Photonic network-on-chip (PNoC) architectures are a potential candidate for communication in future chip multi-processors as they can attain higher bandwidth with lower power dissipation than electrical NoCs. PNoCs typically employ dense wavelength division multiplexing (DWDM) for high bandwidth transfers. Unfortunately, DWDM increases crosstalk noise and decreases optical signal to noise ratio (SNR) in microring resonators (MRs) threatening the reliability of data communication. Additionally, process variations induce variations in the width and thickness of MRs causing shifts in resonance wavelengths of MRs, which further reduces signal integrity, leading to communication errors and bandwidth loss. In this paper, we propose a novel encoding mechanism that intelligently adapts to on-chip process variations, and improves worst-case SNR by reducing crosstalk noise in MRs used within DWDM-based PNoCs. Experimental results on the Corona PNoC architecture indicate that our approach improves worst-case SNR by up to 44.13%.

1. MWSCAS 2015
Fort Collins, Colorado, USA
August 2-5, 2015
Process Variation Aware Crosstalk Mitigation
for DWDM based Photonic NoC Architectures
Sai Vineel Reddy Chittamuru, Ishan Thakkar and Sudeep Pasricha
Department of Electrical and Computer Engineering
Colorado State University, Fort Collins, CO, U.S.A.
{sai.chittamuru, ishan.thakkar, sudeep}@colostate.edu
DOI: 10.1109/ISQED.2016.7479176

2. • Introduction
• Motivation and Contributions
• Related Work
• Impact of Localized Trimming on Crosstalk
• Double-bit Crosstalk Mitigation Technique
• Experimental Results
• Conclusion
Outline
1

3. Introduction
• Execution of modern complex applications necessitates
 Many-core processors
• To enable chip many-core processors (CMPs)
 Efficient communication fabrics are essential
Eletrical buses are no longer scalable
Electrical networks-on-chip (NoCs) are more viable
• With increase in core count, electrical NoC has
 Higher power dissipation
 Reduced performance (increased latency)
2
Mellonox 72-core chip
Intel Xeon Phi 60 core processor
To address drawbacks of electrical NoCs
Several new interconnect technologies are being explored

4. Benefits of Photonic Interconnects
3Source: L. Xu, et al. IEEE-PTL, 2012 and S. V. R. Chittamuru, et al. GLSVLSI 2015
• Photonic interconnects are potential solution to address
drawbacks of copper wire based electrical interconnects
• Advantages of photonic interconnects over copper wires:
 High bandwidth (~40 Gbps) with DWDM (dense
wavelength division multiplexing)
5× or higher compared to copper wires
 Low latency (10.45 ps/mm)
10× faster than copper wires
 Low power (7.9 fJ/bit)
 Better scalability, no pin limits
Photonic links for data communicationNoCs that use photonic interconnects provide higher
bandwidth with lower power consumption
Microring Resonator

5. 4
Introduction to Photonic Elements
Modulator Detector
Electrical
Bit-stream
Electrical
Bit-stream
010101
Modulators and detectors perform E/O and O/E conversion of data
• Microring (MR) resonator operation with ON/OFF keying modulation
 Modulator to write data
 Detector to read data
SiGe
DopedWaveguide
Microring
Resonator
Circular
waveguide with
diameter 5µm
Trans Impedance
Amplifier (TIA)
E/O: Electrical to Optical and O/E: Optical to Electrical
101010010101010010100

6. Ideal Photonic Link Overview
5
Electrical
Bit-stream
Electrical
Bit-stream
Electrical
Bit-stream
Electrical
Bit-stream
MR Modulators
SiGe Doped
MR Detectors Trans Impedance
Amplifier (TIA)
Waveguide
Four DWDM (Dense
Wavelength Division
Multiplexing)
In real world, photonic
link is not ideal
MR: Micro Ring

7. 6
• Existence of process variation also incurs crosstalk in
DMDM based photonic NoCs
MR Modulators
MR Detectors
SiGe Doped
TIA
Waveguide
Process variation causes
resonance wavelength drift
Unable to write on
dedicated wavelengths
Suppose modulation side
successfully writes data
Process variation causes
wavelength drift in detector
Read wrong data
(data corruption)
Process Variation Impact on Photonic Link
MR: Micro Ring

