dB Signal-to-noise Ratio Gain Research Articles

Efficient Optical OFDM System Resilience to Indoor Wireless Multipath Channels

This article presents a developed intensity modulation/direct detection (IM/DD) optical orthogonal frequency division multiplexing (O-OFDM). More precisely, the presented C-O-OFDM is based on the C-transform as a unitary orthogonal transform instead of the state-of-the-art discrete Fourier transform (DFT). Due to the properties of the real C-transform, Hermitian symmetry (HS) is not required to produce real OFDM samples. Therefore, the proposed scheme supports twice the input symbols compared to conventional DFT-based OFDM system. Real data mapping and DC bias technology is considered to evaluate the performance of the presented scheme over optical wireless multipath. The simulation results shows that the proposed C-O-OFDM is more resilience to multipath phenomena than the competitive DFT-O-OFDM and DHT-O-OFDM schemes for similar bit rate. The proposed scheme achieves about 22 dB signal-to-noise ratio (SNR) gain in comparison with the DFT-O-OFDM and about 2.5 dB SNR gain in comparison with the DHT-O-OFDM scheme.

Multipolarization Superposition Beamforming: Novel Scheme of Transmit Power Allocation and Subcarrier Assignment

The 5th generation (5G) new radio (NR) access technology and the beyond-5G future wireless communication require extremely high data rate and spectrum efficiency. Energy-efficient transmission/reception schemes are also regarded as an important component. The polarization domain has attracted substantial attention in this aspect. This paper is the first to propose multi-polarization superposition beamforming (MPS-Beamforming) with cross-polarization discrimination (XPD) and cross-polarization ratio (XPR)-aware transmit power allocation utilizing the 5G NR antenna panel structure. The appropriate orthogonal frequency division multiplexing (OFDM) subcarrier assignment algorithm is also proposed to verify the theoretical schemes via simulations. The detailed theoretical derivation along with comprehensive simulation results illustrate that the proposed novel scheme of MPS-Beamforming is significantly beneficial to the improvement of the performance in terms of the symbol error rate (SER) and signal-to-noise ratio (SNR) gain at the user equipment (UE). For instance, a provided practical wireless channel environment in the simulations exhibits 8 dB SNR gain for 10−4 SER in a deterministic channel, and 4 dB SNR gain for 10−5 SER in abundant statistical channel realizations.

A Novel Weighted LLR Algorithm for Non-Coherent OFDM-MFSK System in Ionospheric Scatter Channel

A novel weighted log likelihood ratio (LLR) algorithm is proposed for the non-coherent orthogonal frequency division multiplexing with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${M}$ </tex-math></inline-formula> -ary frequency shift keying (OFDM-MFSK) system in an ionospheric scatter channel (ISC) with serious frequency-selective fading. In the algorithm, the bit LLR is derived according to the probability distribution of non-coherent received signals in the MFSK symbol and is approximately simplified to a concise expression, which reveals that LLRs cannot indicate the effect of the frequency-selective fading channel on different MFSK symbols. The proposed weighted LLR algorithm maximizes the signal-to-noise ratio (SNR) of the metric combined with LLRs in channel decoding. The weighted factors were estimated blindly from the received MFSK signals. Simulation results show that the bit error rate (BER) of OFDM-MFSK systems with the proposed LLR-weighted algorithm is superior than that of conventional LLR algorithms, and has approximately 1.6 dB SNR gain when BER is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${10^{-4}}$ </tex-math></inline-formula> .

