CN108337047B - Time domain hybrid modulation method based on visible light communication system - Google Patents
Time domain hybrid modulation method based on visible light communication system Download PDFInfo
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- CN108337047B CN108337047B CN201810244156.2A CN201810244156A CN108337047B CN 108337047 B CN108337047 B CN 108337047B CN 201810244156 A CN201810244156 A CN 201810244156A CN 108337047 B CN108337047 B CN 108337047B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
- H04B10/114—Indoor or close-range type systems
- H04B10/116—Visible light communication
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/524—Pulse modulation
Abstract
The invention discloses a time domain hybrid modulation method based on a visible light communication system, wherein the time domain hybrid modulation refers to properly designing a time division multiplexing frame to adjust the time slot duty ratios of different modulation formats in the frame. By taking the application of time domain hybrid modulation in optical fiber communication as reference, the idea of time domain hybrid modulation is provided in visible light communication. The time domain hybrid modulation in the invention adopts common on-off keying modulation and pulse position modulation, solves the problem of adopting modulation multiplexing equipment with complex technology and extremely high cost in VLC, simplifies the complexity of the modulation equipment and ensures better communication performance. In practical applications, especially in the case of VLC as an auxiliary communication system, this is of great significance.
Description
Technical Field
The invention relates to a Time domain Hybrid Modulation method based on a visible light communication system, in particular to a Time domain Hybrid Modulation method (TDHM) which is applied to an indoor visible light communication system and belongs to the technical field of visible light communication.
Background
The LED lamp has the characteristics of long service life, energy conservation and environmental protection, and is a main lighting source of the next generation. And visible light communication with which communication is performed is receiving increasing attention. The advantages of the visible light communication, such as environmental protection, no need of spectrum authentication, and high transmission rate, have led to a hot research trend in the global field.
In fact, visible light communication is being developed vigorously as an effective complementary scheme to radio frequency communication. Currently, the common modulation mode intensity modulation-direct detection for visible light communication system VLC is to directly load a modulation electrical signal onto the intensity of light. The characteristics of the light intensity determine that the electrical signal to be transmitted must also be "positive real". That is, pulse modulation has the advantage of being unique to the choice of signal pulse shaping in visible light systems. The carrier modulation adopted in visible light communication requires that the carrier frequency is higher than the bandwidth, so that the lowest frequency is lost, and because the intensity modulation-direct detection is adopted in visible light communication, the required modulation signal must be a positive real signal, so that the information is easily lost by adopting the carrier modulation. Compared with carrier modulation, baseband modulation meets the requirement that a modulation signal in visible light communication is a positive real signal without losing information, and therefore pulse modulation schemes such as on-off keying modulation (OOK) and Pulse Position Modulation (PPM) are often used in early researches on visible light communication systems.
OOK is the most widely reported IM/DD modulation technique in visible light communications. This is because it is a very straightforward solution in modulation techniques, bit 1 is simply represented by an optical pulse, which may occupy the entire bit period or be a part of the bit period; bit 0 is represented by a bit period without a light pulse. Although the OOK modulation scheme is simple and easy to operate, has low bandwidth requirements, high bandwidth utilization, and strong anti-interference capability, the power utilization and the system transmission rate are low. PPM is the most attractive option for visible light communication because of its significantly better power efficiency. PPM is a quadrature modulation technique and a member of the pulse modulation family, a form of modulation that maps information bits onto pulse positions. In modern usage terminology, a PPM signal for an L-slot, has one and only one slot bit containing one pulse. And the input information bits determine which bit of the L slot is applied. When L >2, the modulation of any L-PPM, i.e. L slots, requires less optical power than OOK, and when L is arbitrarily large, the power required by L-PPM theoretically can be arbitrarily small. And because OOK occupies a very small frequency band, PPM occupies a very large frequency band, so PPM has higher receiving sensitivity compared with OOK. In terms of both positioning and communication, a low rate can satisfy the positioning requirement, while a higher sensitivity is required in communication. VLC is positioned in an indoor auxiliary communication system, simple modulation equipment is additionally arranged on the existing lighting system, indoor wireless data transmission can be realized, and the VLC has the characteristics of low cost, simplicity, energy conservation and the like, and modulation multiplexing equipment with complex technology and extremely high cost is obviously not suitable for the positioning.
Disclosure of Invention
The purpose of the invention is as follows: the technical problem to be solved by the present invention is to provide a time domain hybrid modulation scheme based on a visible light communication system for the deficiency of the background art, which is a modulation technique capable of providing flexible data rate, solves the problem of using modulation multiplexing equipment with complex technique and extremely high cost in VLC, simplifies the complexity of the modulation equipment and ensures better communication performance. In practical applications, especially in the case of VLC as an auxiliary communication system, this is of great significance.
