Data Mining Practical Machine Learning Tools and Techniques

Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications and all cellular systems. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.

Spatially Sparse Precoding in Millimeter Wave MIMO Systems

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts of IBFD wireless. One of the biggest practical impediments to IBFD operation is the presence of self-interference, i.e., the interference that the modem's transmitter causes to its own receiver. This tutorial surveys a wide range of IBFD self-interference mitigation techniques. Also discussed are numerous other research challenges and opportunities in the design and analysis of IBFD wireless systems.

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In-Band Full-Duplex Wireless: Challenges and Opportunities

In-band full-duplex (IBFD) operation has emerged as an attractive solution for increasing the throughput of wireless communication systems and networks. With IBFD, a wireless terminal is allowed to transmit and receive simultaneously in the same frequency band. This tutorial paper reviews the main concepts of IBFD wireless. Because one the biggest practical impediments to IBFD operation is the presence of self-interference, i.e., the interference caused by an IBFD node's own transmissions to its desired receptions, this tutorial surveys a wide range of IBFD self-interference mitigation techniques. Also discussed are numerous other research challenges and opportunities in the design and analysis of IBFD wireless systems.

Orthogonal frequency-division multiple access (OFDMA) has been integrated into emerging broadband wireless systems such as the IEEE 802.16 WirelessMAN. One critical problem in OFDMA is to assign multiple frequency bands (called subcarriers) to different users. Taking advantage of the frequency and multiuser diversities in OFDMA systems, dynamic subcarrier assignment mechanisms have been shown to achieve much higher downlink capacity than static assignment. A rich literature exists, which proposes medium-access-control- and physical-layer (MAC/PHY) schemes, aiming at exploiting the diversity gain with low implementation complexity. In this paper, we propose a cross-layer approach that explores the joint advantage of network coding and dynamic subcarrier assignment. With network coding, it becomes possible to assign the same subcarriers to different downlinks without causing interference. Consequently, our coding-aware assignment scheme improves the bandwidth efficiency and increases the downlink throughput by a substantial margin. In designing the scheme, we identify a tradeoff between diversity gain and the network coding advantage, which is critical to network performance in terms of throughput and fairness. To explore the tradeoff, we formulate the coding-aware assignment scheme as a mixed integer program and design a polynomial-time approximation algorithm that can be used in practical systems. We prove the asymptotic performance bound of the algorithm and demonstrate that it closely approximates the optimum under realistic experimental settings.

Network-Coding-Aware Dynamic Subcarrier Assignment in OFDMA-Based Wireless Networks

Leveraging the redundancy and parallelism from multiple RF chains, MIMO technology can easily scale wireless link capacity. However, the high power consumption and circuit-area cost prevents MIMO from being adopted by energy-constrained wireless devices. In this paper, we propose Halma, that can boost link capacity using multiple antennas but a single RF chain, thereby, consuming the same power as SISO. While modulating its normal data symbols, a Halma transmitter hops between multiple passive antennas on a per-symbol basis. The antenna hopping pattern implicitly carriers extra data, which the receiver can decode by extracting the index of the active antenna using its channel pattern as a signature. We design Halma by intercepting the antenna switching and channel estimation modules in modern wireless systems, including ZigBee and WiFi. Further, we design a model-driven antenna hopping protocol to balance a tradeoff between link quality and dissimilarity of channel signatures. Remarkably, by leveraging the inherent packet structure in ZigBee, Halma's link capacity can scale well with the number of antennas. Using the WARP software radio, we have implemented Halma along with a ZigBee- and WiFi-based PHY layer. Our experiments demonstrate that Halma can improve ZigBee's throughput and energy efficiency by multiple folds under realistic network settings. For WiFi, it consumes similar power as SISO, but boosts throughput across a wide range of link conditions and modulation levels.

Bringing multi-antenna gain to energy-constrained wireless devices

Smart wristband has become a dominant device in the wearable ecosystem, providing versatile functions such as fitness tracking, mobile payment, and transport ticketing. However, the small form-factor, low-profile hardware interfaces and computational resources limit their capabilities in security checking. Many wristband devices have recently witnessed alarming vulnerabilities, e.g., personal data leakage and payment fraud, due to the lack of authentication and access control. To fill this gap, we propose a secure text pin input system, namely Taprint, which extends a virtual number pad on the back of a user's hand. Taprint builds on the key observation that the hand “landmarks”, especially finger knuckles, bear unique vibration characteristics when being tapped by the user herself. It thus uses the tapping vibrometry as biometrics to authenticate the user, while distinguishing the tapping locations. Taprint reuses the inertial measurement unit in the wristband, “overclocks” its sampling rate with the cubic spline interpolation to extrapolate fine-grained features, and further refines the features to enhance the uniqueness and reliability. Extensive experiments on 128 users demonstrate that Taprint achieves a high accuracy (96 percent) of keystrokes recognition. It can authenticate users, even through a single-tap, at extremely low error rate (2.2 percent), and under various practical usage disturbances.

Power Saving and Secure Text Input for Commodity Smart Watches

An unobtrusive and continuous estimation of breathing volume could play a vital role in health care, such as for critically ill patients, neonatal ventilation, post-operative monitoring, just to name a few. While radar-based estimation of breathing rate has been discussed in the literature, estimating breathing volume using wireless signal remains relatively intact. With the presence of patient body movement and posture changes, long-term monitoring of breathing volume at fine granularity is even more challenging. In this work, we propose for the first time an autonomous system that monitors a patient's breathing volume with high resolution. We discuss the key research components and challenges in realizing the system. We also present an initial system design encompassing a continuous wave radar, motion tracking and control system, and a set of methods to accurately derive breathing volume from the reflected signal and to address challenges caused by body movement and posture changes. Our implementation shows promising results in estimating breathing volume with fine granularity.

Poster: Continuous and Fine-grained Respiration Volume Monitoring Using Continuous Wave Radar

Mobile virtual reality (VR) headsets, such as Google Cardboard and Samsung GearVR, can reuse a smartphone as near-eye display to create immersive experience. But such devices barely support any user interaction, even for simple tasks such as menu selection and single-character input. In this paper, we design Inkput, a simple passive interface attached to the unexploited backside of the headset to enable touch sensing. Inkput is a piece of paper substrate with carbon ink patterns printed atop. It leverages the column of electrodes near the edge of the smartphone touchscreen to sense multi-touch on the 2D space, and is even able to locate finger hovering. Our experiments demonstrate that Inkput can precisely detect touch positions with mm-level precision. Our case studies in actual VR applications also verify that Inkput can support common VR interactions and can even outperform high-end handheld controllers in terms of efficiency.

Xinyu Zhang

Papers

Network-Coding-Aware Dynamic Subcarrier Assignment in OFDMA-Based Wireless Networks

Bringing multi-antenna gain to energy-constrained wireless devices

Power Saving and Secure Text Input for Commodity Smart Watches

Poster: Continuous and Fine-grained Respiration Volume Monitoring Using Continuous Wave Radar

Conductive Inkjet Printed Passive 2D TrackPad for VR Interaction