Uppsala universitet

Reuse Within a Cell - Interference Rejection or Multiuser Detection?

Claes Tidestav , Mikael Sternad and Anders Ahlén

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:

  1. Detect the signal from one user at a time while treating the other users as interference. This approach is denoted interference rejection or interference cancellation.
  2. 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 near-far 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 detector.

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.

Related publications:
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 equalization".
Master's Thesis by Claes Tidestav, addressing the impact of antenna correlation.

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Longer report version in Pdf 594K

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