A Azari | KTH Royal Institute of Technology (original) (raw)

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Papers by A Azari

In this paper a unified approach for power allocation (PA) problem in multi-hop orthogonal freque... more In this paper a unified approach for power allocation (PA) problem in multi-hop orthogonal frequency division
multiplexing (OFDM) amplify-and-forward (AF) relaying systems has been developed. In the proposed approach, we
consider short and long-term individual and total power constraints at the source and relays, and devise decentralized
low complexity PA algorithms when wireless links are subject to channel path-loss and small-scale Rayleigh fading.
In particular, aiming at improving the instantaneous rate of multi-hop transmission systems with AF relaying, we
develop (i) a near-optimal iterative PA algorithm based on the exact analysis of the received SNR at the destination;
(ii) a low complexity-suboptimal iterative PA algorithm based on an approximate expression at high-SNR regime;
and (iii) a low complexity-non iterative PA scheme with limited performance loss. Since the PA problem in multi-hop
systems is too complex to solve with known optimization solvers, in the proposed formulations, we adopted a twostage
approach, including a power distribution phase among distinct subcarriers, and a power allocation phase among
different relays. The individual PA phases are then appropriately linked through an iterative method which tries to
compensate the performance loss caused by the distinct two-stage approach. Simulation results show the superior
performance of the proposed power allocation algorithms.

The K-user flat fading MIMO interference channel with J instantaneous relays (KICJR) is consider... more The K-user flat fading MIMO interference channel
with J instantaneous relays (KICJR) is considered. In the
KICJR, the effective channel between sources and destinations
including the relays has certain structure and is non-generic. For
non-generic channels, the achievable degrees of freedom (DoF) is
still unknown. Lee and Wang showed that by using the aligned
interference neutralization scheme 3/2 degrees of freedom is
achievable in a 2IC1R system, which is 50% more than the 2-user
interference channel. But the DoF performance and achievable
schemes for other KICJR networks are not investigated in
literature. In this paper we devise an achievable scheme called
restricted interference alignment for instantaneous-relay aided
interference channels. Also, to find insights to the maximum
achievable degrees of freedom we develop linear beamforming
based on the mean square error (MSE) minimization as an
achievable scheme. Furthermore, we present upper-bounds on
the maximum achievable degrees of freedom by investigating
the properness of the interference alignment equation system.
The numerical results show that the DoF performance of the
proposed restricted interference alignment scheme and the MSEbased
beamforming match the upper-bounds determined from
the properness of the interference alignment equations.

In this paper the scheduling and transmit power control are investigated to minimize the energy c... more In this paper the scheduling and transmit power
control are investigated to minimize the energy consumption
for battery-driven devices deployed in LTE networks. To enable
efficient scheduling for a massive number of machine-type
subscribers, a novel distributed scheme is proposed to let machine
nodes form local clusters and communicate with the base-station
through the cluster-heads. Then, uplink scheduling and power
control in LTE networks are introduced and lifetime-aware solu-
tions are investigated to be used for the communication between
cluster-heads and the base-station. Beside the exact solutions,
low-complexity suboptimal solutions are presented in this work
which can achieve near optimal performance with much lower
computational complexity. The performance evaluation shows
that the network lifetime is significantly extended using the
proposed protocols.

In this paper the uplink scheduling and transmit power control are investigated to minimize the ... more In this paper the uplink scheduling and transmit power
control are investigated to minimize the energy consumption for
battery-driven devices deployed in cellular networks. A lifetime
metric, based on the accurate energy consumption model for
cellular-based machine devices, is provided and used to formulate the
uplink scheduling and power control problems as network-lifetime
maximization problems. Then, lifetime-aware uplink scheduling and
power control protocols which maximize the overall network-lifetime
are investigated based on the different lifetime definitions. Besides
the exact solution, a low-complexity suboptimal solution is presented
in this work which can achieve near optimal lifetime-performance
with much lower computational complexity. The performance evaluation
shows that the network lifetime is significantly extended under
proposed protocols.

