A frequency synchronization method for distributed large-scale multi-antenna multi-carrier communication system

Abstract

The present invention proposes a frequency synchronization method for a distributed large-scale multi-antenna multi-carrier system, which belongs to the field of communication. First, a distributed MIMO communication system consisting of a wireless access point AP and a UE is built, and regionalized AP grouping is performed. The AP closest to the regional geometric center is selected as the main AP. For each AP group, the AP closest to the served UE is selected as the secondary AP. The phase rotation matrix corresponding to each subcarrier carrying a single OFDM symbol on the transceiver link formed by each AP and each UE is calculated. Then, the frequency offset estimation of the distributed MIMO communication system is realized through two stages: blind synchronization or a synchronization algorithm based on data assistance is adopted to realize the frequency synchronization between the main AP and all APs in each AP group and the frequency synchronization between the UE in each AP group and the secondary AP, so as to finally realize the frequency synchronization between all UEs and all APs in the communication system. The present invention greatly reduces the complexity of frequency synchronization.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and in particular relates to a frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system. Background Art

[0002] Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technology. Orthogonal Frequency Division Multiple Access (OFDMA) is a combination of OFDM and Multiple Access. Figure 1 As shown in the figure, in the OFDMA system, multiple access is achieved by dividing the orthogonal subcarriers representing the frequency band used by the channel into several subchannels, and then allocating these subchannels to different user equipment (UE, User Equipment). In other words, after the channel is subcarrierized by OFDMA, each UE can transmit data on the allocated part of the subcarriers. In addition, by dividing the entire frequency band into many orthogonal subcarriers through OFDMA technology, the frequency selective fading channel can be converted into several flat fading subchannels, thereby effectively resisting frequency selective fading in wireless mobile environments. Since each subcarrier partially overlaps in the frequency domain and occupies the spectrum orthogonally, OFDMA technology can provide higher spectrum utilization and higher information transmission rate.

[0003] Multi-antenna communication technology is a technology that uses an array antenna composed of multiple antenna elements to communicate on a wireless communication device. The wireless communication devices here include but are not limited to base stations and UEs in cellular networks, and access points (AP, Access Point) and UEs in wireless local area networks such as Wi-Fi. According to the number of antenna elements used at both ends of the transmission and reception, multi-antenna communication technology can be divided into multiple-input multiple-output (MIMO), multiple-input single-output (MISO), single-input multiple-output (SIMO) and other forms; according to whether the antenna elements that constitute the array antenna are on the same communication device, multi-antenna communication technology can be divided into centralized multi-antenna technology and distributed multi-antenna technology.

[0004] OFDMA technology can be used in conjunction with multi-antenna technology. In the uplink and downlink communication links of a centralized multi-antenna system using OFDMA, the estimation of the carrier frequency deviation (frequency offset for short) between the transmitter and the receiver is relatively simple: in this case, for any single UE, all subcarriers of the uplink or downlink signal have the same frequency offset, and the receiving end only needs to estimate one frequency offset parameter. At this time, the frequency synchronization method is equivalent to the frequency synchronization method of the traditional OFDM system. However, in a distributed multi-antenna uplink and downlink system, there are multiple different frequency offset values ​​between any single UE and multiple distributed radio frequency units of a base station or AP. At this time, for the uplink, the frequency synchronization of the receiving end has changed from a single parameter estimation problem to a multi-parameter estimation problem, and the multiple parameters affect each other; while for the downlink, the frequency synchronization of the receiving end consists of multiple single parameter estimation problems.

[0005] Cell-Free Massive MIMO (CF-mMIMO) technology is an emerging multi-antenna communication technology in recent years. It can effectively reduce the frequent cross-cell switching requirements of signals in traditional cellular networks, and more fully tap the spatial dimension transmission resources provided by large-scale distributed antenna arrays, thereby significantly improving the system's throughput, coverage capability, spectrum efficiency and energy efficiency. Cell-Free Massive MIMO is a special form of the combination of distributed massive MIMO and networked MIMO. Its large number of antennas are not deployed on a single device, but distributed in different locations over a wide area of ​​space, and are connected to the central processing unit (CPU) by wire, so that there are multiple radio frequency units providing services near any UE.

[0006] In the prior art, Chinese invention patent ZL201811246529.6 proposes a frequency synchronization method, AP device, server and distributed MIMO system, such as Figure 2 Specifically, in this distributed MIMO system, each AP M1,,M i ,...,M j , ...M m Any one of them (for example, M i ), and then transmits training frames containing preamble sequences to other APs in turn. Other APs (such as M j ), based on the received training frames, estimate the transmitting AP (e.g., M i ) and the receiving AP (e.g., M j ) between the carrier frequency deviation △F i→j ; and send the carrier frequency offset estimate to the server, which determines the frequency offset matrix H according to the carrier frequency offset estimate m*m. Further, according to the frequency offset matrix H m*m , the relative frequency offset ε between each pair of transmitting and receiving APs can be determined i , used to compensate the transmission frequency of each transmitting AP, and finally achieve frequency synchronization of each AP.

[0007] However, this existing solution still has the following problems.

[0008] First, according to the number of transmitting and receiving APs, the existing scheme needs to send training frames intensively: according to the number of estimated carrier frequency deviation parameters (i.e., i×j), the existing scheme needs to send i×j training frames, and each time a training frame is sent, the system needs to calculate the frequency deviation once, and its calculation complexity is relatively high. Summary of the invention

[0009] In view of the high complexity of frequency synchronization in a distributed large-scale multi-antenna multi-carrier communication system, the present invention proposes a frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system, which achieves high-precision frequency synchronization with lower complexity.

