WO2008000118A1

WO2008000118A1 - A method for transmitting and receiving a cell searching information in orthogonal frequency division multiplexing system

Info

Publication number: WO2008000118A1
Application number: PCT/CN2006/003745
Authority: WO
Inventors: Shuqiang Xia; Yong Li
Original assignee: Zte Corporation
Priority date: 2006-06-21
Filing date: 2006-12-30
Publication date: 2008-01-03
Also published as: CN101094498A; CN101094498B

Abstract

A method for transmitting and receiving a cell searching information in Orthogonal Frequency Division Multiplexing system, the transmitting method includes the following steps: the mapped cell searching information is transformed linearly; the signal which is transformed linearly is differentially encoded; the signal which is differentially processed is modulated by OFDM. The receiving method includes the following steps: the mobile station performs the OFDM demodulation, and then the received cell searching information is extracted from the result of the OFDM demodulation; the received cell searching signal is differentially decoded; the signal which is differentially decoded is reverse transformed linearly. It can reduce the cost of transmitting signal at the transmitter end and the realization complexity of the receiver, and then improve the correct detecting probability of the cell searching information through the invention.

Description

TECHNICAL FIELD The present invention relates to the field of digital communications, and in particular, to a method for transmitting cell search information based on an Orthogonal Frequency Division Multiplexing (OFDM) system. BACKGROUND In a mobile communication system based on Orthogonal Frequency Division Multiplexing (OFDM) technology, when a mobile station initially accesses a system, the mobile station should obtain an optimal time in the surrounding cells in addition to basic time and frequency synchronization. The index information of the cell may even include the number of transmit antennas of the cell, cyclic prefix length information, system bandwidth information, and the like. This process is called the cell search process. Since this information is necessary for the motion to be connected to the system, how the base station transmits this information ensures that the mobile station can correctly detect the information under the circumstance without adding too much overhead. The implementation of OFDM systems is of great significance. In a WiMAX system based on OFDM technology, a base station uses a frequency domain pseudo-random code to characterize index information of a cell. One cell index corresponds to one pseudo random code. The random code has good autocorrelation and cross-correlation properties (the good autocorrelation and cross-correlation properties here have the following meanings: The random code itself has strong autocorrelation properties, while the cross-correlation of pseudo-random codes of different cells is Weak). At the receiving end, the mobile station uses this property to correlate the received data with all possible cell pseudo-random codes, the peak corresponding cell being the cell with the best signal around the mobile station. However, OFDM technology is applied in broadband systems. In broadband environments, the frequency selective fading of channels is very significant. The good autocorrelation and cross-correlation properties of pseudo-random codes are often severely damaged. Correspondingly, mobile stations cannot use them. The good autocorrelation and cross-correlation properties of the pseudo-random code accurately determine the best access cell. In addition, in the existing communication system based on OFDM technology, the cell search information sent by the base station is often limited to the index information of the cell, and other information such as the number of transmitting antennas of the base station, the system bandwidth, and the cyclic prefix length type are Not valued. Still taking the WiMAX system as an example, in the system, at the beginning of a frame, the base station utilizes the entire system bandwidth, and only transmits cell index information in one OFDM time, and does not transmit information such as the number of transmitting antennas. The consequence of this is: The base station can only transmit data with a single antenna before transmitting the number of transmit antennas. This is not conducive to the improvement of system link stability and throughput, and often becomes the bottleneck of system performance improvement. Therefore, a new community is proposed. Search information transmission mode, that is, considering content expansion of cell search information: In addition to including cell index information, it also includes information such as number of transmitting antennas, system bandwidth, and cyclic prefix length type, and also considers factors such as frequency selective fading and overhead of the channel. , necessary. SUMMARY OF THE INVENTION The technical problem to be solved by the present invention is to provide a method for transmitting cell search information based on an OFDM system, which solves the problem that the current cell search information is single and the detection rate is not high under the frequency selective channel. To achieve the purpose of the present invention, a method for transmitting cell search information in an orthogonal frequency division multiplexing system at a transmitting end includes the following steps: Step 1: linearly transform the mapped cell search information; Step 2: For linear The transformed signal is subjected to differential encoding processing; Step 3: Performing 0FOM modulation on the differentially processed signal. The above method, before step one, further comprises the step of mapping one or more cell search information to a transmission sequence. In order to achieve the object of the present invention, a method for receiving cell search information in an orthogonal frequency division multiplexing system is further provided at a mobile station, which includes the following steps: Step 1: The mobile station performs OFDM demodulation, and then from the result of OFDM demodulation Extracting the received cell search signal; Step 2: differentially decoding the received cell search