Showing papers on "Quadrature amplitude modulation published in 2007"

Feedforward Carrier Recovery for Coherent Optical Communications

Abstract: We study a carrier-synchronization scheme for coherent optical communications that uses a feedforward architecture that can be implemented in digital hardware without a phase-locked loop. We derive the equations for maximum a posteriori joint detection of the transmitted symbols and the carrier phase. The result is a multidimensional optimization problem that we approximate with a two-stage iterative algorithm: The first stage is a symbol-by-symbol soft detector of the carrier phase, and the second stage is a hard-decision phase estimator that uses prior and subsequent soft-phase decisions to obtain a minimum mean-square-error phase estimate by exploiting the temporal correlation in the phase-noise process. The received symbols are then derotated by the hard-decision phase estimates, and maximum- likelihood sequence detection of the symbols follows. As each component in the carrier-recovery unit can be separately optimized, the resulting system is highly flexible. We show that the optimum hard-decision phase estimator is a linear filter whose impulse response consists of a causal and an anticausal exponential sequence, which we can truncate and implement as an finite-impulse- response filter. We derive equations for the phase-error variance and the system bit-error ratio (BER). Our results show that 4, 8, and 16 quadrature-amplitude-modulation (QAM) transmissions at 1 dB above sensitivity for BER = 10-3 is possible with laser beat linewidths of DeltanuTb = 1.3 X 10-4, 1.3 X 10-4, and 1.5 x 105 when a decision-directed soft-decision phase estimator is employed.

Multinode Cooperative Communications in Wireless Networks

Abstract: In this paper, a class of cooperative communication protocols with arbitrary N-relay nodes is proposed for wireless networks, in which each relay coherently combines the signals received from m (1lesmlesN-1) previous relays in addition to the signal from the source. Exact symbol-error-rate (SER) expressions for an arbitrary N-node network employing M'ary phase-shift-keying (MPSK) modulation or quadrature-amplitude modulation (QAM) are provided for the proposed class of protocols. Further, approximate expressions for the SER are derived and shown to be tight at high enough signal-to-noise ratio (SNR). Our analysis reveals an interesting result: The class of cooperative protocols shares the same asymptotic performance at high enough SNR and does not depend on m, the number of previous nodes involving in coherent detection, hence, the asymptotic performance of a simple cooperative scenario in which each relay combines the signals from the source and the previous relay is exactly the same as that for a much more complicated scenario in which each relay combines the signals from the source and all the previous relays. The theoretical results also confirm that full diversity equal to the number of cooperating nodes is indeed achievable by the proposed protocols. Finally, we formulate a power-allocation problem in order to minimize the SER of the system. The analysis shows that the optimum power allocation at different nodes follows a certain ordering, and that the power-allocation scheme at high SNR does not depend on the channel quality of the direct link between the source and the destination. Closed-form solutions for the optimal power-allocation problem are provided for some network topologies. Simulation results confirm our theoretical analysis

Relaxed $K$ -Best MIMO Signal Detector Design and VLSI Implementation

Abstract: Signal detector is a key element in a multiple-input multiple-output (MIMO) wireless communication receiver. It has been well demonstrated that nonlinear tree search MIMO detectors can achieve near-optimum detection performance, nevertheless their efficient high-speed VLSI implementations are not trivial. For example, the hardware design of hard- or soft- output detectors for a 4 times 4 MIMO system with 64 quadrature amplitude modulation (QAM) still remains missing in the open literature. As an attempt to tackle this challenge, this paper presents an implementation-oriented breadth-first tree search MIMO detector design solution. The key is to appropriately modify the conventional breadth-first tree search detection algorithm in order to largely improve the suitability for efficient hardware implementation, while maintaining good detection performance. To demonstrate the effectiveness of the proposed design solution, using 0.13-mum CMOS standard cell and memory libraries, we designed a soft-output signal detector for 4 times 4 MIMO with 64-QAM. With the silicon area of about 31 mm2, the detector can achieve above 100 Mb/s and realize the performance very close to that of the sphere decoding algorithm

A Family of Distributed Space-Time Trellis Codes With Asynchronous Cooperative Diversity

