A spread spectrum communication system for urban channels with multipath diversity reception

Abstract

The application discloses a multipath diversity receiving spread spectrum communication system suitable for urban channels and relates to the technical field of signal processing, which solves the problem of passive multipath coherent combination in the prior art. The system comprises the following steps: at the transmitting end, after input data streams pass through channel coding, interleaving and symbol mapping, serial-parallel conversion is performed, a data block is formed by adding block pilots after direct sequence spread spectrum using a spread spectrum code sequence, IFFT transformation is performed to the time domain, a cyclic prefix is introduced, and after serial conversion, the data block is sent into a multipath fading channel; at the receiving end, after serial-parallel conversion, the cyclic prefix is removed, FFT transformation is performed to the frequency domain, pilot channel estimation is performed, the pilots are removed, de-spreading and de-mapping are performed, channel estimation results and de-spreading data are mapped to the time domain by IFFT for RAKE reception, finally, soft information is added, demodulation, de-interleaving and decoding are performed; the multipath coherent combination is changed from passive to active, the multipath components can be better extracted, and the system has more superior performance under urban multipath channels.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of signal processing, and in particular to a multipath diversity receiving spread spectrum communication system suitable for urban channels. BACKGROUND

[0002] In recent years, with the rapid development of technology, unmanned terminals have gradually become an important part of people's urban life. Unmanned terminals have the advantages of easy deployment and improved safety, and are rapidly promoted in the fields of transportation logistics and urban security. The application of unmanned terminals is becoming more and more widespread. For example, in the modern logistics industry, unmanned terminal distribution can not only effectively improve the distribution efficiency and reduce the distribution cost, but also to a certain extent, alleviate traffic congestion and reduce carbon emissions. However, the biggest problem faced by the data communication of unmanned terminals is the multipath fading caused by the time-varying of urban channels. In urban environments, due to the shielding of various building facilities, the radio transmitted by the transmitter is reflected and scattered multiple times, and finally reaches the receiver along multiple paths with different delays and Doppler shifts, resulting in serious multipath fading effects. The unmanned terminal measurement and control link is divided into uplink and downlink. The uplink is used to transmit low-speed remote control information, and the downlink is used to transmit payload and telemetry information. With the development of unmanned technology and the improvement of user demand, the amount of payload data to be transmitted is increasing, and the downlink transmission rate is further improved. The higher the transmission rate, the more serious the influence of multipath effect on the measurement and control link, and the multipath fading will produce obvious time-varying effect with the rapid movement of the unmanned terminal, which will require the communication system used by the unmanned terminal to have certain anti-multipath fading ability. Therefore, it is urgent to design a communication system suitable for urban channels and resistant to the influence of multipath fading.

[0003] At present, the downlink high-speed data link anti-multipath scheme has single carrier frequency domain equalization technology (SC-FDE) and orthogonal frequency division multiplexing technology (OFDM). Compared with the traditional single carrier technology, OFDM technology is an effective frequency domain technology that divides the carrier into multiple orthogonal subcarriers, and at the same time divides the transmission data into multiple sub-data, and each subcarrier undertakes the modulation scheme of a sub-data. This technology takes advantage of the diversity characteristics of the multipath channel, and disperses the information symbols through serial-parallel conversion to multiple sub-channels for parallel transmission. This modulation scheme can increase the symbol period and reduce the interference of multipath effect. At the same time, the cyclic prefix (CP) is used as a protection interval to reduce the symbol interference. Moreover, due to the orthogonality of each subcarrier, the interference between subcarriers is greatly reduced, showing good anti-multipath fading characteristics. And OFDM has strong bandwidth scalability and flexible spectrum resource allocation.

[0004] The widely accepted techniques for processing multipath channels today mainly include coding, spread spectrum, equalization and diversity. Coding mainly uses redundant bits to spread the information carried by several bits to more bits to correct errors and detect errors to achieve anti-interference function; spread spectrum technology, especially direct sequence spread spectrum technology (DSSS), is simple and effective, uses high-rate spread spectrum code to expand the bandwidth of the information signal to be transmitted, has good anti-multipath interference ability, and can realize reliable communication under low signal-to-noise ratio, and is mainly used in scenarios that emphasize communication robustness; because the anti-multipath ability of the spread spectrum communication system is limited in the trade-off of communication efficiency, the channel equalization method is often used with the spread spectrum technology, and the channel equalization value is used for channel equalization after channel estimation to compensate the signal; the diversity technology can efficiently utilize the multipath energy to improve the input signal-to-noise ratio at the demodulator end, resist signal fading, improve signal reception gain, and improve communication quality. The RAKE receiver is actually a new type of communication system combining spread spectrum and diversity, which can effectively overcome the multipath effect in urban channels.

