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C Experimental Results

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Chapter 4 Distributed Multiuser QoS Designs

4.3. C Experimental Results

Packet latency of different vital signals in Fig. 4-7 illustrate how intra and inter WBAN priorities are realized in RACOON. For either high or low priority, latency of vital signals are ranked in order of SP+DP. Signal with higher SD+DP value should have lower latency. Note that SP and DP reflect the intrinsic and emergent data priorities, respectively. Furthermore, due to that the high-priority WBAN contends resources more aggressively than the low-priority WBAN does, same signal with same priority setting in the high-priority WBAN has shorter latency than that in the low-priority WBAN. This meets the QoS requirements of the user priority.

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Fig. 4-7 Packet latency of vital signals in high and low priority WBANs

The latency comparison between RACOON and BodyQoS [42] in mobile WBAN scenarios is shown in Fig. 4-8. RACOON has much lower packet latency than BodyQoS has when WBANs move at either 2m/s or 6m/s. The reason is that RACOON makes WBANs cooperatively share the radio resource when they overlap to each other. On the contrary, BodyQoS does not consider interference interactions between WBANs, which makes improper decisions of bandwidth control and thus induces high transmission delay. In the original ideas of BodyQoS, interference is assumed to be generated by regular co-channel communications or path-loss due to limb movements. These sources of interference have “passive” interference patterns, which means it does not increase or

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decrease its interference level following the bandwidth control of BodyQoS. Therefore, BodyQoS reasonably increases transmission opportunities to overcome bad channel conditions. However, in multi-WBAN scenarios, the increasing transmission opportunities cause serious inter-WBAN interference. It than increases the transmission opportunities again and causes more serious interference, which enters a vicious circle. Collision measurements with RACOON and BodyQoS in Fig. 4-9 echo this observation.

1 2 3 4 5 6 7 8 9 10

100 101 102 103 104

Number of WBANs

Packet Latency(slots)

BodyQos ECG 2m/s BodyQoS ECG 6m/s Racoon ECG 2m/s Racoon ECG 6m/s

Fig. 4-8 Packet Latency with RACOON and BodyQoS

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1 2 3 4 5 6 7 8 9 10

0 100 200 300 400 500 600 700 800 900 1000

Number of WBANs

Packet Collisions (times)

BodyQoS 2m/s BodyQoS 6m/s Racoon 2m/s Racoon 6m/s

Fig. 4-9 Packet collisions with RACOON and BodyQoS controls

The collision measurements also show that collision of WBAN with RACOON is less sensitive to the number of co-existence WBANs, as compared with BodyQoS. Interference between WBANs is overcome by RACOON’s cooperative inter-WBAN resource sharing scheme and proposed probing-based interference detection. Collisions of RACOON are created by out-of-date scheduling.

A WBAN moves and encounters other un-negotiated WBANs with out-of-date inter-WBAN scheduling and thus packet collisions happen. However, besides of collisions created by WBAN mobility, BodyQoS creates extra collision by its problem of the inter-WBAN-interference enhancement. This problem gets worse when number of co-existence WBANs increases and hence

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introduces more collisions. The difference reasons of collisions of RACOON and BodyQoS are also reflected in their energy consumptions, which is depicted in Fig. 4-10. Note that the energy consumption is normalized to transmission throughput. The energy consumption considers both TX and RX according to the definition in performance metrics, section 4.1.B.

