UE Role in LTE Heterogeneous NetworksDino Flore

April 1, 2012

2QUALCOMM Proprietary

Deployment Model Vision: Heterogeneous Networks

• Target coverage with macro eNBs for initial deployments

• Pico cells, Home eNBs and Relay nodes added for incremental capacity growth, richer user experience and in-building coverage

– These low power nodes can offer flexible site acquisition

– Relay nodes provide coverage extension with no incremental backhaul expense

MacroHeNB

Core Network

Internet

Relay

Pico

Backhaul

Relay Backhaul

Pico Pico

Need for Flexible and Low-Cost Network Deployment Using Mix of Macro, Pico, Relay, RRH and Home eNBs

3QUALCOMM Proprietary

Heterogeneous Networks: Goals

• Maximize gains from the addition of new nodes to macro-network

– Looking for cell-splitting gains

• More servers providing service to same UE population

– Enough UE population needs to be associated with new low power nodes

– Performance metrics of interest: mean vs. median throughput

• Enable self-configuration / self-adaptation to

– Different densities of new nodes

– Distribution of UEs in the network

• Number of UEs close to hot-zones, etc.

4QUALCOMM Proprietary

HetNets Deployment choices: co-channel

• High power nodes (macro cells) and low power nodes (picos, HeNBs, relays) share the same resources

– Co-channel deployment

– Large power disparity between high power and low power nodes

• Could have a 30x power disparity (30W vs. 1W)

– Low power nodes in disadvantage

– The capacity gain offered by the deployment of low power nodes is limited

• Very few terminals will associate with the low power nodes

Pico

Macro

Macro and pico cells deploy using same resources

Pico

Pico

5QUALCOMM Proprietary

HetNets Deployment choices: resource partition

• Macro cells limit their transmissions to some set of (time/frequency) resources: S1

– Macro cells “give away” some resources with the expectation that there will be a net network capacity gain

– Resources given away by macro cells experience no interference from macro network and become “prime” resources for low power layer

– Effectively expands the coverage of low power nodes: cell rage expansion

• Shift to the right of the geometry distribution of low power layer

Pico

Macro

S1

S1 S2

No macro tx on resource set S2 reduces interference and increases coverage of Pico

layer on these resources

S2

S1

Pico

Pico

S2

S1

6QUALCOMM Proprietary

How can the resources be partitioned?

• FDM of different power class nodes (multi-carrier)

– f1: for macro cells, f2: for pico cells

– Note that an additional carrier could be required for femto/CSG cells to avoid coverage holes

– FDM (multi-carrier) approach characteristics:

• Coarse granularity

• Semi-static in nature

• TDM of different power class nodes (time-domain partition)

– Set 1 of subframes: for macro cells, Set 2 of subframes: for pico cells

• Use of Set 1 of subframes for pico cells also permitted (limited gain)

– Femto/CSG cell operation may require exclusive resources to avoid coverage holes

– TDM approach characteristics:

• Does not require multiple carriers

• Finer granularity: Can adapt to different partitioning more easily

• Requires network time synchronization

7QUALCOMM Proprietary

Issues to resolve with TDM approach

• Hearibility: initial acquisition, regular control and data reception

– Need to enable acquisition, measurement and reporting of weak cells

• Association: serving cell determination

– Best DL SINR may not be the best association technique

• Smallest pathloss minimizes interference on UL

– Handover biasing providing the network the ability to offload traffic to low power nodes

• How aggressive the biasing can be depends on UE capability (discussed later)

• Resource specific UE feedback

– Channel quality on different subframes can change considerably

• Resource partition: what subframes are given to each power class

– Adaptive resource partitioning

• For different/changing densities of low power nodes across the network

• For different UE distributions across the network or over time

• Resource partitioning and association techniques interact with each other

8QUALCOMM Proprietary

Small Caveat…

• Due to UE legacy support issues subframes can not be blanked out

– Therefore, we have almost blank subframes (ABS)

• Taking a closer look almost blanks subframes are not that blank:

– The following signals are transmitted to ensure backward compatibility

• CRS (pilot signal)

• PSS/SSS (synchronization signals)

• SIB1/MIB (broadcast information)

– CRS/PSS/SSS/SIB1/MIB still cause strong interference

– In a ABS, no unicast data or control is transmitted

9QUALCOMM Proprietary

Small Caveat…

Primary synchronization signal (PSS)

Secondary synchronization signal (SSS)

Physical broadcast channel (PBCH)

Subframe,

0

.

