A Priority-aware Frequency Domain Polling MAC Protocol for OFDMA-based Networks in Cyber-physical Systems

Meng Zheng,Junru Lin,Wei Liang,and Haibin Yu

A Priority-aware Frequency Domain Polling MAC Protocol for OFDMA-based Networks in Cyber-physical Systems

Meng Zheng,Junru Lin,Wei Liang,and Haibin Yu

—Wireless networking in cyber-physical systems (CPSs)is characteristically different from traditional wireless systems due to the harsh radio frequency environment and applications that impose high real-time and reliability constraints. One of the fundamental considerations for enabling CPS networks is the medium access control protocol.To this end,this paper proposes a novel priority-aware frequency domain polling medium access control(MAC)protocol,which takes advantage of an orthogonal frequency-division multiple access(OFDMA) physical layer to achieve instantaneous priority-aware polling. Based on the polling result,the proposed work then optimizes the resource allocation of the OFDMA network to further improve the data reliability.Due to the non-polynomial-complete nature of the OFDMA resource allocation,we propose two heuristic rules, based on which an efficient solution algorithm to the OFDMA resource allocation problem is designed.Simulation results show that the reliability performance of CPS networks is significantly improved because of this work.

Index Terms—Cyber-physical system(CPS),medium access control(MAC)protocol,polling,resource allocation.

I.INTRODUCTION

C YBER-PHYSICAL systems(CPSs)are integrations of abstract computations,networking and physical plants,in which sensors,actuators and embedded devices are networked to sense,monitor and control the physical world[1-6].With the development of wireless communication,networking and embedded system technologies,recent years have witnessed the emergence of CPSs in a broad range of applications, e.g.,industry automation,smart grid,remote healthcare,smart transportation and so forth[7].

The typical architecture of a CPS,as shown in Fig.1,includes physical layer,cyber layer and networking layer[8-10].At the physical layer,sensors and actuators are responsible for information collection and controlling the physical world, respectively.At the cyber layer,upon receipt of the inputs, the real-time decision making system executes the abstract computations to analyze the collected data and then relays the decision to the actuators in the physical world by a sequence of control orders.Networking layer is responsible for the reliable and real-time interconnection of physical layer and cyber layer[11-14].As medium access control(MAC)arbitrates the access to shared precious spectrum resources,the reliable and real-time MAC for CPS networks remains an issue that deserves extensive research efforts.

Junru Lin is with Huawei Technologies Co.,Ltd.,Shenzhen 518129,China (e-mail:linjunru@huawei.com).

Fig.1.A typical CPS architecture.

Deterministic MAC protocols such as time division multiple access(TDMA)[15-18]and polling MAC protocols[19-25]are widely adopted in wireless networks for industrial process monitoring because they could guarantee a deterministic medium access and data transmission.However,existing deterministic MAC protocols[15-25]cannot meet the high communication requirements by CPS applications(closed-loop control)due to the inefficient utilization of precious bandwidth. For example,link-level retransmission is an extensively used method to improve the reliability of MAC protocols.When link-level retransmission is adopted in TDMA scheduling,lots of retransmission slots need to be allocated for high reliability,however they are utilized with a low probability.While in a polling MAC protocol,most polling requests are wasted when only a small fraction of nodes in the polling list have packets to transmit.As the node density in CPS applications is normally large(e.g.,node densities of robot end effector,rotary equipment and proximity switches in typical field applications are 2(m3),10(m3)and 120(cell),respectively[25]),the heavy communication overhead of existing deterministic MAC protocols results in a significant performance degradation.

Aiming to meet the high communication requirements by CPS applications,we need to design a cost-effective MAC protocol which yields high efficiency of bandwidth utilization and consequently ensures reliable and real-time data transmission.Motivated by recent contributions[25-28]leveraging orthogonal frequency-division multiplexing(OFDM)modulation to resolve channel contention of IEEE 802.11 MAC in the frequency domain,this paper proposes a novel priority-aware frequency domain polling MAC(PFDP-MAC)protocol for OFDMA-based CPS networks.PFDP-MAC exploits a priorityaware frequency domain polling method that allows the access point(AP)to simultaneously collect both the responses and transmission priorities from stations.Based on the polling result,PFDP-MAC optimizes the OFDMA resource allocation for reliable communication.In order to address the nonpolynomial(NP)-complete nature of the OFDMA resource allocation problem,we derive two heuristic rules and with them propose a lightweight heuristic algorithm to solve the NP-complete problem.The contributions of this work are summarized as follows:

1)We devise a cost effective MAC protocol(PFDP-MAC) which moves the priority-aware polling procedure from time domain into frequency domain.The polling overhead is thus significantly reduced.