Design and Evaluation of Reconfigurable Intelligent Surfaces in Real-World Environment

Reconfigurable intelligent surfaces (RISs) have promising coverage and data-rate gains for wireless communication systems in 5G and beyond. Prior work has mainly focused on analyzing the performance of these surfaces using simulations or lab-level prototypes. To draw accurate insights about the actual performance of these systems, this paper develops an RIS proof-of-concept prototype and extensively evaluates its potential gains in the field under realistic wireless communication settings. In particular, a 160-element RIS, operating at a 5.8 GHz band, is designed, fabricated, and accurately measured in the anechoic chamber. This surface is then integrated into a wireless communication system and the beamforming gains, pathloss, and coverage improvements are evaluated in realistic outdoor communication scenarios. When both the transmitter and receiver employ directional antennas, the developed RIS achieves 15&#x2013;20 dB gain in the signal-to-noise ratio (SNR) in a range of &#x00B1;60&#x00B0; beamforming angles. In terms of coverage, and considering a far-field experiment with a blockage between a basestation and a grid of mobile users and with an average signal path of 35 m, the RIS provides an average SNR improvement of 6 dB (max 8 dB) within an area &#x003E; 75 m². Thanks to the scalable RIS design, these SNR gains can be directly increased with larger RIS areas. For example, a single 1,600-element RIS with the same design is expected to provide 26 dB SNR gain for a similar deployment (theoretical estimation). These results draw useful insights into the design and performance of RIS systems and provide an important proof for their potential gains in real-world far-field wireless communication environments.

Multiple Complex Symbol Golden Space-Time Labeling Diversity

The Golden code is a full-rate space-time block code (STBC), which achieves a diversity order of $2N_{r}$ , where $N_{r}$ is the number of receive antennas. The multiple complex symbol Golden code (MCS-Golden code) is a generalized Golden code. The MCS-Golden code not only achieves full-rate but also achieves a diversity order of $2^{n}{N_{r}}$ , $n$ , $n\ge {1}$ , is an integer, and $2^{n}$ is the number of multiple complex symbols in the MCS-Golden code system. In this paper, we apply the labelling diversity technique to the MCS-Golden code to form a new type of STBC, named as multiple complex symbol Golden space-time labelling diversity (MCS-GSTLD). The proposed MCS-GSTLD achieves a diversity order of $2\times {2^{n}}{N_{r}}$ , and further improves error performance compared to the MCS-Golden code. We also formulate the theoretical average bit error probability (ABEP) for the proposed MCS-GSTLD system. Furthermore, we apply sphere decoding with sorted detection subset (SD-SDS) to detect the transmitted symbols in the MCS-GSTLD system. Simulation results show that the SD-SDS scheme achieves the theoretical ABEP for $N_{r}>2$ . Both theoretical and simulation results demonstrate that with the same spectral efficiency and four receive antennas, the 2CS-GSTLD achieves at least 1.3 dB signal-to-noise ratio (SNR) gain compared to the 2CS-Golden code at a BER of $3\times {10^{-6}}$ , while the 4CS-GSTLD achieves at least 0.7 dB SNR gain compared to the 4CS-Golden code.

Fast Radix-32 Approximate DFTs for 1024-Beam Digital RF Beamforming

The discrete Fourier transform (DFT) is widely employed for multi-beam digital beamforming. The DFT can be efficiently implemented through the use of fast Fourier transform (FFT) algorithms, thus reducing chip area, power consumption, processing time, and consumption of other hardware resources. This paper proposes three new hybrid DFT 1024-point DFT approximations and their respective fast algorithms. These approximate DFT (ADFT) algorithms have significantly reduced circuit complexity and power consumption compared to traditional FFT approaches while trading off a subtle loss in computational precision which is acceptable for digital beamforming applications in RF antenna implementations. ADFT algorithms have not been introduced for beamforming beyond $N = 32$, but this paper anticipates the need for massively large adaptive arrays for future 5G and 6G systems. Digital CMOS circuit designs for the ADFTs show the resulting improvements in both circuit complexity and power consumption metrics. Simulation results show similar or lower critical path delay with up to 48.5% lower chip area compared to a standard Cooley-Tukey FFT. The time-area and dynamic power metrics are reduced up to 66.0%. The 1024-point ADFT beamformers produce signal-to-noise ratio (SNR) gains between 29.2--30.1 dB, which is a loss of $\le$ 0.9 dB SNR gain compared to exact 1024-point DFT beamformers (worst case) realizable at using an FFT.

Numerical research on partially coherent flat-topped beam propagation through atmospheric turbulence along a slant path.