signal; Step 3: performing inverse linear transformation on the differentially decoded signal. The above method further includes a cell search information inverse mapping step after the third step. In the above method for transmitting cell search information, since the transmission signal is subjected to differential processing, at the receiving end, the receiver can detect the received cell search information without performing channel estimation, that is, the overhead of transmitting the signal at the transmitter end is reduced. It also reduces the implementation complexity of the receiver. When OFDM modulation is performed on the differentially encoded signal, the differential signal sequences are all mapped on adjacent subcarriers, which is advantageous for reducing The effect of the frequency selective fading channel on the received signal, thereby improving the correct detection probability of the cell search information. The linear transformation step suppresses the influence of noise, thereby further improving the correct detection probability of the cell search information. In addition, the linear transformation with fast algorithm, such as convolutional coding, discrete Fourier transform, Hadamard transform, etc., is selected. The rate at which the receiver detects the signal can be increased, and the implementation complexity of the receiver can be reduced. Finally, in the cell mapping step, a plurality of cell search information can be mapped into one transmission sequence, and the content of the transmitted information is improved under the same resource overhead as compared with the conventional method. In summary, the cell search information sending method provided by the present invention has many advantages such as realizing a single ticket, high detection accuracy, and more cell search information can be transmitted under the same overhead. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of transmitting cell search information according to the present invention. 2 is a schematic diagram of subcarrier mapping of bearer cell search information according to the present invention. 1 is a schematic diagram of transmitting cell search information according to the present invention. In the 101 linear transform module, the input cell search information is linearly transformed, and the following factors need to be considered when selecting the linear transform mode: (1) These transforms should have a fast algorithm, thereby facilitating reduction of implementation complexity. (2) These transforms have the ability to resist noise, which is beneficial to improve the detection performance of the cell search signal. Preferred linear transforms include discrete Fourier transform, Hadamard transform, linear coding, linear coding such as convolutional coding, orthogonal coding, and the like. The cell search information here includes information of a cell index, a number of transmitting antennas, a cyclic prefix length, and a system bandwidth. Prior to 101, a mapping step is included, which maps one or more cell search information to a transmission sequence, maps the information to 0-1 information and sends it to the linear transformation module. There are two basic mapping modes that can be arbitrarily used in a certain type of cell search information: (1) Bitmap mapping mode: Taking the number of transmitting antennas as an example, if the number of transmitting antennas is N, then when the transmitter adopts k When transmitting antennas, the information can be mapped to B = (0, 0, ... 1, 0, 0, 0), where the number of elements in B is Q, Q ≥ N, and there is only one element in B Is 1, other elements are all 0. (2) Binary mapping mode: set a certain cell: If the number of information is Μ, when the kth (k=0, 1, 2, 3·'·Μ-1) information is sent, the information can be divided. The other 'j maps to (0, 0, 0, --0) , ( 0, 0, 0, · '· 1) , (0, 0, 0, '·· 1, 0) '·. , here ( 0, 0, 0, · ''0>, ( 0, 0, 0, ·" 1) , (0, 0, 0, ·'· 1, 0) are respectively A binary representation of k=0, 1, 2, 3.... Taking the number of transmitting antennas as an example, the maximum number of transmitting antennas is 4, and when the transmitters use 1, 2, 3, 4 transmitting antennas respectively, the information can be mapped to (0, 0), (0, 1) respectively. , (1,0), (1,1). The differentially encoded processing module is performed on the linearly transformed signal at 102, and the linearly transformed signal is subjected to differential encoding processing. The differential encoding processing may adopt the following two methods. Method 1: Let the linearly transformed output signal be: x = (x[l],....4V-1]), then the step-knee two differential encoding process specifically includes the following steps: y[0] = C , C is a constant; when X is a binary sequence, y[k] = x[k] ©y[kl] , ® means modulo 2 plus, 0<k<N; when x is a non-binary sequence, y[k] = x[k] xy[kl] , 0<k<N, the resulting differentially processed signal is: = (yi°wi--yi ^N . square ii- -~, set linearly transformed The first element x[l] of the output signal X is a constant, and the differential encoding process in the second step specifically includes the following steps: I: y[l] = C, C is a constant; when X is a binary sequence, y [k] =x[k] ©y[kl] , ® means modulo 2 plus, Kk<N; when x is a non-binary sequence, y[k] = x[k] χ y[kl] , Kk<N; The resulting four-word differential processing is: = Cy[l],... [-1]). It can be seen from the comparison that the second method is characterized by saving one symbol than the first one. The differential processing signal is subjected to an OFDM modulation module at 103, and the differentially processed signal is mapped on adjacent subcarriers, and then subjected to inverse discrete Fourier transform processing. The differentially encoded sequence is alternately mapped onto the carriers of the first reference symbol and the second reference symbol. In one subframe, the pilot symbols include a first reference symbol and a second reference symbol, and the pilot carriers of the first reference symbol and the second reference symbol are arranged in a stagger manner. The differentially encoded sequence should be alternately mapped onto the carrier of the first reference symbol and the second reference symbol: there are several implementations:

(1), the sequence is alternately mapped on the second reference symbol of the current subframe and the pilot carrier of the first reference symbol; or

(2) alternately sequenced on a pilot carrier of a second reference symbol of a current subframe and a first reference symbol of a next subframe; or

(3), the sequence is alternately mapped on the first reference symbol of the current subframe and the pilot carrier of the second reference symbol of the previous subframe; for a better understanding of the present invention, FIG. 2 shows the differential processed signal A schematic diagram mapped on adjacent subcarriers, the sequence alternately mapping on the pilot symbols of the second reference symbol of the current subframe and the first reference symbol of the next subframe. In this embodiment, the base station uses a subframe as a basic unit of a transmission signal, each subframe includes 7 OFDM symbols in time, and includes a number of subcarriers in the frequency domain. As shown in FIG. 2, the subcarriers of the third, ninth, and fifteenth... of the first OFDM symbol of the subframe are pilot subcarriers (the lattice of the first column with dots in FIG. 2), which is called The first pilot signal; the 0th, 6th, 12th...th subcarrier of the fifth 0?0^ symbol of the subframe is also a pilot subcarrier, which is called a second pilot signal. The differentially processed signal is now mapped to the previously associated pilot carrier. Let the two signal sequences that need to be mapped after the difference processing are (Xl, x2, x3, x4, x5, x6, x7...) and (yl, y2, y3, y4, y5, y6, y7. . . ) Then, the mapping of the sequence on the subcarriers is as shown in FIG. 2. In FIG. 2, x1 is mapped to the first subcarrier of the second reference symbol of the current subframe, x2 is mapped to the first subcarrier of the first reference symbol of the next subframe, and x3 is mapped to the current subframe second reference. The second subcarrier of the symbol. among them, The order 歹 ' J ( yl , y2, y3, y4, y5, y6, y7... ) and the sequence 歹 'J ( Xl, x2, x3, x4, x5, x6, x7... ) ό々 the same mapping law . Figure 3 shows another mapping method in which the sequence is alternately mapped on the second reference symbol of the current subframe and the pilot carrier of the first reference symbol: χΐ mapped to the first sub-frame of the second reference symbol of the current subframe The carrier, x2 is mapped to the first subcarrier of the second reference symbol of the current subframe, and x3 is mapped to the second subcarrier of the first reference symbol of the current subframe. Among them, the mapping rules of the sequence (yl, y2, y3, y4, y5, y6, y7..) and the sequence 歹 'J (xl, x2, x3, x4, x5, x6, x7...) are the same. According to the above-mentioned cell search signal mapping method, the frequency domain interval of the mapping sequence is generally the frequency domain interval of the usual mapping method, and the mapping sequences are already differentially processed, so that it is very suitable for the channel with significant frequency selectivity. transmission. Comparing the two mapping methods of FIG. 2 and FIG. 3, the time domain interval of the mapping sequence in FIG. 2 is one symbol less than the time domain spacing of the mapping sequence in FIG. 3, therefore, the sequence mapping method provided in FIG. 2 is under the time selective channel. Performance is also superior. At the receiving end, the mobile station performs inverse processing opposite to the transmitting end process, that is, the cell transmitted by the base station can be detected; and the inverse processing can be described as follows: The mobile station first performs OFDM demodulation, and then demodulates from OFDM. The received cell search signal is extracted from the result of the modulation, and the signal is now a signal contaminated by the channel. Then, the received signal is differentially decoded, and finally the inversely transformed cell search information is inversely mapped to the differentially decoded signal, and the output result is an estimation of the cell search information sent by the base station. Here, demodulation and extraction of search signals are both prior art and will not be described again. Differential decoding, briefly described as follows: Let the mobile station receive the cell search signal as x(0), x(1), ...x(N-1). The output of the differential decoding is y(l),... y(N-1). The difference process can be described as: y{\ ]=x [0)x[l]

y[2]=x [\]x[2

Where, represents the conjugate of X [/<:]. There are also a variety of inverse mapping methods. Two are introduced here, which is the inverse of the mapping method introduced earlier. It is to be understood by those skilled in the art that the foregoing description of the present invention is not intended to limit the scope of the present invention. Covered by the scope of patents.

