![]() ![]() To maintain the low PAPR property, the PUSCH and its associated comb-based DM-RS are not frequency multiplexed in the same OFDM symbol but are still time multiplexed as in Fig. However, using a comb structure also increased the sensitivity to channel delay spread, like using CS, so the expected doubling of number of available ports is only valid for channels with very small delay spreads, for example, indoor deployments. This theoretically doubles the number of PUSCH DM-RS ports, with the motivation of MU-MIMO with a larger number of simultaneously scheduled users in the UL. This means that the RS of a DM-RS port is in this case mapped in an OFDM symbol to either odd or even subcarriers instead of every subcarrier. In LTE release 14, the number of available DM-RS ports was increased further by introducing a comb-based DM-RS structure for PUSCH. In release 14, a comb-based mapping to subcarriers was introduced for DM-RS. DM-RS and SRS positions in the subframe for the LTE uplink in release 8. Each scheduling grant of the UL selects a CS and one out of the two available OCC codes, that is, a DM-RS port, for the PUSCH DM-RS.įigure 8.14. As an alternative, port separation using the time-domain OCC can be used instead. In case of a very frequency selective fading channel (large channel delay spread), ports separated by CS will not be orthogonal at the receiver and there will thus be cross-port interference. Due to the properties of the DM-RS sequence, using carefully selected CS, orthogonality between the DM-RS ports is ensured provided that the different RSs are time-aligned at the receiver and that the channel is sufficiently nonfrequency selective. The set of orthogonal DM-RS ports is obtained by a time-domain cyclic shift (CS) of the DM-RS sequence and by using an OCC across the two OFDM symbols carrying the DM-RS in the subframe. Since PUSCH in LTE is based on DFT-spread OFDM, which generates a waveform with low peak-to-average power ratio (PAPR), the PUSCH DM-RS is designed with a low PAPR property as well. Note that even if the UE has four physical antennas, if a single PUSCH layer is transmitted with a precoder across the four antennas, then a single DM-RS port is transmitted by the UE. However, the UL transmission is codebook based, where the eNB selects the precoding matrix the UE should use for the PUSCH transmission. The DM-RS for PUSCH is similar in functionality as the PDSCH DM-RS introduced in release 10, where each layer has its own DM-RS port. Erik Larsson, in Advanced Antenna Systems for 5G Network Deployments, 2020 8.3.1.6 Demodulation reference signals for physical uplink shared channel Read moreģGPP Physical Layer Solutions for LTE and the Evolution Toward NR This CSI-RS structure is used as part of a so-called Tracking Reference Signal (TRS, see further details in Section 8.1.7). ![]() There is also a possibility to configure a single-port CSI-RS with a density of 3 in which case the CSI-RS occupies three subcarriers within each resource block. CSI-RS density equal to 1/2 is not supported for CSI-RS with 4, 8, and 12 antenna ports. In the latter case, the CSI-RS configuration includes information about the set of resource blocks (odd resource blocks or even resource blocks) within which the CSI-RS will be transmitted. However, a CSI-RS may also be configured for transmission only in every second resource block, referred to as CSI-RS density equal to 1/2. Within the configured CSI-RS bandwidth, a CSI-RS may be configured for transmission in every resource block, referred to as CSI-RS density equal to one. In the latter case, the CSI-RS bandwidth and frequency-domain starting position are provided as part of the CSI-RS configuration. The CSI-RS can be configured to cover the full bandwidth of the bandwidth part or just a fraction of the bandwidth. Johan Sköld, in 5G NR (Second Edition), 2021 8.1.2 Frequency-Domain Structure of CSI-RS ConfigurationsĪ CSI-RS is configured for a given downlink bandwidth part and is then assumed to be confined within that bandwidth part and use the numerology of the bandwidth part. ![]()
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