EP1593246A1 - Two-dimensional channel estimation for multicarrier multiple input multiple outpout communication systems - Google Patents
Two-dimensional channel estimation for multicarrier multiple input multiple outpout communication systemsInfo
- Publication number
- EP1593246A1 EP1593246A1 EP03708100A EP03708100A EP1593246A1 EP 1593246 A1 EP1593246 A1 EP 1593246A1 EP 03708100 A EP03708100 A EP 03708100A EP 03708100 A EP03708100 A EP 03708100A EP 1593246 A1 EP1593246 A1 EP 1593246A1
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- European Patent Office
- Prior art keywords
- stage
- channel estimation
- estimator
- pilot
- transmission
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- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- 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
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- 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
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- 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
- H04L25/022—Channel estimation of frequency response
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- 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
-
- 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
-
- 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 present invention relates to channel estimation for multiple input multiple output communication systems, and in particular to a method and related channel estimator for multiple input multiple output communication systems using multi- carrier modulation schemes.
- channel estimation For multiple input multiple output MIMO communication systems using multi-carrier modulation schemes the received signal after multi-carrier demodulation is typically correlated in two dimensions, i.e. in time and frequency.
- orthogonal frequency division multiplexing OFDM will be referred to as one typical example for mutli-carrier moduilation schemes. The reason for this is that orthogonal frequency division multiplexing OFDM and variants thereof are the most popular multi-carrier modulation schemes.
- Communication systems employing multiple transmit and receive antennas can be used with orthogonal frequency division multiplexing OFDM to improve the communication capacity and quality of mobile radio systems.
- orthogonal frequency division multiplexing OFDM communication systems with multiple transmit antennas such as space-time codes as decibed in A. Naguib, N.Seshadri, and A. Calderbank: "Space Time Coding and Signal Processmg for High Data Rate Wireless Communications", IEEE Signal Processing Magazine, pp. 76-92, May 2000 or spacial multiplexing
- different signals are transmitted form different transmit antennas simultaneously. Consequently, the received signal is the superposition of these signals, which implies challenges for channel estimation.
- Channel parameters are required for diversity combining, if space-time codes are used or alternatively for separation of superimposed signals if spatial multiplexing is used.
- estimators based on the least squares LS and minimum mean squared error MMSE criterion for OFDM-MIMO systems have been systematically derived in Y. Gong and K. Leta/ef: "Low .Ran Channel Estimation for Space-Time Coded Wideband OFDM Systems", Proc. IEEE Vehicular Technology Conference (VTC'2001-Fall), Atlantic City, USA, pp. 722-776, 2001.
- Related solutions deal with one dimensional approaches where a known pilot OFDM symbol is followed by L data bearing OFDM symbols. This scheme is applicable for a quasi-static environment where the channel does not change significantly during L OFDM symbols, i.e. indoor systems such as wireless local area networks WLAN.
- the receiver may switch to decision directed channel estimation during the reception of the L data bearing OFDM symbols, as suggested in Y Li, N. Seshadri, and S. Aviyavisitakul: “Channel Estimation for OFDM Systems with Transmitte Diversity in Mobile Wireless Channels", IEEE Journal of Selected Areas on Communications, Vol 17., pp. 461-470, March 1999 and Y. Li: "Simplified Channel estimation for OFDM Systems with Multiple Transmit Antennas'", IEEE Transactions on Wireless Communications, Vol I., pp. 67-75, January 2002.
- decision directed channel estimation which uses prior decisions of data symbols as pilot symbols, is significantly more complex than channel estimation schemes relying on pilots only.
- two dimensional channel estimation utilizing a scattered pilot grid can be employed, which satisfy the sampling theorem in time and frequency.
- pilot-symbol aided channel estimation PACE known pilot symbols are multiplexed into the data stream. Interpolation is used to obtain the channel estimate for the information carrying symbols.
