The IBOC Power Increase, Part I; Understanding the Power Increase Formula

Sid Shumate

On January 27, 2010, the FCC adopted changes in the FM digital audio broadcasting rules to allow a blanket 6 dB, or four times, increase in IBOC transmitted sideband power over the initially authorized 1% insertion level, for most FM IBOC stations.  In addition, the FCC will allow up to a 10 dB, or ten times, increase depending upon the potential of interference to other stations(1).   The FCC adopted a formula, proposed by NPR Labs and agreed to by iBiquity; to determine how much additional increase beyond 6 dB would be allowed.  

This formula, (Formula), presented to the FCC in November of 2009 by NPR Labs in: “Report to the FCC on the Advanced IBOC Coverage and Compatibility Study” (“AICCS Report”), is:

 Allowable IBOC power =  [2.27*(60-(IBOC station F(50,10) dBm))-33.6].

Note that this is the version in the AICCS Project Report, Appendix J, "Procedures for All Stations".   The version in the FCC Order, paragraph 12(ii) has a typo;  it is missing the “(“ in front of the word IBOC, rendering it mathematically nonsensical. 

The AICCS Project Report does not provide a full explanation of the ties between the results in the report, and the origins of the Formula.  We will attempt to fill in the gaps here.

The correct version of this Formula is designed to work within the FCC’s historic system of interference analysis for protected contours, and is to be applied only with regard to first-adjacent channel stations that the IBOC station could interfere with.  The procedure has been simplified to consider only a 60 dBu protected service contour for the victims, the potentially affected first adjacent stations’ analog reception.  The calculation of the victim’s 60 dBu protected contour is to be done using the victim’s licensed analog facilities and the standard FCC contour prediction methodology.

Therefore, we are considering the interference of either the upper or lower IBOC sideband of the station requesting a power increase (proponent) with the analog reception of the victim stations, the stations one channel above or below the proponent’s channel, as one of the proponent’s IBOC sidebands overlaps the analog bandwidth of the victim.  The FCC method limits the maximum IBOC increase based on the highest (50,10) field strength of the proponent to be found along the victim’s (50,50) contour edge.     

The FCC, in the Order at paragraph 20, provides a chart of the results of the Formula, so a calculation is not necessary.  This chart is:

Proponent Analog F(50,10) Field Strength at       Maximum Permissible
Protected Analog 60 dBu F(50,50) Contour          FM Digital ERP
 
  51.2 dBu and above                                            -14 dBc
  50.7 dBu to 51.1 dBu                                          -13 dBc
  50.3 dBu to 50.6 dBu                                          -12 dBc
  49.6 dBu to 50.2 dBu                                          -11 dBc
  49.5 dBu or less                                                 -10 dBc

It is interesting to note that only a 1.76 dB difference (51.2 – 49.5) in the interfering field strength level makes the difference between getting the blanket 6 dB increase and a full 10 dB increase!

To understand the Formula, it helps to start by solving it for IBOC insertion powers of –14 dBc and – 10 dBc.  The allowable IBOC power is stated in dBc, or dB below [analog] carrier.

Allowable IBOC power = –14 dBc =  [2.27*(60 – (IBOC station F(50,10) dBu)) – 33.6].
  (33.6 dB– 14 dBc)/2.27 = 60 – (IBOC station F(50,10) dBu)
        IBOC station F(50,10) dBu = 60 – 19.6 dB/2.27) = 51.37 dBu 

and at a maximum of: –10 dBc = [2.27*(60 – IBOC station F(50,10) dBu)) – 33.6].
  (33.6 dB – 10dBc)/2.27 = 60 – (IBOC station F(50,10) dBu)
        IBOC station F(50,10) dBu =  60 dBu – 10.3964 dB = 49.60 dBu     
 
So using the chart in the FCC Order provides a 0.1 dB advantage over the Formula calculation.

Taking the the Formula apart, starting with the (60 – (IBOC station F(50,10) dBu) term, we see that this term is the difference between the victim station's 60 dBu contour and the proponent’s F(50,10) signal.  The F(L,T) terminology refers to the field strength exceeded at L percent of the locations, for T percent of the time.  Here we are comparing the victim’s 60 dBu protected contour field strength taken from the F(50,50) curve, to the proponent’s field strength taken from the F(50,10) curve.

