Weile Zhang

I am the antenna: accurate outdoor AP location using smartphones

Todays WiFi access points (APs) are ubiquitous, and provide critical connectivity for a wide range of mobile networking devices. Many management tasks, e.g. optimizing AP placement and detecting rogue APs, require a user to efficiently determine the location of wireless APs. Unlike prior localization techniques that require either specialized equipment or extensive outdoor measurements, we propose a way to locate APs in real-time using commodity smartphones. Our insight is that by rotating a wireless receiver (smartphone) around a signal-blocking obstacle (the users body), we can effectively emulate the sensitivity and functionality of a directional antenna. Our measurements show that we can detect these signal strength artifacts on multiple smartphone platforms for a variety of outdoor environments. We develop a model for detecting signal dips caused by blocking obstacles, and use it to produce a directional analysis technique that accurately predicts the direction of the AP, along with an associated confidence value. The result is Borealis, a system that provides accurate directional guidance and leads users to a desired AP after a few measurements. Detailed measurements show that Borealis is significantly more accurate than other real-time localization systems, and is nearly as accurate as offline approaches using extensive wireless measurements.

Distributed Angle Estimation for Localization in Wireless Sensor Networks

In this paper, we design a new distributed angle estimation method for localization in wireless sensor networks (WSNs) under multipath propagation environment. We employ a two-antenna anchor that can emit two linear chirp waves simultaneously, and propose to estimate the angle of departure (AOD) of the emitted waves at each receiving node via frequency measurement of the local received signal strength indication (RSSI) signal. An improved estimation method is further proposed where multiple parallel arrays are adopted to provide the space diversity. The proposed methods rely only on radio transceivers and do not require frequency synchronization or precise time synchronization between the transceivers. More importantly, the angle is estimated at each sensor in a completely distributed manner. The performance analysis is derived and simulations are presented to corroborate the proposed studies.

3D beamforming for wireless data centers

Contrary to prior assumptions, recent measurements show that data center traffic is not constrained by network bisection bandwidth, but is instead prone to congestion loss caused by short traffic bursts. Compared to the cost and complexity of modifying data center architectures, a much more attractive option is to augment wired links with flexible wireless links in the 60 GHz band. Current proposals, however, are severely constrained by two factors. First, 60 GHz wireless links are limited by line-of-sight, and can be blocked by even small obstacles between the endpoints. Second, even beamforming links leak power, and potential interference will severely limit concurrent transmissions in dense data centers. In this paper, we explore the feasibility of a new wireless primitive for data centers, 3D beamforming. We explore the design space, and show how bouncing 60 GHz wireless links off reflective ceilings can address both link blockage and link interference, thus improving link range and number of current transmissions in the data center.

Blind Maximum Likelihood Carrier Frequency Offset Estimation for OFDM With Multi-Antenna Receiver

In this paper, based on the maximum likelihood (ML) criterion, we propose a blind carrier frequency offset (CFO) estimation method for orthogonal frequency division multiplexing (OFDM) with multi-antenna receiver. We find that the blind ML solution in this situation is quite different from the case of single antenna receiver. As compared to the conventional MUSIC-like CFO searching algorithm, our proposed method not only has the advantage of being applicable to fully loaded systems, but also can achieve much better performance in the presence of null subcarriers. It is demonstrated that the proposed method also outperforms several existing estimators designed for multi-antenna receivers. The theoretical performance analysis and numerical results are provided, both of which demonstrate that the proposed method can achieve the Cramér-Rao bound (CRB) under the high signal-to-noise ratio (SNR) region.

One-Shot Blind CFO and Channel Estimation for OFDM With Multi-Antenna Receiver

In this paper, we design a new blind joint carrier frequency offset (CFO) and channel estimation method for orthogonal frequency-division multiplexing (OFDM) with multiantenna receiver. The proposed algorithm requires only one received OFDM block and thus belongs to the category of one-shot estimation methods. Other advantages of the proposed algorithm include 1) it supports fully loaded data carriers and is thus spectral efficient; 2) the channel from the transmitter to each receive antenna can be blindly estimated with only a scaling ambiguity; and 3) the algorithms outperforms the existing methods. Moreover, we derive the Cramér-Rao Bounds (CRB) of joint CFO and channel estimation in closed forms. Numeral results not only show the effectiveness of the proposed algorithm but also demonstrate its closed performance to CRB.

