Showing 3 results for Falahati
,
Volume 1, Issue 1 (January 2005)
Abstract
In an environment such as underwater channel where placing test equipments are
difficult to handle, it is much practical to have hardware simulators to examine suitably
designed transceivers (transmitter/receiver). The simulators of this kind will then allow
researchers to observe their intentions and carry out repetitive tests to find suitable digital
coding/decoding algorithms.
In this paper, a simplified shallow water digital data transmission system is first introduced.
The transmission channel considered here is a stochastic DSP hardware model in which
signal degradations leads to a severe distortion in phase and amplitude (fades) across the
bandwidth of the received signal. A computer base-band channel model with frequency
non-selective feature is derived by the authors [10-11]. This system was based on fullraised
cosine channel modelling and proved to be the most suitable for vertical and shortrange
underwater communication csdfher), with a reflected path (specula component, when
the acoustic hydrophone receives reflected signals from surface and bottom of the sea) and
a random path (diffused component, when the acoustic hydrophone receives scattered
signals from the volume of the sea). The model assumed perfect transmitter-receiver
synchronization but utilized realistic channel time delays, and demonstrated the timevarying
characteristics of an underwater acoustic channel observed in practice. In this
paper, they are used to provide a full system simulation in order to design an adaptive
receiver employing the most advanced digital signal processing techniques in hardware to
predict realizable error performances.
A. Falahati, M.-R. Ardestani,
Volume 4, Issue 1 (April 2008)
Abstract
A low complexity dynamic subcarrier and power allocation methodology for
downlink communication in an OFDM-based multiuser environment is developed. The
problem of maximizing overall capacity with constraints on total power consumption, bit
error rate and data rate proportionality among users requiring different QOS specifications
is formulated. Assuming perfect knowledge of the instantaneous channel gains for all users,
a new simple algorithm is developed to solve the mentioned problem. We compare the sum
capacity, proportionality, and computational complexity of the proposed algorithm with the
one presented by Wong et al. Numerical results demonstrate that the proposed algorithm
offers a performance comparable with Wong’s algorithm, yet complexity remains low and
proportionality constraint will be tightly satisfied. As well, the proposed algorithm can
provide a flexible trade-off between complexity, capacity and proportionality constraint.
M. Rezaei, A. Falahati,
Volume 12, Issue 1 (March 2016)
Abstract
In this paper, a cooperative algorithm to improve the orthogonal space-timefrequency block codes (OSTFBC) in frequency selective channels for 2*1, 2*2, 4*1, 4*2 MIMO-OFDM systems, is presented. The algorithm of three node, a source node, a relay node and a destination node is formed, and is implemented in two stages. During the first stage, the destination and the relay antennas receive the symbols sent by the source antennas. The destination node and the relay node obtain the decision variables employing time-space-frequency decoding process by the received signals. During the second stage, the relay node transmits decision variables to the destination node. Due to the increasing diversity in the proposed algorithm, decision variables in the destination node are increased to improve system performance. The bit error rate of the proposed algorithm at high SNR is estimated by considering the BPSK modulation. The simulation results show that cooperative orthogonal space-time-frequency block coding, improves system performance and reduces the BER in a frequency selective channel.
