How to design an ultra low power radio using 802.15.4 and ZigBee RF4CE technologies by SRSTech


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									How to control power for Wireless sensor networks

The new Communications Controller Centric Transceiver chip design architecture is
central to reducing power consumption for 802.15.4 and ZigBee RF4CE solutions

By Cees Links, CEO of GreenPeak

Wireless Technology is evolving from communications to between people and computers to
communications between machines. There is a third wave of wireless that is following the almost
ubiquitous integration of cell phones and wireless Internet (Wi-Fi) into our lives.

This third wireless wave consists of wireless sense and control networks that can connect and
control all kinds of equipment in our homes and businesses – from freezers to light switches,
from consumer electronics (TV, DVD-player) and remote controls to sensors, for detection or
protection, and to central door locking and window locking in our homes (as we are used to in
our cars).

Unfortunately, using today’s wireless technologies, most of those wireless sensors and controls
require the use of a significant quantity of batteries creating environmental concerns (think toxic
chemicals and heavy metals) as well as a serious maintenance problem (continuously exchanging
batteries). Therefore ultra low power wireless networks that require very little power are of great

This includes systems that can run off of a single cell battery for the life of a device as well as
wireless networks and sensors that can be powered by energy harvesting (sometimes called
energy scavenging). Creating ultra low power wireless networks and systems that can run off the
energy that is available in the environment instead of batteries is a very exciting emerging

Recently, the ZigBee organization partnered with several of the largest consumer electronics
companies in the world (Panasonic, Philips, Sony and Samsung) to form what is known as
ZigBee RF4CE (Radio Frequency for Consumer Electronics). This industry partnership signals
the development of an entire new generation of remote control devices – for TVs, for home and
office automation, for many other types of remote control products that communicate via low
power RF instead of the decades old IR (infrared). By using these new communication
technologies, we soon shall be seeing a wide range of remote devices that are not only
interoperable among brands and models, but require so little power that their batteries will never
have be changed or recharged. It is even possible to design and build remotes that will not
require any batteries at all and will get their power from energy harvesting.

Challenges of wireless sensor networks
The biggest technical challenge for developing these ultra low power sensor networks is
managing the energy consumption without reducing range or functionality, like speed and
standards compliance. The resulting elimination of battery replacement will then simplify
maintenance and provide a higher level of ease of use and safety.

Ultra low power consumption

It is obvious that current consumption – milli-amps – and duty cycling are important in
wireless sensor networks. However, minimizing current consumption is only part of the
solution. There are several essential issues key to developing low power wireless sensor
applications, but it all starts with the development of an ultra low power transceiver radio

By using a communication controller centric chip design instead of a microcontroller centric
design, along with synchronized wake-ups, it is possible to reduce overall power consumption
by 65% or more.

     Diagram 1 – GreenPeak’s Communication Controller-centric Architecture versus
     traditional Microcontroller centric approach. Most transceiver solutions require that the
     MCU be switched on the whole time during the transmission of a package. By using a
     communication controller, the MCU is only required to process the data to be transmitted
     or received.

     Most low power radio networks rely on a processor centric approach that requires a
     microcontroller to handle all the intelligence for the transceiver. This requires the
     microcontroller to be awake the entire time that in turn requires additional power. By
     using a more energy efficient communication controller approach, the transceiver can
     transmit and receive the data independently from the microprocessor and the
     microprocessor is only awakened and used when it is needed to further process the data.

     By using a hardware based scheduler and synchronizer within the chip itself, the radio
     only wakes up as needed to see if there is any data that needs to be sent. If not, it returns
     to sleep. If there is data to be sent, the controller then wakes up the microcontroller. The
     chip then communicates the information and then goes back to sleep until the next time it
     is scheduled to wake. 9999 times out of 10,000 – there is no message to be sent and the
     controller does not need to energize the microprocessor. Every time that data is sent, the
     chips also transmit a synchronization message to ensure that they all wake up together on
     the next duty cycle.

