A Coexistence Mitigation Scheme for IEEE based WBAN

Transcription

1 Thesis for the Degree of Master of Science A Coexistence Mitigation Scheme for IEEE based WBAN Jonghyeon Choi Department of Computer Engineering Graduate School Kyung Hee University Seoul, Korea February, 2015

2 A Coexistence Mitigation Scheme for IEEE based WBAN Jonghyeon Choi Department of Computer Engineering Graduate School Kyung Hee University Seoul, Korea February, 2015

3 A Coexistence Mitigation Scheme for IEEE based WBAN by Jonghyeon Choi Advised by Prof. Jinsung Cho Submitted to the Department of Computer Engineering And the Faculty of the Graduate School of Kyung Hee University in the partial fulfillment Of the requirements for degree of Master of Literature Dissertation Committee : Chairman Prof. Hyonsoo Lee Prof. Seokhee Jeon Prof. Jinsung Cho

4 Contents Abstract v 1. Introduction 1 2. Related Work 3 3. Preliminary Study 7 4. Proposed Scheme Experiments Conclusion 20 References 21 - i -

5 Table Contents Table 1. Experiment contents of channel access method 9 Table 2. Experimental parameter 15 Figure Contents Figure 1. Beacon shifting 3 Figure 2. Channel hopping 4 Figure 3. Active superframe interleaving 4 Figure 4. Superframe structure of IEEE Figure 5. Coexistence period of IEEE based WBAN 6 Figure 6. Packet reception ratio of coexisting 2 WBANs 8 - ii -

6 Figure 7. Retransmission rate of coexisting 2 WBANs 8 Figure 8. Nonbeacon-enabled CAP 10 Figure 9. Beacon-enabled CFP 10 Figure 10. Operating flow chart of proposed scheme 11 Figure 11. CFP shifting 12 Figure 12. Remained coexistence problem after CFP shifting 12 Figure 13. Mini-slot assignment 13 Figure 14. Data latency problem on Mini-slot assignment 13 Figure 15. Solution for data latency problem CFP shifting 14 Figure 16. Example of poor coexistence condition 14 Figure 17. Mode conversion 14 - iii -

7 Figure 18. Experimental result of Standard (IEEE ) 16 Figure 19. Experimental result of Phase 1 (Proposed) 17 Figure 20. Experimental result of All phases (Proposed) 18 Figure 21. Comparison with all schemes 19 - iv -

8 ABSTRACT A Coexistence Mitigation scheme for IEEE based WBAN by Jong Hyeon Choi Master of Literature in Computer Engineering Graduate School of Kyung Hee University WBAN (Wireless Body Area Network) is communication technology which provides both medical and CE (Consumer Electronics) services by connecting wireless devices in/on/around a human body. The IEEE , a typical low-power communication technology, was considered to implement WBAN by reasons that its requirements is similar to WBAN and RF module implementation based on the IEEE is not completed yet. In the case of coexisting many IEEE based WBAN, meanwhile, communication reliability may decrease due to signal interference and collision. However, the IEEE does not provide any solutions to solve coexistence problems. In this paper, I firstly perform preliminary experiments to identify aspects of coexisting IEEE based WBANs. Based on results of preliminary experiment, I also propose a scheme to mitigate the reliability decrease at multiple coexistence WBAN. The proposed scheme can be classified in two phases. Phase 1 is avoidance to collision on the CFP through improving data transmission. Phase 2 is mitigation collision through converting channel access method when phase 1 cannot guarantee pre-defined reliability requirement. To evaluate performance of the proposed scheme, experiments are performed by comparing with the proposed scheme and IEEE based WBAN. Key words Coexistence, Coexistence mitigation, Coexistence WBAN, IEEE , WBAN - v -

9 1. Introduction WBAN (Wireless Body Area Network) is communication technology which provides both medical and CE (Consumer Electronics) services by connecting wireless devices in/on/around a human body. To standardize WBAN, the IEEE Working Group had been published the IEEE Task Group 6 in 2007, and had finished standardization in The IEEE defines three different PHY (Physical) layers and the MAC (Medium Access Control) sub-layer, and has goals that are providing QoS (Quality of Service), extremely low power, high reliability, various data rate up to 10Mbps [1]. In general, WBANs can be deployed in a populated area such as a hospital or a healthcare center. Under this situation, WBAN may suffer from interference between each of them and it can cause degradation of data reliability. To solve this problem, the IEEE defines the coexistence problem and classifies coexistence situations into three statuses: dynamic, semi-dynamic, static. In addition, the IEEE also provides solutions to handle coexistence situations: beacon shifting, channel hopping, and active superframe interleaving. Because WBAN is body-centric network, it has inter-network mobility. By this reason, WBAN may coexist with a varying number of other WBANs during their operation. For instance, coexistence may happen when a group of people gather in the hospital or the healthcare center. In this case, WBAN can occur interference or collision affecting to data reliability by their busy traffic state. Because WBAN is closely related with life of user, data reliability decrease is critical. In order to solve this problem, some solutions like beacon shifting, channel hopping, active superframe interleaving are specified on the IEEE Meanwhile, RF module implementation based on the IEEE is not completed yet. By this reason, there are a number of studies to implement - 1 -

