Research on Communication Technology of OPGW Line in Distribution Network under Interference Environment

. Abstract In optical fiber composite overhead ground wire (OPGW) networks, the current monitoring is mainly through installing electronic sensors on the cable and manually monitoring the video on the cable, where the interference plays an important role in the communication and monitoring based systems. In essence, the interference arises from aggressive frequency reuse, especially in the frequency-limited Internet of Things (IoT) networks. The existence of interference causes a negative e ff ect on the system performance of communication systems and IoT networks including the OPGW networks. Hence, this article investigates the communication technology of OPGW line in distribution network under interference environment, where there is one primary link, one secondary link, and one legitimate monitor listening to the secondary link. The secondary user needs to transmit its message to the secondary receiver under the interference power constrained by the primary node. We firstly define the outage probability of legitimate monitoring based on the data rate, and then analyze the system performance by theoretically deriving a closed-form expression of the outage probability for the OPGW communication under interference environment. Simulation results are finally demonstrated to verify the correctness of the closed-form expression for the OPGW communication under interference environment, and show that the interference has a negative impact on the OPGW communication performance.


Introduction
Promoted by advanced information technology [1,2], especially the scalable information systems [3,4], high voltage overhead power lines have become an important part of the transmission network [5,6].Due to the influence of layout factors, the terrain and ground conditions in the corridor area are complicated and changeable, such as high altitude, windy and frozen areas.These micro-environments cause various kinds of environmental disasters of overhead power lines in a variety of harsh environments, such as icing, mountain fire, lightning strikes, instant gales, etc [7,8].At the same time, the overhead power lines need to operate for a long time.In addition to the common fault defects such as line aging and disconnection, they will also cause serious accidents such as conductor galloping, tower tilt and even collapse due to coupling with environmental problems, which seriously threaten the safe and stable operation of the power transmission system.The terrain of the area where the overhead line passes is complicated, and the span is hundreds of kilometers.The cost of traditional manual or helicopter inspection and monitoring is getting higher and higher.

Bo Li et.al
A lot of problems exist such as long monitoring period, low work efficiency, and the monitoring results are vulnerable to human factors.
Optical fiber composite overhead ground wires (OPGW) is the physical carrier of power communication network and an important infrastructure for safe operation of power grid.During long-term operation, OPGW cable will inevitably encounter extreme weather such as ice coating, wind and sand, as well as tension caused by overhead suspension, which will greatly affect the transmission performance of the cable.With the increase of service life, the number of failures of OPGW optical cable is also increasing.However, the fault location and disposal of OPGW optical cable should be carried out during the line outage, and the average fault disposal time is the longest of all kinds of faults.Therefore, accurate and fast fault location of OPGW optical cable has become the pain point and difficult problem in the power grid safety production.
Currently, for fault location of OPGW optical cable, the method of testing breakpoints with optical time domain reflectometer (OTDR) is used in practical operation and maintenance works.Due to factors such as optical cable sag and excess length of optical fiber, the measured optical fiber length at the optical cable fault point has an average error of about kilometers compared with the total corresponding tower span.This leads to the use of manual line patrol method within the range of several thousand meters after the OTDR test breakpoint, or even open the nearby optical cable junction box for accurate positioning of the breakpoint during the line outage maintenance.Generally, it takes about 1.5 day to locate an OPGW cable fault on average, and the fault location efficiency is low.
The global distribution of clean energy resources is uneven.To realize the large-scale development and utilization of clean energy, it is necessary to build an energy network with clean leadership, electricity as the center, and global configuration capabilities.The energy internet is a strong smart grid with ultra-high voltage power grid as the backbone grid and global interconnection.It is bound to build a large capacity, long-distance and highly reliable power information communication system to provide necessary ways for global energy internet scheduling, operation, management, operation, etc.
The interference plays an important role in the communication and monitoring oriented systems, including the considered OPGW networks [9][10][11].In essence, the interference arises from aggressive frequency reuse, especially in the frequency-limited Internet of Things (IoT) networks [12][13][14].The existence of interference causes a negative effect on the system performance of communication systems and IoT networks including the OPGW networks [15][16][17].For example, the data transmission rate of the system will be deteriorated when the interference exists, which reduces the received signal-to-noise ratio (SNR) at the receiver or monitoring node [18,19].The detection performance such as symbol error rate (SER) can be also deteriorated by the arising interference, as the interference will affect the work mechanism of the detector or monitor.In addition, the interference will affect the transmission latency [20][21][22] and energy consumption [23][24][25], which increases the communication and computing cost for the IoT networks including the OPGW networks.Hence, it becomes an important task to investigate the effect of interference on the IoT based OPGW networks, especially the monitoring performance.
In this article, we investigate the communication technology of OPGW line in distribution network under interference environment, where there is one primary link, one secondary link, and one legitimate monitor listening to the secondary link.The secondary user needs to transmit its message to the secondary receiver under the interference power constrained by the primary node.To evaluate the system performance, we firstly define the outage probability of legitimate monitoring according to the transmission data rate and monitoring data rate, and then study the system outage probability by deriving an analytical expression for the OPGW communication under interference environment.Simulation results are finally presented to verify the correctness of the closed-form expression for the OPGW communication under interference environment, and show that the OPGW communication performance can be severely deteriorated by the interference.

