CN105429922B - The channel estimation method based on Comb Pilot for DDO-OFDM systems - Google Patents
The channel estimation method based on Comb Pilot for DDO-OFDM systems Download PDFInfo
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Abstract
The invention discloses a kind of channel estimation methods based on Comb Pilot for DDO ofdm systems.The present invention includes the following steps：Step 1 finds out pilot frequency locations；Step 2, the pilot frequency locations in the transmission signal of transmitting terminal are inserted into pilot sub-carrier；Step 3 docks in receiving terminal and at same pilot frequency locations is extracted pilot sub-carrier by signal；Step 4, the average frequency response that pilot sub-carrier is estimated using the LS estimations method of average；Step 5, the frequency response that data subcarrier in channel is estimated using linear interpolation method.Under the spending of same pilot tone, algorithm of the invention reduces the bit error rate, improves estimated accuracy, improves snr gain；The algorithm bit error rate of the present invention is influenced small by pilot tone spending, i.e., is paid wages using smaller pilot tone and also can guarantee low error rate, improve band efficiency；The algorithm complexity of the present invention is low, highly practical.
Description
Technical Field
The invention belongs to the technical field of optical communication, and particularly relates to a comb-shaped pilot frequency-based channel estimation method for a DDO-OFDM system.
Background
With the development of communication technology and the increasing demand for communication, optical communication shows two obvious trends: the data rate of single channel transmission is greatly increased and approaches to 100 Gb/s; the network must have a fast dynamic adjustment capability. However, when the data rate reaches 100Gb/s, the conventional fiber segment compensation becomes expensive and time consuming, and the compensation of the system dispersion is difficult to be accurately implemented, whereas OFDM has good calculation characteristics, and can conveniently compensate the fiber dispersion by complex operation in the frequency domain. Specific references are: neighbor Optical OFDM: the Theoryandd De-sign [ J ] Optics Express 2008,16: 41-859.
In 2005, n.e.jolley and t.m.tang et al first proposed the application of OFDM technology to optical fiber transmission systems at the OFC2005 conference and verified that 10Gb/s signals can be transmitted for 1km over multimode optical fiber, from which they began the study of optical orthogonal frequency division multiplexing (O-OFDM). Reference documents: jolley N E, Kee H, Pickaed P, et al. Generation and propagation of a 1550nm 10Gbit/s Optical orthogonal frequency division multiple signal over 1000m of multimode fibre used modulated DFB. in: Optical Fiber Communication Conference,2005.Technical digest. OFC/NFOEC.Harlow, UK,2005, 6-11.
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique, and its basic principle is to perform inverse fourier transform (IFFT) by digital signal processing to generate a set of Orthogonal sub-carriers for parallel transmission of low-rate digital signals, thereby completing the transmission task of high-speed digital signals. The most prominent advantage of this procedure is that the spectrum utilization of the system is improved, and the complexity of calculation is reduced.
O-OFDM has shown many outstanding advantages over conventional optical communications, such as high spectral efficiency, significant resistance to dispersion and nonlinear effects, which will be important in next generation high speed communication systems. Many research results at present also show the potential application value of the O-OFDM in future large-capacity and long-distance optical communication systems. More importantly, the O-OFDM has important application value in future dynamic switching optical networks due to the self-adaptive single-tap frequency domain equalization capability.
In the direct detection of the O-OFDM system, the channel needs to be estimated, and the accuracy of the channel estimation directly influences the performance of the whole system. Channel parameter estimation is a key technique for implementing wireless communication systems. Whether detailed channel information can be obtained or not is an important index for measuring the performance of an optical communication system, so that a transmitting signal can be correctly demodulated at a receiving end.
The research of the prior literature proves that the current channel estimation method mainly comprises an averaging method based on the block pilot frequency and a blind estimation algorithm, but the two algorithms have certain defects. The block pilot frequency-based averaging method has high frequency expense and low frequency spectrum utilization rate; the blind road method has the defects of low precision, high complexity, long statistical time and the like, and is difficult to play a role in practical application. Therefore, there is a need to develop a new algorithm that overcomes the drawbacks of the current channel estimation algorithms.
