Quadrature Amplitude Modulation Symbols Research Articles

Neural-network-based carrier-less amplitude phase modulated signal generation and end-to-end optimization for fiber-terahertz integrated communication system.

In fiber-terahertz integrated communication systems, nonlinear distortion and inter-symbol interference (ISI) will degrade transmission performance. Pre-compensation is an efficient method to handle the channel distortion as it can avoid noise boosting during channel compensation and reduce receiver side signal processing algorithmic complexity at user-end (UE) considering the asymmetric access scenario. In this paper, we propose and experimentally demonstrate a neural-network (NN)-based carrier-less amplitude phase (CAP) modulated signal generation and end-to-end optimization method for a fiber-terahertz integrated communication system. The CAP signal is generated directly from quadrature amplitude modulation symbols and pre-compensated through a transmitter NN, which allows the receiver to demodulate the signal with simple linear digital signal process (DSP). In generating the CAP signal, the NN based transmitter learns a group of filters, which can generate, up-convert, and pre-compensate the signals. Based on the proposed method, a fiber-terahertz integration access system at 220 GHz is demonstrated and a sensitivity gain of 1.2 dB is achieved at a transmission speed of 50 Gbps and the forward error correction (FEC) bit error rate (BER) threshold of 1 × 10-2 compared with the baseline after 10-km fiber transmission and 1-m wireless delivering.

On Symbol Error Performance of Probabilistic Shaping in Noise-Limited and Fading Channels

In this work, we present symbol error performance analysis of probabilistic shaping (PS) for wireless communications in noise-limited and fading channels. Two fading models considered are the Rayleigh and log-normal fading channels. The results are corroborated with simulation and compared with the conventional uniformly distributed input symbols. In all channel conditions, PS results in significant reductions in the SNR required to achieve a specific error probability compared to the conventional uniformly shaped symbols. For example, in a noise-limited channel, PS based quadrature amplitude modulation (QAM) signal results in SNR gains of 1.16 dB, 1.41 dB, and 1.52 dB compared to the uniformly distributed QAM symbols at equal entropy rates of 4, 6, and 8 bit/symbol and a symbol error ratio (SER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 10^{-3}$ </tex-math></inline-formula> .

A simple chaotic base encryption scheme for securing OFDM-PON communications

Abstract Optical communications and their security have been got a great attention. In this work, a simple, low cost and secure optical system integrating orthogonal frequency division multiplexing-based passive optical network (OFDM-PON) and chaotic theory is proposed. In the proposed system, a novel scheme of quadrature amplitude modulation (QAM) encryption method is adopted. For privacy of image and video encryption, the process of encryption consists of two processes and a 3D chaotic system is employed. The chaotic system is used to generate the encryption sequences. In the first stage, the first generated chaotic sequence is XORed with the incoming data. While in the second stage, the QAM symbol is remapped using the other two generated chaotic sequences. Generally, the proposed system is designed to provide secure data, videos, and images transmission. The proposed system is simulated and evaluated with a transmission of a 10 Gb/s along 50 km single-mode fiber (SMF). The performance of the proposed system showed that there is a performance enhancement in terms of Peak to average power ratio (PAPR) and bit error rate (BER). The BER of unauthorized user is around 0.6147, while the PAPR is reduced to approximately half. The proposed system is simple and doesn’t require complex computations. It requires simple analog multiplexers for its implementation.

Extended Space Index Modulation

In this letter, a new scheme called extended space index modulation (ESIM) is proposed and investigated. Different from previous works, both the signal constellation and index domains are exploited, and the performance of the spectral efficiency and the reliability can be further enhanced. Specifically, in the ESIM, an antenna index (AI) vector set is designed to convey more extra index bits, which consists of two AI vector subsets. The two AI vector subsets, which are respectively used to modulate the quadrature amplitude modulation (QAM) and secondary QAM (S-QAM) symbols, include the AI vectors having two non-zero elements from <inline-formula> <tex-math notation="LaTeX">$\{(1,1),(1,j),(j,1),(j,j)\}$ </tex-math></inline-formula> to activate two transmit antennas and the AI vectors having only one non-zero element equaling to &#x201C;1&#x201D; to activate one transmit antenna. Furthermore, the S-QAM constellation is designed to enhance the squared Minimum Euclidean Distance. Simulation results are provided to show a close match with analytical results particularly in the high signal-to-noise ratio (SNR) region and demonstrate that significant performance gain can be achieved in the proposed ESIM.