In this paper, we investigate the propagation of partially coherent flat-topped (PCFT) beams through atmospheric turbulence along a slant path. Using wave optics simulation (WOS), we calculated the on-axis intensity of PCFT beams in both vacuum and turbulence links, finding that PCFT beams will self-focus during propagation and that the beam coherence length has a significant impact on this focusing phenomenon. The effects of the zenith angle and the source coherence length on the scintillation index and mean signal-to-noise ratio (SNR) are analyzed. Furthermore, taking into consideration the limitation of available devices, we examined the impacts of source coherence time and detector integration time on the performance of PCFT beams in slant links. WOS results indicate as much as an extra 2 dB SNR gain from PCFT beams in short links in comparison with Gaussian Schell-model beams.

Improving capacity for physical network coding with lattice strategies in two‐way fading channels

In this study, the capacity problem in a two-way fading channel is considered. The authors propose amplify-and-forward with lattice codes (AF&LC), exploiting a modulo operation at the relay. Without destroying the construct of codebook, the modulo operation reduces the power of the received signals, and thus achieves a larger power-scaled gain than the amplified-and-forward with random codes (AF&RC). It is proved that AF&LC outperforms AF&RC in the high signal-to-noise ratio (SNR) regime by employing theoretical analyses. By simulating one-dimension lattice codes, AF&LC achieves about 1.5 dB SNR gain over AF&RC with error probability 10−2. Compared with decode-and-forward with lattice codes (DF&LC) and fixed modulo-and-forward (FMF), AF&LC achieves larger rate region which is closer to upper bound in some scenarios. Moreover, AF&LC can work in fading two-way channels without feedback schemes, which is easier to be implemented than DF&LC, FMF and compress-and-forward.

On Companding and Optimization of OFDM Signals for Mitigating Impulsive Noise in Power-Line Communication Systems

Generally, the probability density function (PDF) of orthogonal frequency division multiplexing (OFDM) signal amplitudes follow the Rayleigh distribution, thus, it is difficult to correctly predict the existence of impulsive noise (IN) in powerline communication (PLC) systems. Compressing and expanding the amplitudes of some of these OFDM signals, usually referred to as companding, is a peak-to-average power ratio reduction technique that distorts the amplitudes of OFDM signals towards a uniform distribution. We suggest its application in PLC systems, such as IEEE 1901 powerline standard (which uses OFDM) to reduce the impacts of IN. This is because the PLC channel picks up impulsive interference that the conventional OFDM driver cannot combat. We explore, therefore, five widely used companding schemes that convert the OFDM signal amplitude distribution to uniform distribution to avail the mitigation of IN in PLC system receivers by blanking, clipping and their hybrid (clipping-blanking). We also apply nonlinear optimization search to find the optimal mitigation thresholds and results show significant improvement in the output signal-to-noise ratio (SNR) for all companding transforms considered of up to 4 dB SNR gain. It follows that the conventional PDF leads to false IN detection, which diminishes the output SNR when any of the above three nonlinear memoryless mitigation schemes is applied.

Signal-to-Noise Ratio Improvement of MAMR on CoX/Pt Media

The signal-to-noise ratio (SNR) gain of microwave-assisted magnetic recording (MAMR) with a spin-transfer oscillator is analyzed by a micromagnetic model simulating the write process on a CoX/Pt media, by including the Voronoi polycrystalline microstructure built upon a regular mesh of micromagnetic cells. With a fixed head of 40 nm-width main pole and a 10.5 kOe maximum perpendicular field, a 2 dB SNR gain is found for low-coercivity media (0 K $H_{c} kOe) and a 4 dB gain is obtained for high-coercivity media (0 K $H_{c}>14$ kOe). Besides, the MAMR performance considering the finite rise time of single-pole-type head field in a bit is also calculated. It is found that the MAMR SNR gain is obtained in the media with lower anisotropy because of the weak writing capacity during the rise time.

Photoacoustic radar phase-filtered spatial resolution and co-registered ultrasound image enhancement for tumor detection.

