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Testing E-OTD.

Article Excerpt
By Darcy Smith, Agilent Technologies

An emulation-based approach can test 3GPP Enhanced Observable Time Difference capability in GSM/GPRS and W-CDMA handsets.

To ensure that the 3GPP's E-OTD (Enhanced Observable Time Difference) capability found in today's GSM/GPRS and W-CDMA mobile handsets operates as anticipated, you need to test the handset's performance. The simplest way to do that is to emulate only two cell towers with a known, but fixed, offset in timing between them. This approach could serve as a quick check, but it falls far short of a real characterization.

For a detailed characterization of E-OTD operation, manufacturers must emulate multiple cell towers, mobility, and variable channel conditions (see "Location-relevant information and E-OTD," p. 51). One plausible means of accomplishing a thorough characterization is through the use of a test system that emulates 12 cell towers with independent time-delay elements and variable channel conditions. As part of the test setup, a channel simulator, often called a fader, provides the time delay from the cell to a phone as well as the fading channel conditions. Just as a real network must calibrate the actual system time, the test system must calibrate the timing of each cell emulator, including both the timing from frame-to-frame and the offset in frame numbers from cell-to-cell.

Using such a test system, you can easily perform tests for a stationary handset. You can hypothetically place the cell towers at known locations on an x-y grid, representing longitude and latitude, and you can place the handset, either deterministically or randomly, at some spot on the grid. The test system computes the propagation delay from each cell tower to the handset location and enters it into the delay elements of the channel simulator from each cell. In this way, the arrival times at the handset are delayed as if in a real system. The handset makes its E-OTD measurements and computes its location by using the data from the handset and the system calibration values. You can then verify the results...

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