The Need for Better HD Radio Coverage
As HD Radio becomes more important to broadcasters’ business models, there is a greater than ever need for reliable reception on mobile and stationary receivers. HD Radio coverage needs to match the analog host closely. Currently, FM stations are allowed to broadcast digital signals at 1 percent of their analog power. This power level offers usable service within the primary unobstructed coverage area of the station, but reception issues can be encountered inside buildings and areas of low signal coverage. The issue is compounded by HD multicast channels not having the protection of fallback to analog when the edge of the digital signal coverage is reached.
Broadcasters are investigating ways to improve HD Radio coverage of the digital signal, including increasing the digital sideband levels and using multiple transmitters in Single-Frequency Network (SFN) and translator approaches.
On January 29, 2010, the FCC changed the FM HD Radio rules to allow virtually all U.S. FM stations a four-fold increase of their HD Radio digital power from the current -20dB below the FM analog power level to -14dBc. In addition, stations may apply for additional increases beyond 14dBc up to -10dBc if it can be shown the increase will not infringe on the protected contour of an adjacent channel station. The power increase will provide more reliable reception of multicast channels and better building penetration of HD signals to portable and desktop receivers, as well as better mobile and portable HD Radio recep¬tion in suburban areas.
Engineered Solutions for Increased HD Radio Power
The most practical choices for increasing HD Radio power are space combining of separate analog and digital transmitters or common amplification of FM + HD through a single transmitter. High-level combining and mid-level combining are too inefficient for HD Radio power increases of more than 3dB, making space combining or common amplification the only practical methods to reach the -14dBc to -10dBc levels.
Space Combining or Common Amplification?
Space combining is often the most cost-effective way to increase the HD Radio power using the existing antennas and analog FM transmitter. If the new transmission system isolation and power handling requirements can be met, increasing the digital transmitter power is the only equipment change. In many cases, existing high-level combined systems may be converted to separate antenna systems using the existing digital transmitter.
The shortcoming of space combining is the amount of miss-tracking between the analog FM and digital HD signal levels that can occur in the far field. As the HD Radio power level is increased, the relative tracking between the analog FM and HD Radio signals becomes more important to avoid digital-to-host analog interference at some receive locations.
The way to guarantee that both signals will track perfectly is to radiate them together from the same antenna with the same radiation pattern and polarization. Low-level combining of the FM analog and HD Radio signals and common amplification is the most elegant solution, but does have its own challenges in terms of amplifier headroom and linearity.
Upgrading a Space-Combined System
A 6dB increase to -14dBc requires the digital transmitter power be increased to four times its current power. Depending on the current digital transmitter power rating, there may be enough headroom to increase power by some number of dB now.
For stations currently using high-level combining through a 10dB hybrid, the digital transmitter is already producing -10dBc into the combiner and can simply be removed from the combiner and connected to the separate antenna of equal gain with the main antenna.
As the digital power is increased in a space-combined system, the isolation between the analog FM transmitter and the digital HD Radio transmitter feed-points to the two antennas must be increased. The power handling ratings of any multi-station combin¬ing system, the transmission line and the antenna also will need to be reviewed and possibly upgraded.
Upgrading a Common Amplification System
The most practical way to achieve proper far-field tracking with elevated HD Radio sidebands is with a common amplification transmitter where both signals are amplified together in the proper ratio and radiated as a single point of transmission.
The major advantage of common amplification is the ability to use an existing, single antenna to provide identical radiation patterns and polarization for both signal com¬ponents, thereby providing nearly perfect signal level tracking of the FM and HD radio signals at all receive locations.
In cases where the current common amplification transmitter does not have enough headroom to go to -14dBc or -10dBc, the addition of a second, identical transmitter combined with a 3dB hybrid offers several advantages. This system provides full back-up of FM + HD Radio. Nearly full FM analog power is possible on either transmitter alone by reducing the HD sideband power level back to -20dBc.
Increasing the HD Radio sideband levels in a common amplification system can be a challenge because as the HD Radio sideband ratio is increased, the RF inter-modulation products must be sup¬pressed by the same ratio to maintain mask compliance. The more we increase the IBOC carriers, the more we must de-rate the transmitter to allow headroom for the higher peak power requirements of the IBOC carriers.
