WO2006112030A1 - Wireless reception apparatus and wireless reception method - Google Patents

Wireless reception apparatus and wireless reception method Download PDF

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Publication number
WO2006112030A1
WO2006112030A1 PCT/JP2005/007570 JP2005007570W WO2006112030A1 WO 2006112030 A1 WO2006112030 A1 WO 2006112030A1 JP 2005007570 W JP2005007570 W JP 2005007570W WO 2006112030 A1 WO2006112030 A1 WO 2006112030A1
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state information
channel state
reception
section
channel
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PCT/JP2005/007570
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French (fr)
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Rahul Malik
Pek Yew Tan
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Matsushita Electric Industrial Co., Ltd.
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/02Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0613Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
    • H04B7/0615Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
    • H04B7/0619Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal using feedback from receiving side
    • H04B7/0636Feedback format
    • H04B7/0643Feedback on request
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/02Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413MIMO systems
    • H04B7/0417Feedback systems
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/02Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0613Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
    • H04B7/0615Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
    • H04B7/0619Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal using feedback from receiving side
    • H04B7/0621Feedback content
    • H04B7/0626Channel coefficients, e.g. channel state information [CSI]
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/02Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/08Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
    • H04B7/0837Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station using pre-detection combining
    • H04B7/0842Weighted combining
    • H04B7/0848Joint weighting
    • H04B7/0857Joint weighting using maximum ratio combining techniques, e.g. signal-to- interference ratio [SIR], received signal strenght indication [RSS]
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00Arrangements affording multiple use of the transmission path
    • H04L5/14Two-way operation using the same type of signal, i.e. duplex
    • H04L5/1438Negotiation of transmission parameters prior to communication

Abstract

A wireless reception apparatus and a wireless reception method according to the present invention are capable of achieving closed loop MIMO communications in time varying channels, with a reduction in feedback information. In one embodiment of the present invention, after a first channel state information is fed back (step 475), a signal to which a transmission filter has been applied is received (step 430). The transmission filter is corresponding to the first channel state information. Using the signal, a second channel state information is generated (step 480), and then a reception filter corresponding to the second channel state information is applied to the signal (step 490).

Description

DESCRIPTION

WIRELESS RECEPTION APPARATUS AND WIRELESS RECEPTION METHOD

Technical Field

The present invention relates to a wireless reception apparatus and a wireless reception method.

Background Art Of late, there has been a dramatic growth in the capacity of wireless communication networks - Cellular networks have grown from analog "voice-only" systems to current 3rd Generation networks that provide a maximum download capacity of 2Mbps - catering to voice, data and multimedia services; Wireless LANs have evolved from initial data rates of 2Mbps specifiedbythe IEEE 802.11-99 specification [refer to "Local and Metropolitan Area Networks - Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications", IEEE Std 802.11-1999, IEEE, August 1999 ] to the present IEEE 802.11a specification [refer to "Local and Metropolitan Area Networks - Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 5 GHz Band", IEEE Std 802. lla-1999, IEEE, September 1999] that provide link rates of upto 54Mbps. To satiate the need for greater data rates, standardization fora are currently developing the next generation wireless standards.

<MIM0> At the core of enhancing the capacity of several next generation wireless systems is MIMO - multiple-input-iαultiple-output - a technology that when applied to wireless communications employs the use of Nτ transmit antennas and NR receive antennas to better effect communication. The use of multiple antennas offers the flexibility of choosing from spatial-multiplexing gain - where a dramatic increase in spectral efficiency, up to min(NT, NR) times that of a conventional single antenna (SISO) system [refer to "On limits of wireless communications in fading environments when using multiple antennas," Wireless Personal Communications, pp. 36-54, March 1998] can be realized; or, diversity gain - where up to NTNR paths that exist between transmitter and receiver may be used to exploit the diversity in the channel, leading to higher link-reliability in the wireles s channel . In general, there are tradeoffs between increased data-rate (spatial-multiplexing) and increased reliability (diversity) .

MIMO can be applied in conjunction with various transmission schemes, such as for example: time division multiple access (TDMA), frequency division multiple access (FDMA) , code division multiple access (CDMA) , orthogonal frequency division multiplexing (OFDM), among others .

Like in any communication system, there are generally two broad categories of MIMO - (1) open-loop and (2) closed-loop. In open-loop systems, the receiver demodulates the transmitted data after it has passed over the channel. Closed loop systems rely on the feedback of parameters bythe receiver to the transmitter, allowing the transmitter to adapt its transmission to the channel conditions observed at the receiver, facilitating better overall communications. Depending on the mode of operation (diversity or multiplexing) , in general, closed loop systems attain a better performance in terms of bit-error rate, reliability and throughput, over their open loop counterparts .

