The test compared xG's 900 MHz transceivers against an unspecified commercial 900 MHz digital radio modem while carrying digital video from an MPEG-4 encoder. The most relevant part of the test included a short radio link and measured the peak RF power required to "close" each link, i.e., to allow the receiver to function.
There are several problems with this test, the main one being that the commercial digital radio used for comparison was not specified. There are many digital radios on the market. Some are good and others are bad. We aren't even told the comparison radio's data rate; we only know that it's greater than that required to carry a MPEG-4 video stream in real time. If the commercial radio operates at a significantly higher peak data rate than the xG transceiver, it would have required considerably more peak RF power than an equally performing radio operating at the average rate of the MPEG-4 encoder.
A more meaningful measurement would compare the average RF powers of the two radios while operating at the same average data rate. Even then we'd have to ensure that the commercial radio did not "pad" unused bits in a way that would inflate the required RF power.
MiCOM's use of a short RF path in the tests was baffling. This merely complicated the tests and introduced additional factors that had to be controlled. End-to-end cable paths with sufficient attenuation would have been simpler and more stable.
But there's a more fundamental problem with the test. Operating power measurements are useful when comparing complete systems, but they are not meaningful measures of a modulation and coding scheme -- which is where xG claims to have innovated. A higher RF power requirement by one receiver could just as easily reflect a noisier preamplifier or mixer as an inferior modulation and coding scheme. To measure only the performance of the modulation and coding, the effects of receiver noise must be eliminated.
This can be done with an extra piece of test equipment, a RF noise generator. The idea is to add enough noise to the input of the receiver under test to "swamp", or render unimportant, the noise produced internally in the receiver. Then the signal-to-noise ratio (SNR) seen at the demodulator will be the same as at the antenna terminals where it can be controlled.
This test should take the user data rate into account so that the required Eb/N0 can be computed. Eb/N0, the ratio of the energy per bit to the noise power spectral density, is a fundamental figure of merit for every digital modulation and coding scheme, and it is the most meaningful way to compare one scheme to another. There are several ways to perform the measurement, and equipment exists to do it automatically. It can also be measured manually with a spectrum analyzer, a wideband RF noise generator, an RF attenuator, a hybrid combiner and a hybrid splitter. The manual procedure is as follows:
Phil Karn, 4 June 2007