Access point of a massive, distributed, cellless MIMO network; network and method for limiting signal distortion caused by hardware imperfections in access point(s).

Abstract

Access point of a massive distributed and cellless MIMO network, network and method for limiting signal distortion caused by hardware imperfections of access point(s). The invention relates to such an access point (10) comprising: - a module for determining a signal distortion caused by its own hardware imperfections, - a transmission module (14), via a fronthaul link comprising a plurality of access points in series ordered according to a predetermined direction of travel to the central unit of a base station, from said distortion to another access point, or to the central unit, following it within said fronthaul link; and / or - a receiving module (16), and for taking into account said distortion of an access point preceding it within said fronthaul link, to determine the local precoding filters of said access point configured to cancel said distortion of said access point preceding it.Figure for the abbreviation: Figure 1.

Description

Title of the invention: Access point of a massive, distributed, cellless MIMO network, network and method for limiting signal distortion caused by hardware imperfections in access point(s)

[0001] The present invention relates to an access point of a massive distributed and cellless MIMO network.

[0002] The invention also relates to such a massive distributed and cellless MIMO network.

[0003] The invention also relates to a method for limiting signal distortion caused by hardware imperfections in a plurality of access points of a fronthaul link of a massive distributed, cellless MIMO network.

[0004] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement such a method of limiting signal distortion caused by hardware imperfections in a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network.

[0005] The present invention relates generally to the field of wireless communication systems, and more particularly to the transmission of data via the use of a radio frequency signal using multi-carrier modulation, in particular OFDM (Orthogonal Frequency Division Multiplexing) according to a downlink communication channel corresponding to the communication between access points and users.

[0006] Telecommunication systems using multi-carrier modulation for the downlink are well known in the prior art. The principle of such modulation consists of dividing the transmission band into a plurality of frequency sub-channels associated with carriers and modulating each of these carriers with the data to be transmitted on said downlink, also referred to hereafter as the downlink.

[0007] More specifically, the present invention falls within the framework of a cell-free, distributed massive MIMO (Multiple-Input Multiple-Output) network architecture, or CF-mMIMO (cell-free massive MIMO), as notably introduced in the article by S. Buzzi et al. entitled "Cell-free massive mimo: User-centric approach" published in IEEE Wireless Communications Letters, vol. 6, no. 6, pp. 706-709, 2017. Such a cell-free, distributed massive MIMO network architecture has been proposed to address the exponential growth of mobile data traffic and the problems of inter-cell interference and excessive variation in quality of service, in order to meet the challenges of the sixth generation of mobile communications, 6G.

[0008] Such a massively distributed, cellless MIMO network architecture is composed of a large number of geographically distributed access points, or APs (Access Points), which jointly serve user terminals along said downlink, or UEs (User Equipment), which guarantees a macro-diversity gain and offers good spectral efficiency, or SE (Spectral Efficiency), uniformly good in the coverage area.

[0009] Such telecommunication systems based on such a distributed, cellless, massive MIMO network architecture can only be commercially viable if the access points (APs) are deployed using inexpensive, low-power hardware. Consequently, severe hardware imperfections (HWIs) can occur during the transmission of radio frequency signals using multi-carrier modulation, particularly OFDM with a high peak-to-average power ratio (PAPR), resulting in poor transmission quality.

[0010] The aim of the present invention is therefore to propose a solution to effectively limit the effect of such material imperfections in order to allow cost- and energy-efficient deployments of access points within such massive distributed and cellless MIMO networks and to provide communications with high energy efficiency.

[0011] To this end, the invention relates to an access point of a massive distributed and cellless MIMO network comprising at least:

[0012] - a determination module configured to determine information representative of a signal distortion caused by the access point's own hardware imperfections,

[0013] - a transmission module configured to transmit, via a fronthaul link comprising a plurality of access points in series ordered according to a predetermined direction of travel from said fronthaul link to the central processing unit of a base station, said information representing said signal distortion at another access point, or at the central processing unit, directly following it within said fronthaul link; and / or

[0014] - a receiving module configured to receive, via said fronthaul link, said information representative of a signal distortion caused by hardware imperfections of an access point directly preceding it within said fronthaul link, and to take it into account in determining the local precoding filters of said access point configured to cancel said signal distortion caused by hardware imperfections of said access point preceding it.

[0015] The present invention thus aims to take advantage of each fronthaul link connecting in series a plurality of access points up to the central unit of the base station, in other words, the fronthaul link corresponding, by definition according to the present invention, to the interconnection link of the access points with the central unit, and this in order to correct step by step via said fronthaul link the signal distortion caused by the hardware imperfections of each access point during the implementation of communication on the downlink communication channel of the massive distributed and cellless MIMO network, each access point being configured to cancel the distortion caused by the hardware imperfections of the preceding access point within said fronthaul link, and so on from the most distant access point until reaching the central unit of the base station,according to a predetermined direction of travel of said fronthaul connection.

