Classifications

• • 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
• • 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
• • 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
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only

Definitions

• the subject of the present invention is a method of optimal estimation of a propagation channel based solely on the pilot symbols and a corresponding estimator. It finds its application in radiocommunications, in particular for multiple access and modulation techniques of the OFDM type [1], [2], [3] (orthogonal frequency multiplexing or "Orthogonal Frequency Division Multiplexing"), or TDMA (access time division multiple) or CDMA (code division multiple access). It can be applied, among others, to HIPERLAN II, DAB [2] and DVB-T [3] systems for OFDM, IRIDIUM (Registered trademark) and ICO for TDMA, UMTS and CDMA-2000 for CDMA .
• OFDM type [1], [2], [3] orthogonal frequency multiplexing or "Orthogonal Frequency Division Multiplexing"
• TDMA access time division multiple
• CDMA code division multiple access
• TDMA time division
• GSM Global System for Mobile communications
• DECT the GSM and DECT systems for the terrestrial radio and systems IRIDIUM (registered trademark) and ICO for satellite radio communications;
• CDMA systems with, for example, the IS'95, UMTS and CDMA-2000 systems for terrestrial radiocommunications and the GLOBALSTAR (Trademark) system for satellite and time radiocommunications.
• the IS'95, UMTS and CDMA-2000 systems for terrestrial radiocommunications
• the GLOBALSTAR (Trademark) system for satellite and time radiocommunications.
• OFDM systems belonging to the first category use multi-carrier modulation to distribute the users in the time-frequency plane. They allow high speed signals to be transmitted without the need for equalizers. They are widely used in broadcasting contexts such as DVB-T and DAB and in mobile radio contexts such as HIPERLAN II.
• the basic principle of OFDM is to produce a number of narrowband signals all orthogonal to each other. These orthogonality properties are then used by each receiver to find the corresponding transmitted data.
• a typical embodiment of an OFDM system implements an inverse discrete Fourier transform (TFDI) on transmission and a discrete Fourier transform (TFD) on reception.
• FIG. 1 attached illustrates a conventional OFDM transmission chain with a single sensor.
• This chain comprises a series-parallel conversion circuit 10 receiving symbols A, a reverse discrete Fourier transformation circuit 12, transmission means 14, reception means 20, a Fourier transformation circuit 22, a parallel-to-serial converter 24 and finally a decision means 26 which delivers the estimated symbols A.
• a classic OFDM modulator processes the data flow in blocks. It manages this flow in sequences of N symbols and performs the inverse Fourier transform. This amounts to saying that this transform generates N subcarriers, each carrying one of the symbols of the starting sequence.
• This block, called the OFDM symbol can contain both data symbols and pilot symbols used for synchronization and channel estimation purposes. Unlike conventional CDMA or TDMA signals, OFDM often requires a distribution of the pilot symbols over the entire time-frequency plane.
• the radiomobile channel crossed during a communication between a transmitter and a receiver is generally of the multipath type with rapid Rayleigh fading. This phenomenon is generally due to the conjunction of the movement of the mobile and the propagation of the radio wave along several paths. It can also be produced artificially by a broadcasting pattern of "1" used for the digital broadcasting system DVB-T (which is a priori impossible to achieve in analog broadcasting).
• the receiver can be fitted with L sensors providing L branches of diversity as shown in the attached FIG. 2.
• the receiver comprises L sensors 30 1 , 30 2 , ..., 30 L , L Fourier transformation circuits, 32 1 , 32 2 , ..., 32 L , L parallel-series converters 34 1 , 34 2 , ..., 34 L , L channel estimation circuits 36 1 , 36 2 , ..., 36 L , L re-phase circuits 38 1 , 38 2 ,. .., 38 L and an adder 40 delivering the weighted outputs of the symbols to be estimated.
• the channel affecting a time-frequency block can be represented in the form of a time-frequency matrix, or even a surface in time-frequency-amplitude space.
• the problem is therefore treated in a two-dimensional space, unlike TDMA where the problem is one-dimensional.
• Channel estimation is based on the use of pilot symbols. These make it possible to directly obtain an estimate of the channel of each diversity branch at the locations of the pilots with a view to an interpolation or an extrapolation making it possible to estimate the channel affecting the data symbols.
