CN107171989A - Channel estimation methods based on DFT in visible light communication system - Google Patents
Channel estimation methods based on DFT in visible light communication system Download PDFInfo
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- CN107171989A CN107171989A CN201710555831.9A CN201710555831A CN107171989A CN 107171989 A CN107171989 A CN 107171989A CN 201710555831 A CN201710555831 A CN 201710555831A CN 107171989 A CN107171989 A CN 107171989A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/022—Channel estimation of frequency response
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/0224—Channel estimation using sounding signals
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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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Abstract
The invention discloses the channel estimation methods based on DFT in a kind of visible light communication system, DFT channel estimation methods based on pilot tone calculate fairly simple, estimated accuracy is higher, often application is in systems in practice, the presence of virtual carrier causes the problem of DFT channel estimation methods based on pilot tone have energy leakage in visible light communication system, the loss of learning for needing the presence to virtual carrier to cause is compensated, but the influence of virtual carrier can be completely eliminated in not a kind of optimal backoff algorithm.The present invention carries out time domain amendment to the DFT channel estimation methods based on pilot tone that there is virtual carrier, so as to not influenceed completely by virtual carrier, improves the performance of channel estimation.
Description
Technical field
Estimate the present invention relates to the channel based on DFT in technical field of visible light communication, more particularly to a kind of visible light communication
Meter method.
Background technology
Orthogonal frequency division multiplexi (Orthogonal frequency division multiplexing, OFDM) is applied
Intersymbol interference (Inter-symbol interference, ISI) can be effective against in a communications system.Meanwhile, by symbol
Cyclic prefix (Cyclic Prefix, CP) is inserted between number, inter-carrier interference (Inter Carrier can be further eliminated
Interference,ICI)。
Therefore ISI and ICI can be effective against in VLC (visible light communication) system by OFDM technology being applied, and be carried simultaneously
The availability of frequency spectrum of high system.Intensity modulated is used directly to detect (Intensity Modulation/ in VLC systems
Direct Detection, IM/DD), signal is propagated using light intensity as carrier, and the modulation system that the system is used is DCO-
OFDM(Direct Current Optical-OFDM)。
Channel estimation is extremely important step in communication system, only obtains channel information, can just carry out follow-up
Channel equalization, demodulation, decoding etc..Channel estimation method based on pilot tone is fairly simple, and the degree of accuracy is higher, conventional channel estimation
Algorithm has least square (Least Squares, LS), linear minimum mean-squared error (Linear Minimum Mean Squared
Error, LMMSE) and channel estimation method based on DFT.LS algorithms do not account for the influence of noise and poor-performing, LMMSE
Better performances but too high complexity in systems in practice and are not applied to, and the channel estimation method based on DFT calculates simple,
Performance comparision is good, therefore the present invention applies the channel estimation method based on DFT in VLC systems.
The content of the invention
Goal of the invention：There is provided DFT is based in a kind of visible light communication system for the problem of present invention exists for prior art
Channel estimation methods, calculate simple, performance is good.
Technical scheme：It is of the present invention
The channel estimation methods based on DFT comprise the following steps in visible light communication system of the present invention：
(1) the initial frequency-domain channel response of pilot sub-carrier in visible light communication system is obtained using LS algorithmsIts
In,
In formula, YPRepresent the reception signal at pilot frequency locations, XPThe N for the data inserted for transmitting terminal at pilot frequency locationsP×NP
Value at diagonal matrix, corresponding pilot frequency locations of its diagonal element to send signal x N point FFT values, NPFor pilot tone
Number, N is subcarrier number；
(2) it is rightDo the initial time-domain channel response that matrix operation obtains channelOnly take in channel response length
Channel information, i.e.,
B is N in formulaPThe DFT matrixes of × L ranks, L is the maximum length that channel time domain is responded；
(3) initial time-domain channel is respondedAfterbody zero-adding make its length be N, i.e.,
(4) it is rightMake N point DFT transforms, the channel estimatedWherein,
Beneficial effect：Compared with prior art, its remarkable advantage is the present invention：When the time domain correction algorithm of the present invention is exactly
The thought of domain LS algorithms, the process of time domain is transformed to for step 2, is not directly to be IDFT, but uses incomplete IDFT
Transformation matrix, can directly obtain time domain response, it is to avoid caused by the channel information missing that the presence of virtual carrier is caused by mistake
Difference, that is, the problem of it also avoid energy leakage occurs.Therefore the algorithm is no longer influenceed by virtual carrier, is better than simple compensation
Algorithm, and the appearance without error platform.
Brief description of the drawings
Fig. 1 is visible ray DCO-OFDM baseband system block diagrams.
Fig. 2 is VLC baseband system emitter fundamental block diagrams.
Fig. 3 is VLC baseband system receiver fundamental block diagrams.