Neural Network Detection for Bandwidth-Limited Non-Orthogonal Multiband CAP UVLC System

In this paper, we propose a novel sparse data-to-symbol neural network (SDSNN) receiver for bandwidth-limited underwater visible light communication (UVLC) based on non-orthogonal multi-band carrierless amplitude and phase modulation (NM-CAP). Bandwidth limited NM-CAP signals usually carry severe inter-symbol interference (ISI) and inter-band interference (IBI). The SDSNN receiver directly converts the received NM-CAP data with ISI and IBI into quadrature amplitude modulation symbols without distortion for each sub-band. In contrast, the conventional receiver requires the least mean square (LMS) equalizer to cancel ISI, and the subcarrier component extraction with complex independent component analysis (SCE-ICA) to cancel IBI, respectively. SDSNN provides a novel receiving structure to replace post-equalization, matched filtering, and SCE-ICA. A blue-LED based UVLC system has been demonstrated utilizing NM-CAP16 with 3 sub-bands. The experimental results show that NM-CAP with the SDSNN receiver case reaches the highest spectral efficiency, where an enhancement of 43%, 20%, 6% has been measured over the orthogonal multi-band CAP case, NM-CAP with LMS equalizer case, and NM-CAP with joint LMS equalizer and SCE-ICA case, respectively. Compared with joint LMS equalizer and SCE-ICA case, the proposed SDSNN receiver can achieve 98% reduction of computational complexity.

Two-Stage ML Detector Using Absolute Value of IQ Components and SVM for Adaptive OFDM-IM

Among various index modulation technology, adaptive orthogonal frequency division multiplexing with index modulation (A-OFDM-IM) improves reliability by solving the problem of deep fading. However, the maximal likelihood (ML) detector of A-OFDM-IM has high complexity by simultaneously performing active subcarrier detection and quadrature amplitude modulation (QAM) symbol demodulation. A simpler type of ML detector can be applied since the A-OFDM-IM has independent active states of each subcarrier and uses a single QAM constellation. In this paper, we propose two low-complexity detectors for the A-OFDM-IM. The proposed detectors have a two-stage receiving process, active subcarrier detection, and QAM demodulation. In active subcarrier detection, the presence or absence of the QAM symbol is more important than its information. Therefore we derive the thresholds using the absolute value of the in-phase and quadrature components of the received signal in the frequency domain. Using the absolute value, the distribution of noise added to inactive subcarrier is half normal. This is different from noise distribution of the ML detector, and the proposed detectors select the threshold considering the corresponding noise distribution. The first proposed detector uses a threshold derived by the ML estimation method. The second detector estimates the active subcarriers via a support vector machine. The proposed detectors have lower complexity than the ML detector because of the divided receiving process. Moreover, the theoretical analysis and simulation results show that the proposed detectors have better reliability than the ML detector due to different noise distribution and thresholds.

Performance of Orthogonal Frequency Division Multiplexing Based Advanced Encryption Standard