<CLOSED-LOOP SPATIAL MULTIPLEXINO One of the key challenges facing wireless system designers today is the demand for increased data rates over band-limited channels (which translate to the need for higher spectral efficiencies). In order to realize the vision of high rate wireless communications, it is anticipated that next generation systems would couple the benefits of closed-loop modes with spatial multiplexing, thereby yielding higher spectral efficiencies.

An example of a closed loop spatially multiplexed MIMO system is one implementing eigen-mode spatial multiplexing - where the transmitter and receiver, having channel state information (CSI) , use a transform such as the singular value decomposition (SVD) , to convert the MIMO channel into a bank of scalar channels, with no cross-talk between channels [refer to "On limits of wireless communications in fading environments when using multiple antennas," Wireless Personal Communications, pp. 36-54, March 1998] . Having full CSI, the transmitter may further (optionally) perform power-loading, where different amounts of power are allocated to different spatial-modes of the channel, thereby maximizing the capacity of the channel. Eigen-mode spatial multiplexing is an optimal space-time processing scheme in the sense that it achieves full diversity and multiplexing gains of the channel [refer to "Transmitter Strategies for Closed-Loop MIMO-OFDM," PhD thesis submitted to School of Electronic and Computer Engineering, Georgia Institute of Technology, July 2004] .

<EIGEN-MODE SPATIAL-MULTI PLEXING> Inasystembasedon eigen-mode spatial multiplexing, the receiver estimates the channel from the transmitter to the receiver - [H], and performs a transform such as a singular value decomposition (SVD) , as shown in equation (1), to determine the matrix of left and right-handed singular vectors - [U] and [V], respectively.

Figure imgf000006_0001

The receiver feeds-back the channel state information (either [H] or [V] ) to the transmitter, which uses the corresponding right-handed singular vectors of the channel - [V] to pre-filter its data.

To illustrate, assume that the transmitter performs conventional spatial multiplexing, transmitting data [x] . The received signal, [y] , can then be represented by equation ( 2 ) :

Figure imgf000007_0001
(2) where, [n] represents noise, which in the context of wireless systems, is typically modeled as an additive white Gaussian variable with finite power.

An open-loop receiver, for example the zero-forcing (ZF) detector, would determine an estimate of the transmitted data as:

Figure imgf000007_0002
(3)

The problem with such an approach is the noise-enhancement effect, which results in a signal-to-noise-ratio (SNR) degradation at the receiver [refer to "Digital Communications 3ed", McGraw-Hill, March 1995] .

In order to effect eigen-mode spatial-multiplexing, the transmitter pre-filters the data [x] , with a transmit steering matrix - [V] , and the receiver applies a matched-filter - [U]H to the received signal, [y] . Equation (4) represents the received signal, while equation (5) depicts the matched filtering applied by the receiver to estimate the transmitted data.

Figure imgf000008_0001

Expanding [H] as per equation (1) , we obtain equation (6) :

Figure imgf000008_0002
(6)

It can be seen from equation (6) that the eigen-mode spatial-multiplexing method results in perfect decoupling (i.e. no cross-talk) between streams and an SNR gain proportional to the square of the singular-values, [D], of the channel. <FEEDBACK>

In order to realize the benefits of eigen-mode spatial multiplexing a.k.a. eigen-beamforming, CSI is required at the transmitter . An intuitive way of achieving this is to merely feedback the channel estimates to the transmitter. However, feedback detracts from the payload carrying capacity of the channel and is hence an expense that must be minimized.

InUS Patent Application 2003/0139139Al the authors describe a means of reducing the feedback requirements by constructing an indexed set of channel state information, the receiver only feeding back the index values as opposed to the actual CSI. It is expected that the number of bits required for communicating the index would be smaller than that required for representing the absolute CSI, therefore achieving a reduction in overall feedback. The authors describe the operation of their invention with a MIMO system having at least 3 transmit antennae .

The time-varying nature of the wireless channel further compounds the feedback problem - requiring the most current channel state information to be available at the transmitter. In US Patent Application 2003/0125040A1 the authors describe techniques for a multiple access MIMO communications scheme . Specifically, within the context of an eigen-mode spatially multiplexed system, the authors state - "Provided that the channel conditions do not change appreciably in the interval between the time the CSI is measured at the receiver unit and reported and the time it is used to precondition the transmission at the transmitter unit, the performance of the (full-CSI) communications system may be equivalent to that of a set of independent AWGN channels with known SNRs . "

While one of the solutions to the problem of "mismatch" between the CSI estimates at the transmitter and the channel condition at the receiver is the trivial one of increasing the update/refresh rate of CSI (and consequently the overall feedback data-rate requirements) , this is not a good approach for obvious reasons. There thus exists the need, based on prior-art for a means of effecting closed-loop spatial multiplexing, while countering the deleterious effects of channel variation and reducing feedback requirements. Disclosure of Invention

It is an object of the present invention to provide a wireless reception apparatus and a wireless reception method which are capable of achieving closed loop MIMO communications in time varying channels, with a reduction in feedback information.