1 2 3 4 5 6 7 8 9 10

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8

4x 10-3

Number of WBANs

Power Consumption (Joule per bit)

BodyQoS ECG 2m/s BodyQoS ECG 6m/s Racoon ECG 2m/s Racoon ECG 6m/s

Fig. 4-10 Energy consumption of WSN with RACOON and BodyQos

There is an interesting result in Fig. 4-8, 4-9, and 4-10. While mobility of WBAN user is increased, BodyQoS and proposed RACOON have opposite reactions. For BodyQoS, the latency, collision, and energy consumption of a WBAN are decreased. On the contrary, for RACOON, those of a WBAN are increased, which are shown in Fig. 4-8, 4-9, and 4-10. The reason comes from the

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different anti-interference strategies of them. As for BodyQoS, a WBAN increases its transmission opportunities when it suffers inter-WBAN interference. If a collision history of a WBAN is separated into collision and non-collision, it will form an iterative collision / non-collision / collision / non-collision … pattern. Thus, for BodyQoS, a WBAN first senses collision and tries to increase its TX times. When the TX times are increased and the WBAN moves into a non-collision area, the latency, collision, and energy consumption of a WBAN are decreased. And then this WBAN starts to decrease the TX times because it senses no interference in the non-collision area. While the TX times are decreased and the WBAN happens to move into a collision area, some collisions are avoided. As a result, for BodyQoS, mobility helps a WBAN to decrease its latency, collision, and energy consumption. RACOON has an opposite strategy in TX-times control. For RACOON, a WBAN tries to decrease its TX times (through inter-WBAN resource negotiation) to resolve inter-WBAN collision and increase the TX times when there is no collision. RACOON thus has an opposite collision result during the iterative collision / non-collision pattern. As a result, RACOON introduces more collision to a WBAN when its mobility is increased. Although mobility helps the performance of BodyQoS and decreases that of RACOON’s, RACOON still guarantee a WBAN to have lower latency, collision rate, and energy consumption than what BodyQoS does due to RACOON’s cooperative inter-WBAN resource allocation.

User capacity is calculated by counting number of co-existence WBANs that provide delay-bound-satisfied transmissions of corresponding vital signals, which is illustrated in Fig. 4-11.

The proposed QoS protocol, RACOON, provides up to four co-existence WBANs that guarantee delay-bound requirements of all traffics. Its user capacity can be increased to ten co-existence WBANs when only the delay-bound of ECG traffics are satisfied. On the contrary, BodyQoS can support only single WBAN QoS due to its problem of inter-WBAN-interference enhancement.

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BodyQoS-All Traffic BodyQoS-ECG only Racoon-All Traffic Racoon-ECG only 0

2 4 6 8 10 12 14

User Capacity

1

4

10

1

Fig. 4-11 User Capacity with RACOON and BodyQoS

4.4 Summary

This work proposes Random Contention-based Resource Allocation (RACOON) protocol to provide multiuser QoS for wireless body area networks (WBANs). By considering QoS requirements of practical medical applications, the inter-WBAN scheduling should have QoS controls that simultaneously consider three different priorities: (i) intrinsic data priority, (ii) emergent data priority, and (iii) user priority. The proposed RACOON protocol uses a dynamic weighted-random-value-comparison scheme to meet these priority requirements. Furthermore, RACOON utilizes a centralized control and a probing-based inter-WBAN interference detection to simplify QoS controls of wireless sensor nodes (WSNs), which decreases unnecessary energy waste of WSN. Simulation results shows that RACOON has better QoS performance in terms of

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transmission latency, energy consumption, and user capacity, as compared with other WBAN QoS controls that do not consider inter-WBAN interference and priority.

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Chapter 5

Conclusions and Future Work

Wireless Body Area Network (WBAN) retrieving vital signals from our body has been treated as the “last meters” of the ubiquitous healthcare. To provide reliable transmission of body signals, WBAN need to overcome a WBAN-specific issue, “inter-WBAN interference”, which is caused by the mobility of WBAN users. It not only impacts the channel efficiency of WBAN but also raise a complex and WBAN-specific priority scheme.

