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Smartphones
Connect goes LTE
When our yearly network test covered LTE in 2013, it was a premiere.
Now connect even measures smartphones in 4G networks, ­being the first
­European magazine to do so. The equipment is worth a fortune.
ast January we were sure
of a catastrophy. The numbers were so terribly wrong that
there was no doubt about a major bug. More than one colleague cut me a look that said
„Come on, give us the allclear!“ Because if this was to
con­tinue, there would be no
measurements for connect 3/14.
A connect magazine with­out
smartphone tests? Unthink­able!
What had happened?
L
LTE measurement setup
The LTE license auctioning in
April 2010 in Germany had
made it clear: As soon as LTE
phone sales and grid expansion
reached a critical mass, we nee-
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3/2014
ded to be able to measure LTE.
Easier said than done! Surely,
WEKA MEDIA PUBLISHING
owns three wickedly expensive
Universal Radio Communication Tester CMU200 by the
German noble brand Rohde &
Schwarz. These devices can simulate a cellular radio station,
but only in the GSM and UMTS
network, respectively. Since
you cannot upgrade the Rohde
& Schwarz to LTE, it was time
for something new. We started
searching for a suitable LTE
measurement setup in the beginning of 2012; two years
­later, this setup would cause the
aforementioned panic in the
newsroom.
800 Mhz instead of 900
On this journey, our first discovery was: The long waves of
LTE at 800 Megahertz make it
easier for the network operators
to cover large areas, but they’re
too much for the absorber hall
of our laboratory at only 3 x 3,5
x 3,5 meters. The lowest frequency this hall was suitable
for was GSM at 900 MHz. Part
of this setup were absorbing
wedges made of foam; their job
was to prevent outer metal walls
reflexions from falsifying the
direct emission of test aerial
and smartphone. Also, the distance between the two radio
partners was only good for precise measurements as minimal
33
Fotos: Peter Fenyvesi (5)
Smartphones
Wireless headquarters The Rohde & Schwarz CMW500 – nick­
named „Callbox“ by its fans – stands at the center of the LTE measurements.
as 900 MHz. Since both preconditions weren’t easily to be
changed – we couldn’t up the
ceiling height – the only solu­
tion was to enlarge the room
virtually through extremely flat
absorbers. This conversion was
reflected in the books with a
whopping 63 000 euros.
For measurement precision it
was also necessary to lengthen
the distance between antenna
and device under test – which
we achieved with a super flat
device you couldn’t even determine the smartphone’s aerial
sensitivity TIS (Total Isotropic
Sensi­tivity).
Having sounded out the market for mobile communications
measurement technique we
set our eyes on the wideband
radio communication tester
CMW500 by Rohde & Schwarz.
The CMU200’s LTE-ready successor is lovingly being referred to as „Callbox“ by many
testing engineers; every established smartphone manufacturer uses it. You can get the basic
CMW500 for a good 40 000 euros, roughly as much as a Mer-
cedes E-Class is, to stay in the
picture. The callbox works as a
base station and can therefore
establish contact with smartphones running R&S SIM
cards. It can also switch to special measuring modes and read
the data provided. To boot, the
CMW500 can measure by it­
self, but when it’s about accuracy it employs the extremely
precise spectrum analyser Rohde & Schwarz FSP7.
Since the basic CMW500 did
not meet the high connect requirements, we settled for the
MIMO option which is dimensioned for two LTE antennas
cross-polarized test aerial. It is
a big part of the accessory that
at 30 000 euros cost as much as
a brand new Volkswagen Golf
GTI. Still, we could rebuild the
absorber hall so we didn’t have
to find a new one.
The Callbox
But a perfect absorber hall is
nothing without the right
measuring device. Although
„measuring device“ is plain
­understatement – with a single
The measuring tower
Since December 2013, connect has been conducting its wireless measurements with the over the air tower Rohde & Schwarz (R&S) TS 8991. At the
top of the head-high rack there is a Maturo NCD controller: it can place the
turntable – with the smartphone fixed at the artificial head – precisely and
quickly in any position. The same goes for the long boom with the antenna
representing the cellular radio station. Below sits the R&S SMT03, the
measuring system’s standard signal generator. One storey down we find
the spectrum analyzer R&S FSP7, serving for all high-precision performance tests. Next up is the R&S Vector Signal Generator SMBV100A – for
GPS measurements, it can simulate as much as eight satellites simultaneously. It is followed by the LTE measurement’s centrepiece, the wideband
radio communication tester R&S CMW500; it supports many other modern
communication standards, too. Finally, there’s a tried and tested R&S
CMU200 for GSM and UMTS measurements. The R&S switch unit OSP120
takes care of connecting the correct ins and outs during measuring. The
R&S AMS32 software manages the measurements; the going rate for our
version is about 40 000 euros.
