Signal Strength
Signal strength is measured in decibels
- Power is measured in Watts or dBw or dBm
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dBm is most relevant to the IoT domain
- 0 dBm equals 1 mW transmit power
- Example
- Max BLE transmit power for nRF52840: 8 dBm (6.31 mW)
- Min BLE receive sensitivity for nRF52840: -95 dBm (316.2 fW)
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Rules of thumb: +3 dB is double the power, 10 dB is 10x power
-
Bluetooth Low Energy (local area)
- nRF52840 transmit power: 8 dBm (6.31 mW)
- nRF52840 receive sensitivity: -95 dBm (316.2 fW)
-
LoRa (wide area)
- SX127X LoRa transmit power: 20 dBm (100 mW)
- SX127X LoRa receive sensitivity: -148 dBm (1.6 attoWatt)
Degradation
- Propagation degrades RF signals (logarithmic)
- Physical obstacles, interference, reflections
At very long ranges, this becomes one of the central design constraints in Satellite Communication, where orbit distance directly increases path loss and latency.
Signal Frequency and Analysis
- Which channel the signal is sent on
- sum of sinusoids can be reversed
- frequency separation is how different technologies can operate simultaneously
- higher frequency shorter ranger, higher speed
- IoT thrives on unlicensed bands
- Bluetooth operates on sequence of transmit channels.
Signal Modulation (Data Encoding)
- encoding signal data in an analog “carrier signal”
- modulation scheme + data defines the bandwidth
On-Off Keyring
When the carrier is present, send 1. Otherwise there will be no signal.
- Simple
- Spectrum Frugal
- Susceptible to noise
Amplitude-shift Keyring (ASK)
- Modify amplitude of carrier signal
- This is an extreme example of On-Off Keyring.
- Extremely susceptible to noise.
Frequency-shift Keyring (FSK)
This modifies the frequency of the carrier signal. The bandwidth is limited, but still commonly used. This is relatively simple and robust to noise.
Phase-shift Keyring (PSK)
This modifies the phase of the carrier signal. It is energy efficient and robust, but requires more expensive hardware.
There are other more complicated ones like Quadrature Amplitude Modulation.