The IBOC Power Increase, Part II; Other D/U Ratios, and a MP3 Mode Power Increase Formula

In Part I, we explained the origin and inner workings of the NPR Labs power increase formula.  The January 27, 2010, FCC Order provided a blanket four times IBOC power increase for most stations transmitting IBOC, increasing the IBOC sideband power to –14 dB with respect to the analog carrier, (dBc).  The FCC adopted the Formula, as proposed by NPR Labs and agreed to by iBiquity, to determine how much of a possible additional increase up to a maximum –10 dBc level would be allowed, depending upon the potential of interference to other stations(1).     

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].

Part II, as promised, shows the additional, higher D/U values for Hybrid MP1 mode vs. monophonic and stereo reception, and for Multicast MP3 mode vs. mono-of-stereo, monophonic, and stereo reception, and to show the modified Formula[MP3] that should have been required for proponents proposing MP3 mode transmission.

In Part I, it was shown how the 2.27 constant in the Formula is the inverse [U/D] of the slope of the desired-to-undesired (D/U) power ratio line formula, the ratio of analog signal power of the desired FM station, to the analog signal power of an undesired station transmitting MP1 mode Hybrid IBOC, found at the onset of interference to the analog signal.  The D/U ratio line formula shows the change in the D/U ratio as the IBOC insertion power level increases from –14 dBc to –10 dBc, and is:

D/U power ratio for onset of interferenceMP1 = – [(14 dB + (IBOC power in dBc)/2.27] dBc
For low received power levels near -60 dBm, indicative of monophonic reception of a stereo signal.

For an IBOC power of –14 dBc, the D/U power ratio for onset of interference MP1 = 0 dB. 

For an IBOC power of –10 dBc, the D/U power ratio for onset of interference MP1 = (14 –10)/2.27]dB , or 1.76 dB.

The slope of the desired-to-undesired (D/U) power ratio line formula for MP3:
Returning to Figure 37 of the AICCS Report, The D/U ratio for the onset of interference from (undesired) IBOC MP3 mode 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:
      
       SNR loss MP3 at –14 dBc = 5 dB.
       SNR loss MP3 at –10 dBc =  8 dB.

From Figure 37, we see that the slope of the green line is a straight line between –14 and –10 dBc, so the equation takes the form y = a + xb.  The slope is (3/4), or 0.75; since we will later use the inverse of the slope, the inverse of the slope b, or 1/b, is: (20-10)dBc)/(8-1.8 dB) = 4/3 = 1.333 dB/dBc. 

The variable x is: – (14 + IBOC power in dBc), and for the constant a, the SNR loss at the zero crossing, –14 dBc, is 5 dB.    

So the formula is:
D/U power ratio for onset of interferenceMP3 = 5 + [0.75 (14 + IBOC power in dBc)] dB
And it is applicable only between –14 dBc and –10 dBc.
   
Assembling the FormulaMP3:
To provide the same interference protection in MP3 mode as in MP1 mode, the MP3 version of the Formula, Formula MP3, 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 –5 –8.63 IBOC station F(50,10) dBu)), (note the addition of a 5 dB penalty not found in the MP1 Formula, the a constant in the D/U power ratio, which was zero for MP1), and the inverse slope term, (4/3) dBc/dB, to get:

Allowable IBOCMP3 power = –14 dBc  –((60 –5 –8.63 -IBOC station F(50,10) dBu) (4/3) dBc/dB.

Here we can see that in both the NPR Labs/FCC equation, and the suggested MP3 equation, what is actually being compared is the difference between the 60 dBu level of the victim station and the "equivalent average" 60 dBu F(50,50) level, equal to the F(50,10) level + 8.63 average fading ratio, of the proponent station.

To take this to the form of the Formula, we do a little math: The Allowable IBOC MP3
power = [(60 –5 –8.63) – (IBOC station F(50,10) dBu)) (1.333) – 14] dBc;
=[1.333(60 –(IBOC station F(50,10) dBu) –1.333(5+8.63)–14] dBc;

to get the Formula MP3:
Allowable IBOCMP3 power = [1.333(60 –(IBOC station F(50,10) dBu) – 32.17] dBc.

An easier to understand form of the Formula is:

Allowable IBOC power = [1.333(46.37 –(IBOC station F(50,10) dBu)  –14] dBc.

Solving this to get the FCC chart in paragraph 20 of the Order, with a column added for MP3 mode:

Allowable IBOC power = –14 dBc =  [1.333(60– (IBOC station F(50,10) dBu)) – 32.17].
  (32.17 dB– 14 dBc)/1.33 = 60 – (IBOC station F(50,10) dBu)
        IBOC station F(50,10) dBu= 60 –(18.17 dB/1.333) = 46.36 dBu 

and at a maximum of: –10 dBc = [1.333(60 – IBOC station F(50,10) dBu)) – 32.17].
  (32.17 dB– 10dBc)/1.333 = 60 – (IBOC station F(50,10) dBu)
        IBOC station F(50,10) dBu=  60 dBu – 16.63 dB = 43.36 dBu     
 
Using the chart in the FCC Order provides a slight advantage over the Formula calculation.   From the calculated results, as adjusted in the left side of the chart:

FCC, from the Formula:         Added here for MP3 mode:   
Proponent Analog                  Proponent Analog                      Maximum
F(50,10) Field Strength          F(50,10) Field Strength              Permissible
at Protected Analog                at Protected Analog                   FM Digital
60 dBu F(50,50) Contour        60 dBu F(50,50) Contour           ERP
[for MP1 mode IBOC]             [for MP3 mode IBOC]

51.2 dBu and above               46.3 dBu and above                -14 dBc
50.7 dBu to 51.1 dBu             45.4 dBu to 46.2 dBu               -13 dBc
50.3 dBu to 50.6 dBu             44.7 dBu to 45.3 dBu               -12 dBc
49.6 dBu to 50.2 dBu             43.3 dBu to 44.6 dBu               -11 dBc
49.5 dBu or less                     43.2 dBu or less                      -10 dBc

So the protection needs to be 5 to 6 dB stronger from a station operating in MP3 mode, as compared to MP1 mode, based on the NPR Labs test results.

