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LTE Physical Layer services assume multiple port antenna systems are used. Multiple port antenna

systems are implemented for the following reasons:

• Improved transmission reliability

• Greater coverage or range

• Reduced UE power consumption

• Increased transmission throughput

Multiple port antenna systems include the following:

• Single Input Multiple Output (SIMO)

• Multiple Input Single Output (MISO)

• Multiple Input Multiple Output (MIMO)

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In a SIMO configuration the transmitter (usually the UE) has one transmitter and the receiver (the

eNodeB) has two physically separated antenna ports. The receiver picks up multiple versions of the

same signal but separated spatially. SIMO receivers use the following techniques to compute the best

received signal.

Switched Diversity

In Switched Diversity, the input with the best signal is chosen as the best source. The “best” signal

may be based on Signal-to-Noise Ratio (SNR) or Bit Error Rate (BER). Switched diversity is the most

simple and inexpensive SIMO technique.

Equal Gain Combining

Equal Gain Combining is a summation of all available received signals.

Maximum Ratio Combining

In Maximum Ratio Combining (MRC), each received signal has compensation applied to it before being

combined to produce a composite single signal. This technique is particularly effective where the signal

undergoes deep fading. Because fading probably occurs at different frequencies on each antenna port,

the reliability of the radio link is increased.

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A MISO (eNodeB) transmitter has two or more physically separated antenna ports, while the MISO (UE)

receiver has one antenna. Each Tx port transmits the same information bits. In addition to data signals,

reference signals are also transmitted via both antenna ports. The normal reference signal pattern is

sent via the first antenna port and the diversity reference signal pattern via the second antenna port.

In Space-Time Transmit Diversity (STTD) the same data is transmitted simultaneously over both Tx

ports. On each port, the channel-coded data is processed in blocks of four bits, then the bits are time

reversed and complex conjugated. The physical separation of the antenna ports provides the space

diversity, and the time difference derived from the bit-reversing process provides the time diversity. These

features together make the decoding process in the receiver more reliable.

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MIMO systems contain multiple antenna ports at both the transmitter and receiver. The MIMO transmitter

transmits signals using time, frequency, and space diversity. The MIMO receiver recovers the data across

multiple receiving antenna ports.

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Space-Time Coding (STC) provides diversity gain to combat the effects of unwanted multipath

propagation. Similar to STTD, time delayed and coded versions of the same signal are sent from the

same transmitter antenna. The codes that are used are mainly: trellis and block (less complex) codes.

This improves the SNR for cell edge performance.

Spatial Multiplexing (SM)

With Spatial Multiplexing, unique (different) data streams are transmitted over different antenna ports.

Spatial multiplexing can double (2x2 MIMO) or quadruple (4x4 MIMO) capacity and throughput. This

technique gives higher capacity when RF conditions are favorable and users are closer to the eNodeB.

The graphic shows spatial multiplexing with a 2x2 MIMO configuration. The receiver can identify the

transmitting antenna port for each received signal

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MIMO supports single user MIMO and multi-user MIMO. Single User MIMO improves the performance

for a UE (via space time coding), or increases the throughput for a UE (using spatial multiplexing).

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In multi-user MIMO, the data for different users is multiplexed onto a single time-frequency resource, so

the capacity of the cell can increase in terms of users without increasing the system bandwidth.

Switching between SU-MIMO and MU-MIMO is supported on a per UE basis. The

use of codes and reference signals not only allows the receiver to differentiate

between antenna streams and users, but also allows accurate channel estimation

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MIMO supports both open loop and closed loop control. Open loop MIMO transceivers adjust their

transmission based on received (reference signal) measurements. This assumes no rapid feedback

technique is available from the UE receiver back to the eNodeB transmitter. Unfortunately, in open loop

operation, the transmitter receives no feedback regarding antenna port operation or signal strength in

the forward direction.

Closed loop MIMO supports a feedback loop describing eNodeB transmitter operation and UE

recommendations. Both the eNodeB and UE contain a codebook which describes possible RF

parameters, for example, the phase shift between antenna ports. In closed loop MIMO, the UE

describes eNodeB transmitter operation by returning an index into the shared codebook.

Closed loop operation uses the following steps.

