A Systematic Review of Relay-Aided Dual-Hop FSO Device-to-Device Communication Systems Using STLC and STBC Techniques

This part of our study shows a comprehensive classification of FSO Dual Hops between two devices with one deportation assistant. Optical communication systems have become increasingly significant in the past few decades due to their low-cost, high-speed connectivity [20].

Accordingly, we divide the study here into two parts. The first part is that the deportation falls within the FSO dual-hop, where each wireless access technology has advantages and limitations. The second part is to find ways to study the problem resulting from the relay and reduce it as much as possible, which we are working on to suggest a suitable way to obtain the diversity of the transmission device and the future through which we can unify the way to help reduce the delay time in the visual signal processing in RD and then send it to the interface. In addition, we seek to reduce and collect signal consumption for the maximum possible energy that can be transferred, or rather to increase SNR to the maximum. Besides energy consumption, the beam formation is applied in the deportation device to increase the power signal and send it to the recipient. Previous literary articles are classified as shown in Figure 2.

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Figure 2. Research taxonomy of D2D connection with RD assistance

2.1 Related work of relay within FSO

D2D is a strong candidate to meet the requirements of the fifth generation, where the connection is direct without the need for a central management network [21]. Channel state information and decoding at the relay node are preferably not required. However, decoding and re-encrypting the data generates a complex computational process and takes more delay in the relay node [2]. It turns out that DF has an undesirable computational complexity despite its improved results that are superior to AF, including noise reduction and not amplification, as is the case with AF [22]. Conversely, the AF at relay FSO system was implemented using an Erbium-Doped Low Mode Fiber Amplifier (EDFA) [23]. A proposed MIMO system to improve data transmission in a FOS link to improve BER by mitigating the effect of atmospheric fields (such as electric fields) affecting signal transmission [24].

However, various technologies have been introduced to meet the requirements of D2D communication, where Orthogonal Time Frequency Space Modulation (OTFS) has been implemented to provide higher transmission rates. On the other hand, it generates interference rates with data codes that require complex processing in the receiver [25], which leads to unwanted delays.

Research has come to link multiple transmission technologies where Multiple-Input Multiple-Output (MIMO) orthogonal frequency-division multiplexing (OFDM) MIMO- OFDM is considered to have high transmission capacity and low cost. Still, it needs more linearity and simplicity of complexity. This shortcoming is overcome using 2×2 Almaout’s, as detecting when encrypted tokens are received is less complex [26]. 2×2 and 4×4 MIMO-OFDM is a transmission system whose results have been verified by Tang et al. [27] as it adopts STBC codes. However, it requires channel estimation, which is difficult to achieve because it consumes time and processing power when decoding symbols. Using channel pre-parameters, a new approach to detect STBC symbols for OFDM transmissions has been established, processing delays and computational complexity in the generated detection, thus undesirable [28]. To achieve better communication links instead of a direct connection between two devices. An experience is done FSO dual-hop relay system, an all-optical forward regeneration, and forwarding (AORF) technique applied in relay instead of the amplification and forwarding (AF) scheme achieved a significant improvement in the BER instrument but taking more delay than using AF for fixed SNR [29]. In addition, several tech methods have been proposed for the dual-hop link to achieve better reliability. Since we do not want delayed processing on the relay, we suggest using amplification and steering AF for its simplicity of processing instead of DF, although it helps to increase noise as AF has been dispensed with in RD [22].

So far, a paging-based FSO system that adopts modern wireless transmission technologies has yet to be described so we will focus on the two STLC-STBC technologies in full. In this paper, STLC-STBC is fully applied to achieve our purposes of this paper and uses techniques that can reduce delay and power consumption in RD and that achieve high data stream diversity gain.

