FERMI : A Femtocell Resource Management System for Interference Mitigation in OFDMA Femtocell Networks Mustafa Y. Arslan Jongwon Yoon Karthikeyan Sundaresan UC Riverside U Wisconsin Madison NEC Laboratories America Inc. Srikanth V. Krishnamurthy Suman Banerjee UC Riverside U Wisconsin Madison ACM Mobicom 2011
Femtocells • Small cellular base stations deployed indoors. ✓ Use existing cable backhaul and cellular access technology ✓ Short range, high throughput ✓ Clients save power on the uplink • Interference is inevitable among collocated femtocells. ✓ different problem than interference in WiFi • What can we do?
Contributions • FERMI - mitigates interference among OFDMA femtocells deployed in an enterprise. ✓ Centralized algorithms to assign orthogonal frequencies to interfering femtocells. ✓ F l e x i b l e f r a m e fo r m a t t h a t s u p p o r t s heterogeneous client requirements for better spatial reuse. • First solution implemented on an actual OFDMA femtocell testbed with off-the-shelf clients!
Roadmap • WiMAX preliminaries • Interference among femtocells ✓ Can we leverage existing WiFi solutions? ✓ If not, how should the solution look like? • Algorithms for interference management • Evaluation
WiMAX Preliminaries User 1 User 1 FCH MCS 1 : QPSK 3/4 DL Burst Sub-channels UL Burst PREAMBLE MCS 2 : 16QAM 1/2 User 2 UL-MAP MCS 3 : 16QAM 3/4 User 3 DL-MAP UL Burst MCS 4 : 64QAM 1/2 User 2 DL Burst MCS 5 : 64QAM 2/3 DL Burst User 3 MCS 6 : 64QAM 3/4 UL Burst Tile MCS 7 : 64QAM 5/6 DOWNLINK UPLINK Symbol Duration Transition Gap • Multiple users scheduled in the same frame. • BS schedules tiles for both downlink and uplink. • Synchronous MAC (no carrier sensing). ✓ frames sent every 5 ms (1 ms for LTE)
OFDMA vs OFDM • WiMAX uses OFDMA technology at the PHY. WiFi (OFDM) WiMAX (OFDMA) Channel Sub-channels 0 1 2 3
Roadmap • WiMAX preliminaries • Interference among femtocells ✓ Can we leverage existing solutions? ✓ If not, how should the solution look like? • Algorithms for resource management • Evaluation
Existing Solutions for WiFi • Tune interfering WiFi APs to orthogonal channels. 0 Equivalent Solution for Femtocells • Licensed spectrum 1 • Orthogonal sub-channels to interfering femtocells. 2 • Under-utilization for clients who are not subject to interference. 3 • Multiple clients should coexist.
How do we define interference? Sub-channels Time • Degradation of decoding at the clients ( need isolation ).
How do we define interference? Sub-channels Time • Degradation of decoding at the clients ( need isolation ).
How do we define interference? Sub-channels Time • Degradation of decoding at the clients ( need isolation ). • GOAL: Intelligent resource management to improve network utilization (taking into account both clients.)
What should the solution look like? Sub-channels ISOLATION ZONE REUSE ZONE USED BY OTHER CELLS Time (Symbols)
What should the solution look like? Sub-channels ISOLATION ZONE REUSE ZONE USED BY OTHER CELLS Time (Symbols)
What should the solution look like? Sub-channels ISOLATION ZONE REUSE ZONE USED BY OTHER CELLS Time (Symbols)
What should the solution look like? Sub-channels ISOLATION ZONE REUSE ZONE USED BY OTHER CELLS Time (Symbols) ✓ Load-based adjustment of zones.
Roadmap • WiMAX preliminaries • Interference among femtocells ✓ Can we leverage existing solutions? ✓ If not, how should the solution look like? • Algorithms for resource management • Evaluation
Algorithms (Overview) ISOLATION REUSE REUSE ISOLATION
Algorithms (Overview) ISOLATION REUSE allocate & assign (coloring) REUSE ISOLATION
Algorithms (Overview) ISOLATION REUSE determine common reuse zone size REUSE ISOLATION
Sub-channel Allocation • Weighted max-min fair allocation • Need to list all maximal cliques: NP-hard
Sub-channel Allocation • Weighted max-min fair allocation • Need to list all maximal cliques: NP-hard 10 10 30 sub-channels F 10 with equal load A C 10 20 D G B 10 E 20
Sub-channel Allocation • Chordal graphs: no cycles of more than 3. • Triangulation: transform general graph G to a chordal graph G` • All maximal cliques can be listed in polynomial time!
