464 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
Theor
Approximate Ergodic Capacity of a Class of Fading Two-User Two-Hop Networks
The fading AWGN two-user two-hop network is considered where the channel coefficients are independent and identically distributed (i.i.d.) according to a continuous distribution and vary over time. For a broad class of channel distributions, the ergodic sum capacity is characterized to within a constant number of bits/second/hertz, independent of the signal-to-noise ratio. The achievability follows from the analysis of an interference neutralization scheme where the relays are partitioned into M pairs, and interference is neutralized separately by each pair of relays. When M = 1, the proposed ergodic interference neutralization characterizes the ergodic sum capacity to within 4 bits/sec/Hz for i.i.d. uniform phase fading and approximately 4.7 bits/sec/Hz for i.i.d. Rayleigh fading. It is further shown that this gap can be tightened to 4 log pi-4 bits/sec/Hz (approximately 2.6) for i.i.d. uniform phase fading and 4-4 log(3 pi/8) bits/sec/Hz (approximately 3.1) for i.i.d. Rayleigh fading in the limit of large M.(1
Energy Harvesting Wireless Communications: A Review of Recent Advances
This article summarizes recent contributions in the broad area of energy
harvesting wireless communications. In particular, we provide the current state
of the art for wireless networks composed of energy harvesting nodes, starting
from the information-theoretic performance limits to transmission scheduling
policies and resource allocation, medium access and networking issues. The
emerging related area of energy transfer for self-sustaining energy harvesting
wireless networks is considered in detail covering both energy cooperation
aspects and simultaneous energy and information transfer. Various potential
models with energy harvesting nodes at different network scales are reviewed as
well as models for energy consumption at the nodes.Comment: To appear in the IEEE Journal of Selected Areas in Communications
(Special Issue: Wireless Communications Powered by Energy Harvesting and
Wireless Energy Transfer
Approximate Ergodic Capacity of a Class of Fading 2x2x2 Networks
We study a 2-user 2-hop network with 2 relays in which channel coefficients are independently drawn from continuous distributions and vary over time. For a broad class of channel distributions, we characterize the ergodic sum capacity within a constant number of bits/sec/Hz, independent of signal- to-noise ratio. Specifically, we characterize the ergodic sum capacity within 4 bits/sec/Hz for independent and identically distributed (i.i.d.) uniform phase fading and approximately 4.7 bits/sec/Hz for i.i.d. Rayleigh fading. For achievability, we propose ergodic interference neutralization in which the relays amplify and forward their received signals with appropriate delays such that interference can be neutralized at each destination
Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks
An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with antennas receives independent messages from its
serving users, while the second BS with antennas transmits
independent messages to its serving users. That is, the first and second
cells operate as uplink and downlink, respectively. Each user is assumed to
have a single antenna. Under this uplink-downlink setting, the sum degrees of
freedom (DoF) is completely characterized as the minimum of
,
, , and , where denotes
. The result demonstrates that, for a broad class of network
configurations, operating one of the two cells as uplink and the other cell as
downlink can strictly improve the sum DoF compared to the conventional uplink
or downlink operation, in which both cells operate as either uplink or
downlink. The DoF gain from such uplink-downlink operation is further shown to
be achievable for heterogeneous cellular networks having hotspots and with
delayed channel state information.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
Information Theor
Computation Alignment: Capacity Approximation without Noise Accumulation
Consider several source nodes communicating across a wireless network to a
destination node with the help of several layers of relay nodes. Recent work by
Avestimehr et al. has approximated the capacity of this network up to an
additive gap. The communication scheme achieving this capacity approximation is
based on compress-and-forward, resulting in noise accumulation as the messages
traverse the network. As a consequence, the approximation gap increases
linearly with the network depth.
This paper develops a computation alignment strategy that can approach the
capacity of a class of layered, time-varying wireless relay networks up to an
approximation gap that is independent of the network depth. This strategy is
based on the compute-and-forward framework, which enables relays to decode
deterministic functions of the transmitted messages. Alone, compute-and-forward
is insufficient to approach the capacity as it incurs a penalty for
approximating the wireless channel with complex-valued coefficients by a
channel with integer coefficients. Here, this penalty is circumvented by
carefully matching channel realizations across time slots to create
integer-valued effective channels that are well-suited to compute-and-forward.
Unlike prior constant gap results, the approximation gap obtained in this paper
also depends closely on the fading statistics, which are assumed to be i.i.d.
Rayleigh.Comment: 36 pages, to appear in IEEE Transactions on Information Theor
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