ARSAdvances in Radio ScienceARSAdv. Radio Sci.1684-9973Copernicus PublicationsGöttingen, Germany10.5194/ars-15-107-2017Implementation of envelope detection based Wake-Up Receiver for IEEE 802.15.4 WPAN from off-the-shelf componentsArndtJosuajarndt@ias.rwth-aachen.deKrystofiakLukasBonehiVahidWunderlichRalfHeinenStefanRWTH Aachen University, Integrated Analog Circuits and RF Systems, 52074 Aachen, GermanyJosua Arndt (jarndt@ias.rwth-aachen.de)21September2017151071132November20161March201722April2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://ars.copernicus.org/articles/15/107/2017/ars-15-107-2017.htmlThe full text article is available as a PDF file from https://ars.copernicus.org/articles/15/107/2017/ars-15-107-2017.pdf
Power consumption in wireless networks is crucial. In most
scenarios the transmission time is short compared to the idle listening time
for data transmission, the most power is consumed by the receiver. In low
latency systems there is a need for low power wake-up receivers (WuRx) that
reduce the power consumption when the node is idle, but keep it responsive.
This work presents a WuRx designed out of commercial components to
investigate the needs of a WuRx when it is embedded in a Wireless Personal
Area Network (WPAN) system in a real environment setup including WLAN and LTE
communication and considering interferer rejection. The calculation necessary
for the attenuation of those interferers is explained in detail. Furthermore,
a system design is presented that fulfills the requirements for this
environment and is build from off-the-shelf components.
Introduction
Personal Area Networks, as specified by , applications like
lighting control, monitoring temperature, moisture etc. or IoT (Internet of
Things) devices can demand a fast reaction in lighting systems for example.
If the user activates the light it should react in milliseconds as we are
used to the lights switching on instantly. For those applications, the
receiver must always be active to listen to the channel. In beacon-enabled
networks a fast reaction time can be achieved by a big duty-cycle, resulting
in a high beacon number to send. The end devices must wake up frequently
which results in a high-energy consumption. A WuRx designed to support a low
data rate modulation can be very simple and consumes less energy than a high
data rate modulation like BPSK or O-QPSK.
Conventional receiver architectures are the heterodyne or homodyne (zero-IF)
receiver which support complex modulation schemes whereby a high spectral
efficiency and at last a high data rate can be achieved. Both rely on at
least one LO (Local Oscillator) which commonly consists of a LC oscillator
embedded in a power consuming PLL (phase-locked loop) with an operating
frequency in the RF range. A regenerative circuit as alternative receiver
concept is tempting because it only oscillates when there is a signal at the
input, but the bias current of the of the LC circuit is the main problem in
reducing power dissipation. Last there is the direct conversion by an
envelope detector, which shows potential for a very low power consumption.
A WuRx using the conventional heterodyne receiver concept architecture is
presented in , the power consumption splits up as follows:
RF Amplification 22, LO 20, Mixer 8
and the ED 1 µW. A LO-less approach using a double-sampling
technique to suppress the offset and 1/f noise of the down-converting
envelope detector is presented in . The power
consumption can be divided as follows: LNA 27 µW, BB-amplifier
14 µW, ED 5 µW, bias circuit 4 µW, frequency
divider and logic 3 µW, external reference clock 1 µW. In
a direct down conversion concept is shown, the power consumption
on this receiver is as follows: ED 1.25 µW, BB-amplifier
0.4 µW, correlation unit 0.4 µW and the RC lowpass filter
0.2 µW. Comparing the power dissipation of these wake-up receivers
shows that a very low power dissipation can be achieved with the direct
conversion approach.
In this paper, we present a work that was done to gain insight for an
integrated design, that is developed in parallel. The approach to build a PCB
based WuRx which is as near as possible to the targeted system design is
followed to enable measurements of the overall system behavior. The PCB based
WuRx was also developed to be used as a first prototype to implement a full
functioning IEEE 802.15.4 network. The Development of on SOC with design,
implementation, tape-out and design of a measurement PCB, debug and measure
the SOC and develop an application PCB takes up to 3 years. Therefore, a WuRx
out of commercial components is built on PCB to investigate the needs of a
system that includes a WuRx and uses the IEEE 802.15.4 transmitter to
generate the wake-up frame. Challenges will be discussed and solutions
presented.