Specifically, the invention embodies techniques that make use of the most recent channel state information available at the receiver, in conj unction with the channel state information previously fed-back to the transmitter, to better recover the constituent signals from an eigen-mode spatially multiplexed transmission.

According to an aspect of the present invention, a wireless reception apparatus comprises a feedback section that feeds back a first channel state information, a reception section that receives, via a MIMO channel, a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back by said feedback section, a generation section that generates a second channel state information using the signal received by said generation section, and an application section that applies a reception filter corresponding to the second channel state information generated by said generation section to the signal received by said reception section. According to another aspect of the present invention, a wireless reception method comprises a feedback step of feeding back a first channel state information, a reception step of receiving, via a MIMO channel, a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back in said feedback step, a generation step of generating a second channel state information using the signal received in said reception step, and an application step of applying a reception filter corresponding to the second channel state information generated in said generation step to the signal received in said reception step.

Brief Description of Drawings Figure 1 is a chart showing an example of transmission/reception operation between a transmitter and a receiver according to a first embodiment of the present invention;

Figure 2A is a graphical representation showing an example of a SINR simulation result according to a fourth embodiment of the present invention;

Figure 2B is a graphical representation showing another example of the SINR simulation result according to the fourth embodiment of the present invention; Figure 3 is a block diagram showing a configuration of a receiver according to the fourth embodiment of the present invention; Figure 4 is a flow chart showing a detector selection operation according to the fourth embodiment of the present invention; and

Figure 5 is a flow chart showing a feedback reduction operation according to a fifth embodiment of the present invention.

Best Mode for Carrying Out the Invention

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. (First Embodiment)

The present invention relates to transmitter and receiver techniques for effecting eigen-mode spatial multiplexing in a time varying channel, while reducing feedback requirements. While it is envisaged that the methods of the present invention may be applied to any transmission scheme, exemplary of which are TDMA, FDMA, CDMA, OFDM and their variants/hybrids, among others; we shall explain the methods of the present invention in the context of a generic communications system. However, with the addition/modification of processing steps that are required for different transmission schemes and are well known to individuals skilled in the art, the methods of the present invention can be easily adapted to such schemes , and are understood not to be limited to the scheme herein . In the following description, we do not consider the effects of inter-symbol interference, whose effects may be countered by an equalizer; or by using OFDM as a transmission scheme; or by using a RAKE receiver in the context of a CDMA system; or by other means that are dependent on specific transmission scheme adopted, as would be apparent to those skilled in the art.

The present invention employs the use of improved receiver processing techniques to better effect eigen-mode spatial multiplexing, taking into account the effects of channel mismatch and the need to reduce overall feedback, as will become apparent in the ensuing embodiments. Representing the instant of time at which the channel was estimated with a previous transmission by the receiver as to; and the instant of time at which the channel is estimated with a current transmission by the receiver as tn; the corresponding channel matrices and their singular value decompositions, can be represented as per equations (7) and (8), respectively.

Figure imgf000013_0001

It is important to note that there exist infinite combinations of left-handed and right-handed singular vector matrices, [U] and [V] , for any given matrix [H] . It is hence important to note that for the purposes of the present invention, the same reference phase is selected for computing the singular value decompositions of the channel estimate at to and tn. This may be achieved by computing the singular vectors starting with the same 'assumed value', or by making use of a 'phase rotation filter' that aligns the singular vectors computed using any generic approach, to a reference phase.

Figure 1 is a message sequence chart representing the general methods applicable to the present embodiment and subsequent embodiments of the present invention. The receiver 450 estimates the channel in step 460 based on a ^previous transmission' 415. 415 may represent a prior data transmission. As per the convention stated previously, the channel estimate derived in step 460 may be represented as [Hto] • Step 470 computes the CSI to be fed-back to the transmitter, say for instance the transmit steering matrix [Vto] • The transmitter 410, upon receiving the feedback information at step 475 uses it to pre-condition/filter 420 the data to be transmitted 425, generating the transmitted signal transmitted and received in step 430. The receiver 450 on receiving the transmitted signal 430 generates a new channel estimate [Htn] in step 480. The new channel estimate is used in conjunction with the feedback information generated by step 470 and the old channel estimate, generated by step 460, to derive the receive filter 490, which recovers an estimate of the transmitted data 495 from the transmitted signal received in step 430. Assuming the transmitter transmits data [x] in 425, preconditioned by the transmit MIMO processor 420 by the transmit filter [Vto]/ the received signal [y], may be represented as per equation (9) .

