CN114978846A - Unipolar signal head OFDM modulation method for wireless optical communication system - Google Patents
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- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
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Abstract
The embodiment of the invention discloses a unipolar signal head OFDM modulation method for a wireless optical communication system. The method firstly sends positive and negative information heads of the bipolar OFDM signal, then sends the amplitude of the signal, and utilizes M-ary pulse amplitude modulation to encode the signal polarity information in the OFDM signal, thereby greatly reducing the occupation of bandwidth. The invention solves the technical problems that the signal modulation technology in the prior art needs larger bandwidth occupation and performance indexes such as the error rate, the data throughput and the like are not optimized to the maximum extent, achieves the technical effects of better error rate performance and higher data throughput and simultaneously provides higher illumination flexibility.
Description
Technical Field
The present invention relates to the field of optical communication technologies, and in particular, to a method for modulating an Orthogonal Frequency Division Multiplexing (OFDM) with a single-polar signal header for a wireless optical communication system.
Background
Visible Light Communication (VLC) provides a short-range wireless Communication alternative using Light Emitting Diodes (LEDs) as transmitters to meet lighting and downlink data transmission. Due to its technical advantages over Radio Frequency (RF) communications, it has attracted extensive attention in both academia and industry. The VLC system is free from radio frequency interference and influence of spectrum adjustment, has low power consumption except for the lighting system, can provide reliable and safe connection, and can realize high-speed data transmission. Orthogonal Frequency Division Multiplexing (OFDM) has the potential to provide high spectral efficiency for bandwidth-limited optical communication systems due to its good resistance to intersymbol interference (ISI). For VLC systems, only intensity modulation and direct detection (IM/DD) can be used, which requires that the transmitted signal is a non-negative real signal. In conventional RF-OFDM, the transmitted signal is complex. Therefore, the conventional RF-OFDM technology cannot be directly applied to the VLC system, and for this reason, it is necessary to explore the OFDM technology suitable for the VLC system.
In the prior art, dc-biased optical orthogonal frequency division multiplexing (DCO-OFDM) is the most commonly used scheme due to its simplicity. For DCO-OFDM, a constant optical power is added as a dc bias to the bipolar orthogonal frequency division multiplexing signal, making it non-negative. Asymmetric-clipped light orthogonal frequency division multiplexing (ACO-OFDM) produces non-negative signals suitable for VLC systems by modulating only odd frequency subcarriers. Since only half of the sub-carriers are used, the bandwidth utilization efficiency of ACO-OFDM is much lower than that of DCO-OFDM. Unipolar orthogonal frequency division multiplexing (U-OFDM), also known as flipped orthogonal frequency division multiplexing, transmits positive and negative portions of a bipolar orthogonal frequency division multiplexed signal over two adjacent frames to produce a non-negative signal. Similar to U-OFDM, non-dc biased orthogonal frequency division multiplexing (NDC-OFDM) proposes the use of two LEDs to transmit both the positive and negative portions of an orthogonal frequency division multiplexed signal simultaneously. Clipping-enhanced optical orthogonal frequency division multiplexing (CEO-OFDM) uses additional time slots to transmit the clipped portion of the signal. The above-described orthogonal frequency division multiplexing technique generates a real orthogonal frequency division multiplexing signal from a complex data sequence using hermitian symmetric data at the input of an Inverse Fast Fourier Transform (IFFT). In addition, there is a polarity-based optical orthogonal frequency division multiplexing suitable for IM/DD systems that generates a unipolar signal by continuously transmitting amplitude and phase information without using hermitian symmetric data.
However, the signal modulation technique in the prior art requires a large bandwidth occupation, and performance indexes such as the bit error rate and the data throughput are not optimized to the maximum extent.
An effective solution to the above problems has not been proposed.
Disclosure of Invention
The embodiment of the invention provides a unipolar signal head OFDM modulation method for a wireless optical communication system, which is used for at least solving the technical problems that a signal modulation technology in the prior art needs larger bandwidth occupation, and performance indexes such as bit error rate, data throughput and the like are not optimized to the greatest extent.
