Indoor Optical Wireless Communication System Research Articles

Efficient utilization of adaptive LMS channel estimation with activity detection guidance and tap decoupling in indoor optical wireless communication systems

Efficient utilization of adaptive LMS channel estimation with activity detection guidance and tap decoupling in indoor optical wireless communication systems

Digital-filter-aided crosstalk-mitigation for a high spatial resolution AWGR-based 2D IR beam-steered indoor optical wireless communication system.

The growing demand for wireless connectivity is attracting interest in optical wireless communication (OWC) technique. In this paper, a filter-aided crosstalk mitigation scheme, employing digital Nyquist filters, is proposed to eliminate the trade-off between the spatial resolution and the channel capacity for the AWGR-based 2D infrared beam-steered indoor OWC system. By shaping the transmitted signal for narrow spectral occupancy, the inter-channel crosstalk resulting from the imperfect AWGR filtering can be avoided, which enables a denser AWGR grid. In addition, the spectral-efficient signal reduces the bandwidth requirement of the AWGR, which allows a low-complexity AWGR design. Thirdly, the proposed method is not sensitive to the wavelength misalignment between AWGRs and lasers, which relaxes the design of high wavelength stability lasers. Moreover, the proposed method is cost-efficient as we can make use of the mature DSP technique without additional optical components. The 20-Gbit/s data rate OWC capacity using PAM4 format has been experimentally demonstrated over a 6-GHz bandwidth-limited AWGR-based 1.1-m free-space link. The experimental results show the feasibility and effectiveness of the proposed method. By combining our proposed method with the polarization orthogonality technique, a promising capacity per beam of 40 Gbit/s is potentially attainable.

Experimental demonstration of an indoor optical wireless communication system with a waveform index modulated uplink.

The optical wireless communication (OWC) technology has been widely studied to provide high-speed wireless connections in indoor environments. Compared with the downlink, the uplink faces more restrictions, such as the stringent power, complexity, and cost requirements. In this Letter, we propose a waveform index modulated OWC uplink with each waveform seen as a virtual transmitter. Extra data is transmitted via the index of selectively activated virtual transmitter, and the OWC uplink speed can be improved whilst achieving similar bit-error-rate (BER) performance. The waveform index modulated uplink is experimentally demonstrated in a near-infrared OWC system. Results show that up to 15 Gb/s data rate is achieved with the proposed waveform index modulated uplink, which increases the data rate significantly with similar power and BER.

Comparison of Adaptive Equalization Methods for Improving Indoor Optical Wireless Communications Employing Few-Mode Based Uniform Beam Shaping

Few-mode based uniform beam shaping exploiting the use of different spatial modes has been recently shown to improve the system resistance against pointing errors and link outage in indoor optical wireless communications. However, the inter-symbol interference arising from transmission of different spatial modes following different paths and mode crosstalk during the transmission becomes another important challenge. In this paper, we theoretically investigate and experimentally compare the performance of two popular adaptive equalization methods, namely least mean square and recursive least-square in both multi-equalizer and single-equalizer configurations, to improve the performance of indoor optical wireless communication system employing few-mode based uniform beam shaping. For both equalization methods, the multi-equalizer scheme achieves better equalization performance and higher convergence speed than the single-equalizer scheme for a similar cost of required computational complexity for each iteration. Moreover, recursive least-square method has a higher convergence speed than least mean square equalization while least mean square method has a lower computational complexity than recursive least-square method under the same desired equalization performance. More importantly, for a fixed reference BER threshold (KP4 FEC limit), the least mean square based single-equalizer scheme with a step size of 0.0035 and a tap number of 61 has the lowest computational complexity among the four types of equalization schemes. This kind of equalizer with relatively low computational complexity can provide guidelines for selecting an applicable adaptive equalization scheme in optical wireless communications with few-mode based beam shaping based on the comprehensive requirements of BER performance and computational complexity.

Remote-Powered Infrared Indoor Optical Wireless Communication Systems

Near-infrared optical wireless communications (OWCs) have been widely studied in indoor environments, and the beam steering has been widely used to achieve both high-speed and wide coverage. Compared with active steering, the passive method eliminates the need for local power supply at OWC access points. However, current passive beam steering is based on wavelength tuning, which requires precisely controlled and costly tunable lasers. In this letter, we propose a beam-steered indoor OWC system without the need for local power through remote powering, providing the benefits of low power consumption, passive OWC access points, and low-cost. The proposed remote-powered indoor OWC system is experimentally demonstrated with 10 Gb/s OWC provided to users. An indirect remote powering scheme is also proposed and realized to support large-scale OWC access points.

