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Providing Infrastructure for Optical Communication Networks

Providing Infrastructure for Optical Communication Networks. Prof. Michael Green Dept. of EECS Henry Samueli School of Engineering mgreen@uci.edu. EECS 294 Colloquium 4 October 2006. This presentation can be found at: http://www.eng.uci.edu/faculty/green/public/courses/294.

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Providing Infrastructure for Optical Communication Networks

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  1. Providing Infrastructure for Optical Communication Networks Prof. Michael Green Dept. of EECS Henry Samueli School of Engineering mgreen@uci.edu EECS 294 Colloquium 4 October 2006 This presentation can be found at: http://www.eng.uci.edu/faculty/green/public/courses/294

  2. Friday, March 7 2003

  3. Advantages of Optical Fibers over Copper Cable • Very high bandwidth (bandwidth of optical transmission network determined • primarily by electronics) • Low loss • Interference Immunity (no antenna-like behavior) • Lower maintenance costs (no corrosion, squirrels don’t like the taste) • Small & light: 1000 feet of copper weighs approx. 300 lb. • 1000 feet of fiber weighs approx. 10 lb. • Different light wavelengths can be multiplexed onto a single fiber: • Dense Wavelength Division Multiplexing (DWM) • 10Gb/s transmission networks now being deployed; • 40Gb/s will be here soon.

  4. Protocols for High-Speed Optical Networks • Synchronous Optical Network (SONET): • Provides a protocol for long-haul (50-100km) wide-area netework • (WAN) fiber transmission • Basic OC-1 rate is 51.84Mb/s • OC-48 (2.5Gb/s) & OC-192 (10Gb/s) are common • Gigabit/10 Gigabit Ethernet (IEEE Standard 802.3): • Ethernet was invented in 1973 at Xerox PARC • (“ether” is the name of the medium through which E/M waves were thought to travel) • Provides a protocol for local-area network (LAN) copper or fiber • transmission • 1 Gb/s links can be transmitted over twisted-pair copper • 10 Gb/s links can be transmitter over copper (short lengths) or • fiber.

  5. Fiber Channel: • Often used for Storage Area Networks (SAN); allows fast • transmission of large amounts of data across many different • servers. • Currently 1-4 Gb/s is deployed; 8Gb/s will arrive soon.

  6. Some SAN Terminology JBOD: Just a Bunch Of Disks Refers to a set of hard disks that are not configured together. RAID: Redundant Array of Independent (or Inexpensive?) Disks Multiple disk drives that are combined for fault tolerance and performance. Looks like a single disk to the rest of the system. If one disk fails, the systems will continue working properly.

  7. Blade Servers vs. Regular Servers See: http://www.spectrum.ieee.org/WEBONLY/publicfeature/apr05/1106 for full article.

  8. Barcelona, Spain:MareNostrum supercomputer cluster (2282 Blade servers) Housed in Chapel Torre Girona (Technical Univ. of Catalonia)

  9. V V t t0 V Vh Vt Vl t t Characteristics of Broadband Signals & Circuits Primarily digital (i.e., bilevel) operation but high bit rate (multi-Gb/s) dictates analog behavior & design techniques. • Standard analog circuit applications: • Continuous-time operation • Precision required in signal domain (i.e., voltage or current) • Dynamic range determined by noise & distortion • Broadband communication circuits: • Discrete-time (clocked) operation • Precision required in time domain (low jitter) • Bilevel signals processed

  10. Eye diagram Typical broadband data waveform: Length of single bit = 1 Unit Interval (1 UI) An eye diagram maps a random bit sequence to a regular structure that can be used to analyze jitter.

  11. Close-up of eye diagram: trise = tfall voltage swing 1 UI Zero crossings

  12. What is Jitter? Jitter is the short-term variation of the significant instants of a digital signal from their ideal positions in time. Jitter normally characterizes variations above 10Hz; variations below 10Hz are called wander. The effects of these variations are measured in 3 ways: Phase noise (frequency domain) Jitter (time domain) Bit Error-Rate (end result of phase noise & jitter)

  13. Types of Jitter • Random Jitter (RJ) • Originates from external and internal random noise sources • Stochastic in nature (probability-based) • Measured in rms units • Observed as Gaussian histogram around zero-crossing • Grows without bound over time Histogram measurement at zero crossing exhibiting Gaussian probability distribution

  14. Types of Jitter (cont.) • Deterministic Jitter (DJ) • Originates from circuit non-idealities (e.g., finite bandwidth, offset, etc.) • Amount of DJ at any given transition is predictable • Measured in peak-to-peak units • Bounded and observed in various eye diagram “signatures” • Different types of DJ: • Intersymbol interference (ISI) • Duty-cycle distortion (DCD) • Periodic jitter (PJ)

  15. 1UI < 1UI If rise/fall time << 1 UI, then the output pulse is attenuated and the pulse width decreases. a) Intersymbol interference (ISI) Consider a 1UI output pulse from a buffer:

  16. ISI (cont.) Consider 2 different bit sequences: 1 0 0 1 1 0 Steady-state not reached at end of 2nd bit t = ISI 2 output sequences superimposed ISI is characterized by a double edge in the eye diagram. It is measured in units of ps peak-to-peak.