Current-generation HD Radio exciter and transmitter systems — such as the Harris FlexStar HDx exciter, ZHD, ZX and HPX series transmitters — are designed to meet the linearity challenge and higher peak-to-average ratio through the use of advanced amplifier linearization, advanced Real-Time Adaptive Pre-Correction (RTAC) and Hybrid Crest Factor Reduction (HCFR). These advanced signal processing techniques are implemented in the exciter’s firmware. HCFR intelligently clips only the digital signal peaks, while still accounting for the FM analog signal as to when and how much clipping to apply. HCFR measurably improves HD Radio performance in common amplification systems by reducing the PAR (Peak to Average ratio — this is the ratio of peak power to average power in the RF signal) by as much as 2 dB. The power rating of a transmitter after increasing the HD Radio sidebands from -20dBc to -14dBc with standard crest factor reduction will be approximately (70%) of the -20dBc rating and approximately (85%) of the -20dBc rating with HCFR.
The Significance of Asymmetrical Sidebands
In addition to the +6dB or greater increase, operating with asymmetrical digital side¬bands allows many stations to further increase HD power above -14dBc on one side of the station’s channel, while protecting the adjacent channel station on the other side. Due to the redundancy of information transmitted in both the upper and lower digital sidebands, HD Radio is still receivable on standard receivers even if the upper and lower digital sidebands are unequal. While unequal sidebands will not provide the full benefit of increasing both sidebands, this will be a very important technique to maximize HD coverage for stations that cannot implement a full +10dB increase for both sidebands due to short spacing protection requirements.
Coverage Enhancement with Single- and Multiple-Frequency Networks
Additional ways to fill coverage gaps and extend coverage footprint are on-channel SFNs and two-frequency translators. These options are able to take advantage of the asymmetrical sideband techniques discussed previously.
As with their analog counterparts, when properly designed and deployed, coverage is improved without interference to other stations. However, the technical issues required by HD-only and hybrid FM + HD SFNs and translators differ substantially from those of analog FM.
Single-Frequency Networks
Perfect HD Radio reception can be maintained in areas where the difference between the relative strengths of the original digital and secondary digital signals on the same frequency is more than 4 dB. If there is less than 4dB difference in the relative strength, a 75uS guard interval requirement needs to be met for HD radio reception.
HD Radio transmitters used as gap fillers or SFNs must be located with antenna patterns so the guard interval is met in those areas where the primary and secondary (and/or tertiary) signals are all within 4dB of each other. The location of gap fillers that take advantage of terrain shielding and directional antennas can expand the area over which HD reception is possible, even outside of the 65uS guard interval.
Use of HD-only gap fillers is an actively considered option when there is insufficient ter¬rain shielding to protect the primary transmitter host analog FM signal from interference by the secondary FM analog signal radiated by a gap filler. Even with precise time and amplitude alignment of the signals, it is impossible to eliminate significant multipath distortions to the analog FM reception in areas where the two signals overlap and are equal in amplitude at the receiver.
It has been suggested that gap fillers that transmit the digital HD signal only — without the analog FM signal — can solve this problem. This strategy, however, creates a problem for analog FM reception near the digital-only gap filler because the strong digital signal will be far above the normal FM to HD ratio. This can cause digital-to-host analog FM interference in some analog FM receivers when the receiver is in close proximity to the satellite transmitter.
Approaches to SFN Gap Fillers
E2X (Exporter to Exgine protocol–the data stream that contains the HD Radio data for the exciter) Transport Stream to Exgine Modulator
Using multiple Exgine modulators, each site is fed a separate HD Radio data stream. HD-only gap fillers regenerate the digital modulation and error correction. The gap filler signals are time aligned with the host transmitter via GPS to maximize the guard interval within the pro¬tected area. This technique requires separate data transport for the analog and digital program and may require separate licensing for each HD Radio modulator.
— Tim Anderson, Manager, Strategic Market & Product Development, Radio, Harris Broadcast Communications
