CN106160873A - A kind of phase-shift coding method based on pulse amplitude modulation system and realize device - Google Patents

Abstract

The invention belongs to the technical field of visible light communication, and specifically relates to a phase shift encoding method and an implementation device based on a direct modulation direct detection pulse amplitude modulation system. In the present invention, at the transmitting end of the system, a phase-shift encoding method of opposite symbols is adopted for the odd and even symbols of the continuous symbol sequence; at the receiving end of the system, a differential decoding method of delayed subtraction is performed on adjacent symbols of the received symbols. Its corresponding implementation device includes a phase-shift encoder, a PAM mapping module, a serial-to-parallel/parallel-serial conversion module, an up-sampling module, an LED and its driving module at the transmitting end; Post equalization module, PAM demapping module, clock synchronization module, normalization module, downsampling module, focusing lens, filter and receiver. Using the device and the encoding and decoding method of the present invention can effectively eliminate the inherent DC component and crosstalk noise between signals in the visible light communication system based on the pulse amplitude modulation format, increase receiver sensitivity, improve signal transmission quality, and effectively improve system performance. Moreover, the method has a simple implementation process and is easy to integrate.

Description

A Phase Shift Encoding Method and Realization Device Based on Pulse Amplitude Modulation System

technical field

The invention belongs to the technical field of visible light communication, and in particular relates to a phase shift (PS) encoding method and an implementation device based on a pulse amplitude modulation (PAM) system.

Background technique

At present, white light emitting diodes (LED, light emitting diode) have been widely used in various fields, such as lighting, signal emission, display and so on. Compared with other commonly used light sources such as incandescent lamps and fluorescent lamps, white LEDs have a higher modulation bandwidth. In addition, white LEDs also have many advantages such as good modulation performance and high response sensitivity. Using these characteristics of LED, we can modulate the signal to the light emitted by LED for transmission, so as to realize the effective integration of lighting and communication. Visible light communication (VLC, Visible Light Communication) is a new green information technology that utilizes the fast response characteristics of LEDs to achieve high-speed data transmission.

Compared with other wireless communication technologies, VLC technology has the advantages of high transmission rate, good security, strong confidentiality, no electromagnetic radiation, green and environmental protection, etc., and has become a research hotspot in the field of communication in recent years. However, there are also some limiting factors in its development. One of the important challenges is that the limited bandwidth of LEDs limits the transmission rate and system performance of the VLC system. In order to realize the high-speed transmission of the VLC system, researchers generally use advanced high-order modulation techniques to encode and modulate the original data. Among them, pulse amplitude modulation has been widely researched and used as a simple and feasible modulation method. Moreover, at the IEEE 802.1/IEEE 802.3 Joint Conference in 2015, PAM4 became the new standard of IEEE P802.3bs 400Gbs, which shows that PAM modulation technology is a practical transmission scheme with great potential.

However, for the PAM visible light communication system with direct modulation and direct detection, the crosstalk noise between signals will seriously deteriorate the system performance, and the inherent frequency noise caused by direct detection will also have a certain impact on system performance. In view of this problem, the encoding and decoding method proposed by the present invention, by controlling the spectral shape of the encoded signal at the transmitting end, and performing subtraction operations on adjacent symbols at the receiving end, can suppress the DC component of the signal and the quadratic term noise, and reduce the noise Improve system performance against system performance deterioration.

Contents of the invention

The purpose of the present invention is to provide a phase-shift encoding method and implementation device based on direct modulation and direct detection pulse amplitude modulation (PAM) system, which is used to realize visible light high-speed communication based on PAM modulation, and solve the problem of receiving signals in existing PAM modulation technology. The problem of crosstalk noise affects seriously, thus improving the transmission performance of the system.

The phase-shift coding method based on direct modulation and direct detection PAM system provided by the present invention adopts a phase-shift coding method of opposite signs for the parity symbols of the original signal, and adopts a differential decoding method of delay subtraction for the received signal, specifically as follows: baseband The transmitted PAM signal controls the low-frequency component of its spectrum, and suppresses its DC component and crosstalk noise between signals for the received signal.

