fair co-existence of licensed assisted access lte … · 2017-03-22 · fair co-existence of...

Fair Co-Existence of Licensed Assisted Access LTE (LAA-LTE) and Wi-Fi in Unlicensed Spectrum

R. Kwan, R. Pazhyannur, J. Seymour, V. Chandrasekhar – Cisco Inc.

S.R. Saunders, D. Bevan, H. Osman, J. Bradford, J. Robson, K. Konstantinou – Real Wireless Ltd.

IEEE CEEC Thursday 24th September 2015

25/09/2015 Company Confidential © Real Wireless Ltd. 2015

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25/09/2015 © Real Wireless Ltd. 2015

Some Clients

2

Our services

Background and aims of our paper

• Growing wireless data demand has driven demand for spectrum.

• Wi-Fi has successfully helped to offload this growing demand from licensed spectrum.

• Central to Wi-Fi’s success is the premise of fair co-existence in unlicensed spectrum.

• Now Licensed Assisted Access has been proposed for LTE to use unlicensed spectrum also (LAA-LTE).

• Our paper aims to find the best LBT design for LAA-LTE with the key aim of ensuring fairness is maintained with Wi-Fi.

• Results from this paper formed the basis of submissions to the 3GPP LAA study item which concluded in June 2015.

25/09/2015 3Company Confidential © Real Wireless Ltd. 2015

Nov 2014

Feb 2015

April 2015

May 2015

Aug 2015

RAN #79 RAN #80 RAN #80-bis

RAN #81

RAN #82

Timeline of 3GPP LAA study item

Sept 2014

RAN #79

LAA study item approved

LAA study item completion in June 2015

Licensed Spectrum

Primary carrier

(Potentially uplink)

Unlicensed spectrum

Secondary Carriers

Licensed spectrum

Secondary Carrier

Carrier Aggregation

Control + Data

Data

only

LAA-LTE is based on the carrier aggregation framework

Key findings

• Our investigations show that for LAA-LTE:

• Fair coexistence with Wi-Fi cannot be achieved without LBT.

• Choice of LBT scheme matters, with not all LBT schemes providing fair coexistence. • The LBT scheme currently proposed for Load Based Equipment (LBE) operating in unlicensed

spectrum in Europe, as defined in ETSI EN 301 893 v1.7.1, is not adequate for achieving fairness.

• The above scheme must evolve to a more 'Wi-Fi-like' LBT mechanism to ensure fair coexistence even in hidden node conditions.

• Our results show that the proposed 3GPP Category 4 LBT scheme does achieve fair co-existence with Wi-Fi if configured with Wi-Fi like settings for the scenarios that we examined.


Simulation approach

• We developed an event-based simulator of Wi-Fi and LAA-LTE co-existence known as WaLT.

• Generally compliant with requirements of 3GPP study item on LAA-LTE.

• Focused on the indoor scenario of 4 Wi-Fi APs from one operator (victim network) co-existing with either 4 APs from a contending Wi-Fi or LAA-LTE network (aggressor).

• Assessed system throughputs and median, cell edge and peak user perceived throughputs and latencies achieved per operator at different traffic loads.

• Overall assessed if LAA-LTE would be a worse neighbour to Wi-Fi than Wi-Fi itself.


Indoor office environment modelled

WaLT object-based system simulator structure for LAA-LTE and Wi-Fi performance evaluation.

Impact of LAA-LTE on Wi-Fi performance without LBT

• Graph shows how the victim operator’s Wi-Fi throughput changes when the aggressor, Operator 2, deploys either Wi-Fi or LAA-LTE.

• When Operator 2 deploys Wi-Fi, Operator 1’s aggregated throughput (DL + UL) tracks the offered load up to nearly 20 clients/access point (AP) when congestion starts.

• When Operator 2 deploys LAA-LTE without LBT we see congestion occurring at much lighter loads.

• Without LBT LAA-LTE is a worse neighbour to Wi-Fi than Wi-Fi itself.


Parameter

Value

ValueNote

Office dimensions 55.73m x 21.55m

i.e. 3,000 sq ft per

AP, consistent with

real enterprise

deployments

APs per operator 4Evenly spaced along

corridor

System bandwidth 20 MHzBoth licensed and

unlicensed cell

Carrier frequency 5.3 GHz

BS TX power 23 dBm

For both LAA

eNodeBs and Wi-Fi

APs

UE TX Power 23 dBm For Wi-Fi clients

FTP model

parameters

0.5 MB file, l = 0.2

Hz

i.e. 5s mean

interarrival time

Wi-Fi parameters

802.11ac with slot period 9 ms, DIFS=34ms,

CWmin=15, CWmax=1023. Minimum LBT

interval = 34+9=43ms.

Spatial streams: 2 SS on DL, 1 SS on UL

Victim’s (Op1) aggregated network throughput when aggressor (Op2) deploys either LAA (red line) or Wi-Fi (green line). Dashed line shows the offered load.

Parameters for evaluation of LAA impact on Wi-Fi without LBT

What LBT scheme to use?


• Logical start point is LBT mechanism for Load Based Equipment (LBE) from European regulations (ETSI EN 301 893).

• Similar to Wi-Fi, a clear channel assessment (CCA) is continuously performed.