2)We formulate a novel OFDMA resource allocation problem aiming to minimize the network timeout probability and propose a heuristic algorithm to solve it.To the best of our knowledge,this work is the first paper to study the OFDMA resource allocation for the reliable transmission in CPS networks.

3)We demonstrate the performance advantage of PFDPMAC over the IEEE 802.16 MAC through simulations. Additionally,the reliability requirement of typical CPS applications-factory automation is shown to be met by PFDPMAC.

The rest of this paper is organized as follows.Section II reviews related work.Section III presents the protocol architecture of the proposed PFDP-MAC and illustrates the priority-aware frequency domain polling process particularly. In Section IV,the OFDMA resource allocation problem aiming to improve the reliability of PFDP-MAC is formulated, and solved by a heuristic algorithm.In Section V,simulations validate the efficiency of PFDP-MAC.Section VI concludes this paper and points out future directions.

II.RELATED WORK

In the context of TDMA MAC protocols for CPS networks (including wireless sensor networks and industrial wireless networks),Zheng et al.[15]study the question of how many slots to allocate for retransmissions from a reliability-lifetime tradeoff perspective in wireless sensor networks.Gamba et al.[16]consider different retransmission strategies for centralized wireless industrial networks subjected to external interference,by experimentally showing that the straightforward bounded immediate retransmission scheme can be improved in different ways.Zhang et al.[17]propose an adaptive and reliable transmission scheduling algorithm for wireless sensor networks based on low-cost estimation of channel states. Willig and Uhlemann[18]investigate how to schedule the TDMA slots for retransmissions while taking advantage of the relay nodes so that the average number of packets missing their deadlines is minimized.The spirit of TDMA MAC protocols for highly reliable transmission is to reserve sufficient communication resources(e.g.,time slots and relays)to cope with the uncertainty of wireless channel.However,as shown in[25], resources for retransmission are normally utilized with a rather low probability,which reflects the inefficiency of existing TDMA MAC protocols for industrial wireless networks.

Polling MAC protocols are widely adopted in wired industrial communication standards,such as Ethernet powerlink, ProfiBus and WorldFIP.Willig[19]proposes a set of pollingbased MAC protocols for wireless implementation of ProfiBus and results indicate that polling MAC protocols are good substitutes for wireless ProfiBus.Seno et al.[20]propose a wireless extension of Ethernet powerlink based on IEEE 802.11 WLAN and its polling time is theoretically analyzed and experimentally evaluated in[21].Son et al.[22]devise an effective and simple polling scheme for IEEE 802.11e networks(OFDM physical layer)to reduce the number of polling times for station with no packets to transmit by using traffic characteristics and the number of successive polling times for a station with packets to transmit.Perez et al.[23]present an efficient multipolling scheme that improves the IEEE 802.11g (OFDM physical layer)MAC protocol efficiency and does not suffer from the hidden terminal problem or inefficient channel usage problems.Lam et al.[24]present a survey of different polling-based protocols for supporting voice over various types of IEEE 802.11 WLANs.In particular,three key issues are discussed:managing a polling list,determining the polling sequence,and reducing polling overhead.The inefficiency of existing polling schemes lies in the sequential polling of all the stations in the polling list(non-active stations may be removed after a predefined time period)regardless of their actual demands.In CPS networks,the polling cycle should be sufficiently short to precisely trace the control parameters, which effectively limits the number of stations(or network scale)supported by existing polling MAC protocols.

