Reuse Within a Cell - Interference Rejection or Multiuser
IEEE Transactions on Communications
vol. 47, pp. 1511-1522, October 1999.
© 1999 IEEE
Longer version: Report, Signals and Systems Group, Uppsala University.
The high capacity of a wireless cellular communication
system is obtained by the division of a geographical area into cells.
Each communication channel is used in a fraction of the cells, and by
decreasing the cell size, the capacity of the system can be increased.
Reducing the cell size is however expensive. Instead, multi-element
antennas, also known as antenna arrays, can be used at the
receiver to increase the capacity. Antenna arrays can enhance the
desired signal and suppress the interference so that each communication
channel can be used more frequently across the network, thereby
decreasing the so-called reuse factor. When all channels are
utilized in every cell, the system is said to have reuse factor one.
To increase the capacity of an FDMA or a TDMA cellular
system which has
reuse factor one, several users within a cell would have to share each
available channel; the system must support reuse within a cell
This will cause severe co-channel
interference at the receiver. Antenna arrays are then indispensable
tools for separating the signals
from different users. In this paper, we illustrate, compare and
explore two ways of using an antenna array at the receiver to
accomplish channel reuse within a cell:
- Detect the signal from one user at a time while treating the
other users as interference. This approach is
denoted interference rejection or interference
- Detect the signals from all users simultaneously.
This approach is called multiuser detection.
We investigate the use of an antenna array at the
receiver in FDMA/TDMA systems to let
several users share one communication channel within a cell.
A decision feedback equalizer which simultaneously detects all
incoming signals (multiuser detection) is compared to a set of decision
feedback equalizers, each detecting one signal and rejecting the
remaining as interference. We also introduce the existence of a
zero-forcing solution to the equalization problem as an indicator of
resistance of different detector structures. Near-far resistance
guarantees that good performance will be obtained if the channel is
known and the noise level is low.
Simulations show that with an increased number of users in the
cell, the incremental performance degradation is small for the multiuser
We have applied the proposed algorithms to experimental
measurements from an
antenna array testbed, implementing the air interface of DCS-1800. The
results from these experiments confirm that reuse within a cell is
indeed possible, using either an eight-element array antenna or a
two-branch diversity sector antenna.
Multiuser detection will in general provide better performance
than interference rejection, especially when the power levels
of the users differ substantially.
The difference in performance is of crucial importance when the
available training sequences are short.
PhD Thesis by Claes Tidestav.
VTC'99 paper, based on this report.
ICASSP'99 paper on multiuser detection
using a multivariable IIR DFE
ICUPC'98- paper on "Bootstrap
by Claes Tidestav, addressing the impact of antenna correlation.
Paper in Postscript (uncompressed) 2800K
In Postscript (compressed, .gz) 490K
In Pdf 789K
Longer report version in Postscript (uncompressed) 966K
Longer report version in Postscript (compressed, .gz) 281K
Longer report version in Pdf 594K
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