254397

Research ArticleAdaptive Medium Access Control Protocol forWireless Body Area Networks

N. Javaid,1,2 A. Ahmad,1 A. Rahim,3 Z. A. Khan,4 M. Ishfaq,5 and U. Qasim6

1 CAST, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan2 EE Department, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan3 School of Software, Dalian University of Technology, Dalian 116600, China4 Internetworking Program, FE, Dalhousie University, Halifax, NS, Canada B3J 4R25 King Abdulaziz University, Rabigh 21911, Saudi Arabia6University of Alberta, AB, Canada T6G 2J8

Correspondence should be addressed to N. Javaid; [email protected]

Received 19 December 2013; Accepted 10 January 2014; Published 17 March 2014

Academic Editor: Fatos Xhafa

Copyright © 2014 N. Javaid et al.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Wireless Body Area Networks (WBANs) are widely used for applications such as modern health-care systems, where wirelesssensors (nodes)monitor the parameter(s) of interest. Nodes are providedwith limited battery power and battery power is dependenton radio activity. MAC protocols play a key role in controlling the radio activity. Therefore, we present Adaptive Medium AccessControl (A-MAC) protocol for WBANs supported by linear programming models for the minimization of energy consumptionand maximization of dataflow. Our proposed protocol is adaptive in terms of guard band assignment technique and sleep/wakeupmechanism. We focus on specific application to monitor human body with the help of nodes which continuously scan body forupdated information. If the current value is within normal range, nodes do not try to access channel. However, if the current valuerises or falls beyond the permissible range, nodes switch on their transceiver to access channel. Moreover, A-MAC uses TDMAapproach to access channel andwell-defined synchronization scheme to avoid collisions. Furthermore, we conduct a comprehensiveanalysis supported by MATLAB simulations to provide estimation of delay spread. Simulation results justify that the proposedprotocol performs better in terms of network lifetime and throughput as compared to the counterpart protocols.

1. Introduction

Inmodern day life, peoplewant to get information about theirbody. A special purpose of Wireless Sensor Network (WSN)that enables remote monitoring is termed as WBAN. Animportant application ofWBAN is to health caremonitoring.This application enables the patient to be observed, diag-nosed, and prescribed remotely. Hence, WBANs flourishedas promising networks in the field of medical sciences ascompared to traditional health care methods. On large scale,WBAN is classified into invasive and noninvasive networks[1].

Nodes scan the body to gather the required informationand send this information to the respective station. Station isusually equipped with high power; however, nodes are pro-vided with limited power source. In a typical WBAN/WSN,

most of the power is consumed by transceiver. In these net-works, a change in a single physiological parameter triggersmany on-body nodes for data transmission at the same time.This traffic correlation in WBANs leads to high competitionfor medium access. As the nodes are supplied with limitedbattery power, so radio activity of transceiver to accesschannel becomes significant. AsMAC layer controls the radioactivity, therefore, it is obligatory to aim at an energy efficientMAC protocol. For this purpose, many MAC protocols areproposed; however, we only discuss some of these works inrelated work section.

Our proposed A-MAC protocol controls sleep and activemode in a well-organized manner. Nodes sense body regu-larly; however, they do not transmit regularly. Transmissionsdiffer from application to application. For a specific one, theseoccur whenever data fluctuates from normal range. If the

Hindawi Publishing CorporationInternational Journal of Distributed Sensor NetworksVolume 2014, Article ID 254397, 10 pageshttp://dx.doi.org/10.1155/2014/254397

http://dx.doi.org/10.1155/2014/254397

2 International Journal of Distributed Sensor Networks

readings continue to be in normal range, nodes continueto be in idle mode. Moreover, guaranteed time slots forcommunication and adaptive guard band allocation furtherfacilitate energy efficiency. Linear programming models forthe minimization of energy consumption and maximizationof dataflow along with delay spread analysis supported byMATLAB simulations enrich the level of design and under-standing.

The rest of the paper is organized as follows: in Section 2related work is provided, Section 3 contains motivation,Section 4 deals with a brief explanation of our proposedprotocol, Sections 5, 6, and 7 contain energy consumptionanalysis, linear programming based network model, anddelay spread, respectively, Section 8 is provided with thesimulation results, conclusion along with future work is inSection 9, and finally references are given.

2. Related Work

TheIEEE802.11 and its further enhancements like IEEE 802.11b/g/n are designed for medium range high speed wirelessnetworks, like Wireless Local Area Networks (WLANs). Itsupports high data rate.However, IEEE 802.11 has high energyrequirements and deprived bandwidth management, whichmakes it completely inappropriate forWBANs. IEEE 802.15.4has been designed for Wireless Personal Area Networks(WPANs) with a range of 10 to 20 meters. This protocol canbe used in healthcare and consumer electronics applications.However, it does not support devices heterogeneity andlife-critical guaranteed transmission. Beacon enabled modeof IEEE 802.15.4 MAC does not efficiently work in longtermmonitoring applications due to beacon broadcast whichresults in overhead. The nonbeacon enabled mode of IEEE802.15.4MACuses simple CSMA/CAwhich increases energyrequirements for Clear Channel Assessment (CCA).

