How can we,
in our proposed adaptive OFDM downlink,
simultaneously obtain good coverage within cells
and a high average spectral efficiency?
The present report outlines a solution
to this problem.
It also gives a first estimate of
the attainable system capacity for our target
radio interface.
The proposed method uses coordinated scheduling
among sectors on the same site
(radio access point)
to suppress interference. Furthermore,
one frequency band (with reuse 1)
transmits to
near users, while another band, with
orthogonal resource sharing
among clusters of 3 base stations,
transmits to far-off users in the sectors.
The key idea is to apply a frequency reuse factor >1 only
where it is needed most, in the outer part
of the sector, where the signal from
the base station is weak.
It becomes possible to
attain coverage over the whole sector,
with an effective resource reuse 2 and
an average capacity
of 1.24 bit/s/Hz/sector for one Rayleigh fading
user, including overhead, without assuming multiuser
diversity or multiple receiver antennas.
At least 16 QAM
can be used in approximately 33 % of the sector area.
The resulting signal-to-interference ratio and
spectral efficiency has been evaluated as a
function of the position
within the sector,
by summing over
all relevant interferers. The traffic
density is assumed
constant over the area, and hexagonal
coverage areas are assumed for the base stations.
Both triangular 60 degree sectors and
diamond-shaped (30 degree rotated) sectors
are considered.
The antenna pattern and an exponential
path loss is taken into account.
The spectral efficiency is first calculated
for one user per sector (no multiuser diversity)
with adaptive modulation.
Results are presented for static channels
and for flat Rayleigh-fading channels,
both with path-loss.
Situations with K active users within the sectors
who each have L antennas and use maximum
ratio combining are then investigated, for
Rayleigh fading channels with
path loss.
The presented estimates neglect
shadow fading and noise. They therefore
represent only a
first approximation of the true, much
more complicated,
situation, which we will investigate in our system
simulator under construction.
Various ways of improving this basic solution
are discussed briefly,
including the
use of coordinated scheduling over
several base stations, slow power control,
the use of interference rejection by
multiple antennas in user equipment, and
transmission by Trellis-coded
adaptive modulation instead of
uncoded adaptive modulation.
The methods and performance measures presented
here can serve as benchmarks for such more
elaborate solutions.