As IEEE 802.15.4 transmitter we used the AT86RF233 from
which is a high-performance RF-CMOS 2.4 GHz radio transceiver targeted for
IEEE 802.15.4, ZigBee, RF4CE, 6LoWPAN, and ISM applications. It has a
receiver sensitivity of -101 dBm and a programmable TX output power from
-17 to 4 dBm.
In Sect. an overview of the requirements will be given and basics
of path loss and link margin calculations are presented. In
Sect. the system setup, filter design and resulting
link budget are presented. In Sect. the measurements
of the WuRx are shown and Sect. concludes the paper.
Design requirements
As the comparison in shows, on-off-keying (OOK) is the
simplest modulation scheme for digital radio transmission. A tuned RF
receiver is one of the simplest architectures (). It accepts
incoming RF signals which are filtered, amplified and converted from RF to
baseband by an envelope detector (ED). This eliminates the need for a power
consuming local oscillator (LO) completely, which is usually the most power
consuming component in a receiver; as shown in the previews comparison of
state-of-the-art WuRx.
However, this means that this receiver can process amplitude modulated
signals only and the architecture calls for a very high selectivity at RF,
which will be explained later in detail. This is a result of the behavior of
the ED converting all signals directly to baseband without filtering.
The designated transmit distance of our WPAN networks with WuRx is between
5 and 30 m indoors. In our scenario, the WuRx works at the same frequency
as the main transceiver. To reduce hardware complexity, the main transceiver
will be used to generate the OOK signal.
As widely known, a higher carrier frequency enables broader bandwidth and
therefore a high data rate as well as reduced diffraction loss, smaller
antenna size and overall increased level of integration.
However, a higher carrier frequency also increases the path loss and
therefore the necessary sensitivity of the receiver. Moreover, additional
filtering and amplification at high frequencies is more complex and power
hungry. The formula for free space path loss is:
LsdB=10log10(4πdλ)2=20log10(4πfdc)=20log10(d)+20log10(f)-147.55
with the carrier wavelength λ (m), the carrier frequency f (Hz),
the speed of light c (m s-1) and the link distance d (m). To
calculate the path loss, we consider a direct line of sight and no walls and
floors, which is equal to a large room. As shown by the
path loss exponent then is smaller than 2, so for simplicity we can use the
simple free space path loss equation formula.
Hence with a carrier frequency of 2.48 GHz and a link distance of 10 m the
path loss would be 70 dB, at 30 m roughly 80 dB. A better insight of
indoor wireless coverage can be found in . For WLAN at
2.467 GHz and a distance of 1 m the result is Ls=-40 dB which is an
attenuation of 40 dB.
Inserting the system specific transmit power PTX [dB] and the ED
sensitivity SED [dB] we can calculate the resulting link margin
LM [dB]. This can be interpreted as the additional gain
necessary to be able to detect a signal at the inserted frequency and
distance.
LMdB=PTX-SED-20log10(d)-20log10(f)+147.55
Setting the link margin to zero and solving the equation for d gives the
distance at which the signal can be detected, or at which an interferer can
no longer disturb.
d=10(PTX-SED-20log10(f)+147.55)/20
Also, considering the overall gain G [dB] and interferer attenuation
AI [dB] of the system and solving the equation for
AI [dB] we can calculate the necessary attenuation for an
interferer in a dedicated distance.
AIdB=PTX-SED+G-20log10(d)-20log10(f)+147.55
Figure shows the ISM and LTE Band allocation for WLAN,
Bluetooth, WPAN and LTE Band 7. As shown the ISM Band is used by a lot of
participants in different channels. Bluetooth uses 1 MHz channels and
frequency hopping so collision with a Bluetooth device should not occur too
often and only for short times. WLAN uses 20 or 40 MHz channels, e.g.
when a video is streamed the channel is used continuously, this interferer
has to be suppressed as much as possible. LTE Band 7 is used for mobile
communication. It also uses a high bandwidth and a higher transmit power.
ISM and LTE band 7 allocation.
For our test system we choose to use the WPAN channel with fewest
interferences which is channel 26 at 2.48 GHz. Figure
illustrates the down-conversion of the RF band to baseband by an ED
containing the wake-up signal on WPAN channel 26 and the two closest
interferers, WLAN channel 13 and LTE band 7. As shown in the illustration the
interferers potentially have a higher transmit power than our WPAN node,
which is specified for the AT68RF233 as 4 dBm without losses due to the
balloon, the antenna or mismatch.