System model of OPGW communication under interference environment
As shown in Fig. 1, we consider an OPGW communication under interference environment, where there is a cognitive radio network composed of one primary node, one secondary node, and one legitimate monitoring node.The primary user (P U ) transmits message to the primary receiver (P R ), and the secondary user (S U ) transmits message to the secondary receiver (S R ) under the monitoring of the legitimate monitor (LM).Due to the interference power constraint, the transmit power of the secondary user can be given by [26][27][28] where I p is the tolerated peak interference power on the primary receiver, and h p is the channel parameter of the link between S U and P R subject to Rayleigh flat fading.Thus, the signal-to-noise ratios (SNRs) of the secondary link and the monitoring link are given by [29,30] where h and g denote the channel parameters of the secondary link and monitoring link, respectively.Notation σ 2 is the AWGN variance at the S U and LM.
The LM needs to monitor the secondary link, and the unified corresponding monitoring data rate can be given by

Performance analysis of the OPGW communication under interference environment
In this section, we first give the definition on the outage probability of the legitimate monitoring, and then we derive the closed-form expression of the system outage probability.
In order to ensure the legitimate monitoring, we first define the outage probability as = Pr where γ th is the data rate threshold for monitoring the network and it is used to guarantee the quality of monitoring.
Then, we begin to derive the analytical form of P out .The average gain of the link between S U and P R is β I , and according to the cumulative distribution function (CDF) of h p 2 , we can have The average gain of the secondary link and the monitoring link are β h and β g , respectively.Hence, we can then have After some manipulations, we can further have, Finally, we can obtain the closed-form expression for the outage probability, which is given by We now analyze some asymptotic behavior of the system outage probability, when the tolerated interfering power is large.Specifically, when I P becomes large, we can approximate v from (13) as,  By applying this approximation into (15), we can further approximate P out as, From this asymptotic expression of P out , we can find that the system outage probability almost linearly increases with the value of β h , while it almost linearly decreases with the value of β g .
In next section, we will conduct some simulations to verify the correctness of the derived closed-form expression of P out .

