High Speed Circuits & Systems Laboratory Joungwook Moon 2011. 6.13

1. Power Comparison Between High-Speed Electrical and Optical Interconnects for Interchip Communication High Speed Circuits & Systems Laboratory Joungwook Moon 2011. 6.13

2. 1. Introduction 2. Optical Interconnect Power Dissipation 4. 5. 3. Electrical Interconnect Power Dissipation Comparison Between Electrical & Optical Conclusion Contents

3. 1. Introduction 3. 4. 5. 2. Optical Interconnect Power Dissipation Electrical Interconnect Power Dissipation Comparison Between Electrical & Optical Conclusion Contents

4. 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.

5. 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)

6. 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

7. 3. 4. 5. 1. 2. Electrical Interconnect Power Dissipation Comparison Between Electrical & Optical Conclusion Introduction Optical Interconnect Power Dissipation Contents

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 4. 5. 1. 2. Comparison Between Electrical & Optical Conclusion Introduction Optical Interconnect Power Dissipation 3. Electrical Interconnect Power Dissipation Contents

14. 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

15. 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

16. 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

17. 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

18. 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

19. Electrical Interconnect Power Dissipation • Summarizes noise sources in electrical interconnect • Assuming 5% mismatch between termination resistances and the characteristic impedance of PCB trace

20. 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

21. 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

22. 3. 5. 1. 2. Optical Interconnect Power Dissipation Electrical Interconnect Power Dissipation Conclusion Introduction 4. Comparison Between Electrical & Optical Contents

23. 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

24. 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

25. 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

26. 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.

27. 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

28. 3. 4. 1. 2. Optical Interconnect Power Dissipation Electrical Interconnect Power Dissipation Comparison Between Electrical & Optical Introduction 5. Conclusion Contents

29. 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