Underwater visible light communication (UVLC) has gained attention for its effectiveness in undersea exploration. However, due to background noise like sunlight, UVLC has mainly been applied in deep ocean environments. To extend its use to shallow water, a scheme was proposed that applies maximal-ratio combining to an angle diversity receiver (ADR) with multiple photodiodes (PDs) positioned at different angles, focusing on optimizing PD placement. Previous studies fixed each PD’s field of view (FOV) at 90 degrees without optimization, and neglected movement in the Z direction (depth changes). This paper explores optimal FOV settings for PDs to maximize area coverage (AC). Four types of communication areas are considered, optimizing parameters for each under solar noise and receiver movement in the XYZ direction. Results show that uniform optimization, setting all FOVs equally, works well for areas with similar aspect ratios but performs poorly in distinctly vertical or horizontal areas, reducing AC significantly. In contrast, individual optimization, adjusting FOV based on PD installation angle, improves AC by over 8% compared to uniform optimization.

This paper aims to propose a multi-color-based modulation scheme for underwater simultaneous light information and power transfer (USLIPT), which can perform wireless charging while maintaining a desired communication rate. In particular, we proposed an enhanced optimized color shift keying (EOCSK) that can adjust signal space enhancement coefficient$\alpha(0\leq\alpha\leq 1)$to balance communication and energy harvesting performances. We further analyze the spectral efficiency, averaged bit error rate (BER), and harvested energy of USLIPT using time-switching (TS) and power-splitting (PS) receivers. It was found that the average BER performance of the proposed system can be improved when the value of$\alpha$is increased. Conversely, when the value of$\alpha$is decreased, the harvested energy can be increased. Furthermore, focusing on the proposed method's maximum communication distance, it was shown that a larger$\alpha$increases the possible communication distance. Still, the amount of power supplied can be increased at the expense of a slight reduction in communication distance, indicating that the optimal$\alpha$is other than$\alpha=1$, i.e., conventional optimized CSK. For example, when the desired spectral efficiency is 1.5 b/s/Hz, the harvested energy of the proposed system with$\alpha=0.6$is more than twice that with$\alpha=1$at the expense of reduced communication distance within 1 m.

Underwater simultaneous light information and power transfer (USLIPT) has recently attracted attention for the efficient exploration and excavation of international seabed resources. In this paper,$N$parallel code shift keying (N-CSK), in which$\mathrm{N}$pseudo-noise (PN) codes are selected according to the transmitted signal, is adopted as a multiple-access scheme to study USLIPT with multiple access. In this paper, we propose an inverted N-CSK scheme to further improve the power-feeding performance of the USLIPT using N-CSK by constantly turning on$M-N$unselected LEDs, which are not used as transmit signals in N-CSK, to improve the power-feeding performance while maintaining the amount of information transmitted. The performance of the bit error rate (BER) and the power supply are evaluated by theoretical analysis. The results show that the power feeding performance degrades with decreasing$N$in N -CSK, but improves with decreasing$N$in the proposed scheme because the power feeding performance improves with increasing$N$. This is because, in the proposed method, the larger$N$is, the fewer the number of LEDs that are lit. However, no matter how large$N$is, the power-feeding performance of the proposed method is superior to that of N -CSK.

Recently, underwater visible light communications (UVLC) have attracted much attention for searching and excavating undersea resources effectively. While UVLC suffers from background noise from sunlight, the application of UVLC has been limited to the deep ocean. To realize the UVLC in shallow water, UVLC with the maximal-ratio combining scheme applied to angle diversity receiver with multiple Photodiodes (PDs) at different angles was proposed and, in particular, the optimization of PD placement has been studied. However, as the field of view (FOV) of each PD was fixed at 90 degrees, its optimization has not been studied. Therefore, this paper investigates the optimal FOV of PDs that maximize the average SNR of a square communication area of$n$centered at the initial position of the receiver. As a result, in an area of$k$, narrowing the FOV of the receiver with evenly distributed PDs to 25 degrees improved the average SNR by about 3 dB compared to the case where the FOV was set to 90 degrees. In the$i$area, narrowing the FOV of the receiver with PDs placed on the sides to 75 degrees improved the average SNR by about 0.8 dB compared to the case where the FOV was set to 90 degrees.

