Building Robust Wireless LAN for Industrial Control with DSSSCDMA Cellphone Network Paradigm

1
Building Robust Wireless LAN for Industrial
Control with DSSS-CDMA Cellphone Network Paradigm

• Qixin Wang, Xue Liu, Weiqun Chen, Wenbo He, and
  Marco Caccamo
• Real-Time Systems Lab, CS Dept., UIUC
• ECECS, Univ. of Cincinnati
• IEEE RTSS 2005

2
Content

• Demand
• Challenge
• Observation and Solution Heuristics
• Theoretical Results
• Simulation and Comparisons
• Related Work
• Conclusion
• Future Work
• References
• Thank You!

3
Demand

• The demand for Industrial Control WLAN is
  increasing Cavalieri 98Jiang 98 Ye 00Ye
  01Ploplys 04
• More mechanical freedom
• Support Mobility
• Ease of Deployment and Flexibility

4
Challenge

• Real-Time Control requires persistent stable
  duplex communication links Backing off under
  adverse channel conditions is not allowed.
• Wireless medium in industrial environments is
  often adverse
• Worse large scale path-loss
• Worse fading (multipath)
• Persistent Electric-Magnetic Interference (EMI)
  from Electric Welding/Motors
• Possible interference from same-band RF devices
  turned on accidentally or maliciously.

5
Challenge

• Robustness is the top concern for wireless
  industrial real-time control communicationDefini
  tion of Robustness the degree to which a system
  or component can function correctly in the
  presence of invalid inputs or stressful
  environment conditions IEEE 90.

6
Observation

• Interest of deploying wireless is mainly at the
  last hop
• Centralized control of multiple remote machines
  is the widely deployed and economic paradigm.
• Industrial control facilities are mostly
  permanent instead of ad hoc. Wireline backbones
  for connecting base stations are often available
  already.

7
Solution Heuristic

• Heuristic I A cellphone network paradigm / IEEE
  802.11 WLAN with access-point paradigm is what
  interests the industry most.

8
Observation

• Real-time control communications are usually of
  stable low data rate
• Mostly involve 100200 bit/pkt, 101 pkt/sec for
  each direction.
• Higher rate controls are usually carried out
  locally, e.g. using step motor, central control
  node only need to send medium grain control
  packets to the remote step motor.

9
Observation

• Information TheoryLower data rate can be
  exploited to achieve higher robustness.
• The state-of-the-art Direct Sequence Spread
  Spectrum (DSSS) Technology Lower data rate ?
  Higher robustness

10
Tutorial on DSSS
Pseudo Noise Sequence (PN) Stream, a.k.a chip
stream. Chip rate Rc.
Integration gEc for each bit (Ec is the energy
of a chip)
Definition Processing Gain g Rc/rb .
Same PN Sequence
Data stream, a.k.a bit stream. Bit rate rb .
DSSS Modulated Stream, a.k.a Scrambled Stream
DSSS Modulated Stream, a.k.a Scrambled Stream
Original Data
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Tutorial on DSSS
If a different PN Sequence is applied
Integration Gaussian Noise

1
1
1
-1
-1
-1
1
1
1
1
-1
-1
Another scrambled sequence
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Observation
Bit Error Rate
Processing Gain

• DSSS Technology Larger Processing Gain g ?
  Lower data rate ? Lower Bit Error Rate (Higher
  robustness)

13
Solution Heuristic

• Heuristic II Fully exploit low-data-rate feature
  of industrial real-time control communication
  using state-of-the-art DSSS technology can
  achieve higher robustness.

14
Observation

• MAC IS-95-like CDMA paradigm vs IEEE 802.11
  PCF-like paradigm
• CDMA provides better real-time overrun isolation
• CDMA is easier to schedule (isolation)
• Smaller overhead under adverse wireless channel
  conditions
• CDMA paradigm sends packets in a continuous
  stream ? just need to sync (acquisition)
  sender/receiver once at the stream setup stage
• 802.11 PCF paradigm needs to sync (acquisition)
  sender/receiver for every packet. Overhead under
  adverse wireless channel conditions may be
  intolerably high (see TechReport 05 Appendix
  II).

15
Solution Heuristic

• Heuristic III We choose DSSS-CDMA Cellphone
  Network Paradigm to build robust wireless LAN for
  Industrial Real-Time Control

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Theoretical Results

• Question How to configure for maximum robustness
  when the wireless medium is unknown?Answer
  Deploy as slow data rate as possible, or say, as
  large processing gain gn as possible, meanwhile
  not violate the maximum processing gain limit set
  by application and hardware.

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Theoretical Results
Limit set by application
Limit set by hardware
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Theoretical Results

• Question When the wireless medium is known and
  is fixed, a faster data rate can be allowed. What
  is the optimal data rate?Note a faster data
  rate corresponds to higher sampling/actuating
  rate, but also bigger packet error rate (PER).

