Introduction

Antenna Diversity is a RF concept used for improving the quality of Rx Signal. WiFi also uses the same RF concept for improving rx signal strength.

History of Antenna Diversity

This is pretty popular concept utilized by many of legacy WiFi (802.11a/b/g) devices and being used by some of 11n devices as well. Legacy WiFi devices use single antenna (1×1 -SISO single input and single output) for Tx & Rx. But many of you would have noticed 2 antennas on legacy APs/Routers and legacy client devices. That means that device has antenna diversity implemented.

Technical Description

Diversity is built on fact of using two antennas instead of one. At the start receving a WLAN frame (while receiving the preamble), device with antenna diversity capability will switch between two antennas to verify which antenna is providing better signal strength and tune the RF chain to use that particular antenna. Confused ? lets try to simplify with real world example.

Think of a person as Wi-Fi device and two ears are two antennas, brain is the receiver. One assumption is that person can hear sound only with one ear at a time, which can be connected to brain for processing. Lets imagine the each message that person is supposed to receive comes with beep preceeded. Beep represents the preamble in WiFi terminology, where both preamble and beep doesnt carry any data, just it is used for alerting the person(device) about incoming message. When the person hears the beep, person can find which ear is able to hear beep with more intensity and decide to continue to hear the actual message with the same ear. In similar manner WiFi devices use two antennas during preamble and decide which antenna is providing better signal strength(RSSI) and tune to that antenna.

Having good RSSI over the WiFi link always ensures better throughput, with minimal errors.

Tx Diversity

Above description is completely centered around Rx quality. Diversity concept is used even for transmission direction.

Antenna Diversity in 802.11n devices

Few of 802.11n capable devices also has support for antenna diveristy. One such chipset is TI Wilink8, which has 2x2 11n support. But due to SDIO interface limit, 5GHz is restricted to operate in 1x1 mode with 40MHz to keep the peak throughput at 150Mbps. Additional antenna of 2x2 set, is used for antenna diversity in 5GHz.

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