- PACE for single carrier systems was introduced in J.K. Cavers: "An analysis of Pilot Symbol assited Modulation for Rayleigh Fading Channels", IEEE Transactions on Vehicular Technology, Vol. VT-40, pp. 686-693, November 1991. In R. Nils- son, O. Edfors, M. Sandell, and P.
- Boerjesson "An Analysis of Two-Dimensional Pilot-Symbol Assisted Modulation for OFDM", Proc. IEEE Intern. Conf. on Personal Wireless Communications (ICPWCV7), Mumbai (Bombay), India, pp. 71-74, 1997 and P. Hoeher, S. Kaiser, and P. Robertson: "Two-Dimensional Pilot- Symbol-Aided Channel Estimation by Wiener Filtering", Proc. IEEE Intern. Conf. on Acoustics, Speech, and Signal Processing.(ICASSP'97), Kunststoff, Germany, pp. 1845-1848, 1997 two-dimensional 2D filtering algorithms have been proposed for pilot-symbol aided channel estimation PACE. However, such a 2D estimator structure is generally too complex for practical implementation.
- the object of the present invention is to extend the concept ot two dimensional channel estimation to MLMO systems.
- this object is achieved through a method of two dimensional channel estimation for multiple input multiple output transmission systems using multicarrier modulated transmission signals impinging from a plurality of transmit antennas and carrying a two dimensional data sequence with embedded pilot symbols.
- the plurality of transmit antennas is divided into disjoint transmission antenna subsets.
- impinging pilot sequences are seperated in relation to transmission antenna subsets by performing a first stage channel estimation to yield tentative estimates of a channel response in a first dimension of transmission.
- impinging pilot sequences are seperated in relation to antennas in transmission antenna subsets by performing a second stage channel estimation for each antenna in each transmission antenna subset to yield an estimation of the channel response.
- An important advantage of the present invention is increased fiexibilty in channel estimation.
- the reason for this is that by dividing the separation task of the superimposed transmission signals in time and frequency direction, a more efficient usage of the pilot symbols is possible. Hence, either the number of required pilot symbols may be reduced or the performance can be improved.
- another important advantage of the present invention is the increase in the number of transmit antennas which can be estimated with a certain number of pilot symbols through application of a two stage channel estimaiton approach.
- Yet another important advantage of the present invention is that the two stage channel estimation approach allows for tracking of channel variations even at high Doppler frequencies. This is a prerequisite to support high velocities of mobile users and therefore to enable truely mobile multiple input multiple output MLMO communication systems.
- the first stage channel estimation is performed using pilot sequences arranged as a two dimensional grid of pilot symbols, wherein pilot symbols used for first stage channel estimation depend on the first dimension of transmission only and pilot symbols used for second stage channel estimation depend on a second dimension of transmission only.
- pilot sequences are expressed in a product form for achieving seperability of pilot sequences in the first dimension of transmission and the second dimension of transmission.
- An advantage of this preferred embodiment of the present invention is that utilization of a scattered pilot grid allows for efficient use of pilot symbols. Further, by matching the pilot spacing in time and frequency to the worst case channel characteristics higher mobile velocities can be supported with respect to conventional one dimensional schemes.
- pilot spacing having a value of one the first stage channel estimation and/or the second stage channel estimation is achieved in a non-interpolating manner through yield of tentative estimates in relation to pilot symbol grid positions in the dimension of estimation.
- pilot spacing having a value larger than one the first stage channel estimation and/or the second stage channel estimation is achieved in an interpolating manner through yield of tentative estimates for all data sequence grid positions in the dimension of estimation.
- An important advantage of this preferred embodiment is flexible support of different two dimensional pilot grids.
- the present invention may be flexibly applied using any type of pilot spacing, both, in frequncy and time direction the application of suitable interpolation techniques.
- Further preferred embodiments of the present invention relate selection of first dimension of transmission for the first channel estimation stage and the second channel estimation stage - i.e., in frequence direction or in time direction - and further to the selection of channel estimation domain - i.e., frequency domain channel estimation or time domain channel estimation.