GlobalSIP 2014

In Machine-to-Machine (M2M) networks, an energy efficient scalable medium access control (MAC) i... more In Machine-to-Machine (M2M) networks, an energy
efficient scalable medium access control (MAC) is crucial for
serving massive battery-driven machine-type devices. In this
paper, we investigate the energy efficient MAC design to minimize
battery power consumption in cellular-based M2M communications.
We present an energy efficient MAC protocol that not
only adapts contention and reservation-based protocols for M2M
communications in cellular networks, but also benefits from
partial clustering to handle the massive access problem. Then we
investigate the energy efficiency and access capacity of contentionbased
protocols and present an energy efficient contention-based
protocol for intra-cluster communication of the proposed MAC,
which results in huge power saving. The simulation results show
that the proposed MAC protocol outperforms the others in energy saving without sacrificing much delay or throughput. Also, the
lifetimes of both individual nodes and the whole M2M network
are significantly extended.

This paper proposes a synchronization method with a preamble for IEEE 802.16 and other systems th... more This paper proposes a synchronization method with a preamble for IEEE 802.16 and other systems that use Orthogonal Frequency Division Multiplexing (OFDM). The preamble for synchronization is composed of two identical periods, which are polyphase codes, and have an autocorrelation function with one main peak and very small sub peaks. Using this signal repetition and autocorrelation, the correlation for symbol timing detection is calculated with a new metric. The proposed metric uses a filtering structure to find the accurate start point. Proposed method drastically improves the performance of timing synchronization, even with a very low SNR, in both AWGN and multipath fading Rayleigh channels.

Frame synchronization is a critical factor for effective operation of Orthogonal Frequency Divisi... more Frame synchronization is a critical factor for effective operation of Orthogonal Frequency Division Multiplexing (OFDM) schemes. This paper studies the frame synchronization of OFDM based wireless systems and proposes a new method for IEEE 802.16 (WiMAX) to improve its performance. The preamble, which is used in this method, is consists of two identical halves, and our method uses the correlation between these two parts in a new efficient form. Our metric calculates the start point within two levels. It finds some possible points as start point in the first level and removes the irrelevant candidates at the second level. The performance of proposed metric and existing ones is compared in terms of mean square error (MSE) and probability of correct detection in AWGN and multipath fading channels. Simulation results show that the proposed method has better performances than famous methods, which were proposed by Schmidl, Minn, Park, Kim, Meng, and in both AWGN and multipath fading Rayleigh channels. Also, compared to new methods with more accurate performance, proposed method has egual performance with less complexity.

Thesis Chapters by A Azari

Relayed transmission has been proposed as a viable option to improve reliability and to extend wi... more Relayed transmission has been proposed as a viable option to improve
reliability and to extend wireless network coverage. On the other hand, emerging
wireless applications, e.g. sensor networks, have an increasing demand for small
devices having limited battery lifetimes. Thus, it is important to determine the
appropriate transmit power levels for the source and the relays. In this thesis, we
have analyzed some scenarios of cooperative communication systems, and some
new methods for power allocation of these systems, have been proposed.

This thesis investigates the design and analysis of linear interference alignment in K-user flat ... more This thesis investigates the design and analysis of linear interference alignment
in K-user flat fading MIMO interference channel with J instantaneous relays (KICJR).
Prior art in the literature involves aligned interference neutralization for 2-user
interference channel with one instantaneous relay to obtain 3/2 degrees of freedom.
We investigate transceiver design for linear interference alignment based on the
mean square error criterion using an iterative algorithm. Joint design of transmitter,
receiver, and relay filters is considered, and a lower bound on the achievable
DoF in the said channel is obtained. In addition, rank based necessary conditions
for solvability of interference alignment equation system is developed. By evaluating
these necessary conditions, we establish upper bounds that are to be satisfied
by any achievable degrees of freedom tuple for the K users through linear interference
alignment. Using these results, the exact DoF performance of 2IC-JR is
derived. We then extend this to an achievable scheme herein referred to as the restricted
interference alignment which provides improved DoF performance in the
case of KIC-JR. Also for physical layer security, the secure achievable degrees of
freedom, and the achievable scheme for this DoF is investigated. Finally, for systems
without channel state information available at the transmitters, the bound on
achievable DoF, and the achievable scheme for this DoF is presented.

GlobalSIP 2014