Disclosure of Invention
In order to overcome the defects of low frequency spectrum utilization rate, low precision and the like of the traditional channel estimation method, the invention provides a novel channel estimation method based on comb-shaped pilot frequency for a DDO-OFDM system. The invention can reduce pilot frequency cost, improve frequency band utilization rate, reduce bit error rate and improve signal-to-noise ratio gain. The early simulation result shows that the algorithm has good reliability and stability.
The technical scheme adopted by the invention for solving the technical problems is as follows: a channel estimation method based on comb-shaped pilot frequency suitable for a DDO-OFDM system comprises the steps of firstly estimating the frequency response of a pilot frequency subcarrier by using an LS (least square) channel estimation algorithm averaging method, and then estimating the frequency response of a data subcarrier through linear interpolation on the basis of the frequency response of the pilot frequency subcarrier.
The method comprises the following concrete steps:
step 1, pilot frequency positions are obtained.
And 2, inserting pilot subcarriers in pilot positions in the sending signals of the transmitting end.
And 3, extracting pilot frequency sub-carriers at the same pilot frequency position of the received signal at the receiving end.
Step 4, estimating the average frequency response of the pilot frequency subcarrier by using an LS estimation average method;
and 5, estimating the frequency response of the data subcarrier in the channel by using a linear interpolation method.
The solving of the pilot frequency position in the step 1 is specifically as follows:
(1-1) calculating the number p of pilot frequencies, wherein the formula is as follows:
wherein, N is the actual number of effective carriers, and L is the pilot interval. ceil (X) represents taking the smallest integer greater than or equal to the specified expression X.
And (1-2) calculating the position of the pilot frequency subcarrier.
Wherein inIndicating the location of the nth pilot on the frequency axis.
The pilot subcarrier vector inserted in the transmission signal in the step 2 is:
Xp＝[X(i1,：)；X(i2,：)；...；X(in,：)；...；X(ip,：)]p×M
wherein, X (i)nAnd) is the nth pilot subcarrier inserted in the transmission signal.
The step (3) of extracting the pilot frequency subcarrier vector from the received signal is as follows:
Yp＝[Y(i1,：)；Y(i2,：)；...；Y(in,：)；...；Y(ip,：)]p×M
wherein Y (i)nAnd (b) extracting the nth pilot subcarrier from the received data.
The step (4) estimates the average frequency response of the pilot subcarriers by using an LS estimation averaging method, which is specifically as follows:
(4-1) estimating the frequency response of the pilot subcarrier by using an LS estimation method, wherein the formula is as follows:
wherein,is the frequency response of the nth pilot subcarrier.
And (4-2) averaging the frequency response at the pilot frequency position on a time axis to obtain the average frequency response of the pilot frequency sub-carriers. The average frequency response vector of the pilot subcarriers is:
wherein,is the average frequency response of the nth pilot subcarrier.
The step (5) estimates the frequency response of the data subcarrier in the channel by using a linear interpolation method, which is specifically as follows:
(5-1) carrying out linear interpolation on the average frequency response of the pilot frequency sub-carriers on a frequency axis, and estimating the frequency response of the data sub-carriers in the channel. The estimated data subcarrier frequency response is:
and (5-2) extending the frequency response of the data subcarriers on a time axis to obtain the required frequency response of the channel. The channel frequency response after expansion on the time axis is as follows:
wherein M is the number of OFDM symbols.
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. under the same pilot frequency expense, the algorithm of the invention reduces the error rate, improves the estimation precision and improves the signal-to-noise ratio gain.
2. The algorithm error rate of the invention is little influenced by pilot frequency expenses, and low error rate can be ensured even if smaller pilot frequency expenses are used, thereby improving the utilization rate of frequency bands.
