342 research outputs found
Degrees of Freedom of the 3-User Rank-Deficient MIMO Interference Channel
We provide the degrees of freedom (DoF) characterization for the -user
multiple-input multiple-output (MIMO) interference channel (IC)
with \emph{rank-deficient} channel matrices, where each transmitter is equipped
with antennas and each receiver with antennas, and the interfering
channel matrices from each transmitter to the other two receivers are of ranks
and , respectively. One important intermediate step for both the
converse and achievability arguments is to convert the fully-connected
rank-deficient channel into an equivalent partially-connected full-rank MIMO-IC
by invertible linear transformations. As such, existing techniques developed
for full-rank MIMO-IC can be incorporated to derive the DoF outer and inner
bounds for the rank-deficient case. Our result shows that when the interfering
links are weak in terms of the channel ranks, i.e., , zero forcing is sufficient to achieve the optimal DoF. On the other
hand, when , a combination of zero forcing and
interference alignment is in general required for DoF optimality. The DoF
characterization obtained in this paper unifies several existing results in the
literature.Comment: 28 pages, 7 figures. To appear in IEEE transactions on wireless
communication
On the Degrees of Freedom of Asymmetric MIMO Interference Broadcast Channels
In this paper, we study the degrees of freedom (DoF) of the asymmetric
multi-input-multi-output interference broadcast channel (MIMO-IBC). By
introducing a notion of connection pattern chain, we generalize the genie chain
proposed in [11] to derive and prove the necessary condition of IA feasibility
for asymmetric MIMO-IBC, which is denoted as irreducible condition. It is
necessary for both linear interference alignment (IA) and asymptotic IA
feasibility in MIMO-IBC with arbitrary configurations. In a special class of
asymmetric two-cell MIMOIBC, the irreducible condition is proved to be the
sufficient and necessary condition for asymptotic IA feasibility, while the
combination of proper condition and irreducible condition is proved to the
sufficient and necessary condition for linear IA feasibility. From these
conditions, we derive the information theoretic maximal DoF per user and the
maximal DoF per user achieved by linear IA, and these DoFs are also the DoF per
user upper-bounds of asymmetric G-cell MIMO-IBC with asymptotic IA and linear
IA, respectively.Comment: 6 pages, 3 figures, submitted to ICC 201
Interference alignment for the MIMO interference channel
We study vector space interference alignment for the MIMO interference
channel with no time or frequency diversity, and no symbol extensions. We prove
both necessary and sufficient conditions for alignment. In particular, we
characterize the feasibility of alignment for the symmetric three-user channel
where all users transmit along d dimensions, all transmitters have M antennas
and all receivers have N antennas, as well as feasibility of alignment for the
fully symmetric (M=N) channel with an arbitrary number of users.
An implication of our results is that the total degrees of freedom available
in a K-user interference channel, using only spatial diversity from the
multiple antennas, is at most 2. This is in sharp contrast to the K/2 degrees
of freedom shown to be possible by Cadambe and Jafar with arbitrarily large
time or frequency diversity.
Moving beyond the question of feasibility, we additionally discuss
computation of the number of solutions using Schubert calculus in cases where
there are a finite number of solutions.Comment: 16 pages, 7 figures, final submitted versio
Retrospective Interference Alignment for the 3-user MIMO Interference Channel with delayed CSIT
The degrees of freedom (DoF) of the 3-user multiple input multiple output
interference channel (3-user MIMO IC) are investigated where there is delayed
channel state information at the transmitters (dCSIT). We generalize the ideas
of Maleki et al. about {\it Retrospective Interference Alignment (RIA)} to be
applied to the MIMO IC, where transmitters and receivers are equipped with
antennas, respectively. We propose a two-phase transmission scheme
where the number of slots per phase and number of transmitted symbols are
optimized by solving a maximization problem. Finally, we review the existing
achievable DoF results in the literature as a function of the ratio between
transmitting and receiving antennas . The proposed scheme improves
all other strategies when .Comment: Draft version of the accepted manuscript at IEEE ICASSP 1
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