Frequency diversity of the power line time service system

A frequency diversity technique is proposed for OFDM-modulated power line time service systems, and a variety of frequency diversity reception processing schemes are investigated to address the problem of interference caused by power line channel noise. Simulation results show that selecting the appropriate frequency diversity order can effectively reduce the system received signal bit error rate, while using the maximum ratio combining diversity reception scheme has the best effect on improving the system bit error rate performance.


INTRODUCTION
Power line communication technology has gradually attracted the attention of industry and academia since it was first proposed by British scientists in the 20th century to enable remote battery voltage measurement using power line communication technology.After years of research and development, the transmission rate of power line communication technology has increased from less than 100 kbits/s at the beginning to 200 Mbits/s at present [1].At the same time, power line communication technology has also been applied in many fields, power line time service being one of them.
China's time service system is mainly divided into the satellite-based time service system and the land-based time service system.The satellite-based time service system is mainly based on the BeiDou satellite navigation system, The land-based time service system includes a long-wave time service system, low-frequency time code time service system and so on.In addition to the operating time service system, there are also various time service technology means such as fiber optic time service and power line time service under research [2].Among them, power line time service technology has great research value, which can rely on the existing power line network to achieve the construction of a power line time service system, thus saving many infrastructure costs.
Since 2005, when power lines were proposed for time service, research units and universities, including the National Time Service Centre, have produced certain research results.However, in the recent power line time service tests, it was found that many factors still lead to the short transmission distance of the power line time service system [3].After testing and comprehensive analysis, the main factors leading to the short transmission distance of the power line time service system include complex electromagnetic influence, multipath signal interference, channel noise influence, etc. [4].In order to improve the noise immunity of the power line time service system and increase the transmission distance of the system signal, researchers have proposed the use of direct sequence spread spectrum [5], orthogonal frequency division multiplexing modulation, channel cascade coding, and other techniques.In this paper, we propose a frequency diversity processing scheme to improve the noise immunity of OFDM-modulated power line time service technology.

FREQUENCY DIVERSITY TECHNOLOGY
Frequency diversity technology includes Frequency diversity transmission and Frequency diversity combination.

Frequency diversity transmission
Frequency diversity technology can not only effectively overcome multipath fading but also effectively improve the noise immunity of time service systems [6].During the implementation of Frequency diversity transmission, multiple copies of the original sub-signal are copied to achieve multi-order diversity (the order of diversity is the number of copies of the same sub-signal plus one), in which each sub-signal with the same information should be transmitted in the less correlated sub-channel, thus effectively avoiding the interference of noise in a certain frequency band or frequency point.Based on this advantage, this paper presents a frequency diversity scheme based on OFDM modulation for improving the noise immunity of power line time service systems.In this scheme, the high-speed signals are first converted into sub-signal groups smaller than the number of channels in the IFFT, and then the sub-signals are subdivided by several replications.
The OFDM modulation scheme based on frequency diversity processing is compared with the traditional OFDM modulation scheme.The OFDM modulation link has more diverse order planning and signals replication steps, and the frequency diversity transmission process is shown in Fig. 1. (1) where  _ is the signal after frequency diversity, S_m is the signal after Fourier inversion of the diversity signal, i is the original signal sequence number, and  is the sequence number of the signal after diversity.

Frequency diversity combination
In the Frequency diversity scheme, in addition to signal transmission, another important step is the combination of diversity signals.The main purpose is to recombine and restore the diversity signals.Up to now, there have been four main schemes for combining diversity signals in communication systems: selection combination, switch combination, maximum ratio combination, and equal gain combination [7].With reference to the diversity signal combination scheme of the communication system, the frequency diversity combination scheme of the power line time service system is designed and applied.
(1) Selection combination scheme.The received diversity signals are grouped according to the same information, and the signal-to-noise ratio (SNR) of each group of sub-signals is calculated.The sub-signal with the highest instantaneous signal-to-noise ratio in the replica is selected as the frequency diversity combined signal of the power line time service system.
(2) Switch of the combination scheme.After grouping the received subset signals, a signal-to-noise ratio threshold reception value is set.When the instantaneous signal-to-noise ratio of the received subsignals is greater than the set threshold, signal reception is carried out, followed by a weighted combination of the received sub-signals and the instantaneous signal-to-noise ratio of the next set of subsignals.In this method, the maximum instantaneous signal-to-noise ratio is selected as the maximum instantaneous signal-to-noise ratio, and the signal is weighted and combined into the received signal.In this paper, the average instantaneous signal-to-noise ratio value of the previous set of sub-signals is used as the reception threshold for the next set of sub-signals.
(3) Maximum ratio combination scheme.After the sub-information carrying the same information is grouped, the signal-to-noise ratio of each group of sub-signals is calculated.The weight of the sub-signals is assigned according to the signal-to-noise ratio quality; the better the signal-to-noise ratio is, the greater the weight of the signal is.After weighing each sub-signal according to the different weights, it is subsequently combined into the received signal; this method can better circumvent the influence of the worse sub-channel on the corresponding signal.In the process of combining diversity signals in power line time service systems, this scheme's calculation complexity is higher than other schemes.
(4) Equal gain combination scheme.After grouping the signals by carrying the same information, the sub-channel signals are weighted equally.Then each sub-channel signal with the same information is combined to complete the signal recovery.This method can better spread the channel interference of each sub-channel signal, and the process is simple.During the implementation, the data can be added and then divided to ensure higher accuracy.
For OFDM-modulated power line time service systems using a frequency diversity scheme, the selection of a suitable diversity-combination scheme is beneficial for improving the demodulation performance of the system.In addition, considering that the calculation of signal-to-noise ratio in the combination scheme is a process that consumes more hardware resources, in the implementation process, if the amount of calculated data is large, the signal can be selected by calculating the signal power for comparison to reduce the calculation steps and resource consumption.

