# Cooperative Algorithms for MIMO Interference Channels

### Authors:

Steven W. Peters and
Robert W. Heath, Jr.
### Reference:

Submitted to *IEEE Transactions on Vehicular Technology*, December 2009.

### Abstract:

Interference alignment is a transmission technique for exploiting all available degrees of freedom in
the symmetric frequency- or time-selective interference channel with an arbitrary number of users. Most
prior work on interference alignment, however, neglects interference from other nodes in the network
not participating in the alignment operation. This paper proposes three generalizations of interference
alignment for the multiple-antenna interference channel with multiple users that account for colored noise,
which models uncoordinated interference. First, a minimum interference-plus-noise leakage algorithm is
presented, and shown to be equivalent to previous subspace methods when noise is spatially white or
negligible. This algorithm results in orthonormal precoders that are desirable for practical implementation
with limited feedback. A joint minimum mean squared error design is then proposed that jointly optimizes
the transmit precoders and receive spatial filters, whereas previous designs neglect the receive spatial
filter. This algorithm is shown to be a generalization of previous joint MMSE designs for other system
configurations such as the broadcast channel. Finally, a maximum signal-to-interference-plus-noise ratio
algorithm is developed that is proven to converge, unlike previous maximum SINR algorithms. The
latter two designs are shown to have increased complexity due to non-orthogonal precoders, more
required iterations, or more channel state knowledge than the min INL or subspace methods. The sum
throughput performance of these algorithms is simulated in the context of a network with uncoordinated
co-channel interferers not participating in the alignment protocol. It is found that a network with cochannel
interference can benefit from employing precoders designed to consider that interference, but in
extreme cases, such as when only one receiver has a large amount of interference, ignoring the co-channel
interference is advantageous. Adapting between the strategies in their preferred operating regimes is left
to future work.

Available on arXiv