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© The Author 2011. Published by Oxford University Press on behalf of The British Computer Society. All rights reserved.For Permissions, please email: [email protected]

Advance Access publication on June 24, 2011 doi:10.1093/comjnl/bxr057

Fair Coexistence MAC Protocol forContention-Based Heterogeneous

Networks

Eun-Chan Park and Minjoong Rim

Department of Information and Communication Engineering, Dongguk University, Seoul, Republic of Korea∗Corresponding author: [email protected]

This paper proposes a contention-based fair coexistence mechanism among heterogeneous networksthat have different transmission power and/or coverage. First, we show that the existing carriersensing multiple access (CSMA) mechanism, that is a prevailing contention-based protocol, resultsin significant unfairness in channel access when heterogeneous networks coexist; a system with lowertransmission power hardly occupies the shared channel due to interference from a system with highertransmission power. We analyze the causes of unfairness in terms of (i) the asymmetry of carriersensing and (ii) the blindness of binary exponential backoff mechanism and the link adaptationmechanism, and we derive an analytical model of per-system throughput to investigate the effects ofthese causes.To resolve this problem, we propose a fair coexistence CSMA protocol consisting of accessetiquette and interference-aware backoff. The former adaptively controls the contention window sizeso that the high-power system allows transmission opportunities to the low-power system in a fairand efficient manner. The latter differentiates between the response to transmission failure caused bycollision and the response to failure caused by interference. The simulation results confirm that theproposed scheme effectively mitigates the unfairness of channel sharing while attaining high spectral

efficiency.

Keywords: coexistence; CSMA; contention; interference; fairness

Received 21 February 2011; revised 22 April 2011Handling editor: Ing-Ray Chen

1. INTRODUCTION

The Internet is continuously expanding to include many newemerging services and portable devices, and users want toaccess the Internet anywhere, anytime and with any device.To satisfy the diverse requirements of these services and userdemands, wireless communication systems have evolved tosupport high capacity and quality of service and a new advancedcommunication system has appeared. Therefore, the demand forwide bandwidth is continuously increasing, but the frequencyspectrum is essentially a limited resource. Thus, it is imperativeto effectively manage and allocate the frequency spectrum.

Recently, Federal Communications Commission releasedthe 3.65 GHz spectrum for license-exempt non-exclusivecoexistence among heterogeneous networks and mandated thecontention-based protocol to assure fair sharing among them.To satisfy these regulatory requirements, the standards ofIEEE 802.16h and IEEE 802.11y have been established with

coexistence mechanisms for license-exempt operation [1, 2].The studies in [3, 4] investigated the coexistence issue of IEEE802.16 systems and IEEE 802.11 systems in a license-exemptshared frequency band.

This paper deals with coexistence issues arising whenheterogeneous networks that have different transmission powerand/or coverage employ the contention-based protocol for fairchannel sharing. First, we show that the existing carrier sensingmultiple access (CSMA) mechanism, which is a prevailingcontention-based protocol, results in significant unfairness ofchannel access. We analyze the cause of unfair channel accessfrom the viewpoints of (i) the asymmetry of carrier sensingability and (ii) the blindness of the binary exponential backoff(BEB) mechanism and the link adaptation mechanism. Thehigh-power system may fail to detect the ongoing transmissionof the low-power system, and thus the low-power systemsuffers from frequent transmission failures due to interferencecaused by the high-power system. In addition, the BEB

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Fair Coexistence MAC Protocol for Contention-Based Heterogeneous Networks 1383

mechanism and the link adaptation mechanism, which controlsthe contention window size and transmission rate dependingon the transmission status and channel status, respectively,exacerbate the unfair channel access because they are unawareof the interference-driven transmission failure. We also derivean analytical model for per-system throughput to investigate theeffects of the contention window and the transmission rate onthe fairness and efficiency of channel sharing.

To resolve this problem, we propose the fair coexistenceCSMA protocol, which makes a small and simple changeto the existing CSMA mechanism. The proposed mechanismconsists of two schemes: access etiquette and interference-aware backoff. The former reduces the effect of interferencecaused by the high-power system on the low-power system,while the latter enables the low-power system not to blindlydefer the channel access in response to transmission failuredue to interference. The access etiquette scheme is applied tothe high-power system and controls its contention window sizebased on the feedback information about per-system throughputto maximize the given objective function that accounts forthe overall efficiency and fairness of channel sharing. On theother hand, the interference-aware backoff scheme is appliedto the low-power system. The receiver of the low-powersystem determines the cause of transmission failure: intra-system collision, or inter-system interference, and it informsthe corresponding sender of the excessive interference. Thenthe sender does not unnecessarily increase the contentionwindow on the interference-driven transmission failure. Theextensive simulation results confirm that the proposed approachsignificantly reduces the unfairness of channel sharing, whileattaining high efficiency under various network configurations.

The coexistence issue has been widely studied in theliterature, especially for wireless local area network (WLAN)and Bluetooth in the unlicensed band [5–7]. These studiesare different from this work from the viewpoints of channelaccess mechanism and network deployment. As channel accessmechanism, the WLAN uses CSMA while Bluetooth usesfrequency-hopping spread spectrum. Moreover, the coexistenceof the WLAN and Bluetooth is irrelevant to the issue of spatialreuse, which should be carefully considered in the multi-cellnetworks. Recently, the IEEE 802.22 working group has beendeveloping a standard for wireless regional area networks,aiming to provide broadband access in rural areas by allowingsharing of geographically unused spectrum allocated to thetelevision broadcast service on a non-interfering basis usingcognitive radio technology [8]. Also, several approaches havebeen proposed to reduce interference and/or collision and toimprove spatial reuse in contention-based multi-rate multi-hop 802.11 WLANs [9–13]. These approaches focus on thetrade-off between interference mitigation and spatial reuse insetting the physical carrier sensing threshold (CSTH) and/orthe transmit power. These works derived the optimal valueof the physical CSTH and proposed the adaptive control ofthe CSTH, transmission power and/or transmission data rate.

Compared with the previous studies, this work makes thefollowing contributions:

(i) This work focuses on the fair and efficient contention-based coexistence of heterogeneous networks thatare asymmetric in terms of transmission power andcoverage. We identify several problems and their causesarising in this situation.

(ii) Unlike the previous approaches that may be infeasibleor undesirable in heterogeneous networks, the proposedscheme adaptively controls the contention window forfair channel sharing without degrading the efficiency ofchannel sharing.

(iii) The proposed MAC protocol is novel in that it cancontrol the trade-off between the fairness and efficiencyof channel sharing by using a control parameter, and itdifferentiates the response to transmission failure due tocollision from the response to failure due to interference.

The rest of this paper is organized as follows. Section 2resents the motivation of the study; we state the problemand cause of unfair channel sharing among heterogeneousnetworks. Section 3 analyzes the feasibility of CSTH controland derives the analysis model for per-system throughput.Section 4 proposes the access etiquette and interference-aware backoff schemes for achieving fair and efficient channelsharing. Section 5 presents the simulation results to evaluate theperformance of the proposed scheme. The conclusions followin Section 6.

