Spectrum & Connectivity

The center focuses on developing new solutions that enable communication-intensive applications across large numbers of wireless and mobile devices. These solutions include advanced interference management techniques, better coding and transmission schemes that increase spectral efficiency (bits/s/Hz), scalable protocols for heterogeneous radio networks, and novel protocol and software architectures, particularly cross-layer approaches that work across the traditional network layers. The research targets the licensed cellular networks, technologies operating in the unlicensed bands, emerging frequencies, and dynamic spectrum access.

Members:

David Clark
Devavrat Shah
Dina Katabi
Hari Balakrishnan
Jonathan Perry
Keith Winstein
Kermin Elliott Fleming
Peter Iannucci
Swarun Kumar
Yury Polyanskiy

Research topics


Spinal Codes Spinal codes are a new rateless error correcting code designed to efficiently overcome noise and interference plaguing today's wireless networks. Spinal codes are much faster than existing alternative codes, making it possible to provide increased wireless throughput and coverage with existing radio resources. They also work very well over small message sizes, and over highly variable channel conditions, unlike existing alternatives. They are the first rateless code to provably achieve Shannon capacity over both additive Gaussian noise (AWGN) channels and over bit-flip ("BSC") channels with an efficient encoder and decoder. An FPGA hardware implementation of the encoder and decoder achieves over 10 Mbits/s over wireless channels. Investigators: Devavrat Shah Hari Balakrishnan Topics: Spectrum & Connectivity http://nms.csail.mit.edu/spinal
SoftCast: A Cross-Layer Design for Scalable Mobile Video In broadcast and mobile scenarios the bit rate supported by the channel differs across receivers and varies quickly over time. The conventional design however forces the source to pick a single bit rate and degrades sharply when the channel cannot not support the chosen bit rate. SoftCast is a clean-slate design for wireless video where the source transmits one video stream that each receiver decodes to a video quality commensurate with its speciﬁc instantaneous channel quality. To do so, SoftCast ensures the samples of the digital video signal transmitted on the channel are linearly related to the pixels' luminance. Thus, when channel noise perturbs the transmitted signal samples, the perturbation naturally translates into approximation in the original video pixels. Hence, a receiver with a good channel (low noise) obtains a high ﬁdelity video, and a receiver with a bad channel (high noise) obtains a low ﬁdelity video. Investigators: Dina Katabi Topics: Mobile Applications Spectrum & Connectivity http://people.csail.mit.edu/szym/softcast/videos.html
Efficient and Reliable Backscatter Networks There is a long-standing vision of embedding backscatter nodes like RFIDs into everyday objects to build ultralow power ubiquitous networks. A major problem that has challenged this vision is that backscatter communication is neither reliable nor efficient. Backscatter nodes cannot sense each other, and hence tend to suffer from colliding transmissions. Further, they are ineffective at adapting the bit rate to channel conditions, and thus miss opportunities to increase throughput, or transmit above capacity causing errors. This paper introduces a new approach to backscatter communication. The key idea is to treat all nodes as if they were a single virtual sender. One can then view collisions as a code across the bits transmitted by the nodes. By ensuring only a few nodes collide at any time, we make collisions act as a sparse code and decode them using a new customized compressive sensing algorithm. Further, we can make these collisions act as a rateless code to automatically adapt the bit rate to channel quality –i.e., nodes can keep colliding until the base station has collected enough collisions to decode. Results from a network of backscatter nodes communicating with a USRP backscatter base station demonstrate that the new design produces a 3.5× throughput gain, and due to its rateless code, reduces message loss rate in challenging scenarios from 50% to zero. Investigators: Dina Katabi Topics: Low-Power Systems Mobile Applications Spectrum & Connectivity
TIMO : Making 802.11n Robust to Cross-Technology Interference Dina Katabi Cross-technology interference is emerging as a major problem for 802.11 networks. Independent studies in 2010 by the Farpoint Group, BandSpeed, and Miercom all show that highpower interferers like baby monitors and cordless phones can cause 802.11n networks to experience a complete loss of connectivity. Other studies from Ofcom, Jupiter Research, and Cisco report that such interferers are responsible for more than half of the problems reported in customer networks.
802.11n+ : Random Access Heterogeneous MIMO Networks Dina Katabi This project presents the design and implementation of 802.11n+, a fully distributed random access protocol for MIMO networks. 802.11n+ allows nodes that differ in the number of antennas to contend not just for time, but also for the degrees of freedom provided by multiple antennas. We show that even when the medium is already occupied by some nodes, nodes with more antennas can transmit concurrently without harming the ongoing transmissions. Furthermore, such nodes can contend for the medium in a fully distributed way.
MegaMIMO: Scaling Wireless Capacity with User Demands We present joint multi-user beamforming (JMB), a system that enables independent access points (APs) to beamform their signals, and communicate with their clients on the same channel as if they were one large MIMO transmitter. The key enabling technology behind JMB is a new low-overhead technique for synchronizing the phase of multiple transmitters in a distributed manner. The design allows a wireless LAN to scale its throughput by continually adding more APs on the same channel. JMB is implemented and tested with both software radio clients and off-the-shelf 802.11n cards, and evaluated in a dense congested deployment resembling a conference room. Results from a 10-AP software-radio testbed show a linear increase in network throughput with a median gain of 8.1 to 9.4. Our results also demonstrate that JMB’s joint multi-user beamforming can provide throughput gains with unmodified 802.11n cards. Investigators: Dina Katabi Topics: Mobile Applications Spectrum & Connectivity