coexistence mechanisms for interference mitigation in the 2.4-ghz

964 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 2, NO. 5, SEPTEMBER 2003

Coexistence Mechanisms for Interference Mitigationin the 2.4-GHz ISM Band

Carla-Fabiana Chiasserini, Member, IEEE,and Ramesh R. Rao, Senior Member, IEEE

Abstract—Wireless technologies sharing the same frequencyband and operating in the same environment often interferewith each other, causing severe decrease in performance. In thispaper, we propose two coexistence mechanisms based on trafficscheduling techniques that mitigate interference between differentwireless systems operating in the 2.4-GHz industrial, medical, andscientific band. In particular, we consider IEEE 802.11 wirelesslocal area networks (WLANs) and Bluetooth (BT) voice and datanodes, showing that the proposed algorithms can work when thetwo systems are able to exchange information as well as whenthey operate independently of one another. Results indicate thatthe proposed algorithms remarkably mitigate the interferencebetween the IEEE 802.11 and BT technologies at the expense of asmall additional delay in the data transfer. It is also shown that theimpact of the interference generated by microwave ovens on theIEEE 802.11 WLAN’s performance can be significantly reducedthrough the mechanisms presented.

Index Terms—Coexistence mechanisms, quality of service, wire-less local area networks (WLANs), wireless personal area networks(WPANs).

I. INTRODUCTION

T HE NEXT few years will likely bring pervasive deploy-ment of smart wireless devices. To make this vision a re-

ality, devices must be able to share the same frequency bandand move between different wireless systems without the needof any licensing procedure [1]. Although the use of unlicensedbands facilitates spectrum sharing and allows for an open accessto the wireless medium, it also raises serious challenges, such asmutual interference between different radio systems and spec-trum utilization inefficiency.

In this paper, we deal with the problem of mutual interfer-ence between two emerging wireless technologies: wirelesslocal area networks (WLANs) and wireless personal areanetworks (WPANs). In particular, we consider IEEE 802.11WLANs [2], [3] and short-range radio systems based on theBluetooth (BT) specification [4], [5] or, equivalently, IEEE802.15 WPANs [6]. These systems will operate in the 2.4-GHzindustrial, scientific, and medical (ISM) frequency band,i.e., the unlicensed spectrum. BT uses a frequency-hopping

Manuscript received August 10, 2001; revised March 4, 2002 and June 1,2002; accepted June 3, 2002. The editor coordinating the review of this paperand approving it for publication is V. C. M. Leung. This work was supported bythe Centro di Eccellenza per le Radio Comunicazioni Multimediali (CERCOM),Torino, Italy.

C.-F. Chiasserini is with the Dipartimento di Elettronica, Politecnico diTorino, 10129 Torino, Italy (e-mail: [email protected]).

R. R. Rao is with the Electrical and Computer Engineering Department,University of California, San Diego, CA 92093-0407 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/TWC.2003.817417

spread spectrum (FHSS) scheme while IEEE 802.11 can eitheruse an FHSS or a direct-sequence spread spectrum (DSSS)technique. WLANs and WPANs are complementary ratherthan competing technologies, and many application modelshave been envisioned for situations requiring BT and 802.11 tooperate simultaneously and in close proximity [7]. Under theseconditions, interference between 802.11 and BT occurs when-ever the interference energy is sufficient to cause a decreaseof the signal-to-interference ratio at the receiver and the twosystem transmissions overlap both in frequency and in time.

According to the IEEE 802.15 working group, interferencebetween 802.11 and BT causes a severe degradation of the sys-tems’ throughput when the distance between interfering devicesis less than 2 m. A slightly less significant degradation is ob-served when the distance ranges between 2 and 4 m [8]. Inorder to mitigate this effect, the IEEE 802.15 working grouphas created the Task Group 2 (TG2), which is devoted to the de-velopment ofcoexistence mechanisms[6], i.e., techniques thatallow 802.11 and BT to operate in a shared environment withoutsignificantly impacting the performance of each other [9]. Twoclasses of coexistence mechanisms have been defined: collab-orative and noncollaborative techniques [6]. With collaborativetechniques, it is possible for the BT network and the WLAN toexchange information and reduce the mutual interference; how-ever, they can be implemented only when the BT and the 802.11devices are collocated in the same terminal. With noncollabo-rative techniques, there is no way to exchange information be-tween the two systems, and they operate independently.

In this paper, we propose two novel coexistence mechanisms,called overlap avoidance (OLA) schemes, which are based onsimple traffic scheduling techniques. The first mechanism is tobe performed at the IEEE 802.11 in the presence of a BT voicelink, the second mechanism at the BT system in the case of aBT data link. The proposed algorithms have the following ad-vantages: 1) they do not need a centralized traffic scheduler;2) they can be implemented in collaborative or noncollabora-tive mode; 3) they are able to mitigate interference between col-located and noncollocated BT and IEEE 802.11 devices; and4) they have a minor impact on the IEEE 802.11 standard andon the BT specification. Both schemes are based on the assump-tion that 802.11 and BT can detect interference due to other tech-nologies sharing the same environment. This assumption is triv-ially true in a collaborative setting, where information related totraffic transmissions can be directly exchanged between the in-terfering systems. In a noncollaborative setting, this informationcan be acquired through channel sensing and assessment of thereceived signal strength and packet loss rate. This is further dis-cussed in Section IV.

1536-1276/03$17.00 © 2003 IEEE

CHIASSERINI AND RAO: COEXISTENCE MECHANISMS FOR INTERFERENCE MITIGATION 965

By applying the OLA mechanisms, a significant reduction inthe interference between IEEE 802.11 and BT can be achieved.Similar reductions are expected when other interference sourcesare considered. Using the OLA scheme at the 802.11 stations re-duces the impact on the WLAN performance of any interferingtransmission with a periodic pattern. Likewise, the OLA schemeperformed by the BT devices is able to counteract the interfer-ence generated by any wireless system using a limited frequencyspan in the ISM band. To illustrate this, we show how the pro-posed mechanisms improve performance of an 802.11 WLANsuffering the interference caused by microwave ovens.

