ANTENNA DIVERSITY ORINOCO AP-600

How does antenna diversity work?
In an environment with a lot of reflections e.g. offices, storage and manufacturing facilities, antenna diversity can be used as a means to have alternate paths for the radio signals to take to be able to reach through the room. Since it's a matter of two-way communication, one needs diversity both for downlink (from access point to client) and uplink (from client to access point), for full usability. There are two ways of using antenna diversity;

Space diversity - two linearly polarised (usually vertical polarisation) antennas are placed on a distance of 10 wavelengths from each other, i.e. 1.2 m at 2.4 GHz. This method is the most proven and with the lowest cost for antennas.

Polarisation diversity - the antenna has two orthogonal polarisations, usually vertical and horisontal. If the access point has support for diversity on both transmission and reception (TX/RX) the client only needs one antenna, which makes it cheaper.

How to use the antenna ports?

AP-600b (IEEE 802.11b) with chipset from Agere has TX/RX on the connector to the left (see picture), and RX on the connector to the right. If we understand this correct, it primarily listens on the connector to the left, and if it can not hear anything, then listens to the connector to the right. This is a form of basal receiver diversity. In practical tests in a storage environment, with many reflections, we get a clear improvement with antennas on both ports.

AP-600b/g (IEEE 802.11b/g) with chipset from Atheros has TX/RX on both ports, both when using IEEE 802.11b and/or IEEE 802.11g. It transmit on the port where it last received a data packet. In practical tests in a storage environment, with many reflective objects, we get a vast improvement with antennas on both ports.    However, if there is one client on each antenna, we will have rather poor performance, as the access point always tries to transmit on the the "wrong" port.

Is IEEE 802.11g better in a storage environment?

We have not yet done complete tests on IEEE 802.11g in a storage environment, with a lot of reflections and with limited (or none) line of sight. The OFDM technology that is used in IEEE 802.11g is supposedly better for multi path propagation. Unfortunately the client software for Orinoco is a lot worse in the new version for IEEE 802.11g, so we have no good way of making measurements. A whole new measurement platform is being developed for Linux though.    However, the preliminary tests we have done, by sending data packets, and looking at response times and packet loss, show vast improvements compared to IEEE 802.11b.

Orinoco Client Manager is a very good measurement tool, in free space condictions, ie outdoor. The reported signal strength in dBm correspond within a few dB with measurements done in eg LinkBudget. However, when doing measurements in a multi path environment it produces some interesting phenomena.

The bottom line is that the noise floor reported by the Ornioco Client Manager is not to be taken "as fact". If transmitting actual data, using PING, FTP or other application, we get a answer whether it is practically possible to use the network.

Page updated 041103.