KR101618003B1

KR101618003B1 - Apparatus and method for transceiving a signal in a visible light communication

Info

Publication number: KR101618003B1
Application number: KR1020150047714A
Authority: KR
Inventors: 김병욱
Original assignee: 경일대학교산학협력단
Priority date: 2015-04-03
Filing date: 2015-04-03
Publication date: 2016-05-04

Abstract

In this specification, a visible light transmitting apparatus for transmitting a signal in a Visible Light Communication (VLC) system includes a serial / parallel converter (S / P) for converting a plurality of serial data signals into a plurality of parallel data signals; A precoder for precoding a signal output through the serial / parallel converter using cyclic shifted orthogonal polyphase sequences of length M; A pilot inserter for generating a hidden pilot used for channel estimation; And an IFFT unit for performing IFFT (Inverse Fast Fourier Transform) by combining a signal output from the precoder and a signal output through the pilot inserter, wherein the M-1 cyclically shifted orthogonal polyhedral sequences of length M And one cyclic-shifted orthogonal polyphase sequence of length M is used for the hidden pilot.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a method for transmitting and receiving signals in a visible light communication system,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a visible light communication (VLC) system, and more particularly, to a method for transmitting and receiving signals in a visible light communication system using an OFDM scheme and a device supporting the same.

2. Description of the Related Art Recently, as the use of light-emitting diodes (LEDs) as light sources has grown in popularity, Visible Light Communication (VLC) technology capable of communicating with LED lighting devices has been introduced.

VLC technology is a communication technology that transmits data to light of a visible light wavelength band that can be perceived by human eyes and transmits it wirelessly. VLC technology is distinguished from existing wired optical communication technology and infrared wireless communication technology in that it uses light of visible light wavelength band. Also, unlike radio frequency (VRF) communication, VLC technology has the convenience of being freely used without being regulated or licensed in terms of frequency use, excellent physical security, and has a differentiating feature that a user can visually confirm a communication link .

Visible light communication (VLC) technology based on LEDs can provide both light and communication, which is of high interest in both academia and industry.

Recently, the OFDM scheme has been applied to VLC due to its high data rate. However, when OFDM is applied to the VLC, the OFDM signal has a real value of one polarity, so that the system bandwidth can be considerably reduced.

In a practical OFDM-based VLC system, the channel estimation process is required for data demodulation and the channel efficiency reduces the bandwidth efficiency.

Also, due to the large fluctuation of the transmitted OFDM signal, the result is that the quality of the light is deteriorated as the color change occurs.

[1] Jean Armstrong, " OFDM for Optical Communications, " J. Lightw. Technol., Vol. 27, no. 3, Feb. 2009, pp. 189? 204. [2] Z. Yu, R. J. Baxley, and G. T. Zhou, "EVM and Achievable Data Rate Analysis of Clipped OFDM Signals in Visible Light Communication," EURASIP J. Wirel. Comm., Vol. 2012, no. 321, Oct. 2012.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for improving the bandwidth and reducing the flickering of light based on the HP-VLC (Hidden Pilot Aided Precoding Technology for VLC) method.

It is also an object of the present invention to provide a method of transmitting a visible light signal through precoding and hidden pilot vectors using cyclically shifted orthogonal polyphase sequences.

It is another object of the present invention to provide a method for canceling interference caused by a hidden pilot vector and receiving a visible light signal using an MMSE technique.

In the present invention, the precoder performs precoding using Hermitian symmetry so that a signal output from the S / P conversion unit has a real valued signal.

The present invention also includes a DC biasing and clipping unit for converting a signal output through the IFFT unit into a signal having a positive real value and clipping a signal portion out of the driving range of the LED And further comprising:

The present invention further includes an LED light source module for converting a signal output through the DC biasing and clipping unit into a visible light signal and transmitting the converted visible light signal.

The present invention also relates to a method for transmitting a signal in a Visible Light Communication (VLC) system, which comprises converting a plurality of serial data signals into a plurality of parallel data signals through a serial / parallel converter (S / P) ; Precoding the plurality of parallel data signals using cyclic shifted orthogonal polyphase sequences of length M; Generating a hidden pilot used for channel estimation; And performing inverse fast Fourier transform (IFFT) by combining the precoded data signal and the generated hidden pilot signal, wherein the M-1 cyclic shifted orthogonal polyphase sequences of length M are subjected to precoding And one cyclic-shifted orthogonal polyphase sequence of length M is used for the hidden pilot.

In addition, the present specification includes a step of converting the IFFT-processed signal into a signal having a positive real value; And clipping the signal portion beyond the driving range of the LED.

The present specification uses the well-designed precoder and hidden pilot symbols to obtain channel information without loss of bandwidth efficiency and to reduce the flickering that may occur in OFDM-based VLC signals have.

Further, by using the HP scheme, the present invention has an effect of guaranteeing a data transmission rate and color quality superior to the conventional PSAM (pilot symbol aided modulation) method.

