52 August 2017
The frequency corresponding to a wavelength of 0.33
f = 300/l = 300/0.33 = 909 MHz
The wireless path loss formula clearly shows the effect
of higher frequencies:
dB path loss = 37 dB + 20log(f) + 20log(d)
Here, f is frequency in MHz and d is distance in miles.
If all of the above factors are held constant, then range
is directly related to wavelength; the longer the wavelength,
the lower the frequency and therefore the greater the
range. A 433 MHz signal will travel farther than a 2. 4 GHz
signal for a given transmit power, receiver sensitivity, and
There are also some other considerations such as
noise, type of modulation, data rate, encoding, and line of
sight (LOS) conditions:
• If the noise level is high, the signal will get weaker with
distance, so will be smaller compared to the noise and
may be lost in the noise.
• Higher levels of modulation like QAM are more
sensitive to noise as they use more amplitudes and phase
shifts, so are less likely to be received reliably. Simpler
basic methods like FSK (frequency-shift keying) or BPSK
(simplest form of phase shift keying) fare better in noise.
• The higher the data rate, the greater the chance for bit
errors. Low data rate signals go farther. A low data rate
is typically anything less than 100 kb/s, down to a few
hundred bits per second.
• Encoding like forward error correction (FEC) fixes bit
errors on-the-fly, but lengthens transmission time. FEC
really helps extend range in a noisy channel.
• LOS means the transmit and receive antennas can
“see” one another. This is important as the usual Io T
operating frequencies are all UHF or microwave,
and these signals travel in a straight line. The lower
frequencies do better in a non-LOS environment with
some signal blockage by trees, etc. Lower frequencies
penetrate walls better than higher frequencies.
All this adds up to one thing: Lower frequencies,
lower data rates, simpler modulation schemes, use of FEC,
and a clear LOS signal path will greatly extend the range
The Long Range Technologies
The new wireless standards for longer range
incorporate the characteristics mentioned earlier. In
addition, all of them feature low power consumption. Most
of the products like sensor nodes are battery powered and
for industrial applications, batteries must last 5 to 10 years
to maximize replacement intervals. Because of this, these
new standards are called low power wide area networks
(LPWAN). Networking refers to connections of multiple
wireless sensors to a single collection source. Usually, the
star network topology is used.
In no particular order, here are the most popular long
range LPWAN standards:
ISM Band. ISM means industrial-scientific-medical, and
band refers to frequencies set aside for data transmissions
in unlicensed segments of the spectrum. Common US
ISM frequencies as examples are 315 MHz for garage
door openers and remote keyless entry fobs; 433 MHz for
home weather stations; 902-928 MHz for telemetry data
transmission; and 2. 4 -2.483 GHz for networking.
The most popular choice is the 902-928 MHz band.
ISM-based systems are usually single-purpose and simple;
no complex protocols and low data rates. They use
simple ICs and low power. Figure 1 shows a typical IC:
the Microsemi ZL70550. It can be set to operate in the
779 to 965 MHz range with a power level of 0 dBm (one
milliwatt), and a -106 dBm receiver sensitivity. The data rate
choices are 50, 100, or 200 kb/s.
LoRa. This is short hand for long range. It defines the
chip and module products of Semtech Corporation. This
standard uses 915 MHz and an FM chirp spread spectrum
modulation scheme. The use of spread spectrum permits
multiple users to share a common band. Data rates can be
anything from several hundred bits per second to 50 kb/s.
Estimated range is 2 to 15 km depending upon the terrain.
SIGFOX. SIGFOX is a French company that uses the
915 MHz frequency. Their ultra narrow bandwidth BPSK
modulation produces data rates from 100 to 1,000 bps.
Transmissions from sensors are short and infrequent.
Simple modulation and narrow bands of 200 kHz with low
data rates minimize the impact of noise, so a range up to
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FIGURE 2. The ublox SARA-R410M LTE Cat M1 module
consumes low power and is housed in a 16 x 26 mm
package designed to be embedded in Io T or M2M