8,837 research outputs found
Capacity Bounds for the -User Gaussian Interference Channel
The capacity region of the -user Gaussian interference channel (GIC) is a
long-standing open problem and even capacity outer bounds are little known in
general. A significant progress on degrees-of-freedom (DoF) analysis, a
first-order capacity approximation, for the -user GIC has provided new
important insights into the problem of interest in the high signal-to-noise
ratio (SNR) limit. However, such capacity approximation has been observed to
have some limitations in predicting the capacity at \emph{finite} SNR. In this
work, we develop a new upper-bounding technique that utilizes a new type of
genie signal and applies \emph{time sharing} to genie signals at receivers.
Based on this technique, we derive new upper bounds on the sum capacity of the
three-user GIC with constant, complex channel coefficients and then generalize
to the -user case to better understand sum-rate behavior at finite SNR. We
also provide closed-form expressions of our upper bounds on the capacity of the
-user symmetric GIC easily computable for \emph{any} . From the
perspectives of our results, some sum-rate behavior at finite SNR is in line
with the insights given by the known DoF results, while some others are not. In
particular, the well-known DoF achievable for almost all constant real
channel coefficients turns out to be not embodied as a substantial performance
gain over a certain range of the cross-channel coefficient in the -user
symmetric real case especially for \emph{large} . We further investigate the
impact of phase offset between the direct-channel coefficient and the
cross-channel coefficients on the sum-rate upper bound for the three-user
\emph{complex} GIC. As a consequence, we aim to provide new findings that could
not be predicted by the prior works on DoF of GICs.Comment: Presented in part at ISIT 2015, submitted to IEEE Transactions on
Information Theory on July 2015, and revised on January 201
Ergodic Interference Alignment
This paper develops a new communication strategy, ergodic interference
alignment, for the K-user interference channel with time-varying fading. At any
particular time, each receiver will see a superposition of the transmitted
signals plus noise. The standard approach to such a scenario results in each
transmitter-receiver pair achieving a rate proportional to 1/K its
interference-free ergodic capacity. However, given two well-chosen time
indices, the channel coefficients from interfering users can be made to exactly
cancel. By adding up these two observations, each receiver can obtain its
desired signal without any interference. If the channel gains have independent,
uniform phases, this technique allows each user to achieve at least 1/2 its
interference-free ergodic capacity at any signal-to-noise ratio. Prior
interference alignment techniques were only able to attain this performance as
the signal-to-noise ratio tended to infinity. Extensions are given for the case
where each receiver wants a message from more than one transmitter as well as
the "X channel" case (with two receivers) where each transmitter has an
independent message for each receiver. Finally, it is shown how to generalize
this strategy beyond Gaussian channel models. For a class of finite field
interference channels, this approach yields the ergodic capacity region.Comment: 16 pages, 6 figure, To appear in IEEE Transactions on Information
Theor
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