WO2014171551A1 - Wireless receiving device, wireless transmitting device, wireless communication system, wireless receiving method and wireless transmitting method - Google Patents

Abstract

Feedback overhead reduction in a wireless communication system is obtained. The steps for conditioning a first set of angular elements obtained from a channel characteristic and compressing a second set of angular elements obtained from the channel characteristic are provided in a method of receiving data streams through a wireless communication channel.

Description

[DESCRIPTION]

[Title of Invention]

WIRELESS RECEIVING DEVICE, WIRELESS TRANSMITTING DEVICE, WIRELESS COMMUNICATION SYSTEM, WIRELESS RECEIVING METHOD AND WIRELESS TRANSMITTING METHOD

[Technical Field]

[0001]

This invention relates to a wireless receiving device, a wireless transmitting device, a wireless communication system, a wireless receiving method and a wireless transmitting method.

[Background Art]

[0002]

There are many instances in which a transmitting wireless communication access point (from now on AP) benefits from knowing the characteristics of the channel between the antennas at the AP and the antennas at the receiving wireless

communication station (from now on STA).

This information can give the AP the capacity to perform precoding, modifying a signal in a way that it will be perceived by the STA as interference free after going through the channel. Precoding can be used for single antenna operation or multiple-input multiple-output operation (MIMO), either for the single user case (SU-MIMO) or the multi-user case (MU-MIMO), in which the AP transmits information to multiple antennas belonging to different STAs.

The information about the condition of the channel can serve other purposes, e.g. the AP chooses the STAs to which to transmit according to their current channel conditions, etc.

[0003]

In a time-division duplexing (TDD), the measure of the channel can be estimated by the AP. The STA transmits a signal known by the AP, which can extract the channel characteristic observing its alteration. In TDD, this process results in very small overhead. However, it requires a precise calibration of the elements at the AP and the STA, as slight divergences can induce high error.

In a frequency-division duplexing (FDD), the AP and the STA transmit at different frequencies. Therefore, it is not possible for the AP to measure the characteristic of the channel. In FDD, it is the STA that must measure the channel characteristic and transmit that measurement to the AP as feedback.

The feedback can be transmitted in numerous ways. The elements of the computed channel matrix can be quantized and transmitted without further

transformation. Or, the STA can compute the precoding matrix to be used by the AP, quantize it and transmit it. Or, the STA can transmit as feedback only the average SNR of subsets of one or more subcarriers.

[0004]

One especially relevant example is the case of SVD precoding. In this case, the precoding matrix to be used by the AP is unitary and orthonormal. Considering these characteristics, the reduction of the matrix to its fundamental components is advantageous. The STA can compute these components and send them to the AP, which can reconstruct the precoding matrix from them.

The embodiments included in this document are explained taking as a baseline example the 802.1 1 standard in its current form (Non Patent Literature 1) and relevant amendments such as 1 lac (Non Patent Literature 2). This is chosen as reference, and it is noteworthy that the embodiments of the invention are not limited to this standard.

It is important to note that in this document the concept of antenna is taken as the device or a set of devices that allows the transmission or reception of one stream. That is, in this document, a station that can transmit or receive up to one stream is considered as being furnished with one antenna; a station that can transmit or receive up to two streams is considered to be furnished with two antennas; etc.

[Summary of Invention]

[Technical Problem]

[0005]

In an FDD system, the overhead caused by the transmission of feedback from the STA to the AP is an element of concern. This overhead can be especially taxing in the case of MIMO communications, in which the number of possible paths between antennas at AP and STA increases geometrically. Besides, the requirements of accuracy are higher for MIMO transmissions, especially the ones based on non-linear techniques, which can achieve a higher data rate than the relatively simple linear techniques, but are more sensitive to channel estimation errors.

In order to be able to leverage the increasing number of antennas at AP and STA, the feedback overhead must be reduced without compromising its accuracy. [Solution to Problem]

[0006]

According to an embodiment of the process, there are provided the steps for conditioning a set of first angular elements obtained from a channel characteristic, and compressing a set of second angular elements obtained from the channel characteristic in a method of receiving data streams through a wireless communication channel.

[Advantageous Effects of Invention]

[0007]

The feedback overhead for interference free in a wireless communication, especially in a MIMO communication, is reduced without compromising its accuracy. [Brief Description of Drawings]

[0008]

[Fig. 1]

FIG. 1 shows an example MIMO communication system.

[Fig. 2A]

FIG. 2A shows the maximum absolute value obtained in the transformed domain for angle φ.

[Fig. 2B]

FIG. 2B shows the maximum absolute value obtained in the transformed domain for angle ψ.

[Fig. 3]

FIG. 3 shows a flow diagram in which the angles ψ and/or φ are conditioned and compressed in the transformed domain according to the first embodiment.

[Fig. 4]

FIG. 4 shows the flow diagram of the conditioning process.

[Fig. 5A]

FIG. 5A shows an example of the values of the function (p(SC_j).

[Fig. 5B]

FIG. 5B shows the same characteristic after performing conditioning.

[Fig. 6]

FIG. 6 shows the flow diagram of the process that conditions the angles

<Pi(SCj).

[Fig. 7A]

FIG. 7 A shows the example of the values of the function v|/(SC_j).