Aiming to improve the efficiency of IEEE 802.11 MAC, recent works[25-28]take advantage of OFDM modulation to reduce the communication overhead of coordinating multiple stations.Sen et al.[26]reduce the backoff overhead by counting contention using subcarriers.Feng et al.[27]extend the use of OFDM subcarriers to reduce all three overheads(including distributed inter-frame spacing(DIFS),random backoff and acknowledgement(ACK))in IEEE 802.11 DCF.Wang et al.[28]move the cooperative sensing and multi-channel con-tention in cognitive ad hoc networks from time domain to frequency domain by OFDM modulation,which significantly reduces the control overhead caused by cooperation and contention.Xiao et al.[25]combine frequency domain technologies with the polling-based 802.11 MAC protocol(FDP-MAC)for industrial wireless networks in which multiple stations are polled concurrently and a dynamic TDMA scheduling method along with a downlink acknowledgement aggregation method are further proposed to improve the physical layer efficiency. However,there are two main limitations of[25].One is that packet priorities of stations cannot be collected by the AP with the frequency domain polling method.The other is the poor bandwidth efficiency of dynamic TDMA scheduling, especially when the network scales.

Similar to[25],the PFDP-MAC is also a frequency domain polling MAC protocol.However,there are two fundamental differences between PFDP and FDP.First,FDP uses subcarriers to identify stations while PFDP uses subcarriers to identify the priorities of stations.In doing so,priority-aware frequency domain polling is proposed for the first time in this paper.Second,the FDP-MAC[25]is designed for OFDM networks,while PFDP-MAC is designed for OFDMA networks.OFDMA is a multi-user version of OFDM.Specifically, multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users.This allows simultaneous low data rate transmission from several users.In doing so, PFDP-MAC addresses the limitations of[25]by proposing the priority-aware frequency domain polling and performing OFDMA resource allocation.

III.PROTOCOL DESIGN OF PFDP-MAC

A.Illustration on PFDP-MAC

Consider an infrastructured TDMA-OFDMA network with one AP andKstations.The communication channel with bandwidthBis divided into 2Soverlapping narrow-band subcarriers and each of them is with equal bandwidth and constant noise power densityN0.In order to overcome the severe co-channel interference,we only use even subcarriers in this paper,as shown in Fig.2.Time is divided into equal frames.Besides the polling and resource allocation time and one transmission transit gap(TTG),each frame includesDtime slots for OFDMA transmission.Every packet has to be reliably sent to the AP within the specified deadline (e.g.,Tframes),or is discarded otherwise.The function of PFDP-MAC is to poll all stations and perform priorityaware resource(subcarriers,transmission power and physical layer rates)allocation among the loaded stations for reliable transmission.To this end,as shown in Fig.3,PFDP-MAC first designs a preamble phase(the preliminary step for later uplink communication)in which the AP broadcasts the preamble for time synchronization within the OFDMA network.After the preamble phase,the priority-aware polling response in frequency domain collects packets priorities of all stations. Based on the polling result,the AP solves an uplink OFDMA resource allocation problem aiming for reliable transmission. Finally,after a short interframe space(SIFS),all transmissions during the DATA transmission phase will be acknowledged by an aggregate ACK from the AP.The idea of aggregate ACK is the same as in[8],and thus omitted for simplicity.According to the IEEE 802.16 standard[29],one symbol duration in the polling phase is 100µs and one slot in the data phase fits three symbols.Notice that each scheduled station only transmits one packet in one frame,but the transmission of one packet may occupy up toDslots.We will illustrate the priority-aware frequency domain polling in the remaining part of this section and leave the OFDMA resource allocation problem to Section IV.

Fig.2.Frequency division of the communication channel.fi(i∈S) represents the central frequency of thei-th subcarrier of the communication channel.

Fig.3.Architecture of PFDP-MAC protocol.

We assume that the priorities of stations are classified into multiple levels.In PFDP-MAC,station k(k=1,2,...,K)is assigned Tkdifferent subcarriers,i.e.,sk,1,sk,2,...,sk,Tk.We use subcarriers to identify transmission priorities of stations. Without loss of generality,we label priorities of station k,i.e., {Ak,i},in an increasing manner,namely,1=Ak,1≤Ak,2≤···≤Ak,Tk=Tk.Ak,icorresponds to the case when the remaining lifetime of the packet at station k lies in the time interval[tk,i+1,tk,i),where tk,i∈R+,tk,i＞tk,i+1,i=1, 2,...,Tk,and tk,Tk+1=0.For stations whose packets are not delivered successfully in the current frame,their priorities will be increased by 1 in the coming frame.

