Precise localization encompassing diverse indoor spaces is the key to immersive interaction services. In practice, indoor localization often undergoes blind spots as RF is easily blocked by everyday objects ranging from concrete walls, metallic shelves, and partitions to electronics and appliances. This paper presents SuperSight, an NLOS localization for mmWave backscatter that, for the first time, achieves NLOS (non-penetrable) localization without multipath environment profiling/manipulation. The key insight of SuperSight is uniquely exploiting the mmWave features of highly directional and specularly reflected multipath in combination with the triangular tag array to yield sub-cm NLOS localization accuracy over 8 m range - an order of magnitude performance enhancement compared to the competitors. Circularly polarized, 77GHz retro-reflective tag ensures high precision and robustness across diverse reflectors and tag orientations. The prototype was evaluated across six different reflector materials (including metal, concrete, and plaster) and demonstrated in the corridor and office space to reveal x, y, z position accuracy of up to (metal reflector) 5.7 mm, 5.5 mm, 7.7 mm at 8 m range, with Yaw, Pitch, Roll accuracy of 0.22, 0.28, 0.1 degrees respectively.

Despite the potential of vision-based personal monitoring (e.g., healthcare), private data leakage concerns hinder its wide deployment in personal spaces (e.g., bedrooms). A body of data anonymization designs was proposed throughout image processing and federated learning. They commonly store high-quality images and videos locally, which are anonymized via post-processing before cloud upload. However, the recent IoT camera hacking and local data leakage call for anonymized data at the sensing stage. Also, continuous and pervasive monitoring without blind spots in complicated indoor spaces requires a scalable and economic system. This paper present Mosaic, a vision-based end-to-end action recognition framework that (i) intrinsically achieves data anonymity from the sensing stage and (ii) battery-free operation for blind spot-free continuous monitoring. Mosaic leverages an extremely low resolution (eLR) Near-Infrared (NIR) image sensor with 6 × 10 pixels for video anonymity and RFID-compliant fully-passive tag with four solar cells for real-time eLR video streaming under as low as 50 lux (e.g., deep in the shelf without direct light). This is accompanied by light-weight action recognition neural network for real-time inference (18.4ms on Intel(R) Core i7-8700). Mosaic achieves an average of 98% accuracy on 10 action classes, hitting the balance between data anonymity and high-precision action recognition. By taking advantage of NIR (non-visible) frequency, Mosaic also works in dark without disturbing sleep. Lastly, wildfire detection reaching 20m was demonstrated, showcasing the potential for outdoor monitoring.

Accurate and scalable localization is one of the keys to pervasive interaction with the Internet of Things. mmWave backscatter possesses great potential toward this goal - The abundant bandwidth of mmWave enables high-precision localization, and low-cost and ultra low-power backscatter tags enable massive deployment with minimum deployment cost and maintenance efforts. We present Hawkeye, a new mmWave backscatter that offers (i) submillimeter localization accuracy (ii) at over 2 m range, (iii) while consuming 2.25 μW power. At the heart of our design is the new Hawkeye super-resolution, which exploits the interplay between the tag FSK and FMCW radar to improve the localization performance by ×60 over conventional FMCW radar (i.e., c/2BW). Hawkeye readers were implemented on commodity 77 GHz radars and the tags were prototyped on PCB.

Accurate localization of a large number of objects over a wide area is one of the keys to the pervasive interaction with the Internet of Things. This paper presents Hawkeye, a new mmWave backscatter that, for the first time, offers over (i) hundred-scale simultaneous 3D localization at (ii) subcentimeter accuracy for over an (iii) hectometer distance. Hawkeye generally applies to indoors and outdoors as well as under mobility. Hawkeye tag's Van Atta array design with retro-reflectivity in both elevation and azimuth planes offers 3D localization and effectively suppresses the multipath. Hawkeye localization algorithm is a lightweight signal processing compatible with the commodity FMCW radar. It uniquely leverages the interplay between the tag signal and clutter, and leverages the spectral leakage for fine-grained positioning. Prototype evaluations in corridor, lecture room, and soccer field reveal 7 mm median accuracy at 160 m range, and simultaneously localize 100 tags in only 33.2 ms. Hawkeye is reliable under temperature change with significant oscillator frequency offset. Demo video: https://tinyurl.com/4zkwxatu

Traffic from interactive applications demanding low latency has become dominant in cellular networks. However, existing schedulers of cellular network base stations fall short in delivering low latency when prior information (i.e., dedicated Quality of Service (QoS)) is unavailable; they become service agnostic and perform towards maximizing the radio resource utilization or user fairness. We identify a new opportunity of providing a better latency for those latency-sensitive traffic flows by additionally taking the Flow Completion Time (FCT) into account in downlink scheduling at the base stations. However, the key challenges are 1) it can bring a severe cost in optimization metrics of the existing scheduler and 2) it should work without prior knowledge of the traffic.

