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Interference cancellation for shot-code DS-CDMA in the presence of channel fading
Interference from other adjacent users in wireless applications is a major problem
in direct-sequence code-division multiple-access (DS-CDMA). This is also known as the
near-far problem where a strong signal from one user interferes with other users. The
current approach to deal with the near-far problem in DS-CDMA systems is to use strict
transmitter power control. An alternative approach is to use near-far resistant receivers.
The practical near-far resistance receiver structure is the adaptive decorrelating detectors
since it avoids complex matrix inversion.
The existing CDMA standard known as IS-95 uses a long signature code
sequence. However for simplicity, the adaptive multi-user receiver uses short signature
code sequence. The problem is that adaptive receivers lose near-far resistance as the
number of users increases in the system. This thesis describes a novel method of multistage
decision feedback cancellation (DFC) scheme immune from the near-far problem.
The performance of the new DFC structure is constructed using three different adaptive
algorithms: the least mean squared (LMS), the recursive least squared (RLS) and the
linearly constraint constant modulus (LCCM) adaptive algorithms. It is found that LMS
adaptive algorithm provides the best result considering its simple hardware complexity.
It is also found that the LMS adaptive receiver along with the DFC structure provides a
better bit synchronization capability to the over all system. Since the receiver is near-far
resistant, the LMS adaptive receiver along with the decision feedback cancellation
structure also performs better in the presence of Rayleigh fading
Channel estimation relying on the minimum bit-error-ratio criterion for BPSK and QPSK signals
The authors consider the channel estimation problem in the context of a linear equaliser designed for a frequency selective channel, which relies on the minimum bit-error-ratio (MBER) optimisation framework. Previous literature has shown that the MBER-based signal detection may outperform its minimum-mean-square-error (MMSE) counterpart in the bit-error-ratio performance sense. In this study, they develop a framework for channel estimation by first discretising the parameter space and then posing it as a detection problem. Explicitly, the MBER cost function (CF) is derived and its performance studied, when transmitting BPSK and QPSK signals. It is demonstrated that the MBER based CF aided scheme is capable of outperforming existing MMSE, least square-based solutions
Quantum Annealing: An Overview
In this review, after providing the basic physical concept behind quantum
annealing (or adiabatic quantum computation), we present an overview of some
recent theoretical as well as experimental developments pointing to the issues
which are still debated. With a brief discussion on the fundamental ideas of
continuous and discontinuous quantum phase transitions, we discuss the
Kibble-Zurek scaling of defect generation following a ramping of a quantum many
body system across a quantum critical point. In the process, we discuss
associated models, both pure and disordered, and shed light on implementations
and some recent applications of the quantum annealing protocols. Furthermore,
we discuss the effect of environmental coupling on quantum annealing. Some
possible ways to speed up the annealing protocol in closed systems are
elaborated upon: We especially focus on the recipes to avoid discontinuous
quantum phase transitions occurring in some models where energy gaps vanish
exponentially with the system size.Comment: Final version; in pres
Quenching Dynamics of a quantum XY spin-1/2 chain in presence of a transverse field
We study the quantum dynamics of a one-dimensional spin-1/2 anisotropic XY
model in a transverse field when the transverse field or the anisotropic
interaction is quenched at a slow but uniform rate. The two quenching schemes
are called transverse and anisotropic quenching respectively. Our emphasis in
this paper is on the anisotropic quenching scheme and we compare the results
with those of the other scheme. In the process of anisotropic quenching, the
system crosses all the quantum critical lines of the phase diagram where the
relaxation time diverges. The evolution is non-adiabatic in the time interval
when the parameters are close to their critical values, and is adiabatic
otherwise. The density of defects produced due to non-adiabatic transitions is
calculated by mapping the many-particle system to an equivalent Landau-Zener
problem and is generally found to vary as , where is the
characteristic time scale of quenching, a scenario that supports the
Kibble-Zurek mechanism. Interestingly, in the case of anisotropic quenching,
there exists an additional non-adiabatic transition, in comparison to the
transverse quenching case, with the corresponding probability peaking at an
incommensurate value of the wave vector. In the special case in which the
system passes through a multi-critical point, the defect density is found to
vary as . The von Neumann entropy of the final state is shown to
maximize at a quenching rate around which the ordering of the final state
changes from antiferromagnetic to ferromagnetic.Comment: 8 pages, 6 figure
Dynamics of linear polymers in random media
We study phenomenological scaling theories of the polymer dynamics in random
media, employing the existing scaling theories of polymer chains and the
percolation statistics. We investigate both the Rouse and the Zimm model for
Brownian dynamics and estimate the diffusion constant of the center-of-mass of
the chain in such disordered media. For internal dynamics of the chain, we
estimate the dynamic exponents. We propose similar scaling theory for the
reptation dynamics of the chain in the framework of Flory theory for the
disordered medium. The modifications in the case of correlated disordered are
also discussed.Comment: 4 pages, no figure
Effect of long range connections on an infinite randomness fixed point associated with the quantum phase transitions in a transverse Ising model
We study the effect of long-range connections on the infinite-randomness
fixed point associated with the quantum phase transitions in a transverse Ising
model (TIM). The TIM resides on a long-range connected lattice where any two
sites at a distance r are connected with a non-random ferromagnetic bond with a
probability that falls algebraically with the distance between the sites as
1/r^{d+\sigma}. The interplay of the fluctuations due to dilutions together
with the quantum fluctuations due to the transverse field leads to an
interesting critical behaviour. The exponents at the critical fixed point
(which is an infinite randomness fixed point (IRFP)) are related to the
classical "long-range" percolation exponents. The most interesting observation
is that the gap exponent \psi is exactly obtained for all values of \sigma and
d. Exponents depend on the range parameter \sigma and show a crossover to
short-range values when \sigma >= 2 -\eta_{SR} where \eta_{SR} is the anomalous
dimension for the conventional percolation problem. Long-range connections are
also found to tune the strength of the Griffiths phase.Comment: 5 pages, 1 figure, To appear in Phys. Rev.
Adiabatic multicritical quantum quenches: Continuously varying exponents depending on the direction of quenching
We study adiabatic quantum quenches across a quantum multicritical point
(MCP) using a quenching scheme that enables the system to hit the MCP along
different paths. We show that the power-law scaling of the defect density with
the rate of driving depends non-trivially on the path, i.e., the exponent
varies continuously with the parameter that defines the path, up to a
critical value ; on the other hand for , the scaling exponent saturates to a constant value. We show that
dynamically generated and {\it path()-dependent} effective critical
exponents associated with the quasicritical points lying close to the MCP (on
the ferromagnetic side), where the energy-gap is minimum, lead to this
continuously varying exponent. The scaling relations are established using the
integrable transverse XY spin chain and generalized to a MCP associated with a
-dimensional quantum many-body systems (not reducible to two-level systems)
using adiabatic perturbation theory. We also calculate the effective {\it
path-dependent} dimensional shift (or the shift in center of the
impulse region) that appears in the scaling relation for special paths lying
entirely in the paramagnetic phase. Numerically obtained results are in good
agreement with analytical predictions.Comment: 5 pages, 4 figure
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