As defined in , the maximum transmit power of WLAN is 20 dBm
and LTE band 7 user equipment and base stations are limited to 24 dBm as
defined in and . Allready mentioned
earlier a narrowband filter in the RF band is necessary to suppress
interferers and converts down only the band of interest to be able to detect
the signal with the ED. Calculating the attenuation needed to suppress WLAN
with a transmit power of 20 dBm at a distance of 1 m for a system using an
ED with a sensitivity of -30 dBm results in approximately 10 dB and for
LTE with 24 dBm transmit power in 13.7 dB, according to Eq. ().
Down conversion of RF signals to baseband by envelop detection with WLAN and LTE communication as interference.
System designSystem setup
For this work the absolute power consumption of the system is not important
since it cannot compete with integrated solutions anyway. The emphasis is on
the knowledge gained on how parts influence each other, System feasibility
and usability in a real world application.
All parts had to be chosen to allow for manual soldering for fast adaption of
changes and debugging.
Figure shows the different stages of the WuRx. To
achieve a good interferer suppression, we designed the WuRx using multiple
bulk acoustic wave (BAW) filters with different pass characteristics, which
will be explained later.
WuRx system concept with overlapping filters to gain an extremely narrowband filtering.
The WuRx sensitivity suffers heavily from the additional attenuation of the
bandpass filter configuration. Increased gain is the only way to increase the
sensitivity, but amplification comes with the cost of power consumption in
general.
The LNA SKY67159-396LF from Skyworks has promising features but was not
commercially available at the time. Skyworks provided some samples, we
decided to not rely on this LNA only and used another amplifier for the 2 and
3 stage. The SKY67159-396LF is a broadband amplifier, that works from
200 MHz to 3800 MHz, has a gain of 17 dB and a very low noise figure of
approximately 1 dB.
HMC414, PA1 and PA2, manufactured by Hittite Microwave Products (HMC)
designed for frequencies between 2.2 and 2.8 GHz has a gain of up to 20 dB,
a noise figure of 6.5 dB and enable-times in the nanosecond time scale.
Additionally, the output current can be set by dedicated pins and can be
reduced as much as possible without influencing the sensitivity.
The ED, which demodulates the wake-up signal by converting the incoming
carrier frequency to DC, should be implemented similarly to the way it is
done in integrated wake-up receivers. Therefore, the choice is limited to EDs
that consist of a diode followed by a RC low pass filter. The chosen detector
is the LTC5508 produced by Linear Technology and comes only in combination
with a buffer amplifier in one package. It has a frequency operation range of
300 to 7000 MHz and supports data rates up to 2 MHz, which is more than
sufficient for this work. The buffer amplifier has an output voltage range of
around 0.25 up to 1.75 V and omits the use of additional gain after
detection. Unfortunately, the data sheet shows that the output voltage is
between 0.15 and 0.4 V even during the absence of an input signal. For
reliable information, the component must be calibrated after it has been
implemented in the circuit. The output offset also has influence on the
detection threshold for the comparator to work properly. As the data sheet
states it has a sensitivity of -32 to 12 dBm, but between 2 and 3 GHz the
output voltage change at -32 to -25 dBm input power is very small and
measurements have to show how much of that sensitivity range is actually
effective.
The last active component of the front-end is a comparator which serves as
1-bit ADC. The requirements are very low since the wake-up signal is in
baseband at this stage and has a fairly low data rate of 1 kHz. It should
have a pin to latch the output.
The used comparator is a MAX9141 produced by Maxim Integrated. It is a low
power, high speed device and exceeds the needed requirements. The data sheet
states a propagation delay of 140 ns, a latch delay of 16 ns for setup and
hold time and a latch propagation delay of 60 ns.
It features an internal hysteresis of about 1.5 mV; this ensures clean
switching even with slow moving input signals generated by the ED. The
threshold at which the comparator switches from 0.3 to 3 V is adjustable.
Its value depends on the dc offset of the ED, the received signal strength od
the wake-up signal, the gain of the WuRx and the received signal strength of
the interferer.
An extraction of some interesting characteristics of the parts used are
listed in Table , taken from the data sheets.
