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WILLOW Report Summary

Project ID: 648382
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - WILLOW (WIreLess LOWband communications: massive and ultra-reliable access)

Reporting period: 2015-04-01 to 2016-09-30

Summary of the context and overall objectives of the project

Cellular wireless systems from 2G to today’s 4G have been continuously evolving towards offering broadband connectivity to the users. While the trend of reaching even higher rates will continue in the fifth generation of wireless systems, there is a common consensus that 5G will not only be “4G, but faster“, but it will offer new modes of connectivity to a massive number of simple devices and/or support extremely reliable connections. WILLOW has the ambitious objective to create the fundamental wireless transmission schemes and protocols for massive and ultra-reliable lowband connectivity. Here lowband indicates that the target applications and services do not require high rates, but rather low rate represented by short messages from a large number of machines/sensors (e.g. as in the smart grid) and/or ultra-reliable delivery of short critical messages (e.g. among interconnected cars at a crossing). Designing massive and ultra-reliable lowband communication cannot be done by incremental changes to the current protocols, but it requires a fundamental leap in the wireless system architecture, by introducing the capability to handle a massive number of short packets and redefining the relationship between the data and the metadata (control data). In addition to the obvious advantages of broadband communication, massive and ultra-reliable lowband communication is bringing a ground-breaking novelty to the wireless 5G systems, as it will make wireless a true commodity, just like electricity or water, and enable a plethora of new applications and services.

Massive and ultra-reliable lowband connections are essential enablers of the Internet of Things (IoT). The avaliability of IoT connectivity for massive number of devices, as well as support for IoT connections with ultra-high reliability, will introduce disruptive changes in the vertical sectors: energy, transportation, industrial production, health, etc. For example, once ultra-reliable wireless connections are available, the notion of a robot changes from a single physical entity to a distributed group of modules that are wirelessly collaborating towards performing the robotic function. This paradigm fits in the emerging concept of Industry 4.0 taht consists of smart interconnected production facilities.

The two fundamental issues that WILLOW investigates towards creating lowband communication systems are:
1- Efficient communication with short packets, in which the data size is comparable to the size of the metadata, i.e. control information. This is not an issue in broadband communication, where the large data makes the metadata negligible. The considerations of control information are essentially changing the traditional communication model introduced by Shannon by the one on Figure 1, which takes into account protocol information.
2- System architecture in which graceful rate degradation, low latency and massive access can exist simultaneously with the usual, broadband data services.

Considering these two fundamental issues, the overall objective of WILLOW is to investigate the architectural principles of lowband communication and create communication schemes and protocols that are optimized for massive and reliable transmission of short packets.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

"The work has been organized into 5 workpackages (WPs) and they are used to categorize the main achievements.

WP1: Fundamental concepts.
- The first major achievement is described the paper [1]. This paper is the first work that systematically identifies the new tradeoffs that emerge when communication is carried with packets that are short in size and where the data and the metadata are of comparable size. Specifically, the PI contributed three examples of generic protocols where the short packet brings nontrivial tradeoffs.
- Identification o a new fundamental tradeoff between latency and energy consumption in broadcast channels. This tradeoff points out to generalization of the usual transmission schemes that are based on frames. The preliminary work has been presented at two workshops [2] and [3] and is now under revision in a peer-reviewed journal [4].
- Novel generalized framework for Hybrid Automatic Retransmission Request (HARQ) under average latency constraint. The paper is in the second revision round [5].
- Fundamental analysis of transmission of a common short message in a broadcast channel. The preliminary work has been presented at the IEEE ISIT conference [6] and the extended version is under revision in IEEE Transactions on Information Theory [7].
- One work item that has not been envisioned in the original plan, but it emerged as a very important one during the development of the concept of ultra-reliable communications (URC) is propagation/channel modeling for ultra-reliable communications. Many of the works that consider URC utilize the traditional channel models, but they are not suitable to capture the rare events that occur with very low probability, e.g. 10-5 or lower. The initial results are very promising and a research article [8] is currently being prepared.