With the above preparation in hand,we are now ready to illustrate the priority-aware frequency domain polling method. We introduce a bijective mapping:∀k,f(Ak,i)=sk,i,i= 1,2,...,Tk.Then station k could notify the AP that it has a packet with priority i to send by simply transmitting one symbol on subcarrier sk,i.For simplicity,we let Tk=T,k= 1,2,...,K.

Fig.4 shows an example of priority-aware frequency domain polling,in which each station is assigned four subcarriers (i.e.,T=4)and each subcarrier represents one priority.In this example,A(subcarriers{1,2,3,4}),B(subcarriers{5, 6,7,8}),and C(subcarriers{17,18,19,20})have packets to transmit.The AP starts the priority-aware polling with a preamble.After synchronizing with the AP in preamble phase,each station with pending packet immediately selects one allocated subcarrier(which depends on its priority)and sends one symbol over it.In Fig.4,subcarriers 4,8 and 19 are detected active via energy detection,and the AP then knows that stations A,B and C would like to transmit packets with priorities 4,4 and 3,respectively.Notice that,these symbol transmissions occur simultaneously in the time domain.

Remark 1.In the example of Fig.4,for easy illustration, we simply allocate a block of four adjacent subcarriers to each station.However,the ways of identifying stations and priorities with subcarriers are diverse.The comparison between different identifying ways in terms of polling accuracy is out of the scope of this paper.

Remark 2.If there are multiple selfish(or malicious) stations,which always claim the highest priority,the reliability performance of the proposed scheme will definitely degrade. At the moment,the proposed MAC cannot mitigate the harmful influence of selfish(or malicious)stations.However, we believe that the combination of a reputation updating mechanism[30]and this work may help us target the selfish (or malicious)stations.Future work will focus on the design of PFDP-MAC which is robust against selfish(or malicious) stations.

B.Architecture Comparison

In order to highlight the contributions of this paper,this subsection retrospects the architecture of FDP-MAC in[25]. FDP-MAC[25]consists of three phases as shown in Fig.5.In the first phase of FDP-MAC,a frequency domain polling process is performed to achieve simultaneous polling of multiple stations.A dynamic TDMA scheduling is carried out to allocate time slots according to the actual demand of each station in the second phase.At last,a downlink acknowledgements aggregation is proposed to reduce the overhead of wireless physical layer.

As FDP-MAC[25]identifies each station with a unique subcarrier ID,the priority information of each station cannot be attained through frequency domain polling.In[25],one OFDM system(compliant with the IEEE 802.11a/g[31])containing 48 data subcarriers is adopted for frequency domain polling, which means that FDP-MAC can at most poll 48 stations at a time.This is already a huge improvement compared with the polling-based IEEE 802.11 MAC.However,as this work adopts an OFDMA system whose subcarriers quantity is the multiple of 1 024(i.e.,S=1 024m,m∈Z+),the polling efficiency(the number of stations polled at a time)of PFDPMAC is higher than that of FDP-MAC when T＜21m(S＞21m×48).For both FDP-MAC and PFDP-MAC,we have to exploit multiple frames to accomplish one complete polling and transmission scheduling of stations when the number of served stations is larger than the polling efficiency of the employed MAC protocol.

The OFDMA phase in PFDM-MAC allows the concurrent transmission of multiple loaded stations,which saves considerable time in comparison to the dynamic TDMA phase in FDM-MAC when the number of loaded stations are numerous.

IV.OFDMA RESOURCE ALLOCATION

Fig.4.Priority-aware frequency domain polling.

In many factory automation applications,packets missingdeadlines(termed as outdated packets)are of limited use for industrial system and usually discarded by stations[32].Consequently,low packet delivery ratio may lead the automation process to suffer destructive influences,e.g.,an outage of the production line,a damage of the factory machine and even the loss of workers’lives.Different from traditional OFDMA networks desiring high throughput,the OFDMA resource allocation of PFDP-MAC protocol in this paper aims to reduce the number of outdated packets.

Fig.5.Architecture of FDP-MAC protocol[25].

A.Problem Formulation

After the priority-aware polling phase of PFDP-MAC,all the subcarriers can be used for communication.Let SSS ={1, 2,...,S}denote the set of available subcarriers.

