4,820 research outputs found
On the ergodic sum-rate performance of CDD in multi-user systems
The main focus of space-time coding design and analysis for MIMO systems has
been so far focused on single-user systems. For single-user systems, transmit
diversity schemes suffer a loss in spectral efficiency if the receiver is
equipped with more than one antenna, making them unsuitable for high rate
transmission. One such transmit diversity scheme is the cyclic delay diversity
code (CDD). The advantage of CDD over other diversity schemes such as
orthogonal space-time block codes (OSTBC) is that a code rate of one and delay
optimality are achieved independent of the number of transmit antennas. In this
work we analyze the ergodic rate of a multi-user multiple access channel (MAC)
with each user applying such a cyclic delay diversity (CDD) code. We derive
closed form expressions for the ergodic sum-rate of multi-user CDD and compare
it with the sum-capacity. We study the ergodic rate region and show that in
contrast to what is conventionally known regarding the single-user case,
transmit diversity schemes are viable candidates for high rate transmission in
multi-user systems. Finally, our theoretical findings are illustrated by
numerical simulation results.Comment: to appear in Proceedings of 2007 IEEE Information Theory Workshop
(ITW) in Lake Taho
A Survey of Physical Layer Security Techniques for 5G Wireless Networks and Challenges Ahead
Physical layer security which safeguards data confidentiality based on the
information-theoretic approaches has received significant research interest
recently. The key idea behind physical layer security is to utilize the
intrinsic randomness of the transmission channel to guarantee the security in
physical layer. The evolution towards 5G wireless communications poses new
challenges for physical layer security research. This paper provides a latest
survey of the physical layer security research on various promising 5G
technologies, including physical layer security coding, massive multiple-input
multiple-output, millimeter wave communications, heterogeneous networks,
non-orthogonal multiple access, full duplex technology, etc. Technical
challenges which remain unresolved at the time of writing are summarized and
the future trends of physical layer security in 5G and beyond are discussed.Comment: To appear in IEEE Journal on Selected Areas in Communication
Maximum-rate Transmission with Improved Diversity Gain for Interference Networks
Interference alignment (IA) was shown effective for interference management
to improve transmission rate in terms of the degree of freedom (DoF) gain. On
the other hand, orthogonal space-time block codes (STBCs) were widely used in
point-to-point multi-antenna channels to enhance transmission reliability in
terms of the diversity gain. In this paper, we connect these two ideas, i.e.,
IA and space-time block coding, to improve the designs of alignment precoders
for multi-user networks. Specifically, we consider the use of Alamouti codes
for IA because of its rate-one transmission and achievability of full diversity
in point-to-point systems. The Alamouti codes protect the desired link by
introducing orthogonality between the two symbols in one Alamouti codeword, and
create alignment at the interfering receiver. We show that the proposed
alignment methods can maintain the maximum DoF gain and improve the ergodic
mutual information in the long-term regime, while increasing the diversity gain
to 2 in the short-term regime. The presented examples of interference networks
have two antennas at each node and include the two-user X channel, the
interferring multi-access channel (IMAC), and the interferring broadcast
channel (IBC).Comment: submitted to IEEE Transactions on Information Theor
Single-Symbol ML Decodable Distributed STBCs for Partially-Coherent Cooperative Networks
Space-time block codes (STBCs) that are single-symbol decodable (SSD) in a
co-located multiple antenna setting need not be SSD in a distributed
cooperative communication setting. A relay network with N relays and a single
source-destination pair is called a partially-coherent relay channel (PCRC) if
the destination has perfect channel state information (CSI) of all the channels
and the relays have only the phase information of the source-to-relay channels.
In this paper, first, a new set of necessary and sufficient conditions for a
STBC to be SSD for co-located multiple antenna communication is obtained. Then,
this is extended to a set of necessary and sufficient conditions for a
distributed STBC (DSTBC) to be SSD for a PCRC, by identifying the additional
conditions. Using this, several SSD DSTBCs for PCRC are identified among the
known classes of STBCs. It is proved that even if a SSD STBC for a co-located
MIMO channel does not satisfy the additional conditions for the code to be SSD
for a PCRC, single-symbol decoding of it in a PCRC gives full-diversity and
only coding gain is lost. It is shown that when a DSTBC is SSD for a PCRC, then
arbitrary coordinate interleaving of the in-phase and quadrature-phase
components of the variables does not disturb its SSD property for PCRC.
Finally, it is shown that the possibility of {\em channel phase compensation}
operation at the relay nodes using partial CSI at the relays increases the
possible rate of SSD DSTBCs from when the relays do not have CSI
to 1/2, which is independent of N
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