MIMO optical wireless connection achieves reliable connection. However, near channels negatively affect the connectivity, as spatial multiplexing has been proposed to mitigate these effects by using intense LEDs (light-emitting diodes) and PD (photodiodes). It is necessary to detect the optical signals from the receiver except when adjusting the tilt angles of both the LEDs and the PD because signal vector detection is difficult since the channel gain of each LED is the same. Signal detection is achieved through maximum likelihood (ML) detection [23]. This generates computational complexity in detection and causes processing delays, so it is not considered successful, especially after increasing the distance of the link. Generalized MIMO modulation and encrypted modulation have been used to improve the high spectral efficiency in the wireless optical communication system [30].

The nonlinearity of the LED in the channel link increases and decreases in order, and as a result, generates difficulty in detecting the received signal in spatial multiplexing of the MIMO transmitter to achieve the receiver diversity gain and the equal-gain combination (EGC) algorithm is proposed for this lack of detection [31]. Research has developed a new technique based on space-time in which the Khatri-Rao space-time (KRST) coding is proposed for double or triple hop system the MIMO for joint estimation of CSI and symbols. The transmission of the training sequence is used to obtain the matrix information of the channel and the characters in the string [32], which leads to the processing delay and computational complexity of dual hops or triple hops. In addition, as presented by Han et al. [33], KRST coding is proposed to encode symbols in the transmission for MIMO communication of the dual-hop system, and channel estimation is required in the relay. Using the Parallel Factor Interface (PARAFAC) in the relay and PARATUCK2 in the interface is suggested. However, this generates computational complexity and a delay in detecting symbols.

Space-time coding (STC) is a spatial diversity technique proposed in transmission with MIMO to mitigate multipath fading in wireless communication until it reaches the receiver, thus improving SNR. Increasing the number of outputs and inputs reduces the bad BER result. STC is combined with spatial diversity to achieve better results in BER [34]. Li et al. [35] presented a design for a non-coherent space-time system in a non-coherent wireless communication environment with dual outputs at transmit and massive inputs at the exit. To ensure maximum detection, the USTC (unitary space-time code) is proposed as it contributes to mitigating the worst-case double error. Furthermore, the USTC achieves full diversity at the receiver with many inputs.

In Mthethwa and Xu’s study [36], Uncoded Space-Time Labeling Diversity (USTLD) is a strong candidate for ensuring reliable wireless connectivity while maintaining the spectral efficiency of the link STBC with labelling diversity results in a USTLD system-furthermore, a Neural Network Machine Learning (NNML) has been proposed for blind channel estimation required when using the USTLD-MIMO system instead of traditional estimation techniques as it reduces the bandwidth requirement when channel estimation. Spatial diversity and multiplexing are essential in modern communication technologies. USTLD scores better in diversity gains than Alamouti STBC [37] as USTLD achieves spatial diversity and labelling diversity together to obtain the best improvements in system performance [38]. The authors proved by Patel [39] that USTLD does not achieve a high data rate, although it allows any number when sending. Therefore, a high-rate USTLD has been proposed to earn high rates. However, this generates more complexity in detecting processing low complexity detection techniques presented on the HR-USTLD system.

In Choi and Joung’s study [40], the system generalized STLC suggested that CSI must be fully available at the sender, but when receiving, the CSI is optional to be complete. Optimal precoding with linear combining is required to derive generalized STLC where the minimum BER is achieved with optimal precoding. A codebook consisting of a permutation matrix with STLC decoding has been proposed to reduce computational complexity at the receiver without symbol doubling. Implementing the STLC is possible using a simple, straightforward linear encoding and decoding process. These benefits have led to further research on STLC-based communication approaches [40]. Joung et al. [41] proposed STLC to reduce the power consumption of an unmanned aircraft system. In Pang et al.’s study [42], STLC-MIMO gains full diversity for the transmitted symbols. It is done by providing channel information in the transmitter. BER is improved upon antenna selection (AS) for STLC rather than traditional STLC. Additionally, Space-frequency line code (SFLC) has been proposed by Wu et al. [43]. SFLC was combined with orthogonal frequency division multiplexing (OFDM). The results were verified by simulation, and it was found that for multiple paths, the BER performance of SFLC-OFDM is slightly worse than that of STLC-OFDM. By increasing the number of outputs and inputs, the BER performance improves. But STLC-OFDM is better in terms of delay spread. The two schemes were merged so that STLC/SFLC-OFDM achieves complete diversity, as it needs to provide channel information to encode symbols at the transmitter, and CSI is incomplete at the receiver.

