Cellless distributed massive MIMO network access point, network and method for limiting signal distortion due to access point hardware failures

Description

[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 material 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 as described in US patent 2022 / 302967 A1, and more particularly to data transmission via the use of a radio frequency signal employing multi-carrier modulation, notably OFDM (from the English Orthogonal Frequency Division Multiplexing ) according to a downward 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 MIMO (Mixed Information Modeling) type network architecture (from the English Multiple-Input Multiple-Output ) distributed and cellless massive, or CF-mMIMO (from English cell-free massive MIMO ) as notably introduced in the article by S. Buzzi et al. entitled "Cell-free massive mima: User-centric approach" published in IEEE Wireless Communications Letters, vol. 6, no. 6, pp. 706-709, 2017. Such a distributed and cell-free 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 distributed, cellless, massive MIMO network architecture is composed of a large number of access points, or APs (from the English Access Point ) geographically distributed which jointly serve user terminals via said downlink, or UE (from English User Equipement ) , which guarantees a gain in macro-diversity and offers good spectral efficiency, or SE (from the English 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. Therefore, severe hardware imperfections, or HWls (from the English hardware impairments ) can occur during the transmission of radio frequency signals using multi-carrier modulation, particularly OFDM with a high crest factor, or PAPR (Peak-to-Average Power Ratio), 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 enable 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] For this purpose, the invention relates to an access point of a massive distributed and cellless MIMO network according to claim 1.

[0012] 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 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 of the massive distributed and cellless MIMO network, each access point being configured to cancel the distortion caused by the material 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 link.

[0013] 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: to determine said information representative of the signal distortion, said 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 in said massive distributed and cellless MIMO network, determine 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 and cellless MIMO network, precode each data signal suitable for transmission 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 of the transmission chain of its associated transmitter;determine said information representative of the signal distortion caused by the transmitter's own material imperfections by comparing the output of said material model and said input precoded signal; said material model of the transmission chain of each transmitter is modeled using at least one predetermined measurement and / or predetermined optimization; said access point comprises a plurality of simultaneously active transmitters, said information representative of a signal distortion caused by the transmitter's own material imperfections corresponding to the concatenation of the signal distortion caused by the transmitter's own material imperfections of each of the transmitters in said plurality; each transmitter is an OFDM transmitter; each local precoding filter is of the MRT maximum ratio transmission precoding type, FZF zero-forcing, or RZF regularized zero-forcing type.

[0014] The invention also relates to a central unit of a base station in a massive distributed and 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 of said fronthaul link to said central unit,

[0015] said base station central unit comprising at least one receiving module configured to receive, via said fronthaul link, information representative of signal distortion caused by hardware imperfections in 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

[0016] 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.

[0017] 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 according to claim 9.

[0018] 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 inherent 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 and cellless MIMO network, determining 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 and cellless MIMO network, precoding 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, based on said local estimation and associated allocated power; transmitting 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 material imperfections of said transmitter by comparison of the output of said material model and said precoded input signal. ;

[0019] 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.

[0020] 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: [ Fig 1 ] there figure 1 is a schematic representation of an access point according to the present invention; [ Fig 2 ] there figure 2 illustrates an example of the hardware architecture of a part of the access point shown schematically on the figure 1 ; [ Fig 3 ] there figure 3 is a schematic representation of two fronthaul link implementations 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; [ Fig 4 ] there figure 4 is a flowchart of the process 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.

[0021] It should be noted that the following mathematical notation conventions are subsequently used, namely: Matrices are designated by bold capital letters, for example X , Vectors are denoted by lowercase letters in bold, for example x , A matrix transpose, a matrix conjugate transpose, a matrix pseudoinverse, and the trace of a matrix are respectively denoted by X T< , X H< , X †< and tr ( X ) respectively, for a matrix of dimension M × N, the notation X = { x mn } is used, with x n to designate the nth column, and x m t to designate the mth row, the identity matrix N × N and the all-zero matrix M × N are respectively designated by I N And 0 M × N , the notation X = diag { x 1 , ..., x k } denotes a diagonal matrix with elements { x i }, E[.] represents the expectation operator, ! is the factorial operator, and j represents − 1 .

