14 research outputs found
Diversity-Multiplexing Tradeoff in the Low-SNR Regime
An extension of the popular diversity-multiplexing tradeoff framework to the
low-SNR (or wideband) regime is proposed. The concept of diversity gain is
shown to be redundant in this regime since the outage probability is
SNR-independent and depends on the multiplexing gain and the channel power gain
statistics only. The outage probability under the DMT framework is obtained in
an explicit, closed form for a broad class of channels. The low and high-SNR
regime boundaries are explicitly determined for the scalar Rayleigh-fading
channel, indicating a significant limitation of the SNR-asymptotic DMT when the
multiplexing gain is small.Comment: accepted by IEEE Comm. Letter
Level Crossing Rate and Average Fade Duration of the Double Nakagami-m Random Process and Application in MIMO Keyhole Fading Channels
We present novel exact expressions and accurate closed-form approximations
for the level crossing rate (LCR) and the average fade duration (AFD) of the
double Nakagami-m random process. These results are then used to study the
second order statistics of multiple input multiple output (MIMO) keyhole fading
channels with space-time block coding. Numerical and computer simulation
examples validate the accuracy of the presented mathematical analysis and show
the tightness of the proposed approximations
Diversity-Multiplexing Tradeoff via Asymptotic Analysis of Large MIMO Systems
Diversity-multiplexing tradeoff (DMT) presents a compact framework to compare
various MIMO systems and channels in terms of the two main advantages they
provide (i.e. high data rate and/or low error rate). This tradeoff was
characterized asymptotically (SNR-> infinity) for i.i.d. Rayleigh fading
channel by Zheng and Tse [1]. The asymptotic DMT overestimates the finite-SNR
one [2]. In this paper, using the recent results on the asymptotic (in the
number of antennas) outage capacity distribution, we derive and analyze the
finite-SNR DMT for a broad class of channels (not necessarily Rayleigh fading).
Based on this, we give the convergence conditions for the asymptotic DMT to be
approached by the finite-SNR one. The multiplexing gain definition is shown to
affect critically the convergence point: when the multiplexing gain is defined
via the mean (ergodic) capacity, the convergence takes place at realistic SNR
values. Furthermore, in this case the diversity gain can also be used to
estimate the outage probability with reasonable accuracy. The multiplexing gain
definition via the high-SNR asymptote of the mean capacity (as in [1]) results
in very slow convergence for moderate to large systems (as 1/ln(SNR)^2) and,
hence, the asymptotic DMT cannot be used at realistic SNR values. For this
definition, the high-SNR threshold increases exponentially in the number of
antennas and in the multiplexing gain. For correlated keyhole channel, the
diversity gain is shown to decrease with correlation and power imbalance of the
channel. While the SNR-asymptotic DMT of Zheng and Tse does not capture this
effect, the size-asymptotic DMT does.Comment: To appear in 2007 IEEE International Symposium on Information Theory
(ISIT 2007), Nice, June 200
On Outage Probability and Diversity-Multiplexing Tradeoff in MIMO Relay Channels
Fading MIMO relay channels are studied analytically, when the source and
destination are equipped with multiple antennas and the relays have a single
one. Compact closed-form expressions are obtained for the outage probability
under i.i.d. and correlated Rayleigh-fading links. Low-outage approximations
are derived, which reveal a number of insights, including the impact of
correlation, of the number of antennas, of relay noise and of relaying
protocol. The effect of correlation is shown to be negligible, unless the
channel becomes almost fully correlated. The SNR loss of relay fading channels
compared to the AWGN channel is quantified. The SNR-asymptotic
diversity-multiplexing tradeoff (DMT) is obtained for a broad class of fading
distributions, including, as special cases, Rayleigh, Rice, Nakagami, Weibull,
which may be non-identical, spatially correlated and/or non-zero mean. The DMT
is shown to depend not on a particular fading distribution, but rather on its
polynomial behavior near zero, and is the same for the simple
"amplify-and-forward" protocol and more complicated "decode-and-forward" one
with capacity achieving codes, i.e. the full processing capability at the relay
does not help to improve the DMT. There is however a significant difference
between the SNR-asymptotic DMT and the finite-SNR outage performance: while the
former is not improved by using an extra antenna on either side, the latter can
be significantly improved and, in particular, an extra antenna can be
traded-off for a full processing capability at the relay. The results are
extended to the multi-relay channels with selection relaying and typical outage
events are identified.Comment: accepted by IEEE Trans. on Comm., 201
Open-Loop Beamforming Technique for MIMO System and Its Practical Realization
The concept of close-loop beamforming for MIMO system was well known proposed the singular value decomposition on channel matrix. This technique can improve the capacity performance, but the cost of feedback channel and the complexity processing discard the interest of implementation. Therefore, this paper aims to investigate the benefit of using an open-loop beamforming for MIMO system in practical approaches. The low-profile concept of open-loop beamforming which is convenient for implementation is proposed by just inserting Butler matrices at both transmitter and receiver. The simulation and measurement results indicate that the open-loop beamforming with Butler matrix outperforms the conventional MIMO system. Although, the close-loop beamforming offers a better performance than open-loop beamforming technique, the proposed system is attractive because it is low cost, uncomplicated, and easy to implement