[0010] The frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system comprises the following specific steps:

[0011] Step 1: Build a distributed large-scale multi-antenna multi-carrier communication system consisting of wireless access points (APs) and user equipment (UEs), and perform regionalized AP grouping within the coverage area of ​​the system;

[0012] The equipment of wireless access point AP includes: transceiver antenna, signal and information processing device;

[0013] The signal and information processing device includes a wireless communication unit, a storage unit, a frequency deviation calculation unit and a data forwarding unit.

[0014] The wireless communication unit is responsible for the transmission and processing of wireless signals between APs or between AP / UE, including up and down frequency conversion, modulation, demodulation, intermediate radio frequency processing unit, etc.; the storage unit is responsible for the storage of communication data, including demodulation data, sampling data, etc.; the frequency deviation calculation unit is used to implement a distributed large-scale multi-antenna frequency synchronization processing algorithm, whose input is the demodulation and sampling data stored in the receiving end storage unit, and the output is the frequency deviation estimation value between AP and AP (or between AP and UE); the data forwarding unit is a unit that forwards the frequency deviation data between AP and AP (or between AP and UE) to other APs through a backhaul link.

[0015] AP grouping standards are:

[0016] With the UE as the center, all APs serving the same UE are divided into an AP group; when dividing different areas, the number of APs in each group is not limited; each AP has a processing device and a single or multiple transceiver antennas.

[0017] Step 2: Select the AP closest to the geometric center of the coverage area of ​​the communication system as the main AP;

[0018] Step 3: For each AP group in each area, select the AP closest to the served UE as the secondary AP;

[0019] Step 4: Calculate the Φ corresponding to each subcarrier carrying a single OFDM symbol on the transceiver link formed by each AP and each UE k ,The frequency offset estimation of distributed multi-antenna multi-carrier communication system is divided into two stages;

[0020] Matrix Φ k It is expressed as follows:

[0021]

[0022] where Θ k is the normalized frequency offset matrix between APs, represents the normalized frequency offset between the secondary AP and the kth UE,

[0023] In addition, Θ k is defined as:

[0024]

[0025] Where M represents the number of APs in a distributed large-scale multi-antenna multi-carrier communication system. APs can be configured with a single antenna or multiple antennas. N is the total number of subcarriers allocated to the main AP. represents the normalized frequency offset between the mth AP and the kth UE. The UE can be configured with a single antenna or multiple antennas.

[0026] Matrix Φ k Contains redundant information. For each UE, the matrix Φ k Contains two parts of information: the first part indicates the frequency offset between APs, and the second part indicates the frequency offset between the UE and the secondary AP. For different UEs, the frequency offset value of the first part is the same. Therefore, in a multi-user scenario, the frequency offset of the first part only needs to be estimated once.

[0027] The two stages are: 1) the main AP achieves frequency synchronization with all APs in the AP group; 2) the UE in the group achieves frequency synchronization with the corresponding secondary AP.

[0028] Step 5, first stage: frequency synchronization between the master AP and all APs in the AP group is achieved by using blind synchronization, synchronization based on preamble / training / pilot sequences, or other point-to-point link synchronization algorithms;

[0029] The method for achieving frequency synchronization can adopt the first method or the second method;

[0030] The first method for frequency synchronization within an AP group is as follows:

[0031] First, the master AP M1 transmits its dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequence is used) or a signal carrying common payload data (when a blind synchronization algorithm is used) F1 to all other APs in turn;

[0032] Then, the current AP M j After receiving F1, calculate the frequency deviation △F between the main AP M1 1→j , used to compensate its own receiving carrier frequency and achieve frequency synchronization with the main AP.

[0033] Finally, the remaining APs repeat the above process to synchronize their own carrier frequencies with the carrier frequency of the master AP.

[0034] The method for frequency synchronization within the second type of user AP group is as follows:

[0035] First, all other APs (such as M j ) transmits its own dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequence is used) or a signal carrying ordinary payload data (when a blind synchronization algorithm is used) to the main AP M1 in turn. j ; Then, the current AP M j F j The APM is sent to the main AP M1, which receives and calculates the APM of the transmission. j Frequency deviation △F j→1 and feeds it back to the transmitting AP M through the central processing unit CPU and the backhaul link j Compensation is performed to achieve the transmission of AP M j Its own carrier frequency is synchronized with the carrier frequency of the main AP M1.

[0036] Step 6, second stage: select a frequency synchronization algorithm to achieve carrier frequency synchronization between the UE in each AP group and the corresponding secondary AP, thereby achieving frequency synchronization between each UE and all APs in the communication system.

[0037] The specific steps are as follows:

[0038] Step 601: For the current AP group, the UE sends its own dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequence is used) or a signal carrying normal payload data (when a blind synchronization algorithm is used) to the secondary AP.

[0039] Step 602: After receiving the data sent by the UE, the secondary AP inputs the data into a frequency offset calculation unit to estimate the frequency offset between itself and the UE.