Sub-channel Allocation • Chordal graphs: no cycles of more than 3. • Triangulation: transform general graph G to a chordal graph G` • All maximal cliques can be listed in polynomial time! 10 10 F 10 A C 20 D G B 10 10 E 20
Sub-channel Allocation • Chordal graphs: no cycles of more than 3. • Triangulation: transform general graph G to a chordal graph G` • All maximal cliques can be listed in polynomial time! 10 10 F 10 A C 20 D G B 10 10 E 20
Sub-channel Allocation • Chordal graphs: no cycles of more than 3. • Triangulation: transform general graph G to a chordal graph G` • All maximal cliques can be listed in polynomial time! 10 10 F 10 A C 20 D G B 10 10 E 20
Sub-channel Allocation • Chordal graphs: no cycles of more than 3. • Triangulation: transform general graph G to a chordal graph G` • All maximal cliques can be listed in polynomial time! 10 10 F 10 A C 20 D G B 10 10 E 10 20
Sub-channel Assignment • Coloring with multiple colors (sub-channels). • Construct a clique tree for chordal graph G`
Sub-channel Assignment • Coloring with multiple colors (sub-channels). • Construct a clique tree for chordal graph G` Chordal graph F A C D G B E
Sub-channel Assignment • Coloring with multiple colors (sub-channels). • Construct a clique tree for chordal graph G` Chordal graph Clique tree CBD F A C D G B BED ACB CFD E GF
Sub-channel Assignment • Color each level starting from the root.
Sub-channel Assignment • Color each level starting from the root. CBD BED ACB CFD GF
Sub-channel Assignment • Color each level starting from the root. CBD E F A GF
Sub-channel Assignment • Color each level starting from the root. CBD BED ACB CFD G
Sub-channel Assignment • Color each level starting from the root. CBD BED ACB CFD G • FERMI guarantees a feasible coloring!
Zoning • Common reuse zone size: min or max? ISOLATION REUSE REUSE ISOLATION REUSE ISOLATION
Zoning • Common reuse zone size: min or max? ISOLATION REUSE REUSE ISOLATION REUSE ISOLATION
Zoning • Common reuse zone size: min or max? ISOLATION REUSE REUSE ISOLATION REUSE ISOLATION
Zoning • Common reuse zone size: min or max? ISOLATION REUSE AVOID REUSE CASCADES! ISOLATION REUSE ISOLATION
Zoning (avoiding cascades) BS 1 BS 2 BS 3 10 15 5
Zoning (avoiding cascades) BS 1 BS 2 BS 3 10 15 5 5
Zoning (avoiding cascades) BS 1 BS 2 BS 3 10 15 5 5 5 15 Reuse clients (using isolated sub-channels)
Zoning (avoiding cascades) BS 1 BS 2 BS 3 10 15 5 5 10 5 15 Cascade avoided since no Reuse clients interference to BS2’s clients (using isolated sub-channels)
Roadmap • WiMAX preliminaries • Interference among femtocells ✓ Can we leverage existing solutions? ✓ If not, how should the solution look like? • Algorithms for resource management • Evaluation
Evaluation 40 Baseline Throughput (Mbps) Freq. Isolation Freq. Isolation + Zoning 30 20 10 0 1 2 3 4 5 Topology • Zoning provides around 50% throughput gain over pure sub-channel isolation.
Evaluation 250 Baseline FERMI Throughput (Mbps) Cascaded 225 Without cascade 200 175 150 125 100 0.25 0.33 0.5 0.66 0.75 Reuse Load in the Network • Avoiding cascades provides 30% gain over cascaded zoning.
Conclusion • FERMI mitigates interference among femtocells in an enterprise. The distinguishing aspects are: ✓ Identify tolerance of clients to interference. ✓ Flexible Frame structure to support the graceful coexistence of clients (reuse and isolation). ✓ Novel use of chordal graphs to achieve near optimal allocation and feasible assignment. ✓ Intelligent zoning to mitigate interference and leverage reuse at the same time. ✓ Implemented, evaluated on a WiMAX testbed (concepts applicable to LTE as well.).
Thank you! • Questions?
Recommend
More recommend
