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

Table of Contents

• Demand

• Proposed Framework

• Evaluation

• Related work

Demand

Crisis of wired BAN

• Cause frequent falling off of medical sensors

• Limit the movement of patients• Make medical unit untidy

Advantage of WBAN

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

Medical WBAN Features

• Low duty cycle 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 ar-chitecture

WiFi Co-Channel Interference

• WiFi Co-channel inference is a major threat to WBAN

ZigBee channels vs. 802.11b WiFi channels

WiFi Co-Channel Interference

• Power asymmetry Typical WiFi power 30mW Typical ZigBee (Bluetooth, IEEE 802.15.6) power

1mW

• MAC asymmetry Many WiFi device use Carrier Sense (CS) based

Clear Channel Assessment (CCA). Such WiFi de-vices do not back off to ZigBee.

Many ZigBee uses Energy Detection (ED) CCA to assess the channel. ZigBee backs off to WiFi.

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 net-works 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 mo-

bile WiFi devices

The treat of WiFi to WBANs

The treat of WiFi to WBANsWBAN

Monitor: Base stationPolling period: 100ms

Electrode: Client250 samples/sec (4ms/sample)

25 samples/chunk (100ms/chunk)

3chunks/packet, i.e. each chunk is retrans-mitted 3 times (COT-S4ms to send a packet)

The treat of WiFi to WBANs

WBAN

monitor: Base stationpolling period: 100ms

electrode: Client250 samples / sec25 samples / chunk3 chunks / packet, i.e., each chunk is retrans-mitted 3 times

Failure: a chunk fails all of its retransmissions.

a chunk fails all = 3 re-transmissions.

Performance metrics

• Packet Reception Rate (PRR)

• Mean Time To Fail-ure (MTTF)

𝑴𝑻𝑻𝑭=𝑻 𝒑𝒐𝒍𝒍𝒊𝒏𝒈

(𝟏−𝑷𝑹𝑹)𝑵𝒓𝒆

Proposed Framework

Proposed Framework

• “Engineer” temporal white-spaces be-tween WiFi transmissions to allow WBAN transmissions

Busy WiFi leaves no room for WBAN

Goal: create temporal white-spaces in WiFi traffic for WBAN

Proposed Framework

• Policing: prohibit the transmissions of WiFi interferers in a well-controlled man-ner

• Shield WBAN transmissions in space and time

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

Fake-PHY-Header

• Temporal Scheme

It is claims a fake WiFi packet with du-

ration = WBAN active inter-

val

DSSS-Nulling

• Repeated DSSS preamble

Continuously re-peated DSSS Pream-

bles

Band-rejection filtered DSSS-Nulling policing signal

Spectrum illustration of interferer, policing and Zigbee sig-nal

Evaluation

Evaluation

• The policing node implements the two policing mechanisms

Temporal white-spaces due to WiCop

Without Policing

With Policing (5ms temporal white-space/10ms)

Moderate Impact on WiFi traf-fic

WBAN MTTF under different WiFi interference source end data rates

Related Work

WBAN and WiFi coexis-tence

• 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.

DoS attack to WLAN

• Thuente [29] studied several intelligent jam-ming methods with the requirement of Low power Low detection probability Including DIFS waiting jamming ACK corruption jamming Fake RTS jamming

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.

Experiment layout

Conclusion

• WiCop significantly improves WBAN perfor-mance

• 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