Specifically, at the transmitting end, phase shift coding is performed on the PAM modulated signal, and the modulated PAM symbols are divided into different symbol blocks, each group of symbol blocks contains two consecutive symbols, and the second code The element is the inverse of the first symbol. The baseband PAM signal in the kth symbol block at the transmitting end and Can be expressed as:

in, A symbol representing a PAM map.

At the receiving end, the signal is differentially decoded to obtain the photocurrent. The photocurrent in the kth group of symbol blocks at the receiving end Can be expressed as:

Among them, the coefficient Indicates the strength ratio of the PAM baseband signal; and Respectively represent the bias voltage and reverse voltage of the LED; and denote the noise currents in the 2k-1 and 2kth symbols in the k-th symbol block, respectively.

The invention also discloses a device for realizing phase-shift encoding and differential decoding based on direct modulation PAM system, including corresponding transmitting end and receiving end; the transmitting end includes phase-shift encoder, PAM mapping module, serial-parallel/ Parallel-to-serial conversion module, up-sampling module, LED and its driver module; the receiving end includes a differential decoder, a serial-to-parallel conversion module, a post-equalization module, a PAM demapping module, a clock synchronization module, a normalization module, and down-sampling modules, focusing lenses, filters and receivers; of which:

The phase shift encoder is used to phase shift encode the signal;

The PAM mapping module is used for encoding and modulating signals;

The serial-parallel/parallel-serial conversion module is used for sequence conversion before and after signal encoding;

The upsampling module is used for upsampling of signals;

The LED and its driving module are used to load the signal onto the LED and emit it through free space;

The differential decoder is used to differentially decode signals;

The serial-to-parallel conversion module is used for sequence conversion after signal decoding;

The post-equalization module is used to equalize the decoded signal;

The PAM demapping module is used to demodulate the PAM signal into a binary bit stream;

The clock synchronization module is used for clock extraction and synchronization of received signals;

The normalization module is used to normalize the average power of the received signal;

The down-sampling module is used for down-sampling of signals;

The focusing lens, optical filter and receiver are used for receiving optical signals and converting optical signals into electrical signals.

The present invention utilizes wavelength division multiplexing (WDM) technology to transmit PAM signals based on phase-shift coding with different wavelengths on the three bands of red, green and blue, and performs corresponding differential decoding at the receiving end to realize phase-shift coded PAM signals in High-speed transmission in visible light communication systems.

The work flow of device of the present invention is as follows:

On the transmitter side:

(1) The PAM mapping module performs PAM mapping on the original binary bit stream to generate a multi-level PAM signal;

(2) The upsampling module performs 2 times upsampling on the mapped PAM symbols to prepare for the serial-to-parallel conversion of the signal;

(3) The serial-to-parallel conversion module performs serial-to-parallel conversion on the signal, divides the serial signal into two parallel signals, and prepares for phase-shift encoding;

(4) The phase shift encoder inverts the input signal of the second channel and delays it by T/2 bits;

(5) The parallel-to-serial conversion module combines two parallel signals into one signal, so that each symbol signal is followed by its inverse signal;

(6) The up-sampling module up-samples the encoded signal for input into an arbitrary waveform generator (AWG);

(7) The LED and its drive module pass the signal emitted by the AWG through the pre-balance board, the electric amplifier and the AC-DC coupler, and couple the emission signal and bias voltage to the red, green and blue (RGB) LED, and pass through the free space launch out.