[Fig. 7B]

FIG. 7B shows the effect of conditioning.

[Fig. 8]

FIG. 8 shows the flowchart to compute the conditioning of the function

_Vi(SCj).

[Fig- 9]

FIG. 9 shows flow diagram of the exemplary process that conditions the angle [Fig. 10A]

FIG. 10A shows the average absolute values obtained in the transformed domain for the angles φ'.

[Fig. 10B]

FIG. 10B shows the average absolute values obtained in the transformed domain for the angles ψ' .

[Fig. 11]

FIG. 11 shows the flow diagram of an example transformed domain

compression.

[Fig. 12]

FIG. 12 shows the reverse process at AP.

[Fig. 13]

FIG. 13 shows the flow diagram of recovering the angles ψ' and φ'.

[Fig. 14]

FIG. 14 shows an example of block diagram for STA according to the first embodiment.

[Fig. 15]

FIG. 15 shows an example of block diagram for AP according to the first embodiment.

[Fig. 16]

FIG. 16 shows a flow diagram in which the transformed domain compression is performed after selected conditioning according to the second embodiment.

[Fig. 17]

FIG. 17 shows the flow diagram of the detailed transformed domain

compression after selected conditioning.

[Fig. 18]

FIG. 18 shows the flow diagram in which reverse process at AP is performed according to the second embodiment.

[Fig. 19]

FIG. 19 shows an example of block diagram for STA according to the second embodiment.

[Fig. 20]

FIG. 20 shows an example of block diagram for AP according to the second embodiment.

[Description of Embodiments] [0009]

Now, preferred embodiments of the present invention will be explained in detail with reference to the annexed drawings. The embodiments relates to WLAN (wireless Local Area Network), but they are not restricted to the WLAN, but are also applicable to a mobile phone network.

[Embodiments 1]

[0010]

FIG. 1 shows an example wireless system. AP (Access Point) 101 acts as the AP of the basic service set BSS (Basic Service Set) 1. AP 101 can communicate with STAs (Stations) 11 1 to 1 18. An AP may be called a wireless transmitting device, and an STA may be called a wireless receiving device hereinafter.

AP 101 may be endowed with many antennas that are capable of inputting different signals into the medium at the same time and in the same frequency bandwidth through spatial multiplexing. If the STA also has multiple antennas, AP 101 can transmit different data streams from each antenna at AP 101, each stream targeting a different STA antenna (single user MIMO, SU-MIMO). Alternatively, AP 101 can transmit different data streams to different antennas that may belong to different STAs (multiuser MIMO, MU-MIMO).

In this example, AP 101 and STAs 11 1 to 118 belong to BSS 1. AP101 has eight antennas and each STA has one or more antenna.

[0011]

However, the antennas (belonging to the same STA or not) to which the MIMO multiplexed signals are addressed perceive the signal intended to other antennas as interference, which leads to a performance degradation in bandwidth. In order to restrain the interference, STAs 111 to 118 send information about the channel to AP 101. AP 101 performs precoding, altering the signal in a way that after passing through the channel the signal will be seen as interference free by the receiving antennas.

A well-known way of performing precoding is by singular value

decomposition (SVD). Equation 1 shows the singular value decomposition of the channel matrix H.

[0012]

[Equation 1 ]

^^■N_Rxfi_T ⁼ ^N_RxN_R ' ^ N_RxN_T ' ^Ν_τ Ν_τ ( * ) [0013]

In the above equation, U and V are unitary matrices, S is a diagonal matrix of singular values, N_R is the number of antennas at the ST A, Ντ is the number of antennas at AP 101, and V^H is the Hermitian (complex conjugate transpose) of V. V is the matrix sent as feedback from the STA to AP 101.

The signal received by the STA is shown in Equation 2.

[0014]

[Equation 2]

Y = H x + η (2) [0015]

In the above equation, H is the channel matrix between the transmitting antennas and the receiving antennas, x is the signal transmitted by AP 101, η is the noise as seen by the STAs and Y is the signal received by the STAs.

AP 101 pre-multiplies the transmitted signal by the matrix V as in Equation 3.

[0016]

[Equation 3]

Y = H χ'+η = Η V χ + η = (υ S V") V χ + η = ϋ S χ + η (3)

[0017]

In the above equation, ' is the signal to be transmitted by AP 101 after precoding.

At reception, the STA pre-multiplies Y by the hermitian of the matrix U as in Equation 4.

[0018]

[Equation 4]

Y' = U" - Y = V^H•(Η · χ'+η) = υ^/ί H V x + U" η

(4)

= V^H - (U - S - V^H ) - V - x + r '= S - x + i '

[0019]

In the above equation, Y' is the received signal Y after pre-multiplying by U^, and the resulting noise η is equivalent to the original noise factor η, being the matrix U^H unitary and therefore not affecting the magnitude of the noise.

The matrix V is orthonormal. This implies that there is some redundancy into it. An efficient way of transmitting this matrix to the AP 101 is by finding the fundamental elements of V and transmitting only those that are essential to reconstruct the matrix at the AP 101. The fundamental elements of V may be defined as the elements of the matrix V, or as an arbitrary number of extracted elements from V, or as an average of an arbitrary number of elements of V, or the coefficients of a curve fitting the elements of V, or a set of angles obtained as a result of the decomposition of the matrix V (for instance, the angles obtained through Givens rotation, or the angles obtained through Householder reflection).