Co-registered ultrasound (US) and frequency-domain photoacoustic radar (FD-PAR) imaging is reported for the first time in this paper. The merits of ultrasound and cross-correlation (radar) frequency-domain photoacoustic imaging are leveraged for accurate tumor detection. Commercial US imagers possess sophisticated, optimized software for rapid image acquisition that could dramatically speed-up PA imaging. The PAR image generated from the amplitude of the cross-correlation between detected and input signals was filtered by the standard deviation (SD) of the phase of the correlation signal, resulting in strong improvement of image spatial resolution, signal-to-noise ratio (SNR) and contrast. Application of phase-mediated image improvement is illustrated by imaging a cancer cell-injected mouse. A 14-15 dB SNR gain was recorded for the phase-filtered image compared to the amplitude and phase independently, while ~340 μm spatial resolution was seen for the phase PAR image compared to ~840 μm for the amplitude image.

Soft forwarding technique for unitary space‐time modulation

The authors propose two soft forwarding schemes for unitary space-time modulation, namely estimate-and-forward (EF) and mutual information-based forwarding (MIF), which are ideally suited for rapid piecewise-constant fading scenarios with no channel state information both at transmitters and receivers. In the EF scheme, the relay regenerates signal via the conditional expectation of the source unitary space-time (UST) signal given the received matrix at the relay node instead of hard decisions. The MIF scheme employs the instantaneous mutual information between the source UST signal and received matrix at the relay node as a measurement for maximum-likelihood detection. The authors provide the relay forwarding functions, the normalisation factors, and the detection methods at the destination firstly. Above all, the generalised signal-to-noise ratio (SNR) at the destination for our unitary space-time modulated relaying system is analysed and simulated firstly. Simulation results reveal that the proposed EF and MIF schemes can outperform the existing amplify-and-forward (AF) and detect-and-forward (DF) schemes in terms of bit-error-rate performances in most scenarios. For a parallel relay network with two relays, the proposed EF scheme achieves 2 and 3 dB SNR gain over AF and DF schemes, respectively. Moreover, the MIF scheme can further give about 1.5 dB SNR gain.

Power Allocation for Amplify-and-Forward Relaying with Correlated Shadowing

Power allocation is a key technique to exploit the benefits of cooperative relaying. In this paper, we investigate the effect of shadowing on the power allocation of amplify-and-forward cooperative relaying systems. Considering the joint effects of path loss, correlated shadowing and flat Rayleigh fading, the approximate outage probability at high signal-to-noise ratio (SNR) is first derived. Then we solve the power allocation problem by minimizing the approximate outage probability subject to a total power constraint. It is shown by the analytical results that the correlation coefficients and the standard deviations of shadowing have significant impacts on the power allocation. The simulation results show that the proposed power allocation scheme yields about 2 dB SNR gain compared to the equal power allocation in the high SNR regime.

10 Gb/s Indoor Optical Wireless Systems Employing Beam Delay, Power, and Angle Adaptation Methods With Imaging Detection

In this paper, we propose a mobile optical wireless system that employs beam delay adaptation, and makes use of our previously introduced beam angle and power adaptation multi-beam spot diffusing configuration in conjunction with an imaging receiver. Our ultimate goal is to improve the bandwidth, reduce the effect of intersymbol-interference, and increase the signal-to-noise ratio (SNR) when the transmitter operates at a higher data rate under the impact of multipath dispersion, background noise, and mobility. A significant reduction in the delay spread can be achieved compared to a conventional diffuse system (CDS) when an imaging receiver replaces a nonimaging receiver at the room's corner, where the delay spread is reduced from 2.4 ns to about 0.35 ns. Our proposed system, beam delay, angle, and power adaptation in a line strip multibeam spot diffusing configuration (BDAPA- LSMS), offers a reduction in delay spread by a factor of more than 10 compared with only the beam angle and power adaptation LSMS. An increase in channel bandwidth from 36 MHz (CDS) to about 9.8 GHz can be achieved when our methods of beam delay, angle, and power adaptation coupled with an imaging receiver are employed. These improvements enhance our system and enable it to operate at a higher data rate of 10 Gb/s. At a bit rate of 30 Mb/s, our proposed BDAPA-LSMS achieves about 50 dB SNR gain over conventional diffuse systems that employ a nonimaging receiver (CDS). Moreover, our simulation results show that the proposed BDAPA-LSMS at a bit rate of 10 Gb/s achieves about 32.3 dB SNR at the worst communication path under the presence of background noise and mobility while achieving a bit error rate below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> .