Figure imgf000015_0001
(9)

The receive MIMO processor 490 applies a receive filter [Rxfilttn], generated as per the embodiments of the present invention, in order to estimate the transmitted data 495, as per equation (10) .

Figure imgf000015_0002
(10)

<Receive processing based on current CSI> In the present embodiment, the present invention prescribes the synthesis of the receive filter of equation (10) as the left-handed singular matrix of the present channel estimate [Htn] r ie [Utn] . Equation (11) depicts the receive filter as prescribed by the first embodiment, while equation (12) illustrates the estimate of the transmitted signal. (11)

Figure imgf000015_0003
(12)

As can be seen from equation (12), the noise term is multiplied by a unitary matrix, [Utn]/ resulting in no change to its statistical properties [refer to "Transmitter Strategies for Closed-Loop MIMO-OFDM, " PhD thesis submitted to School of Electronic and Computer Engineering, Georgia Institute of Technology, July2004] . The product [Vtn]H[Vto] results in cross-talk between the different spatial channels/modes of [x] , the distortion tending to zero as tn tends to to. The elements of the diagonal matrix [Dtn] results in a corresponding gain to each spatial mode. In the description, a spatial mode is often referred to as "a spatial channel". (Second embodiment)

<Receive processing based on inverse-cancellation>

In a second embodiment, the present invention prescribes the synthesis of the receive filter of equation (10) based on the inverse-cancellation principle. Using this criterion, it would be apparent to one skilled in the art, that the receive filter can be represented as per Equation (13) . (13 )

Figure imgf000016_0001
Applying the receive filter as described by equation (13), the estimate of the transmitted signal, as determined by the receiver, can be represented by Equation (14) .

Figure imgf000016_0002
(14) As can be seen from equation (14), the inverse-cancellation detector of equation (13) results in no cross-talk/distortion of the channel spatial modes , the diagonal singular values of [Dtn] resulting merely in a gain. However, as can be seen from equation (13), the receive filter is now non-unitary and its product with the noise term no longer preserves the statistical properties of noise. While the receive filter approaches a unitary matrix as tn approaches to, in the realistic scenario where to ≠ tn (and where, for reducing the amounts of feedback, it is desired to space to and tn as far apart as possible) , the use of the method of the second embodiment impacts the overall signal to noise ratio (SNR) of the estimated signal. (Third embodiment)

<Receive processing based on MSE criterion> While the method of the first embodiment results in the distortion of the signal estimate, the second embodiment impacts the noise. In a third embodiment of the present invention, the receive filter is synthesized using a mean squared error criterion to balance the effects of signal distortion and noise amplification, resulting in a better overall SINR.

Assuming ideal feedback (zero-delay i.e. tn = to, and not writing the subscript), the estimate of the transmitted signal can be represented by:

Figure imgf000017_0001
Assuming the practical (delayed feedback) case, and a receive filter [Rxfilttn] H, the estimate of the transmitted signal can be represented by equation (16) .

Figure imgf000017_0002
(16)

Deriving an error signal as the difference between the ideal and the practical estimate, in equation (17), we may represent the mean-squared error in equation (18) .

Figure imgf000017_0003
(17)
Figure imgf000018_0001
(18) where, the function E { ... } denotes the expected value of the argument.

For ease of representation and further derivation, we define the matrices [A] and [B] , as per (19) and (20) , such that equation (17) may be expressed as (21) : (19) (20)

Figure imgf000018_0002
(21)

In order to derive the coefficients of the receive filter, the general approach is to minimize the mean squared error criterion of (18) . For the purposes of illustration and subsequent evaluation, we employ the example of a 2Rxx2Tx MIMO system in the remainder of this section, although it is to be understood that the applicability of the present invention is not curtailed by the antenna configuration adopted herein.

Noting that the only time indexed variables of equation (21) are in terms of tn, for convenience of representation, we may drop the index in the remainder of this embodiment. Re-writing equation (21) in expanded notation, we obtain equation (22) :

(22

Figure imgf000018_0003

Noise has a zero-mean nature as per equation (23) and a variance as per equation (24), while the signal has a zero-mean (i.e. no DC value) , as per equation (25) , but may have different powers (if power loading is used) as per equations (26) and (27) . (23) (24) (25) (26) (27)

Figure imgf000019_0001

Expanding equation (18), we obtain a real-valued function for mean-squared error (MSE) , in terms of complex valued variables (not shown here due to considerations of length) . Minimizing this equation by equating the partial derivatives of (18) taken with respect to the conjugates of the individual terms of the receive filter - {r11, r12, r21, r22 } , as per the teachings of the following document: "Stationary points of a real-valued function of a complex variable", Univ. of California at Berkeley, January 2005, we obtain equations (28) through (31), with coefficients specified by equations (32) through (43) :