[0040] The frequency offset calculation in this embodiment adopts the Multiple Signal Classification (MUSIC) algorithm. First, the loss function is calculated:

[0041] Where L is the number of empty subcarriers in the multicarrier signal sent by the UE to the secondary AP; K is the total number of data blocks (such as OFDM symbols) carried by the multicarrier signal sent by the UE to the secondary AP; W is the N-dimensional discrete Fourier transform (IDFT, Inverse Discrete Fourier Transform) matrix W H is the N-dimensional discrete Fourier transform (DFT) matrix, is the (P+i)th row of the N-dimensional Discrete Fourier Transform (DFT) matrix, N is the total number of subcarriers of the secondary AP, and P is the number of subcarriers carrying data (i.e., the number of valid subcarriers remaining after removing the null subcarriers); the diagonal matrix Z = diag(1,z,z 2 ,...,z N-1 ), where z n-1 = Indicates the phase rotation caused by the frequency offset at the (n-1)th sampling point of a single data block (such as a single OFDM symbol) carried by the multi-carrier signal sent by the UE to the secondary AP; k ∈C 1×P represents the received signal at the secondary AP, which is obtained after the multi-carrier signal sent by the UE to the secondary AP passes through the wireless channel, and includes the time domain sample value vector corresponding to the k-th data block (such as the k-th OFDM symbol) (taking the case where both the transmitting and receiving ends select a single antenna to transmit and receive the frequency synchronization signal as an example), and its expression is:

[0042] y k (p) = W p HS k +ω k =W p diag{H(1),H(2),...,H(P)}S k +ωk (p)

[0043] Among them, S k =diag{s k (1),s k (2),....,s k (P)} is a diagonal matrix consisting of the kth data block (e.g., OFDM symbol) carried in the multi-carrier signal sent by the UE to the secondary AP, H(i) (i=1,2...,N) is the frequency domain channel response of the i-th subcarrier in the multi-carrier signal sent by the UE to the secondary AP, ω k ∈C 1×P In addition, in the multi-carrier signal sent by the UE to the secondary AP, there are N subcarriers in total: P subcarriers carry data (ie, non-null subcarriers), and the remaining NP subcarriers do not carry data.

[0044] Step 603: The secondary AP sends the frequency offset to other APs in the group through the backhaul link, thereby completing the frequency synchronization of each UE and all APs in the communication system.

[0045] The advantages of the present invention are:

[0046] 1) A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system. The present invention utilizes regionalized AP grouping to first complete synchronization between APs in the group; secondly, synchronization between UEs in the group and secondary APs is performed. This solution can greatly reduce the complexity of frequency synchronization in a distributed large-scale multi-antenna multi-carrier communication system while ensuring certain accuracy requirements.

[0047] 2) A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system. Different synchronization stages in the synchronization process are compatible with various point-to-point link frequency synchronization algorithms; this makes the frequency synchronization algorithm used in the present invention not limited to a certain type or category, and any algorithm that supports point-to-point link frequency synchronization can be embedded in the first and second stages of the frequency synchronization algorithm proposed in the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of OFDMA technology in the prior art;

[0049] Figure 2 It is a flow chart of a frequency synchronization solution in the prior art;

[0050] Figure 3 A flow chart of a frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system proposed by the present invention;

[0051] Figure 4It is a schematic diagram of the architecture of the signal and information processing device in the wireless access point AP of the present invention;

[0052] Figure 5 This is a schematic diagram of the first type of method for frequency synchronization within an AP group of the present invention;

[0053] Figure 6 This is a schematic diagram of the second type of method for frequency synchronization within an AP group according to the present invention;

[0054] Figure 7 This is an action timing diagram of the frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system according to the present invention;

[0055] Figure 8 This is a schematic diagram of the architecture of the wireless access point AP device of the present invention;

[0056] Fig. 9 It is a schematic diagram of the architecture of a frequency calculation unit in a wireless access point AP of the present invention;

[0057] Fig.10 A schematic diagram of a system model of an embodiment of the present invention;

[0058] Fig.11 A schematic diagram comparing the computational complexity of the embodiment of the present invention and the existing algorithm;

[0059] Fig.12 The figure is a schematic diagram comparing the cumulative distribution function of the mean square error of frequency estimation achievable by the embodiment of the present invention and the existing algorithm. DETAILED DESCRIPTION

[0060] The present invention is further described below with reference to the accompanying drawings and examples.

[0061] The present invention discloses a frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system. The frequency synchronization scheme architecture based on the uplink includes three aspects: AP grouping within a region, AP frequency synchronization within a group, and secondary AP-UE frequency synchronization.

[0062] like Figure 3 As shown, the specific steps of this scheme are as follows:

[0063] Step 1: Build a distributed multi-antenna multi-carrier communication system consisting of distributed APs and UEs, and perform regionalized AP grouping within the coverage of the multi-antenna system;

[0064] The AP equipment includes: a transceiver antenna and a processing device;

[0065] Processing device such as Figure 4 As shown, it includes a wireless communication unit, a storage unit, a frequency deviation calculation unit and a data forwarding unit.

[0066] The wireless communication unit is responsible for the transmission and processing of wireless signals between APs or between APs and UEs, including up and down frequency conversion, modulation, demodulation, and intermediate radio frequency processing units. The storage unit is responsible for the storage of communication data, including demodulation data and sampling data of received signals. The frequency deviation calculation unit is used to implement a distributed large-scale multi-antenna frequency synchronization processing algorithm, whose input is the demodulation and sampling data of the received signal stored in the receiving end storage unit, and its output is the frequency deviation between APs (or between APs and UEs). The data forwarding unit is a unit that forwards the frequency deviation between APs (or between APs and UEs) to other APs through a backhaul link.

[0067] The method of regionalizing AP groups is:

[0068] This application adopts the idea of ​​regional division to regionalize AP groups in the system range covered by the distributed large-scale multi-antenna system. The regional division of AP groups is flexible and relatively speaking, there is no strict division standard, and it only needs to meet the actual requirements. The division standards adopted in this solution are:

[0069] For a certain UE, all APs serving the UE should be in the same AP group;

[0070] When different areas are divided, there is no limit on the number of APs in each group.