WP2: System architecture.
- Random access protocols for very short packets with improved noncoherent detection [9].
- One of the envisioned aspects in this WP is the study of new modes for spectrum usage. The PI has contributed to a paper that explores the utility of spectrum pooling in mmWave frequencies [10].
- Delay model for massive access in wide-area networks based on LoRa interface. The manuscript [11] has been submitted to IEEE ICC 2017.
- Analysis of ultra-dense network architecture for supporting ultra-reliable ubiquitous-rate communication. The manuscript [12] has been submitted to IEEE ICC 2017.

WP3: Algorithmic communication solutions.
- Novel access algorithm for inter-vehicular communications based on coded random access. The journal article has been accepted [13].
- Algorithm for massive Machine-Type Communication based on Bloom filtering. The first work has been accepted at IEEE Globecom [14]. A follow up work has taken the concept a significant step further by integrating the access with the authentication procedure; a paper [15] has been submitted to IEEE ICC 2017 and a journal paper is currently in preparation.
- Investigation of reliable noncoherent detection of signals with massive receive arrays [16]. This serves now as a basis for developing ultra-reliable packet detection with large arrays.
- Application of the concept of reliable service composition to the scenario of ultra-reliable cloud computing [17].

WP4: Reengineering through protocol coding.
The major achievement in reusing the existing systems has been achieved. Namely, an important architectural aspect that has been identified as central to achieving ultra-high reliability and low latency is the use of multiple communication interfaces. The preliminary work has been presented at IEEE SPAWC [18], a followup version has been submitted to IEEE ICC 2017 [19] and a journal version is currently under preparation.

WP5: Proof-of-Concept.
This is an experimental WP and we are currently developing a testbed for massive access and setting up an experimental scneario for testing of LoRa-based access for M2M communication.

