There has been burgeoning interest in wireless technologies that can use wider frequency spectrum. Technology advances, such as 802.11n and ultra-wideband (UWB), are pushing toward wider frequency bands. The analog-to-digital TV transition has made 100-250 MHz of digital whitespace bandwidth available for unlicensed access. Also, recent work on WiFi networks has advocated discarding the notion of channelization and allowing all nodes to access the wide 802.11 spectrum in order to improve load balancing. This shift towards wider bands presents an opportunity to exploit frequency diversity. Specifically, frequencies that are far from each other in the spectrum have significantly different SNRs, and good frequencies differ across sender-receiver pairs. This paper presents FARA, a combined frequency-aware rate adaptation and MAC protocol. FARA makes three departures from conventional wireless network design: First, it presents a scheme to robustly compute per-frequency SNRs using normal data transmissions. Second, instead of using one bit rate per link, it enables a sender to adapt the bitrate independently across frequencies based on these per-frequency SNRs. Third, in contrast to traditional frequency-oblivious MAC protocols, it introduces a MAC protocol that allocates to a sender-receiver pair the frequencies that work best for that pair. We have implemented FARA in FPGA on a wideband 802.11-compatible radio platform. Our experiments reveal that FARA provides a 3.1 × throughput improvement in comparison to frequency-oblivious systems that occupy the same spectrum.

Co-primary authors This paper discusses the design of a single channel full-duplex wireless transceiver. The design uses a combination of RF and baseband techniques to achieve full-duplexing with minimal effect on link reliability. Experiments on real nodes show the fullduplex prototype achieves median performance that is within 8% of an ideal full-duplexing system. This paper presents Antenna Cancellation, a novel technique for self-interference cancellation. In conjunction with existing RF interference cancellation and digital baseband interference cancellation, antenna cancellation achieves the amount of self-interference cancellation required for full-duplex operation. The paper also discusses potential MAC and network gains with full-duplexing. It suggests ways in which a full-duplex system can solve some important problems with existing wireless systems including hidden terminals, loss of throughput due to congestion, and large end-to-end delays.

RSSI is known to be a fickle indicator of whether a wireless link will work, for many reasons. This greatly complicates operation because it requires testing and adaptation to find the best rate, transmit power or other parameter that is tuned to boost performance. We show that, for the first time, wireless packet delivery can be accurately predicted for commodity 802.11 NICs from only the channel measurements that they provide. Our model uses 802.11n Channel State Information measurements as input to an OFDM receiver model we develop by using the concept of effective SNR. It is simple, easy to deploy, broadly useful, and accurate. It makes packet delivery predictions for 802.11a/g SISO rates and 802.11n MIMO rates, plus choices of transmit power and antennas. We report testbed experiments that show narrow transition regions (<2 dB for most links) similar to the near-ideal case of narrowband, frequency-flat channels. Unlike RSSI, this lets us predict the highest rate that will work for a link, trim transmit power, and more. We use trace-driven simulation to show that our rate prediction is as good as the best rate adaptation algorithms for 802.11a/g, even over dynamic channels, and extends this good performance to 802.11n.

This paper proposes to exploit physical layer information towards improved rate selection in wireless networks. While existing schemes pick good transmission rates, this paper takes a step further towards computing the optimal bit rate. The main idea is to capture the channel behavior through symbol level dispersions, and “replay” these dispersions on different rate encodings of the same packet. The “replay ” action can be emulated at the receiver without requiring the transmitter to send the packet at every other rate. The maximum successful rate is likely to be the optimal rate of the received packet, and assuming that the channel remains coherent, the same rate can be prescribed for the next transmission. We design, implement, and evaluate this idea over a small testbed of USRP hardware and GNURadio software. Our proposal, called AccuRate, predicts a packet’s optimal rate 95 % of times when the packet is received correctly. When the packet is received in error, AccuRate computes its optimal rate with 93 % accuracy. In terms of throughput, we show that AccuRate improves over the state-of-the-art scheme SoftRate by around 10%, and is reasonably close to the optimal. 1

With the proliferation of mobile wireless devices such as smartphones and tablets that are used in a wide range of locations and movement conditions, it has become important for wireless protocols to adapt to different settings over short periods of time. Network protocols that perform well in static settings where channel conditions are relatively stable tend to perform poorly in mobile settings where channel conditions change rapidly, and vice versa. To adapt to the conditions under which communication is occurring, we propose the use of external sensor hints to augment network protocols. Commodity smartphones and tablet devices come equipped with a variety of sensors, including GPS, accelerometers, magnetic compasses, and gyroscopes, which can provide hints about the device’s mobility state and its operating environment. We present a wireless protocol architecture that integrates sensor hints in adaptation algorithms. We validate the idea and architecture by implementing and evaluating sensor-augmented wireless protocols for bit rate adaptation, access point association, neighbor maintenance in mobile mesh networks, and path selection in vehicular networks. 1