8. PV-Induced Crosstalk in Photonic Link
7
Electrical
Bit-stream
Electrical
Bit-stream
MR modulators
SiGe doped
MR detectors
Trans Impedance
Amplifier (TIA)
Waveguide
Crosstalk noise in
detector
Crosstalk noise
in waveguide
Electrical bit-streams with noise
• PV-induced crosstalk noise in ring detectors
 Decreases Signal to Noise Ratio (SNR)
 Increases Bit Error Rate (BER)
 Threatens reliable photonic communication
Crosstalk noise
in modulator
MR: Micro Ring

9. PV-Induced Crosstalk in Photonic Link
8
Electrical
Bit-stream
Electrical
Bit-stream
MR modulators
SiGe doped
MR detectors
Trans Impedance
Amplifier (TIA)
Waveguide
Crosstalk noise in
detector
Crosstalk noise
in waveguide
Electrical bit-streams with noise
• PV-induced crosstalk noise in ring detectors
 Decreases Signal to Noise Ratio (SNR)
 Increases Bit Error Rate (BER)
 Threatens reliable photonic communicationPV-induced crosstalk noise in MR detector needs to be mitigated
for reliable photonic communication
Crosstalk noise
in modulator
MR: Micro Ring

10. 9
Voltage Tuning (Trimming):
=VON
VR
Input Port Output Port
n+
p+ n+
Thermal Tuning:
Input Port Output Port
Micro Heater
Wavelength
PowerTransmission
Voltage Tuning
Blue Shift
Wavelength
PowerTransmission
Thermal Tuning
Red Shift
These solutions increase intrinsic optical loss and crosstalk noise in
MRs and motivate new crosstalk mitigation mechanisms
How to Tolerate Process Variations?

11. Our Contributions
10
• Analytical models for PV-aware crosstalk
analysis
Impact of localized trimming on crosstalk
Crosstalk modeling for Corona PNoC
• Double bit crosstalk mitigation (DBCTM)
technique
To reduce crosstalk noise in PV-affected PNoCs
• Explore impact of DBCTM on DWDM-based
PNoCs
Analysis in terms of worst-case SNR
DBCTM performance implications
PNoC: Photonic Networks-on-chip
Corona PNoC

12. • Introduction
• Motivation and Contributions
• Related Work
• Impact of Localized Trimming on Crosstalk
• Double-bit Crosstalk Mitigation Technique
• Experimental Results
• Conclusion
Outline
11

13. Device Level Crosstalk:
• [C. H. Chen WOCC 2012] Crosstalk noise in single waveguide crossings is
shown to be close to -47.58 dB
• [Q. Xu, et al. Opt. Exp. 2006] A cascaded MR-based modulator is proposed
for low-density DWDM waveguides, with an extinction ratio of 13dB
• These works show that crosstalk noise is negligible at device level
Network Level Crosstalk and Mitigation:
• [L.H.K. Duong, et al. IEEE D&T 2014] Crosstalk analysis for the Corona
PNoC, where its data channels are studied and worst-case SNR is
estimated to be 14dB
• [S. V. R. Chittamuru, et al. IEEE D&T 2015] two encoding techniques
PCTM5B and PCTM6B are presented to mitigate the impacts of
crosstalk noise in DWDM based PNoCs.
• These works do not consider process variations and their impact on
crosstalk
Related Work
12
None of these works consider PV-aware crosstalk mitigation