[0016] According to other advantageous aspects of the invention, the access point of a distributed, cellless, massive MIMO network comprises one or more of the following features, taken individually or in any technically possible combination:

[0017] - to determine said information representative of the signal distortion, said The access point determination module is configured to: - obtain a local estimation of the propagation channel between each of its transmitters and each user terminal within its range within said massive, distributed, and cellless MIMO network, - determine the power allocated to each data signal to be transmitted between said access point and each user terminal within its range within said massive distributed and cellless MIMO network, - pre-code each data signal to be transmitted between each of its transmitters and each user terminal within its range within said massive distributed and cellless MIMO network, according to said local estimation and associated allocated power; - transmit each pre-coded input signal from a hardware model of the transmission chain of its associated transmitter; - determine said information representative of the signal distortion caused by the own material imperfections of said transmitter by comparison of the output of said material model and said pre-coded input signal;

[0018] - said hardware model of the transmission chain of each transmitter is modeled using at least one predetermined measure and / or a predetermined optimization;

[0019] - said access point comprises a plurality of transmitters active simultaneously, said information representing a signal distortion caused by the material imperfections corresponding to the concatenation of the signal distortion caused by the inherent material imperfections of each of the emitters of said plurality;

[0020] - each transmitter is an OFDM transmitter;

[0021] - each local precoding filter is of type MRT, FZF or RZF.

[0022] The invention also relates to a central unit of a base station of a a massive, distributed, cellless MIMO network, said central unit being associated with at least one fronthaul link comprising a plurality of said access points in series and ordered according to a predetermined direction of travel from said fronthaul link to said central unit,

[0023] said base station central unit comprising at least one receiving module configured to receive, via said fronthaul link, information representative of a signal distortion caused by hardware imperfections of an access point directly preceding it within said fronthaul link, and to take said information into account to determine the local precoding filters of said central unit configured to cancel said signal distortion caused by the hardware imperfections of said access point preceding it

[0024] The invention also relates to a distributed, cellless, massive MIMO network comprising at least one fronthaul link, said at least one fronthaul link comprising a plurality of access points in series according to the invention as described above and ordered according to a predetermined direction of travel of said fronthaul link.

[0025] The invention also relates to a method for limiting signal distortion caused by material imperfections in a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network, said access points of said plurality being connected in series along said fronthaul link to the central processing unit of a base station and ordered according to a predetermined direction of travel of said fronthaul link, said method being implemented by each access point of said fronthaul link or by said central processing unit, and implemented during communication on the downlink of said massive distributed cellless MIMO network, said method comprising at least the following steps:

[0026] - determination of information representative of a signal distortion caused due to the inherent physical imperfections of said access point, and / or

[0027] - transmission, via said fronthaul link, of said representative information said signal distortion caused to another access point, or to the central processing unit of a base station, directly following it within a fronthaul link comprising a plurality of access points in series ordered according to a predetermined direction of travel of said fronthaul link; and / or

[0028] - reception, via said fronthaul link, and processing of said information representative of a signal distortion caused by hardware imperfections of an access point directly preceding it within said fronthaul link to determine the local precoding filters of said access point or said central unit configured to cancel said signal distortion caused by hardware imperfections of said access point preceding it.

[0029] According to an advantageous optional aspect of the method according to the invention, said determination of information representative of a signal distortion caused by the access point's own material imperfections comprises:

[0030] - obtaining a local estimate of the propagation channel between each of its transmitters and each user terminal within its range within said massive distributed and cellless MIMO network,

[0031] - the determination of the power allocated to each data signal specific to being transmitted between said access point and each user terminal within its range within said massive distributed and cellless MIMO network,

[0032] - the precoding of each data signal specific to be transmitted between each of its transmitters and each user terminal within its range within said massive distributed and cellless MIMO network, according to said local estimation and associated allocated power;

[0033] - the transmission of each precoded input signal to a hardware model of the transmission chain of its associated transmitter;

[0034] - the determination of said information representative of the signal distortion caused by the own material imperfections of said transmitter by comparison of the output of said material model and said precoded input signal.

[0035] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement such a method of limiting signal distortion caused by hardware imperfections of a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network as defined above.

[0036] These features and advantages of the invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

[0037] - [Fig.1] [Fig.1] is a schematic representation of an access point according to the present invention;

[0038] - [Fig.2] [Fig.2] illustrates an example of the material architecture of a part of the point access represented schematically on [Fig.1];

[0039] - [Fig.3] [Fig.3] is a schematic representation of two embodiments fronthaul connection of a massive distributed and cellless MIMO network and the role played by each of the access points of the plurality of serial access points that compose it;

[0040] - [Fig.4] [Fig.4] is a flowchart of the method for limiting distortion of signal caused by hardware imperfections in a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network.

[0041] It should be noted that the following mathematical notation conventions are used thereafter, namely:

[0042] - matrices are designated by bold capital letters, for example X,

[0043] - vectors are designated by lowercase letters in bold, for example x,

[0044] - a matrix transpose, a matrix conjugate transpose, a matrix The pseudo-inverse matrix and the trace of a matrix are respectively denoted by X^' and tï^X) respectively,

[0045] - for a matrix of dimension M x N, the notation X = {xnm} is used, with xn to denote the nth column, and x^ to denote the mth row,

[0046] - the identity matrix NXN and the all-zero matrix M x N are respectively designated by IN and 0MxN,

[0047] -the notation X=diag{x1, elements {Xj}, ,1 denotes a diagonal matrix with

[0048] - E [.] represents the expectation operator, ! is the factorial operator and j represents

[0049] Fig. 1 first schematically illustrates an access point 10 of a massive distributed cellless MIMO network according to the present invention.

[0050] According to the present invention, such an access point 10 of a massive distributed cellless MIMO network first of all comprises a determination module 12 configured to determine information representative of a signal distortion caused by the own material imperfections of said access point 10.