Carrier sense is often used to regulate concurrency in wireless medium access control (MAC) protocols, balancing interference protection and spatial reuse. Carrier sense is known to be imperfect, and many improved techniques have been proposed. Is the search for a replacement justified? This paper presents a theoretical model for average case two-sender carrier sense based on radio propagation theory and Shannon capacity. Analysis using the model shows that carrier sense performance is surprisingly close to optimal for radios with adaptive bitrate. The model suggests that hidden and exposed terminals usually cause modest reductions in throughput rather than dramatic decreases. Finally, it is possible to choose a fixed sense threshold which performs well across a wide range of scenarios, in large part due to the role of the noise floor. Experimental results from an indoor 802.11 testbed support these claims.

Commodity smartphones and tablet devices now come equipped with a variety of sensors, including accelerometers, multiple positioning sensors, magnetic compasses, and inertial sensors (gyros). In this paper, we posit that these sensors can be profitably used to improve the performance of wireless network protocols running on these mobile devices, and introduce the idea of using external sensor hints for this purpose. We focus on mobility hints, including the device’s state of motion, speed, direction of movement, and position. We outline how these hints can be used to: increase throughput by adapting bit rate selection to the state of movement; reduce the bandwidth required for estimating link delivery probabilities; improve the connectivity of routes in vehicular mesh networks using directionality hints; and enable access points to tailor the management of clients to their mobility.

Diversity is an intrinsic property of wireless networks. Recent years have witnessed the emergence of many distributed protocols like ExOR, MORE, SOAR, SOFT, and MIXIT that exploit receiver diversity in 802.11-like networks. In contrast, the dual of receiver diversity, sender diversity, has remained largely elusive to such networks. This paper presents SourceSync, a distributed architecture for harnessing sender diversity. SourceSync enables concurrent senders to synchronize their transmissions to symbol boundaries, and cooperate to forward packets at higher data rates than they could have achieved by transmitting separately. The paper shows that SourceSync improves the performance of opportunistic routing protocols. Specifically, SourceSync allows all nodes that overhear a packet in a wireless mesh to simultaneously transmit it to their nexthops, in contrast to existing opportunistic routing protocols that are forced to pick a single forwarder from among the overhearing nodes. Such simultaneous transmission reduces bit errors and improves throughput. The paper also shows that SourceSync increases the throughput of 802.11 last hop diversity protocols by allowing multiple APs to transmit simultaneously to a client, thereby harnessing sender diversity. We have implemented SourceSync on the FPGA of an 802.11-like radio platform. We have also evaluated our system in an indoor wireless testbed, empirically showing its benefits.

Backscatter communication offers an ultra-low power alternative to active radios in urban sensing deployments — communication is powered by a reader, thereby making it virtually “free”. While backscatter communication has largely been used for extremely small amounts of data transfer (e.g. a 12 byte EPC identifier from an RFID tag), sensors need to use backscatter for continuous and high-volume sensor data transfer. To address this need, we describe a novel link layer that exploits unique characteristics of backscatter communication to optimize throughput. Our system offers several optimizations including 1) understanding of multi-path self-interference characteristics and link metrics that capture these characteristics, 2) design of novel mobility-aware probing techniques that use backscatter link signatures to determine when to probe the channel, 3) bitrate selection algorithms that use link metrics to determine the optimal bitrate, and 4) channel selection mechanism that optimize throughput while remaining compliant within FCC regulations. Our results show upto 3 × increase in goodput over other mechanisms across a wide range of channel conditions, scales, and mobility scenarios.

This paper presents the design, implementation and evaluation of Strider, a system that automatically achieves almost the optimal rate adaptation without incurring any overhead. The key component in Strider is a novel code that has two important properties: it is rateless and collision-resilient. First, in time-varying wireless channels, Strider’s rateless code allows a sender to effectively achieve almost the optimal bitrate, without knowing how the channel state varies. Second, Strider’s collision-resilient code allows a receiver to decode both packets from collisions, and achieves the same throughput as the collision-free scheduler. We show via theoretical analysis that Strider achieves Shannon capacity for Gaussian channels, and our empirical evaluation shows that Strider outperforms SoftRate, a state of the art rate adaptation technique by 70 % in mobile scenarios and by upto 2.8 × in contention scenarios.