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Power Comparison Between High-Speed Electrical and Optical Interconnects for Interchip Communication. High Speed Circuits & Systems Laboratory Joungwook Moon 2011. 6.13. 1. Introduction. 2. Optical Interconnect Power Dissipation. 4 . 5. 3.

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Power Comparison Between High-Speed Electrical and Optical Interconnects for Interchip Communication

High Speed Circuits & Systems Laboratory

Joungwook Moon

2011. 6.13


Contents

1.

Introduction

2.

Optical Interconnect Power Dissipation

4.

5.

3.

Electrical Interconnect Power Dissipation

Comparison Between Electrical & Optical

Conclusion

Contents


Contents1

1.

Introduction

3.

4.

5.

2.

Optical Interconnect Power Dissipation

Electrical Interconnect Power Dissipation

Comparison Between Electrical & Optical

Conclusion

Contents


Introduction
Introduction

  • About Paper

  • Author

  • Presents an optimization scheme to minimize optical interconnect powerand quantify its performance.

  • Examine the power dissipation of a state-of-art electrical interconnection.

  • Comparisons between optical and electrical interconnects, BW at 6Gb/s at 100nm technology.


Introduction1
Introduction

  • Different classes of digital systems impose specific requirements on the communication medium

  • Long-haul systems use optical fibers :

    low attenuation at high bandwidths

  • Shorter systems traditionally use Cu interconnects

  • The mordern IC increases dramatically, and applications are struggling to keep up bandwidth

     Optical medium of communication to penetrate the short distance world

  • Cabinet level(1~100m) - (O)

  • Backplane level between boards (10cm ~1m) –(O)

  • Chip to chip level ( < 10cm) – Optoelectric conversion overhead,

  • Microprocessor ~ DRAM Latency issue (X) BUT...

  • On chip level( < 2cm) – Cheaper, Power, low swing (X)


Introduction2
Introduction

  • Digital systems with communication bandwidth limitation can benefit enormously from the choice of optical medium

    • limitated board space, connector density, pin count, insufficient SNR, ISI, noise , crosstalk, impedance mismatch, package induced reflection, etc...

  • In this paper, more comprehensive view of both Cu and optical systems for short distance, off-chip, bandwidth-sensitive applications

  • Compare power dissipation with relevant parameter

    • Bandwidth, Interconnect length, and bit error rate


  • Contents2

    3.

    4.

    5.

    1.

    2.

    Electrical Interconnect Power Dissipation

    Comparison Between Electrical & Optical

    Conclusion

    Introduction

    Optical Interconnect Power Dissipation

    Contents


    Optical interconnect power dissipation
    Optical Interconnect Power Dissipation

    • Off-chip Laser source @ λ=1.3um

    • CMOS driven MQW(Multiple Quantum Well) modulator

      • (InP-based, hybrid-bonded to Si-CMOS)

    • Reverse-biased PIN quantum-well detector & modulator

    Transmitter

    Receiver

    MQW Bandgap


    Optical interconnect power dissipation1
    Optical Interconnect Power Dissipation

    A. Modulator Power Dissipation

    • Both dynamic & static modulator power dissipation are considered.

      • Dynamic power : Capacitance of modulator and buffer-chain

      • Static power : absorbed optical power in “ON” and “OFF” state

        • (Ideal modulator – “ON” state power absorption = 0 (IL=0) )

    • Power dissipation

      IL = Insertion Loss (optical power absorbed during the “on” state)

      CR = Contrast Ratio (ratio of modulator output optical power in “on” & “off” states)

      Poptrec = average optical power at the receiver

      η = optical power transfer efficiency v = frequency of the laser source ,

      Vbias = DC bias applied to the modulator , Vdd = voltage swing

    from Ref. 21

    Power loss = 0.082 dB/cm @ λ =1.3um


    Optical interconnect power dissipation2
    Optical Interconnect Power Dissipation

    B. Receiver Power Dissipation

    • Optical receiver : photodetector + nonintegratingtransimpedance amplifier + gain stage

      • (Its design and power dissipation is detailed in an early work.)

    • The analytical design model was verified through Spice simulation

    from Ref. 26

    Gain stage


    Optical interconnect power dissipation3
    Optical Interconnect Power Dissipation

    C. Power Dissipation Minimization

    • The increase in optical power increase the modulator power , but decreases the receiver power

       Finding optimal laser power at total interconnect power (receiver and modulator) is minimized

    • Receiver power doesn’t change with laser power beyond a certain point

    • Receiver power is dominant

    • Modulator2 is larger power dissipation than modulator1

    • A higher loss (6dB) : lower reflectivity difference between “on & off” state at the receiver

    Commonly used reflective Modulator

    Ideal Modulator


    Optical interconnect power dissipation4
    Optical Interconnect Power Dissipation

    • Optimum laser power and resulting minimum power dissipation as a function of loss for two different bit rates

    Tech scale down

    • Increase in the power dissipation with bit rate

    •  entirely due to the receiver power at higher bit rate

    • Technology scaling reduce power dissipation (100&50 nm)

    • Detector capacitance of 250fF ( somewhat pessimistic)


    Contents3

    4.

    5.

    1.

    2.

    Comparison Between Electrical & Optical

    Conclusion

    Introduction

    Optical Interconnect Power Dissipation

    3.

