4,068 research outputs found
A Systematic Approach for Interference Alignment in CSIT-less Relay-Aided X-Networks
The degrees of freedom (DoF) of an X-network with M transmit and N receive
nodes utilizing interference alignment with the support of relays each
equipped with antennas operating in a half-duplex non-regenerative mode
is investigated. Conditions on the feasibility of interference alignment are
derived using a proper transmit strategy and a structured approach based on a
Kronecker-product representation. The advantages of this approach are twofold:
First, it extends existing results on the achievable DoF to generalized antenna
configurations. Second, it unifies the analysis for time-varying and constant
channels and provides valuable insights and interconnections between the two
channel models. It turns out that a DoF of \nicefrac{NM}{M+N-1} is feasible
whenever the sum of the
The Degrees-of-Freedom of Multi-way Device-to-Device Communications is Limited by 2
A 3-user device-to-device (D2D) communications scenario is studied where each
user wants to send and receive a message from each other user. This scenario
resembles a 3-way communication channel. The capacity of this channel is
unknown in general. In this paper, a sum-capacity upper bound that
characterizes the degrees-of-freedom of the channel is derived by using
genie-aided arguments. It is further shown that the derived upper bound is
achievable within a gap of 2 bits, thus leading to an approximate sum-capacity
characterization for the 3-way channel. As a by-product, interesting analogies
between multi-way communications and multi-way relay communications are
concluded.Comment: 5 pages, ISIT 201
Opportunistic Relaying in Wireless Networks
Relay networks having source-to-destination pairs and half-duplex
relays, all operating in the same frequency band in the presence of block
fading, are analyzed. This setup has attracted significant attention and
several relaying protocols have been reported in the literature. However, most
of the proposed solutions require either centrally coordinated scheduling or
detailed channel state information (CSI) at the transmitter side. Here, an
opportunistic relaying scheme is proposed, which alleviates these limitations.
The scheme entails a two-hop communication protocol, in which sources
communicate with destinations only through half-duplex relays. The key idea is
to schedule at each hop only a subset of nodes that can benefit from
\emph{multiuser diversity}. To select the source and destination nodes for each
hop, it requires only CSI at receivers (relays for the first hop, and
destination nodes for the second hop) and an integer-value CSI feedback to the
transmitters. For the case when is large and is fixed, it is shown that
the proposed scheme achieves a system throughput of bits/s/Hz. In
contrast, the information-theoretic upper bound of bits/s/Hz
is achievable only with more demanding CSI assumptions and cooperation between
the relays. Furthermore, it is shown that, under the condition that the product
of block duration and system bandwidth scales faster than , the
achievable throughput of the proposed scheme scales as .
Notably, this is proven to be the optimal throughput scaling even if
centralized scheduling is allowed, thus proving the optimality of the proposed
scheme in the scaling law sense.Comment: 17 pages, 8 figures, To appear in IEEE Transactions on Information
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