POWER LINE TIME SERVICE SYSTEMS
As an important means of time service, the power line time service system needs to meet the following basic requirements: (1) It should have good noise immunity, which will be reflected by the received signal bit error rate; (2) It should meet the transmission requirements of message information; (3) It should meet the requirements of the power line application standards [8] for the frequency band.The above requirements will be addressed in detail in this chapter.
3.1 Design of a power line time service system based on frequency diversity technology Reference wireless communication system [9] on the design of power line time service system is based on frequency diversity technology.At the transmitter end of the system, the standard time generated by the time reference source is processed to obtain the data frame signal, which is first channel coded, then OFDM modulated (signal serial and converted to achieve diversity), and finally up-converted and coupled into the power line channel at the front end, where the reference source is traced to UTC (NTSC).At the receiver end of the system, the front end removes the signal from the power line channel.It achieves diversity and combination, followed by demodulation, channel decoding, and finally, data process and time delay measurement.Fig. 2 shows the block diagram of a power line time service system based on frequency diversity technology.

Message design for power line time service systems
The power line time service system message includes synchronization code and data frame, where the synchronization code is mainly used for synchronization estimation at the receiving end.The code length of 127 bits pseudo-random code is selected as the synchronization code in this paper.The data frame includes (1) The header is used to define the data boundary and also can be used to capture information; the length is 6 bits; (2) The address code is the physical address of the repeater or user end of the power line network, the length is set to 32 bits; (3) The time code includes the year, month, day, hour, minute, second and other information, the length is 34 bits; (4) Channel delay measurement is the path time delay measurement is completed to fill, reserved 30 bits;(5) CRC code is 8-bits CRC check code.The message format of the power line time service system is shown in Fig. 3. Combined with the requirements of China's power line frequency band division standard and power line narrow band standard G3-PLC [8], the operating system band is selected as 35.938 kHz~90.625 kHz in the design of the power line time service system.Other parameters of the system are designed as follows: (1) The loop prefix length is 25% of the sub-path signal length [10]; (2) The number of FFT/IFFT channels and sub The number of FFT/IFFT channels and sub-carrier channels are 128 and 36 respectively, where the sub-carrier channel sequence numbers are 23~58; (3) The frequency interval is 1.5625 kHz, and the sampling rate is 200 kHz; (4) The data modulation method is 16-QAM, the maximum multipath time delay is taken as 63 μs [11], and the pilot code length is 4.
The pilot code calculation method as [14]: where ∆ is the sub-carrier frequency interval, and  is the maximum multipath delay.

Power line channel noise simulation analysis
Noise in the power line channel can easily cause distortion of the transmitted signal, resulting in a short transmission distance and a high bit error rate of the received signal, where the main components of the power line channel noise are burst pulse noise and colored background noise.The background noise intensity decreases with the carrier frequency and transforms slowly, usually after a few minutes or hours [12].There are many research results related to it, which can be suppressed by existing means, so it is not discussed further in this paper.Burst pulse noise is characterized by the suddenness of the pulse and the uncertainty of its intensity.Its impact on the transmitted signal is stronger than that of colored background noise, which is mainly generated by the switch closure of equipment in the power line and the transient signal of grid switching [12].Burst pulse noise can seriously affect the quality of the signal at the receiving end and can even cause sudden data errors.After a large number of practical measurements and analytical summaries, burst noise models are mainly categorized into Markov, Middleton, and other classes [13].
Based on the Middleton Class A burst noise model, the article [14] proposes a new burst impulse noise model, namely the Markov-Middleton Class A noise model.It not only retains the simplicity and ease of implementation of the Middleton Class A model but also has better performance in noise samples and time correlation.In view of the advantages of the Markov-Middleton Class A noise model, the Markov-Middleton Class A impulse noise model is used in this paper for the simulation of bursty impulse noise in power line time service systems, and the mathematical expression of the model is [15]: where  is the impulse noise sample, and A is the impulse index, equal to the product of the average number of pulses per unit time and the pulse time.When A≥10, it is very close to the Gaussian distribution,  is the state value of the noise sequence,  denoted as the probability of the  sequence state occurring,  ́ denotes the probability of the transient state to  state,  =   ⁄ is the average power ratio between the Gaussian noise and the impulse noise,  denotes the sequence of noise states, and  is the overall power of the noise.Based on the Markov-Middleton Class A random pulse noise model, the random pulse noise in the power line channel is simulated, including weak random pulse noise and strong random pulse noise, as detailed in Fig. 4. The weak random pulse noise pulse amplitude is small and dense, and the strong random pulse noise pulse intensity is large and sparse, with A taking values of 0.01 and 0.0025, respectively.The Sampling number is 57337.The experimental results show that the noise model has good randomness, and as the pulse index A is adjusted, the number of pulses also changes.This model can be used to simulate the noise components in power line channels.