[0005] In wireless signal transmission, the transmitted signal experiences different propagation paths and is superimposed at the receiving end, which causes inter-symbol interference (ISI) problems and causes signal distortion. Equalization technology uses channel CSI information to calculate and process the received data signal, which can effectively resist channel multipath fading. In 1965, Lucky proposed an adaptive equalization technology in digital communication, which completed adaptive equalization through iterative calculation of known pulse signals transmitted and received, and reduced the influence of inter-symbol interference. In 1973, Walzman and Schwartz proposed a new type of equalizer based on minimum mean square error, which first used DFT to calculate the equalization coefficient, opening up the frequency domain equalization technology. After that, people proposed the carrier modulation technology of OFDM, and through the single-tap frequency domain equalization algorithm, the influence of ISI was effectively resisted, and different channel environments were further studied in the literature based on frequency domain equalization OFDM. In 2002, Falconer et al. proposed the ZF and MMSE linear frequency domain equalization algorithm of the single-carrier frequency domain equalization system. Zhu et al. proposed a decision feedback equalizer based on noise prediction (HDFE-NP) in 2004, which achieved an equalization performance close to the time-frequency domain hybrid equalizer. In 2005, Benvenuto et al. studied the block iteration-based decision feedback equalization (IBDFE). In 2018, Yonglei et al. combined the STBC technology and proposed an improved MMSE-RISIC equalization algorithm, which achieved better performance. At present, the channel equalization method under the OFDM system mostly operates in the frequency domain, which greatly reduces the complexity of the equalizer, but it cannot fully utilize the characteristics of the multipath channel and is passive in multipath coherent combination. SUMMARY

[0006] The application solves the passive problem in the prior art of multipath coherent combination, realizes the change from passive to active of multipath coherent combination, can better extract multipath components, makes the OFDM system have more superior performance under urban multipath channels, and improves the ability of the OFDM system to resist multipath interference.

[0007] The application provides a multipath diversity receiving spread spectrum communication system suitable for urban channels.

[0008] The transmitting end encodes input data to obtain coded data, and performs bit interleaving on the coded data to obtain interleaved data; the interleaved data is modulated to obtain modulated data;

[0009] The transmitting end performs direct sequence spread spectrum on the modulated data by using a spread spectrum code sequence, places the spread spectrum data after spread spectrum on subcarriers, and performs interleaving mapping on the spread spectrum data on the subcarriers to obtain a data symbol block S D ;

[0010] Block pilot is added to the data symbol block S D , and IFFT transformation is performed to the time domain to obtain a transmission signal x;

[0011] The transmitting end adds a cyclic prefix CP with a length of N CP to the transmission signal x, performs parallel-serial conversion on the transmission signal after adding the cyclic prefix to obtain an OFDM signal x cp , and transmits the OFDM signal x cp through the channel to obtain a receiving signal y cp , and transmits the receiving signal y cp to the receiving end;

[0012] The receiving end removes the cyclic prefix from the receiving signal y cp to obtain a time domain signal y, performs Fourier transformation on the signal y to the frequency domain to obtain a frequency domain signal Y, and determines an OFDM data symbol block Y D and a pilot part frequency domain signal Y P in the frequency domain signal Y;

[0013] The receiving end performs despreading on the OFDM data symbol block Y D to obtain a despread symbol block Z, performs diversity mapping on the despread symbol block Z to obtain a plurality of diversities, and performs IFFT transformation on the plurality of diversities to obtain time domain results p(i) corresponding to the plurality of diversities;

[0014] The receiving end performs calculation on the pilot part frequency domain signal Y P to obtain each diversity time domain channel impulse response estimation value

[0015] The receiving end performs calculation on the pilot part frequency domain signal Y to obtain each diversity time domain channel impulse response estimation value D (i), and performs RAKE receiving processing on the each diversity combining signal z D (i) to obtain RAKE receiving result z R ;

[0016] The receiving end performs analysis decoding on the RAKE receiving result to obtain demodulation signal.

[0017] In a possible implementation, the transmitting end performs encoding on input data to obtain encoded data, including: performing encoding on the input data by LDPC encoding to obtain encoded data.