( 2 8 )

( 2 9 )

( 3 0 )

( 3 D

( 32 )

( 3 3 )

( 3 4 )

( 3 5 )

Figure imgf000019_0002
(36)

(37)

(38) (39)

(40)

(41) (42)

(43)

Figure imgf000020_0001
olving equations (28) through (31), the coefficients of the receive filter may be obtained, as specified by (44) though (47) :

(44

!45)

(46)

47)

Figure imgf000020_0002

(Fourth embodiment)

<Receiver with per-spatial channel selective processing>

Figures 2A and 2B illustrate the output SINR of the receiver techniques outlined in the above-mentioned embodiments of the present invention with respect to transmitter update interval, when compared to the performance in the ideal case (i.e. no mismatch) , in the context of a 2Rxx2Tx MIMO system performing eigen-mode spatial multiplexing . The simulations are conducted over a single sub-carrier (having a subcarrier bandwidth of 312.5kHz) of an OFDM system, operating over MIMO Channel Model 'F' , as specified in the following documents: "TGn Channel Models," doc: IEEE 802.11- 03/ 940r4 , May 2004; and "A Stochastic MIMO Radio Channel Model with Experimental Validation", IEEE JSAC, vol. 20, no. 6, August 2002. Model 'F' is des igned to simulate the effects of a hotspot micro-cellular environment, typical of an outdoor open space, such as a city square. Although the primary Doppler component of the scattering environment is 6Hz, the third tap of the Doppler spectrum has a Doppler spread of 100Hz, in order to capture the effects of moving vehicles on the streets. The simulated performances do not make use of power-loading and are performed for two representative signal to noise ratios of 3 and 15dB at the input of the receiver.

In generating the plots of the figures, we assume that the receiver obtains CSI feedback from the transmitter once every reference point (as marked on the abscissa) 505a and 505b. The transmitter uses this feedback information to pre-code subsequent transmitted data, till it receives the next feedback update. On the other hand, the receiver has a constant update of the channel conditions.

For convenience in representation and explanation, Figures 2A and 2B follow the same set of reference numerals , differing only in the suffixed alphabet - 'a' corresponding to Figure 2A, for the first spatial mode; and Λb' corresponding to Figure 2B, for the second spatial mode of the simulated 2Rxx2Tx MIMO system.

In Figure 2A, the output SINR of the various detection schemes when applied to the first spatial-mode, is plotted for representative input signal to noise ratios of {3dB and 15dB}. The detection schemes include a first detection scheme, a second detection scheme and a third detection scheme which are corresponding to the first embodiment, the second embodiment and the third embodiment, respectively. 510a marks the output SINR, assuming no channel mismatch, assuming an input SNR of 3dB. Likewise, 525a, 520a and 515a mark the output SINRs of the first spatial mode of the detectors (i.e. the detection schemes) in embodiment 1 through 3 (i.e. the first through third embodiments), respectively, of the present invention . Correspondingly, 530a, 545a, 540a and 535a respectively mark the performance of the case of no channel mismatch and embodiments 1 through 3, of the present invention, assuming an input SNR of 15dB.

Figure 2B depicts the corresponding set of data as does Figure 2A, for the second spatial mode of the channel, as explained above.

From the results of Figure 2A, it can be seen that in the case of the first spatial mode, the third detection scheme of receive processing, as stated in the third embodiment yields the best performance (515a and 535a) in the case of delayed feedback - the SNR most closely approaching the case of no mismatch (510a and 530a) . Also, it is observed that in the case of lower SNRs (eg: 3dB) , the first detection scheme of the first embodiment 525a outperforms the second detection scheme of the second embodiment 525a, while for high SNRs (eg: 15db) , the second detection scheme of the second embodiment 540a outperforms the first detection scheme of the first embodiment 545a. From the results of Figure 2B, it can be seen that in the case of the second spatial mode, the second and third schemes of receive processing, as stated in embodiments 2 (520b & 540b) and 3 (515b & 535b) , outperform both embodiment 1 (525b & 545b) and the no-mismatch case (510b & 530b); in the case of the second spatial mode. It is further observed that the second detection scheme of embodiment 2 (520b & 540b) consistently yield the highest SINR after receive processing, regardless of the input SNR. From equation (14), we can see that the signal estimate generated by the method of embodiment 2 (i.e. the second detection scheme) comprises the summation of two terms. The first term represents the signal, while the second term represents noise. Recalling that the method of embodiment 2 involved the use of a receive filter that was based on the inverse-cancellation criterion, the signal term has no distortion or cross-talk. However, as described previously, the noise term is no longer statistically white - rather, it is modified by the non-unitary nature of the receive filter.