Omeni et al. in [2] present MAC protocol for single-hopWBANs which has important feature of wakeup/sleep mech-anism along with wakeup fall-back time. Core process of thisprotocol is master-slave relation; when slave node attempts tocommunicate with master node and it fails, slave node goesto sleep mode. Moreover, central control mechanism avoidsoverhearing and continuous time slots avoid collision.

Timmons and Scanlon propose MedMAC in [3] whichuses TDMA based approach. Guard band is introducedbetween two adjacent slots which helps to avoid overlapping,and it depends upon practical situations. Moreover, guaran-teed time slots are used to overcome collisions.

Proposed S-MAC in [4] solves the problem of idle listen-ing by assigning fixed duty cycles. Coordinator node assignsfixed time slots to nodes for wakeup. After wakeup periodnodes go back to sleep mode and collisions are reduced byits synchronization mechanism.

The authors in [5] discuss H-MAC which works onsynchronizationmechanism.This protocol uses natural heartbeats for the synchronization of nodes. So, nodes are inde-pendent in terms of extra energy needed for their synchro-nization. Dedicated time slot assignment is used to overcomecollision.

Proposed McMAC in [6] deals with multi-constrainedQoS in WBANs. This technique introduces a superframestructure that depends on the traffic of a node. A node istransmitted during a particular period of time, if the cor-responding QoS is achievable; moreover it also presents amechanism to handle emergency traffic.

Proposed AR-MAC in [7] uses a star topology with acentral node and for channel access TDMA approach isused. It uses a novel scheme for synchronization, and centralnode uses dedicated time slots for communication. To avoidcollision, an adaptive guard band approach is implemented.

The authors in [8] analyze two models of packet dropacross the link. These are Random Uniformed model andGilbert-Elliott model. Both models are briefly discussed andsimulation results are provided.

3. Motivation

Main objectives to design MAC layer protocols for WBANsare high reliability and less energy consumption. In beaconenabled mode of IEEE 802.15.4, beacon packets are requiredfor broadcast, which results in overhead. The nonbeaconenabled mode of IEEE 802.15.4 and needs extra energy forClear Channel Assessment (CCA) operation. In protocolslike S-MAC, MedMAC, and McMAC sleep schedules areperiodically exchanged resulting in high synchronizationoverhead.Most of the earlier work based on the improvementof MAC protocols for WBANs is just like painting one sideof the picture. Researchers seem to be focused on issuesrelated to synchronization, collision avoidance, time slotsassignment, guard band assignment, emergency data priority,and so forth. In [5] the authors presentWBANMAC require-ments: energy efficiency, support of simultaneous operations,and Quality of Service (QoS). In [9], a comprehensive studyon MAC protocols for WBANs is presented which focuseson energy minimization techniques like low power listening,schedule contention, and TDMA. However, the other sideof the picture, that is, the number of transmissions, remainslike a dark shadow. Let 𝐸cycle be the energy consumed bytransceiver during one cycle:

𝐸cycle = 𝐸sleep + 𝐸active, (1)

where 𝐸sleep is the energy consumed during sleep modeand 𝐸active is the energy consumed during active mode.Transceiver consumes less energy in sleepmode as comparedto active mode.Thus, we emphasize on an adaptive approachto minimizing 𝐸active.

4. A-MAC

Our proposed protocol uses the available resources efficientlybecause it is based on specific application scenario whichhelps in reducing energy consumption. A-MAC uses TDMAtechnique, and Guaranteed Time Slot (GTS) is assigned toeach node for communication.

We assume star topology (shown in Figure 1) for sim-plicity in implementation; individual nodes sense requiredinformation from body and send it to a Coordinator Node

International Journal of Distributed Sensor Networks 3

Node

Endstation

Accesspoint

Figure 1: WBAN topology.

(CN). CN forwards received information to MonitoringStation (MS), directly or indirectly via an Access Point (AP).CN is endowed with larger battery and higher computationalabilities. We assume a single transceiver within CN. Totaltime frame𝑇Frame is divided into three parts: Contention-FreePeriod (CFP) for communicationwith nodes and ContentionAccess Period (CAP) to accommodate on-demand traffic andtime 𝑇MS for transforming node’s data to MS. The followingsubsections elaborate the proposed protocol in detail.

4.1. Adaptive Sleep and Wakeup Mechanism. Nodes sensehuman body to gather required information like temperature,blood pressure, pulse rate, and so forth. Nodes access channelonly if the criterion of interest is satisfied; otherwise, nodescontinue to be in idle mode. Criteria of interest vary fromapplication to application. For example, let us consider thecase of blood pressure; if the current blood pressure sensed isnormal, node continues to be in idle mode.When the currentsensed value drifts from its normal range, node switchesto active mode, where it tries to access channel in orderto transmit data to CN. In this way, nodes minimize thenumber of transmissions by an adaptive sleep and wakeupmechanism, ultimately saving a huge amount of energy.

