What should the solution look like? Sub-channels ISOLATION ZONE - - PowerPoint PPT Presentation

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

• 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)

PREAMBLE Sub-channels FCH DL-MAP UL-MAP 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

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 Femtocells

1 2 3

How do we define interference?

• Degradation of decoding at the clients (need

isolation).

Sub-channels Time

How do we define interference?

• Degradation of decoding at the clients (need

isolation).

Sub-channels Time

How do we define interference?

• Degradation of decoding at the clients (need

isolation).

Sub-channels Time

• GOAL: Intelligent resource management to

improve network utilization (taking into account both clients.)

What should the solution look like?

REUSE ZONE ISOLATION ZONE Sub-channels Time (Symbols) USED BY OTHER CELLS

What should the solution look like?

REUSE ZONE ISOLATION ZONE Sub-channels Time (Symbols) USED BY OTHER CELLS

What should the solution look like?

REUSE ZONE ISOLATION ZONE Sub-channels Time (Symbols) USED BY OTHER CELLS

What should the solution look like?

REUSE ZONE ISOLATION ZONE Sub-channels 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)

REUSE ISOLATION REUSE ISOLATION

Algorithms (Overview)

REUSE ISOLATION REUSE ISOLATION

allocate & assign (coloring)

Algorithms (Overview)

REUSE ISOLATION REUSE ISOLATION

determine common reuse 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 F A B

10 10 10 10 10 20 20

30 sub-channels with 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 F A B

10 10 10 10 10 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 F A B

10 10 10 10 10 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 F A B

10 10 10 10 10 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 F A B

10 10 10 10 10 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 F A B

Chordal graph

Sub-channel Assignment

• Coloring with multiple colors (sub-channels).
• Construct a clique tree for chordal graph G`

D C G E F A 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?

REUSE ISOLATION REUSE ISOLATION REUSE ISOLATION

Zoning

• Common reuse zone size: min or max?

REUSE ISOLATION REUSE ISOLATION REUSE ISOLATION

Zoning

• Common reuse zone size: min or max?

REUSE ISOLATION REUSE ISOLATION REUSE ISOLATION

Zoning

• Common reuse zone size: min or max?

REUSE ISOLATION REUSE ISOLATION REUSE ISOLATION

AVOID CASCADES!

Zoning (avoiding cascades)

5 15 10 BS 1 BS 3 BS 2

Zoning (avoiding cascades)

5 15 10 BS 1 BS 3 BS 2 5

Zoning (avoiding cascades)

5 15 10 5 15 BS 1 BS 3 BS 2 5

Reuse clients (using isolated sub-channels)

Zoning (avoiding cascades)

5 15 10 5 15 BS 1 BS 3 BS 2 5 10

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.

10 20 30 40 1 2 3 4 5

Throughput (Mbps) Topology

Baseline

• Freq. Isolation
• Freq. 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

Throughput (Mbps) Reuse Load in the Network

Baseline FERMI Cascaded Without 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

• ptimal 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?