1. Mitigating Macro-cell Outage in LTE-Advanced Deployments
Rajarajan Sivaraj, PhD Candidate, UC Davis, CA (Presenter)
Ioannis Broustis, AT&T Labs Research, NJ
N. K. Shankaranarayanan, AT&T Labs Research, NJ
Vaneet Aggarwal, Purdue University, IN
Prasant Mohapatra, UC Davis, CA
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Website : 1

2. AGENDA OF THE PRESENTATION
CONCEPTS, DEFINITIONS AND SYSTEM MODEL
OUTAGE AND ISSUES
PROBLEM FORMULATION AND HARDNESS
APPROXIMATIONS, RELAXATIONS AND SOLUTION
PERFORMANCE EVALUATION
CONCLUSION 2

3. DIRECTIONAL eNBs WITH CA AND CELL-SECTORSf1 f2
eNB with directional antennae – Covers a CC-specific cell sector
Typically, 3 sectors per cell – Angular orientations of 00 , 1200 , 2400
Carrier Aggregation – An eNB operates on more than one CC, non-adjacent frequency bands (f1 and f2), serving more than one cell

4. 2D-BEAMFORMING
Signal processing technique for directional signal transmission or reception
Combining elements in a phased array for constructive interference of signals. Achieves spatial selectivity by steering beams at transmit and receive ends.
Parameters : Beamwidth, Horizontal gain, Angular orientation
Causes inter-cell interference along the cell-edges

5. MULTI-USER MIMO-OFDMA BEAMFORMING
EQUAL POWER SPECTRAL DENSITY 5

6. RECEIVED SIGNAL Received signal on UE k at PRB b Transmit Beamformer
Transmitted Data symbol
from eNB m
Receive combiner
Channel gain matrix
Gaussian noise

7. AGENDA OF THE PRESENTATION
CONCEPTS, DEFINITIONS AND SYSTEM MODEL
OUTAGE AND ISSUES
PROBLEM FORMULATION AND HARDNESS
APPROXIMATIONS, RELAXATIONS AND SOLUTION
PERFORMANCE EVALUATION
CONCLUSION 7

8. DEFINITION OF OUTAGE
Application
Server
EPC
MME
S/P-GW
eNB
outage
eNB
eNB
eNB UE UE UE UE UE UE UE UE UE UE
Interruption in the coverage and service of an LTE eNB to associated UEs (based on highest received signal strength) by:
Unplanned failure - bad weather event, power loss, software/hardware failure.
Planned / Scheduled maintenance - software/hardware changes
eNB Transmitter/antenna damage, backbone link cut-off etc.

9. USER RE-ASSOCIATION
Application
Server
EPC
MME
S/P-GW
eNB
eNB
outage
eNB
eNB UE UE UE UE UE UE UE UE UE UE UE UE UE UE
User re-association
Re-association to neighboring eNB cell sectors depending on network planning and resource availability

10. ISSUES IN USER RE-ASSOCIATION
Poor received signal strength for re-associated UEs from neighboring eNBs
Residual resource exhaustion at neighboring eNBs (as eNBs are operating at full resource capacity on current traffic load)
eNB
outage
RESOURCE
EXHAUSTION

11. ADDRESSING RE-ASSOCIATION ISSUES
Configurable parameters to address user re-association issues:
eNB Transmission power – To increase received signal strength
Precoding/Beamforming weights – Steering the direction of signal transmission
eNB
outage
Transmission Power
RESOURCE
EXHAUSTION
H. Dahrouj and Y. Wu, “Coordinated beamforming for the multi-cell multi-antenna Wireless systems”, IEEE Trans. Wireless Communications, vol. 9, no. 4, May 2010

12. CHALLENGES Link stability for re-associated UEs
Meeting QoS requirements of re-associated UEs
Providing service for re-associated UEs without affecting the rest
eNB
outage
Transmission Power
RESOURCE
EXHAUSTION 12

