Showing papers on "Quadrature amplitude modulation published in 2008"

Digital filters for coherent optical receivers.

Abstract: Digital filters underpin the performance of coherent optical receivers which exploit digital signal processing (DSP) to mitigate transmission impairments. We outline the principles of such receivers and review our experimental investigations into compensation of polarization mode dispersion. We then consider the details of the digital filtering employed and present an analytical solution to the design of a chromatic dispersion compensating filter. Using the analytical solution an upper bound on the number of taps required to compensate chromatic dispersion is obtained, with simulation revealing an improved bound of 2.2 taps per 1000ps/nm for 10.7GBaud data. Finally the principles of digital polarization tracking are outlined and through simulation, it is demonstrated that 100krad/s polarization rotations could be tracked using DSP with a clock frequency of less than 500MHz.

Comparison of Asymmetrically Clipped Optical OFDM and DC-Biased Optical OFDM in AWGN

Abstract: We present theoretical and simulation results for the performance of asymmetrically-clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) in AWGN for intensity-modulated direct-detection systems. Constellations from 4 QAM to 1024 QAM are considered. For DCO-OFDM, the optimum bias depends on the constellation size which limits its performance in adaptive systems. ACO-OFDM requires less optical power for a given data rate than DCO-OFDM for all but the largest constellations and is better suited to adaptive systems as the same structure is optimum for all constellations.

MIMO Relaying With Linear Processing for Multiuser Transmission in Fixed Relay Networks

Abstract: In this paper, a novel relaying strategy that uses multiple-input multiple-output (MIMO) fixed relays with linear processing to support multiuser transmission in cellular networks is proposed. The fixed relay processes the received signal with linear operations and forwards the processed signal to multiple users creating a multiuser MIMO relay. This paper proposes upper and lower bounds on the achievable sum rate for this architecture assuming zero-forcing dirty paper coding at the base station, neglecting the direct links from the base station to the users, and with certain structure in the relay. These bounds are used to motivate an implementable multiuser precoding strategy that combines Tomlinson-Harashima precoding at the base station and linear signal processing at the relay, adaptive stream selection, and QAM modulation. Reduced complexity algorithms based on the sum rate lower bounds are used to select a subset of users. We compare the sum rates achieved by the proposed system architecture and algorithms with the sum rate upper bound and the sum rate achieved by the decode-and-forward relaying.

Cooperative Communication Protocols in Wireless Networks: Performance Analysis and Optimum Power Allocation

Abstract: In this paper, symbol-error-rate (SER) performance analysis and optimum power allocation are provided for uncoded cooperative communications in wireless networks with either decode-and-forward (DF) or amplify-and-forward (AF) cooperation protocol, in which source and relay send information to destination through orthogonal channels. In case of the DF cooperation systems, closed-form SER formulation is provided for uncoded cooperation systems with PSK and QAM signals. Moreover, an SER upper bound as well as an approximation are established to show the asymptotic performance of the DF cooperation systems, where the SER approximation is asymptotically tight at high signal-to-noise ratio (SNR). Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the DF cooperation systems. In case of the AF cooperation systems, we obtain at first a simple closed-form moment generating function (MGF) expression for the harmonic mean to avoid the hypergeometric functions as commonly used in the literature. By taking advantage of the simple MGF expression, we obtain a closed-form SER performance analysis for the AF cooperation systems with PSK and QAM signals. Moreover, an SER approximation is also established which is asymptotically tight at high SNR. Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the AF cooperation systems. In both the DF and AF cooperation systems, it turns out that an equal power strategy is good, but in general not optimum in cooperative communications. The optimum power allocation depends on the channel link quality. An interesting result is that in case that all channel links are available, the optimum power allocation does not depend on the direct link between source and destination, it depends only on the channel links related to the relay. Finally, we compare the performance of the cooperation systems with either DF or AF protocol. It is shown that the performance of a systems with the DF cooperation protocol is better than that with the AF protocol. However, the performance gain varies with different modulation types and channel conditions, and the gain is limited. For example, in case of BPSK modulation, the performance gain cannot be larger than 2.4 dB; and for QPSK modulation, it cannot be larger than 1.2 dB. Extensive simulation results are provided to validate the theoretical analysis.