As a method of dimming support for visible light communications using a massive LED array, we consider a hybrid pulse width modulation/digitally controlled pulse amplitude modulation (PWM/DPAM) system. Especially, in this paper, we consider a digital color shift keying (DCSK) using an RGB-LED array as an optical intensity modulation scheme, that transmits data through the ratio of the optical intensities (i.e., color) emitted by red, green, and blue LEDs. In DCSK, since only one color is activated in each RGB-LED at a time, the color can be represented by the combination of the digitally (i.e., linearly) controlled “ON-OFF” LEDs. In general, for the dimming control system of DCSK, two schemes have been considered. One is the dimming control by PWM, which changes the duty cycle of optical transmit signals, and the other is the dimming control by DPAM, which changes the number of active LEDs in the RGB-LED array. In this paper, PWM and DPAM are combined to realize a higher spectral efficiency than PWM and a wider dimming range than DPAM. We evaluate the error performances of the proposed system, DCSK with PWM, and DCSK with DPAM from a simulation analysis under several measured light dimming levels. The results show that DPAM should be used only for low bit rate systems because the effect of inter-symbol-interference (ISI), caused by the LED frequency response, increases at a high bit rate. While, PWM is significantly robust against ISI because the PWM signal duration is limited in a symbol duration at the low dimming levels and the empty duration can mitigate the effect of ISI even if the bit rate is high. When focusing on symbol error rate performances corresponding to the dimming levels, the hybrid dimming control and the PWM dimming control can achieve lower signal energy-to-noise-ratio at the dimming level of low and high, respectively.

In this paper, the extended pseudo orthogonal M-sequence is proposed for the optical-wireless code-shift-keying (CSK) system. The extended pseudo orthogonal M-sequence is a new optical pseudo-noise (PN) code which consists of the modified pseudo orthogonal M-sequence sets (MPOMS) and the bi-orthogonal code. The performance of the CSK systems is enhanced by increasing the number of the optical PN codes. As the conventional optical PN code, the bi-orthogonal code and the pseudo-orthogonal extended prime code set can generate the number of the optical PN codes same as the code length. The extended pseudo orthogonal M-sequence can achieve the performance enhancement of the CSK systems because of its generation of the large number of the codes per the code length. The CSK system using the extended pseudo orthogonal M-sequence has better performance than the CSK systems using the conventional PN code.

To realize the high data transmission rate in the optical wireless communications effectively, a Repeated-permutation Multi-pulse Pulse Position Modulation (RMPPM) system with interference avoidance is considered. The RMPPM system can transmit the bit data by using the repeated-permutation of the slot positions of the orthogonal codes, whereas the conventional Multi-pulse Pulse Position Modulation (MPPM) system can transmit the bit data by using the combination of the slot positions of the orthogonal codes. To avoid Multiple Access Interference (MAI), we consider Reduction Rule (RR) of orthogonal codes at a transmitter. We analyze the Bit Error Rate (BER) performance theoretically. As results, BER can be greatly improved by RR. RR is effective to the RMPPM system in the multi-user case of optical wireless communication.

In this paper, M-CSK with extended prime code sequence to enhance a data transmission rate of CSK is considered. Moreover, the bit error rate (BER) performance of M-CSK/CDMA is evaluated by taking into account scintillation, background-noise, avalanche photo-diode (APD) noise, thermal noise, and signal dependence noise through theoretical analysis. It is shown that the data transmission rate of the M-CSK system is better than that of the CSK system using extended prime code sequences. However, the BER performance of the M-CSK system is degraded by increasing the number of selected extended prime code sequences.

