Traditionally, there has been an isolation of responsibilities of physical layer (PHY) and higher layers of the network stack leading to severe inefficiencies in bandwidth-limited wireless networks. Remedying these inefficiencies requires rethinking and redesigning wireless protocols such that higher layers and PHY work in synergy. This project explores three schemes under the PHY-Informed Networking (PHY-IN) framework. 1) Carrier Sense Multiple Access with Collision Notification (CSMA/CN) mechanism detects and aborts collisions, breaking away from the existing collision avoidance (CSMA/CA) based schemes such as 802.11. 2) Constellation BAsed Rate (CBAR) adaptation scheme jumps to the best feasible rate suitable for a varying channel. 3) Remap scheme permutes bit-to-subcarrier assignment for a retransmission to improve the chances of its success. The PHY-IN framework has the potential to impact the landscape of future wireless networking protocols. Apart from mentoring both undergraduate and graduate students into independent researchers, this project, involving an EPSCoR university and a research lab, will also help broaden the participation and industry collaboration.
The wide adoption of mobile devices such as smartphones has made wireless networks an essential part of our digital life. The amount of traffic generated by mobile devices has been growing exponentially (Cisco predicts a 13 times increase of cellular data traffic between 2012 and 2017). Traditionally there has been an isolation of responsibilities of physical layer (PHY) and higher layers of the network stack — PHY is in charge of encoding/decoding bits whereas the higher layers provide packet-level services abstracting the PHY as a bit-pipe. While such an abstraction mitigates the complexity of developing/upgrading networking software/hardware, it leads to severe inefficiencies that are acutely felt in today's bandwidth-limited wireless networks. Therefore, we argue that instead of keeping PHY out of the higher layer protocol design, PHY should be kept in. We refer to our approach as PHY-Informed Networking (PHY- IN). With PHY-IN, intrinsic features of PHY are accounted for such that higher layers and PHY work in synergy. While PHY-IN can be considered a disruptive technology, it is amenable for incremental adoption. In our project, we have redesigned key wireless protocols based on PHY-IN approach. Currently, a sender when it does not receive an acknowledgement simply repeats the transmission hoping that it will not fail again. If the sender could appropriately alter the encoding of retransmission, it would likely succeeds as interference may not affect it the same way as the original transmission. We have designed Remap, a simple, novel paradigm for handling collisions in OFDM networks with overlapping channels. Remap is different from the existing, passively repeat paradigm, and introduces a novel concept called retransmission permutation to permute the bit-to-subcarrier mapping after each transmission. Retransmission permutation is a powerful diversity technique that can recover frequency selective losses from subsequent retransmissions when there is no collision. When there are collisions, it in essence provides channel-width adaptation and allows bootstrapping of the decoding of collided frames that may otherwise be impossible to decode. Specifically, the foundation of Remap is based on a simple observation and a simple idea. The observation is that when two frames transmitted on overlapping channels collide, only the subcarriers in the intersection of the two channels collide; the bits in other subcarriers are clean and can be collected. However, the non-colliding subcarriers do not contain complete frame information. The idea of Remap is to introduce structured permutation on the mapping from bits to subcarriers after each collision to create structured diversity. This diversity allows either independent decoding or bootstrapping other decoding techniques such as Zigzag decoding. Spectrum sensing, the task of discovering spectrum usage at a given location, is a fundamental problem in dynamic spectrum access networks. While sensing in narrow spectrum bands is well studied in previous work, wideband spectrum sensing is challenging since a wideband radio is generally too expensive and power consuming for mobile devices. Sequential scan, on the other hand, can be very slow if the wide spectrum band contains many narrow channels. We propose a PHY-informed compressed spectrum sensing method, which is much faster than sequential scan and much cheaper than using a wideband radio. The key insight is that, if the sum of energy on a contiguous band is low, we can conclude that all channels in this band are available with just one measurement. Based on this insight, we design an intelligent search algorithm to minimize the number of measurements. We prove the algorithm is asymptotically optimal. We prototype our method using simple analog filters and analog energy detectors. Our evaluation using real TV white space signals shows the effectiveness of our algorithm. Our solution is called QuickSense. Our key finding in Remap improves the efficiency of current 802.11 wireless networks by decoding collisions. Our key finding in QuickSense enables fast detection of unused spectrum. This can potentially have a big impact on how FCC allocates wireless spectrum. These likely will have long-lasting impacts in the field of wireless networking in terms of efficient design and practice. We not only leverage existing physical layer information as in Remap, but also redesign hardware as in QuickSense. These techniques have trained graduate student interns and will be useful in training other graduate students and engineers in the field of wireless networking.