Constant Envelope OFDM

Abstract: This paper describes a transformation technique aimed at solving the peak-to-average power ratio (PAPR) problem associated with OFDM (orthogonal frequency-division multiplexing). Constant envelope OFDM (CE-OFDM) transforms the OFDM signal, by way of phase modulation, to a signal designed for efficient power amplification. At the receiver, the inverse transformation - phase demodulation - is applied prior to the conventional OFDM demodulator. The performance of CE-OFDM is analyzed in additive white Gaussian noise (AWGN) and fading channels. CE-OFDM is shown to achieve good performance in dense multipath with the use of cyclic prefix transmission in conjunction with a frequency- domain equalizer (FDE). By way of computer simulation and hardware realization, CE-OFDM is shown to compare favorably to conventional OFDM.

Digital Communications: A Discrete-Time Approach

Abstract: Contents 1 Introduction 1.1 A brief History of Communications 1.2 Basics of Wireless Communications 1.3 Digital Communications 1.4 Why Discrete-Time Processing is so Popular 1.5 Organization of the Text 1.6 Notes and References 2 Signals and Systems 1: A Review of the Basics 2.1 Introduction 2.2 Signals 2.2.1 Continuous-Time Signals 2.2.2 Discrete-Time Signals 2.3 Systems 2.3.1 Continuous-Time Systems 2.3.2 Discrete- Time Systems 2.4 Frequency Domain Characterization 2.4.1 Laplace Transform 2.4.2 Continuous-Time Fourier Transform 2.4.3 Z Transform 2.4.4 Discrete-Time Fourier Transform 2.5 The Discrete Fourier Transform 2.6 The Relationship Between Discrete-Time and Continuous- Time Systems 2.6.1 The Sampling Theorem 2.6.2 Discrete-Time Processing of Continuous-Time Signals 2.7 Discrete-Time Processing of Bandpass Signals 2.8 Notes and References 2.9 Exercises 3 Signals and Systems 2: Some Useful Discrete-Time Techniques for Digital Communications 3.1 Introduction 3.2 Multirate 3.2.1 Impulse Train Sampling 3.2.2 Downsampling 3.2.3 Upsampling 3.2.4 The Noble Identities 3.2.5 Polyphase Filterbanks 3.3 Discrete-Time Filters Design Methods 3.3.1 IIR Filter Design 3.3.2 FIR Filter Design 3.3.3 Two Important Filters: The Differentiator and the Intergrator 3.4 Notes and References 3.5 Exercises 4 A Review of Probability Theory 4.1 Basic Definitions 4.2 Gaussian Random Variables 4.2.1 Density and Distribution Functions 4.2.2 Product Moments 4.2.3 BivariateGaussian Distribution 4.2.4 Functions of Random Variables 4.3 Multivariate Gaussian Random Variables 4.4 Random Sequences 4.4.1 Power Spectral Density 4.4.2 Random Sequences and Discrete-Time LTI Systems 4.5 Additive White Gaussian Noise 4.5.1 Continuous Time Random Processes 4.5.2 The White Gaussian Random Process: A Good Model For Noise 4.5.3 White Gaussian Noise in a sampled data System 4.6 Notes and References 4.7 Exercises 5 Linear Modulation 1: Demodulation, and Detection 5.1 Signal Spaces 5.1.1 Definitions 5.1.2 The Synthesis Equation and Linear Modulation 5.1.3 The Analysis Equation and Detection 5.1.4 The matched Filter 5.2 M-ary Baseband