• TDMA systems belonging to the second category, are currently limited to the treatment of two extreme cases: channels that are very selective in frequency (very large delay spreading) but not selective in time (negligible Doppler spreading) and channels that are not selective in frequency but can possibly be very selective in time.
• the first case is often encountered in terrestrial radio systems such as GSM.
• the second is rather encountered in the systems of satellite communications such as ICO and IRIDIUM.
• CDMA systems belonging to the third category, introduce the concept of power control period (PCP).
• PCP power control period
• the signal strength sent by the transmitter remains constant during each PCP but can vary from one PCP to another to counteract slow fading (due to loss of space propagation and mask effects) and fading fast due to the effects of multiple paths (time selectivity).
• the conventional receiver of a CDMA system first performs a suitable filtering of the received signal.
• the signal thus obtained is then despread and continued for each of the significant power paths retained by the receiver.
• each path is estimated at the level of each PCP by correlation of the pilot symbols with the despread samples associated with both this path and this PCP. This estimate is then used in the rest of the CFP to demodulate the data symbols for each of the paths and recombine them for decision-making.
• This is the principle of the so-called rake receiver ("RARE receiver" in English terminology).
• the estimation of the paths for a given PCP can be consolidated by weighting with those of a finite number of neighboring PCPs.
• the channel seen by the receiver can vary significantly from one time-frequency block to another for OFDM, from one time interval to another for TDMA and from a power control period to 1 other for the AMRC.
• this variation is mainly due to changes in the propagation conditions between the transmitter and the receiver.
• this variation can be further accentuated in frequency with the artificial increase in the spread of the delays T m generated by the use of a diffusion pattern at "1" (in particular as in the case of the DVB-T system).
• variable character of the channel can be characterized by the product B d T m / where B d represents the Doppler spread. The larger this product, the faster the channel varies in time and frequency. In the case of TDMA and CDMA, this variable character can be characterized by the product B d T s , where T s represents the duration of a symbol. The larger this product, the faster the channel varies in time.
• Reception methods prior to the invention do not seek to optimize the estimation of the channel. They are content to carry out an estimation of this at the positions of the pilot symbols and then to extend, by linear interpolation, this estimation to the data symbols.
• a second method uses a simple form of the MMSE ("Minimum Mean Square Error") criterion. It consists in searching for the constant plane by means of the values of the channel at the level of the pilot symbols and in deducing therefrom a constant estimate in time and in frequency of the channel affecting the transmitted data.
• This channel modeling is well suited to channels varying very little at the level of each block received and therefore having relatively small products B d T m . However, as soon as the channel becomes more selective, the plane modeling shows its limits because of the notable bias affecting the channel estimates at the level of the data symbols.
• a third method also uses another form of the MMSE criterion with, this time, a non-constant sought channel estimation plan. This method therefore adapts better to slowly varying channels, but remains less suitable than the second for almost constant channels.
• a fourth method is based on the use of a two-dimensional discrete Fourier transformation. Only the samples corresponding to the pilot symbols in the places they occupy in the block are kept from the received time-frequency block. All other positions, associated with the data symbols, are set to zero. A two-dimensional discrete Fourier transformation is then performed on this modified block. It is then followed by filtering isolating the channel space from the total information obtained in the Doppler-delay plane. Finally, it ends with an interpolation over the entire block carried out by an inverse two-dimensional discrete Fourier transformation. This technique has undesirable side effects and is therefore not suitable for small blocks. v) Fifth method
• a fifth method consists in a representation of the channel by the eigenvectors of its correlation matrix.
• the projection of the signal received on the basis of the eigenvectors is not optimized since the restriction of the eigenvectors to the pilot symbols does not form an orthogonal basis.
• the first three methods are adapted to very specific propagation cases, but in no way to time and frequency selective channels.
• the last two methods can be used in the case of time and frequency selective channels but quickly show their limits for high values of the product B d T m .
• the object of the present invention is precisely to remedy these drawbacks.
• the method proposed by the invention is based on an optimal estimation of a channel, possibly very selective in time and / or in frequency, carried out from a modeling of its evolution in time and in frequency.
• This approach makes it possible to use the pilot symbols of neighboring blocks without any introduction of bias even for channels varying significantly from one block to another.
• the efficiency of this approach can therefore be put to good use by better fighting against channels that are more and more selective in time and frequency, by reducing the number of pilot symbols or even by reducing their power.