The remainder of the paper is organized as follows. In Sec-tion II, we review previous work on coexistence mechanisms be-tween IEEE 802.11 and BT. Section III briefly describes IEEE802.11 and BT technology, and introduces the model adopted toevaluate the mutual interference between the two systems. Sec-tion IV presents the proposed coexistence techniques. Section Vdescribes the considered simulation scenario. Section VI showsthe improvement obtained in the 802.11 and BT performance.The ability of the proposed schemes to cope with the interfer-ence generated by microwave ovens is studied in Section VII.Finally, Section VIII concludes the paper.

II. RELATED WORK

Several coexistence mechanisms have been proposed withinthe IEEE 802.15 TG2. As mentioned above, they can be clas-sified as collaborative or noncollaborative mechanisms [10].There are two important examples of collaborative coexistencemechanisms: the so-called medium access control (MAC)enhanced temporal algorithm (META) [11] and alternatingwireless medium access (AWMA) [12], [13] schemes.

In the META technique, an 802.11 station and a BT node arecollocated in the same physical unit. META involves the useof a centralized controller that monitors the BT and the 802.11traffic, allowing the exchange of information between the twocollocated radio systems. The controller works at the MAClayer; it provides per-packet authorization of all transmissionsand uses its knowledge of the 802.11 and BT activity to predictcollisions. When a collision might occur, META schedulestransmissions based on simple rules determined by the packettypes. In particular, 802.11 acknowledgment packets havehigher priority than any BT packets, while BT synchronousconnection oriented (SCO) traffic has higher priority than any802.11 data packets.

The AWMA mechanism operates at the MAC layer and isbased on the time-division multiple access scheme. It assumesthat the 802.11 access point and the BT master are collocatedin the same physical unit [12], [13], and that the 802.11 and BTdevices transmit alternately to avoid overlap in time betweentheir transmissions. The 802.11 access point sends out a beaconat a periodic interval and each beacon-to-beacon interval is di-vided into two subintervals: one for 802.11 traffic and one forBT traffic. AWMA cannot be applied in the case of BT SCOlinks [13].

Note that both the META and the AWMA schemes areunable to mitigate interference coming from noncollocated de-vices. Also, since they completely orthogonalize transmissions

of technologies sharing the same radio spectrum, the systems’throughput will be significantly decreased as the number ofwireless technologies operating in the unlicensed bands grows.The two MAC layer techniques can be integrated with a col-laborative physical layer solution, the so-called deterministicfrequency nulling scheme [14]. The key idea is that becausethe BT signal can be considered a narrowband interferer for the802.11 DSSS signal, we can put a null in the 802.11 receiverat the frequency of the BT signal. Since the BT signal hopsin frequency, the 802.11 receiver must know the frequencyhopping pattern, as well as the timing, of the BT transmitter.This again implies that the 802.11 and the BT devices must becollocated in the same physical unit.

Noncollaborative coexistence mechanisms include the adap-tive frequency hopping (AFH) technique, the adaptive packetselection and scheduling, and the transmit power control/ratescaling scheme.

According to the AFH scheme [15]–[17], BT frequency chan-nels are classified as “good” or “bad” and are used intelligentlyto reduce the probability of overlap in frequency with the 802.11signal. Due to FCC restrictions (see Title 47, Part 15.247 (a)[27]), two methods have been defined. In the first method, theso-called Mode , bad channels are removed from the hop-ping sequence; in the second one, the so-called Mode, somegrouping of the bad and good channels is performed so that thehopping sequence can intelligently schedule the use of the badchannels and maximize the use of the good channels.

Adaptive packet selection and scheduling [18] can be effec-tively used to mitigate interference between 802.11 and BT. Byselecting the best BT packet type according to the condition ofthe upcoming frequency hop, BT throughput is improved. Also,BT transmissions can be scheduled in such a way that hops in the802.11 band are avoided, thus, reducing interference betweenthe two radio systems.

In [19], the transmit power control/rate scaling scheme ispresented. This technique is based on the idea that 802.11 andBT devices should reduce their transmission power as much aspossible to obtain satisfactory performance. All 802.11 devicescurrently implement a rate shifting/control algorithm based onthe perceived level of signal quality. The scheme presented in[19] extends this control algorithm to incorporate the highestmandatory rate at lower transmit power, i.e., when possible,the 802.11 devices would shift to the highest rate using lowertransmit power.

III. SYSTEM BACKGROUND

IEEE 802.11 WLANs cover a range of approximately 100 mand can operate at bit rates as high as 11 Mb/s. We focus on sys-tems that use the DSSS scheme and consider their bandwidth tobe roughly equal to 22 MHz [2], [20]. The fundamental buildingblock of the network is the so-called basic service set (BSS),which is composed of several wireless stations using the samespreading sequence and MAC function. Wireless stations candirectly communicate with each other forming anad hocnet-work or through a centralized access point that also providesa connection to the wired network [2]. The two fundamental


Fig. 1. FH/TDD channel in BT.

MAC schemes defined in the IEEE 802.11 standard are the dis-tributed coordination function (DCF) and the point coordinationfunction. The former is based on the carrier-sense multiple ac-cess with collision avoidance protocol and allows for an asyn-chronous data transport; the latter is based on polling controlledby the access point and is able to support real-time traffic [3]. Inthis paper, only the DCF scheme is considered.

BT provides interconnection of devices in the user’s vicinity;its typical use is in a range of roughly 10 m. The basic architec-tural unit in BT systems is the piconet, composed of amasterdevice and seven activeslavedevices at most, which are allowedto communicate with the master only [4], [5]. BT can support upto three SCO links for real-time services, such as voice traffic,and asynchronous connection-less (ACL) links for non real-timeapplications, such as data traffic. The maximum throughput thatcan be provided is equal to 721 Kb/s. An FHSS scheme is usedat the physical level with hop rate equal to 1600 hops/s; eachmaster chooses a different hopping sequence so that piconetscan operate in the same area without interfering with each other.Hopping frequencies range over 79 frequency channels in theISM band, each of the channels being 1 MHz wide. The nom-inal hop dwell time is equal to 625s. A time-division duplex(TDD) technique is used to transmit and receive data in a pi-conet. Each packet transmitted in a slot occupies 366s; slotsare centrally allocated by the master and alternately used formaster and slave transmissions. Master transmissions alwaysbegin at even slots (namely, in slots with ),slave transmissions at odd slots (namely, in slots with

). Fig. 1 shows the FH/TDD channel. The BT spec-ification also allows for multislot data transmissions, i.e., forpackets occupying more than one slot (namely, three or fiveslots). In this case, packets are sent by using a single frequencyhop, which is the hop corresponding to the slot at which thepacket started.

