CN103312652B - A kind of space-frequency coding SFBC MIMO-OFDM system based on F matrix carries out the method for selected mapping method SLM - Google Patents

Abstract

The invention discloses a kind of SFBCMIMO-OFDM system SLM method based on F matrix, solve the height power ratio of SFBCMIMO-OFDM signal in MIMO-OFDM system and the problem of high computation complexity.By studying the orthogonality based on signal on SFBC coding aft antenna in mimo system, a kind of algorithm reducing system peak-to-average ratio and computation complexity is proposed.The present invention utilizes the phase sequence group F matrix of proposition, signal after space-frequency coding SFBC is processed, the optimum angle factor obtained will be carried out orthogonal coding and is acted on the signal of the every root antenna after space-frequency coding SFBC respectively, on so every root antenna, signal to be sent can avoid the IFFT computing carried out repeatedly, thus reduces the computation complexity of system.The use of F matrix, also allows MIMO-OFDM system obtain good peak-to-average force ratio performance simultaneously.

Description

Selective mapping SLM (selective mapping) method for space-frequency coding SFBC (space frequency coding) MIMO-OFDM (multiple input multiple output-orthogonal frequency division multiplexing) system based on F matrix

Technical Field

The invention relates to the field of mobile communication, in particular to a selective mapping (SLM) method for reducing the peak-to-average ratio of a multi-input multi-output-orthogonal frequency division multiplexing (MIMO-OFDM) system based on space frequency coding (SFBC).

Background

Orthogonal Frequency Division Multiplexing (OFDM) is widely used in Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and Wireless Local Area Network (WLAN), and has the advantages of high spectrum utilization rate, multipath interference resistance, and the like, so that signal interference can be effectively eliminated, and thus, the OFDM has attracted more and more attention. Multiple Input Multiple Output (MIMO) is capable of increasing a channel transmission rate and obtaining a high diversity gain or capacity gain by arranging a plurality of antennas at both the transmitter and the receiver. Two technologies in wireless communication are combined into MIMO-OFDM (Multiple-input Multiple-output orthogonal frequency division multiplexing), wherein OFDM can convert a frequency selective MIMO channel into a parallel flat MIMO channel, and realize robust transmission of high-speed data by using multipath fading. Therefore, MIMO-OFDM has become a very promising alternative in fourth generation mobile communication systems.

However, the disadvantage of high peak-to-average power ratio of MIMO-OFDM signals forces High Power Amplifiers (HPAs) to have large back-offs, which reduces the efficiency of the HPAs. The MIMO-OFDM signal generates in-band distortion and out-of-band noise, and thus the performance of the entire system is severely degraded due to spectrum spreading interference and signal distortion. The high peak-to-average ratio is a technical obstacle of MIMO-OFDM, so a proper method must be found to reduce the peak-to-average ratio and improve the fidelity of signals.

For OFDM systems, techniques for reducing PAPR can be roughly divided into three categories: one is a coding technique, documents "Chen, HLiang, HPAPRReducated non OFDMSignalUsing PartialTransmit sequence and Reed-Muller codes, IEEEcommunications letters, vol.11, No.6, pp.528-530, Sep.2007", the algorithm idea is to use different coding methods to avoid the occurrence of symbols which may generate higher PAPR, but the coding process is more complex. The second category is signal predistortion technique, including amplitude limiting method and companding method, which is the simplest and most direct nonlinear method for reducing peak-to-average ratio, but the former introduces in-band distortion and out-of-band radiation which can seriously reduce the performance of the system. The latter uses a companding function and an inverse transform function to achieve the reduction of the peak-to-average ratio, and documents "x.b.wang, t.t.tjhung, and c.s.ng, reduction of peak-to-average power ratio of ofdm systematic amplification of companding technology, ieee trans.broadcast, vol.45, No.3, pp.303-307, sep.1999" propose a scheme for making mu-law companding have good performance based on speech processing, but the reduced PAPR is at the cost of increasing average power. The third type is scrambling code technology, including selective mapping SLM and partial transmission sequence PTS, this technology uses different scrambling code sequences to carry on weighting process to OFDM symbol, through setting up PAPR threshold value condition, choose a group with the smallest PAPR from the transmission sequence to be used for transmission, in this way, reduce the probability that the big peak power signal appears apparently, but because of adopting too much IFFT, the computational complexity increases sharply, and the transmission of the sideband information has caused the loss of the data transmission rate too.