10 WBAN based on the IEEE which has similar characteristics to the IEEE [3-10]. The IEEE is one of the representative short-range wireless communications which defines the PHY layer and the MAC sub-layer specifications for low-complexity, low-data rate, low-power in POS (Personal Operation Space) of 10m [2]. However, degradation of data reliability caused by interference or collision can frequently occur on coexisting IEEE based WBANs because the IEEE does not consider coexistence situations. To identify the effects of coexistence on the operation of IEEE based WBAN, I firstly perform a preliminary study. In addition, I also propose a scheme to mitigate the harmful effects of coexistence which are obtained from the results of preliminary study. The proposed scheme consists 2 phases (Phase 1, 2). In Phase 1, the beginning time of CFP (Contention Free Period) is shifted based on pre-defined sequences and CFP slots are divided into mini-slots to avoid interference or collision. When coexistence problems cannot be solved in Phase 1, the proposed scheme change from Phase 1 to Phase 2 which converts CFP into CAP (Contention Access Period). To validate performance of the proposed scheme, I implement both IEEE based WBAN and the proposed scheme on practical environment, and compare each of them. Contents of this paper are organized as follows. Chapter 2 describes for the previous studies for solving the problem WBAN coexistence. In Chapter 3, discusses the effects and cause of coexistence on the operation of IEEE based WBAN through the actual experiment about overlapped IEEE based WBAN. And Chapter 4 introduces a proposed scheme to solve confirmed problem at Chapter 3. In Chapter 5, to verify performance of the proposed scheme, the experiment is preceded at actual environment by comparing the proposed scheme to standard. Section 6 concludes the paper

11 2. Related Work To deal with problem on situation of coexistence, the IEEE defines 3 avoidance schemes. First, beacon shifting is a scheme which shifts beacon start time to avoid collision with beacons of other WBANs. In beacon shifting, coordinator selects one of the beacon shifting indexes which indicates beacon shifting sequence and beacon shifting phase which is an element of Beacon shifting sequence by BP (Beacon Period). After selecting both beacon shifting index and phase, beacon start time of next superframe is shifted by selected beacon shifting phase. These are depicted in Figure 1. Figure 1. Beacon shifting Second avoidance scheme in standard is Channel hopping which selects different channel hopping sequence with neighbor as shown in Figure 2. Sensor node can obtain candidate channels by 16-bit Galois LFSR (Linear Feedback Shift Register) and channel hopping sequence. Then, it randomly select among candidate channels and hop to the selected channel. Active superframe interleaving is a last avoidance scheme in standard as illustrated in Figure 3 This scheme makes to WBAN using channel divided resource by interaction. Coordinators which are coexisted negotiate how to allocate their communication resources by using pre-defined control frames and their priorities

12 Figure 2. Channel hopping Figure 3. Active superframe interleaving - 4 -

13 As explained above, the IEEE proposes schemes for coexistence problem mitigation. On the contrary, the IEEE does not consider about coexistence. Therefore, IEEE based WBAN should analyze about coexistence environment. Figure 4. Superframe structure of the IEEE standard In the IEEE , Superframe is divided into active and inactive period. Active period consists of CAP (Contention Access Period) and CFP (contention Free Period). In CAP, beacon frame is firstly broadcasted by coordinator to announce start of superframe and inform parameters of superframe. After broadcast beacon frame, each member node access channel to transmit its data or control messages with CSMA/CA. If a node wants to use GTS (Guaranteed Time Slot), it can request GTS allocation to coordinator in CAP. When there are GTS requests, coordinator sets GTS allocation information in beacon frame to configure CFP which operate based on TDMA (Time Division Multiple Access). In inactive period which is next period of active period, every node in the network go to sleep mode to save their power

