LTE Advanced - eCCE/ePDCCH

 

 

Why eCCE/ePDCCH ?

 

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.

decrease interference between control region from different cells

increase the reliability of reception by applying BeamForming

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.

 Eventuall 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)

 

 

 

 eREG to RE Mapping

 

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.

 

< 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 >

 

  

RRC Aspect of eCCE/ePDCCH

 

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.

 

Heterogeneous Networks in LTE

by Jeanette Wannstrom, masterltefaster.com and Keith Mallinson,

WiseHarborEffective network planning is essential to cope with the increasing number of mobile broadband data subscribers and

bandwidth-intensive services competing for limited radio resources. Operators have met this challenge by increasing capacity

with new radio spectrum, adding multi-antenna techniques and implementing more efficient modulation and coding schemes.

However, these measures alone are insufficient in the most crowded environments and at cell edges where performance can

significantly degrade. Operators are also adding small cells and tightly-integrating these with their macro networks to spread

traffic loads, widely maintain performance and service quality while reusing spectrum most efficiently.

One way to expand an existing macro-network, while maintaining it as a homogeneous network, is to “densify” it by adding

more sectors per eNB or deploying more macro-eNBs. However, reducing the site-to-site distance in the macro-network can

only be pursued to a certain extent because finding new macro-sites becomes increasingly difficult and can be expensive,

especially in city centres. An alternative is to introduce small cells through the addition of low-power base stations (eNBs,

HeNBs or Relay Nodes (RNs)) or Remote Radio Heads (RRH) to existing macro-eNBs. Site acquisition is easier and cheaper with

this equipment which is also correspondingly smaller.

Small cells are primarily added to increase capacity in hot spots with high user demand and to fill in areas not covered by the

macro network – both outdoors and indoors. They also improve network performance and service quality by offloading from

the large macro-cells. The result is a heterogeneous network with large macro-cells in combination with small cells providing

increased bitrates per unit area. See Figure 1.

   

Heterogeneous network planning was already used in GSM. The large and small cells in GSM are separated through the use of

different frequencies. This solution is still possible in LTE. However, LTE networks mainly use a frequency reuse of one to

maximize utilization of the licensed bandwidth.

In heterogeneous networks the cells of different sizes are referred to as macro-, micro-, pico- and femto-cells; listed in order of

decreasing base station power. The actual cell size depends not only on the eNB power but also on antenna position, as well as

the location environment; e.g. rural or city, indoor or outdoor . The HeNB (Home eNB) was introduced in LTE Release 9 (R9). It

is a low power eNB which is mainly used to provide indoor coverage, femto-cells, for Closed Subscriber Groups (CSG), for

example, in office premises. See Figure 2.

Specific to HeNBs, is that they are privately owned and deployed without coordination with the macro-network. If the frequency

used in the femto-cell is the same as the frequency used in the macro-cells, and the femto-cell is only used for CSG, then there

is a risk of interference between the femto-cell and the surrounding network.

The Relay Node (RN) is another type of low-power base station added to the LTE R10 specifications. The RN is connected to a

Donor eNB (DeNB) via the Un radio interface, which is based on the LTE Uu interface. See Figure 2. When the frequencies used

on Uu and Un for the RN are the same, there is a risk of self interference in the RN. From the UE perspective the RN will act as

an eNB, and from the DeNB’s view the RN will be seen as a UE. As also mentioned, RRHs connected to an eNB via fibre can be

used to provide small cell coverage.

 

Introducing a mix of cell sizes and generating a heterogeneous network adds to the complexity of network planning. In a

network with a frequency reuse of one, the UE normally camps on the cell with the strongest received DL signal (SSDL), hence

the border between two cells is located at the point where SSDL is the same in both cells. In homogeneous networks, this also

typically coincides with the point of equal path loss for the UL (PLUL) in both cells. In a heterogeneous network, with high-

power nodes in the large cells and low-power nodes in the small cells, the point of equal SSDL will not necessarily be the same

as that of equal PLUL. See Figure 3.

A major issue in heterogeneous network planning is to ensure that the small cells actually serve enough users. One way to do

that is to increase the area served by the small cell, which can be done through the use of a positive cell selection offset to the

SSDL of the small cell. This is called Cell Range Extension (CRE). See Figure 4.

A negative effect of this is the increased interference on the DL experienced by the UE located in the CRE region and served by

the base station in the small cell. This may impact the reception of the DL control channels in particular.

A number of features added to the 3GPP LTE specification can be used to mitigate the above-mentioned interference problem

in heterogeneous networks with small cells:

Inter-cell Interference Coordination: ICIC

ICIC was introduced in R8. The eNBs can communicate using ICIC via the X2 interface to mitigate inter-cell interference for UEs

at the cell edge. The X2AP message used for this is called “Load Information”. See Figure 5. Through the “Load Information”

message an eNB can inform neighbouring eNBs about: UL interference level per Physical Resource Block (PRB);  UL PRBs that

are allocated to cell edge UEs, and hence are sensitive to UL interference; and if DL Tx power is higher or lower than a set

threshold value. The eNBs receiving these messages can use the received information to optimize scheduling for UEs at cell

edges.