Simulation analysis of frequency diversity techniques
In order to verify the effectiveness of the frequency diversity scheme to improve the noise immunity of the power line time service system, a full simulation was carried out on the MATLAB platform to verify, and a detailed comparative analysis was carried out for the signal reception bit error rate (BER) index.The specific simulation environment is set as follows.
(1) Channel model: Markov-Middleton Class A noise model, Section Ⅳ A shows more model details.
(2) Diversity schemes: Selection combination, Switch combination, Maximum ratio combination, Equal gain combination, No frequency diversity.
Fig. 5 The 3rd order diversity receiver signal BER Fig. 6 The 4th order diversity receiver signal BER Fig. 7 The 5th order diversity receiver signal BER Fig. 8 The 6th order diversity receiver signal BER Fig. 9 The 7th order diversity receiver signal BER Fig. 10 The 8th order diversity receiver signal BER Fig. 11 The 9th order diversity receiver signal BER Fig. 12 The 10th order diversity receiver signal BER

Computational complexity
Maximum ratio combination In Table Ⅰ, the computational complexity of the signal reception process is summarized for the four combination schemes and for the signal reception process without the combination scheme (No frequency diversity).M denotes the order of diversity, and N (N<M) denotes the number of signals with signal-tonoise ratios above a threshold in switching and combination.The conventional OFDM modulation scheme without diversity can be considered as the 1st order frequency diversity.The data transmission rate is one OFDM symbol transmitted per unit time, which is recorded as 1 OFDM symbol/time slot.The complexity of receiving one information merge is recorded as O(q).It can be seen that as the order of diversity grows, the difference in combination complexity of different combination schemes increases further.
By comparing and analyzing the simulation results in Fig. 5 to Fig. 12, the following conclusions can be drawn.
(1) Compared with the transmission system without Frequency diversity, the bit error rate of the system with a diversity scheme is lower as a whole.
(2) In the diversity scheme, the transmission system using the maximum ratio scheme has the fastest decrease in bit error rate values and a stable downward trend.In addition, the equal gain combination scheme also performs well in terms of bit error rate, but its stability performance is slightly poor.Although the selection combination scheme and switch combination scheme have certain effects in reducing the bit error rate, their overall performance is not as good as the first two schemes.
(3) When the diversity order increases within a certain range, and the signal-to-noise ratio is low (SNR≤15 dB), the overall bit error rate shows a rapid decline trend.Therefore, Frequency diversity technology can effectively enhance the anti-noise performance of the power line time service system.But as the diversity order increases and the signal-to-noise ratio increases, the rate of bit error rate reduction begins to slow down, indicating that the system gain brought by high-order diversity has certain limitations.In addition, high-order diversity can also slow down signal transmission rates and consume a large amount of system resources.
Based on the above conclusions and the reliability and certain transmission rate requirements of the power line timing system, the diversity order should not be too large.The design scheme of the power line timing system in this article suggests adopting the 6th to 10th order diversity processing.Based on the complexity and signal recovery effect of the combination scheme, it is recommended to use an equal gain combination or maximum ratio combination schemes for signal reception.

CONCLUSIONS
In this paper, a method is proposed to improve the noise immunity performance of a power line time service system based on a frequency diversity technique for the problem of short transmission distance.By comparing the change of bit error rate with different frequency diversity orders and different diversity combination schemes, the effectiveness of the diversity technique in improving the noise immunity performance of the system is analyzed and verified, and the computational complexity of different combination schemes is discussed.The experimental results can provide some reference for the research on power line time service technology or other communication systems adopting frequency diversity techniques.In addition, higher-order diversity means more sub-channels carrying duplicated information, which will reduce the system transfer rate and may also lead to more hardware resources being consumed, which is also an issue to be considered when adopting frequency diversity techniques in engineering applications.

Fig. 2
Fig. 2 Block diagram of power line time service system based on frequency diversity technology

Fig. 3
Fig. 3 Power line time service system message format

Fig. 4
Fig. 4 Weak and strong random pulse noise

TABLE I .
PERFORMANCE ANALYSIS OF THE COMBINATION SOLUTION