On the receiving end:

(1) The receiver distinguishes the red, green and blue transmission signals in three different bands through the focusing lens and optical filter, and then converts the received optical signal into an electrical signal through the receiver, and collects the signal with a high-speed sampling real-time oscilloscope To complete the next step of offline processing;

(2) The clock synchronization module performs clock synchronization on the signal collected by the oscilloscope, and finds the signal head;

(3) The normalization module normalizes the received signal with the average power;

(4) The downsampling module performs 4 times downsampling on the received signal;

(5) The serial-to-parallel conversion module converts the serial signal into two parallel signals to prepare for differential decoding;

(6) The differential decoder delays the parallel second signal by T/2 bits, and subtracts the delayed second signal from the first signal to obtain the decoded signal;

(7) The post-equalization module performs post-equalization on the decoded signal to compensate for the intersymbol interference of the signal;

(8) The PAM demapping module demaps the PAM signal into a binary bit stream.

The beneficial effect of the present invention is that: adopting phase shift coding and differential decoding to carry out visible light transmission of PAM modulated signal can effectively suppress the DC component in the received signal and the crosstalk noise between the signals, and improve the sensitivity of the receiver. The present invention is applicable to the field of visible light communication, and is used to solve the problem that crosstalk noise seriously deteriorates system performance in a visible light communication system based on PAM modulation, and greatly improves the transmission performance of the system.

Description of drawings

FIG. 1 is a schematic diagram of a PAM signal with phase shift coding proposed by the present invention.

Fig. 2 is a schematic diagram of an implementation device for a PAM system based on phase-shift coding proposed by the present invention.

FIG. 3 is a schematic diagram of the application of the present invention in a WDM-based visible light communication system.

FIG. 4 is a spectrum comparison chart of the phase-shift coded PAM signal proposed by the present invention and the uncoded PAM signal.

Fig. 5 Uncoded PAM-8 received signal spectrum.

Fig. 6 is the spectrum of the PAM-8 received signal of the phase-shift coding proposed by the present invention.

Fig. 7 is the signal spectrum of PAM-8 directly decoded by the phase shift coding proposed by the present invention.

Fig. 8 is the signal spectrum after PAM-8 differential decoding of the phase shift coding proposed by the present invention.

Fig. 9 Constellation diagrams of PAM-8 signals without encoding, phase-shift encoding and direct decoding, and phase-shift encoding and differential decoding.

Fig. 10 Study of bit error rate performance as a function of bit rate for phase-shift encoded PAM-8 signals in differential decoding and direct decoding in red light transmission.

Fig. 11 Study of bit error rate performance as a function of bit rate for phase shift encoded PAM-8 signals in differential decoding and direct decoding in green light transmission.

Fig. 12 Study of bit error rate performance as a function of bit rate for phase-shift encoded PAM-8 signals in the case of differential decoding and direct decoding in Blu-ray transmission.

Figure 13 Research on the BER performance of PAM-8 signals with phase-shift encoding and differential decoding in red light transmission as a function of distance.

Fig. 14 Research on the bit error rate performance of PAM-8 signal with phase shift encoding and differential decoding in green light transmission as a function of distance.

Fig. 15 Study of BER performance of PAM-8 signals with phase-shift encoding and differential decoding as a function of distance in Blu-ray transmission.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, various embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

The purpose of the present invention is to provide a phase shift (PS) encoding method of a pulse amplitude modulation system (PAM) based on direct modulation and direct detection and its implementation device in a visible light communication system. The method can reduce the influence of system inherent noise and inter-signal crosstalk noise on the system, improve system performance and obtain higher transmission rate. We will introduce the specific implementation plan in detail from two aspects of principle analysis and experimental transmission.

1. Principle analysis

The coding schematic diagram proposed by the present invention is shown in Figure 1 . In the phase shift encoder at the transmitting end, the continuous symbols are divided into different blocks, and each group of symbol blocks contains two symbols, so that the sign of the second symbol is completely opposite to that of the first symbol. In the symbol group, the first symbol of the first group is , the second symbol in the first group is , and so on, a symbol in the kth group is , the second symbol .

The encoded baseband PAM signal can be expressed as:

(1)

(2)

in, A symbol representing a PAM map.