13. CONTRIBUTIONS Online Outage Mitigation Framework
Closed-loop Service Quality Management
Imperfect channel knowledge of UEs, Statistical distribution of CSI, at the eNBs
DESIGN : LTE-A outage mitigation framework with CA and 2D/3D Beamforming
SERVICE MODEL : Channel- and traffic-aware
OPTIMIZATION : Relaxation via Convex Optimization. Maximizes effective network coverage with higher link stability and QoS guarantees
EVALUATION : Simulations of our reconfiguration procedure using NS3 13

14. OUTAGE AND MITIGATION - ILLUSTRATION14

15. SYSTEM MODEL
LTE-A Single Frequency network supporting MIMO-OFDM, Carrier Aggregation and 2D/3D multi-cell beamforming 15

16. AGENDA OF THE PRESENTATION
CONCEPTS, DEFINITIONS AND SYSTEM MODEL
OUTAGE AND ISSUES
PROBLEM FORMULATION AND HARDNESS
APPROXIMATIONS, RELAXATIONS AND SOLUTION
PERFORMANCE EVALUATION
CONCLUSION 16

17. OPTIMAL MITIGATION STRATEGY
OPTIMIZATION PARAMETERS AT THE BASE STATIONS
Per CC Transmission Power
Maximize
Per PRB Unit-norm Beamforming Vector
Minimize
OBJECTIVE : Minmax product of per PRB unit-norm beamforming vector and per CC transmission power at each serving eNB 17

18. OPTIMAL MITIGATION STRATEGY
Upper-bound on per CC transmission power
Unit norm of the beamforming vector
Per-RB SINR of any associated/re-associated UE is greater than threshold
Sum of PRBs from any CC allocated to each associated/re-associated UE to the eNB for satisfying QoS is upper-bounded by the bandwidth of the CC
Integrality in the allocation of PRBs to UEs
CONSTRAINTS 18

19. NP HARDNESS FOR CCASSIGNMENT
GENERALIZED ASSIGNMENT PROBLEM : NP-Hard problem for assigning inter-band aggregated CCs to UEs B B B B
WEIGHT … …
CC1
CC2
CCi
CCn … …
BINS
Vi,j
Fi,j U1 U2 … Uj … Um
ITEMS 19

20. NP HARDNESS FOR SCHEDULING
SUBSET SUM PROBLEM : NP-Complete problem for scheduling PRBs of assigned CCs to UEs B
PRB1
PRB2
PRBi
PRBn … …
BINS
Fi,j U1 U2 … Uj … Um
ITEMS R1 R2 … Rj … Rm
QoS 20

21. AGENDA OF THE PRESENTATION
CONCEPTS, DEFINITIONS AND SYSTEM MODEL
OUTAGE AND ISSUES
PROBLEM FORMULATION AND HARDNESS
APPROXIMATIONS, RELAXATIONS AND SOLUTION
PERFORMANCE EVALUATION
CONCLUSION 21

22. CARRIER AGGREGATION HEURISTIC
Lowest frequency
CC1
UE1
Cell-center
UE2
CC2
CC3
UE3
CC4
UE4
Highest frequency
CC5
UE5
Cell-edge
R. Sivaraj, A. Pande, K. Zeng, K. Govindan, P. Mohapatra, “Edge-prioritized channel- and traffic-aware uplink Carrier Aggregation in LTE-Advanced systems”, IEEE WoWMoM, 2012 22

23. CONVEX RELAXATION – PHASE I
PHASE I : PER CC TRANSMISSION POWER (With Constant unit-norm Beamforming vector configured during outage)
Approximation from frequency-selective fading to flat fading
A variation of the Proportional-fair heuristic to SCHEDULE PRBs to UEs (associated/re- associated UE)
Allocation of PRBs to satisfy QoS is based on log(1+SINR). This is approximated as log(1+x) ≈ log x (Since, SINR > SINR Th and for practical reasons, SINR Th >> 1 )
A. Wiesel, Y. C. Eldar, S. Shamai, “Linear Precoding via Conic Optimization for fixed MIMO Receivers”, IEEE Trans. Signal Processing”, vol. 54, no. 1, Jan 2006 23

24. CONVEX RELAXATION – PHASE II
PHASE II : PER RB BEAMFORMING WEIGHTS ( With per CC Transmission power obtained in Phase I )
Determination of approximate SINR threshold per PRB, given the expected number of PRBs allocated to the UE to satisfy QoS
Retains the frequency-selective nature of the CC (since, UE’s per-PRB SINR must be greater than per-RB SINR threshold) 24