According to an aspect of the embodiments of the present invention, there is provided a single-polarity signal header OFDM modulation method for a wireless optical communication system, including: step 1: constructing a transmitter; step 1-1: modulating the sub-carriers of the binary data format by M-ary quadrature amplitude modulation M-QAM, wherein X is set i For M-QAM data modulated on the ith subcarrier and creating a Hermite symmetric data vector, wherein X i Is X N-1-i The conjugate of (a) to (b),
n is the number of subcarriers; step 1-2: and obtaining an IFFT output signal after the Hermite symmetric data vector is subjected to Inverse Fast Fourier Transform (IFFT), wherein the mth sample of the IFFT output signal is represented as x [ m ]]，x[m]The expression of (a) is:
wherein β is a modulation index for controlling the amplitude of the bipolar OFDM signal; step 1-3: establishing binary bit number L and M-PAM modulation order M carried by each symbol in the signal head of the M-PAM data h And represents the mth sample in the signal header of the M-PAM data as
X h [m]M is 0,1, …, L-1, wherein M is h ＝2 N/L ，
P max For maximum emission of the light source employedA transmission power; step 1-4: outputting a modulation signal of the unipolar signal head in the wireless optical communication system; step 2: constructing a receiver; step 2-1: obtaining a discrete-time modulated signal, wherein an mth sample of the modulated signal is represented as: y [ m ]]＝ρh[m]*x PHO [m]+n y [m]0,1, 2N + L-1, wherein h [ m ]]Is the m-th sample, x, of the channel impulse response at discrete time PHO [m]For the m-th sample, discrete time convolution, p is the responsivity of the photodetector, n y [m]Is additive noise on the mth sample of the modulated signal; step 2-2: and decoding the signal head of the modulation signal, and reconstructing the original bipolar signal by using the symbol information to obtain a reconstructed signal.
Optionally, the expression of the modulation signal is:
wherein the content of the first and second substances,
the current response function of the adopted light source is modeled as follows:
wherein, U (X, L, P) max )＝u(x-l)-u(x-l-P max ) U (-) is a unit step function, erfc (-) is a complementary error function,
optionally, the method further comprises: when the additive noise is white Gaussian noise, the mean value and the variance of the additive noise are respectively 0,
Wherein R is S For transmitting QAM symbol rates, N o Is the noise power spectral density.
Optionally, the method further comprises: the expression of the mth sample of the reconstructed signal in one symbol is: r [ m ]]＝ρh[m]*(α(β)x[m]+n clip [m])+n sign [m]+n r [m]N-1, where α (β) is a coefficient describing a power loss caused by peak power clipping, and is obtained by the following expression:
where ψ (x) is a non-linear function expressed as:
N r [m]to reconstruct the mth sample in the equivalent noise of the signal, the variance is
n clip [m]To clip the mth sample of the noise, it is modeled as a gaussian distributed variable with mean and variance of zero:
wherein n is sign [m]For noise due to symbol information decision errors transmitted in the signal header, it is modeled as a gaussian random variable with mean and variance of zero
Optionally, the method further comprises: calculating the equivalent noise variance added to the reconstructed signal when the symbol error rate in the signal header is p
Comprises the following steps:
wherein the content of the first and second substances,
calculated from the following formula:
the invention has the following beneficial effects:
in the embodiment of the invention, the unipolar signal header OFDM modulation method for the wireless optical communication system firstly transmits the positive and negative information headers of the bipolar OFDM signal, then transmits the amplitude of the signal, and encodes the signal polarity information in the OFDM by using M-ary pulse amplitude modulation, thereby greatly reducing the occupation of bandwidth, further solving the technical problems that the signal modulation technology in the prior art needs larger bandwidth occupation, and the performance indexes such as the bit error rate, the data throughput and the like are not optimized to the maximum extent, achieving the technical effects of better bit error rate performance and higher data throughput, and simultaneously providing higher illumination flexibility.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:
fig. 1 is a schematic diagram of a transmitter provided by an embodiment of the present invention;
fig. 2 is a schematic diagram of a receiver according to an embodiment of the present invention;
FIG. 3(a) is a schematic diagram of a bipolar OFDM signal according to an embodiment of the present invention;
FIG. 3(b) is a schematic diagram of binary symbol information and signal header provided by an embodiment of the present invention;
fig. 3(c) is a schematic diagram of a single-polarity signal header OFDM signal that can be used for transmission according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of bit error rate comparison of DCO-, ACO-, U-, CEO-, single-polarity signal header OFDM provided by the embodiment of the invention at 20Msps input symbol rate using the optimal modulation index of each case;
FIG. 5 is a diagram illustrating BER performance comparison of DCO-, ACO-, U-, CEO-, CE-PHO-OFDM under 64-QAM modulation according to an embodiment of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that the terms "first", "second", and the like in the description and claims of the present invention and the accompanying drawings are used for distinguishing different objects, and are not used for limiting a specific order.