Blind Carrier Frequency Offset Estimation for Interleaved OFDMA Uplink

In this paper, we develop two novel blind carrier frequency offset (CFO) estimators for interleaved orthogonal frequency division multiple access (OFDMA) uplink transmission in a multiantenna system. The first estimator is the subspace-based one and could blindly estimate multiple CFOs from a rank reduction approach. The second estimator is based on the maximum likelihood (ML) approach and improves the performance as compared to the first one. The higher computational complexity of the ML estimator is alleviated by the alternating projection (AP) method. Both the proposed estimators support fully loaded data transmissions, i.e., all subcarriers being occupied, which provides higher bandwidth efficiency as compared to the existing schemes. The numerical results are then provided to corroborate the proposed studies.

Distributed TDoA Estimation for Wireless Sensor Networks Based on Frequency-Hopping in Multipath Environment

A novel distributed TDoA (Time-Difference-of-Arrival) estimation method for wireless sensor networks (WSN) in multipath environment is proposed in this paper. For the conventional narrow band system usually adopted in the wireless sensors, the frequency-hopping technique can be utilized to perform the estimation of the wide band frequency-domain channel response, by which the super-resolution TDoA estimation is conducted independently at each sensor to detect range difference from the sensor to the two anchors. The proposed method only relies on the conventional narrow band radio transceiver and each sensor conducts range estimation independently without any information exchange or time synchronization requirement, which is a distributed range estimation method more suitable for WSN.

Blind Channel Estimation for MIMO-OFDM Systems With Low Order Signal Constellation

In this paper, we design a new blind channel estimation method for multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems to cope with possible rank deficient signal covariance matrices when symbols are drawn from low order constellations, e.g., BPSK, QPSK, etc. By collecting the signal covariance matrices from a group of carefully selected subcarriers, the proposed method could identify the channel information from a small amount of received blocks. Moreover, the proposed method also outperforms the existing competitor in terms of both estimation performance and computational complexity. The numeral results are provided to corroborate the proposed studies.

Radio interferometric localization of WSNs based on Doppler effect

A novel distributed localization method based on the Doppler effect for wireless sensor networks is proposed in this paper, which only relies on radio transceivers without the need of other auxiliary measurement equipments. Based on radio interference, each sensor node can obtain a multi-group of distance variations from itself to a mobile anchor through the measurement of the cumulative phase variations caused by Doppler effect, by which its position can be determined. The proposed method has the following advantages: the sensor node conducts localization in a completely distributed manner without information exchange or centralized processing, and the measurement result equals the actual distance without modulus operation, which is in reality measuring the distance with a ruler of radio wavelength, and will significantly reduce the system complexity and achieve high location accuracy. Analysis and simulation results demonstrate the effectiveness of our proposed method.

Blind Frequency Synchronization for Multiuser OFDM Uplink With Large Number of Receive Antennas

In this paper, we propose a new blind frequency synchronization scheme for multiuser orthogonal-frequency-division multiplexing (OFDM) uplink transmissions. The spatial multiplexing is supported in the considered model that allows the subcarriers to be simultaneously occupied by multiple users, while the orthogonal frequency division multiple access (OFDMA) can serve as a special case of our discussion. We propose to assign different null subcarriers to different users and design algorithm that can perform blind carrier frequency offset (CFO) estimation for each individual user with the aid of large number of receive antennas, which then removes the necessity of multidimensional searching. After estimating CFO for one user, the orthogonal complement space of the other users can be obtained which could be further utilized for multiuser interference cancelation. Hence, the multiuser OFDM uplink model reduces to an equivalent single user signal model, where the blind channel estimation with scalar ambiguity can be designed and the conventional single-user data detection can be peformed. To make the proposed study complete, the closed-form performance analysis for mean square error (MSE) of the CFO estimation is derived, based on which the discussion about optimal null subcarrier assignment scheme is further provided. Finally, the Cramer-Rao bound (CRB) is derived, and the numerical results are provided to corroborate the proposed studies.