2. PROBLEM STATEMENT

We consider two different wireless networks, the wireless metroarea network (WMAN) and the WLAN, which coexist andoperate in the same frequency band. As illustrated in Fig. 1, theWMANs are deployed as cellular networks with the frequencyreuse factor of 1/3, and they have relatively higher transmissionpower (Ptx,1) and coverage (Drx,1)1, compared with the WLAN.The WLAN is deployed within the coverage of the centralWMAN, WMAN_0, and the other six WMANs surroundingWMAN_0, denoted as WMAN_i, are considered as the first-tierinterfering cells. As interference from WMAN_is is dominant,we neglect the interference from the second-tier or further-awayinterfering cells. In order to focus on the performance anomalyin the contention-based coexistence among heterogeneousnetworks, we consider a general coexistence scenario whereboth the WMAN and the WLAN deploy the CSMA protocolfor channel access.2

1For the simplicity of notation, we use subscript ‘1’ and ‘2’ for WMAN andWLAN, respectively.

2According to IEEE 802.16h [1], the contention-based protocol, which issimilar to the CSMA of the WLAN, is used in the specified frames, calledas coordinated coexistence contention-based interval, to allow non-exclusivecoexistence with non-WiMAX systems (e.g. WLAN).


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1384 E.-C. Park and M. Rim

Ics,2I

cs,1D

Drx,1

cs,2D

WMAN1

WMAN2

WMAN3

WMAN4

WMAN5

WMAN6

WLAN

WMAN0cs,1

FIGURE 1. Coexistence scenario of the WMAN and the WLAN(WMANs are deployed with a frequency reuse factor of 1/3).

2.1. Asymmetry of carrier sensing

Consider that WMAN_0 and WLAN have the physical CSTHs,Pcs,1 and Pcs,2, respectively, and that they have differenttransmission power, that is, Ptx,1 > Ptx,2. Note that thereexist the regulatory restrictions on the CSTH and transmissionpower for non-exclusive coexistence. We consider the energydetection approach to be used as physical carrier sensing. Letus define Dcs and Ics as the carrier sensing range within whichthe transmitter can detect a busy channel due to transmissionby the homogeneous system and by the heterogeneous system,respectively. Accordingly, Dcs and Ics are related to intra-system collision and inter-system interference, respectively. Themaximum transmission range with the most robust transmissionrate, Drx, can be determined by the minimum receiversensitivity, Pmin, and the transmission power. Considering thepath-loss propagation model with the path-loss exponent of α,which is the most dominant factor for the channel model, thesethree ranges for WMAN_0 can be represented as

Dcs,1 =(

G1Ptx,1

Pcs,1

)1/α

,

Ics,1 =(

G2Ptx,2

Pcs,1

)1/α

,

Drx,1 =(

G1Ptx,1

Pmin,1

)1/α

,

(1)

where G1 and G2 are the channel gain for WMAN_0 and WLAN,respectively. Similarly, for WLAN, Dcs,2, Ics,2 and Drx,2 can beobtained using the path-loss model. It is known that Dcs > Drx

since carrier sensing is available with the weaker signal requiredfor successful decoding. As indicated in Fig. 1, we assume that

the cells of the WMAN are well planned so that the spatial reuseis available among WMANs. From (1), it can be shown that

if Ptx,1 > Ptx,2 →{

WMAN: Dcs,1 > Ics,1,

WLAN: Dcs,2 < Ics,2.

if Ptx,1 > Ptx,2 and Pcs,1 = Pcs,2

→ Dcs,1 = Ics,2 > Dcs,2 = Ics,1.

(2)

This asymmetry gives priority to WMAN_0 over WLAN in termsof channel access. Depending on the values of transmissionpower and the locations of WMAN/WLAN nodes, it is possiblethat (i) Ics,1 is small enough not to cover the transmission rangeof WLAN at all, that is, the transmitter of WMAN_0 hardlydetects the channel occupation by WLAN (WLAN becomes ahidden terminal to WMAN_0) and (ii) Ics,2 is large enoughto cover the whole transmission range of WMAN_0, i.e. thetransmitter of WLAN defers its transmission whenever WMAN_0occupies the channel (WMAN_0 becomes an exposed terminal toWLAN ).Accordingly, there occurs a severe unfair channel accessbetween WMAN_0 and WLAN; WMAN_0 can access the channelregardless of whether or not WLAN occupies the channel, whileWLAN is significantly liable to be interfered by WMAN_0.

2.2. BEB and link adaptation due to interference

The BEB mechanism adopted in the IEEE 802.11 WLAN ishelpful to alleviate the intra-system collision by doubling thecontention window size on detecting a transmission failure.However, it degrades the fairness of channel sharing among theWMAN and the WLAN due to the inter-system interference.As shown in Fig. 2, the BEB mechanism of the WMAN and theWLAN works in an asymmetric way, responding to the inter-system interference as follows:

(i) WMAN: The WMAN sender cannot detect theongoing packet transmission by the WLAN due tothe weak WLAN signal, and starts transmitting itspacket. However, the WMAN receiver can tolerate theinterference by the WLAN since the interference signalstrength is relatively small. Thus, the WMAN receivercan successfully receive the packet and the contentionwindow of the WMAN sender does not increase.

undetectableby WMAN

tolerableinterference to WMAN

unrecoverableinterference to WLAN

backoff withincreased CWmin

increased TX timedue to reduced TX rate

retransmissionby WLAN

backoff

WMAN

WLAN

time

FIGURE 2. Responses of WMAN and WLAN to inter-systeminterference.


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(ii) WLAN: The high interference signal from the WMANmay corrupt the WLAN packet transmission, and theWLAN sender may retransmit the packet with theincreased contention window according to the BEBmechanism.

Therefore, the BEB mechanism, which cannot distin-guish collision-driven failure from interference-driven failure,reduces the channel access opportunity of WLAN and deterio-rates fair channel sharing.

Similar to the BEB mechanism, the link adaptationmechanism may also worsen the problem. The automaticrate fallback (ARF) [14], the most common link adaptationalgorithm, adjusts the transmission rate by estimating thechannel condition based on the numbers of successivetransmission successes and failures. If the packet transmissionfails consecutively (regardless of the cause of failure, i.e. intra-system collision, inter-system interference, temporary degradeof channel quality), then the ARF decreases the transmissionrate to the next lower one among the available sets (e.g.54, 48, 36, 24, 18, 12, 9, and 6 Mb/s in the case of IEEE802.11a/g). Then the receiver can correctly decode the packetwith higher probability because the more robust modulationand coding scheme is used in the reduced transmission rate.In a sense, the link adaptation mechanism is beneficial inmitigating interference; however, it has an adverse effect in thecase where the WMAN and the WLAN coexist with differenttransmission power. It is important to note that the transmissiontime required for transmitting a constant-size packet is inverselyproportional to the transmission rate. The reduced transmissionrate of the WLAN increases its transmission time, and theprobability that WLAN packet transmission is interfered byWMAN increases accordingly (Fig. 2), because the WMANsender is unaware of the packet transmission of the WLAN,regardless of the transmission rate of theWLAN. In this case, thereduced transmission rate makes the WLAN more vulnerableto interference from the WMAN. Moreover, the decrease oftransmission rate reduces the throughput accordingly. This logicis still applicable to the link adaptation mechanism based on thesignal to interference and noise ratio (SINR).

2.3. Preliminary simulation

We perform preliminary simulations to identify the problemof unfair channel sharing and to observe the effect of CSTHon the achievable throughput. We implement a simulator usingC programming language. The parameters and their valuesused in the simulations are listed in Table 1, and they arecarefully determined considering regulatory restrictions andrealistic deployment. The wireless channel is modeled bytaking path loss, shadowing and multi-path fading into account.Considering the urban environment, the path-loss exponent isset to 3.7, and the Rayleigh fading model is used. The shadowingis modeled as a log-normal random variable with zero mean

TABLE 1. Parameters used in the simulations.

