified bandwidth or passband as the equivalent of a bandpass filter.


In many instances the velocity of propagation through a filter varies with frequency

• The velocity of propagation may be greater in the center of the filter’s passband, but slower near the band edges

Velocity of propagation versus frequency

The time required for a signal to pass through a filter—or any device for that matter—is called delay

• Absolute delay is the delay a signal experiences passing through the device at some reference frequency


64 QAM

UPSTREAM DEPLOYMENT

Good CNR and C/I


Upstream Carrier-to-interference


Impact of BER on 64QAM upstream signal.

Bit Error Rate (BER) is an important concept to understand in any digital transmission system since it is a major indicator of the quality of the digital system.

As data is transmitted some of the bits may not be reproduced at the receiver correctly. The more bits that are incorrect, the more the signal will be affected.

BER is a ratio of incorrect bits to the total number of bits measured.

Its important to know what portion of the bits are in error so you can determine how much margin the system has before failure.


BER is defined as the ratio of the number of wrong bits over the number of total bits.

BER is measured by sending a known string of bits and then counting the errored bits vs. the total number of bits sent.

This is technically an out of service measurement.

Sent Bits 11 01 10 11 01

Received Bits 11 00 10 11 01

BER = # of Wrong Bits= 1

# of Total Bits =10

------

0.1



CHOICE CABLE SNR REPORT 7/16/2013

Jul-16 SNR Results

Number of ports 953

>31dBc 881 92.4%

<30.99-28.0 62 6.5%

27.99 or < 10 1.0%

West South Number of ports 505

Number of ports 448

>31dBc 466 92.3% >31dBc 415 92.6% <30.99-28.0 35 6.9% <30.99-28.0 27 6.0%

27.99 or < 4 0.8% 27.99 or < 6 1.3%

Checklist DOCSIS 3.0 -capable CMTS and modems

Entire cable network—head-end, distribution network and

subscriber drops—DOCSIS-compliant

Upconverter setup, IF input/RF output levels

Downstream laser input levels

Avoid downstream frequencies near band edges or roll off

Areas

Avoid downstream frequencies that may be susceptible to

ingress from strong over-the-air signals

Forward and reverse properly aligned (coax and fiber)

Frequency response flat

Signal leakage management (<5 μV/m at 3 meters)

Home Certification Program


Final

Questions?


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