When the FCC issued the Report and Order (3.65 R&O<https://apps.fcc.gov/edocs_public/attachmatch/FCC-05-56A1.pdf>) for the 3650-3700 MHz band in March of 2005, it set power limits in a manner not widely understood by many in the WISP community, as it based the limits for fixed operations on a formula relating to power density. Some quickly read "1 watt" and assumed the limit was 36 dB EIRP, just as it is in the 5 GHz UNII rules now (that limit used to be only for the upper 5 GHz UNII, but was harmonized at the higher only recently). Even in the legacy ISM bands of 902-928 MHz, 2.4-2.5835 GHz and 5.725-5.850 GHz we are all familiar with that "1 watt" "36 dB" PMP EIRP limit (with the CPE side up or PTP links also allowing the "3:1" rule.
Well, in 3.65 MHz that's actually not the rule. In this case, the rule applies a power density qualifier whereby that 1 watt limit is PER megahertz of frequency. Here's the language from the R&O (page 19, paragraph 50): "Consequently, [for fixed operations] we believe that EIRP limits should be specified not simply as a maximum power, but rather in terms of power density (i.e., power per unit of occupied bandwidth).... For example, a system using a bandwidth of 25 megahertz may use the full 25 Watts peak EIRP, but a system using only 1 megahertz bandwidth may only use 1 watt peak EIRP; in either case, the power density is equivalent.... Therefore, we adopt a fixed station peak power density of 25 Watts EIRP in any 25 megahertz band." To make things even more clear, and to provide flexibility, the rule adds: "Furthermore, to promote additional flexibility in system design, any combination of transmitter output power and antenna gain will be permitted, so long as the peak 25 Watt/25 megahertz EIRP limit is not exceeded." To drive home the point so as to be crystal clear, the FCC adds a few footnotes: " 100 We note that, at frequency ranges above one-gigahertz, a power density measurement bandwidth of one megahertz would typically be specified. Consistent with that practice, and the intent of the rules adopted here, the maximum peak power density in any one-megahertz slice of spectrum in this band shall not exceed 1 Watt. 101 For example, the Wi-Max standard specifies various bandwidths. 102 For free space propagation, distance is proportional to the square of the distance or in terms of decibels distance doubles for each additional 6 dB of power..." So how does this translate into your world? Well, this means if you are using 10 MHz channels, as those using WiMAX are limited to using, you have a peak EIRP of 40 dB on a 10 MHz channel. For those using 20 MHz, as most are doing with Wi-Fi derivative gear or the largest channel size option of LTE, the EIRP limit is 43 dB. Let's noodle this out further looking at dBm per port: If your radio has a max power of 22 dBm per port, that's 25 dBm when used in a 2x2 MIMO configuration. If you are using a Wi-Fi derived system at 20 MHz, any antenna under 18 dBi means you'd be running BELOW your power maximum. For example, 120 degree antennas are commonly used (many WISPs think three 120 degree sectors is the best way to get 360 degree coverage, which is not always the case, e.g. we use three 65 degree antennas). 13 dBi is a common gain for a 120 degree sector. Used with a 2x2 MIMO with peak power of 25 dBm gets you 38 dB EIRP -- that's 5 dB LESS than your maximum allowed power, or more thoroughly understood, more than 1/2 your total allowed power! Conversely, if your system produces 30 dBm max per port, that's 33 dBm in 2x2 MIMO Mode and 36 dBm in 4x4 MIMO mode if that capability exists. The total limit is still 40 dB EIRP in a 10 MHz channel and 43 dB EIRP in a 20 MHz channel, but antenna flexibility is increased without compromising max EIRP. To be sure, in such a case the operator must configure the base station to reduce to power output of the radio to avoid violating the FCC (or IC in Canada) limits. So when it comes to fixed wireless broadband in 3.65 MHz, stop thinking about "1 watt" or "36 dB" EIRP, as that is only true if you were using 1 MHz channels. Instead, focus on 1 watt PER 1 MHz. Finally, if we really want to have fun, we can talk about this additional verbiage in the FCC R&O, but I'll leave that for another day: " 54. In that light, we conclude that transmitters installed at fixed locations should not be prohibited from using any particular type of antenna design. As a general requirement, the EIRP in any antenna beam must be limited to 25 Watts per 25 megahertz. However, transmitters using sectorized, scanning spot-beam, or other antenna types with multiple beam capability shall be required to limit their EIRP in any direction to no more than the limit we are adopting for fixed systems (i.e., 25 Watts per 25 megahertz). Thus, the aggregate power transmitted simultaneously on overlapping beams will have to be reduced such that the EIRP in the area of overlap does not exceed the limit for a single beam. In addition, to allow flexibility in deployment of advanced antenna systems, including sectorized and adaptive array systems, we will allow systems using these antennas to operate with an aggregate transmit output power transmitted simultaneously on all beams of up to 8 dB above the limit for an individual beam.115 Happy radiating, Patrick Leary National Sales Director | Telrad Networks Ltd. M 727.501.3735 | Skype pleary [cid:image004.png@01CFFCED.069DD750]<http://mkt2.us/TelrdNet> See us on [cid:image007.png@01CECEFE.8A880C70] <http://bit.ly/18nna4j> ************************************************************************************ This footnote confirms that this email message has been scanned by PineApp Mail-SeCure for the presence of malicious code, vandals & computer viruses. ************************************************************************************
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