Massive connectivity is a key to the success of the Internet of Things. While mmWave backscatter has great potential, substantial signal attenuation and overwhelming ambient reflections impose significant challenges. We present OmniScatter, a practical mmWave backscatter with an extreme sensitivity of -115 dBm. The performance is theoretically comparable to the popular commodity RFID EPC Gen2 (900 MHz), and is empirically validated via evaluations under various practical settings with abundant ambient reflections and blockages - e.g., In an office where a tag is locked in a wooden closet 6m away, as well in libraries and retail stores where a tag is placed across two rows of metal shelves. At the heart of OmniScatter is the new High Definition FMCW (HD-FMCW), which interplays with the tag (FSK) signal to disentangle the ambient reflections from the tag signal in the frequency domain, essentially offering immunity to ambient reflections. To further support practical deployment, OmniScatter offers coordination-free Frequency Division Multiple Access (FDMA) that effortlessly scales to thousands of concurrent tags. The readers were built on commodity radars and the tags were prototyped on PCB. The trace-driven evaluation demonstrates concurrent communication of 1100 tags with the BER < 1.5%, paving a pathway towards practical mmWave backscatter for everyday and anywhere use.

Recent research on physical layer cross technology communication (PHY-CTC) brings a timely answer for escalated wireless coexistence and open spectrum movement. PHY-CTC achieves direct communication among heterogeneous wireless technologies (e.g.,WiFi, Bluetooth, and ZigBee) in physical layer and thus brings communication support for coexistence service such as spectrum management and IoT device control. To put PHY-CTC into service, however, there still exists a gap due to its transmission failure and asymmetric link (i.e., one-way PHY-CTC) issues. In this paper, we propose NetCTC – the first networking support design for PHY-CTC to establish feedbacks (e.g., ACKs) and thus meet the upper layer networking requirements in heterogeneous unicast, multicast and broadcast. The core design of NetCTC is a real-time interaction mechanism which achieves reliable, transmission efficient and concurrent interactive communication among heterogeneous devices. We implement and evaluate NetCTC on commodity devices and the USRP-N210 platform. Our extensive evaluation demonstrates that NetCTC achieves reliable bidirectional cross technology communication under a full range of wireless configurations including stationary, mobile and duty-cycled settings.

Overcrowded wireless devices in unlicensed bands compete for spectrum access, generating excessive cross-technology interference (CTI), which has become a major source of performance degradation especially for low-power IoT (e.g., ZigBee) networks. This paper presents a new forward error correction (FEC) mechanism to alleviate CTI, named SafetyNet. Designed for ZigBee, SafetyNet is inspired by the observation that ZigBee is overly robust for environment noises, but insufficiently protected from high-power CTI. By effectively embedding correction code bits into the PHY layer SafetyNet significantly enhances CTI robustness without compromising noise resilience. SafetyNet additionally offers a set of unique features including (i) transparency, making it compatible with millions of readily-deployed ZigBee devices and (ii) zero additional cost on energy and spectrum, as it does not increase the frame length. Such features not only differentiate SafetyNet from known FEC techniques (e.g., Hamming and Reed-Solomon), but also uniquely position it to be critically beneficial for today's crowded wireless environment. Our extensive evaluation on physical testbeds shows that SafetyNet significantly improves ZigBee's CTI robustness under a wide range of networking settings, where it corrects 55% of the corrupted packets.

With the explosive increase in wireless devices, physical-layer signal analysis has become critically beneficial across distinctive domains including interference minimization in network planning, security and privacy (e.g., drone and spycam detection), and mobile health with remote sensing. While SDR is known to be highly effective in realizing such services, they are rarely deployed or used by the end-users due to the costly hardware ~1K USD (e.g., USRP). Low-cost SDRs (e.g., RTL-SDR) are available, but their bandwidth is limited to 2-3 MHz and operation range falls well below 2.4 GHz - the unlicensed band holding majority of the wireless devices. This paper presents SDR-Lite, the first zero-cost, software-only software defined radio (SDR) receiver that empowers commodity WiFi to retrieve the In-phase and Quadrature of an ambient signal. With the full compatibility to pervasively-deployed WiFi infrastructure (without any change to the hardware and firmware), SDR-Lite aims to spread the blessing of SDR receiver functionalities to billions of WiFi users and households to enhance our everyday lives. The key idea of SDR-Lite is to trick WiFi to begin packet reception (i.e., the decoding process) when the packet is absent, so that it accepts ambient signals in the air and outputs corresponding bits. The bits are then reconstructed to the original physical-layer waveform, on which diverse SDR applications are performed. Our comprehensive evaluation shows that the reconstructed signal closely reassembles the original ambient signal (>85% correlation). We extensively demonstrate SDR-Lite effectiveness across seven distinctive SDR receiver applications under three representative categories: (i) RF fingerprinting, (ii) spectrum monitoring, and (iii) (ZigBee) decoding. For instance, in security applications of drone and rogue WiFi AP detection, SDR-Lite achieves 99% and 97% accuracy, which is comparable to USRP.

