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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 f o 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
• 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)
PREA
MBL
E
Sub-
chan
nels
FCH
DL-M
AP
UL-M
AP
User 1 DL Burst
User 2 DL Burst
User 3 DL Burst
Symbol Duration
User 1 UL Burst
User 2 UL Burst
User 3 UL Burst
Transition Gap
MCS 1 : QPSK 3/4 MCS 2 : 16QAM 1/2 MCS 3 : 16QAM 3/4 MCS 4 : 64QAM 1/2 MCS 5 : 64QAM 2/3 MCS 6 : 64QAM 3/4 MCS 7 : 64QAM 5/6
DOWNLINK UPLINK
Tile
OFDMA vs OFDM
• WiMAX uses OFDMA technology at the PHY.
WiFi (OFDM)
Channel
WiMAX (OFDMA)
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.
• Licensed spectrum
• Orthogonal sub-channels to interfering femtocells.
• Under-utilization for clients who are not subject to interference.
• Multiple clients should coexist.
Equivalent Solution for Femtocells0
1
2
3
How do we define interference?
• Degradation of decoding at the clients (need isolation).
Su
b-c
han
ne
ls
Time
How do we define interference?
• Degradation of decoding at the clients (need isolation).
Su
b-c
han
ne
ls
Time
How do we define interference?
• Degradation of decoding at the clients (need isolation).
Su
b-c
han
ne
ls
Time
• GOAL: Intelligent resource management to improve network utilization (taking into account both clients.)
What should the solution look like?
REUSEZONE
ISOLATIONZONE
Su
b-c
han
ne
ls
Time (Symbols)
USED BY OTHER CELLS
What should the solution look like?
REUSEZONE
ISOLATIONZONE
Su
b-c
han
ne
ls
Time (Symbols)
USED BY OTHER CELLS
What should the solution look like?
REUSEZONE
ISOLATIONZONE
Su
b-c
han
ne
ls
Time (Symbols)
USED BY OTHER CELLS
What should the solution look like?
REUSEZONE
ISOLATIONZONE
Su
b-c
han
ne
ls
Time (Symbols)
USED BY OTHER CELLS
✓ 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)
REUSEISOLATION
REUSEISOLATION
Algorithms (Overview)
REUSEISOLATION
REUSEISOLATION
allocate &assign (coloring)
Algorithms (Overview)
REUSEISOLATION
REUSEISOLATION
determine commonreuse zone size
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
D
C
G
E
FA
B
10
10
10 10
10 20
20
30 sub-channelswith equal load
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!
D
C
G
E
FA
B
1010 10
1010
20
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!
D
C
G
E
FA
B
1010 10
1010
20
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!
D
C
G
E
FA
B
1010 10
1010
20
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!
D
C
G
E
FA
B
1010 10
1010
20
20
10
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`
D
C
G
E
FA
B
Chordal graph
Sub-channel Assignment
• Coloring with multiple colors (sub-channels).
• Construct a clique tree for chordal graph G`
D
C
G
E
FA
B
CBD
BED ACB CFD
GF
Chordal graph Clique tree
Sub-channel Assignment
• Color each level starting from the root.
Sub-channel Assignment
CBD
BED ACB CFD
GF
• Color each level starting from the root.
Sub-channel Assignment
• Color each level starting from the root.
CBD
E A F
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?
REUSEISOLATION
REUSEISOLATION
REUSEISOLATION
Zoning
• Common reuse zone size: min or max?
REUSEISOLATION
REUSEISOLATION
REUSEISOLATION
Zoning
• Common reuse zone size: min or max?
REUSEISOLATION
REUSEISOLATION
REUSEISOLATION
Zoning
• Common reuse zone size: min or max?
REUSEISOLATION
REUSEISOLATION
REUSEISOLATION
AVOIDCASCADES!
Zoning (avoiding cascades)
51510
BS 1 BS 3BS 2
Zoning (avoiding cascades)
51510
BS 1 BS 3BS 2
5
Zoning (avoiding cascades)
51510
5 15
BS 1 BS 3BS 2
5
Reuse clients(using isolated sub-channels)
Zoning (avoiding cascades)
51510
5 15
BS 1 BS 3BS 2
510
Reuse clients(using isolated sub-channels)
Cascade avoided since no interference to BS2’s clients
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
• Zoning provides around 50% throughput gain over pure sub-channel isolation.
0
10
20
30
40
1 2 3 4 5
Thr
ough
put
(Mbp
s)
Topology
BaselineFreq. IsolationFreq. Isolation + Zoning
Evaluation
• Avoiding cascades provides 30% gain over cascaded zoning.
100
125
150
175
200
225
250
0.25 0.33 0.5 0.66 0.75
Thr
ough
put
(Mbp
s)
Reuse Load in the Network
Baseline FERMICascadedWithout cascade
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?
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