- channel estimation domain i.e., frequency domain channel estimation or time domain channel estimation.
- the free selectability of dimension of transmission and channel estimation domain is further reason for flexibility of the channel estimation approach according to the present invention. It enables optimal consideration of multi-carrier related transmission parameters, selected pilot grid structure, and also application of com- putationanally most suitable channel estimation techniques.
- the channel estimation approach is applied to a celluar communication system with a frequency reuse factor of one such that base stations and related anntenna arrays form the plurality of transmit antennas and such that transmission antenna subsets and related transmission antennas are defined in relation to this plurality of transmit antennas.
- An important advantage of this preferred embodiment of the present invention is the application of the two stage channel estimation techiques as outlined above to distributed antennas. In particular, it allows to handle a situation where a mobile user roams at a cell border. While data bearing symbols can be protected against interference using a channel code or spreading, this is not possible for pilot symbols. According to the present invention through appropriate definition of subset in relation to cells in the celluar communication system.
- a computer program product directly loadable into the internal memory of a channel estimator for estimating multiple input multiple output transmission channels in two dimensions comprising software code portions for performing the steps of the method of two dimensional channel estimation according to the present invention when the product is run on a processor of the channel estimator.
- the present invention is also provided to achieve an implementation of the inventive method steps on computer or processor systems.
- such implementation leads to the provision of computer program products for use with a computer system or more specifically a processor comprised, e.g., in a channel estimator for estimating multiple input multiple output transmission channels in two dimensions.
- the programs defining the function of the present invention can be delivered to a computer/processor in many forms, including, but not limited to information permanently stored on non-writeable storage media, e.g., read only memory devices such as ROM or CD ROM discs readable by processors or computer I/O attachments; information stored on writable storage media, i.e. floppy discs and hard drives; or information convey to a computer/processor through communication media such as local area network and/or telephone networks and/or Internet or other interface devices. It should be understood, that such media when carrying processor readable instructions implementing the inventive concept represent alternate embodiments of the present invention.
- Fig. 1 shows a schematic diagram of an OFDM based multiple input multiple output MIMO communication system for explanation of the system model under- lying the present invention
- Fig. 2 shows a schematic diagram illustrating OFDM modulation and demodulation, respectively
- Fig. 3 shows a scattered pilot grid suitable for two dimensional channel estimation according to the present invention
- Fig. 4 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of mutlicarrier communication systems according to the present invention
- Fig. 5 shows a flowchart of operation of the channel estimator shown in Fig. 5;
- Fig. 6 shows a schematic diagram illustrating the principle of 2xlD channel estimation underlying the present invention
- Fig. 7 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of mutlicarrier communication systems according to the present invention, wherein channel estimation is performed in frequency direction first;
- Fig. 8 shows a further scattered pilot grid suitable for two dimensional channel estimation according to the present invention
- Fig. 9 shows a further scattered pilot grid corresponding to an digital video broadcast DVB-T application and suitable for two dimensional channel estimation according to the present invention
- Fig. 10 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of mutlicarrier communication systems according to the present invention, wherein channel estimation is per- formed in time direction first;
- Fig. 11 shows a schematic diagram of an estimator stage adapted to achieve channel estimation in the time domain according to the present invention
- Fig. 12 shows a schematic diagram of a further estimator stage adapted to achieve channel estimation in the time domain according to the present invention.
- Fig. 13 shows an application of the two stage channel estimation approach according to the present invention to a cellular communication system with a frequency reuse factor of one.
- the present invention addresses pilot-symbol aided channel estimation PACE schemes for multi-carrier multiple input multiple output MIMO communication systems which are based on the insertion of a pilot grid.
- pilot-symbol aided channel estimation PACE - also referred to as PACE in the following - is applied across subcarriers in frequency direction and over several multi-carrier transmission symbols - e.g., OFDM symbols - in two dimensions, resulting in two dimensional 2D PACE.
