Transmultiplexer Research Articles

Efficiency Analysis of a Truncated Flip-FBMC in Burst Optical Transmission

A novel Flip-filter bank multicarrier (Flip-FBMC)-based transmultiplexer (TMUX) with offset quadrature amplitude modulation is proposed to enhance the transmission performance compared to a conventional Flip-OFDM system. Moreover, the possibility to reduce the TMUX response (latency) and increase spectral efficiency is investigated for the first time through a tail shortening method. The proposed design is based on a biorthogonal form for visible light communication (VLC) to increase the flexibility of design requirements. However, spectral efficiency suffers from the ramp-up and the ramp-down at the beginning and end, respectively, of a data burst. Hence, as a penalty, Flip-FBMC imposes 9 more symbols than a Flip-OFDM packet and two factors compared to a DCO-FBMC burst. Hard truncation of the lowest energy tail minimizes latency and limits the system penalty to 2.5 symbols, which is lower than that of DCO-FBCM by 2 symbols. The results show that the prototype filter of the Heisenberg factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1$ </tex-math></inline-formula> ) is highly effective in reducing the energy loss of truncated tails and reduces the symbol error rate (SER). The Flip-FBMC gain over a direct line-of-sight VLC channel is analyzed, and the channel estimation of a truncated burst, which is based on the interference approximation method (IAM) of IAM-C type, exhibits a superior performance of 1.5 dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> SER over the IAM-R method and 1 dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> SNR over a cyclic prefix of 1 point Flip-OFDM. On the other hand, the analysis reveals that IAM-C is slightly impacted by the truncated burst compared to the nontruncated version.

Neural network principle of implementation of digital filters

Comparative evaluations of the frequency responses (FR) of two types of filters implemented by the classical and neural network methods are carried out. It is shown that the neural network principle of the implementation of digital filters can serve as an alternative to the classical method for specifically defined parameters of FR in the pass bands and attenuation bands of the frequencies of signal spectrum. The simplest method for calculating the parameters of the filters’ difference equations is the neural network approach, regardless of the type of classification of discrete and digital filters. The implementation of TM (transmultiplexer) on a digital element base requires the use of methods of filtering, modulating and demodulating signals that are largely different from traditional analog methods. The frequency responses of non-recursive types of filters presented in the paper are based on the property of the approximable function determined only in the pass bands and attenuation bands of the frequencies of signal spectrum.

Meta-heuristic evolutionary algorithms for the design of optimal multiplier-less recombination filter banks

This paper proposes a design for multiplier-less recombination non-uniform filter banks (RNUFBs) optimized using meta-heuristic algorithms. The structure consists of an M-channel uniform filter bank, with some channels combined by the synthesis filters of a transmultiplexer (TMUX), yielding non-uniform sub-bands. When any structure is realized in hardware, it is necessary to have low power consumption and a small chip area. These can be achieved by replacing the multipliers with shifters and adders. Once the continuous coefficient recombination non-uniform filter bank is designed, the coefficients are converted to the canonic-signed-digit (CSD) space to make the design multiplier-less, so as to reduce the complexity of the hardware implementation. To reduce the number of adders and shifters in the multiplier-less implementation, the filter coefficients are rounded with a restricted number of signed power-of-two (SPT) terms, which may cause degradation in the performance of the RNUFBs. To improve the performance of the CSD rounded filters and filter bank, meta-heuristic algorithms such as the artificial bee colony (ABC) algorithm, harmony search algorithm (HSA) and gravitational search algorithm (GSA) are deployed. Of these meta-heuristic algorithms, GSA is found to give the best performance. The method proposed in this paper results in non-uniform filter banks with rational sampling factors which are multiplier-less and have linear-phase and near-perfect-reconstruction.

Design of near-perfect-reconstructed transmultiplexer using different modulation techniques: A comparative study

In this paper, an efficient iterative method for design of near-perfect reconstructed transmultiplexer (NPR TMUX) is proposed for the prescribed roll-off factor (RF) and stop band attenuation (As). In this method, windowing technique has been used for the design of prototype filter, and different modulation techniques have been exploited for designing multi-channel transmultiplexer (TMUX). In this method, inter-channel interference (ICI) is iteratively minimized so that it approximately reduces to ideal value zero. Design example is given to illustrate the superiority of the proposed method over earlier reported work. A comparative study of the performance of different modulation techniques for designing TMUX is also presented.

