MU-MIMO Multi-User MIMO

- An overview of the basics of MU-MIMO Multi-User MIMO - a form of advanced Multiple Input Multiple Output technology including MIMO-BC and MIMO-MAC.

Multi-user MIMO or MU-MIMO is an enhanced form of MIMO technology that is gaining acceptance. MU-MIMO, Multi-user MIMO enables multiple independent radio terminals to access a system enhancing the communication capabilities of each individual terminal.

MU-MIMO exploits the maximum system capacity by scheduling multiple users to be able to simultaneously access the same channel using the spatial degrees of freedom offered by MIMO.

To enable MU-MIMO to be used there are several approaches that can be adopted, and a number of applications / versions that are available.

MU-MIMO basics

MU-MIMO provides a methodology whereby spatial sharing of channels can be achieved. This can be achieved at the cost of additional hardware - filters and antennas - but the incorporation does not come at the expense of additional bandwidth as is the case when technologies such as FDMA, TDMA or CDMA are used.

When using spatial multiplexing, MU-MIMO, the interference between the different users on the same channel is accommodated by the use of additional antennas, and additional processing when enable the spatial separation of the different users.

There are two scenarios associated with MU-MIMO, Multi-user MIMO:

  • Uplink - Multiple Access Channel, MAC:   The development of the MIMO-MAC is based on the known single user MIMO concepts broadened out to account for multiple users.
  • Downlink - Broadcast Channel, BC :   The MIMO-BC is the more challenging scenario. The optimum strategy involves pre-interference cancellation techniques known as "Dirty Paper Coding", DPC - see below. This is complemented by implicit user scheduling and a power loading algorithm

MU-MIMO Multi-User MIMO advantages

MU-MIMO, Multi-user MIMO offers some significant advantages over other techniques:

  • MU-MIMO systems enable a level of direct gain to be obtained in a multiple access capacity arising from the multi-user multiplexing schemes. This is proportional to the number of base station antennas employed.
  • MU-MIMO appears to be affected less by some propagation issues that affect single user MIMO systems. These include channel rank loss and antenna correlation - although channel correlation still affects diversity on a per user basis, it is not a major issue for multi-user diversity.
  • MU-MIMO allows spatial multiplexing gain to be achieved at the base station without the need for multiple antennas at the UE. This allows for the production of cheap remote terminals - the intelligence and cost is included within the base station.

The advantages of using multi-user MIMO, MU-MIMO come at a cost of additional hardware - antennas and processing - and also obtaining the channel state information which requires the use of the available bandwidth.

MIMO-MAC

This form of MU-MIMO is used for a multiple access channel - hence MIMO and it is used in uplink scenarios.

For the MIMO-MAC the receiver performs much of the processing - here the receiver needs to know the channel state and uses Channel Sate Information at the Receiver, CSIR. Determining CSIR is generally easier than determining CSIT, but it requires significant levels of uplink capacity to transmit the dedicated pilots from each user. However MIMO MAC systems outperform point-to-point MIMO particularly if the number of receiver antennas is greater than the number of transmit antennas at each user.

MIMO-BC

This form of MU-MIMO is used for the MIMO broadcast channels, i.e. the downlink. Of the two channels, BC and MAC, it is the broadcast channel that is the more challenging within MU-MIMO.

Transmit processing is required for this and it is typically in the form of pre-coding and SDMA, Space Division Multiple Access based downlink user scheduling. For this the transmitter has to know the Channel State Information at the Transmitter, CSIT. This enables significant throughput improvements over that of ordinary point to point MIMO systems, especially when the number of transmit antennas exceeds that of the antennas at each receiver.

Dirty Paper Coding, DPC

Dirty Paper Coding, DPC is a technique used within telecommunications scenarios, particularly wireless communications to provide efficient transmission of digital data through a channel that is subject to interference, the nature of which is known to the transmitter.

The Dirty Paper Coding, DPC, technique consists of precoding the data so the interference data can be read in the presence of the interference. The pre-coding normally uses the Channel State Information.

To explain Dirty Paper Coding, DPC, an analogy of writing on dirty paper can be used. Normally black ink would be used, but if the paper is dirty, i.e. black, then the writing cannot be read. However if the writing was in white, although it could not be read on white paper, it would be perfectly legible on black, or dirty paper. The same technique is used on the data transmission, although the nature of the interference must be known so that the pre-coding can be incorporated to counter the effect of the interference.

Multi-user MIMO is still in its infancy, and many developments are underway to determine the optimum formats for its use. Coding types as well as levels of channel state indication are being determined as these use up valuable resource and can detract from the overall data throughput available. However the significant gains that can be made by using MU-MIMO, multi-user MIMO mean that it will be introduced in the foreseeable future.

By Ian Poole


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