The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein may be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.

Hereinafter, a method and apparatus for determining an amount of feedback information needed according to a transmission environment in a Multi-User Multiple-Input Multiple-Output (MU-MIMO) communication system will be described.

FIG. 1 is a flowchart illustrating a feedback process of a Base Station (BS) in a MU-MIMO communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 1, the BS measures or determines a transmission environment in step 100. The transmission environment may be determined according to a number of Mobile Stations (MSs), a Quality of Service (QoS, e.g., a transfer rate required by each MS), an interference level (or a noise level), etc.

In step 102, the BS determines a feedback amount for each MS according to the transmission environment. For example, in a case of using Per-User Unitary Rate Control (PU2RC), if a number of users is small, N-layer feedback may be used, and if the number of users is large, a single-layer feedback is sufficient (see FIG. 2A below). In addition, a feedback amount needs to be regulated so that N layers (or a single layer) are allocated to the MS according to QoS requirements of the MS (see FIG. 2B below). When the MS has two or more Receive (Rx) antennas, there is an advantage in that a Coordinated Beam Forming (CBF) shows an optimized performance in a noise-limited environment. However, the CBF performance is similar to a Zero Forcing (ZF) performance in an interference-limited environment. Therefore, a ZF scheme requiring a smaller amount of feedback is advantageous in the interference-limited environment (see FIG. 2C below).

In step 104, the BS allocates a resource corresponding to the determined feedback amount.

Thereafter, the procedure of FIG. 1 ends.

FIG. 2A is a flowchart illustrating a process of determining a feedback amount depending on the number of users according to an exemplary embodiment of the present invention.

Referring to FIG. 2A, a BS measures a number of MSs which attempt a network access in step 200.

The BS determines if the number of MSs is greater than a preset threshold in step 201. If the number of MSs is greater than the preset threshold, the BS allocates feedback resources for a single layer in step 202. That is, the BS allocates the resources to feed back an index of a precoding matrix and a corresponding layer index.

In contrast, if the number of MSs is less than the preset threshold in step 201, the BS allocates feedback resources for N layers in step 203. That is, the BS allocates the resources to feed back the index of the precoding matrix and corresponding N layer indices.

Thereafter, the procedure of FIG. 2A ends.

FIG. 2B is a flowchart illustrating a process of determining a feedback amount depending on a transfer rate required by an MS according to an exemplary embodiment of the present invention.

Referring to FIG. 2B, a BS measures the transfer rate required by the MS in step 210.

The BS determines if the transfer rate required by the MS is greater than a preset threshold in step 212. If the transfer rate required by the MS is greater than the preset threshold, the BS allocates feedback resources for a single layer in step 213. That is, the BS allocates the resources to feed back an index of a precoding matrix and a corresponding layer index.

In contrast, if the number of MSs is less than the preset threshold in step 212, the BS allocates feedback resources for N layers in step 214. That is, the BS allocates the resources to feed back the index of the precoding matrix and corresponding N layer indices.

Thereafter, the procedure of FIG. 2B ends.

As described above with reference to FIGs. 2A and 2B, cell capacity may be limited due to limited channel information. When a number of users is large, deterioration of system capacity may be minimized by performing MU-MIMO scheduling. However, there is a limit in increasing a QoS of the MS from the perspective of the MS since only a single layer may be allocated by feeding back only a Signal to Interference and Noise Ratio (SINR) for a single layer. In addition, when the number of MSs is small, deterioration of system capacity may increase due to limited channel information of the BS. In this case, performance deterioration may be prevented by increasing a feedback amount (e.g., N-layer feedback) of the MS. For example, if Post Detection (PD)-SINR information for two layers is fed back in a precoding matrix selected by each MS in a 2x2 MIMO system environment, feedback information for each MS increases two times higher than a case of feeding back PD-SINR information for a single layer. However, since two layers may be allocated to a single MS (i.e., allocation in a single user mode), a QoS of the MS may be increased. In addition, if the number of users is small and thus capacity may not be expected to be increased due to a small multi-user diversity gain, a capacity gain may be obtained by using a Maximum Likelihood (ML) scheme or a Successive Interference Cancellation (SIC) scheme.

FIG. 2C is a flowchart illustrating a process of determining a feedback amount depending on an interference environment according to an exemplary embodiment of the present invention.

Referring to FIG. 2C, a BS measures an Interference Over Thermal (IOT) in step 220. Herein, the greater the IOT, the greater the interference.