[0018] In a possible implementation, the transmitting end performs spread spectrum on the modulated data, places spread spectrum data after spread spectrum on subcarriers, and performs interleaving mapping on the spread spectrum data on the subcarriers to obtain data symbol block S D , including:

[0019] determining that the spread spectrum sequence is C=[c(0), c(1),..., c(SF-1)]; wherein SF is the length of the spread spectrum sequence;

[0020] multiplying the spread spectrum sequence and the interleaving data to obtain spread spectrum data; mapping the spread spectrum data on the subcarriers, and performing interleaving mapping on the spread spectrum data on the subcarriers to obtain data symbol block S D .

[0021] In a possible implementation, the channel is represented as:

[0022] h=[h(0), h(1),..., h(L-1)] T ;

[0023] wherein L is the channel impulse response length, and N CP ≥ L, N CP is the cyclic prefix length.

[0024] In a possible implementation, the frequency domain signal Y is specifically represented as:

[0025]

[0026] wherein, represents an N-order discrete Fourier transform matrix; y represents a time domain signal; H T represents a frequency domain channel response; S T represents a frequency domain sending data; w represents a time domain noise vector; Y(0) represents a first symbol of a frequency domain signal sequence; Y(1) represents a second symbol of the frequency domain signal sequence; Y(N-1) represents an Nth symbol of the frequency domain signal sequence; T represents a transpose.

[0027] In a possible implementation, the OFDM data symbol block Y D is represented as:

[0028] Y D = H D S D + w;

[0029] wherein H D represents a data symbol block S D corresponds to a frequency domain channel impulse response matrix; S D represents a data symbol block; w represents a frequency domain noise vector.

[0030] In a possible implementation, the pilot part frequency domain signal Y P is represented as:

[0031] Y P = H P X P + w P ;

[0032] wherein H P represents a frequency domain channel fading coefficient corresponding to the pilot part; X P represents a known pilot sequence output by a transmitting end; w P represents a frequency domain noise corresponding to the pilot part.

[0033] In a possible implementation, the IFFT transform is performed on the multiple diversity respectively to obtain time domain results p(i) corresponding to each diversity, including:

[0034] Q effective subcarriers in each diversity are screened to obtain effective diversity P'(i);

[0035] Q-point IFFT transform is performed on the Q effective subcarriers respectively to time domain to obtain time domain results p(i) corresponding to each diversity.

[0036] In a possible implementation, the pilot part frequency domain signal Y P is calculated by the receiving end to obtain time domain channel impulse response estimation values including:

[0037] According to the pilot part frequency domain signal Y P Calculate the corresponding estimation value of the pilot part frequency domain channel And according to the subcarrier mapping characteristics, obtain each diversity channel estimation value

[0038] For each diversity frequency domain channel response estimation value Perform IFFT transformation to obtain each diversity time domain channel impulse response estimation value

[0039] One or more technical solutions provided in the present application have at least the following technical effects or advantages:

[0040] (1) The transmitting end of the present application performs subcarrier interleaving mapping according to the spreading factor and the number of effective subcarriers, the receiving end performs diversity mapping on the despread data according to the interleaving mapping characteristics, performs IFFT transformation on the diversity blocks to convert to the time domain, and then uses RAKE receiving technology to extract and combine the multipath components with the channel impulse response estimated by the block pilot, so as to change passive combining into active combining to better control the interference of multipath components, improve the ability of the communication system to resist multipath fading, and better adapt to complex wireless communication environment.

[0041] (2) The present application obtains different channel information (CSI) of data carriers according to the equalization method at the receiving end, the data with high CSI is more reliable than the data with low CSI after equalization, and then the soft information is weighted and processed using additional reliability information and provided to the decoder, so as to further improve the detection performance and realize the reliability improvement of the communication system. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 The present application provides a whole scheme block diagram of a multipath diversity receiving spread spectrum communication system suitable for urban channels;

[0043] Figure 2 The present application provides an interleaving mapping schematic diagram;

[0044] Figure 3 The present application provides a diversity mapping characteristic schematic diagram;

[0045] Figure 4 The present application provides a path combining schematic diagram;

[0046] Figure 5 The present application provides a diversity demapping characteristic schematic diagram;

[0047] Figure 6 The present application provides a decoding flowchart of CSI joint RAKE receiving;

[0048] Figure 7The BER curve provided by the embodiment of the present application. DETAILED DESCRIPTION

[0049] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0050] The present application provides a multipath diversity receiving spread spectrum communication system suitable for urban channels, as shown in the accompanying drawings, comprising a transmitting end, a channel and a receiving end. Figure 1 The present application provides a multipath diversity receiving spread spectrum communication system suitable for urban channels, as shown in the accompanying drawings, comprising a transmitting end, a channel and a receiving end.