[0022] There figure 1 firstly schematically illustrates an access point 10 of a massive distributed and cellless MIMO network according to the present invention.

[0023] According to the present invention, such an access point 10 of a massive distributed cellless MIMO network first includes a determination module 12 configured to determine information representative of a signal distortion caused by the own material imperfections of said access point 10.

[0024] 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.

[0025] Furthermore, the access point 10, according to the present invention, also includes a receiving module 16 configured to receive, via said fronthaul link, said 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 it into account to determine the local precoding filters of said access point configured to cancel said signal distortion caused by the material imperfections of said access point preceding it.

[0026] Furthermore, the access point 10 comprises, in a conventional manner, a set of 18 electronic modules dedicated to the classic processing functions typically implemented by an access point in a massive, distributed, 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 hardware imperfections in the 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, via radio, the precoded data, through said precoding filters, to the users, etc., these elements being subsequently described in relation to the figure 2 illustrates an example of the hardware architecture of a part of the access point shown schematically on the figure 1 .

[0027] An example of a determination modulus 12, as represented by the figure 1 .

[0028] Indeed, according to the embodiment illustrated by the figure 1 , the determination module 12 includes for example the elements 20, 22, 24, 26 and 28.

[0029] Element 20 is specifically configured to obtain a local estimate of the propagation channel between each of the transmitters (not shown on the figure 1 ) of access point 10 and each user terminal within its range within said massive distributed and cellless MIMO network.

[0030] Such a local channel estimation is implemented in particular by a channel estimation module of set 18 and element 20 retrieves each channel estimation between each of the emitters (not shown on the figure 1 ) of access point 10 and each user terminal within its range within said massive distributed and cellless MIMO network.

[0031] 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.

[0032] Element 24 is configured to precode each data signal proper to be transmitted between each of the transmitters of access point 10 and each user terminal within its range within said massive distributed cellless MIMO network, based on said local estimation and associated allocated power.

[0033] 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).

[0034] 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.

[0035] According to a variant illustrated as an example by the figure 1 The access point 10 further comprises a processing unit 30, for example, consisting of a memory 32 and a processor 34 associated with the memory 32. The access point 10 is at least partially implemented as software, or a software component, executable by the processor, including 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 performed 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.

[0036] 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 classic 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 Specific integrated Circuit).

[0037] 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.

[0038] There figure 2 illustrates a non-limiting example of the hardware architecture of a part of the access point shown schematically on the figure 1 , particularly in the case where the implementation of massive distributed and cellless MIMO type networks is considered useful, using in particular multi-carrier modulation, notably OFDM, these networks being known by the abbreviated name CF-mMIMO-OFDM.

[0039] In this example, we consider, in particular, N modulated input signals s 1 à s N , N being an integer corresponding to the number of OFDM subcarriers, at the input of a set of 40 electronic modules for digital signal processing.

[0040] The information data to be transmitted to K users on the nth subcarrier is denoted by means of the signal Sn such that s n ∈ ℂ K × 1 include independent unitary power elements, such as E {∥ s n ∥ 2<} = 1.

[0041] It should be noted that OFDM systems typically 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 complement used for the guard band, in which no data is transmitted.

[0042] In particular, we note subsequently h l , k , n ϵ ℂ M × 1 The channel response in the frequency domain between the lth access point and user k on the nth subcarrier with n=0, ..., N-1. The channels are modeled using Rayleigh independent fading, i.e., each channel h l,k,n ~ CN , β l,k IM ) , Or β l,k is the large-scale fading coefficient between the lth< access point AP and the kth< user UE (from the English User Equipment), independent of the antennas or subcarriers implemented.

[0043] Furthermore, we subsequently consider blocks-fading channels, which are constant over a time-frequency interval, known as the coherence interval, and which vary independently between coherence intervals.

[0044] Furthermore, it is also subsequently assumed that large-scale fading coefficients vary slowly, within a range of several consistency intervals, which makes it obvious to consider that the channel gains are known a priori at each access point and are used to estimate current channel responses.

[0045] The set of 40 electronic modules is organized into M digital signal processing chains associated with each of the said M transmitters of the access point 10 considered, each transmitter being optionally an OFDM transmitter.

