CN101710890B - Pulse and OFDMD double-data modulation method - Google Patents
Pulse and OFDMD double-data modulation method Download PDFInfo
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- CN101710890B CN101710890B CN 200910201114 CN200910201114A CN101710890B CN 101710890 B CN101710890 B CN 101710890B CN 200910201114 CN200910201114 CN 200910201114 CN 200910201114 A CN200910201114 A CN 200910201114A CN 101710890 B CN101710890 B CN 101710890B
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Abstract
The invention discloses a double-data modulation method of pulse and OFDM. An optical spectrum of a single chip white light LED is formed by exciting fluorescent powder of yellow color or red-green color and the like by a blue light, and the shape of the whole optical spectrum has two relative independent peak values, wherein the responding speed of the blue light is higher than that of a warm color light. The invention can be used for transmitting data by pulse modulation of the blue light and can be used as a carrier wave for the warm color light OFDM modulation, thus a double channel high-speed data transmission is realized. In addition, the blue light can be used as a reference simultaneously for OFDM symbol synchronization, direct current offset estimation and channel estimation.
Description
Technical field
The invention belongs to communication technical field, what be specifically related to is a kind of pulse and the double data modulation of OFDM and the device and method of demodulation when utilizing illumination LED to realize short-range communication.
Background technology
Visible light communication based on illumination LED is a kind of emerging short-distance wireless communication mode, is expected to replace or additional current short distance RF wireless communication technology, to improve the electromagnetic radiation of traffic rate, reduction cost and environment.At present, tentative confirmation use the feasibility of illuminating LED communication under the varying environment, comprising the application in indoor and intelligent transportation.
One of important technology aspect of communication is the modulation to the communication data signal.For different communication system and application demand, in a broad aspect, the modulation system of employing has various pulse modulations (PM), QAM (quadrature amplitude modulation) modulation, OFDM (OFDM) modulation and CDMA (code division multiple access) etc.Effectively modulation system has a great impact message transmission rate, capacity and the quality of communication system.
Pulse modulation is a kind of relatively modulation system of letter, and is commonly used in wired baseband communication system, and wherein a kind of is exactly OOK (on-off keying) modulation.The OFDM modulation is a kind of multi-carrier modulation of effective elimination channel multi-path delayed impact, its application has more and more widely trend, successful Application has been in xDSL (Digital Subscriber Line) broadband access system and WLAN standard IEEE802.11a, and might be applied in following various wireless communication system.
The wave band of visible light between the 700nm, will be the following main body that realize visible light communication as the large power white light LED that throws light at 400nm.Conventional white light LEDs has two classes: single-chip and multi-chip, this is two kinds of different realization technology.The single-chip technology adopts yellow or the red and green fluorescent material that mixes of the LED excitation of a monochrome band to send the light of secondary colour, and exciting light source commonly used is blue-ray LED (perhaps ultraviolet LED).Therefore, the spectral characteristic of Single chip white light LED completely is divided into the blue light of cool colour and warm colour light two parts of reddish yellow.Figure 1 illustrates the curve of spectrum of a typical Single chip white light LED, greatly about the 420nm-460nm scope, the warm colour light spectrum of light-emitting phosphor is greatly about the 500nm-750nm scope as the blue spectrum of exciting light source.The white light LEDs of multi-chip is made of the LED of three primary colors light.At present, because the power demand in when illumination, most of white light LEDs adopt the structure of single-chips, and multiplex but the occasion that chromaticity have relatively high expectations not too high in power requirement of the white light LEDs of multi-chip structure, such as display screen etc.
The single-chip LED of above-mentioned pattern, blue light belongs to semiconductor property, is that directly electricity drives, so response ratio is very fast, very high bandwidth is arranged.And fluorescent material belongs to quadratic response, and speed is slower, and bandwidth is narrower.Therefore, when communicating with LED, there is the difference of light on response speed of different spectrum.
Present research to visible light communication, the modulation system that adopts has OOK mode, PWM mode, CDMA mode (such as Tim C.W.Schenk, " Optical Wireless CDMA Employing Solid StateLighting LEDs "), OFDM mode (such as Hany Elgala, " Indoor Broadcasting via White LEDsand OFDM ") etc.In view of the radiative incoherence of visible illumination LED and unipolarity, it has been generally acknowledged that the OOK mode may be to realize the fairly simple practical modulation system that can reach again given pace, OFDM then is more complicated but can realizes the modulation format that multi-carrier high-speed transmits at incoherent wave.
If only receive the light of full spectrum frequency range with an optical receiver, the bandwidth of communication system is decided by the warm colour light of slow-response so.The research of most, for being exactly this a kind of situation, the bandwidth that system can realize is probably about 10MHz.Research is also arranged in addition separately for blue light fast response characteristic partly, as in the paper " Discrete Multitone for NovelApplication Areas of Optical Communications " of the paper " 100-Mb/s NRZ Visible Light Communications Using aPostequalized White LED " of Hoa Le Minh and S.C.J.Lee, adopt blue color filter to filter out at a slow speed warm colour light, thereby under simple OOK modulation or in the OFDM modulation, be issued to very high message transmission rate.But this method obviously can't be used the energy of the most of light outside the blue light, and this part light also can reach the message transmission rate of approximately same magnitude when adopting the OFDM modulation.
