This paper describes the design of a programmable QAM transceiver for VDSL applications. A 12-b DAC with 64-dB spurious-free dynamic range (SFDR) at 75-MS/s and an 11-b ADC with 72.3-dB SFDR at 70-MS/s are integrated in this complete physical layer IC. A digital IIR notch filter is included in order to not interrupt existing amateur radio bands. The proposed dual loop AGC adjusts the gain of a variable gain amplifier (VGA) to obtain maximum SNR while avoiding saturation. Using several low power techniques, the total power consumption is reduced to 300-mW at 1.8-V core and 3.3-V I/O supplies. The transceiver is fabricated in a 0.18-µm CMOS process and the chip size is 5-mm 5-mm. This VDSL transceiver supports 13-Mbps data rate over a 9000-ft channel with a BER < 10-7.

General closed-form expressions are derived and analyzed for the exact bit error rate (BER) performance of the arbitrary rectangular Gray coded QAM signal in conjunction with maximal-ratio combining (MRC) diversity on frequency non-selective slow m-distributed Nakagami fading channel. The analyses consider four channel models, independent and identical, independent and nonidentical, identical but correlated, and arbitrary correlated fading. Numerical results demonstrate error performance improvement with the use of MRC diversity reception. The new expressions presented here are suitable for evaluating various cases of practical interest on wireless communication channels.

In this paper, performance of chaos and burst noises injected to the Hopfield Neural Network for quadratic assignment problems is investigated. For the evaluation of the noises, two methods to appreciate finding a lot of nearly optimal solutions are proposed. By computer simulations, it is confirmed that the burst noise generated by the Gilbert model with a laminar part and a burst part achieved the good performance as the intermittency chaos noise near the three-periodic window.

In this study, a modeling method of the intermittency chaos using the Markov chain is proposed. The performances of the intermittency chaos and the Markov chain model are investigated when they are injected to the Hopfield Neural Network for a quadratic assignment problem or an associative memory. Computer simulated results show that the proposed modeling is good enough to gain similar performance of the intermittency chaos.

In this paper, an area efficient VLSI architecture of decision feedback equalizer is derived accommodating 64/256 QAM modulators. This architecture is implemented efficiently in VLSI structure using EDA tools due to its regular structure. The method is to employ a time-multiplexed design scheme, so-called Folding, which executes multiple operation on a single functional unit. In addition, we define a new folding set by grouping the adjacent filter taps with data transfer having the same processing sequence between blocks and perform the internal data-bit optimization. By doing so, the computational complexity is reduced by performance optimization and also silicon area is reduced by using a shared operator. Moreover, through the performance and convergence time comparison of the various LMS (e.g. LMS, data signed LMS, error signed LMS, signed-signed LMS) ) coefficient updating algorithms, we identify an optimum LMS algorithm scheme suitable for the low complexity, high performance and high order (64 and 256) QAM applications for the presented Fractionally Spaced Decision Feedback Equalizer. We simulated the proposed design scheme using SYNOPSYSTM and SPWTM.

In this letter, iterative sequence estimation technique based on expectation-maximization (EM) algorithm is considered for quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) signals. For QAM-OFDM signaling, the optimal EM algorithm requires high computational complexity due to the inversion of complex matrix executed at each iteration. To avoid this problem, we propose a sub-optimal iterative sequence estimation algorithm with some approximations, which results in reduced computational complexity for QAM-OFDM signals. Moreover, we use two different approaches to obtain initial estimate for beginning iteration of proposed algorithm. One is for less time-dispersive but fast fading channel and the other is for highly time-dispersive but relatively slow fading channel. The bit error rate (BER) performances of the proposed algorithm are evaluated using computer simulations. The results show that the proposed algorithm performs nearly as well as the optimal EM algorithm.

