CN105871769B - Dynamic despicking method in a kind of ROF-OFDMA downlinks - Google Patents
Dynamic despicking method in a kind of ROF-OFDMA downlinks Download PDFInfo
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- CN105871769B CN105871769B CN201610458103.1A CN201610458103A CN105871769B CN 105871769 B CN105871769 B CN 105871769B CN 201610458103 A CN201610458103 A CN 201610458103A CN 105871769 B CN105871769 B CN 105871769B
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- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
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- H04L27/2614—Peak power aspects
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Abstract
The invention discloses the dynamic despicking methods in a kind of ROF-OFDMA downlinks, by the way that peak clipping thresholding is arranged, the value that will be greater than the OFDMA signals of threshold value is set as threshold value, signal peak after peak clipping is exactly threshold value, then the signal after peak clipping is amplified, reach the peak value of system permission, at this moment the mean power of signal will be greatly improved；In specific operation, if the selection of peak clipping thresholding is excessive, mean power promotion is smaller, if the selection of peak clipping threshold value is too small, linearly degree is a greater impact, and system performance declines serious, it is therefore desirable to select a suitable peak clipping thresholding；The present invention determines peak clipping thresholding when different modulating format and different sub-carrier number by emulation, has stronger adaptability, can adapt to complicated modulation format and different number of subcarrier number.
Description
Technical field
The invention belongs to fields of communication technology, more specifically, are related to dynamically cutting in a kind of ROF-OFDMA downlinks
Peak method.
Background technology
Under the rapid development of wireless communication and application, user is higher and higher to the bandwidth demand of wireless access, and current is low
The bandwidth of frequency range far can not be met the needs of users, it is therefore desirable to the wireless access technology of higher frequency range is used, such as
60GHz frequency ranges have nearly 7GHz bandwidth, it has also become the focus studied at present.But the signal of this frequency range is decayed greatly in air,
Propagation distance is short, if accessed using traditional honeycomb, needs to dispose a large amount of antenna and base station.The propagation loss of optical fiber is small,
Propagation distance is remote, it can be considered to use the technology of optical fiber and wireless fusion, using optical fiber come transmitting radio frequency signal, original
Reason is as shown in Figure 1.By using optical fiber transmitting radio frequency signal between CS (central station) and RAU (remote antenna), can effectively expand
Transmission range greatly between CS and RAU realizes the increase of CS coverage areas.Since light carries radio frequency (ROF) technology by base station and antenna
Separation, is connected central station and remote antenna using optical fiber, is managed collectively to remote antenna in central station, such system
Maintenance and upgrade just becomes simple.
The frequency spectrum resource of wireless access is valuable, it is therefore desirable to the modulation format for using the availability of frequency spectrum high as far as possible, wherein
Orthogonal frequency division multiplexing access (OFDMA) technology is widely used in the downlink of LTE system.Meanwhile though
So there are many implementation methods for the ROF technologies based on optical fiber transmitting radio frequency signal, but from the angle of cost of implementation, are based on intensity tune
System adds the selection that the ROF technologies of directly detection (IM-DD) are still best.By OFDMA technologies and the ROF technologies based on IM-DD
It is combined, the obtained system block diagram that OFDMA downlink signals are transmitted based on ROF is as shown in Figure 2.In figure, in ROF transmitting terminals, electricity
Absorption modulation device (EAM) realizes the intensity modulated to signal, the linear pass of radio frequency signal amplitude of Output optical power and input
System.In ROF receiving terminals, photoelectric converter (PD) realizes opto-electronic conversion, radio-frequency current and the input optical signal work(of output
Rate is linear, and the radio-frequency current that such PD is detected is just linear with the radio frequency signal amplitude into EAM,
Under conditions of impedance matching, the radio frequency signal amplitude after PD and the radio frequency signal amplitude into EAM are linear.
