CN101997792B - Cyclic prefix-based blind channel length estimation method - Google Patents
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Abstract
The invention relates to a cyclic prefix-based blind channel length estimation method, which is a blind estimation method suitable for maximum time delay spread of a channel of an orthogonal frequency division multiplexing (OFDM) system. The estimation method comprises the following steps of: 1) performing fast Fourier transform (FFT) with the length of N by taking a first, a second, ..., a G+1th sampling point of a receiving symbol as a starting point respectively to acquire G+1 frequency domain signals; 2) performing corresponding phase compensation on each subcarrier of each frequency domain signal according to offset and a subcarrier sequence number; 3) subtracting the front G frequency domain signals with the G+1th frequency domain signal respectively to acquire G difference signals with the length of N; 4) working out the average power of the G difference signals; 5) performing the calculation process of the step 1 to the step 4 on the next M-1 received symbols respectively to acquire M groups of data in all, wherein each group of data consists of G average power; 6) working out the average value of the G average power in the M groups of data to acquire G average values; 7) calculating the change rate of the G average values; and 8) adding 1 to the position corresponding to the maximum change rate to obtain the maximum time delay spread of the channel.
Description
Technical Field
The invention discloses a blind estimation method applied to maximum time delay expansion of an Orthogonal Frequency Division Multiplexing (OFDM) system channel, belonging to the field of channel estimation in mobile communication.
Background
Channel estimation is a prerequisite for coherent demodulation in a mobile communication system. In order to be able to support high-speed data services, future mobile communication systems will employ broadband, multiple (transmitting, receiving) antenna systems, while OFDM is an important candidate for future mobile communication systems. In an OFDM system, a cyclic prefix is inserted in front of each OFDM symbol in order to remove inter-symbol interference (ISI). The cyclic prefix is a copy of the tail of the symbol and is larger than the maximum delay spread of the wireless multipath fading channel. At the receiving end, channel parameters must be estimated for coherent demodulation. In order to achieve a certain spectrum utilization, the OFDM system may employ a blind channel estimation method. When blind channel estimation is performed, the maximum delay spread and the channel length of a multipath channel are very important parameters, which have great influence on the accuracy of an estimation result and the complexity of an algorithm. The invention provides a blind channel length estimation method by utilizing the characteristic that an OFDM symbol has a cyclic structure. The method has low calculation complexity and high estimation precision, and can provide effective prior information for subsequent blind channel estimation.
Disclosure of Invention
The technical problem is as follows:the invention aims to provide a blind parameter estimation method based on cyclic prefix, and further provides a blind channel length estimation method which is fast, reliable, high in estimation precision, low in implementation complexity and suitable for an OFDM system.
The technical scheme is as follows:the blind channel length estimation method based on the cyclic prefix comprises the following steps:
1) respectively to receive symbols
Length of sampling point as starting point
By fast Fourier transform to obtain
A frequency domain signal;
2) carrying out corresponding phase compensation on each subcarrier of each frequency domain signal according to the offset and the subcarrier serial number;
3) front, front
The frequency domain signals are respectively corresponding to
Subtracting the signals of the frequency domains to obtain
Has a length of
A difference signal of (d);
4) to find
The average power of the difference signals;
5) to be next connectedRespectively carrying out the calculation processes from step 1) to step 4) on each received symbol to obtain the total
Group data, each group data is composed ofAverage power composition;
6) to find
Average power is at
Mean value in group data, obtained
(ii) an average value;
7) and calculating
Rate of change of individual mean values;
8) and the position corresponding to the change rate with the maximum value plus 1 is the maximum time delay expansion of the channel.
Wherein,
the number of sub-carriers of the OFDM system;is the length of the cyclic prefix. In order to eliminate the interference between symbols, the length of the cyclic prefix is larger than the maximum delay spread of the wireless multipath fading channel.
Has the advantages that:
1. by using a cyclic structure formed by adding the cyclic prefix into the OFDM symbol, the maximum delay spread information of the channel can be obtained without the assistance of any pilot frequency by estimating the influence of the multipath channel on the received signal.
2. The Fourier forward/inverse transformation has an efficient realization structure and a quick algorithm, and the method reduces the complexity of system realization by adopting a large amount of Fourier forward/inverse transformation.
The blind channel length estimation method provided by the invention can be used for various block transmission systems with cyclic prefixes, including OFDM.
Drawings
Fig. 1 is a schematic structural diagram of a blind channel length estimation method according to the present invention.
Fig. 2 is a schematic structural diagram of an implementation apparatus of the cyclic prefix-based blind channel length estimation method. It includes fast Fourier transform device, device for calculating difference average power after phase compensation, addition device, multiplication device, device for calculating normalized variation value and maximum value search device.