Data sheet specifications at 25 ∘C, 3.3 V, 2.481 GHz.
* Absolute power consumption is not focus of this work.
Narrowband filter
As mentioned earlier the interferers potentially have a higher power than the
wanted signal which leads to the necessity of strong suppression. To achieve
the necessary interferer rejection, a combination of multiple BAW filters are
used, which have an overlapping frequency range in the designated area. For a
cheap WuRx BAW filter will not be feasible, so in the concept of the
integrated version the interferer rejection should be addressed by selective
amplification and low power filter concepts.
More information about BAW filters can be found in
. We choose two BAW filters from Triquint,
BAW 885033 (fc=2.442 MHz, BAW3) and BAW 885009 (fc=2.535 MHz,
BAW1/2). The BAW 885033 has a bandwidth of 79 MHz and for the desired
frequency of 2.481 GHz it has an insertion loss of 1.7 to 2.2 dB as shown
in Fig. . It creates the upper cut-off-frequency for
interferers, especially LTE in Germany, starting at 2.5 GHz. According to
the data sheet the attenuation at 2.5 GHz is around 40 dB at 25∘C.
The BAW 885009 filter used as BAW1/2 shown in Fig.
forms the lower cut-off frequency and has a bandwidth of 70 MHz. The desired
frequency lies at the lower edge of this filter and is attenuated by about
9.5 dB. Undesired frequencies that go up to 2.473 GHz will be attenuated by
at least 33 dB. Unfortunately there were no S-parameter files provided for
the BAW 885033 so the S-parameters of a BAW filter from the same
manufacturer, that comes very close to the used one are shown in
Fig. for simulation and visualization.
Simulated S21-parameter of the BAW filters and their combinations.
Combining the two filters (BAW1+3) results in the characteristic as shown in
Fi. . Calculated values result in an attenuation of
12.6 dB for 2.481 GHz, of 34.8 dB for 2.467 GHz and of 39.1 dB for
2.5 GHz. With a gain of 51.5 dB this would result in approx. 11 m distance
needed to the next WLAN router and 17 m to the next LTE device. This is not
sufficient for our scenario, as coexistence with at least WLAN should be
possible.
Subsequently, an additional filter, BAW2, with a centre frequency at
2.535 GHz was used.
The plots for BAW3, BAW1/2 and BAW1+3 are made using unmatched components
to get a general overview of the behavior. In a last step, a matching network
was designed to increase performance for the frequency used.
For BAW3 the attenuation at 2.481 GHz is already low and at 2.5 GHz
sufficient, so adapting the matching was not necessary. The impedance
matching network provided in the data sheet was used, because there were no
S-parameters provided. For the other filters S11 and S22 were adjusted with
available capacitors and inductors for best performance at 2.481 GHz. These
had to be selected carefully with focus on low resistance to keep losses low
and a high series resonance frequency to maintain the desired properties. The
result is shown as trace BAW1+2+3 in Fig. . The
attenuation at 2.481 GHz decreased by around 3 to 16 dB, it increased
to 63.7 dB at 2.467 GHz and at 2.5 GHz by around 1.5 to 41.5 dB, as
marked in Fig. . Consequently, the distance to WLAN
reduces to 0.7 m and for LTE to 14.8 m. Adding more attenuation for LTE
would have added too much attenuation to the desired signal. With the choosen
3 filter configuration the system will still be error prone for LTE, but
sufficiently robust against WLAN.
Link margin
In Fig. the minimal distances for WLAN, LTE and the
maximum distance for the transmitter and the WuRX are shown.
Min. interferer and max. transmitter distance.
Calculating the link margin has been done repeatedly throughout the work to
evaluate the results of this approach. The final performance presented here
is achieved with simulated data of the architecture and components described
before. With a sensitivity of -25 dBm of the ED, a gain of about 35.2 dB
of the amplifier and filter stage, we get a sensitivity of -60.2 dBm at
2.481 GHz. With an assumed radiated output power of 4 dBm the equation for
path loss gives an ideal maximum link distance of around 15.6 m.
This will be reduced by the small output swing of the ED and noise at the
input of the ED, which effect the configuration of the threshold voltage of
the comparator. Considering the output offset of the ED a reasonable start
value for the threshold of the comparator is 280 mV at the output of the ED.