In this paper, a quasi-synchronous code-division multiple-access (QS-CDMA) is investigated for application in the reverse link of a microcellular or indoor mobile communication environment. In a QS-CDMA system, the relative time delay between the signals of different users is normally restricted within a few chips. Generalized orthogonal (GO) codes added with guard chips are employed as the spreading sequences in this paper and the strict timing error restrictions for BPSK and M-QAM modulation schemes are derived based on the correlation properties of GO code set which determines the multiple access interference (MAI). The results reveal that the system employing GO code set with bigger GO zone can tolerate more serious timing error, and higher order modulation schemes require stricter synchronization. Based on the system model presented, the BER performance for BPSK and M-QAM is evaluated by Gaussian Approximation (GA) and Monte Carlo simulation. The system capacity in terms of acquirable total bit rates are also evaluated, revealing that if system synchronization error is limited within the GO zone, M-QAM scheme can be utilized to improve the system capacity.

We present the channel capacity, specifically the mutual information, of an additive white Gaussian noise (AWGN) channel in the presence of phase noise, and investigate the effect of phase noise impairment on powerful error-correcting codes (ECCs) that normally operate in low signal-to-noise ratio (SNR) regions. This channel-induced impairment is common in digital coherent transmission systems and is caused by imperfect carrier tracking of the phase error detector for coherent demodulation. It is shown through semi-analytical derivation that decreasing the information rate from its ideal capacity to an information rate lower than its inherent capacity significantly mitigates the impairment caused by phase noise, and that operating systems in the low SNR region also lessen the phase noise impairment by transforming typical phase noise behavior into Gaussian-like behavior. We also demonstrate by computer simulation using turbo-trellis coded modulation (TTCM) with high-order quadrature amplitude modulation (QAM) signals that the use of capacity-approaching codes (CACs) makes transmission systems invulnerable to phase noise. To verify the effect of CACs on phase noise, simulation results of TTCM are also compared to that of trellis-coded modulation (TCM), which is used as an example of a conventional ECC operating at a relatively high SNR.

We experimentally demonstrated a remote antenna system based on a millimeter-wave (MMW) over fiber scheme for 622-Mbps broadband fixed wireless access systems. In this system, the format of the RF signal is based on a four-carrier signal in which each carrier is modulated by using 64-QAM, to reduce the complexity of the RF system in comparison with the single-carrier QAM system using many more signal-points than 64. The remote antenna system based on the IF-over-fiber scheme was also experimentally demonstrated, as well as the MMW over fiber scheme for comparison. From the experimental results, we found that the remote antenna system based on the MMW over fiber scheme is effective not only from the viewpoints of miniaturization of the remote antenna station and ability to provide a stable millimeter-wave frequency, but also from the viewpoint of link performances such as allowable dynamic range and power penalty, even though the scheme's E/O and O/E devices have a higher cost.

This paper presents an extension of the database query language SQL to include queries against a database with natural language annotations. The proposed scheme is based on Concept Coupling Model, a language model for handling natural language sentence structures. Integration of the language model with the conventional relational data model provides a unified environment for manipulating information sources comprised of relational tables and natural language texts.

When decision-directed channel estimation is used for QAM-OFDM systems, the optimal filter shape depends on the amplitudes of the modulated symbols as well as the channel characteristics. In this letter we propose a simple channel estimation method for multi-level-amplitude-modulated systems, which can effectively suppress the estimation variances with a small filter. Using the proposed method the implementation cost can be reduced and possibly better results might be obtained by avoiding the estimation bias due to large-sized filtering.

This paper derives a set of orthogonal polynomials for a complex random variable that is uniformly distributed in two dimensions (2D). The polynomials are used in a series expansion to approximate memoryless nonlinearities in digital QAM systems. We also study stochastic identification of nonlinearities using the orthogonal polynomials through analysis and simulations.

In this paper, a hierarchical bit mapping (HBM) scheme suitable for M-level quadrature amplitude modulation (M-QAM) packet transmission is presented. Generally, M-QAM, such as 16QAM or 64QAM, consists of multiple bits with differing bit error-rate quality. Because the packet error rate when using M-QAM is highly dependent on the performance of the "weakest" (poor quality) bits, the throughput of M-QAM packets can dramatically deteriorate especially in a multi-path environment. This paper proposes the use of a bit mapping scheme conceptually similar to hierarchical QAM to improve packet transmission throughput. For 16QAM under this scheme for example, as there are four bits in a 16QAM symbol, four independent packets can be simultaneously transmitted on each of the different bits when HBM is used. In doing so, at least two packets can be transmitted with a high probability of success even under poor transmission conditions, and under good transmission conditions, four packets can potentially be successfully transmitted. An interference cancellation (IC) method is then presented for HBM, with simulation results showing that the combination of HBM with IC results in very good performance. It was also determined that the HBM scheme can be used easily and effectively with hybrid ARQ.