4.2. Channel Selection. At the beginning, CN scans for freeRadio Frequency (RF) channels. If busy, CN switches off thecurrent RF channel and switches on another RF channel.Theprocess continues till CN finds a free RF channel, and thenit broadcasts the channel packet to nodes. Channel packetsinclude information about the address of CN and channelinformation. Parallel to this process, nodes scan for free RFchannel, and if busy, they wait for time 𝑇CP to listen forchannel packet. If the channel packet is not received, nodeswitches off the current RF channel and switches on th nextchannel. When node receives channel packet successfully, itsends an acknowledgment (ACK) packet to CN, as shown inFigure 2.

NodeStart Start

Scan body forcurrent value

(CV)

Yes

Yes

Is CV withinnormal range

No

No

No

No

Scan RFchannel

Free

Free

YesYes

Switch toanother RF

channel

CN

Scan RFchannel

Send channelpacket (CP)

CPreceived

Send ACK

End

Wait fortime TCP

Figure 2: Channel selection.

4.3. Time Slots Assignment. The selection of free RF channelis followed by Time Slot Request (TSR) packet; transmittedby nodes to CN which includes information about the node’sTime Slot (TS) for communication as well the data rate.In order to efficiently utilize the available resources, CNassigns TSs and guard band time (𝑇GB) to nodes accordingto their traffic which is an adaptive approach. Assignment ofvariable TSs and 𝑇GB is followed by Time Slot Request Reply(TSRR) from CN to nodes. To avoid interference betweentwo successive time slots 𝑇GB is inserted. We calculate 𝑇GB

as follows:

𝑇GB𝑛,𝑛+1

=𝐺𝐹

100×1

2(TS𝑛+ TS𝑛+1) ,

𝑇GB1

=𝐺𝐹× TS1

100,

𝑇GB𝑛

=𝐺𝐹× TS𝑛

100,

(2)

where 𝐺𝐹is the guard band factor which depends upon the

average drift value, 𝑇GB1

is inserted before first time slot, and𝑇GB𝑛

is placed before 𝑛th time slot. After the assignment ofTSs, nodes switch into sleep mode; they switch to wakeupmode only when they have data to send within their allocatedTSs. This mechanism provides, almost collision-free andreliable TSs with reduction in energy consumption. 𝑇GB

assignment is shown in Figure 3.

4.4. Synchronization Mechanism. In order to communicateefficientlywithin the assignedTSs, CNneeds synchronizationwith nodes. Within expected TS, CN listens for data packet.Upon the reception of data packet, CN compares expectedreception time with current reception time. Let𝐷 be accept-able delay. Drift value (DV) is calculated from current and


CAP

Guard time

Guaranteed time slot

Communication with MS

Contention access period

TMS· · ·TS1 TSn

Figure 3: Time slots assignment with guard-band time.

expected reception time, which is used for synchronization infuture. If the difference value is greater than 𝐷, then a piggybackmechanism is used; that is, DVwithin SYNChronizationACKnowledgment (SYNC-ACK) packet is sent by CN forfuture synchronization. Else, simple ACK packet is sent byCN.

4.5. Packet Types. A-MAC deals with two types of packets:data packets and control packets. Data packet includes node’ssensed data, and control packets are as follows.

(1) Channel Packet (CP): it includes CN’s address andchannel information.

(2) Time Slot Request (TSR) packet: request informationto CN for GTS is embedded in TSR packet.

(3) Time Slot Request Reply (TSRR) packet: this packetincludes CN’s reply to node along with GTS informa-tion.

(4) SYNChronization-ACKnowledgment (SYNC-ACK)packet: DV along with ACK of the last received dataare coupled in SYNC-ACK packet.

(5) Data Request (DR) packet: CN sends DR packet tonode in order to meet on-demand traffic.

(6) Acknowledgment (ACK) packet for the ACK of datapacket.

5. Energy Consumption Analysis

Energy consumption model is based on the transceiver’sactivity, and we assume constant consumption of energyregarding sensing and processing units. To minimize energyconsumption, sleep and wakeup mechanism play a vital role.Let 𝐸𝑐be the total consumed energy in one cycle, 𝐸

𝑠is energy

consumed in sleepmode, and𝐸𝑎is energy consumed in active

mode. Then,𝐸𝑐= 𝐸𝑠+ 𝐸𝑎. (3)

Total energy consumption for 𝑛 the number of cycles isgiven by

𝐸𝑡=

𝑛

∑

𝑐=1

𝐸𝑐. (4)

Energy is a function of time and power, and power itselfis a function of voltage and current. In sleep mode, nodesconsume less energy as compared to active mode:

𝐸𝑠= 𝑇𝑠× 𝐼𝑠× 𝑉,

𝑇𝑠= 𝑇𝑓− 𝑇𝑎,

(5)

where 𝑇𝑓is the total frame duration and 𝐼

𝑠is the current

drawn in sleep mode from voltage source 𝑉. Let 𝑇𝑎be active

time duration for nodes. In 𝑇𝑎nodes consume switching

energy 𝐸sw, transmission energy 𝐸tx, reception energy 𝐸rx,and time-out energy 𝐸to:

𝐸𝑎= 2 × 𝐸sw + 𝐸tx + 𝐸rx + 𝐸to. (6)

To switch between sleep and active modes, transceiverconsumes energy 𝐸sw:

𝐸sw = 𝑇sw × 𝐼sw × 𝑉, (7)

where nodes draw 𝐼sw current from voltage source during𝑇swswitching time duration.