The input/output relation of EAM modulators is as follows：
Wherein, Ein(t) it is input light, Eout(t) it is output light, α is the chirp coefficient of EAM, into the electric signal of modulator
The expression formula of d (t) is as follows：
D (t)=(1-m)+m × SOFDM(t)
M is the modulation index of EAM.In this way, the Output optical power of EAM becomes：
Pout=Eout(t)×Eout(t)*=Ein(t)×Ein(t)*× d (t)=Pind(t)
Wherein, Eout(t)*And Ein(t)*It is E respectivelyout(t) and Ein(t) conjugation；It can be seen that the Output optical power of EAM
With the amplitude proportional of the d (t) of the power of input electrical signal.But in the signal that PD is received, in addition to the radio frequency of needs is believed
Number SOFDM(t) outside, also include that there are one direct current signals, the amplitude of this direct current signal is 1-m, in order to ensure OFDM AC signals
It is sufficiently large, it needs to ensure that m is big as possible.M maximum occurrences are that 0.5, OFDM radiofrequency signal peak values are up to 1.Since ofdm signal has
There is prodigious peak-to-average force ratio, the power very little of useful radiofrequency signal can be caused, and m is smaller, useful RF signal power is got over
It is small.
In order to ensure the normal modulation of EAM, need to ensure that the amplitude into the radiofrequency signal of EAM between (- 1,1), is
This can use the maximum value of ofdm signal to be normalized.But since ofdm signal has larger peak-to-average force ratio, normalization
The mean power of signal can be caused to become very little, Fig. 3 is that the ofdm signal after normalization uses QPSK, 16QAM and 64QAM tune
When format processed, the relationship of system performance and received optical power.It in figure, is labeled as the curve of " ideal ", is referred to when ROF links not being added
OFDMA transmission performances with received signal to noise ratio variation.
The performance curve of QPSK in observation chart 3 (a), it can be found that if not using peak clipping, when received optical power is relatively low,
The influence of PD noises is serious in ROF links, and system is unable to reach the bit error rate requirement of needs；In order to reach FEC threshold requirements
3.8×10-3The bit error rate, need to improve received optical power to -7dBm.It is -2dBm for the 16QAM received optical powers needed,
It is very poor system performance under the conditions of the received optical power in -2dBm and for 64QAM and 256QAM, it is extremely difficult to 3.8 ×
10-3Requirement.
Invention content
It is an object of the invention to overcome the deficiencies of the prior art and provide dynamically cut in a kind of ROF-OFDMA downlinks
Peak method reduces the peak-to-average force ratio of OFDMA signals using the method for peak clipping, realizes effective transmission to OFDMA signals.
For achieving the above object, the present invention is dynamic despicking method in a kind of ROF-OFDMA downlinks, feature
It is, includes the following steps：
(1), QAM is completed according to the modulation format and subcarrier number n that are distributed to each user information sequence to be transmitted
Mapping, obtains the corresponding symbol sebolic addressing of each subcarrier；
(2), n symbol sebolic addressing is carried out at the same time OFDMA modulation, obtains signal S to be sentOFDM(t)；
(3), according to subcarrier number n and modulation format, signal S is calculatedOFDM(t) maximum amplitude V0, and to signal
SOFDM(t) it is normalized, obtains signal X (t)；
(4), it when modulation format determines, is fitted by the simulation result to different sub-carrier number, determines peak clipping
Thresholding W；
W=a × 10-4n+b
Wherein, a, b are constant, and value is determined by corresponding modulation format；
(5), peak clipping is carried out according to peak clipping thresholding W to signal X (t)；
The signal after peak clipping is normalized again, obtains final output signal
What the goal of the invention of the present invention was realized in：
The present invention is the dynamic despicking method in a kind of ROF-OFDMA downlinks, since OFDMA signals are with prodigious
Peak-to-average force ratio, the probability very little of the appearance peak value of signal, when signal peak reaches the maximum value of system permission, the average work(of signal
Rate very little, but by the way that peak clipping thresholding is arranged, the value that will be greater than the OFDMA signals of threshold value is set as threshold value, after peak clipping
Signal peak be exactly threshold value, then the signal after peak clipping is amplified, reaches the peak value of system permission, at this moment signal
Mean power will be greatly improved.By losing a part of linearity of signal, the mean power of signal is improved.
Peak clipping thresholding selects excessive, and mean power promotion is smaller, and threshold value selection is too small, then linearly degree is a greater impact,
System performance declines serious, it is therefore desirable to select a suitable peak clipping thresholding.The present invention determines different modulating lattice by emulation
Peak clipping thresholding when formula and different sub-carrier number has stronger adaptability, can adapt to complicated modulation format and difference
The subcarrier number of number.
