IEEE Solid-State Circuits Magazine - Spring 2015 - 21

size constrained, then the antenna
dimension will be limited; assuming
that antenna dimensions are constrained to the cm range, it can be
shown that the low GHz region is a
fairly "sweet spot" in terms of operating frequency [1].
As a practical example, assume
transmit and receive antenna gains
are -6 dB, operating frequency is
2.4 GHz, and line of sight range is
50 m. From Friis' equation, the difference between the transmitted
power Pt and received power Pr is
calculated as 86 dB, which is known
as the link budget (since effectively
it is the power lost over the wireless
link between the two antennas). Assume first a receiver design that requires a received power Pr of at least
−76 dBm for successful detection and
demodulation (this value is 6 dB better than the minimum required by
the Bluetooth low energy standard).
Since the link budget is 86 dB, the
transmitted power must be +10 dBm,
which is 10 mW. Assuming the transmitter efficiency is significantly less
than 100%, we can conclude that the
peak power of our transmitter is going to be much greater than 10 mW
-not really consistent with an ultralow power wireless implementation.
So alternatively consider a receiver
which is 10 dB more sensitive such
that Pr = - 86 dBm. Increasing sensitivity can usually be achieved without a significant overhead in power
consumption, especially if high quality passive devices are available. The
required transmitted power similarly
reduces to 0 dBm, or 1 mW. Even with
a modest transmit efficiency, peak
transmitter power can now be below
10 mW, which is more in-line with the
stated low peak power requirements.
In summary for a given range and
link budget, the aim should be to maximize receiver sensitivity. This enables
the transmitted power to be reduced,
resulting in significant power savings
during transmitter operation.
Friis' formula states how the
operating frequency affects the
achievable range, but in reality most
ultra-low-power wireless systems

have to operate within sections of
the frequency spectrum allocated to
short range devices. Devices operating within these bands have to follow certain rules defining maximum
transmit power, bandwidth and

wireless implementation. There are
also worldwide ISM bands at 5, 24
and 60 GHz-these bands are currently not targeted for ULP wireless
because the high frequencies
translate to high power consumption

An ultra-low-power wireless system comprises
a number of functional blocks in addition to the
wireless transceiver.
channel spacing, and duty cycle. The
aim of these rules is to ensure that
as many users as possible can coexist within the same spectrum.
The actual spectrum allocated
for short range wireless devices
tends to vary with geography; as
shown in Figure 8, most countries
have allocation in the low hundreds
of MHz, 300 or 400 MHz, and also
allocation just below 1 GHz, around
800/900 MHz. 2.4 GHz is a worldwide band, and while this band is
crowded, the simplicity of worldwide operation means that it is currently very popular for low power

for the transceiver circuits, but it
is likely that further developments
in technology and techniques will
change this. After all, even 10 years
ago it was unthinkable that you
could design a very low power wireless system operating at 2.4 GHz!

Deep Asleep
A very low power wireless system
which has a low duty cycle may spend
the majority of the time asleep.
In this state, a major function
which remains operational is the
sleep timer, which counts elapsed
time and produces an interrupt when

20 log d = Gr (dB) + Gt (dB) + Pt (dBm) - Pr (dBm) + 20 log

λ
4π

Figure 7: Friis' transmission equation.

5.7 GHz, 24 GHz, 60 GHz...?
EU
433/868 MHz
JP
315/426/950 MHz

USA
315/433/915 MHz

Asia
315/433/780 MHz
SA
343/868 MHz

AU/NZ
433/915 MHz

2.4 GHz-Worldwide
Figure 8: Allocated frequency bands for low power wireless.

IEEE SOLID-STATE CIRCUITS MAGAZINE

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Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Spring 2015