[0071] Step 2: Select the AP closest to the geometric center of the coverage area of ​​the communication system as the main AP;

[0072] After APs are grouped, all APs in the group are divided into three categories: primary AP, secondary AP corresponding to each UE, and other APs. There is no strict distinction between primary and secondary AP functional entities, but they play different logical roles to reduce the calculation of redundant information during synchronization.

[0073] When selecting the primary and secondary APs, this application proposes a robust primary and secondary AP selection mechanism to ensure the frequency offset estimation performance, so as to ensure the low complexity and high precision of this synchronization scheme; in order to obtain better estimation performance in frequency offset estimation, the scheme needs to ensure a higher signal-to-noise ratio when the primary AP communicates with other APs. The signal-to-noise ratio depends on both large-scale fading and small-scale fading. Due to the randomness of small-scale fading, only large-scale fading is used here as the basis for judging the signal-to-noise ratio when the primary AP communicates with other APs. In addition, due to the size of large-scale fading, it depends on the communication distance. Therefore: On average, the AP located at the geometric center of the area has a smaller average distance from the remaining APs. This will result in a higher average signal-to-noise ratio between the AP located at the geometric center of the area and other APs. Therefore, this scheme selects the AP closest to the geometric center of the area as the primary AP.

[0074] Step 3: For each AP group in each area, select the AP closest to the served UE as the secondary AP;

[0075] Similar to the selection mechanism of the primary AP in step 2, in the second stage of the synchronization process, the AP closest to the served UE in the area (with the smallest large-scale fading) will become the secondary AP corresponding to the UE to ensure a high signal-to-noise ratio between the user and the AP.

[0076] Step 4: Calculate the Φ corresponding to each subcarrier carrying a single OFDM symbol on the transceiver link formed by each AP and each UE j , the frequency offset estimation in a distributed multi-antenna system is divided into two stages;

[0077] Matrix Φ k It is expressed as follows:

[0078]

[0079] where Θ k is the normalized frequency offset matrix between APs, represents the normalized frequency offset between the secondary AP and the kth UE,

[0080] In addition, Θ k is defined as:

[0081]

[0082] Where M represents the number of APs in a distributed large-scale multi-antenna multi-carrier communication system; APs can be configured with a single antenna or multiple antennas; N is the total number of subcarriers allocated to the master AP, represents the normalized frequency offset between the mth AP and the kth UE. The UE can be configured with a single antenna or multiple antennas.

[0083] Matrix Φ k Contains redundant information. For each UE, the matrix Φ k Contains two parts of information. The first part indicates the frequency offset between APs, and the second part indicates the frequency offset between the UE and the secondary AP. For different UEs, the frequency offset value of the first part is the same. Therefore, in a multi-user scenario, the frequency offset of the first part only needs to be estimated once.

[0084] The two stages are: 1) the main AP achieves frequency synchronization with all APs in the AP group; 2) the UE in the group achieves frequency synchronization with the corresponding secondary AP.

[0085] Step 5: For the first stage, blind synchronization, synchronization algorithm based on preamble / training / pilot sequence, or other point-to-point link synchronization is used to synchronize the frequency between the master AP and all APs in the AP group.

[0086] In fact, many synchronization algorithms can be selected and applied to this synchronization process, such as Schmid, ML and MUSIC-like algorithms. Among them, the synchronization algorithm based on the preamble / training / pilot sequence will occupy a large number of subcarriers. Therefore, this embodiment considers the unified deployment of blind synchronization algorithms. The synchronization process can be the following first or second category;

[0087] like Figure 5 As shown, the first type of intra-group synchronization method is specifically:

[0088] First, the master AP M1 transmits its dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequence is used) or a signal carrying common payload data (when a blind synchronization algorithm is used) F1 to all other APs in turn;

[0089] Then, the current AP M j After receiving F1, calculate the frequency deviation △F between the main AP M1 1→j , used to compensate its own receiving carrier frequency and achieve frequency synchronization with the main AP.

[0090] Finally, the remaining APs repeat the above process to synchronize their own carrier frequencies with the carrier frequency of the master AP.

[0091] like Figure 6 As shown, the second type of intra-group synchronization method is specifically:

[0092] All other APs (such as M j ) transmits its own dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequence is used) or a signal carrying ordinary payload data (when a blind synchronization algorithm is used) to the main AP M1 in turn. j , current AP M j F j The main AP M1 receives the data and calculates the value of the data with the transmitting AP M j Frequency deviation △F j→1 and feeds it back to the transmitting AP M through the central processing unit CPU and the backhaul link j Compensation is performed to achieve the transmission of AP M j Its own carrier frequency is synchronized with the carrier frequency of the main AP M1.

[0093] Step 6: For the second stage: select a frequency synchronization algorithm to achieve carrier frequency synchronization between the UE in each AP group and the corresponding secondary AP, thereby achieving frequency synchronization between each UE and all APs in the communication system;

[0094] For a certain UE, the AP closest to it (with the smallest corresponding large-scale fading) in the area is selected as the secondary AP. In this case, the signal-to-noise ratio between the UE and its secondary AP can be maintained at a fairly high level. This embodiment selects the MUSIC-like algorithm;

[0095] The specific steps are as follows:

[0096] Step 601: For the current AP group, the UE sends a dedicated preamble / training / pilot sequence for frequency offset estimation (when a synchronization algorithm based on preamble / training / pilot sequences is used) or a signal carrying common payload data (when a blind synchronization algorithm is used) to the secondary AP.

