Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
  • H04L1/0618—Space-time coding
  • H04L1/0637—Properties of the code
  • H04L1/0668—Orthogonal systems, e.g. using Alamouti codes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
  • H04L1/0618—Space-time coding
  • H04L1/0625—Transmitter arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
  • H04L1/0618—Space-time coding
  • H04L1/0631—Receiver arrangements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0204—Channel estimation of multiple channels
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0226—Channel estimation using sounding signals sounding signals per se
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/024—Channel estimation channel estimation algorithms
  • H04L25/0242—Channel estimation channel estimation algorithms using matrix methods

Definitions

• the field of the invention is that of digital communications over the air. More specifically, the invention relates to transmission and reception, and in particular the estimation of the transmission channels in a multi-antenna system of the MIMO (“Multiple Input Multiple Output”) or MISO ( “Multiple Input Single Output”), based on the transmission of signals subject to space-time and / or space-frequency coding. More precisely still, the invention applies to multi-antenna systems implementing several, and in particular more than two transmitting antennas.
• the signals include reference symbols, known to at least one receiver and enabling the latter to estimate the transmission channels corresponding to each of the transmission antennas.
• An example of application of the invention is the field of radiocommunications, in particular for third, fourth generation and following systems.
• the invention can be applied to uplink communications (from a terminal to a base station), as well as to downlink communications (from a base station to a terminal). 2. Solutions of the Prior Art Several techniques for estimating transmission channels are already known in a multi-antenna system comprising several transmission antennas. Most of these estimation techniques are limited to the application of space-time or space-frequency coding in multicarrier systems of the OFDM type. Thus, the first systems proposed all used space-time codes in orthogonal blocks.
• Alamouti in "A Simple Transmit Diversity Technique for Wireless Communications", IEEE Journal on Selected Areas in Communications, pp. 311-335, vol. 6, 1998, presented the first system using an orthogonal space-time block code with a yield of 1 (where the yield is defined as the ratio between the number N of symbols transmitted and the number L of symbol times during which they are transmitted ), for two transmitting antennas.
• a major drawback of Alamouti's orthogonal space-time codes is that they are limited to systems with two transmit antennas, and that it is not possible to generalize them directly to a system with more than two antennae. issue, while maintaining a unit return. Tarokh et al. ("Space-time Block Codes from Orthogonal Designs", IEEE
• a disadvantage of this estimation technique is that the number of transmitting antennas of the transmission system is limited by the use of space-time codes in known orthogonal blocks. Thus, according to the techniques of the prior art, there are no complex orthogonal codes with unit efficiency for systems with more than two transmitting antennas, which reduces the spectral efficiency. 3. Objectives of the invention
• the object of the invention is in particular to overcome these drawbacks of the prior art. More specifically, an objective of the invention is to provide a technique for estimating the transmission channels in a multi-antenna system using more than two transmit antennas. Another objective of the invention is to propose such a technique which is more effective and efficient than known techniques, while having reduced complexity.
• Yet another objective of the invention is to provide a technique for transmitting a signal comprising reference symbols implementing a space-time and / or space-frequency coding matrix.
• an objective of the invention is to provide a unit efficiency coding matrix.
• the invention also aims to provide such a technique which is suitable for multi-antenna systems of the MISO or MIMO type for modulations of the single-carrier or multi-carrier type, combined with the various multiple access techniques (CDMA, from English “ Code Division Multiplex Access ”for“ code-division multiple access ”, FDMA for“ frequency multiple access ”, or TDMA for“ time multiple access ”).
• Another objective of the invention is to propose such a technique which makes it possible to increase the spatial diversity of the systems while minimizing the interference between the different transmission channels and limiting the loss of spectral efficiency.
• the invention aims to provide such a technique, which can be implemented in a practical and inexpensive way in a system implementing a high number of antennas. 4.
• the invention is based on a completely new and inventive approach to the transmission of a digital signal, implementing a coding matrix in a multi-antenna system with more than two transmission antennas. More specifically, the invention proposes to transmit on the n transmit antennas the reference symbols of the coding matrix M with a yield equal to 1, a vector of reference symbols being associated with the coding matrix M by the through a coding function.