Diagram 2 – By letting the microprocessor sleep until it is needed, it is possible to save over 65%
of energy usage as compared to a the typical always on traditional transceiver

Diagram two illustrates that by letting the communications controller decide when to wake up
and check for messages, it is possible to greatly reduce overall energy consumption. Because of
the scheduler and synchronizer inside the communication controller, the system only wakes up
for a brief moment to check to see if there are any messages and goes back to sleep.

If you multiply this individual node power saving by a wireless network of over 100 nodes, it is
obvious that the entire network will be able to operate using vastly less power than a traditional
microprocessor based network.

Peak current savings

Diagram 3 below depicts the current consumption in three typical wireless sensor node states
for a commonly used wireless sensor platform. In state one, the microprocessor and transceiver
are in sleep mode (10µA). In state two, the microprocessor is switched on while the transceiver
is asleep (10 mA). In state three, both the transceiver and the microprocessor are awake (27

                      transceiver on

                                           27 mA

                                           10 mA
                                           ~10 µA

                 microprocessor on
 Sleep current

       Diagram 3 – the three wireless node states and typical power consumption

When closely examining the power consumption behavior of electronic circuits, it
becomes apparent that what initially looks like a flat current curve actually bears more
resemblance to a mountain range with peaks and valleys. When certain functional blocks
become active, they draw peak current. When two functional blocks switch on
simultaneously, the peak amplitude doubles.

The secret to reducing the peak power lies in carefully managing the turn-on and turn-off
time for key functions so that double peaks can be avoided.

Synchronized Wake Up and Sleeping enables reduction of power consumption for low
power mesh networks

One of the most dramatic differences between wireless sensor communications technology and
other well known wireless technologies is the ability of sensor nodes to forward messages from
other nodes located further down the communications chain. This technique, known as mesh
routing or multi-hop networking, provides an effective and reliable means of spanning large
infrastructures, beyond the range of what a single wireless link can do.

For a node to forward a message received from another node, it needs to be in an awake and
receiving mode when the original wireless message arrives. Unfortunately, the reception mode
requires so much power that it can drain batteries in a matter of a few days. As this power
lifespan is too short for most real-life applications, the most straightforward solution, as
specified by most industry standards, is to limit the multi-hop capability to the nodes that are
permanently connected to the main power. In such a framework, low-power devices, which are
assumed to be in a power-down mode most of the time, are not capable of retransmitting
messages from other devices. These low-power devices, known as end-devices, are located at
the end or beginning of the communications chain.

This framework, which combines mains-powered mesh routing devices and low-power end-
devices, works for some applications. Take, for example, an office lighting application utilizing
interconnected wireless lamps and light switches. The lamps, which are connected to the main
power source, house the mesh routing communication nodes. The switches, which are not
mains powered, are a natural place for the end-devices.

Many other applications do not fit well in such a framework. In applications like gas detection,
fire detection, access control, precision farming, battlefield monitoring, perimeter surveillance,
warehouse temperature monitoring, etc., mains power is not readily available or even present.
Running a power cable in these applications would be cost prohibitive, offsetting the benefit of
wireless communication.

To address this class of applications requires low-power multi-hop networking, or low-power
routing, in which all of the nodes, including the mesh routing nodes, operate in low-power

By using a “synchronized wake-up” scheme, it is possible to coordinate receiving activity in a
way that eliminates the need for the mesh routing nodes to continually operate in receive mode,
thereby significantly reducing power consumption. The picture below depicts how low-power-
routing works when Node A wants to send a message to Node C, through Node B. All nodes in
the pictures are low-power nodes, sleeping most of the time.
 Node A wants to pass a
 message to node C                           A                        B                         C

                                    A awake                  B awake
                                     period                   period

                                                                                          B goes to sleep
                                                                        A goes to sleep
 A and B