[0097] Step 602: After receiving the data sent by the UE, the secondary AP inputs the data into a frequency offset calculation unit to estimate the frequency offset between itself and the UE.

[0098] Assume that the number of subcarriers allocated to the UE is N, among which P subcarriers carry data, the remaining L are empty subcarriers, and P+L=N.

[0099] y k ∈C 1×P represents the received signal at the secondary AP, which is obtained after the multi-carrier signal sent by the UE to the secondary AP passes through the wireless channel, and includes the time domain sample value vector corresponding to the k-th data block (such as the k-th OFDM symbol) (taking the case where both the transmitting and receiving ends select a single antenna to transmit and receive the frequency synchronization signal as an example), and its expression is:

[0100] y k (p) = W p HS k +ω k =W p diag{H(1),H(2),...,H(P)}S k +ω k (p),

[0101] W is the N-dimensional discrete inverse Fourier transform, W H is the N-dimensional discrete Fourier transform matrix, W p is the pth row of the N-dimensional discrete inverse Fourier transform matrix, S k =diag{s k (1),s k (2),....,s k (P)} is the diagonal matrix consisting of the kth data block (such as OFDM symbol) carried in the multi-carrier signal sent by the UE to the secondary AP, H(i) (i=1,2...,N) is the frequency domain channel response of the i-th subcarrier in the multi-carrier signal sent by the UE to the secondary AP, ω k ∈C 1×Prepresents the Gaussian white noise vector, W p is the Pth column of the normalized N×N IDFT matrix.

[0102] Define Z = diag(1,z,z 2 ,...,z N-1 ),in Indicates the phase rotation of the sampling point due to frequency offset;

[0103] Then define the loss function:

[0104] K is the total number of OFDM symbols;

[0105] The frequency offset is estimated by using the MUSIC algorithm to obtain A(z).

[0106] However, please note that in actual scenarios, the number of UEs in this step is likely to be greater than 1, that is, there are multiple UEs synchronizing with the same AP at the same time. At this time, if the frequency offsets between each UE and the same AP are different, the synchronization accuracy will be seriously reduced. In order to solve this problem, the OFDMA multi-user frequency offset estimation algorithm can be embedded in the second-stage synchronization process to replace the existing MUSIC-like synchronization algorithm.

[0107] Step 603: The secondary AP sends the frequency offset to other APs in the group via a backhaul link, thereby achieving frequency synchronization between each UE and all APs in the communication system.

[0108] The synchronous preprocessing process of the present invention is as follows: Figure 7 As shown, including:

[0109] 1) Regionalized AP Grouping

[0110] Specifically, the AP group division criteria include: the regional AP groups are divided based on the UE they serve; each regional AP group should cover all APs that serve the same UE; and the number of APs in each group does not have to be strictly consistent when different regions are divided.

[0111] 2) Primary and secondary AP classification

[0112] A communication system includes a master AP and several slave APs. Two synchronization phases are designed in the frequency synchronization process. The main differences between the master and slave APs are:

[0113] The master AP only participates in the first phase of synchronization: completing synchronization between itself and other APs in the area;

[0114] The secondary APs corresponding to different UEs participate in the synchronization between the UE and the secondary AP in the second phase.

[0115] That is, the specific synchronization process is implemented in two stages:

[0116] The main AP is synchronized with all other APs first, and then the UE in the group is synchronized with the corresponding secondary AP. The synchronization between the main AP and the other APs is responsible for the synchronization of all APs in the regional AP group; then, the UE is synchronized with its corresponding secondary AP. During the synchronization process, the secondary AP receives and processes the wireless signal sent by the UE, and realizes the synchronization of the UE and the AP through the synchronization algorithm carried by the secondary AP.

[0117] like Figure 8 As shown, the transceiver antenna and processing device of the master AP include:

[0118] 1) Transceiver antenna

[0119] In the first intra-group synchronization mode, the master AP M1 sends a message to all APs M in the AP group. j Transmit broadcast signals, where j∈(1,m), m is the total number of APs;

[0120] In the second intra-group synchronization mode, the master AP M1 receives data from all APs M in the AP group. j The transmitted signal, where j∈(1,m).

[0121] 2) Processing device

[0122] In the first intra-group AP synchronization mode, the intra-group AP processing device determines the frequency offset △F between the local and the main AP 1→j , where the subscript 1→j represents the frequency offset between the master AP and the APs in the group.

[0123] In the second AP group synchronization mode, the master AP M1 receives the data from other APs M1 in the AP group in turn. j The processing device of the master AP determines the frequency deviation △F from all other APs in the AP group. j→1 , where j→1 represents the frequency offset between each AP in the AP group and the main AP, and j∈(1,m) represents all other APs.

[0124] All AP devices in the AP group except the master AP specifically include:

[0125] In the two phases of synchronization, all APs have no functional difference, but the difference lies in the logical role played by APs in the synchronization architecture. In the first phase of synchronization, all APs in the AP group except the main AP have no functional and conceptual difference; in the second phase of the synchronization process, all other APs in the AP group except the main AP can be specifically divided into secondary APs and other APs according to their functions.

[0126] a. In the first stage of synchronization: frequency synchronization within the AP group

[0127] The transceiver antennas and processing devices of all APs in the AP group:

[0128] 1) Transceiver antenna:

[0129] In the first intra-group synchronization mode, all APs in the AP group M j The receiving antenna receives the signal transmitted from the main AP M1 transmitting antenna in turn.

[0130] In the second intra-group synchronization mode, all APs in the AP group M j The transmitting antennas of transmit signals to the receiving antenna of the main AP M1 in turn to estimate the frequency offset between the main AP and each AP.