The BS determines if the IOT is greater than a preset threshold in step 222. If the IOT is greater than the preset threshold, the BS allocates feedback resources for a single layer in step 223. That is, the BS allocates a vector codebook.

In contrast, if the IOT is less than the preset threshold in step 222, the BS allocates feedback resources for N layers in step 224. That is, the BS allocates a matrix codebook.

Thereafter, the procedure of FIG. 2C ends.

As described above with reference to FIG. 2C, unlike in the PU2RC in which a BS uses a codebook matrix fed back by an MS as a beam weight, in a Zero Forcing-Space Division Multiple Access (ZF-SDMA) scheme of the MU-MIMO scheme, the BS generates the beam weight through additional processing and transmits the generated beam weight to the MS, resulting in performance improvement in comparison with the PU2RC. When the BS has four antennas and the MS has a single Rx antenna, each MS selects a codebook for a 4x1 channel vector and feeds back the selected codebook to the BS. Then, the BS estimates a channel vector from an index of the codebook fed back by each MS and thereafter performs a MU-MIMO operation by generating and transmitting beams orthogonal to all of the four MSs. When the MS has two Rx antennas, the beams orthogonal to all of the four MSs may not be generated by performing a simple ZF operation. Thus, the MS may feed back a codebook for an Rx antenna having a better channel environment between the two Rx antennas and then the BS may allocate four users by performing the same operation as in the case where the MS uses a single Rx antenna. However, since the beam weight has not been generated by considering both of the two Rx antennas of the MS, a quasi-optimized operation is performed. In this case, performance may be optimized since the orthogonal beams may be generated by simultaneously optimizing a beam weight of the BS and a beam weight received by the MS according to a CBF scheme. A feedback amount required by the CBF scheme is two times higher than that of the ZF scheme since a matrix codebook for a 4x2 channel has to be fed back in the CBF scheme.

FIG. 3 is a flowchart illustrating a feedback process of an MS in a MU-MIMO communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the MS receives a feedback resource allocated by a BS in step 300.

The MS performs channel estimation for all layers in step 302, and feeds back a channel estimation result according to the allocated resource in step 304. Table 1 below shows an example of a feedback channel configuration of PU2RC.

Table 1

Herein, if the BS determines a feedback format to be "0010" according to a transmission environment in a process of resource allocation, the MS configures feedback information by including a MU-Channel Quality Indicator (CQI) value, a column index of a corresponding matrix, and a codebook index. If the feedback format is "0011", the MS configures the feedback information by including rank information and an additional CQI for a Single User (SU). That is, the BS may change the feedback format for each MS according to the transmission environment.

Thereafter, the procedure of FIG. 3 ends.

FIG. 4 is a block diagram illustrating a BS for performing a feedback process in a MU-MIMO communication system according to an exemplary embodiment of the present invention.

Referring to FIG. 4, a BS 400 includes a transmission environment measuring unit 410 for measuring a MIMO transmission environment, a feedback amount determining unit 420 for determining a feedback amount according to the MIMO transmission environment, and a feedback channel allocating unit 430 for allocating resources according to the feedback amount.

In an exemplary implementation, the transmission environment measuring unit 410 measures a number of MSs, and the feedback channel allocating unit 430 allocates a feedback channel for a single layer if the number of MSs is greater than a preset threshold. In contrast, if the number of MSs is less than the preset threshold, the feedback channel allocating unit 430 allocates a feedback channel for N layers.

In another exemplary implementation, the transmission environment measuring unit 410 measures a transfer rate required by an MS, and the feedback channel allocating unit 430 allocates a feedback channel for a single layer if the transfer rate required by the MS is greater than a preset threshold. In contrast, if the transfer rate required by the MS is less than the preset threshold, the feedback channel allocating unit 430 allocates a feedback channel for N layers.

In still another exemplary implementation, the transmission environment measuring unit 410 measures an interference environment (i.e., an interference level), and the feedback channel allocating unit 430 allocates a feedback channel for a single layer if the interference level is greater than a preset threshold. In contrast, if the interference level is less than the preset threshold, the feedback channel allocating unit 430 allocates a feedback channel for N layers. The measured interference level may be an Interference Over Thermal (IOT).

As described above, exemplary embodiments of the present invention provide an advantage in that system capacity may be optimized according to a transmission environment by determining a feedback amount requested by an MS according to the transmission environment in a MU-MIMO communication system.

While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.