Pulse Amplitude Modulation (PAM) 5.2.1 Continuous-Time Realization 5.2.2 Discrete-Time Realization 5.3 M-ary Quadrature Amplitude Modulation (MQAM) 5.3.1 Continuous-Time Realization 5.3.2 Discrete-Time Realization 5.4 Offset QPSK 5.5 Multicarrier 5.6 Maximum Likelihood detection 5.6.1 Introduction 5.6.2 Preliminaries 5.6.3 Maximum Likelihood Decision Rule 5.7 Notes and References 5.8 Exercises 6 Linear Modulation 2: Performance 6.1 Performance of PAM 6.1.1 Bandwidth 6.1.2 Probability of Error 6.2 Performance of QAM 6.2.1 Bandwidth 6.2.2 Probability of Error 6.3 Comparisons 6.4 Link Budgets 6.4.1 Received Power and The Friis equation 6.4.2 Equivalent Noise Temperature and Noise Figure 6.4.3 The Link Budget Equation 6.5 Projection White Noise Onto An Orthonormal Basis Set 6.6 Notes and References 6.7 Exercises 7 Carrier Phase Synchronization 7.1 Basics Problem Formulation 7.2 Carrier Phase Synchronization for QPSK 7.2.1 A Heuristic Phase Error Detector 7.2.2 The Maximum Likelihood Phase Error Detector 7.2.3 Examples 7.3 Carrier Phase Synchronization for BPSK 7.4 Carrier Phase Synchronization for MQAM 7.5 Carrier Phase Synchronization for Offset QPSK 7.6 Carrier Phase Synchronization for BPSK and QPSK Using Continuous-Time-Techniques 7.7 Phase Ambiguity Resolution 7.7.1 Unique Word 7.7.2 Differential Encoding 7.8 Maximum Likelihood Phase Estimation 7.8.1 Preliminaries 7.8.2 Carrier Phase Estimation 7.9 Notes and References 7.10 Exercises 8 Symbol Timing Synchronization 8.1 Basic Problem Formulation 8.2 Continuous-Time Techniques for M-ary PAM 8.3 Continuous-Time Techniques for MQAM 8.4 Discrete-Time Techniques for M-ary PAM 8.4.1 Timing Error Detectors 8.4.2 Interpolation 8.4.3 Interpolation Control 8.4.4 Examples 8.5 Discrete-Time Techniques for MQAM 8.6 Discrete-Time Techniques for Offset QPSK 8.7 Dealing with Transition Density: A Parctical Consideration 8.8 Maximum Likelihood Estimation 8.8.1 Preliminaries 8.2.2 Symbol Timing Estimation 8.9 Notes and References 8.10 Exercises 9 System Components 9.1 The Continuous-Time Discrete-Time Interface 9.1.1 Analog-to-Digital Converter 9.2.2 Digital-to-Analog Converter 9.2 Discrete-Time Oscillators 9.2.1 Discrete Oscillators Based on LTI Systems 9.2.2 Direct Digital Synthesizer 9.3 Resampling Filters 9.3.1 CIC and Hogenauer Filters 9.3.2 Half-Band Filters 9.3.3 Arbitrary Resampling Using Polyphase Filterbanks 9.4 CoRDiC: Coordinate Rotation Digital Computer 9.4.1 Rotations: Moving on a Circle 9.4.2 Moving Along Other Shapes 9.5 Automatic gain Control 9.6 Notes and References 9.7 Exercise 10 System Design 10.1 Advance Discrete-Time Architectures 10.1.1 Discrete-Time Architectures for QAM Modulators 10.1.2 Discrete-Time Architectures for QAM Demodulators 10.1.3 Putting It all Together 10.2 Channelization 10.2.1 Continuous-Time Techniques: The Superheterodynd Receiver 10.2.2 Discrete-Time Techniques Using Multirate Processing 10.3 Notes and References 10.4 Exercises

Phase Noise Effects on High Spectral Efficiency Coherent Optical OFDM Transmission