SNR Losses for Higher Signal Received Levels and MP1 and MP3 modes.
In the same manner, we can derive the D/U formulas for MP1 mode for fair-to-good stereo reception, at –45 dBm (field strength approximately 65 dBu) from Figure 38 of the AICCS Report, and for excellent stereo analog reception at –30 dBm (field strength approximately 80 dBu), from Figure 39 of the AICCS Report:  

At –45 dBm receiver signal input level,
        SNR loss MP1 at –14 dBc = 2.9 dB; 
        SNR loss MP1 at –10 dBc = 7.9 dB.
SNR loss MP3 at –14 dBc = 14.2 dB.
         SNR loss MP3 at –10 dBc = 17.9 dB.

D/U power ratio at interference onsetMP1,-45dBm= 2.9+[1.25 (14 + IBOC power, dBc)] dB
D/U power ratio at interference onsetMP3, -45dBm= 14.2+[.925 (14 + IBOC power, dBc)] dB
Applicable only between –14 dBc and –10 dBc.
 
And from the AICCS Report, Figure39, at –30 dBm receiver signal input level,
         SNR loss MP1 at –14 dBc = 9.1 dB.
         SNR loss MP1 at –10 dBc = 14.1 dB.
SNR loss MP3 at –14 dBc = 13 dB.
         SNR loss MP3 at –10 dBc = 18 dB.
 
D/U power ratio at interference onset MP1,-30dBm=9.1+[1.25(14 + IBOC power, dBc)] dB
D/U power ratio at interference onset MP3, -30dBm= 13+[1.25(14 + IBOC power, dBc)] dB
Applicable only between –14 dBc and –10 dBc.

Conclusions:
Studying the AICCS Figures 37 to 39, we can see how the wider bandwidth of the MP3 IBOC sidebands provides 5 to 6 dB higher SNR loss to the analog FM receiver at a received level of –60 dBm, increasing to a 10 to 11 dB higher SNR loss to the analog FM receiver at a received level of –45 dBm.   At a received level of –30, as the receiver bandwidth widens further, the interference from MP3 mode starts to plateau as the interference level from the MP1 mode partially catches up to that from the MP3 mode.

Note the increase in interference, shown as SNR loss, for analog reception against MP1 mode IBOC transmission as the received power increases: from no SNR loss at –14 dBc at a received power of –60 dBm, to 2.9 dB SNR loss at –45 dBm, and up to 9.1 SNR dB at –30 dBm. The SNR loss vs. received power results provide a curve, not a straight line; there is a more rapid increase at higher received power level, between –45 dBm and –30 dBm, than there is between –60 and –45 dBm.

For the MP3 mode, the SNR curve has a different shape.  The SNR loss starts out higher, at 5 dB for –14 dBc IBOC and a received power of –60 dBm.  It increases to 14.2 dB at a received power of –45 dBm, and then plateaus, dropping to 13 dB at a received power of –30 dBm. 

For MP1 mode, for a received power increase of  15 dB, we have an  increased SNR loss of  2.9 dB and for the next additional received power increase of 15 dB,  a total increase of  30 dB, we have an increased SNR loss of  9.1 dB. 

For MP3 mode, for the received power increase of  15 dB between a received level of –30 dBm and –45 dBm,  we have an increase in interference, or increased SNR loss, of  9.2 dB, so against MP3 mode, there is only a 5.8 dB increase in SNR for a 15 dB increase in received power.  The race to stay ahead of the increasing interference is, however, won at the higher received power levels between –45 and –30 dBm.

Swiss-Cheese Reception:
So especially in MP3 mode, for analog FM reception, if a terrain obstacle or building reduces the desired signal level behind it, even within the protected contour, the undesired IBOC interferer can create a Swiss-cheese like “reception hole” on the interfering station’s side of the hill or building, where one would only have had a fade-to-mono occur without the interference.  Where a “reception hole” already exists, it can make an existing reception hole bigger.   A comprehensive market analysis, using an updated, location-specific Longley-Rice prediction program such as the Givens and Bell ITWOM model, and comparing signals from all stations in the market, can show the victim station where these “reception holes” are likely to occur, so they can prepare a petition to deny, and measure the reception at these locations before and after the proponent increases power and/or upgrades from MP1 to MP3 mode. 

Comments:
The commuter who is used to receiving a station all the way to his assigned spot in the parking lot behind the skyscraper he works in, will find the station unlistenable or missing the day after the interfering station increases its IBOC signal power and upgrades to MP3 mode and multicast transmission.  The commuter will only know that his reception has worsened or disappeared; he is unlikely to understand why, or know who to complain to.  But he will never get his reception back.  It is up to the victim station to understand why this will happen, to obtain an analysis of the changes in reception that will occur, and to protect its audience by defending the protected contour against interference from a mode upgrade from MP1 to MP3 accompanying a proponent’s application to increase IBOC insertion power to begin or improve multicasting reception.

(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”.

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

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