Polar Coordinate Based Modulation: Concept, Performance Analysis and System Design

Optical Wireless Communication (OWC) is widely used in current broadband access markets. This is due to the usage of optical radiation through free space for transmission. OWC employs Intensity Modulation with Direct Detection (IMDD) to modulate the intensity of the signal in order to transmit information. This offers a flexible networking solution. OWC systems that operate in the visible band region (390–750 nm) are referred to as Visible Light Communication. VLC has a dual purpose of illumination and communication system. VLC uses On-Off Keying (OOK) and Pulse Position Modulation (PPM). But recently, a noticeable increase in the use of a number of other modulation techniques such as Digital Pulse Interval Modulation (DPIM) and Dual-header Pulse Interval Modulation (DH-PIM) to improve the efficiency in terms of both the bandwidth and power has been observed. This paper concentrates on baseband pulse time modulation techniques that are used to modulate data for OWC and compares OOK, PPM, DH-PIM and DPIM techniques with respect to their spectral efficiency, optical power, bandwidth, transmission capacity, packet transmission rates and peak average power ratio (PAPR).

The paper provides a new perspective on peak- and average-constrained Gaussian channels. Such channels model optical wireless communication (OWC) systems which employ intensity-modulation with direct detection (IM/DD). First, the paper proposes a new, capacity-preserving vector binary channel (VBC) model, consisting of dependent binary noisy bit-pipes. Then, to simplify coding over this VBC, the paper proposes coding schemes with varying levels of complexity, building on the capacity of binary-symmetric channels (BSC) and channels with state. The achievable rates are compared to capacity and capacity bounds, showing that coding for the BSC with state over the VBC achieves rates close to capacity at moderate to high signal-to-noise ratio (SNR), whereas simpler schemes achieve lower rates at lower complexity. The presented coding schemes are realizable using capacity-achieving codes for binary-input channels, such as polar codes. Numerical results are provided to validate the theoretical results and demonstrate the applicability of the proposed schemes.

This paper addresses the design of optimal and near-optimal detectors in an interference channel with fading and with additive white Gaussian noise (AWGN), where the transmitters employ discrete modulation schemes as in practical communication scenarios. The conventional detectors typically either ignore the interference or successively detect and then cancel the interference, assuming that the desired signal and/or the interference are Gaussian. This paper quantifies the significant performance gain that can be obtained if the detectors explicitly take into account the modulation formats of the desired and the interference signals. This paper first describes the optimal maximum-likelihood (ML) detector that minimizes the probability of detection error for a given modulation scheme, and the joint minimum-distance (MD) detector, which is a lower-complexity approximation of the ML detector. It is then demonstrated by analysis and by simulation that in an AWGN channel, while interference-ignorant and successive interference cancellation detectors are both prone to error floors, the optimal ML and joint MD detectors are not. This paper further analyzes the performance of joint detection in a Rayleigh fading environment. It is demonstrated that the joint detector can achieve symbol error rates that have the same dependence on the received signal-to-noise ratio (SNR) as if the channel were interference free. Thus, the performance of joint detection is fundamentally limited by the SNR rather than the signal-to-interference ratio (SIR). Moreover, the joint detector enables the use of transmit diversity schemes to achieve the same diversity order as in the absence of interference. These results show that the use of interference-aware detectors can significantly alleviate the effect of interference thereby improving the achievable rates and the reliability of future wireless systems.

In this paper, antisymmetry-constructed clipped optical OFDM (AC- OFDM) is proposed for intensity modulation with direct detection (IM/DD) systems, in which an antisymmetry property is imposed directly in time domain. AC-OFDM has the same spectral efficiency as traditional asymmetrically clipped optical OFDM (ACO-OFDM) but is less complex to implement. Layered AC-OFDM (LAC-OFDM) is then proposed as an extension to further improve spectral efficiency, where different layers of AC-OFDM signals are added in the time domain and transmitted simultaneously. Computational complexity analysis and numerical results show that LAC-OFDM has the same spectral efficiency as layered asymmetrically clipped optical OFDM (LACO-OFDM) and enhanced unipolar OFDM (eU-OFDM) but is less complex in terms of both the number of real multiplications and real additions required. Specifically, LAC-OFDM requires less than half the multiplication and addition operations compared to the comparable LACO-OFDM scheme. Monte Carlo simulation results show that LAC- OFDM has the same bit error rate (BER) performance as LACO- and eU-OFDM.

This paper is an overview of indoor OFDM (orthogonal frequency division multiplexing)/DMT (discrete multitone) optical wireless (OW) communication systems. Indoor OW OFDM/DMT systems can be classified into two groups. One group produces half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers. Thus, allowing signal clipping at the zero level and avoiding the need of DC bias at the expense of data rate reduction. ACOOFDM (asymmetrically clipped OFDM system) and PAM (pulse amplitude modulation)-DMT are two techniques from the first group. The second group assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED (light emitting diode) intensity. DC-biased OFDM and a novel technique, proposed in this paper, called orthogonal PAM-DMT (OPAM-DMT) that is an extension of the proposed PAM-DMT by using discrete sine transform and discrete cosine transform to transmit two orthogonal signals at the same time, are two techniques from the second group. This paper considerers a practical LED model and studies the performance of all these systems in terms of average electrical OFDM signal power versus bit-error-ratio (BER) in the presence of additive white Gaussian noise channel (AWGN). It is shown that LED clipping has significant impact on the performance of all these systems and the performance of these systems substantially depends on the considered modulation order.