14 Figure 5. Coexistence period of IEEE based WBAN In operation scenario of IEEE based WBAN, a number of networks should be coexisted in narrow area and overlapped IEEE based WBANs transmitted data frame will be collided because they probably interfere each other as shown in Figure 5. In general, communication reliability in CAP is influenced barely because it operates based on CSMA/CA, ACK, and retransmission. On the other hand, successful data transmission in CFP cannot be guaranteed due to recovery process of failed transmission. Different from the IEEE , however, the IEEE does not consider about coexistence. To solve this problem, there are two studies about coexistence between WBANs; they solve interference through exchange of time schedule between TDMA-based WBAN [11, 12]. These schemes have an advantage that coexisting WBANs efficiently use time slot. However, coordinator manages scheduling information of its neighbor WBANs as the number of coexisting WBAN due to synchronization among them. Another study proposes beacon replacement and channel switching to avoid interference or collision when IEEE based WBAN detects coexistence situation [12]. This scheme replaces beacon transmission time to another time and switches its channel. However, this scheme consumes amount of power because it should monitor channel state during much times whenever WBAN detects coexistence

15 3. Preliminary Study As explained above, the IEEE proposes schemes for coexistence problem mitigation. On the contrary, the IEEE does not consider coexistence problems among multiple networks. Therefore, IEEE based WBAN cannot solve coexistence problems and analyzing coexistence environment of IEEE based WBAN is necessary. In this chapter, I perform a preliminary study to analyze problems which can be occurred by coexisting IEEE based WBAN. To perform preliminary study, I constructed experimental environment which composes 2 WBANs and each WBAN consists of 1 coordinator and 4 sensor nodes. Node platform is ZigbeX-II which is developed by the HANBACK electronics, and each sensor node has 14.4 kbps data rates. In first experiment, PRR (Packet Reception Ratio) and RR (Retransmission Rate) are measured according to distance between WBANs to find specific problem on coexistence environment. Figure 6 and Figure 7 show the result of first experiment. As shown in Figure 6 and 7 lower PRR and higher RR can be observed on the closer coexisting WBANs, the distant WBANs shows higher PRR and lower RR. These results mean that coexistence effects influence transmission reliability according to distance between coexisting WBANs

16 Figure 6. Packet reception ratio of coexisting 2 WBAN Figure 7. Retransmission rate of coexisting 2 WBAN - 8 -

17 Table 1. Experiment contents of channel access method Overlapped Non-overlapped Overlapped Non-overlapped Measurement (Scenario 1) Nonbeacon-enabled (Scenario 2) Nonbeacon-enabled (Scenario 3) Beacon-enabled (Scenario 4) Beacon-enabled WBAN Interference WBAN CAP Nonbeacon-enabled CAP CAP - CFP nonbeacon-enabled CAP CFP - To measure PRR according to channel access method, I perform second experiment which consists of 4 different scenarios as shown in Table 1. In scenario 2 and 4, interfering WBAN generates constantly interference on Nonbeacon-enabled CAP mode. Measuring WBANs in scenario 1 and 2 operate based on Nonbeacon-enabled CAP mode. On the contrary, measuring WBANs in scenario 3 and 4 operate based on beacon-enabled CFP mode. Figure 8 shows results of scenario 1 and 2, and Figure 9 illustrates results of scenario 3 and 4. When there is no interfering WBAN (scenario 1 and 3), measuring WBANs shows more than 90% of PRR. When interfering WBAN exists, however, PRR is decreased as shown in Figure 8 and 9. Especially, results scenario 4 shows dramatically decreasing PRR. Based on these experimental results, we can confirm that WBAN based on CAP mode less influence to interference or collision through using CCA, CSMA/CA or Acknowledge frame. Contrastively, WBAN based on CFP mode is sensitive to other coexisting WBAN because it is designed without considering to external interference or collision. In summary, through a preliminary study, I found that coexistence problems of IEEE based WBAN can occur low transmission reliability. Especially, external interference seriously influence transmission reliability when WBAN operates based on CFP mode

18 Figure 8. Nonbeacon-enabled CAP Figure 9. Beacon-enabled CFP

19 4. Proposed scheme As mentioned in Section 3, we recognize that transmission reliability on CFP mode is suffered from external interference. To solve this problem, I propose a coexistence mitigation scheme on IEEE based WBANs. The proposed scheme consists of two phases, as shown in Figure 10. Figure 10. Operating flow chart of proposed scheme First, Phase 1 opportunistically avoids collisions on overlapped CFP between WBANs by performing the CFP Shifting and Mini-slot assignment. If the PRR is less than pre-defined threshold when Phase 1 have been performed, WBAN change from Phase 1 to Phase 2 which convert channel access mode from CFP to CAP