• Key differences include the addition in Wi-Fi of:

• Enforced ECCA

• Contention window growth

• This means that if LAA-LTE used ETSI EN 301.893 V1.7.1 for LBT Wi-Fi would be at a disadvantage with Wi-Fi the politer of the two.

Illustration of politeness controls in Wi-Fi (after 1st transmission)

Illustration of politeness controls in ETSI EN 301 893 V1.7.1

ECCA back-off: Random number of empty slots n = U(1…N)N doubles when there is a collision on the mediumNote ECCA term not used in Wi-Fi but used here to aid comparison with ETSI approach above.

ECCA back-off: Random number of empty slots n = U(1…N)N stays fixed (related to max transmission size so that devices with long transmissions back-off for longer)

Set up for LBT scheme experiments


ParameterValue

ValueNote

Office dimensions 120m x 50mFor consistency with

[6]

APs per operator 4Evenly spaced along

corridor

System bandwidth 20 MHzBoth licensed and

unlicensed cell

Carrier frequency 5.3 GHz

BS TX power 18 dBm

UE TX Power 18 dBm

FTP modelparameters

0.5 MB file,l = {0.25, 0.5,1,1.5,

2.5,3.5}Hz (both victim andaggressor)

Medium reservationCTS-to-self sent byLAA

Detection threshold

-82dBm for virtualcarrier sensing-62dBm for physicalcarrier sensing

• In the next sequence of results we examine:• “ETSI”: LBT according to ETSI ETSI EN

301 893 V1.7.1.• “+ECCA”: As “ETSI” but the ECCA

process is always enforced after every successful transmission.

• “+Exponential”: As “+ECCA” plus a contention window that grows if medium access is unsuccessful (as per ETSI EN 301 893 V1.7.2 option A) rather than having a fixed back-off mechanism.

• Simulation assumptions updated slightly to the table on the right in line with 3GPP at the time. Parameters for evaluation of LAA impact on Wi-Fi with LBT

Impact of enforced ECCA and allowing contention window growth


These results confirm that:• LAA with basic “ETSI” LBT is not polite, and results in significant degradation of Wi-Fi once load grows.• With “+ECCA”, LAA is more polite improving Victim throughput. But victim throughout is still lower

than the aggressor LAA network’s throughput.• With an exponential back-off “+Exponential” further improves LAA politeness, giving better victim

throughput at a small cost to the aggressor LAA network’s throughput.

Victim Wi-Fi network throughput variation with traffic loading for three LBT schemes

Aggressor LAA network throughput variation with traffic loading for three LBT schemes

Impact of changing ICCA and ECCA durations

• “ETSI+ ECCA+ Exponential” scheme (or “Scheme B” for short) on the previous slide arguably makes LAA-LTE too polite.• In above graphs we examine refining the ICCA and ECCA duration settings within this scheme to achieve better fairness.• The best balance between victim and aggressor system throughputs is achieved when a Wi-Fi like setting of 34µs is used for the

ICCA period and 9 µs is used for the ECCA slot duration.


Victim network throughput variation with traffic loading for Wi-Fi co-existing with LAA-LTE using “Scheme B” LBT with different ICCA and ECCA settings

Aggressor network throughput variation with traffic loading for Wi-Fi co-existing with LAA-LTE using “Scheme B” LBT with different ICCA and ECCA settings

Impact of hidden nodes and changing the trigger for contention window growth

• We next introduce hidden nodes to the previous scenario modelled.• If two nodes are hidden and transmitting on top of each other we want them to detect this and back-off from the medium.• Therefore the trigger mechanism for growing the contention window for back-off is key in hidden node situations.• Above results show that Scheme B, which grows its contention window based on unsuccessful accesses to the medium performs, poorly in

hidden node conditions as it does not detect these “deadlock” situations.• Proposed 3GPP category 4 LBT grows contention window based on collisions (determined via ACK/NACK responses) similar to Wi-Fi and

achieves better fairness.• Note Wi-Fi uses RTS/CTS to avoid hidden node situations and in our simulations we extend this concept to LAA by applying a CTS-to-self.


Victim Wi-Fi network throughput variation with traffic loading for Wi-Fi co-existing with Wi-Fi, Wi-Fi with LAA-LTE using Scheme B and Wi-Fi with LAA-LTE using 3GPP's 'Cat 4 LBT' scheme (with hidden nodes included)

Aggressor network throughput variation with traffic loading for Wi-Fi co-existing with Wi-Fi, Wi-Fi with LAA-LTE using Scheme B and Wi-Fi with LAA-LTE using 3GPPs Cat 4 LBT scheme (with hidden nodes included)

Key findings

• Our investigations show that for LAA-LTE:

• Fair coexistence with Wi-Fi cannot be achieved without LBT.

• Choice of LBT scheme matters, with not all LBT schemes providing fair coexistence. • The LBT scheme currently proposed for Load Based Equipment (LBE) operating in unlicensed

spectrum in Europe, as defined in ETSI EN 301 893 v1.7.1, is not adequate for achieving fairness.

• The above scheme must evolve to a more 'Wi-Fi-like' LBT mechanism to ensure fair coexistence even in hidden node conditions.

• Our results show that the proposed 3GPP Category 4 LBT scheme does achieve fair co-existence with Wi-Fi if configured with Wi-Fi like settings for the scenarios we examined.


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