This paper presents the symbol error rate (SER) vs. signal-to-noise (SNR) ratio performance of OFDM system with QAM modulation. It has been observed that the optimal power allocation between pilot and information symbols can improve the performance gain by 2.3 dB relative to the equal power allocation between pilot and information symbols, for a system with N=3D512 sub-carriers, and a channel with L=3D40 taps. The SER performance is then compared with Rayleigh-faded as well as Rician-faded channels. It has been further concluded that the SER vs. SNR performance graph in Rayleigh-faded channel environment is better than that in Rician-faded channel. This idea implies that in OFDM case, the SER vs. SNR performance is better in scattered rich channel environments. The SER vs. SNR performance graph is degraded by equal magnitude for both equal-power and optimal-power cases, which implies that the difference between the equal-power and optimal power case is independent of the channel fading model being used. It is also observed that by varying the standard-deviation of the channel, the SER vs. SNR performance graph is not going to be affected if the channel is considered Rayleigh-faded.

In this paper the performance of an indoor wireless communication system is investigated. Adaptive decision feedback equalisers (DFE) are used to mitigate the intersymbol interference (ISI) caused by multipath propagation in the indoor radio channel. The system can be described as follows. It is a 16 quadrature amplitude modulation (16-QAM) system with raised cosine matched filtering with a roll-off factor of 0.50. A realistic channel model is used in the simulations by interpreting complex measured impulse responses of indoor radio channels. The measurements used were done in different types of rooms, and for line-of-sight (LOS) and obstructed (OBS) situations at the frequency of 2.4 GHz. After the signals have gone through the multipath channel additive white Gaussian noise (AWGN) was added. Symbol error rate (SER) performance simulations were done for different signal-to-noise ratios (SNR) and different data rates. On the average the SER performance was a few orders of magnitude (2-4) better than was the case without adaptive equalisation, depending on the SNR.

The combination of orthogonal frequency division multiplexing (OFDM) and spatial modulation (SM) can enhance the capacity of optical wireless communication (OWC) systems with low complexity. In this paper, we propose a novel SM scheme for intensity modulation/direct detection (IM/DD) OWC systems by employing discrete Hartley transform based OFDM (DHT-OFDM). Due to the use of DHT with one-dimensional constellations, the Hermitian symmetry constraint, which is generally imposed in conventional discrete Fourier transform based OFDM (DFT-OFDM) to obtain a real-valued output signal, is not required in DHT-OFDM. As a result, DHT-OFDM based SM can achieve much higher spectral efficiency than that of DFT-OFDM based SM in OWC systems. Simulation results show that, for an indoor 4×4 SM-OWC system with a spectral efficiency of 6 bits/s/Hz, DHT-OFDM achieves a remarkable 4.5-dB transmit signal-to-noise ratio reduction for an overall bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in comparison to conventional DFT-OFDM.

Intensity modulation with direct detection (IM/DD) provides a low-cost solution for data center optical interconnects. To achieve higher data rate, Tomlinson-Harashima precoding (THP) is adopted to increase the operating symbol rate beyond the Nyquist rate defined by the system bandwidth. By applying THP, the signal is precoded by a feedback filter at the transmitter for the mitigation of the intersymbol interference caused by the faster-than-Nyquist signaling and the spectral power fading induced by the interaction of chromatic dispersion and direct detection. In this paper, high-performance THP-based IM/DD transmission is demonstrated, increasing the achievable data rate using a limited system bandwidth. In the experimental demonstration, we achieve a net bit rate of 185 Gb/s for up to 2-km fiber link, transmitting 74 GBd precoded eight-level pulse amplitude modulation (PAM-8) using an IM/DD system with 33 GHz brick-wall electrical bandwidth limitation.

A Comparative analysis of Asymmetrically clipped orthogonal frequency division multiplexing (ACO-OFDM), Flip orthogonal frequency division multiplexing (Flip-OFDM) and Unipolar orthogonal frequency division multiplexing (U-OFDM) for intensity modulation (IM) and direct detection (DD) indoor optical wireless communication (OWC) is presented. These techniques are differentiated based on the higher order modulated information symbols assigned to the subcarriers and conversion of real bipolar signal to non negative unipolar signal. In ACO higher order QAM symbols are assigned only to the odd subcarriers where as in Flip-OFDM and U-OFDM data is assigned to all the subcarriers. In ACO-OFDM bipolar signals are converted to unipolar by clipping the negative real time domain samples, in Flip-OFDM by separation of bipolar signals in to positive and negative frames and then flipping is incorporated where as in U-OFDM concept of representing each positive and negative time domain sample by a pair of samples is incorporated. Analytical approach is discussed and the implementation of these techniques is performed Performance of ACO-OFDM and Flip-OFDM is evaluated for various higher order modulation techniques in terms of bit error rate (BER) as a function of electrical energy per bit to noise ratio for additive white Gaussian noise (AWGN) channel. A comparative analysis of ACO, Flip and U-OFDM for 4-QAM is illustrated. It is observed that the BER performance is same but they vary in spectral efficiency, transmit power, bandwidth and clipping noise