Abstract: There are three major advantages for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission using digital signal processing. First, coherent detection is realized by digital phase estimation without the need for optical phase-locked loop. Second, OFDM modulation and demodulation are realized by the well-established computation-efficient fast Fourier transform (FFT) and inverse FFT. Third, adaptive data rates can be supported as different quadrature amplitude modulation (QAM) constellations are software-defined, without any hardware change in transmitter and receiver. However, it is well-known that coherent detection, OFDM, and QAM are all susceptible to phase noise. In this paper, theoretical, numerical, and experimental investigations are carried out for phase noise effects on high spectral efficiency CO-OFDM transmission. A transmission model in the presence of phase noise is presented. By using simulation, the bit error rate floors from finite laser linewidth are presented for CO-OFDM systems with high-order QAM constellations. In the experiments, the phase noise effects from both laser linewidth and nonlinear fiber transmission are investigated. The fiber nonlinearity mitigation based on receiver digital signal processing is also discussed.

Centralized Lightwave WDM-PON Employing 16-QAM Intensity Modulated OFDM Downstream and OOK Modulated Upstream Signals

Abstract: We have proposed and experimentally demonstrated a novel architecture for orthogonal frequency-division- multiplexing (OFDM) wavelength-division-multiplexing passive optical network with centralized lightwave. In this architecture, 16 quadrature amplitude modulation intensity-modulated OFDM signals at 10 Gb/s are utilized for downstream transmission. A wavelength-reuse scheme is employed to carry the upstream data to reduce the cost at optical network unit. By using one intensity modulator, the downstream signal is remodulated for upstream on-off keying (OOK) data at 2.5 Gb/s based on its return-to-zero shape waveform. We have also studied the fading effect caused by double-sideband (DSB) downstream signals. Measurement results show that 2.5-dB power penalty is caused by the fading effect. The fading effect can be removed when the DSB OFDM downstream signals are converted to single sideband (SSB) after vestigial filtering. The power penalty is negligible for both SSB OFDM downstream and the remodulated OOK upstream signals after over 25-km standard single-mode-fiber transmission. Index

The History of Orthogonal Frequency Division Multiplexing

Abstract: This paper depicts the development of Orthogonal Frequency Division Multiplexing from a historical perspective. A summary of major research milestones are noted that contributed to modern-day OFDM. These contributions include the use of discrete Fourier transforms replacing the analog implementation and addition of cyclic extensions to ensure orthogonality among the sub-channels. Also, channel equalization algorithms to suppress inter-symbol interference and inter-carrier interference, channel estimation through the insertion of pilot tones among data blocks, peak-to-average power ratio reduction, and synchronization techniques are discussed.

Digital Lock-In Detection for Discriminating Multiple Modulation Frequencies With High Accuracy and Computational Efficiency

Abstract: We introduce a novel digital lock-in detection technique for simultaneously measuring the amplitude and phase of multiple amplitude-modulated signals. Using particular modulation and sampling constraints and averaging filters, we achieve optimal noise reduction and discrimination between sources of different modulation frequencies. Furthermore, it is shown that the digital lock-in technique can be performed as a simple matrix multiplication, which considerably reduces the computation time. The digital lock-in algorithm is described and analyzed under certain sampling and modulation conditions, and results are shown for both numerical and experimental data.

2 dB Better Than CP-OFDM with OFDM/OQAM for Preamble-Based Channel Estimation

Abstract: OFDM/OQAM is a special type of multi-carrier modulation that can be considered as an alternative to conventional OFDM with cyclic prefix (CP) for transmission over multi-path fading channels. Indeed, as it requires no CP, it has the advantage of a theoretically higher spectral efficiency. Furthermore, efficient pulse shaping can also be easily implemented with OFDM/OQAM. However, the classical channel estimation methods used for OFDM cannot be directly applied to OFDM/OQAM. In this paper we present an analysis of this problem and we introduce a new preamble-based channel estimation method. The performance results are obtained either by considering an IEEE802.22 channel model or regarding to the channel delay spread variation of a two-tap channel. The proposed OFDM/OQAM channel estimation method is evaluated, in both scenarios, using different pulse shaping and taking conventional CP-OFDM as reference.

