CN104539363B - Self-adaptive transmission method for multicolor LED visible light communication - Google Patents

Abstract

The invention discloses an adaptive transmission method for multi-color LED visible light communication. When the available transmission bandwidth of the multi-color LED is unknown and the change of the signal-to-interference ratio exceeds a threshold during normal communication, the configuration negotiation state is entered; in the configuration negotiation state, by sending The preamble sequence estimates the available bandwidth and effective signal-to-interference ratio, adopts the orthogonal frequency division multiplexing virtual subcarrier method to realize unequal bandwidth transmission on different colors, selects the error correction coding rate according to the equivalent signal-to-interference ratio of the system, and selects the code rate according to the equivalent signal-to-interference ratio of the system, and according to The equivalent signal-to-interference ratio of each subcarrier within the available bandwidth is selected by an appropriate modulation method to achieve adaptive transmission close to the channel capacity; after the configuration negotiation is completed, the system enters the normal communication state for communication transmission. The present invention introduces virtual subcarriers to enable OFDM modulation with the same subcarrier spacing on different colors, which simplifies implementation complexity, selects coding code rate and subcarrier modulation through equivalent signal-to-interference ratio, and effectively improves frequency band utilization and power efficiency.

Description

A self-adaptive transmission method for multi-color LED visible light communication

technical field

The invention relates to a multi-color LED visible light communication adaptive transmission method, which belongs to the visible light communication technology.

Background technique

Wireless Optical Communication (OWC) has attracted the attention of researchers in the field of communication due to its rich and unlimited spectrum resources, high transmission power can improve the acceptance signal-to-noise ratio, low electromagnetic radiation level, and good security, and has gradually become a research hotspot. . In recent years, visible light communication technology has developed rapidly. IEEE has formulated the IEEE802.15.7 standard, and carried out the implementation methods and parameter indicators of its physical layer (PHY, Physics) and medium access control layer (MAC, Medium Access Control). standardization. In order to improve the transmission rate of the wireless optical communication system and combat multipath interference, the OFDM modulation method was introduced. The researchers found that OFDM is an effective way to improve the performance of the wireless optical communication system from theory and experiments. Most of the current wireless optical OFDM communication systems are designed for a specific channel to ensure normal communication even under the worst channel conditions, but the communication rate is not different between the bad channel environment and the good channel environment. The system cannot turn good channel conditions into a transmission rate gain.

Since the current wireless optical communication system generally adopts intensity modulation/direct detection (IM/DD), it is necessary to ensure the authenticity of the OFDM time domain signal. Two optical communication OFDM structures widely used now are asymmetric clipped optical OFDM (ACO-OFDM) and DC biased optical OFDM (DCO-OFDM). ACO-OFDM is an improvement of traditional OFDM in optical communication special channel transmission. It was proposed by Australian scientist Jean Armstrong in 2006. Its basic idea is to use more subcarriers to exchange some properties of Fourier transform for transmitting symbols in the time domain. Satisfies the characteristics of the optical channel, its frequency domain symbols satisfy conjugate symmetry and the values on the even subcarriers are all 0, only 1/4 of the subcarriers are used to send effective symbols, so the spectrum utilization rate is relatively low; but it does not Adding DC bias can improve energy utilization. The frequency domain symbols of DCO-OFDM also satisfy conjugate symmetry, but it does not require the value on even subcarriers to be 0, 1/2 subcarriers are used to transmit effective symbols, and the spectrum utilization rate is high, but it is used to make the time domain The signal is a positive value, and an additional DC bias needs to be added to the signal output by the IFFT, which reduces the energy utilization rate.

White LEDs can be divided into monocrystalline and polycrystalline according to the number of light-emitting diodes. The common monocrystalline white light LED uses a single piece of blue light LED to excite yellow phosphor to produce white light. The efficiency is lower. Common polycrystalline white light LEDs are produced by mixing red, green, blue, yellow and other colors of LEDs to produce white light. Compared with monocrystalline white light LEDs, their color rendering index and luminous efficiency are greatly improved, but its The technology is relatively complex and the cost is high at this stage. These two kinds of LEDs can be used for visible light communication, and can also modulate the diodes of various colors of polycrystalline LED lamps separately, and then add filters at the receiving end to obtain multiple relatively independent channels, realizing multi-band visible light Communication, improve the transmission rate. However, the channel characteristics of the red, green, blue, and yellow sub-channels of polycrystalline LED lights are different, and the bandwidth of these sub-channels can be fully utilized through adaptive modulation technology.