According to an aspect of the embodiments of the present invention, there is provided a single-polarity signal head OFDM modulation method for a wireless optical communication system, the method including the steps of:
step 1: constructing a transmitter; fig. 1 is a schematic diagram of a transmitter according to an embodiment of the present invention, as shown in fig. 1, it should be noted that the transmitter has a so-called transmitting end;
step 1-1: modulating the sub-carriers of the binary data format using M-ary quadrature amplitude modulation M-QAM, wherein X is set i Is the ith subcarrierM-QAM data modulated on a wave and creating a hermitian symmetric data vector, wherein X i Is X N-1-i The conjugate of (a) to (b),
n is the number of subcarriers;
step 1-2: and after the Hermite symmetric data vector is subjected to Inverse Fast Fourier Transform (IFFT), obtaining an IFFT output signal, wherein the mth sample of the IFFT output signal is represented as x [ m ]]，x[m]The expression of (a) is:
wherein β is a modulation index for controlling the amplitude of the bipolar OFDM signal;
step 1-3: establishing binary bit number L and M-PAM modulation order M carried by each symbol in signal head of the M-PAM data h And represents the mth sample in the signal header of the M-PAM data as X h [m]M is 0,1, …, L-1, wherein M is h ＝2 N/L ，
P max The maximum emission power of the adopted light source;
step 1-4: outputting a modulation signal of the unipolar signal head in the wireless optical communication system; the modulation signal is also called as a transmission signal at a transmitting end and is also called as a receiving signal at a receiving end;
step 2: constructing a receiver; fig. 2 is a schematic diagram of a receiver according to an embodiment of the present invention, as shown in fig. 2, it should be noted that the receiver is referred to as a receiving end;
step 2-1: obtaining a discrete-time modulation signal, wherein the mth sample of the modulation signal is represented as: y [ m ]]＝ρh[m]*x PHO [m]+n y [m]0,1, 2N + L-1, wherein h [ m ]]Is the m-th sample, x, of the channel impulse response at discrete time PHO [m]The modulation signal of the m-th sample is discrete time convolution, rho is responsivity of the photoelectric detector, and n y [m]For the above-mentioned adjustmentAdditive noise on the mth sample of the signal;
step 2-2: and decoding the signal head of the modulation signal, and reconstructing the original bipolar signal by using the symbol information to obtain a reconstructed signal.
It should be noted that, in the embodiment of the present invention, first, positive and negative information headers of a bipolar OFDM signal are sent, then, the amplitude of the signal is sent, and M-ary pulse amplitude modulation is used to encode signal polarity information in the OFDM signal, so that occupation of bandwidth is greatly reduced, and further, technical problems that a signal modulation technique in the prior art needs large bandwidth occupation, and performance indexes such as bit error rate and data throughput are not optimized to the greatest extent are solved, so that a technical effect of having better bit error rate performance and higher data throughput is achieved, and meanwhile, higher illumination flexibility is provided.
Furthermore, the above method is applicable to wireless optical communication systems for intensity modulation and direct detection (IM/DD), such as visible light systems, infrared communication systems, etc.
