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Overview. What is QAM? Why Use QAM? Quadrature Amplitude Modulation Bits and Symbols QAM Encoding and Implementation QAM Measurement What Constellations Tell Us Modulation Error Ratio (MER) BER FEC. Why Go Digital?. Cable and Terrestrial TV signals are going digital

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overview
Overview
  • What is QAM?
    • Why Use QAM?
    • Quadrature Amplitude Modulation
    • Bits and Symbols
    • QAM Encoding and Implementation
  • QAM Measurement
    • What Constellations Tell Us
    • Modulation Error Ratio (MER)
    • BER
    • FEC
why go digital
Why Go Digital?
  • Cable and Terrestrial TV signals are going digital
    • Digital Cable - Now; Terrestrial Xmit - 2006
    • Standard Definition TV (SDTV)
    • High Definition TV (HDTV)
  • Better Picture and Sound Quality
  • Cable Modems transmit and receive digital data
  • Digital signals can be less susceptible to noise
  • Data Compression, error detection and correction is done with digital data
  • Datacasting easily multiplexed into digital signal
  • Higher Data Security
analog vs digital
Analog signal components are visibly discernable using a spectrum analyzer

Digitally modulated signals only show a “haystack” on a spectrum analyzer regardless of modulation or content – (more tools needed)

Analog vs. Digital
effect of noise on analog systems
Effect of Noise on Analog Systems

(Gradually poorer C/N)

effect of noise on digital systems

No FEC

Effect of Noise on Digital Systems

(Gradually poorer MER)

Noise has very little affect on digital systems until the system fails completely

what is qam
What is QAM?
  • Quadrature Amplitude Modulation – pronounced as “kwam”)
  • Modulation Scheme where Phase and Amplitude are modulated to represent data
  • Similar to QPSK which is robust and has been used for years (QPSK is the same as 4QAM)
  • By providing different levels of amplitude and phase modulation, groups of bits can be represented as a symbol.
  • Additional levels of modulation provide higher data capacity (16QAM, 64QAM, 256QAM, 1024QAM)
why use qam
Why Use QAM?
  • QAM is the standard for DOCSIS and DVB-C
  • Improves spectral efficiency thereby providing more channels within a limited bandwidth
    • 64 QAM can transmit 27Mbps or the equivalent of 6 to 10 analog channels or 1 HDTV signal over one 6MHz bandwidth
    • 256 QAM can transmit 38.8 Mbps or the equivalent of 11 to 20 analog channels or 2 HDTV signals over one 6MHz bandwidth
      • An SD signal requires 2 to 3.5Mbps (depending on quality) and an HD signal requires 19.2 Mbps.
      • New compression techniques can provide up to 3 HD signals on a 256 QAM carrier
data over cable
Data over Cable

11100100100

Mod.

Demod.

11100100100

quadrature amplitude modulation
Quadrature Amplitude Modulation
  • Both I and Q are at the same frequency but amplitude and phase are modulated.
    • I = Incidental or in-phase Axis
    • Q = Quadrature Axis (90 degrees to I)
  • Modulated Amplitude Levels
    • Four different levels for 64 QAM
    • Eight different levels for 256 QAM
  • I and Q can be in phase (I = 0 degrees, Q = 90 degrees) or out of phase (I =180 degrees, Q = 270 degrees)
slide13

4 Level

Linear

Attenuator

0/180°

I-Channel

(0)

(10)

S

RF-In

(010)

90°

4 Level

Linear

Attenuator

0/180°

Q-Channel

(0)

(11)

Bit stream in

(011)(010)

(011)

Quadrature Amplitude Modulation

RF-Out

64-QAM

64 qam waveforms
64 QAM Waveforms
  • I and Q are in phase or 180 degrees out of phase
  • I and Q are four discrete independent levels
quadrature modulation
Quadrature Modulation
  • Simply measuring the carrier level relative to the noise level does not take into account any phase noise that may also be present on the signal

Carrier

Amplitude

Modulation

Carrier

Amplitude

Modulation

Analog Video AM Modulation

Carrier Phase Shift

QAM Modulation

bits and symbols
Bits and Symbols
  • A Symbol is a waveform that represents one or more bits
  • Data is encoded into symbols for transmission
  • Symbol Rate = Bit Rate/Number of Bits per Symbol
    • Assume a 8 bit sampler at 10kHz (voice) -Bit rate is 80Kbps
forward error correction fec
Forward Error Correction (FEC)
  • Adds redundant information to the data stream
  • Trade-off of data size vs error correction
  • Trellis Encoding
  • Randomization
  • Interleaving
  • Reed Solomon
fec made easy
FEC Made Easy