However, the PAPR reduction algorithm for the MIMO-OFDM system is less studied, and the method for solving the PAPR problem in the MIMO-OFDM system is summarized in two aspects: on one hand, the method in the OFDM system is directly transplanted to each antenna of the MIMO-OFDM system, for example, documents "ByungMooLee, ruij.p. defuieired.sidelnformationpowerallocationfor MIMO-OFDM paprpredepredbymeasurement mapping, IEEE, international communications acoustics, spechandsignalprocessing, 2007" can effectively reduce the PAPR of the system but with a slightly higher complexity; and on the other hand, the method takes flexible method to process in consideration of the characteristics of the MIMO-OFDM system. The simplified PTS and SLM algorithms proposed by the documents Joo-HeeMoon, Young-HwanYou, Won-GiJeon, Ki-WonKwoon, Hyoung-Kyuson, Peak-to-Averagepowercontrolfor multiple-antenna HIPLAN/2 and IEEE 802.11estims IEEETrans. OnConsumer electronics,2003,49(4):1078-1083 "use the same sideband information for each antenna, and have slightly reduced PAPR effect but improved error rate performance compared with independent PTS and SLM algorithms. The patent "zhangyang et al, a peak-to-average ratio control method, a receiving end and a transmitting end" describes a method for reducing PAPR of a system by a combination of linear and nonlinear methods, but the algorithmic process is too complicated. Patent "CN 102075222A, jiang tao, li kodao, a method for reducing the peak-to-average power ratio of space-frequency coded MIMO-OFDM signal", although it is not necessary to transmit side-band information, the algorithm is computationally expensive. The CARI and the improved algorithm thereof proposed in patent "CN 101073217A, TANM, zonalz, yeheskelbn, stbcmmo-ofdm peak-to-averagepowerratereduction-based cross-interference-antenna rotation and inversion" have no complex multiplication operation per iteration, and the computational complexity is reduced, but if the algorithm is directly extended to a system with more transmitting antennas, the additional degree of freedom provided by the added antennas cannot be fully utilized.

Aiming at the algorithm for reducing the PAPR of the MIMO-OFDM system, the invention provides an SFBCMIMO-OFDM system SLM method based on an F matrix. An F matrix is set as a phase factor, an SLM algorithm is adopted for scrambling signals before SFBC coding, after symbol peak-to-average ratio is calculated, the selected optimal phase factor sequence and the orthogonal coding sequence thereof are used as the optimal phase factor after SFBC coding. Thus each antenna is used as an IFFT only once in one symbol period, which greatly reduces the computational complexity, and the use of F matrix also greatly improves the peak-to-average ratio of the system.

Disclosure of Invention

To overcome the above-mentioned drawbacks of the MIMO-OFDM system more effectively, the present invention aims to provide a method that can reduce the peak-to-average power ratio in the MIMO-OFDM system and can be applied to an actual communication system more effectively.

The innovation of the invention is to provide an effective phase factor called F matrix for reducing the peak-to-average ratio of the MIMO-OFDM system.

The innovation of the invention is that an F matrix is provided as a phase factor, an SLM algorithm is adopted for scrambling signals before SFBC coding, after symbol peak-to-average ratio is calculated, the selected optimal phase factor sequence and the orthogonal coding sequence thereof are used as the optimal phase factor of signals to be sent on each antenna after SFBC coding. When OFDM modulation is carried out on each antenna, one symbol period is only used as IFFT once, and the calculation complexity is greatly reduced.

The invention relates to an SLM method of an SFBCMIMO-OFDM system based on an F matrix, wherein T transmitting antennas are T (T is more than 0), and the specific process of the method comprises the following steps:

step 1, inputting binary data bit stream, modulating to obtain mapping signal, serial-parallel connectionAfter replacement, X is obtained_iFrequency domain signal, X_iRepresents the ith frequency domain symbol:

X_i＝[X₀,X₁,…X_N-1]^T(1)

wherein N represents the number of subcarriers, and the upper limit value of i is set to P, P > 0 [. cndot. ]]^TRepresents a transpose of a matrix;

step 2, generating an initial phase factor set by using the F matrix in the formula (2):