[0046] Such a set 40 includes, in particular, the example of digital architecture of the figure 2 , a module 42 for precoding specific to implementing distributed linear local precoding.

[0047] More specifically, the precoding implemented by the precoding module 42 is necessary to be implemented at each access point in order 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 preserving scalability. scalability, a scalable network being a network whose computational complexity remains finite by geographically extending the network and increasing the number of users) of system, this is why subsequently the notion of "local precoding" (i.e. locally within the access point) is used.

[0048] 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.

[0049] Each local precoding filter is of type MRT (from English Maximum Ratio Transmission ) namely maximum ratio transmission precoding, FZF (from English Full pilot zero forcing ) namely zero forcing for pilots or RZF the regularized version of FZF namely regularized zero forcing, the precoded vector in the frequency domain proper to be transmitted by the access point of index I on the subcarrier of index n is expressed in the following form: x l , n = W l , n P l s n Or : W l , n ϵ ℂ M × K is the precoding matrix associated with the subcarrier of index n for the access point of index I, P l ϵ ℂ K × K , which is frequency-independent, represents a diagonal matrix whose elements η l , k , such that k=1,..., K are the normalized transmission powers allocated to the K users. s n ϵ ℂ K × 1 is the data vector to be transmitted to the K users on the subcarrier n.

[0050] More specifically, the precoding vector w l , n , k ϵ ℂ M × 1 used by the lth access point to the kth user on subcarrier n is expressed in the form: w l , n , k = H ^ l , n H ^ l , n H H ^ l , n − 1 C l e k , pour un filtrage local FZF H ^ l , n H ^ l , n H H ^ l , n + P l − 1 − 1 C l e k pour un filtrage local RZF Or : C l ϵ ℂ K × K is a normalization matrix satisfying { E {∥ w l,n , k ∥ 2<} = 1, ∀ k}, H ^ l , n ϵ ℂ M × K is the channel estimation matrix between the access point of index I (i.e., the lth access point) and the K users on the subcarrier n, which can also be expressed as: Ĥ l,n = [ ĥ l,n, 1 , ..., ĥ l,n,K ], and represented by the downward vertical arrow entering the precoding module 42, particularly considering 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 ĥ l,k,n of the canal h l,k,nis 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. P l ϵ ℝ K × K is a diagonal regularization matrix, e k is the kth column of the identity matrix of the identity matrix I k, which subsequently amounts to taking the kth column of Ĥ l,n ( Ĥ l,n H< Ĥ l,n ) - 1< C l .

[0051] It should be noted that classically precoding schemes are known to the person skilled in the art, such as those 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, which is why these schemes are not detailed further as they are known and not the subject of the invention as such.

[0052] As illustrated by the figure 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.

[0053] In particular, each chain typically includes, firstly, at the output of the precoding module 42, a module 44 applying an inverse fast Fourier transform (IFT) of size M FFT , OFDM subcarriers is less than or equal to the size M FFT 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 the figure 2 .

[0054] Then, at the output of module 44, which applies an inverse fast Fourier transform, each chain typically includes a multiplexer 46

[0055] Specifically according to the present invention, each chain is further supplemented by elements 48, 50 and 52 enabling the determination of the signal distortion caused by material imperfections in each transmission chain.

[0056] Indeed, each radio frequency transmission chain also classically includes a digital-to-analog converter (DAC) 54, a transmitter 56, in particular an OFDM transmitter marked RF Tx on the figure 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.

[0057] For example, the signal transmitted by the antenna with index m from the access point with index I is expressed in the following form: z l , m t = f a l , m t 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 modulated signals a l , m t , ∀ m transmitted to each antenna of the access point through the M radio frequency transmission chains.

[0058] Note 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: z l , m t = K 0 a l , m t + d l , m t Or K 0 is a frequency-independent complex gain and d l , m t is a distortion (i.e., noise) with a mean of zero and a variance σ d 2 decoupled from each modulated signal a l , m t not Gaussian at transmission but becoming Gaussian at reception after OFDM demodulation.

