The objective of WILLOW (Wireless Lowband Communications: Massive and Ultra-Reliable Access) is to make wireless communication a true commodity by supporting low-rate links for massive number of devices and ultra-reliable connectivity. Lowband communication is the key to enabling new applications, such as massive sensing, ultra-reliable vehicular links and wireless cloud connectivity with guaranteed minimal rate. The research in WILLOW is centered on two fundamental issues. First is the efficient communication with short packets. Second is the system architecture in which graceful rate degradation, low latency, and massive access can exist simultaneously with the broadband services.
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 [1,2] 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).
Figure 1, featured from [5G1], provides a diagram of the population size (number of connected devices) versus the data rate with indicative percentages. The region (R1) reflects the operating range of today’s wireless systems, outlining that the data rate of each user decreases as the user population increases. (R2) is the region that reflects the broadband wireless research agenda: 60 GHz spectrum use, full duplex wireless, sub-Nyquist sampling, processing, interference coordination, etc. Operation in (R5) is impossible due to basic physical and information-theoretic limits. The research of WILLOW project is focused in communication systems that operate in regions (R3-4), and addresses two fundamental issues:
- 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.
- System architecture in which graceful rate degradation, low latency and massive access can exist simultaneously with the usual, broadband data services.
The principles from WILLOW will be applied to: (a) clean-slate wireless systems; (b) reengineer existing wireless systems. Option (b) is unique to lowband communication that does not require high physical-layer speed, but can reuse the physical layer of an existing system and redefine the metadata/data relationship to achieve massive/ultra-reliable communication. WILLOW is poised to make a breakthrough towards lowband communications and create the technology that will enable a plethora of new wireless usage modes.
Objectives and Work packages
The scientific objectives of WILLOW are:
- Investigate the fundamental methods and trade-offs for metadata and data for sending short packets;
- Design protocols that support short packet transmission to/from a massive number of devices;
- Design component algorithms that can trade off energy for reliable low-latency transmissions;
- Revise system architecture for lowband communication and coexistence with broadband traffic;
- Use the principles and algorithms from (Ob1-Ob3) to reengineer an existing wireless system;
- Demonstrate the operation of the designed system/algorithms through a proof-of-concept implementation using Software-Defined Radio (SDR) platforms and cellular system simulators.
The work is organized in five work packages (WP):
- WP1: Fundamental concepts;
- WP2: System Architecture;
- WP3: Algorithmic communication solutions;
- WP4: Reengineering through protocol coding;
- WP5: Proof-Of-Concept.
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Trillingsgaard, Kasper Fløe, and Petar Popovski. “Downlink Transmission of Short Packets: Framing and Control Information Revisited.” arXiv preprint arXiv:1605.01829 (2016).
Trillingsgaard, Kasper Fløe, et al. “Variable-length coding with stop-feedback for the common-message broadcast channel in the nonasymptotic regime.” arXiv preprint arXiv:1607.03519 (2016).
Utkovski, Zoran, Tome Eftimov, and Petar Popovski. “Random Access Protocols with Collision Resolution in a Noncoherent Setting.” IEEE Wireless Communications Letters Vol. 4, Issue 4, pp. 445-448, Aug. 2015.
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“Latency Analysis of Systems with Multiple Interfaces for Ultra-Reliable M2M Communication”. IEEE 17th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2016.
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