6 research outputs found
Network Coherence Time Matters - Aligned Image Sets and the Degrees of Freedom of Interference Networks with Finite Precision CSIT and Perfect CSIR
This work obtains the first bound that is provably sensitive to network
coherence time, i.e., coherence time in an interference network where all
channels experience the same coherence patterns. This is accomplished by a
novel adaptation of the aligned image sets bound, and settles various open
problems noted previously by Naderi and Avestimehr and by Gou et al. For
example, a necessary and sufficient condition is obtained for the optimality of
1/2 DoF per user in a partially connected interference network where the
channel state information at the receivers (CSIR) is perfect, the channel state
information at the transmitters (CSIT) is instantaneous but limited to finite
precision, and the network coherence time is T_c= 1. The surprising insight
that emerges is that even with perfect CSIR and instantaneous finite precision
CSIT, network coherence time matters, i.e., it has a DoF impact.Comment: 19 pages, 4 figure
Degrees of Freedom for the MIMO Interference Channel
We explore the available degrees of freedom (DoF) for the two user MIMO interference channel, and find a general inner bound and a genie aided outer bound that give us the exact # of DoF in many cases. We also study a share-and-transmit scheme and show how the gains of transmitter cooperation are entirely offset by the cost of enabling that cooperation so that the available DoF are not increased
Degrees of Freedom of Wireless X Networks
We explore the degrees of freedom of user wireless networks,
i.e. networks of transmitters and receivers where every transmitter has
an independent message for every receiver. We derive a general outerbound on
the degrees of freedom \emph{region} of these networks. When all nodes have a
single antenna and all channel coefficients vary in time or frequency, we show
that the \emph{total} number of degrees of freedom of the network is equal
to per orthogonal time and frequency dimension.
Achievability is proved by constructing interference alignment schemes for
networks that can come arbitrarily close to the outerbound on degrees of
freedom. For the case where either M=2 or N=2 we find that the outerbound is
exactly achievable. While networks have significant degrees of freedom
benefits over interference networks when the number of users is small, our
results show that as the number of users increases, this advantage disappears.
Thus, for large , the user wireless network loses half the
degrees of freedom relative to the MIMO outerbound achievable
through full cooperation. Interestingly, when there are few transmitters
sending to many receivers () or many transmitters sending to few
receivers (), networks are able to approach the degrees
of freedom possible with full cooperation on the MIMO channel.
Similar to the interference channel, we also construct an example of a 2 user
channel with propagation delays where the outerbound on degrees of freedom
is achieved through interference alignment based on a simple TDMA strategy.Comment: 26 page