Let 𝑙 be length of packet (control or data), let 𝑇𝑏be

time needed for single byte transmission, and let 𝐼tx theamount of current drawn from 𝑉 during transmission.Energy consumed during transmission is given by

𝐸tx = 𝑙 × 𝑇𝑏 × 𝐼tx × 𝑉. (8)

Similarly, energy consumed at the receiver end 𝐸rx iscalculated as

𝐸rx = 𝑙 × 𝑇𝑏 × 𝐼rx × 𝑉. (9)

Time interval, after the transmission of an ACK packetand before its reception, is called time-out period (𝑇to).Energy consumed during 𝑇to is termed as time-out energy(𝐸to):

𝐸to = 𝑇to × 𝐼to × 𝑉, (10)

where 𝐼to is the current drawn from voltage source 𝑉 during𝑇to.

6. Problem Formulation and Modeling viaLinear Programming

We consider a WBAN scenario where nodes collect datafrom the body and send the collected information to the sinkdirectly or indirectly via an access point. So, WBAN consistsof two types of nodes: monitoring nodes and sink nodewhichcollects the information at receiving end. In WBAN model,the position of nodes and sink is predetermined accordingto the application. Let 𝑆 be the set of nodes, where 𝑆 =

[𝑖, 𝑗, . . . , 𝑛] and the sink node is 𝑘. Each node establishesa link with its neighbouring access point or sink if locatedin its communication range 𝑍

𝑐. According to the topology

representation, the following are connectivity parameters:

𝐴(𝑖,𝑗)

={1, if node 𝑖 establishes a link with access point 𝑗,0, otherwise,

𝐵(𝑖,𝑘)

={1, if node 𝑖 establishes a link with sink 𝑘,0, otherwise,

(11)

where 𝑖 ∈ 𝑆. The link between any node and sink dependsupon the closeness and distance between nodes and sink.


We use the propagation radio model used in [10]. Thepath loss coefficient between 𝑖 and 𝑗 (or 𝑖 and 𝑘) is denotedby 𝜂(𝑖,𝑗)

(or 𝜂(𝑖,𝑘)

), and its value is equal to 3.38 for humanbody for Line of Sight (LOS) and 5.9 for Non-Line-of-Sight (NLOS). To calculate energy consumption of nodes, weassume transmission energy, reception energy, and amplifi-cation energy in case if node is not in the transmission rangeand we want to send critical data through this node. Thesensing and processing energy are assumed to be negligiblewith respect to transmission and reception energy. Radio dis-sipation energy (transmission and reception) while turningon the circuitry is denoted by ETX

(elec) and ERX(elec). 𝜖amp

is the radio amplification factor and 𝐷(𝑖,𝑗)

is the distancebetween 𝑖 and 𝑗. 𝑦

(𝑖,𝑗)is coverage parameter. If 𝑦

(𝑖,𝑗)= 1, then

𝑖 is in coverage range of 𝑗.

6.1. Energy Consumption Minimization. The main problemis to maximize the network lifetime. To maximize networklifetime, energy consumption of all the nodes needs to bebalanced. Nodes equipped with less residual energy shoulddecrease their energy consumption.

We considerWBANmodel as a directed graph𝐺 = (𝑆, 𝐸),where |𝑆| = 𝑛 is and𝐸 is a set of links (directed graphs). (𝑖, 𝑗) ∈𝐴 represents an arc between two nodes within minimum ormaximum transmission range. 𝑖 ∈ 𝑉 represents the numberof nods equipped with initial energy 𝐸

0. The goal of the

proposed topology model is to efficiently transmit the senseddata to the destination by consuming minimum energy:

Min [( ∑

𝑖∈𝑆,𝑘∈𝑁

𝑑(𝑖,𝑘)

𝑦𝑖,𝑘(ETXelec + 𝜖amp𝐷𝑖,𝑘𝜂𝑖,𝑗)

+ ∑


(𝑑𝑖,𝑘) 𝑦𝑖,𝑘ERX(elec))

+ ( ∑


𝑓𝑘

𝑖,𝑗(ETXelec + 𝜖amp𝐷𝑖,𝑘𝜂𝑖,𝑘) + ERXelec)

+( ∑


𝑓𝑡

𝑗,𝑘(ETXelec + 𝜖amp𝐷𝑗,𝑘𝜂𝑗,𝑘) + ERXelec)]

(12)

s.t.