[0051] S101, the transmitting end encodes input data to obtain encoded data, and performs bit interleaving on the encoded data to obtain interleaved data; and modulates the interleaved data to obtain modulated data.

[0052] Specifically, at the transmitting end, input data is encoded to obtain encoded data, including: the input data is encoded by LDPC to obtain encoded data. LDPC code has strong error correction capability, can effectively correct bit errors generated in the channel transmission process, and has anti-interference and anti-fading capability. The encoded data is block interleaved, the data is rearranged to change the distribution of the data in time to reduce the probability of continuous error transmission. The bit interleaved data is modulated, considering the multi-order QAM modulation, at time n, a QAM complex modulation symbol block is D=[d(0),d(0),...,d(N D -1)] T , wherein each data symbol is independent and identical, and N D is the symbol length.

[0053] S102, the transmitting end performs direct sequence spread spectrum on the modulated data using a spread spectrum code sequence, places the spread spectrum data after spread spectrum on subcarriers, and performs interleaving mapping on the spread spectrum data on the subcarriers to obtain a data symbol block S D .

[0054] Specifically, the transmitting end performs direct sequence spread spectrum on the modulated data using a spread spectrum code sequence, places the spread spectrum data after spread spectrum on subcarriers, and performs interleaving mapping on the spread spectrum data on the subcarriers to obtain a data symbol block S D , including:

[0055] (1) determining the spread spectrum sequence as C=[c(0),c(1),...,c(SF-1)]; wherein SF is the length of the spread spectrum sequence;

[0056] (2) multiplying the spread spectrum sequence with the interleaved data to obtain spread spectrum data; mapping the spread spectrum data to subcarriers, and interleaving and mapping the spread spectrum data on the subcarriers to obtain a data symbol block S D .

[0057] For example, the modulated data is spread spectrum, the system adopts direct sequence spread spectrum, and the spread spectrum process is completed by multiplying the generated QAM signal with the selected spread spectrum sequence. The spread spectrum scheme spreads the same symbol information in the frequency domain direction, transmits the same information symbol on multiple subcarriers, so that the system can obtain frequency diversity gain. Assuming that the spread spectrum sequence length is SF, the spread spectrum sequence is C = [c(0), c(1),..., c(SF-1)], and the spread spectrum operation on the signal is:

[0058] d(i) · C = [d(i) · c(0), d(i) · c(1),..., d(i) · c(SF-1)] T ;

[0059] Wherein, i ∈ [0, N D -1].

[0060] The spread spectrum data symbol block with a length of N can be obtained:

[0061] [d(0) · C, d(1) · C,..., d(N D -1) · C] T ;

[0062] Wherein, N = N D · SF, serial-parallel conversion to N-way data, and mapping to N subcarriers.

[0063] The present application utilizes the interleaving mapping characteristics, the transmitting end maps the frequency domain resources to the subcarriers with uniform intervals, and the spread spectrum data signals are placed at intervals on the subcarriers. Here, in combination with the spread spectrum operation, the continuous data in one spread spectrum period is expanded to the entire frequency domain, and is placed discontinuously with M as the interval, as shown in Figure 2 , taking N = 6, N D = 3, and SF = 2 as an example. The symbol block length of the symbol block [d(0), d(1), d(2)] D with N T = 3 is multiplied by the spread spectrum sequence [c(0), c(1)] with a length of SF = 2, and is mapped to N = 6 subcarriers, and the result is [d(0)c(0), d(0)c(1), d(1)c(0), d(1)c(1), d(2)c(0), d(2)c(1)] T, then the result of mapping according to interleaving mapping characteristic is [d(0)c(0), d(1)c(0), d(2)c(0), d(0)c(1), d(1)c(1), d(2)c(1)] T .

[0064] Through the mapping operation, we have:

[0065] c(i)·D = [c(i)·d(0), c(i)·d(1),..., c(i)·d(M-1)] T ;

[0066] wherein, i∈[0, SF-1]. Thus, the data symbol block after interleaving mapping is:

[0067] S D = [s D (0), s D (1),..., s D (N-1)] T = [c(0)·D, c(1)·D,..., c(SF-1)·D] T .

[0068] S103, add the block pilot into the data symbol block S D , and perform IFFT transformation to time domain to obtain the transmission signal x;

[0069] For example, the block pilot is used as the pilot pattern, the block pilot is distributed in time domain (OFDM symbol) interval and in frequency domain (subcarrier) continuously, and here the pilot occupies one OFDM symbol length and all available subcarriers N on the OFDM symbol.