[0059] K 0 and σ d 2 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: by transmitting the precoded modulated signal of each chain, obtained at the output of the multiplexer 46, into the input of a hardware model 48 of the transmission chain considered, called HWI_M, and by subtracting, via the element 50, the output of the hardware model 48 from the precoded modulated signal at the input of the multiplexer 46, then by demodulating, via the module 52 applying a Fast Fourier Transform (FFT) to obtain the distortion expressed in the frequency domain.

[0060] As an optional complement, the said hardware 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 allows the development of models for characterizing hardware imperfections.

[0061] As an optional addition, as illustrated by the figure 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.

[0062] In other words, for a given sample, the distortion generated by a transmitter is represented by a coefficient d l , m t . Therefore, the distortion generated by M emitters is a vector of dimension M comprising the M distortion coefficients generated by the M emitters.

[0063] There figure 3 is a schematic representation of two fronthaul link embodiments of a massive distributed cellless 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.

[0064] More specifically, time-division duplex (TDD) allows for the separation of downlink (DL) and uplink (UL) transmissions, assuming perfect channel reciprocity, which can be ensured by precise calibration methods known to those skilled in the art. Furthermore, frame transmission using TDD within a CF-mMIMO-OFDM network is implemented within the coherence interval, and the physical resource block (RB) width is smaller than the coherence bandwidth.

[0065] Subsequently, to comply with the 5G NR standard, let us consider, in particular, a radio frame whose time-frequency resource is divided into N rb resource blocks RB. Each resource block comprises N sc = N N rb consecutive subcarriers. We note ( t, n ) l,m the resource unit RU (from the English resource unit) which represents the smallest time-frequency resource of the nth< subcarrier of the tth< OFDM symbol corresponding to the mth< antenna of the lth< access point.

[0066] For example, such a TDD frame typically comprises N c OFDM symbols, which corresponds to the shortest coherence interval of all users, and to the transmission of: τ c = N sc N c units of resource RU per block of resource RB, where τ p resource units among the τ c Resource units are used as drivers that are distributed within the transmission payload on the uplink communication channel (UL). Such drivers are typically used to estimate MxK channels in the frequency domain, per resource block, within each access point. Also, N D = N sc N c - τ p Resource units are reserved, per resource block, for the data needed in the samples, which are split between the transmissions of the downlink (DL) and those of the uplink (UL) into two complementary parts. εN D and (1 - ε ) N D respectively with 0 < ε < 1.

[0067] More specifically, on the figure 3 , two embodiments A and B of fronthaul links are represented, 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.

[0068] On the figure 3 , according to embodiment A, the fronthaul link has one end consisting of an access point 10 1 and another end consisting of the central unit 62 of the base station 64, the L access points 10 1 , 10 2 , 10 3 , ...,10 L 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.

[0069] According to embodiment B, the fronthaul link forms a frame around K users, for example K=6 for the users illustrated by terminals p1, p2, p3, p4, p5, p6 distributed arbitrarily in the coverage area, and also includes L access points, such that 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).

[0070] Note that in general, a CF-mMIMO network is likely to be divided into several fronthaul links (i.e. segments) corresponding to access points linked in series via a fronthaul link, and the processing is applied to each segment, for example a segment according to embodiment A and / or a segment according to embodiment B in a way that is independent of one segment to another, or according to any other known network topology.

[0071] The signal received at the kth user via the nth subcarrier is expressed in particular in the following form: y k , n = ∑ l = 1 L h l , k , n H z l , n + b k , n = ∑ l = 1 L h l , k , n H K 0 x l , n + ∑ l = 1 L h l , k , n H d l , n + b k , n Or z l , n ϵ ℂ M × 1 denotes the amplified signal, expressed in the frequency domain, transmitted by the access point of index I on the subcarrier n, d l , n ϵ ℂ M × 1 refers to signal distortion (i.e., distortion noise) caused by hardware imperfections. K 0 is the diagonal MxM matrix whose elements are equal to K0 as introduced previously, and b k,n ~ CN identically distributed (ieiid).

