| dc.description.abstract | Simultaneous operation of different wireless applications in the same geographical region and
the same frequency band gives rise to undesired interference issues. Since licensed (primary)
applications have been granted priority access to the frequency spectrum, unlicensed (secondary)
services should avoid imposing interference on the primary system. In other words, secondary
system’s activity in the same bands should be in a controlled fashion so that the primary system
maintains its quality of service (QoS) requirements.
In this thesis, we consider collocated point-to-multipoint primary and secondary networks that
have simultaneous access to the same frequency band. Particularly, we examine three different
levels at which the two networks may coexist: pure interference, asymmetric co-existence, and
symmetric co-existence levels.
At the pure interference level, both networks operate simultaneously regardless of their interference
to each other. At the other two levels, at least one of the networks attempts to mitigate its
interference to the other network by deactivating some of its users. Specifically, at the asymmetric
co-existence level, the secondary network selectively deactivates its users based on knowledge
of the interference and channel gains, whereas at the symmetric level, the primary network also
schedules its users in the same way.
Our aim is to derive optimal sum-rates (i.e., throughputs) of both networks at each co-existence
level as the number of users grows asymptotically and evaluate how the sum-rates scale with the
network size. In order to find the asymptotic throughput results, we derive two propositions; one
on the asymptotic behaviour of the largest order statistic and one on the asymptotic behaviour of
the sum of lower order statistics.
As a baseline comparison, we calculate primary and secondary sum-rates for the time division
(TD) channel sharing. Then, we compare the asymptotic secondary sum-rate in TD to that under
simultaneous channel sharing, while ensuring the primary network maintains the same sum-rate in
both cases.
Our results indicate that simultaneous channel sharing at both asymmetric and symmetric
co-existence levels can outperform TD. Furthermore, this enhancement is achievable when user
scheduling in uplink mode is based only on the interference gains to the opposite network and not
on a network’s own channel gains. In other words, the optimal secondary sum-rate is achievable
by applying a scheduling strategy, referred to as the least interference strategy, for which only the
knowledge of interference gains is required and can be performed in a distributed way. | en |
