Summary

At a Glance

6TiSCH schedules radio resources in time and frequency. When few channels are free (Wi-Fi leaves only four 802.15.4 channels untouched), that is not enough. Switchable directional antennas add space as a third dimension.

1 · Introduction

Why Add Space to 6TiSCH?

2 · Related Work

How Much Can Directional Antennas Help?

3 · 6TiSCH · Fig. 1

6TiSCH: Schedules in Time and Frequency

• A slotframe (here 31 cells × 10 ms = 310 ms) repeats; each cell is a (time slot, channel offset) pair. Distant pairs may reuse a cell.
• 6top, between 6LoWPAN and the TSCH MAC, adds, deletes and relocates cells between neighbours (6P); the standard leaves the scheduling policy open.
• Enhanced Beacons in hard slots carry slotframe size, slot duration, hopping pattern and the network-wide slot counter.
• Longer slotframes save energy; shorter ones cut delay.

4 · System Model · Fig. 2

A Switchable Directional Antenna Model for Cooja

The arrival direction is passed up the stack and stored in the MAC neighbour table, so unicast frames leave over the antenna facing the receiver.

4 · Space Time Scheduling · Fig. 3

Omni vs Space Time Scheduling

Nine nodes, root 1, two channels, each node sends one packet. Omni antennas need a 10-slot schedule; with directional antennas, pairs facing away from each other share slots and the same packets arrive in 8 slots. Cells become (slot, channel, antenna) triples.

4 · Space Time Scheduling · Interactive

Scheduler Playground: Add Antennas, Add Channels

Simplified greedy scheduler on the Fig. 3 tree, not the paper's SF-DA: 40 m spacing, 100 m interference, half-duplex nodes; a receiver is disturbed only by a same-channel transmitter whose active antenna covers it and that its own listening antenna faces.

4 · Space Time Scheduling · Algorithms 1–2

SF-DA: Directional-Antenna-Aware Scheduling

EB frames carry the arrival angle; the paper assumes it can be estimated accurately.

5 · Results · Table 1, Figs. 4–5

Setup, and Antennas in Shared Slots

Minimal configuration and Orchestra only use shared cells. Sending on the right antenna already raises PRR 2–3× as traffic grows, although receivers still listen on all antennas.

Grid / area	25 nodes, 200 × 150 m
Antennas	2 × 180° or 3 × 120°
Tx / interference range	50 / 100 m
Timeslot / slotframe	15 ms / 15
Retransmissions / buffer	5 / 5
Test duration	2 h

5 · Results · Figs. 6–7

SF-DA: PRR, Delay and Energy

PRR · 100% linksPRR · 80% linksDelayEnergy

With more than 8 channels the antennas add little on this topology: space helps most when channels are scarce, traffic is heavy and links are poor. The 1-antenna case is the standard SF0 baseline.

5 · Results · Table 2

Scaling with Network Size

Bigger networks mean longer paths: PRR falls, delay and energy rise. Three antennas add about 5–10 points of PRR, roughly halve the delay (1,267 → 536 ms at 25 nodes) and cut energy by about 60% (3.21 → 1.26 mWh).

6 · Conclusion

Conclusion and Future Work

• A low-complexity switchable directional antenna lets 6TiSCH schedule in time, frequency and space.
• Even schedulers unaware of antennas (minimal, Orchestra) gain from sending on the right antenna.
• SF-DA brings up to 100% higher PRR under heavy traffic with few channels, about half the delay and much lower energy.
• Gains are largest in dense, interference-limited networks; with many free channels they shrink.

Limits and next steps

• Simulation with an idealized arrival-angle estimate and UDGM propagation
• Real antenna hardware and realistic channel models

6 · Conclusion

Check Questions

1. Why does limited spectrum (four Wi-Fi-free channels) make space a valuable resource for 6TiSCH?
2. What is the capacity gain factor for three 120° antennas at both ends?
3. Why do minimal configuration and Orchestra improve only partly with directional antennas?
4. When does SF-DA reject a candidate cell?
5. Why do directional antennas also save energy?

References

Selected References

• P. Gupta, P. R. Kumar, "The capacity of wireless networks," IEEE Transactions on Information Theory, 46(2), pp. 388–404, 2000.
• S. Duquennoy et al., "Orchestra: Robust mesh networks through autonomously scheduled TSCH," ACM SenSys, 2015.
• X. Vilajosana, K. Pister, T. Watteyne, "Minimal IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH) configuration," IETF RFC 8180, 2017.
• T. Voigt, L. Mottola, K. Hewage, "Understanding link dynamics in wireless sensor networks with dynamically steerable directional antennas," EWSN, 2013.
• P. Li, C. Zhang, Y. Fang, "The capacity of wireless ad hoc networks using directional antennas," IEEE Transactions on Mobile Computing, 10(10), pp. 1374–1387, 2011.

See the full article for the complete reference list. Figures are reproduced from the article (CC BY 4.0); some are cropped or split for interaction.