1 is a schematic diagram illustrating an example of a visible light communication system to which the methods proposed herein can be applied.
2 is a block diagram showing an example of an internal block diagram of a visible light transmitting apparatus proposed in this specification.
3 is a diagram showing an example of an internal block diagram of a visible light receiving apparatus proposed in this specification.
FIG. 4 is a diagram illustrating a change amount of an OFDM signal truncated according to a biasing ratio proposed in the present specification.
5 is a diagram illustrating performance of channel estimation according to SNR with dynamic range limitation of the LED input signal proposed in the present specification.
6 is a diagram illustrating a channel estimation performance according to the biasing ratio and SNR proposed in the present specification.
7 is a diagram illustrating a data rate that can be obtained according to a change in a biasing ratio proposed in the present specification.
8 is a diagram illustrating a data transmission rate obtained with respect to the biasing ratio and SNR proposed in the present specification.
FIG. 9 is a diagram showing the overall clipping amplitude according to the biasing ratio proposed in the present specification.
Figure 10 is a comparison of the percent flicker applied in this specification with the prior art in comparison to the quality of the light.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification and claims, when a section is referred to as "including " an element, it is understood that it does not exclude other elements, but may include other elements, unless specifically stated otherwise.

Recently, VLC systems have received much attention due to high-speed indoor optical wireless communication.

Compared to conventional wireless communication, LED optical communication has many advantages.

As an example, LED optical communication is an unregulated wide bandwidth for a high data rate, and there is no interference with other channels in the adjacent space.

For faster data modulation in VLC systems, we use a light modulation technique with direct detection (IM / DD).

Pulses in visible light wireless communications using such light modulation techniques are simple and low cost, and are popular.

However, the presence of wireless multipaths results in multipath distortion and inter-symbol interference (ISI) of the signal, thereby requiring a complicated equalization technique for using IM / DD do.

Accordingly, it is known that a multi-subcarrier modulation (MSM) technique is strong against distortion of signals due to multipath, and is effective for fluorescent-light noise close to DC.

As one example of the MSM scheme, an OFDM scheme is used.

The OFDM scheme is an efficient data transmission scheme for high-speed mobile communication and ISI.

Currently, methods such as ACO-OFDM, DCO-OFDM, and DMT-OFDM are being discussed to ensure OFDM signals with real-valued unipolar in VLC systems.

Considering an actual OFDM system, the biggest problem of the OFDM signal having a single real value is as follows.

The first is the low efficiency of bandwidth caused by Hermitian symmetry conditions for generating real-valued signals and pilot symbol insertion for channel estimation.

Second, the VLC signal based on OFDM has a large fluctuation.

Therefore, flickering is caused thereby, and deterioration of the illumination quality which can be noticed by the human eye, such as chromaticity change.

Pilot symbol-aided modulation (PSAM) is a widely used method in actual implementation for channel tracking.

The loss of significant bandwidth occurs because the number of reserved subcarriers for pilot tones is greater than the length of the channel taps.

Thus, blind and semiblind methods are provided to reduce this bandwidth loss.

However, these methods complicate implementation and are inefficient because of the relatively low accuracy of channel estimation.

Therefore, in order to solve the foregoing problems, the present invention provides a Hidden pilot-aided precoding VLC (HP-VLC) method using precoders and hidden pilots.

The Hidden pilot-aided precoding (HP) method refers to a method using an added pilot vector based on a linear precoder and orthogonal polyphase sequences.

The HP method has a relatively low signal dynamic range because it powers the pilot vector as well as in terms of bandwidth efficiency.

Also, the HP method can reduce the flickering of light caused by the large fluctuation of the OFDM-based VLC signal.

In the past, there have been no documents dealing with the problem of the quality of light and efficiency of bandwidth of VLC signals based on OFDM.

As described above, the present invention provides a method of applying hidden pilot aided precoding (HP) in a VLC system, that is, an HP-VLC method.

When the OFDM scheme is applied to a VLC system, there is a limitation due to an OFDM symbol having a positive real value.

However, the method of transmitting a signal by additionally designing (or designing) the HP-VLC scheme proposed in this specification, i.e., linear precoder and hidden pilot, can successfully avoid interferences between signals that may necessarily occur.

That is, the HP-VLC method proposed in the present specification can efficiently estimate a channel without loss of bandwidth and can provide a good quality of light because the signal changes dynamically in a low range.

Also, as can be seen from the computer simulation described below, it can be easily seen that the HP-VLC method proposed herein is superior to other conventional methods in terms of data rate and light quality.

1 is a schematic diagram illustrating an example of a visible light communication system to which the methods proposed herein can be applied.

The visible light communication system 10 includes a visible light transmitting apparatus 200 for transmitting data through a light source and a visible light receiving apparatus 100 receiving the light transmitted through the visible light transmitting apparatus and receiving data.