[0051] Furthermore, the access point 10 according to the present invention comprises a transmission module 14 configured to transmit, via a fronthaul link comprising a plurality of access points in series ordered in a predetermined direction of travel of said fronthaul link to the central unit, said information representative of said signal distortion to another access point, or to the central unit of a base station, following it directly within said fronthaul link.

[0052] Furthermore, the access point 10, according to the present invention, also comprises a receiving module 16 configured to receive, via said fronthaul link, said information representative of a signal distortion caused by imperfections hardware of an access point directly preceding it within said fronthaul link, and to take it into account in determining the local precoding filters of said access point configured to cancel said signal distortion caused by hardware imperfections of said access point preceding it.

[0053] In addition, the access point 10, includes, in a conventional manner, a set 18 of electronic modules dedicated to the conventional processing implemented conventionally by an access point of a massive distributed and cellless MIMO network, such as, for example, a channel estimation module configured to estimate the channel, a first precoding module configured to calculate the local precoding filters (i.e. conventionally without regard to signal distortion caused by the hardware imperfections of said preceding access point), a second precoding module configured to precode each of the data vectors to be transmitted to a user by applying said local precoding filters, electronic elements of the radio interface configured to transmit, by radio, the precoded data, via said precoding filters, to the users, etc., these elements being subsequently described in relation to [Fig.2] illustrates an example of the hardware architecture of part of the access point shown schematically in [Fig. 1].

[0054] An example of a determination modulus 12 as represented by [Fig.1] is described in more detail below.

[0055] Indeed, according to the embodiment illustrated by [Fig.1], the determination module 12 includes, for example, the elements 20, 22, 24, 26 and 28.

[0056] Element 20 is specifically configured to obtain a local estimation of the propagation channel between each of the transmitters (not shown in [Fig.1]) of the access point 10 and each user terminal within its range in said massive distributed and cellless MIMO network.

[0057] Such a local channel estimation is notably implemented by a channel estimation module of the assembly 18 and the element 20 retrieves each channel estimation between each of the transmitters (not shown in [Fig.1]) of the access point 10 and each user terminal within its range within said massive distributed cellless MIMO network.

[0058] Element 22 is configured to determine the power allocated to each data signal to be transmitted between said access point and each user terminal within its range in said massive distributed cellless MIMO network.

[0059] Element 24 is configured to precode each data signal to be transmitted between each of the transmitters of access point 10 and each user terminal within its range in said cellless distributed massive MIMO network, according to said local estimation and associated allocated power.

[0060] Element 26 is configured to transmit each precoded signal into input of a hardware model of the transmission chain of its associated transmitter (i.e. associated with said precoded signal).

[0061] Element 28 is configured to determine said information representative of the signal distortion caused by the own material imperfections of said transmitter by comparison of the output of said material model and said precoded input signal.

[0062] According to a variant illustrated by way of example in [Fig. 1], the access point 10 further comprises a processing unit 30 formed, for example, of a memory 32 and a processor 34 associated with the memory 32, and the access point 10 is at least partly implemented in the form of software, or a software component, executable by the processor, in particular the determination module 12, the transmission module 14, the reception module 16, and one or more electronic modules from the set 18 of electronic modules dedicated to the conventional processing typically implemented by an access point in a massive distributed, cellless MIMO network. The memory 32 of the access point 10 is then capable of storing such software or software components, and the processor 34 is then capable of executing them.

[0063] In an alternative not shown, the determination module 12, the transmission module 14, the reception module 16, and one or more electronic modules of the set 18 of electronic modules dedicated to the conventional processing carried out conventionally by an access point of a massive distributed and cellless MIMO network are each made in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or in the form of a dedicated integrated circuit, such as an ASIC (Application Specified Integrated Circuit).

[0064] When at least a portion of the access point 10 according to the present invention is implemented in the form of one or more software programs, i.e., in the form of a computer program, this portion is also capable of being stored on a computer-readable medium (not shown). The computer-readable medium is, for example, a medium capable of storing electronic instructions and being connected to a bus of a computer system. By way of example, the readable medium is an optical disc, a magneto-optical disc, a ROM, a RAM, any type of non-volatile memory (e.g., EPROM, EEPROM, FLASH, NVRAM), a magnetic card, or an optical card. A computer program containing software instructions is then stored on the readable medium.

[0065] [Fig. 2] illustrates a non-limiting example of the hardware architecture of a part of the access point shown schematically in [Fig. 1], particularly in the case where the implementation of massively distributed MIMO networks is considered useful and cellless, using in particular multi-carrier modulation, notably OFDM, these networks being known by the abbreviated name CF-mMIMO-OFDM.

[0066] According to this example, we consider in particular N input signals modulated if at &n, N being an integer corresponding to the number of OFDM subcarriers, at the input of a set 40 of electronic modules for digital signal processing.

[0067] The information data to be transmitted to K users on the nth subcarrier, denoted by means of the signal Sn such that gn → X^xl, comprises independent elements of unit power, such that || = 1-

[0068] It should be noted that OFDM systems conventionally define a guard band of unused subcarriers, located at each end of the used spectral band. Thus, the set of available subcarriers is divided into two complementary sets, one used for data transmission and its complementary set used for the guard band in which no data is transmitted.