Value

Parameter WMAN WLAN

Transmission power (Ptx ) 1000 mW 50 mWminimum receiver sensitivity (Pmin) −80 dBm −80 dBmcell radius (Drx) 750 m 100 mnumber of users per cell (N ) 10 10path-loss exponent, α 3.7distance between the centers of WMAN_0

and WLAN, Dint-sys

300 m

TABLE 2. Minimum threshold values of SINR for each data rate.

Modulation and Data rate Minimumcoding rate (Mb/s) SINR (dB)

64QAM 3/4 56 24.5664QAM 2/3 48 24.0516QAM 3/4 36 18.8016QAM 1/2 24 17.04QPSK 3/4 18 10.79QPSK 1/2 12 9.03BPSK 3/4 9 7.78BPSK 1/2 6 6.02

and standard deviation of 8 dB. The thermal noise power is setto −100 dBm. The SINR-based link adaptation mechanism isimplemented in the simulation and the corresponding minimumvalue of the SINR for each data rate is given in Table 2 [15]. Thepacket error is modeled according to [16]. The IEEE 802.11aMAC/PHY parameters are used in the simulation. Ten usersare uniformly distributed per cell, and the simulation does notconsider node mobility. For each user, a 1000-byte packet israndomly generated such that its inter-arrival time follows aPoisson random variable with a mean value of 5 ms. This packetgeneration rate is high enough to investigate saturated networkcapacity. Users send/receive packets to/from the base station(BS) of the WMAN and the access point (AP) of WLAN, andso both uplink and downlink communications exist.

Figure 3 shows per-system throughput and total networkcapacity for the following four cases with different values ofCSTH. Here, Pcs,i is defined as the CSTH for WMAN_i:

(i) PCS1: (Pcs,1, Pcs,i , Pcs,2) = (−90, −90, −90) dBm;(ii) PCS2: (Pcs,1, Pcs,i , Pcs,2) = (−100, −90, −90) dBm;

(iii) PCS3: (Pcs,1, Pcs,i , Pcs,2) = (−100, −100, −90) dBm;(iv) PCS4: (Pcs,1, Pcs,i , Pcs,2) = (−100, −100, −100) dBm.

In Fig. 3, the throughput of WMAN_is is represented as anaverage value, while the total network capacity is calculated


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FIGURE 3. Per-system throughput and aggregate network capacityfor four cases with different values of the CSTH.

as the throughput sum of WMAN_0, WLAN and six WMAN_is.The case of PCS1 is set as a baseline scenario where we canevaluate the degree of unfairness. In this case, Dcs,1(= Ics,2)

and Dcs,2(= Ics,1) are ∼1400 and 186 m, respectively, andthere occurs a severe unfairness problem; the throughput ofWMAN_0 is about five times higher than that of WLAN. As anaive solution to this problem, we consider the case of PCS2where Pcs,1 is decreased to detect the channel occupation byWLAN, i.e. Pcs,1 (= −100 dBm) < (G2Ptx,2)/(D

αint-sys). In

this case, Ics,1 (= 347 m) becomes larger than Dint-sys andthe transmitter of WMAN_0 can detect a busy channel due toWLAN transmission. Note that all the senders of WMAN_0 cannotcompletely detect WLAN’s packet transmission. Here, Pcs,i andPcs,2 are not changed from −90 dBm. Figure 3 shows that thisapproach cannot improve the fairness of channel sharing at all;the throughput of WMAN_0 becomes almost zero while that ofWLAN is increased to about 10 Mb/s.The reason is as follows: thedecrease of Pcs,1 increases Dcs,1 (≈ 2600 m) as well as Ics,1, thenthe senders of WMAN_0 are likely to defer transmission duringWMAN_i’s packet transmission (WMAN_is are less interferedby WMAN_0) and WMAN_0’s transmission will encounter moreinterference by WMAN_is. Next, we consider the case of PCS3where Pcs,1 = Pcs,i = −100 dBm for all WMANs, but Pcs,2 forWLAN remains unchanged from −90 dBm. In this configuration,the throughput of WLAN is about five times higher than thatof WMAN_0. The unfair channel sharing between WMAN_0and WLAN appears in the reverse aspect, compared with thecase of PCS1. Meanwhile, the decrease of Pcs,i creates anotherproblem; the overall network capacity is reduced by about 25%compared with those of the PCS1 and PCS2 cases, because thespatial reuse cannot be fully employed. Lastly, in the case ofPCS4, Pcs for all the systems are set to −100 dBm. As shownin Fig. 3, the total capacity is further decreased (by more than35% compared with those in the cases of PCS1 and PCS2), thethroughput of WLAN is less than that of WMAN_0 by more than

four times, and the cell where both WMAN_0 and WLAN coexisthas about four times lower throughput than that in the case ofPCS1. None of these four cases achieves fair channel sharing,and the performance is quite sensitive to the value of the CSTH.

3. ANALYSIS OF UNFAIR CHANNEL SHARING

In this section, we first focus on the feasibility of CSTH controland obtain the upper and lower bounds of the CSTH from therequirements of fair channel sharing and spatial reuse. Next,we derive a simple analytical per-system throughput model tonumerically evaluate the degree of unfair channel sharing andto observe the effect of several parameters on the fairness andefficiency of channel sharing.

3.1. Feasible condition of CSTH control

To achieve fair and efficient channel sharing, the value ofPcs,1 should satisfy two requirements of mutual detection andspatial reuse: (i) it should be small enough for the WMAN_0sender to detect the transmission of WLAN and (ii) at thesame time, it should be large enough for the WMAN_0 sendernot to defer its channel access during the transmission ofWMAN_i. Let us denote dtx1−tx2 and dtx1−txi as the distancebetween transmitters of WMAN_0 and WLAN and that betweentransmitters of WMAN_0 and WMAN_i, respectively, and defineK1 and K2 as the relative distance of dtx1−tx2 and dtx1−txi ,respectively, normalized by Drx,1, i.e. K1 = dtx1−tx2/Drx,1

and K2 = dtx1−txi/Drx,1. Note that 0 ≤ K1 ≤ 2 under thecondition of overlapping deployment of WMAN_0 and WLANand 1 ≤ K2 ≤ 5 considering the deployment of WMANs withthe frequency reuse factor of 1/3. These two requirements canbe represented in terms of Ics,1 and Dcs,1 as

Ics,1 > K1Drx,1,

Dcs,1 < K2Drx,1.(3)

Assuming Pmin,1 = Pmin,2, we can derive the upper/lowerbounds on Pcs,1, defined as Pcs,1 and P cs,1, respectively, from(1) and (3);

Pcs,1(dBm) = Pmin,1(dBm) − 10α log10(K1/Ka),

P cs,1(dBm) = Pmin,1(dBm) − 10α log10(K2).(4)

Here, Ka is defined as the coverage asymmetry factor, i.e.Ka = Drx,2/Drx,1.

It is worthwhile to note that Pcs,1 satisfying (4) does notalways exist; it only exists with the following condition on K1,K2, and Ka:

K1

K2< Ka = Drx,2

Drx,1. (5)

This analysis means that the proper range of the CSTHdepends on the node placement and cell deployment and thatit is not always possible to find the proper range of the CSTH


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achieving mutual detection and spatial reuse at the same time.Thus, if a WLAN coexists with WMANs that are deployedas cellular networks and there exists an asymmetry in thetransmission power, then the approach of adjusting the CSTH(or transmission power) is neither desirable nor feasible toassure fair sharing between the WMAN and the WLAN whileattaining spatial reuse.

