An example embodiment will now be described based on a wireless multi-antenna transmission system, such as—but not limited to—a MIMO system with a general uplink (UL) feedback scheme for MIMO downlink (DL) transmission for an example case of four available transmission antennas at a transmitter unit of a base station device, such as a Node B. However, it will be apparent from the following description and is therefore explicitly stressed that the present disclosure can be applied to other embodiments, such as, for example, another network architecture with different radio access technologies involving multi-antenna transmitter devices (e.g. base station devices, access points or other access devices) capable of being operated in different operating modes.
In transmission systems where forward and reverse channels are not reciprocal, MIMO systems may require coarse quantization of the channel and a beamforming vector to accommodate the limited bandwidth of the feedback channel. To support such limitations of the feedback channel, codebooks of possible beamforming vectors can be used, which are known to both the transmitting and receiving ends. The codebook is restricted to have fixed cardinality and may be designed off-line. The receiving end (e.g. mobile station) is assumed to select from the available codebook the best beamforming vector or matrix and to convey it over the feedback channel to the transmitting end (e.g. base station). More specifically, the receiving end learns the CSI from received downlink information and selects a transmit beamforming vector or matrix from the available codebook. An index of the selected beamforming vector or matrix is then fed back to the transmitting end. Having received the index, the transmitting end looks up the corresponding codebook and selects the beamforming matrix or vector according to the index. The selected matrix or vector can then be used for MIMO precoding operation.
According to an example embodiment, a new 6-bit codebook is disclosed (below) for the four transmission antennas 201 to 204, which may provide better performance than conventional codebooks, considering different correlation and scenarios. The 6-bit codebook may comprise a combination of a first codebook (or first codebook subset) for phase-only transmission control and at least one of two other codebooks (or second and third codebook subsets) for antenna subset selection and single-antenna selection, respectively. As an example, the first codebook may be a Hochwald codebook or any other type of codebook which provides phase-only transmission control of the transmission beams generated by the transmission antennas.
Additionally, in a specific example of four transmission antennas, a size-48 Hochwald codebook may be used, which may be enhanced by a size-16 codebook comprising the second and third codebook subsets. The second codebook subset may comprise twelve codebook elements (e.g., precoding or beamforming vectors) for antenna subset selection, and the third codebook may comprise four codebook elements for single antenna selection.
Corresponding other codebook sizes may be utilized for a different number of transmission antennas, considering, for example, the tradeoff between overhead and performance.
In an example embodiment, the transmitter and receiver may maintain or store a common codebook, such as a finite collection of precoding vectors (codewords). In this example embodiment, the receiver determines which vector(s) are selected to be used from the codebook and then feeds its index back to the transmitter via a feedback channel. After receiving the codeword index, the transmitter determines the corresponding beamforming or precoding vector(s) for data transmission. The selection of proper beamforming or precoding weights from the codebook may follow some criterion, such as maximizing the post-processing SNR or maximizing the sum of the throughput of all streams, as non-limiting examples.
The transceiver unit 14 may be in communication with a signal processing stage 12, the latter of which may be responsible for receiver-related processing, such as demodulating, descrambling, decoding etc. of received downlink data, and/or for transmitter-related processing, such as modulating, scrambling, coding etc. of uplink data to be transmitted, and which may also be configured to add feedback information for precoding or beamforming to the uplink data stream. This feedback information may comprise an index to an element of a codebook 18, which may maintain or store an uplink feedback circuit 16. The uplink feedback circuit 16 may generate uplink feedback index information 70 based on a corresponding control information issued by the signal processing stage 12. The uplink feedback index information 70 may comprise an index to an element of a codebook 28 (shown in
In an alternative example embodiment, the codebook 28 may be maintained or stored at the signal processing unit 26, wherein the feedback index information 70 may be supplied by the feedback extraction unit 27 to the signal processing unit 26.
The generation of the enhanced codebooks 18 and 28 is now described with reference to
According to the example embodiment shown in
w
l
=Q
l−1
w
1
, l=2,3, . . . L
where L is the size of the Hochwald codebook (48 in the present example of four antennas 201 to 204), w1 is the first element, which can be chosen to be one column of Mt×Mt IDFT (Inverse Digital Fourier Transformation) matrix, for example
where Mt is the number of transmit antennas, and the above rotation matrix Q is a diagonal matrix constructed by an integer rotation vector u=└u1 u2 . . . uM
The choice of the rotation vector may minimize the maximum correlation between elements in the codebook. The exemplary 48 elements may all lead to phase-only adaptation from the four antennas 201 to 204, providing good performance in strong correlated channel.