[0069] In particular, we subsequently denote 'the channel response in the domain frequency between the lth access point and user k on the nth subcarrier with n=0, ..., Nl. The channels are modeled using Rayleigh independent fading, i.e. each channel ~ CIV (fl 5 I} oa eSt coe®c^ent l,k,n \ ' * IJi MJ large-scale fading between the thieth access point AP and the kieth user UE (from the English User Equipment), independent of the antennas or subcarriers implemented.

[0070] Furthermore, subsequently we consider blocks fading channels, which are constant during a time-frequency interval, known as the coherence interval, and which vary independently between coherence intervals.

[0071] In addition, it is also subsequently assumed that the large-scale fading coefficients vary slowly, within a range of several coherence intervals, which makes it obvious to consider that the channel gains are known a priori at each access point and are used to estimate the channel current responses.

[0072] The set 40 of electronic modules is organized into M digital signal processing chains associated with each of said M transmitters of the access point 10 considered, each transmitter being optionally an OFDM transmitter.

[0073] Such an assembly 40 includes, in particular, on the example of digital architecture of [Fig.2], a precoding module 42 suitable for implementing distributed linear local precoding.

[0074]

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083] More specifically, the precoding implemented by the precoding module 42 is necessary at each access point to eliminate multi-user interference (MUI) at the receivers. Within CF-mMIMO-OFDM networks, the local nature of the precoders (i.e., precoding module 42) is crucial for maintaining system scalability (a scalable network being one whose computational complexity remains finite as the network expands geographically and the number of users increases); therefore, the term "local precoding" (i.e., precoding within the access point) is subsequently used. More specifically, we subsequently consider a distributed implementation by the precoding module 42 of each access point, without information sharing between access points, and, at the level of this precoding module 42 as such, without worrying about signal distortion caused by hardware imperfections of the access point preceding the current access point under consideration. Each local precoding filter is of type MRT (Maximum Ratio Transmission), FZF (Fullpilot Zero Forcing) or RZF, the regularized version of FZF. The precoded vector in the frequency domain to be transmitted by the access point of index 1 on the subcarrier of index n is expressed in the following form: Xlp = WjpPjSn (1) where: - WlneXMxK is the precoding matrix associated with the subcarrier of index n for access point index 1, - PjeXKxK^, which is frequency independent, represents a matrix diagonal whose elements , such that k=l,..., K are the powers of Standardized transmissions allocated to K users. - Sn€XKxlest is the data vector to be transmitted to the K users on the sub- carrier n. More precisely, the precoding vector wj n ^FX^*! used by the lieme access point to the kieme user on subcarrier n is expressed in the form: (2) Or : for local FZF filtering for local RZF filtering

[0084] - (June normalization matrix satisfying {£■{ 1^11^ = 1 v*}'

[0085] - is the channel estimation matrix between the index access point 1 (i.e., the liith access point) and the K users on the subcarrier n, which can also be expressed in the form: x ] 'ct mpæ^cntéc by the downward vertical arrow entering the precoding module 42, considering in particular frequency-selective channels, channel estimation is indeed implemented locally at each access point to preserve network scalability; this is why the local minimum mean-squared error (MMSE) of channel estimation of the channel is suitable for calculation as described by G. Interdonato et al. in the article "Local Partial Zero-Forcing Precoding for Cell-Free Massive MIMO" IEEE Transactions on Wireless Communications, vol. 19, no. 7, pp. 4758-4774, 2020 with pilot transmission on the uplink.

[0086] - is a diagonal regularization matrix,

[0087] - is the kth column of the identity matrix of the identity matrix Ik which amounts to subsequently to take the kth column of H v1 C1

[0088] It should be noted that classically precoding schemes are known to the person skilled in the art, such as those described in particular by G. Interdonato et al. in the article "Local Partial Zero-Forcing Precoding for Cell-Free Massive MIMO" IEEE Transactions on Wireless Communications, vol. 19, no. 7, pp. 4758-4774, 2020, which is why these schemes are not further detailed thereafter as they are known and not the subject of the invention as such.

[0089] As illustrated by [Fig.2], at the output of the precoding module 42, M radio frequency transmission chains are implemented, each chain being associated with a separate transmitter, in particular an OFDM transmitter.

[0090] In particular, each chain classically includes first of all at the output of the precoding module 42 a module 44 for applying an Inverse Fast Fourier Transform of size Mppp, such that N the number of OFDM subcarriers is less than or equal to the size Mppp of the Inverse Fast Fourier Transform, so that in the case of strict inferiority the inputs of the Inverse Fast Fourier Transform are padded with zeros as illustrated by [Fig.2].

[0091] Then, at the output of module 44, which applies an inverse fast Fourier transform, each chain conventionally includes a multiplexer 46

[0092] Specifically according to the present invention, each chain is further supplemented with elements 48, 50 and 52 enabling the determination of the signal distortion caused by material imperfections in each transmission chain.

[0093] Indeed, each radio frequency transmission chain, which also classically includes a digital-to-analog converter (DAC) 54, a transmitter 56, in particular an OFDM transmitter marked RF Tx on [Fig.2], and a power amplifier 58, is classically non-linear due to material imperfections, such as those caused by low-resolution quantization or by non-linear power amplifiers.

[0094] For example, the signal transmitted by the antenna with index m from the access point with index 1 is expressed in the following form:

[0095] zU=f(aU)(3)

[0096] where f(.) represents the nonlinear operation assumed to be memoryless and identical for all antennas of all access points of the fronthaul link considered, for reasons of simplicity applied to the modulated signals | V transmitted to each antenna of the access point through the M radio frequency transmission chains.

