3,446 research outputs found
MIMO Assisted Space-Code-Division Multiple-Access: Linear Detectors and Performance over Multipath Fading Channels
In this contribution we propose and investigate a multiple-input multiple-output space-division, code-division multiple-access (MIMO SCDMA) scheme. The main objective is to improve the capacity of the existing DS-CDMA systems, for example, for supporting an increased number of users, by deploying multiple transmit and receive antennas in the corresponding systems and by using some advanced transmission and detection algorithms. In the proposed MIMO SCDMA system, each user can be distinguished jointly by its spreading code-signature and its unique channel impulse response (CIR) transfer function referred to as spatial-signature. Hence, the number of users might be supported by the MIMO SCDMA system and the corresponding achievable performance are determined by the degrees of freedom provided by both the code-signatures and the spatial-signatures, as well as by how efficiently the degrees of freedom are exploited. Specifically, the number of users supported by the proposed MIMO SCDMA can be significantly higher than the number of chips per bit, owing to the employment of space-division. In this contribution space-time spreading (STS) is employed for configuring the transmitted signals. Three types of low-complexity linear detectors, namely correlation, decorrelating and minimum mean-square error (MMSE), are considered for detecting the MIMO SCDMA signals. The BER performance of the MIMO SCDMA system associated with these linear detectors are evaluated by simulations, when assuming that the MIMO SCDMA signals are transmitted over multipath Rayleigh fading channels. Our study and simulation results show that MIMO SCDMA assisted by multiuser detection is capable of facilitating joint space-time de-spreading, multipath combining and receiver diversity combining, while simultaneously suppressing the multiuser interfering signals
Generic Multiuser Coordinated Beamforming for Underlay Spectrum Sharing
The beamforming techniques have been recently studied as possible enablers
for underlay spectrum sharing. The existing beamforming techniques have several
common limitations: they are usually system model specific, cannot operate with
arbitrary number of transmit/receive antennas, and cannot serve arbitrary
number of users. Moreover, the beamforming techniques for underlay spectrum
sharing do not consider the interference originating from the incumbent primary
system. This work extends the common underlay sharing model by incorporating
the interference originating from the incumbent system into generic combined
beamforming design that can be applied on interference, broadcast or multiple
access channels. The paper proposes two novel multiuser beamforming algorithms
for user fairness and sum rate maximization, utilizing newly derived convex
optimization problems for transmit and receive beamformers calculation in a
recursive optimization. Both beamforming algorithms provide efficient operation
for the interference, broadcast and multiple access channels, as well as for
arbitrary number of antennas and secondary users in the system. Furthermore,
the paper proposes a successive transmit/receive optimization approach that
reduces the computational complexity of the proposed recursive algorithms. The
results show that the proposed complexity reduction significantly improves the
convergence rates and can facilitate their operation in scenarios which require
agile beamformers computation.Comment: 30 pages, 5 figure
Fundamental Limits in Correlated Fading MIMO Broadcast Channels: Benefits of Transmit Correlation Diversity
We investigate asymptotic capacity limits of the Gaussian MIMO broadcast
channel (BC) with spatially correlated fading to understand when and how much
transmit correlation helps the capacity. By imposing a structure on channel
covariances (equivalently, transmit correlations at the transmitter side) of
users, also referred to as \emph{transmit correlation diversity}, the impact of
transmit correlation on the power gain of MIMO BCs is characterized in several
regimes of system parameters, with a particular interest in the large-scale
array (or massive MIMO) regime. Taking the cost for downlink training into
account, we provide asymptotic capacity bounds of multiuser MIMO downlink
systems to see how transmit correlation diversity affects the system
multiplexing gain. We make use of the notion of joint spatial division and
multiplexing (JSDM) to derive the capacity bounds. It is advocated in this
paper that transmit correlation diversity may be of use to significantly
increase multiplexing gain as well as power gain in multiuser MIMO systems. In
particular, the new type of diversity in wireless communications is shown to
improve the system multiplexing gain up to by a factor of the number of degrees
of such diversity. Finally, performance limits of conventional large-scale MIMO
systems not exploiting transmit correlation are also characterized.Comment: 29 pages, 8 figure
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