Currently, there are a lot of secure communication schemes have been proposed to hide secret contents. In this work, one of the methods deploying encryption to cipher data is represented. The primary object of this project is applying Advanced Encryption Standard (AES) in communications based Orthogonal Frequency Division Multiplexing (OFDM). This article discusses the security of the method encrypting directly QAM symbols instead of input bit-stream. This leads to improving the security of transmitting data by utilization of authentication key between the mobile and base station. The archived results demonstrate that the performance of the AES-OFDM system is completely acceptable to compare with the criteria for 4G.&#x0D; Keywords:&#x0D; Orthogonal Frequency Division Multiplexing (OFDM), Advanced Encryption Standard (AES), Quadrature Amplitude Modulation (QAM), Authentication Key, Cellular Network, Encryption, Physical Layer, 4G, LTE.&#x0D; References&#x0D; [1] M.A. Jessen, “Wireless communication security: Physical-Layer techniques exploiting radio and propagation characteristics”, Wireless Information Technology and Systems (ICWITS), IEEE International Conference, 2012.[2] M. Kim, M. Lee, S. Kim, D. Won, “Weakness and Improvements of a One-time Password Authentication Scheme”, International Journal of Future Generation Communication and Networking, 2009.&#x0D; [3] Alabaichi, Ashwaq, Salih, Adnan, “Enhance security of advance encryption standard algorithm based on key-dependent S-box”, 2015, pp. 44-53.&#x0D; [4] S. Xiao, W. Gong, D. Towsley, “Secure Wireless Communication with Dynamic secrets”, IEEE INFOCOM, 2010.[5] N.U. Rehman, L. Zhang, M.Z. Hammad, “ICI cancellation in OFDM system by frequency offset reduction”, Journal of Information Engineering and Applications, 2014.&#x0D; [6] Nikita Agrawal, Neelesh Gupta, “Security of OFDM through Steganography”, International Journal of Computer Applications 121(20) (2015) 41-43.&#x0D; [7] A. Al-Dweik, M. Mirahmadi, A. Sharmi, Z. Ding, R. Hamila, “Joint Secured and Robust technique for OFDM systems”, Western University, Canada, IEEE ICC 2013.&#x0D; [8] G.R. Tsouri, D. Wulich, “Securing OFDM over Wireless Time-varying channel using subcarrier overloading with Joint signal constellations”, Hindawi Publishing Corporation, EURASIP Journal on Wireless Communication and Networking, 2009.&#x0D; [9] D. Rajaveerappa, A. Almarimi, A., “RSA/Shift secured IFFT/FFT based OFDM wireless system,” Fifth International Conference on Information Assurance and Security, 2009.&#x0D; [10] M. Hilmey, S. Elhalafwy, M. Zein Eldin, “Efficient transmission of chaotic and AES encrypted images with OFDM over an AWGN channel”, 2009 International Conference on Computer Engineering &amp; Systems, Cairo, 2009, pp. 353-358.&#x0D; [11] B.V. Naik, N.L.K. Sai, C.M. Kumar, “Efficient transmission of encrypted images with OFDM system”, 2017 IEEE International Conference on Power, Control, Signals and Instrumentation Engineering (ICPCSI), Chennai, 2017, pp. 2383-2388.&#x0D; [12] S.M.S. Eldin, “Optimized OFDM Transmission of Encrypted Image Over Fading Channel”, An International Journal on Sensing and Imaging 15(1) (2014), pp. 1-14.&#x0D; [13] C. Akbar, H. Mahmood, Q. Minhas, I. Mustafa, “Secure AES OFDM with channel reciprocity exploitation through relative calibration”, 2016 International Conference on Open Source Systems &amp; Technologies (ICOSST), Lahore, 2016, pp. 54-61.&#x0D; [14] Y. Liang, J. Ren, T. Li, “Secure OFDM System Design and Capacity Analysis Under Disguised Jamming”, in IEEE Transactions on Information Forensics and Security 15 (2020) 738-752.&#x0D; [15] Westlund, B. Harold, “NIST reports measurable success of Advanced Encryption Standard”,Journal of Research of the National Institute of Standards and Technology, 2002.&#x0D; [16] B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C. Hall, N. Ferguson, Performance Comparison of the AES Submissions, Proceedings of the Second AES Candidate Conference, 1999.&#x0D; [17] S. He, A. Tang, H. Zhang, “A high-performance Implementation of OFDM-MIMO base-band in wireless video system”, Information Technology Journal 13 (2014), pp. 1678-1685.

Exact Bit Error-Rate Analysis of Two-User NOMA Using QAM With Arbitrary Modulation Orders

This letter considers the exact bit error rate (BER) analysis of a two-user non-orthogonal multiple access (NOMA) system using square quadrature amplitude modulation (QAM). Unlike existing work, no constraints are imposed on the modulation order of the QAM symbols for any user. Closed-form expressions are derived for the BER of the successive interference cancellation (SIC) receiver in Raleigh fading channels. The analytical BER results corroborated by Mote Carlo simulation show that the power control becomes challenging for high order QAM. Moreover, the BER of each user is approximately independent of the modulation scheme used by the other user for certain power settings.