[0131] 2) Processing device

[0132] In the first synchronization mode, the processing device of all APs in the AP group determines the frequency offset ΔF between itself and the main AP according to the transmission signal received from the main AP M1. 1→j .

[0133] In the second intra-AP group synchronization mode, the processing devices of all APs in the AP group sequentially receive the frequency offset data forwarded from the master AP via the backhaul link, determine the frequency offset with the master AP and complete synchronization.

[0134] b. In the second stage of synchronization: synchronization between UE and secondary AP

[0135] In this stage, all APs except the master AP can be specifically divided into secondary APs and other APs according to their logical roles in the architecture.

[0136] The secondary AP and the remaining APs are only conceptually different. All APs in the AP group can act as secondary APs to complete synchronization with the corresponding UE, or they can act as remaining APs and not participate in the UE synchronization process. All APs in the regional AP group can become secondary APs subordinate to any UE.

[0137] Specific classification criteria: For different UEs in the area, the AP closest to a UE in the area will become the secondary AP corresponding to the UE, and all APs except the primary and secondary APs are other APs;

[0138] The secondary AP includes:

[0139] The transceiver antenna completes signal transmission interaction with the corresponding UE or main AP, including transmitting signals to the UE or main AP and receiving transmitted signals from the UE or main AP.

[0140] The processing device determines the UE to which it corresponds, estimates the frequency offset with respect to the corresponding UE according to the multi-carrier signal received from the UE, and sends the frequency offset data to the remaining APs via the backhaul link.

[0141] The present invention is based on the design of the frequency deviation calculation unit, such as Fig. 9 As shown, it specifically includes: a baseband signal buffer, a calculation module and a frequency deviation information buffer.

[0142] The first type of intra-group synchronization method: the main AP transmits a signal to all other APs through its antenna. All other APs receive the signal and estimate the frequency offset between them and the main AP locally.

[0143] Frequency deviation information buffer: used for the main AP to store the frequency deviation data obtained by the calculation module, or to store the frequency deviation △F between the secondary AP and the UE received through the backhaul link UE→j , where j∈1,m, m is the total number of APs in the area; or used to store data obtained by the storage operation module of the remaining APs except the main and secondary APs, or to store the frequency deviation △F between itself and the UE to be sent to the main AP UE→j , so that other APs in the AP group can synchronize with the UE; or the secondary AP can store the frequency offset between itself and the UE to be sent to other APs.

[0144] Baseband signal buffer: used by each AP to store received signals for frequency offset estimation; or used to store frequency offset information output by the operation module (or frequency offset information buffer).

[0145] Operation module: Determine the frequency deviation between the AP and the main AP according to the wireless signal received by each AP and transmitted from the main AP and stored in the baseband signal buffer, so as to achieve synchronization with the main AP; or be used for the secondary AP to determine the frequency deviation between the secondary AP and the corresponding UE according to the signal received from the UE.

[0146] The second type of intra-group synchronization method: All APs except the main AP transmit signals to the main AP in turn. The main AP receives the wireless signals of other APs and estimates the frequency deviation between itself and other APs. Then, the main AP sends frequency deviation data to the other APs in the AP group except the main AP via the backhaul link and CPU to complete the synchronization within the AP group.

[0147] Frequency deviation information buffer: used for each AP (except the main AP) to store the frequency deviation data obtained by the calculation module, or to store the frequency deviation △F between the main AP and itself received through the backhaul link j→1 ; or for the main AP to store the frequency deviation between the secondary AP and the corresponding UE received through the backhaul link, where j∈(1,m), m is the total number of APs in the area; or for the main AP to store the frequency deviation △F between the main AP and all other APs to be sent through the backhaul link j→1 , or used by the secondary AP to store the frequency offset between itself and the UE to be sent to other APs.

[0148] Baseband signal buffer: used by each AP to store received signals for frequency offset estimation; or used to store frequency offset information output by the operation module (or frequency offset information buffer).

[0149] Operation module: Determine the relative frequency deviation between the main AP and each of the APs according to the signals received by the main AP from the other APs to achieve synchronization with the main AP; or use the secondary AP to determine the frequency deviation between the secondary AP and the corresponding UE according to the signals received from the UE.

[0150] Example:

[0151] like Fig.10 As shown, the present invention considers the uplink synchronization process of a distributed large-scale multi-antenna multi-carrier communication system. The base station is equipped with a CPU, M distributed APs, each AP has a separate oscillator; there are K UEs in the system, and the user is equipped with a single antenna; it is assumed that multiple APs can communicate with each other.

[0152] represents the normalized CFO between the mth AP and the kth UE. Assuming that the CFO has completed the initial synchronization, the range of the normalized CFO is within (-0.5, 0.5);

[0153] The channel from the kth user to the mth AP is expressed as: Each element has a mean of 0 and a variance of The total channel gain between the kth user and the base station satisfies Furthermore, it is assumed that the channel and CFO remain constant during one OFDM symbol.

[0154] The total number of subcarriers at the base station is N, and the number of subcarriers allocated to each UE is N d Without loss of generality, assume that P of the subcarriers are data subcarriers and the remaining N d -P subcarriers are empty subcarriers, and N g is the cyclic prefix length. j ∈C P×N Represents the subcarrier allocation matrix, assuming that it is located at coordinates (γ k ,i) is 1, i∈1,2,...,P, and the other elements are 0. γ k is a set of data subcarrier indexes of the kth user arranged in ascending order;

[0155] assumed k th The frequency domain data of the g-th OFDM symbol of a UE, and there is In this system, the g-th OFDM symbol transmitted by the k-th user can be represented by Description, F represents the normalized DFT matrix, is the cumulative phase offset between the kth user and the mth AP caused by CFO. At the same time, CFO will cause phase offset for each different sample.

