DTV vs. Analog Coverage

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The Effect of Channel Assignment on Transmitter and Receiver Requirements for Equivalent
HDTV / NTSC Coverage

0. Bendov

Dielectric Communications Antennas

Voorhees, NJ

Introduction

Broadcasters, especially those who will be moving from VHF-NTSC transmission to UHF-HDTV transmission, arc concerned with providing equivalent HDTV coverage to their NTSC viewers. This concern can be summarized by the following questions: (a) What will it take to provide reliable HDTV picture and sound to the viewers who now receive a passable (grade 3) picture and sound? (b) How will the UHF-HDTV channel assignment affect the station's ability to provide equivalent coverage relative to other UHF-HDTV stations and relative to their own present NTSC coverage?

This paper will define the concept of equivalent and reliable coverage and will show the derivation of the correct planning factors, including the effects of sky temperature and Low Noise Amplifier (LNA) at the receiver end. Two approaches to the determination of the planning factors and transmitter size will be presented. One is based on absolute power requirements and the other is based on power levels relative to the existing NTSC transmission. Finally, the areas where knowledge is presently lacking will be highlighted.

Equivalent Reliable Coverage

Equivalent coverage is established when the HDTV channel provides reliable pictures and sound through the Grade B contour of the existing NTSC channel. In this paper, the coverage area is not considered limited by interference from other stations. That is, the coverage and service areas are seen as equal. Grade B contour is defined here as the contour of a Passable (grade 3) picture. This is equivalent to (peak-of-sync) carrier-to-noise ratio (CNR) of 28dB⁽¹⁾ with at least 90% of time availability at the best 50% of the locations. To replace a grade 3 picture with HDTV, a CNR of 16dB is sufficient. However, there arc three twists to the 16dB CNR level for HDTV.

First, unlike NTSC, the CNR for HDTV is a threshold value below which picture and sound become unavailable. Because it is a threshold value, it must be maintained close to 100% of the time for reliable service. In this paper, 99% availability was selected.

Second, a carrier penalty, also known as "Implementation margin," must be added to the theoretical threshold-CNR to compensate for the residual distortions at the transmission plant. In NTSC, these distortions will appear as picture impairments but will not affect the coverage. The reverse is true for HDTV. In HDTV, the distortions will not affect picture quality but will raise the "noise" level. The result is a reduced coverage area unless the carrier level is increased to maintain the 16dB threshold CNR, Simply put, two transmitter/antenna systems, classified as equal and acceptable using standard NTSC measurements, may provide two unequal HDTV coverage contours.

Third, in NTSC, the CNR is defined by the RMS power during sync pulses. Therefore, the power during sync pulses serves as a useful definition for both transmitter size and coverage contours. In HDTV, the coverage contour is defined in terms of the average operating power, and the transmitter size is defined by the peak instantaneous power (maximum amplitude). The difference between the average operating HDTV power and the peak operating NTSC power is expected to be 10±3dB⁽²⁾. The definition of equivalent and reliable HDTV/NTSC coverage is embodied in Figure 1.

Frequency Dependent Factors

Even though the FCC propagation curves arc not channel-specific within the UHF hand, the CNR at the TV receiver depends significantly on channel assignment. There are three elements that contribute to this dependence:

Receive antenna effective interception area. This area is defined as a multiplier of the wavelength squared. The larger the area, the higher is the intercepted power level. Therefore, for a given physical size, the interception area decreases with the square increasing frequency. For a constant-gain receive antenna, this factor implies that the effective radiated power at channel 69 will have to be at least 4dB (2.5 times) higher than that at channel 14 for equivalent coverage. This effect can be mitigated by a high gain (>14dB @ ch. 69) receive antenna, the largest dimension of which will be at least 4 feet.
Downlead cable loss. The loss of a coaxial cable increases with the square root of the frequency. The increase in carrier loss from channel 14 to 69 is approximately 2dB. The high downlead loss and its variation with frequency can be mitigated by adding a low-noise-amplifier (LNA) at the terminals of the receive antenna.
Antenna Noise. There arc two sources of thermal noise at the receiver. One is the ambient temperature of the LNA, downlead cable and the receiver. The second source is antenna noise. Antenna noise is due partly to the surrounding ground temperature and partly to sky temperature. Sky temperature decreases with increasing frequency, and, as shown in Figure 2, its maximum value at channel 14 is equal to 290°K, the ambient temperature.

Receiver Model

The receiver model is shown in Figure 3 along with the expression for the overall noise figure referenced to the terminals of the receive antenna. With a high gain LNA and a short cable between the antenna and the LNA, the system noise figure is the same as that of the LNA alone. The receive antenna, being directional and broadside, acquires half its noise from the sky temperature and the other half from the ground temperature. The ground temperature is assumed to be the same as the ambient temperature.