Without loss of generality,multiple quadrature amplitude modulation(M-QAM)is adopted by the physical layer of the OFDMA network.Given a signal-to-noise ratio(SNR)γ and modulation factor M(the number of points of the QAM constellation),the bit error rate(BER)of M-QAM is given as follows[33]:

Similarly,the BER of station k if communicating over subcarrier s∈ SSS with Mk,s-QAM is given by

Let L denote the packet size and then the packet error rate (PER)of station k is calculated as:

We introduce a set of binary variables{δk,s}.The assignment indicator δk,s=1 if subcarrier s is assigned to station k,and δk,s=0 otherwise.The probability that packets of station k cannot arrive the AP within D time slots(which is termed as timeout probability in this paper)is given by

where βPk(0≤βPk≤1)denotes the priority coefficient of station k which estimates the timeout probability of packets with priority Pk.We will defer the way of updating βPkto Section IV-B.

Consequently,the OFDMA resource allocation problem aiming to minimize the overall outdated packets is formulated as follows:

Constraint(5b)limits each subcarrier to be assigned to at most one station in one time slot.Constraint(5c)requires the total transmission power of each station to be upper bounded by pmax,where pmaxis specified according to some hardware or regulatory limitations.Constraints(5d)-(5f)are simple physical constraints.

Since problem(5)is NP-complete due to the discrete nature of subcarrier allocation,the complexity of finding the optimal solution is extremely high[34].Section IV-B will devote to the derivation of a heuristic solution with low computational complexity to problem(5).

B.Heuristic Solution

The objective function of problem(5)is so complex that existing works on OFDMA resource allocation[34]do not apply to this work.For this,Section IV-B first proposes two important heuristic rules,which then facilitate us to design a heuristic algorithm to solve the problem(5).

For reliably transmitting one packet with M-QAM modulation,we could either transmit the modulated symbols one time with a small modulation factor M(W1)or many times with a big M(W2)ahead of the specified deadline.In the following,we will show that W1 yields a higher transmission reliability than W2.For any Mi1and Mi2,Mi1,Mi2∈F and Mi1＜Mi2,the BER of Mj-QAM(j∈{i1,i2})modulation is given by

Note that for the fair comparison,the same SNR γ is used in (6).Then the PER of Mj-QAM is given by

We define qi1,i2=e(Mi1)/e(Mi2)and in Fig.6 plot the curves of qi1,i2for different M-QAM schemes.From Fig.6, we obtain the following heuristic.

Fig.6.qi1,i2for different M-QAM schemes(L=160 bits).

Heuristic 1.Low rate M-QAM modulation schemes are adopted for high reliability when time slots are sufficient.

Fig.7.q(ω)for different weights(Sh=2,8-QAM).

Heuristic 2.For reliable transmission,each scheduled station in any time slot communicates over only one subcarrier with the maximum transmit power.

where θ(0≤θ≤1)represents the degree of weighting decrease.With(10),the priority coefficient of priority i is updated as follows

Combining the polling result and(11),the AP then knows the priority coefficient of each loaded station.Based on Heuristics 1 and 2,we propose a heuristic algorithm,termed as timeout probability minimized resource allocation(TPMA) algorithm.The rough idea of TPMA can be summarized as follows:1)The best subcarrier is assigned to the station with the largest timeout probability(which varies across time slots); 2)The maximum transmit power is adopted by each matched station-subcarrier pair;3)The Mmin-QAM modulation is usedby the station whose matched subcarriers in adjacent time slots are the same.Please see Algorithm 1 for details.

Algorithm 1.TPMA algorithm

Input.∀k,s:hk,s,pmax,βPk,Mmin,Mmax,D,K,S,DDD={1,2The traffic in CPS networks(typical machine to machine communication)mainly consists of periodic small packets including plant state or control sequence with fixed size.Therefore,throughput is not a suitable performance metric for CPS networks.,...,D},KKK={1,2,...,K}.

1.Initialization:∀k,s,t,αk=βPk,δk,s(t)=0,pk,s(t)=0,Mk,s(t)=0.

4.repeat

5.from station set with the largest time output probability, i.e.,k∗∈arg maxk∈KKKαk,randomly select one stationk∗.

8.stationk∗allocates all its power to the subcarriers∗,i.e.,

9.ifsubcarriers∗is not allocated to stationk∗in slott-1, i.e,δk∗,s∗(t-1)=0then

10.stationk∗selectsMmax-QAM for transmission in slott,i.e.,Mk,s(t)=Mmax,and setαk=αke(Mmax), wheree(·)is defined in(7).