• a coding matrix M with a yield equal to 1 corresponds either to a non-orthogonal matrix, or to an orthogonal matrix by blocks, the yield being defined as the ratio between the number of symbols transmitted and the number of symbol times during which they are issued.
• the reference symbols are distributed in space and in time and / or in space and in frequency.
• the coding matrix then implements space-time and / or space-frequency coding.
• the coding matrix comprises at least two blocks, each of the blocks being orthogonal.
• each of the blocks of reference symbols is transmitted separately, each of the blocks being transmitted on certain transmit antennas, the other transmit antennas being off.
• the transmission method comprises a step of selection between a frequency distribution and a time distribution. In particular, this selection step can take account of the characteristics of a transmission channel.
• the reference symbols are transmitted on all the transmission antennas after mathematical transformation by the coding matrix M.
• the coding matrix M is a globally non-orthogonal matrix.
• the coding matrix M can be obtained by a Jafarkhani type coding, and is of the form:
• the invention also relates to a corresponding transmission device.
• the invention also relates to a digital signal formed by n vectors transmitted respectively using n transmit antennas, n being strictly greater than 2.
• the signal comprises coded reference symbols, obtained after mathematical transformation of reference symbols by a coding matrix M of unit efficiency, so as to allow, in a receiver, to estimate at least three transmission channels corresponding respectively to each of the transmitting antennas.
• the invention also relates to a method for estimating transmission channels in a multi-antenna system using n transmit antennas, where n is strictly greater than 2, and at least one receive antenna.
• such an estimation method comprises a step of receiving a received reference vector, corresponding to an emitted reference vector obtained by the multiplication of reference symbols by said coding matrix M, and modified by at minus one transmission channel for each of the transmitting antennas.
• the reference vector received undergoes a mathematical transformation by a decoding matrix, inverse to the coding matrix and taking into account the effect of a transmission channel associated with the reception antenna, for provide an estimate of the effects of the transmission channels on the reference symbols.
• the invention is based on a completely new and inventive approach to channel estimation in a multi-antenna system with more than two transmit antennas. Note that this approach is also new in a system with two transmitting antennas. Indeed, the estimation of the different transmission channels is implemented from reference symbols known from at least one receiver, a vector of reference symbols being associated with the coding matrix M via a coding function.
• the decoding matrix is an inverse matrix integrating an equalization within the meaning of the criterion MMSE (in English “Minimum Mean Squared Error” for “minimization of the mean square error") or ZF (in English “Zero Forcing” for "forcing to zero ").
• the criterion used may be the MMSE criterion.
• the criterion used can also be the criterion ZF.
• the decoding matrix is then formed of the elements: r M H ⁇ - 7; - r, M H M
• the estimation method comprises an interpolation step, delivering an estimation of the transmission channels for each of the useful data, from the estimation of the reference symbols.
• the interpolation step is remarkable in that it implements a temporal interpolation and / or a frequency interpolation.
• This interpolation step can belong to the group comprising: - linear interpolations; - Wiener's interpolations. 5.
• FIGS. 1A and 1B present a system for estimating channels in a multi-antenna system with 4 transmitting antennas, with symbols distributed in the frequency (FIG. 1A) or time domain (FIG. 1B) according to a first mode of carrying out the invention
• FIGS. 2A and 2B show a particular distribution of the symbols of the channel estimation system of FIGS. 1A and 1B
• FIGS. 3A and 3B illustrate a system for estimating channels in a multi-antenna system with 4 transmit antennas, with symbols distributed in the frequency (FIG.
• the general principle of the invention is based on the association of a coding matrix M with a vector of reference symbols, known to at least one receiver, so to allow, in the receiver, to estimate the different propagation channels between more than two transmit antennas and a receive antenna.