In Lee et al.’s study [44], a new technique known as space-time line coded spatial modulation (STLC-SM) is proposed with MIMO. Constellation rotated STLC for the transmission codes, after which two modulation codes are sent through one of the outputs of the transmitter, where CSI must be available when transmitting but without the total need for it when decoding the codes at the receiver. A maximum-likelihood detector placed at the receiver to detect received symbols STLC-SM is of lower complexity and better performance in BER than STLC performance. Nevertheless, integrating quasi-orthogonal STBCs (QOSTBCs) with SM further improved the system's spectral efficiency [19].

Double space time line code (DSTLC) was designed with many transmitting outputs and four receiving inputs, as it needs CSI when encoding the symbols to be sent. It increases spatial diversity and better performance of BER and data rate compared with spatial multiplexing and STBC. Thus, the pre-coded (SM) gain is negligible when the modulation sizes are identical. It has been proven that (D-STLC) is a promising candidate at high SNR and has spatial diversity and multiplexing. It transmits two parallel STLC streams to achieve additional multiplexing gains [45]. The diversity of codes for transmission helps to reduce the degradation of wireless signal propagation, and the reception of the signal to the receiver is less corrupt. Space-Time Trellis Codes (STATs) were introduced to achieve diversity and increase communication reliability, and decoding is done in the receiver using Viterbi decoding [46]. Space-Time Trellis Code Modulation (STTCM) is suggested to achieve high data flow, power efficiency, and access to increase spatial diversity significantly. Gain diversity grows as the number of inputs and outputs rises. The receiver uses the iterative decoder for various input conditions and numbers. It makes use of the Viterbi algorithm and feed-forward encoder construction. As BER is preferable to using Space-Time Trellis Codes, the recursive decoder (STTCM) is recommended [47].

STLC is proposed by Mavares et al. [48] to achieve the full rate by using the orthogonality of STLC and the quasi-orthogonality of STLC. It realizes it is adequate for a full transmission rate instead of other techniques. Average Likelihood Ratio Test (ALRT) technology was used to achieve Low-Complexity in the STBC borrowing system for a little SNR, as it turned out that the system achieved an accurate performance against CSI estimation error variance [49].

This part of the paper included a focused review of transmission technologies that meet the high need for high data flow requirements and simplicity in the computational complexity of encoding data codes or decoding them upon reception. Most of the techniques mentioned above come to me in several matters, the most important of which is the computational complexity of encoding and the difficulty in decoding. Secondly, some require CSI when receiving or sending, which will delay processing and lead to more power drain. Therefore, technologies that cause these two things will be dispensed with. When we decided to use a relay device between two devices, the shortcoming we referred to in the above two points must be eliminated, requiring minimum processing of relay/power/time resources in RD. Therefore, to achieve minimum processing in the RD device, we must make RD. it does not need CSI estimation, and there is no decoding when sending and receiving the signal in the RD device.