A Time Sharing Hybrid Probabilistic Shaping Scheme for Nonbinary LDPC Codes

A time sharing (TS) hybrid probabilistic shaping (HPS) scheme for nonbinary (NB) LDPC codes is proposed. For the conventional probabilistic shaping (CPS) scheme, the demodulator uses bit-metric decoding (BMD) for binary LDPC codes and NB LDPC codes. However, the demodulator for NB LDPC codes adopts symbol-metric decoding (SMD) on the AWGN channel to avoid the loss of symbol-to-bit mapping and obtain better performance than binary LDPC codes. In order to take advantage of SMD demodulation for NB LDPC codes, the NB-LDPC information symbols use constant composition distribution matcher (CCDM) to generate quadrature amplitude modulation (QAM) symbols with unequal probability distribution, while the NB-LDPC parity symbols are utilized to yield QAM symbols with approximately uniform distribution. The SMD demodulation of the proposed NB-LDPC-coded TS HPS scheme has lower demodulation complexity than the NB-LDPC-coded CPS scheme. The information-theoretical analysis of the CPS system and the TS HPS system turns out that the proposed NB-LDPC-coded TS HPS scheme is superior to the NB-LDPC-coded CPS scheme for 8QAM and 32QAM format. Simulation results verify the theoretical average mutual information (AMI) analysis and show that the proposed NB-LDPC-coded TS HPS scheme has up-to-0.24 dB and 1.10 dB performance gain compared with the NB-LDPC-coded CPS scheme for 8QAM and 32QAM format, respectively. Compared with the binary LDPC-coded regular Nyquist scheme for 8QAM and 32QAM format, the proposed NB-LDPC-coded TS HPS scheme has up-to-1.31 dB and 2.04 dB performance gain, respectively. Meanwhile, the LDPC code rate of the proposed TS HPS scheme is more flexible than the CPS scheme. Therefore, the proposed NB-LDPC-coded TS HPS scheme is more reliable, efficient, and flexible for 8QAM and 32QAM, and thus suitable for the sixth generation (6G) communication system.

PAPR and security in OFDM-PON via optimum block dividing with dynamic key and 2D-LASM.

An approach for peak-to-average-power ratio (PAPR) reduction and security improvement in an orthogonal frequency division multiplexing passive optical network (OFDM-PON) is proposed by using optimum block dividing with 2-dimensional logistic adjusted sine map (2D-LASM) and dynamic key assignment technique. One frame of OFDM signal can be regarded as a symbol matrix. All the divisors of length and width of matrix can be calculated out, and each divisor of length and width can be arbitrarily combined to divide the matrix into blocks. A 4D hyperchaotic system is applied to generate a cipher book for 2D-LASM. And we assign different dynamic key groups from the cipher book for 2D-LASM to encrypt different block dividing situations. Different encrypted divisor blocks can obtain different values of PAPR. The optimum dividing situation is obtained by calculating out the minimum value of PAPR (VPAPR). The values of optimum encryption signal (VPAPR-op), the original signal (VPAPR-or) and the optimization ratio (η) are gradually equal to 146, 269 and 1.82, respectively, with the number of quadrature amplitude modulation (QAM) symbols in each subcarrier increasing. Simulating 1000 sets of 120×200 QAM symbols, the distribution of η approximately meets the Rayleigh distribution and its central distribution is around 1.7. The processing time decreases with the number of QAM symbols increasing in each block. The performance of PAPR reduction is more than 3 dB between the secure optimum signal and the original signal. In addition, the hyper-threading technique with two algorithms is applied to improve the performance of the encryption method. They promote the processing time by 38.6% and 50%, respectively. Finally, a 22.06 Gb/s optimum encryption OFDM signal transmits through a back-to-back (BTB) system and a 25-km standard single mode fiber (SSMF). These experimental results verify that the proposed approach is a promising candidate for solving both of PAPR reduction and security improvement in access network systems.