In order to define mechanisms for the coexistence of IEEE802.11 and BT devices operating in a common area, it isimperative to develop an appropriate model for their mutualinterference.

A. Interference Model

Interferencebetween IEEE802.11andBTariseswhenever theinterfering power from a BT (802.11) transmitter causes a signif-icant decrease in the carrier to interference power margin at the802.11 (BT) receiver [21]–[25]. By using the method presentedin [23]–[25], the number of interfering devices and the associatedcarrier to interference power margin can be derived from the fol-lowing system parameters: 1) distance between transmitters andreceivers; 2) average density of the transmitters in the consid-ered spatial area; 3) transmission power of the interfering sys-tems; and 4) signal attenuation factor due to propagation.

In this work, we assume that the set of BT devices having suf-ficient power to cause interference to 802.11 is given, as well asthe set of 802.11 stations that cause interference to BT. We alsoconsider thatwhenever transmissionsby theseBTand802.11de-vices overlap both in time and in frequency, the signals involvedin the collision are received in error with probability equal to one.The average number of symbols “hit” because of a collision be-tween 802.11 and BT can be, therefore, computed as follows.

We denote the BT time slot with , the actual BT transmis-sion time per slot with , and the 802.11 packet time durationwith . Let be the time period from the beginning of thefirst overlapping BT slot to the beginning of the 802.11 packet[ ranges in the time interval ]. The number of BT slotsthat overlap the 802.11 packet in time depends onand can bederived as [22]

if

else(1)

Fig. 2 shows an example with and packet lengthequal to one slot. Variables indicate theportion of the th BT slot that actually interferes with the 802.11packet. For the generic time slot , we havethat if no BT transmission occurs in interval; otherwiseas shown in (2) at the bottom of the page [22]. By fixing the

(2)


Fig. 2. Overlap between IEEE 802.11 and BT packets.

value of for , we define as the probabilitythat BT traffic is transmitted in slot

if the th slot is idleotherwise

(3)

By considering that the 802.11 stations use a DSSS schemeand neglecting out-of-band interference, the probability that BTand 802.11 overlap in frequency is equal to the probability thatBT hops on the 22-MHz WLAN DSSS band. From the proce-dure used to generate the BT hopping sequences [5], it followsthat the BT hopping on the WLAN band can be approximatedby an independent identically distributed process with param-eter . When no coexistence mechanism is applied, we canwrite [26]: , where 22 and 79 MHz arethe 802.11 and the BT bandwidth, respectively.

The average number of symbols “hit” because of a collisionbetween BT and 802.11 can, therefore, be written as

(4)

where we denote with the ratio ,with being the symbol time duration. From (4), it isclear that in order to mitigate the mutual interference be-tween BT and 802.11, we need to make either or

small. A small can be obtainedby using short WLAN packets, which, however, increases the802.11 transmission overhead. A small requires reducingthe probability that 802.11 and BT transmissions overlapinfrequency, while a small implies a low probability of overlapin timebetween the two systems transmission.

IV. OLA COEXISTENCEMECHANISMS

Based on the previous findings, we develop two coexistencealgorithms, named OLA mechanisms that use simple trafficscheduling techniques at the MAC layer.

The first algorithm, denoted by voice-OLA (V-OLA), is usedin the case of BT SCO links. This scheme avoids overlap intime between the BT SCO traffic and the 802.11 packets byperforming a proper scheduling of the traffic transmissions atthe WLAN stations. In a BT network, each SCO link occu-pies FH/TDD channel slots according to a deterministic pattern.

Thus, an 802.11 station shall start transmitting when the BTchannel is idle and adjust the length of the WLAN packet so thatit fits between two successive BT transmissions. The second al-gorithm, denoted by data-OLA (D-OLA), is suitable for BT datalinks. As described in Section III, the length of the BT packetscan be equal to one, three, or five time slots. In the case of multi-slot transmissions, packets are sent by using a single frequencyhop, which is the hop corresponding to the slot at which thepacket started. The key idea of the D-OLA algorithm, describedin more detail below, is to use the variety of packet lengths thatcharacterize the BT system to avoid overlap in frequency be-tween 802.11 and BT transmissions. Within each interfering pi-conet, the D-OLA algorithm induces the BT master device toschedule data packets with the proper duration (i.e., one, three,or five slots) in order to skip the frequency locations of the hop-ping sequence that are expected to drop on the 802.11 band. Thetwo proposed mechanisms are jointly applied when both SCOand ACL links are active over the BT channel.

The proposed schemes are based on the assumption that both802.11 and BT devices can detect interference due to other tech-nologies sharing the same environment and using the same fre-quency band. This assumption is trivially true in a collaborativesetting where BT and 802.11 can directly exchange informationrelated to their traffic transmissions. In a noncollaborative set-ting, this information can be acquired through channel sensingand assessment of the received signal strength and of the packetloss rate. This issue is further discussed below for each of theproposed schemes.

A. V-OLA Mechanism

In the case of BT SCO traffic, slots are allocated according toa deterministic pattern; for instance, for each SCO connectionusing a HV3-type link [5], a single-slot packet is transmitted pe-riodically in both directions every six time slots. Whenever a BTpacket hops in the 802.11 frequency band, an 802.11 station inreceive mode1 senses the BT transmission as colored noise, i.e.,as a signal with a specific behavior in time and in frequency.In a noncollaborative setting, an 802.11 station can detect thetime intervals that are occupied by interfering transmissions byusing the clear channel assessment (CCA) procedure. In partic-ular, the primitive PMDED.INDICATE allows an 802.11 device

1802.11 and BT are half-duplex systems (i.e., devices cannot simultaneouslytransmit and receive).


to detect any radio frequency signal, that is above a predefinedthreshold. More details on the primitives used by 802.11 sta-tions to detect interference on the radio channel can be found in[3, pp. 214–215].