[0072] Furthermore, per resource block and per user, the spectral efficiency (SE) can be calculated 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 SE k = ξ 1 − τ p N sc N c N sc Δ flog 2 1 + SINR k , n with SINR k,n The signal-to-interference-plus-noise ratio (SNR) of the kth user using the nth subcarrier is expressed in the following form: SINR k , n = ∑ l = 1 L η l , k E h l , k , n K 0 <none / > <mprescripts / > <none / > H w l , k , n 2 ∑ t = 1 K E ∑ l = 1 L η l , t h l , k , n K 0 <none / > <mprescripts / > <none / > H w l , n , t 2 − ∑ l = 1 L η l , k E h l , k , n K 0 <none / > <mprescripts / > <none / > H w l , k , n 2 + Ψ k , n + 1 with : Ψ k , n = E ∑ l = 1 L h l , k , n d l , n <none / > <mprescripts / > <none / > H − ∑ l = 1 L − 1 C l ′ h l + 1 , k , n H w l + 1 , k , n h ^ l , k , n H d l , n 2 And C l ′ is a normalization factor.

[0073] As previously mentioned in relation to the figures 1 And 2The 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 in the fronthaul link considered. These imperfections are 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 the figure 2 .

[0074] There figure 3 illustrates more precisely the information transmitted on each fronthaul segment, allowing the limitation, or even elimination, step by step along the fronthaul link of signal distortions caused by hardware imperfections at each access point.

[0075] 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 antennas of the user terminals and this advantageously limits, or even cancels, signal distortions caused by the hardware imperfections of each access point.

[0076] In other words, as illustrated by the figure 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.

[0077] On the examples of fronthaul segment A and B of the figure 3 The first access point 101 of the fronthaul link, following a predetermined path 66 or 68 to the central unit 62, first determines, conventionally, local precoding filters based on (i.e., using) local channel estimates between this first access point 101 and each user k, each illustrated, for example, by the terminals using the drivers p1, p2, p3, p4, p5, p6 of the figure 3 .

[0078] As previously mentioned, 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 of index I on the subcarrier of index n being, as previously mentioned, expressed in the following form: x l,n = W l,n P L s n.

[0079] Then, as previously mentioned in relation to the figures 1 And 2 , the first access point 10 1 , is also suitable, specifically according to the present invention, for approximating (i.e. determining) the signal distortion caused by its own material imperfections { d 1, n , ∀ n}, this distortion d 1, n being de facto also transmitted by the first access point 10, to each user k via radio.

[0080] On the user side, signal distortions caused by the first access point's own hardware imperfections 10 1, effectively received after the pre-coded signals in the frequency domain { x 1, n , ∀ n} 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 H ^ 1 , n H d 1 , n , ∀ n , and as illustrated by the figure 3 , particularly on the type A fronthaul segment, the first access point 10 1 transmits these signal distortions H ^ 1 , n H d 1 , n , ∀ n at the next access point 10 2 along the 66 direction of travel of said fronthaul link, and this via said fronthaul link. The transmission modules 14 and reception modules 16 previously mentioned in relation to the figure 1 These are therefore transmission and / or reception 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.

[0081] Then, when the access point index I with l ∈ [1, L ] , For example l = 2 for the second access point 10 2, receives, via the fronthaul link, the signal distortions H ^ l − 1 , n H d l − 1 , n , ∀ n received on the user side from the previous access point indexed I-1, along the serial fronthaul link, this access point indexed I uses this information H ^ l − 1 , n H d l − 1 , n , ∀ n together with its own local channel estimates H ^ l , n H , ∀ n 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 access point of index I-1 preceding it.

[0082] Thus, the signals x l,n The optimal precoded values ​​in the frequency domain suitable for transmission by the lth access point on subcarrier n to K users are expressed by the unique solution: x l , n = W l , n P l s n − W l , n C l ′ H ^ l − 1 , n H d l − 1 , n Or C l ′ ϵ ℂ K × K is a normalization matrix satisfying E H ^ l , n H w l , n 2 = I k , ∀ k which can be optimized using an optimal diagonal matrix whose elements are 1 M − K β l , k , k = 1 , … , K .

[0083] As illustrated on the figure 3 , the lth access point, for example for l= 2, the second access point 10 2, 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 H ^ l , n H d l , n , ∀ n , (which amounts to H ^ 2 , n H d 2 , n , ∀ n for the second access point, index 10 2 l = 2) at the access point with index I+1, for example for l = 2, access point 10 3 index l + 1 = 3, and so on repeated serially (i.e. step by step) along the fronthaul link until reaching the central unit 62.