HARQ-Chaotic: Analog Chaotic Code Applied in HARQ Scheme of Wireless Communication System

This paper proposes a novel symbol-level combining hybrid automatic repeat request (HARQ) scheme based on analog chaotic code and named HARQ-Chaotic. The transmitter of HARQ-Chaotic adopts analog chaotic code to encode Quadrature Amplitude Modulation (QAM) symbols of retransmission packets to combat fading and noise. As the analog chaotic code can only handle sources with amplitudes in the range of [-0.5, 0.5], QAM symbols must be scaled into this range. We derived the optimal scaling factor through theoretical analysis. A joint algorithm combining with novel soft chaotic decoder and novel soft QAM demapper is proposed for the receiver to enhance the performance of the whole communication system. We implemented HARQ-Chaotic with LDPC codes and 16-QAM/64-QAM to carry out simulations in both AWGN channels and multipath fading channels. Massive simulation results demonstrate that the proposed HARQ-Chaotic has 1dB-4dB gain over traditional HARQ-Chase combining (HARQ-CC) scheme in block error rate (BLER) performance.

Performance analysis of the reference signal reconstruction for DVB-T passive radars

Abstract The present work investigates the reference signal reconstruction method for passive coherent location (PCL) radars exploiting Digital Video Broadcasting-Terrestrial (DVB-T) signals. The reference signal reconstruction improves the signal-to-noise ratio (SNR) of the reference signal, which enhances the detection probability for DVB-T based PCL systems. The existing approach for the reference signal reconstruction is performed by demodulating the received reference signal to retrieve the transmitted quadrature amplitude modulation (QAM) symbols and modulate them to obtain the reconstructed signal. For low SNR values, the QAM symbol detection is accompanied with a significant error, which leads to a mismatch between the reconstrued signal and the exact one. Consequently, the expected improvement due to the signal reconstruction is limited. In this work, we propose an optimum reconstruction method which filters the detected QAM symbol prior to the remodulation step. To validate the proposed method, we retrieved analytic expressions for the detection and false-alarm probabilities and verified them through Monte-Carlo simulations. In addition, real-data sets were used to evaluate the proposed method performances. The results show that the proposed method outperforms the conventional one by extending the feasibility of the reference signal reconstruction for low SNR values.

Chaotic distribution of QAM symbols for secure OFDM signal transmission

Abstract This paper proposes and demonstrates a novel dynamic quadrature amplitude modulation (QAM) mapping scheme to provide a secure physical-layer transmission in orthogonal frequency division multiplexing passive optical network (OFDM-PON). In the proposed scheme, permutated QAM symbols are independently and randomly mapped onto chaotically selected new locations on the complex plane. With the proposed encryption scheme these new locations are within the dimensions of a conventional QAM. A hyper digital chaos is applied to perform a multi-fold encryption in the proposed scheme which involves dynamic QAM mapping as well as time domain and frequency domain permutations on QAM symbols. As a result, a noise-like constellation with a key space of ∼10310 is achieved to enhance the high-level security in physical-layer transmission. A successful transmission experiment of 9.4-Gb/s encrypted 16-QAM OFDM signals is demonstrated over 25-km standard single mode fiber (SSMF), where the robust security and transmission performances are validated.

Analysis of a Multibeam Optical Beamforming Network Based on Blass Matrix Architecture

We present an extensive analysis of an optical Blass-matrix architecture as a beamforming network with potential for multibeam operation in wireless systems. Its design relies on the use of phase shifters and Mach-Zehnder Interferometers (MZIs) inside an $M\times N$ matrix, and enables the generation of M beams by N-element antenna arrays. We start our analysis from an optical signal with amplitude modulation by discrete microwave tones, and confirm the possibility to translate its optical phase shifts inside the matrix into equivalent phase shifts in the microwave domain. We show this is possible when the input is an optical single-side band signal and the optical carrier is reinserted before photodetection. We extend the conclusions to the case of an optical signal carrying a microwave with quadrature amplitude modulation (QAM) and the case of simultaneous inputs at the M input ports. Based on this analysis, we propose a Blass-matrix configuration algorithm taking into account the properties of the MZIs. Through simulations, we validate the potential for multibeam operation, and evaluate the beamforming performance at 28.5 GHz with respect to the QAM order, symbol rate, and pulse shaping parameters. In all cases with rate up to 3 Gbaud, the bit-error rate remains lower than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , showing that the beam squinting effect, which is present in our design, can be tolerated. Finally, we study the frequency dependence of the beamforming performance due to inevitable asymmetries of the MZIs and length variations of the waveguides, and evaluate the impact of the imperfections in the couplers inside the MZIs and the phase shifters. We show that in all cases the performance degradation is negligible for realistic fabrication and operation conditions.

