Amir Eghbali

Reconfigurable Nonuniform Transmultiplexers Using Uniform Modulated Filter Banks

This paper introduces reconfigurable nonuniform transmultiplexers (TMUXs) based on fixed uniform modulated filter banks (FBs). The TMUXs use parallel processing where polyphase components, of any user, are processed by a number of synthesis FB and analysis FB branches. One branch represents one granularity band, and any user can occupy integer multiples of a granularity band. The proposed TMUX also requires adjustable commutators so that any user occupies any portion of the frequency spectrum. The location and width of this portion can be modified without additional arithmetic complexity or filter redesign. This paper considers both cosine modulated and modified discrete Fourier transform FBs. It discusses the filter design, TMUX realization, and the parameter selection. It is shown that one can indeed decrease the arithmetic complexity by proper choice of system parameters. For the critically sampled case and if the number of channels is higher than necessary, we can reduce the arithmetic complexity. In case of an oversampled system, the arithmetic complexity can be reduced by proper choice of the number of channels and the roll-off factor of the prototype filter. The proposed TMUX is compared to existing reconfigurable TMUXs, and examples are provided for illustration.

A method for the design of Farrow-structure based variable fractional-delay FIR filters

This paper proposes a method to design variable fractional-delay (FD) filters using the Farrow structure. In the transfer function of the Farrow structure, different subfilters are weighted by different powers of the FD value. As both the FD value and its powers are smaller than 0.5, our proposed method uses them as diminishing weighting functions. The approximation error, for each subfilter, is then increased in proportion to the power of the FD value. This gives a new distribution for the orders of the Farrow subfilters which has not been utilized before. This paper also includes these diminishing weighting functions in the filter design so as to obtain their optimal values, iteratively. We consider subfilters of both even and odd orders. Examples illustrate our proposed method and comparisons, to various earlier designs, show a reduction of the arithmetic complexity. Highlights? We use a new weighting function (and hence a new distribution of subfilter orders) which has not been treated before in the literature. ? We find the optimal value of this weighting function during the filter design. ? This has also not been treated in the literature. ? Our proposed method results in savings of arithmetic complexity over a wide range of existing specifications as well as both even and odd order subfilters. ? We deal with another issue, namely, the choice of order to be odd or even in order to further reduce the arithmetic complexity.

A Multimode Transmultiplexer Structure

This paper introduces a multimode transmultiplexer (TMUX) structure capable of generating a large set of user-bandwidths and center frequencies. The structure utilizes fixed integer sampling rate conversion (SRC) blocks, Farrow-based variable interpolation and decimation structures, and variable frequency shifters. A main advantage of this TMUX is that it needs only one filter design beforehand. Specifically, the filters in the fixed integer SRC blocks as well as the subfilters of the Farrow structure are designed only once. Then, all possible combinations of bandwidths and center frequencies are obtained by properly adjusting the variable delay parameter of the Farrow-based filters and the variable parameters of the frequency shifters. The paper includes examples for demonstration. It also shows that, using the rational SRC equivalent of the Farrow-based filters, the TMUX can be described in terms of conventional multirate building blocks which may be useful in further analysis of the overall system.

Optimal Least-Squares FIR Digital Filters for Compensation of Chromatic Dispersion in Digital Coherent Optical Receivers

This paper proposes optimal finite-length impulse response (FIR) digital filters, in the least-squares (LS) sense, for compensation of chromatic dispersion (CD) in digital coherent optical receivers. The proposed filters are based on the convex minimization of the energy of the complex error between the frequency responses of the actual CD compensation filter and the ideal CD compensation filter. The paper utilizes the fact that pulse shaping filters limit the effective bandwidth of the signal. Then, the filter design for CD compensation needs to be performed over a smaller frequency range, as compared to the whole frequency band in the existing CD compensation methods. By means of design examples, we show that our proposed optimal LS FIR CD compensation filters outperform the existing filters in terms of performance, implementation complexity, and delay.