- the present invention as explained in the following is not restricted to a par- ticual type of multi-carrier multiple input multiple output MLMO communication system, and may be applied, e.g., to orthogonal frequency divsion multiplexing OFDM, discrete multitone transmission DMT, filtered multitone transmission FMT, or biorthogonal frequency division multiplexing BFDM.
- multi-carrier multiple input multiple output MLMO communication system is multi-carrier code divsion mutliple access MC-CDMA where spreading in frequency and/or time direction is introduced in addition to the multi-carrier modulation.
- multi-carrier multiple input multiple output MIMO communication system is a multi-carrier code divsion mutliple access MC-CDMA system with a variable spreading factor, namely variable spreading factor orthogonal frequency and code division multiple access VSF-OFCDM.
- pilot symbol aided channel estimation PACE may be applied to all multi-carrier multiple input multiple output MLMO communication systems operating with transmission signals being correlated in two dimensions. Therefore, all these multi-carrier multiple input multiple output MLMO communication systems may employ the different embodiments of the present invention as explained in the following.
- Fig. 1 shows a schematic diagram of an OFDM based multiple input multiple • output MLMO communication system - also referred to as OFDM system in the following - for explanation of the system model underlying the present invention.
- Fig. 2 shows a schematic diagram illustrating OFDM modulation and demodulation in correspondence to the OFDM based multiple input multiple output MIMO communication system shown in Fig. 1.
- the signal stream is divided into N c parallel substreams, typically for any multi-carrier modulation scheme.
- the i th substream, also referred to as subcarrier in the following, of the £ th symbol block, named OFDM symbol in the following, is denoted by X iti .
- An inverse DFT with Nppr points is performed on each block, and subsequently the guard interval having noisy samples is inserted to obtain x i ⁇ n .
- the signal x(t) is transmitted over a mobile radio channel with response h(t, r).
- the received signal at receive antenna v consists of superimposed signals from N ⁇ transmit antennas.
- the received signal of the equivalent baseband system impinging at receive antenna v at sampling instants t — [n + lN Sym ⁇ T Sp i is in the form
- the guard interval is removed and the information is recovered by performing an DFT on the received block of signal samples, to obtain the output ofthe OFDM demodulation Y ⁇ -.
- the received signal at receive antenna ⁇ after OFDM demodulation given by
- the present invention considers a time- variant frequency selective fading channel.
- the number of non-zero taps is typically smaller or equal to the maximum delay of the channel, Q 0 ⁇ Q.
- the channel impulse response CIR between transmit antenna ⁇ and receive antenna v is defined by
- h ⁇ ⁇ ) (t) and r ⁇ - ⁇ ) are the complex amplitude and delay of the q th channel tap.
- the channel taps h ⁇ » ' ⁇ ) (t) are zero-mean complex independent identically distributed (i.i.d.) Gaussian random variables. Due to the motion of the vehicle h ⁇ , v) (t) will be time-variant caused by the Doppler effect.
- the g th channel tap h£' v) (t) is a wide sense stationary WSS Gaussian process, being band-limited by the maximum Doppler frequency v max .
- T l "'" represents the transformation matrix which transforms h ⁇ '" 5 into the frequency domain, defined by
- the guard interval is longer than the maximum delay of the channel, i.e. N GI > Q, where Q > Q 0 denotes the total number of channel taps, the orthogonality at the receiver after OFDM demodulation is maintained, and the received signal of equation (2) is obtained.
- the channel estimation according to the present invention will be performed independently for each antenna. Therefore, in the description to follow the marker for the receive antenna v is omitted.
- a channel is defined to be sample spaced if the tap delays ⁇ q are multiples of the sample instant T sp , i.e.
- Hf- ⁇ can be expressed in matrix notation
- F represents the DFT-matrix of dimension N G ⁇ x N c , defined by
- ⁇ F ⁇ n> . e ⁇ j2 ⁇ ni/NpFr ; 0 ⁇ i ⁇ N c - 1 , 0 ⁇ n ⁇ N GI ⁇ 1 (9)
- Fig. 3 shows a scattered pilot grid suitable for two dimensional channel estimation according to the present invention.