Performance Evaluation of 5 Gbit/s and 10 Gbit/s Mobile Optical Wireless Systems Employing Beam Angle and Power Adaptation with Diversity Receivers

Over the last two decades, indoor optical wireless (OW) systems have operated typically at 30 Mbit/s to 155 Mbit/s. Here, we propose and evaluate for the first time a mobile OW system that operates at 5 Gbit/s and 10 Gbit/s. The improvements in data rates and channel bandwidth are achieved through the introduction of three approaches: angle diversity, beam angle and beam power adaptation. We propose a mobile OW system that employs beam angle and beam power adaptation in a line strip multibeam spot diffusing system configuration in conjunction with an angle diversity receiver (APA-LSMS) to mitigate the degradation due to ambient light noise, multipath dispersion and mobility. The performance of our proposed system was investigated through channel and noise modelling. Our results show that the proposed APA-LSMS at a bit rate of 30 Mbit/s achieves about 45 dB signal-to-noise ratio (SNR) gain over a power adaptive line strip multibeam system (PA-LSMS) and offers 60 dB SNR gain over a conventional diffuse system (CDS) at a 6 m horizontal separation between the transmitter and the receiver. The SNR is independent of the transmitter location and when our methods (angle diversity, beam angle and beam power adaptations) are implemented, the SNR can be maximized at the receiver for every transmitter location. The results show an increase in channel bandwidth from 36 MHz (CDS) to approximately 7.2 GHz in our proposed system (APA-LSMS). These improvements enhance our system and enable it to operate at higher data rates of 5 Gbit/s and 10 Gbit/s.

Parallel Detection Algorithm for Fast Frequency Hopping OFDM

Fast frequency hopping OFDM (FFH-OFDM) exploits frequency diversity in one OFDM symbol to enhance conventional OFDM performance without using channel coding. Zero-forcing (ZF) and minimum mean square error (MMSE) equalization were first used to detect FFH-OFDM signal with a relatively poor bit error rate (BER) performance compared to QR-based detection algorithm. This paper proposes a parallel detection algorithm (PDA) to further improve the BER performance with parallel interference cancelation (PIC) based on MMSE criterion. Our proposed PDA not only improves the BER performance at high signal to noise ratio (SNR) regime but also possesses lower decoding delay property with respect to QR-based detection algorithm while maintaining comparable computation complexity. Simulation results indicate that at BER = 10–3 the PDA achieves 5 dB SNR gain over QR-based detection algorithm and more as SNR increases.

Fast Link Adaptation for MIMO OFDM

We investigate link-quality metrics (LQMs) based on raw bit-error-rate, effective signal-to-interference-plus-noise ratio, and mutual information (MI) for the purpose of fast link adaptation (LA) in communication systems employing orthogonal frequency-division multiplexing and multiple-input-multiple-output (MIMO) antenna technology. From these LQMs, the packet error rate (PER) can be estimated and exploited to select the modulation and coding scheme (MCS) among a class of candidate MCSs that achieves the maximum throughput for the current channel state under a specified target PER objective. We propose a novel MI-based LQM and compare the PER-estimation accuracy obtained with this LQM with that resulting from using other LQMs by means of comprehensive Monte Carlo simulations. Search methods for the MCS in the class that is most suitable for a given channel state are presented. An algorithm for obtaining a practical upper bound on the throughput of any LA scheme is proposed. The investigated LQMs are applied to the IEEE 802.11n standard with a 2 × 2 MIMO configuration and practical channel estimation. The proposed MI-based LQM yields the highest PER estimation accuracy, and its throughput shows only 1.7 dB signal-to-noise-ratio (SNR) loss with respect to the upper bound but up to 9.5 dB SNR gain, compared to the MCS maximizing the throughput for the current noise variance.