[0020]

The 802.11 standard gives an example in which this can be accomplished by the use of Givens rotation. The Givens rotation operation can be used to reduce the columns of a matrix so all elements other than the diagonal elements are zero.

However, this operation can only be performed over real numbers. The process described in the standard (Non Patent Literature 2) obtains two types of fundamental angles that represent the matrix, i.e. the angles needed to transform the elements of a given column to the real domain, and the Givens rotation angles themselves, that show the common planar rotation of two dimensions to reduce the elements other than the diagonal element of the column under consideration. In the rest of the document, we will refer to the rotation angles as φ, and to the Givens rotation angles as ψ.

The number of angles N_angi_es of each kind required to represent the matrix V varies depending on the size of the parts to be given as feedback, as shown in Equation 5.

[0021]

[Equation 5] , , = F{V)- N_R = 2 - N_T - N_R - N_R ² - N_R (5)

Νχ means the number of antennas AP 101 has, and N_R means the number of antennas an STA has.

[0022]

The rotation experienced by the streams arriving at the receiving antennas of the STA can be estimated by the STA, therefore this information can be omitted. F(V) represents the degrees of freedom of the matrix V. For an MU-MIMO configuration according to FIG.l, in which Ντ =8 and N_R=1, the number of feedback elements transmitted from each STA is reduced from 16 elements to 14 by Givens rotation. For an SU-MIMO configuration, in which N_T =8 and N_R=8, the reduction is from 128 elements to 56. The level of compression attained through this process is high, and it is indeed optimal from the point of view of lossless compression. The matrix V cannot be compressed further without incurring into losses. However, other steps of the feedback transmission process introduce losses into the system, such as the quantization step, denying part of the benefits of lossless compression techniques.

It is worth noting that similar results could be obtained through other methods instead of Givens rotation, for example Householder reflections. The document gives an exemplary procedure for the elements obtained through Givens rotation, but other methods that follow the spirit of reducing the precoding matrix V to its fundamental angular elements are also included.

It is possible to further reduce the feedback overhead by applying transformed domain compression techniques to the obtained angles (introducing losses).

An example transformation is the DCT transformation (Discrete Cosine Transformation) as shown in Equation 6.

[0023]

[Equation 6] X, [n] = DCT{ _i - n (6)

[0024]

In the above equation, 'n' represents a sample of the transformed domain, 'k' represents a sample of the frequency domain, v|/i[k] represents the signal in the frequency domain. Xi[n] is the transformed domain representation of v|/i[k], and N_DCT is the number of points of the DCT operation. In OFDM, the sample k represents a subcarrier.

The inverse DCT operation is performed as shown in Equation 7:

[0025]

[Equation 7]

(7)

[0026]

FIG. 2A shows the average absolute values obtained in the transformed domain for the angles φ for an exemplary case of a 4x4 transmission with a 40 MHz bandwidth under channel model D (defined for the 802.11 standard (Non Patent Literature 1) to simulate the conditions of a typical office). In FIG. 2A, the abscissa means transformed domain samples n and the ordinate represents the value of the samples (DCT(cp (SC_j)). It can be seen in this figure that there is significant energy remaining in the high samples.

FIG. 2B shows the average absolute values obtained in the transformed domain for the angles ψ for the exemplary case mentioned above. In FIG. 2B, the abscissa means transformed domain samples n and the ordinate represents the value of the samples (DCT(v|/ (SC_j)). In the case of the angles ψ, the energy quickly drops to nearly zero for the medium to high samples.

[0027]

FIG. 3 shows a flow chart in which the channel matrix H is compressed to be fed back to AP 101. The STA performs singular value decomposition to the channel matrix H (301), and extracts the fundamental elements (a set of angles φ and ψ) of the orthonormal matrix V (302). In our example explanation, this is done following the procedure described in [Non Patent Literature 1].

Each subcarrier presents one channel matrix H that is decomposed into N_angi_es elements. There are N_Sc subcarriers, and accordingly there are Nsc sets of N_angies elements. The elements corresponding to the same fundamental element (where i = 1, 2, ..., N_angi_es) can be grouped as functions ψ, (SC) and φ^ΞΟ^), where j represents the subcarrier position (j = N_start, N_st_art + 1 , ..., N_en ) as shown in Equation 8. A different frequency domain function can be computed for each of these groups.

[0028]

[Equation 8]

(8) i— {,2,...N_rmgjes

SC_J = N„_m, N_tm + l,.. # _ill

[0029]

N_slart and N_ena- could be the first and the last subcarrier respectively, or there could be multiple regions, for example one for N_start = 1 and N_ena- = Nsc /2, and another for Nstart = Nsc /2+1 and N_ena- =Nsc, where Nsc is the total number of subcarriers.

Conditioning operations (explained in detail later) are performed to either or both of functions ψ and φ and obtain angles ψ' and/or angles φ' to avoid sudden spikes or discontinuities that cause high energy spread in the transformed domain (303).

After computing either or both of the angles ψ' and the angles φ', transformed domain compression is performed to either or both of them (304). The resulting elements of that transformed domain compression are given to block 305, which quantizes their values to prepare the beamforming report (the feedback or the feedback information).