• This coding matrix M is either non-orthogonal or orthogonal by blocks and has a yield equal to 1, the yield being defined as the ratio between the number of symbols transmitted and the number of symbol times during which they are transmitted.
• the symbols of the coding matrix M are then distributed in time and / or in frequency on each of the transmit antennas.
• the received signal is multiplied by the inverse matrix (incorporating an equalization technique within the meaning of the MMSE or ZF criterion) of the coding matrix M, possibly taking into account the noise introduced by the receiver.
• the result is an n-dimensional vector which represents the n transmission channels between the n transmit antennas and this receive antenna.
• This n-dimensional vector is then used by the receiver to estimate the transmission channel, in particular by repeating this operation periodically and by performing a temporal and / or frequency interpolation between two reference symbols estimated during this operation.
• the interpolation is for example of linear or Wiener type.
• the coding matrix M is a block matrix, each of the blocks comprising n reference symbols.
• An orthogonal space-time Alamouti coding is then applied to each block of the coding matrix M.
• Each of the blocks of n reference symbols is then orthogonal.
• the Alamouti coding is applied to the reference symbols used for the estimation of the channel, then these reference symbols coded are transmitted on a pair of transmitting antennas, while keeping the other pair of antennas off.
• the coding matrix M associated with it via the coding function is: x l x 2 0 0 ⁇ - —x 2 0 0 M ⁇ 0 0 x 3 x 4 0 0 -x 4 x 3
• x . is a reference symbol, ⁇ t . a conjugate reference symbol, with i a relative integer and 1 ⁇ i ⁇ 4, and 0 means that no symbol is emitted on the antenna concerned.
• Each block of the coding matrix being coded according to an Alamouti code we have M - M H ⁇ I, with / the unit matrix, and w the conjugate transpose.
• the reference symbols of the coding matrix M are then transmitted after space-frequency distribution (FIG. 1A) or space-time (FIG. 1B) on the various transmission antennas, the spatial axis representing the columns of the matrix Met l 'frequency ( Figure 1A) or time ( Figure 1B) axis representing the lines of the matrix M. It is understood that other space-time or space-frequency distributions of symbols can be envisaged, as well as a combination of the distributions space-time and space-frequency.
• each block of the coding matrix M is transmitted independently on its respective antennas, while the other blocks of the coding matrix are not transmitted.
• each block of reference symbols is transmitted separately, each of the blocks being transmitted on certain transmission antennas while the other antennas are off.
• FIG. 1A thus presents the symbols emitted by the 4 antennas 11, 12, 13, 14 of a multi-antenna system with 4 transmitting antennas, the symbols emitted being distributed in the frequency domain (ordinate axis), with X t a reference symbol referenced 15, X * a conjugate reference symbol (i a relative integer and 1 ⁇ i ⁇ A), x a data symbol, referenced 16, and 0 means that no symbol is emitted.
• the symbols emitted by the 4 antennas 11, 12, 13, 14 are distributed in the space-frequency domain (FIG. 1A) or space-time (FIG. 1B) according to the parameters ⁇ F, ⁇ f l5 ⁇ f 2 (FIG. 1A), ⁇ T , ⁇ t l5 ⁇ t 2 ( Figure 1B), representing the repeating patterns of the reference symbols.
• ⁇ f ⁇ F, ⁇ f 1; ⁇ f 2 ⁇
• ⁇ t ⁇ T, ⁇ t l5 ⁇ t 2 ⁇
• the decoding matrix is formed of the elements: M 'h ⁇ -r, M H M + - Y with: r said reference vector received; M said coding matrix; / a unit matrix; ⁇ the signal to noise ratio; H the conjugate transpose.
• the receiver can determine the coefficients of the propagation channel at the carriers k, k + 1, k + 2, ..., k + ⁇ f r l, k + ⁇ f t .
• Interpolation can also be carried out in the time domain, considering that the reference symbols are transmitted at time p on the carrier k, at time p + ⁇ t on the same carrier k, ....
• the receiver can then determine the coefficients of the propagation channel at times p, p + 1, p + 2, ..., p + ⁇ t-1, p + ⁇ t and so on.
