Symbol Detection and Channel Estimation using Neural Networks in Optical Communication Systems

In order to improve the delay quality-of-service aware throughput performance of the optical wireless communications, a joint power allocation and relay selection scheme is proposed for a relay assisted multichannel coherent optical wireless communication system. The proposed joint power allocation and relay selection scheme provides the delay aware quality-of-service guarantee by maximizing the end-to-end effective capacity under the average transmit power constraints at both source as well as relay nodes. The joint power allocation and relay selection problem is formulated as a non-convex and combinatorial optimization problem, and the solution of this optimization problem is obtained using a time-sharing relaxation technique. Subsequently, a fast convergent algorithm is proposed for the joint power allocation and relay selection by considering the delay aware quality-of-service requirements of the services transmitted over the optical wireless communication channels. Numerical results demonstrate that the proposed joint power allocation and relay selection scheme provides the strict statistical delay aware quality-of-service with an improved throughput without increasing the transmit power requirements.

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.

Guest Editorial

발광 다이오드와 같은 광원을 이용하여 조명과 통신의 두가지 기능을 동시에 이용할 수 있는 가시광 무선 통신은 최근 초고속 개인용 무선 네트워크에서 각광을 받고 있다. 광원의 제한된 대역폭으로 인하여 무선 광채널에서 전송률을 향상시키는 것이 이슈로 대두되는 가운데, 본 논문에서는 전송률을 향상시키는 기법으로써 다중 입출력 무선 광채널에서 두 개의 데이터 스트림을 지원하기 위해 고유값 분해를 이용한 공간 다중화 시스템을 설계한다. 채널의 변화에 따라 적응적 변조 기법을 적용하여 주파수 효율을 높이기 위해 펄스 진폭 변조 기법을 이용한다. 본 논문에서는 세기 변조를 이용한 변조 기법의 세 가지 제한 조건인 비음수성, 총 광출력값, 비트 오류율에 대하여 주파수 효율성을 향상시키기 위해 광출력값, 오프셋값, 변조 크기를 적응적으로 할당하기 위한 기법을 이론적으로 제안한다. 모의 실험 결과를 통해 본 논문의 제안 기법이 각 데이터 스트림에 광출력값을 동일하게 할당하는 기법에 비해 성능 향상을 이룰 수 있다는 것을 보인다. Visible light communication (VLC) using optical sources which can be simultaneously utilized for illumination and communication is currently an attractive option for wireless personal area network. Improving the data rate in optical wireless communication system is challenging due to the limited bandwidth of the optical sources. In this paper, we design the singular value decomposition (SVD)-based multiplexing multi-input multi-output (MIMO) system to support two data streams in optical wireless channels. In order to improve the spectral efficiency, the rate adaptation using multi-level pulse amplitude modulation (PAM) is applied according to the channel condition and we propose the method to allocate the optical power, the offset and the size of modulation scheme theoretically under the constraints of the nonnegativity of the modulated signals, the aggregate optical power and the bit error rate (BER) requirement. The simulation results show that the proposed allocation method gives the better performance than the method to allocate the optical power equally for each data stream.

In this paper, the performance of current digital pulse modulation schemes for optical wireless communications was examined by simulation, and we concluded that PPM is suitable for long distance optical wireless communication systems especially for military use due to its best power efficiency and also suitable for optical wireless communication systems working near sea surface; DPIM and DH-PIM are suitable for normal outdoor applications, for example, being used in city or suburb, for its moderate performance and no need of symbol-level synchronization; MPPM is especially suitable for indoor optical wireless communication systems with diffused channel due to its best bandwidth efficiency. (4 pages)

The quasi-static nature of the optical wireless channel means that the channel state information (CSI) can be readily available at the transmitter and receiver prior to data transmission. This implies that electrically band-limited optical wireless communication (OWC) systems can make use of optimal channel partitioning or vector coding based multi-channel modulation (MCM) to achieve high throughput by mitigating the non-linearities arising from the optical and electrical channel. This paper proposes a pulse amplitude modulation (PAM) based DC-biased optical vector coding (DCO-VC) MCM scheme for OWC. The throughput performance of DCO-VC is evaluated and compared to the well known DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) over hybrid (line-of-sight and diffuse) and diffuse (non line-of-sight only) visible light communication (VLC) channels with additive white Gaussian noise. For the completeness of the VLC physical layer, the performance comparison is based on an uncoded and a forward error correction transmission mode using well-known convolutional codes with Viterbi decoder. The results show that the coded DCO-VC outperforms DCO-OFDM system by achieving up to 2 and 3 dB signal to noise ratio gains over hybrid and diffuse VLC channels, respectively.

When the optical signal is transmitted in atmospheric channel in the wireless optical communication (WOC) systems, the inter-symbol interference (ISI) will be caused by the atmospheric motion and the fading characteristics of the atmospheric channel. In addition, device noises such as photodetectors noises can also affect the signal in the real communication systems. It is difficult to eliminate both types of interference by the single equalization algorithm at the same time, therefore, a cascaded equalization algorithm is proposed. First of all, we use the MATLAB to simulate the cascaded equalization algorithm, the convergence performance of the algorithm is analyzed, the bit error rate (BER) under different signal-to-noise ratios (SNR) are also compared. Then, the algorithm is verified under laboratory conditions, we calculate the BER of the receiving signal, which is processed by the cascade equalization algorithm. The simulation analysis and experiments results show that, the convergence performance of the cascaded equalization algorithm is improved effectively, the ISI is decrease, the BER can reduce to about 10-8 , decreased by 4 orders of magnitude than without equalization. It can improve the communication performance of the WOC systems, the validation of the cascaded equalization algorithm is verified.