[0097] It should be noted that, according to Bussgang's theorem as introduced by R. Price in the article entitled "A useful theorem for nonlinear devices having Gaussian inputs" IRE Transactions on Information Theory, vol. 4, no. 2, pp. 69-72, 1958, the OFDM signal in the time domain is expressed at the output of the nonlinear function by the following unique decomposition:

[0098] = K al +

[0099] where Kq is a frequency-independent complex gain and is a distortion (i.e. noise) of zero mean with a variance (j^ uncorrelated with each modulated signal 3.^n non-Gaussian at transmission but becoming Gaussian on the receiving side after OFDM demodulation.

[0100] Kq and O'j constitute the parameters of material imperfections and are suitable for being determined analytically, so that the present invention proposes to determine the associated signal distortion:

[0101] - by transmitting the modulated precoded signal of each chain, obtained at the output of the multiplexer 46, at the input of a hardware model 48 of the transmission chain under consideration, called HWI_M, and

[0102] - by subtracting, via element 50, the output of the hardware model 48 from the precoded signal modulated at the input of multiplexer 46, then

[0103] - by demodulating, via module 52, the application of a Fourier transform Fast FFT (from the English Fast Fourier Transform) to obtain the distortion expressed in the frequency domain.

[0104] As an optional complement, said material model 48 of the transmission chain of each transmitter is modeled using at least one predetermined measurement and / or a predetermined optimization, in particular mathematical, which makes it possible to develop models for characterizing material imperfections.

[0105] As an optional complement, as illustrated by [Fig.2], when the access point includes a plurality of simultaneously active transmitters, said information representing a signal distortion caused by the own material imperfections corresponds to the concatenation of the signal distortion caused by the own material imperfections of each of the transmitters of said plurality.

[0106] In other words, for a given sample, the distortion generated by an emitter is represented by a coefficient • Therefore, the distortion generated by M emitters is a vector of dimension M comprising the M distortion coefficients generated by the M emitters.

[0107] Fig. 3 is a schematic representation of two fronthaul link embodiments of a distributed, cellless, massive MIMO network and of the role played by each of the access points, according to the present invention, of the plurality of serial access points that make up such a fronthaul link.

[0108] More specifically, the time-division duplex (TDD) technique allows the transmissions of the downlink (DL) communication channel to be separated from those of the uplink (UL) communication channel, assuming perfect channel reciprocity, which can be ensured by precise calibration methods known to those skilled in the art. Furthermore, the transmission of a frame using the TDD technique within a CF-mMIMO-OFDM network is implemented within the coherence interval, and the physical resource block (RB) width is less than the coherence bandwidth.

[0109] Subsequently, to comply with the 5G NR standard, consider in particular a radio frame whose time-frequency resource is divided into Nrb resource blocks RB. Each resource block comprises = ...N... consecutive subcarriers. We denote (t, lî)} the resource unit RU (from the English resource unit) which represents the smallest time-frequency resource of the nth subcarrier of the thieth OFDM symbol corresponding to the thieth antenna of the thieth access point.

[0110] For example, such a TDD frame classically comprises Nc OFDM symbols, which corresponds to the shortest coherence interval of all users, and the transmission of: tc = NSCNC resource units RU per resource block RB, where TP resource units among the Tc resource units are used as pilots that are distributed within the transmission payload on the uplink UL. Such pilots are classically used, in particular, to estimate MxK channels in the frequency domain, per resource block, within each access point. Also, = NSCNC - TP resource units are reserved, per resource block, for the useful data in the samples, which are split between the transmissions of the downlink DL and those of the uplink UL into two complementary parts END and res respectively, with 0 < Σ < 1.

[0111] More specifically, in [Fig.3], two embodiments A and B of fronthaul links are shown, a fronthaul link corresponding, by definition according to the present invention, to the interconnection link of the access points with the central unit 62 of a base station 64, and this to correct step by step, via said fronthaul link, the signal distortion caused by the material imperfections of each access point during the implementation of communication on the downlink communication channel DL of the massive distributed and cellless MIMO network.

[0112] On [Fig.3], according to embodiment A, the fronthaul link has one end consisting of an access point 10i and another end consisting of the central unit 62 of the base station 64, the access points 10i, 102, 103, ..., 10L constituting it being in series ordered according to a predetermined direction 66 of the path of said fronthaul link to the central unit 62 of the base station.

[0113] According to embodiment B, the fronthaul link forms a frame around K users, for example K=6 for the users illustrated by terminals pl1, p2, p3, p4, p5, p6 arbitrarily distributed in the coverage area, and also includes L access points, such L » K, in series ordered according to a predetermined direction 68 of said fronthaul link starting for example with access point 101, then 102, 103>..., up to access point 10L then the central unit 62 of the base station (note that according to another case the direction 68 could be reversed).

[0114] It should be noted that, generally speaking, a CF-mMIMO network is typically divided into several fronthaul links (i.e., segments) corresponding to access points connected in series via a fronthaul link, and the processing is applied to each segment, for example, one segment according to embodiment A and / or one segment according to the embodiment B in a manner independent of one segment to another, or according to any other known network topology.