20 Detailed operation of the CFP shifting in Phase 1 is illustrated in Figure 11. If coexisting WBAN detects interference or collision, next superframe uses CFP shifting. Then, each sensor node opportunistically avoids collisions by random shifting CFP. Figure 11. CFP shifting Shifting sequence is a set of shifting time value, In the case of being detected collision, one of the shifting sequences is selected randomly and one time value of shifting sequence is selected by modulo N check of current beacon sequence to shift CFP. Selected shifting sequence will be changed after several beacon transmissions. Figure 12. Remained coexistence problem after CFP shifting In CFP shifting, WBANs does not exchange information each other, and they selects randomly shifting sequence. Therefore, If only CFP shifting is performed, interference or collision can occur as shown in Figure 12. To deal with this problem, Mini-slot assignment must be operated with CFP shifting

21 at the same time. Figure 13 shows Mini-slot assignment. A time slot is divided into mini-slots, and then data communication is performed on selected one of the mini-slots. Figure 13. Mini-slot assignment Mini-slot assignment uses mini-slot sequence similar to CFP shifting. Mini-slot sequence is a set of available mini-slot number, and it generated as the number of mini-slots. Sensor node should transmit frames during assigned mini-slot. Mini-slot sequence is randomly selected per beacon. Figure 14. Data latency problem on Mini-slot assignment As shown in Figure 14, however, Mini-slot assignment causes large latency because it makes to use only on smaller mini-slot. To reduce latency, proposed scheme uses CFP extension until inactive period as illustrated in Figure 15. CFP can be extended up to the number of mini-slots, and it prevents increase of latency

22 Figure 15. Solution for data latency problem CFP shifting Meanwhile, Using CFP Shifting and Mini-slot assignment cannot completely solve coexistence problems as shown in Figure 16. When a number of WBAN densely coexist, each of them cannot guarantee communication reliability through performing Phase 1. In this case, WBANs perform Mode conversion from Phase 1 to Phase 2. Condition of Mode conversion is as follows. As shown in Figure 17, WBAN monitors its PRR value in Phase 1. When PRR was lower than pre-defined threshold value, the condition of Mode conversion is satisfy and WBAN change the channel access method from CFP to CAP. Because CAP is endurable against to external interference, it can guarantee reliability on coexisting WBANs situation than Phase 1. Figure 16. Example of poor coexistence condition Figure 17. Mode conversion

23 5. Experiments To verify performance of the proposed scheme operating along Phase 1 (CFP shifting and Mini-slot assignment) and Phase 2 (Mode conversion), I perform three different experiments as follow. 1. WBAN applying the IEEE WBAN applying until Phase 1 of proposed scheme. 3. WBAN applying All phases of proposed scheme. In all experiments, WBAN consists of 1 coordinator and 5 sensor nodes on ZigbeX-II mote. To construct coexistence environment, up to three WBANs were deployed at 1m interval. In addition, experimental parameters are set as shown in Table 2. Based on these environments, each experiment was performed 4 times during 120 minutes according to data rate. Table 2. Experimental parameters Standard (The IEEE ) Phase 1 (Proposed) Number of WBANs 1, 2, 3 Data rate(bps) 128, 256, 384, 512 Phase 1 & 2 (Proposed) BO:SO 7 : 6 ( 1 beacon / 2 second ) Mini-slots None 2 2 M-C threshold None None 95 Experiment time 120 minutes

24 Figure 18. Experimental result of Standard (The IEEE ) Figure 18 shows performance of IEEE based WBAN. In non-coexisting WBAN, most frames arrived to coordinator without loss regardless of data-rate. In contrast, PRR decrease according to data-rate when 2 or more WBANs coexist due to interference or collision

25 Figure 19. Experimental result of Phase 1 (Proposed) On the other hand, result of using only Phase 1 presents a relatively higher rate of successfully received frames than previous result in coexisting WBANs as shown in Figure 19. The reason of this results is that WBAN using Phase 1 can avoid interference or collision through performing CFP shifting or Mini-slot assignment. When three WBANs are coexisting, PRR is less than 95% because it cannot completely solve coexistence problem by using only Phase

26 Figure 20. Experimental result of All phases (Proposed) Figure 20 illustrates result of WBAN which uses All phases. As mentioned above, operating only Phase 1 cannot guarantee reliability when a number of WBANs are coexisting. On the other hand, when All phases are performed, PRR hardly decreased although networks were coexisting due to Mode conversion which helps guaranteeing high reliability by using converting to CAP when PRR is lower than pre-defined threshold