Equation (48) represents the noise term, [n'], of equation (14) .

Figure imgf000024_0001

As has been described previously, the product [Vto] H [Vtn] → [I] as tn → to, resulting in [n' ] having the same statistical characteristics as [n] (i.e. statistically white) . Further, as is characteristic of most 2x2 wireless channels (and also verified for Channel Model 'F' , used in the simulations of Figure 2A and 2B) , the principle eigen-mode gain di is typically larger than 1, while the secondary eigen-mode d2 is smaller than 1. These properties , when coupled with the product [Vto]H[Vtn] when to ≠ tn, results in a 'noise-suppression' effect on the noise power at the output of the second spatial mode, resulting in the behavior of 520b and 540b in Figure 2B. A similar reasoning holds true for the behavior of 515b and 535b.

Using the results outlined above, the fourth embodiment of the present invention advocates the selective use of different detection schemes, as outlined in the previous embodiments of the present invention, to obtain the maximum signal to interference and noise ratio, and consequently performance, of both spatial-modes of the channel.

Figure 3 represents a MIMO receiver 600 that is capable of performing eigen-mode spatial multiplexing in conj unction with a MIMO transmitter. 610a through 61 Om represent elements of the receive antenna array which are respectively connected to their corresponding RF units - 620a through 620m which are responsible for signal down conversion. The down-converted signals from the RF units are fed to their respective demodulator units, represented by 630a through 630m, respectively. The demodulators perform inverse functions of their modulator counterparts of the transmitter, corresponding to the transmission scheme in use.

Each of the 'm' demodulators is connected to a channel estimator 640 through data-path 635, in addition to being connected to a MIMO Rx Processor 650. Channel estimator 640 is responsible for generating the present estimate of the MIMO channel, as seen by the receive array elements 210a through 210m. The MIMO Rx processor 650 uses the present channel estimates (i.e. at time instant tn) and the channel estimates that were previously fed-back to the transmitter (i.e. at time instant to) to pre-condition its transmission, a copy of which is stored in memory 651, to develop a receive steering matrix using the detection schemes of embodiments 1 through 3, for example. The MIMO Receive Processor is further coupled to the detector selection logic 654 that uses the singular values of the channel and the knowledge of the channel estimates at to and at tn, as well as knowledge of the signal to noise ratio (as could be measured/estimated by channel estimator 640, for example) to determine which of the detection schemes is optimal, maximizing the output SINR (and consequently capacity) of each spatial mode. The results of the determination performed by logic 654 are feedback to MIMO Receive Processor 650 which uses the selected detection scheme to recover estimates of the individual spatial channels. The estimates of the recovered signals on each of the 'n' spatial channel are then fed to individual de-mapping, de-interleaving and decoding units, represented by 670a through 67On, the outputs of which, are connected to a multiplexer 680, which combines them to recover estimates of the composite transmitted data bit stream, output as 685.

The MIMO receiver 600 also includes a CSI generator 660, which takes input from the channel estimator 640 and the MIMO Receive Processor 650 to generate feedback information 665 to the transmitter. Based on the description of the methods of the present invention, this feedback could comprise of the right-handed singular vector matrix [V], or its variants, as described in prior-art .

Figure 4 is a flow diagram detailing the steps of selecting the best detector, so as to optimize the performance of each spatial mode. It is anticipated that the steps detailed in Figure 4 may be partitioned into either hardware or software and may be partitioned among the various functional units of the receiver 600. As an input, the process uses an estimate of the noise variance at the input to the receiver, the power distribution between different spatial-modes, the CSI fed-back to the transmitter previously (at time to) and the estimate of the present CSI (at time tn) , as per step 710. Step 730 estimates the output SINR for the various detectors, for each spatial-mode, while step 740 selects the best performing detector for each channel, which is output in step 750. It is expected that the output of step 750 is used by the MIMO Rx processor 650 to process the demodulated signals emanating from the units 630a through 630m to generate inputs to the functional blocks 670a through 67On.

In the fourth embodiment the usage of SINR is explained as a performance metric, but other performance metrics may be used instead of SINR. For example, if an error rate is estimated instead of SINR, a detection scheme corresponding to a lower error or the lowest error rate is selected in a preferred embodiment. (Fifth embodiment) <Reduced feedback generation>

As mentioned in background to the present invention, feedback is an over-head to effecting efficient eigen-mode spatial mutliplexing between transmitter and receiver, utilizing valuable communications resources . An important problem, thus facing designers, is that of minimizing the amount of feedbac. signaling.

