Infrastructure Mesh Broadband Wireless System: Example of Cooperative WirelessByoung-Jo Kim, N. K. Shankaranarayanan

AT&T Labs

April 2007

(This is work done jointly with: Amit Saha, Rice University )

An operator perspective

• Operators seek to apply wireless technology/network advances to aid deployment (control costs) and/or improve performance (additional capacity, revenue)

• Areas where cooperative wireless could help in cellular systems:– Improve coverage/outage, especially at cell edge– Improve capacity / spectral efficiency– Improve interference avoidance

• Some examples: – Use of infrastructure mesh Base Stations (mBS) a.k.a. Relay

Repeaters– Inter-Base Station cooperation– Cooperation at radio as well as network level

Infrastructure Mesh example• Mesh Base Stations (mBS) are strategically deployed and

controlled by the operator to forward user traffic between Base Stations (BS) and user terminals (SS)

• Similar to Relay Repeater

• Mesh BS and BS are good examples of cooperative wireless nodes:

• PHY/MAC level

• Network level

• Dynamic resource sharing

Why Infrastructure Mesh ?

• Mesh BS can be installed closer to cell edge and mitigate coverage and outage issues

• Mesh BS avoids cost and complexity of wired backhaul provision to a new BS

• Mesh/relay architecture aggregates traffic at the main BS which has wired backhaul

– Lower $/bit for fatter wired pipes

• Changes the economics of achieving “smaller cells” where needed

Technical Issues for Infrastructure Mesh

• Radio Capacity exhaustion at Wired Base Station for Access Applications

– Optimization problem over depth of mesh, cost, management

• Radio Resource Sharing between Client Traffic & Forwarding Traffic

• Self-Organization and Adaptation– Routes, Load, QoS, Latency, Channel Adaptation– Consideration for MIMO & Directivity– Minimum installation effort

• Unique Security issue– Trust Relationship Management

• Standards Support– 802.11s: Near Completion– 802.16j Multihop Relay: Infrastructure focus

Assumptions in this Analysis

• Mesh BS can be lower complexity/cost• Smaller and lower antenna height than BS

• Single radio communicating with both SS and BS

• Omni-directional antenna to serve SS

• Similar complexity as SS except– Directional antenna for backhaul – Alternate between antennas using simple switch

• Time-shared “centralized MAC” packet radio system• Examples: 802.16/WiMAX, UMTS HSDPA

• Equal time per SS under uniform infinite offered traffic load

Analysis approach: Take 6-sector cell ..

Add mBS at cell edge

BS

mBS mBS

600

Analysis on pair of opposing sectors

A

BS1

BS2

• 6-sector cell

• 1 mBS per sector per cell

• 2 mBS at cell-edge boundary of each sector

• mBS1 belonging to & cooperating with BS1

• mBS2 belonging to & cooperating with BS2

mBS1

mBS2

BS to mBSBS to mBS BS to SSBS to SS BS to SSBS to SS

mBS to SSmBS to SS mBS to SSmBS to SS

BackhaulSimultaneous

SchedulingDedicatedScheduling

Time

Intra-Sector Scheduling Approach

Compatible with 802.16 structure• A possible downlink frame structure

Simulation Parameters

Frequency reuse (1,6,6)

Cell size 1000 m

BS gain 20 dBi

mBS gain 0 dBi

BS height 30 m

mBS height 15 m

SS height 2 m

Transmit power 30 dBm

Power control No

Fading model Erceg-Greenstein (Terrain A)

Simulation Parameters - Rates

• 6 MHz effective channel

• Representative values from 802.16/WiMAX

• Discrete levels• Continous capacity curves with no SNR limits

exaggerate the benefits of mesh

Modulation Code RateRequired SINR (dB)

Data Rate (Mbps)

QPSK 1/2 6.6 6.0

16-QAM 1/2 10.5 12.0

64-QAM 2/3 15.3 24.0

64-QAM 3/4 20.8 27.0

Multi-Cell Scenario without mBS

No log normalfading

Multi-Cell Scenario without mBS

With log normal fading

Multi-Cell Scenario with mBS

Multi-Cell Scenario with mBS

No log-normal fading for illustration

Simultaneousscheduling regions

Multi-Cell Scenario with mBS

No log-normal fading

Dedicated scheduling regions

Throughput Comparison

Sector Throughput

(Mbps)

0

5

10

15

20

25

Terrain A Terrain C

Without MBS

With MBS

16.22 16.02

21.25 20.44

QPSK ½ Outage Comparison

Percentage of SS below QPSK ½

0

5

10

15

20

25

Terrain A Terrain C

Without MBS

With MBS

24.3222.36

5.16 4.17

SS Data Rate Comparison

Conclusions and Future Work

• (1,6,6) system with 6 mBS per cell shows:• Outage improvement around 80 %• Overall sector throughput improves from 16 Mbps to

21 Mbps• Uplink and (1,3,6) system shows similar trends

• Capacity improvement in mesh system more than compensates for radio resources diverted towards mBS - BS backhaul

• Power control, measurement/scheduling scheme, technology-specific issues