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each period. The duration of the awake period is fixed by the user, while its position is
initialized randomly within the frame for each node. These active slots will be shifted
as a result of the learning, which will coordinate nodes' wake-up schedules in order to
ensure high data throughput and longer battery life. Each node will learn to be in ac-
tive mode when its parents and children are awake, so that it forwards messages faster
(synchronization), and stay asleep when neighboring nodes on the same hop are com-
municating, so that it avoids collisions and overhearing (desynchronization).
The routing protocol is not explicitly part of the learning algorithm and therefore
any multi-hop routing scheme can be applied without losing the properties of our ap-
proach. In the experimental results, presented in Section 3, the routing is achieved using
a standard shortest path multi-hop routing mechanism. The forwarding nodes need not
be explicitly known, as long as they ensure that their distance to the sink is
lower
than
the sender. Communication is done using a Carrier Sense Multiple Access (CSMA)
protocol. Successful data reception is acknowledged with an ACK packet. We would
like to note that the acknowledgment packet is necessary for the proper and reliable
forwarding of messages. Our algorithm does use this packet to indicate a “correct re-
ception” in order to formulate one of its reward signals (see Subsection 3.1). However,
this signal is not crucial for the RL algorithm and thus the latter can easily function
without acknowledgment packets. Subsection 2.3 will further elaborate on the use of
reward signals.
It is noteworthy that the communication partners of a node (and thus the formation
of coalitions) are influenced by the communication and routing protocols that are in use
and not by our algorithm itself. These protocols only implicitly determine the
direction
of the message flow and not
who
will forward those messages, since nodes should find
out the latter by themselves.
Fig. 2.
Examples of routing and coalition formation
Depending on the routing protocol, coalitions (e.g., synchronized groups of nodes)
logically emerge across the different hops, such that there is, if possible, only one agent
from a certain hop within a coalition. Figure 2 illustrates this concept in three differ-
ent topologies. It shows as an example how coalitions form as a result of the routing
protocol. Intuitively, nodes from one coalition need to synchronize their wake-up sched-
ules. As defined by the routing protocol, messages are not sent between nodes from the
same hop, hence these nodes should desynchronize (or belong to separate coalitions) to
avoid communication interference. The emergence of coalitions will be experimentally
illustrated for different topologies in Section 3.