Alternates odd even, sum is 100

how fec works

1011 1

1000 1

1011 1

0100 1

1100 0

1011 1

1000 1

1001 1

0100 1

1100 0

How FEC works
  • Video Stream 1011100010110100
  • Stream with FEC 1011100010010100111111000

After Transmission with bit error

slide21

Digital Modulation Stream

Reed-Solomon

Encoder

Reed-Solomon Coding provides block encoding and decoding to correct up to three symbols within an RS block

Interleaver

Interleaving evenly disperses the symbols, protecting against a burst of symbol errors from being sent to the RS decoder

Digital Modulation Stream

Randomizer

Randomizes the data on the channel to allow effective QAM demodulation synchronization

Trellis Encoder

Trellis Coding provides convolutional encoding and with the possibility of using soft decision trellis decoding of random channel errors

Modulation

qam measurements
QAM Measurements
  • Spectrum & Digital Average Power Level
  • MER
  • BER
  • Constellation Display
  • QAM Ingress
  • Group Delay
  • In-Channel Frequency Response
  • Equalizer Stress
  • Sweep
digital average power level measurements
Digital Average Power Level Measurements
  • Digital Average Power Measurements and Measurement Bandwidth
    • The spectrum analyzer view is an excellent tool to see discreet RF-carriers.
    • Caution is needed when viewing digital modulated signals (noise mountain). The signals level is depended from the selected measurement bandwidth (resolution bandwidth). At a RBW = 300 kHz, a 64QAM - 6 MHz wide digital signal reads in the spectrum analyzer trace 3 dB to low.
    • The Average Power principle takes little slices from the integrated RF-energy, summing them together to one total power reading in the LEVEL-mode.

Summing slices of the total integrated energy

Analog and digital (broadcast) signal.

The delta in level should be 10 dB.

spectrum analyzers can cause confusion
Spectrum analyzers can cause confusion
  • The spectrum analyzer’s different resolution-bandwidth filter give different results for power level measurements.
slide25
Level meters that use correction factors can be inaccurate; Averaging over time. Unreliable method, not according to the standard

t

level measurements on digital video channels
Level measurements on digital video channels
  • Average Power Level according to standards
  • Scanning the level envelope of the channel using a 280 kHz IF-filter and summing the values of all samples.
  • Can be used on all digital channels QPSK, QAM, 8-VSB

> 10 dB

modulation error ratio mer
Modulation Error Ratio (MER)
  • Analogous to S/N or C/N
  • A measure of how tightly symbols are recorded with respect to desired symbol location
  • MER(dB) = 20 x log RMS error magnitude average symbol magnitude
  • Good MER
    • 64 QAM: 23 dB MER
    • 256 QAM: 29 dB MER

Average symbol magnitude

RMS error

magnitude

slide29
MER
  • Modulation Error Ratio (MER) in digital systems is similar to S/N or C/N used in analog systems
  • MER determines how much margin the system has before failure
  • Analog systems that have a poor C/N show up as a “snowy” picture
  • A poor MER is not noticeable on the picture right up to the point of system failure - “Cliff Effect”
  • Can’t use the TV as a piece of test equipment anymore
effect of noise on analog systems1
Effect of Noise on Analog Systems

(Gradually poorer C/N)

mer modulation error ratio db evm error vector magnitude
MER? Modulation Error Ratio (dB)(EVM? Error Vector Magnitude) (%)

Amplitude and phase error

  • Equivalent to analog C/N
  • The bigger the number the closer to the target.
  • Field test ~ 32 - 35dB.
  • Set top boxes ~ 28dB.
  • Headend > 40dB.
  • Bad MER = Bad BER
what is a good mer
What is a Good MER?
  • A 64-QAM signal requires better than 23 dB MER at the set top box or CM to operate
  • A 256-QAM requires better than 28 dB MER at the set top box or CM to operate
  • A 1024-QAM signal requires better than 33 dB MER at the set top box or CM to operate)
  • To allow for degradation a margin (or headroom) of at least 3 to 4dB is preferred
error vector magnitude

Q

Target Symbol

Error Vector

Transmitted Symbol

I

Error Vector Magnitude
error vector magnitude evm

Average error magnitude

Max symbol magnitude

X 100%

Error Vector Magnitude (EVM)
  • EVM is defined as follows:

Expressed in percentage

Error Magnitude

Ideal Symbol

Max Symbol Magnitude

ber introduction
BER Introduction
  • Bit Error Rate is a major indicator of system health
  • As data is transmitted some of the bits may not be received correctly
  • The more bits that are incorrect, the more the signal will be affected
  • It’s important to know what portion of the bits are in error
  • Need to know how much margin the system has before failure
  • The harder FEC is working, the closer the system is to failure (“The Cliff”)
slide36
BER
  • Good signal: BER 10-10
  • Threshold for visible degradation: BER 10-6
  • FEC can improve BER from 10-4 to 10-10
    • BER before FEC: correctable + uncorrectable errors
    • BER after FEC: uncorrectable errors
  • Bit Error Tester (BERT)
    • Inject known signal
ber example
BER Example
  • A 256QAM channel transmits at a symbol rate of 5M symbols per second
  • Bit rate = 8 bits per symbol X 5M symbol per second =40M bits per second
  • Error Incident = Bit rate X BER = Errors Per Second
pre and post fec ber

Pre FEC BER

(before correction)

Post FEC BER

(after correction)

Pre and Post FEC BER
  • FEC - Corrected Errors Estimated uncorrected Errors
  • Pre FEC = corrected + uncorrected errors
  • Post FEC = uncorrected errors
  • Pre and Post FEC BER indicate how hard the FEC is working to correct errors
bit error rate provides benefit for commissioning
Bit Error Rateprovides benefit for commissioning
  • Number of bad bits for every good bit.
  • Forward Error Correction when working will output >10-11
    • 1 error in 100 billion bits
    • 1 error every 42 minutes
    • MPEG-2 likes good BER
  • FEC will work to about 10-4
    • 1 error in 10000 bits
    • 1 error every 276 uses
  • FEC causes Cliff Effect
fec causes cliff effect

1.10-1

BER

4QAM

16QAM

64QAM

256QAM

4.10-4

1.10-9

40

MER

2

23.5

FEC causes Cliff Effect
  • A small variation in MER (+/- 1 dB) will cause a large variation in BER measurement.
  • Using BER for trouble-shooting and fault location is not repeatable and very inaccurate.
constellation basics
Constellation Basics
  • The constellation display shows both I and Q
  • A symbol is the smallest piece of information transmitted - plotted as a point representing a digital bit(s)
  • It is the digital equivalent of a Vectorscope display
  • Useful for determining modulation problems:
    • Amplitude Imbalance
    • Quadrature Error
    • Phase Error
    • Modulation Error Ratio
slide44

4 Level

Linear

Attenuator

0/180°

I-Channel

(0)

(10)

S

RF-In

(010)

90°

4 Level

Linear

Attenuator

0/180°

Q-Channel

(0)

(11)

Bit stream in

(011)(010)

(011)

Quadrature Amplitude Modulation

RF-Out

64-QAM

typical constellations

Q

I

Typical Constellations

DecisionBoundary

16 QAM

64 QAM

256 QAM

constellations symbols and digital bits
Constellations, Symbols, and Digital Bits
  • Each “dot” on constellation represents a unique symbol
  • Each unique symbol represents unique digital bits
  • Digital data is parsed into data lengths that encode the symbol waveform.

16 QAM

gain compression
Gain Compression
  • If the outer dots are pulled into the center while the middle ones are not affected, the signal has gain compression
  • Gain compression can be caused by IF and RF amplifiers and filters, up/down converters and IF equalizers

Outer edges pulled in

system noise
System Noise
  • A constellation displaying significant noise
  • Dots are spread out indicating high noise and most likely significant errors
    • An error occurs when a dot is plotted across a boundary and is placed in the wrong location
  • Meter will not lock if too much noise present

Dots are spread out showing error

phase noise

Rotation

Rotation

Constellation with Phase Noise

Zoomed Constellation with Phase Noise

Phase Noise
  • Display appears to rotate at the extremes
  • HE down/up converters can cause phase noise
  • Random phase errors cause decreased transmission margin
  • Caused by transmitter symbol clock jitter
  • Bad LO in meter can cause phase noise Constellation
coherent interference

Circular “donuts”