The coded PAM signal is modulated to the LED lamp through the AC-DC coupler, and the signal emitted by the LED can be expressed as:

(3)

in, and Represents the bias current and reverse current of the LED, respectively; the coefficient Indicates the strength of the PAM signal; Indicates the center frequency of LED light; Indicates the unit coefficient; Indicates time.

After the signal emitted by the LED is transmitted in free space, the received signal can be expressed as:

(4)

in, Represents additive white Gaussian noise.

The receiver converts the optical signal into an electrical signal. Since the receiver uses square law detection, the signal current in the kth group of symbols can be approximately expressed as:

(5)

(6)

(7)

in, is the noise introduced by the receiver. The first term in formula (5) represents the DC component of the received signal, the second term represents the signal component, the third term represents the crosstalk noise between signals, and the fourth term is the Other noises listed. Formula (6) represents the beat frequency noise of AWGN to AWGN, and the second term represents the beat frequency noise of signal to AWGN.

The second photoelectric current in the kth symbol block can be represented by formula (7). Through the subtraction operation of Equation (5) and Equation (7), the photocurrent in the kth symbol block can be obtained as follows:

(8)

Among them, the first term represents the signal, and the second term represents the noise. Comparing Equation (8) and Equation (5), it can be found that the DC component of the signal and the crosstalk noise of the quadratic term can be completely eliminated, and the strength of the received signal is doubled, that is, the receiver sensitivity can be increased by 3dB. It can be seen that the use of phase-shift coding and differential decoding methods can effectively improve the signal-to-noise ratio of the system, thereby improving system performance.

2. Transmission experiment

In order to test the feasibility of this method in the visible light communication system, we carried out a transmission verification experiment using the implementation device of the direct modulation PAM system based on phase-shift coding. The device schematic diagram and system framework diagram of the system are shown in Figure 2 and Figure 3 respectively. The realization device includes a transmitting end and a receiving end.

The transmitting end includes a phase shift encoder, a PAM mapping module, a serial-parallel/parallel-serial conversion module, an LED and its driving module, and the like. Contains the following steps:

Step 101: the PAM mapping module performs PAM mapping on the original binary bit stream to generate a multi-level PAM signal;

Step 102: the upsampling module performs 2 times upsampling on the mapped PAM symbols, preparing for the serial-to-parallel conversion of the signal;

Step 103: the serial-to-parallel conversion module performs serial-to-parallel conversion on the signal, divides the serial signal into two parallel signals, and prepares for phase-shift coding;

Step 104: a phase shift encoder, inverting the input second signal and delaying it by T/2 bits;

Step 105: the parallel-to-serial conversion module merges the two parallel signals into one signal, so that each symbol signal is followed by its inverse signal;

Step 106: an up-sampling module, which up-samples the coded signal for input into an arbitrary waveform generator (AWG).

Step 107: LED and its driver module, the signal emitted by the AWG passes through the pre-equalization board, the electric amplifier and the AC-DC coupler, and the emission signal and bias voltage are coupled to the red, green and blue (RGB) LED, and passed through the free Space launch out.

The receiving end includes a differential decoder, a PAM demapping module, a serial-to-parallel conversion module, a receiver module, and the like. Contains the following steps:

Step 201: The receiver module distinguishes the red, green, and blue transmission signals in three different bands through the focusing lens and optical filter, and then converts the received optical signal into an electrical signal through the receiver, and uses a high-speed sampling real-time oscilloscope to analyze the signal Acquisition to complete the next step of offline processing;

Step 202: the clock synchronization module performs clock synchronization on the signal collected by the oscilloscope, and finds the signal head;

Step 203: a normalization module, which normalizes the received signal with the average power;

Step 204: the down-sampling module performs 4 times down-sampling on the received signal;

Step 205: the serial-to-parallel conversion module converts the serial signal into two parallel signals to prepare for differential decoding;

Step 206: The differential decoder delays the parallel second signal by T/2 bits, and subtracts the delayed second signal from the first signal to obtain a decoded signal;

Step 207: the post-equalization module performs post-equalization on the decoded signal to compensate for the intersymbol interference of the signal;

Step 208: The PAM demapping module demaps the PAM signal into a binary bit stream.

