Clearing the RF Smog: Making 802.11 Robust to Cross-Technology Interference

Clearing the RF Smog: Making 802.11 Robust to Cross-Technology Interference Shyamnath Gollakota † Fadel Adib† Dina Katabi† Srinivasan Seshan⋆ † ⋆ Massachusetts Institute of Technology Carnegie Mellon University {gshyam, fadel, dina}@csail.mit.edu [email protected] ABSTRACT frequency band as wide as 802.11, and all of them emit power that is Recent studies show that high-power cross-technology interfer- comparable or higher than 802.11 devices [17]. Further, the number ence is becoming a major problem in today’s 802.11 networks. De- and diversity of such interferers is likely to increase over time due vices like baby monitors and cordless phones can cause a wire- to the proliferation of new technologies in the ISM band. less LAN to lose connectivity. The existing approach for dealing Traditional solutions that increase resilience to interference by with such high-power interferers makes the 802.11 network switch making 802.11 fall down to a lower bit rate are ineffective against to a different channel; yet the ISM band is becoming increasingly high-power cross-technology interference. As a result, the most crowded with diverse technologies, and hence many 802.11 access common solution today is to hop away to an 802.11 channel that points may not find an interference-free channel. does not suffer from interference [6, 38, 31, 32]. However, the ISM This paper presents TIMO, a MIMO design that enables 802.11n band is becoming increasingly crowded, making it difficult to find to communicate in the presence of high-power cross-technology an interference-free channel. The lack of interference-free channels interference. Unlike existing MIMO designs, however, which re- has led WiFi device manufacturers [6, 11, 3] and researchers [29] to quire all concurrent transmissions to belong to the same technol- develop signal classifiers that inform the 802.11 user about the root ogy, TIMO can exploit MIMO capabilities to decode in the pres- cause of the problem (e.g., Bluetooth, microwave, baby monitor). ence of a signal from a different technology, hence enabling diverse However, these classifiers put the burden of addressing the problem technologies to share the same frequency band. We implement a on the user and cannot solve the problem on their own. prototype of TIMO in GNURadio-USRP2 and show that it enables In this paper, we ask whether it is possible to use the MIMO capa- 802.11n to communicate in the presence of interference from baby bility inherent to 802.11n to address high-power cross-technology monitors, cordless phones, and microwave ovens, transforming sce- interference. MIMO achieves most of its throughput gains by en- narios with a complete loss of connectivity to operational networks. abling multiple concurrent streams (e.g., packets). Current MIMO decoding, however, fails if any of these concurrent streams belongs Categories and Subject Descriptors C.2.2 [Computer to a different technology. Nonetheless, if MIMO can be made to Systems Organization]: Computer-Communications Networks work across technologies, a 3×3 802.11n transmitter can then treat General Terms Algorithms, Design, Performance, Experimen- the signal from a baby monitor or microwave as one stream and still tation deliver two concurrent streams to its receiver. The challenge in harnessing MIMO across different technolo- Keywords Cognitive MIMO, Cross-Technology Interference gies stems from the fact that MIMO decoding hinges on estimat- ing the channel between all transmit and receive antennas. These 1. INTRODUCTION estimates rely on understanding the signal structure and assume a Cross-technology interference is emerging as a major problem known preamble. Hence, it has been infeasible to use MIMO across for 802.11 networks. Independent studies in 2010 by the Farpoint different and potentially unknown technologies. Group [8], BandSpeed [17], and Miercom [10] all show that high- We present TIMO,1 an 802.11n receiver design robust to high- power interferers like baby monitors and cordless phones can cause power cross-technology interference. TIMO introduces a MIMO 802.11n networks to experience a complete loss of connectivity. technique that enables a receiver to decode a signal of interest, even Other studies from Ofcom [7], Jupiter Research [1], and Cisco [14] when the channel from other concurrent transmissions is unknown. report that such interferers are responsible for more than half of the The intuition underlying TIMO is best explained via an example. problems reported in customer networks. Today’s high-power non- Consider a pair of 2-antenna 802.11n nodes that want to communi- WiFi sources in the ISM band include surveillance cameras, baby cate in the presence of a high-power unknown interferer. Let s(t) be monitors, microwave