Meanwhile dynamic despicking method also has the advantages that in a kind of ROF-OFDMA downlinks of the present invention：
(1) single modulation format is covered and the case where different modulating format is used in mixed way, wide coverage, to actually making
OFDMA signals have very strong adaptability；
(2) method choice peak clipping thresholding of the invention is simple.Peak clipping door is fitted according to subcarrier number and modulation format
Formula is limited, although not being optimal peak clipping thresholding, can obtain, close to optimal peak clipping thresholding, finally implementing simply,
User need not table look-up to determine the peak clipping thresholding of different situations, directly bring subcarrier number into formula and calculate；
(3) can peak clipping thresholding be adjusted according to the subcarrier number of every frame data and modulation format in real time in the present invention,
Real-time is good, will not be since it is desired that adjusting peak clipping thresholding and stopping transmission data or influence the transmission of data.
Description of the drawings
Fig. 1 is optical fiber and wireless fusion system schematic based on ROF；
Fig. 2 is ROF-OFDMA downlink block diagrams；
Fig. 3 is the performance curve of different received optical powers in ROF links；
Fig. 4 is dynamic despicking method flow chart in a kind of ROF-OFDMA downlinks of the present invention；
Fig. 5 is the relationship and its linear fit of QPSK peak clippings thresholding and subcarrier；
Fig. 6 is signal schematic representation before and after peak clipping；
Fig. 7 is System performance profiles under different peak clipping thresholdings；
Fig. 8 is radio frequency amplifier input and output voltage relation curve；
Fig. 9 is that block diagram is realized in analog domain peak clipping；
Figure 10 is numeric field peak clipping system block diagram；
Figure 11 is the Performance Simulation Results figure of QPSK peak clippings and not peak clipping；
Figure 12 is 16QAM, 64QAM, 256QAM, the Performance Simulation Results figure before and after peak clipping.
Performance Simulation Results figure when Figure 13 is 4QAM and 16QAM hybrid modulations.
Specific implementation mode
The specific implementation mode of the present invention is described below in conjunction with the accompanying drawings, preferably so as to those skilled in the art
Understand the present invention.Requiring particular attention is that in the following description, when known function and the detailed description of design perhaps
When can desalinate the main contents of the present invention, these descriptions will be ignored herein.
Embodiment
For the convenience of description, first being illustrated to the relevant speciality term occurred in specific implementation mode：
OFDM(Orthogonal Frequency Division Multiplexing)：Orthogonal frequency division multiplexing；
OFDMA(Orthogonal Frequency Division Multiple Access)：Orthogonal frequency division multiple access connects
Enter；
CS：(Central Station) central station；
RAU：(Remote Antenna Unit) remote antenna unit；
ROF(Radio Over Fiber)：Light carries radio frequency；
LTE：(Long Term Evolution) long term evolution；
IM-DD：(Intensity Modulation-Direct Detection) intensity modulated-directly detects；
EAM：(Electro-Absorption Modulator) electroabsorption modulator；
PD：(Photo-Detector)；
QPSK(Quadrature Phase Shift Keying)：Quadrature phase shift keying；
QAM(Quadrature Amplitude Modulation)：Quadrature amplitude modulation.
Dynamic despicking method flow chart in a kind of ROF-OFDMA downlinks of Fig. 4 present invention.
In the present embodiment, as shown in figure 4, dynamic despicking method in a kind of ROF-OFDMA downlinks of the present invention, including
Following steps：
S1, each user information sequence to be transmitted is reflected according to the modulation format and subcarrier number n completions QAM that are distributed
It penetrates, obtains the corresponding symbol sebolic addressing of each subcarrier；
In the present embodiment, with reference to the OFDMA signals in LTE system, workable modulation format includes QPSK, 4QAM,
The QAM hybrid modulations of 16QAM, 64QAM, 256QAM and different rank, such as 4QAM+16QAM hybrid modulations.
