FREQUENCY MODULATED CONTINUOUS
TECHNOLOGYFOR RADIO CHANNEL MEASUREMENTS IN
THE 60 GHZ BAND
Stuart M Feeney and Sana Salous
School of Engineering, University of Durham, South Road, Durham, U.K
Keywords: 60 GHz, channel sounder, ISM band, MIMO, UWB and FMCW.
Abstract: The architecture of an UWB multi-band channel sounder is presented. The sounder architecture provides an
FMCW source to enable measurements in the frequency range up to 1 GHz (which covers the TV white
space / digital dividend), the 2.2 – 2.9 GHz band (ISM and LTE) and the 4.4 – 5.9 GHz band (ISM /
WiLAN). Additional frequency converters support operation in the 16 GHz and 60 GHz bands. Here we
have configured a 2 by 2 MIMO system in the 60 GHz band specifically targeting channel measurements to
support the development of on-body networks and short range backhaul communication networks.
Performance results in the 2.4 GHz ISM band demonstrate the resolution of the sounder.
1 INTRODUCTION
The availability of broad blocks of spectrum in the
60 GHz band provides the opportunity to support
very high data rate systems. This includes in-flight
entertainment content delivery to the seat-back in
high capacity passenger aircraft (Garcia, et al.,
2009). In addition the 60 GHz band is attractive for
on-body networks since antennas can be physically
small and the excess absorption assists covert
operation of equipment for military applications.
Initial on-body measurements (Hall, Hao and
Cotton, 2010) at 60 GHz were performed with a 60
GHz SISO channel sounder with ~ 1 GHz of channel
width (Feeney and Salous, 2008). These initial
measurements stimulated the development of a new
system which has 2 by 2 MIMO capability and 6
GHz channel width. The new sounder is also able to
support higher sweep repetition rates ( more than 2.5
kHz) which provides an unambiguous Doppler
measurement to +/- 1.25 kHz. The architecture of
the sounder also facilitates the generation of sweep
signals which can be used in a broad range of bands
which are appropriate for short range wireless
applications in cluttered environments.
Additionally the new architecture also addresses
the physical size and power limitation of the original
sounder which limited operation to either a
laboratory environment or within a vehicle
(Landrover) that had been specifically adapted to
support the operation of the sounder receiver. The
new sounder can be operated directly from a 12 V
vehicle (or battery pack) supply and is contained
within a single 3U rack. Each of the transmitter and
receiver consume ~90 W.
2 FMCW CHANNEL SOUNDING
The equipment has been designed to perform
channel sounding using the FMCW technique. The
FMCW sounder transmitter stimulates the channel
with CW signal which is swept across the channel
with a constant rate of change of frequency with
respect to time. At the receiver a similar sweep
provides the local oscillator input to a frequency
mixer which is operated as a correlator. The output
from the mixer is a beat frequency between the local
oscillator and the received signals. Different
multipath components produce different beat
frequencies. Thus the multipath structure is
delineated via spectral analysis using either a
87
Feeney S. and Salous S.
FREQUENCY MODULATED CONTINUOUS TECHNOLOGYFOR RADIO CHANNEL MEASUREMENTS IN THE 60 GHZ BAND.
DOI: 10.5220/0005414400870093
In Proceedings of the First International Conference on Telecommunications and Remote Sensing (ICTRS 2012), pages 87-93
ISBN: 978-989-8565-28-0
Copyright
c
 2012 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

spectrum analyser for online monitoring or FFT
processing following digitisation. A second stage of
spectral analysis over a number of frequency sweeps
for each time delayed multipath component gives the
delay-Doppler function which can be used to
estimate the power delay profile and the Doppler
spectrum of the channel. Other channel functions
such as the time variant frequency function can be
also evaluated via a second FFT on each sweep or
envelope detection of the received output of the
detector.
3 SYSTEM DESCRIPTION
The system block diagram of the 2 by 2 MIMO 60
GHz sounder is shown in Figure 1.
Figure 1: Block diagram of 60 GHz sounder.
At the transmitter and receiver identical sweep
generation systems are used as shown in Figure 2.
Figure 2: Sweep Generator Block Diagram.
