881 research outputs found
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
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MIMO Multiway Relaying with Pairwise Data Exchange: A Degrees of Freedom Perspective
In this paper, we study achievable degrees of freedom (DoF) of a
multiple-input multiple-output (MIMO) multiway relay channel (mRC) where
users, each equipped with antennas, exchange messages in a pairwise manner
via a common -antenna relay node. % A novel and systematic way of joint
beamforming design at the users and at the relay is proposed to align signals
for efficient implementation of physical-layer network coding (PNC). It is
shown that, when the user number , the proposed beamforming design can
achieve the DoF capacity of the considered mRC for any setups. % For
the scenarios with , we show that the proposed signaling scheme can be
improved by disabling a portion of relay antennas so as to align signals more
efficiently. Our analysis reveals that the obtained achievable DoF is always
piecewise linear, and is bounded either by the number of user antennas or
by the number of relay antennas . Further, we show that the DoF capacity can
be achieved for and
, which
provides a broader range of the DoF capacity than the existing results.
Asymptotic DoF as is also derived based on the proposed
signaling scheme.Comment: 13 pages, 7 figure
Opportunistic Interference Mitigation Achieves Optimal Degrees-of-Freedom in Wireless Multi-cell Uplink Networks
We introduce an opportunistic interference mitigation (OIM) protocol, where a
user scheduling strategy is utilized in -cell uplink networks with
time-invariant channel coefficients and base stations (BSs) having
antennas. Each BS opportunistically selects a set of users who generate the
minimum interference to the other BSs. Two OIM protocols are shown according to
the number of simultaneously transmitting users per cell: opportunistic
interference nulling (OIN) and opportunistic interference alignment (OIA).
Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As
our main result, it is shown that DoFs are achievable under the OIN
protocol with selected users per cell, if the total number of users in
a cell scales at least as . Similarly, it turns out that
the OIA scheme with () selected users achieves DoFs, if scales
faster than . These results indicate that there exists a
trade-off between the achievable DoFs and the minimum required . By deriving
the corresponding upper bound on the DoFs, it is shown that the OIN scheme is
DoF optimal. Finally, numerical evaluation, a two-step scheduling method, and
the extension to multi-carrier scenarios are shown.Comment: 18 pages, 3 figures, Submitted to IEEE Transactions on Communication
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