Aakanksha Chowdhery

The Design and Implementation of a Wireless Video Surveillance System

Internet-enabled cameras pervade daily life, generating a huge amount of data, but most of the video they generate is transmitted over wires and analyzed offline with a human in the loop. The ubiquity of cameras limits the amount of video that can be sent to the cloud, especially on wireless networks where capacity is at a premium. In this paper, we present Vigil, a real-time distributed wireless surveillance system that leverages edge computing to support real-time tracking and surveillance in enterprise campuses, retail stores, and across smart cities. Vigil intelligently partitions video processing between edge computing nodes co-located with cameras and the cloud to save wireless capacity, which can then be dedicated to Wi-Fi hotspots, offsetting their cost. Novel video frame prioritization and traffic scheduling algorithms further optimize Vigils bandwidth utilization. We have deployed Vigil across three sites in both whitespace and Wi-Fi networks. Depending on the level of activity in the scene, experimental results show that Vigil allows a video surveillance system to support a geographical area of coverage between five and 200 times greater than an approach that simply streams video over the wireless network. For a fixed region of coverage and bandwidth, Vigil outperforms the default equal throughput allocation strategy of Wi-Fi by delivering up to 25% more objects relevant to a users query.

Greener Copper with Dynamic Spectrum Management

Power-saving benefits of Dynamic Spectrum Management (DSM) result from its increased robustness to channel and noise changes, as in this paper. A proposed intermittent noise model enables evaluation of the performance of DSM algorithms. Polite Level-1 DSM algorithm consumes much less power than non-polite DSL management algorithms in the presence of intermittent noise. Secondly, a proposed robust level- 2 DSM algorithm compares favorably with methods that change the PSDMASK only upon modem initialization or retraining. The proposed robust level-2 DSM algorithm significantly reduces power consumption. This DSM reduction of DSL transmit power can be even more significant when considering the additional loss of robustness and stability of non-DSM methods.

Cooperative Wireless Multicell OFDMA Network with Backhaul Capacity Constraints

This papers considers the downlink of a wireless multicell downlink orthogonal frequency division multiple-access (OFDMA) system where neighboring base-stations (BSs) can jointly encode and transmit data signals to their users using zero-forcing (ZF) precoding in each frequency tone, but only a limited backhaul-capacity is available for each BS to share its users data streams with the neighboring BSs. A numerical algorithm is proposed to maximize the network-wide utility of this system subject to backhaul capacity constraints. The proposed algorithm first selects a subset of frequency tones for each BS to share its users data streams with the neighboring BSs and then, jointly schedules users and adapts the precoding coefficients and the power spectra of the BSs to effectively utilize the limited backhaul capacity. Numerical results show that using the proposed algorithm, the gain in downlink sum-rate per cell can be made to scale linearly with the available backhaul capacity per BS between the no-cooperation and the full-cooperation scenarios.

Dynamic cooperation link selection for network MIMO systems with limited backhaul capacity

Base-station (BS) cooperation in wireless cellular networks offers a promising approach for interference mitigation. However, the implementation of practical network multi-input multi-output (MIMO) system also faces the challenge of high capacity cost for sharing the user data over the backhaul connections. This paper considers a downlink multi-cell orthogonal frequency-division multiple-access (OFDMA) network where the capacities of the backhaul links between the BSs are limited, and extends the single-antenna BS multi-cell system model considered in our previous work to the multiple-antenna BS case. The BSs use zero-forcing precoding to spatially multiplex multiple users within each cell and to pre-subtract the interference from cooperating BSs that share user data with them. An iterative algorithm that maximizes the downlink network utility is proposed. The algorithm iteratively selects the cooperation links, schedules the users, and optimizes the precoding coefficients and the power spectra for each frequency tone. Numerical results suggest that the use of dynamic cooperation link selection can provide a better trade-off between the downlink sum-rate gain and the backhaul capacity than the earlier fixed link-selection algorithm.

Optasia: A Relational Platform for Efficient Large-Scale Video Analytics

Camera deployments are ubiquitous, but existing methods to analyze video feeds do not scale and are error-prone. We describe Optasia, a dataflow system that employs relational query optimization to efficiently process queries on video feeds from many cameras. Key gains of Optasia result from modularizing vision pipelines in such a manner that relational query optimization can be applied. Specifically, Optasia can (i) de-duplicate the work of common modules, (ii) auto-parallelize the query plans based on the video input size, number of cameras and operation complexity, (iii) offers chunk-level parallelism that allows multiple tasks to process the feed of a single camera. Evaluation on traffic videos from a large city on complex vision queries shows high accuracy with many fold improvements in query completion time and resource usage relative to existing systems.