Abstract: In current cooperative communication schemes, to achieve cooperative diversity, synchronization between terminals is usually assumed, which may not be practical since each terminal has its own local oscillator. In this paper, based on the stack construction proposed by Hammons and El Gamal, we first construct a family of space-time trellis codes for BPSK modulation scheme that is characterized to possess the full cooperative diversity order without the synchronization assumption. We then generalize this family of the space-time trellis codes from BPSK to higher order QAM and PSK modulation schemes based on the unified construction proposed by Lu and Kumar. Some diversity product properties of space-time trellis codes are studied and simplified decoding methods are discussed. Simulation results are given to illustrate the performance of the newly proposed codes

Slow Adaptive $M$ -QAM With Diversity in Fast Fading and Shadowing

Abstract: This paper investigates the performance of adaptive M-ary quadrature amplitude modulation (QAM) with antenna subset diversity. We consider a slow adaptive modulation (SAM) technique that adapts the constellation size to the slow variation of the channel due, for example, to shadowing. The proposed SAM technique is more practical than conventional fast adaptive modulation (FAM) techniques that require adaptation to fast-fading variations. Our results show that the SAM technique can provide a substantial increase in throughput with respect to fixed schemes while maintaining an acceptable low bit-error outage. We also compare SAM and FAM techniques, showing that the throughput of SAM can be, in many practical cases, close to that of FAM, despite the fact that SAM is less complex and requires a lower feedback rate. For example, using a set of possible modulations {4,16,64}-QAM with dual-branch maximal ratio combining reception, 5% outage at a bit-error probability of 10-2 and a median signal-to-noise ratio of 22 dB, SAM is capable of improving the mean spectral efficiency of fixed schemes from about 1.9 to 4.7 b/s/Hz, which is close to the 5.5 b/s/Hz achieved by FAM

An Automatic Digital Modulation Classifier for Measurement on Telecommunication Networks

Abstract: This paper presents a method for the automatic classification of digital modulations without a priori knowledge of the signal parameters. This method can recognize classical single- carrier modulations such as M-ary phase-shift keying, M-ary frequency-shift keying, M-ary amplitude-shift keying, and M-ary quadrature amplitude modulation, as well as orthogonal frequency-division multiplexing modulations such as discrete mul- titone that is used for asymmetric digital subscriber line and very high speed digital subscriber line standards and for power-line carrier transmissions. After identification of the modulation type, the method automatically estimates some parameters characterizing the modulation. To evaluate the method performance, several simulations have been carried out in different operating conditions that should be particularly critical by varying the values of signal- to-noise ratio and the main parameters of each identifiable modulation. To assess the advantages, comparison with other classification methods has been given. To validate the assumption that is made, experimental tests have been performed.

Signal Design and Detection in Presence of Nonlinear Phase Noise

Abstract: In optical fiber transmission systems using inline amplifiers, the interaction of a signal and amplifier noise through the Kerr effect leads to nonlinear phase noise that can impair the detection of phase-modulated signals. We present analytical expressions for the maximum-likelihood (ML) decision boundaries and symbol-error rate (SER) for phase-shift keying and differential phase-shift keying systems with coherent and differentially coherent detection, respectively. The ML decision boundaries are in the form thetas(r) = c2r2 + c1r + c0, where thetas and r are the phase and the amplitude of the received signal, respectively. Using the expressions for the SER, we show that the impact of phase error from carrier synchronization is small, particularly for transoceanic links. For modulation formats such as 16-quadrature amplitude modulation, we propose various transmitter and receiver phase rotation strategies such that the ML detection is well approximated by using straight-line decision boundaries. The problem of signal constellation design for optimal SER performance is also studied for a system with four signal points.

An Introduction to Adaptive QAM Modulation Schemes for Known and Predicted Channels

Abstract: A major disadvantage with fixed modulation (nonadaptive) on channels with varying signal-to-noise ratio (SNR) is that the bit-error-rate (BER) probability performance is changing with the channel quality. Most applications require a certain maximum BER and there is normally no reason for providing a smaller BER than required. An adaptive modulation scheme, on the contrary, can be designed to have a BER which is constant for all channel SNRs. The spectral efficiency of the fixed modulation is constant, while it, in general, will increase with increasing channel SNRs for the adaptive scheme. This in effect means that the average spectral efficiency of the adaptive scheme is improved, while at the same time the BER is better suited to the requirement of the application. Thus, the adaptive link becomes much more efficient for data transmission. The major disadvantage is that the transmitter needs to know the channel SNR such that the best suitable modulation is chosen and the receiver must be informed on the used modulation in order to decode the information. This leads to an increased overhead in the system as compared with a fixed modulation system. In this paper, we introduce adaptive modulation systems by presenting some of the simpler adaptive quadrature amplitude modulation schemes and their performance for both perfectly known and predicted channels.