11.else

12.stationk∗selectsMmin-QAM for transmission in slot

15.end for

Output.optimized configuration:{δk,s(t),pk,s(t),Mk,s(t)}.

Remark 3.In PFDP-MAC,the priority-aware frequency domain polling is prone to the background noise and external interference.The negative effect of the imperfect polling could be alleviated either by allocating more subcarriers to stations or incorporating the probability of polling errors into the formulation of the OFDMA resource allocation.We will leave the design of an improved PFDM-MAC,which is robust to the imperfectness in polling,as a future work.

V.SIMULATION

In this section,simulations are employed to evaluate the performance of PFDP-MAC.We compare PFDP-MAC(TPMA) with unsolicited grant service(UGS)and real-time polling service(rtPS)of IEEE 802.16[29].The critical performance metrics2The traffic in CPS networks(typical machine to machine communication)mainly consists of periodic small packets including plant state or control sequence with fixed size.Therefore,throughput is not a suitable performance metric for CPS networks.of CPS networks are considered as follows:1) Percentage of outdated packets:it is defined as the ratio of outdated packets to the packets sent by senders.The lower the value of this parameter,the higher the data reliability. 2)Average delay:it is defined as the average delay of the successfully delivered packets.The lower the delay,the better the real-time performance.

Considering the traffic characteristic of industrial CPS applications,we assume a cyclic traffic with periodTperiodfor each station,which implies that all stations are saturated in each simulated frame.However,due to synchronization issues, jitters may happen during the packet generating process and for this we assume the packet generating time is uniformly distributed in the time interval[Tperiod-Tjitter,Tperiod+Tjitter],whereTjitterrepresents the largest duration of traffic jitters.The simulation parameters are selected in compliance with the IEEE 802.16[16](see Table I for details).For fair comparison,we only consider the uplink communication.We have conducted our simulations on a Lenovo Thinkstation with a 3.4 GHz CPU and 16 GB RAM.The simulation platform is programmed by C++.

TABLE I PARAMETER VALUES USED IN THE SIMULATION

Figs.8 and 9 depict the comparison results between TPMA and two services of IEEE 802.16(UGS and rtPS)from different perspectives.IEEE 802.16 defines UGS to support real-time service flows that generate fixed size data packets periodically and defines rtPS to support variable bit-rate flows that have specific bandwidth requirements as well as themaximum latency.

Fig.8.Percentage of outdated packets versus number of stations.

Fig.9.Average delay versus number of stations.

Fig.8 shows that the reliability performance(percentage of outdated packets)is ranked as UGS＜rtPS＜TPMA,no matter how many stations are involved in the simulation.Different from UGS and rtPS,the reliability of TPMA decreases as the number of stations increases.However,we notice that TPMA could still guarantee the percentage of outdated packets as low as 2.33×10-6whenK=300.We can also observe that the reliability of TPMA whenK=100 meets the reliability requirement of typical industrial automation applications(e.g., 10-9for 100 nodes per roundtable production cell and 40 nodes per robot manufacturing cell)[25].

Fig.9 shows that the average delay is ranked as UGS＜TPMA＜rtPS,where whiskers denote the 95%confidence interval.As rtPS is designed specifically to improve the realtime performance,rtPS presents the best delay performance among all compared schemes.However,the poor reliability performance of rtPS prohibits its use in CPS networks.In contrast,TPMA presents a high reliability performance while preserving a rather low average delay(2.7-3.3 ms)which is quite close to the average delay of rtPS.

Fig.10 depicts the comparison results among TPMA,UGS and rtPS in terms of running time.It is clearly shown that the running time is ranked as rtPS＜UGS＜TPMA.Specifically, the running time of TPMA grows with the number of nodes, which verifies the scalability and the complexity analysis of Algorithm 1.Combining Figs.8-10,we can conclude that the proposed PFDP-MAC with the TPMA algorithm is a good candidate MAC for CPS networks.

Fig.10.Running time versus number of stations.

Finally,we carry out the performance evaluation of real-time under different SNRs(γk,svaries from-10 dB to 10 dB)and find similar comparison results as Figs.8-10.Therefore,in this paper we omit the comparison results under differentγk,ss for concise presentation.