• the object of the present invention is to improve the performance of existing or future OFDM, TDMA and CDMA systems.
• This improvement obtained by optimizing the channel estimation, makes it possible to significantly increase capacity and coverage of these systems. It is generated by an optimization of the operation of the conventional receiver in the case of slow fading but also in the more difficult case of very rapid fading. It is further increased if the pilot symbols are judiciously distributed in time for TDMA and CDMA systems and in time-frequency for OFDM systems.
• An object of the invention is to improve the quality of the information received by implementing an optimal method of channel estimation for OFDM, TDMA or CDMA systems. This method can be used regardless of how the pilot symbols are introduced or distributed in the flow of information transmitted.
• the invention makes it possible to reduce, at constant reception quality, the relative number and / or the power of the pilot symbols. This object is achieved by optimally taking into account an arbitrary number of pilot symbols of time-frequency blocks or consecutive time intervals in the channel estimation.
• the invention can be used independently. It can also advantageously be used to optimally initialize certain iterative semi-blind channel algorithms (using both the pilot symbols and the data symbols) often very sensitive to the initial conditions [4], [5], [6], [7].
• This technique also allows, thanks to a theoretical performance formulation, to optimize by exhaustive research the position of the pilot symbols in the time-frequency blocks.
• the subject of the invention is a method for optimal estimation of a propagation channel in which:
• a signal having passed through said channel is received, this signal comprising blocks of symbols either one-dimensional in time or in frequency, or two-dimensional in time and in frequency, each block comprising N digital symbols with N P pilot symbols and N D data symbols ,
• the propagation channel is modeled by a discrete multiplicative channel vector C with N components
• restriction vector R P of this vector limited to the N P pilot symbols is calculated from the signal vector R, - from the discrete multiplicative channel vector C, we define a restriction vector C P limited to the pilot symbols,
• the subject of the invention is also an estimator which comprises the means capable of fulfilling the functions of the method thus defined.
• FIG. 3 illustrates the calculation of the flexible outputs for the data symbols from the estimation of the equivalent channels of the L diversity branches
• FIG. 4 shows the following of the treatments carried out according to the invention for estimating the equivalent channel in - a branch of diversity;
• FIG. 5 shows the main normalized eigenvalues corresponding to the positions of the pilot symbols given in Figure 8B;
• the principle of the invention, shown here for OFDM is to use 'samples pilot symbols in the received signal to achieve an optimal estimate of the multipath channel correspondan.
• the OFDM receiver obtained performs block-by-block processing each time a given number of OFDM symbols is available. It achieves an optimal estimate of the channel only from the pilot symbols. This method is optimal within the meaning of the criterion Due a posteriori (MAP). It can be reformulated simply by using an appropriate weighting of the projections of the samples received corresponding to the pilot symbols on an extended orthonormal basis. This basis is obtained by extending the Karhunen-Loève orthogonal decomposition from the pilot symbols to the remaining data symbols.
• MAP a posteriori
• This method can be implemented as is to obtain an estimate of the channel based solely on the pilot symbols. It can also be used as an initialization phase for an iterative semi-blind channel estimator taking into account the samples of the data symbols. It can obviously be used for a multisensor system by placing such an estimator behind each of the sensors.
• the optimization of the operation of the OFDM receiver is obtained through the use of a multi-path channel estimator with Rayleigh fading exclusively based on the pilot symbols.
• the optimal structure of the estimator is based on the representation of the channel obtained thanks to the Karhunen-Loève orthogonal decomposition theorem.
• that of the invention uses the eigenvectors of the correlation matrix of the channel observed only at the locations of the pilot symbols. This makes it possible to have a set of orthogonal vectors at the locations of these pilot symbols. These vectors are then suitably extended to the data symbols, which allows optimal interpolation of the channel estimation to the data symbols.
• This technique also allows, thanks to a theoretical performance formulation, to optimize by exhaustive research the position of the pilot symbols in the time-frequency blocks.
• the proposed receiver has L independent diversity branches with L sensors (or receiving antennas) sufficiently spaced.
• This receiver processes block by . blocks the signals received in these branches.
• the size of each block processed does not necessarily depend on the number of carriers of the OFDM system and can take into account all or part of one or more OFDM symbols.
• the shape and size of the block processed on reception is free, so as to best adapt to the system.