In this paper, to increase normalized throughput of the optical wireless code-division multiple-access (CDMA) system, modified pseudo orthogonal M-sequences (MPOMS) are considered as optical pseudo-noise (PN) code. Moreover, the bit error rate (BER) performance of the optical wireless CDMA system using multi-pulse pulse position modulation (MPPM) is also derived in the presence of the scintillation, background-noise, avalanche photo-diode (APD) noise, thermal noise and signal dependence noise. Consequently, the BER performance in consideration of normalized throughput of the MPPM system with MPOMS is superior to that of the MPPM system with conventional pseudo orthogonal M-sequences in the single-user case and multi-user case.

The advanced wireless communications such as the sensor networks and ad-hoc networks requires the communications system that is flexible about the transmission power control and robust over multiple access interference. We have showed that the throughput of Code Shift Keying (CSK) random access network is high with the transmission power control. However, it is difficult to use transmission power control into the communication between users. This paper presents the throughput performance of the CSK random access network without transmission power control by theoretical analysis. This paper also presents the effect of dynamic assignment of PN sequence. In particular, we derive the throughput when each user select one of the plural PN sequences randomly. Consequently, the throughput of CSK random access network is better than that of the conventional system.

In this paper, optical wireless multi-pulse pulse position modulation (MPPM) system with modified pseudo orthogonal M-sequence sets is proposed. Moreover, the symbol error rates of the proposed system are analyzed by taking into account the scintillation, background-noise, avalanche photo- diode (APD) noise, thermal noise, and signal dependence noise in the single-user case. Consequently, the normalized throughput in consideration of SER of the proposed system is superior to that of the conventional MPPM systems. Furthermore, the normalized throughput of the proposed system can achieve about 1.25 [bit/chip].

The purpose of this paper is to compensate loss packet in the broadcasting system using road to vehicle communications (RVC). Authors have proposed and evaluated the push-type and pull-type packet compensation protocol. These protocols are the loss packet compensation protocol using inter-vehicle communications (IVC). In this paper, the push/pull-type packet compensation protocol with common spreading sequences for broadcasting system is proposed. The average packet acquisition time is analyzed. By comparing with the pull-type packet compensation protocol, the push/pull-type packet compensation protocol is more efficient than the pull-type packet compensation protocol.

In our work, the channel estimation-oriented access control scheme has been investigated. In the system, each user estimates the channel based on the received power, and then decides whether to transmit the packet or refrain from transmission. However, each user can not estimate the channel completely because it is not practical that all users receive the signals with equal power. The purpose of this paper is to investigate the throughput performance of the scheme in consideration of free space loss. It is found that the scheme shows better performance than the conventional scheme even when free space loss occurs. It is also found that the probability of decision, which shows the best throughput, is different depending on offered load.

In this paper, the atmospheric optical code-division multiple- access (CDMA) system with Pseudo-orthogonal M-sequences is proposed. The bit error rate (BER) of the proposed system using pulse position modulation (PPM) by taking into account the effects of the scintillation, avalanche photo-diode (APD) noise, thermal noise, and multiple access interference (MAI) are analyzed. By comparing with the optical orthogonal code (OOC) and the prime code, the proposed system with pseudo- orthogonal M-sequences is better than the systems with other PN codes. But, the scintillation has an enormous effect on the performance of the proposed system.

The purpose of this paper is to compensate loss packets in the broadcasting services using road to vehicle communications (RVC). In particular, the push-type packet compensation protocol with multi-code sense code division multiple access (MCS/CDMA) (push scheme) and the pull-type packet compensation protocol with MCS/CDMA (pull scheme) are considered. The blocking probability and the average number of packet transmissions are analyzed. As the result, the average number of packet transmissions of the pull scheme is smaller than that of the push scheme. Therefore the pull scheme is superior to the push scheme in the power consumption