Contents of the invention

Purpose of the invention: In order to overcome the deficiencies in the prior art, the present invention provides an adaptive transmission scheme suitable for multi-color LED visible light communication systems. Improve the transmission rate while ensuring the reliability of system communication.

Technical scheme: in order to achieve the above object, the technical scheme adopted in the present invention is:

An adaptive transmission method for multi-color LED visible light communication. The system includes two working states of configuration negotiation and normal communication. When the available transmission bandwidth of the multi-color LED is unknown and the signal-to-interference ratio change exceeds a threshold during normal communication, the system enters the configuration negotiation state. ; In the configuration negotiation state, the available bandwidth and effective SIR are estimated by sending the preamble sequence, and the Orthogonal Frequency Division Multiplexing (OFDM) virtual subcarrier method is used to realize unequal bandwidth transmission on different colors, according to the equivalent SIR of the system Select the code rate of error correction coding, and select the appropriate modulation method according to the equivalent signal-to-interference ratio of each subcarrier in the available bandwidth of each color to achieve adaptive transmission close to the channel capacity; after the configuration negotiation is completed, the system enters the normal communication state and performs communication transmission .

The present invention introduces virtual subcarriers so that OFDM modulation with the same subcarrier spacing is used on different colors, which greatly simplifies the implementation complexity, selects the coding code rate and subcarrier modulation through the equivalent signal-to-interference ratio, and effectively improves the frequency band utilization and power efficiency .

In the configuration negotiation state, it includes the following steps: first, the transmitter sends a specific pilot sequence known to the receiving end to the receiver; then, the receiver estimates the signal-to-interference ratio on each subcarrier according to the received pilot sequence , get the transmission parameters that adapt to the current system channel conditions, and send the transmission parameters to the receiver; then, after receiving the transmission parameters, the transmitter sets the transmitter according to the transmission parameters, and sends ACK to the receiver after the setting is successful; again, After the receiver receives the ACK signal, it sets the receiver according to the transmission parameters, and sends ACK to the transmitter after the setting is successful. The transmitter receives the ACK, indicating that the parameter configuration is completed, and the system enters the normal communication state; finally, the receiver and the transmitter communicate normally. data transmission.

The method specifically includes the following steps:

(1) After the system is turned on, it enters the configuration negotiation state. The transmitter first puts a specific ZC sequence known to the receiving end on each subcarrier on each color (such as RGB three-color) sub-channel. The ZC sequence is conjugated and symmetrical and fast Fourier transform (IFFT transform) and convert it into a ZC sequence time-domain signal and send it out; where the ZC sequence is the leading sequence, and its expression is:

In the formula: r is the root index of the ZC sequence, which is mutually prime with the length N _ZC of the ZC sequence; k=0,1,...,N _ZC -1, q is any integer; for simplicity, q can be 0, The possible value of N _ZC is the number of subcarriers N _SC of the OFDM system;

(2) Each color sub-channel of the receiver uses the same ZC sequence time-domain signal to correlate with the received signal sequence, and the obtained correlation peak is the synchronization peak. The formula is as follows:

In the formula: y(n) represents the received signal sequence, x _ZC (n) represents the known ZC sequence time domain signal, p(d) is the cross-correlation function of y(n) and x _ZC (n), d is the independent variable of the cross-correlation function, N is the length of the ZC sequence, and n represents the position of the received signal; since the ZC sequence has good autocorrelation characteristics, the peak value of p(d) is used as the synchronization head of the ZC sequence, according to the synchronization head from Extract the ZC sequence time domain signal y _ZC (n) through the channel from the received signal sequence;

(3) Perform fast Fourier transform (FFT transform) on the y _ZC (n) received by each color sub-channel of the receiver to obtain the frequency domain signal Y _ZC (n) of the ZC sequence, and use the LS algorithm to obtain the frequency domain of the channel response:

In the formula: H(n) is the channel coefficient corresponding to the nth subcarrier, and X _ZC and Y _ZC represent the frequency-domain ZC sequences that have not passed the channel and passed the channel, respectively;

(4) Reconstruct the estimated value of the received sequence according to the channel parameters H and X _ZC of each color sub-channel:

Then the noise interference variance on each subcarrier is The estimated signal-to-interference ratio of each subcarrier is obtained as There is a certain gap between the estimated value of this SINR and the real value, and it needs to be corrected according to the prior information to obtain a more accurate estimated value of SINR; the modulation order of each subcarrier is selected according to the bit error rate requirement of the predetermined channel coding rate , setting the subcarriers whose SINR value is lower than the predetermined lower limit of SINR as virtual subcarriers, that is, no data is transmitted on these subcarriers, and then the receiver sends the configuration information to the transmitter through a signaling channel;

(5) After the transmitter receives the configuration information and confirms that the configuration information is correct through CRC, it sends an ACK signal to the receiver;

(6) The receiver returns an ACK to the transmitter after receiving the ACK, and the transmitter confirms that the configuration negotiation between the two parties is completed after receiving the ACK, enters the normal communication stage, and starts sending user data with the negotiated configuration;

(7) The receiving end receives user data normally, and tracks and monitors the signal-to-interference ratio (SINR) of the system. If the SINR changes significantly, it re-enters the configuration negotiation state, negotiates the configuration again, and realizes the adaptive update of transmission parameters and channels.