Channel estimation with scattered pilots in OFDM/OQAM

Abstract: OFDM/OQAM is a multi-carrier modulation scheme that can be considered as an alternative to conventional OFDM with cyclic prefix (CP) for transmission over multi-path fading channels. As it requires no CP, it has the advantage of a theoretically higher spectral efficiency. Furthermore, efficient pulse shaping can also be easily implemented. However, the classical channel estimation methods used for OFDM cannot be directly applied to OFDM/OQAM. In this paper, we present an analysis of this problem and we introduce a channel estimation method using scattered pilots. The performance results are evaluated for a slow varying multi-path channel model. For both scenarios, the results of OFDM/OQAM, using 2 different pulse shaping, are compared to those of CP-OFDM.

Real-time Modulation Classification Based On Maximum Likelihood

Abstract: This paper describes a likelihood test based modulation classification method for identifying the modulation scheme of a software-defined radio (SDR) in real-time without pilot symbols between transmitters and receivers. Unlike the prior art, the paper converts an unknown signal symbol to an address of the look-up table (LUT), loads the pre-calculated values of the test functions for the likelihood ratio test, and produces the estimated modulation scheme in real-time. The statistical performance of the LUT based classifier is studied. Simulation results are presented to confirm the theoretical analysis.

Full-Diversity Codes for MISO Systems Equipped With Linear or ML Detectors

Abstract: In this paper, a general criterion for space-time block codes (STBC) to achieve full diversity with a linear receiver is proposed for a wireless communication system having multiple transmitter and single receiver antennas [multiple-input-single-output (MISO)]. Particularly, the STBC with Toeplitz structure satisfies this criterion, and therefore, enables full diversity. Further examination of this Toeplitz STBC reveals the following important properties: (1) the symbol transmission rate can be made to approach unity; (2) applying the Toeplitz code to any signalling scheme having nonzero distance between the nearest constellation points results in a nonvanishing determinant. In addition, if quadratic-amplitude modulation (QAM) is used as the signalling scheme, then for independent MISO flat-fading channels, the Toeplitz codes is proved to approach the optimal diversity-versus-multiplexing tradeoff with a zero-forcing (ZF) receiver when the number of channel uses is large. This is, so far, the first nonorthogonal STBC shown to achieve the optimal tradeoff for such a receiver. On the other hand, when maximum-likelihood (ML) detection is employed in a MISO system, the Toeplitz STBC achieves the maximum coding gain for independent channels. When the channel fading coefficients are correlated, the inherent transmission matrix in the Toeplitz STBC can be designed to minimize the average worst case pairwise error probability.

End-to-end BER analysis for dual-hop OSTBC transmissions over Rayleigh fading channels

Abstract: In this letter, a BER study is presented for the end- to-end performance of dual-hop wireless communication systems employing transmit diversity with orthogonal space-time block codes (OSTBCs), where a nonregenerative or regenerative relay is equipped with a single antenna operating over flat Rayleigh fading channels. More specifically, we provide probability density functions (PDFs) and moment generating functions (MGFs) for the end-to-end SNR of the dual-hop OSTBC transmissions and then present its BER performance over M-ary QAM and PSK modulations, respectively. Numerical investigation shows that the analytic BER provided in the letter makes an exact match with the simulation result in various multiple-antenna transmission scenarios. The result also shows how the number of antennas equipped at the source and destination affects the end-to-end performance.

Quadrature amplitude demodulator and demodulation method

Abstract: A quadrature amplitude demodulator demodulates a modulated signal on which quadrature amplitude modulation is performed. Oscillators generate an in-phase carrier signal having a rectangular wave, a trapezoidal wave or a waveform similar to these, and a quadrature carrier signal, the phase of which is shifted by ¼ cycle relative to the in-phase signal. First and second mixers respectively perform mixing of the modulated signal with the in-phase signal and the quadrature carrier signal. First and second integrators respectively integrate output signals of the first and the second mixers, for a predetermined period in accordance with the cycle of the in-phase carrier signal and the quadrature carrier signal. First and second A/D converters respectively convert outputs of the first and the second integrators into digital values.

