Uplink MIMO Transmission Schemes [10]

Again this section is mainly taken from [10].

3.7.1 Uplink MIMO Architecture and Data Processing

The architecture of uplink MIMO at the transmitter side is shown in Fig. 3.1. The MIMO encoder block maps a single MIMO layer (L = 1) onto Mt (Mt≥ L) MIMO streams, which are fed to the precoder block. For SU-MIMO and collaborative spatial multiplexing (MU-MIMO), only one FEC block exists in the allocated RU (vertical MIMO encoding at transmit side).

The precoder block maps MIMO stream(s) to antennas by generating the antenna-specific

data symbols according to the selected MIMO mode. The MIMO encoder and precoder blocks shall be omitted when the AMS has one transmit antenna. The subcarrier mapping blocks map antenna-specific data to the OFDM symbol.

3.7.2 MIMO Layer to MIMO Stream Mapping

MIMO layer to MIMO stream mapping is performed by the MIMO encoder. The uplink MIMO encoder is identical to the downlink MIMO encoder.

Horizontal encoding (MEF = 0b10) is not supported for uplink transmissions. Collabo-rative spatial multiplexing (CSM) is achieved with vertical encoding (MEF = 0b01) at the AMS. The STC rate per AMS for uplink SU-MIMO and MU-MIMO (CSM) transmissions is defined as R = M/NF. An AMS with 1 transmit antenna shall use vertical encoding (MEF

= 0b01) for uplink transmissions.

Uplink SFBC encoding is identical to the downlink SFBC encoding. SFBC encoding format shall not be allocated to an AMS with 1 transmit antenna.

Uplink vertical encoding is identical to the downlink vertical encoding. Vertical encoding with 1 MIMO stream (M_t = 1) format shall be allocated to an AMS with 1 transmit antenna.

3.7.3 MIMO Stream to Antenna Mapping

MIMO stream to antenna mapping is performed by the precoder. The uplink mapping is identical to the downlink mapping.

Non-adaptive Precoding

There is no precoding if there is only one transmit antenna at the MS. With non-adaptive precoding, the precoding matrix is an Nt× M^t matrix W(k), where Nt is the number of transmit antennas, M_t is the numbers of MIMO streams, and k is the physical index of

‘

Figure 3.20: Codebook subsets used for non-adaptive precoding in UL DLRU and NLRU (from [10, Table 921]).

²³´ µ¶·¸¹º²¹»¸¼

Figure 3.21: Codebook subsets used for non-adaptive precoding in UL SLRU (from [10, Table 922]).

the subcarrier where W(k) is applied. The matrix W is selected from a subset of size Nw

precoders of the base codebook for a given rank. W belongs to one of the subsets of the base codebook, according to the type of allocation, MEF, Nt and Mt, as specified in Figs. 3.20 and 3.21.

In an RU allocated in an AAI subframe and non-adaptive precoding, the matrix W changes every N1PSC contiguous physical subcarriers according to Equation, and it does not depend on the AAI subframe number. The Nt× M^t precoding matrix W(k) applied on subcarrier kin physical subband s is selected as the codeword of index i in the open-loop codebook subset of rank Mt, where i is given by

i = s mod N_w, s = 0, . . . , N_sub− 1, (3.30)

with Nsub denoting the number of physical subbands across the entire system bandwidth.

Adaptive Precoding

There is no precoding if there is only one transmit antenna at the AMS. With adaptive precoding, the precoder W is derived at the ABS or at the AMS, as instructed by the ABS.

With 2Tx or 4Tx at the AMS in FDD and TDD systems, unitary codebook based adaptive precoding is supported. In this mode, an AMS transmits a sounding signal on the uplink to assist the precoder selection at the ABS. The ABS then signals the uplink precoding matrix index to be used by the AMS. With 2Tx or 4Tx at the AMS in TDD systems, adaptive precoding based on the measurements of downlink reference signals is supported. The AMS chooses the precoder based on the downlink measurements. The form and derivation of the precoding matrix does not need to be known at the ABS.

3.7.4 Uplink MIMO Transmission Modes

There are five MIMO transmission modes for UL MIMO transmission as listed in Fig. 3.22 and where the allowed values of the parameters for each UL MIMO mode are shown in Fig. 3.23. Mt refers to the number of MIMO streams transmitted from one AMS. In modes 3 and 4, Nt refers to the number of transmit antennas at one AMS involved in CSM.

à

Figure 3.22: Uplink MIMO modes (from [10, Table 923]).

"#$

Figure 3.23: UL MIMO parameters (from [10, Table 924]).

NONPQ

R S TU VTW VTW XR

NONPQ

R S TY

VTW

Z[\

]^N

_` a

VTW VTW

NONPQ

R S T a

VTW

Z[\

]^

N

_` a

VTW VTW

NONPQ

R S Tb VTW

Z[\

]^

N

_c

Y VTW VTW

NONPQ

R S Td VTW

Z[\

]^N

_c

Y VTW VTW

Figure 3.24: Supported permutation for each UL MIMO mode (from [10, Table 925]).

Consecutive symbols for each antenna at the output of the MIMO precoder are mapped in a frequency domain first order across LRUs of the allocation, starting from the data sub-carrier with the smallest OFDM symbol index and smallest subsub-carrier index, and continuing to subcarrier index with increasing subcarrier index. When the edge of the allocation is reached, the mapping is continued on the next OFDM symbol.

Fig. 3.24 shows the permutations supported for each MIMO mode.

Downlink Signaling Support of UL-MIMO Modes

The ABS shall send parameters necessary for UL MIMO operation in a unicast message. The parameters may be transmitted depending on the type of operation. The unicast information is carried in the UL basic Assignment A-MAP IE, UL Subband Assignment A-MAP IE, and UL Persistent A-MAP IE. Fig. 3.25 specifies the DL control parameters required for UL MIMO operation.

Codebooks for Closed-Loop Transmit Precoding

The notation C_{base,U L}(N_t, M_t, NB) denotes the rank-M_t uplink base codebook, which con-sists of 2^{N B} complex matrices of dimension N_t by M_t, and M_t denotes the number of MIMO streams. The notation Cbase,U L(Nt, Mt, NB, i) denotes the i^th codebook entry of

e

Figure 3.25: UL MIMO control parameters (from [10, Table 926]).

Cbase,U L(Nt, Mt, NB).

The base codebooks of SU-MIMO with two transmit antennas consist of rank-1 codebook C_{base,U L}(2, 1, 4)and rank-2 codebook C_{base,U L}(2, 2, 3). Fig. 3.15 is included to illustrate the rank-1 base codebooks. The rank-2 base codebook Cbase,U L(2, 2, 3) for uplink 2 Tx is the same as the downlink 2 Tx rank-2 base codebook

The base codebook for UL collaborative spatial multiplexing MIMO is the same as the base codebook for SU-MIMO.

Chapter 4

Equalization and Closed-Loop MIMO