In Release 10, SU-MIMO transmission with up to four spatial layers is introduced to increase the data rate for the PUSCH, and the reliability of the control signalling on the PUCCH is increased using transmit diversity.
29.4.1 Uplink SU-MIMO for PUSCH
In order to support uplink SU-MIMO, the concept of transmission modes is introduced for the PUSCH in Release 10. The transmission modes are as follows:
PUSCH Transmission Mode 1: Transmission from a single antenna port;13
PUSCH Transmission Mode 2: Transmission from multiple antenna ports; within this mode, the UE can be configured to transmit from either 2 or 4 antenna ports.
If a UE is configured in PUSCH transmission mode 2, it can transmit up to two transport blocks per subframe.14 The transport blocks are mapped onto one or more layers according to the same rules as are used in the downlink in Release 8, as shown in Table 11.2.
Each transport block is independently acknowledged using the Physical HARQ Indicator CHannel (PHICH). The PHICH index for the first codeword is the same as is used in Release 8 for single-codeword PUSCH transmission (i.e. associated with the lowest PRB index of the PUSCH resource allocation); the PHICH index for the second codeword is related to the first by an offset.
The PDCCH resource grant for uplink MIMO transmissions includes two independent New Data Indicators (NDIs) to indicate whether a retransmission is expected.
Closed-loop codebook-based precoding is used for the PUSCH in a very similar way to the Release 8 PUSCH in transmission mode 4 (see Section 11.2.2.2). The UE is instructed by the eNodeB as to which rank and precoding matrix to use, by means of a dynamic precoding matrix indicator transmitted in the uplink grant on the PDCCH.
In designing the codebooks for uplink SU-MIMO precoding, the primary considerations were preserving the single-carrier waveform at each of the transmit antennas and achieving as much precoding gain as possible. In order to comply with the former, there can be at most one non-zero entry in each row of the precoding matrix. For the rank-1 codebook, some vectors are introduced to turn offcertain transmit antennas to save power.
The number of codewords in the codebook is a trade-off between performance and signalling overhead. As a result, for 2 transmit antennas, 6 precoding vectors are defined for rank-1, and a single identity precoding matrix is defined for rank-2; this can be signalled using 3 bits. For 4 transmit antennas, there are 24 precoding vectors for rank-1, 16 for rank-2, 12 for rank-3, and a single identity matrix for rank-4, requiring 6-bit signalling. Full details of the PUSCH precoding codebooks can be found in [1, Section 5.3.3A].
In view of the performance of closed-loop precoding for the PUSCH, transmit diversity is not considered useful. In Figure 29.9, the throughputs using Space-Time Block Coding (STBC) and long-term closed-loop precoding are compared at 3 km/h and 120 km/h under different transmit antenna correlations. Even with uncorrelated transmit antennas, STBC only
13Two configurations exist within this mode: one is the Release 8 PUSCH transmission scheme, while the other supports both contiguous and non-contiguous resource allocation (see Section 28.3.6.2).
14Per component carrier if carrier aggregation is employed – see Chapter 28.
shows better link throughput at high SINRs, corresponding to a BLock Error Rate (BLER) below 10% (which is an unlikely operating point). Furthermore, as the transmit antennas become more correlated, long-term precoding exhibits more beamforming gain.
Figure 29.9: Link throughput performance of STBC vs long-term precoding [9].
If uplink control signalling is carried on the PUSCH15in a subframe when two codewords are transmitted, any Acknowledgement/Negative Acknowledgement (ACK/NACK) and RI are replicated across all layers of both codewords before channel coding, with the coded modulation symbols for ACK/NACK and RI being time-domain multiplexed with the data and time-aligned across all layers. CQI and PMI signalling is mapped only to the codeword with the highest MCS as indicated by the initial uplink grant. When the two codewords have the same MCS, codeword 0 is always selected. The same multiplexing and channel interleaving mechanisms are used as in Release 8, with the control information being mapped to the same REs on the two spatial layers if the codeword is transmitted on two layers. In Figure 29.10, the placement of the Uplink Control Information (UCI) is illustrated for the case when the uplink PUSCH transmission is of rank 2 and codeword 0 has the higher MCS.
15See Section 16.4.
−6 −2 2 6
0 50 100 150 200 250 300 350 400 450 500 550
Average subcarrier SNR per Rx Ant (dB)
Throughput (kbps)
UE mobility: 3 km/h
−60 −2 2 6
50 100 150 200 250 300 350 400 450 500 550
Average subcarrier SNR per Rx Ant (dB) UE mobility: 120 km/h
STBC TxCorr: 0.0 Long−Term Prec TxCorr: 0.0 STBC TxCorr: 0.8 Long−Term Prec TxCorr: 0.8
Layer 0: SC-FDMA symbol
Layer 1:
slot 0 slot 1
slot 0 slot 1
PUSCH data DM-RS CQI/PMI ACK/NACK RI
Figure 29.10: Multiplexing of control signalling with PUSCH data in the case of rank-2 SU-MIMO.
29.4.2 Uplink Transmit Diversity for PUCCH
The transmit diversity scheme introduced for the PUCCH in Release 10 is designed to ensure backward compatibility with the Release 8 PUCCH design (see Chapter 16).
For PUCCH formats 1/1a/1b (see Section 16.3.2), Space Orthogonal-Resource Transmit Diversity (SORTD) [10] is used, whereby the UE transmits the same control information from different transmit antennas with different orthogonal resources, including cyclic shifts and OCCs. For PUCCH formats 1a/1b, the two orthogonal resources for SORTD transmission are derived from nCCE and nCCE+1, where nCCE is the number of the first Control Channel Element (CCE) used for the transmission of the corresponding uplink grant on the PDCCH. SORTD is also defined for PUCCH formats 2/2a/2b and 3. Transmit diversity is not supported in Release 10 for PUCCH format 1b with channel selection (see Section 28.3.2.1).
If the transmit antennas are uncorrelated, SORTD provides the best diversity performance, at the expense of consuming multiple orthogonal resources which potentially reduces the PUCCH multiplexing capability.