Coherent Interference
  • If the accumulation looks like a “donut”, the problem is coherent interference
    • CTB, CSO, spurs and ingress
  • Sometimes only a couple dots will be misplaced
    • This is usually laser clipping or sweep interference
ingress under the carrier
Ingress Under the Carrier
  • Interference will cause poor MER
    • Noise
    • Discreet Signal
      • Ingress
      • Bad Modulator
      • CSO/CTB (TV)
    • CSO/CTB Digital
qam ingress ingress under the carrier
QAM Ingress (Ingress Under the Carrier)
  • Meter knows how much error is in signal from measuring Constellation points
  • Meter uses this error to plot Ingress Under the Carrier
cso and ctb under qam 256 carrier
CSO and CTB under QAM 256 carrier
  • Using ingress under the carrier, the SDA can uncover CSO and CTB that are not visible using standard spectrum analysis.
group delay
Group Delay
  • Definition: Group delay is the measure of the slope of the phase shift with frequency.
  • Effects: If there are group delay variations in the network, then signals of one frequency can make it through the network faster than signals at another frequency.
  • For analog signals this typically can cause misregistration of the chrominance to luminance since the chrominance subcarrier is 3.58MHz higher than the luminance carrier. The visible effect is that the colors are not within the outline of the subject.
group delay1
Group Delay
  • For digital signals the effect can lead to QAM symbol misinterpretation. The net effect is that short duration pulses that are input into the network will exit the network having a longer duration. This spreading leaves energy from one pulse in the time slot of other pulses. This causes the BER to degrade.
  • For downstream carriers, the DOCSIS 1.0 spec requires the group delay ripple to be less than 75nS.
  • Bad filters are a typical cause of group delay
in channel frequency response
In-Channel Frequency Response
  • In-Channel Frequency response is amplitude ripple. This means that signals at one frequency are attenuated relative to signals at another frequency.
  • For downstream digital carriers DOCSIS 1.0 specifies a max ripple of 0.5dB in 6MHz. DOCSIS 1.1 has relaxed this specification to 3.0dB in 6MHz.
equalizer stress
Equalizer Stress
  • Digital demodulation receivers utilize adaptive equalizers to negate the effects of signals arriving other than the desired signal.
  • Signals can arrive ahead of or after the desired signal. In a cable system, the majority of signals are reflections and micro-reflections that arrive after the desired signal.
  • Cable modems and digital set top boxes must be able to handle pre and post signals at levels defined by DVB standards. If the equalizer is pushed beyond those limits, errors will occur.
  • By using the Velocity of Propagation, the distance to the source of the reflection can sometimes be located. If the reflections occur before the next upstream amplifier, they are simply amplified and passed downstream thereby eliminating the ability to perform fault detection based on reflection time.
  • Equalizer stress is used more as a figure of merit for the margin available to the set top box or cable modem.
equalizer stress1
Equalizer Stress

Signal arriving about 0.8usec before desired carrier

Signal arriving about 2usec after desired carrier

what faults cause catv signals to fail 80 90 of the time the same faults

5% Spectrum Analyzers

7% Visual TV-picture inspection

11% BER Digital Analyzers

23% Signal Level Meters

21% Reverse Ingress

72% Forward & Reverse Sweep

What faults cause CATV signals to fail ?(80-90% of the time, the same faults…)
  • Success rate of finding and fixing the following problems using:
    • Signal Levels
    • TILT
    • Gain / Loss
    • Suck-outs (notches)
    • C/N
    • HUM
    • CTB/CSO Intermodulation
    • CPD - Forward and Reverse
    • Reverse Ingress
    • BER / MER
    • Reflections / Standing waves

Source: Research 11/97-2/98 Market survey with 200 US and European CATV operators

sweep is the best way to prepare the network for 256 qam

NODE 1

Sweep is the best way to prepare the network for 256 QAM
  • Standing waves, suck-outs, intermodulation distortion and non-linear performance effect digital performance

Bad Forward Sweep Trace

reflections causes by bad terminations
Reflections causes by bad terminations

f

  • Reflections or standing waves caused by any defective, miss-matching devise
  • Damages cable, connectors ground block, splitters, etc.
    • A sweep signal is transmitted by the SDA 5500 over coaxial cable (the medium). A portion of the transmitted sweep signal on the cable will be reflected back to the transmitter if the load is not a perfect 75Ohm impedance match. The reflected energy will be the same frequency as the incident (sweep) signal but different in phase. The resulting signal (incident + reflected) will appear as standing waves on a frequency sweep (see figure). The reflection is such that the peaks of the individual cycles can be translated to distance to the fault (impedance mismatch) through the following equation:

D = 491*Vop/f

Where D=distance to fault, Vop=velocity of propagation of the cable, and f = frequency of 1 cycle of the standing wave.

Bad Forward Sweep Trace - Standing waves

suck outs
Suck-outs
  • Bad taps or connectors are mostly causing a suck-out (notch) in frequency response.
  • It generates individual channel errors, Sweep is a very efficient way to locate bad taps or connectors. Scanning the channels works too, but the error is less apparent.
  • Causes are:
    • Humidity problems
    • Small RF leaks to mass.
    • Bad mounted connectors

Bad Forward Sweep Trace - Suck-out

Bad Level SCAN-Trace Trace - Suck-out

terms
Terms
  • QAM - Quadrature Amplitude Modulation
  • Symbols - Collection of Bits
  • Symbol Rate - Transmission Speed
  • I & Q - Components of QAM data
  • Constellation - Graph of QAM Data
  • MER - Modulation Error Ratio
  • BER - Bit Error Rate
  • FEC - Forward Error Correction