The FCC F(L,T) curves used here are from FCC Report No. R-6602, circa 1966.  Report R-6602 builds on a concept found in the work of John Egli, published in 1957(2).  Egli derived, from FCC reception data collected by Harry Fine, a Formula and set of average curves for the power received at half of the locations for a given frequency and distance from the transmitter for line of sight reception, and a set of variation curves that could be added or subtracted from the 50% of location curves for other percentages of locations. 

Egli stated in his paper that his equations are only valid for “those geographical areas which are similar to plane earth, such as relatively short over-water and very flat barren land paths.”   It should also be noted that Egli’s work only determines the F(L,) percentage of location, and does not have anything to do with determining the F(,T) percentage fade margins. 

Ten years later, in Report R-6602, the FCC retained only the concept of the master curve and variation charts, derived them from the FCC’s reception data, extended the charts to include beyond the line of sight and fade margins, and combined the lot to create the FCC(L,T) curves found in the FCC part 73 rules.   Therefore, the FCC (50,50) and (50,10) charts are empirically (test data only) based charts, based on curves regressively line-fitted to the data, and one should no longer refer to Egli’s formulas, especially when working in the beyond-the-horizon range.

The difference between F(50,50) and F(50,10) is the fading ratio, which we can get from Figure 10 of the R-6602 report.    R-6602 notes, on page 15, that they “vary both with distance and antenna height, as shown in Figure 10”, and were empirically derived.  In Figure 10, for all transmitter heights, the fading ratios plateau at a maximum of 10.8 dB.  For a transmitter height of 500 feet, the plateau is between 80 and 120 miles from the transmitter.  PBS Labs made a study of the average distance from transmitters to victim service areas, and then stated that, working from R-6602’s Figure 10, “a fading ratio of 8 dB is appropriate”(3).  In fact, the Formula is based on an 8.63 dB fading ratio, as from the FCC chart for allowing an increase to –14 dBc, we get: 60 – 8.63 dB = 51.37 dB.   This provided the basis for the allowable power increase up to –14 dBc.

D/U value(s) for interference of IBOC into analog reception.
How much interference does a 1% (-20 dBc) IBOC insertion level create into an adjacent channel analog channel?  In 2001-2002, the Advanced Television Test Center (ATTC) performed a series of initial laboratory tests on some of the best analog receivers(4).  For the two auto receivers tested, a Delphi OEM receiver (Delphi builds the receivers for GM products, including Chevy), and a Pioneer aftermarket automobile receiver, the D/U ratio at which the received audio quality deteriorated to the iBiquity-determined 30 dB WQPSNR tune-out point occurred at an average 3 dB D/U ratio.  After later subjective listening field tests produced results that indicated a lower level, a follow-up report(5) noted that the lab tests were performed at a strong -47.5 dBm receiver input signal level, for which stereo analog reception is expected.  The majority of the field test data was from areas that received lower signal levels at which the analog receivers had switched to monophonic reception.  In monophonic reception, the receiver narrows the IF filter bandwidth to the 150 kHz monophonic bandwidth, or, if a stereo pilot is found, to the even narrower 100 kHz monophonic bandwidth of a stereo transmission, to reduce the received noise.  This lowers the D/U ratio required to achieve a listenable signal in weak signal areas versus the interfering IBOC sidebands, including near the protected contour.  However, additional receiver laboratory tests were not ordered to find the variation from a 3 dB D/U SNR ratio that occurs for weak signal reception.

That 1.76 dB difference:
More extensive tests of more recent receivers by NPR Labs, performed for the DRCIA report, but not published until Nov. 2009 in the AICCS report, section 7.5, show that the IBOC interference effect to 1st adjacent analog reception varies with the signal input level to the receiver.  The interference effect also varies with the mode of operation of the IBOC sidebands; the wider sidebands of the MP3 mode also provide a significantly higher interference earlier on as the signal levels increase.   From the analysis of subjective listening tests shown in Figure 37 of the AICCS report, at the low 60 dBu signal levels expected at the protected contour, roughly equivalent to a –50 dBm receiver input level and bandwidth-of-stereo-monophonic-signal IF, and for MP1 IBOC sideband mode, the interference penalty is zero until the IBOC insertion level reaches –14 dBc.  The interference penalty between –14 dBc and –10 dBc rises from zero to 1.76 dB for MP1 mode.  MP1 mode is normally used for Hybrid-only, single channel IBOC broadcasting.  It appears that, due to the fact that only approximately 50% of the IBOC stations are currently multicasting (for which MP3 mode is appropriate), the power increase equation only considers the absolute very best case of only a 1.76 dB increase in interference between –14 dBc and –10 dBc at weak signal reception levels.  This is therefore, the source of the 1.76 dB difference in the FCC IBOC power increase chart above.

