codebook-based precoding and non-codebook-based precoding

LTE supports both beam-forming and spatial multiplexing as part of moregeneralantenna precoding. There are two approaches to downlink antenna precoding, which differdepending on whether cell-specific reference signals or demodulation reference signals are used forchannel estimation. These two approaches can also be referred to ascodebook-based precodingandnon-codebook-based precodingrespectively [refer from 4G LTE/LTE-Advance for mobile broadband]

1.codebook-based precoding

Figure 10.17also indicates how the cell-specific reference signals are applied after antenna pre-coding. Channel estimation based on the cell-specific reference signals will thus reflect the channelfor each antenna port not including the precoding. As a consequence, the terminal receiver must have explicit knowledge about what precoding has been applied at the transmitter side in order to prop-erly process the received signal and recover the different layers. Once again, the figure should notbe interpreted such that cell-specific reference signals are inserted specifically for a given PDSCHtransmission. 

2. non-codebook-based precoding

The main difference inFigure 10.19compared to Figure 10.17(codebook-based precoding) isthe presence of demodulation reference signals (DM-RS) before the precoding. The transmissionof precoded reference signals allows for demodulation and recovery of the transmitted layers at thereceiver side without explicit receiver knowledge of the precoding applied at the transmitter side. Putsimply, channel estimation based on precoded demodulation reference signals will reflect the chan-nel experienced by the layers,including the precoding, and can thus be used directly for coherentdemodulation of the different layers. There is thus no need to signal any precoder-matrix information to the terminal, which only needs to know the number of layers – that is, the transmission rank. As aconsequence, the network can select an arbitrary precoder and there is no need for any explicit code-book to select from. This is the reason for the term non-codebook-based precoding. It should be notedthough that non-codebook-based precoding may still rely on codebooks for the terminal feedback, asdescribed below. 

The possibility to select an arbitrary precoder matrix for the transmission is also the reason whythe precoder is shaded inFigure 10.19. The precoder part ofFigure 10.19is not visible in the LTEspecification and, strictly speaking, in the case of non-codebook-based precoding the antenna map-ping defined according toFigure 10.14consists of only the layer mapping. This also means that theantenna ports defined inFigure 10.14correspond to the different layers inFigure 10.19or, expresseddifferently, precoding occursafter the antenna ports.

Still, there must be a way for the network to select a suitable precoder matrix for the transmis-sion. There are essentially two ways by which this can be done in the case of non-codebook-basedprecoding.

The network may estimate the uplink channel state, for example based on transmission of uplinksounding reference signals as described in the next chapter, and rely on uplink/downlink channel reci-procity when selecting the precoder matrix to use for the downlink transmission. This is especially ofinterest for TDD operation for which the use of the same frequency for uplink and downlink trans-mission typically leads to a higher degree of downlink/uplink channel reciprocity. It should be notedthough that if the terminal uses multiple receive antennas, it also has to transmit on multiple antennasin order for uplink measurements to fully reflect the downlink channel state.

Alternatively, the network may rely on terminal feedback for precoder-matrix selection. Fortransmission modes 8 and 9, this feedback is actually very similar to the corresponding feedback forclosed-loop codebook-based precoding. In particular, the feedback is based on the same precodermatrices as for codebook-based precoding (seeTable 10.3for an example) but extended to handle upto eight antenna ports. Furthermore, for transmission mode 9, the terminal measurements should bebased on CSI-RS, as described in Section 10.2.3, rather than the cell-specific reference signals.

Thus, despite the name, non-codebook-based precoding may also use defined codebooks.However, in contrast to codebook-based precoding, the codebooks are then only used for the terminalPMI reporting and not for the actual downlink transmission.