For the rest of the document, the assumed exemplary operation is the transformed domain compression of the angles ψ' and φ', although the case in which only one of the types is compressed is also included in the embodiments. Therefore, the combination of conditioned φ' and the transformed domain compression of ψ is also included in the embodiments. Also, transformed domain compression of either or both of ψ and φ without the operation of any conditioning is included in the embodiments.

[0030]

FIG. 4 shows an example of the conditioning operations in block 303, where both angles (pi(SCj) and v|/i(SCj) are conditioned. An index 'i' indicating the current angle position is initialized to a value of 1 (401). The function (pi(SCj) corresponding to that angle is conditioned (402). The function \| i(SCj) is also conditioned (403). The index 'i' is compared to the total number of angles N_angies (404). If it is lower, it is updated by incrementing it in one unit (405), and the process is iterated. If it is not lower, the process ends.

[0031]

FIG. 5A shows an example of the values of the function cp(SCj). The abscissa means subcarriers. The ordinate represents (p(SQ). The range of this function is [-π, π]. Occasionally, the difference in value between consecutive samples is larger than π. These sudden changes are reflected in the transformed domain characteristic as a spread of the energy towards higher samples.

FIG. 5B shows an example of the same characteristic after performing conditioning to it as explained below. In FIG. 5B, the sudden changes are eliminated.

[0032]

FIG. 6 shows an exemplary conditioning process to follow with each of the functions ⁽Pi(SCj), corresponding to the block 402 in FIG. 4, where i = 1, 2, N_angies- An index 'j' indicating the current sample is initialized to a value of 2 (601). The difference between the current sample and the previous sample is compared to π (602). If the difference is bigger than π, 2·π is subtracted from the current sample and the subsequent samples (603). If it is not, the difference is compared to -π (604). If the difference is lower than -π, 2·π is added to the current sample and the subsequent samples (605). If there remain samples to be computed, the index j is updated (607) and the process is iterated. The process finishes when all the samples have been computed (606). The result is φ' .

[0033]

FIG. 7A shows an example of the values of the function v|/(SCj). The range of this function is [0, π/2]. The abscissa means subcarriers. The nature of the operation performed to obtain these values introduces an ambiguity in whether conditioning is appropriate or not. In FIG. 7 A, the value of point B is larger than that of point A, and the value of point C is lower than that of point B. The curve going through the points A, B and C is depicted. There are two possible reasons for which the value of point C is lower than the value of point B. The first possible reason is that, due to v|/(SCj) being restricted to the range [0, π/2], the value of C, that was originally higher than the value of B, is folded back into the range [0, π/2] by the cosine operations performed to compute the givens rotation angles. The second possible reason is that its value is naturally lower.

[0034]

FIG. 7B shows the effect of conditioning, through which the point C is replaced with the point D. The point D is symmetrical to the point C with respect to the straight line \|/(SCj) = π/2. In the following, such straight line may be referred to as a border of the region. The resulting curve going through the points A, B and D is depicted. The curve going through the points A, B and D is smoother than the curve going through the points A, B and C, and therefore its transformed domain

representation presents a higher percentage of its energy in the lower samples.

[0035]

FIG. 8 shows the flowchart of a possible way to compute the conditioning of the function \|/i(SCj), corresponding to block 403 in FIG. 4. First, the candidate points for which conditioning might be advantageous are identified and stored in a variable (in the flowchart, variable G(p), where p = 1, 2, .. . , N_candidate, and N_candidate is the total number of candidate points) (801). A series of operations will be done for each of these points as follows. The point for which conditioning is currently evaluated is denoted by an index p, which is initialized with the value 1 (802).

The conditioned value ψ' of the candidate point indicated by G(p) is computed (803). The smoothness of the resulting characteristic is evaluated and compared with the smoothness of the characteristic if no change is performed (804). If the

conditioning of the point results in a smoother characteristic, the point G(p) and all subsequent points are conditioned (805). Otherwise, no change is effected (806). The variable G is checked (807). If all candidates have been evaluated, the process finishes. If there remain candidates to be evaluated, the counter p is incremented (808) and the process is iterated.

[0036]

The smoothness resulting from the conditioning process and the smoothness of the original signal can be compared by many different ways. For example, the angle of the straight line joining the conditioned point and the previous point can be compared with the angle of the straight line joining the two points immediately before the conditioned point. If the conditioning process results in a difference between angles smaller than before the conditioning, the conditioning process is appropriate and can be applied. Another example would be to compute the second derivative in the point previous to the candidate point when the point is conditioned and when it is not. If the conditioning results in a lower second derivative, the conditioning process is performed.

[0037]

FIG. 9 shows an exemplary conditioning process in which the means to evaluate the smoothness of the characteristic is based on the slopes of the lines joining the candidate point and the two points immediately prior to it.

First, the slope of the straight line linking each pair of points is computed. The positions B (as defined in figures 7A and 7B) for which the previous slope_A→B has a different sign than the posterior slope_B-»c are detected, and these positions B are stored in an array called "anchor" (901). The positions B are potential positions in which the change in slope may be large. A recoverable modification to the function immediately after some of the positions B (that is, a modification to the positions C and subsequent positions) may result in a lower energy spread in the transformed domain.