3.2. Derivation of per-system throughput model

There are several well-known throughput models for the CSMAprotocol (e.g. distributed coordination function of IEEE 802.11)that consider the dynamics of the contention window and/or linkadaptation [17–19]. However, they cannot be straightforwardlyapplied to this study where the transmission failure mainlyresults from inter-system interference, instead of collision orchannel error. The purpose of deriving an analytic throughputmodel is to evaluate the effect of several parameters (e.g.contention window, the number of nodes, transmission rateand transmission coverage) on the achievable throughput andto find a clue to solving the problem of unfair channelsharing. For this reason, we do not intend to capture thedynamic characteristics of contention window control and linkadaptation, i.e. the contention window size and the transmissionrate are intentionally set to be constant.

Consider that there are N1 WMAN senders within thetransmission coverage of WMAN and N2 WLAN senderscoexist with the WMAN nodes. We define CW1 and CW2 asthe contention window size of the WMAN and WLAN senders,respectively, and define transmission round as the time intervalbetween two consecutive packet transmissions. We make thefollowing assumptions.3

(A1) All WMAN and WLAN senders always have data tosend and compete for channel access.

(A2) Pcs,1 is configured to satisfy the requirement of spatialreuse, i.e. Pcs,1 > P cs,1.

(A3) The WMAN senders can detect the WLAN’s packettransmission with a probability of pd , while the WLANsenders can detect the WMAN’s packet transmissionwith a probability of 1.

(A4) At an initial time slot of transmission round,WMAN andWLAN nodes select their own random backoff counters,b1 and b2, respectively.

3The detection probability pd depends on several parameters including theCSTH, transmission power, channel model and location of WMAN and WLANtransmitters. In this study, we consider that it is constant and approximate it as(Ics,1/Drx,1)

2. Moreover, the backoff counter is either selected randomly atthe beginning of transmission round or decreased from the one selected inthe previous transmission round, depending on the transmission status. Theassumptions (A3) and (A4) are made for the tractability of analysis but droppedin the simulation. The comparison between the analysis results and simulationresults in the next subsection validates these assumptions.

The backoff probability of the WMAN sender, denoted as pbo,1,becomes

pbo,1 � Pr{b1 = k} = 1

CW1, 0 ≤ k ≤ CW1 − 1. (6)

Similarly, pbo,2 can be represented with CW2 as (6). Letus define nbo,1 and nbo,2 as the minimum value of backoffcounters among N1 WMAN senders and N2 WLAN senders,respectively. Depending on the values of nbo,1 and nbo,2, aWMAN or WLAN sender can start packet transmission. Weconsider the following two cases:

(i) CASE 1: nbo,1 < nbo,2 (interference-free channeloccupation by WMAN)

(ii) CASE 2: nbo,1 ≥ nbo,2 (interference-prone channeloccupation by WLAN)

First, we focus on CASE 1 where a WMAN sender startsto transmit a packet at the (nbo,1 + 1)th time slot, while aWLAN sender defers its transmission. Let us denote p(1)

a (k1)

as the probability that a WMAN sender accesses the channelwithout intra-system collision and inter-system interferenceafter nbo,1 = k1 backoff time. Then, p(1)

a (k1) is represented as

p(1)a (k1) = N1pbo,1(1−(k1+1)pbo,1)

N1−1(1−(k1+1)pbo,2)N2 ,

(7)and the average throughput achieved by WMAN nodes becomes

TH(1)1 =

CWm−1∑k1=0

p(1)a (k1)(1 − ber1)

L L

k1ts + L/R1 + toh, (8)

where CWm = min(CW1, CW2), L denotes the packet sizewhose unit is bits. R1 is the transmission rate of the WMANnodes, toh accounts for several overhead time (e.g. inter-framespaces, transmission time of PHY/MAC header andACK frame)and ts is the slot time. The bit error rate (BER), ber1 in (8),can be obtained with a path-loss model and a geometric modelspecifying the locations of nodes and the assumption of additivewhite Gaussian noise channel [20]. It is noteworthy that inCASE 1 (nbo,1 < nbo,2), WLAN defers the channel access,and thus its throughput is zero.

Next, we consider the CASE 2 where nbo,1 ≥ nbo,2, i.e. theWLAN sender makes the channel access attempt before theWMAN sender does. In this case, we need to distinguish thecase where the WMAN sender detects WLAN’s transmissionfrom the case where the WMAN sender does not (there occursinter-system interference). Let us define TH(2)

2,n and TH(2)2,i as the

WLAN throughput in the former case and in the latter case,respectively. Similarly, TH(2)

1,n and TH(2)1,i denote the throughput

achieved by the WMAN node for these two cases. Note thatTH(2)

1,n is zero. Then the throughputs of the WMAN and theWLAN in CASE 2 become

WMAN : TH(2)1 = (1 − pd)TH(2)

1,i ,

WLAN : TH(2)2 = pdTH(2)

2,n + (1 − pd)TH(2)2,i .

(9)


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The value of TH(2)2,n, i.e. the WLAN throughput without

interference by the WMAN, can be calculated similarly to(8). Now, we derive TH(2)

1,i and TH(2)2,i , the throughput of the

WMAN and the WLAN with the inter-system interference. Thechannel access probability of WLAN and WMAN senders whennbo,1 = k1 and nbo,2 = k2(≤ k1) is

p(2)a (k1, k2) = N1pbo,1(1 − (k1 + 1)pbo,1)

N1−1

N2pbo,2(1 − (k2 + 1)pbo,2)N2−1,

(10)

because nbo,1 and nbo,2 are independent. We define nf,1 as thenumber of time slots elapsed from the start of transmissionround to the end of WMAN’s packet transmission:

nf,1 = (k1ts + L/R1 + toh)/ts. (11)

In the same way, nf,2 can be represented. Depending on thevalues of nbo,1, nf,1 and nf,2, there exist the following threesub-cases as depicted in Fig. 4:

(i) CASE 2.1 (full overlapping): nf,1 ≤ nf,2

(ii) CASE 2.2 (partial overlapping): nf,1 > nf,2 and nbo,1 <

nf,2

(iii) CASE 2.3 (non-overlapping): nf,1 > nf,2 and nbo,1 ≥nf,2

Note that CASE 2.1 may occur if R1 > R2. For these threesub-cases, we can calculate the number of time slots with inter-system interference, si , during which both WMAN and WLANsenders transmit packets.Also, we can obtain the total number oftime slots occupied by the WMAN and WLAN senders withoutinterference, which are defined as s1,n and s2,n, respectively.We assume that the BERs of the WMAN during the si and thes1,n intervals do not significantly change from ber1,i and ber1,n,respectively. Similarly, we denote ber2,i and ber2,n as the BERs

WMAN

nbo,1+1

nbo,1+1

nbo,1+1

nf,2

WLAN

time slotnbo,2+1

WMAN

time slot

time slot

time slot

CASE 2.2(partial overlapping)

CASE 2.1(full overlapping)

CASE 2.3(non−overlapping)

WMAN

nf,1

nf,1

nf,1

FIGURE 4. Three sub-cases of inter-system interference whennbo,1 ≥ nbo,2.

of WLAN during si and s2,n intervals. The effective packet errorrates (PERs) of WMAN and WLAN can be represented as

per(2)1 = 1 − (1 − ber1,n)

s1,ntsR1(1 − ber1,i )si tsR1 ,

per(2)2 = 1 − (1 − ber2,n)

s2,ntsR2(1 − ber2,i )si tsR2 .