If at the optical sender end, can control separately the light of different spectrum, just can tell the signal of different spectrum at receiver end with filter so, thereby realize multichannel communication.To the multi-chip white light LEDs, the white light LEDs that is packaged together such as R, G, B three-color LED, if can control separately, a kind of exactly method that may realize 3 tunnel parallel communicationss.But for common Single chip white light LED, be merely able to exciting light (normally blue light) is controlled, by the excitation corresponding warm colour luminous intensity of light intensity acquiring size (this ratio is specifically matched colors by the inside of LED and formed decision), both are actually closely-related, can't independently use.
For the problems referred to above, this patent has proposed a kind of pulse modulated " PM-OFDM " two-way high speed transmission data method of realizing simultaneously warm colour light OFDM modulation of blue light of realizing.
Summary of the invention
Initial purpose of the present invention is spectral characteristic and the response characteristic that takes full advantage of white light LEDs, realizes a kind of high speed data modulations and demodulation based on white light LEDs.
Basic ideas of the present invention are: during take blue light as the LED driving source, time dimension in the driving pulse sequence carries out a kind of pulse modulation, be matched with the modulation of warm colour light OFDM waveform in its amplitude, realize thus the parallel transmission of two paths of data information on different spectrum.
At the optical sender end, send data flow and be divided into two-way: D
PM(n) and D
OFDM(n).
Data flow D
PM(n) through conventional coding, interweave, after the preliminary treatment measure that the baseband communication system such as scrambler is commonly used, be Φ to clock frequency
PPulse train carry out the OOK pulse modulation, form the constant amplitude modulated pulse trains P carry data message
PM(n), suppose pulse representative data " 1 " is arranged, no pulse representative data " 0 ", the amplitude of pulse is 1.This pulse train is delivered to two places simultaneously as signal, and one is the excitation compensating unit, and one is the switching circuit of control LED current switching.The effect of excitation compensating unit is to judge current modulating pulse P
PM(n) afterwards modulating pulse P
PM(n-1) whether be zero.If be zero, then after current pulse amplitude 1 increases a Δ, export, otherwise keep former amplitude 1 output.This new pulse train has same frequency Φ
P, but no longer be constant amplitude pulse, be designated as P
Δ PM(n).P
Δ PM(n) be sent to modulator M
OOK-OFDMFirst input port.Deliver to the modulating pulse P of LED break-make control
PM(n), postpone T
dAfter, work as P
PMO'clock (n)=1, open LED, work as P
PM(n)=0 o'clock, close LED.Open LED and namely allow LED to send light signal, close LED and namely do not allow LED to send light signal.And the size of the light signal of transmission that allows is decided by to be applied to the bias voltage V on the LED
LED
Data flow D
OFDM(n) through conventional coding, interweave, after the treatment measures that the ofdm communication systems such as QAM modulation, pilots insertion, mapping, IFFT conversion are commonly used, the formation sampling frequency is Φ
OFDMTime domain discrete OFDM real signal.Clock frequency Φ
PWith Φ
OFDMBe the integral multiple relation.Be Φ with the sampling point frequency
OFDMDiscrete ofdm signal according to frequency Φ
PCarry out interpolation arithmetic, obtaining a sampling frequency is Φ
PDiscrete ofdm signal S
OFDM(n).This discrete signal is carried out a prebias V
bBe converted to unipolar signal, deliver to modulator M
OOK-OFDMSecond input port.The output signal of modulator be exactly the amplitude frequency being modulated and compensated by ofdm signal be Φ
PPulse train S
Δ OFDM(n).Then this pulse train be sent to a DAC transducer, is converted to analog signal and become forward bias voltage (operating voltage) V of LED after the power amplification
LED
White light LEDs is subjected to the synchronous effect of two signals: one is the operating voltage that applies thereon, and its size has reflected signal S
Δ OFDM(n) size, the electric current when determining the LED conducting; One is the pulse-modulated signal P of control LED current switching
PM(n).Coefficient result is the light signal of LED output, and owing to its fast response characteristic, the light signal of output has reflected pulse-modulated signal P at blue wave band
PM(n), at implicit OOK modulation data information and the clock information that sends of time shaft, the implicit OFDM Data Modulation information that sends on the amplitude axis; In warm optical band since the larger response of fluorescent material often rapid pulse rush that information is smoothed to be fallen, thereby only implied OFDM Wave data information.
At the optical receiver end, two light-receiving passages are arranged: one is the blue light receive path, and one is warm light-receiving passage.