14. Impact of Localized Trimming on Crosstalk
13
𝜱 𝒊, 𝒋 =
𝜹 𝟐
(𝒊 − 𝐣)
𝑭𝑺𝑹
𝒏
𝟐
+ 𝜹 𝟐
𝑯𝒆𝒓𝒆 𝜹 =
𝝀𝒋
𝟐𝑸′
𝝺 𝑛𝝺 𝑛+1 𝝺 𝑛−1
TRANSMISSION
1
0
Ideal condition of MR passbands (without PV)
Increase in resonance wavelength
• We model passband overlap
with coupling factor (𝚽)
• With PV, passband shifts due
to change in refractive index
• Suppose PV induces red shift
• Trimming is used to
compensate this resonance
drift
• Passband overlap increases
with trimming of MRs
Passband overlap region
𝝺 𝑛𝝺 𝑛+1 𝝺 𝑛−1
TRANSMISSION
1
0
MR passbands with PV
Increase in resonance wavelength
Red Shift
𝝺 𝑛𝝺 𝑛+1 𝝺 𝑛−1
TRANSMISSION
1
0
MR passbands with PV after trimming
Increase in resonance wavelength
Increase in passband overlap region
Coupling factor increases with trimming of MRs

15. • With localized trimming
 Q-factor (Q’) of MR decreases
 Coupling factor (𝚽) and crosstalk noise increases
Impact of Localized Trimming on Crosstalk
14
Our work decreases crosstalk noise and improves
SNR in DWDM based PNoCs
0
2000
4000
6000
8000
10000
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
Q-factor
Increaseincouplingfactor
(φ)
Compensated PV-induced resonance shift (in nm)
increase in coupling factor
Q-factor

16. Double-Bit Crosstalk Mitigation Technique
15
• Crosstalk noise in PNoCs increases with
 Coupling factor (𝚽)
 Signal strength of adjacent non-resonant
wavelengths
• Localized trimming increases 𝚽
• DBCTM reduces crosstalk noise
 Modulates zero on alternate wavelengths
 Modulated zeros are shield bits
 Reduces signal strength of adjacent
non-resonant wavelengths
• Resonance shift has linear dependency on
length and width variation
Divide MRs in each
detecting node into
groups of 8 MRs
Determine the
thickness and width
variation in each MR
using SE and CD-SEM
Determine maximum
PV-induced resonance
red shifts (Δ𝛌max) in
each MR Group
Yes
Enable DBCTM
encoding in this
MR Group
Disable DBCTM
encoding in this
MR Group
No
Δ𝝺max> Δ𝝺th
DBCTM Technique

17. • We analyzed our DBCTM technique by porting it to Corona PNoC
 [D. Vantrease et al. MICRO 2009] Corona architecture with token slot
arbitration and 64×64 multiple write single read (MWSR) crossbar
• CMP configuration for implementation for Corona PNoC
Experimental Setup
16
Chip Many Core Configuration
Number of cores 256
Technology node 22nm
Memory controllers 32
Main memory 32GB; DDR4@30ns
Per Core:
L1 I-Cache size/Associativity 16KB/Direct Mapped Cache
L1 D-Cache size/Associativity 16KB/Direct Mapped Cache
L2 Cache size/ Associativity 128KB/ Direct Mapped Cache
L2 Coherence MOESI
Frequency 5 GHz
Issue Policy In-order

18. • Built a cycle accurate photonic network simulator in SystemC
• Trace driven simulations using GEM5 simulator (PARSEC benchmarks)
• 12 multithreaded application workloads from PARSEC benchmark
• Model and estimate PV in MRs using the VARIUS tool
• 100 process variation maps are considered for our evaluation
• Performance modeling using DSENT, CACTI 6.5, and circuit-level analysis
• Static and dynamic power/energy for photonic devices:
Source: [P. Grani, et al. JETC 2014] and [L.H.K. Duong, et al. IEEE Design and Test, 2014]
17
Energy consumption type Energy
Edynamic 0.42 pJ/bit
Elogic−dyn 0.18 pJ/bit
Photonic loss type Loss (in dB)
Propagation loss -0.274 per cm
Bending loss -0.005 per 90o
Inactive modulator through loss -0.0005
Active modulator power loss -0.6
Passing detector through loss -0.0005
Detecting detector power loss -1.6
Active modulator crosstalk coefficient -16
Detecting detector crosstalk coefficient -16
Performance and Energy Models