The function ψι has values in the range [0, π/2]. For easier operability, the function \|/_sym = ψ_ί- π/4 is created (902). The range of this function \|/_sym is [-π/4, π/4].

An index 'm' indicating the current element of "anchor" is initialized to a value of 1 (903).

[0038]

The variable "point" represents the candidate point, and is the position starting from which the function would be altered if deemed necessary according to the conditions explained in the next modules. The variable "point" corresponds to the position anchor(m) + 1. The "previous_slope" is the slope corresponding to the line linking the position "anchor(m)" with the position previous to it (anchor(m)-l) (904).

In the case of the function \|/_sym, all the values are in the range [-π/4, π/4]. Initially, the conditioned value of the point is the symmetrical value with regard to either -id 4 or π/4. In the event of performing conditioning, the candidate point and all subsequent points are shifted to a different interval [-π/4+ζ·π/2, π/4+ ζ·π/2], where z is an integer representing the total shift from the original range. In order to obtain the value of the conditioned point, the border that must be crossed must be computed, the border being either -π/4+ζ·π/2 or π/4+ ζ·π/2.

[0039]

The sign of "previous slope" is compared with the sign of v|/_sym (point) (905).

If the signs are the same, the "border", which marks the reference to the modified resulting point is the one that is farther from the origin, and can be computed with a ceiling operation as shown in Equation 9 (906). The origin is the line in which the ordinate is zero. The ceiling function fx] gives the largest integer larger than or equal to x.

[0040]

[Equation 9] border = sign(y/_sym (poinij- (9)

[0041]

If the signs are different, the "border" is the one that is closer to the origin, and can be computed with a floor operation as shown in Equation 10 (907). The floor function LxJ, also called the greatest integer function or integer value, gives the largest integer less than or equal to x.

[0042]

[Equation 10]

(10)

[0043]

The value of the modified point is computed as "new_value" according to Equation 11 , and the "new slope" is computed as the slope between the point "new value" and \|/_sym(anchor). The value of the slope of the line linking the position "anchor" with the original v|/_sym (point) is stored as "old_slope" (908).

[0044] [Equation 11]

π

new value— {border + sign(previoas _ slope)^■ V_sAjPoint) (11)

4 J

[0045]

A comparison is made to see if the difference between "new_slope" and "previous slope" is smaller than the difference between "old_slope" and

"previous_slope" (909).

In the case that assumption is true, the positions starting from the variable "point" of the function \|/_sym are updated to be symmetric to "border" as shown in equation 10, and the slopes are updated accordingly (910).

[0046]

A check is performed to see if there remain elements in "anchor" to iterate this process for (91 1). In that case, the index 'm' is updated as m = m + 1 and the process is repeated (912). If not, the resulting ψ' is obtained by returning the final \|/_sym to the range [0, π/2)] (913).

[0047]

FIG. 10A shows the average absolute values obtained in the transformed domain for the angles φ' for an exemplary case of a 4x4 transmission with a 40 MHz bandwidth under channel model D (Non-patent reference 1). After performing conditioning to the angles, the energy spread is highly diminished, and most of the energy is now in the first few samples, allowing for higher compression.

FIG. 10B shows the average absolute values obtained in the transformed domain for the angles ψ'. Compared to FIG. 2B, the average absolute value of the first few samples is larger. As the energy is proportional to the second power of this value, the proportion of energy that remains in the first few samples is increased. The gain is not as large as in the case of the angles φ', but still allows for a higher degree of compression by concentrating more energy in the first few samples.

[0048]

FIG. 11 shows the flow diagram of an example transformed domain

compression technique that can be performed in block 304 in FIG. 3. This operation is performed for the angles ψ' and φ'.

The angles are divided into two regions (1101). A first region, for example a lower region, comprises the angles corresponding to the subcarriers that are positioned before the DC subcarriers. A second region, an upper region, comprises the subcarriers that are positioned after the DC subcarriers. First, the working set W is taken as the first region (1 102). The angles corresponding to the set W are, according to equation 8, those between N_start = 1 and Nend ⁼ Nsc /2. X[n] is obtained by performing DCT operation to the working set W (1103).

The compression is attained by truncating the function X[n] and transmitting only the first L samples, where most of the energy is concentrated (1104). L can be any value from 1 to N_Sc/2, which is the number of subcarriers in each one of the regions. Equation 12 shows this process.

[0049]

[Equation 12]

Xf^ee<iback[l] = Xft] l =\,2,.:L ( 12) [0050]

If all the regions have been computed, the operation ends (1105). If the second region, the upper region, has not been computed yet, the working set W is updated to the second region (1106), for which N_start = Nsc /2+1 and N_end = N_Sc , and the process is iterated from block 1 103.

[0051]

FIG. 12 shows the reverse process at AP 101. AP 101 first extracts the values of the angles ψ' and φ' from the feedback sent by the STA (1201). Reverse

quantization is applied to these values for further operations (1202). AP 101 needs to perform additional operations to recondition the values of ψ', for example as shown in equation 13 (1203).

[0052]

[Equation 13]

[0053]

In the above equation, '^uniuant represents the angles ψ obtained after

performing reverse quantization.

