Multiple input multiple output (MIMO)
technology is an integral part of 3GPP
E-UTRA long term evolution (LTE). As
part of MIMO, beamforming is also used in
LTE.
This white paper discusses the basics of
beamforming and explains the eight MIMO
transmission modes in LTE Release 9.
Introduction Modern communications networks use MIMO technology to achieve high data rates. As a special MIMO technique, beamforming also permits targeted illumination of specific areas, making it possible to improve transmission to users at the far reaches of cell coverage. Like other communications standards such as WLAN and WiMAXTM, LTE also defines beamforming. Beamforming is particularly important for the time division duplex (TDD) mode in LTE. This white paper describes the eight available MIMO transmission modes in LTE as specified in 3GPP Release 9, as well as how beamforming is used in LTE. 2 MIMO and Beamforming Technologies 2.1 MIMO This paper discusses the MIMO concepts only to the extent that they apply to LTE transmission modes (see 3.2). Refer to [3] for a more detailed description of the MIMO concept as well as for a look at how MIMO is used in various communications systems. MIMO systems are used to improve the robustness of data transmission or to increase data rates. Typically, a MIMO system consists of m transmit antennas and n receive antennas (Figure 1).
Transmission matrix H contains the channel impulse responses hnm, which reference the channel between the transmit antenna m and the receive antenna n. Many MIMO algorithms are based on the analysis of transmission matrix H characteristics. The rank (of the channel matrix) defines the number of linearly independent rows or columns in H. It indicates how many independent data streams (layers) can be transmitted simultaneously. > Increasing the robustness of data transmission – transmit diversity When the same data is transmitted redundantly over more than one transmit antenna, this is called TX diversity. This increases the signal-to-noise ratio. Spacetime codes are used to generate a redundant signal. Alamouti developed the first codes for two antennas. Today, different codes are available for more than two antennas. > Increasing the data rate – spatial multiplexing Spatial multiplexing increases the data rate. Data is divided into separate streams, which are then transmitted simultaneously over the same air interface resources. The transmission includes special sections (also called pilots or reference signals) that are also known to the receiver. The receiver can perform a channel estimation for each transmit antenna’s signal. In the closed-loop method, the receiver reports the channel status to the transmitter via a special feedback channel. This enables fast reactions to changing channel circumstances, e.g. adaptation of the number of multiplexed streams. When the data rate is to be increased for a single user equipment (UE), this is called Single User MIMO (SU-MIMO). When the individual streams are assigned to various users, this is called Multi User MIMO (MU-MIMO)
Introduction Modern communications networks use MIMO technology to achieve high data rates. As a special MIMO technique, beamforming also permits targeted illumination of specific areas, making it possible to improve transmission to users at the far reaches of cell coverage. Like other communications standards such as WLAN and WiMAXTM, LTE also defines beamforming. Beamforming is particularly important for the time division duplex (TDD) mode in LTE. This white paper describes the eight available MIMO transmission modes in LTE as specified in 3GPP Release 9, as well as how beamforming is used in LTE. 2 MIMO and Beamforming Technologies 2.1 MIMO This paper discusses the MIMO concepts only to the extent that they apply to LTE transmission modes (see 3.2). Refer to [3] for a more detailed description of the MIMO concept as well as for a look at how MIMO is used in various communications systems. MIMO systems are used to improve the robustness of data transmission or to increase data rates. Typically, a MIMO system consists of m transmit antennas and n receive antennas (Figure 1).
Figure 1: MIMO system with m TX and n RX antennas
Simply stated, the receiver receives the signal y that results when the input signal
vector x is multiplied by the transmission matrix H.
y = H * x
Transmission matrix H contains the channel impulse responses hnm, which reference the channel between the transmit antenna m and the receive antenna n. Many MIMO algorithms are based on the analysis of transmission matrix H characteristics. The rank (of the channel matrix) defines the number of linearly independent rows or columns in H. It indicates how many independent data streams (layers) can be transmitted simultaneously. > Increasing the robustness of data transmission – transmit diversity When the same data is transmitted redundantly over more than one transmit antenna, this is called TX diversity. This increases the signal-to-noise ratio. Spacetime codes are used to generate a redundant signal. Alamouti developed the first codes for two antennas. Today, different codes are available for more than two antennas. > Increasing the data rate – spatial multiplexing Spatial multiplexing increases the data rate. Data is divided into separate streams, which are then transmitted simultaneously over the same air interface resources. The transmission includes special sections (also called pilots or reference signals) that are also known to the receiver. The receiver can perform a channel estimation for each transmit antenna’s signal. In the closed-loop method, the receiver reports the channel status to the transmitter via a special feedback channel. This enables fast reactions to changing channel circumstances, e.g. adaptation of the number of multiplexed streams. When the data rate is to be increased for a single user equipment (UE), this is called Single User MIMO (SU-MIMO). When the individual streams are assigned to various users, this is called Multi User MIMO (MU-MIMO)
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