Fade Margin and Reliability

The signal availability throughout the coverage area is statistical in nature. The signal level varies from noon to midnight, as weather changes and as summer turns into winter. To overcome these and other variations, a fade margin is introduced. Just how big this margin is depends on the required signal availability. In UHF-NTSC, the required availability is at least 90% of time for Grade B coverage. For HDTV a larger percentage is required because the critical parameter, CNR=16dB, is a threshold value which may not go lower at any time, else picture and sound may disappear. In this paper, the required availability was set at 99%. Figure 4 provides the fade margins, in dBu over FCC(50,50), as a function of radiation center height and distance from the transmitter.

Link Equation

The required average Effective Radiated Power (ERP) to provide CNR=16dB at least 99% of the time at the best 50% of the locations is:

where:

G_R is the peak gain of the receive antenna relative to

dipole.

M is the fade margin given in Figure 4.

N/N_o is the relative system noise for 6 MHz bandwidth. It is defined by:

where the system's noise figure, F, is defined in Figure 3 and the sky temperature, T, is defined in Figure 2.

Figure 5 is a graphical depiction of the link equation. The receiver parameters (typical) used in Figure 5 are:

TV receiver noise figure=10 dB.

Receive antenna gain=l0 dB any channel.

LNA gain=20 dB any channel.

LNA noise figure=4 dB.

No mismatches and no baluns.

Choosing a height of 1200 feet above the average terrain, and 56 miles to the Grade B contour, the required ERP would be 21.9dBk at channel 14 and 26.2dBk at channel 69 for a constant downlead loss of 6dB. A downlead loss of 6dB is equivalent to 50 feet of RG-59A cable at mid-UHF band with approximately ±1 dB variation to upper and lower ends. Therefore, the required ERP will vary from 20.9dBk for channel 14 to 27.2dBk for channel 69 with 50 feet of downlead cable. Simply put, the average ERP at channel 69 will have to be four times that of channel 14 for equal coverage unless the gain of the receive antenna increases by 6dB from channel 14 to channel 69.

Figure 5 can be developed for various parameters such as lower fade margins, and without LNA. Some comparative results arc given in Tables 1 and 2. For Grade B contour of less than 56 miles, subtract 1dB/mile from the calculated ERP.

Transmitter Size

With the ERP known, the transmitter size can be defined by:

where:

F_T is the carrier penalty, due to residual distortions. A typical value of 1dB, varying by manufacturer, is expected.

P_BO is the power backoff level of the final amplifier, below the NTSC operating point, to the average power of the HDTV signal. The backoff level is a tradeoff between the carrier penalty and adjacent channel spillover.

G_T is the gain of the transmitting antenna relative to L/2 dipole.

is the transmitter-to-antenna efficiency.

For example, assuming an omnidirectional transmitting antenna gain of 14dB(x25), a line loss of -1dB, a carrier penalty of 1dB and final amplifier power backoff of -7dB to accommodate peak power levels, the transmitter size can be determined from:

Applying equation (4) to the example, the required transmitter size, with and without LNA at the receive antenna, is given in Tables I and 2.

56 mile contour HAAT=1200' 50' Downlead G_T=14dB G_R=10dB P_BO=-7dB F_T=1dB	% of TIME AVAILABILITY HDTV
	90	99
CHANNEL 14	ERP=360 TX=114	ERP=2022 TX=639
CHANNEL 69	ERP=1029 TX=325	ERP=5787 TX=1830

Table 1. ERP (kW) and TX (kW) for equivalent UHF-HDTV/UHF-NTSC coverage
Receive antenna without LNA.

56 mile contour HAAT=1200' 50' Downlead G_T=14dB G_R=10dB P_BO=-7dB F_T=1dB	% of TIME AVAILABILITY HDTV
	90	99
CHANNEL 14	ERP=25 TX=8	ERP=141 TX=44
CHANNEL 69	ERP=65 TX=21	ERP=367 TX=116

Table 2. ERP (kW) and TX (kW) for equivalent UHF-HDTV/UHF-NTSC coverage
Receive antenna with LNA.

The results of Tables 1 and 2 show that, for the example chosen, the gain of the receive antenna, even with LNA incorporated in it, will have to be raised from l0dB to at least 13dB at channel 69, two times higher than currently used roof-top antennas, for reliable coverage to 56 miles from an omnidirectional antenna at 1200 feet above average terrain and a transmitter size of less than l00kW.

Power- Relative to NTSC

For Grade B NTSC, equation (1) becomes:

The differences between equations (1) and (la) are in noise bandwidth (4.2 vs. 6 MHz), CNR (28 vs. 16dB) and fade margin (90 vs 99%).

By subtracting equation (la) from (1), and assuming in-band (UHF) transition from NTSC to HDTV, no changes in the basic receiver parameters, and assuming no change in the location of the transmitting antenna, the differential ERP, over that required for NTSC, is:

The differential fade margin, M(99)-M(90), is (from Figure 4) approximately 7.5dB at 56 miles for antenna height of 1200' above the average terrain. Therefore, if f_HDTV=f_NTSC, the average ERP_HDTV for 99% availability is -2.95dB below that of NTSC. For 90% availability, the average ERP_HDTV is -10.45dB below that of peak-of-sync ERP_NTSC.

