EFFECT OF ASE NOISE ON M-QAM FIBER OPTIC LINK PERFORMANCE

1. EFFECT OF ASE NOISE ON M-QAM FIBER OPTIC LINK PERFORMANCE Jennifer Doris James

2. Overview • Fiber Optic Basics • Erbium Doped Fiber Amplifiers • Digital Communications • Experimental Procedure • Experimental Results • Conclusions

3. Why Fiber Optics? • Advantages of Fiber Optic Communications: • Low loss • Large bandwidth • Immunity to electromagnetic interference • High propagation delay stability • No Radiation • Reliability • Economy • Parallel transmission • Flexibility & Ruggedness

4. Optical Waveguides • Total Internal Reflection • Operates in 800 to 1600 nm range • Transmission windows with low attenuation

5. Basic Fiber Optic Link • 4 Major components required: • Light source • Modulator • Optical fiber • Photodetector

6. WDM Fiber Optic Communications • Transmission of multiple wavelengths simultaneously on a single fiber. • Allows for multiple data formats to be transmitted simultaneously.

7. Purpose of Research • Evaluation of the performance of M-QAM fiber optic links. • Primary source of noise under investigation: Amplified Spontaneous Emission noise

8. Overview • Fiber Optic Basics • Erbium Doped Fiber Amplifiers • Digital Communications • Experimental Procedure • Experimental Results • Conclusions

9. Erbium Doped Fiber Amplifier (EDFA) • Incoming optical signal is coupled with light from a pump laser. • Both are transmitted over a length of doped optical fiber • The amplified optical signal is output after any residual light from the pump laser is filtered.

10. Operation of EDFA • Erbium ions are excited into the pump band by a pump laser • Amplification is achieved by stimulated emission of photons from the excited Er3+ ions in the fiber.

11. Amplifier Gain • EDFA gain is non-uniform and dependent upon the input power. Ps,out = signal output power Ps,in = signal input power Pp,in = input pump power lp = pump wavelength ls = signal wavelength

12. Amplified Spontaneous Emission (ASE) • Amplified Spontaneous Emission (ASE) noise arises from the spontaneous recombination of electrons and holes in the amplifying medium. • Emitted photons resulting from this transition has random phase and amplitude.

13. Power Spectral Density of noise resulting from ASE • Single side band PSD of ASE noise: G = amplifier gain hf = photon energy c = a parameter used to account for the non-uniform carrier density due to gain saturation hsp = population inversion factor n1 = fractional density of electrons in state 1 n2 = fractional density in state 2

14. PSD of RF Phase noise due to ASE • In communications schemes that transmit information in both the amplitude and phase of the signal, such as M-QAM, RF phase noise must also be taken into consideration. Go = small signal gain h = quantum efficiency of the EDFA Pin is the input power to the EDFA Psat and a are parameters associated with gain saturation of the EDFA Fn is the noise figure of the EDFA in dB

15. RF Phase noise due to ASE • As shown in the previous equation, RF phase noise is also dependent upon the gain of the amplifier. • Experimental results previously obtained also confirm this as shown in this graph.

16. Overview • Fiber Optic Basics • Erbium Doped Fiber Amplifiers • Digital Communications • Experimental Procedure • Experimental Results • Conclusions

17. The Digital Cliff • Digital signals are not as affected by distortion and interference as analog signals. • In fact, the quality of the signal does not degrade until the signal to noise ratio passes a certain threshold. • Unfortunately, once it passes the threshold, the signal degrades quickly, leading to a phenomenon referred to as the “digital cliff”.

18. Quadrature Amplitude Modulation (QAM) • Quadrature Amplitude Modulation (QAM) modifies the phase and amplitude of the signal simultaneously. • QAM offers increased data throughput and spectral efficiency. • QAM is the standard for • Data Over Cable Service Interface Specification (DOCSIS) • Digital Video Broadcasting – Cable (DVB-C) • Digital television, as standardized by the Society of Cable Television Engineers (SCTE)

19. QAM Transmitter • QAM is accomplished by varying the amplitude of two sinusoidal waveforms that are in quadrature (a 90 degree phase shift) to each other respectively and summing them together. • This allows the effective transmission of two channels at the same frequency, thereby doubling the bandwidth or rate at which the data is transmitted.

20. QAM Signal Representation • Can be represented as a linear combination of two orthonormal waveforms, f1(t) & f2(t). • Amc and Ams, are the amplitudes of the signals of the quadrature carriers • g(t) is the signal pulse • eg is the energy of the pulse

21. QAM Signal Impairments • Common impairments • Additive white Gaussian noise (AWG) • RF phase noise • Coherent interference • Gain compression • I/Q phase error • I/Q gain error

22. QAM Performance Analysis • In determining the performance of the QAM signal, there are several tools available: • Constellation Diagram • Eye Diagrams • Bit Error Rate • Modulation Error Ratio • Error Vector Magnitude • Magnitude and Phase Error

23. Constellation Diagrams • QAM signals can be represented as vectors, in terms of the inphase, I, and quadrature, Q, components. • I and Q data can be displayed on a 2 dimensional graphs known as constellation diagrams. • Each point on the I/Q plane represents a symbol, which in turn represents a specific set of bits. • By increasing the number of symbols, M, the number of bits represented by each symbol is increased as well. M = 2N, where N = number of bits per symbol.