Simulation results and discussions on OPGW communication under interference environment
In this section, we show some simulations of the OPGW communication under interference environment, to verify the proposed studies.If not specified, the environment setup of the simulations is set as follows: the variance of AWGN is set to σ 2 = 0.01, the interference power constraint is set to I p = 5dB, the outage threshold is set to γ th = 0.1, and β I = 1, β h = 1, β g = 1.Fig. 2 shows the simulation and analytical result of the monitoring outage probability of the OPGW communication under interference environment versus the threshold γ th , where the threshold γ th varies from 0.1 bps to 1 bps, and the β h is set to 1 or 3, the value of β g is set to 1. From Fig. 2, we can see that the monitoring link's outage probability increases as the outage threshold increases, showing that a smaller γ th leads to an increase in the outage  performance.Moreover, the analytical results overlap with the image of the simulation results, which verifies the closed-form solution.In further, the outage probability with β h = 3 is higher than that with β h = 1.This is because that better secondary link channel will deteriorates the performance of the legitimate monitoring.Fig. 3 depicts the simulation and analytical outage probabilities of the OPGW communication under interference environment, versus the average channel gain of β g , where β g varies from 1 to 10, and the γ th is set to 0.1 bps and 0.5 bps.From Fig. 3, we can observe that with the increase of β g , the outage probability shows a significant downward trend, which is because when β g increases, the SNR at the  legitimate monitor's end will be better than that at the receiver's end.And it can be observed that the increase of the outage threshold γ th increases the outage probability, which corresponds to the conclusion of Fig. 3. Additionally, the analytical curve perfectly replicates the simulated outcome, demonstrating the validity of the derived analytical expression.
Fig. 4 shows the simulation and analytical outage probabilities of the OPGW communication under interference environment, versus the average channel gain of β h .In this simulation, the γ th is set to 0.1 bps and 0.5 bps.The Fig. 4 shows that as β h grows, the outage probability shows an increasing trend.This is because when h increases, the channel condition at the receiver become better than it at the legitimate monitor.In addition, the analytical conclusions fit well to the simulation results, which support the effectiveness of the derived closed-form outage probability.Fig. 5 shows the simulation and analytical outage probabilities of the OPGW communication under interference environment, versus the interference power constrain I p , where the interference power constrain I p varies  from 1 dB to 10 dB.From Fig. 5, we can see that the outage probability of the monitoring link does not increase due to the increase in I p , indicate that I p does not have an impact on the outage probability.Moreover, the analytical results match the simulation results, which validates the closed-form solution.In further, the outage probability with γ th = 0.5 bps is higher than that with γ th = 0.1 bps.This is because that bigger threshold γ th will deteriorates the performance of the legitimate monitoring.

Conclusions
For the monitoring of optical fiber overhead ground wire (OPGW) cable, the current common methods mainly include monitoring by installing electronic sensors on the cable and manually monitoring the video on the cable, where the interference plays an important role in the communication and monitoring based systems, including the considered OPGW networks.In essence, the interference arises from aggressive frequency reuse, especially in the frequency-limited Internet of Things (IoT) networks.The existence of interference causes a negative effect on the system performance of communication systems and IoT networks including the OPGW networks.Hence, this article investigates the communication technology of OPGW line in distribution network under interference environment, where there is one primary link, one secondary link, and one legitimate monitor listening to the secondary link.The secondary user needs to transmit its message to the secondary receiver under the interference power constraint.Specifically, we define the outage probability of legitimate monitoring.Then, by analyzing the considered network, we theoretically derive a closed-form expression for the outage probability for the OPGW communication under interference environment.Simulation results finally verify the correctness of the closed-form expression for the OPGW communication under interference environment, and shows that the interference has a significant impact on the OPGW communication performance.

Copyright
The Copyright was licensed to EAI.

Figure 1 .
Figure 1.System model of the OPGW communication under interference environment.

Figure 2 .
Figure 2. Monitoring outage probability of the OPGW communication under interference environment versus the threshold γ th .

Figure 3 .
Figure 3. Monitoring outage probability of the OPGW communication under interference environment versus the average channel gain β g .

Figure 4 .
Figure 4. Monitoring outage probability of the OPGW communication under interference environment versus the average channel gain β h .

Figure 5 .
Figure 5. Monitoring outage probability of the OPGW communication under interference environment versus the interference power constrain I p .

Table 1
Data for Fig.2