Mitigating Multipath Fading Through Channel Hopping in Wireless Sensor Networks Thomas Watteyne ∗ , Steven Lanzisera † , Ankur Mehta ∗ , Kristofer S.J. Pister ∗ ∗ BSAC, University of California, Berkeley, USA {watteyne,mehta,pister}@eecs.berkeley.edu † Lawrence Berkeley National Laboratory, USA, smlanzisera@lbl.gov Abstract—Wireless communication between a pair of nodes can suffer from self interference arising from multipath propagation reflecting off obstacles in the environment. In the event of a deep fade, caused by destructive interference, no signal power is seen at the receiver, and so communication fails. Multipath fading can be overcome by shifting the location of one node, or by switching the communication carrier frequency. The effects of such actions can be characterized by the coherence length (L) and coherence bandwidth (B), respectively, given as the amount of shift necessary to transition from a deep fade to a region of average signal strength. Experimental results for a representative 2.4GHz wireless link indicate L =5.5cm and B can vary from 5MHz at long ranges up to 15MHz for short links. For wireless sensor networks (WSNs), typically operating under the IEEE802.15.4 standard, multipath effects are therefore best handled by a channel hopping scheme in which successive communication attempts are widely spread across available carrier frequencies. I. I NTRODUCTION In an indoor environment, every wall, person, and piece of furniture acts as a reflector for RF signals. As a result, on top of the signal following the direct line-of-sight (LOS) path, a node receives multiple echoes which have bounced off nearby elements. Because the paths those echoes follow are necessar- ily longer than the LOS path, they arrive later, typically with several ns delay. This is an unwanted phenomenon, partic- ularly in narrowband communication. If the different signals are phased appropriately, they can destructively interfere, and the receiver will be unable to decode the signal even when physically close to the transmitter. In Fig. 1, we show the effects of such multipath fading (the experimental details will be presented in Section III). This figure shows how the packet delivery ratio (PDR, the ratio of number of received to sent packets; 0≤PDR≤1) varies as only the position of the transmitter changes. While in most locations reception is good (i.e. PDR> 0.9), multipath fading causes the PDR to drop to 0 in certain locations, called deep fades. Multipath fading is a well known phenomenon, and depends strongly on the communication carrier frequency. It is often combated (such as in IEEE802.15.1, Bluetooth) through chan- nel hopping, a Medium Access Control (MAC) layer scheme wherein the nodes’ radios constantly “hop” among different frequency channels. In IEEE802.11b the desired signal is spread across a wide bandwidth to avoid narrow bandwidth fading, and this direct sequence technique is a physical layer approach to defeating narrowband fading. 0 5 10 15 20 x (cm) 0 5 10 15 20 25 30 35 y (cm) 0 0.2 0.4 0.6 0.8 1 PDR 0 0.2 0.4 0.6 0.8 1 Fig. 1. Witnessing multipath fading. The x and y coordinates represent the position of the transmitter on a 20cm × 34cm area; the receiver is static. The z axis (and the shade) represent the Packet Delivery Ratio, PDR. Results obtained for sender and receiver communicating on IEEE802.15.4 channel 20 (2.450GHz) while separated by 1m; transmission power is set to -16dBm. Channel hopping has been somewhat overlooked in WSNs as the vast majority of MAC protocols are single channel. The widespread adoption of IEEE802.15.4 radios capable of rapidly switching between multiple channels opens the possibility for exploiting channel hopping in WSNs. The goal of this paper is to present the reader with experimental results gathered in a number of scenarios, and to use this data to discuss how a channel hopping MAC protocol can efficiently fight multipath fading in the context of WSNs. By mitigating the impact of multipath fading, individual links and the network as a whole becomes more reliable; network power consumption is lowered as energy is no longer wasted in inefficient retransmissions along lossy links. Several papers have studied the relationship between an environment and coherence length and bandwidth [2], [9], [10], and this paper is intended to add to this body of knowledge, while being aimed at the wireless sensor network implementer rather than the communication theorist. To that end, we propose a non-conventional approach to character- izing channel coherence 1 that uses motes like those used in today’s deployments, and we provide design parameters of interest to the WSN engineer. It is known that the coherence 1 We actually measure the change required to see a significant difference in channel conditions. 978-1-4244-6404-3/10/$26.00 ©2010 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2010 proceedings Authorized licensed use limited to: Univ of Calif Berkeley. Downloaded on August 10,2010 at 22:56:39 UTC from IEEE Xplore. Restrictions apply.