$F=\frac{1}{k}I\left(N\right)+\frac{1}{N}rand\left(N\right)---\left(2\right)$

wherein, i (N) represents an N × N unit matrix, k represents an attenuation factor, and rand (x) represents an N × N square matrix composed of uniformly distributed numbers within an open interval (0, 1); the F matrix is an N x N square matrix, k is larger than 0, the PAPR performance of the system is reduced along with the increase of the k value, and the k value range is related to the number of subcarriers and the number of antennas;

step 3, adopting space-frequency coding to X_iCarrying out orthogonal coding on the signals to obtain signals capable of being transmitted on the T antennas;

step 4 of adding X_iMultiplying the initial phase factor group generated by the F matrix, calculating the PAPR after the weighted signals pass through IFFT, and selecting the optimal phase factor which leads the symbol to have the minimum PAPR from the PAPR, and recording the optimal phase factor as F_m：

F_m＝[p_0,p_1,.......,p_2m,p_2m+1]^T(3)

Wherein, $m\∈\[0,\frac{N}{2}-1\];$

step 5 for the optimal phase factor F_mTake the conjugation as F_m ^*Then to F_mAnd F_m ^*Carrying out orthogonal coding to obtain T phase factors; step 6, the obtained T phase factors and signals transmitted on the corresponding T antennas are subjected to complex multiplication, respective PAPR values are calculated after IFFT, and max (PAPR) is selected₁,PAPR₂…PAPR_T) As a signal to be transmittedThe signal is x to E after adding guard interval_iThe guard interval is obtained by cyclic extension of the OFDM symbol;

step 7 output signalLater reading in the next signal X_i+1And judging whether i +1 is equal to the upper limit value P, wherein P is more than 0, if not, going to the step 2, and if so, ending the circulation.

The invention has the beneficial effect of providing a phase factor group which effectively reduces the peak-to-average power ratio of the MIMO-OFDM system and is called as an F matrix. The optimal phase factor with the optimal PAPR can be selected by scrambling the symbols before space-frequency coding by using the F matrix, orthogonal coding is carried out on the optimal phase factor, and the optimal phase factor acts on corresponding signals after space-frequency coding respectively, so that signals to be sent on each antenna can be prevented from carrying out IFFT operation for many times, and the calculation complexity of the system is reduced. Meanwhile, due to the use of the F matrix, the MIMO-OFDM system can obtain good peak-to-average ratio performance.

Drawings

Figure 1 is a basic block diagram of a transmitting end of a MIMO-OFDM system,

in the figure, mapping symbols need to be subjected to space-time/frequency coding, then are subjected to OFDM modulation, and finally are transmitted by a plurality of antennas;

FIG. 2 is a block diagram of a SLM method of the SFBCMIMO-OFDM system based on an F matrix,

in the figure, the figure shows an algorithm block diagram under two antennas, proposes that a mapping symbol must pass through space-frequency coding SFBC, and finally adopts the proposed SLM method to reduce the peak-to-average ratio of the MIMO-OFDM system;

figure 3 CCDF plots for different phase factor set numbers V (same attenuation factor K-5),

in the figure, when the attenuation factor K is 5, the algorithm of the invention is compared with the original algorithm and the independent SLM algorithm under the condition of different phase factor sets, wherein the abscissa represents different peak-to-average ratio papr (db) values, and the ordinate represents complementary cumulative function CCDF value;

figure 4 CCDF plots of different attenuation factors K (same phase factor group number V-16),

in the figure, the algorithm of the invention is compared with the original algorithm and the independent SLM algorithm under the condition of the same phase factor group number, and the algorithm of the invention also changes an attenuation factor K and is compared with the original algorithm and the independent SLM algorithm, wherein the abscissa represents different PAPR (dB) values, and the ordinate represents a complementary cumulative function CCDF value; figure 5 CCDF graph with attenuation factor K at 16,

in the figure, when the attenuation factor K is set to 16, the algorithm of the present invention is compared with the CCDF curves of the original algorithm and the independent SLM algorithm, wherein the abscissa represents different values of the peak-to-average ratio papr (db), and the ordinate represents the value of the complementary cumulative function CCDF.

