Could broadcasters increase IBOC injection levels higher than -20dBc by some means other than purchasing more transmitter power? Nautel has developed a technology called HD Power Boost that uses an intelligent peak to average power ratio (PAPR) technique to address this issue. This technology squeezes more hybrid power from a given transmitter and also achieves increased hybrid-mode efficiency. It wouldn't be fair to expect that -10dBc will be achievable with this technology alone, but this patent-pending technique is currently in development and is being tested on-air at Nautel's own experimental radio station. Initial release is targeted for the NV series products, which provide up to 44kW of solid-state power.
IBOC uses orthogonal frequency division multiplexing (OFDM) to broadcast the HD Radio signal. This scheme uses multiple simultaneously transmitted data carriers, which can provide a more robust signal in multi-path environments. However, these multiple carriers require highly linear signal amplification to minimize carrier intermodulation and ensure spectral compliance. The amplifier also requires significant input back-off to handle large power peaks inherent in the IBOC signal. The addition of more carriers drops the average power of the signal by several decibels while maintaining the same power peaks. This can be expressed as the peak-to-average-power ratio (PAPR). With FM IBOC's minimum of 382 carriers, the PAPR can reach 12dB; this would require broadcasters to add 12dB of average transmitter power to handle the load of HD Radio.
Figure 1. FM + IBOC instantaneous signal envelope power fluctuations at 1W average power
Ibiquity Digital provides an optional PAPR reduction algorithm as part of the standard IBOC modulator, which reduces signal peaks from 12dB to 8dB. It has been found that peaks can be reduced further by driving the signal into compression. Depending on the transmitter, this could yield a final PAPR of 5.5dB. What this means is that to achieve a 3kW digital transmitter power output you must install a transmitter capable of delivering 10.6kW of instantaneous power. Without standard PAPR reduction, a much larger transmitter would need to be installed to handle the signal peaks, even with considerable compression.
With standard PAPR reduction, the algorithm inputs a single-modulated IBOC symbol at a time. Peaks are detected by computing the absolute value of each sample point and comparing it against a pre-defined threshold value. Once a peak is identified, the standard PAPR reduction algorithm clips the peak to a given threshold while maintaining its instantaneous phase value. This clipping introduces distortion into the signal, which must be corrected. To do so, the standard PAPR reduction algorithm demodulates the distorted IBOC signal and digitally processes the signal to basically rebuild it, but with only a partial restoration of the peak. Additionally, a mask is applied to the signal to suppress errors in the non-carrier frequency bins. This allows varying amounts of noise to subside in the IBOC signal without violating the spectral emission mask.
Figure 2. Standard PAPR reduction
The recent proposals to the FCC that suggest an increase of up to 10dB in digital carriers has generated a great deal of interest among broadcasters. Increasing digital carriers by 10dB only increases the average IBOC signal power from 1 percent to 10 percent of the transmitted FM signal. However, this can have considerable implications for the transmitter, which is limited by its peak-power capability and not the average power capability. Figure 1 (above) shows a power envelope comparison of an analog-modulated FM signal, a digital-only signal, and a hybrid signal at -10dBc and -20dBc injection levels all at the same average power of 1W. While at -20dBc about 40 percent of transmitter overhead was sufficient, with -10dBc carriers we now require more than 160 percent of transmitter power and cannot use amplifier compression to the same degree as is possible with the -10dBc signal. Almost all of the signal must now fall into a linear amplification region, so to produce 8kW of FM power requires a transmitter capable of handling 22kW for -10dBc hybrid operation. An 11kW transmitter would suffice for -20dBc.
Every FM IBOC system in use today uses the standard PAPR reduction scheme developed by Ibiquity Digital. The scheme Nautel uses is almost identical to the standard scheme and transmits an IBOC signal of equal quality to HD Radio receivers while more effectively reducing peak power requirements for a hybrid, low-level combined transmitter.
The Power Boost method
The major difference between Nautel's PAPR reduction scheme and the standard method is a difference in peak detection. Figures 2 and 3 depict a complex plane where the X axis reflects the baseband signal's real (phase - I) component and the Y axis represents the signal's imaginary (quadrature - Q) component. Figure 2 illustrates standard PAPR reduction, which only operates on the digital signal and then adds the result to the analog signal. Figure 3 shows Nautel's method of taking the analog signal into account in detecting a peak.
Figure 3. Nautel's Power Boost applied to PAPR reduction
The output of the FM modulation process produces a constant envelope signal with varying phase. This signal is represented by the white circle. Standard PAPR can only detect a peak based on the digital signal and it does not know whether this peak adds constructively or destructively to the analog signal. This means that this PAPR reduction method unnecessarily performs potentially large peak reductions on destructively added peaks — when the reductions are not needed.
The Nautel PAPR reduction method offers a different approach for determining the correction vector. In Figure 3 the analog vector A is first added to the digital vector D. The resultant hybrid vector H is then limited to the maximum desired peak threshold. Only if the digital signal adds constructively to the analog signal is a large correction required. If the vector addition falls close to the maximum desired peak, a smaller correction is applied and no correction is needed if the result is below the maximum desired peak. By introducing a lower amount of correction, our algorithm can achieve the same maximum desired peak value with a lower degree of distortion in the original signal. This allows us to reduce the signal's peaks further compared to the standard PAPR reduction method.
By not simply clipping the hybrid signal, but keeping the correction vector C separate and only applying it to the digital component, allows us to use the established correction techniques of standard PAPR reduction. It also uses the FM signal only during the clipping decision process and therefore maintains the FM portion of the signal until it is finally added to the digital component to form the hybrid signal stream.
Our initial tests show an encouraging reduction in the PAPR from 4.51dB down to 3.19dB at an IBOC injection ratio of -10dBc. This translates to a significant reduction in peak transmitter power requirements. For a station using a 8kW TPO and Nautel's PAPR reduction technique the transmitter would only need to handle peaks of 16.9kW rather than the 22.6kW peaks using standard PAPR reduction. These gains help make it more affordable for broadcasters to adopt the higher injection levels.