- pilot aided channel estimation PACE is based on periodically inserting known symbols, termed pilot symbols in the data sequence. If the spacing of the pilot symbols is sufficiently close to satisfy the sampling theorem, channel estimation and interpolation for the entire data sequence is possible.
- pilot aided channel estimation in the sense of the present invention it must be taken into account that for OFDM the fading fluctuations are in two dimensions, in time and frequency. In order to satisfy the two-dimensional sampling theorem, the pilot symbols are therefore scattered throuout the time- frequency grid, which yields a two-dimensional pilot grid. Another reason for selecting scattered pilot grids is to maximize spectral efficiency. Description of Pilot Grids in Two Dimensions
- Np ⁇ £ • Ng
- the following notation is used: given a matrix describing a 2D structure X, the subsets which describe the dimension corresponding to the frequency and time directions are denoted by X' and X", respectively.
- p [z, £ ⁇ ⁇ denotes the index of the i th and I th pilot in frequency and time direction, respectively, and g 0 defines a pilot grid offset.
- every transmission signal at transmit antenna may to use its own pilot grid. This enables the receiver to separate the superimposed transmission signals from different transmission antennas.
- the structure of the pilot grid is defined by G which is
- A denotes the so-called pilot spacing in frequency and time, respectively.
- the pilots may be multiplied by an outer pilot sequence ⁇ X ⁇ .- ⁇ which is identical for all transmit antennas to yield the transmitted pilot sequence
- the outer pilot sequence ⁇ J 0 - , ⁇ is chosen to have a low peak to power average ratio in the time domain and/or to have good correlation properties for synchronization.
- the cyclic prefix is removed and an FFT is performed to yield the received signal after OFDM demodulation.
- the received signal Y ⁇ of equation (2) is obtained.
- the received signal at the pilot positions are demultiplexed from the data stream, and after removing the outer pilot sequence, by dividing through Xo-cured the received pilot is obtained according to
- Fig. 4 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of mutlicarrier communication systems according to the present invention.
- the channel estimatior 10 comprises a first estimator stage 12 and a second estimator stage 14. Further, the channel estimator comprises a transmission antenna subset memory 16.
- Fig. 5 shows a flowchart of operation of the channel estimator shown in Fig. 4.
- operation of the channel estimator according to the presen invention relies on a method where in a firsat step S10 the plurality of transmit antennas into disjoint transmission antenna subsets. Then, in a step S 12 impinging pilot sequences in relation to transmission antenna subsets are seperated by performing a first stage channel estimation to yield tentative estimates of a channel response in a first dimension of transmission. Then, in a step S14 impinging pilot sequences in relation to antennas in transmission antenna subsets by performing a second stage channel estimation for each antenna in each transmission antenna subset to yield an estimation of the channel response.
- Fig. 6 shows a schematic diagram illustrating the principle of channel estima- tiong according to the present invention.
- step S12 channel estimation is performed in one dimension, yielding tentative estimates for all subcarriers of these OFDM symbols. These tentative estimates are then used in step S14 as new pilots, in order to estimate the channel for the entire frame.
- the second stage does not only interpolate between OFDM symbols having pilots, but it does also improve the accuracy of the tentative esti- mates.
- Either channel estimation in frequency direction or time direction may be performed first.
- Reference to the case where channel estimation in frequency direction is performed first will be made by 2 x ID-PACE type I in the following. Further, reference to the case where channel estimation in time direction is performed first will be mde by 2 x ID-PACE type II in the following.
- the received pilot of OFDM symbol £D t is considered at the (z ⁇ D/) th subcarrier
- Ni Dtji jo f accounts for additive white Gaussian noise AWGN.
- L represents the number of OFDM symbols per frame
- N c is the number of subcarriers per OFDM symbol
- N ⁇ is the number of transmit antennas.
