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Wireless How To

Throughput vs. Path Loss

Figure 2 shows the Azimuth ACE test setup for testing throughput vs. signal level. But since the Azimuth system can control only the loss or signal attenuation between AP and STA and not the signal level directly, this test is more correctly called Throughput vs. Path Loss.

Note that direct connection is made to the antenna connectors for both AP (access points and wireless routers) and STA (client) devices. This is required so that the ACE can be in total control of what happens to the signals between AP and STA.

Azimuth ACE Test Setup

Figure 2: Azimuth ACE Test Setup

For APs without removable antennas, we remove all or part of the AP enclosure. Since most STAs don't have external antenna connectors, we do what's needed to get at the miniature antenna connectors that are now found on most STA devices. This usually involves cutting away part of the plastic housing of the CardBus cards that we usually receive for testing with APs.

APs are then placed into one of Azimuth's RF-tight enclosures and connected to the ACE. The STA card is inserted into a 1.2 GHz Pentium M notebook running WinXP Pro and Azimuth software that enables communication between the Azimuth system and the STA under test, which is placed in another Azimuth "Radio Proof" enclosure and also connected to the ACE.

We then perform a complete Throughput vs. Path Loss test in 3dB increments starting at a level of -55 dBm down to where the product loses connection. The ACE is set to use its Butler emulation mode. According to this article,

a Bulter matrix is commonly used to provide a static emulation of a real environment to evaluate multi-signal/multipath transmission. A Butler Matrix consists of some Hybrids, some Phase shifters and some cross couplers, with multiple inputs and multiple outputs.

An Azimuth application note adds:

When this mode is selected, the channel is configured to have a unity gain, non-fading channel with channel (phase) matrix (Figure 3). Generically, the Butler mode provides a channel with a high degree of linear independence.

Butler matrix

Figure 3: Butler matrix phase conditions

For our purposes, the key thing to know about the Butler matrix is that it presents a best-case condition for throughput measurement.

Traffic generation and measurement for the test is done by IxChariot, running the throughput.scr script using TCP/IP. The only modification we make to the script is to adjust the file size from its default of 100,000 Bytes. We typically use 300,000 Bytes for 802.11g products and 3,000,000 Bytes for draft 802.11n.

The products under test are run with their default settings unless otherwise noted, to emulate the "out of the box" performance that buyers will experience.

The script is usually run for 30 seconds at each programmed attenuation, with a 5 second pause between tests.We record the average throughput reported by IxChariot at each signal level. We test both uplink and downlink profiles in separate runs.

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