The visible light transmitting apparatus 200 generates a modulated signal through various modulation schemes of transmission data and transmits the modulated signal using a light source through a wavelength band matching the characteristics of the modulated signal.

The visible light transmitting apparatus may be implemented by including the components shown in FIG.

That is, the visible light transmitter may include a signal modulator 210, a light source 220, and the like.

The visible light receiving apparatus 100 receives a light source and acquires transmission data by demodulating data transmitted through the received light source through a demodulation scheme corresponding to various modulation schemes.

The visible light receiving apparatus may be implemented by including the components shown in FIG.

That is, the visible light receiving apparatus may include a photoelectric conversion unit 110, a signal demodulation unit 120, and the like.

VLC System Signaling Model

First, a signaling model related to signal reception in a VLC system will be briefly described.

The visible light reception signal received through the visible light receiving device can be expressed by Equation 1 below.

here,

Represents a convolution,

Represents a photodetector responsivity.

Also,

Represents an intensity-modulated signal transmitted from a visible light transmitting device,

Represents a channel impulse response.

Also,

Represents a noise process,

Is a photocurrent received by the visible light receiving apparatus and represents a value by which the optical signal is converted into an electrical signal through the photoelectric conversion unit.

remind

May represent a general linear filter channel model with added noise.

In the case of a VLC system, an input signal

Is different from an input signal in a general wireless communication system in that it has an instantaneous optical power and a non-negative real value (a value greater than 0 or 0).

Here, assuming that there is no loss,

= 1,

Can be expressed simply.

This is because the shot noise generated around the high light intensity corresponds to the noise source dominant in the VLC channel,

Because it can be modeled as an additional white Gaussian that is distributed to

OFDM based VLC System

Next, a brief description will be given of a VLC (OFDM based VLC) system using OFDM.

In an OFDM system, a discrete time-domain signal,

Is generated by applying an inverse discrete Fourier transform (IDFT) to a frequency-domain signal X. The inverse discrete Fourier transform (IDFT)

The discrete time domain signal

The

, The frequency domain signal X is

. &Lt; / RTI >

The discrete time domain signal output via IDFT

Can be expressed by the following equation (2).

here,

Represents the IDFT size.

Also, in the case of a VLC system using an LED, the electric signal input to the LED must have a real valued unipolar.

The real number value of the one pole can be regarded as a real number value larger than 0, that is, a positive real number value.

Depending on the nature of the IDFT, a real-valued time-domain signal,

Is a Hermitian symmetric frequency domain signal

.

The hermetic symmetry can be expressed as Equation (3) below.

Here, * denotes a complex conjugate,

silver

in,

The

Lt; / RTI >

HP-VLC (Hidden Pilot VLC) System

Hereinafter, a signal transmission / reception method (hereinafter referred to as 'HP-VLC method') in the HP-VLC (Hidden Pilot VLC) system proposed in the present specification will be described in detail with reference to FIG. 2 to FIG.

The HP-VLC method proposed in the present invention uses a linear precoder and a hidden pilot vector (VLC) to transmit and receive visible light signals while ensuring high bandwidth efficiency and good light quality in an OFDM-based VLC system. ) Is newly defined.

The HP-VLC method also includes a precoder based on cyclic shifted orthogonal polyphase sequences to remove interference that may occur between data symbols and pilot symbols to predict an accurate channel. And a pilot vector.

As will be seen from the following equations, any orthogonal polyphase sequence among the orthogonal polyphase sequence sets is assigned to the hidden pilot vector, and the remaining orthogonal polyphase sequences are assigned to the precoder.

The orthogonal polyphase sequences have good periodic autocorrelation and cross-correlation.

Since the cyclic shift values of the orthogonal polyphase sequence have orthogonality with other orthogonal polyphase sequences, the interference that may occur between the data symbol and the pilot symbol can be reduced.

In order to design the precoder and hidden pilot vectors proposed in this specification, a length M (

Quot;). &Lt; / RTI >

here,

Represents the prime number,

Represents an integer value.

M sets of orthogonal polyphase sequences of length M,

Can be expressed as Equation (4) below.

here,

The precoder matrix < RTI ID = 0.0 >

Wow

(Hidden pilot vector)

Can be expressed by Equation (5) below. That is, one of the M orthogonal polyphase sequences corresponds to the hidden pilot vector, and the remaining M-1 corresponds to the precoder.

here,

Denotes a discrete Fourier transform (DFT) matrix,

The

Represents a vector.

Therefore, the transmission symbol (convex) in the frequency domain, which is output using the precoder matrix and the hidden pilot vector,

The

Lt; / RTI >

here,

The

And a precoder matrix < RTI ID = 0.0 >

Can be expressed as Equation (6) below.

here,

Lt; / RTI >

1

Represents a zero vector.

The pilot vector

The

Lt; / RTI >

Of the components have a reserved order

Of complex conjugate.