This paper presents GateScatter, the first backscatter-based gateway connecting commodity IoT to WiFi. The backscatter design of GateScatter is an economic option towards pervasive Internet connectivity for ever-growing IoT. The carefully designed tag optimally reshapes ZigBee IoT packets with an arbitrary payload into an 802.11b WiFi packet over the air, such that the payload can be reliably retrieved at the WiFi receiver (hence a gateway). Gate-Scatter is highly compatible - it works with a wide range of IEEE 802.15.4-compliant systems, is agnostic to upper layer proprietary protocols, and does not require any modification to the commodity IoT platforms. GateScatter is extended to BLE IoT for generality. We prototype GateScatter hardware on FPGA where the wide applicability is demonstrated through evaluations on five popular IoT devices including Samsung SmartThings sensor, Philips smart bulb, and Amazon Echo Plus. Further extensive evaluations show that GateScatter consistently achieves throughput above 200 kbps and range of over 27 m under diverse practical scenarios including a corridor, dormitory room, and under user mobility.

Multi-user MIMO (MU-MIMO) is a widely-known, fundamental technique to significantly improve the spectrum efficiency. While there is a great demand for spectrum efficiency and massive scalability under explosively increasing IoT, hardware limitations make it particularly challenging for the mechanism to be transferred to the IoT (e.g., ZigBee) domain. This paper presents X-MIMO, a zero-cost, software-only cross-technology MU-MIMO for commodity ZigBee. As the first work to shed the light on the feasibility of MU-MIMO on commodity IoT, X-MIMO leverages on cross-technology communication (CTC) to turn the pervasively-deployed WiFi AP into MU-MIMO transmitter, delivering different packets to multiple ZigBees in parallel. X-MIMO uniquely exploits WiFi CSI to extract the accurate physical layer signal of the ZigBee packet and the WiFi-ZigBee channel coefficient. Rigorous derivation shows that X-MIMO's precoding is inherently immune to the uncertainties of the commodity devices, making X-MIMO highly reliable in practice. Lastly, spectrum-efficient emulation is proposed to maximize the spectrum reuse. We implement and comprehensively evaluate the performance of X-MIMO on commodity devices (Atheros AR9334 WiFi NIC and TelosB CC2420) as well as on USRP B210 for in-depth analysis. Results reveal that X-MIMO achieves 495 Kbps with <1% symbol error rate (SER) and 704.24 Kbps with 6.1% SER for two and three streams, respectively. Near-linear increase of the throughput effectively demonstrates the feasibility of X-MIMO.

Gas monitoring, often as a part of safety and health systems, is widely used in a variety of sectors such as manufacturing, auto- mobiles, medical, households, food and beverages, environments, and HVACs. Existing gas monitoring approaches such as the use of catalytic and infrared sensors and electrochemical and metal oxide semiconductor technologies, typically require expensive hardware or are power hungry, and thus not suitable for long-term and large scale deployments. We propose a gas intensity measuring system with cost-effective commercial off-the-shelf (COTS) RFID devices. Our key intuition is that the changes in phase and strength of the signal result from not only the signal propagation but also from the hardware (i.e., tag's circuit). To this end, we propose a method to in- tegrate RFID with a chemiresistor, called Carbon Nanotubes (CNTs), whose electrical property varies with the nearby gas concentration. Our system shows the potential of low-cost, easy-to-make, and wireless sensing based gas monitoring systems.

The cross-technology communication (CTC) is a promising technique proposed recently to bridge heterogeneous wireless technologies in the ISM bands. Existing solutions use only the coarse-grained packet-level information for CTC modulation, suffering from a low throughput (e.g., 10 b/s). Our approach, called BlueBee, explores the dense PHY-layer information for CTC by emulating legitimate ZigBee frames with the Bluetooth radio. Uniquely, BlueBee achieves dual-standard compliance and transparency for its only modifying the payload of Bluetooth frames, requiring neither hardware nor firmware changes at either the Bluetooth sender or the ZigBee receiver. Our implementation on both USRP and commodity devices shows that BlueBee can achieve standard ZigBee bit rate of 250 kb/s at more than 99% accuracy, which is over 10000 x faster than the state-of-the-art packet-level CTC technologies.

To mitigate the issue of cross-technology interference (CTI) under dense wireless, cross-technology communication (CTC) was recently proposed, which enables direct communication among heterogeneous wireless technologies. We present SymBee, a novel ZigBee to WiFi CTC with symbol-level encoding for performance breakthrough from packet-level state-of-the-arts. SymBee is uniquely built on the new insight on ZigBee-WiFi physical layer cross-observability - i.e., the output on WiFi when fed with ZigBee signal (due to frequency overlap). This is analyzed experimentally and theoretically through rigorous derivations, from which the key innovation in SymBee design, i.e., payload encoding, stems; Conveying data across technologies is as simple as putting specific symbols in ZigBee packet payload, such that they yield unique and easily detectable patterns when cross-observed at WiFi. This symbol-level encoding is fully compatible with any commodity ZigBee device. Decoding at WiFi is a light-weight function that recycles the output from idle listening, thereby minimizing the computation while keeping compatibility to WiFi standard. SymBee is extensively evaluated both theoretically and experimentally through testbed evaluations on six distinct locations including outdoor. The result demonstrate that SymBee reaches the throughput of up to 31.25kbps, 145.4× faster than the state-of-the-art.