The division of each step in this embodiment is only for clarity of description. During implementation, some steps can be combined into one step or split into multiple steps. As long as they contain the same logical relationship, they are all within the scope of protection of the present invention. Inside.

Those of ordinary skill in the art can understand that the above-mentioned embodiments are specific examples for realizing the present invention, and in practical applications, various changes can be made to it in form and details without departing from the spirit and spirit of the present invention. scope.

We tested the signal transmission of the above experimental setup in free space with a distance of 1~2m. FIG. 4 is a spectrum comparison chart of the phase-shift coded PAM signal proposed by the present invention and the uncoded PAM signal. It can be seen that the low-frequency components in the coded spectrogram are suppressed, and the high-frequency components are retained as much as possible. This spectral shape is conducive to resisting low-frequency attenuation in visible light channel transmission.

Figure 5 shows the uncoded PAM-8 received signal spectrum after 1m free space transmission. It can be seen that the spectrum has a large DC component, and there is strong signal fading and distortion at the low frequency of the spectrum. This is because visible light channels have significant signal attenuation at low frequencies. Moreover, the AC-DC coupler in this experimental device cannot pass the DC component of the signal, and the DC component in the received spectrum is a fixed-frequency noise component introduced by the square-law detection of the receiver. Therefore, the performance of the uncoded received signal is severely impaired and cannot be directly transmitted in the visible light channel.

Figure 6 shows the uncoded PAM-8 received signal spectrum after 1m free space transmission. The spectrum is relatively flat in the 400MHz bandwidth.

FIG. 7 is a signal spectrum of the PAM-8 directly decoded by the phase shift coding proposed by the present invention. This spectrum has a DC component introduced by the square-law detection of the receiver, and richer low-frequency noise.

Fig. 8 is the signal spectrum after PAM-8 differential decoding of the phase shift coding proposed by the present invention. There is no DC component in this frequency spectrum, and the noise intensity is greatly suppressed.

Fig. 9 is a constellation diagram of a PAM-8 signal without encoding, phase shift encoding and direct decoding, and phase shift encoding and differential decoding. It can be seen that the performance of the uncoded signal is the worst, and the original signal cannot be solved at all; the signal performance of the phase-shifted S-coded and directly decoded signal is next; the signal performance of the phase-shifted coded and differential decoded signal is the best, and the original signal can be recovered.

The present invention also tests the bit error rate performance of the signals transmitted by red, green, and blue colors under the conditions of direct decoding and differential decoding at different rates, as shown in Fig. 10, Fig. 11, and Fig. 12 respectively. It can be seen that under the 7% FEC threshold, the total rate of differential decoding signals can reach 3.375Gb/s (red light: 1.35Gb/s; green light: 0.975Gb/s; blue light: 1.05Gb/s); direct decoding The total signal rate can reach 1.8Gb/s (the rates of the three colors of light are all 600Mb/s). Therefore, through differential decoding, the rate is increased by 1.575Gb/s.

In addition, the change of bit error rate performance at different distances for signals transmitted by red, green, and blue colors was also tested, as shown in Figure 13, Figure 14, and Figure 15, respectively. Under the transmission distance of 1m, 1.5m and 2m, the transmission rates are 3.375Gb/s, 3.225-Gb/s and 2.925-Gb/s respectively.

The above transmission experiments have verified the feasibility of the phase-shift coding scheme and implementation device in the visible light communication system based on PAM modulation and the suppression effect on system noise, and its system performance can be greatly improved.