[1] G. Durisi, T. Koch and P. Popovski, "Toward Massive, Ultrareliable, and Low-Latency Wireless Communication With Short Packets", Proceedings of the IEEE, vol. 104, no. 9, pp. 1711-1726, Sept. 2016.
[2] K. F. Trillingsgaard and P. Popovski. “Design Considerations for Downlink Broadcast Frame with Short Data Packets.” International Zurich Seminar on Communications, 2016.
[3] K. F. Trillingsgaard, and P. Popovski, ”Encoding of Control Information and Data for Downlink Broadcast of Short Packets“, in Proc. Information Theory and Applications Workshop, San Diego, CA, 2016.
[4] K. F. Trillingsgaard, and P. Popovski, “Downlink Transmission of Short Packets: Framing and Control Information Revisited.”, second revision round in IEEE Transactions on Communications, arXiv preprint arXiv:1605.01829 (2016).
[5] K. F. Trillingsgaard, and P. Popovski, “Generalized HARQ Protocols with Delayed Channel State Information and Average Latency Constraints”, second revision round in IEEE Transactions on Information Theory, available at, 2016.
[6] K. F. Trillingsgaard, W. Yang, G. Durisi and P. Popovski, ”Variable-Length Coding with Stop-Feedback for the Common-Message Broadcast Channel”, in Proc. IEEE International Symposium on Information Theory (ISIT), Barcelona, Spain, July 2016.
[7] K. F. Trillingsgaard, W. Yang, G. Durisi and P. Popovski, “Variable-Length Coding with Stop-Feedback for the Common-Message Broadcast Channel in the Nonasymptotic Regime”, submitted to IEEE Transactions on Information Theory, arXiv preprint arXiv:1607.03519 (2016).
[8] P. Eggers and P. Popovski, “Wireless Channel Modeling Perspectives for Ultra-Reliable Communications”, in preparation, 2016.
[9] Z. Utkovski, T. Eftimov, and P. Popovski, “Random Access Protocols with Collision Resolution in a Noncoherent Setting“, IEEE Wireless Communications Letters, May 2015.
[10] F. Boccardi, H. Shokri-Ghadikolaei, G. Fodor, E. Erkip, C. Fischione, M. Kountouris, P Popovski, and M. Zorzi, “Spectrum Pooling in MmWave Networks: Opportunities, Challenges, and Enablers”, IEEE Communications Magazine, accepted, 2016.
[11] R. B. Sørensen, D. M. Kim, J. J. Nielsen, and P. Popovski, “Analysis of Duty Cycling in Long Range (LoRa) Low Power Wide Area (LPWA) Networks”, submitted to IEEE ICC 2017.
[12] J. Park, D. M. Kim, P. Popovski and S.-L. Kim, “Revisiting Frequency Reuse towards Supporting Ultra-Reliable Ubiquitous-Rate Communication”, submitted to IEEE ICC 2017.
[13] M. Ivanov, F. Brännström, A. Graell i Amat, and P. Popovski, “Broadcast Coded Slotted ALOHA: A Finite Frame Length Analysis”, IEEE Transactions on Communications, accepted, 2016.
[14] N. Pratas, C. Stefanovic, G. C. Madueño, and P. Popovski, “Random Access for Machine-Type Communication based on Bloom Filtering”, in Proc. IEEE Globecom, Washington, DC, USA, December 2016.
[15] N. Pratas, S. Pattathil, C. Stefanovic, and P. Popovski, “Massive Machine-Type Communication (mMTC) Access with Integrated Authentication”, submitted to IEEE ICC 2017.
[16] L. Jing, Z. Utkovski, E. De Carvalho, and P. Popovski, ”Performance Limits of Energy Detection Systems with Massive Receiver Arrays”, in Proc. IEEE CAMSAP, Cancun, Mexico, December 2015.
[17] S. M. Azimi, O. Simeone, O. Sahin, and P. Popovski, ”Ultra-Reliable Cloud Mobile Computing with Service Composition and Superposition Coding”, CISS 2016: 50th Annual Conference on Information Sciences and Systems, Princeton, USA, March 2016.
[18] J. J. Nielsen and P. Popovski, “Latency Analysis of Systems with Multiple Interfaces for Ultra-Reliable M2M Communication”, in Proc. 2016 IEEE International Workshop on Signal Processing Advances for Wireless Communications (SPAWC), Edinburgh, UK, July 2016, invited paper.
[19] J. J. Nielsen, R. Liu, and P. Popovski, “Latency-Optimized Interface Diversity for Ultra-Reliable Low Latency Communication (URLLC)”, submitted to IEEE ICC 2017."

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The research plan for the WILLOW project was made in 2014 and it is very rewarding to see how relevant it has become with respect to the global research on wireless communications. Namely, since 2014 there has been a growing consensus that 5G wireless systems will support three generic services, which, according to the ITU-R nomenclature, are classified as: Enhanced mobile broadband (eMBB), Massive machine-type communications (mMTC) and Ultra-reliable and low-latency communications (URLLC). It is noted that two of the he three services envisioned in 5G are exactly the ones that represent the main subject of WILLOW, which makes the potential for impact very significant.

The technology aspects investigated in WILLOW are central to the upcoming revolution of the Internet of Things. Indeed, in traditional broadband services the transmitted data volume is very large, which makes the size of metadata not significantly large in order to be a target for highly optimal transmission methods. The results achieved so far have clearly advanced the understanding of communication protocols for sending short data packets and have revealed new aspects of encoding control information. The papers published within the WILLOW project are already cited and very much in the focus of many ongoing 5G projects. Furthermore, the standardization of ultra-reliable communications will be carried out within the coming years and it is expected that it will proliferate in 2025, which makes the results of WILLOW perfectly timed in order to have a very visible impact on the upcoming wireless communication technology.

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