Tuesday, October 30, 2012

TV whitespace vs. cellular power limit anomalies

In the previous post, I considered interference between cellular base stations and TV receivers. What about interference between cellular handsets and TV? Considering this case highlights striking contradictions between the low power allowed for TV whitespace devices and the high power the FCC proposes for cellular operation: 20 dBm for whitespace personal devices but 37 dBm for cellphones, in both cases with a 6MHz guard band.

Related posts:
Post-auction cellular interference into TVs? (29 Oct 2012)
The FCC's TV/cellular guard bands don't compute (3 Nov 2012)
The FCC NPRM [1] proposes a guard band between the cellular handsets (the uplink) and TVs, of 6-11 MHz depending on how the auction turns out. From the NPRM:

The Ofcom technical analysis of interference to DTV from cellular operations [2] did not study this case since uplinks do not adjoin TV broadcast in the European band plan. However, time-division duplex (TDD) cellular service is a good approximation, since it includes handset transmissions. In this case, the European band plan requires a 7 MHz band plan (Figure 2 from [2]), suggesting that a 6-11 MHz guard band in the US is probably adequate.

However, note that the FCC's white space rules allow personal/portable devices to operate at up to 100 milliwatts EIRP (20 dBm; 47 CFR 15.709 (2)) when there is a gap of at least 6 MHz to the nearest TV channel. On the other hand, the NPRM ([1] at para 194) proposes 3 watts ERP for cellular portables and mobiles beyond the 6 MHz guard band; that's 34.8 dBm ERP, i.e. 36.9 dBm EIRP.

20 dBm for whitespace personal devices but 37 dBm for cellphones - that doesn't seem to make sense, particularly since the unlicensed devices can be shut down using the whitespace database if there are interference problems, just like cellphones are under the control of a licensee. What's good for the goose should be good for the gander; will whitespace devices now be allowed 37 dBm of power, or will cellular handsets be reduced to 20 dBm?

Base station anomaly

In fact, this contradiction applies to base station operation too. "Fixed" whitespace devices are limited to a maximum of 36 dBm EIRP in 6 MHz (47 CFR 15.709 (1); that's 28.2 dBm per MHz), but may not operate adjacent to occupied TV channels; i.e. they must observe a 6 MHz guard band. On the other hand, the NPRM ([1] para 193) proposes an ERP limit on urban base stations of 60 dBm per MHz, that is EIRP of 62.15 dBm per MHz. So: 28 dBm/MHz for whitespace base stations, but 62 dBm/MHz for cellular.

These are huge differences; I must be missing something big...


[1] FCC, Expanding the Economic and Innovation Opportunities of Spectrum Through Incentive Auctions, Notice of Proposed Rulemaking, docket 12-268, released 2 October 2012 (pdf)

[2] Ofcom, Technical analysis of interference from mobile network base stations in the 800 MHz band to digital terrestrial television: Further modeling, Technical report, 23 February 2012 (pdf)

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