Base Station Antennas

Smart Antennas

Definition: Antennas capable of dynamically adjusting.

Goal:1. Improve performance. For example, by frequency reuse.2. Improve signal quality. For example, reducing multipath fading.

Antenna Diversity: when the terminal or environment is time-changing, multicopies of signal are received by different antennas withno or little correlation such that some of them with or withoutcombining are good in quality. 1. Spatial diversity.2. Angle diversity3. Polarization diversityDiversity combining techniques:1. Selection combining.2. Maximal ratio combining.3. Equal-gain combining.Example: spatial diversity. Two antennas separated by . Frequency842 MHz. Selective combining.

All three types of diversity perform about the same experimentally.Diversity gain at 1 % probability:1. Spacial: 8 to 10 dB.2. Polarization: 6 to 10 dB. 3. Angel: 5 to 9 dB.

Adaptive beamforming: steer main beam toward a user and nullstoward interferers. Dual or multiple polarized smart antennas can beused to reject interferers that differ in polarization.

MIMO (multiple-input multiple-output): a transmit beam is spread over

Input Output

3-dB Coupler

Crossovers

3-dB Coupler

3-dB Coupler

3-dB Coupler

45 °phase shifter

45 °phase shifter

1

2

3

4

5

6

7

8

Butler Matrix

Port5 Port6 Port7 Port8

Port1 -45° -90° -135° -180°

Port2 -135° 0° -225° -90°

Port3 -90° -225° 0° -135°

Port4 -180° -135° -90° -45°

a wide angular sector such that diversity is even more.

Adaptive and Spatial Filtering (beam forming) Antennas1. Phased array.2. Sectorized.3. Switched beam.4. Adaptive array.

Switched beam antennas

1. Feedline switching.2. Butler matrix.3. Reconfigurable antenna: alter pattern by opening and closing

switches implemented by PIN diodes, FETs or MEMS.

Reconfigurable beam-switching cavity-slot antenna

Frequency: 2.4 GHz.Beam-direction: 10 at interval.Gain: ~ 1dBi.

Feedline switching

Reconfigurable polarization-switching Antenna

Frequency: 2.4 GHz.Polarization: 12 at interval.Gain: ~3 dBi.