4,899 research outputs found
Downlink MBER beamforming transmitter based on uplink MBER beamforming receiver for TDD-SDMA systems
The downlink minimum bit error rate (MBER) transmit beamforming is directly derived based on the uplink MBER receive beamforming for time division duplex (TDD) space-division multiple-access (SDMA) multiple-input multiple-output systems, where the base station (BS) is equipped with multiple antennas to support multiple single-antenna mobile terminals (MTs). It is shown that the relationship between multiuser detection and multiuser transmission can still be applied for the rank-deficient system where the number of users supported is more than the number of transmit antennas available at the BS, if the MBER design is adopted. The proposed MBER transmit beamforming scheme is capable of achieving good performance for rank-deficient TDD-SDMA systems with the support of low-complexity and high power-efficient MTs, and its robustness to the downlink and uplink noise or channel mismatch is verified using simulation
Robust Monotonic Optimization Framework for Multicell MISO Systems
The performance of multiuser systems is both difficult to measure fairly and
to optimize. Most resource allocation problems are non-convex and NP-hard, even
under simplifying assumptions such as perfect channel knowledge, homogeneous
channel properties among users, and simple power constraints. We establish a
general optimization framework that systematically solves these problems to
global optimality. The proposed branch-reduce-and-bound (BRB) algorithm handles
general multicell downlink systems with single-antenna users, multiantenna
transmitters, arbitrary quadratic power constraints, and robustness to channel
uncertainty. A robust fairness-profile optimization (RFO) problem is solved at
each iteration, which is a quasi-convex problem and a novel generalization of
max-min fairness. The BRB algorithm is computationally costly, but it shows
better convergence than the previously proposed outer polyblock approximation
algorithm. Our framework is suitable for computing benchmarks in general
multicell systems with or without channel uncertainty. We illustrate this by
deriving and evaluating a zero-forcing solution to the general problem.Comment: Published in IEEE Transactions on Signal Processing, 16 pages, 9
figures, 2 table
Outage Constrained Robust Secure Transmission for MISO Wiretap Channels
In this paper we consider the robust secure beamformer design for MISO
wiretap channels. Assume that the eavesdroppers' channels are only partially
available at the transmitter, we seek to maximize the secrecy rate under the
transmit power and secrecy rate outage probability constraint. The outage
probability constraint requires that the secrecy rate exceeds certain threshold
with high probability. Therefore including such constraint in the design
naturally ensures the desired robustness. Unfortunately, the presence of the
probabilistic constraints makes the problem non-convex and hence difficult to
solve. In this paper, we investigate the outage probability constrained secrecy
rate maximization problem using a novel two-step approach. Under a wide range
of uncertainty models, our developed algorithms can obtain high-quality
solutions, sometimes even exact global solutions, for the robust secure
beamformer design problem. Simulation results are presented to verify the
effectiveness and robustness of the proposed algorithms
Optimal Multiuser Transmit Beamforming: A Difficult Problem with a Simple Solution Structure
Transmit beamforming is a versatile technique for signal transmission from an
array of antennas to one or multiple users [1]. In wireless communications,
the goal is to increase the signal power at the intended user and reduce
interference to non-intended users. A high signal power is achieved by
transmitting the same data signal from all antennas, but with different
amplitudes and phases, such that the signal components add coherently at the
user. Low interference is accomplished by making the signal components add
destructively at non-intended users. This corresponds mathematically to
designing beamforming vectors (that describe the amplitudes and phases) to have
large inner products with the vectors describing the intended channels and
small inner products with non-intended user channels.
While it is fairly easy to design a beamforming vector that maximizes the
signal power at the intended user, it is difficult to strike a perfect balance
between maximizing the signal power and minimizing the interference leakage. In
fact, the optimization of multiuser transmit beamforming is generally a
nondeterministic polynomial-time (NP) hard problem [2]. Nevertheless, this
lecture shows that the optimal transmit beamforming has a simple structure with
very intuitive properties and interpretations. This structure provides a
theoretical foundation for practical low-complexity beamforming schemes.
(See this lecture note for the complete abstract/introduction)Comment: Accepted for publication as lecture note in IEEE Signal Processing
Magazine, 11 pages, 3 figures. The results can be reproduced using the
following Matlab code: https://github.com/emilbjornson/optimal-beamformin
Interference-Aware Scheduling for Connectivity in MIMO Ad Hoc Multicast Networks
We consider a multicast scenario involving an ad hoc network of co-channel
MIMO nodes in which a source node attempts to share a streaming message with
all nodes in the network via some pre-defined multi-hop routing tree. The
message is assumed to be broken down into packets, and the transmission is
conducted over multiple frames. Each frame is divided into time slots, and each
link in the routing tree is assigned one time slot in which to transmit its
current packet. We present an algorithm for determining the number of time
slots and the scheduling of the links in these time slots in order to optimize
the connectivity of the network, which we define to be the probability that all
links can achieve the required throughput. In addition to time multiplexing,
the MIMO nodes also employ beamforming to manage interference when links are
simultaneously active, and the beamformers are designed with the maximum
connectivity metric in mind. The effects of outdated channel state information
(CSI) are taken into account in both the scheduling and the beamforming
designs. We also derive bounds on the network connectivity and sum transmit
power in order to illustrate the impact of interference on network performance.
Our simulation results demonstrate that the choice of the number of time slots
is critical in optimizing network performance, and illustrate the significant
advantage provided by multiple antennas in improving network connectivity.Comment: 34 pages, 12 figures, accepted by IEEE Transactions on Vehicular
Technology, Dec. 201
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