12 Gbit/s indoor optical wireless communication system with ultrafast beam-steering using tunable VCSEL.

Optical wireless communication (OWC) using line-of-sight connections has great application potential for indoor scenes due to its advantages of high data transmission speed and privacy. In our proposed system, we use infrared tunable vertical-cavity surface-emitting laser (VCSEL) as light source, array waveguide gratings (AWG) combined with 6 × 6 integrated optical fiber arrays as a router to realize ultrafast beam-steering and high capacity point-to-point data transmission, which makes the indoor OWC system compact, low-cost, and easy to install. The high tuning rate of VCSEL enables the channel switching to be completed within 1.7 µs. Based on the modulation format of non-return-to-zero on-off keying (NRZ-OOK), a data rate of 12 Gbit/s per channel can be realized with high sensitivity through 3.1 m free space when the detected optical power is low. The system is proved to be a flexible link with high-speed communication for mobile terminals in a limited space.

A VCSEL Array Transmission System With Novel Beam Activation Mechanisms

Optical wireless communication (OWC) is considered to be a promising technology which will alleviate traffic burden caused by the increasing number of mobile devices. In this study, a novel vertical-cavity surface-emitting laser (VCSEL) array is proposed for indoor OWC systems. To activate the best beam for a mobile user, two beam activation methods are proposed for the system. The method based on a corner-cube retroreflector (CCR) provides very low latency and allows real-time activation for high-speed users. The other method uses the omnidirectional transmitter (ODTx). The ODTx can serve the purpose of uplink transmission and beam activation simultaneously. Moreover, systems with ODTx are very robust to the random orientation of a user equipment (UE). System level analyses are carried out for the proposed VCSEL array system. For a single user scenario, the probability density function (PDF) of the signal-to-noise ratio (SNR) for the central beam of the VCSEL array system can be approximated as a uniform distribution. In addition, the average data rate of the central beam and its upper bound are given analytically and verified by Monte-Carlo simulations. For a multi-user scenario, an analytical upper bound for the average data rate is given. The effects of the cell size and the full width at half maximum (FWHM) angle on the system performance are studied. The results show that the system with a FWHM angle of $4^\circ$ outperforms the others.

Filter-Enhanced Spatial Modulation and Multiplexing in Indoor Optical Wireless Systems

Optical wireless communication (OWC) systems have been widely studied to provide high-speed connections in indoor environments. In this paper, we propose a filter-enhanced indoor OWC system with hybrid spatial modulation (SM) and spatial multiplexing (SMUX) to further increase the data rate. The filter-enhanced SM enables extra information be transmitted via the spatial indices of OWC transmitters, and the SMUX further increases the data rate via the parallel transmission of multiple signals even under highly-similar channels. The proposed scheme is experimentally demonstrated. Up to 35 Gb/s data rate can be achieved with 2.5 GBaud/s symbol rate and four transmitters. Compared with the filter-enhanced SM scheme, the achievable data rate can be increased by 100%. Compared with the conventional SM and SMUX, the bit-error-rate (BER) is improved by over two orders-of-magnitude.

Optical Wireless Communications Using Signal Space Diversity with Spatial Modulation

A signal space diversity (SSD) scheme was proposed to be incorporated with spatial modulation (SM) in an intensity-modulation/direct-detection-based multiple-input-single-output (MISO) indoor optical wireless communication (OWC) system to improve bit-error-rate (BER) performance and system throughput. SSD was realized via signal constellation rotation and diversity interleaving using different channel gains to improve the BER. With SM incorporated, the MISO-OWC system throughput increased. Theoretical BER expressions of the SSD scheme were established for the first time by investigating the distance of neighboring constellation symbols upon maximum-likelihood detection. Such BER expressions were further verified by numerical results. The results showed that, except for the slightly-lower-accuracy performance brought by comparable distances of neighboring constellation symbols in cases of low signal-to-noise ratios, these BER expressions were accurate in most scenarios. Moreover, theoretical investigations of channel gain distributions were performed at different signal constellation rotation angles to show the capability of the SSD scheme to improve the BER. The results showed that a significantly improved BER by two orders of magnitude could be achieved using a reasonably high channel-gain ratio and a larger constellation rotation angle. The SSD-SM scheme provides a promising option to achieve transmitter diversity with an enhanced throughput in high-speed indoor OWC systems.

