Patent Application
20110013603
In wireless communications, beamforming with matrix feedback has been used to provide significant improvements. Previously, when beamforming has been used, there was only one beamforming matrix feedback per frequency subband. This causes an approximate 10% performance degradation due to frequency selectivity across the subband. The beamforming matrix is then used for the transmit beamforming for the whole subband. This causes performance degradation because the channel response and thus the ideal beamforming matrix vary across the subcarriers within the subband. This problem gets severe as the subband bandwidth increases.
Thus, a strong need exists for improved techniques for MIMO beamforming for frequency selective channels in wireless communication systems.
The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the preset invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulate and/or transform data represented as physical (e.g., electronic) quantities within the computer's registers and/or memories into other data similarly represented as physical quantities within the computer's registers and/or memories or other information storage medium that may store instructions to perform operations and/or processes.
Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. For example, “a plurality of stations” may include two or more stations.
Embodiments of the present invention provide schemes that feed back a plurality, such as two, beamforming matrixes per subband and interpolate the beamforming matrixes across the subband. In an embodiment of the present invention, a novel interpolation scheme is provided, which minimizes the interpolation error. A gain of 4.1% is achieved for typical channels under the same feedback overhead. Depending on the system configuration, the whole frequency band may consist of one or multiple subbands.
As set forth above, in existing systems, only one beamforming matrix is fed back per frequency subband. The beamforming matrix is then used for the transmit beamforming for the whole subband. This causes performance degradation because the channel response and thus the ideal beamforming matrix vary across the subcarriers within the subband. This problem gets severe as the subband bandwidth increases.
For multiuser multiple input multiple output (MIMO), a large subband width is used to increase the chance of user pairing. Therefore, the subband usually has 72 subcarriers i.e. about 800 kHz. The variation of the channel response within the subband causes the ideal beamforming angle to vary for about 60 degrees for typical channels, which are spatially uncorrelated and spatially weakly correlated MIMO channels. An example of the real part of the channel response is shown in
In embodiments of the present invention, instead of one beamforming matrix, the present invention provides feeding back a plurality, such as two, beamforming matrixes. This is particularly useful, if uplink feedback width is available or one user's rough beamforming causes strong interference to the others. It can be an optional configuration for the mobile user to generate two feedbacks per subband. Since the feedback channel can indeed carry more bits for strong users, this option allows the strong users to benefit from their good channels. The two beamforming matrixes are for each of the two ends of subband, respectively. Interpolation may be made for all the beamforming matrixes in the subband using the two fed back matrixes. The applied beamforming matrixes vary across the subband and some embodiments of the present invention select the feedback indexes of the two beamforming matrixes at the two subband ends jointly, taking the interpolation into account. Turning now to
There are multiple ways to interpolate the beamforming matrixes between the two fed back beamforming matrixes. Note that the beamforming matrix is unitary and it is on the Grassmann manifold as shown in
Let M=AHB, where A and B are the fed back beamforming matrixes; A and B are Nt×Ns unitary matrixes, i.e. AHA=I and BHB=I; Nt is the number of transmit antennas and Ns is the number of beamformed streams. Particularly, a single spatial stream is sent and the beamforming matrixes A and B are Nt×1 vectors when Ns=1. The singular value decomposition of M is given by
M=QAΣQBH (1)
where QA and QB are Ns×Ns orthogonal matrixes and Σ is a diagonal matrix. Let Ã=AQA and {tilde over (B)}=BQB. Then;
Let σi=cos θi for i=1, . . . , Ns. θi is the angle between the i-th column of Ã, denoted by ãi, and the i-th column of {tilde over (B)}, denoted by {tilde over (b)}i, as illustrated on the right in
θi(k)=akθi, for i=1, . . . , Ns (3)
where
is inversely proportional to the frequency spacing between A's subcarrier and B's subcarrier, i.e. |fA-fB| and is proportional to the frequency spacing between A's subcarrier and the k-th subcarrier, i.e. |fk-fA|. After the angle is interpolated, a vector {tilde over (c)}i(k) interpolated between the i-th column of Ã, ãi, and the i-th column of {tilde over (B)}, {tilde over (b)}i, is computed as illustrated on the right in
{tilde over (C)}(k)=[{tilde over (c)}1(k) . . . {tilde over (c)}N
If {tilde over (C)}(k) is not a unitary matrix, it can be converted to a unitary matrix that spans the same subspace using algorithms such as QR decomposition or Grant-Schmidt operation. In order to minimize the phase transition of the beamforming matrixes across the subband, an Ns×Ns orthogonal matrix Q(k) can be multiplied from the right to each beamforming matrix including A, B, and {tilde over (C)}(k)s. For example, {tilde over (C)}(k) may be converted to C(k) as
C(k)={tilde over (C)}(k)Q(k), (6)
where Q(k) may be equal to QAH; C(k) is used for actual beamforming.
Looking now at
For complexity reduction and performance enhancement, the receiver may select two beamforming matrixes close to the two ends of the subband and interpolate the beamforming matrixes only for a selected subset of subcarriers. For example, the receiver may partition the 72 subcarriers within the subband in 18-subcarrier group. The 18 subcarriers in each group are contiguous. The beamforming matrixes of the group center subcarriers are fed back or interpolated. The fed and interpolated beamforming matrixes are used for each group without further interpolation.
Looking now at
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
