Performance of WLAN RSS-based SON for LTE/WLAN access network selection

Yu Wang+, Relja Djapicφ, Andreas Bergström+, István Z. Kovács*, Daniela Laselva*, Kathleen Spaeyx, Bart Sasx

+Ericsson Research, Stockholm, Sweden; φTNO, Delft, Netherlands; *Nokia, Aalborg, Denmark; x iMinds/University of Antwerp, Antwerp, Belgium

§  Why SON for LTE/WLAN access selection - Motivation

§  What is it – Access selection rule and SON function

§  If it works – Simulation results in a heterogeneous network scenario

§  How does it work – Load balancing leading to user throughput improvement

§  If it really works – Various traffic model parameters

§  How dose it work in practice – Implementation issues

§  How to make it better – Summary and future work

Introduction

2 SEMAFOUR

§  Operators are using WLAN for offloading –  “Carrier grade Wi-Fi”

§  Great interests in integrating cellular and WLAN networks

§  One of the key technology component: access selection / traffic steering

§  SON for access selection –  SON: proved working in related areas, e.g. load balancing –  A good candidate to address the LTE/WLAN access selection

§  Task: design a SON function and prove if it works (or not) by simulation

Why SON for LTE/WLAN Access Selection?

3 SEMAFOUR

§  Control parameter –  Wi-Fi RSS admission threshold

§  RSS-based access selection rule –  Connect to WLAN if RSS > RSS_Thr –  Access selection at session start

§  Monitoring KPIs –  LTE: PRB utilization percentage –  Wi-Fi: Channel busyness percentage

–  Busy: at least one node is transmitting or attempts to transmit

–  Limitation: may not reflect load especially with full buffer users

–  Not comparable between them

What Is It – Control Parameters and Monitoring KPIs

4 SEMAFOUR

UEs are served by Wi-Fi in locations with RSS > RSS_thr

LTE macro LTE micro &Wi-Fi

What Is It – SON Algorithm

5 SEMAFOUR

LTE load measurement (period: pL)

Wi-Fi load measurement (period: pW)

Filtering (αW) Filtering (αL)

Yes

Yes

Yes

Yes

No

No

No

No

repeat every pSON seconds

KPI

mon

itor

Off

load

ing

Onl

oadi

ng

RSS threshold regulation

RSS threshold initialization

Decrease RSS threshold

Wi-Fi high load

LTE low load?

LTE high load

Increase RSS threshold

Increase RSS threshold

Decrease RSS threshold

LTE low load

Wi-Fi low load

Yes

No

LTE & Wi-Fi congested

)(L)1(L)1()(L nnn measured∗+−∗−= αα

§  SON function –  Overload protection –  Offloading –  Onloading

§  Important parameters –  Filtering factor –  SON update period and step size –  Congestion/High/low load thresholds

§  Operator policy fulfillment –  Utilize LTE network until highly loaded –  Balanced LTE/WLAN user

throughput

§  Environment: 1.1 km2 urban area located in city center of Hannover §  Deployment: 5 LTE Macros (46 dBm), 28 co-located outdoor LTE Micro (33

dBm) / AP (20 dBm) §  6 dB cell extension offset for Micros §  Spatial traffic distribution: Outdoor busy hour traffic + a hotspot with 70%

traffic outdoor §  Offered traffic: Fixed size UDP downloading / uploading (DL: 5 MB; UL: 0.8

MB; 12 arrival/sec)

If It Works – Simulation Scenario

6 SEMAFOUR

LTE Macro

LTE Micro & WLAN AP

1 Micro/AP Index

Micro: 20MHz@1.8GHz AP: 20MHz@5GHz

Macro Index

1

1200

1200

0

0

Met

er

Meter

Macro: 20MHz@1.8GHz

User Location

§  Indoor users are located on the ground floor only

§  No external interference §  Mobility: Stationary users only

§  Highest average and 5th percentile user throughput

–  Fixed RSS threshold = -65 dBm. –  User session split:

–  WLAN: 42% –  LTE Micro: 37% –  LTE Macro: 21%

If It Works – Fixed RSS Threshold

-92 -85 -75 -65 -55 -450

20

40

60

80

100

RSS Threshold (dBm)

User

Ses

sion

Per

cent

age

(%)

LTE MacroLTE MicroWi-Fi

-92 -85 -75 -65 -55 -450

20

40

Aver

age

User

Thro

ugph

ut (M

bps)

-92 -85 -75 -65 -55 -450

5

10

RSS Threshold (dBm)

5%-il

e Us

erTh

roug

phut

(Mbp

s)

5%-ile / Avg. user thrp.