.

.

.

.

.

Subframe,

5

Subframe,

1,2,3,4,6,7,

8,9

Slot

Subframe

Control resource element (subcarrier)

Data resource element (subcarrier)

CRS antenna port 0 resource element (subcarrier)

CRS antenna port 1 resource element (subcarrier)

10QUALCOMM Proprietary

Implications of ABS

• The fact that ABS subframes are not really empty poses new challenges

– The interference caused by the transmission of legacy signals affects the reception of weak signals in ABS

• Limited cell range expansion regime unless this interference is removed at the UE

• A UE can convert an ABS subframe into blank by cancelling the interference

– This becomes the main role of the UE for HetNets

• How aggressive the handover biasing can be at the network depends on how well the UE can cope with the interference from legacy signals

– This determines the cell range expansion regime

– 3GPP recently agreed on 9dB handover bias for which UE performance requirements will be standardized as part of Rel-11

11QUALCOMM Proprietary

Conclusions

• Heterogeneous Networks are a cost effective approach to significantly enhance capacity of LTE cellular networks

– Macro cells ensure broad coverage and low power nodes provide additional capacity

• Three design features are crucial for HetNets

– Interference management as severe interference limits the coverage area of low power nodes

– Cell range expansion through adaptive resource partitioning as it enables traffic load balancing between high and low power cells (traffic offloading)

– Interference cancellation receiver in the terminal as it ensures that weak cells can be detected and legacy transmissions can be removed

• All three components together are needed to exploit the full potential of HetNets

12QUALCOMM Proprietary

Annex (some simulation results)

13QUALCOMM Proprietary

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

Serving cell C/I (dB)

Thr

ough

put (

Kbp

s)CollidingRS-TxMode3

CellID0-TxMode3

CellID0-TxMode3-CellID96-TxMode3-16dB

CellID0-TxMode3-CellID96-TxMode3-IC-16dB

CRS IC Gains for colliding RSin non-MBSFN ABS

Throughput Gains by CRS IC – Colliding RS

9 dB Gain

14QUALCOMM Proprietary

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4 2 6 2 8 30 3 20

5 0 0

10 0 0

15 0 0

20 0 0

25 0 0

30 0 0

35 0 0

40 0 0

45 0 0

50 0 0

55 0 0

60 0 0

S e rv i ng c e l l C/ I (d B )

Th

rou

gh

pu

t (K

bp

s)N on c o l li d in g RS -T x Mo d e 3

Ce ll ID0 -T x Mo d e 3

Ce ll ID0 -T x Mo d e 3 -Ce l lID1 2 1 -T x Mo d e 3 -1 6 d B

Ce ll ID0 -T x Mo d e 3 -Ce l lID1 2 1 -T x Mo d e 3 -IC-1 6 d B

CRS IC Gains for Non-colliding RSin non-MBSFN ABS

Throughput Gains by CRS IC – Non-colliding RS

3.5 dB Gain

15QUALCOMM Proprietary

Reliability of PBCH with PBCH IC

SINR=-18dB

16QUALCOMM Proprietary

• Simulation Assumptions

– Hotspot scenario with two pico cells per macro cell randomly placed

– 2/3 of the UEs located within 40m radius of the two pico cells

– User arrivals follow Poisson process

– 1 Mbytes download file size

– Cell range expansion of SIR = -18 dB by CRS/PSS/SSS/PBCH IC

– Served throughput = total amount of data for all users / total amount of observation time / number of cells

• Gains up to 130% at 75% load achievable by aggressive cell range expansion, adaptive ABS allocation and advanced receivers

Performance with IC Receivers – Hotspot Scenario

0

5

10

15

20

25

30

35

40

30 40 50 60 70 80 90

Se

rv

ed

Th

ro

ug

hp

ut [

Mb

ps]

Macro Cell Resource Utilization [%]

Served Throughput in Hotspot Scenario

Macro only 2 Picos, no Resource Partitioning 2 Picos, Resource Partitioning

130% Gain