Cross-technology Communication (CTC) is a key technique to explore the full capacity of heterogeneous wireless. The latest CTC designs explore the PHY-layer to reach the standards' maximum rate, but leaving a critical gap to practicality -- existing PHY-layer CTCs are commonly transmitter-side techniques requiring a high-end transmitter (with a high degree of freedom in signal manipulation) to emulate the receiver signal closely. This inherently limits the reverse direction (low-end to high-end) communication. We present XBee, a unique receiver-side CTC that fills in the gap and makes a critical step towards achieving CTC bidirectionality. XBee is demonstrated as a ZigBee to BLE communication, where the key innovation lies in the unique mechanism of cross-technology decoding, or cross-decoding in short, which interprets a ZigBee frame only by carefully observing the bit patterns obtained at the BLE receiver. Technically, XBee counterintuitively explores the sampling offset to overcome the intrinsic challenge due to BLE's narrower bandwidth (1MHz) than ZigBee (2MHz). Extensive implementation and evaluation on USRP and commodity devices reach 250 kbps under 85% reliability, a 15,000x improvement over state-of-the-art ZigBee to BLE communication, and comparable with the latest PHY-layer CTCs to achieve CTC bidirectionality.

Under significant coexistence in the ISM band, the impact of cross-technology interference (CTI) has become a major threat to low-power IoT. This paper presents ECC that uniquely enables explicit channel coordination among heterogeneities via cross-technology communication (CTC) introduced in the latest studies, while maintaining full compatibility to commodity devices. Unlike any implicit coordination designs adopting statistical models to probabilistically predict white spaces, ECC generates the white space using WiFi CTS, which is then explicitly notified to ZigBee through CTC for immediate use. Technical highlight of ECC lies in ensuring ZigBee communication under CTI, without disrupting WiFi operation. This is effectively achieved by the dynamic adjustment of CTS duration with respect to traffic amount and spectrum availability, which essentially enables ECC to be generally applied to various scenarios without prior knowledge. Lastly, ECC significantly reduces delay and energy in low duty cycled ZigBee, by waking them up upon channel availability (via CTC). We evaluate ECC on commercial platforms: Atheros AR2425 WiFi card and TelosB motes. Experiment results show that ECC achieves 1.8x ZigBee packet reception ratio, and cuts down delay and energy by 98.6% and 51% under the low duty cycle.

Low power IoT suffers from performance degradation due to severe cross-technology interference (CTI) such as WiFi. In this demo, we present a novel ZigBee system that effectively maintains high reliability even under saturated WiFi traffic. This is achieved by placing a ZigBee packet on the guard band of ongoing, ambient WiFi traffic. Guard band is designed to be kept clear of interference from other WiFi, thereby safeguarding the ZigBee within. Our system effectively captures WiFi (802.11b) guard band on the fly, using physical layer information accessible on commodity ZigBee RF. We demonstrate real-time guard band detection and robust ZigBee communication, showcasing a practical pathway to operating low power IoT under excessive CTI.

Cross-technology interference (CTI) from dense and prevalent wireless has become a primary threat to low-power IoT. This paper presents G-Bee, a CTI avoidance technique that uniquely places ZigBee packet on the guard band of ongoing WiFi traffic, which effectively safeguards the packet from WiFi interference. Such design ensures reliable ZigBee communication even under saturated WiFi traffic where traditional ZigBee is considered inoperable. Technical highlight is in lighweight WiFi guard band capture mechanism using ZigBee PHY layer samples directly accessible in various commercial ZigBee chip. Another exclusive feature of G-Bee is spectrum-synchronized low duty cycling - by utilizing guard bands of periodic WiFi beacons, active slots are effectively synchronized to spectrum availability (i.e., guard band) for significant delay improvement. Extensive evaluations on our prototype system demonstrates G-Bee PRR over 95% where legacy ZigBee drops to below 15% under significant interference with hundreds WiFi users and reduction of low duty cycle delay by 87.5%, all of which are achieved with a light computational overhead of 0.3%.