Linear detection for the nonorthogonal amplify and forward protocol

In this paper, we present three new detection schemes for the nonorthogonal amplify and forward (NAF) protocol in a cooperative diversity system based on a multiuser detection approach, namely, a channel inversion type detector, a maximal-ratio combining (MRC) type detector, and a biased maximum likelihood (ML) detector. The decision variables in these schemes are linear functionals of the transmitted symbol vector and therefore they are termed as ldquolinearrdquo detectors. A cooperative relay network using the NAF protocol with N - 1 relays is considered where the source transmits symbols in N - 1 blocks of two time slots in a Rayleigh fading environment. For the case of a general linear space-time block coding scheme using K symbols, we present the performance of this protocol in terms of the code symbol error probability when the symbols come from an M-ary phase-shift keying constellation. The approach is further extended to the transmission in N - 1 blocks of L time slots. Numerical results show that the performance of the MRC type detector is superior to that of the channel inversion type detector, while the biased ML detector has the best performance. However, the MRC type detector has the lowest complexity while the biased ML detector the highest.

A Polite Cross-Layer Protocol for Contention-Based Home Power-Line Communications

In typical home power-line communication (PLC) networks using contention-based access methods, providing Quality-of-service (QoS) to high-priority users often comes at the expense of reducing the throughput of low-priority users. This paper proposes a cross-layer protocol which involves interaction between the Physical (PHY) and the Medium-access-control (MAC) layers, for ensuring politeness of the high-priority users toward the low-priority users for uplink transmission in home PLC networks. This protocol modifies the contention-based CSMA protocol of the MAC layer to exploit the cyclostationarity of the noise in home PLC networks. The PLC noise spectrum has been shown in literature to be periodic with the period of AC line cycle. Using this periodicity, the proposed protocol allows longer medium-access times for low-priority users in every AC line cycle, while meeting the high throughput requirements of the high-priority users. The proposed cross-layer protocol, termed Opportunistic CSMA, improves the throughput of the low-priority users by as much as 300% compared to the current CSMA protocols in home PLC networks.

Compatibility of vectored and non-vectored VDSL2

Vectoring cancels the crosstalk between multiple VDSL2 lines and can greatly improve performance, particularly on short loops. Crosstalk cancellation can only be performed on lines within a vector group and vectored lines may experience uncancelled crosstalk from nearby lines that are non-vectored or in a separate vector group. This paper shows that a substantial consensus exists on the possibility of mitigating the impact of this uncancelled crosstalk on the vectored lines. Among the possible mitigation techniques, we show here that very good levels of compatibility between vectored and non-vectored VDSL2 can be achieved with Dynamic Spectrum Management (DSM]).

A centralized multi-level water-filling algorithm for Dynamic Spectrum Management

In digital-subscriber-line (DSL) networks, the interference, or crosstalk, between multiple DSL lines can severely limit the data-rate of the lines. Level-2 Dynamic Spectrum Management (DSM) optimizes the transmit spectra of multiple DSL lines to mitigate such mutual interference. This paper proposes a centralized multi-level water-filling (MLWF) Level-2 DSM algorithm for optimizing the transmit spectra for multiple DSL lines at a Spectrum Management Center (SMC). The algorithm is a practical approximation to the more complex Optimal Spectrum Balancing (OSB), which is known to present computational issues in practice. The intuition behind the algorithm is that a DSL user should first try to use frequency bands where less interference is emitted unless its target data rate cannot be met. The frequency bands are characterized by one or more cut-off frequencies that separate the bands. MLWF uses a centralized gradient-descent algorithm to search for such cut-off frequencies. After obtaining the cut-off frequencies, MLWF first uses a water-filling algorithm to compute the power spectrum density (PSD) of each user, and then moves bits from the users high-interference band to its low-interference bands to minimize its crosstalk to the adjacent DSL lines. The PSDs obtained by MLWF are subsequently passed to each DSL user as PSDMASKs for transmit spectra shaping. Simulations showed that MLWF achieves close to optimal performance with significantly less computational complexity and much faster convergence.

TxMiner: Identifying transmitters in real-world spectrum measurements

Knowledge about active radio transmitters is critical for multiple applications: spectrum regulators can use this information to assign spectrum, licensees can identify spectrum usage patterns and better provision their future needs, and dynamic spectrum access applications can leverage such knowledge to pick operating frequency. Despite the importance of transmitter identification the current work in this space is limited and requires prior knowledge of transmitter signatures to identify active radio transmitters. More naive approaches are limited to detecting power levels and do not identify characteristics of the active transmitter. To address these challenges we propose TxMiner; a system that identifies transmitters from raw spectrum measurements without prior knowledge of transmitter signatures. TxMiner harnesses the observation that wireless signal fading follows a Rayleigh distribution and applies a novel machine learning algorithm to mine transmitters. We evaluate TxMiner on real-world spectrum measurements between 30MHz and 6GHz. The evaluation results show that TxMiner identifies transmitters robustly. We then make use of TxMiner to map the number of active transmitters and their frequency and temporal characteristics over 30MHz-6GHz, we detect rogue transmitters and identify opportunities for dynamic spectrum access.