Presentation on theme: "2-14-2002 Constellations Demystified Presented by Sunrise Telecom Broadband … a step ahead."— Presentation transcript:
2-14-2002 Constellations Demystified Presented by Sunrise Telecom Broadband … a step ahead
2-14-2002 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.
2-14-2002 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.
2-14-2002 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
2-14-2002 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.
2-14-2002 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.
2-14-2002 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 0 0000111 Carrier Off = 0 Carrier On = 1 000100 001 101 010 110 011111 Four Levels of Amplitude Modulation With 180° Carrier Phase Reversal Transmits Eight Levels Data 000 to 111 180° Phase Reversal of Carrier
2-14-2002 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. 101 010 Local Osc 8 Level AM Modulator 8 Level AM Modulator Bit Stream Oscillator Shifted 90° Combiner 64 QAM Signal Q Component I Component
2-14-2002 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
2-14-2002 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
2-14-2002 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.
2-14-2002 64 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.
2-14-2002 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.
2-14-2002 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.
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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
2-14-2002 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