1.??????CCE是PDCCH的資源單元 (resourceunit)。

A PDCCH is transmitted on one CCE or anaggregation of several consecutive CCEs, where a CCE corresponds to 9 Resource ElementGroups (REGs). In PDCCH transmission, only those REGs are used which are notassigned to PCFICH or PHICH. Each REG contains 4 Resource Elements (REs).

1 CCE = 9 REGs 1REG = 4 REs

2.??? CCE AggregationLevels

一共4個等級: 1, 2, 4, 8. 下圖來自于3GPP 36.211

通過上圖可以看到不同PDCCHformat (CCE level) 可以適配不同數(shù)量的數(shù)據(jù)bits, 而PDCCH中承載的是DCI，所以PDCCH所傳輸?shù)腄CI的長度應(yīng)該小于上面表的第4列數(shù)據(jù)(Number of PDCCH bits)。舉例，假如傳輸DCI IA ，總長度 = 25? + 16bit CRC = 41 bits,41小于上面所有的PDCCH format,最終網(wǎng)絡(luò)決定用哪一個CCE level來傳輸，取決于如下因素：

a.??????最壞的情況

Common的DCI (SIB,RAR, Paging)會被所有的UE來接受，無論UE距離eNodeB 遠還是近。這種情況下，eNodeB最常用的就是會用最高的level (4或8)來調(diào)度這些DCI。比如上述DCI 1A, 41bits 用CCE Level 8來調(diào)度，41bits經(jīng)過rate match 會膨脹到576bits, 這樣能夠提供很好的前向糾錯保護；

下圖來自36.213

b.?????基于CQI

網(wǎng)絡(luò)根據(jù)UE上報的CQI來安排對應(yīng)的CCE level, 如果CQI比較差，網(wǎng)絡(luò)會給以較高的CCE Level。

c.??????RNTI 適配

This again mandates the scheduler to always use aggregation level 4 or 8 for common RNTIs such as SI, P, RA and allows the scheduler to choose any aggregation levels for dedicated RNTIs such as C, TC etc

通過以上得出結(jié)論：CCE aggregation Level越高，UE就越容易解碼出DCI.

3.??????參考

3GPP 36.211/36.213

https://ltebasics.wordpress.com/2015/11/02/aggregation-levels-and-pdcch/

http://bbs.c114.net/thread-585503-1-1.html

http://www.sharetechnote.com/html/Handbook_LTE_CCE_Index.html

http://lteuniversity.com/get_trained/expert_opinion1/b/hongyanlei/archive/2011/05/20/pdcch-construction.aspx

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