Fig. 4 is visible ray DCO-OFDM system physical layer frame structures.
Fig. 5 is the NMSE of DFT backoff algorithms based on pilot tone and time domain correction algorithm.
Embodiment
First, technical problem is analyzed
The system model of the present invention is specific as shown in figure 1, Fig. 2 sends out for VLC baseband systems according to DCO-OFDM system designs
Penetrate the fundamental block diagram of machine.The information received from MAC layer reaches FPGA baseband processing portion by network interface module, to Ether frame
Data are scrambled, channel coding, intertexture, constellation mapping, IFFT are converted, add the operation such as CP, obtain time domain discrete data.In number
According to entering before DAC module, frame head data, i.e. short training sequence (Short Training Sequence, STS), length are added
The data such as training sequence (Long Training Sequence, LTS), physical header (PHY Header), for the same of receiving terminal
Step, channel estimation etc..Fig. 3 is the main handling process of receiver of visible ray DCO-OFDM systems.Receiver data turn by AD
After changing, frame detection, synchronization etc. are carried out first, and then the data to reception remove CP, then obtain frequency domain data by FFT operations,
Channel estimation is carried out by the data of the training sequence data before extraction and the pilot frequency sequence received, line frequency is next entered again
The subsequent operations such as domain is balanced, demodulation, deinterleaving, channel decoding, descrambling.The processing of base band is realized that Fig. 4 is can in physical layer
See the physical layer frame structure of light DCO-OFDM systems.The design method of frame refers to IEEE802.11a, a frame data in VLC systems
In include STS, LTS, PHY Header, physical layer load (PHY Payload) and frame period (IFS).Wherein, STS is used for frame
Detection and frame synchronization, LTS after STS, for being detected in frame after carry out channel estimation.Followed by physical header part, it is used for
The parameter that the demodulation of some receiving terminals needs is transmitted, such as order of modulation, length is an OFDM symbol.It is transmission after physical header
Valid data, are finally frame period parts, and length is one or two OFDM symbol, the knot for indicating the frame data of physical layer one
Beam.
For DCO-OFDM systems, the reception signal at pilot frequency locations can be expressed as
YP=XPHP+WP
X in formulaPFor NP×NPDiagonal matrix, its diagonal element leads to send the correspondence of signal x N point FFT values
Value at frequency position, HPFor NPThe channel column vectors of × 1 dimension, WPFor NPThe noise vector of × 1 dimension.
The problem of channel estimation process based on pilot tone mainly considers three below aspect：The choosing of transmitting terminal pilot frequency design
Select, obtain channel information at pilot tone, whole channel information is obtained by the channel information at pilot tone.
In OFDM transmission system, conventional pilot frequency design has Comb Pilot and Block-type pilot.Comb Pilot is applied to decline soon
Fall channel, and Block-type pilot is applied to slow fading channel.Because visible light channel is slow fading channel, therefore led using block
Frequency mode carries out the calculating of channel estimation.
Channel estimation method based on DFT is easily achieved, better performances, so receiving much concern.In order to prevent other systems
Interference, VLC systems reserve a part of subcarrier in the both sides of transmission bandwidth as guard band, referred to as virtual subcarrier
(Virtual Carriers, VC).
In it there is the system of virtual carrier, typically directly the frequency response value at virtual subcarrier is set after the estimation of LS algorithms
For 0.On frequency domain, the presence of virtual carrier adds two rectangular window functions equivalent to channel frequency response；In time domain, equivalent to
Convolution is done in the time domain response of the time domain impulse response and rectangular window function of system.Due in time domain with including the sinc of two main lobes
Function does convolution, and causing channel energy is concentrated on L point above, but can be leaked on all sampled points,
Cause channel CIR elongated, cause the leakage of energy.
For influence of the analysis virtual carrier to system, it is assumed that there is also pilot tone at virtual carrier position, its regularity of distribution with effectively
It is identical at data subcarrier.IfThe valid data in respectively on the right side of intermediate virtual subcarrier and left side are sub
The pilot number that pilot number, virtual subcarrier planted agent in carrier wave have, NPFor total pilot number, i.e.,
IIn、IOutThe set of pilot frequency locations, has respectively in valid data subcarrier and virtual subcarrier
Channel response after the value at the pilot frequency locations in virtual subcarrier then is set into zeroFor
In formula,Expression uses the channel response value at the pilot frequency locations that LS channel estimation methods are estimated.
The then channel estimation based on DFT transformFor
N in formulauFor valid data sub-carrier number,For Nu×NPDFT transform matrix,For valid data subcarrier
The actual frequency response at place,For channel estimation errors, the data in its virtual carrier are 0,To be virtual
There should be the channel frequency response of pilot tone at subcarrier.Evaluated error in the valid data subcarrier for the system that then there is virtual carrier
For
As can be seen from the above equation, the channel estimation method error is by the channel information missing in noise and virtual subcarrier
Caused by, and this two parts is separate.