(12)

Then, TH(2)1,i and TH(2)

2,i are represented as

TH(2)1,i =

CWm−1∑k2=0

CW1−1∑k1=k2

p(2)a (k1, k2)(1 − per(2)

1 )L

nf,1ts,

TH(2)2,i =

CWm−1∑k2=0

CW1−1∑k1=k2

p(2)a (k1, k2)(1 − per(2)

2 )L

nf,maxts,

(13)

where nf,max = max(nf,1, nf,2). Finally, the per-systemthroughput of WMAN and WLAN can be written as

TH1 = TH(1)1 + TH(2)

1 ,

TH2 = TH(2)2 ,

(14)

which can be obtained from (8), (9) and (13).

3.3. Effect of contention window and transmission rate

Using the derived throughput model, we investigate the effectof the contention window and the transmission rate on fairchannel sharing. We consider a simple geometry model, wherethe distance between the transmitter and receiver of the WMANis 100 m and that of the WLAN is 10 m and the distance betweenWMAN and WLAN transmitters is 300 m, as shown in Fig. 1.The other configurations are the same as those in Table 1.

The contention window and the transmission rate arecontrolled by the BEB mechanism and the link adaptationmechanism, which are reasons of unfair channel sharing asaddressed in Section 2.2. To evaluate the performance in termsof fairness and efficiency of channel sharing, we establishtwo performance indices, throughput ratio (γratio) and totalthroughput (ηeff ); the former is defined as the ratio of the averagethroughput achieved by a WMAN node to that achieved by aWLAN node and the latter is defined as the total throughputachieved by all the nodes, i.e.

γratio = TH1/N1

TH2/N2,

ηeff = TH1 + TH2.

(15)

Figure 5 represents γratio and ηeff for several pairs of R1 andR2 that were derived from the analysis in Section 3.2, alongwith those obtained from the simulation. Here, we set CW2 to128 and change CW1 from 16 to 1024, in order to observe theeffect of the contention window size. In general, CW2 becomesrelatively higher than CW1 because the WLAN suffers moretransmission failures than the WMAN, which triggers the BEB


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anal: (R1,R2)=(24,24)Mb/s

anal: (R1,R2)=(24,6)Mb/s

anal: (R1,R2)=(6,24)Mb/s

sim: (R1,R2)=(24,24)Mb/ssim: (R1,R2)=(24,6)Mb/s

sim: (R1,R2)=(6,24)Mb/s

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FIGURE 5. Effect of the contention window on the fairness and efficiency of channel sharing. (a) Throughput ratio (γratio) and (b) total throughput(ηeff ).

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anal: (CW1,CW2)=(32,32)anal: (CW1,CW2)=(128,32)anal: (CW1,CW2)=(32,128)

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anal: (CW1,CW2)=(32,32)anal: (CW1,CW2)=(128,32)anal: (CW1,CW2)=(32,128)

sim: (CW1,CW2)=(32,32)sim: (CW1,CW2)=(128,32)sim: (CW1,CW2)=(32,128)

FIGURE 6. Effect of the transmission rate on the fairness and efficiency of channel sharing. (a) Throughput ratio (γratio) and (b) total throughput(ηeff ).

mechanism and doubles the contention window size. Figure 5ashows thatγratio decreases toward 1 as CW1 increases, regardlessof the values of R1 and R2. The increase of CW1 contributesto improving fairness since the WLAN gets more channelaccess opportunities as CW1 increases. However, fairness issignificantly degraded when R2 < R1 and CW1 is small,e.g. γratio is about 30 when (R1, R2) = (24, 6) Mb/s, and(CW1, CW2) = (16, 128). From Fig. 5b, we observe the effectof the contention window size on ηeff . As CW1 increases up to acertain value, ηeff increases accordingly, which results from thedecreased probability of transmission failure due to collisionor interference. However, if CW1 exceeds a certain value (e.g.180 when R1 = R2 = 24 Mb/s), ηeff decreases because of theincreased average backoff time. This result confirms the trade-off between collision probability and overhead time related tothe contention window size. Also, we observe that ηeff in thecase of (R1, R2) = (6, 24) Mb/s is much lower than that in thecase of (R1, R2) = (24, 6) Mb/s. This means that the WLANmakes a minor contribution to ηeff .

The effect of the transmission rate on γratio and ηeff canbe observed in Fig. 6, where R1 is set to 24 Mb/s and R2

changes from 6 to 54 Mb/s. As shown in Fig. 6a, γratio decreases(i.e. fairness is improved) as R2 increases. Although the robustmodulation and coding scheme can reduce the BER at the cost ofa decreased transmission rate, the decrease in the transmissionrate rather increases the packet transmission time and causesmore interference to the WLAN, as explained in Section 2.2.Therefore, as R2 decreases, the throughput of the WLANdecreases and γratio increases accordingly. This result impliesthat the link adaptation mechanism is not effective to deal withthe inter-system interference; it may rather worsen the fairness.Also, Fig. 6a shows that γratio is almost immune to the valueof R2 as long as R2 > R1 = 24 Mb/s. On the other hand, weobserve from Fig. 6b that ηeff increases as R2 increases up toR1 (= 24 Mb/s). The increase of R2 decreases the probability ofthe interference-driven transmission failure, as well as decreasesthe transmission time, and so it contributes to the increase of ηeff .However, this positive effect is balanced with the negative effect


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(i.e. increased BER); ηeff no longer increases once R2 > R1,as is shown in Fig. 6b. By comparing three cases with differentvalues of CW1 and CW2 in Fig. 6, we observe that the case of(CW1, CW2) = (128, 32) outperforms the other cases in termsof fairness and efficiency; γratio is closer to 1 and ηeff has a highervalue.

Moreover, Figs 5 and 6 show that there is no significantdifference between the analysis results and the simulationresults, which confirms the validity of the analysis. Finally, wecan make the following conclusions from the results in Figs 5and 6, which provide a clue to the solution for achieving fairand efficient channel sharing.

(i) Increasing the contention window size of the interferer(WMAN), CW1, improves fairness since it providesmore interference-free channel access opportunities tothe victim (WLAN). Moreover, the total throughputincreases as CW1 increases up to a certain value.

(ii) Reducing the transmission rate of the victim, R2,not only worsens fairness but also reduces thetotal throughput. Maintaining R2 comparable to R1

contributes to improving both fairness and efficiency.

4. FAIR COEXISTENCE CSMA PROTOCOL

Considering the causes of unfair channel sharing discussed inSection 2 and the analysis results in Section 3, this sectionproposes the fair coexistence CSMA protocol consisting oftwo schemes: access etiquette and interference-aware backoff.The key idea of the proposed mechanism is 2-fold. First, theaccess etiquette scheme is designed to alleviate the unfairnessof channel sharing and to mitigate interference by decreasing thechannel access attempt of the interferer (WMAN) and allowingmore channel access opportunities to the victim (WLAN)accordingly. This idea can be realized by the dynamic controlof WMAN’s contention window size. Next, the interference-aware backoff scheme is intended to remove the blindness ofthe BEB mechanism, which plays as one cause of unfair channelsharing as discussed in Section 2.2. This scheme recognizes thereason of transmission failure, and disables the BEB mechanismof the WLAN, i.e. the channel access attempt of the WLAN isnot necessarily decreased, in response to the interference-driventransmission failure. The details are described in the followingsubsections.