Here, we propose to use a suitable algorithm represented by Space-Time Line Code (STLC) and Space-Time Block Code (STBC) to reduce RD processing and time resources. So that there is no need for CSI to be available when receiving symbols in RD encoded with STLC technology. Also, there is no need when send symbols encoded with STBC technology when they are sent. Orthogonal STBCs are special cases of STBCs that allow for simplified linear decoding of a single symbol. It is noted, however, that OSTBCs in FSO systems are capable of a maximal coding rate of one. Therefore, higher transmission data rates require STBCs with a high coding rate [19]. Therefore, orthogonality in STLC and STBC contributes to reducing processing, i.e., low latency, but it does not mitigate the effects of energy consumption. Consequently, we are working on using semi-orthogonality for STLC and STBC, where these two technologies eliminate shortcomings in energy consumption. We can achieve the goal of this paper. On the other hand, this increases the overlap between symbols. Hence, the MIMO contributes to reducing the effects of this, and the beamforming technique maximizes the total SNR, which we propose to use in the RD of a dual-hop FSO system.

2.2 Proposed system model

We propose a dual-hop FSO communication system in which, at a particular distance, two devices are connected without the use of a base station (BS). Through an optical relay device outfitted with (k) receiving PDs to constitute a $D_1$-ORD transmission at the first hop and (j) transmitter of LEDs of ORD- $D_2$ transmission at the second hop, the first device $\left(D_1\right)$ with (M) transmitting LEDs communicates with a second device $\left(D_2\right)$ with $(\mathrm{N})$ receiving PDs. The primary goal of this study is to use STLC at the first hop and STBC at the second hop as two coding approaches to achieve low time/energy processing at the ORD. According to Figure 3, the AWGN is modelled as a received noise with zero mean and variance $\sigma^2$ and independent for each receiver.

Figure 3. Proposed system model

On-Off Keying (OOK) modulation is employed for the initial hop from $D_1$. It is one of the most significant modulations that is frequently employed in optical communication in space. When there is no optical intensity during transmission as zero, and one occurs for (M) LEDs from $D_1$, no optical intensity is communicated during that time. The driver will next transform the electrical signal into an optical signal, after which even subcarriers will employ DC-biased, where before transmitting the signal to RD in parallel, each LED receives it in series. The signal will then be received by the PDs at ORD, where LED illumination can achieve lower power consumption and a longer lifespan; however, to demodulate the receiving signal, we apply coherent demodulation for the OOK modulation method. ORD merely needs to combine all the received signals to extract symbols. However, it uses the maximum ratio combining (MRC) technique, which maximizes SNR at RD and combines all received signals for k-M routes. Therefore, the received signals multiply the beamforming factor (W) at stage (Beamforming). Then, the signal is retransmitted as a block of symbols loaded in j of LEDs to reach the second device equipped with N of PDs. Then, the fate will be towards the detection process of signal optical (i.e., IM/DD detection) for decided such as original signal transmitted. The effect of the amount of noise added during the two jumps should not be neglected, in addition to the existing fading of the signal and its effect on it due to weather effects.

Ali et al. [50] presented that to enhance overall SNR, linear beamforming is suggested and created for dual-hop amplify-and-forward (AF) systems with relay nodes. A computationally effective Fukunaga Koontz Transform is employed to tackle the beamforming optimization problem when CSI is applied between source-relay nodes and relay nodes-destination. In addition, Beamforming technology has been used in a wide range of radio communications in RD to achieve maximum signal-to-noise ratio (SNR). The M-ary phase shift modulation (M-PSK) scheme is adopted for beamforming [51].

The beamforming technique maximizes the overall SNR ratio of the double-hop FSO system in our proposed system, where the quality of the link between RD and $D_2$ is enhanced.

The optical signal is prepared to be sent again through several LEDs using STBC encoding, as it does not require the provision of CSI, so the energy consumption is reduced. Point-to-point System Tool Analysis of the FSO System with Migration Assistant is fundamental. Turbulence attenuation and routing error attenuation in a channel link is essential for evaluating communication quality. A near-ideal disturbance condition is challenging, but the FSO link is often exposed to moderate to severe conditions. The MIMO technique used here increases diversity and thus reduces signal reception losses. Our proposed system is by placing the relay device between the D2D connection. There are two links for the first link (first hop) between $D_1$ and RD. and the second link (second hop) between RD and $D_2$. Each has a different channel state from the other, where the SNR1 will be at RD and SNR2 at $D_2$.