Whole information data block (

) &Lt; / RTI >

And a Hermitian symmetry matrix of data vectors such as < RTI ID = 0.0 > This is to ensure that the signal output through the IFFT has a real value.

Therefore, the signal output through the IFFT unit of FIG. 2 has a negative real value.

here,

Is a data vector

Of complex conjugate.

Thereafter, a guard interval (CP) is inserted in each OFDM symbol in the time domain output through the IFFT.

Time-domain OFDM symbol into which the CP is inserted (

) Can be expressed by the following Equation (7).

A time domain OFDM symbol (or signal)

Has a large range of negative amplitudes and positive amplitudes and tends to have elements of real-valued bipolar. Here, the real value of the anode may mean a negative real value and a positive real value.

The dynamic range (or drive range) of the LED, that is, the linear section of the LED

If it is an interval, the out-of-gap portion is clipped to maintain linearity.

In other words,

The signal corresponding to the upper part of

The signal corresponding to the lower part of the signal is clipped.

For example,

Is referred to as a DC bias / clipping signal,

Is a signal output from the DC biasing / clipping unit, that is, an LED input signal,

Can be expressed by Equation (8) below.

Is a non-negative real value,

to be. here,

The dynamic range of

. &Lt; / RTI >

DC bias

The LED input signal

Is a value required to satisfy the limit value of the non-negative magnitude of.

The high distortion of the OFDM signal that may be generated by the clipping procedure may depend on the DC bias level.

Therefore, in a VLC system in which high average optical power is required for high target dimming, a general OFDM scheme can be limitedly used.

To quantify the DC biasing associated with the dynamic range of the LED, the biasing ratio

Can be defined as shown in Equation (9) below.

Here, the DC bias is removed and the light source unit, that is, the signal input to the LED

Can be expressed as Equation (10) below.

here,

Variance

Lt; RTI ID = 0.0 > additive < / RTI > white Gaussian noise.

Visible light send Device

2 shows an example of an internal block diagram of a visible light transmitting apparatus proposed in this specification.

The visible light transmitting apparatus 200 may include a signal modulator 210, a light source 220, and the like.

The signal modulator 210 modulates the source data according to an Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme.

The signal modulator 210 includes a S / P converter 211, a precoder 212, a pilot insertion unit 213, an inverse fast Fourier transform unit (IFFT) 214, A CP insertion unit 215, a P / S conversion unit 216, a signal transmission unit 217, a DC biasing / clipping unit 218, and the like.

The serial / parallel converter (S / P) 211 converts a plurality of serial data signals into a plurality of parallel data signals when a plurality of serial data signals corresponding to bit-type input data are input.

In order to reduce high PAPR, flickering, and the like caused by multicarrier transmission in an OFDM-based VLC system, the precoder 212 adds a cyclic shifted orthogonal polyphase to a signal output from the S / Precoding using orthogonal polyphase sequences.

Here, among the M orthogonal polyphase sequences, M-1 orthogonal polyphase sequences are used for the precoding.

The signal output from the S / P includes only data symbols. That is, the pilot signal is not included in the signal output from the S / P.

In the conventional case, both the data symbol and the pilot symbol are included in the signal output from the S / P.

However, the visible light transmitting apparatus proposed in this specification positions the pilot insert after the precoder and the precoder to alleviate the high PAPR and flickering caused by the OFDM application.

That is, S / P outputs only data symbols that do not include pilot symbols, and performs precoding using orthogonal polyphase sequences cyclically shifted through the precoder to the output signals.

Then, the visible light transmitting apparatus combines the pilot symbol generated through the pilot inserting unit with the signal output through the precoder, and performs IFFT.

Here, the pilot generated through the pilot inserting unit is expressed as a hidden pilot.

One orthogonal polyphase sequence out of the M orthogonal polyphase sequences is used for the hidden pilot.

The pilot inserter 213 generates at least one pilot signal to be used for channel estimation and channel equalization in the visible light receiver, and outputs the generated at least one pilot symbol and a plurality of data symbols output through the precoder to a channel So as to facilitate estimation.

Here, the signal output from the S / P, i.e., the entire information data block

Quot ;, and "

Orthogonal polyphase sequences < RTI ID = 0.0 >

.

in this case,

Length

Lt; / RTI >

Lt;

to be.

here,

Represents the prime number,

Represents an integer.

The orthogonal polyphase sequences have very good periodic autocorrelation and cross-correlation.

Since the cyclically shifted values of the orthogonal polyphase sequence have orthogonality with other orthogonal polyphase sequences, the interference between the data symbol and the pilot symbol is reduced.

As you can see,

A precoder matrix

However,

The hidden pilot vector of

(5) "

). &Lt; / RTI >

here,

Denotes a discrete Fourier transform (DFT) matrix,

The

Represents a vector.

That is, the precoder matrix of the precoder and the frequency domain transmission symbol

The

Lt; / RTI >

here,

Is outputted from the S / P

Lt; / RTI >

In this case, the precoder matrix

Can be designed as shown in Equation (6).