Based on Figures 2A and 2B, it is observed that the detection schemes of embodiments 1 through 3 yield output SINRs for the first and second spatial-mode, as depicted by 525x, 52Ox and 515x for an SNR of 3dB and 545x, 54Ox and 535x for an SNR of 15dB; where 'x' is substituted with 'a' to represent the first spatial-mode; and 'b' for the second-spatial mode. When compared with the case of 'no~mismatch' ( equivalent to feedback with zero-delay) , represented by 51Ox and 53Ox for input SNRs of 3dB and 15dB respectively, it can be seen that one may make a tradeoff between the feedback interval 505x and the output SINR that impacts the system capacity.

In the fifth embodiment, the present invention advocates a mode of operation wherein the feedback interval 505x is altered depending on the channel variation, as can be measured by the apparatus specifically comprising the units 640, 650, 651 and 654 and used by the CSI Generator 660 to generate feedback information 665 to the transmitter, on an xas needed' basis, resulting in a tradeoff between system performance and the overall feedback data rate. For example, it is expected that the CSI generator 660 would update the transmitter with a new transmit steering matrix, only upon estimating sufficient variation between the currently used transmit steering matrix and the transmit steering matrix derived based on present channel estimates, it being expected that a quality metric of the detected signal, being available at the receiver is also used to make such decision.

Figure 5 is a flow diagram depicting the steps to be carried out in determining when to feedback CSI for the transmitter, as per the methods of embodiment 5. As an input, the process uses an estimate of the noise variance at the input to the receiver, the power distribution between different spatial-modes, the CSI fed-back to the transmitter previously (at time to) and the estimate of the present CSI (at time tn) , as per step 810, to estimate the performance degradation due to channel variation, instep820. In step 830 the performance degradation is compared to a threshold value, based on which path 840 is traversed if the degradation is determined to be worse than the threshold, while path 835 is traversed if the degradation is tolerable (i.e. within the bounds of the threshold) . In step 850 the receiver feeds back new CSI estimates to the transmitter and updates its record of the CSI estimate in use by the transmitter at step 860.

The methods and the apparatuses of the present invention have described update of the transmitter with channel state information (CSI) in a single instant - for example in the context of Figure 2A and 2B, points on the x-axis 505a and 505b represent the time difference between consecutive transmitter updates , it being assumed that at said instance the transmitter is provided with a new CSI update. However, it is to be understood that practical implementations may make use of differential signaling schemes - allowing more frequent updates to transmitter, albeit with smaller quanta of information. As such, it is to be appreciated that the methods and the apparatuses of the present invention are not limited to a specific means or format for signaling the CSI.

Also, any one of the above embodiments of the present invention may be implemented in combination with another embodiment .

A wireless reception apparatus according to one of the above-mentioned embodiments, adopts a configuration comprising: a feedback section that feeds back a first channel state information; a reception section that receives, via a MIMO channel, a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back by said feedback section; a generation section that generates a second channel state information using the signal received by said reception section; a synthesis section that synthesizes a reception filter corresponding to the second channel state information generated by said generation section; and an application section that applies the reception filter to the signal received by said reception section.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration wherein: said reception section receives a first signal at a first reception timing, and a second signal at a second reception timing later than the first reception timing, the second signal being the signal to which the transmission filter corresponding to the first channel state information has been applied, and said generation section generates the first channel state information as a channel state information corresponding to the first reception timing, and generates the second channel state information as a channel state information corresponding to the second reception timing.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the second channel state information instead of the first channel state information. A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the first channel state information and the second channel state information.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the first channel state information, the second channel state information, an estimate of the noise-level at the input of the wireless reception apparatus, and information on the power distribution among spatial modes of the MIMO channel.

A wireless reception apparatus according to one of the above-mentioned configuration, a configuration further comprising: a synthesis section that synthesizes a plurality of reception filters corresponding to a plurality of schemes; an estimation section that estimates a plurality of values of a performance metric, the plurality of values corresponding to the plurality of schemes for each spatial-mode; and a selection section that selects one of the plurality of schemes which is corresponding to one of the plurality of values estimated by said estimation section for each spatial mode of the MIMO channel, and said application section applies one of the plurality of reception filters synthesized by said synthesis section, the applied reception filter corresponding to the scheme selected by the selection section .

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration wherein : said synthesis section synthesizes a first reception filter corresponding to a first scheme using the second channel state information, synthesizes a second reception filter corresponding to a second scheme using the first channel state information and the second channel state information, and synthesizes a third reception filter corresponding to a third scheme using the first channel state information, the second channel state information, an estimate of the noise-level at the input of the wireless reception apparatus, and information on the power distribution among spatial modes of the MIMO channel.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration wherein : said selector selects one of the plurality of reception filters for each of a plurality of spatial modes of the MIMO channel.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration wherein: said feedback section feeds back a new first channel-state information to the transmitter in accordance with a performance degradation.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration wherein: said feedback section feeds back a new first channel state information to the wireless transmission apparatus in a case where the performance degradation is worse than a threshold.