It is necessary to analyze the system's performance and discuss the tools used in its analysis. We used important parameters such as SNR and Capacity C to evaluate the system's performance. Measuring their amount (SNR and C) at each hop and their total at $D_2$ is necessary to evaluate the system's overall performance. We consider a cooperative transmission from a $D_1$ to $D_2$ via an ORD that uses AF relaying to amplify the received signal and retransmit the data to the intended recipient. Let $\mathrm{SNR}_1$ represent the signal-tonoise ratio (SNR) of the first FSO hop from $D_1$ to ORD, and $\mathrm{SNR}_2$ represent the SNR of the second FSO hop from ORD to $D_2$. The end-to-end SNR can then be expressed as $\mathrm{SNR}_o=\frac{\mathrm{SNR}_1 \mathrm{SNR}_2}{\mathrm{SNR}_2+\mathrm{G}}$, where G is a fixed relay gain [52].

The two FSO hops (i.e., $D_1$-ORD and ORD-$D_2$) are assumed to be subject to independent but not necessarily identically distributed type of fading. Where that distance is approximately equal between the $D_1$-ORD and the $D_2$-ORD, we assume that a high-energy FSO system's performance is constrained by shot and thermal noise. In this instance, the noise can be accurately modelled as AWGN. Bits are modulated into symbols at the $D_1$ before being sent to the STLC encoder. The transmitted signal of STLC for the first device is denoted by:

$\bar{S}_{D_1}=\sqrt{\frac{P_T}{M}} \sum_{i=0}^k s(i)$          (1)

where, $\mathrm{S}_{\mathrm{D}_1} \in \mathrm{R}^{\mathrm{M} \times 1}$ and $P_T {}$ denote the transmitted power Let's assume that there are (M) LEDs at $D_1$ with (K=2) PDs.

In the first hop, the SNR is denoted by:

$S N R_1=\sum_{m=1}^M \sum_{k=1}^2 \frac{\left\lceil H_{D_{1-} k, m}\right]^2 \cdot \sigma_S^2}{2 \sigma_n^2}$          (2)

Here, $\left\lceil H_{D_{1_{-}} k, m}\right\rceil^2$ denotes a channel gain between the K-th ORD receiving PDs and the M-th $D_1$ LEDs, where, $E\left\{\lceil s\rceil^2\right\}=$ $\sigma_S^2, C N\left(0, \sigma_S^2\right)$ and because the sum of two independent AWGNs is also an AWGN, i.e., $\operatorname{CN}\left(0,2 \sigma_{\mathrm{w}}^2\right)$.

In the second hop, the SNR is denoted by:

$S N R_2=\frac{R P_T\left|\bar{W}^H H_{O R D-D_{2, j}}\right|^2 \cdot \sigma_s^2}{8\|W\|^2 \cdot \sigma_n^2 I}$          (3)

where, R is PDs responsivity, W is the beamforming factor, $H$ is denoted to the Hermitian matrix for W, and $H_{O R D-D_2, j}$ denotes a channel gain between the J-th ORD LEDs the N-th $D_1$ receiving PDs. We concede J=8 LEDs and N=8 PDs. The overall SNR of the first and second hops for the proposed system is calculated by Zedini et al. [52]:

$S N R_O=\frac{S N R_1 S N R_2}{S N R_2+G}$          (4)

The i-th parallel channel capacity for MIMO channels is shown by:

$C_i=\log _2\left(1+S N R_{O i}\right)$          (5)

Figure 4 simulates and compares the capacity of several different STBC-MIMO and QOSTBC-MIMO with beamforming techniques under various SNR conditions. 2×2 STBC provides better performance. It also achieves diversity gain but not spatial multiplexing. 4×4 STBC offers both diversity gain and spatial multiplexing. The capacity increases with SNR due to the combined effects of spatial multiplexing and improved SNR. 8×8 QOSTBC provides the highest capacity due to its significant spatial multiplexing potential, especially at higher SNR levels. The QOSTBC versions generally outperform their STBC counterparts. We can increase the channel capacity by increasing the full transmitter diversity using QOSTBC and increasing the number of LEDs and PDs.