If SCO and ACL links are simultaneously active on the BTchannel, the D-OLA scheme is also applied and, as explainedlater, the probability that an ACL packet hops on the 802.11band becomes negligible. This implies that an 802.11 stationis likely to detect interference due to the BT SCO traffic only.Due to the periodicity and the predefined time duration of theBT SCO packets, the 802.11 device can estimate the BT signalpattern. However, two problems still need to be solved. First,the BT signal hops in the 802.11 band with probability equalto . Thus, even if an 802.11 station can start applying theV-OLA mechanism as soon as the BT interference is sensed, itcan hardly detect the time instant when the interference due tothe BT SCO transmission ends. We assume that an 802.11 sta-tion considers the BT SCO transmission terminated if it does notdetect the interference signal for a time interval equal to 10 ms(which corresponds to 16 BT time slots). Second, once the BTinterference is detected, the timing of the BT packet transmis-sion may drift, thus, resulting in an inaccurate prediction of theBT signal by the 802.11 device. Although this is an interestingsubject for future research, we neglect the effect of the timingdrift of the BT transmissions on the performance of the V-OLAscheme.

Whenever an 802.11 station is ready to transmit, it acts ac-cording to the information acquired on the pattern of the BTSCO traffic. If the channel is idle and no interference is expectedfor a time period equal to the next BT slot duration, the802.11 station transmits a data packet with payload size equalto the minimum of and 1500 B. The minimum payloadhas been set to 500 B to make the corresponding 802.11 packettransmission time comparable to the duration of a single-slot BTpacket. Conversely, if the channel is occupied by an interferingsignal, the WLAN station can either: 1) send a packet with a500-B payload (shortened transmission (ST) mode) or 2) refrainfrom transmitting (postponed transmission (PT) mode).

With the ST mode, the 802.11 transmission does not neces-sarily overlap in time with the BT packets because a one-slotBT packet lasts only slightly longer than half the duration ofone time slot. Besides, even in the case of time overlap, 802.11and BT packets collide only if BT packets hop on the WLANfrequency band.

When a WLAN station refrains from transmitting, i.e., itacts in PT mode, the 802.11 transmission is postponed bycomputing a new backoff time. In this case, two oppositeeffects take place: on the one hand, a lower overlap probabilityis achieved than in the case where a short packet is transmitted;on the other hand, the WLAN stations’ access delay increasesand the WLAN channel utilization decreases with respect tothe case where the ST mode is applied.

B. D-OLA Mechanism

We consider a BT data link and assume that the BT masterdevices are aware of which frequency channels are occupiedby the interfering 802.11 stations. An 802.11 system does nottypically move from its 22-MHz frequency band. Therefore, in anoncollaborative setting, a BT device can identify the frequency

channels that are occupied by the WLAN by using any of thefollowing methods [17].

1) BT devices gradually determine which channels are oc-cupied based on the observed packet loss.

2) BT devices assess the received signal strength (RSSI)across the radio environment before they start operating.

3) BT devices transmit “test” packets across the frequencyspectrum, observe the packet loss rate over the channels,and discover the band used by an interfering system.

Notice that Methods 1 and 2 are already included in the BT spec-ification; while, POLL–NULL messages, which are exchangedbetween master and slave, can be used as “test” packets to im-plement Method 3 [17].

Let us focus on the FH/TDD channel of one BT piconet. Re-call that a master transmission always begins in even slots whileslaves can start transmitting in odd slots only. For the sake ofsimplicity, we assume that default data packets are one-slot long.Let us denote by the frequency location of the hopping se-quence at the generic time slot and let the current time slotbe equal to .

Consider first that following hops on the 802.11band. Notice that and shall correspond to a masterand a slave transmission, respectively. According to the D-OLAalgorithm, if enough data are buffered at the master for the in-tended slave, the master schedules a multislot packet instead of asingle-slot packet. In this way, frequency hop is skipped;for instance, if a three-slot packet is sent, the next slave trans-mission will use . If too little data is available, the masteracts by default and sends a single-slot packet.

Next, assume that among the frequency locations followinghops on the 802.11 band. Notice that frequency lo-

cation corresponds to a master transmission. In this case,at time slot , the master asks the slave, transmitting in the nextslot, to send a multislot packet so that is skipped. If theslave has enough data to send, for example, a three-slot packet,the slave transmission extends from slot to slotby using frequency only. The next slot allocated for themaster transmission will, therefore, hop on frequency location

. A similar mechanism is applied when default data trans-missions use three- or five-slot packets.

The scheduling algorithm could also prevents the master(slave) from transmitting in the time slot corresponding toa frequency that hops on the 802.11 band whenever there isnot enough data in the buffer at the master (slave) to send amultislot packet. In this case, the collision probability is furtherreduced but the BT throughput decreases as well.

C. Remarks

The OLA schemes do not require a centralized controller be-cause they do not perform precise time scheduling of the 802.11and BT packet traffic. They can either operate as collaborativeor noncollaborative coexistence mechanisms and, hence, areable to reduce interference both in the case of collocated andnoncollocated devices. If interfering systems other than BT and802.11are present, the beneficial effectsof the OLAmechanismsstill hold as long as BT and 802.11 can estimate the interferencepattern of the other systems with sufficient accuracy. Using theOLA scheme at the 802.11 stations can reduce the impact on


Fig. 3. Timing of a successful IEEE 802.11 packet transmission.

the WLAN performance of any interfering transmission with aperiodic pattern. Likewise, the OLA scheme performed by theBTdevices is able tocounteract the interference generatedbyanywireless system using a limited frequency span in the ISM band.On the other hand, when interfering devices are collocated in thesame physical unit, we expect that collaborative schemes, suchas META, give higher throughput than the OLA schemes do.