[0070] Then, IFFT transformation is performed on the spread symbol block to the time domain to obtain the transmission signal:

[0071] x = F N S T = [x(0), x(1),..., x(N-1)] T ;

[0072] wherein, F N is an N-order discrete Fourier inverse transformation matrix.

[0073] S104, the transmitting end adds a cyclic prefix (CP) with a length of N CP to the transmission signal x, performs parallel-serial conversion on the transmission signal with added cyclic prefix to obtain the OFDM signal x cp , and transmits the OFDM signal x cp through a channel to obtain a receiving signal y cp , and performs parallel-serial conversion on the receiving signal y cpTransmitted to the receiving end. Here, the channel is represented as:

[0074] h=[h(0),h(1),...,h(L-1)] T ;

[0075] Where L is the length of the channel impulse response, and N CP ≥L, N CP is the cyclic prefix length.

[0076] For example, in order to avoid inter-symbol interference, the length is increased to N CP The cyclic prefix (CP) is then converted into a single channel of data by parallel-serial conversion to form an OFDM signal x cp Sent out by the transmitting antenna. The signal is transmitted through the channel, assuming h=[h(0),h(1),...,h(L-1)] T Represents the channel impulse response of length L, where N CP ≥L.

[0077] S201, the receiving end receives the signal y cp Remove the cyclic prefix to obtain the time domain signal y, perform Fourier transform on the signal y to the frequency domain to obtain the frequency domain signal Y, and determine the OFDM data symbol block Y in the frequency domain signal Y D and the pilot frequency domain signal Y P .

[0078] Here, the frequency domain signal Y is specifically expressed as:

[0079]

[0080] in, represents the N-order discrete Fourier transform matrix; y represents the time domain signal; H T represents the frequency domain channel response; S T represents the frequency domain transmitted data; w represents the time domain noise vector; Y(0) represents the first symbol of the frequency domain signal sequence; Y(1) represents the second symbol of the frequency domain signal sequence; Y(N-1) represents the Nth symbol of the frequency domain signal sequence; T represents transpose.

[0081] OFDM data symbol block Y D , expressed as:

[0082] Y D =H D S D +W;

[0083] Among them, H D Represents data symbol block S D The frequency domain representation of the corresponding channel impulse response matrix; S DY represents a data symbol block; W represents a frequency domain representation of a noise vector.

[0084] Y represents a pilot part frequency domain signal P , represented as:

[0085] Y P = H P X P + W P ;

[0086] wherein H P represents a frequency domain channel fading coefficient corresponding to the pilot part; X P represents a known pilot sequence output by the transmitting end; W P represents a frequency domain noise corresponding to the pilot part.

[0087] Exemplarily, at the receiving end, a received signal y cp is obtained, under the condition that N CP ≥ L, linear convolution is equivalent to cyclic convolution, thus after removing the CP, a received signal expression with a length of N is obtained as:

[0088] y = F N H T S T + w;

[0089] wherein,

[0090] Then, a discrete Fourier transform is performed on the received signal to the frequency domain, and the following is obtained:

[0091]

[0092] S202, the receiving end performs despreading on the OFDM data symbol block Y D to obtain a despread symbol block Z, and performs diversity mapping on the despread symbol block Z to obtain a plurality of diversities, and respectively performs IFFT transform on the plurality of diversities to obtain a time domain result p(i) corresponding to each diversity.

[0093] Specifically, respectively performing IFFT transform on the plurality of diversities to obtain a time domain result p(i) corresponding to each diversity includes:

[0094] (1) selecting Q effective subcarriers in each diversity to obtain an effective time domain result P'(i);

[0095] (2) respectively performing Q-point IFFT transform on the Q effective subcarriers to the time domain to obtain a time domain result p(i) corresponding to each diversity.

[0096] Exemplarily, the receiving end despreads the information data accordingly, and the spread spectrum symbol c(i) is copied into M identical copies, i∈[0,M-1], to obtain the despread sequence:

[0097] C′=[c′(0),c′(1),...,c′(N-1)]=[c(0),...,c(0),c(1),...,c(1),c(2),...,c(N-1)];

[0098] Despreading the signal yields:

[0099] Z=(C′) H ⊙Y D ;

[0100] Wherein, the symbol “⊙” is the Hadamard product.

[0101] Thus, the despread symbol block is Z = [Z(0), Z(0), ..., Z(N-1)] T .