Detailed Description
Consider an OFDM system comprising as many subcarriers as
The length of the cyclic prefix is
Average length ofOne OFDM symbol. Assume a sampling period of
The period of the whole symbol including the cyclic prefix is
. Respectively at 1 st to 1 st of each received symbol
Taking a sampling point as a starting point, performing
Fast fourier transform of the points. After phase compensation of the signal on the subcarrier, the front is calculated
Group data andaverage power of group data difference values. Then, the calculation is performed by an adding device and a multiplying device
The average of the individual inputs. Next, the normalized variation is calculated at these pointsAbsolute value of the value. And finally, estimating the maximum time delay expansion information of the channel by a maximum value searching device.
The specific method is described as follows:
to receive
A number of OFDM symbols
Each sampling point is a starting point, and after FFT (fast Fourier transform), a frequency domain expression can be written as follows:
Wherein,indicating a subcarrier number;
represents a mean value of
Variance isAdditive white gaussian noise of (1);
is the frequency domain response of the channel, which can be expressed as:
[ formula two]
Here, theRepresents the channel resolvable path number;representing the time delay of each path.
And solving the average power of the phase-compensated frequency domain signal after subtraction of the corresponding subcarriers as follows:
Then, pass through
On average of the symbols, we get:
[ equation four ]]
Calculate the absolute change value of each point normalization:
Looking for the maximum
Its corresponding position
I.e. gives an estimate of the channel length.
According to the above description, the implementation steps of the timing synchronization algorithm based on the noise subspace can be obtained as follows:
1) respectively to receive symbolsLength of sampling point as starting point
By fast Fourier transform to obtain
A frequency domain signal;
2) carrying out corresponding phase compensation on each subcarrier of each frequency domain signal according to the offset and the subcarrier serial number;
3) front, front
The frequency domain signals are respectively corresponding to
Subtracting the signals of the frequency domains to obtainHas a length of
A difference signal of (d);
4) to find
The average power of the difference signals;
5) to be next connectedRespectively carrying out the calculation processes from step 1) to step 4) on each received symbol to obtain the total
Group data, each group data is composed of
Average power composition;
6) to find
Average power is at
Mean value in group data, obtained
(ii) an average value;
7) and calculating
Rate of change of individual mean values;
8) and the position corresponding to the change rate with the maximum value plus 1 is the maximum time delay expansion of the channel.
Wherein,
the number of sub-carriers of the OFDM system;
is the length of the cyclic prefix. In order to eliminate the interference between symbols, the length of the cyclic prefix is larger than the maximum delay spread of the wireless multipath fading channel.
The fast Fourier transformation device completes the operation contained in the formulas [ I ] and [ II ], the device for calculating the difference average power after phase compensation completes the formula [ III ], the device for adding and multiplying completes the formula [ IV ], the device for calculating the normalized variation value completes the formula [ V ], and the maximum value searching device completes the search of the maximum value and the determination of the channel length.
Claims (1)
1. A blind channel length estimation method based on cyclic prefix is characterized in that the method comprises the following steps:
1) taking the 1 st, 2 th, 1 st and G +1 th sampling points of the received symbols as starting points to perform fast Fourier transform with the length of N to obtain G +1 frequency domain signals;
2) carrying out corresponding phase compensation on each subcarrier of each frequency domain signal according to the offset and the subcarrier serial number;
3) subtracting the first G frequency domain signals from the G +1 frequency domain signals respectively to obtain G difference signals with the length of N;
4) calculating the average power of the G difference signals;
5) respectively carrying out the processes from the step 1) to the step 4) on the next M-1 received symbols to obtain total M groups of data, wherein each group of data consists of G average powers;
6) solving the average value of G average powers in M groups of data to obtain G average values;
7) calculating the change rate of the G average values;
8) the position corresponding to the change rate with the maximum value plus 1 is the maximum time delay expansion of the channel;
wherein, N is the subcarrier number of the OFDM system; g is the length of the cyclic prefix; m is the average length of the OFDM system; in order to eliminate the interference between symbols, the length of the cyclic prefix is larger than the maximum delay spread of the wireless multipath fading channel.
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Non-Patent Citations (4)
| Title |
|---|
| OFDM系统多径时延及信道估计理论研究和算法实现;李敏;《中国优秀硕士学位论文全文数据库》;20091130;I136-337 * |
| OFDM通信系统盲信道估计算法研究;张瑞齐;《中国优秀硕士学位论文全文数据库》;20050228;I136-324 * |
| 张瑞齐.OFDM通信系统盲信道估计算法研究.《中国优秀硕士学位论文全文数据库》.2005,I136-324. |
| 李敏.OFDM系统多径时延及信道估计理论研究和算法实现.《中国优秀硕士学位论文全文数据库》.2009,I136-337. |
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