In an alternative embodiment, the expression of the modulation signal is:
wherein the content of the first and second substances,
the current response function of the adopted light source is modeled as follows:
x h [m]the probability density function of (a) is:
the probability density function for the remaining signals in a frame is:
wherein, U (X, L, P) max )＝u(x-l)-u(x-l-P max ) U (-) is a unit step function, erfc (-) is a complementary error function,
in an optional embodiment, the method further comprises: when the additive noise is white Gaussian noise, the mean and variance of the additive noise are 0,
Wherein R is S For transmitting QAM symbol rates, N o Is the noise power spectral density.
In an optional embodiment, the method further comprises: the expression of the mth sample of the reconstructed signal in one symbol is: r [ m ]]＝ρh[m]*(α(β)x[m]+n clip [m])+n sign [m]+n r [m]N-1, where α (β) is a coefficient describing a power loss caused by peak power clipping, and is obtained by the following expression:
where Ψ (x) is a nonlinear function represented by:
N r [m]to reconstruct the mth sample in the equivalent noise of the signal, the variance is
n clip [m]To clip the mth sample of the noise, it is modeled as a gaussian distributed variable with mean and variance of zero:
wherein n is sign [m]For noise due to symbol information decision errors transmitted in the signal header, it is modeled as a gaussian random variable with mean and variance of zero
In an optional embodiment, the method further comprises: when the symbol error rate in the signal header is p, the equivalent noise variance added to the reconstructed signal is calculated
Comprises the following steps:
wherein the content of the first and second substances,
calculated from the following formula:
an alternative embodiment of the invention is described in detail below.
In VLC systems, intensity modulation and direct detection are used, only positive real-valued signals can be transmitted. In order to make the transmitted signal positive, the embodiment of the invention adopts a unipolar signal head OFDM modulation technology, which firstly transmits the positive and negative information heads of the bipolar OFDM signal, and then transmits the amplitude of the signal. And coding the signal polarity information in the OFDM by using M-ary pulse amplitude modulation, thereby greatly reducing the occupation of bandwidth. Compared with the current mainstream OFDM technology for the wireless optical communication system, the single-polarity signal head OFDM modulation technology provided by the invention has better error rate performance and higher data throughput, and simultaneously provides higher illumination flexibility.
First, transmitting end signal description
In order to reduce the clipping distortion of the peak power, the scheme modulates each frame of OFDM modulation signalIs divided into two parts, namely a unipolar signal header and an amplitude part of the unipolar OFDM signal. For the proposed OFDM technique, the binary signal will modulate the data using M-ary quadrature amplitude modulation (M-QAM), thereby improving the modulation efficiency. Suppose X i Is M-QAM data of the ith subcarrier. In order to make the transmitted signal a real signal, a hermitian symmetric data vector is created, where X i Is obligate to be X N-1-i The conjugate of (a) to (b),
where N is the number of subcarriers. After this vector has undergone an Inverse Fast Fourier Transform (IFFT), the m-th sample of its bipolar output is denoted x m]Namely:
where β is the modulation index used to control the amplitude of the bipolar OFDM signal. The signal-to-noise ratio of the signal can be optimized by adjusting the modulation index, taking into account the peak power constraint.
The binary bit number L carried by each symbol in the signal head determines the M-PAM modulation order M h Wherein, M is h ＝2 N/L . Thus, the mth sample in a signal header using M-PAM may be represented as X h [m]M is 0,1, …, L-1, wherein,
therefore, the transmission signal in the single-polarity signal header OFDM modulation system can be expressed as:
signal X PHO [m]Can be seen as a random process. Wherein x is h [m]Has a probability density function of
The probability density function for the remaining signals in a frame is:
wherein the function U (X, L, P) max )＝u(x-l)-u(x-l-P max ) And U (-) is a unit step function. The function erfc (-) is a complementary error function, defined as
Second, receiver side signal description
At the receiving end, the mth sample of the discrete-time received signal may be represented as:
y[m]＝ρh[m]*X PHO [m]+n y [m],m＝0,1,...,2N+L-1
wherein, h [ m ]]The mth sample of the channel impulse response at discrete time. The symbol "-" denotes a discrete-time convolution, and ρ is the responsivity of the Photodetector (PD). n is y [m]Assuming that the additive noise is white Gaussian noise, the mean and variance are 0,
Wherein R is S For transmitting QAM symbol rates, N o Is the noise power spectral density.