[0102] The subcarrier mapping feature is used to perform diversity mapping on the symbol block Z. Assuming that the number of N subcarriers is divided into P sets, each set contains Q subcarriers, N = PQ, such as Figure 3 As shown, N = 12, P = 3, Q = 4 are used as an example. The number of N = 12 subcarriers is divided into P = 3 diversity groups, each of which contains Q = 4 subcarriers. Diversity 0 starts at subcarrier position 0 and places subcarrier data at positions 0, 3, 6, and 9 at intervals. Diversity 1 starts at subcarrier position 1 and places subcarrier data at positions 1, 4, 7, and 10 at intervals. Diversity 2 starts at subcarrier position 2 and places subcarrier data at positions 2, 5, 8, and 11 at intervals.

[0103] The subcarrier mapping characteristics can be expressed as:

[0104]

[0105] Where i∈[0,P-1], P(i) represents the result of the i-th diversity subcarrier mapping. This diversity mapping method ensures that in the i-th diversity, the subcarriers with data are arranged starting from the i-th position and arranged at intervals of P to occupy N subcarriers, while the remaining subcarriers are left empty. In other words, each diversity contains Q valid subcarriers.

[0106] Then, each diversity extracts its own Q effective subcarriers to obtain the effective diversity P′(i), and performs Q-point IFFT transformation to the time domain. Then, the time domain result of the i-diversity data p(i) = F Q P′(i).

[0107] S203, the receiving end receives the pilot frequency domain signal Y PThe calculation is performed to obtain the estimation value of each diversity time domain channel impulse response

[0108] Specifically, in step S203, the receiving end performs calculation on the pilot part frequency domain signal Y P The calculation is performed to obtain the estimation value of each diversity time domain channel impulse response comprises:

[0109] (1) According to the pilot part frequency domain signal Y P The estimation value of the pilot part frequency domain channel is calculated And according to the subcarrier mapping characteristics, the estimation value of each diversity channel is obtained

[0110] (2) The estimation value of each diversity frequency domain channel response is subjected to IFFT transformation to obtain the estimation value of each diversity time domain channel impulse response

[0111] Exemplarily, the pilot part frequency domain signal Y P The estimation value of the pilot part frequency domain channel response is calculated by using the channel estimation algorithm:

[0112]

[0113] According to the subcarrier mapping characteristics, the estimation value of the channel corresponding to the diversity part is obtained

[0114] Then, the estimation value of each diversity frequency domain channel response is subjected to IFFT transformation to obtain the estimation value of the time domain channel impulse response

[0115] The block pilot mode is adopted in the application, and the diversity mapping characteristics can make the data on the effective subcarriers in each diversity correspond to the pilot channel estimation result accurately after being converted to the time domain, and the multipath characteristics are better utilized to facilitate subsequent RAKE diversity reception. Considering the spread spectrum, here P=M and Q=SF.

[0116] S204, the receiving end performs RAKE reception processing on the time domain result p(i) corresponding to each diversity and the estimation value of each diversity time domain channel impulse response to obtain the combined signal z D (i) of each diversity, and the combined signal z D (i) of each diversity is subjected to inverse mapping to obtain the RAKE reception result z R ;

[0117] ​Specifically, since the multipath components all carry the same information, the traditional single-branch receiver is prone to error decision when the signal is in deep fading due to only considering the energy of a certain signal, resulting in the decline of system performance. Therefore, the RAKE receiving technology with multipath diversity is considered to improve the performance of the system. The scheme is converted to the time domain after IFFT transformation according to the diversity block after the data part is despread and diversity mapping in the frequency domain, and is converted to a serial. Let the time domain signal of the ith diversity be represented as:

[0118] p(i) = [p(i,0), p(i,1),...,p(i,Q-1)];

[0119] The set of channel coefficients corresponding to the time delay is represented as:

[0120]

[0121] Suppose that the RAKE receiver is composed of N' parallel correlators and combiners, and the received signal receives the fading information of N' paths. Here, the number of combined paths N' can be adjusted, and is generally not less than the maximum multipath time delay L of the channel. If Q < L, N' should be set to N' = Q. According to the estimated time domain impulse response, the time delays of the N' paths are [T(0), T(1),...,T(N'-1)], and the amplitude information corresponding to each path of the N' paths is as shown in Figure 4 .

[0122] Suppose that the receiver is ideally locked on the 0th path, then the expression of the signal received by the RAKE receiver is :

[0123]

[0124] In the above formula, the first term is the expected signal value, the second term represents the multipath interference and multiple access interference, and the last term represents the noise contained in the diversity. According to the ratio of the total energy of the correlator to the energy of the branch, the coefficient gain of the path j is:

[0125]

[0126] where j ∈ [0,Q-1].