The technical scheme is as follows: a time domain hybrid modulation method based on a visible light communication system specifically comprises the following steps:
step 1, generating a string of random original bit streams;
step 2, converting the original bit stream into an M + K system bit stream through serial-to-parallel conversion;
step 3, the K system bit stream in each group of M + K system bit streams is subjected to L-PPM coding;
step 4, respectively passing the result subjected to L-PPM coding mapping and the M-system bit stream through a transmitting filter, generating signal pulses on corresponding time slots, integrating the signal pulses to form a TDM frame, and transmitting the TDM frame, wherein the TDM frame is composed of N time slots, and N is a natural number greater than 2;
and 5, changing the values of M and K in the steps 2, 3 and 4, repeating the steps 1-4, and comparing the signal-to-noise ratio and the bit error rate of the signal obtained by the system using different time domain hybrid modulation receiving ends.
Further, in step 2, the values of M and K in the M + K binary bit stream obtained by serial-to-parallel conversion of the original bit stream can be set.
Further, in step 3, the K-ary bit stream in the M + K-ary bit streams of each group obtained by serial-to-parallel conversion is subjected to L-PPM coding, and the K-ary bit stream is mapped into an L-PPM slot sequence.
Further, the time slot number L of the L-PPM time slot sequence is 2^ K.
Further, in step 4, the transmit filter outputs a rectangular impulse response waveform.
Further, in step 4, "1" in the M-ary bit stream represents a signal, and "0" represents no signal.
Has the advantages that:
1. the invention provides the idea of forming time domain hybrid modulation by two modulation modes of on-off keying modulation (OOK) and Pulse Position Modulation (PPM) on the basis of single modulation of an indoor visible light communication system. Compared with OOK and PPM which are simple to operate, the time domain hybrid modulation has the advantages of OOK frequency spectrum efficiency and PPM power utilization rate, the transmission rate of the system is effectively improved, and the anti-interference capability of the system is improved.
2. The method solves the problem of adopting modulation multiplexing equipment with complex technology and extremely high cost in VLC, simplifies the complexity of the modulation equipment, ensures better communication performance, and has important significance in practical application, especially under the condition that VLC is used as an auxiliary communication system
Drawings
FIG. 1 is a schematic block diagram of an application of time domain hybrid modulation in an indoor visible light communication system;
FIG. 2 is a practical block diagram of time domain hybrid modulation;
fig. 3 is a schematic block diagram of time domain hybrid modulation.
Detailed Description
The invention is further explained below with reference to the drawings.
The invention provides a time domain hybrid modulation method based on a visible light communication system, which is an improvement on the modulation scheme of the existing visible light communication system. The time domain hybrid modulation here refers to properly designing a time division multiplexing frame to adjust the duty ratio of time slots in the frame for different modulation formats. By taking the application of time domain hybrid modulation in optical fiber communication as reference, the idea of Time Domain Hybrid Modulation (TDHM) is proposed in visible light communication. The time domain hybrid modulation applied in the optical fiber communication uses Quadrature Amplitude Modulation (QAM), but the time domain hybrid modulation based on the visible light communication system in the present invention uses the common on-off keying modulation (OOK) and Pulse Position Modulation (PPM). The method solves the problem of adopting modulation multiplexing equipment with complex technology and extremely high cost in VLC, simplifies the complexity of the modulation equipment and ensures better communication performance. In practical applications, especially in the case of VLC as an auxiliary communication system, this is of great significance.
The dotted line in fig. 1 indicates the details of fig. 2, and the OOK/PPM time domain hybrid modulation electrical signal is generated after the time domain hybrid modulation of fig. 2, and then the time domain hybrid modulation signal is converted into an optical signal through a driving circuit and an LED and transmitted in an indoor wireless channel, and the optical signal is received by a PD at a receiving end, and finally the optical signal is converted into an electrical signal and then passes through a matched filter, and then the original bit stream information is recovered through a decision or the like.
FIG. 2 is a block diagram of a time-domain hybrid modulation, which is implemented by generating a random original bit stream, converting the original bit stream into M + K bit streams by serial-to-parallel conversion, L-PPM encoding the K bit streams in each set of M + K bit streams, respectively passing the L-PPM encoded and mapped result and M bit streams through an emission filter, generating signal pulses in corresponding time slots, and integrating the signal pulses to form a TDM frame for transmission.
Fig. 3 is a schematic block diagram of time domain hybrid modulation, i.e. a time division multiplexing frame is properly designed, and an OOK/PPM time domain hybrid modulation signal is formed by adjusting the time slot duty ratios of OOK and PPM in the frame.