A three-dimensional encryption orthogonal frequency division multiplexing passive optical network based on dynamic chaos-iteration

Orthogonal frequency-division multiple passive optical network (OFDM-PON) has emerged as one of the most promising solutions to meet the requirements for the next-generation wide-band optical access network with high capacity, strong fiber dispersion tolerance, and flexible resource allocation. However, like other optical access network in which the downstream signal is broadcasted to all the optical network units (ONUs), OFDM-PON is vulnerable to being eavesdropped. Thus the security of OFDM-PON should be taken seriously into consideration. Recently, some chaos based encryption methods, including chaotic scrambling and permutation, hyper-chaotic system and fractional Fourier transformation, chaos based IQ encryption method and chaos based two-dimensional scrambling, have been proposed to enhance the physical layer security of OFDM-PON system. Owing to the special chaos-related characteristics, such as ergodicity, pseudo randomness, and high sensitivity to the initial values, etc., these encryption methods are of high physical layer security. However, in most of these schemes, key distribution is not considered. In this paper, we propose a three-dimensional encryption OFDM-PON based on dynamic chaos-iteration. The key distribution is implemented through the dynamic chaos synchronization between the transmitter and receiver. The receiver tries to synchronize his chaos system with the transmitters' by calculating the correlation index of the synchronization sequence, which comes from the transmitter and is controlled by dynamic parameters in the parameter sets. The calculation is not very complex because the transmitter and receiver are acquainted with the parameter sets. The synchronized chaos system is used to generate keys for both encryption and decryption. In the proposed encryption scheme, one ONU is connected with four users, and the message is irrelevant to the users. Quadrature amplitude modulation (QAM) symbols from the users are mapped randomly onto the subcarriers in a flame based on the chaotic matrix M1. For the M1 is changeable, the number and position of subcarriers for different users are dynamically varying. Then the matrix M2 generated from chaos system is utilized to mask all QAM symbols. Finally the QAM symbol matrix is multiplied by an invertible chaotic matrix M3 to realize subcarrier perturbation. These three key matrixes are generated from the two-dimension logistic iteration chaos system, to which the initial sensitivity increases up to 10-15. The output sequence of the chaos system after quantification process is of good self-correlation and cross-correlation characteristic and can pass all NIST SP800-22 randomness tests. The key space of the encryption scheme is over 1086, which would be against exhaustive attack effectively. Specifically, a proof-of-principle experiment is conducted to demonstrate the aforementioned proposed scheme. In the experiment, a 13.3 Gb/s encrypted 64QAM OFDM signal transmits over 25 km standard single mode fiber in an OFDM-PON and successfully decrypts at the legal receiver. For an eavesdropper lacking correct keys, the received QAM constellation is totally in disorder and the bit error rate increases up to 0.46, which indicates that not any useful message is eavesdropped. The proposed scheme provides a promising candidate for the next-generation secure optical access networks.

Dynamic QAM Mapping for Physical-Layer Security Using Digital Chaos

This paper proposes and experimentally demonstrates a physical-layer data encryption scheme using multi-fold chaotic mapping of quadrature amplitude modulation (QAM) symbols in orthogonal frequency-division-multiplexed passive optical network (OFDM-PON). Dynamic radius and phase offsets are added in the standard QAM during QAM mapping on constellation using digital chaos. Due to the dynamic chaotic radius and phase offsets, the QAM symbols are mapped over the entire constellation plane, and thus the random, flexible QAM mapping is reconstructed to provide high-level security during data transmission. Since the chaotic offsets along with the data permutations are independently predetermined by a hyper digital chaos, the multi-fold data encryption creates an overall key space of ~10340 to enhance the physical-layer security. Successful transmission experiment of 9.4-Gb/s, 16-QAM encrypted OFDM data is demonstrated over a 22-km standard single-mode fiber in OFDM-PON. Moreover, with the use of dynamic mapping of QAM symbols, the transmission performance is not significantly degraded after implementing the proposed multi-fold secure data encryption.