ovens, digital and analog cordless phones, and the signal of interest and i(t) the interference signal. The 802.11n outdoor microwave links. Some of these technologies transmit in a receiver node will receive the following signals on its two antennas:2 y1(t) = hii(t) + hss(t) (1) Permission to make digital or hard copies of all or part of this work for ′ ′ y2(t) = h i(t) + h s(t), (2) personal or classroom use is granted without fee provided that copies are i s not made or distributed for profit or commercial advantage and that copies where h and h′ are the channels from the interferer to the 802.11n bear this notice and the full citation on the first page. To copy otherwise, to i i republish, to post on servers or to redistribute to lists, requires prior specific 1Technology Independent Multi-Output (TIMO) receiver design. permission and/or a fee. 2 SIGCOMM’11, August 15–19, 2011, Toronto, Ontario, Canada. The equations here are for single-tap channels. Subsequent sec- Copyright 2011 ACM 978-1-4503-0797-0/11/08 ...$10.00. tions extend these equations to multi-tap channels. 170 121 Feet ′ receiver, and hs and hs are the channels from the 802.11n sender to 2 the 802.11n receiver. The 802.11n receiver has to solve these equa- 10 1 3 tions to obtain its signal of interest s(t). It knows the received sam- B ′ 4 ples, y1(t) and y2(t), and the channels from its transmitter, hs and h , s 9 5 which can be computed in the presence of interference (see §6.4). 6 A The receiver, however, cannot compute the channels from the in- 8 ′ 7 terferer, hi and hi , because it does not know the interferer’s signal structure or preamble. Hence, it is left with two equations in three ′ 3 unknowns (s(t), hii(t), and hi i(t)), which it cannot solve. Note that the receiver can cancel the interference if it knows the Figure 1—Testbed. An 802.11n transmitter located at A is com- hi municating with an 802.11n receiver at B. The interferer is placed interferer’s channel ratio h′ . In particular, the receiver can rewrite i in one of the locations 1 to 10. equations 1 and 2 to express the signal of interest as: y1(t) − βy2(t) hi TIMO, when the USRP2 nodes are less than 31 feet away from s(t) = ′ for β = ′ . (3) the cordless phone or the baby monitors, they cannot deliver any hs − βhs h i packets. In contrast, in the presence of TIMO, and for the same The only unknown in the above equation is β. Thus, though the locations, their throughput increases to 13-23 Mb/s. We also imple- 802.11n receiver cannot compute the exact channels of the inter- ment cross-technology interference nulling and show that it enables ferer, it can still cancel its interference using only its channel ratio. a MIMO node to significantly reduce the packet loss at the receiver Still, how do we obtain this ratio given no support from the in- of a competing technology, with the reduction in packet loss being terferer? The receiver can obtain this ratio as follows: Say that for as high as 14x in some locations. some time instance t = t0, our transmitter sends a known symbol s(t0). Our receiver can then substitute in equations 1 and 2 to obtain: 2. IMPACT OF CROSS-TECHNOLOGY IN- hi y (t ) − hss(t ) = 1 0 0 , (4) TERFERENCE ON 802.11N h′ y t h′s t i 2( 0) − s ( 0) We study the interaction between high-power interferers and hi 802.11n and compare against the interaction between a low power where all terms are known except for the ratio h′ . In §6, we develop i interferer, Bluetooth, and 802.11n. We focus on three high-power this idea further and eliminate the need for having the transmitter send a known symbol, which makes the scheme applicable to exist- technologies that are prevalent in today’s environments [7]: DSSS ing 802.11n frames. We further generalize the solution to address cordless phones, baby monitors, and microwave ovens. scenarios in which different frequencies have different interferers, Experimental Setup: We use the Netgear N-300 USB-adapter and or the interferer hops across frequencies. the Netgear N-300 router as the 802.11n client and AP respectively. A MIMO transmitter can also encode its signal using interference Both devices support 2 × 2 MIMO. We place the AP and the client nulling [36] so that it does not interfere with a concurrent transmis- at positions A and B in Fig. 1. In each run, we place the interferer sion from a competing technology. However, using a similar com- at one of the marked locations in Fig. 1. Our experiments include putation, we show that it is necessary to obtain the ratio hs1 , where line-of-sight and non-line-of-sight situations, and show scenarios in hs2 hs1 and hs2 are the channels from the MIMO transmitter to the re- which the interferer is within one foot of the 802.11n client as well ceiver of the competing technology.

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