Recent physical-layer designs capable of decoding multi-packet collisions demonstrate great potential for improving the network performance. Current network protocols, however, tailor the traditional physical layer to avoid collisions and thus cannot fully exploit the benefits of concurrent transmission techniques. In this paper, we propose an innovative generic concurrent transmission aware routing design called mShare. The mShare design exploits the benefits of these techniques by scheduling concurrent senders to utilize co-owned receiver(s) in parallel. This design significantly increases the available number of routing choices and thus improves the network performance. To illustrate the versatility of our design, we test mShare in three settings: unicast, opportunistic routing, and data collection (convergecast). The performance of mShare is evaluated with physical testbed experiments running on USRP and simulations. The experimental results show that compared to conventional designs, mShare: 1) improves 277% of the throughput in unicast; 2) saves 78% of transmissions in opportunistic routing; and 3) reduces 70% of the delivery delay in data collection.

In wireless communication, heterogeneous technologies such as WiFi, ZigBee and BlueTooth operate in the same ISM band.With the exponential growth in the number of wireless devices, the ISM band becomes more and more crowded. These heterogeneous devices have to compete with each other to access spectrum resources, generating cross-technology interference (CTI). Since CTI may destroy wireless communication, this field is facing an urgent and challenging need to investigate spectrum efficiency under CTI. In this paper, we introduce a novel framework to address this problem from two aspects. On the one hand, from the perspective of each communication technology itself, we propose novel channel/link models to capture the channel/link status under CTI. On the other hand, we investigate spectrum efficiency from the perspective by taking all heterogeneous technologies as a whole and building crosstechnology communication among them. The capability of direct communication among heterogeneous devices brings great opportunities to harmoniously sharing the spectrum with collaboration rather than competition.

Cross-technology communication (CTC) techniques are introduced in recent literatures to explore the opportunities of collaboration between heterogeneous wireless technologies, such as WiFi and ZigBee. Their applications include context-aware services and global channel coordination. However, state-of-the-art CTC schemes either suffer from channel inefficiency, low throughput, or disruption to existing networks. This paper presents the CTC via data packets (DCTC), which takes advantage of abundant existing data packets to construct recognizable energy patterns. DCTC features (i) a significant enhancement in CTC throughput while (ii) keeping transparent to upper layer protocols and applications. Our design also features advanced functions including multiplexing to support concurrent transmissions of multiple DCTC senders and adaptive rate control according to the traffic volume. Testbed implementations across WiFi and ZigBee platforms demonstrate reliable bidirectional communication of over 95% in accuracy while achieving throughput 2.3x of the state of the art. Meanwhile, experiment results show that DCTC has little and bounded impact on the delay and throughput of original data traffic.

Recent research on CTC (cross-technology communication) demonstrates the viability of direct coordination among heterogeneous devices (e.g., WiFi and ZigBee) with incompatible physical layers. Although encouraging, current solutions suffer from either severe inefficiency in channel utilization or low throughput using limited beacons. To address these limitations, this paper presents C-Morse, which leverages all traffic (such as through data packets, beacons and other control frames) to achieve a high cross-technology communication throughput. The key idea of C-Morse is to slightly perturb the transmission timing of existing WiFi packets to construct recognizable radio energy patterns without introducing noticeable delays to upper layers. At the receiver side, ZigBee captures such patterns by sensing the RSSI value, and then decodes the transmitted symbols. C-Morse also introduces a novel timing-based multiplexing technique to allow the coexistence of multiple C-Morse access points and reject other interference, showing a reliable symbol delivery ratio. As a result, C-Morse achieves a free side-channel, whose CTC throughput is as much as 9 χ of the present state of the art, while maintaining the through traffic within a negligible delay that goes unnoticed by applications and end-users.

Cross-Technology Communication is a promising solution proposed recently to the coexistence problem of heterogeneous wireless technologies in the ISM bands. The existing works use only the coarse-grained packet-level information for cross-technology modulation, suffering from a low throughput (e.g., 10bps). Our approach, called BlueBee, proposes a new direction by emulating legitimate ZigBee frames using a Bluetooth radio. Uniquely, BlueBee achieves dual-standard compliance and transparency by selecting only the payload of Bluetooth frames, requiring neither hardware nor firmware changes at the Bluetooth senders and ZigBee receivers. Our implementation on both USRP and commodity devices shows that BlueBee can achieve a more than 99% accuracy and a throughput 10,000x faster than the state-of-the-art CTC reported so far.

Enabling direct communication between wireless technologies immediately brings significant benefits including, but not limited to, cross-technology interference mitigation and context-aware smart operation. To explore the opportunities, we propose FreeBee-a novel cross-technology communication technique for direct unicast as well as cross-technology/channel broadcast among three popular technologies of WiFi, ZigBee, and Bluetooth. The key concept of FreeBee is to modulate symbol messages by shifting the timings of periodic beacon frames already mandatory for diverse wireless standards. This keeps our design generically applicable across technologies and avoids additional bandwidth consumption (i.e., does not incur extra traffic), allowing continuous broadcast to safely reach mobile and/or duty-cycled devices. A new interval multiplexing technique is proposed to enable concurrent broadcasts from multiple senders or boost the transmission rate of a single sender. Theoretical and experimental exploration reveals that FreeBee offers a reliable symbol delivery under a second and supports mobility of 30 mph and low duty-cycle operations of under 5%.