As already mentioned, we did not consider out-of-band inter-ference. Although adjacent channel interference may lead to anincrease of , it would not affect the OLA schemes. Any fre-quency channel, where interference is detected, can be simplyincluded in the set of frequencies that BT devices can avoid byusing the D-OLA mechanism.

The proposed algorithms have a minor impact on the 802.11standard and on the BT specification. According to the 802.11standard, a station shall defer its transmission if it detects abusy channel during the CCA procedure. There are three dif-ferent CCA modes [3]: 1) a busy channel is reported upon detec-tion of any energy above a certain threshold; 2) a busy channelis reported only upon detection of a DSSS signal, which canbe either above or below the energy threshold; and 3) a busychannel is reported upon a DSSS signal with energy above thethreshold. Thus, in the V-OLA mechanism, both the PT and theST modes are compliant with the 802.11 standard and can be im-plemented by using the appropriate CCA mode among the pos-sible three. The BT specification involves that BT devices dy-namically adapt their hopping sequence to the interference con-ditions by scheduling ACL packets with different length. Thus,the D-OLA scheme exploits a behavior of the BT devices al-ready existing in the specification, and we do not need to changethe procedure of hop selection that is performed in the BT hard-ware. In a piconet, however, the master can only indicate to theslaves the maximum number of slots to use; while, according tothe D-OLA mechanism, a slave should interpret the indicationfrom the master as the suggested packet length.

Finally, the D-OLA mechanism is compliant with the FCCregulation Title 47, Part 15.247 (h).2 [27] The FCC regulationPart 15.247 (a) states that frequency hopping systems in the2.4-GHz band shall use 75 hopping frequencies and that the av-erage time of occupancy of any frequency shall not be greaterthan 0.4 s within a 30-s period. BT is compliant with this regu-lation. The D-OLA algorithm does not change the BT hoppingpattern; however, since the D-OLA scheme prevents BT fromhopping on the 802.11 22-MHz band, it may happen that a setof frequencies less than 75 is actually used, and that the averagetime of occupancy of any frequency is greater than the above

215.247(h):“The incorporation of intelligence within an FHSS system thatpermits the system to recognize other users within the spectrum band so thatit individually and independently chooses and adapts its hopsets to avoid hop-ping on occupied channels is permitted. The coordination of frequency hoppingsystems in any other manner for the express purpose of avoiding the simulta-neous occupancy of individual hopping frequencies by multiple transmitters isnot permitted.”

limitation. We notice that the same problem occurs when theAFH mechanism is used and that the AFH Mode H has beenintroduced on purpose to overcome this problem. To make theD-OLA scheme fully compliant with the FCC requirements, asolution similar to the AFH Mode H could be adopted.

V. SIMULATION SCENARIO

We consider an IEEE 802.11ad hocnetwork providing aninstantaneous rate equal to 11 Mb/s and using the DCF MACscheme. All the stations operate as a self-contained BSS and areable to directly communicate with each other; all stations are as-sumed to be asynchronous data users with a finite transmissionbuffer. The arrival of frames from a station’s higher layer pro-tocol to the MAC sublayer is modeled with exponential interar-rival times and a truncated geometric distribution for the framelengths [28]. The mean value of the truncated geometric distri-bution is set to 1500 B, while the maximum frame length is setto the maximum length of the MAC service data unit establishedby the IEEE 802.11 standard (i.e., 2304 B). The parameter of theexponential distribution is fixed in such a way that the average802.11 traffic load normalized to the channel capacity is equalto , a varying parameter in the simulations.

In order to reduce the complexity of the simulation model,the following further assumptions have been introduced.

1) Possible values for the WLAN packet length, if not other-wise specified in the following, have been limited to 500,1000, and 1500 B.

2) The request to send/clear to send (RTS/CTS) mechanismis considered always active.

3) No interference is considered from nearby BSSs using thesame DSSS spreading sequence.

4) Propagation delay is neglected, which is a reasonable as-sumption due to the small distance between stations.

5) A two-state Markov model is used to represent the biterror process due to the effect of fading. In stategood,the bit error rate is equal to ; in statebad, it is equalto [28]. The transition probability fromgoodto badis equal to 0.01, frombad to goodis equal to 0.1.

An 802.11 transmission is considered to be successful if nocollision occurs on the RTS frame and both the data packet andthe corresponding acknowledgment sent by the receiver are cor-rectly received. Fig. 3 shows the 802.11 traffic timing in the caseof successful packet transmission. If a packet is not correctly re-ceived, retransmission will take place according to the backoffprocedure defined by the IEEE 802.11 standard. The number ofretransmissions before the packet is discarded from the stationbuffer is limited and set to the LongRetry Limit. The values ofthe IEEE 802.11 parameters used in the simulation model arelisted in Table I.

For the BT system, we consider a single piconet wheredevices are polled by the master on the basis of a round-robin


TABLE IPARAMETERS USED IN THESIMULATION OF THE IEEE 802.11 SYSTEM

TABLE IIPARAMETERS OF THEBLUETOOTH SYSTEM

scheme. Each device has a finite transmission buffer; assump-tion (4), introduced above for the IEEE 802.11 simulationmodel, holds also for the BT network. The packet error processover the wireless channel is assumed to be Bernoulli, and theaverage packet error probability is set to . We assumethat BT SCO traffic is transmitted by using an HV3-type link,which is expected to be the most popular link type for SCOservices [8]. With the HV3-type link for each active connection,a packet is transmitted in both directions every six time slots.In the case of data traffic, a DH1-type link is assumed to be thedefault operating mode, and therefore, single-slot data packetsare used. When the D-OLA scheme is applied, we consider thatone- and three-slot packets are used. In the case of three-slotpackets, a DH-3 type link is adopted. Notice that in the HV3-,DH1-, and DH3-type link, information in the payload is notFEC encoded [5]. The values of the BT system parameters arereported in Table II.

