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4G/LTE - eCCE/ePDCCH |
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eCCE/ePDCCH is a new type of resource allocation for contron channel information. In engineering, when we introduce anything new, usually we would have some reason (or motivation) on why we need the new things. Same thing applies to eCCE/ePDCCH. So our first question would be 'why we need this new type of resource allocation ?'. Main reason (motivation) would be illustrated as below and we may have some additional advantage as a result of adopting this new method.
By adopting this method, we may enjoy some additional advantage and followings are those advantage.
How can we descrease interference between control region from different cells ? With ePDDCH, we can allocate control information for each user in such a way that ePDCCH for different users locate far away from each other to minimize the interference. What does it mean by 'increase the reliability of reception by applying BeamForming' ? Since ePDCCH is in the area where PDSCH is located and each ePDCCH is UE specific, we may apply BeamForming technology and it would increase the reliability of signal reception.
Can we complete remove the CFI overhead ?
If we can move the PDCCH data to PDSCH area, can we completely remove the overhead caused by the symbol 0 (or 1,2) ? The answer is 'NO'. There are a couple of reason for this. First, we still need the symbol 0 for PCFICH and PHICH. We don't have any new mechanism to move this part to other area. Second, even with ePDCCH we cannot move the control information allocated in Common Search Space. So we still need to allocate a certain amount of the space in conventional way.
Resource Allocation for ePDCCH
Following diagrams are from TDoc R-112517 Discussion on ePDCCH Design Issues (3GPP TSG-RAN1#66 meeting). This shows some possible idea of designing ePDCCH. It doesn't mean that all of these concept will be adopted by TS specification. But these can be a good reference for you to get general idea. Important thing to notice is that ePDCCH is allocated in those symbols allocated for PDSCH for conventional LTE.
At last, 3GPP Rel 11 adopted ePDCCH. It seems that 3GPP adopted the (a) Pure FDM (Refere to 36.211 36.211 6.2.4A/6.8A and)
36.211 6.2.4A describes as follows :
There are (1)16 EREGs, numbered from 0 to 15, per physical resource block pair. Number all resource elements, except resource elements carrying DM-RS for antenna ports p = {107,108,109,110} for normal cyclic prefix or p ={107,108} for extended cyclic prefix, in a physical resource-block pair cyclically from 0 to 15 in an increasing order of first frequency, then time. All resource elements with number i in that physical resource-block pair constitutes EREG number i .
If you translate this 3GPP statement into an illustration, it would look as follows. At the first glance, it would be hard to correlate this statement with the following illustration.. but read this statement several times and try to illustrate on your own. You may use following arrows and markers as a guideline to construct the eREG mapping. (I hope I am not confusing you rather than helping :) i) Number all resource elements, except resource elements carrying DM-RS for antenna ports ==> Number from 0 through 15 and restart with 0 when it hits 15, but skip the resource element reserved for reference signal ii) When you numbering, follow the path/direction marked by red arrows shown below.
< eREG to RE Mapping for Normal Subframe >
There are two different ways of mapping eREG to RE. These mapping method are called Transmission type which is configured by IE transmissionType-r11. One is called 'localized' and the other is called 'distributed'. Following is one example that shows the 'Localized' transmission type. Even though the RE location for each eREG is pretty much scrambled, you would recognize the pattern relatively easily and in this way you would notice that the REs in the same eREG would clustered in the same frequency. It implies that this kind of RE mapping would be vulnerable noise or fading. Due to this, 3GPP defines another mapping algorithm called 'distributed'. In distributed mapping, REs in an eREG is scattered in much random fashion so that they can have more resistance to noise and fading.
Following is an example of localized mapping (transmission type) for FDD and normal subframe of TDD.
Following is an example of localized mapping (transmission type) special subframe config 1 and 6 of TDD.
< eREG to RE Mapping for Special Subframe Config 1 and 6 >
Following is the overall RRC message structure to configure ePDCCH. Some of these are straightforward and some of them would need a lot of effort to completely understand the concept down to the physical layer. I will keep updating as I get more understanding on details.
EPDCCH-SetConfig : Provides EPDCCH configuration set. See TS 36.213 - 9.1.4. E-UTRAN configures at least one EPDCCHSetConfig when EPDCCH-Config is configured.
setConfigId : Indicates the identity of the EPDCCH configuration set.
subframePatternConfig : Configures the subframes which the UE shall monitor the UE-specific search space on EPDCCH, except for predefined rules in TS 36.213 9.1.4. If the field is not configured when EPDCCH is configured, the UE shall monitor the UE-specific search space on EPDCCH in all subframes except for pre-defined rules in TS 36.213-9.1.4.
transmissionType : Indicates whether distributed or localized EPDCCH transmission mode is used as defined in TS 36.211-6.8A.1.
< EPDCCH Starting Position : 36.213 9.1.4.1>
startSymbol : Indicates the OFDM starting symbol for any EPDCCH and PDSCH scheduled by EPDCCH on the same cell, see TS 36.213-9.1.4.1. If not present, the UE shall release the configuration and shall derive the starting OFDM symbol of EPDCCH and PDSCH scheduled by EPDCCH from PCFICH. Values 1, 2, and 3 are applicable for dl-Bandwidth greater than 10 resource blocks. Values 2, 3, and 4 are applicable otherwise. E-UTRAN does not configure the field for UEs configured with tm10.
< PRB-pair indication for EPDCCH : 36.213 9.1.4.4>
numberPRB-Pairs : Indicates the number of physical resource-block pairs used for the EPDCCH set. Value n2 corresponds to 2 physical resource-block pairs; n4 corresponds to 4 physical resource-block pairs and so on. Value n8 is not supported if dl-Bandwidth is set to 6 resource blocks.
resourceBlockAssignment : Indicates the index to a specific combination of physical resource-block pair for EPDCCH set. See TS 36.213 - 9.1.4.4. The size of resourceBlockAssignment is specified in TS 36.213 9.1.4.4 and based on numberPRB-Pairs and the signalled value of dl-Bandwidth.
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