19. 18
Worst-Case SNR Sensitivity Analysis
• Corona DBCTM X%
 Has X% ratio of shielding
bits to data bits
 Shielding bits are zeros
between data bits
 Shielding bits increase
laser and static power
• In Corona DBCTM X%
 Increase in shielding bits to data bits ratio
 reduces crosstalk noise
 Increases SNR
 Increases power consumption
• Worst SNR of Corona with DBCTM compared to its baseline
 25% shielding bits - 8.1% higher
 50% shielding bits – 19.67% higher
 75% shielding bits - 26% higher
 100% shielding bits – 40.5% higher
Corona: D. Vantrease et al. MICRO 2009
Increase in shielding bits of DBCTM
• Power consumption of Corona with DBCTM compared to its baseline
 25% shielding bits - 14% higher
 50% shielding bits - 20.1% higher
 75% shielding bits - 63.9% higher
 100% shielding bits - 104.1% higher

20. 19
Worst-Case SNR Sensitivity Analysis
• Corona DBCTM X%
 Has X% ratio of shielding
bits to data bits
 Shielding bits are zeros
between data bits
 Shielding bits increase
laser and static power
• In Corona DBCTM X%
 Increase in shielding bits to data bits ratio
 reduces crosstalk noise
 Increases SNR
 Increases power consumption
• Worst SNR of Corona with DBCTM compared to its baseline
 25% shielding bits - 8.1% higher
 50% shielding bits – 19.67% higher
 75% shielding bits - 26% higher
 100% shielding bits – 40.5% higher
Corona: D. Vantrease et al. MICRO 2009
Increase in shielding bits of DBCTM
• Power consumption of Corona with DBCTM compared to its baseline
 25% shielding bits - 14% higher
 50% shielding bits - 20.1% higher
 75% shielding bits - 63.9% higher
 100% shielding bits - 104.1% higher
• To balance crosstalk reliability and power overheads
 DBCTM uses 50% shielding bits to data bits

21. 20
• Worst-case SNR improvements of Corona with DBCTM
 19.28 to 44.13% compared to baseline
 12.44 to 34.19% compared to PCTM5B
 4.5 to 31.30% compared to PCTM6B
Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Worst-case SNR comparison

22. 21
• Worst-case SNR improvements of Corona with DBCTM
 19.28 to 44.13% compared to baseline
 12.44 to 34.19% compared to PCTM5B
 4.5 to 31.30% compared to PCTM6B
• Corona DBCTM (with 50% shielding bits)
 Reduces crosstalk noise in the detectors by using shielding bits between data bits
 Considers the PV profile of MRs to select MRs for shielding
Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Worst-case SNR comparison

23. 22
• Average packet latency of Corona with DBCTM has
 12.6% higher compared to baseline
 3.4% higher compared to PCTM5B
 2.1% higher compared to PCTM6B
Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Corona Average Packet Latency

24. 23
• Average packet latency of Corona with DBCTM has
 12.6% higher compared to baseline
 3.4% higher compared to PCTM5B
 2.1% higher compared to PCTM6B
Delay due to encoding and decoding of data with DBCTM
contributes to increase in average latency
Corona: D. Vantrease et al. MICRO 2009PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Corona Average Packet Latency

25. 24
Corona: D. Vantrease et al. MICRO 2009
• Corona with the DBCTM technique
 Has 31.6% higher EDP compared to the baseline
 Increase in average latency and bits (increase in photonic hardware)
 Has 16.4% lower EDP compared to the best known crosstalk mitigation
technique PCTM6B
 Considerable laser, static power savings due to lower photonic hardware
PCTM5B and PCTM6B: S. V. R. Chittamuru et al. IEEE D&T 2015
Results: Corona Energy Delay Product

26. • Our proposed DBCTM technique with Corona PNoC
Reduces crosstalk noise in its detectors
Improves SNR by up to 44.13% compared to baseline
• Our proposed DBCTM technique compared to the best known
prior work
 Improves SNR by up to 31.30%
 Reduces EDP by 16.4%
• DBCTM technique is effective in overcoming trimming-induced
crosstalk in PNoCs to improve reliability
25
Conclusions

27. • Questions / Comments ?
Thank You
26