The main goal of CSI-RSs is to obtain channel state feedback for up to eight transmit antenna ports to assist the eNodeB in its precoding operations. Release 10 supports transmission of CSI-RS for 1, 2, 4 and 8 transmit antenna ports. CSI-RSs also enable the UE to estimate the CSI for multiple cells rather than just its serving cell, to support future multicell cooperative transmission schemes (see Section 29.5.1).
The following general design principles can be identified for the CSI-RS:
• In the frequency domain, uniform spacing of CSI-RS locations is highly desirable, as explained in Chapter 8.
• In the time domain, it is desirable to minimize the number of subframes containing CSI-RS, so that a UE can estimate the CSI for different antenna ports and even different cells with a minimal wake-up duty cycle when the UE is in Discontinuous Reception (DRX) mode, to preserve battery life.
• The overall CSI-RS overhead involves a trade-offbetween accurate CSI estimation for efficient operation and minimizing the impact on legacy pre-Release 10 UEs which are unaware of the presence of CSI-RS and whose data are punctured by the CSI-RS transmissions. Figure 29.3 shows that a CSI-RS density of one RE per RB per antenna port is a good choice, as the throughput degradation compared to ideal CSI estimation is negligible.
• CSI-RSs of different antenna ports within a cell, and, as far as possible, from different cells, should be orthogonally multiplexed to enable accurate CSI estimation.
• To ensure backward compatibility, CSI-RSs should avoid REs used for cell-specific RSs and control channels, as well as avoiding REs used for the Release 10 UE-specific RSs.
−5 0 5 10 15 20 25
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Throughput (kbps)
1.0 RE/RB/AP Perfect
Average subcarrier SNR per Rx Antenna (dB) TU channel, 3 km/h, 8x2
Figure 29.3: Throughput performance with a CSI-RS density of 1 RE per RB per antenna port: uncorrelated 8×2 SU-MIMO, 5 MHz, Typical Urban channel model, 3 km/h.
Taking these considerations into account, the CSI-RS patterns selected for Release 10 are shown in Figure 29.4. CDM codes of length 2 are used, so that CSI-RSs on two antenna ports share two REs on a given subcarrier.
The pattern shown in Figure 29.4(a) can be used in both frame structure 1 (FDD)3 and frame structure 2 (TDD).4In Figure 29.4, the REs used for CSI-RSs are labelled using two letters, the first indicating the cell index and the second referring to the antenna ports of the CSI-RS transmitted on that RE. These patterns follow a ‘nested’ structure, meaning that the REs used in the case of two CSI-RS antenna ports are a subset of those used for four and eight antenna ports; this helps to simplify the implementation. The total number of supported antenna ports is 40, which can be used to give a frequency-reuse factor of 5 between cells with 8 antenna ports per cell, or a factor of 20 in the case of 2 antenna ports. It can be seen that collisions may occur with REs used for the UE-specific RS antenna ports defined in Release 8 for PDSCH transmission mode 7 (see Section 8.2.2); it may therefore be desirable to avoid scheduling UEs in transmission mode 7 in subframes containing CSI-RS.
The pattern shown in Figure 29.4(b) can only be used for frame structure 2 for TDD operation. This pattern is designed to avoid collisions with the Release 8 UE-specific RS antenna port, as this port is more suited to TDD operation where channel reciprocity can be more effectively exploited to support the beamforming. However, this pattern only offers frequency reuse factors of between 3 and 12 depending on the number of antenna ports per cell.
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Similar CSI-RS patterns are provided for the case of the extended CP and the DwPTS field of the special mixed downlink-uplink subframe in frame structure 2 (see [1, Section 6.10.5]).
The CSI-RS configuration is UE-specific. When configured, CSI-RSs are present only in some specific subframes following a given duty cycle and subframe offset. The duty cycle and offset of the subframes containing CSI-RSs and the CSI-RS pattern used in those subframes are provided to a Release 10 UE through RRC signalling. The duty cycle and subframe offset are jointly coded, while the CSI-RS pattern is configured independently of these two parameters.
It should be noted that, in subframes containing CSI-RSs, rate-matching (see Section 10.3.2.4) for PDSCH transmissions to Release 10 UEs assumes that the CSI-RS REs are not available for PDSCH data, and the coded PDSCH data is only mapped to the surrounding REs. However, PDSCH transmissions to Release 8 and 9 UEs are punctured by CSI-RS transmissions. The density and periodicity of the CSI-RSs should therefore be chosen such that the impact of puncturing on the performance of these users is acceptable.
In the context of cooperative MIMO, it may be possible to improve the performance of channel estimation, and especially interference estimation, by coordinating CSI-RS
transmissions across multiple cells. In Release 10 it is therefore possible to ‘mute’ a specific set of REs in data transmissions from a cell. The locations of these REs, known as the ‘muting pattern’, can be chosen to avoid colliding with CSI-RS transmissions from other cells and hence improve the inter-cell measurement quality. The muting pattern is indicated to the UEs by a 16-bit bitmap, where each bit corresponds to a 4-port CSI-RS configuration for frame structure 1 or 2 as shown in Figure 29.4(a). All the REs in an indicated 4-port CSI- RS configuration are muted, and the UE can assume zero transmission power on those REs unless they are used to transmit CSI-RSs.
In subframes in which muting is configured, data transmissions for Release 10 UEs are rate-matched around the muted REs in the same way as for CSI-RSs. Data transmitted to Release 8 and 9 UEs is punctured in the muted REs.5