3. The algorithm of the invention has low complexity and strong practicability.
Drawings
FIG. 1 is a system block diagram of O-OFDM.
Fig. 2 is a flow chart of an implementation method.
Fig. 3 is a diagram illustrating a data frame structure of the transmitting end O-OFDM.
FIG. 4 is a graph of the bit error rate (ber) -signal-to-noise ratio (SNR) of the previous experiment based on the block-shaped pilot LS estimation averaging method and the algorithm of the present invention when the pilot overhead is 1/512 in the example.
Detailed Description
The present invention is described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.
The invention mainly relates to a direct detection channel estimation problem of an optical orthogonal frequency division multiplexing O-OFDM system. As shown in fig. 1, O-OFDM includes a plurality of steps of S/P, QAM mapping, data subcarrier modulation, pilot subcarrier insertion, IFFT, CP addition, M sequence removal, CP removal, FFT, pilot subcarrier extraction, channel estimation, data subcarrier demapping, QAM demapping, P/S, and the like. Wherein the inserting pilot subcarriers, extracting pilot subcarriers and channel estimation belong to a channel estimation module.
The steps of the comb-pilot based LS channel estimation algorithm of the present invention are described in detail below with reference to fig. 2 and 3.
(1) Let total N be 2KAnd the number of the pilot subcarriers is p when the number of the pilot subcarriers is L.
(2) As shown in fig. 3, pilot subcarriers are inserted into pilot positions in the transmission signal at the transmitting end, and the position of the nth pilot subcarrier on the frequency axis is recorded as inLet the nth pilot sub-carrier be X (i)nRecording the pilot sub-carrier vector as Xp。
Xp＝[X(i1,：)；X(i2,：)；...；X(in,：)；...；X(ip,：)]p×M
(3) Receiving signals at the receiving end at the same pilot frequency position inExtracting pilot frequency sub-carrier, recording the n-th pilot frequency sub-carrier as Y (i)nRecording the extracted pilot frequency vector as Yp. And estimating the frequency response of the pilot frequency subcarrier in the channel by using a least square method, and recording the frequency response of the nth pilot frequency subcarrier asRecording the frequency response vector as
Yp＝[Y(i1,：)；Y(i2,：)；...；Y(in,：)；...；Y(ip,：)]p×M
(4) Frequency response vector to pilot locationAveraging on the time axis (by rows) to calculate the average frequency response of the pilot subcarriers, and recording the average frequency response of the nth pilot subcarrier asNote that the average frequency response vector of the pilot subcarriers is Hmean。
(5) Average frequency response H to pilot subcarriersmeanLinear interpolation is carried out on the frequency axis to estimate the frequency response H of the data subcarriers in the channellinear。
(6) And extending on a time axis to obtain H, wherein H is the channel frequency response obtained by final estimation.
Wherein M is the number of OFDM symbols.
Example (b):
fig. 4 shows a prior experimental noise ratio-SNR comparison chart of the LS estimation averaging method based on block pilots in this embodiment and the LS channel estimation method based on comb pilots in the present invention. It sets 1023 OFDM symbols for O-OFDM transmission, 1024 carriers (where the first row does not contain data, i.e. the number of active sub-carriers is 1023 from the 2 nd to the 1024 th, i.e. the actual number of active sub-carriers) and the pilot overhead is 1/512. The method of the invention is utilized as follows:
(1) the pilot overhead is 1/512, i.e. the pilot interval L is 511, the number of pilot subcarriers is 3, which is 2, 513, 1024 subcarriers respectively, and the pilot vector is marked as X3。
X3＝[X(2,:)；X(513,:)；X(1024,:)]3×M
The first sub-carrier does not contain data, and the rest sub-carriers are all data sub-carriers.