The blue light receive path at first passes through blue filter, filters out the light energy outside the blue light.The blue light that enters receiver carries out opto-electronic conversion, amplification, filtering etc. process after, signal is sent to two data treatment channel: burst process passage and OFDM treatment channel.The burst process passage is directly processed the pulse signal of input, obtains synchronizing information, the DC biased level of pulse and demodulates the data of modulation.The OFDM treatment channel is at first carried out envelope detection with pulse signal, obtains the profile information of pulse amplitude.In fact this information be exactly ofdm signal at transmitter terminal, and this signal is carried out the ADC conversion again, and the ofdm signal that obtains dispersing is designated as S
B-OFDM(k), then removing direct current biasing is converted to bipolar signal and delivers to a combiner MRC.
Warm light-receiving passage filters out the light energy outside the warm light at first through warm light filter.Then, after the warm light that enters receiver carried out opto-electronic conversion, amplification, filtering etc. and process, at the lower synchronously of synchronizing signal signal is carried out the ADC conversion, the ofdm signal that obtains dispersing is designated as S
W-OFDM(k), then removing direct current biasing is converted to bipolar signal and also delivers to combiner MRC.Combiner MRC selects certain weighting algorithm, such as the ratio merge algorithm two-way ofdm signal is synchronously merged into one road ofdm signal, then remove Cyclic Prefix, carry out the FFT conversion, go mapping, the QAM demodulation, deinterleave, decode, obtain the OFDM modulating data.
Said method can be regarded as the OFDM modulation (when modulating with PWM, can be called the OFDM modulation based on the pwm pulse carrier wave) based on the OOK pulse carrier.For the white light LEDs of single-chip, the result of dual modulation can realize two-way light sendaisle: blue light pulsed light passage and warm optical continuous wave passage, and used LED still only needs one (or one group, but control port only needs one).Utilize two light-receiving passages at receiver end, can access three circuit-switched data information: one tunnel independently pulse modulation data, the OFDM modulating data of two-way association.This method has obtained frequency (blue light, warm colour light) diversity gain simultaneously except increasing message transmission rate.This transceiver can flexible configuration, according to channel conditions and transmission quality requirements, and data that three channel transfer is same or only to select some passages all be to realize easily.
Description of drawings
Fig. 1: typical Single chip white light LED spectrogram.
Fig. 2: visible light communication scene signal.
Fig. 3: LED realizes that the transceiver of visible light communication forms.
The pulse signal of Fig. 4: OOK and PWM modulation.
Fig. 5: the visible light communication system that adopts the OFDM modulation.
Fig. 6: OFDM modulated process and output waveform take pulse as carrier wave.
Fig. 7: OOK-OFDM dual modulation process and corresponding output waveform.
Fig. 8: adopt the transceiver of the visible light communication system of dual modulation to form.
Fig. 9: the detection threshold of the OOK modulation signal of reception.
Figure 10: the synchronizing signal of OOK pulse modulation passage a kind of, adopted the dipulse Barker code.
Figure 11: three-channel OOK-OFDM dual modulation optical transceiver system realizes block diagram.
Figure 12: the realization of OOK-OFDM dual modulation and excitation compensation thereof.
Figure 13: pulse excitation compensation schematic diagram.
Figure 14: two optical channel receiving front-ends.
Embodiment
Utilize the scene of illumination LED realization communication generally as shown in Figure 2, LED (2-1) single or that become array, cluster becomes optical sender simultaneously with the illumination that the various forms realization needs.The most common being mounted on the indoor roof made desk lamp and is placed on the desk, perhaps is installed in the top, seat of automobile, aircraft etc.And optical receiver (2-2) is usually located at the below of illumination LED.Be different from the RF radio transmission, generally there is certain directivity (2-3) in illumination light, and when particularly adopting LED, the scope of illumination ratio is easier to control.This directivity is favourable to realizing multiple access/multichannel communication to a certain extent.
The transceiver of visible light communication system is general to be consisted of as shown in Figure 3.Transmitter is made of controller (3-1), modulator (3-2), driver (3-3) and light-emitting diode (3-4), and the optical system that receiver is made of simple lens (3-5), filter (3-6), optical-electrical converter (3-7), signal deteching circuit (3-8), signal demodulating circuit (3-9), signal processor (3-10) consist of.Except electrical-optical, light-electric conversion portion replace the radio frequency unit of RF radio communication, its total communication forms communicates by letter identical with RF.
The modulation system of visible light communication need to be suitable for noncoherent unipolar signal, and commonly used is on-off keying (OOK) modulation, pulse duration (PWM) modulation etc.Figure 4 illustrates binary OOK and PWM modulation system.Under the triggering of clock pulse (4-0), data sequence (4-1) is the impulse form of signal (4-2) according to the OOK modulation conversion, and same data sequence is the impulse form of (4-3) according to the PWM modulation conversion.
Visible light OFDM modulation not only has the advantage that has when the conventional wireless communication system adopts OFDM by frequency band division being become a plurality of parallel orthogonal subbands, has indirectly realized the quadrature modulation that the incoherent light signal can't be realized simultaneously, such as QPSK.Shown in Fig. 5 chief component and the data manipulation of OFDM modulation flow to.