This process is not needed for the values of φ'. They are operated through cosines in further modules and the result is not affected by the total value of φ' (that is, cos(cp') = cos(cp)). With the values of ψ and φ', AP 101 computes the precoding matrix V (1204). [0054]

FIG. 13 shows the flow diagram of recovering the angles ψ' and φ' that can be performed in block 1201 in FIG. 12. The process is explained for the angles ψ', but it is identical for the angles φ'.

First, the values corresponding to the first region, for example the lower region, are assigned to the variable X (1301). The samples that were truncated by the ST A are considered as of value '0' by AP 101 (1302). The inverse DCT operation is performed to X (1303).

[0055]

The recovered values of ψ' are stored (1304). If not all the regions have been computed yet (1305), X is updated with the values of the second region (1306), and the process is iterated from block 1302. Also, the above mentioned process is performed for the second region.

Once all the regions have been iterated, the stored angles corresponding to each stage are arranged back together in the appropriate order (1307).

Some exemplary options have been described in detail in the precedent paragraphs, but they don't preclude the utilization of other techniques following the same spirit, which is the compression of the fundamental elements through transformed domain transformation and selection of the most representative points therein.

Apart from DCT, other transformation techniques such as DHT (Discrete Hartley Transform) or DFT (Discrete Fourier Transform) can be employed.

[0056]

Figure 14 shows one example of the configuration of STA 110. STA 1 10 designates generally STAs 11 1 to 1 18.

Signal reception 1401 receives the data streams from AP 101 through antenna 1412, and after down-conversion to the baseband and AD conversion applies Fast Fourier Transformation (FFT) to change the signal to the frequency domain.

Pilot demultiplexing 1402 extracts the pilot symbols from the frequency domain signal. It conveys these pilot signal values to Channel estimation 1406 and the signal without the pilot symbols to Un-rotation 1403. The subcarriers in training fields and the subcarriers that appear in a prefixed configuration in data fields are considered as pilot symbols in this document.

Channel estimation 1406, based on the values of the extracted pilot symbols, estimates the channel characteristics and the SINR (Signal to Interference Noise Ratio) perceived by each receiving antenna 1412 from each transmitting antenna at AP 101. Receiving antenna 1412 represents one or more antennas. Decomposition computation 1409 performs the singular value decomposition (SVD) of the channel estimated by Channel estimation 1406 and obtains two

fundamental angles φ and ψ. It outputs the two fundamental angles φ and ψ to the Elements conditioning 1410.

Elements conditioning 1410 performs the conditioning operations described above to the angles φ and ψ obtained in Decomposition computation 1409. It outputs the resulting φ' and ψ' to Transformed domain compression 1410.

[0057]

Rotation estimation 1405 estimates the rotation experienced by the signal as shown in Equation 4 by observing the long training field pilot signals, and gives the result to Un-rotation 1403.

Un-rotation 1403 pre-multiplies the received data symbols by the inverse of the rotation matrix estimated by rotation estimation 1405.

Data extraction 1404 demodulates the data symbols output by Un-rotation 1403 and performs error correction to the demodulated data streams, retrieving the reception data streams.

Transformed domain compression 1411 receives the angles ψ' and φ' from Elements conditioning 1410 and performs compression of the rotation angles ψ' and φ' through transformed domain transformation as explained above.

[0058]

Feedback creation 1407, according to the indications of Control 1413, creates the feedback to be transmitted to AP 101, using the results from Channel estimation 1406, and the elements corresponding to the compression in the transformed domain of the angles ψ' and φ' as computed by Transformed domain compression 141 1. In Feedback creation 1407, the truncated samples in the transformed domain for angles ψ' and φ' transferred from Transformed domain compression 141 1 are quantized and sent to Wireless transmission 1408 to be used as feedback information. Feedback creation 1407 gives feedback information to Wireless transmission 1408.

Wireless transmission 1408 transmits the prepared feedback through antenna 1412 to AP 101 after DA conversion and up-conversion to the wireless frequency band.

Control 1413 processes the necessary actions for the above mentioned units.

[0059]

FIG. 15 shows one example of block diagram for AP 101.

Wireless reception 1506 receives the data transmitted from each STA arriving at antennas 1505-1 to 1505-n after down-conversion to the base band and AD

conversion. Feedback analyzer 1507 extracts the feedback information from each of the data streams received from STAs. Feedback analyzer 1507 gives the transformed domain compressed angles ψ' and φ' to Elements recovery 1508 and the SINR information to Selection 1510 after performing reverse quantization.

Elements recovery 1508 receives the transformed domain compressed samples corresponding to the angles ψ' and φ', and recovers the value of these angles by any of the procedures described above.

ψ reconditioning 151 1 receives the angles ψ' from Elements recovery 1508 and recovers the real value of the angles ψ obtained from the precoding matrix V.

V-retriever 1509 obtains the precoding matrix (Equations 1 to 4) from the angles φ' given by Elements recovery 1508 and the angles ψ given by ψ reconditioning 1511.

Selection 1510, based on the SINR feedback information given by Feedback analyzer 1507 and the V matrices obtained by V-retriever 1509, selects a group with potentially many affiliated STAs.

[0060]

Transmission buffer 1501 stores the transmission data streams coming from upper layers that is intended to be transmitted to STAs, and conveys it to Signal creation 1502-i as indicated by Selection 1510, which takes care of addressing each data stream 1 to n to its corresponding STA. The number 'i' means either of 1 to n.