The 2.27 multiplier constant:
The D/U ratio for the onset of interference from (undesired) IBOC sidebands into the (desired) analog FM reception, as determined in the AICCS Report, Section 5, for the special case of the low received power levels found for monophonic reception of a transmitted stereo FM analog signal, when comparing the analog carrier power of the two signals, is 14 dB less the IBOC insertion level in dBc, or:

 D/U ratio for onset of interference at –14 dBc is: 14 dB – 14dBc = 0 dB.

This D/U ratio increases at a rate of 1.76/4 dB/dBc, or  .44 dB, for each dBc of IBOC power increase above 14 dBc.  The allowable IBOC power can be increased 4/1.76 dBc/dB = 2.27 dBc for each dB of field strength that the applicant station’s maximum F(50,10) field strength, plus the F(L,10) fade margin, is below 60 dBu, at any point on the victim’s 60 dBu service contour line.   

This slope can more accurately seen and read on Figure 28 of the AICCS report as the slope of the “Wide Space Allowance” line; an increase of 1.76 dB in the “F(50,50) – F(F50,10) D/U Radio (dB)”, measured as 8.62 dB at the –14 dBc line, up to 10.38 dB at the –10 dBc line at the top.   There is an IBOC insertion level increase of 4 dBc for this field power increase of 1.76 dB, for a slope of  (4 dBc/1.76) dB, or 2.27 dBc/dB.  The 2.27 multiplier in the Formula comes from the onset of interference to analog reception D/U ratio, adjusted to be relative to the F(50,50) analog power levels of the two stations.

Assembling the Formula:
For the special case of the average transmitter height and the average distance from the applicant’s transmitter to the victim’s F(50,50) 60 dBu service contour,  the fading ratio is 8.63 dB; and the IBOC station F(50,10) dBu =  F(50,50) dBu – fading ratio (in dB):

The Formula starts with a straight-line form, y = a + xb, where a is a constant, x is a variable, and b is the slope value.  The constant is the –14 dBc that results if the x variable cancels out and the result is a horizontal line at -14 dBc.  Add the variable x term, –((60 –8.63 IBOC station F(50,10) dBu), and the slope term, (4/1.76) dBc/dB, to get:

Allowable IBOC power = –14 dBc  –((60 –8.63 IBOC station F(50,10) dBu)) (4/1.76) dBc/dB.

To prove that this is the Formula, we do a little math:
Allowable IBOC power = [(60 – 8.63 – (IBOC station F(50,10) dBu)) (2.27) – 14] dBc;
                              = [2.27(60 –(IBOC station F(50,10) dBu)) –2.27( 8.63)–14] dBc;
to get the Formula:  = [2.27(60 –(IBOC station F(50,10) dBu) – 33.6] dBc.

An easier to understand form of the Formula is:

Allowable IBOC power = [2.27(51.37 –(IBOC station F(50,10) dBu)  –14] dBc.

Comments:
One of the main reasons put forth to justify a power increase was to provide more reliable multicast reception, especially indoors.  Therefore, the power increases are intended and expected to increase the number of IBOC multicasters operating in the MP3 mode. 

The interference penalties from Figure 37, for MP3 mode at low signal reception levels, are significantly higher than for MP1 mode; the interference penalty at –20 dBc is 1.7 dB, and rises to 8 dB for –10 dBc IBOC.  I therefore respectfully disagree with the FCC’s comment in paragraph 19 of the Order, and note that the Formula is, instead, significantly under predictive of the potential for interference from the expansion of MP3 multicast mode operation.  The maximum, not minimum, of these two sets of interference penalties, or at least a weighted average of the two, should have been considered for the Formula, or, even better, the FCC should require the reporting of the intended HD Radio™ mode of transmission in the application to increase power, add the mode of IBOC transmission to the CDBS database, and require the use of an appropriately modified version of the Formula for a proponent proposing MP3 mode IBOC transmission.   NPR Labs would probably not have negotiated iBiquity’s approval if they had recommended the second formula for MP3 mode. No formal comment period was opened after the  Nov., 2009 reveal of the Formula in the AICCS report Appendix J, and no one submitted any ex parte suggestion to this effect after the publishing of the Formula and before the issuance of the Order.  So the FCC will probably just fix the typo.

