# A Light Signaling Approach to Node Grouping for Massive MIMO IoT Networks
## 8 Conclusions and Future Work  
In this paper we have developed a new method for node grouping in massive MIMO systems based on directional channel characteristics. The two channel properties we use, dominant direction and angular spectrum spread, are typically stable over large timescales, when compared to the full channel information that should be updated at the rate of the coherence time of the channel. Our approach hence enables the grouping of low-power IoT devices with a minimal signaling overhead, even when the devices' measurement and transmission periods are long. We provided both a mixed-integer optimization formulation and an efficient approximation algorithm to perform the node grouping. In our numerical evaluation, we demonstrated that our approach provides good performance, in terms of the minimum SINR achieved for any node in any group, comparable to a reference  
Figure 5: Performance of the optimization and approximation algorithms. (**a**) Objective function value for partitioning with the optimization problem and with the approximation algorithm, for different numbers of nodes. (**b**) Solution time vs. number of nodes. method exploiting the full CSI. Our assessment also highlights that a partitioning method that groups nodes with similar directional properties together can have a particularly detrimental effect on link quality due to the interference generated by MRC precoding. A light signaling user grouping approach, as we have demonstrated in this paper, is valuable for any situation where the signaling overhead is significant. This will be the case when there is a very large number of nodes, as in IoT scenarios. For the end devices, reducing the signaling overhead also reduces energy usage, which is critical for battery powered IoT devices. Although we have thus far only tested cases with up to 36 nodes, and in real IoT deployments the number of nodes is expected to be much greater, the approximation algorithm we have presented shows good scalability and should thus be suitable even in larger scenarios. In future work, we aim to close the loop by performing measurements in which we measure nodes' channels and then apply our solutions and measure the performance of the resulting node groups. We also plan to extend our method to take into account errors in the directional channel information. This will not only make the method more robust in case of measurement errors, but will also cater to mobile nodes, since as a node moves, its directional information changes, thus effectively producing an error compared to the last time this information was measured. The range of scenarios investigated can also be expanded in future work. By focusing on heuristic methods, larger problem instances can be tested, as well as more frequency bands and different numbers of antennas at the base station. Multi-cell and distributed cell-free systems are also an interesting direction in future, in particular the latter, as cell-free architectures are likely to feature prominently in 6G systems, and this further complicates the node grouping problem, since nodes cannot simply be grouped within each cell. Conceptualization, E.F., M.P., H.T., G.C. and L.V.d.P. ; Data curation, E.F., G.C. and S.G.; Formal analysis, E.F. and M.P. ; Funding acquisition, E.F. and L.V.d.P. ; Investigation, E.F., G.C., S.G. and L.V.d.P. ; Methodology, E.F., M.P., H.T., G.C., S.G. and L.V.d.P. ; Software, E.F., H.T. and G.C. ; Supervision, L.V.d.P. ; Validation, M.P., H.T., G.C., S.G. and L.V.d.P. ; Visualization, E.F.; Writing--original draft, E.F.; Writing--review & editing, E.F., M.P., H.T., G.C., S.G. and L.V.d.P. All authors have read and agreed to the published version of the manuscript. The work of [PERSON] was supported by the Celtic-Next project IMMINENCE, the SSF project SEC4 FACTORY under grant no. SSF RIT17-0032, and the strategic research area ELLIIT. The work of [PERSON] was supported by the National Science Centre, Poland, under the grant no.