Claims (4)

1. A phase-shift coding method based on direct modulation and direct detection of pulse amplitude modulation systems, characterized in that the parity symbols of the original signal adopt the phase-shift coding method of opposite signs, and the received signal adopts the differential decoding method of delay and subtraction; The specific performance is: control the low-frequency component of the spectrum of the PAM signal transmitted in the baseband, and suppress the DC component and the crosstalk noise between the received signals. 2. phase-shift coding method as claimed in claim 1, is characterized in that: At the transmitter, the continuous transmitted signal is divided into different symbol blocks, each symbol block contains two adjacent symbols, and the second symbol is the inverse of the first symbol; where, the first symbol at the transmitter Baseband PAM signal within k symbol blocks and Respectively expressed as: in, A symbol representing a PAM map; At the receiving end, the photocurrent in the k-th symbol block Expressed as: Among them, the coefficient Indicates the strength ratio of the PAM baseband signal; and Respectively represent the bias voltage and reverse voltage of the LED; and denote the noise currents in the 2k-1 and 2kth symbols in the k-th symbol block, respectively. 3. A realization device of a phase-shift coding method based on direct modulation and direct detection of a pulse amplitude modulation system, including a corresponding transmitter and a receiver; it is characterized in that: a phase-shift encoder is included in the transmitter, and a PAM mapping module , serial-parallel/parallel-serial conversion module, up-sampling module, LED and its drive module; the receiving end includes a differential decoder, a serial-parallel conversion module, a post-equalization module, a PAM demapping module, a clock synchronization module, and a normalization module, downsampling module, focusing lens, optical filter and receiver; wherein: The phase shift encoder is used to phase shift encode the signal; The PAM mapping module is used for encoding and modulating signals; The serial-parallel/parallel-serial conversion module is used for sequence conversion before and after signal encoding; The upsampling module is used for upsampling of signals; The LED and its driving module are used to load the signal onto the LED and emit it through free space; The differential decoder is used to differentially decode signals; The serial-to-parallel conversion module is used for sequence conversion after signal decoding; The post-equalization module is used to equalize the decoded signal; The PAM demapping module is used to demodulate the PAM signal into a binary bit stream; The clock synchronization module is used for clock extraction and synchronization of received signals; The normalization module is used to normalize the average power of the received signal; The down-sampling module is used for down-sampling of signals; The focusing lens, optical filter and receiver are used for receiving optical signals and converting optical signals into electrical signals. 4. The realization device according to claim 3, characterized in that: the working process of the device is as follows: On the transmitter side: (1) The PAM mapping module performs PAM mapping on the original binary bit stream to generate a multi-level PAM signal; (2) The upsampling module performs 2 times upsampling on the mapped PAM symbols to prepare for the serial-to-parallel conversion of the signal; (3) The serial-to-parallel conversion module performs serial-to-parallel conversion on the signal, divides the serial signal into two parallel signals, and prepares for phase-shift encoding; (4) The phase shift encoder inverts the input signal of the second channel and delays it by T/2 bits; (5) The parallel-to-serial conversion module combines two parallel signals into one signal, so that each symbol signal is followed by its inverse signal; (6) The up-sampling module up-samples the encoded signal for input into an arbitrary waveform generator (AWG); (7) The LED and its drive module pass the signal emitted by the AWG through the pre-balance board, the electric amplifier and the AC-DC coupler, and couple the emission signal and bias voltage to the red, green and blue (RGB) LED, and pass through the free space launch out; On the receiving end: (1) The receiver distinguishes the red, green and blue transmission signals in three different bands through the focusing lens and optical filter, and then converts the received optical signal into an electrical signal through the receiver, and collects the signal with a high-speed sampling real-time oscilloscope To complete the next step of offline processing; (2) The clock synchronization module performs clock synchronization on the signal collected by the oscilloscope, and finds the signal head; (3) The normalization module normalizes the received signal with the average power; (4) The downsampling module performs 4 times downsampling on the received signal; (5) The serial-to-parallel conversion module converts the serial signal into two parallel signals to prepare for differential decoding; (6) The differential decoder delays the parallel second signal by T/2 bits, and subtracts the delayed second signal from the first signal to obtain the decoded signal; (7) The post-equalization module performs post-equalization on the decoded signal to compensate for the intersymbol interference of the signal; (8) The PAM demapping module demaps the PAM signal into a binary bit stream.