A 56 mW Continuous-Time Quadrature Cascaded $\Sigma\Delta$ Modulator With 77 dB DR in a Near Zero-IF 20 MHz Band

Abstract: A quadrature cascaded modulator with continuous-time loop filters is presented for a digital multi-stream FM radio receiver. The ADC achieves a dynamic range of 77 dB and 20 MHz bandwidth centered on an intermediate frequency of 10.5 MHz and is sampling at 340 MHz. The cascaded modulator comprises programmable analog second-order quadrature filters and a digital quadrature noise cancellation filter. The 0.5 chip in 90 nm CMOS consumes 56 mW from a 1.2 V supply.

An Introduction to Adaptive QAM Modulation Schemes for Known and Predicted Channels Relatively simple Quadrature Amplitude Modulation (QAM) systems illustrate the advantages, in error performance and spectral efficiency, of adaptive modulation over fixed modulation.

Abstract: A major disadvantage with fixed modulation (nonadaptive) on channels with varying signal-to-noise ratio (SNR) is that the bit-error-rate (BER) probability performance is changing with the channel quality. Most applications require a certain maximum BER and there is normally no reason for providing a smaller BER than required. An adaptive modulation scheme, on the contrary, can be designed to have a BER which is constant for all channel SNRs. The spectral efficiency of the fixed modulation is constant, while it, in general, will increase with increasing channel SNRs for the adaptive scheme. This in effect means that the average spectral efficiency of the adaptive scheme is improved, while at the same time the BER is better suited to the requirement of the application. Thus, the adaptive link becomes much more efficient for data transmis- sion. The major disadvantage is that the transmitter needs to know the channel SNR such that the best suitable modulation is chosen and the receiver must be informed on the used modulation in order to decode the information. This leads to an increased overhead in the system as compared with a fixed modulation system. In this paper, we introduce adaptive modulation systems by presenting some of the simpler adaptive quadrature amplitude modulation schemes and their performance for both perfectly known and predicted channels.

Blind Frequency-Offset Estimation for OFDM/OQAM Systems

Abstract: This paper deals with the problem of blind carrier-frequency offset (CFO) estimation in orthogonal frequency-division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OFDM/OQAM). Specifically, by assuming that the number of subcarriers is sufficiently large, the received signal is modeled as a complex Gaussian random vector (CGRV). Since the OFDM/OQAM signal results to be a noncircular (NC) process (i.e., its relation function is different from zero), by exploiting the generalized probability density function for NC-CGRVs, the unconditional maximum-likelihood algorithm for CFO estimation in nondispersive channel is derived. Moreover, due to its computational complexity, a more feasible estimator is considered. The performance of the proposed estimators, assessed via computer simulation, is compared with that of recently proposed CFO estimators exploiting second-order cyclostationarity and with the Gaussian Crameacuter-Rao bound

Using LDPC-Coded Modulation and Coherent Detection for Ultra Highspeed Optical Transmission

Abstract: We propose the coded modulation schemes for ultrahigh-speed transmission (100 Gb/s and above) by using commercially available components operating at 40 gigasymbols/s. The bit-interleaved coded modulation in combination with the low-density parity-check codes that are used as component codes has been applied. The modulation is based on either M-ary quadrature-amplitude modulation or M-ary phase-shift keying. bits are mapped into the corresponding signal constellation point using either Gray or natural mapping. The coherent detection scheme has been found to outperform the direct detection one and to provide an additional margin much needed for longer transmission distances or for application in an all-optical network scenario.

Multiuser Communications Using Passive Time Reversal

Abstract: A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).

Performance of BICM-ID with Signal Space Diversity

Abstract: This paper presents a performance analysis of bit-interleaved coded-modulation with iterative decoding (BICM-ID) and complex N-dimensional signal space diversity in fading channels to investigate its performance limitation, the choice of the rotation matrix and the design of a low-complexity receiver. The tight error bound is first analytically derived. Based on the design criterion obtained from the error bound, the optimality of the rotation matrix is then established. It is shown that using the class of the optimal rotation matrices, the performance of BICM-ID systems over a Rayleigh fading channel approaches that of the BICM-ID systems over an AWGN channel when the dimension of the signal constellation increases. Furthermore, by exploiting the sigma mapping for any M-ary QAM constellation, a very simple sub-optimal, but yet effective iterative receiver structure suitable for signal constellations with large dimensions is proposed. Simulation results in various cases and conditions indicate that the proposed receiver can achieve the analytical performance bounds with low complexity