This paper starts from an empirical observation on the existence of spatiotemporal correlation among nearby wireless links within short time intervals. The phenomenon due to correlated interference has become pervasive with densely deployed wireless devices, causing potential errors in existing popular metrics built upon the assumption of link independence. To this end, we propose correlated ETX (cETX), which generalizes the widely-adopted ETX to maintain the accuracy under correlated links. To the best of our knowledge, this is the first work to introduce a unified metric embracing both temporal and spatiotemporal correlations. As a generalized metric, the highlight of our work is the broad applicability and effectiveness-extensive evaluations on ZigBee (802.15.4) and Wi-Fi (802.11b/g/n) testbeds deployed in a lab, corridor, and on a bridge reveal that: simply replacing ETX with cETX: 1) cuts down the error by 62.1%-70.2% and 2) saves averages of 22% and 37% communication cost in three unicast and nine broadcast protocols, respectively, under only 0.7% additional overhead.

Recent research has shown that network coding has great potential to improve network performance in wireless communication. The performance of network coding in real-world scenarios, however, varies dramatically. It is reported that network coding brings negligible improvements but extra coding overhead in some scenarios. In this article, for the first time, we analyze the impact of link correlation on network coding and quantify the coding benefits. We propose correlated coding, which encodes packets only when performance improvement is achieved. Correlated coding uses only one-hop information, which makes it work in a fully distributed manner and introduces minimal communication overhead. The highlight of the design is its broad applicability and effectiveness. We implement the design with four broadcast protocols and three unicast protocols, and we evaluate them extensively on one 802.11 testbed and three 802.15.4 testbeds. The experimental results show that (i) more coding operations do not lead to fewer transmissions, and (ii) compared to existing network coding protocols, the number of transmissions is reduced with lower coding overhead.

Internet of Things (IoT) is founded on the unprecedented proliferation of heterogeneous wireless technologies. However, dense deployment of such devices intensify the interference, which has become a major cause of performance degradation. This paper first introduces schemes for harmonious coexistence. Our designs effectively enhance diverse existing protocols via a complementing middle layer that enables real-time adaptation to the spatial and temporal dynamics of interferences. Further, a technique to explore the opportunity behind heterogeneous wireless technologies is introduced. By extending direct connectivity between heterogeneous technologies, the design uniquely enables their collaboration to reach beyond the capabilities of each technology.

Recent studies on CTC (cross-technology communication) have demonstrated the possibility of building direct communication between heterogeneous wireless communication technologies, such as WiFi and ZigBee. However, current solutions suffer from significant spectrum usage or limited throughput. To address the issues, this paper presents Transparent Cross-technology Communication (TCTC), a novel CTC technique that establishes a side channel by exploiting today's abundant wireless traffic. The key idea of TCTC is to embed messages in the timings of on-going legacy traffic (e.g., HTTP packets in WiFi), with small and bounded impact on upper-layer applications. Feasibility and effectiveness of our design has been validated via test-bed implementations and experiments on bidirectional communication between WiFi and ZigBee platforms.

Recent pioneer work has shown that packet receptions on adjacent links are correlated, which contradicts the long held assumption that wireless links are statistically independent. Since wireless link correlation affects a wide range of protocol designs, it is essential to quantify the impact generically. In particular, this paper focuses on a unified transmission cost metric for diversity-based routing schemes, including opportunistic routing, network coding, and hybrid routing. This paper covers both unicast and broadcast. Compared with the legacy metrics, our metric provides a direct and accurate estimation of the transmission cost in the presence of link correlation. The new metric helps a wide range of routing algorithms determine when they can benefit from reception diversity, and how to maximize the benefit, at negligible costs. We evaluate the metric on one 802.11 test bed and three 802.15.4 test beds running TelosB, MICAz, and GreenOrbs nodes. The experimental results show that our metric 1) reduces 92% and 94% of the estimation error of the transmission cost in unicast and broadcast and 2) outperforms the link independent metric in both unicast and broadcast applications.

Contradicting the widely believed assumption of link independence, recently the phenomenon of reception correlation among nearby receivers has been revealed and exploited for varieties of protocols [3], [8], [17], [21], [23], [24]. However, despite the diversified correlation-aware designs proposed up to date, they commonly suffer from a shortcoming where link correlation is inaccurately measured, which leads them to sub-optimal performance. In this work we propose a general framework for accurate capturing of link correlation, enabling better utilization of the phenomenon for protocols lying on top of it. Our framework uses SINR (Signal to Interference plus Noise Ratio) to detect correlations, followed by modeling the correlations for in-network use. We show that our design is light-weight, both computation and storage-wise. We apply our model to opportunistic routing and network coding on a physical 802.15.4 test-bed for energy savings of 25% and 15%.