∑

𝑖∈𝑆

𝑦(𝑖,𝑗)

= 1, ∀ (𝑖, 𝑗) ∈ 𝐴, (13a)

∑

𝑖∈𝑆

𝑦(𝑖,𝑗)

= 0, otherwise, (13b)

∑

𝑖∈𝑆

𝑑(𝑖,𝑘)

, 𝑦(𝑖,𝑘)

+∑

𝑖∈𝑆

(𝑓(𝑖,𝑗)

− 𝑓(𝑗,𝑖)) − 𝑓𝑡

(𝑖,𝑘)= 0, ∀𝑖, 𝑗 ∈ 𝑆,

(13c)

𝑓𝑘

𝑖,𝑗≤ ∑

𝑖∈𝑆

𝑑(𝑖,𝑗)𝑋(𝑖,𝑗), ∀𝑖, 𝑗 ∈ 𝑆, (13d)

∑

𝑖∈𝑆

𝑑(𝑖,𝑘)

, 𝑦(𝑖,𝑘)

+∑

𝑖∈𝑆

𝑓𝑘

(𝑖,𝑗), ≤ ]𝑡, ∀𝑖, 𝑗 ∈ 𝑁. (13e)

The objective function (12) denotes the total energyconsumption of the sensor network. First term showsthe energy consumed by the nodes during transmission(∑𝑖∈𝑆,𝑘∈𝑁

𝑑(𝑖,𝑘)

𝑦(𝑖,𝑘)

(ETXelec + 𝐸amp𝐷(𝑖,𝑘)𝜂(𝑖,𝑗)). Thesecond term ∑

𝑖∈𝑆,𝑘∈𝑁(𝑑𝑖,𝑘)𝑦𝑖,𝑘ERX(𝑖,𝑗)) of the objective

function (12) represents the total energy consumedby nodes to receive the transmitted data. The term[∑𝑖∈S,𝑘∈𝑁𝑓

𝑘

𝑖,𝑗(ETXelec + 𝐸amp𝐷𝑖,𝑘𝜂𝑖,𝑘) + ERXelec] +

[∑𝑖∈𝑆,𝑘∈𝑁

𝑓𝑡

𝑗,𝑘(ETXelec + 𝐸amp𝐷𝑗,𝑘𝜂𝑗,𝑘) + ERXelec] comes

under consideration if and only if data is relayed throughrelay node (s); that is, it indicates the total energy consumedby relay nodes to transmit data from source nodes to sink (incase of A-MAC the value of this term is zero; however, we willadress this in future). Finally the total energy consumed bysink to receive the transmitted data is ∑

𝑖∈𝑆,𝑘∈𝑁𝑓𝑡

𝑗,𝑘,ERXelec.

Constraints (13a) and (13b) indicate the full coverage.Constraint (13c) provides the flow balance of traffic fromnode 𝑖 to sink. The term ∑

𝑖∈𝑆𝑑(𝑖,𝑘)

, 𝑦(𝑖,𝑘)

describes the datagenerated by the nodes routed towards the sink 𝑘, and theterm∑

𝑖∈𝑆(𝑓(𝑖,𝑗)

−𝑓(𝑗,𝑖))−𝑓𝑡

(𝑖,𝑘)= 0 shows the total flow balance

of total traffic fromnodes towards the sink. If the link betweenthe communicating parties is established, then constraint(13d) defines that the total flow of traffic is always less thanthe data generated by nodes because the protocol operationof A-MAC does not allow nodes to transmit normal datawhile sensing. Constraint (13e) describes that the total trafficgenerated by the nodes does not exceed the total capacity ]

𝑡

of the link.

6.2. Maximum Flow Problem. Consider 𝑉 is a set of verticesand 𝐸 is a set of edges between two nodes, where 𝑆 is a setof nodes. Each node has a capacity ]

𝑡. The goal here is to

maximize the total flow of traffic from source to destination.The max flow problem is given as follows:

Max ∑

(𝑖,𝑘)

𝑓(𝑖,𝑘) (14)

s.t.∑

(𝑖,𝑘)∈𝐸

𝑓𝑖,𝑘≥ ∑

(𝑘,𝑖)∈𝐸

𝑓𝑘,𝑖, ∀𝑖 ∈ 𝑆, (15a)

0 ≤ 𝑓𝑖,𝑘≤ ](𝑖,𝑘)

, ∀ (𝑖, 𝑘) ∈ 𝐸, (15b)

𝑦(𝑖,𝑘)

= 1, (15c)

𝑦(𝑖,𝑘)

= [0, 1] , (15d)

𝐸𝑖(rem) ≥ 𝐸

(min), ∀𝑖 ∈ 𝑆 (15e)

𝐷(𝑖,𝑘)

≤ 𝐷(min-trans), ∀𝑖 ∈ 𝑆 (15f)

∑

𝑖∈𝑆

Col(𝑖,𝑘)

= 0, ∀𝑖 ∈ 𝑆 (15g)

∑

𝑖∈𝑆

Pd(𝑖,𝑘)

= 0, ∀𝑖 ∈ 𝑆. (15h)

The objective function (14) defines the total flow of trafficfrom sender to receiver. Constraint (15a) shows the flow con-servation of traffic from node 𝑖 to sink 𝑘. The total traffic


from node 𝑖 to sink 𝑘 ≥ the total traffic from sink 𝑘

to node 𝑖 meaning that the uplink data traffic is morethan that of downlink. Constraint (15b) describes that thenet flow is always less than the total capacity of the link.Constraints (15c) and (15d) describe about the existence ofthe link between 𝑖 and 𝑘. Constraint (15e) describes thata node only transmits data when it has sufficient energygreater than the minimum energy required for transmission.Constraints (15f) describe theminimumdistance required fordata transmission. Finally, constraints (15g) and (15h) statethat the total flow is maximum when packet drops Pd andcollisions Col from all nodes to sink are zero because thepacket is then retransmitted.

