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... of concurrent spatial streams and parallel transmission of multiple users’ data. The optimal method of constructing W from H to concurrently serve multiple users is known as Dirty Paper Coding (DPC) =-=[4]-=-, [5]; however, in practice this method is difficult to implement due to its complexity. Instead, ZFBF is a suboptimal W construction method that is simpler to implement yet still achieves a performan...

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...oncurrent spatial streams and parallel transmission of multiple users’ data. The optimal method of constructing W from H to concurrently serve multiple users is known as Dirty Paper Coding (DPC) [4], =-=[5]-=-; however, in practice this method is difficult to implement due to its complexity. Instead, ZFBF is a suboptimal W construction method that is simpler to implement yet still achieves a performance al...

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...smitting “orthogonally blinding” streams (defined in § II-B) everywhere else. ZFBF is a precoding method that enables a multi-antenna access point (AP) to create multiple simultaneous spatial streams =-=[3]-=-. Recent wireless standards such as 802.11n or the This research is sponsored in part by HP Labs’ Innovation Research Program and by NSF Grants CNS-1012831 and CNS-1126478. upcoming 802.11ac 1 employ ...

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...ormation theoretic studies that examine the theoretical performance of multi-antenna based security methods. In particular, these works define the fundamental limits of secrecy capacity. For example, =-=[16]-=- proves that a nonzero rate of communication can be guaranteed to be secret for any eavesdropper position. In contrast, our focus is on protocol design and experimental evaluation with alternate schem...

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... nonzero rate of communication can be guaranteed to be secret for any eavesdropper position. In contrast, our focus is on protocol design and experimental evaluation with alternate schemes. Likewise, =-=[17]-=- explores how eavesdroppers can be thwarted by a cooperative communication scheme. IX. CONCLUSION We design and experimentally characterize STROBE, a method of enhancing wireless security using ZFBF. ...

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...dropping location. VIII. RELATED WORK Wireless security exploiting CSI-based secret. One method of guarding a transmitter from an eavesdropper is CSI-based secret sharing. For example, the authors of =-=[12]-=- directly use intended user CSI as a secret generation method while the authors of [13] use intended user CSI to actively disrupt OFDM subcarriers to confuse eavesdroppers. In contrast, our implementa...

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... achieves a performance almost equivalent to DPC [3]. ZFBF selects weights that cause zero inter-user interference (the effect of one beamformed stream on another is “forced” to zero). The authors of =-=[6]-=- have shown that the optimal selection of W to satisfy this zero-interference condition is the pseudoinverse of H as shown in Eq. (1). W = H † = H ∗ (HH ∗ ) -1 The use of the pseudo-inverse is how the...

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... W calculation in MATLAB, and beamforming weight multiplication in the WARP node. For the characterization of STROBE, we use the ZFBF experimental framework, built on top of WARPLab by the authors of =-=[10]-=-. We employ one transmit antenna for the receivers and all four for the transmitter (thus N = 4 for our characterization). Our antenna array is circular with the antennas spaced one wavelength apart f...

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...e method of guarding a transmitter from an eavesdropper is CSI-based secret sharing. For example, the authors of [12] directly use intended user CSI as a secret generation method while the authors of =-=[13]-=- use intended user CSI to actively disrupt OFDM subcarriers to confuse eavesdroppers. In contrast, our implementation uses the CSI to beamform a signal toward the intended user while simultaneously bl...

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...wo methods are complementary. Beamforming-based multiple AP cooperation. Beamforming schemes that rely on groups of cooperating APs have also been proposed to secure wireless networks. The authors of =-=[14]-=- propose a method of securing wireless communications using a collection of phased arrays working in tandem to serve an intended user with a data stream. Each AP provides partial information of the ov...

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...pend on the predictable behavior of transmitted beam patterns or that are agnostic to the entire eavesdropper environment fail to prevent eavesdropping as confirmed by our own experiments and also in =-=[2]-=-. To address this problem, we design a new, multi-antenna, 802.11-compatible scheme that adaptively sends a beam toward an intended user while “blinding” (actively interfering with) potential eavesdro...

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... Omni SUBF STROBE CE SINR at IU and E1-3 for inline receivers. VI. IS MULTI-PATH ESSENTIAL? STROBE’s efficacy relies on multi-path effects in an indoor environment as described in § V. The authors of =-=[11]-=- experimentally show that receiver separation distances of 70 m are required to serve users in parallel with ZFBF in outdoor environments. Multi-path is the hypothesized explanation for the ability of...

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...d relies on precise directionally-based transmissions with tight AP synchronization to ensure the intersection of all partial streams at a particular geographic location. Additionally, the authors of =-=[15]-=- propose a set of multiple AP methods that allow energy to be focused toward an intended user but away from an eavesdropper. Although the authors propose using adaptive array beamformers, the weight c...

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...ers from intercepting transmissions directed toward an intended user. While encryption mitigates this vulnerability, even industry standard encryption methods such as WEP and WPA could be compromised =-=[1]-=- and readily available software packages enable malicious users to defeat secure networks. One method of enhancing the security of wireless transmissions is to prevent the eavesdropper from receiving ...

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...≤ M. (2) The Gram-Schmidt process is simple to integrate into a ZFBF enabled 802.11n/ac AP. The first step of calculating the required pseudo-inverse is implemented in hardware using QR decomposition =-=[8]-=-, an operation that decomposes a matrix into an upper triangular (R) and a unitary matrix (Q). The GramSchmidt process can also be executed using the QR method. Thus, the silicon in the PHY of a ZFBF ...