4 research outputs found
Semidefinite Relaxation and Approximation Analysis of a Beamformed Alamouti Scheme for Relay Beamforming Networks
In this paper, we study the amplify-and-forward (AF) schemes in two-hop
one-way relay networks. In particular, we consider the multigroup multicast
transmission between long-distance users. Given that perfect channel state
information is perceived, our goal is to design the AF process so that the
max-min-fair (MMF) signal-to-interference-plus-noise ratio (SINR) is optimized
subject to generalized power constraints. We propose a rank-two beamformed
Alamouti (BFA) AF scheme and formulate the corresponding AF design problem as a
\emph{two-variable} fractional quadratically-constrained quadratic program
(QCQP), which is further tackled by the semidefinite relaxation (SDR)
technique. We analyze the approximation quality of two-variable fractional SDRs
under the Gaussian randomization algorithm. These results are fundamentally new
and reveal that the proposed BFA AF scheme can outperform the traditional BF AF
scheme, especially when there are many users in the system or many generalized
power constraints in the problem formulation. From a practical perspective, the
BFA AF scheme offers two degrees of freedom (DoFs) in beamformer design, as
opposed to the one DoF offered by the BF AF scheme, to improve the receivers'
SINR. In the latter part of this paper, we demonstrate how this extra DoF leads
to provable performance gains by considering two special cases of multicasting,
where the AF process is shown to employ a special structure. The numerical
simulations further validate that the proposed BFA AF scheme outperforms the BF
AF scheme and works well for large-scale relay systems
Quartic Perturbation-based Outage-constrained Robust Design in Two-hop One-way Relay Networks
In this work, we study a classic robust design problem in two-hop one-way
relay system. We are particularly interested in the scenario where channel
uncertainty exists in both the transmitter-to-relay and relay-to-receiver
links. By considering the problem design that minimizes the average
amplify-and-forward power budget at the relay side while satisfying SNR outage
requirements, an outage-constrained robust design problem involving quartic
perturbations is formulated to guarantee the robustness during transmission.
This problem is in general difficult as it involves constraints on the tail
probability of a high-order polynomial. Herein, we resort to moment inequality
and Bernstein-type inequality to tackle this problem, which provide convex
restrictions, or safe approximations, of the original design. We also analyze
the relative tightness of the two safe approximations for a quadratic
perturbation-based outage constrained problem. Our analysis shows that the
Bernstein-type inequality approach is less conservative than the moment
inequality approach when the outage rate is within some prescribed regime. To
our best knowledge, this is the first provable tightness result for these two
safe approximations. Our numerical simulations verify the superiority of the
robust design and corroborate the tightness results
Unraveling the Rank-One Solution Mystery of Robust MISO Downlink Transmit Optimization: A Verifiable Sufficient Condition via a New Duality Result
This paper concentrates on a robust transmit optimization problem for the
multiuser multi-input single-output (MISO) downlink scenario and under
inaccurate channel state information (CSI). This robust problem deals with a
general-rank transmit covariance design, and it follows a safe rate-constrained
formulation under spherically bounded CSI uncertainties. Curiously, simulation
results in previous works suggested that the robust problem admits rank-one
optimal transmit covariances in most cases. Such a numerical finding is
appealing because transmission with rank-one covariances can be easily realized
by single-stream transmit beamforming. This gives rise to a fundamentally
important question, namely, whether we can theoretically identify conditions
under which the robust problem admits a rank-one solution. In this paper, we
identify one such condition. Simply speaking, we show that the robust problem
is guaranteed to admit a rank-one solution if the CSI uncertainties are not too
large and the multiuser channel is not too poorly conditioned. To establish the
aforementioned condition, we develop a novel duality framework, through which
an intimate relationship between the robust problem and a related maximin
problem is revealed. Our condition involves only a simple expression with
respect to the multiuser channel and other system parameters. In particular,
unlike other sufficient rank-one conditions that have appeared in the
literature, ours is verifiable. The application of our analysis framework to
several other CSI uncertainty models is also discussed.Comment: To appear in IEEE Trans. Signal Process. This version combines the
main manuscript and its accompanied supplementary report into one single
articl
Symbol-level and Multicast Precoding for Multiuser Multiantenna Downlink: A Survey, Classification and Challenges
Precoding has been conventionally considered as an effective means of
mitigating the interference and efficiently exploiting the available in the
multiantenna downlink channel, where multiple users are simultaneously served
with independent information over the same channel resources. The early works
in this area were focused on transmitting an individual information stream to
each user by constructing weighted linear combinations of symbol blocks
(codewords). However, more recent works have moved beyond this traditional view
by: i) transmitting distinct data streams to groups of users and ii) applying
precoding on a symbol-per-symbol basis. In this context, the current survey
presents a unified view and classification of precoding techniques with respect
to two main axes: i) the switching rate of the precoding weights, leading to
the classes of block- and symbol-level precoding, ii) the number of users that
each stream is addressed to, hence unicast-/multicast-/broadcast- precoding.
Furthermore, the classified techniques are compared through representative
numerical results to demonstrate their relative performance and uncover
fundamental insights. Finally, a list of open theoretical problems and
practical challenges are presented to inspire further research in this area.Comment: Submitted to IEEE Communications Surveys & Tutorial