92 research outputs found
Robust Symbol Level Precoding for Overlay Cognitive Radio Networks
This paper focuses on designing robust symbol-level precoding (SLP) in the
downlink of an overlay cognitive radio (CR) network, where a primary base
station (PBS) serving primary users (PUs) and a cognitive base station (CBS)
serving cognitive users (CUs) share the same frequency band. When the PBS
shares data and perfect channel state information (CSI) with the CBS, an SLP
approach which minimizes the CR transmission power and satisfies symbol-wise
Safety Margin (SM) constraints of both PUs and CUs, is obtained in a
low-complexity quadratic formulation. Then for the case of imperfect CSI from
the PBS to CBS, we propose robust SLP schemes. First, with a norm-bounded CSI
error model to approximate uncertain channels at the PBS, we adopt the max-min
philosophy to conservatively achieve robust SLP constraints. Second, we use the
additive quantization noise model (AQNM) to describe the statistics of the
quantized PBS CSI, and we employ a stochastic constraint to formulate the
problem, where the SM constraints are converted to be deterministic. Simulation
results show that the proposed robust SLP schemes help enable PUs to mitigate
negative effect of the quantization noise and simultaneously offer CR
transmission with significant improvements in energy efficiency compared to
non-robust methods.Comment: 30 pages, 13 figures, journa
Robust Vehicular Communications for Traffic Safety---Channel Estimation and Multiantenna Schemes
Vehicular communications, where vehicles exchange information with other vehicles or entities in the road traffic environment, is expected to be a part of the future transportation system and promises to support a plethora of applications for traffic safety and efficiency. In particular, vehicle-to-vehicle (V2V) communication promises to support numerous traffic safety applications by enabling a vehicle to broadcast its current status to all the other vehicles in its surrounding.\ua0 \ua0 Vehicular wireless channels can be highly time- and/or frequency-selective due to high mobility of the vehicles and/or large delay spreads. IEEE 802.11p has been specified as the physical layer standard for vehicular communications, where the pilots are densely concentrated at the beginning of a frame. As a consequence, accurate channel estimation in later parts of the frame becomes a challenging task. In this thesis, a solution to overcome the ill-suited pilot pattern is studied; a cross-layered scheme to insert complementary pilots into an 802.11p frame is proposed. The scheme does not require modifications to the 802.11p standard and a modified receiver can utilize the complementary pilots for accurate channel estimation in vehicular channels.\ua0 \ua0 The metallic components of present-day vehicles pose a challenge in designing antenna systems that satisfy a minimum required directive gain in the entire horizontal plane. Multiple antennas with contrasting directive gain patterns can be used to alleviate the problems due to low directive gains. A scheme that combines the output of L antennas to the input of a single-port receiver is proposed in the thesis. The combining scheme is designed to minimize the probability of a burst error, i.e., an unsuccessful decoding of K consecutive packets from a transmitter arriving in the direction of low directive gains of the individual antennas. To minimize complexity, the scheme does not estimate or use any channel state information. It is shown using measured and simulated directive gain patterns that the probability of burst errors for packets arriving in the direction of low directive gains of the individual antenna elements can be minimized.\ua0 \ua0 The enhanced distributed channel access (EDCA) scheme is used in V2V communications to facilitate the sharing of allocated time-frequency resources. The packet success ratio (PSR) of the broadcast messages in the EDCA scheme depends on the number of vehicles and the packet transmission rate. The interference at a receiving vehicle increases due to multiple simultaneous transmissions when the number of vehicles grows beyond a limit, resulting in the decrease of the PSR. A receiver setup with sector antennas, where the output of each antenna can be processed separately to decode a packet, is described in the thesis with a detailed performance analysis. A significant increase in the PSR is shown in a dense vehicular scenario by using four partially overlapping sector antennas compared with a single omnidirectional antenna setup
Interference mitigation using group decoding in multiantenna systems
fi=vertaisarvioitu|en=peerReviewed
Robust transmit beamforming design using outage probability specification
Transmit beamforming (precoding) is a powerful technique for enhancing the channel capacity
and reliability of multiple-input and multiple-output (MIMO) wireless systems. The optimum
exploitation of the benefits provided by MIMO systems can be achieved when a perfect channel
state information at transmitter (CSIT) is available. In practices, however, the channel knowledge
is generally imperfect at transmitter because of the inevitable errors induced by finite
feedback channel capacity, quantization and other physical constraints. Such errors degrade the
system performance severely. Hence, robustness has become a crucial issue.
Current robust designs address the channel imperfections with the worst-case and stochastic approaches.
In worst-case analysis, the channel uncertainties are considered as deterministic and
norm-bounded, and the resulting design is a conservative optimization that guarantees a certain
quality of service (QoS) for every allowable perturbation. The latter approach focuses on the
average performance under the assumption of channel statistics, such as mean and covariance.
The system performance could break down when persistent extreme errors occur. Thus, an
outage probability-based approach is developed by keeping a low probability that channel condition
falls below an acceptable level. Compared to the aforementioned methods, this approach
can optimize the average performance as well as consider the extreme scenarios proportionally.
This thesis implements the outage-probability specification into transmit beamforming design
for three scenarios: the single-user MIMO system and the corresponding adaptive modulation
scheme as well as the multi-user MIMO system. In a single-user MIMO system, the transmit
beamformer provides the maximum average received SNR and ensures the robustness to the
CSIT errors by introducing probabilistic constraint on the instantaneous SNR. Beside the robustness
against channel imperfections, the outage probability-based approach also provides a
tight BER bound for adaptive modulation scheme, so that the maximum transmission rate can
be achieved by taking advantage of transmit beamforming. Moreover, in multi-user MIMO
(MU-MIMO) systems, the leakage power is accounted by probability measurement. The resulting
transmit beamformer is designed based on signal-to-leakage-plus-noise ratio (SLNR)
criteria, which maximizes the average received SNR and guarantees the least leakage energy
from the desired user. In such a setting, an outstanding BER performance can be achieved as
well as high reliability of signal-to-interference-plus-noise ratio (SINR).
Given the superior overall performances and significantly improved robustness, the probabilistic
approach provides an attractive alternative to existing robust techniques under imperfect
channel information at transmitter
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