On the BER Performance of Space-Frequency Block Coded OFDM Systems in Fading MIMO Channels

Abstract: Closed-form expressions for the bit error rate (BER) performance of space-frequency block coded OFDM (SFBC-OFDM) systems are derived and evaluated for frequency-selective fading channels. In the performance analysis, both M-ary phase shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) are considered, and the effects of channel estimation errors on the BER performance are studied. Numerical results and comparisons are provided for several forms of SFBC-OFDM. It is shown that the results obtained from the closed-form formulae are very close to the ones using the exact expressions and to simulation results of the SFBC-OFDM model. The BER deterioration that results from channel estimation errors in the SFBC-OFDM systems is also illustrated. Using the provided results, the amount of degradation can be quantified.

Precoding for the Multiantenna Downlink: Multiuser SNR Gap and Optimal User Ordering

Abstract: This paper develops a practical design method for implementing Tomlinson-Harashima precoding (THP) in a downlink channel with multiple antennas at the transmitter and a single antenna at each receiver. A two-step design process is proposed for minimizing the total transmit power while satisfying every user's minimum data rate and maximum bit-error rate (BER) requirements. First, the BER and rate requirements are converted to "virtual rate" requirements, which account for the gap-to-capacity introduced by practical quadrature amplitude modulation (QAM) and THP. The second step is to determine the transmit covariance matrices (which specify the entire THP system) that will provide these virtual rates at the minimum total transmit power. As one of the main features in the proposed scheme, an algorithm for finding the optimal user encoding (or presubtraction) order in polynomial time is proposed. In addition, we also propose an algorithm that finds a near-optimal order, but which is much less complex. The proposed method outperforms existing zero-forcing-based THP systems in term of power efficiency

BER analysis for decode-and-forward relaying in dissimilar Rayleigh fading channels

Abstract: In this letter, a BER study is presented for decode-and-forward (DF) relaying in independent but not identically distributed Rayleigh fading channels. For an arbitrary number of relays, exact and closed-form expressions of the BER are provided for M-ary PAM, QAM and PSK, respectively. It is also shown that the analytic results are perfectly matched with the simulated ones

Gray Coding for Multilevel Constellations in Gaussian Noise

Abstract: The problem of finding the optimal labeling (bit-to-symbol mapping) of multilevel coherent phase shift keying (PSK), pulse amplitude modulation (PAM), and quadrature amplitude modulation (QAM) constellations with respect to minimizing the bit-error probability (BEP) over a Gaussian channel is addressed. We show that using the binary reflected Gray code (BRGC) to label the signal constellation results in the lowest possible BEP for high enough signal energy-to-noise ratios and analyze what is "high enough" in this sense. It turns out that the BRGC is optimal for PSK and PAM systems whenever the target BEP is at most a few percent, which covers most systems of practical interest. New and simple closed-form expressions are presented for the BEP of PSK, PAM, and QAM using the BRGC

24-Gb/s Transmission over 730 m of Multimode Fiber by Direct Modulation of an 850-nm VCSEL Using Discrete Multi-Tone Modulation

Abstract: Using discrete multi-tone modulation with up to 64-QAM mapping, 24-Gb/s transmission is experimentally demonstrated over 730 m of MMF by direct modulation of an 850-nm VCSEL and direct detection with a MMF receiver.

Optical Systems With High-Order DPSK and Star QAM Modulation Based on Interferometric Direct Detection

Abstract: Increased interest in novel modulation formats for optical transmission has come up in recent years. Receivers with interferometric direct detection can be used to detect arbitrary modulation formats with differentially encoded phases as differential phase-shift keying (DPSK) and differentially phase-encoded star-shaped quadrature amplitude modulation (Star QAM). In this paper, two novel 16-ary modulation formats, which are the 16 DPSK and the Star 16 QAM (ASK-8 DPSK), are characterized for optical transmission for the first time. To be able to identify clear performance tendencies for high-order optical modulation, the novel formats are compared to a wide range of already investigated formats by conducting comprehensive calculations in a uniform simulation environment. The influence of different transmitter structures and decision schemes is considered, and all the systems are characterized with respect to the optical signal-to-noise ratio requirements, dispersion tolerance, and self-phase-modulation (SPM) performance for nonreturn-to-zero (NRZ) and RZ pulse shapes. Moreover, an inherent problem of Star QAM transmission concerning SPM is illustrated, and compensation techniques are examined. The results give a substantial insight into the properties of high-order optical modulation formats.