The present invention is applicable not only to multi-color LED visible light communication but also to single-color LED and infrared visible light communication systems. When the color participating in the transmission at the same time is selected as one, the scheme automatically becomes a single-color LED self-adaptive transmission scheme.

Beneficial effects: The self-adaptive transmission method for multi-color LED visible light communication provided by the present invention has the following advantages: 1. Bandwidth self-adaptation, and a transmission rate close to the channel capacity can be obtained; 2. The transmission bandwidth is determined by virtual subcarrier technology without changing The hardware greatly simplifies the system design; 3. It can adapt to different LED lights and different application scenarios, expanding the scope of use of visible light communication; 4. When the channel characteristics change, it can automatically re-configure and negotiate to restore normal communication , which enhances the robustness of the system.

Description of drawings

Fig. 1 is a system working state transition diagram of an adaptive transmission scheme suitable for a multi-color LED visible light communication system proposed by the present invention;

Figure 2 is a schematic diagram of the response sequence of the transmitter and receiver of the adaptive transmission scheme applicable to the multi-color LED visible light communication system;

Fig. 3 is the input and output schematic diagram of adaptive parameter selection algorithm of the present invention;

Figure 4 shows the signal-to-interference-noise ratio of a group of RGB tricolors tested on the hardware platform. Frequency in the figure represents the frequency; 4(a) represents red, 4(b) represents green, and 4(c) represents blue.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figure 1, it is an adaptive transmission method for multi-color LED visible light communication. The system includes two working states: configuration negotiation and normal communication. When the available transmission bandwidth of multi-color LED is unknown and the normal communication occurs, the change of signal-to-interference ratio exceeds the threshold. In the configuration negotiation state, the available bandwidth and effective signal-to-interference ratio are estimated by sending the preamble sequence, and the orthogonal frequency division multiplexing (OFDM) virtual subcarrier method is used to realize unequal bandwidth transmission on different colors. The system equivalent signal-to-interference ratio selects the error correction code rate, and selects the appropriate modulation method according to the equivalent signal-to-interference ratio of each subcarrier in the available bandwidth of each color to achieve adaptive transmission close to the channel capacity; after the configuration negotiation is completed, the system enters normal Communication state, carry out communication transmission.

Figure 2 shows the detailed response sequence diagram of the transmitter and receiver of the system: first, the transmitter sends a specific pilot sequence known to the receiving end to the receiver; then, the receiver estimates the The signal-to-interference ratio on each subcarrier is obtained to adapt to the transmission parameters under the current system channel conditions, and the transmission parameters are sent to the receiver; then, after the transmitter receives the transmission parameters, it sets the transmitter according to the transmission parameters, and after the setting is successful Send ACK to the receiver; again, after the receiver receives the ACK signal, set the receiver according to the transmission parameters, send ACK to the transmitter after the setting is successful, and the transmitter receives the ACK, indicating that the parameter configuration is completed, and the system enters the normal communication state; finally , the receiver and transmitter perform normal data transmission.

As shown in Figure 1, the method specifically includes the following steps:

(1) After the system is turned on, it enters the configuration negotiation state. The transmitter first puts a specific ZC sequence known to the receiving end on each subcarrier on each color (such as RGB three-color) sub-channel. The ZC sequence is conjugated and symmetrical and fast Fourier transform (IFFT transform) and convert it into a ZC sequence time-domain signal and send it out; where the ZC sequence is the leading sequence, and its expression is:

In the formula: r is the root index of the ZC sequence, which is mutually prime with the length N _ZC of the ZC sequence; k=0,1,...,N _ZC -1, q is any integer; for simplicity, q can be 0, The possible value of N _ZC is the number of subcarriers N _SC of the OFDM system;

(2) Each color sub-channel of the receiver uses the same ZC sequence time-domain signal to correlate with the received signal sequence, and the obtained correlation peak is the synchronization peak. The formula is as follows:

In the formula: y(n) represents the received signal sequence, x _ZC (n) represents the known ZC sequence time domain signal, p(d) is the cross-correlation function of y(n) and x _ZC (n), d is the independent variable of the cross-correlation function, N is the length of the ZC sequence, and n represents the position of the received signal; since the ZC sequence has good autocorrelation characteristics, the peak value of p(d) is used as the synchronization head of the ZC sequence, according to the synchronization head from Extract the ZC sequence time-domain signal y _ZC (n) through the channel from the received signal sequence;

(3) Perform fast Fourier transform (FFT transform) on the y _ZC (n) received by each color sub-channel of the receiver to obtain the frequency domain signal Y _ZC (n) of the ZC sequence, and use the LS algorithm to obtain the frequency domain of the channel response:

In the formula: H(n) is the channel coefficient corresponding to the nth subcarrier, and X _ZC and Y _ZC represent the frequency-domain ZC sequences that have not passed the channel and passed the channel, respectively;

(4) Reconstruct the estimated value of the received sequence according to the channel parameters H and X _ZC of each color sub-channel:

Then the noise interference variance on each subcarrier is The estimated signal-to-interference ratio of each subcarrier is obtained as There is a certain gap between the estimated value of this SINR and the real value, and it needs to be corrected according to the prior information to obtain a more accurate estimated value of SINR; the modulation order of each subcarrier is selected according to the bit error rate requirement of the predetermined channel coding rate , setting the subcarriers whose SINR value is lower than the predetermined lower limit of SINR as virtual subcarriers, that is, no data is transmitted on these subcarriers, and then the receiver sends the configuration information to the transmitter using a signaling channel;

(5) After the transmitter receives the configuration information and confirms that the configuration information is correct through CRC, it sends an ACK signal to the receiver;

(6) The receiver returns an ACK to the transmitter after receiving the ACK, and the transmitter confirms that the configuration negotiation between the two parties is completed after receiving the ACK, enters the normal communication stage, and starts sending user data with the negotiated configuration;

(7) The receiving end receives user data normally, and tracks and monitors the signal-to-interference ratio (SINR) of the system. If the SINR changes significantly, it re-enters the configuration negotiation state, negotiates the configuration again, and realizes the adaptive update of transmission parameters and channels.

Fig. 3 shows a flow chart of a transmission system using RGB three colors. As shown in Figure 3, the adaptive modulation parameter optimization algorithm is determined by the SINR on each subcarrier, the BER limit of the system, the rated transmission power, and the RGB color mixing requirements. The light should be guaranteed to be white light, which is also the basic requirement of visible light communication.

Figure 4 shows a set of RGB three-color signal-to-interference-noise ratios tested on the hardware platform. If we set the working bandwidth of the system to 50M (the actual sampling frequency is 100M, the first 50M is the working frequency band, and the latter 50M is Conjugate symmetrical frequency band), assuming that the modulation method is limited to 256QAM, 64QAM, 16QAM, 4QAM. The system has a total of 128 subcarriers. In order to simplify the system hardware implementation, the subcarriers are divided into 8 groups, and each group has 16 subcarriers. Because the DC blocking capacitors on the analog circuit will affect the performance of several low-frequency subcarriers, the fixed settings are 0~3 The numbered subcarriers are virtual subcarriers. The modulation scheme obtained by the present invention is shown in Table 1. This is because the low-frequency signal-to-interference-noise ratio of the red light channel is significantly higher than that of green light and blue light, and a higher-order modulation method can be used, and the SINR of the frequency band from 40M to 50M is too low, so it should be set as a virtual subcarrier.

Table 1 RGB three sub-channel modulation schemes

subcarrier number 0～3 4～15 16～63 64～95 96～111 112～127 Red virtual subcarrier 256QAM 64QAM 16QAM 4QAM virtual subcarrier Green virtual subcarrier 64QAM 64QAM 16QAM 4QAM virtual subcarrier Blue virtual subcarrier 64QAM 64QAM 16QAM 4QAM virtual subcarrier

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also possible. It should be regarded as the protection scope of the present invention.

