The benefit of this approach is that MAC and wakeup do not interfere
and therefore can be optimized independently resulting in a more e?±cient solution.
The drawback is that creating these di?®erent channels typically comes
with an overhead of its own. If both are separated in time, synchronization is
needed, which has to be updated at regular intervals due to clock drifts [8].
Using di?®erent frequency band works best if also two radios are present, one
for each band [13]. The cost is additional hardware in this case, or the inability
to use these solutions on existing hardware platforms that only posses a
single radio.
2.2 Paging, Synchronous and Asynchronous Wakeup
An orthogonal classification of possible wakeup solutions divides them into ondemand
paging, synchronous schemes and asynchronous ones. Other related,
although slightly di?®erent, nomenclatures have been proposed as well [14].
In on-demand paging, a physically separate device is used for the wakeup,
where this other device has a much lower standby power then the main radio.
This is analogous to using a traditional pager to contact someone who
is otherwise unreachable. In terms of the terminology used in Section 1.2,
this solution can be viewed as one where Tperiod = 1 or Tactive = 0, and
the contribution of the paging device has to be added to the energy consumption
in Section 1.3. Various alternatives of on-demand paging have been
proposed, which di?®er mainly in the technology used and the resulting design
choices [15] [16] [17] [18] [19].
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