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"What's the frequency, Kenneth?" Interpreting your engineering screen. Part two.




by Andrew J. Shepherd
Sprint 4G Rollout Updates
Friday, May 10, 2013 - 12:35 PM MDT


Welcome back to S4GRU's continuing series focused on understanding many of the signal metrics displayed on your devices' engineering screens. If you missed part one a few weeks ago, that is a good place to start for background info.

Last time out, we covered 3GPP2 band class 1 PCS 1900 MHz, in which Sprint has long operated its CDMA2000 network, and 3GPP band 25 PCS 1900 MHz, in which Sprint is currently deploying its LTE network.

Today, let us begin with the last of Sprint's current native spectrum usage -- 3GPP2 band class 10 SMR 800 MHz. This is what 3GPP2 also calls the "Secondary 800 MHz band," and we will understand why when we finish up with band class 0 Cellular 850 MHz a bit later today.

First, take a look at the following CDMA1X engineering screenshot:




This handset is camped on Sprint's brand new band class 10 CDMA1X 800 overlay, which is replacing iDEN 800 and is currently available in select markets around the country.

Now, as we did last time, we can take into account the band class and carrier channel number, then use the appropriate formulas to calculate both uplink and downlink center frequencies:

     uplink center frequency (MHz) = 806 + (0.025 × carrier channel)

     downlink center frequency (MHz) = 851 + (0.025 × carrier channel)

In other words, the spacing in between potential carrier channel assignments in band class 10 is 0.025 MHz (or 25 kHz). This is due to the SMR 800 MHz band's legacy of dispatch and iDEN, both of which conform to 25 kHz channelization. And the band class 10 range of channel numbers extends from 0-719.

So, using our formulas, band class 10 carrier channel 476 in the included screenshot has an uplink center frequency of 817.9 MHz, a downlink center frequency of 862.9 MHz. This is the one and only band class 10 carrier channel that Sprint will employ across most of the country. In parts of the Southeast where SouthernLINC also operates in rebanded SMR 800 MHz spectrum, Sprint users will instead see band class 10 channel 526, which has uplink and downlink center frequencies of 819.15 MHz and 864.15 MHz, respectively, just as S4GRU detailed in an article a year ago.

As for band 26 LTE 800, well, that should be coming online in the next several months, but no devices are yet available. So, for both of those reasons, we cannot post any engineering screenshots. What we can anticipate, however, based on SMR 800 MHz spectrum constraints, is that Sprint's 5 MHz FDD LTE 800 carrier likely will be centered somewhere in the 821.1-821.5 MHz x 866.1-866.5 MHz ranges, translating to uplink and downlink EARFCN ranges of 26761-26765 and 8761-8765, respectively.

I will be out in the field with my spectrum analyzer in the coming months, ready to capture and publish a first peek at the LTE 800 carrier. And expect a follow up article on LTE 800 engineering later this year.

Now, let us conclude with a look at Sprint roaming service in 3GPP2 band class 0 Cellular 850 MHz. Or this is what 3GPP2 has traditionally referred to as the "800 MHz band." And that, as I piqued earlier, is why the "Secondary 800 MHz band" name comes into play for band class 10 SMR 800 MHz.

In more recent years, the "800 MHz" nomenclature has become problematic, as it makes distinguishing between band class 0 and band class 10 difficult for less informed users. For a good example of this, see the iPhone 4S tech specs, which mislead many into thinking that it supports Sprint's band class 10 CDMA1X 800 overlay. For this reason, I have long advocated using "Cellular 850 MHz" as distinct terminology.

That background aside, let us examine a CDMA1X engineering screen of a Sprint device roaming on VZW:




This handset is idling on channel assignment 425. Again, we can use the appropriate formulas to calculate both uplink and downlink center frequencies:

     uplink center frequency (MHz) = 825 + (0.03 × carrier channel)

     downlink center frequency (MHz) = 870 + (0.03 × carrier channel)

So, that VZW channel 425 is centered at 837.75 MHz x 882.75 MHz, which is toward the bottom of the Cellular B block license, as we will see in just a moment.

First, in the Cellular 850 MHz band, channelization is 0.03 MHz (or 30 kHz), as that dates back to the original analog AMPS standard, which used 30 kHz FM channels and got us all started on this cellularized wireless network journey.

Second, we encounter a complication with band class 0. The above formula works only for a subset of channel assignments, 1-799. For channel assignments 991-1023, we have to use slightly modified formulas:

     uplink center frequency (MHz) = 825 + [0.03 × (carrier channel − 1023)]

     downlink center frequency (MHz) = 870 + [0.03 × (carrier channel − 1023)]

The reason for this complication is complicated itself. When the FCC originally created the Cellular 850 MHz band plan in the 1980s, it was 825-845 MHz x 870-890 MHz, divided into two equal 10 MHz FDD (10 MHz x 10 MHz) licenses: Cellular A block (825-835 MHz x 870-880 MHz) and Cellular B block (835-845 MHz x 880-890 MHz). Each block consists of 333 AMPS channels, A block covering 1-333, B block running 334-666.

Not long after, the FCC expanded the Cellular 850 MHz band, but it could not do so by simply adding spectrum exclusively at the bottom or the top of the band plan. Because of spectrum constraints and equal license bandwidth, the FCC had to add a sliver at the bottom of the band plan and two at the top of the band plan. The additions became known as "A low," "A high," and "B high." See my band plan graphic below:


Since "A high" (1.5 MHz FDD) and "B high" (2.5 MHz FDD) continue as upper end extensions of the band plan, they follow the original center frequency formula, adding channels 667-799. "A low" (1 MHz FDD) tacked on at the bottom of the original plan is the anomaly. It requires its own center frequency formula and adds channels 991-1023. Also, note the missing channels 800-990. Those are a mystery, unbeknownst even to me.

Additionally, because it is only 1 MHz FDD, "A low" is not frequently used for CDMA2000 carrier channels, which are always 1.25 MHz FDD in bandwidth. So, many of the carrier channel assignments in "A low" are invalid, since they would cause the CDMA1X or EV-DO carrier to extend off the lower edge of the band. If "A low" is utilized, the only permissible channel assignments are 1013-1023, all of which cause the CDMA2000 carrier to extend into the original A block. So, if you ever encounter a band class 0 channel assignment in the 1013-1023 range, you have found something of a rare bird.

Well, that covers the relationships among bands, band classes, carrier channel assignments, EARFCNs, and center frequencies. Next time, we will turn our attention to another signal metric. I am thinking maybe SIDs and NIDs or PN offsets but have not decided yet. See you then...


Sources: 3GPP, 3GPP2, FCC

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You bet. I have decided on a topic and started data collection for the next installment in the article series. Look for it in about a week.



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Only one minor thing: In the VzW roaming calculation, my numbers differ from yours by .25 MHz (my numbers BOLD):


So, that VZW channel 425 is centered at 837.5 MHz (837.75 MHz) x 882.5 MHz (882.75 MHz),  which is toward the bottom of the Cellular B block license, as we will see in just a moment.


--> Can you check?


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Only one minor thing: In the VzW roaming calculation, my numbers differ from yours by .25 MHz (my numbers BOLD):


So, that VZW channel 425 is centered at 837.5 MHz (837.75 MHz) x 882.5 MHz (882.75 MHz),  which is toward the bottom of the Cellular B block license, as we will see in just a moment.


--> Can you check?



Hmm, the center frequency formula remains correct, but I must have had a calculator glitch or mental lapse when I crunched the numbers while writing the article.


Thanks, and good catch.  I have made the appropriate corrections to those two figures.



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