MULTI-USER MIMO

'Multi-user MIMO (MU-MIMO)' is advanced MIMO that transmits signals to multiple users in a same band and time, simultaneously and so, it is called as Space-division multiple access (SDMA). Similar to the relationship between OFDM and OFDMA, MU-MIMO is the extended technique of MIMO to be used as a multiple access method. One interesting difference is that MU-MIMO highly requires precoding relative to OFDMA, whre precoding enhances the system capacity of MU-MIMO. Multiple access MIMO, MIMO-SDMA, MAIMO-SDMA are all family terminologies of MU-MIMO where MAIMO abbreviates many-input and multiple-output.

Contents

Technology

MIMO BC

MIMO MAC

See also

Technology

Multi-user MIMO can exploit multiple users as spatial resources at the cost of advanced transmit or receiver processing, while conventional, or single-user, MIMO uses only multiple antenna dimensions. Multi-user MIMO algorithms are developed to consider a MIMO system when the number of users is more than one. Multi-user MIMO can be categorized into two cases: MIMO broadcast channels (MIMO BC) and MIMO multiple access channels (MIMO MAC) for downlink and uplink situations, respectively. Similarly, single-user MIMO can be represented as point-to-point MIMO.
Note that to remove ambiguity of the two words receiver and transmitter we add two new terms access point (AP) and user in this section. AP is a transmitter and user is each receiver for downlink environments and AP is a receiver and user is each transmitter for uplink environments.

MIMO BC

MIMO BC represents a MIMO downlink case in the single-point to multi-points wireless network. Examples of advanced transmit processing for MIMO BC are interference aware Precoding and SDMA-based downlink user scheduling. For advanced transmit processing, the transmitter has to know the channel state information at the transmitter (CSIT). That is, no knowledge of CSIT prevents the throughput improvement in MIMO systems so that how to obtain CSIT is an important matter. MIMO BC systems has an outstanding advantage over point-to-point MIMO systems especially when the number of transmit antennas at the transmitter, or AP, is larger than the number of receiver antennas at each receiver (user).

MIMO MAC

MIMO MAC represents a MIMO uplink case in the multi-points to single-point wireless network. Examples of advanced receive processing for MIMO MAC are joint interference cancellation and SDMA-based uplink user scheduling. For advanced receive processing, the receiver has to know the channel status information at the receiver (CSIR). Knowing CSIR at the receiver is easier than knowing CSIT at the transmitter. However, to know CSIR at the receiver costs a lot of uplink resources to transmit dedicated pilots from each user to AP. MIMO MAC systems can have an outstanding performance advantage over point-to-point MIMO systems especially when the number of receiver antennas at AP is larger than the number of transmit antennas at each user.

See also

★ Multiple-input multiple-output communications

★ Advanced MIMO communication

★ Precoding

★ Spatial multiplexing