The arrival of data to a BT device’s MAC sublayer is modeledwith exponential interarrival times and a truncated geometricdistribution for the data unit length. The mean value of the trun-cated geometric distribution is set to 1500 B while the maximumdata unit length is set to 2800 B, corresponding to the total in-formation carried by 100 DH1 packets. The parameter of theexponential distribution is determined in such a way that the av-erage BT traffic load normalized to the channel capacity is equalto , a varying parameter in the simulations. Packets that arenot correctly received are retransmitted according to the fast au-tomatic repeat request scheme [4], where the sender is notifiedof the transmission outcome in the first possible slot followingthe packet transmission.

We model the mutual interference between 802.11 and BTas described in Section III-A and assume a noncollaborativesetting. As mentioned in Section III-A, we assume that when-ever the transmissions by the BT piconet and the 802.11 sta-tions overlap both in time and in frequency, the signals involvedin the collision are received in error. Therefore, we assess the

performance of the OLA schemes when the distance betweeninterfering devices is such that the bit error probability due tointerference is equal to one. This is a worst-case analysis. Wealso studied the performance of the proposed schemes in dif-ferent settings, i.e., by varying the bit error probability experi-enced by BT and 802.11 when a collision occurs; however, theresults that we obtained do not qualitatively differ from thosepresented below.

VI. PERFORMANCERESULTS

Results showing the performance of the OLA mechanismsare derived by using the simulation scenario described in theprevious section. The values of the traffic parameters of the BTand 802.11 systems are summarized in Table III.

Fig. 4 presents the 802.11 goodput as a function of the 802.11traffic load in the case where the BT channel supports one SCOlink (left plot) and two SCO links (right plot). Performanceof the V-OLA scheme in PT mode and in ST mode are com-pared with the results obtained in the absence of any coexistencemechanism (indicated in the figure by label N-CM). Goodput isdefined as the fraction of transmitted information that is suc-cessfully transferred over the radio channel. As expected, thebehavior of the 802.11 goodput slightly varies as the WLANtraffic load increases. In contrast, comparing the two plots inFig. 4, we observe a significant reduction in the 802.11 goodputwhen we pass from one to two SCO links. However, in the caseof one SCO link, by applying the V-OLA PT scheme, we obtainan improvement of 10% with respect to the case where no co-existence mechanism is implemented. In the case of two SCOlinks, the improvement is equal to 23%. When the V-OLA STscheme is used, slightly worse performance than in the case ofthe V-OLA PT scheme is achieved. In fact, in ST mode, the802.11 stations do not stop transmitting during the BT busy slotsand, thus, the probability to overlap BT SCO packets is higher.

Fig. 5 shows the BT goodput as a function of the 802.11traffic load for the two V-OLA schemes and in the absence ofany coexistence mechanism. The left and the right plots referto the case where BT supports one and two SCO links, respec-tively. Clearly, as grows, the BT goodput decreases due to thegreater interference level. The improvement achieved by usingthe V-OLA PT scheme can be up to 15% in the case of oneBT SCO link and up to 20% in the case of two SCO links. Inthese plots, the gap between the performance obtained throughthe PT mode and the ST mode is much greater than in Fig. 4,due to the interference caused by unsuccessful RTS and CTSframes. This effect becomes more evident as the 802.11 trafficload grows and the number of BT idle slots decreases, i.e., thecollision probability between WLAN stations increases.

Fig. 6 presents the behavior of the 802.11 average packetdelay, with the packet delay being the period from the instanta packet is generated to the instant the packet is successfullytransmitted. Results are presented as functions of the 802.11traffic load for the PT and the ST schemes and in the absenceof any coexistence mechanism. For very low values of, themajor delay contribution is due to the assumption that 802.11packets must have a minimum payload equal to 500 B. As

grows, this effect becomes less relevant, thus, resulting in


TABLE IIIVALUES OF THETRAFFIC PARAMETERS USED IN THE SIMULATION OF THE OLA MECHANISMS

(a)

(b)

Fig. 4. (a) IEEE 802.11 goodput when BT supports one SCO link and (b) twoSCO links. Performances obtained through the V-OLA schemes and when nocoexistence mechanism is applied (N-CM) are compared.

smaller traffic delay. For high values of traffic load, delay ismainly due to collisions between WLAN stations and, in thecase of the PT mode, to the lack of BT idle slots. The delayobtained in the case of the ST mode is slightly greater than thedelay experienced when no scheme is applied and remains loweven when two SCO links are considered. When the PT modeis applied, a low delay is obtained only for one SCO link and

less than 0.6. When two SCO links are supported and thenumber of BT idle slots decreases, for almost any value of,the PT mode gives a delay one order of magnitude higher thanin the case of the ST mode.

(a)

(b)

Fig. 5. (a) BT goodput versus the IEEE 802.11 traffic load when BT supportsone SCO link and (b) two SCO links. Performances obtained through theV-OLA schemes and in the case where no coexistence mechanism is applied(N-CM) are compared.

Figs. 7–9 compare the performance of the D-OLA schemewith the performance obtained in the absence of any coexis-tence mechanism. Results shown in Figs. 7 and 8 were derivedby setting the payload of the 802.11 packets to be equal to1500 B. The left plot in Fig. 7 presents the 802.11 goodputas a function of the BT traffic load for and . Inthe case of the D-OLA scheme, the 802.11 goodput remainsalmost constant as the BT traffic load increases, but when noscheme is implemented, a significant degradation is observed.The improvement in performance achieved through the pro-posed coexistence algorithm is as high as 50% for BT traffic


(a)

(b)

Fig. 6. (a) IEEE 802.11 average packet delay versus traffic load� in thepresence of one BT SCO link and (b) two BT SCO links. Performances obtainedthrough the V-OLA schemes and in the case where no coexistence mechanismis applied (N-CM) are compared.

load equal to 0.8. As expected, results slightly change as the802.11 traffic load varies.

Similar considerations hold for the results presented in theright plot in Fig. 7 where the BT goodput is shown as a functionof and for different values of the BT traffic load. In this case,the improvement in performance obtained through the D-OLAscheme is equal to 24% for .

Fig. 8 shows the BT average packet delay versus the BT trafficload for and . For , the delay experi-enced when the D-OLA algorithm is applied is slightly higherthan the delay achieved in the absence of any coexistence mech-anism; for , the two curves overlap. This shows thatthe D-OLA scheme greatly mitigates the mutual interference be-tween 802.11 and BT without causing a reduction in the BTthroughput.

