MIMO In 4G Wireless Communications(1)

MIMO In 4G Wireless Communications(1)

4G Communications 

Definition:

 Multiple-input and Multiple-Output, or (MIMO), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology.

Importance:

C=B log₂(1+SNR) …………. Shannon's law

Two major limitations in communications channels can be multipath

interference, and the data throughput limitations as a result of Shannon's Law.

MIMO provides a way of utilizing the multiple signal paths that exist between a

transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth.

By using multiple antennas at the transmitter and receiver along with some complex digital signal processing,

MIMO technology enables the system to set up multiple data streams on the

same channel, thereby increasing the data capacity of a channel.

MIMO is being used increasingly in many high data rate technologies

including Wi-Fi and other wireless and cellular technologies to provide improved levels of efficiency.

Essentially MIMO employs multiple antennas on the receiver and transmitter to utilize the multi-path effects that always exist to transmit additional data, rather than causing interference.

The schemes employed in LTE again vary slightly between the uplink and downlink. The reason for this is to  keep the terminal cost low as there are far more terminals than base stations and as a result terminal works cost price is far more sensitive.

 For the downlink, a configuration of two transmit antennas at the base station and two receive antennas on the mobile terminal is used as baseline, although configurations with four antennas are also being considered.

Applications:

It is used in Mobile radio telephone standards such as recent 3GPP and 3GPP2. In 3GPP, High-Speed Packet Access plus (HSPA+) and Long Term Evolution (LTE)standards take MIMO into account.

It also can be used in non-wireless communications systems, Wi-Fi and other wireless and cellular technologies.                                       

DOWNLINK SU-MIMO in LTE: 

           

The SU-MIMO scheme is applied to the Physical Downlink Shared Channel (PDSCH), which is the physical layer channel that carries the information data from the network to the UE. With SU-MIMO spatial multiplexing, the LTE system provides a peak rate of 150 Mbps for two transmit antennas and 300 Mbps for four transmit antennas.

Modes of SUMIMO:

 There are two operation modes in SUMIMO spatial multiplexing:

The closed-loop spatial multiplexing mode and the open-loop spatial multiplexing mode.

In the closed-loop spatial multiplexing mode, the base station (also known as E-Node B) applies the spatial domain precoding on the transmitted signal taking into account the pre-coding matrix indicator (PMI) reported by the UE so That the transmitted signal matches with the spatial channel experienced by the UE.

MU-MIMO in LTE:

MU-MIMO scheme is supported in both the uplink and downlink of the LTE

standard.

In the uplink, the eNodeB can always schedule more than one UEs to

transmit in the same time frequency resource, which forms a MU-MIMO

transmission configuration However, in order for the eNodeB to be able to

correctly differentiate and demodulate these UEs’ signals eNodeB needs to

assign orthogonal reference signals for these UEs scheduled for the MU-MIMO transmission.

        

The following Figure shows the uplink slot structure where the reference signal is transmitted using the fourth symbol and the data is transmitted using the  others. For a given slot and sub-frame in each cell, a Zadoff-Chu sequence is defined as the base sequence for uplink reference signals.

The cyclically shifted versions of a given Zadoff-Chu sequence form an orthogonal set of sequences.

Each UE scheduled for MUMIMO transmission is assigned a distinctive cyclic

shift value, and the UE combines this cyclic shift value with the knowledge of the base Zadoff-Chu sequence to form a reference signal sequence that is

orthogonal to other UEs’ reference signal sequences. It is noted that the cyclic

shift value is always contained in the control signaling, which the UE has to

receive for data transmission on uplink, regardless whether the MU-MIMO is

operated or not.

In the downlink, if a UE is configured to be in the MU-MIMO transmission

mode, only rank-1 transmission can be scheduled to the UE. The eNodeB can

schedule multiple UEs, which are configured to be in the MU-MIMO transmission mode, in the same time-frequency resource using different rank-1 pre-coding matrices  at LTE Standard for two transmit antennas and another matrix at LTE Standard for four transmit antennas. Note that the UE receives only the information about its own pre-coding matrix.

The scheduled UE then decodes the information data utilizing the common reference signal together with the precoding information obtained from the control signaling.

The UE generates the PMI/CQI (pre-coding matrix indicator / Channel Capacity indicator) feedback without any knowledge about other simultaneously scheduled UEs. Hence, there could be mismatch between

The UE’s CQI report and the actual CQI experienced  due to lack of knowledge of interference caused by another UEs scheduled simultaneously.

In LTE, for support of receiving higher-order modulation signal such as 16QAM and 64QAM without causing too much complexity in the UE, the transmit power level for each UE is configured in a long-term manner.

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