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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?. Cable and Terrestrial TV signals are going digital

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Overview

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  1. 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

  2. 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

  3. 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

  4. Digital TV Waterfall Graph

  5. Effect of Noise on Analog Systems (Gradually poorer C/N)

  6. No FEC Effect of Noise on Digital Systems (Gradually poorer MER) Noise has very little affect on digital systems until the system fails completely

  7. Modulation formats in Cable

  8. 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)

  9. 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

  10. Data over Cable 11100100100 Mod. Demod. 11100100100

  11. 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)

  12. 4 Level Linear Attenuator 0/180° I-Channel (0) (10) 0° 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

  13. 64 QAM Waveforms • I and Q are in phase or 180 degrees out of phase • I and Q are four discrete independent levels

  14. 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

  15. QAM

  16. 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

  17. 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

  18. FEC Made Easy Alternates odd even, sum is 100

  19. 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

  20. 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

  21. QAM Measurements • Spectrum & Digital Average Power Level • MER • BER • Constellation Display • QAM Ingress • Group Delay • In-Channel Frequency Response • Equalizer Stress • Sweep

  22. 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.

  23. Spectrum analyzers can cause confusion • The spectrum analyzer’s different resolution-bandwidth filter give different results for power level measurements.

  24. Level meters that use correction factors can be inaccurate; Averaging over time. Unreliable method, not according to the standard t

  25. 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

  26. Make sure you setup the right measurement bandwidth

  27. 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

  28. 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

  29. Effect of Noise on Analog Systems (Gradually poorer C/N)

  30. 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

  31. 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

  32. Q Target Symbol Error Vector Transmitted Symbol I Error Vector Magnitude

  33. 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

  34. 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”)

  35. 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

  36. 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

  37. 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

  38. 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

  39. 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.

  40. No FEC C/N vs. BER vs. MER

  41. Constellations

  42. 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

  43. 4 Level Linear Attenuator 0/180° I-Channel (0) (10) 0° 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

  44. Q I Typical Constellations DecisionBoundary 16 QAM 64 QAM 256 QAM

  45. 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

  46. 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

  47. 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

  48. 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

  49. 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

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