Fig. 9 presents the 802.11 goodput as the payload of the802.11 packet varies for and . Results werederived from simulations where the 802.11 payload wasfixed to a constant value. The plot confirms the improvement

Fig. 7. Goodput of the IEEE 802.11 and the BT system in the presence ofBT data links. Performances obtained through the D-OLA scheme and when nocoexistence mechanism is applied (N-CM) are compared.

Fig. 8. BT average packet delay versus BT traffic load in the presence of BTdata links. Performances obtained through the D-OLA scheme and when nocoexistence mechanism is applied (N-CM) are compared for different values ofthe IEEE 802.11 traffic load.

achieved through the D-OLA algorithm. As expected, whenno coexistence mechanism is used, lower values of WLAN


Fig. 9. IEEE 802.11 goodput versus the 802.11 packet payload in the presenceof BT data links and BT traffic load equal to 0.4. Performances obtained throughthe D-OLA scheme and when no coexistence mechanism is applied (N-CM) arecompared for different values of the IEEE 802.11 traffic load.

Fig. 10. IEEE 802.11 goodput in presence of one interfering BT devicesupporting data traffic. Performances obtained through the D-OLA algorithm,the META scheme, and in the case where no coexistence mechanism isapplied (N-CM) are compared. Results obtained in the scenario with andwithout collocated interfering devices are indicated by labels Col and N-Col,respectively.

payload give higher 802.11 goodput because the packet errorprobability decreases. It is interesting to notice that in the caseof the D-OLA scheme, the WLAN payload has a negligibleimpact on the performance.

Finally, we present results showing that collaborativeschemes, such as META [11], outperform the OLA mech-anisms when interfering devices are collocated in the samephysical unit. The opposite is true when noncollocated devicesinterfere with each other.

Fig. 10 presents the 802.11 goodput achieved through theD-OLA algorithm and through the META scheme comparedwith the performance obtained when no coexistence mecha-nism is applied. The plot is obtained by assuming a BT datalink with normalized traffic load equal to 0.4 and setting theWLAN payload to be equal to 1500 B. Two different scenariosare studied: 1) a BT device interferes with one collocated 802.11device only and 2) a BT device interferes with one collocated

Fig. 11. BT average packet delay when one IEEE 802.11 and one BTcollocated devices are considered. The BT device supports data traffic.Performances obtained through the D-OLA algorithm, the META scheme and,when no coexistence mechanism is applied (N-CM) are compared.

and other noncollocated 802.11 devices. Clearly, in the first sce-nario, META gives the best performance. However, the METAgoodput greatly decreases as noncollocated 802.11 devices areconsidered while the performance of the D-OLA algorithm isalmost unchanged.

Fig. 11 presents the average packet delay for a BT device.The curves in the plot refer to the case where only two col-located devices interfere; results do not significantly vary asnoncollocated devices are considered. When META is applied,the 802.11 traffic load significantly affects the BT packet delay.When the D-OLA algorithm is adopted, the packet delay is con-stant as increases. Also, by using META, we obtain a BTpacket delay that is one order of magnitude greater than in thecase of the D-OLA scheme.

VII. I MPACT OF MICROWAVE OVEN INTERFERENCE ON

IEEE 802.11 PERFORMANCE

Microwave ovens operate in the ISM bands and are largelyused in residential and commercial environments. As shown in[29]–[32], the effect of the microwave ovens interference on thebit-error rate of WLANs operating at 2.4 GHz is quite signifi-cant.

We consider residential transformer-type microwave ovens,which include a single magnetron tube and whose power con-sumption is roughly equal to 600 W. Typically, they are activefor a time period of about 8 ms over a power cycle of 20 ms whenthe power supply frequency is equal to 50 Hz or of 16 ms whenthe power supply frequency is equal to 60 Hz. Since the mi-crowave oven interference has a periodic pattern, we can applythe V-OLA schemes and study their ability to reduce the im-pact of the microwave oven interference on the 802.11 perfor-mance. The following two scenarios are considered [29], [30],[32]: 1) the microwave oven is located 3 m away from the 802.11receiver causing a bit error probability equal to 0.01 and 2) themicrowave oven is located 0.5 m away from the 802.11 deviceresulting in a bit error probability equal to one.


(a)

(b)

Fig. 12. IEEE 802.11 goodput in presence of the interference generated by amicrowave oven. (a) The distance between the microwave oven and the 802.11receiver is roughly equal to 3 and (b) 0.5 m. Performances obtained through theV-OLA schemes and when no coexistence mechanism is applied (N-CM) arecompared.

Fig. 12 illustrates the 802.11 goodput obtained through theV-OLA algorithms with that achieved when no coexistencemechanism is applied. Results in the left and right plots arederived by assuming the 802.11 bit-error probability to be equalto 0.01 and 1.0, respectively; the power cycle of the microwaveoven is assumed to be equal to 20 ms. In the first scenario, boththe V-OLA schemes are able to mitigate the microwave oveninterference. In the second scenario, two effects take place:1) the 802.11 goodput is higher than in the first scenario and2) an improvement in performance is obtained only when theVOLA PT scheme is applied. This is because the WLAN usesthe RTS/CTS access protocol and errors in the reception ofeither the RTS or the CTS message prevent the 802.11 stationsfrom transmitting. Therefore, while the microwave oven isactive (an erroneous reception of the handshaking messagesoccurs with probability equal to one), no packet is transmittedover the 802.11 channel, and the performance of the WLANin absence of any coexistence mechanism is the same as thatobtained through the V-OLA ST scheme.

VIII. C ONCLUSION

In this paper, the problem of mutual interference betweendifferent wireless technologies operating in the 2.4-GHz ISMbands was addressed. We considered IEEE 802.11 WLANsand BT-based WPANs. Two different coexistence mechanismsbased on traffic scheduling techniques were proposed: one to beapplied at the WLAN stations to avoid overlap between 802.11traffic and BT voice packets and the other to be executed atthe BT devices to avoid overlap in frequency between 802.11traffic and BT data packets.