Efficient Mobility Management for Indoor Optical Wireless Communication System

Beam-steered infrared light communication (BS-ILC) is a promising technology to establish high data rate links for next-generation indoor optical wireless networks. However, practical limitations such as localization latency and handover delay make challenging to implement a reliable BS-ILC system for mobile user devices. This letter studies mobility management, enabling the user devices to stay connected while they are on the move. Our BS-ILC system uses a wavelength domain where each wavelength covers one wireless cell. A camera-based localization determines the position of user devices relative to access points. With the help of a prediction algorithm, we demonstrated real-time localization with a <; 4mm tracking error. Furthermore, we propose and evaluate experimentally hard and soft handover mechanisms to support inter-cell user mobility. This is the first demonstration of seamless transmission in a BS-ILC system to the best of our knowledge.

Recurrent neural network FPGA hardware accelerator for delay-tolerant indoor optical wireless communications.

The optical wireless communication (OWC) system has been widely studied as a promising solution for high-speed indoor applications. The transmitter diversity scheme has been proposed to improve the performance of high-speed OWC systems. However, the transmitter diversity is vulnerable to the delay of multiple channels. Recently neural networks have been studied to realize delay-tolerant indoor OWC systems, where long-short term memory (LSTM) and attention-augmented LSTM (ALSTM) recurrent neural networks (RNNs) have shown their capabilities. However, they have high computation complexity and long computation latency. In this paper, we propose a low complexity delay-tolerant RNN scheme for indoor OWC systems. In particular, an RNN with parallelized structure is proposed to reduce the computation cost. The proposed RNN schemes show comparable capability to the more complicated ALSTM, where a bit-error-rate (BER) performance within the forward-error-correction (FEC) limit is achieved for up to 5.5 symbol periods delays. In addition, previously studied LSTM/ALSTM schemes are implemented using high-end GPUs, which have high cost, high power consumption, and long processing latency. To solve these practical limitations, in this paper we further propose and demonstrate the FPGA-based RNN hardware accelerator for delay-tolerant indoor OWC systems. To optimize the processing latency and power consumption, we also propose two optimization methods: the parallel implementation with triple-phase clocking and the stream-in based computation with additive input data insertion. Results show that the FPGA-based RNN hardware accelerator with the proposed optimization methods achieves 96.75% effective latency reduction and 90.7% lower energy consumption per symbol compared with the FPGA-based RNN hardware accelerator without optimization. Compared to the GPU implementation, the latency is reduced by about 61% and the power consumption is reduced by about 58.1%.

Indoor optical wireless communication system with continuous and simultaneous positioning.

The optical wireless communication (OWC) technology has been widely studied to provide high-speed communications in indoor environments. The indoor OWC-based positioning function is also highly demanded and the received signal strength (RSS) method has attracted intensive interests, where multiple transmitters are used and the positioning information is provided by estimating the channel gain from each transmitter with known location. However, this process normally requires dedicated positioning time slots, RF carriers or codewords, which limit the system data rate and throughput. To solve this limitation, in this paper we propose a filter-enhanced indoor OWC positioning scheme, where spatial waveform shaping filters are applied to the transmitters, thus enabling the signals from different transmitters to be separated via matched filters at the receiver side. Hence, this approach allows the RSS information to be obtained from the wireless communication signal directly and the positioning function realized without affecting the wireless communication data rate or throughput. In addition, since positioning is realized using the communication signal, continuous positioning can be achieved for real-time tracking. The proposed filter-enhanced positioning scheme is experimentally demonstrated in a near-infrared indoor OWC system with laser transmitters. Results show that an average positioning accuracy of 5.41 cm and 2.5 Gb/s wireless communication are achieved simultaneously. The proposed filter-enhanced positioning scheme can also be applied in visible light communication (VLC) systems with LED transmitters, and the feasibility is verified via simulations. The proposed filter-enhanced scheme provides a promising positioning method in indoor OWC systems without affecting the wireless data communication.

Full-duplex high-speed indoor optical wireless communication system based on a micro-LED and VCSEL array.

We built a full-duplex high-speed optical wireless communication (OWC) system based on high-bandwidth micro-size devices, for which micro-LED and VCSEL arrays are implemented to establish downlink and uplink, respectively. The high-capacity downlink based on a single-pixel quantum dot (QD) micro-LED can reach a data rate of 2.74 Gbps with adaptive orthogonal frequency division multiplexing (OFDM). VCSEL-based line-of-sight (LOS) and non-line-of-sight (NLOS) uplinks are designed with lens-free receiving functions for a 2.2-m communication distance. Experimental results have been demonstrated and confirmed that both downlink and uplinks are capable of providing sufficient bandwidth for a multi-gigabit OWC. Besides, the lens-free uplink receiver can alleviate requirements for aligning and improve the mobility of the transmitter. The VCSELs implemented for both systems work with low driving currents of 140-mA and 190-mA under consideration of the human eye safety. For non-return-to-zero on-off keying (NRZ-OOK), both uplinks can achieve 2.125 Gbps with bit-error-rate (BER) lower than the forward error correction (FEC) threshold of 3.8×10-3 for Ethernet access.