After the signal head is decoded, the original bipolar signal is reconstructed by utilizing the symbol information, and the signal model is as follows:
r[m]＝ρh[m]*(α(β)x[m]+n clip [m])+n sign [m]+n r [m],m＝0,1,...,N-1
where α (β) is a coefficient describing a power loss caused by the peak power clipping, and can be obtained by the following equation:
ψ (x) is a non-linear function, which can be expressed as:
N r [m]represents the m-th sample in the equivalent noise of the reconstructed data with a variance of
n clip [m]The mth sample, representing clipping truncation noise, can be modeled as a gaussian distribution variable with mean and variance of zero:
n sign [m]representing noise due to symbol information decision errors transmitted in the signal header, can be modeled as gaussian random variables with mean and variance of zero
Assuming a symbol error rate p in the signal header, the equivalent noise variance added to the reconstructed signal can be approximately calculated
Comprises the following steps:
wherein the content of the first and second substances,
can be calculated from:
third, performance analysis
From the above analysis, the signal-to-noise ratio (SINR) of its reconstructed signal can be calculated as follows:
where W denotes a Minimum Mean Square Error (MMSE) equalizer that filters a signal header at a receiving end. The error rate of the binary symbol information in the signal head is approximately as follows:
considering the limitation of the channel bandwidth, the signal-to-noise ratio of the ith sub-carrier after FFT can be calculated as:
where H is the Fourier transform of H, and Hj represents the channel frequency response of the ith subcarrier. A tapped equalizer is used at each subcarrier of the receiver to compensate for phase distortion caused by the dispersive channel. Given the signal-to-noise ratio (SINR), the Bit Error Rate (BER) of the data on the ith subcarrier can be approximated as:
wherein, M [ i ] is the modulation order of M-QAM data modulated by the ith subcarrier.
Fig. 3(a) is a schematic diagram of a bipolar OFDM signal according to an embodiment of the present invention, as shown in fig. 3(a), showing a bipolar real signal generated by IFFT; fig. 3(b) is a schematic diagram of binary symbol information and a signal header provided by an embodiment of the present invention, and as shown in fig. 3(b), the positive and negative binary information thereof can be modulated into the signal header by M-PAM; fig. 3(c) is a schematic diagram of a single-polarity signal header OFDM signal that can be used for transmission according to an embodiment of the present invention, and shows that the single-polarity signal header OFDM signal is constructed as shown in fig. 3 (c). Therefore, fig. 3(a), 3(b) and 3(c) respectively show the parts of the transmitting end constructing the single-polarity signal head OFDM modulation signal.
Fourthly, comparing simulation and test results
The performance results of the present invention were compared to ACO-, DCO-, U-and CEO-OFDM. Without loss of generality, the simulation parameters are set as shown in table 1.
TABLE 1 simulation parameters
Fig. 4 is a schematic diagram illustrating bit error rate comparison of DCO-, ACO-, U-, CEO-, and unipolar signal header OFDM provided in the embodiment of the present invention using the optimal modulation index of each case at an input symbol rate of 20Msps, as shown in fig. 4, the minimum bit error rate of the tested algorithm is compared, and the corresponding bandwidth efficiency is compared. Where L-32 may provide the best BER for single-polarity signal header OFDM (CE-PHO-OFDM). When L is small compared to N, a larger order of M-PAM is used for modulation in the signal header. Since a larger order of M-PAM requires a larger SNR, the overall performance of the system is limited at limited transmit power. When L is larger, transmitting the same symbol rate requires a longer signal header and also requires a larger bandwidth, and therefore, introduces more additional noise. As can be seen from fig. 4, the present invention has the lowest bit error rate among all the tested techniques, since the required low bandwidth and lower clipping distortion results in a higher signal-to-noise ratio. With larger QAM modulation orders used, single polarity signal header OFDM using 128-QAM and L ═ 16 can provide 15% higher data rates than DCO-, ACO-, U-, CEO-, polarity-based OFDM using 64-QAM.
