Multilevel Modulation System Research Articles

Nonorthogonal CSK/SS ALOHA system under MANET environment

Spread ALOHA systems that combine the spread spectrum techniques with the ALOHA system (a simple random access protocol without carrier sensing function) have been investigated as Mobile Ad-hoc Network (MANET) protocols. This paper presents the effect of the nonorthogonal Code Shift Keying Spread Spectrum (CSK/SS) ALOHA system on improving the throughput and delay performances of the spread ALOHA system under the MANET environment. The nonorthogonal CSK/SS system, which uses systematically-constructed nonorthogonal sequences, is one of the multilevel modulation systems for the spread spectrum technique with increasing the number of bits per frame. Optimum parameters can be used to obtain the best system performance; thus, this paper gives analytical expressions of the data transmission efficiency and delay property of the proposed scheme as a function of the system parameters under the assumption of no power control. Numerical results show that the nonorthogonal CSK/SS ALOHA system is useful in the MANET environment, compared with the conventional spread ALOHA systems.

Nonlinear phase noise tolerance for coherent optical systems using soft-decision-aided ML carrier phase estimation enhanced with constellation partitioning

A novel soft-decision-aided maximum likelihood (SDA-ML) carrier phase estimation method and its simplified version, the decision-aided and soft-decision-aided maximum likelihood (DA-SDA-ML) methods are tested in a nonlinear phase noise-dominant channel. The numerical performance results show that both the SDA-ML and DA-SDA-ML methods outperform the conventional DA-ML in systems with constant-amplitude modulation formats. In addition, modified algorithms based on constellation partitioning are proposed. With partitioning, the modified SDA-ML and DA-SDA-ML are shown to be useful for compensating the nonlinear phase noise in multi-level modulation systems.

Frequency-Domain Iterative SDFE for MIMO-OFDM Systems

This paper proposes a frequency-domain iterative soft decision feedback equalization (SDFE) scheme for multiple-input---multiple-output orthogonal frequency division multiplexing communication systems in multi-path fading channels. Error propagation can seriously affect the performance of the adaptive decision feedback equalization. We propose an iterative SDFE to not only combat error propagation but also be suitable for multilevel modulation systems. Under a Gaussian approximation of the a priori information and the SDFE output, the output of an equivalent AWGN channel and expressions for the a posteriori log likelihood ratio (LLR) and the extrinsic LLR are derived for multilevel modulation. In addition, we also investigate the iterative receivers by employing irregular low-density parity check codes (LDPC) and turbo codes. The performance of the LDPC-coded iterative receiver is better than that of the turbo-coded iterative receiver. Some simulation examples are given to show the effectiveness and comparisons of the proposed receiver.

Soft-Decision Feedback Turbo Equalization for Multilevel Modulations

Error propagation phenomena is the major drawback for existing hard-decision feedback turbo equalizers. In this paper, we propose a new soft-decision feedback equalizer (SDFE) suitable for multilevel modulation systems employing turbo equalization. The proposed SDFE offers a low computational complexity growing only linearly with the number of equalizer coefficients, as opposed to the quadratic complexity of MMSE-based linear turbo equalizer with time-varying coefficients (Exact-MMSE-LE). The performance and convergence property of the proposed SDFE are analyzed using extrinsic information transfer (EXIT) chart and verified by simulations in a severe intersymbol interference channel set by Proakis. Results show that our approach performs close to Exact-MMSE-LE for BPSK/QPSK modulation. And for 8PSK/16QAM modulations, the proposed SDFE performs much better. It exhibits lower SNR threshold (SNR required for “waterfall” BER) and much faster convergence than Exact-MMSE-LE.

Application of capacity approaching coding techniques to digital subscriber lines

The use of coding error control is an integral part of the design of modern communication systems. Capacity-approaching codes such as turbo and LDPC codes, discovered or rediscovered in the past decade, offset near-Shannon-limit performance on the AWGN channel with rather low implementation complexity and are therefore increasingly being applied for error control in various fields of data communications. This article describes a generic multilevel modulation and coding scheme based on the use of turbo-like, or LDPC codes for DSL system. It is shown that such codes provide significant gains in performance and allow an increase in data rate and/or loop reach that can be instrumental to the widespread deployment of future DSL services. Such techniques are also suitable for general multilevel modulation system in other application areas.

Design of an all‐digital multilevel modulator and its application to variable‐capacity transmission

AbstractWith the advent of the radio relay system and multilevel modulation, the multicarrier system and variable capacity transmission system are proposed to realize excellent performance in the fading channel. Accordingly, it is desired to realize a highly functioned MODEM which can cope with those systems in a flexible way.On the other hand, intensive studies have been made on digital signal processing in the modulation and demodulation in the field of data modem. However, these methods consider the low‐rate system of less than 100 Kbaud and are difficult to apply directly to the system where the transmission rate of 10 Mbaud or above is required.For the foregoing reasons, this paper considers the realization of the all‐digital modulator which can realize the high‐capacity transmission of over 10 Mbaud for the multilevel modulation system, such as 256 QAM.This paper discusses the design of the digital filter with excellent intersymbol interference. The condition for the carrier frequency for a given clock rate is presented. Then a variable multilevel modulator with a simple correction circuit is proposed which can be applied to the variable‐capacity transmission system.Finally, the proposed circuit is constructed and it is confirmed that an excellent modulation performance is obtained without adjustment in various multilevel modulation systems.

Fully digital cross polarization interference canceller for multilevel digital radio

AbstractIn the digital radio system using dual polarizations, an interference canceller is needed that cancels excessive interference. In this paper, a fully digital cross‐polarization interference canceller is proposed in which digital signal processing (DSP) technology and LSI technology are applied. By means of DSP technology, the cancellation characteristics are determined mainly by the quantization accuracy and the number of taps. The only degradation factor is the rounding‐off error of the operations. High accuracy and large cancellation effects are expected. Also, an application of an advanced control algorithm is possible. Flexible applications are possible for any symbol rate. In this paper, the estimation method for the fully digital cross‐polarization interference canceller is presented. Based on this method, the relationships among the cancellation characteristics, the input signal quantization accuracy, tap coefficient quantization accuracy, and number of taps are analyzed theoretically for the case where the method is applied to the 256 QAM system. Next, the configuration of the canceller is presented. From the results of the experiments using the symbol rate of 12.5 MBaud and 256 QAM signals, good agreement with the theoretical prediction is obtained and, hence, the validity of the present estimation method is demonstrated. It is demonstrated that a high‐performance cross‐polarization interference canceller is realized which can be applied to a multilevel modulation system for a high‐speed region, such as 12.5 MBaud.