The overall pilot vector is

Lt; / RTI >

Lt; RTI ID = 0.0 >

Of complex conjugate.

Whole information data block

The data vector

And a Hermitian symmetry matrix of data vectors such as < RTI ID = 0.0 >

here,

Is a data vector

Of complex conjugate.

The inverse fast Fourier transform unit (IFFT) 214 inverse fast Fourier transforms the parallel modulated data symbols. In this case, the parallel modulated data symbols are transformed from the frequency domain to the time domain.

The guard interval inserter (CP inserter 215) inserts a cyclic prefix (CP) into the serial modulated data symbols. At this time, the CP and the modulated data symbol are combined to form an OFDM symbol.

That is, the CP is inserted into each OFDM symbol in the time domain through the guard interval inserting unit. The OFDM symbol in the time domain in which the CP is inserted

Is expressed by Equation (7).

Here, the time domain signal

Tends to have large dynamic ranges of negative and positive amplitudes and real-valued bipolar components.

The P / S converter (P / S) 216 converts the parallel modulated data symbols converted into the time domain into serial modulated data symbols.

The signal transmitter 217 converts an OFDM symbol from a digital signal to an analog radio frequency signal.

The DC biasing / clipping unit 218 converts a signal output from the signal transmitter into a signal of a real-valued unipolar signal, that is, a signal of a positive real value, (Linear section of the LED) of the input signal.

Specifically, the DC biasing of the DC biasing / clipping unit 218 converts the signal into a signal having a positive real value, and the clipping function clips a signal out of the linear region of the LED.

In a VLC system using an LED, a signal input to an LED must have a real and a unipolar value. Therefore, when the OFDM scheme is applied to the VLC system, the DC biasing / clipping function is required.

Also, LED dimming in a VLC system based on OFDM can be controlled via the DC biasing. That is, the DC biasing can be given to match the brightness of the LED dimming value.

A process in which a specific signal has a positive real value through the DC biasing / clipping unit 218 may be as follows.

Specifically, a real-valued time-domain signal having a real value according to the characteristics of the IDFT,

Is a Hermitian symmetric frequency domain signal

.

An example of the hermetic symmetry can be expressed as Equation (3).

The process of clipping the signal by the DC biasing / clipping unit 218 may be as follows.

IFFT and DC processed time domain signals

Tend to have large dynamic ranges of negative and positive amplitudes and real-valued bipolar elements.

However, VLC systems are limited by the dynamic range that can maintain the linearity of the LEDs. Thus, the nonlinear characteristics of the LED can be compensated by a digital predistortion process. That is, the signal outside the dynamic range of the LED is clipped.

The dynamic range of the LED, that is, the linear section of the LED

If it is an interval, the part out of the interval is cut off.

In other words,

The signal corresponding to the upper part of

The signal corresponding to the lower part of the signal is clipped.

For example,

A signal input to the DC biasing / clipping unit,

Can be expressed as Equation (8).

Is a non-negative real value,

to be. here,

The dynamic range of

. &Lt; / RTI >

DC bias

The LED input signal

Therefore, in a VLC system in which a high average optical power is required for high target dimming, a general OFDM scheme can be limitedly used.

Can be defined as Equation (9).

Can be expressed as Equation (10).

The light source 220 may include a light source driver 221 and a light-emitting diode (LED) light source module 222.

The light source unit 220 converts an OFDM signal into a visible light modulation signal and transmits the converted visible light modulation signal.

The light source driver 221 drives the LED light source module 222. The LED light source module may include one or more LEDs.

The light source driving unit 221 generates driving signals for controlling on and off states of the plurality of LEDs according to the OFDM modulated signal and outputs the driving signals to the LED light source module 222.

The LED light source module 222 is turned on in response to a driving signal of the light source driving unit, and generates and transmits a visible light modulation signal corresponding to light emitted from the plurality of LEDs.

Next, a method of receiving a visible light signal through a visible light receiving apparatus in an HP-VLC system will be described.

The vector form of the LED signal received through the DC biasing removing unit and the CP removing unit shown in FIG. 3, that is, the LED signal received by the visible light receiving device after the DC bias is removed,

Can be expressed as

To reduce the effect of DC-biasing, the average amplitude of the LED signal received by the visible light receiver through the DC biasing canceller is eliminated.

Thereafter, the DC-bias is removed, the guard interval (CP) is removed from the received LED signal, and the OFDM block from which the CP is removed is subjected to a discrete Fourier transform (DFT)

Lt; / RTI >

The demodulated

Symbol vector

Can be expressed by Equation (11) below.

here,

The first column

With

column-wise circulant matrix,

Length

Domain channel vector of < RTI ID = 0.0 >

silver

represents a zero vector.

Also,

The

Having the main diagonal components with the components of

Diagonal channel matrix,

The

, &Lt; / RTI >

The

Having

.