Quasi-Passive Indoor Optical Wireless Communication Systems

Optical wireless communications (OWCs) have been widely studied to provide high-speed wireless services in indoor environments, and the combination of beam steering with localization has been explored to extend the coverage area and to provide user mobility. However, beam steering typically consumes continuous power, even when the user is stationary. In this paper, we propose and demonstrate a quasi-passive indoor OWC system with zero power consumption when the user is stationary or quasi-stationary. It also supports the generation of multiple signal beams for multi-user access. 10 Gb/s transmission is demonstrated and users at different locations are served simultaneously. Experimental results show that no power penalty is observed compared to traditional systems consuming continuous power. The proposed quasi-passive system provides a multi-user access and low-power consumption solution for indoor applications.

Indoor optical wireless communication system with filters-enhanced generalized spatial modulation and carrierless amplitude and phase (CAP) modulation.

The spatial modulation principle has been widely studied in indoor optical wireless communication (OWC) systems to provide high-speed connections to users by encoding part of data in the spatial domain. To further increase the speed and to eliminate the requirements of accurate channel state information (CSI) and low channel correlation, in this Letter we propose and demonstrate an indoor OWC system with a filters-enhanced generalized spatial modulation (FE-GSM) scheme and 16-carrierless amplitude and phase (16-CAP) modulation. The 16-CAP increases the data rate in symbol domain, and the FE-GSM scheme allows activating multiple transmitters simultaneously to increase data rate in the spatial domain. Experimental results show that using a 2.5 GBaud/s indoor OWC system with three transmitters, 15 Gb/s data rate is achieved even under identical channels without pre-known CSI. The demonstrated scheme provides a promising solution to increase data rate in indoor OWC systems.

Delay-Tolerant Indoor Optical Wireless Communication Systems Based on Attention-Augmented Recurrent Neural Network

The optical wireless communication (OWC) system has been widely studied as a promising candidate for indoor high-speed wireless communications. In particular, the spatial diversity scheme has shown to be effective to improve the performance of OWC systems. However, the delay amongst multiple channels in this scheme may result in severe inter-symbol-interference (ISI) and degradations of the system performance. Recent studies have shown that the symbol decision schemes based on recurrent neural networks (RNNs) can mitigate the impact of delays. However, their performances are limited when the channel delays are long. In this paper, we propose a delay-tolerant indoor OWC system, which utilizes an attention-augmented long-short term memory (ALSTM) RNN decision scheme to handle long channel delays. A 10 Gb/s repetition-coded indoor OWC system with the proposed ALSTM RNN decision scheme is experimentally demonstrated. Compared with traditional OWC systems which add cyclic prefix (CP) /zero-postfix (ZP) to combat the impact of channel delays, the proposed system does not reduce the effective data rate or throughput. Compared with the previous study which adopts the vanilla RNN decision scheme, the proposed ALSTM RNN decision scheme can better learn dependencies amongst received symbols with the help of attention mechanism, especially amongst non-neighboring symbols. Hence, it is more robust against the ISI induced by long channel delays. Experimental results show that compared with the previously studied vanilla RNN, over one-order-of-magnitude bit-error-rate improvement is achieved when the channel delay is more than 4.7-symbol-period.

Generalized Time Slot Index Modulation for Optical Wireless Communications

A novel modulation scheme for an indoor optical wireless communication (OWC) system referred to as generalized time slot index modulation (GTIM) is proposed for single-carrier frequency domain equalization (SC-FDE) systems. This scheme is further referred to as SC-GTIM. In SC-GTIM, information bits are flexibly encoded onto both the amplitudes and positions of pulses. A set partitioning algorithm is proposed to construct a codebook for the SC-GTIM system. In this paper, SC-GTIM will be compared with the conventional pulse amplitude modulation-based SC-FDE (SC-PAM) and time slot index modulation-based SC-FDE (SC-TIM). By considering a multipath optical wireless channel and the limited dynamic range of a light emitting diode (LED), a considerable bit error ratio (BER) gain can be achieved by SC-GTIM. It is concluded that SC-GTIM is a promising technique for low to medium spectral efficiency transmission and uplink schemes.