7. Delay Spread Analysis

To develop some general guidelines for wireless systems, dif-ferent multipath channels are compared and due to variationin path lengths the impulse response of a wireless channellooks like a series of pulses. Time domain analysis fromthe measured frequency domain transfer function 𝑆

21(𝑓)

via Inverse Fast Fourier Transform (FFT/IFFT) results inimpulse response ℎ(𝑡) = 𝑠

21(𝑡). Practically, distinguishable

pulses that depend on time domain resolution of the com-munication system are very large in number and most of theenergy is received via a direct path with multipath compo-nents after some time. Thus, delay spread is a measure of themultipath richness of a communication channel or the arrivaltime difference between the earliest multipath componentand the latestmultipath component of the received signal [11].

For evaluation purpose, mostly we focus on a class ofchannels rather than a single impulse response. Delay spreadis generally quantified by different metrics like maximumdelay spread 𝜏max, rms delay spread 𝜏rms, and so forth.𝜏max is probably the most important single measure for thedelay time extent of a multipath radio channel. Since the𝑠21(𝑡) and the 𝑆

21(𝑓) of a channel are related by the (IFFT),

it is intuitively understandable that the transfer functionmagnitude shows more fades per bandwidth, by increasingthe length of impulse response.

(1) Maximum excess delay 𝜏max defines the temporalextent of the multipat; that is above a particularthreshold the received signal can be neglected whichis known as 𝜏max.

(2) Propagation delay relative to that of the shortest pathand characterized by the first centralmoment is calledmean excess delay 𝜏

0.

(3) Mostly, root mean square rms value of the delayspread 𝜏rms is used instead of 𝜏max.

𝜏0acts as a linear function of antenna separation and is

mathematically modelled as in [12] as follows:

𝜏0(𝑑) = 𝐴𝑑 + 𝐵, (16)

where 𝑑 is the distance between transmitter and receiverin cm and 𝐴 and 𝐵 are the model parameters in [ns/cm]and [ns], respectively. Figure 4 shows the fitted 𝜏

0model for

30 40 50 60 70 80

10

20

30

40

50

60

70

Distance (cm)

MeasuredFit model

𝜏0

(ns)

Figure 4: Mean excess delay spread.

30 40 50 60 70 80

12

14

16

18

20

22

24

Distance (cm)

MeasuredFit model

𝜏rm

s(n

s)

Figure 5: rms delay spread.

our proposed protocol based on the measurements, usingGaussian fit model.

Rms delay spread is given as a piecewise function whosefirst part is modelled by an exponential fit and second by alogarithmic fit, with a break point 𝑑bp given below:

𝜏rms (𝑑) =

{{{{

{{{{

{

𝐶(𝑒𝐷𝑑

− 1) , for 𝑑 ≤ 𝑑bp,

𝐸 + 𝐹 ln( 𝑑

𝑑bp) , for 𝑑 > 𝑑bp,

(17)

where 𝐶, 𝐷, 𝐸, and 𝐹 are the model parameters in [ns],[1/cm], [ns], and [ns], respectively. Figure 5 shows the fitted


Table 1: Parameter values of the models for 𝜏0and 𝜏rms.

Model Parameter Arm Leg Back Torso

𝜏0(𝑑) = 𝐴𝑑 + 𝐵

𝐴 0.35 0.41 0.72 0.76𝐵 −1.1 −1.8 −1.9 −2.7

𝜏rms(𝑑) = 𝐶(𝑒𝐷𝑑

− 1)𝐶 1.41 0.67 1.88 0.58𝐷 0.09 0.15 0.15 0.23

𝜏rms(𝑑) = 𝐸 + 𝐹 ⋅ ln(𝑑

𝑑bp)

𝐸 9.97 6.40 13.01 12.23𝐹 5.88 16.72 3.03 6.13𝑑bp 22.3 16.2 14 13.2

Table 2: Simulation parameters.

Parameter Value𝑇Frame 1 second

𝑉 3 volts𝐼𝑠

1 micro-A𝐼idle 20.0mA𝐼tx 17.4mA𝐼rx 19.7mA

𝜏rms model for our proposed A-MAC based on the mea-surements. The parameter values [10] used in fit models forsimulation are provided in Table 1.

