# A Light Signaling Approach to Node Grouping for Massive MIMO IoT Networks
## 4 Scenario and Problem Definition  
We consider a single-cell network with the base station equipped with a large, central antenna array and a number of single-antenna IoT nodes. Here we consider static node devices, and they could be placed in different environments (urban, indoors, rural, etc. ), with the cell size varying according to the environment. Although the devices themselves do not move, there may nonetheless be some movement in the environment, for example, people walking around. Here, we provide a brief overview of massive MIMO transmission, the technology underlying our work. For more details, see [3]. In a massive MIMO system, transmission is established according to the coherence blocks of the channel. A coherence block is a time-by-frequency region in which the channel is constant to within some small margin of error. Within each coherence block, nodes transmit pilot signals in order for the base station to determine the CSI and thus allow data transmission and reception. We consider a time-division duplex (TDD) massive MIMO system, relying on channel reciprocity within each coherence block, and hence not requiring downlink pilots. In each coherence block, a number of nodes are served simultaneously, for either uplink or downlink transmission, or both. The number of nodes that may be served in each coherence block is limited by either the number of available pilot signals--since each node must be allocated a pilot in order to transmit or receive in the block--or by the inter-node interference, or both. The interference between nodes depends on both the combination of their channels and the precoding scheme used. In this work, we focus on MRC as precoding as it can realize power-efficient spatial multiplexing of devices [3], based on the CSI that is acquired in the TDD-based massive MIMO operation. In scenarios in which the number of nodes exceeds the number that can be served simultaneously in one coherence block, node grouping becomes necessary, such that each group can be accommodated in a single coherence block. After grouping, node groups can then be scheduled by assigning coherence blocks to them such that the traffic demands of all nodes in the group are satisfied. In this paper, we do not consider scheduling, but rather focus on the node partitioning problem. In this work, we perform node grouping based on two directional characteristics of the node channels. The first is the dominant propagation direction, that is, the direction from which the dominant component of a given node's channel arrives at the base station. The second characteristic we use is the angular spectrum spread--how much the power of the node's channel, as received by the base station, is spread over the angular domain. A low angular spectrum spread thus implies a channel where the power is largely focused in the dominant direction, whereas a large angular spectrum spread indicates a channel with either multiple strong components or a main component that is spread over a wide angle, or both. Examples of channels with low and high angular spectrum spreads are shown in Figure 2.  
For static nodes, these two characteristics of the channel will remain stable over a long period of time, as demonstrated in [28; 29]. If there is a line-of-sight component present, this will typically constitute the dominant component, and its direction will not change unless shadowing occurs to block it. Even in cases without a line-of-sight component, there may be one or more strong multipath components, and their directions will similarly be stable in the absence of variable shadowing. The angular spectrum spread is influenced by the number and variety of multipath components. If there are many, the channel is effectively \"spread out\" over a wider range of angles, leading to a high angular spectrum spread. This means that even if the angles of arrival of individual multipath channel components change, the overall angular spectrum spread will again be relatively stable.