4.1. Access etiquette

The objective of the access etiquette is to assure fair channelsharing between the WMAN and the WLAN while attaininghigh efficiency of channel usage. The access etiquette schemeis applied to WMAN senders and controls their contentionwindow size in order for the WLAN to occupy the channel in afair manner without interference or with minimal interference.The analysis results in Section 3.3 (Fig. 5) indicate that we

can control the trade-off between fairness and efficiency byadjusting the size of the WMAN’s contention window, CW1,i.e. the large value of CW1 improves fairness by giving morechannel access opportunities to WLAN, but it reduces efficiencydue to the increased backoff time, and that there exists anoptimal value of CW1 that strikes a balance between fairnessand efficiency.

Under this rationale, we establish an objective function forcontrolling CW1 as a linear combination of ηeff/Cmax andτfair, i.e.

F(CW1) = wηeff

Cmax+ (1 − w)μfair, (16)

where w is a weight factor (0 ≤ w ≤ 1), Cmax is theideal maximum capacity and μfair is Jain’s fairness index [21]considering the average per-node throughput of the WMAN andthe WLAN,

μfair = (TH1/N1 + TH2/N2)2

2((TH1/N1)2 + (TH2/N2)2). (17)

The value of Cmax can be calculated under the assumption thatthere is neither collision nor interference so that the contentionwindow has the minimum value of CWmin and the transmissionrate has the maximum value of Rmax, i.e.

Cmax = L

toh + ((CWmin − 1)/2)ts + L/Rmax. (18)

The first term in the right-hand side of (16), ηeff/Cmax,represents the normalized efficiency of channel sharing and isless than 1. The second term μfair in (16) is used to considerthe fairness of channel sharing;4 it has the largest value of1 when both the WMAN and the WLAN have the same per-node throughput, i.e. TH1/N1 = TH2/N2 (best case), and theminimum value of 1/2 when either the WMAN or the WLANcannot occupy the channel at all, i.e. TH1 or TH2 is zero(worst case). The problem can be formulated as an optimizationproblem, i.e.

maximizeCW1 F(CW1)

subject to CW1 ∈ �,(19)

where � = {CW1|CWmin ≤ CW1 ≤ CWmax}, CWmin andCWmax are the minimum and maximum values of contentionwindow, respectively, and the optimal value of CW1 is

CW∗1 = argmaxCW1∈�F(CW1). (20)

Figure 7 shows an example of F(CW1) with w = 0.5, which isobtained from the analysis in Section 3.3 with CW2 = 128,

4In Section 3.3, we have already introduced γratio as an intuitive measureof fairness, however, we make use of another fairness index in the objectivefunction of (16), whose range is comparable to that of ηeff/Cmax and whosevalue increases as the fairness improves.


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0.5

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0.7

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16 32 64 128 256 512 1024contention window of WMAN (CW1) (CW2=32)

�heff/cmaxmfair

objective function

FIGURE 7. Objective function (weighted sum ofηeff/Cmax andμfair)versus CW1.

R1 = R2 = 24 Mb/s and N1 = N2 = 10. From Fig. 7,we observe that the objective function F(CW1) is a unimodaland concave function with respect to CW1 so that there existsa unique optimal value of CW1 (e.g. 270 in this case) thatmaximizes F .5

Next, we propose a control rule of CW1 to maximizethe objective function. For every monitoring interval of Tae,the AP of WLAN measures its aggregate throughput TH2

and keeps track of the number of associated nodes N2. Weassume that a signaling path is established between coexistingBS and AP for the license-exempt coexistence according tothe legal requirements. The AP sends a control message forresource management, which contains TH2 and N2, to theBS of the WMAN. Similarly, the BS measures the aggregatethroughput and manages the number of WMAN nodes. Usingthis information, BS estimates the objective function F(CW1)

from (16) to (18) for every Tae. On the basis of the measurementof F(CW1), the value of CW1 is updated toward CW∗

1 usingthe golden section search algorithm [22], which is a simplealgorithm for finding the local minimum (or maximum) of aone-dimensional unimodal function in a closed interval withoutrequiring the derivative of function. The pseudo code for theupdate rule of CW1 is given in Fig. 8. Initially, the value ofF is evaluated at an intermediate point of cw_int (= CWmin +ρ(CWmax −CWmin)), and the next probe point is set as cw_opt.At every interval of Tae, the value of F is evaluated at thepoint of cw_opt, and cw_opt (= CW1) is updated withinthe narrower search space of [min (cw_bound1, cw_bound2),max (cw_bound1, cw_bound2)] to be converged to CW∗

1. Thevalue of cw_opt is broadcasted via a control message so that allthe WMAN senders set their contention window size as cw_optuntil the next update interval of Tae.

5We assume that the objective function is unimodal and concave from theintuition of the trade-off between fairness and efficiency as well as from thenumerical analysis results in Figs 5 and 7, even though it is hard to prove itmathematically.

Remark 1. The proposed access etiquette scheme is flexiblein that (i) the trade-off between fairness and efficiency canbe controlled by assigning the value of w(0 ≤ w ≤ 1) inthe objective function of (16), i.e. emphasis can be put on theefficiency or on fairness, as w is close to 1 or 0, respectively,and (ii) the weighted fairness can also be supported, i.e. γratio

can be attained close to an arbitrary value of K by replacingTH2 with KTH2 in (17).

Remark 2. The only requirement for finding the (local)optimal value of CW1 with the golden section search algorithmis that F is unimodal in the given interval. If this requirementis met, the value of CW1 converges to its optimal value CW∗

1 atthe rate of (1 − ρ), i.e. the search space is reduced by (1 − ρ) atevery stage [22]. Therefore, the error between CW1 and CW∗

1becomes less than ε, i.e. |CW1 − CW∗

1| < ε, as long as thenumber of iterations exceeds Nc,

Nc = log(ε/(CWmax − CWmin))

log(1 − ρ). (21)

For example, when CWmax = 1024, CWmin = 16 and ε = 1,Nc becomes 14.37.

Remark 3. The update rule of CW1 in Fig. 8 requires thatthe objective function should be evaluated at every iteration,which can be obtained based on the measurement of per-nodethroughput from (16) to (18). Therefore, the proposed methoddoes not resort to a specific system model and it is free fromthe system modeling error. However, we need to cope with thecase where the measurement error is not negligible so that thevalue of CW1 may be converged to a non-optimal value. Forthis purpose, we consider three points. First, the update periodTae should be long enough so that the change in CW1 canbe reflected in the objective function.6 Next, the per-systemthroughputs TH1 and TH2 are averaged in the form of anexponentially weighted moving average when evaluating F , sothat the temporary abnormal change or the measurement errorcan be relieved. Lastly, in order for the value of CW1 to escapefrom the non-optimal value, the procedure of updating CW1

needs to be periodically re-initialized or re-initialized upon thecrucial change of system configuration.

4.2. Interference-aware backoff

The interference-aware backoff mechanism is applied tothe WLAN senders, and it differentiates the response totransmission failure due to the inter-system interference fromthe response to that due to the intra-system collision. It freezesthe contention window size of the WLAN, CW2, i.e. not

6As Tae increases, the signaling overhead decreases and the stability can beimproved at the cost of convergence time. The purpose of the access etiquettescheme is to provide long-term fairness, instead of short-term fairness, whileattaining high efficiency. Therefore, we consider that the proper value of Tae isthe time required for transmitting several hundreds of packets.


