2,082 research outputs found
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
Integer-Forcing Linear Receivers
Linear receivers are often used to reduce the implementation complexity of
multiple-antenna systems. In a traditional linear receiver architecture, the
receive antennas are used to separate out the codewords sent by each transmit
antenna, which can then be decoded individually. Although easy to implement,
this approach can be highly suboptimal when the channel matrix is near
singular. This paper develops a new linear receiver architecture that uses the
receive antennas to create an effective channel matrix with integer-valued
entries. Rather than attempting to recover transmitted codewords directly, the
decoder recovers integer combinations of the codewords according to the entries
of the effective channel matrix. The codewords are all generated using the same
linear code which guarantees that these integer combinations are themselves
codewords. Provided that the effective channel is full rank, these integer
combinations can then be digitally solved for the original codewords. This
paper focuses on the special case where there is no coding across transmit
antennas and no channel state information at the transmitter(s), which
corresponds either to a multi-user uplink scenario or to single-user V-BLAST
encoding. In this setting, the proposed integer-forcing linear receiver
significantly outperforms conventional linear architectures such as the
zero-forcing and linear MMSE receiver. In the high SNR regime, the proposed
receiver attains the optimal diversity-multiplexing tradeoff for the standard
MIMO channel with no coding across transmit antennas. It is further shown that
in an extended MIMO model with interference, the integer-forcing linear
receiver achieves the optimal generalized degrees-of-freedom.Comment: 40 pages, 16 figures, to appear in the IEEE Transactions on
Information Theor
Degrees of Freedom of Wireless X Networks
We explore the degrees of freedom of user wireless networks,
i.e. networks of transmitters and receivers where every transmitter has
an independent message for every receiver. We derive a general outerbound on
the degrees of freedom \emph{region} of these networks. When all nodes have a
single antenna and all channel coefficients vary in time or frequency, we show
that the \emph{total} number of degrees of freedom of the network is equal
to per orthogonal time and frequency dimension.
Achievability is proved by constructing interference alignment schemes for
networks that can come arbitrarily close to the outerbound on degrees of
freedom. For the case where either M=2 or N=2 we find that the outerbound is
exactly achievable. While networks have significant degrees of freedom
benefits over interference networks when the number of users is small, our
results show that as the number of users increases, this advantage disappears.
Thus, for large , the user wireless network loses half the
degrees of freedom relative to the MIMO outerbound achievable
through full cooperation. Interestingly, when there are few transmitters
sending to many receivers () or many transmitters sending to few
receivers (), networks are able to approach the degrees
of freedom possible with full cooperation on the MIMO channel.
Similar to the interference channel, we also construct an example of a 2 user
channel with propagation delays where the outerbound on degrees of freedom
is achieved through interference alignment based on a simple TDMA strategy.Comment: 26 page
Interference alignment at intermediate SNR with perfect or noisy CSI
Interference alignment is a new technique combining transmitter precoding and receiver interference suppression to achieve the optimal multiplexing gain in interference networks by exploiting knowledge of channel state information of all transmission links. So far closed form solutions for the transmit filters have only been found in certain cases. Also the feasibility of interference alignment schemes based on symbol extensions, over a limited number of signalling dimensions, is still an open problem.
In this work we investigate the performance in terms of bit error rates, of interference alignment schemes at intermediate signal-to-noise ratios, through Monte Carlo simulations. We focus our attention on the three and four users time-varying interference channel, using both the closed form solutions
known at present as well as iterative algorithms. We then investigate the impact of noisy channel state information on the performances of some of the interference alignment systems considered.
In the single input single output interference channel the closed form solutions of the interference alignment cause considerably different bit error rates for the different nodes in the network. In the multiple input multiple output interference channel we exhibit that bit error rate saturates at moderate signal-to-noise ratios when interference alignment schemes are infeasible
and even when they are feasible, some of the analyzed algorithms show unpredictable behaviors by deteriorating the performance as the signal-to-noise ratio exceeds a threshold. Further renements are necessary in order to obtain better bit error rates in these cases. We evince that additional
improvements are also needed to the original interference alignment schemes in order to mitigate their sensitivity to noisy channel state informatio
Smart antennas: state of the art
Aim of this contribution is to illustrate the state of the art of smart antenna research from several perspectives. The bow is drawn from transmitter issues via channel measurements and modeling, receiver signal processing, network aspects, technological challenges towards first smart antenna applications and current status of standardization. Moreover, some future prospects of different disciplines in smart antenna research are given.Peer Reviewe
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