A design of IFIR prototype filter for Cosine Modulated Filterbank and Transmultiplexer

This paper proposes a computational efficient approach for the design of M-band Cosine Modulated Filterbank (CMFB) and Transmultiplexer (TMUX) in Near Perfect Reconstruction (NPR) condition. The required prototype filter is formulated by using interpolated FIR (IFIR) technique. Suitable stretch factor is used for the reduction in computation cost and obtained minimum value of distortion parameters. The minimization of these parameters has been achieved by applying a single variable bisection type optimization technique. Design examples are included to prove superiority of performance over the existing work.

Zero-padding or cyclic prefix for MDFT-based filter bank multicarrier communications

In this paper, the problem of channel equalization in modified discrete Fourier transform (MDFT)-based filter-bank multicarrier system (FBMC) is addressed. Two different system models based on padding zeros or using cyclic prefix in each transmitted block are presented. The proposed systems utilize IFFT/FFT blocks embedded in an MDFT-FBMC system. In the first one, an equalization block similar to single-carrier systems is included as the first stage in the receiver. The second system basically consists of an OFDM embedded in the MDFT-based transmultiplexer (TMUX). For both systems, the channel equalization is performed at the receiver in the frequency-domain by using a one-tap per subcarrier equalizer. Finally, several simulation results obtained in different scenarios are presented to assess the performance of the proposed systems compared to the standardized DFT-based systems (OFDM).

A Stabilized Multichannel Fast RLS Algorithm for Adaptive Transmultiplexer Receivers

The transmultiplexer (TMUX) system has been studied for its application to multicarrier communications. The channel impairments including noise, interference, and distortion draw the need for adaptive reconstruction at the TMUX receiver. Among possible adaptive methods, the recursive least squares (RLS) algorithm is appealing for its good convergence rate and steady state performance. However, higher computational complexity due to the matrix operation is the drawback of utilizing RLS. A fast RLS algorithm used for adaptive signal reconstruction in the TMUX system is developed in this paper. By using the polyphase decomposition method, the adaptive receiver in the TMUX system can be formulated as a multichannel filtering problem, and the fast algorithm is obtained through the block Toeplitz matrix structure of received signals. In addition to the reduction of complexity, simulation results show that the adaptive TMUX receiver has a convergence rate close to that of the standard RLS algorithm and the performance approaches the minimum mean square error solution.

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.

Biorthogonal Recombination Nonuniform Cosine-Modulated Filter Banks and their Multiplier-Less Realizations

This paper studies the theory, design and multiplier-less (ML) realization of a class of perfect reconstruction (PR) low-delay biorthogonal nonuniform cosine-modulated filter banks (CMFBs). It is based on a recombination (or merging) structure previously proposed by the authors. By relaxing the original CMFB and the recombination transmultiplexer (TMUX) in the recombination structure to be biorthogonal, nonuniform CMFBs with lower system delay can be obtained. This also increases the possible choices of the prototype filters to meet different design objectives. A matching condition is introduced to suppress the spurious response resulting from the mismatch in the transition bands of the two biorthogonal CMFBs. A complete factorization of biorthgonal CMFB using the lifting scheme is employed to obtain structurally PR biorthogonal nonuniform filter banks (FBs), which are robust to coefficient quantization. In addition, by approximating the lifting coefficients and the modulation matrices by the sum of powers-of-two (SOPOT) coefficients, ML realization with very low implementation complexity is obtained. Design examples and comparison are given to illustrate the effectiveness of the proposed method.