This paper presents FreeBee, which enables direct unicast as well as cross-technology/channel broadcast among three popular wireless technologies: WiFi, ZigBee, and Bluetooth. Our design aims to shed the light on the opportunities that cross-technology communication has to offer including, but not limited to, cross-technology cooperation and coordination. The key concept of FreeBee is to modulate symbol messages by shifting the timing of periodic beacon frames already mandatory for wireless standards without incurring extra traffic. Such a generic cross-technology design consumes zero additional bandwidth, allowing continuous broadcast to safely reach mobile and/or duty-cycled devices. A new \emph{interval multiplexing} technique is proposed to enable concurrent broadcasts from multiple senders or boost the transmission rate of a single sender. Theoretical and experimental exploration reveals that FreeBee offers a reliable symbol delivery under a second and supports mobility of 30mph and low duty-cycle operations of under 5%.

In this work, we experimentally observe the existence of spatiotemporal correlation among adjacent wireless links within short time intervals. Such an observation calls attention to potential errors in existing popular metrics built upon the assumption of link independence. Specifically we propose cETX (correlated ETX), a generalized metric, to compensate for estimation errors suffered by the widely-adopted ETX in the presence of correlated interference. To the best of our knowledge, this is the first work to introduce a unified metric embracing both temporal and spatiotemporal correlations. The highlight of the cETX metric is its broad applicability and effectiveness. Evaluations on ZigBee (802.15.4) and Wi-Fi (802.11b/g/n) testbeds deployed in a lab, corridor, and on a bridge reveal that: Simply replacing ETX with cETX (i) cuts down the error by 70.2% and 62.1%, respectively, and (ii) saves averages of 22% and 37% communication cost in three unicast [4, 13, 17] and nine broadcast protocols [7, 18, 23, 24, 27, 30, 40] at the price of only 0.7% additional overhead.

Recent work has shown that wireless links are not independent, and that transmissions from a transmitter to multiple receivers are correlated. This finding has profound implications for the performance of network protocols such as broadcast, multicast, opportunistic routing, and network coding. In this paper, we show how link correlation can significantly impact broadcast. We present the design and implementation of CorLayer, a general supporting layer for energy-efficient reliable broadcast that carefully blacklists certain poorly correlated wireless links. The design uses only one-hop information, which makes it work in a fully distributed manner and introduces minimal communication overhead. The highlight of our work is CorLayer's broad applicability and effectiveness. We integrate CorLayer transparently with 16 state-of-the-art broadcast protocols specified in 13 publications on three physical testbeds running TelosB, MICAz, and GreenOrbs nodes, respectively. The experimental results show that CorLayer significantly improves energy efficiency across a wide spectrum of broadcast protocols and that the total number of packet transmissions can be reduced consistently by 47% on average.

Recent empirical studies have shown clear evidence that wireless links are not independent and that the packet receptions on adjacent wireless links are correlated. This finding contradicts the widely held link-independence assumption in the calculation of the core metric, i.e., the expected number of transmissions to the candidate forwarder set, in opportunistic routing (OR). The inappropriate assumption may cause serious estimation errors in the forwarder set selection, which further leads to underutilized diversity benefits or extra scheduling costs. We thus advocate that OR should be made aware of link correlation. In this paper, we propose a novel link-correlation-aware OR scheme, which significantly improves the performance by exploiting the diverse low correlated forwarding links. We evaluate the design in a real-world setting with 24 MICAz nodes. Testbed evaluation and extensive simulation show that higher link correlation leads to fewer diversity benefits and that, with our link-correlation-aware design, the number of transmissions is reduced by 38%.

In wireless networks, duty-cycling operations have been widely used to reduce the energy cost of RF idle listening at wireless receivers. Such operations, however, introduce delays in data forwarding because a sender has to wait for a targeted receiver to wake up. To reduce end-to-end delivery delays, researchers have proposed scheduling techniques [1], [2], [3] to wake up nodes along the data forwarding path at the right moment. However, these techniques consider only one-way delivery from a sink to nodes (or vice versa), failing to optimally support round-trip network operations such as (i) query and response, (ii) command and control and (iii) data fetching. In this work, we interestingly reveal that the optimal roundtrip delay in a low-duty-cycle network depends only on (i) duty cycle period and (ii) the number of 2-connected network components between the source and destination nodes. We prove that optimality in the round-trip delay can be achieved by establishing a simple circular path and its related cords, in which nodes are assigned wake-up slots in an ascending order in each network component, and connecting these paths into a circular pipeline. We compared our Circular Pipelining (CP) algorithm with the state-of-art solutions [3], and the experimental results show that without using circular forwarding, existing solutions have a round-trip delay proportional to the network diameter, while CP remains a minimal constant delay of T as long as the network is 2-connected. We also implement the Circular Pipelining (CP) algorithm in a test bed consisting of 30 MICAz nodes, achieving significant delay reduction compared to three baseline solutions in the literature.

