LTE-Unlicensed. Sreekanth Dama, Dr. Kiran Kuchi, Dr. Abhinav Kumar IIT Hyderabad

Transcription

1 LTE-Unlicensed Sreekanth Dama, Dr. Kiran Kuchi, Dr. Abhinav Kumar IIT Hyderabad

2 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.

3 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

4 Offloading to Unlicensed Bands DATA LTE-U LTE/WiFi Interworking Licensed Primary Carrier Unlicensed Secondary Carrier LTE WiFi

5 LTE-U (LAA) Unlicensed (DATA) Licensed (DATA+CONTROL) 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

6 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

7 WiFi (IEEE ) 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.

8 DCF IDLE Need to TX Need to TX Yes ICCA 34us C=randi(0,CW-1) CW=CW*2 If(CW>CWmax) CW = CWmax CW=CWmin NO ICCA 34us Ack? TX CCA 9us C=C-1 IS C=0?

9 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.

10 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)

11 ETSI Option B Need to TX ICCA 34us IDLE Need to TX Yes NO TX C=randi(0,CW-1), ICCA 34us IS C=0? C=C-1 CCA 9us

12 ETSI Option A Need to TX ICCA 34us IDLE Need to TX CW=CWmin TX C=randi(0,CW-1), Q=CW Q=Q-1 ICCA 34us Yes CW=CW*2; if(cw>cwmax) CW=CWmin; Q<=0& C>0? IS C=0? CCA 9us NO C=C-1,Q=Q-1

13 Recent ACK Method IDLE Need to TX Need to TX Yes ICCA 34us C=randi(0,CW-1) CW=CW*2 If(CW>CWmax) CW = CWmax CW=CWmin NO ICCA 34us Recent Ack? TX CCA 9us C=C-1 IS C=0?

14 Simulations

15 Indoor Layout

16 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.

17 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

18 Low Load

19 Low Load

20 Medium Load

21 Medium Load

22 High Load

23 High Load

24 Served Load

25 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.

26 Questions Time