The original data stream of transmitting terminal at first passes through channel encoder (5-1) and carries out necessary error correction coding, then passes through interleaver (5-2) encoded data stream is interweaved, to improve the error correcting capability to bursty interference.The modulated device of data after interweaving (5-3) converts one group of modulating data of corresponding QPSK planisphere to, then according to the quantity of sub-band convert to parallel symmetrical complex conjugate number according to and be mapped on the sub-band of regulation by (5-4a) with pilot signal (5-4b).The parallel data that are equivalent to frequency domain are input to the real discrete signal that Fourier inversion device IFFT (5-5) converts time domain to.During according to system for the time-delay size of optical channel, CP insert module (5-6) is inserted into the front portion of signal at the afterbody Extraction parts signal sampling point of this signal as Cyclic Prefix (CP), forms a complete discrete OFDM symbol.Discrete OFDM symbol is converted to continuous OFDM time-domain signal waveform through digital to analog converter DAC (5-7), apply and pass through analog line driver (5-8) driving LED after direct current biasing is converted to unipolar signal again, the signal of telecommunication is converted to light intensity signal and sends in the visible light channel (5-9) the most at last.
At OFDM visible light receiver end, light signal by optical channel (5-9) is detected and is converted to the signal of telecommunication by photodetector (5-10), through necessary amplification and filtering (5-11), continuous signal under the triggering of OFDM symbol sync signal (5-13), is converted to discrete signal through ADC transducer (5-12).This signal is corresponding with the discrete ofdm signal of transmitter terminal time domain, remove cyclic prefix CP and remove direct current biasing through (5-14), deliver to fast Fourier transformer FFT (5-15) conversion and return one group of signal of each subcarrier of correspondence of frequency domain, after (5-16) went mapping, demodulator (5-17) decomposed the bit stream that QPSK is converted to serial.And then through deinterleave (5-18), error correction decoding (5-19), the actual data stream that output receives.
Above-mentioned modulation system all is single baseband modulation, is merely able to the transmission of one line data.The method that the present invention proposes is to utilize simultaneously pulse and OFDM modulation, realizes the two-way transfer of data.Principle is as follows:
For visible light communication, the time-domain signal that the ofdm system that adopts base band directly to move transmits is exactly the light form of OFDM waveform.And because the non-coherent nature of visible light, be difficult to carry out the carrier modulation based on light wave, but be based on subcarrier (with respect to light wave) or the pulse wave modulation still is feasible.Figure 6 illustrates a kind of implementation procedure with impulse wave Modulation OFDM signal.Signal S (ofdm) supposes to post the ofdm signal waveform (6-1) that has carried data message, S (p) is pulse carrier (6-2), behind two signals process modulators (6-3), the waveform S (p-ofdm) of output is exactly the OFDM waveform (6-4) after the modulation.This process is equivalent to a kind of amplitude modulation(PAM), and the amplitude envelops of output pulse signal has been put down in writing the shape information of original ofdm signal fully.Certainly, the S (p) here can be configured to regular sinusoidal wave form equally.
The modulator approach of this conventional effect does not comprise data message in pulse carrier, the implicit information of output waveform (7-4) is one dimension, the just change of transmission system frequency spectrum that it can bring and the improvement of performance.In the visible LED communication system, its actual output effect is different from the RF system.At blue wave band, what see will be the such impulse waveform (6-4) of S (p-ofdm) as shown in Figure 6, and in the warm colour optical band, what see then is the such continuous wave (6-5) of S among Fig. 6 (led).Because warm colour is only encouraged by blue light, has a definite response relation and delay (T
d), therefore when the pulse excitation frequency ratio of blue light was higher, warm colour light was equivalent to the rectification of excitation input pulse.
So, when adopting the receiver of 2 optical bands, at blue wave band, can obtain simultaneously the time synchronization information of pulse carrier and the data message of OFDM modulation, and can obtain to have the more OFDM modulation data information of macro-energy in the warm colour optical band.Dual information source is equivalent to obtain frequency diversity gain after merging, and the optical receiver performance is improved.And the independence of blue light receives the OFDM sign synchronization demodulation that the burst length information of obtaining can be used for assisting warm colour light, and this is a kind of effective measure that improves time synchronized.
When the time response of blue light was very fast, another method of promoting data transmission capabilities was that the employing dual modulation namely utilizes the carrier pulse amplitude information to post when carrying the OFDM modulating data, utilizes the temporal information of blue light pulse carrier to put down in writing an other circuit-switched data.
Figure 7 illustrates the process of this modulation.Signal S (ook) as pulse carrier is the OOK modulation of determining continuous impulse ripple under the clock frequency.Supposed a circuit-switched data (7-4) among Fig. 7, its corresponding OOK modulation output waveform is (7-3).The pulse signal of this constant amplitude is sent to modulator (7-5) as modulated carrier.Simultaneously, other circuit-switched data stream adopts the OFDM modulation to form OFDM waveform (7-1), also delivers to modulator (7-5).The output of modulator is equivalent to the amplitude of pulse signal and interval by dual modulation.