The signal output through the FFT of FIG. 3, that is,

Is input to the interference elimination of FIG. 3, the influence of the interference due to the hidden pilot is removed, and the channel value estimated by the channel estimator of FIG. 3

(13), a data symbol, that is, a signal transmitted from the visible light transmitting apparatus

As shown in FIG.

Hereinafter, channel estimation and symbol detection in a visible light receiving apparatus will be described in more detail.

The channel estimation is performed in a channel estimator of FIG. 3, and the symbol detection is performed in a symbol detector of FIG.

In a practical VLC-OFDM system, performance must take into account that there is an error in channel estimation.

The HP-VLC method proposed herein uses a minimum mean square error (MMSE) channel estimator in channel estimation.

In the visible light receiver, the channel state information can be estimated by post-processing the received visible light signal using the characteristics of the designed precoder and hidden pilot.

The post-processing is performed through the post-processing unit of FIG. 3, and the post-processing is a process of eliminating a data symbol (that is, a transmission signal) precoded in order to estimate a channel through hidden pilots.

That is, it is possible to prevent the channel estimation from being inaccurate due to the transmission signal during the channel estimation using the hidden pilot through the post-processing.

Processing for post-processing through the post-processing unit will now be described.

(Equation 10)

(Block) output through the post-processing unit can be expressed by Equation (12) below.

As can be seen from equation (12)

Serves as an interference which deteriorates the performance of the channel estimation.

Thus, for more accurate detection of data symbols,

.

for teeth,

Is the first column,

Having

column-wise circulant matrix.

here,

The

&Lt; / RTI >

Based on the commutativity characteristic of the circular convolution,

The

(Or switched) to < / RTI >

Due to the periodic cross-correlation of the precoder and hidden pilots designed in the visible light transmitter,

And the interference vector

All decrease. Thus, the channel can be successfully estimated.

The signal output through the post-

Using the Minimum Mean Square Error (MMSE) method for channel estimation in Equation (12), the channel estimation value (

) Can be expressed as shown in Equation (13) below.

In order to detect a data symbol using the channel estimation result, the interference caused by the hidden pilot vector should be removed. That is, the interference caused by the hidden pilot on the transmission signal must be removed in order to accurately detect the transmission signal (data symbol). This can be done through the interference canceller of FIG.

Specifically,

(11)

, It is possible to obtain a received symbol having a reduced effect due to the interference of the pilot.

here,

The

Lt; RTI ID = 0.0 > diagonal < / RTI >

Frequency channel

Respectively.

Therefore, the interference due to the hidden pilot is removed, and a received symbol (i.e., a signal output through the interference canceller

) Can be expressed by the following Equation (14).

here,

Can be expressed by Equation (15) below.

here,

The

The diagonal matrix with the main diagonal elements of.

, The signal output through the interference canceller

) To the detected data symbol (< RTI ID = 0.0 >

) Can be expressed by the following Equation (16).

Visible light reception Device

3 shows an example of an internal block diagram of a visible light receiving apparatus proposed in this specification.

The visible light receiving apparatus 100 may include a photoelectric conversion unit 110, a signal demodulation unit 120, and the like.

The photoelectric conversion unit (or photodiode) 110 converts an input visible light signal into an electric signal, and converts the converted electric signal to a digital signal for digital signal processing.

The photoelectric conversion unit 110 may include a photodiode or an image sensor.

The signal demodulator 120 demodulates the OFDM symbol corresponding to the digital signal and restores the original data.

The signal demodulation unit 120 includes a signal receiving unit 121, a S / P converter 122, a DC biasing and CP removing unit 123, a fast Fourier transform unit (FFT) 124, a post- 125, a channel estimator 126, an interference suppressor 127, a signal detector or a symbol detector 128, a P / S converter 129, and the like. can do.

The signal receiving unit 121 detects an OFDM symbol of a visible light wireless communication transmission apparatus and converts the OFDM symbol from an analog signal to a digital signal.

The S / P converter 122 converts the modulated data symbols output from the signal receiver into a parallel signal from a serial signal.

The DC biasing and CP removing unit 123 reduces the effect of DC biasing and removes the guard interval (CP) from the OFDM symbol. The DC biasing and the CP removing unit 123 may be separately configured by a DC biasing removing unit and a CP removing unit.

The fast Fourier transformer (FFT) 124 performs fast Fourier transform on the parallel modulated data symbols. In this case, the parallel OFDM symbols are transformed from the time domain to the frequency domain.

The CP is removed by the guard interval removing unit, and the CP is demodulated through the FFT unit

Symbol vector

Can be expressed as Equation (11).

The post-processing unit 125 estimates a transmission signal (i.e., a pilot signal) from a signal input to the channel estimator

, &Lt; / RTI > Equation (10)).

That is, the post-processing unit adds the transmission signal (

, Equation (10)), that is, to prevent the channel estimation from being inaccurate due to the inclusion of the precoded and received data symbol, the transmission signal is eliminated or reduced.