High-Speed Spot Diffusing Mobile Optical Wireless System Employing Beam Angle and Power Adaptation and Imaging Receivers

The spot-diffusing geometry is one of the attractive configurations considered in the literature. It provides a better signal-to-noise ratio (SNR) than the conventional diffuse system (CDS), but its SNR can be degraded due to shadowing, signal blockage and mobility. Three methods: imaging reception, beam angle and beam power adaptation are introduced to the design of spot-diffusing OW systems to effectively mitigate the degradation due to mobility in the presence of ambient light noise, multipath propagation, and shadowing. The performance of our systems was evaluated through channel and noise modeling. The CDS SNR performance improves by more than 20 dB when an imaging receiver with maximum ratio combining (MRC) replaces a non-imaging receiver. A 24 dB SNR gain can be achieved when spot-diffusing is employed with an imaging MRC receiver instead of the imaging MRC CDS. In an imaging spot-diffusing system, the SNR is independent of the transmitter position and can be maximized at all receiver locations when our new methods (beam angle and beam power adaptation) are implemented. Regardless of the transmitter position, beam angle adaptation can target the spots at the optimum location that yields the best SNR at the receiver. A significant SNR improvement of 36 dB in the imaging spot-diffusing performance can be achieved when angle adaptation is introduced. Further SNR improvement of 4 dB can be obtained if the power is adaptively distributed among the spots. Furthermore, an increase in the channel bandwidth from 43 MHz (non-imaging CDS) to 8.19 GHz can be achieved through the combination of these methods (imaging reception, spot-diffusing, beam angle and beam power adaptation). The increase in channel bandwidth and SNR can enable the OW system to achieve higher data rates and 2.5 Gbit/s and 5 Gbit/s mobile OW systems are shown to be feasible. The results also prove that the influence of shadowing and signal blockage can be sufficiently combated through the use of these methods.

The effect of a trailing shield for perpendicular write heads

The effect of a magnetic trailing shield located in close proximity of the pole tip for a perpendicular write head has been studied in a perpendicular recording system. For a 150nm wide write pole, the write field gradient is improved yielding a 40% decrease in jitter and 2.2dB increase in the signal-to-noise ratio (SNR). As the pole width is narrowed further, modeling and experiments show that the trailing shield leads to a tradeoff between maintaining a high write field (writeability) and achieving an optimal write field gradient (jitter). For a 70nm writer, the addition of a trailing shield results only in a small 0.5dB SNR gain despite a 25% decrease in jitter as a result of the concomitant loss in writeability. The latter results in an increased dc noise and becomes more significant with trailing shield throat thickness.

On the Design of MIMO Block-Fading Channels With Feedback-Link Capacity Constraint

In this paper, we propose a combined adaptive power control and beamforming framework for optimizing multiple-input/multiple-output (MIMO) link capacity in the presence of feedback-link capacity constraint. The feedback channel is used to carry channel state information only. It is assumed to be noiseless and causal with a feedback capacity constraint in terms of maximum number of feedback bits per fading block. We show that the hybrid design could achieve the optimal MIMO link capacity, and we derive a computationally efficient algorithm to search for the optimal design under a specific average power constraint. Finally, we shall illustrate that a minimum mean-square error spatial processor with a successive interference canceller at the receiver could be used to realize the optimal capacity. We found that feedback effectively enhances the forward channel capacity for all signal-to-noise ratio (SNR) values when the number of transmit antennas (n/sub T/) is larger than the number of receive antennas (n/sub R/). The SNR gain with feedback is contributed by focusing transmission power on active eigenchannel and temporal power waterfilling . The former factor contributed, at most, 10log/sub 10/(n/sub T//n/sub R/) dB SNR gain when n/sub T/>n/sub R/, while the latter factor's SNR gain is significant only for low SNR values.