VI.CONCLUSION AND FUTURE WORK

Wireless technology is a promising solution to CPS networking.However,due to the low ef ficiency of conventional MAC protocols,existing wireless technologies cannot meet the rigorous performance requirement of typical CPS applications, such as factory automation.To this end,this paper proposed a PFDP-MAC protocol for OFDMA-based CPS networks.By using OFDM subcarrier modulation,PFDP-MAC could poll multiple stations and collect their priorities concurrently in the time domain.In addition,we optimized the resource allocation in the OFDMA network for reliable communication.In order to address the NP-completeness of the OFDMA resource allocation problem,we proposed a lightweight heuristic algorithm (termed TPMA)to solve the OFDMA resource allocation, based on two heuristic rules.Extensive simulations showed that the reliability requirements of typical CPS applications could be met by PFDP-MAC.The performance gain of PFDPMAC over IEEE 802.16 MAC was demonstrated under different scenarios as well.

Apart from the future works in Remark 2 and Remark 3, we will also implement the proposed PFDP-MAC on real CPS network platforms in the near future.

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Meng Zheng received the B.S.degree in information and computing science,and the M.S.degree in operational research and cybernetics from Northeastern University,Shenyang,China,and the Ph.D. degree in mechatronic engineering from Shenyang Institute of Automation,Chinese Academy of Sciences,Shenyang,China,in 2005,2008 and 2012,respectively.He had been a visiting student from 2010 to 2012 at the Fraunhofer Institute for Telecommunication,Heinrich-Hertz-Institute,Berlin,Germany. Currently,he is serving as an associate professor at Shenyang Institute of Automation,Chinese Academy of Sciences.His research interests include wireless ad hoc and sensor networks,cognitive radio network, and security in smart grid.

Junru Lin received the B.E.degree from University of Science and Technology of China,Hefei,China, in 2007,and the Ph.D.degree from University of Chinese Academy of Sciences,Beijing,China,in 2013.He is currently working as a researcher at Huawei Technologies Co.,Ltd.,Shenzhen,China. His research interests include industrial wireless communication,software defined network,and network function visualization.

Wei Liang received the Ph.D.degree in mechatronic engineering from Shenyang Institute of Automation,Chinese Academy of Sciences,in 2002.She is currently serving as a professor at Shenyang Institute of Automation.As a primary participant, she developed the WIA-PA standard for industrial wireless networks,which is specified by IEC 62601 and Chinese GB/T 26790.1-2011.As an expert of industrial wireless network technology and standard, she is a member of ISA,IEC TC65 WG16,and WG17.She is also a member of IEEE and ACM.She is the council member of Digital-city Measurement and Control Technology TC of China Instrument and Control Society.She is also the council member of China Computer Federation Technical Committee on Sensor Network Instrument and Control Society.Her research interests include wireless sensor network,dynamic scheduling theory,and system simulation.Corresponding author of this paper.

Haibin Yu received the Ph.D.degree in control theory and control engineering from Northeastern University in 1997.He has been with Shenyang Institute of Automation,Chinese Academy of Sciences since April 1993.He is recognized worldwide for his contributions in fieldbus technologies and has recently been elected as an ISA Fellow.Prof.Yu has also been very active in serving professional communities.He is the Deputy Editor-in-Chief of Journal of Information and Control.He is also the Vice-Chairman of China National Technical Committee for Automation Systems and Integration Standardization,and the council member of Chinese Association of Automation and China Instrument and Control Society.He is also one of the expert members for the Intelligent Manufacturing Fund of the Ministry of Science and Technology of the People’s Republic of China.His research interests include industrial communication and control systems,and the development of industrial sensor network technology and system.

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September 23,2014;accepted May 17,2015.This work was supported by National Natural Science Foundation of China (61304263,61233007),and the Cross-disciplinary Collaborative Teams Program for Science,Technology and Innovation of Chinese Academy of Sciences-Network and System Technologies for Security Monitoring and Information Interaction in Smart Arid.Recommended by Associate Editor Jiming Chen.

:Meng Zheng,Junru Lin,Wei Liang,Haibin Yu.A priority-aware frequency domain polling MAC protocol for OFDMA-based networks in cyber-physical systems.IEEE/CAA Journal of Automatica Sinica,2015,2(4): 412-421