Signal creation 1502-i performs forward error correction to the data coming from transmission buffer 1501. Furthermore, it performs rate matching (through puncturing) to adapt the coding rate to the rate selected by Selection 1510 for each of the data streams. Afterwards, modulated streams are created by modulating the resulting streams and the pilot symbols are multiplexed with them.

Precoding 1503, having as an input the modulated streams and the matrix V from V-retriever 1509, performs precoding to those streams.

Transmission 1504-i performs IFFT (Inverse Fast Fourier Transform) to each of the precoded streams (OFDM (Orthogonal Frequency Division Multiplexing) streams), carry out DA (digital to analog conversion) and frequency conversion and transmits from Antenna 1505-i each of the streams.

Control 1512 processes the necessary actions for the above mentioned units.

[0061]

According to the first embodiment, less overhead for interference free feedback prepared by the receiving device may be used in the communication system, especially in the MIMO communication system. [Embodiments 2]

[0062]

Another embodiment of this invention is a system in which the STA 1 10a only performs conditioning operations to a subset of the total number of functions ψ and φ.

[0063]

FIG. 16 shows the flow of the process followed by the station to feed back the fundamental elements of the channel. It is similar to FIG. 3, but in this case each stream of ψ and φ is conditioned only if the energy spread of their transformed domain representation is over a certain threshold (1601).

[0064]

FIG. 17 shows the flow diagram of an example transformed domain compression that can be performed in block 1601 in FIG. 16. This operation is similar to the one described in FIG. 1 1. Therefore, the explanation of blocks 1 101 to 1 103, 1 105 and 1 106 is abbreviated.

After performing the DCT operation of W, the energy spread of each stream of ψ or φ is computed and evaluated (1701).

A measure 5, of the energy spread of the i^th stream of ψ or φ can be obtained as expressed in equation 14:

[0065]

[Equation 14]

[0066]

In the above equation, 'n' represents a sample of the transformed domain, and Xi(n) is the transformed domain expression of the i^th stream of either ψ or φ.

A threshold over which the conditioning takes place can be a prefixed threshold, or can be related to the conditions of each situation. For example, the conditioning may be performed for each stream of ψ that presents an energy spread S at least 10% higher than the average of the energy spreads of all streams of ψ. It may be performed for each stream of φ under the same conditions over the average of all streams of φ. Equation 15 shows the value of the threshold for the case of ψ or φ.

[0067] [Equation 15]

[0068]

If the energy spread of a stream W is evaluated as too high, the stream is conditioned following the steps explained in the first embodiment (1702). The DCT operation of the resulting W is performed (1703), and the truncation is performed over it (1704). The streams for which the energy spread is not deemed too high are not conditioned, and the truncation is performed over the DCT operation of W (1704).

[0069]

FIG. 18 shows the reverse process at AP 101. This process is similar to the one described in FIG. 12. Therefore, the explanation of blocks 1201 1202 and 1204 is abbreviated. After computing the reverse quantization of ψ', the range of each of its streams is compared to the nominal range [0, π/2] (1801). If any of the elements of ψ is out of said range, reconditioning is performed to that stream, following the steps explained in the first embodiment (1802).

[0070]

FIG. 19 shows one example of the configuration of STA 110a. It is similar to FIG. 14. Therefore, the explanation of blocks 1401 to 1412 is abbreviated.

The fundamental elements of the matrix V obtained in Decomposition computation 1409 are given to Energy spread estimation 1901. At the moment, switch 1902 occupies its neutral position. Energy spread estimation 1901 receives the transformed domain expression of ψ and φ from Decomposition computation 1409. If the threshold passed, the output of Energy spread estimation 1901 controls the switch 1902 to pass the output of Decomposition computation 1409 to Element conditioning 1410. If the threshold is not passed, the output of the Energy spread estimation 1901 controls Switch 1902 to pass the output of Decomposition computationl409 to

Transformed domain

compression 141 1 directly.

[0071]

FIG. 20 shows one example of the configuration of AP 101a. It is similar to FIG. 15. Therefore, the explanation of blocks 1501 to 1507 and 1509 to 1511 is abbreviated.

Feedback analyzer 1507 gives the recovered values of ψ' and φ' to Check and recover 2001. Check and recover 2001 receives the transformed domain compressed samples corresponding to the angles ψ' and φ', and recovers the value of these angles by any of the procedures described above. If the resulting streams ψ' present a range broader than [0, π/2], those streams are given to ψ reconditioning 1511. The streams of φ' that don't require reconditioning and the angles ψ are given to V-retriever 1509. [Embodiments 3]

[0072]

In another embodiment of the invention, STAs 1 11 to 118 in FIG. 1 are able to estimate the channel from a sounding frame sent by AP 101. In the embodiment, STAs 111 to 113 use the standard procedures, in which fundamental angles φ and ψ are sent back to AP 101, for example. Other STAs 114 to 118 send the feedback according to the first or second embodiments. AP 101 in FIG. 1 supports all kind of feedback transmission, and is able to recover the channel information corresponding to each of STA 111 to 118.