The minimum distance is also limited by the ED with an upper limit of
12 dBm, that can be detected. This leads to a minimum distance of around
0.2 m.
The input compression of the amplifiers is not be reached at this point with
these components. With the maximum radiation power of 20 dBm and an
attenuation of 12.5 dB WLAN signals will not disturb at a distance of around
0.4 m.
Depending on the distance of the base station the radiation power for mobile
phones can reach up to 24 dBm as specified in the 3G standard, to which LTE
belongs (3.9G). The uplink frequencies for LTE vary from 2.5 GHz up to
2.57 GHz in Germany. With a total gain of 10 dB for LTE frequencies, these
signals overshadow the desired signal significantly at the same distance to
the receiver.
A minimum distance of 8.3 m to the LTE transmitter is necessary prevent
corruption of the wake-up sequence. To coexist closer to LTE, we can adjust
the threshold of the comparator, which results in a reduced maximum distance
of the wake-up receiver to the transmitter.
Implementation and measurements
To debug the WuRx every stage was soldered and measured successively. As
expected, the comparison of simulated and the measured values differ and each
stage had to be matched with regard to the measured values again. Some
selected measurements are listed in Table .
1 Absolute power consumption is not focus of this work. 2 With optimized output current configuration.
All stages showed lower performance than listed in the data sheet, which is
caused by the FR4 substrate of the PCB. The data sheets and application notes
are based on Roger substrate and the data sheet values also take board loss
deembedding into account. Our measurements include all losses only
deembedding the cable losses by calibration.
Measured gain of the complete WuRx
Getting the HMC414 running was a big challenge because of oscillations which
were caused by crosstalk from the output to the input. This could be reduced
by inserting a barrier of silver wire between the input and output pin, but
still the maximal gain could not be used. Thus, the gain of the HMC414 is not
as high as expected.
The Application note from the addressing this problem
helped to solve this issue but was found while debugging. The solutions and
improvements shown there could not be implemented on the board to gain the
best performance.
Wake-up receiver PCB.
Figure shows a measurement of the whole circuit from
the SMA connector to the output of the last amplifier. The increase of 3 dB
from single component to whole system measurement can be explained by better
matching of the stages to each other than to the measurement equipment.
Considering optimum matching and operating power amplifiers the general
functionality of the circuit was tested. For the baseband output voltage of
the ED we get a value of about 275 mV with no incoming RF signal. With
regard to the input hysteresis and offset of the comparator the voltage is
adjusted to make sure it switches only if a signal is present. This threshold
is found to be at 284 mV. With a signal generator directly connected to the
board, the sensitivity at 2.481 GHz is -20.0 dBm. With a gain of 26 dB
this results in an ideal range of about 3 m with an output power of 4 dBm
of the transmitter. For 2.467 GHz we have an attenuation of 19 dB and for
2.5 GHz a gain of 3 dB.
Table shows the calculated distance using the
measured gain and sensitivity values and the measured distance achieved with
a signal generator and a transmitter board with an AT86RF233. Interferer
rejection was tested by applying the signal from the generator to an antenna.
Conclusions
In this work, we showed the implementation of a wake-up receiver with
commercial components to investigate the needs of such a system. It uses
discrete off-the-shelf components and was built on a FR4 PCB to have the
possibility to measure and analyze the state of the signal after every step.
It will be mounted on a board with a RF transceiver and a microcontroller.
Calculated and measured range and interferer rejection.
Due to the lack of selectivity of the components used in the circuit, high-Q
bandpass filtering had to be implemented to make sure that the receiver works
reliably in the 2.4 GHz band. These three successive BAW filters were used
to form a narrow bandpass filter and reject even strong out-of-band
interferers. The increased losses had to be compensated with additional gain
stages. We designed the circuit with a sensitivity of -60 dBm, while still
being able to detect a signal in close approximation to interferers sources.
Due to mismatch and oscillation problems, measurements showed a reduced
sensitivity of only -47 dBm. However, the attenuation of out-of-band
interferers stayed in the same relation to the gain for the desired signal.
This work shows an implementation of an envelope-detection-based WuRx with
off-the-shelf components which fulfills the requirements of a real-world
application. The further development will be the implementation of a network
with this WuRx and investigate how the AT86RF233 can be used to generate the
wake-up signal, to investigate the system network setup. The goal of this
work was to get better insight in the overall system and the obstacles,
gained knowledge by the development is included to the integrated designs.