  17. Effect of ISI on eye diagram: Double-edge

  18. Crossing offset from nominal threshold b) Duty cycle distortion (DCD) Occurs when rising and falling edges exhibit different delays Caused by circuit mismatches Nominal data sequence Data sequence with early falling edges & late rising edges t = DCD Eye diagram with DCD

  19. Clock source with duty cycle t1 t0 c) Periodic Jitter (PJ) Timing variation caused by periodic sources unrelated to the data pattern. Can be correlated or uncorrelated with data rate. Synchronized data exhibiting correlated PJ Uncorrelated jitter (e.g., sub-rate PJ due to supply ripple) affects the eye diagram in a similar way as RJ.

  20. Jitter and Bit Error Rate R 0 T Probability of sample at t > t0 from left-hand transition: Probability of sample at t < t0 from right-hand transition:

  21. Total Bit Error Rate (BER) given by:

  22. Example: T = 100ps log(0.5) log BER t0 (ps) (64ps eye opening) (38ps eye opening)

  23. Bathtub Curves The bit error-rate vs. sampling time can be measured directly using a bit error-rate tester (BERT) at various sampling points. Note: The inherent jitter of the analyzer trigger should be considered.

  24. Benefits of Using Bathtub Curve Measurements Curves can easily be numerically extrapolated to very low BERs (corresponding to random jitter), allowing much lower measurement times. Example: 10-12 BER with T = 100ps is equivalent to an average of 1 error per 100s. To verify this over a sample of 100 errors would require almost 3 hours! t0 (ps)

  25. Deterministic jitter and random jitter can be distinguished and measured by observing the bathtub curve.

  26. Advantages of Using CMOS Fabrication Process • Compact (shared diffusion regions) • Very low static power dissipation • High noise margins (nearly ideal inverter voltage transfer • characteristic) • Very well modeled and characterized • Inexpensive (?) • Mechanically robust • Lends itself very well to high integration levels • SiGe BiCMOS has many advantages but is a generation behind • currently available standard CMOS

  27. CMOS gates generate and are sensitive to supply/ground bounce. Series R & L cause supply/ground bounce. Resulting modulation of transistor Vt’s results in jitter.

  28. data in data out clock in clock out Rs = 5WLs= 5nH clock out Rs = 0 Ls= 0 clock out Rs = 5W Ls= 5nH data out

  29. Inverter based on differential pair: • Differential operation • Inherent common-mode rejection • Very robust in the presence of common-mode • disturbances (e.g., VDD/VSS bounce) “Current-mode logic (CML)”

  30. data in data out clock in clock out Rs = 5WLs= 5nH clock out Rs = 0 Ls= 0 clock out Rs = 5W Ls= 5nH data out

  31. Research Topics • BiCMOS 10Gb/s Adaptive Equalizer • A Novel CDR with Adjustable Phase Detector Characteristics • A Distributed Approach to Broadband Circuit Design

  32. Research Topics • BiCMOS 10Gb/s Adaptive Equalizer Evelina Zhang, Graduate Student Researcher • A Novel CDR with Adjustable Phase Detector Characteristics • A Distributed Approach to Broadband Circuit Design

  33. Cable Model magnitude (dB) +10 0 shorter cable -10 Copper Cable longer cable -20 -30 10G 1G 100M f phase (deg) 0 shorter cable -100 Where: L is the cable length a is a cable-dependent characteristic longer cable -200 -300 100M 10G 1G f

  34. Reduce ISI Improve receiver sensitivity Motivation input waveform (V) input eye 0.5 0.5 0 0 -0.5 -0.5 300 200 100 0 43 42 41 39 40 t (ps) t (ns) output waveform (V) output eye 0.3 0.3 0 0 -0.3 -0.3 300 200 100 0 43 42 41 39 40 t (ps) t (ns)

  35. Adaptive Equalizer • Implemented in Jazz Semiconductor SiGe process: • 120GHz fT npn • 0.35m CMOS

  36. Equalizer Block Diagram

  37. Feedforward Path

  38. FFE Frequency Response Vcontrol f (Hz)

  39. ISI & Transition Time VFFE 0.3 teq= 45ps PW = 108ps teq= 60ps PW = 100ps 0 teq= 75ps PW = 86ps -0.3 2.4 2.5 2.6 2.7 2.8 t (ns) • Simulations indicate that ISI correlates • strongly with FFE transition time teq. • Optimum teq is observed to be 60ps.

  40. Slicer

  41. Feedback Path

  42. Transition Time Detector DC characteristic: Transient Characteristic: (b) (a) (b) (a) t • Rectification & filtering done in a single stage.

  43. Integrator

  44. Detector + Integrator FFE transition Time tFFE From Slicer tslicer=60ps From FFE tFFE Vcontrol (mV) 90ps 60 slope detector slope detector 40 75ps 20 60ps 0 -20 45ps -40 15ps -60 0 10 20 30 40 50 _ + t (ns) Vcontrol

  45. System Analysis integrator feedforward equalizer Vcontrol tslicer teq detector + Kd H(s) Keq _ detector Kd Keq = 1.5 ps/mV Kd = 2.5 mV/ps tint = 75ns

  46. Measurement Setup EQ inputs Die under test EQ outputs 231 PRBS signal applied to cable

  47. Eye Diagrams EQ output EQ input 4-foot RU256 cable 4.0ps rms jitter 15-foot RU256 cable 3.9ps rms jitter

  48. Summary of MeasuredPerformance

  49. Ongoing Research • Investigate transition detector more thoroughly • Understand trade-off between ISI reduction and random jitter generation • Investigate compensation of PMD in optical fiber

  50. Random noise in Analog Equalizer output eye ISI: 6.2ps p-p input eye (no noise added)

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