Electronic post-compensation for nonlinear phase fluctuations in a 1000-km 20-Gbit/s optical quadrature phase-shift keying transmission system using the digital coherent receiver.

Abstract: We demonstrate electronic post-compensation for nonlinear phase fluctuation in a 1000-km 20-Gbit/s optical quadrature phase-shift keying (QPSK) transmission system, where group-velocity dispersion is well managed. The inter-symbol interference (ISI) at the transmitter induces the nonlinear phase fluctuation through self-phase modulation (SPM) of the signal transmitted through a fiber. However, when the optimized phase shift proportional to the intensity fluctuation is given to the complex amplitude of the signal electric field by using a digital coherent receiver, the nonlinear phase fluctuation can be reduced effectively.

A novel selected mapping technique for PAPR reduction in OFDM systems

Abstract: Selected mapping (SLM) is a well-known method for reducing the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems. The main drawback of this technique is that, for each data block, it requires the transmission of several side information bits, which results in some data rate loss. These redundant bits are so critical to the error performance of the system that they need in practice to be protected by a powerful channel code. This increases the system complexity and transmission delay, and decreases the data rate even further. In this paper, we propose a novel SLM method for which no side information needs to be sent. By considering the example of an OFDM system using 16-QAM modulation, it is shown that the proposed method performs very well both in terms of PAPR reduction and bit error rate at the receiver output.

Legendre-FLANN-based nonlinear channel equalization in wireless communication system

Abstract: In this paper, we present the result of our study on the application of artificial neural networks (ANNs) for adaptive channel equalization in a digital communication system using 4-quadrature amplitude modulation (QAM) signal constellation. We propose a novel single-layer Legendre functional-link ANN (L-FLANN) by using Legendre polynomials to expand the input space into a higher dimension. A performance comparison was carried out with extensive computer simulations between different ANN-based equalizers, such as, radial basis function (RBF), Chebyshev neural network (ChNN) and the proposed L-FLANN along with a linear least mean square (LMS) finite impulse response (FIR) adaptive filter-based equalizer. The performance indicators include the mean square error (MSE), bit error rate (BER), and computational complexities of the different architectures as well as the eye patterns of the various equalizers. It is shown that the L-FLANN exhibited excellent results in terms of the MSE, BER and the computational complexity of the networks.

Digital self-coherent detection.

Abstract: We review recent progresses on digital self-coherent detection of differential phase-shift keyed (DPSK) signal using orthogonal differential direct detection followed by high-speed analog-to-digital conversion and digital signal processing (DSP). Techniques such as data-aided multi-symbol phase estimation for receiver sensitivity enhancement, unified detection scheme for multi-level DPSK signals, and optical field reconstruction are described. The availability of signal field information brings the possibility to compensate for some linear and nonlinear transmission impairments through further DSP. An adaptive DSP algorithm for simultaneous electronic polarization de-multiplexing and polarization-mode dispersion compensation is also presented.

Improved DSP algorithms for coherent 16-QAM transmission

Abstract: We propose new algorithms for adaptive mitigation of coherent 16-QAM constellation distortions. We show that 16-QAM is a potential candidate for the implementation of 100 Gb/s transmission.

Rotated QAM Constellations to Improve BICM Performance for DVB-T2

Abstract: A technique intended to increase the diversity order of Bit-Interleaved Coded Modulations (BICM) over non Gaussian channels is presented. It introduces simple modifications to the mapper and to the corresponding demapper. They consist of a constellation rotation coupled with signal space component interleaving. Iterative processing at the receiver side can provide additional improvement to the BICM performance. This method has been shown to perform well over fading channels with or without erasures. It has been adopted for the 4-, 16-, 64- and 256-QAM constellations considered in the DVB-T2 standard. Resulting gains can vary from 0.2 dB to several dBs depending on the order of the constellation, the coding rate and the channel model.