• the receiver can therefore perform time interpolation and / or frequency interpolation.
• This interpolation step implements an interpolation technique well known to those skilled in the art, such as for example a linear type interpolation, or of Wiener.
• the other pair of antennas not transmitting on the same carriers and at the same times, as illustrated in FIGS. 1A, 1B, 2A and 2B, the signal emitted by the first pair of antennas is not disturbed.
• Each pair of antennas then alternately transmits the reference symbols distributed over its antennas, so as to estimate all of the transmission channels of the mutli-antenna system.
• orthogonal space-time codes it is thus possible to apply orthogonal space-time codes to systems having a greater number of transmitting antennas, using a coding matrix M retaining an efficiency equal to 1. It is possible to thus apply an Alamouti coding, of yield equal to 1, to systems having 4, 6, 8, ..., transmission antennas (whereas according to the state of the art, the number of antennas d transmission is limited due to the use of orthogonal space-time codes).
• a second embodiment of the invention is then presented, in which the reference symbols are transmitted on all of the transmit antennas after mathematical transformation by the coding matrix M, the coding matrix M being non-orthogonal.
• a non-orthogonal space-time coding of the Jafarkhani type, is applied, as presented in "A Quasi-Orthogonal Space-Time Block Code ”(IEEE Transactions on Communications, Vol. 49, No.
• x i is a reference symbol
• x i a conjugate reference symbol
• i a relative integer, 1 ⁇ ⁇ 4.
• All the reference symbols of the coding matrix M are then emitted after space / frequency distribution on the set of transmission antennas, the spatial axis representing the columns of the matrix M and the frequency or time axis representing the lines of the matrix M.
• h corresponds to a modeling of the transmission channel and n is a white Gaussian noise vector.
• This matrix is of full rank, so that it can be inverted during the estimation of the different channels.
• the reference symbols of the coding matrix M are transmitted after space / frequency distribution over all of the transmission antennas, and the reference vector received at the level of a reception antenna, modified by the channel of transmission, can be written in the form
• a mathematical transformation is applied to the received reference vector by a decoding matrix, corresponding to the inverse matrix incorporating an equalization technique within the meaning of the MMSE or ZF criterion of the coding matrix M , to estimate the transmission channel h.
• an equalization technique is used within the meaning of the MMSE criterion, we obtain: n, with ⁇ the signal to noise ratio.
• the missing coefficients of a transmission channel can then be determined by applying a time or frequency interpolation (or both) at the receiver, using a conventional interpolation technique.
• FIG. 3A illustrates the transmission of 4 reference symbols and their conjugates temporally spaced, in a multi-antenna system with 4 transmission antennas, with X t a reference symbol referenced 15, X * a reference symbol conjugate (a relative integer and 1 ⁇ i ⁇ 4), x a data symbol, referenced 16.
• FIG. 3B illustrates the emission of 4 reference symbols and their frequency-spaced conjugates, in a multi-antenna system with 4 antennas resignation.
• the reference symbols once coded by means of the coding matrix M, are distributed along the time axis or the frequency axis as a function of the properties of the propagation channel. It is then possible to switch from space-time coding to space-frequency coding. It will be recalled that the values chosen for ⁇ f (spacing between two reference carriers) and ⁇ t (spacing between two reference symbols at known times) are not specific to the system proposed but depend respectively on the band and the coherence time of the channel of transmission. In general, the distribution in the time domain is rather applied in the case of a time-varying channel, while the frequency distribution is more applied for a frequency-varying channel.
• the various transmitting antennas transmit, on the same carrier and at the same instant, a signal characterized by space-time and / or space-frequency coding, which limits the loss in spectral efficiency. This signal therefore intrinsically comprises the characteristics of the invention.
• a receiver can estimate each of the transmission channels between the different transmit and receive antennas on the basis of this specific coding and of the appropriate processing described above.
• the particular technique of channel estimation proposed according to the invention can moreover be applied to the case of a system having two transmit antennas.

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• 2004
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