[0127] From the above formula, it can be seen that when the energy of a certain branch signal is strong, the ratio g j will be large, and it will play a larger role in the system. If the energy of a certain branch signal is small, the ratio g j will be small, and it will play a smaller role.

[0128] Let each path corresponding to the correlator target item output is The first term of the RAKE receiver received signal expression The signal output after weighted combination is:

[0129]

[0130] The Rake receiving result z D (i) of the i-th diversity is obtained D (i,0), z D (i,1),..., z D (i,Q-1) The obtained Rake receiving result of each diversity is reflected back according to the diversity mapping mode, as shown in Figure 5 The legend corresponds to the diversity mapping, N=12, P=3, Q=4, P=3 diversities are reflected in turn to N=12 subcarriers, and the subcarrier data of diversity 0, diversity 1 and diversity 2 are stored in turn on the total subcarriers. The final total data part Rake receiving result is represented as:

[0131] z R =[z R (0), z R (1),..., z R (N D -1) T =[z D (0), z D (1),..., z D (P-1) T ;

[0132] S205, the receiving end processes the Rake receiving result soft information auxiliary, and obtains a demodulation signal.

[0133] Specifically, before soft decoding at the receiving end, soft information needs to be obtained first, and the soft information is generated by demapping soft decision. Compared with hard decision, soft decision can further improve the detection performance. In a multi-carrier OFDM system, the attenuation of each carrier is different, and each data carrier has different CSI. The CSI information of each carrier can be provided to the decoder by weighting the soft information. Therefore, the CSI of the Rake receiving data in this scheme is weighted, that is, the soft information is added, and then the original signal is recovered by demodulation, deinterleaving and other operations.

[0134] The CSI auxiliary Rake receiving flowchart in the present scheme is as shown in Figure 6The shown includes FFT demodulation, the output of which is applied to the input of channel estimation and CSI-assisted MRC equalization. The output of channel estimation is applied to another input of CSI-assisted RAKE reception, while considering the adjustment of the spread spectrum period to the CSI calculation, the utilization of CSI is independent of the deinterleaving, demapping, deinterleaving and decoder. Assuming x T = [x(0), x(1),..., x(N-1)] represents the actual transmitted signal matrix, based on the soft output signal detection process of the time domain RAKE, after the received signal is converted to the time domain for RAKE multipath combination processing, the estimated value of the transmitted signal vector can be obtained as:

[0135]

[0136] wherein, is the change corresponding to the AWGN,

[0137] Because w T (n) is a Gaussian random variable with mean 0 and variance , then is also a Gaussian random variable, that is, Here

[0138] Under the condition of known transmitted symbols, the conditional PDF of the received symbols is:

[0139]

[0140] Therefore, according to the Max-Log-MAP criterion, The LLR of the bth coded bit in is:

[0141]

[0142] Wherein S1(i) and S0(i) represent the QAM modulation symbol set of the bth bit being 0 and 1 respectively. According to the formula, the soft information is weighted, and then demodulation, deinterleaving and decoding operations are performed to finally restore the original signal.

[0143] In one specific simulation experiment provided by the present application, MATLAB is used as a simulation platform to simulate and analyze a multipath diversity reception spread spectrum communication system suitable for urban channels proposed by the present application. The simulation experiment parameters are shown in Table 1.

[0144] Table 1 System parameter table

[0145] Simulation parameters Value System bandwidth 16.384MHz Total number of subcarriers 1024 Effective number of subcarriers 768 Modulation mode QPSK Coding mode LDPC Number of channel paths 6 Maximum delay offset 53

[0146] The simulation experiment adopts NLOS channel and noise is full-bandwidth AWGN. When the transmission rate is 32 kbps, the coding rate is 1 / 16 and the spreading factor is 48; when the transmission rate is 512 kbps, the coding rate is 1 / 12 and the spreading factor is 4; when the transmission rate is 10 Mbps, the coding rate is 1 / 2 and the spreading factor is 2. The bit error rate performance curves of 32 kbps, 512 kbps and 4 Mbps are shown in Fig. 4, wherein the labels are named according to "transmission rate-coding rate-spreading factor". Figure 7

[0147] The above examples are only used to illustrate the technical solutions of the present application, and are not limited to the present application; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the present application.​