8. Simulation Results

This section provides a brief description related to MATLABsimulations of the proposed A-MAC protocol as well as IEEE802.15.4 andAR-MAC.We consider star network of 10 nodes,where a single node is implanted on the body of each patient.As our approach is application specific, we consider a specificapplication here, that is, BloodPressure (BP).Different rangesfor BP are provided in Figure 6. We use energy model fromthe data sheet of Crossbow MICAz as shown in Table 2. Forthis purpose, we execute our protocol 5 times and calculate itsmean value with possible deviation frommean value in termsof upper and lower bounds. The interval or range of valueswithin upper and lower bounds is the confidence interval. Forour simulation, there is 90% probability that the outcomes ofinterest lie within the error bars. We use Random Uniformedmodel for packets drop calculation.

From Figure 7 it is clear that A-MAC performs betterthan IEEE 802.15.4 and AR-MAC. In CSMA/CA operation ofIEEE 802.15.4, with an increase in average packet error rateprobability, the number of back-offs increases. With everyadditional back-off, extra energy is consumed to performclear channel assessment operation leading to more energybeing consumed. AR-MAC uses adaptive guard band andadaptive TS allocation, to decrease the number of collisionswhich results in relatively less energy consumed. A-MACfurther minimizes energy consumptions by minimizing thenumber of channel access tries, adaptive guard band alloca-tion, and GTSs for communication. When the current valueof blood pressure is within normal range (systolic BP = 90–120mmHg and diastolic BP = 60–80mmHg), transceiver

190

180

170

160

150

140

130

120

110

100

90

80

70

Systo

lic B

P (m

mH

g)

40 50 60 70 80 90 100

Diastolic BP (mmHg)

High BP

Pre-high BP

Ideal BP

Low BP

Figure 6: Blood pressure range.

0 5 10 15 208.6

8.8

9

9.2

9.4

9.6

Packet error rate (%)

Ener

gy co

nsum

ptio

n (m

J)

AR-MACA-MAC

IEEE 802.15.4

Figure 7: Energy consumption analysis for 1000 cycles.


0 10 20 30 40 50 600

1

2

3

4

5

6

7

8

9

10

Number of cycles

Num

ber o

f dea

d no

des

AR-MACA-MAC

IEEE 802.15.4

Figure 8: Network lifetime.

associatedwith each node is off and nodes do not try to accesschannel.

For lifetime and throughput analysis, we assume thateach node is initially provided with 0.5 J of energy. A-MACshows extension in the network lifetime as compared to thegiven counterpart protocols, as can be seen from Figure 8. InIEEE 802.15.4 and AR-MAC, nodes sense data from humanbody and transmit it to CN periodically, whether the data isnormal or not. Irrespective of the data weight, IEEE 802.15.4assigns the same guard band to each node, whereas AR-MAC assigns guard band according to the weight of data. Inour proposed scheme, nodes sense data regularly but thesedo not try channel access on a regular basis. Attempt toaccess channel only occurs when data fluctuates from itsnormal range. Moreover, the assignment of guard bands isaccording to the weight of data and GTSs are assigned tonodes for data communication to overcome packet collisionand overhearing. In short, adaptive approach with extendedsleep time mechanism in A-MAC results in network lifetimeextension.

IEEE 802.15.4 uses CSMS/CA approach, in which ifafter the maximum number of back-offs the channel is stillbusy, packet is discarded, whereas AR-MAC uses TDMAapproach, that is, guaranteed time slots assignment to nodes.Moreover, the minimum number of transmissions in A-MAC should result in lower number of packets sent to CN.However, witnessing A-MAC’s curve in Figure 9 really grabsthe attention. A-MAC sends more packets to CN than IEEE802.15.4 and the same number of packet as AR-MAC. Whythis kind of behaviour occurs? Let us justify this behaviourby taking cycle number 26 under consideration. Packets sentto CN at cycle number 26 are A-MAC 234, IEEE 802.15.4250, and AR-MAC 250. At this cycle, A-MAC shows theminimum number of packets sent which is justification of itsminimized number of transmissions. In IEEE 802.15.4, after

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

300

350

Number of cycles

Num

ber o

f pac

kets

sent

to C

N

AR-MACA-MAC

IEEE 802.15.4

Figure 9: The average number of packets sent to CN (aggregated).

cycle number 26, no more packets are sent to CN becauseafter this particular cycle all nodes are dead. Similarly, in AR-MAC packet sending ends after cycle number 34. At cyclenumber 34, AR-MAC’s sent packets to CN aremore than thatof A-MAC.However, inA-MACpacket sending continues tillcycle number 41. These additional cycles compensate for thelower number of packets sent to CN.

In real scenarios, the total number of packets sent is notequal to the total number of packets received. Packets arealways dropped. For our simulations to be more realistic,we choose two states of link reliability, that is, good (having70% probability) and bad (having 30% probability). If thelink status is good, packet is received successfully, else itis dropped. According to Figure 10 packets dropped orderis IEEE 802.15.4 > AR-MAC > A-MAC. Reasons are asfollows: CSMA/CA and fixed guard band assignment in IEEE802.15.4 means high contention leading to more packetsbeing dropped, adaptive guard band assignment in AR-MACmeans relatively low contention leading to less number ofpackets being dropped, and adaptive guard band assignmentas well as minimizing the number of transmissions meansminimizing the contention for channel access thereby furtherdecreasing the number of packets being dropped. Figure 11shows the network throughput, that is, the number of packetsreceived at CN. In this regard, A-MAC’s performance issuperior to the other two protocols due to well-definedsynchronization mechanism as well as the reasons stated forpacket drops.