Optical direct-detection OFDM signal generation for radio-over-fiber link using frequency doubling scheme with carrier suppression

Abstract: This investigation demonstrates the generation of OFDM-RoF signal using frequency doubling technique for the first time, to the author’s best knowledge. The 4-Gb/s OFDM signal using 16-QAM format modulated on each subcarrier at a center frequency of 19GHz is experimentally demonstrated. Benchmarked against the OOK format, the 16-QAM OFDM format has the higher spectral efficiency with a sensitivity penalty of less than 2.6 dB. After transmission over 50-km single mode fiber, the power penalties of RF OOK and OFDM signals are less than 0.5dB.

Methodology and method and apparatus for signaling with capacity optimized constellations

Abstract: Communication systems are described that use geometrically shaped constellations that have increased capacity compared to conventional constellations operating within a similar SNR band. In several embodiments, the geometrically shaped is optimized based upon a capacity measure such as parallel decoding capacity or joint capacity. In many embodiments, a capacity optimized geometrically shaped constellation can be used to replace a conventional constellation as part of a firmware upgrade to transmitters and receivers within a communication system. In a number of embodiments, the geometrically shaped constellation is optimized for an Additive White Gaussian Noise channel or a fading channel. In numerous embodiments, the communication uses adaptive rate encoding and the location of points within the geometrically shaped constellation changes as the code rate changes. One embodiment of the invention includes a transmitter configured to transmit signals to a receiver via a communication channel, wherein the transmitter, includes a coder configured to receive user bits and output encoded bits at an expanded output encoded bit rate, a mapper configured to map encoded bits to symbols in a symbol constellation, a modulator configured to generate a signal for transmission via the communication channel using symbols generated by the mapper. In addition, the receiver includes a demodulator configured to demodulate the received signal via the communication channel, a demapper configured to estimate likelihoods from the demodulated signal, a decoder that is configured to estimate decoded bits from the likelihoods generated by the demapper. Furthermore, the symbol constellation is a capacity optimized geometrically spaced symbol constellation that provides a given capacity at a reduced signal-to-noise ratio compared to a signal constellation that maximizes dmin.

64 and 128 coherent QAM optical transmission over 150 km using frequency-stabilized laser and heterodyne PLL detection.

Abstract: We describe 1 Gsymbol/s, 64 and 128 coherent quadrature amplitude modulation (QAM) transmissions over 150 km, in which we employ a frequency-stabilized C2H2 fiber laser, an optical phase-looked loop (OPLL), and a heterodyne detection circuit.

Improving BICM performance of QAM constellations for broadcasting applications

Abstract: A technique intended to improve the performance of bit-interleaved coded modulations over non Gaussian channels is presented. It introduces signal space diversity by the means of modifications to the constellation mapper and to the corresponding demapper. A rotation of the constellation is followed by a signal space component interleaving. Iterative processing at the receiver side is shown to provide additional error correction. This method outperforms state-of-the-art systems over flat fading channels and substantial gains with respect to bit-interleaved coded modulation are obtained for severe channel conditions. It has been adopted by the Digital Video Broadcasting European consortium in the upcoming next generation of digital terrestrial television, DVB-T2, for the 4-, 16-, 64- and 256-QAM constellations. The resulting improvement in performance can vary from 0.2 dB to several dBs depending on the order of the constellation, the coding rate and the channel model.