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FIGURE 8. Pseudo code of access etiquette scheme based on the golden section search algorithm.

increasing CW2 unnecessarily, in the case of the interference-driven transmission failure. For this purpose, it is essential todetermine the cause of transmission failure. Note that there aretwo independent error detection codes in the WLAN [15]; onein the PHY header [e.g. 16-bit cyclic redundancy check (CRC)code in the physical layer convergence procedure (PLCP)header] and the other one in the MAC protocol data unit(MPDU) (e.g. 32-bit CRC in frame control sequence field). If thereceiver of the WLAN successfully receives the PLCP headerbut fails to decode the MPDU correctly, it determines that thefailure is due to interference. Otherwise, if neither the PLCP

header nor MPDU is received successfully, then the failure isconsidered to be due to collision. The underlying rationale isthat the WMAN sender usually starts to cause interference inthe middle of the WLAN’s transmission rather than at the startof the WLAN’s transmission (Fig. 2), so the WLAN receiverpossibly decodes the PHY header, in spite of interference bythe WMAN. We consider that the transmission failure due tothe temporary degradation of channel quality is insignificant ifa proper link adaptation mechanism along with the proposedmechanism is employed. To inform the WLAN senders of aninterference-driven transmission failure, we introduce a binary


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control flag, called the severe interference notification (SIN)flag, which needs to be newly added in the acknowledgement(ACK) frame. If a WLAN node fails to receive packets due tointerference, then it transmits a negative ACK frame where theSIN flag is set. The transmission of such a negative ACK frameis deferred until the channel becomes idle to avoid transmissionfailure due to interference by the WMAN. As the conventionalCSMA mechanism, the WLAN sender doubles the contentionwindow if it does not receive any positive ACK frame untilretransmission time-out is reached. However, if the WLANsender receives a negative ACK frame with the SIN flag, thenit reduces CW2 by half. With the aid of explicit interferencenotification, the WLAN sender disables the BEB mechanism inthe case of interference-driven failure, but it is only activated inthe case of collision-driven failure.

5. SIMULATION

In this section, we conduct extensive simulations to validatethe performance of the proposed mechanism. First, we focuson the stability and controllability of the proposed mechanism;we observe how the access etiquette algorithm convergesthe contention window size and how it controls the trade-off between fairness and efficiency in channel sharing. Next,we compare the performance of the proposed mechanismwith those of several approaches in relation to the variationsin certain system configurations, such as the degree ofoverlapping deployment between the WMAN and the WLAN,the asymmetry of transmission power/coverage, and the numberof nodes. We consider the following four approaches for fairchannel sharing and interference mitigation, as well as theproposed mechanism:

(i) BASE: This does not employ any coexistence mecha-nism, and it is considered as a baseline mechanism tocompare the performance of other approaches.

(ii) CS1-: Compared with BASE, this approach reducesthe physical CSTH of coexisting WMAN (WMAN_0)to increase the probability of detecting the WLAN’spacket transmission: Pcs,1 = −97.65 dBm. Note that thevalue of the CSTH of the first-tier interfering WMANs(WMAN_i), Pcs,i , is not changed from −90 dBm.With this configuration, the carrier sensing range ofWMAN_0 for detecting the inter-system interference,Ics,1, increases up to 300 m, which is the default valueof Dint,sys, while that for detecting the intra-systemcollision, Dcs,1, also increases to 2250 m.

(iii) TR2-: In this approach, the WLAN uses the mostrobust modulation and coding scheme to overcomeinterference from WMANs, i.e. R2 is fixed to 6 Mb/s.

(iv) CW1+: By increasing the minimum contention windowof WMAN_0, the channel access probability of theWLAN can be increased and interference to the WLANcan also be alleviated. This approach sets the minimum

contention windows of WMAN_0 to the maximum valueof 1024, i.e. CWmin = CWmax = 1024 for WMAN_0.

(v) AE-IB: This is the proposed mechanism deployingthe access etiquette algorithm to WMAN_0 and theinterference-aware backoff algorithm to WLAN. Thedesign parameters of the access etiquette algorithm areset as; Tae = 1 s, w = 0.5 and ε = 1.

We use the same simulation configuration in Section 2.3,unless otherwise stated. To evaluate the performance in termsof fairness and efficiency, two performance indices are used,respectively; γratio = (TH1/N1)/(TH2/N2) and ηeff = TH1 +TH2. We do not represent the throughput of the six first-tier interfering WMAN_is, because there was no notabledifference among the different approaches, except for CS1-.The simulation time is set to 10 million time slots, and thesimulation results are averaged over ten simulation runs, eachof which has the random placement of nodes.

5.1. Stability and controllability of the proposedmechanism

The objective of the first simulation is to validate the stabilityand controllability of the access etiquette scheme, whichcontrols CW1 to maximize the objective function accordingto the golden section search algorithm. Figure 9 shows howCW1 is updated for several cases that have different values ofweight in the objective function, w. Recall that we can controlthe trade-off between fairness and efficiency by assigning thevalue of w(0 ≤ w ≤ 1) in (16); as the value of w is close to 0,fairness is much attainable at the cost of efficiency; otherwise,the efficiency can be improved by setting w close to 1.

As shown in Fig. 9, CW1 converges to the value CW∗1 for

each case: CW∗1 ≈ 765, 251 and 187 when w = 0.1, 0.5 and 0.9,

respectively. This simulation result agrees with the intuition that

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TABLE 3. Several performance indices of the proposed mechanism with different values of weight in the objectivefunction.

Per-system throughput (Mb/s)

Weight WMAN WLAN Total throughput (Mb/s) Throughput ratio

0.1 (fairness-centric) 7.31 4.62 11.93 1.580.5 (balanced) 9.15 4.36 13.51 2.100.9 (efficiency-centric) 12.09 3.04 15.13 3.98Baseline 10.91 2.33 13.24 4.68

CW∗1 becomes higher as w becomes smaller (i.e. higher priority

to fairness). On the other hand, Fig. 9 shows that the convergencetime is independent of w, as already stated in Section 4.1. Table 3lists the performance indices of the proposed mechanism forthe three cases of w = 0.1, 0.5 and 0.9, along with those ofBASE. In the case when w = 0.1 the throughput ratio (γratio)is 1.58, which is smaller than the case when w = 0.9 by morethan 2.5 times, but the total throughput (ηeff ) is smaller than thecase when w = 0.9 by about 21%. Compared with BASE, theproposed mechanism can improve fairness (e.g. γratio decreasesby about three times when w = 0.1), or it can enhance efficiency(e.g. ηeff increases by about 15% when w = 0.9), dependingon the value of w. These results confirm that CW1 is controlledand converged to maximize the given objective function andthat the proposed mechanism can control the trade-off betweenthe fairness and efficiency of channel sharing by assigning aproper value to w.

5.2. Effect of the degree of overlapping deploymentbetween WMAN and WLAN

This simulation compares the performance of severalapproaches under different degree of overlapping deploymentbetween the WMAN and the WLAN; Dint,sys ranges from 0 to750 m (the cell radius of WMAN). Here, the other simulationparameters are unchanged from those in Table 1.

