IEEE Solid-States Circuits Magazine - Fall 2019 - 17

receive-and-transmit current on the
order of 10 mA, the average current
drawn from the battery can be fewer
than 10 μA, enabling years of battery
life with a commonly used 220-mAh
coin-cell battery.
As depicted in Figure 1, during
each event the node must go through
a start-up sequence where the realtime operating system (RTOS) wakes
up and the radio is configured,
including the locking of the RF phaselocked loop (PLL). The next state is
the guard time, the period when the
receiver is active and waiting for
data. The guard time is a function
of the connection interval and the
sleep-clock accuracy, which can be
fixed in the BLE stack to one of eight
settings between 20 and 500 parts/
million (ppm) [2]. The guard time, TG ,
is given by

Power

TG = CI $ SCA,

Master

Slave
Connection
Interval

Time

Power
Rx to Tx Tx
Rx

Sleep
Power

R
x

Ti
m
e
Po
R s
TO tp
S roc
Sh es
ut sin
do g
w ,
n

Time

Ti
m
e
R Pre
TO pr
S oce
W s
ak sin
e g,
U
R
p
ad
io
S
PL e
L tU
Lo p,
ck
G
ua
rd
Ti
m
e,
T

In a synchronous MAC protocol, periods of wake and sleep are defined so
that two connected nodes are awake at
the same time to exchange data. That
minimizes the time spent and, therefore, the radio power wasted waiting
for data to be received. The connected
nodes must exchange synchronization
packets to maintain coordination, and
each must have an accurate real-time
clock (RTC) source that keeps track of
the current time. For cellular applications, each handset receives information indicating the precise current
time from the base station. That information is used to calibrate the handset
RTC, creating a very accurate timer.
However, in low-power wireless
networks, there is often no central
external source available to provide
the time. Each node in the network
must have its own accurate RTC that
is operational even when the node is
asleep and used to generate a signal to
wake up the radio to synchronize data
reception and transmission with other
nodes. The clock source is most commonly 32.768 kHz because that is 215
clock cycles in 1 s and convenient to
generate with simple low-power, lowfrequency counter circuits. The synchronization process itself requires
some energy overhead but still enables
a system that is much more power efficient than a continuously active radio
for transmitting small amounts of
data. Many variations of synchronous

MAC protocols are used depending on
the optimizations needed for different
wireless protocols. Here, BLE is used as
an example.
Figure 1 shows a power profile
for two connected wireless nodes
using synchronized data transmission. At every connection interval,
the master node initiates the connection event, and the slave listens. To
maintain the link, connection events
occur even if neither side has any
data to send. BLE devices use a connection interval between 7.5 ms and
4 s. The transmission time depends
on how much data must be sent but
is often a few hundred microseconds,
giving a duty cycle of lower than
1%. During sleep, the current drawn
from the battery is ideally lower
than 1 μA at room temperature and
is consumed by the synchronization
clock, power management, memory
retention, and leakage. Even with a

ee
p

The Synchronous MAC Protocol

The projected number of sensor nodes is so
large that battery replacement for all of them
is impractical.

average power consumption, but the
radio's active time must align such that
it is on at the same time as the node
with which it must communicate.
That coordination can be accomplished in various ways and is a part
of the medium-access control (MAC)
protocol [1]. MAC protocols can be
synchronous or asynchronous. In this
article, synchronous protocols and
their advantages and limitations are
described, wake-up radios (WuRs) are
presented as an alternative method
for coordinating data transfer, and the
use of WuR for reducing power in the
Bluetooth low-energy (BLE) and Wi-Fi
standards is analyzed.

FIGURE 1: The power profile of two connected wireless nodes using synchronized data transmission. Rx: receiver; Tx: transmitter.

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IEEE Solid-States Circuits Magazine - Fall 2019

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