Diversity-based protocols such as network coding and opportunistic routing have been proposed in recent years to exploit spatial diversity in wireless communication. By utilizing concurrent links, these protocols achieve significantly better performance than traditional approaches. However, they explicitly or implicitly assume that wireless links are independent, which overestimates the true spatial diversity in reality. For the first time, this paper analyzes the impact of link correlation on network coding and introduces Correlated Coding, a link correlation-aware design that seeks to optimize the transmission efficiency by maximizing necessary coding opportunities. Correlated coding uses only one-hop information, which makes it work in a fully distributed manner and introduces minimal communication overhead. The highlight of our design is its broad applicability and effectiveness. We implement our design with four broadcast protocols and three unicast protocols, and evaluate them extensively with one 802.11 test bed and three 802.15.4 test beds running TelosB, MICAz, and Green Orbs nodes. The experiment results show that (i) more coding opportunities do not lead to more transmission benefits, and (ii) compared to coding aware protocols, the number of coding operations is reduced while the transmission efficiency is improved.

Low-duty-cycle radio operations have been proposed for wireless networks facing severe energy constraints. Despite energy savings, duty-cycling the radio creates transient-available wireless links, making communication rendezvous a challenging task under the practical issue of clock drift. To overcome limitations of prior work, this paper presents PSR, a practical design for synchronous rendezvous in low-duty-cycle wireless networks. The key idea behind PSR is to extract timing information naturally embedded in the pattern of radio duty-cycling, so that normal traffic in the network can be utilized as a “free” input for drift detection, which helps reduce (or even eliminate) the overhead of traditional time-stamp exchange with dedicated packets or bits. To prevent an overuse of such free information, leading to energy waste, an energy-driven adaptive mechanism is developed for clock calibration to balance between energy efficiency and rendezvous accuracy. PSR is evaluated with both test-bed experiments and extensive simulations, by augmenting and comparing with four different MAC protocols. Results show that PSR is practical and effective under different levels of traffic load, and can be fused with those MAC protocols to improve their energy efficiency without major change of the original designs.

Wireless communication essentially occurs in a broadcast medium with concurrent receptions. Recent works [34, 41] have shown clear evidence that wireless links are not independent and that transmissions from a transmitter to multiple receivers are correlated, a phenomenon that has profound implications for the performance of network protocols such as broadcast, multi-cast, opportunistic forwarding and network coding. In this paper, we show how link correlation can significantly impact broadcast. We present the design and implementation of CorLayer, a general supporting layer for energy efficient reliable broadcast that carefully blacklists certain poorly correlated wireless links. This method uses only one-hop information, which makes it work in a fully distributed manner and introduces minimal communication overhead. The highlight of our work is CorLayer's broad applicability and effectiveness. Our system effort is indeed significant. We integrate CorLayer transparently with sixteen state-of-the-art broadcast protocols specified in thirteen publications [1, 3, 18, 19, 23, 25--27, 32, 36, 38--40] on three physical testbeds running TelosB, MICAz, and GreenOrbs nodes, respectively. The experimental results show that CorLayer remarkably improves energy efficiency across a wide spectrum of broadcast protocols and that the total number of packet transmissions can be reduced consistently by 47% on average.

By exploiting reception diversity of wireless network links, researchers have shown that opportunistic routing can improve network performance significantly over traditional routing schemes. However, recently empirical studies indicate that we are too optimistic, i.e. diversity gain can be overestimated if we continue to assume that packet receptions of wireless links are independent events. For the first time, this paper formally analyzes the opportunistic routing gain under the presence of link correlation considering the loss of DATA and ACK packets. Based on the model, we introduce a new link-correlation-aware opportunistic routing scheme, which improves the performance by exploiting the diverse uncorrelated forwarding links. Our design is evaluated using simulation where we show (i) link correlation leads to less diversity gain, (ii) and with our link-correlation-aware design; improvement can be gained. We also provide a unique model to generate strings of randomly correlated receptions.

Flooding in low-duty-cycle wireless sensor networks is very costly due to asynchronous schedules of sensor nodes. To adapt existing flooding-tree-based designs for low-duty-cycle networks, we shall schedule nodes of common parents wake up simultaneously. Traditionally, energy optimality in a designated flooding-tree is achieved by selecting parents with the highest link quality. In this work, we demonstrate that surprisingly more energy can be saved by considering link correlation. Specifically, this work first experimentally verifies the existence of link correlation and mathematically proves that the energy consumption of broadcasting can be reduced by letting nodes with higher correlation receive packets simultaneously. A novel flooding scheme, named Correlated Flooding, is then designed so that nodes with high correlation are assigned to a common sender and their receptions of a broadcasting packet are only acknowledged by a single ACK. This unique feature effectively ameliorates the ACK implosion problem, saving energy on both data packets and ACKs. We evaluate Correlated Flooding with extensive simulations and a testbed implementation with 20 MICAz nodes. We show that Correlated Flooding saves more than 66% energy on ACKs and 15%-50% energy on data packets for most network settings, while having similar performance on flooding delay and reliability.