Figure 10 shows γratio and ηeff for various values of Dint-sys.Except for the case of fully overlapping deployment (Dint-sys =0), CW1+ shows the best performance in terms of fairness;γratio lies between 0.94 and 1.47 when Dint-sys ≥ 150 m;however, CW1+ improves fairness at the cost of efficiency;ηeff of CW1+ is smaller than that of BASE by up to 20%.In the cases of BASE and TR2-, as Dint-sys increases from0 to 750 m, γratio increases from 2.2 and 1.5 to 4.7 and 5.6,respectively. Except for the case of Dint-sys = 0, γratio of TR2-is higher than that of BASE, but ηeff of TR2- is lower thanthat of BASE, implying that reducing the transmission rate ofthe WLAN is not effective for fairness and efficiency. Theseresults conform to the analysis results in Fig. 6 of Section 3.3.Contrary to the other approaches, CS1− gives the preferenceof channel sharing to the WLAN, rather than to the WMAN;

γratio is not greater than 0.36. Moreover, ηeff of CS1- is lessaffected by Dint-sys compared with the other approaches, andit is quite larger than the others when Dint-sys = 0, but lowerthan the others when Dint-sys ≥ 150 m. These results can beexplained as follows: CS1- cannot fully employ spatial reuseamong WMANs, i.e. the sender of WMAN_0 unnecessarilydefers its channel access when the senders of WMAN_is occupythe channel. Therefore, the approach of reducing the CSTHnot only fails to assure fairness but also reduces efficiency.However, AE-IB succeeds in alleviating the unfair sharing ofchannel without impairing the efficiency; γratio is maintainedbetween 0.54 and 2.17 for the entire range of Dint-sys, whichis smaller than that of BASE by up to 2.7 times, and at thesame time, ηeff of AE-IB is higher than that of CW1+ byabout 25%. The outstanding performance of AE-IB stemsfrom (i) the adaptive control of contention window to enforcefairness and to improve efficiency (due to the access etiquettescheme) and (ii) the prevention of unnecessary increase ofcontention window in response to the interference-driventransmission failure (due to the interference-aware backoffscheme).

5.3. Effect of power/coverage asymmetry betweenWMAN and WLAN

We investigate the effect of the asymmetry of transmissionpower or coverage between the WMAN and the WLAN oncoexistence performance. Recall that the coverage asymmetryfactor Ka was defined in Section 3.1 as Ka = Drx,2/Drx,1, whichis also related to the asymmetry of transmission power. We fixDrx,1 to 750 m and change Ptx,2 such that Drx,2 ranges from 75to 750 m, i.e. Ka changes from 0.1 to 1.0. Here, Dint,sys is set to300 m.

Figure 11 shows the throughput ratio and total throughputfor various values of Ka. As was expected, the problem ofunfair sharing becomes alleviated as the degree of asymmetrydecreases, i.e. in the cases of BASE and TR2-, γratio

monotonically decreases to 1 as Ka increases to 1; however,ηeff decreases until Ka increases to 0.6 and slightly changesonce Ka > 0.6. The decrease of ηeff results from theincreased interference power of the WLAN, which decreases


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FIGURE 11. Performance comparison of several approaches with different values of the WLAN’s coverage. (a) Throughput ratio (γratio) and(b) total throughput (ηeff ).

the WMAN transmission rate or increases its transmissionfailure accordingly. Among the five approaches, TR2- suffersremarkable decreases of ηeff , e.g., it is lower than those ofBASE and CS1- by about 40% and 100%, respectively, whenKa = 1.0. As Ka increases, the channel occupation by theWLAN can be detected by the WMAN, and the WLAN cantransmit packets without interference from the WMAN. Theseresults reconfirm that, compared with BASE, the approachof TR2- does not achieve any gain, but decreases thethroughput. On the other hand, in the cases of CS1- andCW1+, as Ka increases, γratio significantly decreases from0.24 and 1.40 to 0.014 and 0.05, respectively. Unlike theother approaches, ηeff of CS1- is almost immune to Ka,because the throughput of the WLAN is mostly unrelated toits transmission power or coverage, and its contribution to ηeff

is dominant. The proposed AE-IBmechanism outperforms theother mechanisms; compared with BASE, γratio decreases from

5.84 to 2.20 when Ka = 0.1, and ηeff increases by up to 36%when Ka = 1.0.

5.4. Effect of the number of nodes

The number of nodes is one of the most important factorsaffecting performance, since it is related to the degree of intra-system collision and inter-system interference. We observe itseffect on γratio and ηeff from Fig. 12. Here, we change the numberof WMAN nodes (N1) from 2 to 20, while that of WLAN nodes(N2) is fixed at 10, and Dint-sys = 300 m, Drx,1 = 750 m andDrx,2 = 100 m. Note that the number of nodes is considered inevaluating γratio as (15).

As shown in Fig. 12a, expect for CS1-, fairness is quiteimpaired when N1 is small, e.g. in the cases of BASE and TR2-,γratio is about 19 and 24 when N1 = 2, respectively, but it isimproved as N1 increases. However, fairness achieved by the


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FIGURE 12. Performance comparison of several approaches with different values of the number of WMAN nodes. (a) Throughput ratio (γratio)and (b) total throughput (ηeff ).

CS1- mechanism is deteriorated as N1 increases because thechannel access is biased against WMAN in CS1-, as alreadyobserved in Figs. 10 and 11. As N1 increases from 2 to 20, γratio

of CW1+ and AE-IB decreases from 2.26 and 3.38 to 1.12 and1.06, respectively.

Fig. 12b shows ηeff when N1 has different values. In theBASE and TR2- mechanisms, ηeff decreases as N1 increases,which results from the increase of intra-system collision of theWMAN. Similarly, ηeff of AE-IB decreases as N1 increases.When N1 < 10, AE-IB achieves a lower total throughput thanBASE to enforce fairness; however, when N1 ≥ 10, its totalthroughput becomes higher than those of BASE and TR2-. Thetotal throughput of CS1- is insensitive to the change of N1,as already observed and explained in the previous simulations.As N1 increases, CW1+ improves both fairness and efficiency,from which we can infer that the size of CW1 used in CW1+is close to the optimal value. It is important to note thatthe value of CW∗

1 depends on many design parameters andnetwork configurations, so it is quite difficult to determineeither by analysis or by experiment. However, the proposedAE-IB mechanism updates the value of CW1 to maximize thegiven objective function represented in terms of fairness andefficiency, only by measuring the objective function withoutrequiring the exact information about network configurations.Therefore, the proposed mechanism is effective for a wide rangeof network configurations, as confirmed in Figs 10–12.

6. CONCLUSION

In this study, we have addressed the issues of coexistence arisingwhen heterogeneous networks with different transmissionpower employ the contention-based MAC protocol for non-exclusive coexistence. Since the high-power system failsto detect the ongoing packet transmission by the lower-power system, the high-power system attempts to accessthe channel even when the lower-power system has already

occupied the shared channel, creating interference that causesthe low-power system to suffer from frequent transmissionfailures. Therefore, there occurs a severe unfairness of channelsharing between the two systems. We have shown that, inaddition to the asymmetry of carrier sensing ability, the BEBmechanism and the link adaptation mechanism exacerbate thefair channel sharing. As a solution to mitigate interferenceand achieve fair channel sharing, we have proposed thefair coexistence CSMA mechanism, which consists of twoschemes, the access etiquette and interference-aware backoffschemes. The proposed mechanism controls the contentionwindow size to maximize the given objective function, whichis represented in terms of fairness and efficiency, based on thefeedback information about per-system throughput. Moreover,it differentiates the response to collision-driven transmissionfailure from that to interference-driven failure. In this way,the proposed mechanism assures the fairness of channelsharing without impairing efficiency, which is confirmed inthis paper by extensive simulations performed under variousnetwork configurations.

FUNDING

This work was supported in part by Electronics andTelecommunications Research Institute, Korea; and BasicScience Research Program through the National ResearchFoundation of Korea (KRF), funded by the Ministry ofEducation, Science, and Technology [Grant No. 2010-0008711].

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