In this paper, a system based on pulse spacing modulation and code shift keying is considered. The system uses quadriphase even-shift orthogonal sequences (E-sequences) as orthogonal sequence. The auto-correlation value of the quadriphase sequence is better than that of the biphase E-sequence. In this paper, the probability of block error and efficiency are shown. The probability of block error can improve and the efficiency is greater than that of the SS-PPM system

In this paper, a code selecting/spacing ALOHA system is proposed. The proposed system not only use Nmspreading codes but also M time slots. The proposed system varies in packet size from KNmTcto KMNmTc, where K is the number of symbols per packet and Tcis the chip duration. So, the length of the packet for the proposed system changes according to the source data although the message bits per packet are constant. The number of symbols per packet of the proposed system is smaller than that of the conventional direct sequence spread spectrum multiple access (DS/SSMA) ALOHA system. Therefore, the proposed system is expected for improving the throughput performance of the conventional DS/SSMA ALOHA system. In the result of the throughput performance evaluation on the additive white Gaussian noise (AWGN) channel, it is shown that the throughput performance of the proposed system is about 2 times higher than that of the DS/SSMA ALOHA system

The present paper discusses a new construction of UWB pulses within the framework of soft-spectrum adaptation. The employed basis functions are B-splines having the following properties: (i) the B-splines are time-limited piecewise polynomials; (ii) the first-order B-splines are rectangular pulses and they converge band-limited functions at the limit that their order tends to infinity; (iii) there is an analog circuit and a fast digital filter for the generation of B-splines. A constrained approximation technique is proposed for adaptively designing pulses so that they approximate target frequency characteristics. At the cost of using eleven shifted B-splines, an example set of four pulses conforming to the FCC spectral mask is obtained.

This paper discusses the influence of the erroneous feedback to the M-ary spread spectrum multiple access (Mary/SSMA) ALOHA system with the traffic control. In particular, the throughput performance which takes into account the detection error of the transmission control signal for channel load sensing protocol (CLSP) and the automatic repeat request (ARQ) notifying signal is evaluated by theoretical analysis. As a result, it is shown that the detection error of the traffic control signal and the ARQ notifying signal causes serious performance degradation.

In this paper, the nonorthogonal multilevel spread ALOHA system, which combines the spread ALOHA system with the nonorthogonal multilevel system is investigated. In particular, the throughput performance of the nonorthogonal multilevel spread ALOHA system is evaluated by theoretical analysis. It is found that the throughput performance of the nonorthogonal multilevel spread ALOHA system is greater than that of the conventional spread ALOHA systems when the network traffic is not busy. It is found that application of the traffic control to the nonorthogonal multilevel spread ALOHA system is effective in improvement of the performance.

A time hopping modified PSM system using pseudo-noise (PN) codes is proposed. The time hopping modified PSM system uses the several PN codes, and it transmits information by the interval between the successive PN codes. The bit error rate performance and the synchronization performance of the proposed system on the additive white Gaussian noise (AWGN) channel are evaluated by theoretical analysis. As a result, there is the optimum preamble pattern for bringing the probability of block error that takes into account the synchronization performance close to the lower bound. The efficiency of the time hopping modified PSM system is greater than that of the conventional SS-PPM system.

A spread spectrum pulse spacing modulation (SS-PSM) system using E-sequences and a preamble is considered. The preamble signal is used for block synchronization. The following performances are evaluated by theoretical analysis: (1) the data transmission rate; (2) the block error rate when the receiver detects the preamble correctly; (3) the synchronization performances (the probability of detection and the probability of false alarm); (4) the block error rate considering the synchronization performances; and (5) the efficiency. Consequently, the data transmission rate of the SS-PSM system is much higher than that of the conventional spread-spectrum pulse position modulation (SS-PPM) system. The block error rate considering the synchronization performance can reach the lower bound by making use of the preamble. The efficiency of the SS-PSM system is 2 times greater than that of the conventional SS-PPM system.

