Per-user unitary rate control (PU²RC) is a practical multi-user MIMO solution. PU²RC allows a base station to transmit different data streams to multiple users simultaneously. The base station selects target users from candidate users based on the information fed by users. Transmission data are multiplied by a pre-coding matrix selected from the set of predefined matrices before transmission. The selection of a pre-coding matrix is determined based on the information provided by users. The selection of both target users and a pre-coding matrix according to the information provided by mobiles enables the utilization of multi-user diversity and data multiplexing at the same time. Moreover, using predefined precoding matrices reduces feedback overhead from users to the base station. Pre-coding matrices used in this scheme is unitary. The use of unitary pre-coding matrices facilitates the estimation of interference from other users' data to the unintended user.

The operation of PU²RC is mathematically described for the transmitter and receiver sides, respectively.

Base station edit

It is assumed that the base station employs ${\displaystyle N_{t}}$  transmission antennas. The ${\displaystyle N_{t}\times 1}$  transmission signal vector is given by

${\displaystyle \mathbf {x} =\sum _{i=1}^{K}\mathbf {w} _{i}P_{i}s_{i}}$ 

where ${\displaystyle \mathbf {w} _{i}}$  is the ${\displaystyle N_{t}\times 1}$  linear precoding vector. PU²RC generates ${\displaystyle \mathbf {w} _{i}}$  based on the received finite channel status information, which is delivered to the base station from the user equipment (UE) through uplink feedback signaling. The feedback signal consists of index in a look-up table of a precoding codebook.

Receiver side edit

Every receiver has a receive antenna array with ${\displaystyle N_{r}}$  elements. The receive signal vector at user ${\displaystyle k(=1,2,\ldots ,K)}$  is modeled as follows:

${\displaystyle \mathbf {y} _{k}=\mathbf {H} _{k}\mathbf {x} +\mathbf {n} _{k}}$ 

where ${\displaystyle \mathbf {y} _{k}}$  and ${\displaystyle \mathbf {n} _{k}}$  are the ${\displaystyle N_{r}\times 1}$  received symbol and noise, respectively, and ${\displaystyle \mathbf {H} _{k}}$  is the ${\displaystyle N_{r}\times N_{t}}$  matrix with the channel coefficients.

Throughput performance edit

The figure illustrates the throughput advantage of PU²RC over the conventional single-user and no scheduling scheme, assuming that the codebook size is one, i.e., ${\displaystyle (G=1)}$ . For larger codebook sizes the performance can be better than the performance of the unit-size codebook. Because of codebook-based multi-user scheduling, PU²RC outperforms the conventional single-user and no scheduling scheme when the number of users is larger than one. Note that the performance plotted in the figure for the two systems were obtained assuming linear receiver.

• Multiple-input multiple-output communications
• Multi-user MIMO as the advanced MIMO communication technology
• Precoding
• Spatial multiplexing

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