Transmission modes and Beamforming in LTE
Transmission modes (TM) in LTE downlink
1MA186_2e Rohde & Schwarz LTE Beamforming 13
For two antennas, a frequency-based version of the Alamouti codes (space frequency
block code, SFBC) is used, while for four antennas, a combination of SFBC and
frequency switched transmit diversity (FSTD) is used.
3.2.3 TM 3 – Open loop spatial multiplexing with CDD
This mode supports spatial multiplexing of two to four layers that are multiplexed to two
to four antennas, respectively, in order to achieve higher data rates. It requires less UE
feedback regarding the channel situation (no precoding matrix indicator is included),
and is used when channel information is missing or when the channel rapidly changes,
e.g. for UEs moving with high velocity.
In addition to the precoding as defined in Table 4, the signal is supplied to every
antenna with a specific delay (cyclic delay diversity, or CDD), thus artificially creating
frequency diversity.
Figure 9: TM 3: Spatial multiplexing with CDD; the individual subcarriers are delayed artificially.
Figure 9 shows the CDD principle. For two transmit antennas, a fixed precoding
(codebook index 0 is used as defined in Table 4), while for four antennas, the
precoders are cyclically switched.
3.2.4 TM 4 – Closed loop spatial multiplexing
This mode supports spatial multiplexing with up to four layers that are multiplexed to up
to four antennas, respectively, in order to achieve higher data rates. To permit channel
estimation at the receiver, the base station transmits cell-specific reference signals
(RS), distributed over various resource elements (RE) and over various timeslots. The
UE sends a response regarding the channel situation, which includes information
about which precoding is preferred from the defined codebook. This is accomplished
using an index (precoding matrix indicators, or PMI) defined in the codebook, a table
with possible precoding matrices that is known to both sides.