Detailed Description

The following provides a specific implementation method of the present invention, taking the transmitting antenna T as 2 as an example:

step 1, inputting binary data bit stream, modulating by PSK/QAM to obtain mapping signal, and obtaining X after serial-parallel conversion_iFrequency domain signal, X_iRepresents the ith frequency domain symbol:

X_i＝[X₀,X₁,.......,X_N-1]^T(1)

where N represents the number of subcarriers, the upper limit value of i is set to P (P > 0), [. cndot.]^TRepresents a transpose of a matrix;

step 2, generating an initial phase factor set by using the F matrix in the formula (2):

$F=\frac{1}{k}I\left(N\right)+\frac{1}{N}rand\left(N\right)---\left(2\right)$

wherein i (N) represents an N × N identity matrix, k represents an attenuation factor, and rand (x) represents an N × N square matrix composed of random numbers within an open interval (0, 1);

step 3, SFBC is adopted to X_iThe signals are orthogonally coded to obtain signals, respectively X, that can be transmitted on two antennas a, b_i,a′，X_i,b', the expression is as follows:

an antenna a: ${X_{i,a}}^{\′}={\[X_i\left(0\right),-X_i^{*}\left(1\right),X_i\left(2\right),-X_i^{*}\left(3\right),\mathrm{.......},X_i\left(2m\right),-X_i^{*}(2m+1)\]}^T---\left(3\right)$

an antenna b:

wherein,()^*represents the conjugate of the signal in parentheses;

step 4 of adding X_iMultiplying the initial phase factor group generated by the F matrix, calculating the PAPR after the weighted signal is subjected to IFFT, and selecting the optimal phase factor which leads the symbol to have the minimum PAPR from the PAPR, and recording the optimal phase factor as F_m：

F_m＝[p_0,p_1,.......,p_2m,p_2m+1]^T(5)

Wherein, $m\∈\[0,\frac{N}{2}-1\];$

step 5 for the optimal phase factor F_mTake the conjugation as F_m ^*Then, two vectors are orthogonally encoded to obtain two phase factors f_m,a，f_m,bThe expression is as follows:

f_m,a＝[p₀,-p₁ ^*,.......,p_2m,-p_2m+1 ^*]^T(6)

f_m,b＝[p₁,p₀ ^*,.......,p_2m+1,p_2m ^*]^T(7)

wherein the phase factor f_m,a，f_m,bRespectively correspond to the signals X_i,a′，X_i,b′；

Step 6, multiplying the obtained phase factor and the corresponding signal in complex, calculating the respective PAPR value after IFFT and recording as PAPR_aAnd PAPR_bChoose to have max (PAPR)_a,PAPR_b) As a signal to be transmittedAfter adding a guard interval signal ofThe guard interval is obtained by cyclic extension of the OFDM symbol;

step 7 output signalLater reading in the next signal X_i+1And judging whether the i +1 is equal to the upper limit value, if not, turning to the step 2, and if so, ending the circulation.

The drawings are further described in conjunction with the detailed description above.

Fig. 2 is a system block diagram of an SLM method of an SFBCMIMO-OFDM system based on an F matrix, and the block diagram describes in detail the specific implementation steps of the system of the present invention when the number of antennas T is 2. In the figure, "choose the largest PAPR" corresponds to step 6, and steps 1 to 6 indicate that the processing of one OFDM symbol is completed, i.e., the entire contents of the system block diagram of fig. 2 are completed.

Fig. 3 is a graph of CCDF for different phase factor group numbers V (same attenuation factor K-5). The simulation process parameters are set as follows: in the MIMO system adopting the SFBC coding of the Alamouti scheme, the number of transmitting antennas is 2, the number of subcarriers N is 128, the system adopts an oversampling coefficient L is 4, and binary signals adopt PSK modulation. The figure shows the comparison of CCDF curves with the original algorithm, the independent SLM algorithm, at different phase factor group numbers, when the inventive algorithm uses the attenuation factor K-5. As seen from the CCDF curve in the figure, when V is 4, the PAPR of the algorithm of the present invention is 10 compared with that of the original algorithm^-3There is an improvement of about 6.5dB compared to the independent SLM algorithm at 10^-3There is an improvement in PAPR of about 4.5 dB. When V is 16, the PAPR of the algorithm of the invention is 10 compared with that of the original algorithm^-3There is an improvement of about 6.6dB,and at 10 compared to the stand-alone SLM algorithm^-3There is an improvement in PAPR of about 2.8 dB. And the increase of the number of the phase factor groups enables the independent SLM algorithm to have 1.6dB improvement, but hardly influences the PAPR of the algorithm, which shows that the F matrix has stronger stability when used for reducing the PAPR.

