Commodity software steerable antennas for mesh networks

Jim Kajiya

MSR

Why software steerable antennas?

• High gain antenna significantly improves mesh networks.

• Narrow beam antennas are key to network topology control for minimizing interference.

• High gain, low sidelobe design enables indoor placement

• Software antenna control integrates routing and propagation.

• Much nearer term than smart antennas/MIMO.

Challenges for steerable antennas in mesh networks

• Exacerbates hidden terminal problem

• FCC regulations for unlicensed bands limit ERP– Aimed at access point abuse, current

regulations preclude neighborhood mesh

• Cost increment over omni– High gain antenna is currently very expensive

First Prototype

• 8x8 dipole array

• Designing feed network was the hard part

• Induce time delays

• Phase shifters are currently expensive

Phased array Antenna

d 2d 3d

Coplanar Waveguide (CPW) Phase shifter

Dielectric

CPW Phase shifter uses elementary transmission line physics

• Delay is given by

• Impedance is given by

• As distance varies relative permittivity changes

Z

0 0r r r

t L v

v c c

Simulation Results

Phase shifter design is best done with dielectric penetrating the air gap in a coplanar waveguide.

r dr

r

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

1

2

3

4

5

6

7

8

9

10

Effective dielectric constant ( ee)

distance( mm ) from the substrate

ee

Simulation Results, cont’d

• High dielectric constant material not necessary: alumina with =10 is fine.

• Variable characteristic impedance does not materially affect antenna pattern or VSWR.

r

Simulation of 4 element patch Beam

Patterns

Next Design

• Coplanar Waveguide phase shifters compatible with corporate feed.

• Form factor for indoor wall mounting