The performance of optical inter‐satellite communications using microwave photonics technologies employing intensity modulation with direct detection (IMDD) and phase modulation (PM) with interferometric detection are investigated and demonstrated with a numerical example. The theoretical model considering the saturated gain and amplifier spontaneous emission noise of optical booster amplifier and optical preamplifier is constructed for both architectures. Furthermore, the authors compare the performance of PM with interferometric detection to optimum biased IMDD in terms of radio frequency (RF) gain, RF noise figure and compression dynamic range (CDR). The numerical results show that the PM with interferometric detection can achieve better RF gain, RF noise figure and CDR than the optimum biased IMDD link as the laser output power is lower than 14.34, 12.45 and 4.2 dBm, respectively.

A novel adaptive beamforming (BF) receiver is proposed for a multiple-input–multiple-output communication system over multipath fading channels. By dividing a large number of receive antennas into many adaptive beamformers (ABFs), the proposed system reduces the array dimension in order to simplify the signal detection process and enjoys the benefit of BF techniques to improve the symbol-error-rate (SER) performance. The effectiveness of the proposed system depends on the number of ABFs and the number of antennas per ABF. The statistical properties of complex Wishart matrices are applied to analyse the SER performance of the proposed system with both maximum-likelihood (ML) detection and zero-forcing detection. Furthermore, the authors have derived an upper bound for SER of the ML detection based on the smallest eigenvalue distribution. Through the authors analysis, it has been shown that the proposed system using ML detection provides comparable SER performance, but with a lower complexity as compared with that of a conventional ML receiver because of reduced array dimensions.

Free-space optical (FSO) communication systems enhanced with optical intelligent reflecting surfaces (OIRS) offer promising solutions to overcome line-of-sight limitations and mitigate atmospheric turbulence effects. This paper presents a comprehensive analysis of OIRS-assisted FSO systems with phase errors utilizing both heterodyne and intensity modulation with direct detection (IM/DD) methods, which provides a more tractable alternative to existing methods. We first derive exact closed-form formulas for the mean and variance of the squared sum of products of independent, identically distributed Gamma-Gamma random variables, incorporating both pointing errors and IRS phase errors. Based on these results, we approximate the square envelope of the OIRS-assisted FSO channel with a Gamma distribution, with shape and scale terms determined from the derived mean and variance. This approximation allows us to obtain closed-form expressions for key performance metrics, including the average bit-error rate for different modulations, the outage probability, and the ergodic capacity, expressed using the Meijer’s G function. Our approach demonstrates excellent agreement with Monte Carlo simulations while offering superior computational efficiency compared to methods based on multivariate Fox’s H functions. Furthermore, we conduct asymptotic analysis at high signal-to-noise ratios and investigate the system’s diversity order for both heterodyne and IM/DD techniques.

This paper deals with the theoretical symbol error rate (SER) performance analysis of coherent square M-QAM in fading channels. The channels in this paper are modeled as frequency non-selective slowly fading channels corrupted by additive white Gaussian noise (AWGN). Closed form expressions for average SERs for square M-QAM in fading channels are derived and analyzed. The solutions presented are in terms of a single finite integral with an integrand composed of elementary (exponential and trigonometric) functions. Because of their simple forms, these expressions readily allow numerical evaluations for cases of practical interest. The dependence of SER performance of M-QAM on Rician parameter K and Nakagami fading parameter m are plotted and examined. The results presented are expected to provide useful information to communication systems design engineers for design and performance evaluation of radio systems over fading channels.

This paper presents asymptotic multiuser efficiency (AME) performance analysis of the decorrelator based successive interference cancellation (DBSIC) DS-CDMA multiuser receiver DBSIC is similar to the conventional SIC receiver except that a decorrelator is used at each detection stage prior to cancellation. AMEs of DBSIC are compared with those of single-user matched filter, optimum, decorrelating and conventional SIC receivers over both additive white Gaussian noise (AWGN) and Rayleigh fading channels. It is shown that over AWGN channels, DBSIC AME is lower than the optimum receiver AME when the interfering user is weaker than the user of interest but approaches the unity optimum receiver AME as the interferer becomes stronger than the user of interest. For all spreading codes cross-correlation considered, DBSIC has a similar AME curve shape as the conventional SIC receiver but has a higher AME which remains positive for all user energy ratios, illustrating its near-far resistance over AWGN channels. The AME of DBSIC is better than that of the decorrelating detector when the interferer is much weaker or slightly stronger than the user of interest and has worse performance when the interferer is not much weaker. It is shown that DBSIC is completely near-far resistant over Rayleigh fading channels with the same AME as the decorrelating scheme for user 1 and as the optimum scheme for user 2, both AMEs being independent of the user energy ratio. This shows that DBSIC is preferable compared to the decorrelating detector, while still being less complex than the optimum scheme over Rayleigh fading channels