[0115] The signal received at the kth user via the nth subcarrier is expressed in particular in the following form: [onô] y = 5^-^2^^ + = h^ kn K o x ljl +^ b k p is a Gaussian noise whose variables (5)

[0117] where 2> eXMx^ denotes the amplified signal, expressed in the frequency domain, transmitted by the access point of index 1 on the subcarrier n, eXMxl denotes the signal distortion (i.e., the distortion noise) caused by hardware imperfections, Ko is the diagonal MxM matrix whose elements are equal to Kq as introduced previously, and && random variables are independent and identically distributed (ieiid).

[0118] Furthermore, per resource block and per user, the spectral efficiency (SE) is suitable for calculation as described by W. Jiang in the article "Cell-Free Massive MIMO-OFDM Transmission Over Frequency-Selective Fading Channels" IEEE Communications Letters, vol. 25, no. 8, pp. 2718-2722, 2021 in the form

[0119] 8¾ = f (1) Nsc A flog 2(1+ SINR^) (6)

[0120] with SINRk^ the signal-to-interference ratio plus noise (from the English signal-to interference-plus-noise ratio) of the kieme user using the n subcarrier expressed in the following form:

[0121] SINR = ______________

[0122]

[0123] with :

[0124] and ç j is a normalization factor.

[0125] As previously indicated with reference to Figures 1 and 2, the access points, adapted (i.e., modified) according to the present invention and connected in series via a fronthaul link, offer significant limitations (i.e., reductions) in the event of severe material imperfections, or even advantageously, cancellation (or compensation) of the material imperfections caused by each access point of the fronthaul link considered, these imperfections being largely caused by the power amplifiers 58 and digital-to-analog converters 54 of each transmission chain associated with each transmitter 56 of each access point 10 as illustrated previously in relation to [Fig.2].

[0126] Fig. 3 illustrates more precisely the information transmitted on each fronthaul segment, allowing the limitation, or even the elimination, step by step along the fronthaul link of signal distortions caused by the material imperfections of each access point.

[0127] More specifically, according to the present invention, it is first considered that each access point conventionally calculates local channel estimates from the pilot sequences transmitted by each user, and uses them specifically according to the present invention to precode the data useful to the users served, so that the signals transmitted by all the access points add up constructively at the level of the antennas of the user terminals and this advantageously limiting, or even canceling, signal distortions caused by the material imperfections of each access point.

[0128] In other words, as illustrated by [Fig.3], the present invention exploits the serial architecture of the serial fronthaul link (i.e. segment) to implement a limitation (in case of severe hardware imperfections), or even a suppression of signal distortions caused by hardware imperfections of each access point.

[0129] On the examples of fronthaul segment A and B of [Fig.3], the first access point 10i of the fronthaul link in a predetermined direction of travel 66 or 68 to the central unit 62 first determines, classically, local precoding filters based on (i.e. using) the local channel estimates between this first access point 10i and each user k, illustrated each for example by the terminals using the drivers pl, p2, p3, p4, p5, p6 of [Fig.3].

[0130] As previously stated, these local precoding filters are of the MRT, FZF, or RZF type, the precoded vector in the frequency domain to be transmitted by radio by the access point index 1 on the subcarrier index n being, as previously stated, expressed in the following form: I,n ~ W\,nPlSn-

[0131] Then, as previously indicated in relation to Figures 1 and 2, the first access point 10i is also suitable, specifically according to the present invention, for approximating (i.e. determining) the signal distortion caused by its own material imperfections { d^ VH}, this distortion d^ being de facto also transmitted by the first access point 10i to each user k by radio.

[0132] On the user side, the signal distortions caused by the first access point's own hardware imperfections 10i, actually received after the pre-coded signals in the frequency domain V 71} have been created, modulated, passed through each radio frequency chain of the first access point and then transmitted via the user channels, can be expressed in the form { V il}' and as illustrated by Figure 3, particularly on the type A fronthaul segment, the first access point 10i transmits these signal distortions |Vn} to The next access point 102, following the direction 66 of the fronthaul link, is transmitted via the fronthaul link. The previously mentioned transmit 14 and receive 16 modules in relation to [Fig. 1] are therefore transmit and / or receive modules via the fronthaul link and not via a radio channel such as that used to communicate with users. In other words, advantageously, K complex samples for each subcarrier with index n will be transmitted from the first access point 101 to the second access point 102, using the fronthaul segment.

[0133] Then, when the access point index 1 with 1 ∈ [1, L], for example 1=2 for the second access point 102, receives, via the fronthaul link, the signal distortions {j&p d V 22} received on the user side from the previous access point index 1-1, along the serial fronthaul link, this access point index 1 exploits this information |V 22} jointly with its own estimates of local channel | Va} To in turn determine its local precoding filters, so that these can allow on the side of the K users a coherent construction of useful signals while suppressing signal distortions caused by hardware imperfections of the preceding 1-1 index access point.

[0134] Thus, the optimal precoded signals xl,n in the frequency domain suitable for transmission by the liith access point on subcarrier n to the K users is expressed by the unique solution: > H Xiji— where is a normalization matrix satisfying {f 11 h Ipl 1 which can be optimized using a matrix [=Ik, VkJ optimal diagonal whose elements are . k=l, ..., K.