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.

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

Underwater optical wireless communication (UOWC) is attracting significant attention due to global climate monitoring, military applications, and exploration to study marine biology. Optical wireless communication (OWC) can provide the benefits of higher transmission data rate and bandwidth by using unlicensed bandwidth and light-weight transceivers having low power requirements. However, OWC suffers from many limitations such as absorption, scattering, and turbulence, which become harder when the channel includes a water-to-air (W2A) interface. In this paper, we focus on the modeling impulse response of a W2A-OWC channel under different turbulent water surface conditions. We empirically evaluate the statistical behavior of the channel and show fits to probabilistic distributions to understand the nature of the W2A-OWC channels.

High-speed wireless communications in personal areas are now highly demanded, given that broadband connectivity has now reached the doorstep of the users' premises. Optical wireless technology has been considered as a promising solution due to the availability of huge unregulated bandwidth. However, the coverage area of optical wireless communication systems is limited because of safety issues and localization of users is needed. Previously, we proposed a localization system for personal areas based on optical wireless technology, so the same system was used for both localization and high-speed wireless communications. However, the height difference between the fiber transceiver and subscriber was fixed in the investigation. In real applications, the height of subscriber unit is always changing and the localization system should have the capability of estimating the height difference. In this paper, we propose a novel optical wireless based localization system for personal areas with the capability of height estimation. The localization and height estimation functions are realized by the combinational usage of received signal power and angle of arrival information. Experimental results show that a localization accuracy of ∼ 4.73 cm can be achieved and the height estimation accuracy is ∼ 4.86 cm. When this localization system with height estimation is incorporated with the previously proposed optical wireless communication system, high-speed wireless connectivity can be provided in personal areas.

In this paper, a multipoint-to-point system consisting K users and a central node over wireless optical communication (WOC) channel is analyzed. The scenario focused on is that there is simultaneous communication from a number of users to the central node. As a powerful solution, we utilize non-orthogonal multiple access (NOMA) technique in the system. Although the superiority of NOMA in radio frequency (RF) system has been greatly considered, the NOMA in WOC still needs further research due to the special features of WOC, especially the non-broadcast nature of optical beam and the vulnerable turbulence channel. With the special features of WOC in mind, system is evaluated in terms of outage probability, bit error rate and ergodic sum rate. In addition, we theoretically prove that NOMA outperforms orthogonal multiple access (OMA), and the performance gain increases with the increase of turbulence strength. Hence, NOMA is more suitable for WOC, especially in strong turbulence channel. Moreover, we also analyze the user pairing scheme. Monte Carlo simulations have been done, which match quite well with the theoretical analysis.

Optical Wireless Communication Systems (OWCSs) are becoming more popular each day, especially after numerous mobile applications are being employed within the concept of Internet of Things (IoT). OWCSs are largely used in both terrestrial and non-terrestrial environments, like underwater, air, and space scenarios. Due to the large applicability of OWCS, it represents one of the main candidate technologies for the future 6G wireless communication systems. Naturally, this market trend forces the system designers to reach the best performance in their designs, as well as optimize the cost. In this survey paper, we intend to provide information to the researchers working in this field on the statistical models adopted in OWCS, the methods and techniques used to improve their performances, mainly in outdoor environment like air, space, and underwater. In this respect, the background on theoretical aspects of OWCS, together with their benefits, limitations and challenges are presented. Performance improvement techniques employed in OWCSs, such as power increase, partial coherence, beamforming, aperture averaging, spatial diversity, and intelligent reflecting surfaces, are also introduced. Finally, we discuss the open challenges that researchers are still facing, together with future directions on next steps for a large-scale adoption of OWCS.

Investigation of digital video broadcasting application employing the modulation formats like QAM and PSK using OWC, FSO, and LOS-FSO channels

The performance of signal detection in a multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) optical wireless communication (OWC) system is significantly affected by channel fading and atmospheric turbulence. Accurate channel state information (CSI) is crucial for reliable channel estimation, but obtaining it typically requires a high pilot overhead. To address this issue, we propose a joint channel estimation and signal detection (JCESD) algorithm that reduces the pilot overhead while improving the detection performance with a moderate increase in computational complexity. By leveraging convolutional neural network (CNN) and long short-term memory (LSTM) networks, the JCESD algorithm learns the mapping relationship between atmospheric turbulence channels and signals, thereby mitigating the challenges associated with high-complexity signal processing at the receiver. Additionally, layer normalization is applied during data pre-processing to enhance overall performance. Simulation results demonstrate that the proposed JCESD algorithm outperforms the LS-ZF algorithm and achieves an approximate 1dB gain over the LS-MMSE algorithm under weak turbulence conditions at a bit error rate (BER) of 10-5 in the 2×2 system when using eight pilots, with a 0.9dB gain when using four pilots-highlighting its capability to maintain superior performance while reducing pilot overhead. Additionally, the JCESD algorithm exhibits superior robustness in atmospheric turbulence channels while effectively reducing pilot overhead and maintaining manageable computational complexity.