Claims

Method for transmitting cell search information in an orthogonal frequency division multiplexing system, characterized in that it comprises the following steps

Step 1: linearly transform the mapped cell search information;

Step 2: performing differential coding processing on the linearly transformed signal;

Step 3: The differentially processed signal is subjected to OFD modulation.

The method of claim 1 wherein step one further comprises the step of mapping one or more types of cell search information to a transmission sequence.

The method according to claim 1 or 2, wherein the step of using a linear transform method having a fast algorithm and a noise capability.

The method according to claim 3, wherein the linear transformation method may be a discrete Fourier transform or a Hadamard transform or a convolutional code or an orthogonal code or a pseudo orthogonal code.

The method according to claim 4, wherein the linearly transformed output signal is:

Then, the differential encoding process of step 2 specifically includes the following steps: y[0] = C, C is a constant; when X is a binary sequence, y[k] = x[k] ©y[kl] , ® represents Modulo 2 plus, 0<k<N;

When X is a non-binary sequence, y[k] = x[k] χ y[kl] , 0<k<N, and the resulting differentially processed signal is: ^ ⁼ ( loiyUl- ― 1]). The method according to claim 4, wherein the first element ¹ ] of the linearly transformed output signal X is a constant, and the differential encoding process in the second step is specifically performed as follows: y[l] - C, C is a constant; when X is a binary sequence, y[k] = x[k] ten y[k - 1], ® represents the modulo 2 force port, Kk <N; When x is a non-binary sequence, y[k] = x[k] X y[kl], Kk<N; The resulting differentially processed signal is: y = Mi],....^[Ni ]). The method according to claim 2, wherein the cell search information comprises: an index of a cell, a number of transmit antennas, a cyclic prefix length, and a system bandwidth.

The method according to claim 2 or 7, wherein there are two mapping methods that can be used for any kind of cell search information:

(1) Bitmap mapping mode: If the number of certain cell search information is N, then when the kth information is transmitted, the information can be mapped to B = (0,0, ...1, 0, 0, 0). , where the number of elements in B is Q, Q ≥ N, and only one element in B is 1, the other elements are all 0, and the "Γ" element is the kth element in the last ;; (2) Binary mapping Mode: If the number of certain cell search information is M, then when the kth (k=0, 1, 2, 3---M-1) information is sent, the information can be mapped to (0, 0, 0 respectively). , ···()), ( 0, 0, 0, ···1), (0, 0, 0, ·'·1, 0)... , here (0, 0, 0, '··0) , ( 0, 0, 0, · '·1), (0, 0, 0, "·1, 0) are binary representations of k = 0, 1, 2, 3, respectively.

The method according to claim 1 or 2, wherein the step 3 comprises: mapping the differentially processed signal on adjacent subcarriers, and then performing inverse discrete Fourier transform processing.

The method according to claim 9, wherein the differentially processed signals are alternately mapped onto the carriers of the first reference symbol and the second reference symbol, specifically:

(1) mapping between the second reference symbol of the current subframe and the pilot carrier of the first reference symbol;

Or

(2) mapping alternately between the second reference symbol of the current subframe and the pilot carrier of the first reference symbol of the next subframe;

Or

(3) mapping alternately on a pilot carrier of a first reference symbol of a current subframe and a second reference symbol of a previous subframe;

Wherein, in one subframe or frame, the pilot symbols include a first reference symbol and a second reference symbol, and pilot carriers of the first reference symbol and the second reference symbol are staggered.

A method for receiving cell search information in an Orthogonal Frequency Division Multiplexing system, comprising the following steps: Step 1: A mobile station performs OFDM demodulation, and then extracts a received cell from a result of OFDM demodulation. Search signal

Step 2: differentially decoding the received cell search signal;

Step 3: Perform inverse linear transformation on the differentially decoded signal.

The method according to claim 11, wherein the step 3 further comprises a cell search information inverse mapping step.

The method according to claim 11 or 12, wherein the cell search signals received by the mobile station are χ(0), _Χ (1), ...χ(Ν-1), the output of the differential decoding. For y(l), .... y(Nl), the differential decoding process of the second step is specifically: 1]=χ*[0]χ[1]

2] [l]x[2]

]=x*[2]x[3] where represents the conjugate of X 1 1 .