- the overall object underlying the present invention is to estimate H for all ⁇ £, i, ⁇ within the frame.
- the basic concept underlying the present invention is to divide the separation task into two stages, in the way that in step S12 channel estimation is performed in one dimension, at OFDM symbols £ — £D t , yielding tentative estimates for all subcarriers of these OFDM symbols.
- the second step S14 uses these tentative estimates as new pilots, in order to estimate the channel for the entire frame.
- the second stage estimation does not only interpolate between OFDM symbols having pilots, but it does also improve the accuracy of the tentative estimates. Therefore, the different embodiments of the present invention as explained in detail in the following have significantly reduced complexity while there is little degradation in performance.
- channel estimation in frequency direction is performed first, to reverse the order such that channel estimation in time direction is performed first is straightforward.
- channel estimation in frequency direction is performed first by 2 x ID-PACE type I.
- reference to the case where channel estimation in time direction is performed first will be made to as 2 x ID-PACE type II.
- prediction type filtering does not require any buffering, however, the performance with respect to smoothing degrades.
- channel estimation in frequency direction on the other hand, all pilots of one OFDM symbol are being received together, so no buffering is required. However, the accuracy of the channel estimates typically degrades near the band edges.
- Fig. 7 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of mutlicarrier communication systems according to the present invention, wherein channel estimation is performed in frequency direction first.
- the present invention it is prposed to extend 2 x ID-PACE to OFDM-based MLMO channel estimation.
- MIMO channel estimation it is necessary to separate the impinging signals from N ⁇ transmit antennas.
- the MIMO system having N ? transmit antennas be denoted by set the A.
- N transmit antennas For a MIMO system having N transmit antennas, according to the present invention it is proposed to group the signals corresponding to N transmit antennas into one set, to form N ⁇ subsets of A. Without restricting scope of protection, one may assume that all sets have the same number of transmit antennas, and the subsets A ⁇ are disjoint, i.e. each transmit antenna can only be in one set.
- the concept underlying tne present invention is to divide the separation task into two stages, in the way that we first separate a subset of the N ⁇ ⁇ N T signals together with channel estimation in the first estimation stage.
- the remaining N - 2 superimposed signals are seperated for each of the N ⁇ ⁇ signals of the first estimation stage, together with channel estimation in the second dimension, to yield the estimate of the frequency response H ..
- Fig. 7 illustrates the basic idea for type I of the proposed scheme.
- the buffer shown in Fig. 7 is used in order to apply smoothing type filtering in time direction.
- the present invention is applicable to, both, smoothing and prediction type filtering, so the buffer shown in Fig. 7 is optional and depends on the particular channel estimation algorithm being used.
- Fig. 8 and 9 show pilot sequence designs which may be used to support the two stage approach for OFDM-based MLMO channel estimation accordance to the present invention.
- the pilot grid shown in Fig. 9 corresponds to the DVB-T pilot grid according to ETSI EN 300 744, V 1.4.1 (2001-01).
- Further standards - however, not to be considered as restricting scope of protection - would be IEEE 802.1 la or ETSI TS 101 475 HIPERLAN/2.
- the pilot sequence of transmit antenna ⁇ is defined by ⁇ Xf! ⁇ .
- pilot symbol of the first stage X[ ⁇ ° only depends on the subcarrier index i, while the pilot symbol of the second stage X 2 . 2) only depends on OFDM symbol I.
- the pilot sequences ⁇ X ⁇ 1 ⁇ and ⁇ X 2 . 2) ⁇ are chosen from orthogonal designs, e.g., Walsh sequences or phase shifted sequences.
- H Li is the frequency response of transmit antenna ⁇ .
- the task of the first estimation stage is to estimate ⁇ °; that is to separate the Nyi groups, and then to estimate and interpolate the channel in frequency direction.
- pilot sequence X 2 t 2) is constant with respect to the subcarrier index i.