In this modulated process, the data of OOK modulation are at first carried out scrambler, make 0,1 bit probability of occurrence close, avoid occurring 0 continuous bit as far as possible.Such 0 impact that occurs reduces, and the frequency that on average is equivalent to modulating pulse is compared with pulse clock and reduced half.For keeping the mean intensity of output optical signal, can adopt the average amplitude that improves the blue light driving pulse to compensate.
Because the output of warm colour light is by the output drive of blue light, affect the OFDM waveform of warm colour light output for reducing blue light pulse inhomogeneous excitation, take in addition a kind of Pulse by Pulse indemnifying measure, the blue light driving pulse of compensation OOK modulation does not namely make the excitation density of warm colour light influenced when the blue light pulse lacks because bit 0 the weakening of warm colour light when vacant occurring.
Suppose that the data flow and the corresponding pulse train that enter the OOK modulation are:
D(0)、D(1)、...、D(n-1)、D(n)、D(n+1)、D(n+2)、D(n+3)...
P(0)、P(1)、...、P(n-1)、P(n)、P(n+1)、P(n+2)、P(n+3)...
Data D (n)=0 or 1 wherein, (pulse duration refers to that in amplitude be 0 or 1 fixedly the time to pulse P (n)=0 or 1; Amplitude is fixedly the time, and finger widths is T0 or T1 (T0 ≠ 0, T1 ≠ 0, T0 ≠ T1).The frequency of supposing pulse is constant under the clock of determining).When excitation does not compensate, in the OOK modulated process: P (n)=D (n).
The way of Pulse by Pulse compensation is in the pulse modulated OOK signal of blue light, when finding to have D (n+k)=0 (k=0,1 ..., K makes P (n-i)=1+ Δ i, (i=1,2 ..., I; Δ i 〉=0).The value of Δ i is decided by the response characteristic of warm colour fluorescent material among the LED and the energy of blue light driving pulse, can be determined by experiment.Compensate the PAM (pulse amplitude modulation) that is equivalent in advance in pulse amplitude, and compensate PWM (pulse duration) modulation that is equivalent in advance at the width of pulse.Signal S (ook-ofdm) in Fig. 7 is exactly the modulating pulse output (7-6) behind the Pulse by Pulse Amplitude Compensation (7-8).Wherein amplitude exceeds the amplitude that the pulse amplitude of OFDM modulation waveform (being shown in dotted line) is exactly compensation, such as Δ v1 and the Δ v2 that marks among the figure.(7-6) be exactly the output signal of blue light.Under such signal excitation, the warm colour optical output signal of generation is shown in the S among Fig. 7 (led-ofdm) (7-7), and it should be identical with the signal waveform (7-1) of input, and Vb wherein is direct current biasing.
The formation that can represent with Fig. 8 optical transceiver.The controller of optical sender (8-1) is encoded data, interweave, is divided into two-way after the preliminary treatment such as scrambler and delivers to respectively OOK modulator (8-2) and OFDM modulator (8-3), and the signal of formation is delivered to driver (8-4) driving LED (8-5) and sent the light signal of modulation.At receiver end two receive paths are arranged, one is the blue light receive path, and one is warm colour light-receiving passage.At the blue light receive path, comprised the information of two-way modulating data: OOK pulse modulation passage and OFDM modulation channels.After light signal was through optical lens (8-6), warm colour light was by filter (8-7) filtering, and only blue light is converted to the signal of telecommunication by optical-electrical converter (8-8).In receiver (8-12) be divided into two-way thereafter.One the tunnel is the processing of pulse signal, and the common communication pulse signal such as filtering, detection, OOK demodulation is processed, and can obtain synchronizing information and one road receive data of pulse.Envelope detection is carried out to input pulse first in another road, then carries out the processing such as demodulation of ofdm signal, obtains another circuit-switched data, and the data that this circuit-switched data and warm colour optical channel obtain are sources, therefore can carry out joint-detection.
At warm colour light-receiving passage, after light signal was through optical lens (8-9), blue light was by filter (8-10) filtering, and only warm colour light is converted to the signal of telecommunication by optical-electrical converter (8-11).Thereafter, signal will be processed according to the handling process of OFDM receiver in (8-12).The synchronizing information that the blue light receive path obtains can be used for the auxiliary synchronous detection of OFDM symbol herein, and the estimation of data also can be united with blue channel and carried out, and reduces the error rate.
A kind of method of joint-detection that two-way contains the ofdm signal of same data message is that the discrete signal after the synchronous ADC conversion of each road signal is weighted merging, and then carries out all the other operations of OFDM.Suppose the output S of blue channel ADC
b(n), the output of warm colour optical channel ADC is S
w(n), wherein n is the discrete sampling point of an OFDM symbol, unites so to be output as:
S
∑(n)＝K
b(n)×S
b(n)+K
w(n)×S
w(n) n＝0，2，...，N-1
Wherein, K
b(n), K
w(n) be respectively the weight coefficient of blue channel and warm optical channel, N is the number of samples of an OFDM symbol.Definite signal to noise ratio according to each channel of weight coefficient, adopting maximum ratio to merge is a kind of effective method.