Then the mean square error (Mean Square Error, MSE) of the channel estimation method is
β is the constant related to modulation system in formula, and SNR is the average signal-to-noise ratio of input signal,
For the autocorrelation matrix of channel frequency response.
As can be seen that there is being made up of based on DFT channel estimation errors two parts for virtual carrier：One is influence of noise institute band
The error come, it is believed that DFT channel estimation errors are equal during with the absence of virtual carrier；Two be due to the information that virtual carrier is brought
The information lacked at the error for lacking and causing, virtual carrier is more, so that the channel estimation caused is also bigger, particularly in height letter
Make an uproar than when, the presence of virtual carrier causes energy to leak on adjacent useful subcarrier, but the operation blocked in time domain
So that the energy loss of useful signal, causes floor effect.
, can be from the aspect of two in order to reduce virtual carrier to the influence based on DFT channel estimation methods, one is to reduce to make an uproar
The influence of sound, two be to reduce the error caused by virtual carrier loss of learning, that is, needs to mend the channel value at virtual carrier
Repay.The influence for reducing noise is basic based on DFT channel estimation methods, in order to improve the precision of channel estimation, it is necessary to void
The error that carrier wave missing is caused is compensated.
2nd, technical method
The influence caused for reduction virtual carrier to system, common backoff algorithm has repetition marginal value method and portion subcarriers
Method.This two kinds of algorithms calculate fairly simple, but application condition is big, and void is completely eliminated without a kind of optimal backoff algorithm
The influence of carrier wave.Method of the present invention amendment based on DFT, improves estimation performance, comprises the following steps that：
(1) the initial frequency-domain channel response of pilot sub-carrier in visible light communication system is obtained using LS algorithmsIts
In,
(2) it is rightDo the initial time-domain channel response that matrix operation obtains channelOnly take in channel response length
Channel information, i.e.,
B is N in formulaPThe DFT matrixes of × L ranks, L is the maximum length that channel time domain is responded；
(3) initial time-domain channel is respondedAfterbody zero-adding make its length be N, i.e.,
(4) it is rightMake N point DFT transforms, the channel estimatedWherein,
FNFor DFT transform matrix.
3rd, simulating, verifying
Floating-point emulation is carried out on MATLAB to the DFT channel estimation methods based on pilot tone in visible ray DCO-OFDM systems
Com-parison and analysis, because visible light channel is slow time-varying channel, therefore using the research of Block-type pilot progress channel estimation.It is at this
In system, in the case where transmission data frame is not grown, it is believed that a frame in channel keeps constant.The two long instruction of design generally in frame head
Practice sequence, take both to be averaged to improve the quality of channel estimation, can typically increase precision by estimating to be averaging twice
3dB.Main simulation parameter is：FFT/IFFT points N=512, subcarrier spacing Δ f=312.5KHz, signal bandwidth BW=
242 × 312.5KHz=75.625MHz, pilot subcarrier positions k=2~243,271~512, virtual sub-carrier positions k=1,
244~270, CP length NCP=64, CIR length NCIR=25, OFDM symbol number N in a frame dataOFDM=32.To return in emulation
One changes mean square error (Normalized MSE, NMSE) as the performance indications of channel estimation method, and NMSE is defined as follows
H represents the real response value of channel in formula,Represent the estimate of channel.
The DFT channel estimation methods based on pilot tone during to there is virtual carrier in visible ray DCO-OFDM systems are imitated
True analysis, being repaiied based on DFT channel estimation methods, two kinds of backoff algorithms and time domain when Fig. 5 gives LS algorithms, there is virtual carrier
The NMSE simulation results of normal operation method.The performance of time domain correction algorithm has very big compared to backoff algorithm it can be seen from simulation result
Lifting, and the appearance without error platform.
Above disclosed is only a kind of preferred embodiment of the invention, it is impossible to the right model of the present invention is limited with this
Enclose, therefore the equivalent variations made according to the claims in the present invention, still belong to the scope that the present invention is covered.
Claims (1)
1. the channel estimation methods based on DFT in a kind of visible light communication system, it is characterised in that：This method includes
(1) the initial frequency-domain channel response of pilot sub-carrier in visible light communication system is obtained using LS algorithmsWherein,
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| CN109547103A (en) * | 2018-12-17 | 2019-03-29 | 武汉邮电科学研究院有限公司 | A kind of monitoring method and system of visible light channel |
| CN109617851A (en) * | 2019-03-05 | 2019-04-12 | 中国人民解放军国防科技大学 | Channel estimation method and device based on DFT smooth filtering |
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Application publication date: 20170915 |