24. Eye Diagrams • Constructed by superimposing the signal repetitively on itself several times. • Eye diagrams can also give indications of timing deviations and distortions. Time variation at zero crossing Distortion SNR at sampling point

25. Bit Error Rate (BER) • Defined as the ratio of bits received in error to the number of bits transmitted. • Bit Error Rate for M-QAM with Gray coding applied: Where: And: gs is the average carrier to noise ratio of the link and M is the number of possible QAM symbols

26. Modulation Error Ratio (MER) • MER is the ratio of the RMS error magnitude over the average symbol magnitude. • National Instruments defines MER over N number of symbols as I & Q components of the j-th received symbol ideal I & Q components of the j-th symbol

27. POOR MER GREAT BER GOOD MER GREAT BER GREAT MER GREAT BER Why use MER? • Measures the mean deviation of received symbols from the ideal symbol values • Incorporates all impairments of QAM • Chosen as the preferred measurement for cable TV by the SCTE • Declared an international standard by the DVB Measurement Group • MER provides an early indication of the performance of the signal and is a good indicator of non-transient impairments of the system. • Analysis of the BER alone would not give any indication that the system is approaching the digital cliff and imminent failure A signal with a poor MER would still be showing excellent BER results.

28. Error Vector Magnitude (EVM) • Expressed as a percentage, provides the same information as MER. • EVM is the ratio of the error vector and the ideal vector, e/v , where e = w – v .

29. Magnitude and Phase Error • Magnitude error is the difference between the ideal magnitude and the measured magnitude. • Magnitude errors affect I and Q equally, resulting in movement of the symbols away or toward the origin on a constellation diagram. • Similarly, phase errors, shown as q in the previous diagram, are the difference between the ideal phase and the measured phase of the received symbol. • Both measurements are represented as percentages, similar to the EVM.

30. Overview (cont.) • Fiber Optic Basics • Erbium Doped Fiber Amplifiers • Digital Communications • Experimental Procedure • Experimental Results • Conclusions

31. QAM Signal Generation & Analysis

32. QAM Signal Generation & Analysis (cont.) • QAM Generation VI • The data stream is a pseudo-random sequence of bits continually repeated until the program is stopped by the user. • The baseband signal is then upconverted to the appropriate frequency by the vector generator. • Capable of generating 4, 8, 16, 32, 64, 128, and 256 QAM. • Pulse shaping filter, resampling of the data, and assurance of the phase continuity of the data is also handled in the program. • QAM Demodulation VI • Analog signal is acquired and downconverted • Downconverted signal, represented as a complex waveform, is resampled and passed along to the demodulator • Output of the demodulator gives us the “recovered waveform” with the frequency and phase offset removed

33. Fiber Optic Link Test Setup

34. Link Configurations

35. Lab Equipment • National Instruments PXI-5670, 2.7 GHz RF Vector Generator • Ando Electric Corporation AQ4321D Tunable Laser • JDS Uniphase AM-150 APE microwave analog intensity modulator • Corning SMF-28 Optical fiber • Fiber Instrument Sales Erbium Doped Fiber Amplifier (FIS EDFA) • New Focus 3.5 GHz InGaAs Photoreceiver, Model 1592 • Inline Variable Attenuators, Blocking style

36. Gain Characteristic of FIS EDFA • As mentioned previously, the gain of the EDFA is dependant upon the input power of the device.

37. Overview (cont.) • Fiber Optic Basics • Erbium Doped Fiber Amplifiers • Digital Communications • Experimental Procedure • Experimental Results • Conclusions

38. Eye Diagrams16 QAM

39. Constellation Diagrams16 QAM Performance

40. Visual Comparisons 16 QAM Gain = 44 dB, MER =19.7 dB Gain = 27 dB, MER = 35.7 dB

41. Visual Comparisons 64 QAM Gain = 27 dB, MER = 34.8 dB Gain = 42 dB, MER =22.3 dB

42. Visual Comparisons 256 QAM Gain = 27 dB, MER = 34.4 dB Gain = 36.9 dB, MER =27.9 dB

43. Modulation Error Ratio Performance

44. Modulation Error Ratio Performance (cont.)

45. Modulation Error Ratio Performance (cont.)

46. Error Performance

47. BER Performance

48. Comparative ResultsWavelength Comparison

49. Comparative ResultsM-level Comparison

50. Comparative ResultsLink Configuration Comparison