S2, n symbol sebolic addressing is carried out at the same time to OFDMA modulation, obtains signal S to be sentOFDM(t)；
S3, according to subcarrier number n and modulation format, calculate signal SOFDM(t) maximum amplitude V0, further according to V0It is right
Signal SOFDM(t) it is normalized, obtains signal X (t)；
S4, when modulation format determines, be fitted by the simulation result to different sub-carrier number, determine peak clipping door
Limit W；
W=a × 10-4n+b
Wherein, a, b are constant, and value is determined by corresponding modulation format；
In the present embodiment, it by taking QPSK modulation formats as an example, can be confirmed using different sub-carrier said conditions by emulation
Under best peak clipping thresholding it is as shown in table 1.
Table 1 is the corresponding best peak clipping thresholding of QPSK different sub-carrier numbers；
Table 1
Simultaneously, it is contemplated that when peak clipping value is near best peak clipping value, the performance close to best peak clipping value can be obtained,
Therefore more simple linear fit can be carried out to above-mentioned simulation result, as shown in Figure 5.QPSK modulation can be obtained from Fig. 5
The linear fit formula of peak clipping thresholding and subcarrier number, W=3.1 × 10 under mode-4n+0.030。
The best peak clipping door of corresponding 16QAM, 64QAM and 256QAM can also be similarly obtained by the method for emulation plus fitting
Limit list and its fitting formula.
The fitting formula of 16QAM：
W=3.9 × 10-4n+0.024
The fitting formula of 64QAM：
W=3.12 × 10-4n+0.040
The fitting formula of 256QAM：
W=2.68 × 10-4n+0.041
When user use mixed modulation formats when, due to higher order modulation formats to the linearity of system than low order
Height, it is also harsher to the selection of peak clipping thresholding, therefore order of modulation highest modulation format can be taken when peak clipping
Peak clipping rule selects peak clipping thresholding, such as 4QAM+16QAM that the peak clipping rule of 16QAM can be used to select as current rule
Peak clipping thresholding is selected, is not repeated herein.
S5, peak clipping is carried out according to peak clipping thresholding W to signal X (t) and is normalized；
Wherein, | X (t) | amplitudes of the expression X (t) in t moment；The signal after peak clipping is normalized again, is obtained
Final output signal
In the present embodiment, normalized signal waveform when giving peak clipping thresholding W=0.5 such as Fig. 6 before and after peak clipping
Figure, wherein V0S before expression does not normalizeOFDM(t) peak value of signal.It is obvious that before without peak clipping, into the normalization of EAM
Average power signal is smaller, and increases 4 times using the mean power of signal after peak clipping and normalization.
From Fig. 6 it is obvious that when peak clipping thresholding is close to 1, system mean power is promoted very after normalization
Small, system performance is poor；But when peak clipping thresholding is close to 0, most of signal is eliminated, system linear degree is very poor, leads
Cause system performance that can also be deteriorated, therefore there are a best peak clipping values between (0,1).Fig. 7 is depicted in no peak clipping, is cut
Peak thresholding is excessive, it is moderate and it is too small it is several in the case of System performance profiles, the modulation format used in emulation receives for QPSK
Luminous power is set in -17dBm.
In Fig. 7 it can be seen that peak clipping thresholding choosing it is excessive, too small when, system performance is all very poor, and moderate peak clipping thresholding
The performance of system can be significantly increased.It can be seen that the optimum choice of peak clipping thresholding, is the key that efficiently use peak clipping.
Example
Implementation method one：Analog domain realizes peak clipping
Analog domain realizes that the key of peak clipping is the saturation output characteristics using radio frequency amplifier, and the input of radio frequency amplifier is defeated
It is as shown in Figure 8 to go out characteristic：
Input signal amplitude is that B points are in the linearly interval of amplifier, therefore are linearly amplified, corresponding output in Fig. 8
Signal amplitude is C points；And A points are in the saturation interval of amplifier, therefore by after amplifier, being satisfied for amplifier by peak clipping
With amplitude output signal D points.
Analog domain realizes the system block diagram of peak clipping as shown in figure 9, according to step S1-S5 the methods, obtains final defeated
Go out signal
Implementation method two：Numeric field realizes peak clipping
By the discussion of front it is found that the essence of peak clipping is that operation is normalized according to peak clipping threshold value.Once peak clipping door
Limit value determines, normalization can be realized by simple mathematical operation in numeric field.