[0156] The phase rotation matrix is ​​defined as follows:

[0157]

[0158] For the g-th OFDM symbol, the data received by the m-th AP can be expressed as

[0159]

[0160] Among them, F L ∈C N×L represents the first L columns of the matrix F, ω m,g ∈C N×1 Corresponding to additive white Gaussian noise (AWGN), the covariance matrix of the Gaussian white noise is

[0161] Considering the data received by M APs:

[0162] Y g =[y {1,g} ,y {2,g} ,...,y {M,g} ] T (3)

[0163] In order to generalize the received signal model of a single AP to a distributed MIMO system, the channel response between the kth user and M APs is defined as:

[0164]

[0165] Based on this, we can get the M×N data matrix received by multiple antennas:

[0166]

[0167] where Φ k ∈C M×N is the phase rotation matrix in a multi-AP system, where the (m,n) element is

[0168] As shown in formula (5), the matrix Φ k There are M×N unknown parameters that need to be estimated, and the computational complexity is extremely high. However, the matrix Φ k Contains redundant information: For each UE, the matrix Φ kContains two parts of information: the first part indicates the frequency offset between the main AP and the remaining APs, and the second part indicates the frequency offset between the UE and the corresponding secondary AP.

[0169] In summary, consider the matrix Φ k It is expressed as follows:

[0170]

[0171] where Θ k is the normalized frequency offset matrix between the main AP and the rest of the APs, Indicates the secondary AP and k th The normalized frequency offset between UEs. In addition, Θ k is defined as:

[0172]

[0173] Note Θ k The (m,n)th element of can be further expressed as:

[0174]

[0175] In the formula They represent the normalized frequency differences between the mth AP and the kth UE and their nominal frequencies respectively; represents the Doppler frequency offset between the kth UE and the mth AP. In fact, when the UE speed is low, equation (8) is approximately equal to That is Θ k It has nothing to do with the kth UE.

[0176] Substituting equation (6) into equation (5), we can obtain

[0177]

[0178] Where (°) represents the Hadamard product. According to formula (6), if Θ and That is, the received data Y can be obtained g ,The specific acquisition process is completed by two stages in this proposal.

[0179] The following is a brief introduction to the MUSIC-like algorithm; assuming that the number of subcarriers allocated to the primary AP is N, of which P subcarriers contain data and the rest are empty subcarriers. The input of the receiver at the kth OFDM symbol can be expressed as:

[0180] y k (p) = W p HS k +ω k =W p diag{H(1),H(2),...,H(P)}Sk +ω k (p) (10)

[0181] y k ∈C 1×P Represents the received signal at the secondary AP, which is obtained by the multi-carrier signal sent by the UE to the secondary AP through the wireless channel, and contains the time domain sample value vector corresponding to the kth data block (such as the kth OFDM symbol). k =diag{s k (1),s k (2),....,s k (P)} is the diagonal matrix consisting of the k-th data block (such as OFDM symbol) carried in the multi-carrier signal sent by the UE to the secondary AP, H(i) (i=1,2...,N) is the frequency domain channel response of the i-th subcarrier in the multi-carrier signal sent by the UE to the secondary AP, ω k ∈C 1×P represents a Gaussian white noise vector.

[0182] When φ=0, ICI disappears. Define Z=diag(1,z,z 2 ,...,z N-1 ),in The loss function can then be defined based on this:

[0183]

[0184] By using the MUSIC algorithm, the frequency offset can be estimated by obtaining P(z) along the unit circle.

[0185] When the loss function is minimum, the frequency offset between the secondary AP and the UE is output;

[0186] like Fig.11 As shown in the figure, the complexity comparison between the distributed large-scale multi-antenna multi-carrier communication system frequency synchronization algorithm considered by the present invention and the existing algorithms. Among them, the existing baseline algorithms are: PBEE and MUSIC-like synchronization algorithms; the vertical axis represents the number of basic operations required by the processor to complete a synchronization algorithm. Fig.11 It can be seen that when the number of distributed APs and the number of UEs served are small, the complexity of the algorithm of the present invention is equivalent to that of the baseline algorithm; as the number of UEs and APs increases, the computational complexity of the baseline algorithm increases rapidly, while the algorithm of the present invention remains stable at a relatively low level.

[0187] like Fig.12As shown in the figure, the cumulative distribution function of the mean square error (MSE) estimated by the frequency synchronization algorithm of the distributed large-scale multi-antenna multi-carrier communication system considered in the present invention is compared with that of the existing algorithm. Among them, the existing baseline algorithms are: PBEE and MUSIC-like synchronization algorithms; the vertical axis represents the cumulative distribution of the mean square error of the synchronization algorithm. Fig.12 The simulation conditions are set as M=16 / M=64, N=32, P=20, K=1, and the cumulative distribution function data comes from 1000 Monte Carlo experiments. Fig.12 It can be seen that the MSE of the algorithm of the present invention is much smaller than that of the baseline algorithm: it is about two orders of magnitude lower than that of the PBEE algorithm; it is about one order of magnitude lower than that of the MUSIC-like algorithm. In addition, the simulation results show that the synchronization accuracy of this scheme increases with the increase in the number of distributed APs.

