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Why is Frequency Channel Diversity so Beneficial in Wireless Sensor Networks?

Liviu-Octavian Varga 1, 2 Martin Heusse 1 Roberto Guizzetti 2 Andrzej Duda 1
1 Drakkar
LIG - Laboratoire d'Informatique de Grenoble
Abstract : In dense wireless sensor networks, a multichannel MAC is a good means to reduce channel contention and increase frame reception probability. In this paper, we report on experiments with transmissions on various channels in the 2.4GHz ISM band and find more channel diversity than expected: this effect is particularly exacerbated at a short range, but it also has a significant impact at any distance. Moreover, we find that wireless sensor nodes have a radiation pattern that changes significantly with the frequency channel. This feature is inherent to the size of the sensor node, in which the antenna necessarily interferes with other components. The first consequence of this finding is that frequency diversity in sensor networks is even more effective than generally thought, and conversely, single channel communication schemes should be avoided as long as the power budget is not very comfortable. Keywords—802.15.4, coherence band, channel hopping, radio propagation I. INTRODUCTION In the context of a growing number of connected devices forming potentially dense wireless networks, using the whole available spectrum becomes unavoidable. IEEE 802.15.4 defines 16 channels in the 2.4GHz band with the width of 2 MHz and 5 MHz interchannel spacing. Besides segregating the transmissions, changing the channel allows to benefit from the diversity due to the sensitivity of multipath fading to the central frequency. In other words, there can be a channel between two nodes that works well, but during the same time frame, this channel gives very bad performance with other nodes. For all these reasons, the IEEE 802.15.4e-2012 amendment [1] specified channel hopping protocols such as TSCH (Time Synchronized Channel Hopping) or DSME (Deterministic and Synchronous Multi-channel Extension). Even though a lot of research aims at harnessing channel diversity, most of the studies invoke multipath fading as the reason of bad communication on certain channels between two given nodes. Two solutions exist to improve performance: either change the position of a node or the antenna, or change the frequency. The first solution is inapplicable in most cases because access to the node might not always be possible. Taking advantage of antenna diversity is also challenging to implement for cost and space constraints. Fortunately, even for short range and line of sight communications , our experiments show that there is a lot of diversity when changing channels, more than multipath fading itself can explain. We posit that the entire wireless sensor board has a role in radio propagation. On certain frequencies and in many directions, its presence either attenuates the radio signal or amplifies it. This effect is unavoidable for compact nodes and it has a noticeable impact within the band of interest. We start the paper with a simple radio propagation model and present selected real world measurements of commu
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Submitted on : Monday, March 14, 2016 - 9:36:21 AM
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Liviu-Octavian Varga, Martin Heusse, Roberto Guizzetti, Andrzej Duda. Why is Frequency Channel Diversity so Beneficial in Wireless Sensor Networks?. IFIP Wireless Days, IFIP, Mar 2016, Toulouse, France. ⟨hal-01287518⟩



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