122 research outputs found
Generalized discrete Fourier transform with non-linear phase : theory and design
Constant modulus transforms like discrete Fourier transform (DFT), Walsh transform, and Gold codes have been successfully used over several decades in various engineering applications, including discrete multi-tone (DMT), orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) communications systems. Among these popular transforms, DFT is a linear phase transform and widely used in multicarrier communications due to its performance and fast algorithms. In this thesis, a theoretical framework for Generalized DFT (GDFT) with nonlinear phase exploiting the phase space is developed. It is shown that GDFT offers sizable correlation improvements over DFT, Walsh, and Gold codes. Brute force search algorithm is employed to obtain orthogonal GDFT code sets with improved correlations. Design examples and simulation results on several channel types presented in the thesis show that the proposed GDFT codes, with better auto and cross-correlation properties than DFT, lead to better bit-error-rate performance in all multi-carrier and multi-user communications scenarios investigated. It is also highlighted how known constant modulus code families such as Walsh, Walsh-like and other codes are special solutions of the GDFT framework. In addition to theoretical framework, practical design methods with computationally efficient implementations of GDFT as enhancements to DFT are presented in the thesis. The main advantage of the proposed method is its ability to design a wide selection of constant modulus orthogonal code sets based on the desired performance metrics mimicking the engineering .specs of interest.
Orthogonal Frequency Division Multiplexing (OFDM) is a leading candidate to be adopted for high speed 4G wireless communications standards due to its high spectral efficiency, strong resistance to multipath fading and ease of implementation with Fast Fourier Transform (FFT) algorithms. However, the main disadvantage of an OFDM based communications technique is of its high PAPR at the RF stage of a transmitter. PAPR dominates the power (battery) efficiency of the radio transceiver. Among the PAPR reduction methods proposed in the literature, Selected Mapping (SLM) method has been successfully used in OFDM communications. In this thesis, an SLM method employing GDFT with closed form phase functions rather than fixed DFT for PAPR reduction is introduced. The performance improvements of GDFT based SLM PAPR reduction for various OFDM communications scenarios including the WiMAX standard based system are evaluated by simulations. Moreover, an efficient implementation of GDFT based SLM method reducing computational cost of multiple transform operations is forwarded. Performance simulation results show that power efficiency of non-linear RF amplifier in an OFDM system employing proposed method significantly improved
Design of Block Transceivers with Decision Feedback Detection
This paper presents a method for jointly designing the transmitter-receiver
pair in a block-by-block communication system that employs (intra-block)
decision feedback detection. We provide closed-form expressions for
transmitter-receiver pairs that simultaneously minimize the arithmetic mean
squared error (MSE) at the decision point (assuming perfect feedback), the
geometric MSE, and the bit error rate of a uniformly bit-loaded system at
moderate-to-high signal-to-noise ratios. Separate expressions apply for the
``zero-forcing'' and ``minimum MSE'' (MMSE) decision feedback structures. In
the MMSE case, the proposed design also maximizes the Gaussian mutual
information and suggests that one can approach the capacity of the block
transmission system using (independent instances of) the same (Gaussian) code
for each element of the block. Our simulation studies indicate that the
proposed transceivers perform significantly better than standard transceivers,
and that they retain their performance advantages in the presence of error
propagation.Comment: 14 pages, 8 figures, to appear in the IEEE Transactions on Signal
Processin
Group Frames with Few Distinct Inner Products and Low Coherence
Frame theory has been a popular subject in the design of structured signals
and codes in recent years, with applications ranging from the design of
measurement matrices in compressive sensing, to spherical codes for data
compression and data transmission, to spacetime codes for MIMO communications,
and to measurement operators in quantum sensing. High-performance codes usually
arise from designing frames whose elements have mutually low coherence.
Building off the original "group frame" design of Slepian which has since been
elaborated in the works of Vale and Waldron, we present several new frame
constructions based on cyclic and generalized dihedral groups. Slepian's
original construction was based on the premise that group structure allows one
to reduce the number of distinct inner pairwise inner products in a frame with
elements from to . All of our constructions further
utilize the group structure to produce tight frames with even fewer distinct
inner product values between the frame elements. When is prime, for
example, we use cyclic groups to construct -dimensional frame vectors with
at most distinct inner products. We use this behavior to bound
the coherence of our frames via arguments based on the frame potential, and
derive even tighter bounds from combinatorial and algebraic arguments using the
group structure alone. In certain cases, we recover well-known Welch bound
achieving frames. In cases where the Welch bound has not been achieved, and is
not known to be achievable, we obtain frames with close to Welch bound
performance
Quantum algorithms for highly non-linear Boolean functions
Attempts to separate the power of classical and quantum models of computation
have a long history. The ultimate goal is to find exponential separations for
computational problems. However, such separations do not come a dime a dozen:
while there were some early successes in the form of hidden subgroup problems
for abelian groups--which generalize Shor's factoring algorithm perhaps most
faithfully--only for a handful of non-abelian groups efficient quantum
algorithms were found. Recently, problems have gotten increased attention that
seek to identify hidden sub-structures of other combinatorial and algebraic
objects besides groups. In this paper we provide new examples for exponential
separations by considering hidden shift problems that are defined for several
classes of highly non-linear Boolean functions. These so-called bent functions
arise in cryptography, where their property of having perfectly flat Fourier
spectra on the Boolean hypercube gives them resilience against certain types of
attack. We present new quantum algorithms that solve the hidden shift problems
for several well-known classes of bent functions in polynomial time and with a
constant number of queries, while the classical query complexity is shown to be
exponential. Our approach uses a technique that exploits the duality between
bent functions and their Fourier transforms.Comment: 15 pages, 1 figure, to appear in Proceedings of the 21st Annual
ACM-SIAM Symposium on Discrete Algorithms (SODA'10). This updated version of
the paper contains a new exponential separation between classical and quantum
query complexit
Biangular vectors
viii, 133 leaves ; 29 cmThis thesis introduces unit weighing matrices, a generalization of Hadamard matrices.
When dealing with unit weighing matrices, a lot of the structure that is held by Hadamard
matrices is lost, but this loss of rigidity allows these matrices to be used in the construction
of certain combinatorial objects. We are able to fully classify these matrices for many small
values by defining equivalence classes analogous to those found with Hadamard matrices.
We then proceed to introduce an extension to mutually unbiased bases, called mutually unbiased
weighing matrices, by allowing for different subsets of vectors to be orthogonal. The
bounds on the size of these sets of matrices, both lower and upper, are examined. In many
situations, we are able to show that these bounds are sharp. Finally, we show how these sets
of matrices can be used to generate combinatorial objects such as strongly regular graphs
and association schemes
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