Obtain S
∑(n) after, carry out according to the demodulation process of OFDM, remove the CP Cyclic Prefix, deliver to FFT and carry out time domain-frequency domain conversion and obtain the data terminal signal.
The overall formation that the present invention realizes described OOK pulse carrier OFDM modulation as shown in figure 11.
Optical sender end, the data flow that is sent out are assigned as two-way and deliver to respectively pulse modulation passage (11-1) and OFDM modulation channels (11-2) in (11-0).Two passages are modulated the data that send respectively, then deliver to together driver (11-3) driving LED and send light signal to spatial light channel (11-4).At the optical receiver end two light-receiving passages are arranged, one is blue light receive path (11-5), and another is warm light-receiving passage (11-6).The blue light receive path is mainly finished the detection of OOK pulse-modulated signal and demodulation, and warm light-receiving passage is mainly finished joint-detection and the demodulation to the ofdm modulation signal that comes from blue light and warm light.Three data passages finish together input signal synchronously, the processing such as channel estimating and equilibrium (11-7).At last, the data flow of two receive paths is merged into a data flow output at (11-8).
OFDM modulates sendaisle: it is the convolution code of R=2/3 that chnnel coding (11-2-1) adopts speed, the degree of depth of interleaver elects 1024 as, data adopt 16-QAM in the modulation (11-2-3) of frequency domain, per 4 bits consist of a data symbol, form 64 data points (number of corresponding subcarrier is 64) behind the zero padding of per 60 data symbols and the pilots insertion, be mapped on 128 frequency domain sampling points of IFFT (11-2-5) through (11-2-4) Conjugate extended.After the IFFT conversion, export 128 time domain sampling points, inserted again discrete OFDM symbol of the rear formation of CP (Cyclic Prefix) of 8 sampling point length by (11-2-6), then encourage LED luminous through OOK modulation, DAC conversion (11-2-7) to driver (11-3).The total bandwidth of OFDM modulation is made as 8MHz, and the subband width is 125KHz, and the length of information symbol is 8us, and the length of CP is 0.5us, and length enough adapts to the multidiameter under any environment, and total OFDM symbol lengths is 8.5us, sampling rate Φ
OFDMBe 16MHz.Data rate can reach 18.8Mbps.
The blue light sendaisle: the frequency of pulse carrier is 48MHz, and this also is the clock frequency Φ of system
P, be 3 times of OFDM sampling frequency, pulse duty factor is taken as 50%.Consider except channel fading affects the error rate of the transmission of data, also to be subject to the impact of OFDM modulation, select the lower R=1/2 convolution code of chnnel coding (11-1-1) speed, the interleave depth of interleaver (11-1-2) is taken as 1024, and scrambler (11-1-3) is selected pseudo random sequence.Data Modulation (11-1-4) adopts the OOK mode, and pulse " is opened " during bit " 1 ", pulse " pass " during bit " 0 ".During spread spectrum, data rate can not reach nearly 48Mbps, and when adopting 4 times of spreading factors (encode with 4 digit pulses represent a data " 1 " or " 0 "), data rate can reach 12Mbps.If adopt the PWM modulation, as among Fig. 4 by the pulse train (4-3) after data sequence (4-1) modulation, also can reach same speed, but follow-up processing mode is slightly different.
The pulse signal that the blue light receive path receives is through shaping (11-5-2) squaring after amplifying, and one the tunnel delivers to (11-5-3) carries out OOK demodulation, (11-5-4) and go the processing such as scrambler/deinterleave, (11-5-5) decoding to obtain receive data; Other one the tunnel delivers to envelope detector (11-6-3), detects the OFDM symbol waveform of modulation on it and removes direct current biasing.Then under (11-7) definite synchronizing signal, carry out the discrete OFDM sampling point of ADC (11-6-4) conversion output.OFDM symbol after this burst and the synchronous ADC of process (116-5) conversion that comes from warm optical channel carry out ratio merge obtain having diversity gain and value sequence, carry out subsequently other inverse transformation with OFDM emission system symmetry: Cyclic Prefix removes count identical FFT (11-6-9) conversion, reflection of (11-6-8) and transmission channel IFFT and penetrates (11-6-10), QAM demodulation (116-11), deinterleave (11-6-12), decoding (11-6-13).The data of the at last data of output and blue channel output merge in (11-8), form final whole receive datas.