(Mbps)

WLAN users (%)

Reference: WLAN if coverage Fixed RSS_Thr = -92 dBm 0.1 / 11 87 Reference: Best Fixed Threshold Fixed RSS_Thr = -65 dBm 7.2 / 31.6 42 SON: reference settings PSON/PL/PW=1s, RSS_Thr_Step=1dB, α=0.9 8.2 / 31.7 41 SON: smooth load filtering PSON/PL/PW=1s, RSS_Thr_Step=1dB, α=0.5 7.5 / 33.1 45 SON: slow RSS_Thr update PSON/PL/PW=2s, RSS_Thr_Step=0.5dB, α=0.9 3.7 / 28.2 54

SON: fast RSS_Thr update PSON/PL/PW=0.5s, RSS_Thr_Step=2dB, α=0.9 7.1 / 31.2 38

If It Works – SON Function to Improve User Throughput

8 SEMAFOUR

0 5 10 15 20 25 30 35 40 450

10

20

30

40

50

60

70

80

90

100

CDF

[%]

User Throughput (Mbps)

All (DL)Macro (DL)Micro (DL)WLAN (DL)All (UL)Macro (UL)Micro (UL)WLAN (UL)

§  14% gain in 5th percentile user throughput comparing to the best fixed threshold reference case

§  Sensitivity to SON parameter configuration –  Not very sensitive to the load filtering factor –  Need to be fast enough to capture traffic load variation

§  Balanced DL user throughput among users

§  UL user throughput is not optimized with the DL KPI driven algorithm

§  A more balanced load between the two system –  Load was kept in the configured range by the SON function

§  Convergence of the RSS threshold –  ‘Warming-up’ and stabilization

How Does It Work – Load Balancing

9 SEMAFOUR

1 2 3 4 5 1 2 3 4 5 6 7 8 9 1011121314151617180

20

40

60

80

100

Cell

load

-80-75-70-65-60-55-50-45-40

RSS

thre

shol

d (d

Bm)

1 2 3 4 5 1 2 3 4 5 6 7 8 9 1011121314151617180

20

40

60

80

100

Cell

load

-80-75-70-65-60-55-50-45-40

RSS

thre

shol

d (d

Bm)

Fixed Threshold = -65 dBm

SON function

20 40 60 80 100 1200

20

40

60

80

100

Load

(%)

-80-75-70-65-60-55-50-45-40

RSS

thre

shol

d (d

Bm)

Time (sec)

RSS thresholdWLAN AP loadLTE cell load

SON with the reference settings RSS_Thr = -65 dBm

[File size: DL/UL, User arrival rate]

5%-ile / Avg. user

thrp. (Mbps)

WLAN users (%)

5%-ile / Avg. user

thrp. (Mbps)

WLAN users (%)

Reference [5 MB/0.8 MB, 12] 8.2/31.7 41 7.2/31.6 42

[2.5 MB/0.4 MB, 24] 7.2/27.8 39 6.6/26.8 42 [1.25 MB/0.2 MB, 48] 5.8/23.1 34 3.9/21.8 42 [10 MB/1.6 MB, 6] 6.6/33.0 40 7.2/35.6 42 [5 MB/0.8 MB, 6] 10.7/38.1 48 8.3/40.1 42

If It Really Works – Various Traffic Parameters

10 SEMAFOUR

§  Smaller file size and higher user arrival rate: More simultaneously active users à Lower WLAN efficiency and higher WLAN load à SON moves more users to LTE

§  Larger file size and lower user arrival rate: Approaching to full buffer traffic à Resource utilization is not a good load measure

§  Lower offered traffic: SON outperforms the fixed RSS threshold in the 5th percentile user throughput

§  The RSS-based SON algorithm is primarily a distributed solution because it may be difficult for the central controller, e.g. ANDSF, to execute an algorithm which updates parameters every few seconds.

§  Implementation of the RSS-based access selection rule –  Executed in terminals:

–  RSS_Thr is send to terminals via broadcasted or dedicated signalling channels being standardized in 3GPP

–  Executed in network: –  Uplink RSS is monitored at the network node and the node controls the access

selection

§  Information exchange between LTE and WLAN –  Standardization of such an interface is being discussed in 3GPP –  Proprietary interfaces –  Terminals as relays

§  WLAN RSS measures –  RSSI: Received Signal Strength Indicator –  RCPI: Received Channel Power Indicator

How it Works in Practice

11 SEMAFOUR

§  Simulation results showed a RSS-based SON algorithm for access network selection between the LTE and WLAN systems successfully

–  Balanced the load between the two systems –  Optimized user throughput statistics with various traffic parameter settings.

§  Future work –  The optimization of the SON parameters with respect to the traffic parameters is

subject to further study. –  Better load measure –  Evaluate the SON function in more dynamic scenarios: mobility and and/or

mixed traffic types. –  More understanding of the coexistence of distributed and centralized SON

functions is needed to achieve a holistic solution and further optimize the performance.

How to Make It Better

12 SEMAFOUR