Foreword
Downlink MIMO is undoubtedly a key technical component in LTE R8. Transmission modes for 1, 2 and 4 eNodeB antenna ports that provide peak data rates in excess of 300 Mbit / s have been specified. In LTE-Advanced, the next step is naturally to continue ambitious goal setting to ensure its status as the leading wireless access technology. To ensure this, LTE-Advanced supports up to 8 transmit antenna ports for data transmission on the downlink. The reference symbol structure in the downlink is analyzed, the working principle of the codebook design is described, the system performance of the downlink multi-antenna enhancement scheme is verified, and finally the LTE-Advanced downlink multi-antenna technology in LTE-Advanced R11 is predicted Development trend.
1 Reference symbol structure in the downlink
In LTE R8 and R9, MIMO operation is mainly based on the common reference symbols (CRS) related to cellular. Between the antenna port and the scale, the reference symbol pattern is orthogonal, depending on the number of transmit antenna port configurations. Channel state information (CSI) measurement and data demodulation are usually performed using CRS. The exception is TDD beamforming transmission mode 7, in which case the reference symbol (URS) related to the UE is used for demodulation. The simple solution in LTE-Advanced has defined another cellular-related RS for the case of 8 transmit (TX) antennas, which implies that both CSI measurement and demodulation can use CRS. However, the backward compatibility of using R8 terminals will generate a problem that the existence of the new RS is not known. In these cases, due to the continuous collision between the data and the new RSS, the performance of the traditional terminal will inevitably deteriorate. Another disadvantage of 8-TX CRS is that, given the fact that most terminals usually cannot enjoy the advantages of layer 8 transmission, the overhead of reference symbols is too high.
To address these challenges, LTE R10 decided to adopt another reference symbol paradigm. The key idea is to decouple the reference signals used for CSI measurement from those used for data demodulation, as follows:
a) In the case of 2/4/8 transmit antennas, CSI (ie, CQI, PMI, and RI) measures and reports incoming channel state information reference symbols (CSI-RS).
b) With the support of up to 8 spatial layers, the precoding orthogonal reference symbols related to the UE are used for data demodulation. Mainly prove the rationality of this choice from three aspects. First, the reference symbols related to the UE support flexible transmission precoding at the eNodeB, which can be seen as an engine for competitive downlink MU-MIMO. Second, according to the transmission level, the reference symbol overhead increases, so some high-level capable terminals cannot punish the entire system, mainly because the reference symbol overhead is high, which is similar to the case of using CRS. Third, the reference symbols related to the UE will benefit from transmitting the precoding gain, which in turn will lead to reliable channel estimation. The LTE R10 URS mode of level 1-2 corresponds to the URS mode in LTE R9, and the LTE R10 URS mode of level 3-8 can be regarded as an extension.
CSI-RS is sparse in time and frequency because CSI measurement requirements are less stringent than data demodulation. Normally, CSI-RS uses very low density (1RE / port / PRB) for periodic transmission (for example, every 10ms). The period of the CSI-RS is configurable, and its duty cycle value varies from 5 to 80 ms, because the target of the downlink MIMO enhancement scheme is mainly a low mobility scenario. This means that the impact on traditional LTE R8 / 9 terminals is limited to subframes, and CSI-RS is transmitted in these subframes. At other times, the traditional terminals can work without any penalty. At the same time, the relatively low density of CSI-RS considers the use of CSI-RS to transmit data to R8 terminals in subframes, although the performance is reduced. However, the MCS level needs to be reduced accordingly to support the UE to deal with additional interference.
Although the main driving force for the introduction of CSI-RS is support for eight transmit antennas at the eNodeB, the CSI-RS mode is also defined for other antenna configurations. Overall, the specifications are very sensible, and CSI-RS and CRS can be configured independently. In Figure 1, the R10 CSI-RS situation is described for 8, 4, and 2 transmit antenna ports, respectively. The CSI-RS mode has a nesting feature-the mode for a few antenna ports is a subset of the mode for a large number of antenna ports. In addition to the modes in Figure 1, other possible configurations are supported, and independent CSI-RS configurations for normal and extended cyclic prefixes are defined. For frame structure types 1 and 2, different modes are also available-that is, for FDD and TDD, there is a slight change in TDD, that is, collision with antenna port 5 can be avoided. Another major difference of CSI-RS is the high reuse factor. For example, in the case of 2 antenna ports, the reuse factor is 20. In contrast, in the case of 2 antenna ports, the CRS reuse factor is 3. A higher reuse factor makes network planning easier, and collisions from CSI-RS to CSI-RS can be largely avoided, which is very beneficial in the case of local network load.
In Figure 2, two examples of URS configuration are given. The specification supports the use of 12/24 resource elements (RE), which can be used for URS, depending on the transmission level. For example, as in the case of R9, Layer 1 and Layer 2 can work with 12 resource elements and an overlay orthogonal code (OCC) of length 2, while 24 resource elements and an overlay orthogonal code of length 4 are available Located on floors 3-8. The use of frequency division multiplexing plus variable length OCC supports the effective expansion of RS overhead based on the transmission level. We have noticed that by using antenna virtualization (eg, down to a certain CRS port), CSI-RS provides an opportunity to efficiently reduce CRS overhead, as shown in the right half of Figure 2, while the UE is still able to connect via CSI-RS Up to 8 antenna ports.
The basic principle of system operation using CSI-RS is shown in Figure 3. The terminal estimates the CSI based on the CSI-RS and transmits the CSI feedback to the eNodeB, and the eNodeB, in turn, can use the CSI for selecting the precoder and modulation and coding scheme (MCS) for the data. Data can be transmitted together with user-specific (dedicated) demodulation reference symbols (URS, also known as DM-RS), and like data, the same physical resource block is extended. The same transmission precoding can be used for the data layer and related DM-RS ports. Contrary to the situation in LTE R8, it considers the case where the eNodeB uses any precoding, because the precoding used is still transparent to the terminal and does not need to be transmitted to the user.
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