964 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 2, NO. 5, SEPTEMBER 2003Coexistence Mechanisms for Interference Mitigationin the 2.4-GHz ISM BandCarla-Fabiana Chiasserini, Member, IEEE, and Ramesh R. Rao, Senior Member, IEEEAbstract—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-lesslocalareanetworks(WLANs),wirelesspersonal area networks(WPANs).I. INTRODUCTIONTHE 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-hoppingManuscript 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 CentrodiEccellenzaper 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.2003.817417spread 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 of coexistence mechanisms [6], i.e., techniques thatallow802.11 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-tionthat 802.11 and BT can detect interference due to othertech-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 IEEECHIASSERINI AND RAO: COEXISTENCE MECHANISMS FOR INTERFERENCE MITIGATION 965By 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