One of dual modulation implementation of transmitter front ends is the break-make that adopts the constant amplitude pulse control LED after the OOK modulation, and the OFDM Waveform Control of the size of the drive current of LED after being compensated.Figure 12 illustrates this scheme.(12-1) be sequences of pulsed signals after the OOK modulation, comprised data message.It is divided into two-way: the one tunnel is sent to excitation compensation operation unit (12-6) calculates the amplitude that each driving pulse should have; Other one tunnel delayed unit (12-11) postpones a time T
dBreak-make by gate drive circuit (12-12) output drive signal control switch mosfet pipe Q1.The time T that postpones
dGuarantee that OOK sequence implicit in the driving signal of OOK sequence and OFDM passage output is synchronous.Q1 has good switch performance, can open fast/turn-off the LED current channel.The resistance R 2 that is connected between Q1 and the ground is current limliting and the sample resistance of LED, and current sampling signal feeds back on the Driver amplifier (12-10) of LED, realizes the closed loop current adjustment.In the OFDM modulation channels, the discrete signal (12-3) that transforms to time domain through IFFT is applied in a direct current biasing V
b(12-2) be converted to the clock frequency Φ that provides according to system behind the unipolar signal
P(12-5) export after (12-4) interpolation, the sampling point frequency of exporting after the interpolation is identical with pulse modulated clock frequency, and the algorithm of interpolation can adopt simple linear interpolation method.The function of Amplitude Compensation unit is according to 0 position in the input traffic pulse amplitude of front to be adjusted, and then removes the OFDM symbol sampling point after modulation (12-7) interpolation.Output after the modulation is transformed into the luminous intensity of drive circuit (12-10) control LED through DAC (12-9).
Figure 13 has illustrated the process of interpolation and compensation, the time scale of 3 reference axis is identical among the figure, (13-1) shows and the time series of clock synchronous order in figure below (X1), (13-2) is (13-3) data sequence is carried out pulse train after the OOK modulation.Upper figure (X2) shows the sampling value of the OFDM symbol after the interpolation, and the solid sampling point of similar (13-6) is original OFDM symbol sampling point, and the hollow sampling point of similar (13-5) namely is the sampling point after the interpolation.Supposed among the figure that time series is in the of 0 from the right side.Middle graph (X0) is the output behind the Amplitude Compensation, i.e. the output of functional block (12-6) among Figure 12.3 dotted lines among the figure have represented 3 kinds of amplitudes of compensation.A dotted line height is 1 bottom, is the value that does not have balanced sampling point, and a middle dotted line height is the sample value that has compensated behind the unit, and topmost dotted line height is the sample value that has compensated behind 2 units.The amplitude value of compensation also can be continuous.Having compensated 1 unit at No. 2 sampling point among the figure, is because No. 3 sampling point thereafter is data " 0 ", with the excitation of disappearance to LED; Compensation on No. 7 sampling point get 2 units be because thereafter followed by the 8th, No. 9 sampling point on continuous 2 data " 0 " occur thereby lack 2 times to the excitation of LED.The algorithm of compensation is not limited to this simple regulation, can determine according to the actual output effect of LED.
The front end of optical transceiver adopts structure shown in Figure 8.The LED of optical sender (8-5) adopts the common white light high-capacity LED of single-chip, and its emission spectrum and Fig. 1 are similar.Corresponding such spectrum, photoreceiver front-end has two optical channels.The lens of two optical channels (8-6,8-9) can have identical spectral characteristic and geometry, light energy can be converged on the optical-electrical converter.But the spectral characteristic of two groups of filters (8-7,8-10) must can be separated the different wave length signal, and has the effect of certain filtering environmental stray light, so just requires the transmitance of filter and the luminescent spectrum of LED to mate as far as possible.The transmitance that blue filter can be designed as in the 420nm-460nm scope surpasses 70%, and the mean transmissivity in other optical frequencies is less than 20%.The transmitance that warm colour light filter is designed in the 520nm-650nm scope surpasses 70%, and the mean transmissivity in other optical frequencies is less than 20%.Such design comparison realizes easily, and ambient interferences that can all the other optical frequenciess of filtering, as disturbing at the visible light between the 460nm-520nm and the infrared light more than the 650nm.
The receiving unit that figure 14 illustrates optical receiver consists of.The optical-electrical converter of two passages (14-1,14-2) can adopt PIN photodiode commonly used or silicon photoelectric diode.Their response speed is all very high, and the response speed of LED can satisfy bandwidth requirement head and shoulders above.But their spectral response characteristic has number of different types, should select the peak value of response of spectrum and the type that LED luminescent spectrum peak value is complementary as far as possible.The peak in the spectral response of this class photodiode is generally at 550nm, spectral response range between 350nm-800nm, photodiode that can be different according to channel selecting of living in.Connect transreactance amplifier (TIA, 14-3,14-4) behind the optical-electrical converter, photo-signal is converted to voltage signal, voltage signal after amplifying (14-5,14-6), is delivered to follow-up treatment circuit again.The bandwidth of dual-stage amplifier will enough adapt to the bandwidth of light signal, can get here 〉=200MHz.