27 Figure 21. Comparison with all schemes Graph comparison among results of three experiments is presented in Figure 21. Communication reliability of IEEE based WBAN dramatically decrease according to data-rate and the number of coexisting WBANs. On the contrary, proposed scheme guaranteed reliability despite of coexistence or high data-rate. In summary, IEEE based WBAN cannot guarantee reliability in coexistence situation. Contrastively, the proposed scheme can guarantee higher reliability than IEEE based WBAN because Phase 1 provide opportunistic transmission through CFP shifting and Mini-slot assignment. In addition, the proposed scheme outperforms when both Phase 1 and Phase 2 are applied due to the fact that Mode conversion change channel access mode from CFP to CAP which can help to mitigate high complex coexistence situation

28 6. Conclusion When many WBAN are coexisting, signal interference or collision occur and it can cause decrease of communication reliability. However, the IEEE does not define solution for these variety problems. In this paper, I analyzed problems of coexisting IEEE based WBAN through a preliminary study, and discovered that CFP can influence to reliability. Based on experimental result, we introduced the scheme which modifies CFP over two phases to mitigate coexistence problem. In order to guarantee reliability, proposed scheme tries to avoid from interference or collision by shifting and dividing to mini-slot from the CFP, and converts channel access mode to CAP from CFP. To evaluate performance of the proposed scheme, three different experiment were performed by comparing with IEEE based WBAN. The experimental results show that the proposed scheme can guarantee reliability on coexistence situation

29 References [1] IEEE Standard for Local and metropolitan area networks-part 15.6: Wireless Body Area Networks, IEEE Standard-2012, Feb , pp [2] IEEE Standard for Local and metropolitan area networks-part 15.4: Low-Rate Wireless Personal Area Networks, IEEE Standard-2011(Revision of IEEE ), Sept , pp [3] Changle Li, Huan-Bang Li, Ryuji Kohno, Performance Evaluation of IEEE for Wireless Body Area Network(WBAN), in IEEE International Conference on Communication Workshops (ICC Workshops) 2009, Jun. 2009, pp. 1-5 [4] Ruben de Francisco, Li Huang, and Guido Dolmans, Coexistence of WBAN and WLAN in Medical Environments, in IEEE 70th on Vehicular Technology Conference Fall (VTC 2009-Fall) 2009, Sept. 2009, pp. 1-5 [5] Ullah S, Kyungsup Kwak, Performance study of low-power MAC protocols for Wireless Body Area Networks, in IEEE 21st International Symposium on Personal, Indoor and Mobile Radio Communications Workshops (PIMRC Workshops), 2010, Sept. 2010, pp [6] Flavia Martelli, Roberto Verdone, Coexistence Issues for Wireless Body Area Networks at 2.45 GHz, in 18th European Wireless Conference on European Wireless 2012, Apr. 2012, pp. 1-6 [7] Sungwoo Weon, Seokwon Lee, Yosub Park, Haesoon Lee, Daesik Hong, Energy Efficient Interference Avoidance Strategy in Wireless Body Area Network, in IEIE Conference on The Institute of Electronics and Information Engineers, 2013, Jul. 2013, pp [8] Jinseok Han, Taehoon Kim, Yonghwan Lee, Robust Transmission of

30 the IEEE Signal in the Presence of Co-channel Interference, in 2013 IEEE on Wireless Communications and Networking Conference (WCNC) 2013, Apr. 2013, pp [9] Byoungseon Kim, Beomseok Kim, Jingsung Cho, A Partial Loss Retransmission Scheme in IEEE based WBAN, Journal of KIISE, Vol. 41, No. 1, Feb. 2014, pp [10] Sarra E, Moungla H, Benayoune S, Mehaoua A, Coexistence improvement of wearable body area network WBAN in medical environment, in IEEE International Conference on Communications (ICC) 2014, Jun. 2014, pp [11] Mahapatro J, Misra S, Manjunatha M, Islam N, Interference mitigation between WBAN equipped patients, in International Conference on Wireless and Optical Communications Networks (WOCN) 2012, Sept. 2012, pp. 1-5 [12] Yongsuk Park, Distributed Time Division Piconet Coexistence Using Local Time Offset Exchange, Journal of KIICE, Vol. 18, No. 6, Jun. 2014, pp [13] Deylami M, A Distributed Scheme to Manage The Dynamic Coexistence of IEEE Based Health Monitoring WBANs, IEEE Journal of Biomedical and Health Informatics, Vol.18, No. 1, Aug. 2014, pp