In many designs of modulation schemes for visible light communication, simple pulse modulation schemes such as OOK and PPM are especially common. In the literature about visible light communication, the literature proposes a single modulation concept from the time domain perspective, but the advantages and disadvantages of multiple modulation schemes cannot be combined for the single modulation. Although the OOK modulation scheme is simple and easy to operate, low in bandwidth requirement, high in frequency band utilization rate and strong in anti-interference capability, the power utilization rate and the system transmission rate are low; PPM is the most attractive option for visible light communication because of its significantly better power efficiency, whereas PPM modulation techniques require slot and symbol synchronization for the receiver to demodulate the signal, albeit at an increased power efficiency over OOK, but at the cost of increased bandwidth requirements and more complexity. The invention combines the advantages of two modulation schemes, adopts time domain hybrid modulation, namely properly designs a time division multiplexing frame, and forms time domain hybrid quadrature amplitude modulation by adjusting the time slot duty ratio of different modulation formats in the frame. The invention provides the idea of forming time domain hybrid modulation by two modulation modes of OOK and PPM on the basis of single modulation of an indoor visible light communication system. Compared with OOK and PPM which are simple to operate, the time domain hybrid modulation has the advantages of OOK frequency spectrum efficiency and PPM power utilization rate, the transmission rate of the system is effectively improved, and the anti-interference capability of the system is improved.
The invention proposes the idea of time-domain hybrid modulation, as shown in fig. 2. Under the condition of ensuring that the system has certain anti-interference capability, the transmission rate of the system is maximized.
A time domain hybrid modulation method based on a visible light communication system, as shown in fig. 2: the method specifically comprises the following steps:
step 1, generating a string of random original bit streams;
step 2, converting the original bit stream into an (M + K) system bit stream through serial-to-parallel conversion; the values M and K of M and K may not be fixed values, and M and K determine the ratio of OOK and PPM in a TDM frame, for example, M may be 2, and K may also be 2;
step 3, the K-system bit stream in each group of (M + K) -system bit streams is subjected to L-PPM coding; the L-PPM encoder maps the K-system bit stream into an L-PPM time slot sequence, and the number of the L-PPM time slots is L ^ 2^ K;
step 4, integrating the result of L-PPM coding mapping and M-ary bit stream through an emission filter to form a TDM frame for transmission, as shown in FIG. 3; the transmit filter is a filter with a rectangular impulse response waveform, where a "1" in the M-ary bit stream represents a signal and a "0" represents no signal.
And 5, changing the values of M and K in the steps 2, 3 and 4, repeating the steps 1-4, and comparing the signal-to-noise ratio and the bit error rate of the signal obtained by the system using different time domain hybrid modulation receiving ends.
From the above processes, it can be known that a new modulation scheme, time-domain hybrid modulation, in the visible light communication system proposed by the present invention can flexibly obtain different spectrum efficiencies according to the allocation of the proportions of two different modulation formats in a TDM frame, and the complexity of the time-domain hybrid modulation is much reduced compared to the multi-carrier modulation capable of achieving a high communication data rate. The method solves the problem of adopting modulation multiplexing equipment with complex technology and extremely high cost in VLC, simplifies the complexity of the modulation equipment and ensures better communication performance. In practical applications, especially in the case of VLC as an auxiliary communication system, this is of great significance.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (6)
1. A time domain hybrid modulation method based on a visible light communication system is characterized by comprising the following steps:
step 1, generating a string of random original bit streams;
step 2, converting the original bit stream into an M + K system bit stream through serial-to-parallel conversion;
step 3, the K system bit stream in each group of M + K system bit streams is subjected to L-PPM coding;
step 4, respectively passing the result of L-PPM coding mapping and the M-system bit stream through a transmitting filter, generating signal pulses on time slots corresponding to the L-PPM coding mapping result and the M-system bit stream, integrating the signal pulses to form a TDM frame for transmission, and calculating the signal-to-noise ratio and the bit error rate, wherein one TDM frame is composed of N time slots, and N is a natural number greater than 2;
and 5, changing the values of M and K in the steps 2, 3 and 4, repeating the steps 1-4, and comparing the signal-to-noise ratio and the bit error rate of the signal obtained by the system using different time domain hybrid modulation receiving ends.
2. The time domain hybrid modulation method based on the visible light communication system according to claim 1, wherein: in step 2, the values of M and K in the M + K binary bit stream obtained by serial-to-parallel conversion of the original bit stream can be set.
3. The time domain hybrid modulation method based on the visible light communication system according to claim 1, wherein: in step 3, the K-ary bit stream in each group of M + K-ary bit streams obtained by serial-to-parallel conversion is subjected to L-PPM coding, and the K-ary bit stream is mapped into an L-PPM time slot sequence.
4. The time domain hybrid modulation method based on the visible light communication system according to claim 1, wherein: and the time slot number L of the L-PPM time slot sequence is 2^ K.
5. The time domain hybrid modulation method based on the visible light communication system according to claim 1, wherein: in step 4, the transmit filter outputs a rectangular impulse response waveform.
6. The time domain hybrid modulation method based on the visible light communication system according to claim 1, wherein: in step 4, "1" in the M-ary bit stream represents a signal, and "0" represents no signal.
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