FIG. 5 is a graph showing the comparison of BER performance of DCO-, ACO-, U-, CEO-, CE-PHO-OFDM under 64-QAM modulation according to an embodiment of the present invention, and FIG. 5 is a graph comparing the BER performance of DCO-, ACO-, U-, CEO-OFDM and CE-PHO-OFDM under different illumination requirements, wherein the average light power (P/P) is normalized max ) Determines the illumination intensity of the system. For all compared algorithms, increasing the average power reduces the system bit error rate; if the average power is further increased by increasing the modulation index, more clipping distortion is introduced due to the peak radiated power limitation, so that the system BER performance is rapidly reduced. For DCO-OFDM, the dc component is constant, so the normalized average power is constant. CE-PHO-OFDM may provide better minimum BER than DCO-, ACO-, U-, polarity-based OFDM.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention.
Claims (5)
1. A single-polarity signal header OFDM modulation method for a wireless optical communication system, comprising:
step 1: constructing a transmitter;
step 1-1: modulating the sub-carriers of the binary data format by M-ary quadrature amplitude modulation M-QAM, wherein X is set i For M-QAM data modulated on the ith subcarrier and creating a Hermite symmetric data vector, wherein X i Is X N-1-i The conjugate of (a) to (b),
n is the number of subcarriers;
step 1-2: and after the Hermite symmetric data vector is subjected to Inverse Fast Fourier Transform (IFFT), obtaining an IFFT output signal, wherein the mth sample of the IFFT output signal is represented as x [ m ], and the expression of the x [ m ] is as follows:
wherein β is a modulation index for controlling the amplitude of the bipolar OFDM signal;
step 1-3: establishing binary bit number L and M-PAM modulation order M carried by each symbol in the signal head of the M-PAM data h And represents the mth sample in the signal header of the M-PAM data as X h [m]M is 0,1, …, L-1, wherein, M h ＝2 N/L ，
P max The maximum emission power of the adopted light source;
step 1-4: outputting a modulation signal of the unipolar signal head in the wireless optical communication system;
step 2: constructing a receiver;
step 2-1: obtaining a discrete-time modulated signal, wherein an mth sample of the modulated signal is represented as:
y[m]＝ρh[m]*x PHO [m]+n y [m],m＝0,1,...,2N+L-1
wherein, h [ m ]]Is the m-th sample, x, of the channel impulse response at discrete time PHO [m]For the modulation signal of the m-th sample, discrete-time convolution, p is the responsivity of the photodetector, n y [m]Is additive noise on the mth sample of the modulated signal;
step 2-2: and decoding the signal head of the modulation signal, and reconstructing the original bipolar signal by using the symbol information to obtain a reconstructed signal.
2. The method of claim 1, wherein the modulated signal is expressed as:
wherein the content of the first and second substances,
the current response function of the adopted light source is modeled as follows:
x h [m]the probability density function of (a) is:
the probability density function for the remaining signals in a frame is:
wherein, U (X, L, P) max )＝u(x-l)-u(x-l-P max ) U (-) is a unit step function, erfc (-) is a complementary error function,
3. the method of claim 1, further comprising:
when the additive noise is white Gaussian noise, the mean value and the variance of the additive noise are respectively 0,
Wherein R is S For transmitting QAM symbol rates, N o Is the noise power spectral density.
4. The method of claim 1, further comprising:
the expression of the mth sample of the reconstructed signal in one symbol is:
r[m]＝ρh[m]*(α(β)x[m]+n clip [m])+n sign [m]+n r [m],m＝0,1,...,N-1
where α (β) is a coefficient describing a power loss caused by the peak power clipping, which is obtained by the following expression:
where ψ (x) is a non-linear function expressed as:
N r [m]to reconstruct the mth sample in the equivalent noise of the signal, the variance is
n clip [m]To clip the mth sample of the noise, it is modeled as a gaussian distributed variable with mean and variance of zero:
wherein n is sign [m]For noise due to symbol information decision errors transmitted in the signal header, it is modeled as a gaussian random variable with mean and variance of zero
5. The method of claim 4, further comprising:
calculating the equivalent noise variance added to the reconstructed signal when the symbol error rate in the signal header is p
Comprises the following steps:
wherein the content of the first and second substances,
calculated from the following formula:
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