25. OPTIMIZATION Iteration for dual variables
Primal variables : Per CC transmit power and per RB beamformer
Both relaxed sub-problems are convex and so, strong duality
Derive Lagrangian dual variables
Derive complementary slackness and KKT conditions
Derive iterative convex descent procedure and solve for KKT saddle points
Iteration for dual variables 25

26. AGENDA OF THE PRESENTATION
CONCEPTS, DEFINITIONS AND SYSTEM MODEL
OUTAGE AND ISSUES
PROBLEM FORMULATION AND HARDNESS
APPROXIMATIONS, RELAXATIONS AND SOLUTION
PERFORMANCE EVALUATION
CONCLUSION 26

27. NS3 SIMULATIONS
Network simulations on discrete-event NS3 LENA (LTE/EPC network simulator)
Complete implementation of uplink/downlink PHY and MAC layers, AMC, path loss models, channel state information
Our custom additions:
RSRP-based association of UEs to eNBs
Log-normal shadowing between each UE-eNB pair
Frequency-dependent path loss exponent for computation in inter-band CA
21-sector hexagonal grid with 3 collacted sector eNB sites with directional antennae on MIMO channels
Scenarios:
A – outage of a single cell-sector at the center of the hexagon
B – outage of a collocated set of eNBs at the center
C – outage of 3 cell sectors randomly distributed over the network
Evaluation parameters : Percentage of successfully-reassociated outage UEs and the net achievable throughput, against the number of UEs 27

28. NS3 SIMULATIONS 28

29. NS3 SIMULATIONS
Comparison : With SINR only (re-associating UEs to eNBs purely based on highest SINR) and residual PRBs only (re-associating UEs to eNBs purely based on highest number of residual PRBs )
Scenario A
Our method : 45%
SINR only : 26%
Residual PRB only : 23%
Scenario B
Worst-case re-association
Our method : 10%
SINR only : 4%
Residual PRB only : 1%
Scenario C
Worst-case re-association
Our method : 15%
SINR only : 4%
Residual PRB only : 2% 29

30. NS3 SIMULATIONS
Comparison : With SINR only (re-associating UEs to eNBs purely based on highest SINR) and residual PRBs only (re-associating UEs to eNBs purely based on highest number of residual PRBs )
Scenario A
Scenario B
Scenario C
Improvements : 20-25% half-way through, and upto 75% with further decrease in UE density in successful re-association
On average, 20% throughput improvement on all 3 scenarios 30

31. CONCLUSIONS
Outage mitigation framework for dynamic outages in LTE-Advanced deployments
Discussed Carrier Aggregation, 2D/3D beamforming, OFDMA-MIMO
Impacts of outage – Channel quality assessment of outage UEs and other UEs
Issues in re-association without re-configuring parameters
Optimal mitigation strategy to serve for QoS and NP-hardness
Essential parameters to be reconfigured :
Per CC transmission power
Per RB transmit beamformer (And hence, receive beamformer based on Linear MMSE)
Convex relaxation and approximation guarantees
Convex Optimization. Iterative procedure and descent algorithm to re-configure per CC transmission power and per RB transmit beamformer until KKT optimality
NS3 simulations and performance evaluation of proposed technique against SINR only and residual RB only 31

32. THANK YOU AND QUESTIONS !!!!32

33. 2D-BEAMFORMING
Signal processing technique for directional signal transmission or reception
Combining elements in a phased array for constructive interference of signals. Achieves spatial selectivity by steering beams at transmit and receive ends.
Parameters : Beamwidth, Horizontal gain, Angular orientation
Causes inter-cell interference along the cell-edges

34. 3D-BEAMFORMING
Introduces a vertical dimension at the transmitter by accounting for the heights of the eNB and UE, and vertical differences of 3D antennae.
Beam steering is scattered in 3-D space, in contrast to 2-D plane.
Parameters : Vertical gain, downtilt
Reduces the inter-cell interference as the transmission is divided into both horizontal and vertical directions.