Constellations Demystified Presented by Sunrise Telecom Broadband … a step ahead

Introduction  Understanding the constellation display and how it is derived will help you understand QAM Modulation and how the digital signal is transmitted.  The Constellation Display on a test instrument is can be an extremely valuable tool for determining the health of a digital signal.  Understanding how the various impairments can affect the constellation display will help to troubleshoot the source of the problem.

Bi-Phase Shift Keying (BPSK)  BPSK is the simplest method of digital transmission.  Data is transmitted by reversing the phase of the carrier.  The amplitude of the carrier remains constant.  Is a very robust transmission method but consumes significant bandwidth.

Bits and Symbols  A higher data rate can be achieved by adding amplitude modulation to the carrier in addition to the phase modulation.  By having multiple levels of amplitude, groups of bits can be transmitted.  A group of bits that is represented by a particular level and phase of the carrier is called a symbol. Two Levels of Amplitude Modulation and Bi-Phase Modulation Makes Four Possible Symbols

Bits and Symbols  Bits are grouped into pairs or symbols and the appropriate phase and amplitude is transmitted.  A negative amplitude indicates a phase reversal of the carrier.

Quadrature Modulation  Quadrature Modulation is a method of amplitude modulation that allows two channels to be carried at the same frequency effectively doubling the bandwidth that can be carried.  A form of Quadrature Modulation has been used for many years in analog television to carry the two components of the color subcarrier.  By modulating two carriers at exactly the same frequency but shifted by 90° both the amplitude and phase of the carrier is modulated.

Quadrature Amplitude Modulation (QAM)  Eight levels of modulation are achieved on both the I and Q channels by using four AM modulation levels and by reversing the phase 180°. Two Levels of Amplitude Modulation (Carrier On or Off) Data 1 or Carrier Off = 0 Carrier On = Four Levels of Amplitude Modulation With 180° Carrier Phase Reversal Transmits Eight Levels Data 000 to ° Phase Reversal of Carrier

Quadrature Amplitude Modulation (QAM)  Both the I and Q channels are AM modulated at the same frequency on carriers shifted by 90°.  The two signals are combined to make up the 64 QAM signal Local Osc 8 Level AM Modulator 8 Level AM Modulator Bit Stream Oscillator Shifted 90° Combiner 64 QAM Signal Q Component I Component

Quadrature Modulation

Quadrature Modulation  The phase and amplitude of the carrier at any given time determine the location on the Constellation and the I and Q channels can be derived from this information. Phase of Carrier Amplitude of Carrier Location On Constellation

Constellation  The location on the Constellation determines the I and Q components amplitude.  The amplitudes of the I and Q channel are derived from the rectangular coordinates of the carriers amplitude and phase. I Channel Q Channel

Gray Coding  The symbol which each location on the constellation represents is chosen using a technique known as Gray Coding.  Gray coding insures that any adjacent location will only be one bit different, reducing the effect of an error. Adjacent Locations are only different by one bit.

and 256 QAM Constellations  By adding more levels to the I and Q channels, higher data rates can be carried.  The higher the number of levels, the more effect there will be from noise or interference.  64 QAM uses 8 levels in the I direction and 8 levels in the Q direction for a total of 8 squared or 64 symbols.  256 QAM uses 16 levels in the I direction and 16 levels in the Q direction for a total of 16 squared or 256 symbols.

and 256 QAM Constellations 64 QAM Constellation 256 QAM Constellation

Decision Boundaries  Each location on the constellation is framed by decision boundaries.  If the signal falls within these boundaries, the correct data will be received.  If because of noise or other interference it falls in an adjacent area the data will be in error.

Constellation Buildup  On a test instrument the locations on the constellation build up over time and the shape and distribution can tell you much about the health of the signal and any problems that it has.

Clean Constellation Display  Example of a relatively good 64 QAM constellation.  Dots are reasonably well defined and positioned in a square, indicating good gain, phase noise and Modulation Error Ratio. Well Defined and away from the decision boundaries Well Positioned Dots in a Square

System Noise  A constellation displaying significant noise.  Dots are spread out indicating high noise and most likely significant errors. Dots are spread out causing errors to occur

Phase Noise  A display that appear to be rotating a the extremes indicates excessive phase noise.  Phase noise can be caused by headend down/up converters. Constellation With Phase Noise Zoomed Constellation With Phase Noise Rotation

Coherent Interference  If build up takes on a circular look, the problem is coherent interference.  Examples of coherent interference are CTB, CSO, spurs and ingress. Circular Cluster

Intermittent Interference  Isolated dots away from the main cluster indicate interference that is intermittent.  Examples of intermittent interference are intermittent ingress and laser clipping. Random Dots Away from the Cluster

Gain Compression  It the outer dots are pulled into the center while the middle ones are not effected, the signal has gain compression.  Gain Compression can be caused by IF and RF amplifiers and filters, up/down converters and IF equalizers. Outer Dots Pulled In

I Q Imbalance  I Q Imbalance is caused by a difference between the gain of the I and Q channels.  The display is taller than wide.  This indicates a problem with headend baseband amplifiers or filters. Taller than Wide

Automatic Constellation Diagnosis*  Analyzer automatically analyzes the constellation and displays the type of distortion present  Minimizes training and eliminates guess work.  By quickly type of distortion you can quickly find the source of the problem. Analysis of distortion *Patent Pending

Conclusions  Understanding the constellation display will go along way towards helping you understand QAM Modulation.  The constellation display can tell you many things about the health of the signal and be a valuable tool for locating and troubleshooting problems. CM1000 Cable Modem System Analyzer AT2000 Spectrum Analyzer