    Electrical Interconnect Power Dissipation

    Contents


    Electrical interconnect power dissipation
    Electrical Interconnect Power Dissipation

    • Full-duplex channel, provides higher BWover smaller number of pins

    • Transmitter replica usedto isolate the received and transmitted signal

    • Low swing current mode, bipolar, differential signaling scheme – maximum noise immunity


    Electrical interconnect power dissipation1
    Electrical Interconnect Power Dissipation

    • High performance GETEK board : expensive than FR4

      • Provide lower dielectric loss, lower signal attenuation

    • Using a transmitter side pre-emphasis equalization

      (multi-tap FIR filter)

    • Several Assumption : small rise time for lower noise, reduce channel crosstalk due to PKG. reflection

    1mil = 1/1000 inch

    Full consideration was given to maximize electrical interconnect performance

    for fair comparison with its optical counterpart


    Electrical interconnect power dissipation2
    Electrical Interconnect Power Dissipation

    • Stark difference between electrical & optical media :

      • Power dissipated in the termination resistors related to current swing requirement

      • This power critically depends on the attenuation and noise characteristics of interconnects

    • Modeling the attenuation and noise source

      : function of the bit rate and length

    • The net required noise margin for adequate BER

    VSNR : voltage SNR

    Vnm : the net noise margin (difference of half the signal swing and the sum off all worst-case noise source at the receiver)

    VGaussian : Standard deviation of all the statistical noise source


    Electrical interconnect power dissipation3
    Electrical Interconnect Power Dissipation

    • Netavailable noise margin at the receiver

      (1) the attenuated signal swing

      (2) the sum of all worst-case noise sources

      • Proportional to signal swing :

        • Transmitter –end : attenuated by the trace

          • (KA – trace crosstalk, impedance mismatch, PKG. reflection)

        • Receiver-end : not attenuated by the board trace

          • (KU – reverse channel crosstalk, transmitter replica mismatch, PKG reflec)

      • Fixed noise source (VNF) :

        • Receiver offset and its sensitivity

    • The available net noise margin should be greater than

      the required net noise margin

    Vswtrans : Swing at the transmitter

    A : attenuated fraction of the signal


    Electrical interconnect power dissipation4
    Electrical Interconnect Power Dissipation

    • The required one way swing Current (I0)

    • Total power dissipated in the termination resistance

    • The other sources of power dissipation

      • Transmitter and receiver logic circuit power ( about 100uA )

      • Equalization power – neglected ( # of taps are matter)

      • Additional transmitter for canceling the PKG. reflections

      • Clock and timing circuits for clock recovery – not considered

    Replica transmitter circuit

    Each power of termination resistance


    Electrical interconnect power dissipation5
    Electrical Interconnect Power Dissipation

    • Summarizes noise sources in electrical interconnect

      • Assuming 5% mismatch between termination resistances and the characteristic impedance of PCB trace


    Electrical interconnect power dissipation6
    Electrical Interconnect Power Dissipation

    • Multi-gigabit data rate, attenuation due to the skin effect loss, and dielectric loss become extremely important

    • Dielectric loss becomes

    • more limiting at high

    • frequency


    Electrical interconnect power dissipation7
    Electrical Interconnect Power Dissipation

    • For a given interconnect length, there is a maximum allowed bit rate(10cm ~ 100cm)

    • The power dissipation will become much higher before this limit is reached

    • The maximum bit rate for two

    • different swing requirements

    • is shown


    Contents4

    3.

    5.

    1.

    2.

    Optical Interconnect Power Dissipation

    Electrical Interconnect Power Dissipation

    Conclusion

    Introduction

    4.

    Comparison Between Electrical & Optical

    Contents


    Comparison between electrical and optical interconnects
    Comparison Between Electrical and Optical Interconnects

    • Electrical interconnect power rises with length and

      bit rate due to a larger attenuation

    • Beyond a critical length, optical interconnect yields lower power

    • This critical length reduces at higher bit rates


    Comparison between electrical and optical interconnects1
    Comparison Between Electrical and Optical Interconnects

    • Quantify the impact of critical device/system parameters

      • Optical interconnect : detector/modulator capacitance,

        coupling loss, ideal modulator1

    • Coupling loss and detector capacitance play a pivotal role in dictating critical length


    Comparison between electrical and optical interconnects2
    Comparison Between Electrical and Optical Interconnects

    • Modulator 2, this length gradually reduces to about 40cm at 15Gb/s

    • Apparent saturation of critical length at high bit rate

    • the electrical interconnection :

    • With bit rate increase, the impact of worsening trace attenuation on power dissipated in the termination resistance


    Comparison between electrical and optical interconnects3
    Comparison Between Electrical and Optical Interconnects

    • Different BER is demanded indifferent system applications

    • High BER can be tolerated if explicit error correction schemes are utilized

    • For small BER values, the critical lengths are smaller and optical interconnects have advantage over electrical interconnects.


    Comparison between electrical and optical interconnects4
    Comparison Between Electrical and Optical Interconnects

    • Sensitivity of critical length on the mismatch between termination impedances and the characterization impedance of the PCB trace

    • Critical length increase with small reduction in the impedance mismatch


    Contents5

    3.

    4.

    1.

    2.

    Optical Interconnect Power Dissipation

    Electrical Interconnect Power Dissipation

    Comparison Between Electrical & Optical

    Introduction

    5.

    Conclusion

    Contents


    Conclusion
    Conclusion

    • Extensive power dissipation comparison between electrical and optical interconnects for bandwidth sensitive application in 10cm to 1m range interconnects

    • Beyond a critical length, power optimized optical interconnects dissipate lower power

    • At higher bitrates and lower BER, the critical length reduces and optics becomes more power favorable

    • Optical interconnects are superior; lower attenuation and lower noise

    • Their downside ; need extra power for conversion from electronics to optics and vice versa


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