That is, the signal output through the post-processing unit is expressed by Equation (12)

. &Lt; / RTI >

The channel estimator 126 estimates a channel through which the visible light signal is transmitted using the hidden pilot signal.

The channel estimation through the channel estimator uses an MMSE method. A channel estimation value estimated through the channel estimation unit

) Can be expressed as Equation (13).

The interference suppressor 127 removes interference generated by a hidden pilot inserted through a visible light transmitter, thereby detecting an accurate transmission signal, i.e., a data symbol.

That is, in the interference elimination, the signal output through the FFT

(11)

To obtain a received symbol with a reduced effect due to the interference of the hidden pilot.

And a signal output through the interference canceller

) Can be expressed by Equation (14).

The signal detector 128 estimates a channel estimate value

And a signal output from the interference eliminator

) Transmitted from the visible light transmitting apparatus to the transmission data symbol (

).

And a signal output through the interference canceller

) To the detected data symbol (< RTI ID = 0.0 >

) Can be expressed as Equation (16).

The P / S converter 129 converts the parallel data into serial data to restore the source data.

Next, let's look at the Illumination Quality Assessment.

Since VLC is a communication technology that uses LED lighting, it is necessary to satisfy the light intensity requirements generally required for various illumination sources.

Since it is not desirable for a person to sense a change in light caused by the flicker of a VLC signal, it needs to be implemented carefully considering the task of the light source.

In addition, the chromaticity of the LED may be changed as the driving current of the LED is changed.

Therefore, it is desirable to minimize the range of flicker in an OFDM-based VLC signal to reduce the color change of the LED caused by the fluctuation of the signal.

As we have seen, the HP-VLC method proposed herein has the advantage of reducing the range of blinking in OFDM-based VLC signals.

That is, in order to demonstrate the advantages of the HP-VLC method as described above, metrics for measuring the amount of flickering of the HP-VLC signal will be described below.

To quantify the quality of light that a person can perceive, the commonly used metrics are flicker index and percent flicker.

The flicker index represents a value for measuring the amount of change in the source output of the light considering the output waveform of light.

Also, the percent flicker represents a relative measure of the cyclic variation in the source output of light.

The metric

. &Lt; / RTI > here,

Is the maximum output value,

Represents the minimum output value.

The HP-VLC method proposed here uses a percent flicker as a metric to measure the amount of flicker.

FIG. 4 is a diagram illustrating a change amount of an OFDM signal truncated according to a biasing ratio proposed in the present specification.

To provide a sufficient range for DC-biasing in terms of LED dynamic range, ie drive range, the range of biasing ratio is considered from 0.1 to 0.9.

As shown in FIG. 4, clipping becomes important when the biasing ratio approaches 0.1 or 0.9, and the signal variance decreases.

Next, a simulation result of an OFDM-based VLC signal under various average optical power and dynamic range of light will be described with reference to FIG. 5 to FIG.

The HP-VLC of DCO-OFDM can be confirmed by computer simulation.

The normal power on the hidden pilot is set to 0.5, the number of subcarriers is 256 (N = 256), and the data modulation uses QPSK.

The length of the channel tap (

&Lt; / RTI > is 8, taking into account the multipath channel environment in which the signal is diffused.

In order to compare channel estimation performance, a pilot symbol aided modulation (PSAM) corresponding to the conventional method, that is, a comparison method of the HP-VLC method proposed in the present specification, is exemplified.

PSAM is the most commonly used method, and designated pilot tones are required.

5 is a diagram illustrating performance of channel estimation according to SNR with dynamic range limitation of the LED input signal proposed in the present specification.

As shown in FIG. 5, the HP-VLC method shows good performance in all SNR regions.

Furthermore, even when clipping occurs due to the limited dynamic range, it can be seen that the HP-VLC method proposed herein can more accurately estimate the channel than the conventional PSAM method.

In addition, since the HP-VLC method proposed in the present specification uses hidden pilot symbols in the entire sub-carrier bands, even if a symbol distortion occurs in some sub-carriers, the performance does not become a big problem.

6 is a diagram illustrating a channel estimation performance according to the biasing ratio and SNR proposed in the present specification.

As shown in FIG. 6, when the biasing ratio is 0.5, the clipping is minimized, and the error of the channel estimation is the smallest.

However, when the biasing ratio is close to 0 or 1, the amplitude loss of the optical signal due to clipping occurs, and the SNR performance deteriorates. Therefore, accurate channel estimation becomes difficult.

7 is a diagram illustrating a data rate that can be obtained according to a change in a biasing ratio proposed in the present specification.

For SNR values, it can be seen that the HP-VLC method exhibits better performance than the conventional method.

Also, as the SNR value increases, the performance difference between the HP-VLC method and the conventional method becomes larger.

8 is a diagram illustrating a data transmission rate obtained with respect to the biasing ratio and SNR proposed in the present specification.