Claims (3)

1. A multi-color LED visible light communication adaptive transmission method, characterized in that: the system includes two working states of configuration negotiation and normal communication, and the signal-to-interference ratio change exceeds a threshold when the available transmission bandwidth of the multi-color LED is unknown and normal communication occurs In the configuration negotiation state, the available bandwidth and effective signal-to-interference ratio are estimated by sending the preamble sequence, and the orthogonal frequency division multiplexing virtual subcarrier method is used to realize the transmission of different bandwidths on different colors, which is equivalent to the system Signal-to-interference ratio Select the error-correcting code rate, and select the appropriate modulation method according to the equivalent signal-to-interference ratio of each subcarrier in the available bandwidth of each color to achieve adaptive transmission close to the channel capacity; after the configuration negotiation is completed, the system enters the normal communication state, Carry out communication transmission. 2. The multi-color LED visible light communication adaptive transmission method according to claim 1, characterized in that: in the configuration negotiation state, it comprises the following steps: first, the transmitter sends a specific pilot sequence known to the receiving end to the receiver ; Then, the receiver estimates the signal-to-interference ratio on each subcarrier according to the received pilot sequence, obtains the transmission parameters adapted to the current system channel conditions, and sends the transmission parameters to the receiver; then, the transmitter receives After receiving the transmission parameters, set the transmitter according to the transmission parameters, and send ACK to the receiver after the setting is successful; again, after the receiver receives the ACK signal, set the receiver according to the transmission parameters, and send ACK to the transmitter after the setting is successful, and the transmitter receives When ACK is received, it means that the parameter configuration is completed, and the system enters the normal communication state; finally, the receiver and the transmitter perform normal data transmission. 3. The multi-color LED visible light communication adaptive transmission method according to claim 1, characterized in that: the method specifically comprises the following steps: (1) After the system is turned on, it enters the configuration negotiation state. The transmitter first puts a specific ZC sequence known to the receiver on each subcarrier on each color subchannel. The ZC sequence is conjugated and symmetric and fast Fourier transformed. After that, it is converted into a ZC sequence time-domain signal and sent out; where the ZC sequence is the leading sequence, and its expression is: In the formula: r is the root index of the ZC sequence, which is mutually prime with the length N _ZC of the ZC sequence; k=0,1,...,N _ZC -1, q is any integer; (2) Each color sub-channel of the receiver uses the same ZC sequence time-domain signal to correlate with the received signal sequence, and the obtained correlation peak is the synchronization peak. The formula is as follows: $\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; d</span>}}{\overset{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}\mathrm{<span\; class="notranslate">\; the\; y</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span>$ In the formula: y(n) represents the received signal sequence, x _ZC (n) represents the known ZC sequence time domain signal, p(d) is the cross-correlation function of y(n) and x _ZC (n), d is the independent variable of the cross-correlation function, N _ZC is the length of the ZC sequence, and n represents the position of the received signal; the peak value of p(d) is used as the synchronization head of the ZC sequence, and the channel is extracted from the received signal sequence according to the synchronization head ZC sequence time domain signal y _ZC (n) after that; (3) Perform fast Fourier transform on the y _ZC (n) received by each color sub-channel of the receiver to obtain the frequency domain signal Y _ZC (n) of the ZC sequence, and use the LS algorithm to obtain the frequency domain response of the channel: $\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; Y</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span>}$ In the formula: H(n) is the channel coefficient corresponding to the nth subcarrier, X _ZC (n) and Y _ZC (n) represent the frequency-domain ZC sequence without channel and channel, respectively; (4) Reconstruct the estimated value of the received sequence according to the channel parameters H(n) and X _ZC (n) of each color sub-channel: Y _ZC (n)＝H(n) X _ZC (n) Then the variance of noise interference on each subcarrier is σ ² (n)=|Y _ZC (n)-Y _ZC (n)| ² , and the estimated value of the signal-to-interference ratio of each subcarrier is SINR(n)=σ ² (n)·|Y _ZC (n)| ^-2 , there is a certain gap between the estimated value of the SINR and the real value, and it needs to be corrected according to the prior information to obtain a more accurate estimated value of the SINR; according to the predetermined channel coding code The bit error rate of the bit error rate requires the selection of the modulation order of each subcarrier, and the subcarriers whose SINR value is lower than the predetermined SINR lower limit are set as virtual subcarriers, that is, no data is transmitted on these subcarriers, and then the receiver uses the configuration information as The signaling channel is sent to the transmitter; (5) After the transmitter receives the configuration information and confirms that the configuration information is correct through CRC, it sends an ACK signal to the receiver; (6) The receiver returns an ACK to the transmitter after receiving the ACK, and the transmitter confirms that the configuration negotiation between the two parties is completed after receiving the ACK, enters the normal communication stage, and starts sending user data with the negotiated configuration; (7) The receiving end receives user data normally, and tracks and monitors the signal-to-interference ratio (SINR) of the system. If the SINR changes significantly, it re-enters the configuration negotiation state, negotiates the configuration again, and realizes the adaptive update of transmission parameters and channels.