1.      CCE是PDCCH的资源单元 (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个等级: 1, 2, 4, 8. 下图来自于3GPP 36.211

通过上图可以看到不同PDCCHformat (CCE level) 可以适配不同数量的数据bits, 而PDCCH中承载的是DCI，所以PDCCH所传输的DCI的长度应该小于上面表的第4列数据(Number of PDCCH bits)。举例，假如传输DCI IA ，总长度 = 25  + 16bit CRC = 41 bits,41小于上面所有的PDCCH format,最终网络决定用哪一个CCE level来传输，取决于如下因素：

a.      最坏的情况

Common的DCI (SIB,RAR, Paging)会被所有的UE来接受，无论UE距离eNodeB 远还是近。这种情况下，eNodeB最常用的就是会用最高的level (4或8)来调度这些DCI。比如上述DCI 1A, 41bits 用CCE Level 8来调度，41bits经过rate match 会膨胀到576bits, 这样能够提供很好的前向纠错保护；

下图来自36.213

b.     基于CQI

网络根据UE上报的CQI来安排对应的CCE level, 如果CQI比较差，网络会给以较高的CCE Level。

c.      RNTI 适配

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

通过以上得出结论：CCE aggregation Level越高，UE就越容易解码出DCI.

3.      参考

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