1. The eNodeB transmits a DL pilot channel as a reference signal on all antenna ports.

2. he UE evaluates various codebook options that specify the RF parameters.

3. The UE transmits its recommendations in the form of a codebook index to the eNodeB.

4. The eNodeB adjusts its DL transmission to the UE based on the recommended parameters.

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In case of open loop spatial multiplex two cases have to be distinguished. If the transmitted rank indication (TRI) = 1 the transmission mode corresponds to transmit diversity.

If TRI >1 large delay CDD is used. The number of layers is 2, 3 or 4.

In case of closed loop spatial multiplexing feedback from the UE it is used.

The UE feedbacks values of the RI = Rank Indicator and PMI = Precoding Matrix Indicator.

In case of 2 antenna ports the codebook consists of 2 matrices, in case of 4 antenna ports there are 16 entries. A restriction may be signaled so that only a subset thereof can be used.

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Release 10 has enhanced the reference signal design with user specific reference symbols for signal demodulation and common reference symbols for feedback purposes in downlink and more orthogonal reference signal structure in uplink. The enhanced design enables better performance when the number of antenna branches is high.

Downlink MIMO has already been included in LTE Release 8. The LTE Release 8 codebook and reference symbol design was found to be quite optimum for two and four transmit antennas (2x2, 2x4 and 4x4 antenna configurations), but the channel state information feedback from UE to eNB could have been more accurate. This limitation is overcome by the new reference symbol design of Release 10, which is also more effective when the number of transmit antennas is higher. Based on the studies and numerous contributions in 3GPP, it can be safely concluded that the higher the number of antennas, the higher is the gain that Release 10 MIMO provides in downlink. With two eNB and two UE antennas, Release 10 downlink MIMO provides no improvements over release 8 in SU-MIMO mode but small performance improvements have been gained in MU-MIMO mode. In most cases it is best to operate two TX antenna eNBs in Release 8 SU-MIMO mode. When eNB has four transmit antennas, Release 10 downlink MIMO gain is more than 20% over Release 8 and with eight transmit antennas a bit higher. Reference symbol overhead effects on system performance are significant with four and eight transmit antennas. Therefore the selection of MIMO operating modes and system parameters for both Release 8 and 10 UE is a critical network optimization task.

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An important point worth remembering is that the network should also support Release 8 and 9 UE which does not benefit from the Release 10 enhancements. The capacity gain from Release 10 downlink MIMO enhancements could even be negative since new reference symbols create overhead for all UE. However, these overheads can be decreased by decreasing the Release 8 and 9 specific reference symbols, but this would prevent non-LTE-A UE to operate in MIMO mode and thus lower their data rates. Additionally, there would be negative effects on common control channel performance. Consequently, the timing of the introduction of the new features and the configuration of the system parameters are essential for an optimum performance of the LTE network.

CSI - For downlink channel sounding / Sparse, low overhead (configurable)

CSI = PMI(precoding matrix indicator) + RI(rank indicator) + CQI (channel quality indicator)

DM - UE-specific DM-RS, which is precoded, makes it possible to apply non-codebook-based precoding (precoding based on CSI feedback and/or UL sounding)- UE-specific DM-RS will enable application of enhanced multi-user beamformingsuch as zero forcing (ZF) for, e.g., 4-by-2 MIMO - DM RS pattern for higher numbers of layers is extended for 2-layer format for transmission mode 8 in Rel-9 //CDM between RS of two layers// E.g. for 4 antenna ports:

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Uplink MIMO provides significantly higher peak rates and improved spectrum efficiency in uplink direction. SU-MIMO provides mainly increased data rates in lightly loaded networks for high-end multi-transmitter UE, whereas MU-MIMO can offer significant improvement of spectrum efficiency even with single transmitter UE. This can boost network capacity at low costs The LTE-A system can operate in both SU and MU-MIMO modes at the same time using dynamic user specific MIMO transmission configuration.

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Physical Multicast Channel (PMCH) is used instead of PDSCH.

Special RS pattern with higher density in frequency domain supports longer “delay spread”

from multi-cell transmission.

Multimedia Broadcast Single Frequency Network(MBSFN) mode of operation is supported by E‐UTRAN to enable efficient multi‐cell transmission of E‐MBMS services