Preamble-based channel estimation techniques for OFDM/OQAM over the powerline

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 guard interval, it has the advantage of a theoretical higher spectral efficiency. Furthermore, if the pulse shape is well-localized in frequency, the resulting OFDM/OQAM signal satisfies stringent spectrum requirements. 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 describe a method for perfect channel estimation in OFDM/OQAM. Then we introduce two new realistic preamble-based channel estimation methods. The performance results are obtained by considering the transmission of an OFDM/OQAM signal over a multi-path power line communication (PLC) channel model. The proposed channel estimation methods are compared to the ones obtained with CP-OFDM.

(Gray) Mappings for Bit-Interleaved Coded Modulation

Abstract: Mappings for square quadrature amplitude modulation (QAM) are investigated in terms of the achievable capacity in bit-interleaved coded modulation (BICM). In G. Caire et al. (1998) it is conjectured that Gray labelings maximize this capacity. However, Gray mappings are not unique and it can be shown that the conjecture cannot hold in general for QAM constellation sizes larger than 16. Based on the bit level capacities of mappings, structures determining capacity are analyzed. Construction guidelines leading to capacity maximizing Gray mappings are presented.

V-BLAST-Based Virtual MIMO for Distributed Wireless Sensor Networks

Abstract: A virtual multiple-input multiple-output (MIMO) communications architecture based on vertical Bell Laboratories layered space-time (V-BLAST) receiver processing is proposed for wireless sensor networks (WSNs). The proposed scheme does not require transmitter-side node cooperation unlike previously proposed virtual MIMO schemes. The energy and delay efficiencies of the proposed virtual MIMO scheme are derived for networks with both single-and multiple-antenna data gathering nodes (DGNs). Numerical results show the significant energy savings offered by the proposed method. These results also indicate that rate optimization over transmission distance is not essential as in virtual MIMO systems based on Alamouti scheme. In most scenarios, a fixed-rate virtual MIMO system with 4-quadrature amplitude modulation can achieve performance very close to that of an optimized, variable-rate system. In the case of single-antenna DGNs, the proposed scheme typically incur larger delay values compared to traditional single-input single-output communication, making it a good candidate for energy-starved but delay-tolerant WSNs.

Effect of Channel Estimation Errors on $M$ -QAM With MRC and EGC in Nakagami Fading Channels

Abstract: We study the effect of imperfect channel estimation (ICE) on the error probability performance of M-level quadrature amplitude modulation ( M-QAM) with maximal-ratio combining and equal-gain combining diversity formats in Nakagami fading channels. We provide a novel formulation of the bit-error rate (BER) of M-QAM with ICE in terms of the signal constellation-dependent effective signal-to-noise ratios (SNRs) or amplitudes, which allows us to derive the general, accurate, and easy-to-evaluate BER formulas for square and rectangular diversity M-QAM with channel estimation errors. Our result shows that the performance loss caused by ICE may be manifested by the signal decision space distortion and a scaling of the effective SNR. Using our analytical result, we evaluate the performance of M-QAM with pilot-symbol assisted modulation and present some insightful findings

ML-based Tracking Algorithms for MIMO-OFDM

Abstract: This article addresses the problem of tracking the carrier frequency offset (CFO) and sampling frequency offset (SFO) in burst MIMO-OFDM systems. The goal is to accomplish those tasks with the smallest possible piloting overhead (highest spectral efficiency). For that, we derive a precise mathematical model that describes the joint effect of a CFO and SFO on the received MIMO-OFDM subcarriers. The model unifies simpler, well-known results found in the literature. We use it for deriving maximum likelihood (ML) estimators for both impairments based on observations of received pilot subcarriers at the output of the FFTs of the receiver branches. This approach yields estimators that are independent of the type of MIMO decoder, located further downstreams. At the same time, the estimators are the ML-optimal processors of pilot information at the MIMO channel's output. A pair of corresponding tracking algorithms based on the estimators is proposed and evaluated by simulation. The results show that the variance of our estimators decreases with larger MIMO configurations, allowing for increased synchronization accuracy at low SNR, or for reducing the number of pilot subcarriers to maintain equal estimator variance. We also show that the proposed tracking algorithms operate robustly under imperfect channel state information and with modulation sizes ranging from 4-QAM to 64-QAM. The SNR loss of the proposed algorithms is below 0.1 dB in all the cases, while a conventional tracking approach is shown to have an SNR loss between 0.8 dB and 1.2 dB.