[0135] As illustrated in Figure 3, the second access point, for example for 1 = 2, the second access point 102, in turn transmits, via the fronthaul link, the signal distortions actually received by the user K and caused by its own imperfections expressed in the form |V12}' (this refers to |V n} For the second access point 102 of index 7 = 2) to the access point of index 1+1, for example for 1 = 2, the access point 103 of index 1+ 1 = 3, and so on repeated in series (i.e. from one to the next) along the fronthaul link until reaching the central unit 62.

[0136] According to the present invention, the central unit 62 of a base station 64 of a massive distributed and cellless MIMO network, associated with at least one fronthaul link comprising a plurality of access points in series, as described above in relation to Figures 1 and 2, and ordered in a predetermined direction of travel from said fronthaul link to said central unit, also includes, specifically according to the present invention, at least one receiving module configured to receive, via said fronthaul link, representative information dyn J of a signal distortion caused by the material imperfections of the access point of index Ll directly preceding it within said fronthaul link,and to take into account said information to determine the local precoding filters of said central unit configured so as to allow on the side of the K users a coherent construction of the useful signals {sn} V12 while suppressing said signal distortion | V n | caused by the , material imperfections of said access point of index Ll preceding it.

[0137] Thus, the central unit 62 of the base station (or last access point in the fronthaul link) contributes to the compensation of distortions generated by its predecessor access point.

[0138] With regard to the distortion generated by base station 64 itself, it should be noted that, according to a first variant, this distortion generated by base station 64 itself is considered acceptable because it has little impact on the overall performance of the system, or according to a second variant, because the hardware used in the base station is more efficient (more expensive and more energy-consuming) the distortion generated by base station 64 itself is also considered negligible.

[0139] An example of an embodiment of the operation, according to the present invention, of an access point of a fronthaul link of a massive distributed and cellless MIMO network according to the present invention is described below in relation to [Fig.4].

[0140] More specifically, the method 70 for limiting signal distortion caused by material imperfections of a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network is implemented by each access point of said fronthaul link or by said central unit, and implemented during communication on the downlink communication channel of said massive distributed cellless MIMO network.

[0141] The method 70 includes, when implemented by each access point 10i to Kf idc said fronthaul link, firstly a step 72 of determining information representative of a signal distortion caused by the own material imperfections of said access point.

[0142] Furthermore, when the method 70 is implemented by each access point 10i to 10Li of said fronthaul link, the method further comprises a step 74 of transmitting, via said fronthaul link, said information representative of said signal distortion caused to another access point, or to the central unit of a base station, directly following it within a fronthaul link comprising a plurality of access points in series ordered in a predetermined direction of travel of said fronthaul link.

[0143] Finally, when the method 70 is implemented by each access point of said fronthaul link, of index 2 < 1 < L -1 102 to 10L i, or when it is implemented by the central unit 62 (last access point in the fronthaul link), the method 70 further includes a receiving step 76, via said fronthaul link, and taking into account said information representative of a signal distortion caused by the material imperfections of an access point directly preceding it within said fronthaul link in order to determine the local precoding filters of said access point or of said central unit configured to cancel said signal distortion caused by the material imperfections of said access point preceding it.

[0144] As an optional complement as illustrated by the example in [Fig.4], said determination step 72 includes obtaining OBT a local estimation of the propagation channel between each of its transmitters and each user terminal within its range in said massive distributed cellless MIMO network.

[0145] Furthermore, said determination step 72 includes the determination D_P of the power allocated to each data signal suitable for transmission between said access point and each user terminal within its range within said massive distributed cellless MIMO network.

[0146] Furthermore, said determination step 72 includes the PRECOD precoding of each data signal specific to be transmitted between each of its transmitters and each user terminal within its range in said massive distributed MIMO network and without a cell, depending on said local estimate and associated allocated power.

[0147] Said determination step 72 also includes the transmission T_M0D of each precoded input signal of a hardware model of the transmission chain of its associated transmitter.

[0148] Finally, said determination step 72 includes the determination D_Id of said information representative of the signal distortion caused by the own material imperfections of said transmitter by comparison of the output of said material model and said precoded input signal.

[0149] A person skilled in the art will understand that the invention is not limited to the embodiments described, nor to the particular examples of the description, the embodiments and variants mentioned above being capable of being combined with each other to generate new embodiments of the invention.

[0150] The present invention thus makes it possible to propose an effective solution for distributed / cellless MIMO technologies which are currently the most promising technologies to support the future of industry 4.0 in which private / industrial networks and applications with rapid deployment of wireless access infrastructure and low energy consumption are important issues.

[0151] The sequential processing proposed according to the present invention, by exploiting the fronthaul link, takes advantage of both the serial connections of this fronthaul link and the latest advances in terms of distributed precoding schemes to offer radical performance improvements in mitigating the hardware imperfections of each access point of said fronthaul link, which makes practical and useful the implementation of distributed massive MIMO type networks without cells, using in particular multi-carrier modulation, in particular OFDM, these networks being known by the abbreviated name CF-mMIMO-OFDM.

[0152] Finally, the present invention aims to meet at least in part the strong demand for "green" signal processing solutions due to concerns related to the sustainability of the telecommunications sector and, correspondingly, the reduction of associated carbon dioxide emissions, thereby saving energy and reducing environmental pollution.