[0084] According to the present invention, the central unit 62 of a base station 64 of a massive distributed, cellless MIMO network, associated with at least one fronthaul link comprising a plurality of serial access points, as described above in relation to the figures 1 And 2, and ordered according to 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 H ^ L − 1 , n H d L − 1 , n , ∀ n of a signal distortion caused by hardware imperfections in the L-1 index 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 so as to allow on the K users' side a coherent construction of the useful signals { s n } Vn while removing said signal distortion H ^ L − 1 , n H d L − 1 , n , ∀ n caused by the material imperfections of said access point of index L-1 preceding it.

[0085] Thus, the central unit 62 of the base station (or last access point in the fronthaul link) contributes to compensating for distortions generated by its predecessor access point.

[0086] Regarding the distortion generated by Base Station 64 itself, it should be noted that, according to one 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.

[0087] The following is described in relation to the figure 4 , 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.

[0088] More specifically, the method 70 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 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.

[0089] The method 70 includes, when implemented by each access point 10 1 to 10 L-1 of 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.

[0090] Furthermore, when the method 70 is implemented by each access point 10, at 10 L-1 of said fronthaul link, the method further includes 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.

[0091] Finally, when process 70 is implemented by each access point of said fronthaul link, with index 2 ≤l ≤ L - 1 10 2 to 10 L-1, or when 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 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.

[0092] As an optional addition, as illustrated by the example of the figure 4, said step 72 of determination includes obtaining OBT a local estimation of the propagation channel between each of its transmitters and each user terminal within its range within said massive distributed cellless MIMO network.

[0093] Furthermore, said step 72 of determination 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.

[0094] In addition, said determination step 72 includes the PRECOD precoding of each data signal to be transmitted between each of its transmitters and each user terminal within its range within said massive distributed cellless MIMO network, based on said local estimation and associated allocated power.

[0095] The said determination step 72 also includes the T_MOD transmission of each precoded input signal of a hardware model of the transmission chain of its associated transmitter.

[0096] Finally, said determination step 72 includes the determination D_I d 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.

[0097] A person skilled in the art will understand that the invention is not limited to the embodiments described, nor to the specific examples in the description. It is limited only by the scope of the claims.

[0098] 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.

[0099] 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, making it practical and useful to implement distributed massive MIMO and cellless networks, using in particular multi-carrier modulation, especially OFDM, these networks being known by the abbreviated name CF-mMIMO-OFDM.

[0100] Finally, the present invention aims to respond at least in part to the strong demand for "green" signal processing solutions due to concerns related to the sustainability of the telecommunications sector and, correspondingly, to the reduction of associated carbon dioxide emissions, thereby saving energy and reducing environmental pollution.

Claims

1. An access point (10) of a cell-free distributed massive MIMO network, comprising at least the three following modules: - a determination module (12) configured to determine an information item representative of signal distortion caused by the own hardware impairments of said access point, - a transmission module (14) configured to transmit, via a fronthaul link comprising a plurality of access points in series ordered along a predetermined direction of travel of said fronthaul link to the central unit of a base station, said information item representative of said signal distortion to another access point, or to the central unit, directly following it within said fronthaul link; - a reception module (16) configured to receive, via said fronthaul link, said information item representative of a signal distortion caused by the hardware impairments of an access point directly preceding it within said fronthaul link, and to take into account the information item so as to determine the local precoding filters of said access point configured to cancel said signal distortion caused by the hardware impairments of said access point preceding it.

2. The access point (10) according to claim 1, characterized in that, to determine said information item representative of signal distortion, said access point determination module (12) is configured to: - obtain a local estimation of the propagation channel between each of the transmitters thereof and each user terminal within the range thereof within said cell-free distributed massive MIMO network, - determine the power allocated to each data signal suitable for being transmitted between that access point (10) and each user terminal within the range thereof within said cell-free distributed massive MIMO network, - precode each data signal suitable for being transmitted between each of the transmitters thereof and each user terminal within the range thereof within said cell-free distributed massive MIMO network based on said associated local estimation and on said allocated power; - transmit each precoded signal as input to a hardware model (48) of the transmission chain of the associated transmitter thereof; - determine said information item representative of the signal distortion caused by the own hardware impairments of the transmitter, by comparing the output of said hardware model and said input precoded signal.