                                      B wakes up
                       A wakes up
                                                     A and B
                                                             B awake                       C awake
                                                              period                        period
 B and C

                                                                                               B goes to sleep

                                                                                                                 C goes to sleep
                                      B wakes up

                                                         C wakes up


                                                                        B and C

Diagram 4 – Wireless nodes communicate most efficiently via synchronized wake up and sleep

By synchronizing the sleep/wake-up cycles of the nodes to each other, nodes wake up when
they expect a message from a neighboring node. This enables the routing nodes to operate in a
nearly powerless sleeping state most of the time, thereby achieving ultra-low-power operation.
Clearly, more wake-ups will occur than strictly required to carry the data, as neighboring nodes
will not always have data to transmit. However, the additional power required for periodic
wake-ups and synchronization is more than offset by the power saved by eliminating the need
for continuous receive mode operation.

Since its inception, wireless sensor technology has been linked with low-power electronics.
Most low-power wireless sensor networks have been designed for low power, meaning that
they consume little power when switched on. That is not enough. By using communication
centric transceiver chips, wireless mesh networks, and synchronized wake up and sleep cycles,
developers can now create systems that don’t even need batteries and instead, can utilize
energy harvesting to power the sensor network from environmental power sources.

The wireless sensor network standard – IEEE 802.15.4
For wireless sensor transceivers the dominant and probably only real standard is the IEEE
802.15.4 specification. However, there have been efforts to use Bluetooth and Wi-Fi for low
power sensor applications. In most of the cases reported, Bluetooth and W-Fi were used in a non-
standard way, in fact weaving the principles of IEEE 802.15.4 in their native implementation. It
is nowadays widely accepted that the IEEE 802.15.4 offers the best basis for wireless sensor
network applications.

Besides the IEEE 802.15.4 standard, a number of technology suppliers have chosen to build
proprietary transceivers. The main motivation seems to be a reduction of the complexity and thus
a potential lower cost point. However, it remains to be seen if a proprietary solution will ever
reach sufficient volumes to actually reach that theoretically lower cost point. Additionally,
reducing the complexity automatically goes hand in hand with sacrificing performance and thus
limiting the applicability.

Proprietary technologies are vulnerable, for two reasons: (1) the owner of the technology
controls the specification and thus also the price, and (2) the customer depends on the technology
owner for upgrades and uninterrupted sourcing.

Even within the boundaries of standards, technology providers can discover and leverage
differentiation opportunities.

As an example GreenPeak has developed transceiver and network stack technology that is
compliant to the IEEE 802.15.4/2.4 GHz standard but includes additional functionalities that
enable its use for ultra low power applications. An ultra-low-power application is defined as an
application that is able to live off a coin cell battery or off energy harvested from the
environment through a solar cell, a vibration energy harvester or any other environment energy

CAPTION: GreenPeak is fully committed to development based on open industry standards.
Designs using the GP500C communications controller are fully IEEE 802.15.4 compliant,
running in the 2.4 GHz, which allows worldwide certification for single products. The
GreenPeak technology also supports the open global standards of the ZigBee Alliance.

                               Cees Links, Founder & Chief Executive Officer of GreenPeak

                               Cees Links is a pioneer of the wireless data industry, a visionary
                               leader bringing the world of mobile computing and continuous
networking together. Under his responsibility, the first wireless LANs were developed which
ultimately became a house-hold technology integrated into the PCs and notebooks we are all
familiar with. He also pioneered the development of Wi-Fi access points, home networking
routers and hotspot base stations, all widely used today.

Cees was involved in the establishment of the IEEE 802.11 standardization committee and the
Wi-Fi Alliance. He was also instrumental in helping to establish the IEEE 802.15 standardization
committee that became the basis for the ZigBee sense and control networking technology and

Cees’ vision is to build a smarter world by developing a communications platform between
devices sensing and enabling us to control our lives.

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