Claims

Demands

1. Access point (10) of a distributed, cellless, massive MIMO network, characterized in that it comprises at least: - a determination module (12) configured to determine information representative of a signal distortion caused by the access point's own hardware imperfections, - a transmission module (14) configured to transmit, via a fronthaul link comprising a plurality of access points in series ordered in a predetermined direction of travel of said fronthaul link to the central unit of a base station, said information representative of said signal distortion to another access point, or to the central unit, directly following it within said fronthaul link; - a receiving module (16) configured to receive, via said fronthaul link, information representative of a signal distortion caused by the hardware imperfections of an access point directly preceding it within said fronthaul link, and to take it into account to determine the local precoding filters of said access point configured to cancel said signal distortion caused by the hardware imperfections of said access point preceding it.

2. Access point (10) according to claim 1, characterized in that, to determine said information representative of the signal distortion, said access point determination module (12) is configured to: - obtain a local estimation of the propagation channel between each of its transmitters and each user terminal within its range in said massive distributed and cellless MIMO network, - determine the power allocated to each data signal to be transmitted between said access point (10) and each user terminal within its range in said massive distributed and cellless MIMO network, - precode each data signal to be transmitted between each of its transmitters and each user terminal within its range in said massive distributed and cellless MIMO network, according to said local estimation and said allocated power associated with it; - transmit each precoded signal as input to a hardware model (48) of the transmission chain of its associated transmitter;- determine said information representative of the signal distortion caused by the own material imperfections of said transmitter by comparing the output of said material model and said pre-coded input signal.;

3. Access point (10) according to claim 2, characterized in that said hardware model (48) of the transmission chain of each transmitter is modeled using at least one predetermined measurement and / or predetermined optimization.

4. Access point (10) according to any one of claims 1 to 3, characterized in that it comprises a plurality of simultaneously active emitters, said information representing a signal distortion caused by the own material imperfections corresponding to the concatenation of the signal distortion caused by the own material imperfections of each of the emitters of said plurality.

5. Access point (10) according to any one of claims 2 to 4, characterized in that each transmitter is an OFDM transmitter.

6. Access point (10) according to any one of the preceding claims, characterized in that each local precoding filter is of type MRT, FZF or RZF.

7. Central unit (62) of a base station (64) of a massively distributed, cellless MIMO network, said central unit being associated to at least one fronthaul link comprising a plurality of access points in series according to any one of the preceding claims and ordered in a predetermined direction of travel of said fronthaul link to said central unit, said central base station unit being characterized in that it comprises at least one receiving module configured to receive, via said fronthaul link, information representative of a signal distortion caused by the material imperfections of an access point directly preceding it within said fronthaul link, and to take into account said information in determining the local precoding filters of said central unit configured to cancel said signal distortion caused by the material imperfections of said access point preceding it.

8. Massive distributed cellless MIMO network comprising at least one fronthaul link, characterized in that said at least one fronthaul link comprises a plurality of access points in series, according to any one of the preceding claims 1 to 6, and ordered in a predetermined traversal direction of said fronthaul link.

9. A method (70) for limiting signal distortion caused by material imperfections in a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network, said access points of said plurality being connected in series along said fronthaul link to the central processing unit of a base station and ordered in a predetermined direction of travel of said fronthaul link, said method being implemented during communication on the downlink of said massive distributed cellless MIMO network, said method comprising at least the following steps implemented by the first access point (10x) of said link in said direction of travel: - determining (72) information representative of signal distortion caused by the own material imperfections of said access point, and - transmitting (74), via said fronthaul link,of said information representative of said signal distortion caused to another access point, or to the central processing unit of a base station, following it directly within a fronthaul link comprising a plurality, of access points in series ordered in a predetermined direction of travel of said fronthaul link; and comprising the following steps implemented by each access point distinct from said first access point: determination (72) of information representative of a signal distortion caused by the own material imperfections of said access point, and transmission (74), via said fronthaul link, of said information representative of said signal distortion caused to another access point, or to the central processing unit of a base station, directly following it within a fronthaul link comprising a plurality of access points in series ordered in a predetermined direction of travel of said fronthaul link;and - reception (76), via said fronthaul link, and consideration of information representative of a signal distortion caused by the hardware imperfections of an access point directly preceding it within said fronthaul link to determine the local precoding filters of said access point configured to cancel said signal distortion caused by the hardware imperfections of said access point preceding it, and comprising the next step implemented by said central unit - reception (76), via said fronthaul link, and consideration of information representative of a signal distortion caused by the hardware imperfections of an access point directly preceding it within said fronthaul link to determine the local precoding filters of said central unit configured to cancel said signal distortion caused by the hardware imperfections of said access point preceding it.;

10. Method (70) according to claim 9, wherein said determination of information representative of a signal distortion caused by the own material imperfections of said access point comprises: - obtaining a local estimation of the propagation channel between each of its transmitters and each user terminal within its range in said massive distributed cellless MIMO network, - determining the power allocated to each data signal suitable for transmission between said access point and each user terminal within its range in said massive distributed cellless MIMO network, - the precoding of each data signal to be transmitted between each of its transmitters and each user terminal within its range within said massive distributed and cellless MIMO network, according to said local estimation and said allocated power associated; - the transmission of each precoded signal into the input of a hardware model of the transmission chain of its associated transmitter; - the determination of said information representative of the signal distortion caused by the own hardware imperfections of said transmitter by comparison of the output of said hardware model and said precoded input signal.

11. A computer program comprising software instructions which, when executed by a computer, implement a method for limiting signal distortion caused by hardware imperfections in a plurality of access points of a fronthaul link of a massive distributed cellless MIMO network in accordance with claims 9 or 10.

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