A comparison of the M-ary spread spectrum multiple access (M-ary/SSMA) ALOHA system and the direct sequence spread spectrum multiple access (DS/SSMA) slotted ALOHA system is evaluated when using the channel load sensing protocol (CLSP) as the traffic control. In particular, the throughput performance and the delay property of the both systems under additive white Gaussian noise and Rayleigh fading are derived. As a result, it is shown that the CLSP takes effect to improve the system performance of the M-ary/SSMA slotted ALOHA system and the DS/SSMA slotted ALOHA system under Rayleigh fading channel. The performance improvement of the M-ary/SSMA ALOHA system by using CLSP is larger than that of the DS/SSMA ALOHA system. The M-ary/SSMA slotted ALOHA system with CLSP is superior to the DS/SSMA slotted ALOHA system with CLSP.

A bi-orthogonal modulation system using a delay lock loop and a differential detector is proposed. Moreover, the bit error rate (BER) performance under complete synchronization, the synchronization performance, and the BER performance by taking account of the tracking error are evaluated by theoretical analysis. The proposed system uses concatenated sequences which consist of the two separate sequences, that is, an anterior sequence and a posterior sequence, as the spreading codes. Because the particular sequence always appears by multiplying the anterior sequence by the posterior sequence, the proposed system can extract the timing information from the received signal itself. Consequently, the BER performance of the proposed system is the same as the lower bound of the bi-orthogonal modulation system. It is found that the proposed system is superior to the conventional bi-orthogonal modulation system using the particular synchronizing code.

A spread ALOHA system using the spread-spectrum communication system with constrained spreading codes (SS-CSC/SSMA ALOHA system) is proposed. The throughput performance and the delay property of the proposed system are investigated. Moreover, the proposed system is compared with the direct sequence spread spectrum multiple access (DS/SSMA) ALOHA system and the M-ary spread spectrum multiple access (M-ary/SSMA) ALOHA system. Consequently, it is found that the maximum throughput of the proposed system is greater than those of the DS/SSMA ALOHA system and the M-ary/SSMA ALOHA system. It is also found that the delay property of the proposed system is also better than those of the DS/SSMA ALOHA system and the M-ary/SSMA ALOHA system. Furthermore, it is shown that the performance of the proposed system is improved by increasing E/sub b//N/sub 0/ and the length of spreading code.

The spread spectrum pulse spacing modulation (SS-PSM) system using even-shift orthogonal sequences (E-sequences) and a preamble is considered. The probability of block error that takes into account the synchronization performance is evaluated by theoretical analysis. In particular, the effects of the probability of detection of a preamble and the probability of false alarm on the probability of block error are discussed. Consequently, it is found that the probability of detection and the probability of false alarm depend on the pattern of preamble. The probability of block error considering the synchronization performance can reach the lower bound by making use of the preamble with good correlation function.

In this paper, a frame synchronization method using the differential detector and racing counters is proposed. Moreover, the synchronization performance of the proposed system is discussed. In particular, the lose lock time, the recovery time and the BER performance considering the synchronizing performance are analyzed. In consequence, the BER performance considering the synchronization performance can approach the lower bound of the spread spectrum constrained spreading code (SS-CSC) system by tuning the number of the stages in the racing counters.

The performance of M-ary spread spectrum (M-ary/SS) communication systems is discussed. Firstly, the initial acquisition time is evaluated. Secondly, the retention time, which is the average number of frames holding correct frame timing, and the recovery time, which is the average number of frames required to establish synchronization, are derived. Lastly, the bit-error rate (BER) performance is evaluated. M-ary/SS communication systems, which have more than one spreading code, can improve the BER performance under conditions in which there is additive white Gaussian noise (AWGN). However, the synchronization of M-ary/SS communication systems is difficult because they have several spreading codes. The frame synchronization method uses a Hadamard matrix and "racing counters." As a result, the retention time becomes longer than the recovery time when the size of the lower counter differs greatly from that of the upper counter in the racing counters. Then the BER gets close to the performance which is achieved under complete synchronization.