This paper presents an adaptive burst-mode M-QAM modem architecture suitable for variable rate broadband wireless packet data networks. The core signal processing functions for the modem are common to all constellations resulting in an efficient hardware architecture for field programmable gate array (FPGA) implementation.

When M-ary QAM (MQAM) signals experience the Rician fading channels, diversity schemes can minimize the effects of these fadings since deep fades seldom occur simultaneously during the same time intervals on two or more paths. The symbol error probability of MQAM systems using L-branch maximum ratio combining (MRC) diversity reception is derived theoretically over frequency-nonselective slow Rician fading channels with an additive white Gaussian noise (AWGN). This derived evaluation is expressed as the infinite series composed of hypergeometric and gamma functions. These performance evaluations allow designers to determine M-ary modulation methods for Rician fading environments.

Recently, neural networks (NNs) have been extensively applied to many signal processing problem due to their robust abilities to form complex decision regions. In particular, neural networks add flexibility to the design of equalizers for digital communication systems. Recurrent neural network (RNN) is a kind of neural network with one or more feedback loops, whereas self-organizing map (SOM) is characterized by the formation of a topographic map of the input patterns in which the spatial locations (i.e., coordinates) of the neurons in the lattice are indicative of intrinsic statistical features contained in the input patterns. In this paper, we propose a novel receiver structure by combining adaptive RNN equalizer with a SOM detector under serious ISI and nonlinear distortion in QAM system. According to the theoretical analysis and computer simulation results, the performance of the proposed scheme is shown to be quite effective in channel equalization under nonlinear distortion.

The performances of M-ary PSK (MPSK) and QAM (MQAM) systems using L-branch selection combining (SC) diversity reception in frequency-nonselective slow Nakagami fading channels are derived theoretically. For integer values of the Nakagami fading parameter m, the general formula for evaluating symbol error rate (SER) of MPSK signals in the independent branch diversity system comprises numerical analyses with the integral-form expressions. An exact closed-form SER performance of MQAM signals under the effect of SC diversity via numerical integration is presented.

A signal suffers from nonlinear, linear, and additive distortion when transmitted through a channel. Linear equalizers are commonly used in receivers to compensate for linear channel distortion. As an alternative, novel equalizer structures utilizing neural computation have been developed for compensating for nonlinear channel distortion. In this paper, we propose a neural detector based on self-organizing map (SOM) in a 16 QAM system. The proposed scheme uses the SOM algorithm and symbol-by-symbol detector to form a neural detector, and it adapts well to the changing channel conditions, including nonlinear distortions because of the topology-preserving property of the SOM algorithm. According to the theoretical analysis and computer simulation results, the proposed scheme is shown to have better performance than traditional linear equalizer when facing with nonlinear distortion.

This paper considers the design of quadrature amplitude modulation (QAM) transceivers for fixed wireless communications. We propose the use of power control in the QAM transmitter (Tx) to obtain BER performance robust to fading. The gain of the Tx is adaptively adjusted to keep the power of the received signal nearly constant despite of the short term fading and the second multipath. The BER performance of the proposed scheme is analytically evaluated in fixed wireless channels with flat fading and frequency selective fading. Analytic and simulation results show that the use of power control in the Tx can provide the BER performance only about 1 dB inferior to that in additive white Gaussian noise (AWGN) channel.

We have investigated the feasibility of the 16QAM-OFDM transmission scheme by calculating its transmission performance by means of some evaluation models. These models have been proposed by standardization projects such as ETSI-BRAN or ARIB-MMAC project. We evaluated the effects of the following four factors on transmission performance: (1) modulation scheme of each OFDM sub-carrier-channel; (2) FEC scheme, especially coding rate and coding scheme; (3) multipath fading environments; and (4) phase noise and nonlinear amplifier. By these evaluations, we have compiled a fundamental data in order to realize multimedia mobile access communication system based on 16QAM-OFDM transmission scheme.