1. WiCop: Engineering WiFi Temporal White-Space for Safe Operations of Wireless Body Area Networks in Medical Applications
Yufei Wang, Qixin Wang, Zheng Zeng, Guanbo Zheng, Rong Zheng
Dept. of Computing, The Hong Kong Polytechnic Univ.
Dept. of Computer Science UIUC
Dept. of Computer Science, Univ. of Houston

2. Table of Contents
Demand
Proposed Framework
Evaluation
Related work

3. Demand

4. Crisis of wired BAN Cause frequent falling off of medical sensors
Limit the movement of patients
Make medical unit untidy

5. Advantage of WBAN WBAN solves the problem of wired BAN
Sensors unlikely fall off
Patient feel more comfortable
Medical units are more tidy

6. Medical WBAN Features Low duty cycle Low data rate ~500Kbps
Typical sampling rate < 300Hz
Wakeup on demand
Low data rate ~500Kbps
Low transmit power <1mW
Disparate delay requirements
ECG: <500ms
Body temperature monitoring: several seconds
Single-Hop centralized WBAN is the preferred architecture

7. WiFi Co-Channel Interference
WiFi Co-channel inference is a major threat to WBAN
ZigBee channels vs b WiFi channels

8. WiFi Co-Channel Interference
Power asymmetry
Typical WiFi power ≈ 30mW
Typical ZigBee (Bluetooth, IEEE ) power ≤ 1mW
MAC asymmetry
Many WiFi device use Carrier Sense (CS) based Clear Channel Assessment (CCA). Such WiFi devices do not back off to ZigBee.
Many ZigBee uses Energy Detection (ED) CCA to assess the channel. ZigBee backs off to WiFi.

9. The existing Solutions
Operate WBAN over RF channels sufficiently away from the active WiFi RF channels
limits the RF spectrum that WBANs can use
Revise current WBAN or WiFi standards adding intelligent coexistence schemes
Hard to use Commercially-Off-The-Shelf (COTS)
Try to spatially separate WBANs from WiFi networks via careful configuration-time planning
Often difficult as WiFi networks may not be under the same administration domain as WBANs
Cause spurious outages in WBANs by unintended usage of mobile WiFi devices

10. The treat of WiFi to WBANs

11. The treat of WiFi to WBANs
Monitor: Base station
Polling period: 100ms
Electrode: Client
250 samples/sec (4ms/sample)
25 samples/chunk (100ms/chunk)
3chunks/packet, i.e. each chunk is retransmitted 3 times (COTS≤4ms to send a packet)

12. The treat of WiFi to WBANs
monitor: Base station
polling period: 100ms
electrode: Client
250 samples / sec
25 samples / chunk
3 chunks / packet, i.e., each chunk is retransmitted 3 times
Failure: a chunk fails all of its retransmissions.
a chunk fails all 𝑁𝑟𝑒 = 3 retransmissions.

13. Performance metrics 𝑴𝑻𝑻𝑭= 𝑻 𝒑𝒐𝒍𝒍𝒊𝒏𝒈 (𝟏−𝑷𝑹𝑹) 𝑵𝒓𝒆
Packet Reception Rate (PRR)
Mean Time To Failure (MTTF)
𝑴𝑻𝑻𝑭= 𝑻 𝒑𝒐𝒍𝒍𝒊𝒏𝒈 (𝟏−𝑷𝑹𝑹) 𝑵𝒓𝒆

14. Proposed Framework

15. Proposed Framework
“Engineer” temporal white-spaces between WiFi transmissions to allow WBAN transmissions
Busy WiFi leaves no room for WBAN
Goal: create temporal white-spaces in WiFi traffic for WBAN

16. Proposed Framework
Policing: prohibit the transmissions of WiFi interferers in a well-controlled manner
Shield WBAN transmissions in space and time
The basic idea is to exploit the WiFi CCA mechanisms; sending WiFi compliant signals to refrain WiFi stations from transmitting.

17. WiCop
Effectively controlling the temporal white-spaces (gaps) between consecutive WiFi transmissions
Two mechanisms: Utilizing the carrier sensing mechanisms in WiFi
Fake-PHY-Header
DSSS Nulling

18. It is claims a fake WiFi packet with duration
Fake-PHY-Header
Temporal Scheme
It is claims a fake WiFi packet with duration
= WBAN active interval

19. Continuously repeated DSSS Preambles
DSSS-Nulling
Repeated DSSS preamble
Continuously repeated DSSS Preambles

20. Band-rejection filtered DSSS-Nulling policing signal
Spectrum illustration of interferer, policing and Zigbee signal

21. Evaluation

22. Evaluation
The policing node implements the two policing mechanisms

23. Temporal white-spaces due to WiCop
Without Policing
With Policing (5ms temporal white-space/10ms)

24. Moderate Impact on WiFi traffic
WBAN MTTF under different WiFi interference source end data rates

25. Related Work

26. WBAN and WiFi coexistence
Huang [11] argued that the performance degradation of ZigBee in the WiFi interference is caused by two main reasons.
Namely power asymmetry
Carrier sense based CCA
Hou [4] uses the duration field of the RTS MAC header to reserve time.
Before broadcasting a beacon, a ZigBee base station first send an RTS packet to reserve a channel.
Hou’s approach requires sending a whole packet to reserve the channel.

27. DoS attack to WLAN
Thuente [29] studied several intelligent jamming methods with the requirement of
Low power
Low detection probability
Including DIFS waiting jamming
ACK corruption jamming
Fake RTS jamming

28. Experimental Evaluation in Medical Environments
Garudadri [32] applied Compressed Sensing to ECG.
This approach uses the redundancy in periodic ECG trace, to mitigate distortion under high packet losses.
This approach is orthogonal to WiCop and can be used in conjunction with WiCop to further improve the robustness of ECG monitoring.

29. Experiment layout

30. Conclusion WiCop significantly improves WBAN performance
Controlled impact on WiFi
DSSS-Nulling is more effective than Fake-PHY-Header in improving MTTF, mainly due to repeated transmissions of DSSS preamble
Fake-PHY-Header incurs much less overhead than DSSS-Nulling