9. Conclusion and Future Work

Nodes keep updating their readings and on the arrival offresh information they access the channel. In order to accesschannel, nodes switch on their transceiver which consumesenergy. Our approach does not allow nodes to access channel


0 10 20 30 40 500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Number of cycles

Num

ber o

f Pkt

s dro

pped

IEEE 802.15.4AR-MACA-MAC

Figure 10: The average number of packets dropped (per round).

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

Number of cycles

Num

ber o

f pac

kets

rece

ived

at C

N

IEEE 802.15.4AR-MACA-MAC

Figure 11: The average number of packets received at CN (aggre-gated).

after every fresh reading. Furthermore, GTSs for commu-nication, adaptive guard band allocation, and well-definedsynchronization mechanism are used to overcome collisionand overhearing. A linear programming approach is adoptedto maximize data flow and minimize energy consumption.Time domain analysis of the proposed protocol in termsof delay spread is also conducted. From simulation results,we conclude that all of the mentioned features throughoutthe paper enable A-MAC to outperform the counterpartprotocols.

Our future directions will focus on working on jointphysical and MAC model as well as the effect of temperatureon link quality [13].

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

References

[1] M. M. Alam, O. Berder, D. Menard, and O. Sentieys, “Latency-energy optimized mac protocol for body sensor networks,” inProceedings of the 9th International Conference onWearable andImplantable Body Sensor Networks (BSN ’12), pp. 67–72, 2012.

[2] O. Omeni, A. C. W. Wong, A. J. Burdett, and C. Toumazou,“Energy efficient medium access protocol for wireless medicalbody area sensor networks,” IEEE Transactions on BiomedicalCircuits and Systems, vol. 2, no. 4, pp. 251–259, 2008.

[3] N. F. Timmons and W. G. Scanlon, “An adaptive energy effi-cient MAC protocol for the medical body area network,” inProceedings of the 1st International Conference on WirelessCommunication, Vehicular Technology, Information Theory andAerospace and Electronic Systems Technology, Wireless (VITAE’09), pp. 587–593, May 2009.

[4] W. Ye, J. Heidemann, andD. Estrin, “A flexible and reliable radiocommunication stack on motes,” Tech. Rep. ISI-TR-565, USCInformation Sciences Institute, 2002.

[5] S. Ullah, B. Shen, S. M. Riazul Islam, P. Khan, S. Saleem, and K.S. Kwak, “A study of MAC protocols for WBANs,” Sensors, vol.10, no. 1, pp. 128–145, 2009.

[6] H.-S. W. So, J. Walrand, and J. Mo, “McMAC: a parallelrendezvous multi-channel MAC protocol,” in Proceedings ofthe IEEE Wireless Communications and Networking Conference(WCNC ’07), pp. 334–339, March 2007.

[7] A. Rahim, N. Javaid, M. Aslam, U. Qasim, and Z. Khan,“Adaptive-reliable medium access control protocol for wirelessbody area networks,” in Proceedings of the 9th Annual IEEECommunications Society Conference on Sensor,Mesh andAdHocCommunications and Networks (SECON ’12), pp. 56–58, 2012.

[8] Q. Zhou, X. Cao, S. Chen, and G. Lin, “A solution to errorand loss in wireless network transfer,” in Proceedings of theInternational Conference on Wireless Networks and InformationSystems (WNIS ’09), pp. 312–315, December 2009.

[9] S. Hayat, N. Javaid, Z. A. Khan, A. Shareef, A. Mahmood, andS. H. Bouk, “Energy efficient mac protocols,” in Proceedings ofthe IEEE 14th International Conference on High PerformanceComputing and Communication and IEEE 9th InternationalConference on Embedded Software and Systems (HPCCICESS’12), pp. 1185–1192, 2012.

[10] E. Reusens, W. Joseph, B. Latre et al., “Characterization of on-body communication channel and energy efficient topologydesign for wireless body area networks,” IEEE Transactions onInformation Technology in Biomedicine, vol. 13, no. 6, pp. 933–945, 2009.

[11] 2013, http://en.wikipedia.org/wiki/Delayspread/.[12] E. Reusens, W. Joseph, G. Vermeeren, and L. Martens, “On-

body measurements and characterization of wireless commu-nication channel for arm and torso of human,” in Proceedingsof the 4th international workshop on wearable and implantablebody sensor networks (BSN ’07), pp. 264–269, Springer, 2007.


[13] M. Tahir, N. Javaid, A. Iqbal, Z. Khan, and N. Alrajeh, “Onadaptive energy-efficient transmission in wsns,” InternationalJournal of Distributed Sensor Networks, vol. 2013, Article ID923714, 10 pages, 2013.

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