ICIC has evolved to better support heterogeneous network deployments -- especially interference control for DL control

channels. Enhanced ICIC (eICIC) was introduced in LTE R10. The major change is the addition of time domain ICIC, realized

through use of Almost Blank Subframes (ABS). ABS includes only control channels and cell-specific reference signals, no user

data, and is transmitted with reduced power. When eICIC is used, the macro-eNB will transmit ABS according to a semi-static

pattern. During these subframes, UEs at the edge, typically in the CRE region of small cells, can receive DL information, both

control and user data. The macro-eNB will inform the eNB in the small cell about the ABS pattern. See Figure 6.

ICIC is evolved in LTE R11 to further enhanced ICIC (feICIC). The focus here is interference handling by the UE through inter-cell

interference cancellation for control signals, enabling even further cell range extension.

eICIC and feICIC are especially important when Carrier Aggregation (CA) is not used.

Carrier Aggregation with cross-carrier scheduling

Carrier Aggregation (CA) is introduced in R10, with backward compatibility to R8, to increase the total bandwidth available to

UEs and hence their maximum bitrates. When CA is used a number of R8 carriers, referred to as Component Carriers (CC), are

aggregated and any CA-capable UE can be allocated resources on all CCs, while R8/R9 UEs can only be allocated resources on

one CC. Cross-carrier scheduling is an important feature in heterogeneous networks. Using cross-carrier scheduling it is

possible to map the Physical DL control channels (PDCCH) on different CCs in the large and small cells. See Figure 7. The

PDCCH, carrying DL Control Information (DCI) with scheduling information, must be received by the UEs at the cell edge;

PDCCH may be transmitted with higher power than the traffic channels. Hence, using different carriers for the PDCCH in the

large and small cells reduces the risk of PDCCH interference.

From LTE R11 onwards it is possible to handle CA with CCs requiring different timing advance (TA); for example, combining CCs

from eNBs with CCs from RRHs. See Figure 8.

CoMP – Coordinated Multi Point

One way to ensure that a UE is using both the best DL and the best UL carrier in a heterogeneous network is to use CoMP,

introduced in LTE R11. With CoMP a number of transmission/reception points (i.e. eNBs, RNs or RRHs) can be coordinated to

provide service to a UE – for example, data can be transmitted at the same time in the same PRBs from more than one

transmission point to one UE, or data can be received from one transmission point in one subframe and from another

transmission point in the next subframe. CoMP can be used both in DL and UL. When CoMP is used in a heterogeneous network

a number of macro-cells and small cells can be involved in data transmission to and from one UE. Especially useful in

heterogeneous networks is the possibility for a UE in the cell range extension region to utilize the best UL in the small cell and

the best DL in the macro-cell. See Figure 9. This, however, requires that the macro-eNB and the base station in the small cell

are synchronized, and most likely it will require a combination of macro-eNB with RRHs in the small cell.

Further enhancements regarding heterogeneous network and small cells are coming in future 3GPP Releases. At the time of

writing, Release 12 is still in the process of being formulated with some features in the study phase and others, such as work

on interference management for neighbour TDD cells, dual connectivity between the macro cell and small cells, mobility

planning within hyper-dense environments and advances in carrier aggregation combinations already in the normative phase

(specifications).

Future updates to this paper will include what has been achieved with the completion of Release 12 during the second half of

2014.

Further reading

A new HetNet paper by Keith Mallinson, WiseHarbor; August 18, 2014

"...Coordinating the low-power layer of small cells with the macro network improves performance across the entire

network while also further boosting efficiencies in spectrum use and power consumption, automating network

configuration and optimization. The upcoming 3GPP Release 12 (due to be frozen September 2014) standardises various

capabilities in these developments including dual connectivity, small cell on/off and 256 QAM..." ...Read the full article

TR 36.806 Evolved Universal Terrestrial Radio Access (E-UTRA); Relay architectures for E-UTRA (LTE-Advanced) 

TR 36.808 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation; Base Station (BS) radio transmission and

reception 

TR 36.814 Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects 

TR 36.815 Further Advancements for E-UTRA; LTE-Advanced feasibility studies in RAN WG4 

TR 36.823 Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation Enhancements; UE and BS radio transmission and

reception 

TR 36.826 Evolved Universal Terrestrial Radio Access (E-UTRA); Relay radio transmission and reception 

TS 22.220 Technical Specification Group Services and System Aspects; Service requirements for Home Node B (HNB) and Home eNode

B (HeNB) 

TS 36.101 Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception 

TS 36.212 Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding 

TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures 

TS 36.216 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer for relaying operation 

TS 36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN);

Overall description; Stage 2 (R8, R10, R11) 

TS 36.423 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) (R8, R10, R11)