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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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slide1

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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