5.1 Conclusion Remarks

In chapter 2, throughput and energy consumption models for multiuser WBAN are built. These models show how inter-WBAN interference degrades the performance. It also shows the performance sensitivity to the intra / inter-WBAN interference by two popular resource scheduling skills: CSMA-CA and beacon (contention free) modes. CSMA-CA properly handles the inter-WBAN interference by its random access scheme. However, it sacrifices great channel efficiency. On the other hand, beacon mode optimizes the intra WBAN scheduling but has no resistance of inter-WBAN interference. This hints the possible WBAN MAC should be a hybrid way of these schemes.

Chapter 3 further considers the dynamic topology of WBANs due to user mobility. It points out the fast response time of WBAN scheduling is another key of WBAN MAC design. Nevertheless, according to traditional graph coloring theory, channel efficiency and scheduling efficiency are tradeoffs. Chapter 3 proposes a relaxed coloring scheme combining random coloring skill to skip

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these tradeoffs. Surprisingly, The proposed method not only simultaneously realize high channel efficiency and scheduling speed but also has even higher channel efficiency than traditional methods.

Besides of experimental results, analytical models of the proposed coloring based scheduling are also provided in chapter 3.

Finally, in chapter 4, issue of complex WBAN priorities are further analyzed and suggested solution is provided. Proposed QoS controls simultaneously consider three different priorities: (i) intrinsic data priority, (ii) emergent data priority, and (iii) user priority. Experiment results show that the proposed method not only properly deals the complex user priorities, it can also support high user capacity than that without considering the interference from inter-WBANs.

5.2 Future Works

Based on above results of this study, some future works are suggested:

Establishing Multiuser WBAN channel model

This study only applies simple shading models for the calculation of interference. In real world, multiple human bodies may cause very complicated shadowing effect and hence the proposed methods in this study might need further adjustments. However, most existing WBAN channel models can only calculate the attenuation caused by one human body [56]. The bottleneck of multi-user channel model study is numerous combinations of input parameters. Different user positions, angles, gestures, and body shapes all affect attenuation level. Furthermore, there are many imperfect effects on reflection, scattering, radiation, and coupling of human body, which make the rule of attenuation hard to be summarized and simplified. To our best understanding, existing channel models can consider only up to two human bodies [57]. It is still complicated and far from

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the n-user model required in our study. For this reason, radio attenuation due to human body is temporally ignored in this study and hence the multi-user WBAN resource allocation problem can be more focused.

Integrating medical and legal knowledge for WBAN QoS control

In the proposed multi-WBAN QoS control, the real priorities between various vital signals and alarm threshold (priority enhance) still relies on medical experiences. Also, a complete WBAN QoS might be accompanied with dynamic signal quality scheme. For example, with very limited wireless resource, an ECG sensor might optionally switch to lower resolution mode. However, what is the limit of the degradation of signal quality? The answer may vary with difference target syndromes.

This relies on more practices and understanding of working flow of medical scenes. Finally, will the priority controls between involve legal issues? Since priority scheme implies sacrifices among measured signals in some critical situations. The prioritizing procedure may involve human life.

Hence, related designs might involve technology, medical, and legal knowledge. Of course, backup procedure may be necessary in such sensitive applications.

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List of Publications

Key Technology One: Multiuser Inter WBAN Scheduling Publications:

1. ShihHeng Cheng,ChingYao Huang, “Power model for wireless body area network,”Biomedical Circuits and Systems Conference, Baltimore, MD, Nov, 20-22, 2008.

2. ShihHeng Cheng,MeiLing Liu, ChingYao Huang,“Radar Based Network Merging for Low Power Mobile Wireless Body Area Network,”4th International Symposium on Medical Information and Communication Technology, Taipei, Mar, 22-25, 2010.

3. C. Y. Huang, M. L. Liu, and S. H. Cheng, “WRAP: A Weighted Random Value Protocol for Multiuser Wireless Body Area Network,” in International Symposium on Information Theory

3. C. Y. Huang, M. L. Liu, and S. H. Cheng, “WRAP: A Weighted Random Value Protocol for Multiuser Wireless Body Area Network,” in International Symposium on Information Theory

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