LTE-UnlicensedSreekanth Dama, Dr. Kiran Kuchi, Dr. Abhinav Kumar

IIT Hyderabad

Unlicensed Bands

• Shared spectrum

• Huge available spectrum

• Regulations• Dynamic frequency selection

• Restrictions over maximum transmit power

• Strict out of band emission limits

• Limited maximum transmit opportunity

• WiFi is the main technology in the unlicensed spectrum.

5GHz U-NII Bands

• Approx. 500 MHz Bandwidth is available

• Regulations revised to allow outdoor operation with 1W maximum fixed power.

• Less occupied when compared to 2.4GHz

Offloading to Unlicensed Bands

DATA

LTE-U

Licensed Primary Carrier

Unlicensed Secondary

Carrier

LTE/WiFiInterworking

LTE WiFi

LTE-U (LAA)

• Unlicensed Secondary Carrier is aggregated with Licensed Primary carrier.

• Licensed carrier provides seamless mobility and control while unlicensed secondary carrier is used to boost data rates.

• A tighter integration solution

• Leverages the existing features.

PIPE

UE

Licensed (DATA+CONTROL)

Unlicensed (DATA)

Cont..

• LTE is not designed for a shared spectrum.

• So modifications are needed for LTE to operate in the unlicensed spectrum.

• 3GPP has completed a feasibility study on LAA. And LAA is expected to be part of Rel. 13 of LTE.

• Following functionalities are required for LAA to use unlicensed spectrum• Listen-before-talk (Clear channel assessment)

• Discontinuous transmission on a carrier with limited maximum transmission duration

• Dynamic Frequency Selection for radar avoidance in certain bands/regions

• Carrier selection

• Transmit Power Control

WiFi (IEEE 802.11)

• WiFi is the most popular wireless technology in the unlicensed bands.

• It uses DCF as the MAC protocol.

• DCF uses CSMA/CA with an exponential back off to access the channel.

• DCF performs worse when the network is highly populated with nodes. So WiFi is limited by DCF.

IDLE

Need to TX

C=randi(0,CW-1)

CCA 9us

IS C=0?C=C-1

Yes

NO

CW=CWmin

ICCA 34us

TX

CW=CW*2If(CW>CWmax)CW = CWmax

Ack?

ICCA 34us

Need to TX

DCF

How to maintain fairness with WiFi?

• LAA should not impact Wi-Fi services (data, video and voice services) more than an additional Wi-Fi network on the same carrier; these metrics could include throughput, latency, jitter etc.

• A plain co-existence will result in LAA monopolizing as WiFi uses DCF.

• LAA must use LBT with a back off mechanism to maintain fairness.

Possible Candidates

• LAA is going to be centralized as BS contends for the UEs’ and schedules them both in DL or UL.

• In WiFi the stations contend for themselves resulting in collisions so they use DCF to minimize the collisions.

• Spectrum regulations allow devices to do short control signaling without doing a CCA. DCF uses this to update its contention window.

• LAA can’t use DCF as the ACKs are not immediately transmitted in LTE.

• 3GPP is considering three schemes.1. ETSI Option B (fixed Window)2. ETSI Option A (Open loop Variable window)3. Recent Ack Method (Closed loop Variable Window)

IDLE

Need to TX

ICCA 34us

TX

C=randi(0,CW-1),

CCA 9us

IS C=0?

C=C-1

Yes

NO

ICCA 34us

Need to TX

ETSI Option B

IDLE

Need to TX

ICCA 34us

TX

C=randi(0,CW-1), Q=CW

CCA 9us

IS C=0?

C=C-1,Q=Q-1

Q<=0&C>0?

CW=CW*2;if(CW>CWmax)

CW=CWmin;

Yes

NO

CW=CWmin

Q=Q-1

ICCA 34us

Need to TX

ETSI Option A

IDLE

Need to TX

C=randi(0,CW-1)

CCA 9us

IS C=0?C=C-1

Yes

NO

CW=CWmin

ICCA 34us

TX

CW=CW*2If(CW>CWmax)CW = CWmax

Recent Ack?

ICCA 34us

Need to TX

Recent ACK Method

Simulations

Indoor Layout

Test Scenarios

• Case 1: Both Operators deploy WiFi

• Case 2: Operator 1 replaces WiFi with LAA having ETSI Option A as the channel access mechanism and with energy detect thresholds of -82dBm and -62dBm.

• Case 3: Operator 1 replaces WiFi with LAA having Recent Ack Method as the channel access mechanism and with energy detect thresholds of -82dBm and -62dBm.

Some System parameters

• Carrier frequency 5GHz

• System Bandwidth 20MHz

• BS and UE transmit power 18dBm

• 20 users per operator

• Traffic is modelled as FTP Model 3

• File size 0.5MB.

• WiFi Sensing Threshold -62 dBm

• WiFi uses preamble detection along with energy detection

Low Load

Low Load

Medium Load

Medium Load

High Load

High Load

Served Load

Conclusions

• Sensing threshold and Channel access scheme plays an important role in maintaining fairness in the network.

• We can observe that closed loop channel access schemes can perform well even with a poor choice of sensing thresholds than open loop schemes.

• Open loop schemes can maintain fairness only if the parameters are chosen appropriately.

• We can see a tradeoff between WiFi performance and network performance when the sensing thresholds are varied.

Questions Time