上行有两种参考信号：DM-RS 和SRS。
DM-RS与PUSCH和PUCCH的发送相关联，用作求取信道估计矩阵，帮助这两个信道进行解调。
SRS独立发射，用作上行信道质量的估计与信道选择，计算上行信道的SINR。
下行有五种参考信号：
CRS（小区特定的参考信号，也叫公共参考信号）是用于除了不基于码本的波束赋形技术之外的所有下行传输技术的信道估计和相关解调。小区特定是指这个参考信号与一个基站端的天线端口（天线端口0-3）相对应。
MBSFN-RS是用于MBSFN的信道估计和相关解调。在天线端口4上发送。
UE-specific RS（移动台特定的参考信号）用于不基于码本的波束赋形技术的信道估计和相关解调。移动台特定指的是这个参考信号与一个特定的移动台对应。在天线端口5上发送。
PRS是R9中新引入的参考信号。
CSI-RS是R10中新引入的参考信号。
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When
When
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CSI-RS (LTE-Advanced)
CSI-RS is the Channel State Information Reference Signal and is used by the UE to estimate the channel and report channel quality information (CQI) to the base station.
In Release 8, Cell-specific RS (C-RS) was designed for use in channel estimation for up to 4-layer spatial multiplexing, with separate C-RS sequences for each antenna port (0-3). With the addition of up to 8-layer spatial multiplexing in Release 10 came the need for 8-layer channel estimation. However, extending C-RS to 8 layers would add more signaling overhead than was desired, so the CSIReference Signal was added.
Since Release 8/9 UEs are not aware of CSI-RS and will see CSI-RS as interference (which can be placed in PDSCH resource elements), the placement of CSI-RS was designed to be sparse in both the time and frequency domains to minimize the effect on these UEs. And, although the sparse placement of CSI-RS means that CQI will be reported over longer time intervals than C-RS CQI, the target UE devices for higher-layer spatial multiplexing are static or low-mobility devices, so this should not be a major issue.
CSI-RS is transmitted on different antenna ports (15-22) than C-RS (although likely sharing physical antennas with other antenna ports), and instead of using only time/frequency orthogonality like C-RS, CSI-RS uses code-domain orthogonality as well.
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CSI (Channel State Information)
CSI stands for Channel State Information and it is pretty confusing concept. First, it confused me because of too many parameters for configuration and then it became even more confusing with introduction of CA (Carrier Aggregation). It would be very hard to understand every details without having some hands-on (e.g, testing or developing on your own). My recommendation is to go through the basic defintions and additional pages linked in < What is CSI > section and then go through the test procedures briefly described in this page in order to get better sense on the concept.
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网摘地址
https://wenda.so.com/q/1460350606723603?src=140
http://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte/Content/lte_dlg_fmt_rs-prs.htm
http://rfmw.em.keysight.com/wireless/helpfiles/89600b/webhelp/subsystems/lte-a/Content/lte_sym_tbl_framesummary_csi-rs.htm
http://www.sharetechnote.com/html/Handbook_LTE_CSI.html