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

WiWavelength

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by Andrew J. Shepherd
Sprint 4G Rollout Updates
Monday, February 3, 2014 - 8:47 AM MST

Yes, it has been a while, but welcome to S4GRU's third installment in an ongoing series about the many signal metrics available on those engineering screens hidden inside most mobile devices. Both part one and part two date back to last spring, so check those out if you have not already or if you need a refresher.

Part three has been a long time coming mostly for lack of a really relevant topic. But a question was just recently posed in The Forums here at S4GRU about EARFCNs and center frequencies for band 41 TD-LTE 2600. Previously, we covered that 3GPP relationship for band 25 LTE 1900 and touched upon it for band 26 LTE 800, but when we did so, band 41 had not yet made its domestic debut. So, now that band 41 -- christened Sprint Spark -- is being overlaid on Clearwire WiMAX sites in the top 100 markets and tri band LTE handsets are finding their way into more and more Sprint users' hands, it is due time for an educational look at those 20 MHz TDD carriers being deployed across the massive BRS/EBS 2600 MHz band.

First, let us take a look at the BRS/EBS band plan itself. Both it and band 41 encompass 2496-2690 MHz for a total of 194 MHz. The BRS spectrum is licensed -- mostly but not entirely in every market to Sprint subsidiaries. The EBS spectrum is also licensed but to educational institutions, which may then choose to lease the spectrum to commercial entities. So, even though band 41 is maximally 194 MHz wide, Sprint does not necessarily control all of that spectrum. And some of that spectrum -- such as the EBS J block and BRS/EBS K block -- is not intended for broadband uses. In other words, contiguity is periodically interrupted. Plus, WiMAX carriers still occupy much of that BRS/EBS spectrum. All told, band 41 in the US is not quite the huge blank slate that some make it out to be for Sprint to deploy 20 MHz TDD carriers.

For reference, see the BRS/EBS band plan:

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Next, we will examine a couple of band 41 engineering screenshots drawn from The Forums:

30tta42.png

Just as we did for band 25 in part one of this series, we can extract the channel numbers (i.e. EARFCNs) and enter them into an equation to calculate the band 41 center frequencies:

uplink/downlink center frequency (MHz) = 2496 + [0.1 × (EARFCN - 39650)]

Because this is TDD, not FDD, we need to use only the "DL" channel number. In TDD, there are no separate frequencies for uplink and downlink. The LG screenshot on the left properly indicates the same EARFCN for both uplink and downlink. But good old Samsung "enginerring" on the right registers a different channel for the uplink, EARFCN 58978, a number which is an invalid value. So, when working with TDD, disregard any spurious "UL" channel number.

To finish up our calculations, the range for band 41 EARFCNs is 39650-41589, so EARFCN 39991 is toward the low end of the the band, equating to a center frequency of 2530.1 MHz. And EARFCN 40978 comes out to a center frequency of 2628.8 MHz. Separated by nearly 100 MHz, the former is in the lower EBS segment, while the latter is in the contiguous BRS segment, as depicted in the aforementioned band plan graphic.

Now, that 20 MHz TDD carrier at EARFCN 40978 is the one that we have documented most commonly across Sprint Spark markets. This was not surprising, since it is deployed in the up to 55.5 MHz of contiguous BRS spectrum that Sprint is licensed, not EBS spectrum that Sprint just leases. That said, we are seeing more and more reports of other EARFCNs, such as EARFCN 39991 detailed above. In other words, the band 41 EARFCN -- unlike the one and only PCS G block band 25 EARFCN -- can vary from market to market because of differences in spectrum licensing/leasing and remaining WiMAX carriers.

Sprint's ultimate plan is to deploy multiple 20 MHz TDD carriers per market, putting it in an enviable position for satisfying the public's rapidly growing appetite for mobile data. However, do not misinterpret the multitude of current EARFCNs. We have no evidence to this point that the various EARFCNs indicate multiple 20 MHz TDD carriers in the same market. That is coming but probably will not be widespread prior to the WiMAX sunset slated for no earlier than 2015.

In conclusion, S4GRU has created a tracking thread for the various band 41 EARFCNs as they pop up from market to market. Additionally, in our DL Center, we have made available a comprehensive WiMAX/TD-LTE carrier bandwidth and center frequency spreadsheet (screenshot below) that is continually updated as new EARFCNs get reported. If you are interested, we hope that many of you will continue to help us "crowdsource" this band 41 data so that we can get a clearer picture on Sprint Spark and BRS/EBS spectrum utilization.

 

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Sources: 3GPP, FCC

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