Numeric field peak clipping system block diagram is as shown in Figure 10, according to step S1-S5 the methods, obtains final output signal
Simulating, verifying
In the present embodiment, as shown in figure 11, when modulation format is QPSK, different sons are carried in OFDMA-ROF systems
When carrier number, peak clipping is compared with the performance of non-peak clipping.Wherein, in simulation process, what OFDMA-ROF systems received
Luminous power is -17dBm, and when the signal-to-noise ratio of radio frequency link is 10dB, peak clipping threshold value is calculated by matched curve.By with not
Ber curve comparison when peak clipping, the system cisco unity malfunction if without peak clipping, if mistake can be made by carrying out peak clipping
Code check drops under FEC thresholdings.It is therefore found that using peak clipping thresholding selection algorithm proposed by the present invention carry out peak clipping it
Afterwards, the performance boost of system is apparent.
Similarly, for 16QAM, 64QAM, 256QAM, the Performance Simulation Results before and after peak clipping are as shown in figure 12：
Wherein, Figure 12 (a), Figure 12 (b), Figure 12 (c) modulation format correspond to 16QAM, 64QAM, 256QAM, Figure 12 respectively
Middle modulation format corresponding received optical power respectively -7dBm, -2dBm, -2dBm when being 16QAM, 64QAM, 256QAM.From figure
In it can be seen that for 16QAM, 64QAM, 256QAM, peak clipping thresholding selection algorithm proposed by the present invention is equally effective.
For the transmission mode of different modulating format mixing, apparent performance boost can equally be obtained by peak clipping, such as
Shown in Figure 13, half user is modulated using 4QAM, and such as Figure 13 (a), half user is modulated using 16QAM, such as Figure 13 (b),
When carrying different user number in OFDMA-ROF systems using peak clipping compared with the performance of unused peak clipping.In emulation, light is received
Power is -7dBm, and the calculating of peak clipping threshold value is on the basis of 16QAM is modulated, then calculates peak clipping threshold value.It can from figure
Go out, it is proposed that peak clipping thresholding selection algorithm the case where being used in mixed way for modulation format be equally also suitable.
Although the illustrative specific implementation mode of the present invention is described above, in order to the technology of the art
Personnel understand the present invention, it should be apparent that the present invention is not limited to the range of specific implementation mode, to the common skill of the art
For art personnel, if various change the attached claims limit and determine the spirit and scope of the present invention in, these
Variation is it will be apparent that all utilize the innovation and creation of present inventive concept in the row of protection.
Claims (2)
1. a kind of light carries radio frequency orthogonal frequency division multiple access access ROF-OFDMA (Radio Over Fiber-Orthogonal
Frequency Division Multiple Access) dynamic despicking method in downlink, which is characterized in that including following
Step：
(1), QAM mappings are completed according to the modulation format and subcarrier number n that are distributed to each user information sequence to be transmitted,
Obtain the corresponding symbol sebolic addressing of each subcarrier；
(2), n symbol sebolic addressing is carried out at the same time OFDMA modulation, obtains signal S to be sentOFDM(t)；
(3), according to subcarrier number n and modulation format, signal S is calculatedOFDM(t) maximum amplitude V0, further according to V0To signal
SOFDM(t) it is normalized, obtains signal X (t)；
(4), it when modulation format determines, is fitted by the simulation result to different sub-carrier number, determines peak clipping thresholding
W；
W=a × 10-4n+b
Wherein, a, b are constant, and value is determined by corresponding modulation format；
(5), peak clipping is carried out according to peak clipping thresholding W to signal X (t)；
Wherein, | X (t) | indicate the amplitude of t moment；The signal after peak clipping is normalized again, obtains final output
Signal
2. dynamic despicking method in a kind of ROF-OFDMA downlinks according to claim 1, which is characterized in that described
Modulation format be QPSK or 4QAM or 16QAM or 64QAM or 256QAM or different rank QAM hybrid modulations；
When modulation format is the QAM hybrid modulations of different rank, on the basis of the highest modulation format of QAM exponent numbers, according still further to
Step (4) the method, determines peak clipping thresholding W.
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