A farrow-structure-based multi-mode transmultiplexer

This paper introduces a multi-mode transmultiplexer (TMUX) consisting of Farrow-based variable integer sampling rate conversion (SRC) blocks. The polyphase components of general interpolation/decimation filters are realized by the Farrow structure making it possible to achieve different linear-phase finite-length impulse response (FIR) lowpass filters at the cost of a fixed set of subfilters and adjustable fractional delay values. Simultaneous design of the subfilters, to achieve overall approximately Nyquist (Mth-band) filters, are treated in this paper. By means of an example, it is shown that the subfilters can be designed so that for any desired range of integer SRC ratios, the TMUX can approximate perfect recovery as close as desired.

Dynamic Frequency-Band Reallocation and Allocation: from Satellite-Based Communication Systems to Cognitive Radios

This paper discusses two approaches for the baseband processing part of cognitive radios. These approaches can be used depending on the availability of (i) a composite signal comprising several user signals or, (ii) the individual user signals. The aim is to introduce solutions which can support different bandwidths and center frequencies for a large set of users and at the cost of simple modifications on the same hardware platform. Such structures have previously been used for satellite-based communication systems and the paper aims to outline their possible applications in the context of cognitive radios. For this purpose, dynamic frequencyband allocation (DFBA) and reallocation (DFBR) structures based on multirate building blocks are introduced and their reconfigurability issues with respect to the required reconfigurability measures in cognitive radios are discussed.

An arbitrary-bandwidth transmultiplexer and its application to flexible frequency-band reallocation networks

In this paper, we introduce a non-uniform transmultiplexer capable of generating arbitrary-bandwidth user signals. The transmultiplexer consists of linear-phase finite-length impulse response (FIR) filters and Farrow structures for arbitrary-rate interpolation/decimation. By applying the FIR rational sampling rate conversion (SRC) equivalent of the Farrow structure, we model the behavior of the transmultiplexer and derive the conditions under which the system can approximate perfect reconstruction. Furthermore, we illustrate the functionality of the proposed transmultiplexer and we analyze the performance and functionality of a flexible frequency-band reallocation (FFBR) network using this transmultiplexer.

A Class of Multimode Transmultiplexers Based on the Farrow Structure

This paper introduces multimode transmultiplexers (TMUXs) in which the Farrow structure realizes the polyphase components of general lowpass interpolation/decimation filters. As various lowpass filters are obtained by one set of common Farrow subfilters, only one offline filter design enables us to cover different integer sampling rate conversion (SRC) ratios. A model of general rational SRC is also constructed where the same fixed subfilters perform rational SRC. These two SRC schemes are then used to construct multimode TMUXs. Efficient implementation structures are introduced and different filter design techniques such as minimax and least-squares (LS) are discussed. By means of simulation results, it is shown that the performance of the transmultiplexer (TMUX) depends on the ripples of the filters. With the error vector magnitude (EVM) as the performance metric, the LS method has a superiority over the minimax approach.

On the Complexity of Multiplierless Direct and Polyphase FIR Filter Structures

This paper discusses the complexity trend in different finite length impulse response (FIR) filter structures when using multiplierless (shift-and-add) realization. We derive the total number of adders required by the transposed direct form, polyphase, and reduced-complexity polyphase FIR filter structures. A comparison of the arithmetic complexities of these structures for different filter characteristics is performed. The simulation results show that considering both the high level structure and the algorithm used to realize the subfilters gives a more accurate measure of complexity comparison between different FIR filter structures.

On Two-Stage Nyquist Pulse Shaping Filters

This correspondence outlines a method for designing two-stage Nyquist filters. The Nyquist filter is split into two equal and linear-phase finite-length impulse response spectral factors. The per-time-unit multiplicative complexity, of the overall structure, is included as the objective function. Examples are then provided where Nyquist filters are designed so as to minimize the multiplicative complexity subject to the constraints on the overall Nyquist filter. In comparison to the single-stage case, the two-stage realization reduces the multiplicative complexity by an average of 48%. For two-stage sampling rate conversion (SRC), the correspondence shows that it is better to have a larger SRC ratio in the first stage.