• the channel estimation is performed block by block for each of the diversity branches taken separately.
• a block is made up of N symbols a ⁇ .
• the multi-path channel associated with each diversity branch seen by the transmitted OFDM signal exhibits temporal and frequency variations due to the Doppler effect and to the multiple paths.
• Each path is characterized by an average power and a Doppler power spectrum (SPD) depending on both the environment and the speed of the mobile.
• SPD Doppler power spectrum
• the multi-path channel of each branch is generally characterized by its time-frequency autocorrelation function ⁇ ( ⁇ £, ⁇ t), where ⁇ f is the frequency spacing and ⁇ t the time spacing.
• ⁇ f the frequency spacing
• ⁇ t the time spacing
• the signal received from each diversity branch is first demodulated by a discrete Fourier transform (TFD).
• TFD discrete Fourier transform
• j j th oxanc ⁇ e ⁇ corresponding to the min symbol is written: "min ⁇ 4n ⁇ rmn" " ⁇ mn m
• c l n is the gain factor of the discrete spindle th channel seen by _ a m th symbol
• N ⁇ is a white Gaussian noise variance of N 0 complex additive.
• the gain factors within the same branch of diversity are correlated with each other in time and frequency. However, the gain factors belonging to branches of different diversity are uncorrelated with each other.
• the object of the invention is to estimate the gain factors ( ⁇ min of the channels of all the diversity branches as well as to optimize the positions (mF, nT), of the pilot symbols.
• indexing function ⁇ P (k) between the one-dimensional set the two-dimensional indexing set S P corresponding to the only pilot symbols.
• indexing function ⁇ D (k) between the one-dimensional set and the two-dimensional indexing set S D corresponding to the only data symbols.
• RJ ⁇ (k) C ⁇ (k) A ⁇ (k) + N J ⁇ [k) where C ( y is the k th component of the equivalent multiplicative discrete channel vector: c ••• >
• MDP4, MDP8, Considering their real parts for two-state MDP2 modulation. These operations are illustrated in FIG. 3 where we see L branches respectively receiving signals R °, ..., R 3 ' , • • -, R L_1 , L estimation circuits of the equivalent channel referenced 36 1 ,. .., 36 3 , ..., 36 L_1 and delivering the estimates C ⁇ y, L 38 ° phase resetting circuits, ..., 38 3 , ..., 38 L delivering the products:
• FIG. 4 illustrates more precisely the operations implemented for optimally estimating the equivalent channel for each branch of diversity.
• the letter R represents the signal vector received and applied to a circuit 50 for restricting this vector to the only pilot symbols. This circuit therefore delivers a vector denoted R P.
• a circuit 52 projects this vector onto a base comprising N P vectors and denoted ⁇ Bp ⁇ ⁇ 1 . The projection shows the components: These N P components are applied to a circuit 54 to reconstruct a channel estimation vector noted.
• the circuit 54 finally delivers the estimated channel vector C.
• G —B p j R P , K. - U, J, ..., N p —1, represents the decomposition of the restriction of the vector received, R P , in the orthonormal base ⁇ Bp ⁇ ⁇ 1 and the weighting factors w f , £ -0, 1, ..., N P -1, are given by:
• Q ⁇ N P be the number of significant normalized eigenvalues associated with weighting factors close to unity.
• the estimator calculates in a simple way and with very good precision the approximation C of C given by:
• the optimal estimation of the channel according to the invention is based solely on the pilot symbols of a received block. It can be used as is to demodulate and re-phase the contributions of all the diversity branches. It can also be used as an optimal initialization for several iterative semi-blind channel estimation algorithms based on all the symbols of a received block (pilot symbols and data symbols) [4], [5], [6], [7]. v) Optimization of the positions of the pilot symbols
• V k - P ⁇ D (k))> ••• > V E ⁇ p (Np-l) E ⁇ D (k) ⁇ (P ⁇ p (N P -l) ⁇ P ⁇ D (k))) T
• the optimal estimation method according to the invention can be applied to a system
• FIG. 8A presents the results obtained for the criterion of average gross bit error rate.
• Figure 8B presents the same results for the worst gross bit error rate criterion.
• the same figure also shows the performances obtained with perfect knowledge of the channel (curve 84).

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• Mobile Radio Communication Systems (AREA)
• Time-Division Multiplex Systems (AREA)

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