Claims

1. A multipath diversity reception spread spectrum communication system suitable for urban channels, characterized in that: include: Transmitter, channel and receiver; The transmitting end encodes the input data to obtain encoded data, and performs bit interleaving on the encoded data to obtain interleaved data; interleave the data and perform modulation to obtain modulated data; The transmitting end performs direct sequence spread spectrum on the modulated data using a spread spectrum code sequence, places the spread spectrum data after spread spectrum on the subcarrier, and interleaves and maps the spread spectrum data on the subcarrier to obtain a data symbol block S D ; Add block pilots to the data symbol block S D And perform IFFT transformation to the time domain to obtain the transmission signal x; The transmitting end increases the length N of the transmission signal x CP The cyclic prefix CP is added, and the transmission signal with the cyclic prefix is ​​converted into a parallel-serial signal to obtain the OFDM signal x cp , and the OFDM signal x cp After passing through the channel, the received signal y is obtained cp , and the received signal y cp transmitting to the receiving end; The receiving end receives the received signal y cp Remove the cyclic prefix to obtain the time domain signal y, perform Fourier transform on the signal y to the frequency domain to obtain the frequency domain signal Y, and determine the OFDM data symbol block Y in the frequency domain signal Y D and the pilot frequency domain signal Y P ; The receiving end receives the OFDM data symbol block Y D Despreading is performed to obtain a despread symbol block Z; diversity mapping is performed on the despread symbol block Z to obtain multiple diversity components, and IFFT transformation is performed on the multiple diversity components to obtain a time domain result p(i) corresponding to each diversity component; The receiving end receives the pilot frequency domain signal Y P Calculate and obtain the impulse response estimation value of each diversity time domain channel The receiving end compares the time domain result p(i) corresponding to each diversity with the time domain channel impulse response estimation value of each diversity Perform RAKE reception processing to obtain the combined signal z of each diversity D (i) and the combined signal z of each diversity D (i) Perform inverse mapping to obtain the RAKE reception result z R ; The receiving end analyzes and decodes the RAKE reception result to obtain a demodulated signal.

2. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The transmitting end encodes the input data to obtain encoded data, including: encoding the input data through LDPC encoding to obtain encoded data.

3. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The transmitting end spreads the modulated data, places the spread data evenly at intervals on the subcarriers, and interleaves and maps the spread data on the subcarriers to obtain a data symbol block S D ,include: Determine the spreading sequence as C = [c(0), c(1), ..., c(SF-1)]; where SF is the length of the spreading sequence; The spread spectrum sequence is multiplied by the interleaved data to obtain spread spectrum data; the spread spectrum data is mapped to the subcarrier, and the spread spectrum data on the subcarrier is interleaved and mapped to obtain a data symbol block S D .

4. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The channel is represented as: h=[h(0),h(1),...,h(L-1)] T ; Where L is the length of the channel impulse response, and N CP ≥L, N CP is the cyclic prefix length.

5. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The frequency domain signal Y is specifically expressed as: in, represents the N-order discrete Fourier transform matrix; y represents the time domain signal; H T represents the frequency domain channel response; S T represents the frequency domain transmitted data; w represents the time domain noise vector; Y(0) represents the first symbol of the frequency domain signal sequence; Y(1) represents the second symbol of the frequency domain signal sequence; Y(N-1) represents the Nth symbol of the frequency domain signal sequence; T represents transpose.

6. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The OFDM data symbol block Y D , expressed as: Y D =H D S D +W; Among them, H D Represents data symbol block S D The corresponding frequency domain channel impulse response matrix; S D represents a data symbol block; W represents a frequency domain noise vector.

7. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The pilot frequency domain signal Y P , expressed as: Y P =H P X P +W P ; Among them, H P represents the frequency domain channel fading coefficient corresponding to the pilot part; X P represents the known pilot sequence output by the transmitter; W P Represents the frequency domain noise corresponding to the pilot part.

8. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The performing IFFT transformation on the plurality of diversity components to obtain a time domain result p(i) corresponding to each diversity component includes: By screening the Q effective subcarriers in each diversity, the effective diversity P′(i) can be obtained; Perform Q-point IFFT transformation on the Q effective subcarriers to the time domain, and obtain the time domain result p(i) corresponding to each diversity.

9. The multipath diversity reception spread spectrum communication system suitable for urban channels according to claim 1, characterized in that: The receiving end receives the pilot frequency domain signal Y P Calculate and obtain the impulse response estimation value of each diversity time domain channel include: According to the pilot frequency domain signal Y P Calculate the estimated value of the frequency domain channel corresponding to the pilot part And according to the subcarrier mapping characteristics, the estimation value of each diversity channel is obtained The frequency domain channel response estimation value of each diversity Perform IFFT transformation to obtain the impulse response estimation value of each diversity time domain channel

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