  A key feature of the packet scheduler in LTE system is that it can allocate
resources both in the time and frequency domain. Furthermore, the scheduler is
acquainted with channel state information periodically reported by user
equipments either in an aggregate form for the whole downlink channel, or
distinguished for each available subchannel. This mechanism allows for wide
discretion in resource allocation, thus promoting the flourishing of several
scheduling algorithms, with different purposes. It is therefore of great
interest to compare the performance of such algorithms in different scenarios.
A very common simulation tool that can be used for this purpose is ns-3, which
already supports a set of well known scheduling algorithms for LTE downlink,
though it still lacks schedulers that provide throughput guarantees. In this
work we contribute to fill this gap by implementing a scheduling algorithm that
provides long-term throughput guarantees to the different users, while
opportunistically exploiting the instantaneous channel fluctuations to increase
the cell capacity. We then perform a thorough performance analysis of the
different scheduling algorithms by means of extensive ns-3 simulations, both
for saturated UDP and TCP traffic sources. The analysis makes it possible to
appreciate the difference among the scheduling algorithms, and to assess the
performance gain, both in terms of cell capacity and packet service time,
obtained by allowing the schedulers to work on the frequency domain.