- Z - is a superposition of N 2 waveforms H multiplied with a constant phase term X 2 ⁇ A
- the channel is estimated in time direction to separate the remaining N ?2 signals per subset to yield the estimate of the frequency response H l-
- Fig. 10 shows a schematic diagram of a channel estimator for estimating multiple input multiple output transmission channels of multicarrier communication systems according to the present invention, wherein channel estimation is performed in time direction first.
- the major difference of 2 x ID-PACE type II over 2 x ID- PACE type I is that that the separation JV- ⁇ subsets in the first estimation stage is performed in conjunction with channel estimation in time direction. This yields for the pilot symbols of 2 x ID-PACE type II:
- the received pilot sequence of subset A ⁇ x is defined by
- the task of the first estiamtion stage is to estimate Z ⁇ that is to separate the N ⁇ groups, and then to estimate and interpolate the channel in time direction, i.e. over the £ variable.
- the first step is to estimate the channel in the frequency direction.
- the received pilot sequence of OFDM symbol £ becomes
- the transmitted pilot sequence, the received pilot sequence of subset A ⁇ , and the additive noise term, of OFDM symbol £ transmitted from antenna ⁇ are given by
- channel estimation in the frequency domain is preferably performed with an FIR interpolation filter, which can be expressed for the first stage in frequency direction
- the filter W ( ⁇ , [i] depends on the location of the desired symbol, i.e. the subcarrier index i. This means that not only for every transmit antenna but also for every subcarrier a different filter is required.
- Wiener interpolation filter for W' C ⁇ ) [i].
- a Wiener filter minimizes the means squard error MSE between the pilots sequence and the desired response. It is also known as the minimum MSE or equivalently MMSE estimator.
- the covariance matrix of the pilots in frequency direction is defined by R ⁇ .
- ⁇ E Y j ' Y ⁇ ].
- the entry of the th row and n th column of the covariance matrix is given by
- Equation (20) and equation (21) are necessary to evaluate the Wiener interpolation filter.
- the optimum solution in the MMSE sense may be determined using the Wiener-Hopf according to
- a positive A£ imposes a time delay of AE symbols at the receiver output.
- the estimation filter is a smoothing type filter.
- ZJ / t ⁇ [Z[ ⁇ f, ⁇ ⁇ , Z ) ⁇ denotes the block Ng' outputs of stage one of subcarrier i.
- the optimum approach to estimate H is to use a Wiener interpolation filter for w" i ⁇ ) [£, A£].
- the entry of the th row and n th column of the covariance matrix is given by
- Equation (20) and equation (21) are necessary to evaluate the Wiener inte ⁇ olation filter.
- Equation (20) and equation (21) are necessary to evaluate the Wiener inte ⁇ olation filter.
- the optimum solution in the MMSE sense is derived using the Wiener-Hopf equation according to
- Fig. 11 shows a schematic diagram of an estimator stage adapted to achieve channel estimation in the time domain according to the present invention.
- an alternative approach to determine the frequency response He t i is to estimate the channel impulse response (CIR), ti i ⁇ Tl ⁇ ) in the time domain first.
- CIR channel impulse response
- F denotes an N p -point DFT matrix defined by
- the received pilot sequency after OFDM demodulation is given by
- the received pilot sequency after OFDM demodulation is transformed into the time domain.
- time domain channel estimation we choose to pre-multiply ⁇ by the transmitted pilot sequence X and then to transform the result into the time domain via an Np-point IDFT, that is
- the least squares (LS) estimator may be determined according to
- the estimator depends on the transmitted signal, the pilot sequence should be properly chosen.
- the LS estimator exists if D ⁇ is full rank, unfortunately this is not always the case. A necessary condition for the LS estimator to exist is
- two times over- sampling provides a good trade-off between minimizing the system overhead due to pilots and optimizing the performance, i.e. Np « 2N T ⁇ Q. It is assumed that Nor ⁇ Q, i-e. the guard interval is longer than the maximum delay of the channel.
- the LS estimator for more than one transmit antenna does only exist in the time domain.