The sweep generator contains three sub-modules:
reference unit, sweep source and auxiliary converter.
These modules are housed in 3U 19” rack cases and
can be operated from mains or 12 V battery power.
The reference unit contains a Rubidium
disciplined OCXO with an uninterruptable supply
with internal battery back-up (~1 hour). This enables
continuous operation of the Rubidium standard
should the system be temporarily moved from the
mains supply as for example to be set up in a vehicle
or moved to location. The reference module also
includes a distribution module which provides
multiplication and buffering of the 10 MHz internal
standard. Outputs are provided at 10 MHz, 20 MHz,
40 MHz and 80 MHz. These signals are used as
references for the various phase locked loops in the
sweep generator, for system synchronisation and for
clock for the analogue to digital converter in the data
acquisition unit. Figure 3 shows the UPS and the
reference multiplier and distribution boards located
on the underside of the reference module.
Figure 3: Reference module internal view.
The sweep source module consists of several
stages of up-conversion to cover the various
frequency bands. At baseband, a Direct Digital
Frequency Synthesiser, DDFS which is configured
via a USB interface uses a 2.25 GHz PLL clock
source (f
ck
) derived from the 10 MHz reference. This
enables the generation of frequency sweeps up to 1
GHz. The DDFS has a frequency update rate equal
to f
ck
/32 which enables the generation of short
duration sweeps or equivalently high waveform
repetition rates for high Doppler coverage. The
DDFS has the capability of either to free run or to be
synchronised to an external trigger signal. An
additional marker signal is available to confirm the
start of the sweep which can be used to synchronise
the data acquisition unit or to trigger other DDFS
units. The DDFS output is band-limited to eliminate
alias signals using a 15 pole low-pass filter which
has been fabricated as a distributed design on a
Rogers 4003 substrate. The filter and its measured
response are displayed in Figure 4. Taking the
output of the DDFS directly, measurements in the
Digital dividend band around 800 MHz and the VHF
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88

band can be performed using suitable RF front ends
at the transmitter and at the receiver.
Figure 4: DDFS low pass filter and its frequency response.
To cover the LTE band and ISM bands the
frequency range 2.2-2.95 GHz was chosen as the
first intermediate frequency (IF) following the
DDFS output. This gives an overall 750 MHz
instantaneous bandwidth when the DDFS is
programmed to sweep from 250 MHz up to 1 GHz.
This IF is realised by up-converting the DDFS
output using a heterodyne converter which uses a
high-side local oscillator at 3.2 GHz derived from a
40 MHz reference. The lower sideband output of
the heterodyne converter is selected using a nine
pole band-pass filter. This filter provides more than
50 dB of attenuation to the upper sideband and local
oscillator residual response.
The 3.2 GHz PLL in the up-converter and the
2.25 GHz PPL used as a clock to the DDFS have a
similar PCB design as shown in Figure 5 where the
frequency band is selected via appropriate choice of
the VCO and the loop filter parameters. This
provides an efficient design and fabrication
advantage for reproducibility of the sounder. The
first heterodyne up-converter is shown in Figure 6.
Figure 5: 2.25 and 3.2 GHz sources.
Figure 6: First heterodyne up-converter.
The second IF frequency is generated by
doubling the frequency output of the first heterodyne
up-converter. When swept across the 2.2-2.95 GHz
band, this generates a 1.5 GHz frequency sweep
from 4.4-5.9 GHz which provides high resolution of
multipath. This frequency range also covers the
second ISM band and the C band at 5.2 GHz. The
realised frequency doubler consists of an amplifier
and then a MMIC balanced doubler as shown in
Figure 7. A band-pass filter provides attenuation of
the fundamental and other harmonic outputs from
the doubler. The 5 GHz band-pass filter is shown in
Figure 8.
Figure 7: Frequency doubler to 4.4-5.9 GHz.
Frequency Modulated Continuous Technologyfor Radio Channel Measurements in the 60 Ghz Band
89

Figure 8: 4.4-5.9 GHz Band Pass Filter.