4.png

Figure 4. Capacity and SNR of 2×2 and 4×4 STBC, and 2×2, 4×4, and 8×8 QOSTBC

A single relay is sufficient to reduce the communication distance in the FSO link for relatively long distances, as we explained earlier, instead of having a multi-relay between the two devices, as this does generate high and complex parameters that must be observed to preserve the data as it reaches its final destination ($D_2$). We are concerned about the loss of the path at the propagation time of the light beam due to strong atmosphere conditions between $D_1$-ORD or ORD-$D_2$, as well as increasing Hops, which reduces data flow as it consumes data power. We have contributed to proposing a solution for this by using the two techniques in Section 3.2. in RD, all the handling is optical to reduce the delay in processing, as it will dispense with converting the light signal into electrical and then vice versa, and also will reduce the complexity and side effects of that, and beamforming of the optical signal in the RD, a critical proposal to achieve maximum energy of signal at RD.

Here, we propose new directions for developing this approach and fill in the possible gaps. In the case of weather conditions, they are at their best (almost ideal). Here, the presence of RD is not necessary, so it requires sensing the conditions at RD to stop its work. Where we can suggest some sensing mechanisms, including but not limited to Sensor at ORD. Here, the sensor has provided information about the region's parameters that will be monitored near ORD. features of this sensor device are operated and controlled based on their sensitivity as well as the range of sensing; necessary sensing and controlling actions will be taken depending upon the conditions, like fixing the threshold value of BER, SNR, periodicity of sensing, etc. Based on the data analysis, the parameter threshold values or normal working conditions are turned on/off ORD. Also, can be used in Adaptive Learning, implementing machine learning algorithms to predict future channel conditions and performance based on historical data. This adaptive learning can aid in making informed decisions about activating the RD or maintaining the D2D connection. Reinforcement learning techniques can be employed to learn optimal policies for activating the RD or maintaining the D2D connection based on the historical performance of different decisions [61]. Q-learning and Deep Q Networks (DQN) are examples of reinforcement learning algorithms. Also, Recurrent Neural Networks (RNNs) are well-suited for sequential data, making them suitable for time-series predictions like channel conditions. Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU) are popular variants of RNNs that can capture temporal dependencies in the data.

Achieving a direct D2D connection without using a RD can be challenging due it requires LOS. There is a gap in that case, that RD must be on the same line as the two devices or the possibility of modifying the adaptation of the relationship between the two devices automatically within specific algorithms to ensure the direct connection of D2D. Here, after monitoring the ATs at $D_1$ by the sensor node. If communication conditions change or become unstable, communication switching is managed without RD, but there are some limitations and considerations to take into account, such as range and proximity. D2D link is usually limited to short ranges due to the physical limitations of wireless technologies. Interference: The lack of intermediate RD means that devices may have to deal with higher levels of interference, which affects communication quality. So the connection is directed to the second device. For the reliability of a stable FSO connection, we have to focus on a better tool for the paging device from several things, the most important of which are:

1- A wireless place suitable for the RD position between the two devices and free from everything that impedes communication, such as buildings, trees, or even thick smoke, providing line-of-sight communication.

2- Increasing the application of diversity in RD, which increases the reception and transmission of data streams but requires finding better technologies to mitigate the impact of complex processing and significant delays.

3- Making the communication process a full duplex; Achieving the exchange of transmission and reception between the two devices may increase the complexity, but it is helpful in essential applications.

4- We suggest another developmental trend: An additional RD parallel to the primary RD when communication with the main RD device is interrupted or the line of sight is lost.