The gain simulation and measurement results are available at Zenodo (Arndt, 2017).
The authors declare that they have no conflict of interest.
Acknowledgements
The authors acknowledge the support of the German Federal Ministry of
Education and Research (BMBF) through the “TreuFunk” project (FKZ:
16KIS0234). Edited by:
J. Anders Reviewed by: two anonymous referees
ReferencesArndt, J.: Implementation of envelope detection based Wake-Up Receiver for IEEE 802.15.4 WPAN from off-the-shelf components,10.5281/zenodo.582796, 2017.Atmel: AT86RF233 Low Power, 2.4 GHz Transceiver for ZigBee, RF4CE, IEEE
802.15.4, 6LoWPAN, and ISM Applications,
http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-8351-MCU_Wireless-AT86RF233_Datasheet.pdf,
2014.
ETSI TS 136 101, T. S.: ETSI TS 136 101 V12.5.0 (2014-11), LTE;Evolved
Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio
transmission and reception (3GPP TS 36.101 version 12.5.0 Release 12), 2014.
ETSI TS 136 104, T. S.: ETSI TS 136 104 V9.4.0 (2010-07), LTE; Evolved
Universal Terrestrial Radio Access (E-UTRA); Base Station (BS) radio
transmission and reception (3GPP TS 36.104 version 9.4.0 Release 9), 2010.
Griggs, J. D.: Ultra-Low Power Wake up Receiver for Medical Implant
Communications Service Transceiver, Ph.D. thesis, North Carolina A&T State
University, 2012.
Gutierrez, J. A., Callaway, E. H., and Barrett, R.: IEEE 802.15.4 Low-Rate
Wireless Personal Area Networks: Enabling Wireless Sensor Networks, IEEE
Standards Office, New York, NY, USA, 2003.Hambeck, C., Mahlknecht, S., and Herndl, T.: A 2.4 µW Wake-up
Receiver for
wireless sensor nodes with -71 dBm sensitivity, in: 2011 IEEE International
Symposium of Circuits and Systems (ISCAS), 534–537,
10.1109/ISCAS.2011.5937620, 2011.Heereman, F., Joseph, W., Tanghe, E., Plets, D., and Martens, L.: Prediction
of
range, power consumption and throughput for IEEE 802.11n in large conference
rooms, in: Proceedings of the 5th European Conference on Antennas and
Propagation (EUCAP), 692–696, 2011.
Hittite Microwave Corporation, A. D.: Designing w/The HMC414MS8G PA Utilizing
a Low Cost Laminated Printed Circuit Board, 2004.Huang, X., Rampu, S., Wang, X., Dolmans, G., and de Groot, H.: A
2.4 GHz/915 MHz
51 µW wake-up receiver with offset and noise suppression, in: 2010 IEEE
International Solid-State Circuits Conference – (ISSCC), 222–223,
10.1109/ISSCC.2010.5433958, 2010.IEEE 802.11b: IEEE Standard for Information Technology –
Telecommunications
and information exchange between systems – Local and Metropolitan networks –
Specific requirements – Part 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) specifications: Higher Speed Physical Layer (PHY)
Extension in the 2.4 GHz band, IEEE Std 802.11b-1999, 1–96,
10.1109/IEEESTD.2000.90914, 2000.
Mahon, S. and Aigner, R.: Bulk Acoustic Wave Devices – Why, How, and Where
They
are Going, in: CS MANTECH Conference, 2007.Oetting, J.: A Comparison of Modulation Techniques for Digital Radio, IEEE
T. Commun., 27, 1752–1762,
10.1109/TCOM.1979.1094370, 1979.Pletcher, N. M., Gambini, S., and Rabaey, J. M.: A 2 GHz 52 µW
Wake-Up
Receiver with -72 dBm Sensitivity Using Uncertain-IF Architecture, in: 2008
IEEE International Solid-State Circuits Conference – Digest of Technical
Papers, 524–633, 10.1109/ISSCC.2008.4523288, 2008.Plets, D., Joseph, W., Vanhecke, K., Tanghe, E., and Martens, L.: Simple
Indoor
Path Loss Prediction Algorithm and Validation in Living Lab Setting, Kluw.
Commun., 68, 535–552, 10.1007/s11277-011-0467-4, 2013.