Thus, if the present NTSC contour of CNR=28dB is ascertained at key locations, the average ERP_HDTV for equivalent coverage can be determined for any percentage of reliability.

Equation (5) is a special application of the technique. In general, all the terms of equations (1) and (la) will apply.

Similarly, since the size of the NTSC transmitter is:

then, by subtracting (3a) from (3) and assuming no changes in the gain of the transmitting antenna and line efficiency, the differential power, in dB, of the HDTV transmitter for equivalent coverage is:

For the example chosen earlier with F_T=1dB and P_BO= -7dB, the differential transmitter size relative to the NTSC operating point reduces to:

Applying equations (5) and (7) to a UHF station with Grade B at 56 miles, the differential ERPs and transmitter sizes are given in Table 3.

To overcome the requirement for increase in the HDTV transmitter size over that used for NTSC, as indicated in Table 3, an LNA could be added to the HDTV receive antenna. In the latter case, equation (5) becomes:

The differential ERPs and transmitter sizes for various channels and availability for a typical outdoor HDI'V receive antenna with LNA are give in Table 4.

56 mile contour HAAT=1200' P_BO=-7dB F_T=1dB	% of TIME AVAILABILITY HDTV
56 mile contour HAAT=1200' P_BO=-7dB F_T=1dB	90	99
NTSC channel 69 HDTV channel 14	ERP=-15.05 TX=-7.05	ERP=-7.55 TX=+0.45
Same channel for HDTV and NTSC	ERP=-10.45 TX=-2.45	ERP=-2.95 TX=+5.05
NTSC channel 14 HDTV channel 69	ERP=-5.85 TX=+2.15	ERP=+1.65 TX=+9.65

Table 3. Differential ERP (dB) and TX (dB), over NTSC for equivalent UHF-HDTV/UHF-NTSC coverage. HDTV and NTSC receive antennas
without LNA.

56 mile contour HAAT=1200' P_BO=-7dB F_T=1dB	% of TIME AVAILABILITY HDTV
56 mile contour HAAT=1200' P_BO=-7dB F_T=1dB	90	99
NTSC channel 69 HDTV channel 14	ERP=-26.59 TX=-18.59	ERP=-19.89 TX=-11.09
Same channel for HDTV and NTSC	ERP=-22.43 TX=-14.43	ERP=-14.93 TX=-6.93
NTSC channel 14 HDTV channel 69	ERP=-17.87 TX=-9.87	ERP=-10.37 TX=-2.37

Table 4. Differential ERP (dB) and TX (dB), over NTSC for equivalent UHF-HDTV/UHF-NTSC coverage. HDTV receive antenna with LNA and NTSC receive antenna without LNA.

Missing Knowledge

The procedures derived in this paper can be improved once certain information becomes available. The areas where information is lacking are:

A. FCC Propagation Curves and Fade Margins

The present curves apply to narrow-band, NTSC transmission, where most of the picture information is near the carrier. In HDTV, the picture information is evenly spread over the entire channel. The wider bandwidth required for HDTV may affect the conversion factor from incident field at the receiver to transmitted power.

B. Receiver CNR Penalty

As the channel equalizer at the receiver attempts to eliminate the effect of multipath, it reduces the CNR margin. For heavy multipath, the equalizer may force the CNR below threshold. The passband performance of the path between the transmitter and the receiver, sometimes called "channel characterization," is required for urban, suburban and rural environments in order to determine the CNR penalty at the receiver.

C. Transmitter CNR Penalty.

Reliable information on the tradeoff, as a function of backoff level, between CNR penalty and adjacent channel spillover is not publicly available.

D. Reliability and Performance of Outdoor LNA.

Besides protection against adverse weather and electrical storms, an acceptable LNA should exhibit minimum of linear and non-linear distortions and provide proper isolation against man-made noise. Not meeting these criteria may incur CNR penalty.

Conclusion

The significance of Tables 1-4 is that they demonstrate the desirability of adding an LNA to the HDTV receive antenna for stations wishing to provide reliable HDTV coverage for at least 56 miles from a transmitting antenna 1200' above the average terrain. Further, the calculations show that in the case of HDTV, which does not exhibit graceful degradation similar to NTSC, proper coverage and system analyses must be channel-specific.

⁽¹⁾ This value is consistent with TASO data (1959) based on random noise. TASO's definition of Passable picture is based on CNR of 27dB with 6MHZ random noise. Translated to 4.2 MHz, the equivalent CNR is 28.5dB.

⁽²⁾ For 32QAM modulation, a maximum peak RF (not "envelope") to average power ratio between 11 and 13dB, depending on channel filters, is expected.

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