262 research outputs found
Degrees of Freedom for the MIMO Multi-way Relay Channel
This paper investigates the degrees of freedom (DoF) of the L-cluster, K-user
MIMO multi-way relay channel, where users in each cluster wish to exchange
messages within the cluster, and they can only communicate through the relay. A
novel DoF upper bound is derived by providing users with carefully designed
genie information. Achievable DoF is identified using signal space alignment
and multiple-access transmission. For the two-cluster MIMO multi-way relay
channel with two users in each cluster, DoF is established for the general case
when users and the relay have arbitrary number of antennas, and it is shown
that the DoF upper bound can be achieved using signal space alignment or
multiple-access transmission, or a combination of both. The result is then
generalized to the three user case. For the L-cluster K-user MIMO multi-way
relay channel in the symmetric setting, conditions under which the DoF upper
bound can be achieved are established. In addition to being shown to be tight
in a variety of scenarios of interests of the multi-way relay channel, the
newly derived upperbound also establishes the optimality of several previously
established achievable DoF results for multiuser relay channels that are
special cases of the multi-way relay channel.Comment: submitted to IEEE Transactions on Information Theor
Secure Degrees of Freedom of MIMO X-Channels with Output Feedback and Delayed CSIT
We investigate the problem of secure transmission over a two-user multi-input
multi-output (MIMO) X-channel in which channel state information is provided
with one-unit delay to both transmitters (CSIT), and each receiver feeds back
its channel output to a different transmitter. We refer to this model as MIMO
X-channel with asymmetric output feedback and delayed CSIT. The transmitters
are equipped with M-antennas each, and the receivers are equipped with
N-antennas each. For this model, accounting for both messages at each receiver,
we characterize the optimal sum secure degrees of freedom (SDoF) region. We
show that, in presence of asymmetric output feedback and delayed CSIT, the sum
SDoF region of the MIMO X-channel is same as the SDoF region of a two-user MIMO
BC with 2M-antennas at the transmitter, N-antennas at each receiver and delayed
CSIT. This result shows that, upon availability of asymmetric output feedback
and delayed CSIT, there is no performance loss in terms of sum SDoF due to the
distributed nature of the transmitters. Next, we show that this result also
holds if only output feedback is conveyed to the transmitters, but in a
symmetric manner, i.e., each receiver feeds back its output to both
transmitters and no CSIT. We also study the case in which only asymmetric
output feedback is provided to the transmitters, i.e., without CSIT, and derive
a lower bound on the sum SDoF for this model. Furthermore, we specialize our
results to the case in which there are no security constraints. In particular,
similar to the setting with security constraints, we show that the optimal sum
DoF region of the (M,M,N,N)--MIMO X-channel with asymmetric output feedback and
delayed CSIT is same as the DoF region of a two-user MIMO BC with 2M-antennas
at the transmitter, N-antennas at each receiver, and delayed CSIT. We
illustrate our results with some numerical examples.Comment: To Appear in IEEE Transactions on Information Forensics and Securit
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