As shown in FIG. 8, it can be seen that the performance of the maximum data rate obtained by the HP-VLC method is when the biasing ratio is 0.5 and the SNR is high.

FIG. 9 is a diagram showing the overall clipping amplitude according to the biasing ratio proposed in the present specification.

As shown in FIG. 9, the maximum and minimum values of the total clipping size are found when the biasing ratio is close to 0.1 or 0.9 and 0.5, respectively.

For various biasing ratio values, the clipping size of the HP-VLC method is always smaller than the conventional method.

This is because a certain amount of power is given to the hidden pilot.

Therefore, it can be seen that the size change with respect to the OFDM signal becomes small in the HP-VLC technique.

Figure 10 is a comparison of the percent flicker applied in this specification with the prior art in comparison to the quality of the light.

As shown in FIG. 10, the HP-VLC method can be seen to show a lower value percent flicker than the percent flicker in the conventional method.

In particular, when the biasing ratio is greater than 0.3, the HP-VLC method will be less noticeable for the blinking of the LED light when applied.

Therefore, the HP-VLC method proposed in the present specification can reduce the color change and deteriorate the quality of the light.

In summary, in the present specification, the hidden pilot-aided precoding method and its performance that have been proposed in the present specification have been described from the viewpoint of the data rate and the quality of light obtained in visible light communication.

When using the hidden pilot and precoding method proposed in the present specification, significant loss can be avoided in bandwidth efficiency and large fluctuation in OFDM-based VLC.

As can be seen from FIGS. 5 to 10, it can be confirmed that the HP-VLC method proposed in this specification has better performance than the conventional method in terms of accurate channel estimation and data rate that can be obtained.

In addition, in terms of light quality, the HP-VLC method proposed herein can provide flickering performance and the ability to further increase the stable emission of LED light.

The embodiments of the present invention are not limited to the above-described apparatuses and / or methods, but may be implemented through a program for realizing functions corresponding to the configuration of the embodiment of the present invention or a recording medium on which the program is recorded, Such an embodiment can be readily implemented by those skilled in the art from the description of the embodiments described above.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, It belongs to the scope of right.

The present disclosure relates to a method for transmitting and receiving signals in a visible light communication system using an OFDM scheme.

10: Visible light communication system
100: visible light receiver
200: visible light transmitting device

Claims

A visible light transmitting apparatus for transmitting a signal in a Visible Light Communication (VLC) system,
A serial / parallel converter (S / P) for converting a plurality of serial data signals into a plurality of parallel data signals;
A precoder for precoding a signal output through the serial / parallel converter using cyclic shifted orthogonal polyphase sequences of length M;
A pilot inserter for generating a hidden pilot used for channel estimation; And
And an IFFT unit for performing Inverse Fast Fourier Transform (IFFT) by combining a signal output from the precoder and a signal output through the pilot inserting unit,
Wherein the M-1 cyclic-shifted orthogonal polyphase sequences having the length M are used for precoding, and one cyclic-shifted orthogonal polyphase sequence having the length M is used for the hidden pilot.

The method according to claim 1,
Wherein the precoder performs precoding using Hermitian symmetry so that a signal output from the S / P conversion unit has a real valued signal.

The method according to claim 1,
And a DC biasing and clipping unit for converting a signal output through the IFFT unit into a signal having a positive real value and clipping a signal portion out of the driving range of the LED To the visible light.

The method of claim 3,
Further comprising an LED light source module for converting a signal output through the DC biasing and clipping unit into a visible light signal and transmitting the converted visible light signal.

The method according to claim 1,
The signal input to the IFFT unit

) Is defined by the following equation.

here,

The

&Lt; / RTI >

Represents a precoder matrix,

Represents a hidden pilot vector (Hidden Pilot Vector).

6. The method of claim 5,
The precoder matrix (

) And the hidden pilot vector (

) Is defined by the following equation.

here,

Lt; / RTI >

1

Represents a zero vector,

Of the components have a reserved order

Of complex conjugate.

6. The method of claim 5,
The entire data symbol (

) Is composed of a Hermitian symmetry, and is defined by the following equation.

A method for transmitting a signal in a Visible Light Communication (VLC) system,
Converting a plurality of serial data signals into a plurality of parallel data signals through a serial / parallel converter (S / P);
Precoding the plurality of parallel data signals using cyclic shifted orthogonal polyphase sequences of length M;
Generating a hidden pilot used for channel estimation; And
And performing inverse fast Fourier transform (IFFT) by combining the precoded data signal and the generated hidden pilot signal,
Wherein M-1 cyclic-shifted orthogonal polyphase sequences of length M are used for precoding, and one cyclic-shifted orthogonal polyphase sequence of length M is used for hidden pilots.

9. The method of claim 8,
Transforming the IFFT processed signal into a signal having a positive real value; And
Further comprising clipping a portion of the signal that is outside the operating range of the LED.