Claims

1. A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system, characterized in that: The specific steps include: First, a distributed MIMO communication system consisting of wireless access points (APs) and users is built, and APs are grouped into regional groups. Then, the AP closest to the geometric center of the communication scenario area is selected as the main AP; for each regional AP group, the AP closest to the served user is selected as the secondary AP corresponding to the UE; Next, the phase rotation matrix Φ corresponding to each subcarrier carrying a single OFDM symbol on the transceiver link formed by each AP and each UE is calculated k , the frequency offset estimation of distributed MIMO is divided into two stages: for the first stage, blind synchronization or data-assisted synchronization algorithm is used to achieve frequency synchronization between the main AP and all APs in each AP group; for the second stage, a frequency synchronization algorithm is selected to achieve frequency synchronization between the UE in each AP group and the corresponding secondary AP; finally, the frequency synchronization of each UE and all APs in the communication scenario is achieved.

2. A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system as claimed in claim 1, characterized in that: The equipment of the wireless access point AP includes: a transceiver antenna and a processing device; The processing device includes a wireless communication unit, a storage unit, a data forwarding unit, and a frequency deviation calculation unit; The communication unit is responsible for wireless communication between APs or between AP and UE, including up / down conversion, modulation, demodulation, intermediate / radio frequency processing, etc. The storage unit is responsible for the storage of communication data, including sending and receiving data and sampling data; The data forwarding unit is a unit that forwards the frequency deviation between an AP and other APs or users to other APs through a backhaul link; The frequency deviation calculation unit is used to implement a distributed large-scale multi-antenna frequency synchronization processing algorithm, whose input is the demodulated and sampled data stored in the receiving end storage unit, and the output is the frequency deviation between APs and APs, or between APs and UEs; The frequency deviation calculation unit includes: a baseband signal buffer, a calculation module and a frequency deviation information buffer; The baseband signal buffer is used by each AP to store received signals for frequency deviation estimation; or to store frequency deviation information output by the operation module or the frequency deviation information buffer; The operation module is responsible for determining the frequency deviation between the AP and the main AP or UE according to the wireless signal received by each AP and transmitted by the main AP and stored in the baseband signal buffer; The frequency deviation information buffer is used for each AP to store the frequency deviation data obtained by the operation module, or to store the frequency deviation △F between the AP and other APs or UEs received through the backhaul link. j→1 .

3. A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system as claimed in claim 1, characterized in that: The AP grouping standard is: With users as the center, all APs serving the same user form an AP group. When different areas are divided, the number of APs in each group is not limited. Each AP has a processing device that can support single or multiple transceiver antennas.

4. A frequency synchronization method for a distributed large-scale multi-antenna system according to claim 1, characterized in that: The phase rotation matrix Φ k It is expressed as follows: where Θ k is the normalized CFO between APs, represents the normalized CFO between the secondary AP and the kth UE, In addition, θ k is defined as: M represents the number of distributed APs; N is the total number of subcarriers allocated to the main AP. represents the normalized CFO between the mth AP and the kth UE; Matrix Φ k Contains redundant information: For each UE, the matrix Φ k Contains two parts of information: the first part indicates the frequency offset between the main AP and the remaining APs, and the second part indicates the frequency offset between the UE and the corresponding secondary AP; The corresponding two stages are: 1) the master AP achieves frequency synchronization with all APs in each AP group; 2) Synchronization of UEs in the group with the corresponding secondary APs.

5. A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system as claimed in claim 1, characterized in that: For the first stage, the method for achieving frequency synchronization between the main AP and all APs in each AP group can adopt either the first category or the second category; The first type of intra-group synchronization methods are as follows: First, when the synchronization algorithm based on preamble / training / pilot sequence is adopted, the master AP M1 transmits its own dedicated preamble / training / pilot sequence for frequency offset estimation to all other APs in turn, or when the blind synchronization algorithm is adopted, the master AP M1 carries the signal F1 of the normal payload data; Then, the current AP M j After receiving F1, calculate the frequency deviation △F between the main AP M1 1→j , used to compensate its own receiving carrier frequency and achieve frequency synchronization with the main AP; Finally, the remaining APs repeat the above process to synchronize their carrier frequencies with the carrier frequency of the master AP. The second type of intra-group synchronization methods are as follows: When the synchronization algorithm based on the preamble / training / pilot sequence is adopted, all other APs transmit their own dedicated preamble / training / pilot sequences for frequency offset estimation to the master AP M1 in turn; Or when the blind synchronization algorithm is used, all other APs carry signals F with normal payload data. j ; Then, the current AP M j F j The data is sent to the main AP M1, which receives and calculates the data. j Frequency deviation △F j→1 and feeds it back to the transmitting AP M through the central processing unit CPU and the backhaul link j Compensation is performed to achieve the transmission of AP M j Its own carrier frequency is synchronized with the carrier frequency of the main AP M1.

6. A frequency synchronization method for a distributed large-scale multi-antenna multi-carrier communication system as claimed in claim 1, characterized in that: The specific steps for achieving carrier frequency synchronization between the UE in each AP group and the corresponding secondary AP in the second stage are as follows: Step 601: When a synchronization algorithm based on a preamble / training / pilot sequence is used, for the current AP group, the UE sends its own dedicated preamble / training / pilot sequence for frequency offset estimation to the secondary AP; or when a blind synchronization algorithm is used, the UE carries a signal of normal payload data; Step 602: After receiving the data sent by the UE, the secondary AP inputs the data into a frequency deviation calculation unit, and estimates the frequency deviation between itself and the UE using an existing data-assisted synchronization or blind synchronization frequency estimation algorithm; Step 603: The secondary AP sends the frequency offset to other APs in the group through the CPU and the backhaul link, so as to achieve frequency synchronization between each UE and all APs in the communication system.

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