Increasing the symbol rate in QAM system using a new set of orthonormal basics functions

This paper proposes a new technique to increase the symbol rate in Quadrature Amplitude Modulation (QAM) using a new set of orthonormal functions. The proposed technique increases the rate of QAM symbols without increasing the bandwidth of the modulated signal or increasing the modulation order of the QAM symbol. The set of orthonormal functions consists of four shaped-sinusoidal functions. The shaping functions are Hilbert-Transform pair. The proposed QAM modulator modulates two independent complex symbols in the same time. Each modulated symbol uses two different functions from the proposed set. The paper shows the structure of the modulator and demodulator of the proposed system. The paper presents different shaping pulses, which can be used with the proposed system.

Maximum Likelihood Detection With a Closed-Form Solution for the Square QAM Constellation

A low complexity maximum likelihood (ML) detection method is proposed for the square quadrature amplitude modulation (QAM) constellation. In order to reduce the implementation complexity of ML detection, set-partition, subset-selection, and subset-shift are recursively applied to the square QAM constellation. The estimated square QAM symbol is finally given in a closed form as a function of the received signal and the estimated channel. Complexity analysis and numerical simulation confirm that the proposed detection method reduces the implementation complexity substantially and achieves exactly the same performance as the conventional ML detection irrespective of the channel estimation error.

Synthesis of multidimensional signal constellations with quadrature amplitude modulation

The multidimensional constellations of quadrature amplitude modulation (QAM) signals are proposed. Each symbol incorporates the blocks of QAM signals not only carrying the same information content but also composed of the symbols of different higher order constellations (in contrast to the simple 1D modulation where each QAM symbol carries its intrinsic information). The multidimensional constellations of QAM signals are synthesized using the optimization algorithm based on the criteria for enhancing the energy or spectral efficiency of digital data transmission systems. The characteristics of interference immunity of the proposed signals are estimated with the help of analytical calculations and simulation. The relationships making it possible to consider faster-than-Nyquist signaling technology as a particular case of the multidimensional constellations of QAM signals are derived. It is demonstrated that the obtained signals provide an energy gain with respect to traditional 1D signals, in particular, ensure the operability at SNRs smaller than those inherent to binary phase-shift keying signals.

OFDM symbol identification by an unsupervised learning system under dynamically changing channel effects

Orthogonal frequency-division multiplexing (OFDM) is one of the most successful digital communication techniques. Nevertheless, the decrease in inter-symbol interference in quadrature amplitude modulation (QAM) over dispersive channels is still challenging. Different researches recently proposed the idea of using unsupervised learning as an alternative to the classic approaches to equalization of OFDM channels. In those purposes, the identification of a received QAM symbol is possible by the comparison of its position on the in-phase/quadrature (IQ) plane relative to the positions of previously arrived symbols, generally processed by the Kohonen’s Self-Organizing Map (SOM) algorithm. This work presents the SOM unsupervised learning method executed on an embedded system applied to QAM symbols identification. The system is implemented on an FPGA, a configurable digital circuit able to meet the low power and parallel process requirements of mobile applications. Also, in order to extend the classical set of experiments to evaluate our system, this paper proposes a theoretical model of the time-varying scheme representing the transition between different channel characteristics, obtained from real measurements available on a public repository. The model is employed to verify our purpose under dynamically both changing and realistic conditions. On the assumption that it is provided enough IQ symbols for the initial training process, the hardware implementation of SOM is able to track and identify the time-varying distorted QAM constellation. No knowledge of channel characteristics is necessary. The system spends only some microseconds at start-up to reach about 100% performance, and no dedicated training phase is needed afterward.