A wireless reception apparatus according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration further comprising: a degradation estimation section that estimates the performance degradation with the first channel state information, the second channel information, an estimate of the noise-level at input of the wireless reception apparatus, and information on the power distribution among spatial modes of the MIMO channel, the performance degradation being detected by comparing a performance based on the first channel state information with a performance based on the second channel state information.

Awireless reception method according to one of the above-mentioned embodiments adopts a configuration comprising : a feedback step of feedingback a first channel state information; a reception step of receiving, via a MIMO channel, a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back in said feedback step; a generation step of generating a second channel state information using the signal received in said reception step; and an application step of applying a reception filter corresponding to the second channel state information generated in said generation step to the signal received in said reception step.

A wireless reception method according to one of the above-mentioned embodiments adopts, in the above-mentioned configuration, a configuration further compris ing a step corresponding to any of the constituents mentioned with respect to a wireless reception apparatus .

Industrial Applicability

A wireless reception apparatus and a wireless reception method of the present invention are applicable in wireless communications via a MIMO channel.

Claims

1. A wireless reception apparatus comprising: a feedback section that feeds back a first channel state information; a reception section that receives , via a MIMO channel , a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back by said feedback section ; a generation section that generates a second channel state information using the signal received by said reception section; and an application section that applies a reception filter corresponding to the second channel state information generated by said generation section to the signal received by said reception section.
2. The wireless reception apparatus according to claim 1, wherein: said reception section receives a first signal at a first reception timing, and a second signal at a second reception timing later than the first reception timing, the second signal being the signal to which the transmission filter corresponding to the first channel state information has been applied, and said generation section generates the first channel state information as a channel state information corresponding to the first reception timing, and generates the second channel state information as a channel state information corresponding to the second reception timing.
3. The wireless reception apparatus according to claim 2, further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the second channel state information instead of the first channel state information.
4. The wireless reception apparatus according to claim
2, further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the first channel state information and the second channel state information.
5. The wireless reception apparatus according to claim
2, further comprising: a synthesis section that synthesizes the reception filter to be applied to the second signal, using the first channel state information, the second channel state information, an estimate of a noise-level in an input of the wireless reception apparatus, and an information on a power distribution among spatial modes of the MIMO channel .
6. The wireless reception apparatus according to claim
2, further comprising: a synthesis section that synthesizes a plurality of reception filters corresponding to a plurality of schemes ; an estimation section that estimates a plurality of values based on a performance metric, the plurality of values corresponding to the plurality of schemes for each spatial-mode; and a selection section that selects one of the plurality of schemes which is corresponding to one of the plurality of values estimated by said estimation section for a spatial mode of the MIMO channel, and said application section applies one of the plurality of reception filters synthesized by said synthesis section, the applied reception filter corresponding to the scheme selected by the selection section.
7. The wireless reception apparatus according to claim 6, wherein: said synthesis section synthesizes a first reception filter corresponding to a first scheme using the second channel state information, synthesizes a second reception filter corresponding to a second scheme using the first channel state information and the second channel state information, and synthesizes a third reception filter corresponding to a third scheme using the first channel state information, the second channel state information, an estimate of a noise-level in an input of the wireless reception apparatus, and an information on a power distribution among spatial modes of the MIMO channel.
8. The wireless reception apparatus according to claim 6, wherein: said selector selects one of the plurality of reception filters for each of a plurality of spatial modes of the MIMO channel.
9. The wireless reception apparatus according to claim 2, wherein: said feedback section feeds back a new first channel state information in accordance with a performance degradation .
10. The wireless reception apparatus according to claim 9, wherein: said feedback section feeds back the new first channel state information in a case where the performance degradation is worse than a threshold.
11. The wireless reception apparatus according to claim 9, further comprising: a degradation estimation section that estimates the performance degradation using the first channel state information, the second channel information, an estimate of a noise-level in an input of the wireless reception apparatus, and an information on a power distribution among spatial modes of the MIMO channel, the performance degradation being detected by comparing a performance based on the first channel state information with a performance based on the second channel state information .
12. A wireless reception method comprising: a feedback step of feeding back a first channel state information; a reception step of receiving, via a MIMO channel, a signal to which a transmission filter has been applied, the transmission filter being corresponding to the first channel state information fed back in said feedback step; a generation step of generating a second channel state information using the signal received in said reception step; and an application step of applying a reception filter corresponding to the second channel state information generated in said generation step to the signal received in said reception step.
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