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Theoretical Results
Inverted Pendulum utility loss curve, derived
from Monte Carlo
Processing gain gn Data rate Sampling/actuati
ng rate f Packet correct rate (1 - Pper) f
(1 - Pper) ? There is a balancing point for
achieving maximum f (1 - Pper).
20
Theoretical Results
Problem Formalization
21
Theoretical Results
The optimization problem has closed form solution
when Un are of following shapes
or
22
Theoretical Results
23
Simulation and Comparisons

• Nowadays dominant WLAN scheme is IEEE 802.11
  (a/b)
• Objective of simulation and comparisonsShow by
  fully exploiting low-data-rate feature of
  real-time control loops, DSSS-CDMA cellphone
  network paradigm is more robust than IEEE 802.11.

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Simulation and Comparisons

• 802.11 only have fixed robustness levels
• 802.11b (DSSS) 1, 2, 5.5, 11Mbps
• 802.11a (OFDM) 6, 9, 12, 18, 24, 36, 48, 54Mbps
• Under adverse channel conditions, 802.11 backoff
  (DCF), or keeps retransmitting (PCF).

• Deploy as large processing gain g as the
  application allows.
• Keep transmitting even under adverse channel
  conditions.

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Simulation and Comparisons

• Simulation I Demonstrative comparison on a
  distributed Inverted Pendulum scenario using
  DSSS-CDMA paradigm and IEEE 802.11b PCF paradigm

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Simulation and Comparisons
Wireless medium model complies with typical
settings for industrial environments Rappaport
02
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Simulation and Comparisons
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Simulation and Comparisons

• Simulation II Monte-Carlo comparison btw
  DSSS-CDMA paradigm and IEEE 802.11a/b
• A indoor area of 20m?20m
• For each given number of remote nodes n, 200
  trials are carried out, each with a random layout
• DSSS-CDMA fully exploits low-data-rate to
  achieve max robustness (Proposition 1)
• IEEE 802.11a/b uses most robust mode retransmit
  as many times as possible within the real-time
  deadline.

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Simulation and Comparisons
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Simulation and Comparisons
Figure 4. Robustness comparison. Jmin(watt) is
the minimum external RF interference power needed
to break down at least one of the wireless
control loops. n is the number of wireless
control loops. Note the curves for DSSS-CDMA are
lower bounds for Jmin, while the curves for IEEE
802.11b/a are upper bounds.
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Related Work

• Can be easily build on top of existing 1.5-G, 3-G
  DSSS Cellphone schemes IS 95CDMA
  2000QualComm 05Td-scdma 05Umts
  05Korowajczuk 04, although current DSSS
  Cellphone schemes have not yet focused on
  robustness, but rather higher data throughput.
• If Proposition 1 is enforced, 802.11 PCF paradigm
  IEEE 802.11 may still be a possible way to
  build robust wireless LAN for industrial
  real-time control. But it has three disadvantages
  compared to CDMA as pointed out previously. A
  more quantitative study is our future work.
• IEEE 802.15.1 (Bluetooth) and IEEE 802.15.4 (PHY
  and MAC for Zigbee) exploit low data rate for
  power saving instead of robustness. IEEE 802.15.4
  is very similar to IEEE 802.11b, including its
  robustness.
• FHSS and DSSS are often interchangeable
  technologies, but FHSS often incurs higher
  hardware cost and system complexity.

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Conclusion

• DSSS-CDMA Cellphone Network Paradigm which fully
  exploits low-data-rate feature of industrial
  real-time control communication provides better
  robustness than nowadays dominant IEEE 802.11
  WLAN schemes.

33
Future Work

• Q-RAM and Dynamic Adaptation power,
  sampling/actuating rate, number of control loops,
  channels/loop, utility etc.
• Co-existance real-time steady loops bursty ad
  hoc links.
• Multiple Cells.

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References

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• CDMA 2000 TIA/EIA/IS CDMA 2000 Series, Release
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• IS 95 TIA/EIA/IS Std. 95. 1992.
• IEEE 802.11 IEEE Std. 802.11. 1997.
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• Korowajczuk 04 L. Korowajczuk, B. de Souza
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• QualComm 05 Qualcomm cdma technologies.
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• Rappaport 02 Theodore S. Rappaport, Wireless
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  Ed.), Prentice Hall, 2002.
• Td-scdma 05 Td-scdma forum. http//www.tdscdma-f
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• TechReport 05 Q. Wang, X. Liu, W. Chen, W. He,
  and M. Caccamo, Technical Report on Building
  Robust Wireless LAN for Industrial Control with
  DSSS-CDMA Cellphone Network Paradigm,
  http//www-rtsl.cs.uiuc.edu/papers/dsss_cdma_tr.pd
  f , 2005.
• Umts 05 Umts forum. http//www.umts-forum.org ,
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• Ye 00 H. Ye, G. Walsh, and L. Bushnell.
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Thank You!