Blue channel is pulse signal owing to what receive, and the impact that is disturbed is less, also relatively easily recovers synchronizing information.Fig. 9 is the normal data-signal that receives, because OFDM has modulated amplitude, optical channel decline, and the interference relationships of bias light, exist some pulse amplitudes to be lower than detection threshold (V
t) sampling point, this situation can reduce the error rate by strengthening the chnnel coding error correcting capability.In order to determine frame boundaries and the auxiliary OFDM sign synchronization of OOK signal, insert periodically a synchronous head at the blue emission passage by (12-8), structure is as shown in figure 10.Here continuous 2 pulse combined by whole range and half amplitude point become 1 information.Can define " high-low " pulse combined representative " 1 ", " low-Gao " pulse combined representative " 0 " is configured to a Barker code thus again.Shown in Figure 10 is exactly 7 Barker codes of " 1110010 ".According to the stability of clock frequency and channel, in the data sequence that sends, insert periodically such pulse train.Purpose with whole range and half amplitude point structure synchronizing sequence is to obtain easily the intermediate level that receives signal in this signal, can determine the direct current biasing of ofdm signal with this, also easily channel is estimated simultaneously, the non-linear of equalization information and compensation LED is provided.
So far, the basic performing step and the main design parameters that realize the optical transceiver of one " OOK-OFDM " modulation have been narrated complete.
Although above described be communication system for white light LEDs, the method for this dual modulation comprises that twin-channel ofdm signal merges, the assist in synchronization of three passages, the modulation compensated method of OFDM etc., can't only be confined to this application.Such as, in ultra-wideband communication system or optical fiber telecommunications system, through the suitable parameter adjustment, also can use.
Claims (6)
1. the method for a pulse and OFDM dual modulation/demodulation, it is characterized in that: at transmitter terminal, the data flow that sends is divided into two-way, one circuit-switched data is carried out pulse modulation and is formed a modulated pulse trains, another circuit-switched data is carried out the OFDM modulation and is formed a time domain OFDM sampling point sequence, OFDM sampling point sequence according to the synchronous interpolation of the clock frequency of modulated pulse trains after, again the amplitude of modulated pulse trains is modulated the pulse train that forms dual modulation, send in the channel and go; At receiver end, the signal that receives is delivered to two signal processing channels, one is the burst process passage, directly detect to received signal, pulse modulated demodulation, obtain one road receive data and clock signal, another is the OFDM treatment channel, carry out to received signal first envelope detection, recover the time domain OFDM symbolic information, then carry out the ofdm signal demodulation process, obtain another road receive data.
2. the method for pulse according to claim 1 and OFDM dual modulation/demodulation, pulse modulation wherein are OOK modulation or PWM modulation, and its clock frequency is the integral multiple of described OFDM modulation sampling frequency.
3. method that in LED-based visible light communication system, realizes pulse and OFDM dual modulation/demodulation, it is characterized in that: at the optical sender end, the data flow that sends is divided into two-way, one circuit-switched data is carried out pulse modulation and is formed the break-make that a modulated pulse trains removes to control the LED electric current, another circuit-switched data is carried out the OFDM modulation and is formed a time domain OFDM sampling point sequence, OFDM sampling point sequence is according to the clock frequency interpolation of modulated pulse trains and carry out again the amplitude of the modulated pulse trains behind the Amplitude Compensation being modulated the operating current that the pulse train that forms dual modulation removes to control LED after direct current biasing is converted to unipolar signal; Seen through spectrum and be divided into two light-receiving passages with filter that the spectrum of optical sender emission is complementary by difference at receiver end, one is the high-speed light receive path, and one is low speed light-receiving passage; The high-speed light receive path receives pulsed optical signals, after opto-electronic conversion, amplification filtering, deliver to two signal processing channels, one is the burst process passage, directly detects to received signal, pulse modulated demodulation, obtains one road receive data and clock synchronization information, another is the OFDM treatment channel, carry out to received signal first envelope detection, recover the ofdm signal waveform, remove direct current biasing after the ADC conversion and deliver to a combiner; Low speed light-receiving passage receives continuous light signal, delivers to same combiner through removing direct current biasing after opto-electronic conversion, amplification filtering, the ADC conversion; Carry out the ofdm signal demodulation after the two paths of signals merging of combiner with input and obtain receive data.
4. the method that in LED-based visible light communication system, realizes pulse and OFDM dual modulation/demodulation according to claim 3, Amplitude Compensation wherein refers to the amplitude that occurs the one or more pulses before 0 in the modulated pulse trains is promoted, and the target of compensation is to make final emission output keep the OFDM shape information of modulation.
5. the method that in LED-based visible light communication system, realizes pulse and OFDM dual modulation/demodulation according to claim 3, wherein the control of LED is realized the light output of dual modulation signal, modulated pulse trains is controlled the electric current of LED with the break-make of switch form control LED current channel after the pulse train process DAC conversion of dual modulation, the power amplification; Both control sequential are synchronous with modulation.
6. the method that in LED-based visible light communication system, realizes pulse and OFDM dual modulation/demodulation according to claim 3, combiner wherein is weighted merging with the signal of input, and the size of weights is proportional to the signal to noise ratio of input signal.
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