Spectral modulation detection as a function of modulation frequency, carrier bandwidth, and carrier frequency region.

Abstract: The present study investigates the nature of spectral envelope perception using a spectral modulation detection task in which sinusoidal spectral modulation is superimposed upon a noise carrier. The principal goal of this study is to characterize spectral envelope perception in terms of the influence of modulation frequency (cycles/octave), carrier bandwidth (octaves), and carrier frequency region (defined by lower and upper cutoff frequencies in Hz). Spectral modulation detection thresholds measured as a function of spectral modulation frequency result in a spectral modulation transfer function (SMTF). The general form of the SMTF is bandpass in nature, with a minimum modulation detection threshold in the region between 2 to 4 cycles/octave. SMTFs are not strongly dependent on carrier bandwidth (ranging from 1 to 6 octaves) or carrier frequency region (ranging from 200 to 12800Hz), with the exception of carrier bands restricted to very low audio frequencies (e.g., 200–400Hz). Spectral modulation detectio...

Classic OFDM Systems and Pulse Shaping OFDM/OQAM Systems

Abstract: In this report, we provide a comparative study of state-of-the-art in Orthogonal Frequency Division Multiplexing (OFDM) techniques with orthonormal analysis and synthesis basis. Two main categories, OFDM/QAM which adopts base-band Quadrature Amplitude Modulation (QAM) and rectangular pulse shape, and OFDM/OQAM which uses baseband offset QAM and various pulse shapes, are intensively reviewed. OFDM/QAM can provide high data rate communication and effectively remove intersymbol interference (ISI) by employing guard interval, which costs a loss of spectral efficiency and increases power consumption. Meanwhile it remains very sensitive to frequency offset which causes inter-carrier interference (ICI). In order to achieve better spectral efficiency and reducing combined ISI/ICI, OFDM/OQAM using well designed pulses with proper Time Frequency Localization (TFL) is of great interest. Various prototype functions, such as rectangular, half cosine, Isotropic Orthogonal Transfer Algorithm (IOTA) function and Extended Gaussian Functions (EGF) are discussed and simulation results are provided to illustrate the TFL properties by the ambiguity function and the interference function.

Semidefinite programming relaxation approach for multiuser detection of QAM signals

Abstract: A semidefinite programming (SDP) relaxation approach is proposed to solve multiuser detection problems in systems with M-ary quadrature amplitude modulation (M-QAM). In the proposed approach, the optimal M-ary maximum likelihood (ML) detection is carried out by converting the associated M-ary integer programming problem into a binary integer programming problem. Then a relaxation approach is adopted to convert the binary integer programming problem into an SDP problem. This relaxation process leads to a detector of much reduced complexity. A multistage approach is then proposed to improve the performance of the SDP relaxation based detectors. Computer simulations demonstrate that the symbol-error rate (SER) performance offered by the proposed multistage SDP relaxation based detectors outperforms that of several existing suboptimal detectors.

Phase Coherent Digital Communications for Wireless Optical Links in Turbid Underwater Environments

Abstract: Previous studies by the authors have included a theoretical and experimental investigation of the spatial distribution of an optical signal used for communications in underwater scattering environments. Presented here is an experimental study of how scattering affects the temporally encoded information bearing component of the optical signal. Short range underwater optical links employing BPSK, QPSK, 8- PSK, 16-QAM, and 32-QAM modulation are implemented in a laboratory setting, yielding data rates up to 5 Mb/s. The effect of link quality is examined versus water turbidity.

Volterra Behavioral Model for Wideband RF Amplifiers

Abstract: This paper proposes a behavioral modeling approach for the description of nonlinearities in wideband wireless communication circuits with memory. The model is formally derived exploiting the dependence on frequency of the amplifier nonlinear transfer functions and reduce the number of parameters in a general Volterra-based behavioral model. To validate the proposed approach, a commercial amplifier at 915 MHz, exhibiting nonlinear memory effects, has been widely characterized using different stimuli, including two tones, quadrature phase-shift keying wideband code division multiple access, and 16-quadrature amplitude modulation signals with rectangular and root-raised cosine conforming pulses. The theoretical results have been compared with experimental data demonstrating that the model performance is comparable to the well-established memory polynomial model. Calculated and measured baseband waveforms, signal constellation, spectral regrowth and adjacent channel power ratio are tightly coincident in all cases, emphasizing the relevance of the proposed model