Wireless Network Design: Optimization Models and Solution ...

14 Cross Layer Scheduling in Wireless Networks 329
14.3 A Framework for Cross Layer Scheduling
In the above sections, we have discussed information theoretic capacity notions for
multiuser wireless system. These results indicate that significant performance gains
can be obtained at the link and network layers by exploiting physical layer information.
In this section, we discuss how information from physical layer can be exploited
and opportunities created for making scheduling decisions in order to satisfy
certain QoS measures.
14.3.1 Opportunistic Scheduling
As we have already studied, for the symmetric case, sum capacity is maximized by
scheduling the user with the best channel state in a time slot. This suggests that users
should transmit at opportunistic time. This leads to the foundation of opportunistic
scheduling. Since the radio channel conditions vary independently for each user, in
a given slot, there is a high probability of having a user whose channel state is near
its peak. Scheduling such a user leads to high sum throughput. The gains are larger
if the channel variations are larger which in turn are indeed larger if the number of
users is large. Thus, the traditional view that rapid variations in the wireless channel
pose a significant challenge for efficient communication has been converted into an
opportunity for exploiting multiuser diversity.
Thus, we have a scheduling scheme where the scheduler picks up the user in in
slot n such that
in = argmaxx
j j n. (14.24)
This ‘pure’ opportunistic scheduling, though, maximizes overall sum throughput, is
not necessarily fair. It may starve the users who have poor average channel states.
For example in Figure 14.4, user 2 is starved. This problem can be addressed by
imposing fairness constraints in (14.24). We will review such algorithms later in the
chapter.
Channel state
Time
Fig. 14.4 User with perennially poor channel condition may be starved by a pure opportunistic
scheduler
User 1
User 2