Optical Wireless Communications Adopting Delay-Tolerant Repetition-Coding With Orthogonal-Filters and On-Demand Equalization

Transmitter diversity schemes, such as repetition coding (RC), can tackle the optical channel blockage issue in high-speed line-of-sight link based optical wireless communication (OWC) systems, and channel synchronization is required to realize the benefit. In this article, an orthogonal-filters-based repetition coding (OF-RC) transmitter diversity scheme with on-demand equalization is proposed to address time delays amongst multiple optical wireless channels in the indoor OWC systems. The OF is used for the separation and recovery of the overlapped multipath signals from the single received data stream, and hence, the proposed novel OF-RC scheme can relax the channel synchronization requirement without frequent channel state feedback. The proposed OF-RC principle is firstly investigated via both numerical analysis and experimental demonstration in the two-transmitters scenario, where the OF is constructed using the square root RC filter with orthogonal sine/cosine function sets. Results show that a bit-error-rate (BER) performance within the 7% forward-error-correction (FEC) limit is achieved using the proposed OF-RC scheme together with on-demand fast recursive-least-squares (RLS) equalization. Furthermore, both fractional and integer OF length delays can be handled by the proposed OF-RC scheme. In addition to the two-transmitters scenario, the generalized OF-RC scheme supporting more transmitters is also theoretically investigated, and multidimensional OFs are constructed via the multiband approach. The performance and feasibility of OWC system with the generalized OF-RC scheme is numerically simulated in the three-transmitters scenario, and results demonstrate the capability of the generalized OF-RC scheme. The proposed OF-RC scheme with on-demand equalization provides a promising solution to tackle the fundamental time delay limitation in high-speed indoor OWC systems.

Optimum resource allocation in optical wireless systems with energy-efficient fog and cloud architectures.

Optical wireless communication (OWC) is a promising technology that can provide high data rates while supporting multiple users. The optical wireless (OW) physical layer has been researched extensively, however, less work was devoted to multiple access and how the OW front end is connected to the network. In this paper, an OWC system which employs a wavelength division multiple access (WDMA) scheme is studied, for the purpose of supporting multiple users. In addition, a cloud/fog architecture is proposed for the first time for OWC to provide processing capabilities. The cloud/fog-integrated architecture uses visible indoor light to create high data rate connections with potential mobile nodes. These OW nodes are further clustered and used as fog mini servers to provide processing services through the OW channel for other users. Additional fog-processing units are located in the room, the building, the campus and at the metro level. Further processing capabilities are provided by remote cloud sites. Two mixed-integer linear programming (MILP) models were proposed to numerically study networking and processing in OW systems. The first MILP model was developed and used to optimize resource allocation in the indoor OWC systems, in particular, the allocation of access points (APs) and wavelengths to users, while the second MILP model was developed to optimize the placement of processing tasks in the different fog and cloud nodes available. The optimization of tasks placement in the cloud/fog-integrated architecture was analysed using the MILP models. Multiple scenarios were considered where the mobile node locations were varied in the room and the amount of processing and data rate requested by each OW node was varied. The results help to identify the optimum colour and AP to use for communication for a given mobile node location and OWC system configuration, the optimum location to place processing and the impact of the network architecture. This article is part of the theme issue 'Optical wireless communication'.

Digital Orthogonal-Filters Enhanced Spatial Modulation for High-Speed Indoor Optical Wireless Communications

Spatial modulation (SM) has been proposed and studied in high-speed indoor optical wireless communication systems to improve the spectral efficiency and system capacity by exploring the spatial domain information of transmitters. However, the correct detection of active transmitter(s) is critical, and it typically requires precise channel state information (CSI) and low correlation between channel gains. To overcome this limit, in this paper an orthogonal-filters enhanced SM scheme is proposed for high-speed indoor optical wireless communication systems, where the use of orthogonal-filters can improve the correct detection of active transmitter(s) significantly. The proposed scheme is experimentally demonstrated, and results show that with 4-pulse amplitude modulation (PAM-4) and two transmitters, 7.5 Gb/s optical wireless data transmission is achieved with 2.5 GS/s symbol rate. The power penalty of proposed scheme on the receiver sensitivity is negligible, and the active transmitter(s) can be correctly detected even under high channel correlations and without CSI. The scalability of proposed orthogonal-filters enhanced SM scheme is also studied, and it is shown that higher dimensions can be realized to achieve higher spectral efficiency, which provides a promising solution in indoor optical wireless communications to achieve ultra-high bit rate.