A mobile test was designed and conducted to help better understand mobile fading and its effect on our software DTV receiver. On the roof and sides of a minivan, we placed seven magnetic‐mounted antennas and connected to seven radio channels (Ch1–Ch7) of our data acquisition system. As shown in Figures 40.18(a) and (b), a small patch antenna, marked “1,” is connected to Ch1 for GPS. A whip antenna, marked “2,” is connected to Ch2. The remaining five antennas, marked “3” through “7,” are identical and are connected to Ch3 through Ch7, respectively. Ant3 is placed in the middle section on the right‐hand side (the passenger side), while Ant4 is placed horizontally above the right rear wheel. Ant5 repeats the placement of Ant3 but on the left‐hand side (the driver side). Ant6 is similar to Ant4, but placed far to the left. Ant7 is in the middle on the back.
Figure 40.17 Test environment with ATSC‐8VSB signals on Google Earth.
Figure 40.18 Test setting for study of mobile fading.
The mobile test lasted about 70 s, in which all channels were tuned to the station centered at 653 MHz. In this run, the van was initially stationary for 10 s and then moved for 10 s. It next stopped for 10 s and moved for 10 s. It repeated the stop and move sequence for 10 s each before finally stopping for the last 10 s.
Figures 40.19(a)–(g) show the correlation peak, peak to average ratio, code delay error, carrier phase error, TOA error, and pseudorange as a function of the field number for all six DTV antennas (from left to right, Ant2 and Ant3 on the top, 4 and 5 in the middle, and 6 and 7 on the bottom of each subplot in Figure 40.19), respectively. It is clear from the figures that the signal strength at stop has less variations than in motion, but the peak value during the stops is not necessarily larger. There are peaks and dips during motion. When transitioning from stationary to moving and back to stationary, the signal level could be either high or low, depending on the particular location where the transition took place. The swing of signal strength during motion is due to fading.
Figure 40.19 Fading study with six antennas in a stop‐move‐stop sequence.
The performance ranking among the six DTV antennas is 4 > 3 > 2 > 5 > 6 > 7. That is, the horizontally placed antenna on the side above the right rear wheel outperformed the rest. It happens that the DTV station at 653 MHz uses a horizontally polarized antenna and the signal comes from the right, which is in direct sight of Ant4 with matched polarization.
Mobile Test 2: Clock Errors and Calibration. Six radio channels are assigned to six DTV stations for simultaneous data collection: Ch1 @ 551 MHz (data not shown) and Ch2 @ 635 MHz on San Bruno Mountain, Ch3 @ 563 MHz and Ch4 @ 617 MHz on Sutro Tower, Ch5 @ 605 MHz on Monument Peak, and Ch6 @ 683 MHz on Mt. Allison. A passive UHF whip antenna, magnetically mounted on the roof of a minivan, is split to drive the six radio channels for data acquisition. During the test, the van was stationary for about 40 s and was driven up to about 20 miles per hour for the remaining 50 s.
As shown in Figures 14.20(a)–(e), prior to field number 2000, the minivan was stationary. The reference ranges stayed constant. Except for some small variations (oscillatory), the calibrated ranges were rather close to the reference values, indicating that the calibration algorithms were able to find the offset between the clocks of the receiver and DTV stations.
The transmitters in San Bruno and Sutro Tower are in the north (San Francisco), whereas those in Monument Peak and Allison are in the south (Freemont). Since the minivan was traveling from north to south, it was expected that the pseudoranges to the northern stations would increase (see Figures 14.20(a) and (b)), while those to the southern stations would decrease (see Figure 40.20(c)). However, this is not obvious for the two stations in Figures 14.20(d) and (e) that exhibit large variations.
In Figure 40.20(d), the linearly calibrated pseudorange shows a parabolic shape, meaning that the range rate is not constant but under a certain range acceleration. Taking the difference between two successive TOAs for Ch2 provides measurements of the field length, as shown in Figure 40.20(f). Ideally, the nominal field length is 241971.9818 samples. However, it is clear from Figure 40.20(f) that the field length for Ch2 not only differs from the nominal value (a bias in frequency, meaning a clock drift) but also varies over time. A line is fit to the data as the red curve in Figure 40.20(f). Removing this slope from the original data leads to the second‐order calibrated pseudoranges as shown in Figure 40.20(g), which now has no visible drift any more. Since this station is in the north while the minivan was going from north to south, the range increases after field number 2000.
Figure 40.20(e) is an example of oscillatory behavior in pseudoranges (smaller oscillations are observed in Figures 40.20(a) and (b) as well). Before field number 2000, there are about two cycles with an upward trend and seemly increasing amplitude. Although a polynomial can fit nicely to the measurements, it cannot be extrapolated beyond the fitting interval; that is, it cannot predict the remaining data. Alternatively, a constant amplitude sine wave is fit to the first 1940 data points, which misfits the first cycle due to the omission of the linearly increasing amplitude, but the second cycle has a better fitting. Removing it from the data leads to the calibrated pseudoranges as shown in Figure 40.20(h), where the blue‐colored curve is the original one and the green‐colored curve is the calibrated one. After the nonlinear calibration, the errors during the stationary period are within 20 m. Obviously, a better fit could have been achieved if the change in amplitude was taken into account. Since Ch6 was in the south and the minivan was moving from the north to south, the range decreased as it was moving toward the transmitter.
This example with a small number of DTV transmitters reveals a vast difference in the quality of transmitter clocks. The observed clock errors include (i) clock timing bias, (ii) clock frequency drift, (iii) slow change in clock drift (parabolic), (iv) fast change in clock drift (oscillation), and (v) a combination thereof. Oscillatory clock errors were also observed in GSM signals and attributed to local intermittent clock adjustment [31]. Higher‐order calibration can be applied to estimate both quadratic and sinusoidal clock error components [27]. Figure 40.20(i) shows a histogram of the clock drift rate generated from a survey of 159 ATSC channels [84]. It shows a mean clock drift rate of −0.8 ppm and a standard deviation of 3.6 ppm. Some worst cases include −17.8 ppm and 23.9 ppm, to name a pair. The transmitter clocks generating