Additionally, the last sixteen elements in the codebook may cover the second and third subsets 120 and 130, and may include twelve elements of the second subset 120 for antenna subset (e.g., antenna pair) selection with zero or π relative phase rotation, and an additional four elements of the third subset 130 for single antenna selection. This selection of codebook elements may help the proposed codebook to improve the performance in uncorrelated channel in addition to phase-only weighting achieved by the incorporated Hochwald codebook of the first subset 110.
The example codebook described with reference to
As more general examples, improved Hochwald or other phase-only adaptation codebooks combine the Hochwald-type or other phase-only adaptation codebooks with antenna subset selection with phase rotation. In a general expression “x+y+z” means a codebook including size-x Hochwald or phase-only adaptation codebook, y elements of two or more antenna selection, and z elements of single antenna selection. In the specific but non-limiting case of a 6-bit codebook, the sum of x, y and z is sixty-four. The number z of single-selection codebook elements is thus the number of transmit antennas, while the number y of antenna-subset selection codebook elements depends also on the number of possible relative phases given two or more selected antennas. For example, for a “48+12+4” codebook, two antennas are selected and the two relative phases are zero or π, so that y=2*C(4,2)=12. For a “42+18+4” codebook, the three different relative phases are zero, 2*π/3 and 4*π/3. For a “36+24+4” codebook, relative phases are zero, π/2, π and 3*π/2, and so on. This can be basically written as phases φi=(i−1)·2π/L, 1≦i≦L having L different phase states.
As another example, “(64−m)+y+z” means a codebook including a size-64 Hochwald codebook, in which m elements have been left out, y elements of two antenna selection, and z elements of the single antenna selection. The sum of 64−m, y and z is 64, white y and z have the same meaning as in the above “x+y+z” codebook.
The above “48+12+4” codebook example provides weights for both single antenna selection and antenna subset selection. It includes twenty-four orthogonal pairs (eighteen pairs from weights for antenna subset selection and six pairs from weights for single antenna selection) which can be used for two stream transmission. The number of additional orthogonal pairs in the first subset of the codebook is dependent on the selected phase-only adaptation codebook or Hochwald codebook. Pairs of weights for single antenna selection can be used, for example, for 4×2 S-PARC (Selective-Per Antenna Rate Control) systems. The weights for antenna subset selection corresponds to a generalization of a 1-bit TxAA mode 1 and antenna selection. Some orthogonal pairs of weights for antenna subset selection can also be used for Double TxAA, DSTTD-SGRC (Double STTD—Sub Group Rate Control) or GS-PARC (Group Selective Per Antenna Rate Control).
The combination of elements of the at least one of the second and third codebook subsets with the first codebook subset may help the disclosed codebook to improve the performance in uncorrelated channel in addition to phase-only weighting from first codebook subset. A structured approach may thus, for example, be used to generate the codebook. This structured approach allows generation of the codebook when necessary, which means that codebook elements (e.g., codeword, vectors or matrices) do not have to be stored all the time, which is advantageous over some random-searched codebooks, e.g., Grassmannian and Xia's codebooks.
In an embodiment, the third subset of elements for single antenna selection may comprise a number of elements corresponding to the number of transmission antennas. The second subset of elements may comprise elements for antenna subset selection with L different relative phase rotations φi=(i−1)·2π/L, 1≦i≦L between selected antenna elements. The first subset of beamforming elements may comprises a Hochwald-type codebook with a circular correlation property.
In specific implementation example, the multi-antenna transmitting end may comprise four antennas. The second subset of elements may then comprise twelve elements for antenna subset selection of two selected antennas with zero or π relative phase rotation. The Hochwald-type codebook of the first subset may have a size of 48 elements.
Other combinations according to
As mentioned above, a structured approach may be used to generate the codebook. The generation of the codebook may be performed when the codebook is needed, which means that there may be no need to store the codebook elements all the time.
Better performance may be achieved for the proposed codebook considering different correlation and scenarios.
An example process which may be performed at the receiving end, for example, at the mobile station 10 (shown in
An example process which may be performed at the transmitting end, for example, at the base station device 20 (shown in
The processes described with reference to
These results also indicate that two antennas selection achieved by the above second codebook subset 120 (i.e., parameter y) may improve performance in a weak- or medium-correlated channel in addition to single antenna selection.
It is to be noted that the present disclosure is not restricted to the embodiments described above, but can be implemented, for example, in another network environment involving multi-antenna transmission controlled by feedback signaling. Another signaling format or means can be used for feeding back the feedback information, which may be an index information or even the codebook element itself. Moreover, another kind of codebook structure may be used for arranging the first codebook subset and the at least one of the second and third codebook subsets. Alternative embodiments may thus vary within the scope of the attached claims.
Number | Date | Country | Kind |
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06022252.8 | Oct 2006 | EP | regional |