Performance comparison of square root raised‐cosine and lerner filters for the MDFT‐TMUX filter bank

AbstractThis paper deals with prototype filters, namely, square root raised — cosine (SRC) and Lerner filters in the modified discrete Fourier transform (MDFT) transmultiplexer (TMUX) filter bank. The error obtained in the reconstruction of the original signal is compared for various filter orders. These filters can be employed in multicarrier modulation system‐ overlapped discrete multitone (overlapped DMT) or discrete wavelet multitone (DWMT) system. However, with the SRC filter, a large number of computations are required. By employing Lerner filter, we can reduce this by atleast 3‐ folds. We have carried out simulation studies for an 8‐channel MDFT‐ TMUX filter bank and the results obtained are presented.

Modulation techniques for on-board processing satellite communications

The uplink channel from a terminal to a regenerative satellite repeater is considered in which a digital transmultiplexer (TMUX) is used as a frequency-division multiple-access (FDMA) to time-division-multiplexed (TDM) converter for group demodulation. Linear modulation techniques were considered to determine which would make the most efficient use of this on-board processing (OBP) satellite system. The modulation techniques examined were quadrature phase-shift keying (QPSK), /spl pi//4-QPSK, and offset QPSK (OQPSK), all with Nyquist pulse shaping, and minimum-phase keying (MSK). The search for the optimum linear modulation technique took into account the performance criteria of power efficiency, bandwidth efficiency, and especially the complexity of the TMUX algorithm for OBP.

Low-delay distortion recursive (IIR) transmultiplexer

A new scheme based on using an IIR design algorithm is presented to demultiplex critically sampled QPSK channels for onboard processing (OBP) satellites. Comparisons with an existing nonrecursive (FIR) transmultiplexer (TMUX) show that the presented IIR TMUX is also a suitable candidate for consideration in future OBP applications.

Development of a 60-Channel Fdm-Tdm Transmultiplexer

Development of a 60-channel FDM-TDM transmultiplexer The main objective of this paper is to describe and discuss the use of the Fast Hartley Transform (FHT) in the digital transmultiplexer (TMUX) design and implementation process. A 60 channel TMUX has been simulated using a reformulation of an algorithm including the FHT, which was programmed using the OCCAM2 language; performance and results of this approach are shown in this paper.

Fast transform based complex transmultiplexer algorithm for multiband quadrature digital modulation schemes

Onboard digital transmultiplexers and regenerative transponders in satellite systems employing multiple spot-beam antennas will play an important role in realising future cost-effective communications networks for use in developing countries and where isolated user terminals require access to the global telecommunications network. In the paper, a new fast algorithm for the realisation of a complex input/complex output fast transform based digital transmultiplexer (TMUX) is presented with application to a quadrature multicarrier modulation and demodulation system. The novel approach adopts a splitting of the transform processor into two conjugate operations, and a decomposition of the phase offsets is performed leading to a saving in the number of arithmetic operations required. A reduction in the amount of control circuitry required for implementation is also facilitated. An experimental four-channel QPSK modulator based upon the new technique has been implemented for hardware simulation purposes using a TMS320C25 DSP based system, and performance evaluation results are presented where 16 bit integer DSP processing is employed.

Digital group demodulation system for multiple PSK carriers

In the near future, the on-board regenerative repeating system will be employed to convert many low speed SCPC signals to a high speed TDM signal. Similar situation can be considered in a star-network, where many small earth stations communicate with terrestrial telephone stations through a large size central earth station. In both cases, many SCPC signals are concentrated and demodulated by a central station. Therefore, hardware and power consumption minimization is necessary. This demand resulted in a group modulator/demodulator concept. A group demodulator employs the same hardware effectively in common in demodulating multiple PSK/FDM carriers. Actually, a transmultiplexer(TMUX) is used as a combination of a filterbank and down converters. Although a TMUX usually markedly reduces hardware, it introduces a new problem in demodulating an individual channel signal from the TMUX output. This problem is caused by a difference between a TMUX operation rate and each SCPC symbol rate. In this paper, an adaptive rate conversion filter is proposed to resolve this problem. In addition, each channel clock timing difference and carrier frequency offset are compensated for by channel demodulators attached individually to each TMUX channel output. The channel demodulator has been actually developed for 64kbps QPSK signal and its performance has been evaluated. Using this technique, a 16 channel group demodulator has been manufactured.