Fig. 4 is a graph of CCDF for different attenuation factors K (same number of sets of phase factors). The simulation process parameters are set as follows: in the MIMO system adopting the SFBC coding of the Alamouti scheme, the number of transmitting antennas is 2, the number of subcarriers N is 128, the system adopts an oversampling coefficient L is 4, and binary signals adopt PSK modulation. The figure simulates the effect of different attenuation coefficients on the algorithm of the present invention when the phase factor number V is 16, i.e. the corresponding CCDF curves when K takes different values. As can be seen from the CCDF curves in the figure, the PAPR improves as the attenuation coefficient decreases. When k is 5, it is 10^-2The PAPR is about 3.7 dB; when k is 2.5, it is 10^-2The PAPR is about 2 dB; when k is 1, 10^-2The PAPR is about 0.9 dB. Thus, the application of the F matrix improves the peak-to-average ratio of the MIMO-OFDM system.

Table 1 is a comparison table of algorithm calculated quantities. It can be seen from the table that as the number of antennas increases, the IFFT times on each antenna of the independent SLM algorithm will increase greatly, whereas the IFFT times of the algorithm of the present invention increase less under the condition of multiple antennas and multiple modulations of the MIMO-OFDM system, thereby effectively reducing the system computation complexity.

TABLE 1 comparison of algorithmic calculations

Name of algorithm	Algorithm calculated quantity (IFFT times)
		SLM algorithm	count1＝(M*T)*G
Patent algorithm	count2＝(M+T)*G

Where M denotes the number of phase factors (the dimension of the F matrix), T denotes the number of antennas of the MIMO-OFDM system, and G denotes the number of symbols included in one frame signal. Therefore, the algorithm of the invention can reduce the calculation complexity under the condition of multi-antenna modulation of the MIMO-OFDM system.

Fig. 5 is a graph of CCDF at attenuation factor K-16. The simulation process parameters are set as follows: in the MIMO system adopting the SFBC coding of the Alamouti scheme, the number of transmitting antennas is 2, the number of subcarriers N is 128, the system adopts an oversampling coefficient L is 4, and binary signals adopt PSK modulation. The original algorithm, the independent SLM algorithm and the algorithm of the invention in the figure represent the CCDF curve with the attenuation factor K being 16 under the condition of the parameters. After K >16, the peak-to-average ratio performance of the invention is worse than that of the traditional independent SLM algorithm. And the selection of the K value is related to the number of the antennas and the number of the subcarriers.

Claims (1)

1. A method for selective SLM mapping of space-frequency coding SFBCMIMO-OFDM system based on F matrix is characterized in that:

step 1, inputting binary data bit stream, modulating to obtain mapping signal, and obtaining X after serial-to-parallel conversion_iFrequency domain signal, X_iRepresents the ith frequency domain symbol:

X_i＝[X₀,X₁,…X_N-1]^T(1)

wherein N represents the number of subcarriers, and the upper limit value of i is set to P, P > 0 [. cndot. ]]^TRepresentation matrixTransposing;

step 2, generating an initial phase factor set by using the F matrix in the formula (2):

$F=\frac{1}{k}I\left(N\right)+\frac{1}{N}rand\left(N\right)---\left(2\right)$

wherein, i (N) represents an N × N unit matrix, k represents an attenuation factor, and rand (x) represents an N × N square matrix composed of uniformly distributed numbers within an open interval (0, 1); the F matrix is an N x N square matrix, k is larger than 0, the PAPR performance of the system is reduced along with the increase of the k value, and the k value range is related to the number of subcarriers and the number of antennas;

step 3, adopting space-frequency coding to X_iCarrying out orthogonal coding on the signals to obtain signals capable of being transmitted on the T antennas;

step 4 of adding X_iMultiplying the initial phase factor group generated by the F matrix, calculating the PAPR after the weighted signals pass through IFFT, and selecting the optimal phase factor which leads the symbol to have the minimum PAPR from the PAPR, and recording the optimal phase factor as F_m：

F_m＝[p₀,p₁,.......,p_2m,p_2m+1]^T(3)

Wherein, $m\∈\[0,\frac{N}{2}-1\];$

step 5 for the optimal phase factor F_mTake the conjugation as F_m ^*Then to F_mAnd F_m ^*Carrying out orthogonal coding to obtain T phase factors;

step 6, the obtained T phase factors and signals transmitted on the corresponding T antennas are subjected to complex multiplication, respective PAPR values are calculated after IFFT, and max (PAPR) is selected₁,PAPR₂…PAPR_T) As a signal to be transmittedAfter adding a guard interval signal ofThe guard interval is obtained by cyclic extension of the OFDM symbol;

step 7 output signalLater reading in the next signal X_i+1And judging whether i +1 is equal to the upper limit value P, wherein P is more than 0, if not, going to the step 2, and if so, ending the circulation.