3. The 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 a predetermined optimization.

4. The access point (10) according to any of claims 1 to 3, characterized in that it comprises a plurality of simultaneously active transmitters, said information item representative of a signal distortion caused by the own hardware impairments corresponding to the concatenation of the signal distortion caused by the own hardware impairments of each of the transmitters of said plurality.

5. The access point (10) according to any of claims 2 to 4, characterized in that the transmitter is an OFDM transmitter.

6. The access point (10) according to any of the preceding claims, characterized in that each local precoding filter is of the maximum ratio transmission (MRT) precoding, full pilot zero forcing (FZF), or regularized full pilot zero forcing (RZF) type.

7. A central unit (62) of a base station (64) of a cell-free distributed massive MIMO network, said central unit being associated with at least one fronthaul link comprising a plurality of access points in series, according to any of the preceding claims, and ordered along a predetermined direction of travel of said fronthaul link to said central unit, said central unit of a base station being comprising at least one reception module configured to receive, via said fronthaul link, an information item representative of signal distortion caused by hardware impairments of an access point directly preceding it within said fronthaul link, and to take into account said information item so as to determine the local precoding filters of said central unit configured to cancel said signal distortion caused by the hardware impairments of said access point preceding it.

8. A cell-free, distributed bulk 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 of the preceding claims 1 to 6, and ordered along a predetermined direction of travel of said fronthaul link.

9. A method (70) of limiting the signal distortion caused by hardware impairments of a plurality of access points of a fronthaul link of a cell-free distributed massive MIMO network, said plurality of access points being connected in series along said fronthaul link to the central unit of a base station and ordered along a predetermined direction of travel of said fronthaul link, said method being implemented during the communication on the downlink communication channel of said cell-free distributed massive MIMO network, said method comprising at least the following steps, when implemented by the first access point of said fronthaul link: - determination (72) of an information item representative of a signal distortion caused by the own hardware impairments of the access point, and - transmission (74), via said fronthaul link, of said information item 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 along a predetermined direction of travel of said fronthaul link; and or, when implemented by an access point on said fronthaul link that is separate from said first access point and separate from said central unit: - determination (72) of an information item representative of a signal distortion caused by the own hardware impairments of the access point, and - transmission (74), via said fronthaul link, of said information item 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 along a predetermined direction of travel of said fronthaul link; and - reception (76), via said fronthaul link, and taking into account an information item representative of a signal distortion caused by the hardware impairments of an access point directly preceding it within said fronthaul link so as to determine the local precoding filters of said access point or of said central unit configured to cancel said distortion signal caused by hardware impairments of the access point preceding it, or, when executed by the central unit: - reception (76), via said fronthaul link, and taking into account an information item representative of a signal distortion caused by the hardware impairments of an access point directly preceding it within said fronthaul link so as to determine the local precoding filters of said access point or of said central unit configured to cancel said distortion signal caused by hardware impairments of the access point preceding it.

10. The method (70) according to claim 9, wherein said determination of an information item representative of signal distortion caused by said own hardware impairments of said access point comprises: - obtaining a local estimate of the propagation channel between each of the transmitters thereof and each user terminal within the range thereof within the said cell-free distributed massive MIMO network, - the determination of the power allocated to each data signal suitable for being transmitted between said access point and each user terminal within the range thereof within said cell-free distributed massive MIMO network, - the precoding of each data signal suitable for being transmitted between each of the transmitters thereof and each user terminal within the range thereof within said cell-free distributed massive MIMO network based on said associated local estimation and allocated power; - the transmission of each precoded signal at the input of a hardware model of the transmission chain of the associated transmitter thereof; - the determination of said information item representative of the signal distortion caused by the own hardware impairments of the transmitter, by comparing the output of said hardware model and said input precoded signal.

11. A computer program including software instructions which, when executed by a computer, implement a method of limiting signal distortion caused by hardware impairments of a plurality of access points of a fronthaul link of a cell-free distributed massive MIMO network according to claims 9 or 10.