The basis for the Formula, only applies for the “very best case” of MP1 Hybrid-only mode IBOC interference into monophonic reception of a station transmitting stereo, where the better receivers, such as the Chevy (Delphi OEM) auto receiver used for the NPR Labs testing, have narrowed down the final IF filters, either by switching ceramic filters, or by adaptive filtering, gradually narrowing a set of software based digital Finite Impulse Response (FIR) filters after the signal has been digitized, to process only the 100 kHz bandwidth of the L+R monophonic signal of the stereo transmission in weak signal areas.  The Formula, in its current form, is therefore not appropriate to apply against reception of stations that transmit in monophonic with a 150 kHz bandwidth signal, such as news-talk WTOP-FM and its satellite stations in and near Washington, DC, nor does it necessarily apply to stereo reception from the new, sophisticated generation of FM receivers now on the market, an example being the Sony XDR-F1HD home HD Radio™ tuner, which uses the adaptive noise reduction circuits described in U. S. patent 7,110,549 and 7,292,694 to produce a 50 dB quieting sensitivity for both mono and stereo of 13.5 dBf, (–106.5 dBm).  The new technology in this receiver offers a 20 dB improvement in stereo over conventional consumer receivers, so this Sony receiver does not have to, and does not, switch from stereo to mono reception at low signal levels(6).   

Conclusions:
The Formula is a good compromise, in that it provides a relatively simple “bright line” yes/no answer that allows stations to proceed with needed, and now authorized, IBOC power increases to –14 dBc, and perhaps to –10 dBc.  The –10 dBc level is where the original iBiquity design concept appears to have intended Hybrid mode to be.  It works toward providing enough power for reliable multicast reception.   Stations with directional antennas, including beam tilt and mechanical tilt that reduces interference toward the victim station(s), and terrain shielding advantages, can make a showing to allow a higher level if the simple Formula calculation initially does not allow.

For the victim station(s), the Formula may not protect well enough.  I have noted its applicability limitations, and why it is generous in allowing power increases.  It is the nature of receivers to be more subject to reception interference from MP3 mode multicast transmissions, when receiving stereo, and when receiving true monaural-only transmission.  These are good reasons for victim stations to have their consulting engineers analyze and possibly develop a defense in order to petition to deny a neighbor stations’ applications for higher than –14 dBc power.  

Next:
In Part II, I will show the higher D/U values for Hybrid MP1 mode vs. stereo reception, and for Multicast MP3 mode vs. mono-of-stereo and stereo reception. This will provide the background to show why significant Swiss-cheese spot interference can occur well inside of the protected contour.   And I will show the modified Formula[MP3] that should be used for proponents proposing MP3 mode transmission.

Permission is granted to RBR-TVBR to publish this article, in its online and print editions, subject to draft review by the author, Sid Shumate.  All other rights reserved. © March, 2009 Givens & Bell, Inc. 

1 “Order”, DA 10-208, adopted 1/27/2010, released 1/29/2010, FCC MM Docket No. 99-325, “Digital Audio Broadcasting Systems And their Impact on the Terrestrial Radio Broadcast Service”.
2 “Radio Propagation Above 40 MC Over Irregular Terrain”, John J. Egli, Proc. IRE, vol. 45, No. 10, Oct. 1957, pp. 1383-1391.  Available from, and © the IEEE Xplore online library.
3 AICCS report, Section 5, page 30.
4 “Digital Audio Broadcasting:  Analog Main Channel Compatibility and Digital Performance of the iBiquity Digital IBOC System in the FM Band: Summary of Test Results” by Advanced Television Technology Center (ATTC) Document #01-13 Revision 1.1 July 31, 2001.
5 “Appendix H – Discussion of Differences Between Laboratory and Field Subjective Evaluation Results, DAB Subcommittee Evaluation of the iBiquity Digital Corporation IBOC System, Part 1 –FM IBOC”  presented by CEA and NAB before the National Radio Systems Committee, 2002.
6 “Sony XDR-F1HD”, an in-depth technical review by Brian Beezley, published online at http://ham-radio.com/k6sti/xdr-f1hd.htm, and “Sony’s Tuner Rich in Features”, Ira A. Wilner, RadioWorld, 12/18/2008, www.rwonline.com/article/71710.

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