- the MMSE estimator is given by an FIR filter which is for time domain channel estimation
- the Wiener filter is determined by the Wiener-Hopf equation
- the Wiener filter w' i ⁇ [n] depends on the location of the desired symbol n.
- the correlation matrices R ⁇ and R' ⁇ 1 J [ra] are required
- Furtermore, R' ⁇ 'f ] is row n+ ( ⁇ x -1)Q of R z5 .
- the covariance matrix in the time domain R is related to the covariance matrix in the frequency domain R ⁇ g by
- the separation of the N ⁇ signals which is performed by the LS estimator, can be separated from the filtering task.
- the MMSE estimator is in general dependent on the choice of the pilot symbols. However, choosing orthogonal pilot sequences X ⁇ 1 ' the estimator becomes independent on the transmitted pilots. For orthogonal pilots where ⁇ ⁇ jTn denotes the Kronecker symbol, it can be shown
- Fig. 11 shows a block diagram of channel estimation and interpolation in the time domain using orthogonal pilot sequences.
- the received pilot sequence is split into N r ⁇ branches and each branch pre-multiplied by X x ⁇ .
- each branch of the received pilot sequence is transformed to the time domain.
- zero padding of the first stage estimate extends its lenght to N c samples.
- the estimate of the CTF of an entire OFDM symbol (pilots and data), is obtained by an N c -point FFT of the CIR estimate
- Fyy T1 is a N Ti N c x N ⁇ N c block diagonal matrix, consisting of N ⁇ blocks of N c -point DFT matrices F.
- the output of the first stage Z' ' can be fed to the second stage estimator in equation (23).
- z' ( ⁇ ) may be fed into (23) to yield the CIR estimate h j which is then transformed into the frequency domain with N Tl FFTs.
- channel estimation of the second stage: in the Doppler domain is possible, that is Z' 1 or z ( ⁇ ) are transformed into the Doppler domain using equivalent algorithms as in the time domain.
- Fig. 12 shows a schematic diagram of an estimator stage adapted to achieve channel estimation in the time domain according to the present invention.
- the channel estimation of the second stage may also be performed with DFT-inte ⁇ olation cooresponding to the first estimation stage discribed above. However, it may be computationally more efficient to perform the second estimation stage in the time domain as well, i.e., before zero padding and the N c -point FFT.
- Fig. 13 shows an application of the two stage channnel estimation approach according to the present invention to a cellular communication system with a frequency reuse factor of one.
- the proposed scheme can be applied to distributed antennas as well.
- an application is to employ 2xlD-PACE to a celluar system with a frequency reuse factor of one.
- the user will receive the desired signal from one base station and one or several interfering signals from other base stations.
- each base station has N T2 antenna elements. While the data bearing symbols can be protected against interference using a channel code or by spreading, the pilot symbols cannot be protected in this way. Accurate channel estimation, however, is most important for the system to work efficiently. One solution is to boost the pilots; this however will increase the interference to users served by other base stations, and thus limits the system capcity.
- 2 x ID-PACE can be applied to this scenario as follows: the base stations form N ⁇ subsets, each subset having an antenna array with N T2 antenna elements, to form an resulting array of N ⁇ — Nr ⁇ N T 2 elements. This would require inter-cell synchronization.
- IDFT Inverse discrete Fourier transform
- IFFT Inverse fast Fourier transform
- LS Least squares MIMO
- N c Number of subcarriers.
- N GI Number of samples of the guard interval.
- L Number of OFDM symbols per frame.
- T sp ⁇ Sample interval, given by T spi T/N F F ⁇ -
- T sym Total OFDM symbol duration including the guard interval T spl — T+NGIT SP1 .
- QQ Number of non-zero channel taps Q Total number of channel taps.
- NR Number of receive antennas NT Number of transmit antennas.
Abstract
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PCT/EP2003/001495 WO2004073276A1 (en) | 2003-02-14 | 2003-02-14 | Two-dimensional channel estimation for multicarrier multiple input outpout communication systems |
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