The second heterodyne stage up-converts the
4.4-5.9 GHz to 14.5-16 GHz using a high side local
oscillator (LO) injection at 20.48 GHz. The LO
signal is derived from a 5.12 GHz PLL with 80 MHz
reference followed by two doubler stages. The
output signal is available at the front panel at a
nominal level of +17 dBm and is connected via
external cables to the final stage of up-conversion to
the 60 GHz band. This approach avoids the use of
cables at 60 GHz with associated high loss and
reduced dynamic range. This is particularly crucial
for on-body networks where it is necessary to have
the 60 GHz units on the user when performing
measurements.
The 60 GHz transmitter module shown in figure
9 takes the 14.5-16.0 GHz signal and routes it to two
separate times four (X4) multiplier modules via a
two way PIN switch. This approach avoids the use
of a 60 GHz switch which would provide limited
isolation and incur losses at 60 GHz. This approach
has demonstrated very high (more than 100 dB) of
channel isolation at the transmitter. Each transmitter
channel provides an active output power of
nominally +7 dBm (5 mW).
Figure 9: Two channel 60 GHz transmitter module.
The receiver uses a single X4 multiplier to
produce the reference signal for the correlator signal
in the 60 GHz band. The output of the multiplier is
routed to two separate mixers via a 3 dB multi-hole
directional coupler. Each down-converter
mixer/correlator has a signal-conditioning pre-
amplifier on the mixer output. The directivity of the
directional coupler in addition to the LO / RF
isolation provided within the fundamental mode
balance mixers provides a measured channel
isolation of more than 50 dB. The pre-amplifier has
a noise figure of ~5 dB, the mixer conversion loss is
~7 dB and including image noise the total receiver
noise figure is ~15 dB.
4 SWEEP OUTPUT
The measured sweep output from the DDFS (after
low-pass filtering) is shown in Figure 10 where the
slope due to the sinx/x response of the DDFS can be
observed. Although the DDFS does not have a
compensation circuit for the sinx/x function, the
slope does not impact on the compression of the
received signal. However, if frequency variations
across the swept bandwidth are of interest, then the
output can be compensated for in processing. The
plot in Figure 10 demonstrates excellent attenuation
of the DDFS clock (2.25 GHz) and the primary alias
response (1.25-2 GHz).
Figure 10: Output of DDFS after LPF.
The output from the sweep generator in the 14.5
GHz to 16 GHz band is shown in Figure 11 which
demonstrates a substantially flat signal. This has
been achieved through driving the amplifier close to
compression. This approach can be used at any
stage of the sweep generator if desired. However, in
general the main sources of distortion for the
compression of the chirp signal are phase non-
linearity, and amplitude ripple in the pass band.
Phase non-linearity reduces the resolution by
broadening the width of the compressed pulse and
amplitude ripple produces sidebands in the
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compressed signal which reduce the dynamic range
of the measured impulse response of the channel.
Figure 11: Frequency sweep after upconversion to the
14.5-16 GHz band.
5 MIMO ISOLATION
The measured isolation between the transmit
channels and the receive channels is shown in Figure
12. The primary response between the active
transmitter and the active receiver is shown in the
blue trace. The green trace indicates the residual
response from the non-active receiver channel. This
demonstrates more than 50 dB of receiver channel
isolation.
The red trace indicates the output from the
receiver channel when the transmitter output
channels are switched. This demonstrates ~100 dB
of transmitter isolation.
Figure 12: Measured MIMO isolation.
6 PHASE NOISE
The instantaneous dynamic range for an FMCW
sounder is determined by the close to carrier phase
noise of the sweep signals. Close in phase noise
broadens the skirts of the compressed pulse and thus
limits the resolution of multipath components. In
addition close synchronisation between the
transmitter and receiver sweeps is required to
support Doppler discrimination. A time drift
between the transmitter and receiver sources reduces
the time interval over which Doppler analysis can be
carried out which is particularly crucial for high
resolution sounders.
This system uses 10 MHz Rubidium standards.
The synthesisers within the equipment use digital
phase / frequency comparators. This type of device
has a phase noise floor which is proportional to
frequency. The phase noise degradation due to
division within the PLL is proportional to the
frequency squared. Subject to the actual noise floor
of the 10 MHz reference and the noise performance
of the PLL digital devices the net phase noise
degradation can be minimised by exploiting
multiplication of the reference. Multiplication of the
reference reduces the available range of PLL
frequencies which can be realised. Here we have
been able to exploit this technique at 3.2 GHz (40
MHz reference) and 5.12 GHz (80 MHz). The net
improvement in the phase noise at 3.2 GHz is ~6 dB
and ~8 dB at 5.12 GHz.