The main advantages of the proposed mechanisms are asfollows.

1) They do not require a centralized traffic scheduler.2) They can be implemented either when 802.11 and BT are

able to exchange information (collaborative coexistencemechanism) or when they acquire this information by de-tecting interfering transmissions over the radio channel(noncollaborative coexistence mechanism).

3) They are able to mitigate interference between collocatedand noncollocated BT and 802.11 devices.

4) They have a minor impact on the IEEE 802.11 standardand the BT specification.

Results indicate that the proposed mechanisms reduce the in-terference between 802.11 and BT, outperforming collaborativemechanisms when noncollocated devices interfere with eachother. Results showing the ability of the proposed schemes tocope with interference caused by microwave ovens also werepresented.

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[2] B. P. Crow, I. Widjaja, J. G. Kim, and P. T. Sakai, “IEEE 802.11 wirelesslocal area networks,”IEEE Commun. Mag., vol. 35, pp. 116–126, Sept.1997.

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[6] IEEE 802.15 WPAN Task Group 2 (TG2) [Online]. Available:http://www.ieee802.org/15/pub/TG2.html

[7] WiFi (802.11b) and Bluetooth: An Examination of Coexistence Ap-proaches (2001, Mar.). [Online]. Available: http://www.mobilian.com/whitepaper_frame.htm

[8] S. Shellhammer. (2001, Mar.) SCORT—An Alternative to the Blue-tooth SCO Link for Operation in an Interference Environment 01/145r1IEEE 802.15-01/145r1. [Online]. Available: http://www.ieee802.org/15/pub/TG2-Coexistence-Mechanisms.html

[9] D. Cypher. (2000, Mar.) Coexistence, Interoperability, and Other TermsIEEE 802.15-99/134r1. [Online]. Available: http://www.ieee802.org/15/pub/TG2-Technical-Presentations.html

[10] V. Arunachalam and B. Treister. (2001, July) Clause 5.3—Overview ofCoexistence Mechanisms. IEEE P802.15 Working Group for WPANs.[Online]. Available: http://www.ieee802.org/15/pub/TG2-Draft.html

[11] J. Lansford and A. P. Stephens. (2001, July) TG2 Mobilian Draft Text.IEEE P802.15 Working Group for WPANs IEEE 802.15-01/300r1. [On-line]. Available: http://www.ieee802.org/15/pub/TG2-Draft.html

[12] S. Shellhammer. (2001, Jan.) Collocated Collaborative CoexistenceMechanism: TDMA of 802.11 and Bluetooth IEEE 802.15-01/025r0.[Online]. Available: http://www.ieee802.org/15/pub/TG2-Coexistence-Mechanisms.html

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[14] R. E. Van Dyck and A. Soltanian. (2001, July) IEEE 802.15 Clause14.1—Collaborative Co-Located Coexistence Mechanism. IEEEP802.15 Working Group for WPANs IEEE 802.15-01/364r0. [Online].Available: http://www.ieee802.org/15/pub/TG2-Draft.html

[15] H. Gan and B. Treister. (2000, Nov.) Adaptive Frequency HoppingImplementation Proposals for IEEE 802.15.1/2 WPAN IEEE 802.15-00/367r0. [Online]. Available: http://www.ieee802.org/15/pub/TG2-Co-existence-Mechanisms.html

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[28] B. P. Crow, I. Widjaja, J. G. Kim, and P. Sakai, “Investigation of the IEEE802.11 medium access control (MAC) sublayer functions,” inProc. IN-FOCOM’97, Mar. 1997, pp. 126–133.

[29] S. Miyamoto, Y. Yamanaka, and T. Shinozuka, “A study of the effect ofmicrowave oven interference on the performance of digital radio com-munications systems,”Electron. Commun. Japan, vol. 80, no. 12, pp.58–67, 1997.

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[32] S.-T. Sheu, Y.-H. Lee, M.-H. Chen, Y.-C. Yu, and Y.-C. Huang, “PLFC:The packet length fuzzy controller to improve the performance ofWLAN under the interference of microwave oven,” inProc. IEEEGlobecom, San Francisco, CA, Nov. 2000, pp. 1427–1431.

Carla-Fabiana Chiasserini (S’98–M’00) receivedthe electrical engineering degree (summa cum laude)from the University of Florence, Florence, Italy, in1996 and the Ph.D. degree from the Politecnico diTorino, Torino, Italy, in 1999.

Since 1999, she has been with the Departmentof Electrical Engineering at Politecnico di Torino,where she is currently an Assistant Professor. Shewas at the Center for Wireless Communications,University of California, San Diego, CA, as aVisiting Researcher from 1999 to 2001. Her research

interests include architectures, protocols, and performance analysis of wirelessnetworks for integrated multimedia services.

Ramesh R. Rao(M’85–SM’90) received the Bachelor’s degree in electrical andelectronics engineering from the University of Madras, Madras, India, in 1980,and the M.S. and Ph.D. degrees from the University of Maryland, College Park,in 1982 and 1984, respectively.

Since 1984, he has been on the faculty of the Department of Electrical andComputer Engineering at the University of California, San Diego (UCSD),where he is currently Professor and Director of the San Diego Division ofthe California Institute of Telecommunications and Information Technology.Prior to this appointment, he served as the Director of the UCSD Center forWireless Communications and was the Vice Chair of Instructional Affairs inthe Department of Electrical and Computer Engineering. His research interestsinclude architectures, protocols, and performance analysis of wireless, wireline,and photonic networks for integrated multimedia services. He served as theEditor of the INFORMATION THEORY SOCIETY NEWSLETTERfrom 1993 to 1995and is the founding Web Editor of the Information Theory Society web site.

Dr. Rao was elected to the IEEE Information Theory Society Board of Gover-nors (from 1997 to 1999 and 2000 to 2002). He is the Editor for Packet MultipleAccess of the IEEE TRANSACTIONS ONCOMMUNICATIONS and is a member ofthe Editorial Board of the ACM/BALTZER WIRELESSNETWORK JOURNAL, aswell as IEEE Network Magazine.

coexistence mechanisms for interference mitigation in the 2.4-ghz

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