Multiple Antenna SystemsMultiple Antenna Systems

SIMO MISO MIMO

Improved Transmission

Reliability

Greater Coverage or

Range

Reduced UE Power

Consumption

IncreasedTransmission Throughput

Multiple Antenna Systems

Figure 3-1: Multiple Antenna Systems

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)

Single Input Multiple Output (SIMO)Single Input Multiple Output (SIMO)

• Switched Diversity

• Equal Gain Combining

• Maximum Ratio Combining

Rx Tx

Switched Diversity

Equal Gain Combining

Maximum Ratio Combining

Figure 3-2: Single Input Multiple Output (SIMO)

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.

Multiple Input Single Output (MISO)Multiple Input Single Output (MISO)

• Space-Time Transmit Diversity

Tx Rx

Space-Time Transmit Diversity

Figure 3-3: Multiple Input Single Output (MISO)

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.

Space-Time Transmit Diversity (STTD)

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.

Multiple Input Multiple Output (MIMO)Multiple Input Multiple Output (MIMO)

• Improved Transmission Reliability

• Increased coverage or range

• Reduced UE power consumption

RxTx

Improved Transmission Reliability

Increased Coverage or Range

Reduced UE Power Consumption

Figure 3-4: Multiple Input Multiple Output (MIMO)

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.

MIMO antenna systems are not unique to LTE; WiMAX, WiFi, and some cellular networks also use MIMO. The techniques described in this topic apply to any MIMO system; they are not restricted to LTE.

MIMO Techniques MIMO Techniques

• Space-Time Coding (STC) – 1 Data Stream

• Spatial Multiplexing – 2 Data Streams

RxTx

Data Stream 2

Data Stream 1

Space-Time Coding (STC) - 1 Data Stream

Spatial Multiplexing - 2 Data Streams

Figure 3-5: MIMO Techniques

Space-Time Coding (STC)

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 Signal to Noise Ratio 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.

A combination of spatial multiplexing and space-time coding may be implemented. Depending on the RF conditions, a device may dynamically switch between the two MIMO techniques.

Single User MIMO (SU–MIMO)

Improved Performance (STC), or

Improved Throughput (SM) for Single UE

Figure 3-6: Single User MIMO (SU–MIMO)

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).

Multi-User MIMO (MU–MIMO)

Improved Number of UEs

No Increase in System Bandwidth

Figure 3-7: Multi-User MIMO (MU–MIMO)

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.

Open Loop vs Closed Loop MIMO

Figure 3-8: Closed Loop MIMO

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. The 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.