The PLLs use relatively wide closed loop
bandwidths (~100 kHz) with high phase margin
(~75˚). This approach minimises “noise bumps” in
the phase noise response and is highly tolerant of
loop parameter changes.
The DDFS sources were replaced with
synthesised signal generators at 650 MHz and
650.125 MHz to produce a constant beat note at the
output of the receiver at 1 MHz. This response is
shown in Figure 13. Data have been recorded using
300 Hz resolution bandwidth to provide comparison
with the prior 60 GHz system. The total system
signal to noise ratio due to phase noise is ~52 dB at
300 Hz bandwidth. This represents an improvement
of ~7 dB to the prior system (Feeney and Salous,
2008).
With 2.5 k sweeps / second this should provide
more than 40 dB instantaneous dynamic range with
Frequency Modulated Continuous Technologyfor Radio Channel Measurements in the 60 Ghz Band
91

an effective system bandwidth of 3 kHz (2.5 kHz
FFT bin-width).
Figure 13: Complete system phase noise.
7 PERFORMANCE RESULTS IN
THE ISM BAND
The sounder was tested from back to back
measurements and in an indoor environment to
verify its performance. Figure 14 shows the impulse
response of the sounder in the 2.2-2.95 GHz band
with both the overall 750 MHz swept bandwidth and
for 375 MHz obtained by dividing the sweep into
two 375 MHz sections. The figure shows that a
dynamic range of about 60 dB is achieved with
minimum distortion to the impulse response.
Figure 14: Back to back performance of channel sounder
in the ISM band.
The sounder performance was also tested on
the air in an indoor environment and Figure 15
shows the corresponding power delay profile
obtained across 750 MHz bandwidth and across 375
MHz bandwidth. Although the reduced time delay
resolution is evident in the lower bandwidth, the
overall shape of the power delay profile shows a
similar trend.
(a)
(b)
Figure 15: Indoor measurement in the ISM band with (a)
750 MHz swept bandwidth, (b) 375 MHz swept
bandwidth.
8 CONCLUSIONS AND
FURTHER WORK
A new compact FMCW sweep generator platform to
support measurements in multiple frequency bands
has been developed. The sounder has programmable
bandwidth which at baseband can extend up to 1
GHz. With various stages of up-converters it can
generate signals with bandwidths up to 750 MHz in
the 2.2-2.95 GHz, and 1.5 GHz bandwidth in the
4.4-5.9 GHz band. Additional equipment to provide
2 by 2 MIMO capability within the 58-64 GHz band
has been produced. Enhanced phase noise and
excellent MIMO channel isolation has been
demonstrated. Integration to the data acquisition
system to complete the system is under way along
with multiple transmitters and multiple receivers to
enable simultaneous measurements in the two ISM
bands in addition to the 60 GHz band. The sounder
will be used in various indoor environments
representative of test-beds in the CREW consortium
to generate a typical channel model and to study the
benefits of cooperative sensing. Other applications
of the sounder include studies such as on-body
network and car to car communication.
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ACKNOWLEDGEMENTS
The 60 GHz sounder has been supported by an
EPSRC grant within the PATRICIAN project. The
ISM band application is currently being supported
by the EU CREW Open Call 1 project.
REFERENCES
Garcia, A. P., Kotterman, W., Thomä, R. S., Trautwein,
U., Brückner, D., Wirnitzer, W. and Kunisch, J.;
(2009); 60 GHz in-Cabin Real-Time Channel
Sounding. Communications and Networking in China,
2009. ChinaCOM 2009.
Hall, P. S., Hao, Y. and Cotton, S. L.; (2010); Progress in
Antennas and Propagation for Body Area Networks.
Proceedings of International Symposium on Signals,
Systems and Electronics (ISSSE2010).
Feeney, S. and Salous, S.; (2008); Implementation of a
channel sounder for the 60 GHz band. URSI General
Assembly; Chicago, 2008.
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