Monthly Archives: January 2020
In the US, the Federal Communications Commission (FCC) regulates the use of frequencies for wireless communication. The FCC rules and regulations are codified in Title 47 of the Code of Federal Regulations (CFR). Part 15 of this code applies to radio frequency devices operating at unlicensed frequencies and is often colloquially referred to as FCC Part 15.
In the 902-928 MHz band, are no restrictions to the application or the duty cycle which makes this band very popular for unlicensed short range applications such as general wireless sensing and control whether periodic or even driven.
Wireless nodes operating in the 902 – 928 MHz ISM band are classified as intentional radiators and their emissions are subject to the limits given in FCC section 15.209. The maximum power allowed for unlicensed short range applications is –1.23 dBm (EIRP) or -3.38 dBm (ERP). This limit is applicable to PHYs such as FSK and GFSK.
Even higher output power can be used if the system employs some form of spread spectrum such as frequency hopping or direct sequence spread spectrum. The reason such allowances are made is that spread spectrum systems are less likely to interfere with other systems than are single frequency transmitters. They also have the advantage in that they are often more immune to interference from other systems. The limitations and qualifications of a spread spectrum transmitter are defined in FCC section 15.247
Frequency Hopping Spread Spectrum
- ≥ 50 channels: +36 dBm (EIRP)
- < 50 channels: +30 dBm (EIRP)
- The transmitter pseudo-randomly hops between frequencies that are separated from each other by at least the 20-dB bandwidth of the data channel, but not less than 25 kHz.
- The 20-dB bandwidth of the hopping channel is not larger than 500 kHz.
- If the 20-dB bandwidth of the hopping channel is less than 250 kHz, the system must use at least 50 hopping frequencies. The average time of occupancy at any frequency (dwell time) must not be larger than 0.4 seconds within any 20 second period.
- If the bandwidth of the hopping channel is larger than 250 kHz, the system must use at least 25 hopping frequencies. The average time of occupancy at any frequency must not be larger than 0.4 seconds within any 10 second period.
Spread Spectrum using Direct Sequence or Spread Spectrum using High Data Rate Digital Modulation
- +36 dBm (EIRP)
- The 6 dB bandwidth of the modulated signal is not less than 500 kHz.
- The peak power spectral density conducted to the antenna must not be greater than +8 dBm within any 3 kHz bandwidth. This corresponds to distributing the +30 dBm output power uniformly over the 500-kHz bandwidth.
As the numbers clearly show, FHSS and DSSS radios are allowed to transmit with much higher power.
Spurious emission requirements for intentional radiators in the 902-928 MHz band.
For non spread spectrum radios, the spurious emission limit is -41.2 dBm (EIRP).
For spread spectrum radios, the emission in any 100-kHz bandwidth outside the 902–928 MHz band must only be at least 20 dB below the emission in the 100-kHz bandwidth within the band that contains the highest power. There is a catch though. For fundamental signals in the 902–928 MHz band, the 3rd , 4th , and 5th harmonics all fall into restricted bands. Section 15.209 puts a more stringent limit of -41.2 dBm (EIRP) on these harmonics leading to some design constraints on the output filtering in these systems.
Restricted frequency bands relevant to radiators in the 902-928 MHz Band (Section 15.205)
|Harmonic #||Harmonic Frequency Range||Overlapping Restricted Band Frequency Range|
|1st Harmonic / Fundamental Freq||902 MHz – 928 MHz||None|
|2nd Harmonic||1.804 – 1.856 GHz||None|
|3rd Harmonic||2.7 – 2.78 GHz||2.6 – 2.90 GHz|
|4th Harmonic||3.6 – 3.71 GHz||3.6 – 4.40 GHz|
|5th Harmonic||4.5 – 4.64 GHz||4.5 – 5.15 GHz|
- ISM-Band and Short Range Device Regulatory Compliance Overview
- Rules & Regulations for Title 47
- PART 15—RADIO FREQUENCY DEVICES
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