Per-user unitary rate control

Per-User Unitary Rate Control (PU²RC) is the advanced multi-user MIMO technique which utilizes the concept of both pre-coding matrices and scheduling to enhance the system performance of multiple antenna wireless networks.

Background technologies: A single-user MIMO was initially developed to improve the spectral efficiency of point-to-point wireless transmission link. A multi-user MIMO was developed for cellular systems where the base station simultaneously communicates with multiple users.
Principle: The concept of Per-User Unitary Rate Control (PU²RC) was proposed in US Patent No. 7,324,480, Mobile communication apparatus and method including base station and mobile station having multi-antenna by James S. Kim, Kwang Bok Lee, Kiho Kim and Changsoon Park.

Recently, PU²RC has been adopted in the IEEE 802.16m system description documentation (SDD) and the concept of this scheme was included in 3GPP LTE standard.

Technology

Per-User Unitary Rate Control (PU²RC) is the first practical multi-user MIMO scheme. In this scheme different data may simultaneously be transmitted to multiple users from the base station.

Users to be served are selected from the set of service-requesting users by the base station using the information provided by users.

Data transmitted to mobile users are multiplied by a pre-coding matrix selected from the set of predefined matrices before transmission. The selection of a pre-coding matrix is made based on the information provided by users.

The selection of users and a pre-coding matrix using the information provided by mobiles enables the utilization of multi-user diversity and 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.

Mathematical description

File:Pu2rc over single user.png

Throughput performance of PU²RC over the single-user & no scheduling scheme

The operation is described mathematically below for the transmitter and receiver side.

Transmitter Side

The transmitter antenna array has $N_{t}$ elements. The transmit signal vector is modeled as follows:

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

where $\mathbf {x}$ is the $N_{t}\times 1$ vector of transmitted symbols and $\mathbf {w} _{i}$ is the $N_{t}\times 1$ linear precoding vector. PU²RC generates ${\mathbf {w} _{i}}$ based on the received finite channel status information, which is delivered to the base station by the user equipment (UE) using a coded look-up table index.

Receiver Side

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

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

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

Throughput performance

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., $(G=1)$ . 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.

PU²RC Source and Background Materials

Source Materials

James S. Kim, K. B. Lee, et al., Mobile communication apparatus and method including base station and mobile station having multi-antenna, US PTO 7,324,480
S. J. Kim, H. J. Kim, C. S. Park, and K. B. Lee, "On the Performance of Multiuser MIMO Systems in WCDMA/HSDPA: Beamforming, Feedback and User Diversity," IEICE Transactions on Communications, vol. E98-B, no. 8, pp. 2161–2169, Aug. 2006.
3GPP TSG RAN WG1#31 R1-030354, Per unitary basis stream user and rate Control (PU2RC), 3GPP TSG-R1-030354, Tokyo, Fed 18-21, 2003. (See also R1-030130)
Samsung, SNU, “Downlink MIMO for EUTRA,” in 3GPP TSG RAN WG1 # 44/R1-060335

Enhanced Methods

Multiuser MIMO Scheme for Enhanced 3GPP HSDPA
Kim, J.S., Hojin Kim, Yongxing Zhou, Jianjun Li, ``Multi-Mode Multi-User MIMO System with Finite Rate Feedback, IEEE Wireless Communication Systems, 2006. ISWCS '06. 3rd International Symposium on, 6-8 Sept. 2006

Background References

R. W. Heath, Jr., M. Airy, and A. J. Paulraj, "Multiuser Diversity for MIMO Wireless Systems with Linear Receivers," Proc. of the IEEE Asilomar Conf. on Signals, Systems, and Computers, pp. 1194 -1199, vol.2, Pacific Grove, California, Nov. 4 - 7, 2001.
A. Kogiantis and L. Ozarow, "Downlink best-effort packet data with multiple antennas," ICC’03, Volume 1, 11-15 May 2003 Page(s):715 - 719.
K.K. Wong, R.D. Murch and K.B. Letaief, "Performance Enhancement of Multiuser MIMO Wireless Communication Systems," IEEE Transactions on Communications, Vol 50 No 12 , Dec. 2002, pp 1960 –1970
Further benefits of the revised definition of CQI for TxAA, 3GPP TSG-R1-030130, San Diego, Jan. 7-10, 2003.

MU-MIMO Standard Contributions

The following documents are standard contributions about both PU2RC related and general MU-MIMO issues.

General MU-MIMO issues

PU2RC related issues

MIMO Literature

Per-user unitary rate control

Technology