(2) Extracting pilot frequency sub-carrier from pilot frequency position in received data, forming pilot frequency vector as Y3。
Y3＝[Y(2,:)；Y(513,:)；Y(1024,:)]3×M
(3) Estimating frequency response of pilot frequency subcarrier in channel by LS channel estimation algorithm
(4) Frequency response to pilot positionAveraging over time axis (by rows) to estimate average frequency response H of pilot subcarriersmean。
(5) To estimateAverage frequency response H of pilot subcarriers ofmeanLinear interpolation is carried out on the frequency axis to estimate the frequency response H of the data subcarriers in the channellinear。
(6) And extending on a time axis to obtain H, wherein H is the channel frequency response obtained by final estimation.
Comparing the two bit error rate-SNR curves in FIG. 4, it can be seen that the bit error rate is controlled to 10-3In order of magnitude, compared with the traditional channel estimation based on the block pilot frequency, the algorithm has the signal-to-noise ratio gain of about 5 dB.
The channel estimation algorithm based on LS estimation and linear interpolation of comb pilots described in the present invention is described in detail above, and the above description is only for the purpose of facilitating understanding the algorithm and the core idea of the present invention, and not for the purpose of limiting the same, and any other changes, modifications, combinations, and simplifications that do not depart from the spirit and principle of the present invention should be regarded as equivalent substitutions, which are included in the scope of the present invention.
Claims (4)
1. The channel estimation method based on comb-shaped pilot frequency for DDO-OFDM system is characterized by comprising the following steps:
step 1, solving a pilot frequency position;
step 2, inserting pilot frequency sub-carriers in pilot frequency positions in the sending signals of the sending end;
step 3, extracting pilot frequency sub-carriers from the received signals at the same pilot frequency position at the receiving end;
step 4, estimating the average frequency response of the pilot frequency subcarrier by using an LS estimation average method;
step 5, estimating the frequency response of the data subcarrier in the channel by using a linear interpolation method;
the solving of the pilot frequency position in the step 1 is specifically as follows:
(1-1) calculating the number p of pilot frequencies, wherein the formula is as follows:
wherein, N is the number of actual effective carriers, and L is the pilot frequency interval; ceil (X) represents taking the smallest integer greater than or equal to the specified expression X;
(1-2) calculating the position of the pilot frequency subcarrier;
wherein inIndicating the position of the nth pilot on the frequency axis;
the pilot subcarrier vector inserted in the transmission signal in the step 2 is:
Xp＝[X(i1,：)；X(i2,：)；...；X(in,：)；...；X(ip,：)]p×M
wherein, X (i)nAnd M is the number of OFDM symbols, wherein the n is the nth pilot subcarrier inserted in the transmission signal.
2. The comb-based channel estimation method for DDO-OFDM system as claimed in claim 1, wherein said step 3 extracts pilot subcarrier vectors from the received signal as:
Yp＝[Y(i1,：)；Y(i2,：)；...；Y(in,：)；...；Y(ip,：)]p×M
wherein Y (i)nAnd (b) extracting the nth pilot subcarrier from the received signal.
3. The method of claim 2, wherein the step 4 estimates the average frequency response of the pilot subcarriers by using an LS estimation averaging method, specifically as follows:
(4-1) estimating the frequency response of the pilot subcarrier by using an LS estimation method, wherein the formula is as follows:
wherein,frequency response of the nth pilot subcarrier is estimated; a represents YpAnd XpDividing the elements at the corresponding positions;
(4-2) averaging the frequency response at the pilot frequency position on a time axis to obtain the average frequency response of the pilot frequency sub-carriers; the average frequency response vector of the pilot subcarriers is:
wherein,is the average frequency response of the nth pilot subcarrier.
4. The method of claim 3, wherein the step 5 estimates the frequency response of the data subcarriers in the channel by linear interpolation, as follows:
(5-1) carrying out linear interpolation on the average frequency response of the pilot frequency subcarrier on a frequency axis, and estimating the frequency response of the data subcarrier in the channel; the estimated data subcarrier frequency response is:
(5-2) extending the frequency response of the data subcarriers on a time axis to obtain the required frequency response of a channel; the channel frequency response after expansion on the time axis is as follows:
wherein M is the number of OFDM symbols.
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