Downlink Cell Load

In the downlink, the determination of cell loading uses the same basic
approach as for the uplink. The same approach is applicable because the
ability of a given mobile to recover a signal that is destined for that mobile
is dependent upon how many other signals are being sent to other mobiles
in the cell. In other words, for a given user, j, the signals that are being sent
from the base station to other users are simply interference. The more such
signals, the greater the interference. As is the case for the uplink, the effect
of the interference is dependent on the Eb/No requirement needed at the
mobile. There is also interference caused by downlink common channels
and interference caused by other base stations. In the case of interference
from other base stations, the amount of interference will depend upon the
individual user’s location. A user that is close to the serving base station is
less likely to experience as much interference from neighboring cells as a
user that is near the border between cells.
Finally, we need to factor in orthogonality. In the downlink, for a given
scrambling code, transmissions to different users are sent using different
channelization codes, which are chosen such that the codes are orthogonal.
If the transmission from the base station to a single user arrives over multiple
paths, however, and the delay spread across those paths is sufficiently
large, the mobile will directly recover only a part of the signal from the base
station. The other part of the signal, which arrives over a long delay path,
will be seen as interference. This phenomenon needs to be accounted for in
our calculation of downlink loading.
Because the same methodology for downlink load factor calculation
applies in the uplink, then Equation 12-10 still applies, but with some modifications
to account for orthogonality and the fact that interference from neighboring cells is different for each user. Thus, the equation for downlink
load factor becomes
(Equation 12-14)
where aj is the orthogonality factor related to user j and ij is the interference
from neighboring cells experienced by user j.
As in the case in the downlink, for most services, the term C/(aj  Rj(Eb/No)j)
is far greater than 1, which means that the equation can be simplified. Moreover,
it is not realistic to determine the orthogonality factor for each mobile
in the cell as this will depend on the exact user location and multipath profile.
Nor is it realistic to determine the intercell interference experienced by
each user as that will also depend on the user’s exact location.Thus, we need
to consider average values of orthogonality (a) and intercell interference (i).
A typical value for a is 0.4 and a typical value for i is 0.5.
Including these considerations, the load equation becomes