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

Paradigm

Qixin Wang, Xue Liu, Weiqun Chen*, Wenbo He, and Marco CaccamoReal-Time Systems Lab, CS Dept., UIUC

*ECECS, Univ. of CincinnatiIEEE RTSS 2005

Content

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

Demand

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

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.

Challenge

• Robustness is the top concern for wireless industrial real-time control communication

Definition 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].

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.

Solution Heuristic

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

Observation

• Real-time control communications are usually of stable low data rate:– Mostly involve 100~200 bit/pkt, 10~1 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.

Observation

• Information Theory:

Lower 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

Tutorial on DSSSPseudo Noise Sequence (PN) Stream, a.k.a chip stream. Chip rate: Rc.

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

Definition: Processing Gain g := Rc/rb .

Original Data

Integration = gEc for each bit (Ec is the energy of a chip)

Same PN Sequence

Tutorial on DSSS

Integration = Gaussian Noise

+1 +1

-1 -1 -1

+1 …

If a different PN Sequence is applied

+1

-1

+1

-1

+1 +1

Another scrambled sequence

Observation

• DSSS Technology:

Larger Processing Gain g Lower data rate Lower Bit Error Rate (Higher robustness)

Bit Error Rate

Processing Gain

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.

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).

Solution Heuristic

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

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.

Theoretical Results

Limit set by application

Limit set by hardware

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).

Theoretical Results

Processing gain gn –

Data rate +

Sampling/actuating rate f +

Packet correct rate

(1 - Pper) –

f (1 - Pper) + – ?

There is a balancing point for

achieving maximum

f (1 - Pper).

Inverted Pendulum utility loss curve, derived from Monte Carlo

Theoretical Results

Problem Formalization:

Theoretical ResultsThe optimization problem has closed form solution when Un are of following shapes:

or

Theoretical Results

Simulation and Comparisons

• Nowadays dominant WLAN scheme is IEEE 802.11 (a/b)

• Objective of simulation and comparisons:

Show by fully exploiting low-data-rate feature of real-time control loops, DSSS-CDMA cellphone network paradigm is more robust than IEEE 802.11.

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.

Simulation and Comparisons

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

Simulation and ComparisonsWireless medium model complies with typical settings for industrial environments [Rappaport 02]:

Simulation and Comparisons

Simulation and Comparisons

• Simulation II: Monte-Carlo comparison btw DSSS-CDMA paradigm and IEEE 802.11a/b– A indoor area of 20m20m– 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.

Simulation and Comparisons

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.

Related Work• Can be easily build on top of existing 1.5-G, 3-G DSSS Cellphone

schemes [IS 95][CDMA 2000][QualComm 05][Td-scdma 05][Umts 05][Korowajczuk 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.

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.

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.

References[Cavalieri 98] S. Cavalieri and D. Panno. A novel solution to interconnect fieldbus systems using IEEE

wireless LAN technology. Comput. Standards Interfaces, 20(1):9–23, 1998.[CDMA 2000] TIA/EIA/IS CDMA 2000 Series, Release A (2000). 2000.[IS 95] TIA/EIA/IS Std. 95. 1992.[IEEE 802.11] IEEE Std. 802.11. 1997.[IEEE 90] Institute of Electrical and Electronics Engineers. IEEE Standard Computer Dictionary: A

Compilation of IEEE Standard Computer Glossaries. New York, NY: 1990. [Jiang 98] S. Jiang. Wireless communications and a priority access protocol for multiple mobile terminals in

factory automation. IEEE Trans. Robot. Automat., 14:137–143, 1998.[Korowajczuk 04] L. Korowajczuk, B. de Souza Abreu Xavier, A. M. F. Filho, et al. Designing cdma2000

Systems. Wiley, 2004.[Ploplys 04] N. J. Ploplys, P. A. Kawka, and A. G. Alleyne. Closedloop control over wireless networks. IEEE

Control Systems Magazine, 24(3):58–71, June 2004.[QualComm 05] Qualcomm cdma technologies. http://www.cdmatech.com, 2005.[Rappaport 02] Theodore S. Rappaport, Wireless Communications: Principles and Practice (2nd Ed.), Prentice

Hall, 2002.[Td-scdma 05] Td-scdma forum. http://www.tdscdma-forum.org , 2005.[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.pdf , 2005.

[Umts 05] Umts forum. http://www.umts-forum.org , 2005.[Ye 00] H. Ye, G. Walsh, and L. Bushnell. Wireless local area networks in the manufacturing industry. Proc.

American Control Conf., pages 2363–2367, 2000.[Ye 01] H. Ye and G. Walsh. Real-time mixed-traffic wireless networks. IEEE Trans. Ind. Electron., 48(5),

2001.

Thank You!