[0073]

The embodiments include any kind of program that, exerting control over AP or STAs, realizes the functions related to the invention described in the embodiments by, for example, controlling the operation of a CPU (Central Processing Unit) in the

Control units. The information used by this terminal, as well as the results of its processing, can be stored in RAM (Random Access Memory) to be later stored in a more permanent solution such as Flash ROM (Read Only Memory) or other kinds of ROMs or HDDs (Hard Disk Drives). Said information can be read from that memory as needed by the CPU, which has the ability to correct or overwrite that data.

In order to realize all the functions described in the embodiments of the invention, the information is registered in a storage medium that can be read by a computer. The computer is able to access this information and load it into the computer system, carrying out the processing identified with each unit. Furthermore, in this text, "computer system" includes the operating system and all the required hardware.

[0074]

"A storage medium that can be read by a computer" can be a flexible disk, a magnetic or optic disk, a ROM, a CD-ROM, a portable device with storage capabilities, etc. It is any kind of storage device that can be connected to the computer system. Furthermore, "storage medium that can be read by a computer" also includes any way of sending the program in a sufficiently short time through the internet, a network, a telephone circuit, etc. in a way such that the program is dynamically maintained in the pair server - client. The above stated program includes any device conceived in order to perform part of the previously mentioned functions, as well as any computer system in which the previously mentioned functions are already embedded, providing the capability of performing any combination of them. The embodiment also includes any integrated circuit that can carry out part or all of the functionalities described in the communications systems above (related to either transmission or reception). A microchip being able to perform part or all of the above described individual diagram blocks of communications system is also considered. This description is not limited to specific purpose integrated circuits (for instance LSI or VLSI), but also includes more general purpose devices that perform these operations. It is also possible to substitute the semi-conductors present in the integrated circuits with any other material that allows the above described operations. That is also included in the present invention.

[0075]

Although the above text references the figures to give a detailed explanation of an example configuration of this invention, it is not limited to it. Any possible configuration that changes the blocks but does not deviate from the main point and idea of this document is included,

[industrial Applicability]

[0076]

This invention can be used in a wireless communication, especially in a MIMO communication.

[Reference Signs List]

[0077]

101 : wireless transmitting devices

1 11 to 118: wireless receiving device

101a: wireless transmitting device

101b: wireless receiving devices

[Citation List]

[Non Patent Literatures]

[0078]

[NPL 1]

"IEEE 802.11-2012 (Revision of IEEE Std 802.1 1-1999)", March 2012

[NPL 2]

"IEEE P802.1 lac/D3.0", June 2012

Claims

[CLAIMS]

[Claim 1]

A wireless receiving device comprising: a decomposition computation unit to reduce channel characteristics of the wireless communication into fundamental elements; a conditioning unit to transform the fundamental elements into conditioned elements in a recoverable fashion; and a transformed domain compression unit to transform input signals into a transformed domain and compress them, wherein the transformed domain compression unit receives the conditioned elements from the conditioning unit as its input.

[Claim 2]

A wireless receiving device according to Claim 1, wherein the fundamental elements consist of a first set of angles in a real domain and a second set of angles, the wireless receiving device performing operations to reduce the number of elements of one or both sets of angles.

[Claim 3]

A wireless receiving device according to Claim 2, wherein the conditioning unit performs the conditioning on either one or both of the first and second sets of angles, and the transformed domain compression unit performs the compression on either or both of the first and second sets of angles.

[Claim 4]

A wireless receiving device according to Claim 2, wherein the conditioning unit performs the conditioning on the first sets of angles, and the transformed domain compression unit performs the compression on both the first and the second sets of angles.

[Claim 5]

A wireless receiving device according to Claim 1, further comprising: an energy spread estimation unit and a switch unit, wherein the energy spread estimation unit evaluates the output of decomposition computation unit from an energy spread perspective, and the switch unit transfers the output of decomposition computation unit to the conditioning unit or to the transformed domain compression unit.

[Claim 6]

A wireless communication system comprising: a wireless transmitting device and wireless receiving devices, wherein some wireless receiving devices feed back the channel characteristic of their wireless communication channel, and the others feed back the transformed domain compressed elements of their wireless communication channel, these elements having been conditioned or not, to the wireless transmitting device.

[Claim 7]

A method of receiving data streams through a wireless communication channel: comprising the steps of decomposing a channel characteristic to its fundamental elements; conditioning the fundamental elements to reduce their energy spread; and compressing the conditioned elements in the transformed domain.

[Claim 8]

A method of receiving data streams through a wireless communication channel: comprising the steps of conditioning either or both of a first and second set of angular elements obtained from a channel characteristic; and compressing either or both of the first and second angular elements.

[Claim 9]

A method of receiving data streams through a wireless communication channel: comprising the steps of conditioning a first set of angular elements obtained from a channel characteristic; and compressing both the conditioned elements and a second set of angular elements obtained from the channel characteristic.

[Claim 10]

A wireless transmitting device: comprising a V-retriever unit to retrieve an orthonormal matrix obtained after the singular value decomposition of a channel characteristic; and a precoder to perform precoding on data streams to be transmitted using the orthonormal matrix.

[Claim 11]

A method of transmitting data streams through a wireless communication channel: comprising the steps of retrieving an orthonormal matrix obtained after the singular value decomposition of a channel characteristic; and precoding on the data streams using the orthonormal matrix.