On enhancing hierarchical modulation

Abstract: Hierarchical modulation offers an important coverage/throughput tradeoff for wireless communication, however it has received relatively little attention to date. Regular hierarchical modulation suffers from the interference between layers. It results in both achievable capacity decrease and bit- error rate increase. In this paper, an enhanced hierarchical modulation technique along with three optimization schemes, in which the enhancement-layer signal constellation is rotated, are presented and analyzed. The first optimization scheme is proposed to maximize the achievable spectral efficiency with rotating the enhancement-layer signal constellation. The second scheme is proposed to lower the demodulation symbol-error rate with maximizing the modulation efficiency and asymptotic modulation efficiency. The parameters, modulation efficiency and asymptotic modulation efficiency, are formulated for quantizing inter-layer interference. The last one is proposed to maximize RF power amplifier efficiency with reducing the peak-to-average- power ratio of modulated symbols. This perspective is especially important for multi-carrier communications. All proposed schemes are simple and efficient. They can help recover the performance loss of regular hierarchical modulation due to inter-layer interference with little complexity increase. Computer simulations are provided to support our conclusions.

Performance Analysis of Adaptive M-QAM for Rayleigh Fading Cooperative Systems

Abstract: The use of constant-power, rate-adaptive M-QAM transmission with an amplify-and-forward cooperative system is proposed. The upper bound expressions are derived for the outage probability, achievable spectral efficiency, and error rate performance for the amplify-and-forward cooperative system over both independent and identically distributed (i.i.d.) and non-i.i.d. Rayleigh fading environments. The analysis is based on an accurate upper bound on the total effective signal-to- noise ratio SNR at the destination. Adaptive continuous rate M-QAM achieves a capacity that comes within a constant gap of the Shannon capacity of the channel, but adaptive discrete rate M-QAM suffers additional performance penalties.

On the Use of Hexagonal Constellation for Peak-to-Average Power Ratio Reduction of an ODFM Signal

Abstract: One of the main drawbacks of orthogonal frequency division multiplexing (OFDM) is the high peak-to-average power ratio (PAPR) of the OFDM signal. In this paper, we propose the use of hexagonal constellation for PAPR reduction of an OFDM signal. Because hexagonal constellation is the densest packing of regularly spaced points in two dimensions, we can have more signal points in a given area with hexagonal constellation than with quadrature amplitude modulation (QAM) constellation. We can exploit these extra degrees of freedom provided by the hexagonal constellation for PAPR reduction of an OFDM signal. We will apply the proposed technique to eliminate data rate loss due to the side information in partial transmit sequence (PTS) technique and selected mapping (SLM) technique.

On the Relationship Between MMSE-SIC and BI-GDFE Receivers for Large Multiple-Input Multiple-Output Channels

Abstract: A minimum mean-square error (MMSE)-based iterative soft interference cancellation (MMSE-SIC) receiver has been proposed to mitigate the interferences of the multiple-input multiple-output (MIMO) channels, with reduced complexity as compared to maximum-likelihood (ML) detection. On the other hand, the block-iterative generalized decision-feedback equalizer (BI-GDFE) attains close to the performance of the MMSE-SIC receivers with further reduced complexity. The BI-GDFE, however, requires an accurate estimate of the input-decision correlation (IDC), which is a statistical reliability metric of earlier-made decisions. To date, the BI-GDFE receiver is applicable only to phase-shift-keying (PSK) modulations due to the absence of a method to estimate the IDC for higher order quadrature amplitude modulations (QAMs). In this paper, we establish the relationship between the MMSE-SIC and BI-GDFE receivers and propose an algorithm to determine the IDC for BI-GDFE from the unconditional MMSE-SIC (U-MMSE-SIC). We further analyze and compare the asymptotic performances of the two receivers for large random MIMO channels and prove that for the limiting case, the output signal-to-interference-plus-noise ratios (SINRs) at each iteration for both receivers converge in probability to their respective deterministic limits. Our simulation results have shown that the bit error rate (BER) performance of the BI-GDFE receiver with the proposed IDC selection method achieves close to that of the U-MMSE-SIC receiver with similar convergence behavior and reaches the single-user matched filter bound (MFB) with several iterations for high enough signal-to-noise ratio (SNR).