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3/2014
Fotos: © Peter Fenyvesi (6)
connect has bought plenty of measuring equipment in order
to be able to determine a smartphone’s wireless qualities.
each on the transmitter’s and
the receiver’s side. This option
is about 13 000 euros – you can
buy a Volkswagen Polo for that
kind of money.
Running out of time
Another massive challenge for
our measurement setup renovation were the many LTE wave
bands. Wireless carriers are
happy with this range because
they can use up to three bands:
Rural areas with less customers
are served best with 800 MHz
while highly frequented places
like metropolitan train stations
profit from small 2,6 GHz cells.
Every­where else you can operate with a combination of both
or sometimes with 1800 MHz.
Which means that we must
be able to measure all three
bands as to not put people at a
disadvantage – either in cities,
on motorways or in the country.
The consequence of this:
Instead of measuring the three
bands GSM 900, GSM 1800
and UMTS 2100 like we used
to we now have to care for six.
Trying to wrest another absorber hall plus a second CMW500
from the publishing director
was hopeless from the start. But
there are more phones coming
in every day, and as the utilization was high even before, we
had to look for new ways.
Power positioner
The many measuring points
surrounding the smartphone
when it is fixed on the artificial
head used to be a hassle because
each and everyone of these
points had to be approached
precisely with a positioner, vertically and horizontally, before
measuring could start. To cover
every measurement angle we
used to rely on a turning device
that could spin the artificial
head itself around two axises.
The new Conical Cut Positioner NCD by Maturo (45 000
euros) solves part of this time
consuming problem: It spins
the artificial head only vertically. The test aerial sits on a
long boom on the same level as
the smartphone can be turned
around the phone in a large
­circle to change the other angle
level. A precise high performance motor provides quick
Radiation diagram
How well does a smartphone radi­
ate LTE signals in which direction?
Our radiation diagram shows it.
Red symbolizes the highest level,
green stands for median power
and blue for the lowest.
changes of position. To supply
this motor with enough power
we placed a three phase current
in the absorber hall. A laser target device projects a hairline
cross onto the artificial head as
to place the phone precisely at
the intersection of the two turning axises.
Software update
To speed up the measurements
even more we needed to update
the software and purchase a se-
quencer option. This enabled us
to measure a variety of frequency bands automatically and in
rapid succession.
As the LTE measurement set­
up evaluation was completed in
March 2013, it called for an investment of 300 000 euros.
Such a sum required close scrutiny and ­approval at the highest
level at WEKA MEDIA PUBLISHING. Purchasing less
expen­sive technology was discussed, outsourcing the measurements was considered. But
finally the renovation got the
nod.
The renovation starts
As soon as that we had to find
two weeks time between producing issues of connect maga­
zine; we started at the end of
November 2013. Shortly before
Christmas the new system was
in place, and connect engineers
were trained to operate the
measuring equipment and the
software.
Come January, we firstly put
established UMTS phones to
the test – and the measurement
reports scared us to death. The
Turned in In order to be able to measure the radiated power under almost any angle we turn
the artificial head horizontally; the antenna for sending and receiving is vertically turned on a
boom in a large circle around the artificial head (see left). The control unit (above) schematically
shows the turning device in its current position on the screen.
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Smartphones
new sensitivity figures differed
radically from the ones we had
gathered previously. We changed the measurement bandwidth, the measurement channels, the measured subcarrier –
nothing did much of anything.
Was ­there a system error?
After quite a lot of experiments, redemption came: In
order to speed up the measurements a new technique to determine the bit error rate in rela­
tion to the smartphone’s signal
level was used. Here, experts
talk about a combination of
measurements – Receiver Signal Strength (RSS) over RSS
Patern Measurement, RSS Pattern Linearization and Sensitivity Search.
This method’s big advantage:
The important but time consuming bit error rate measuring
has to be conducted at one
measuring point only. All other
measuring points – there are
hundreds – can be calculated
from this result. That saves
plenty of time, but it requires a
connection between Callbox
and smartphone that we previously had not used. After ha-
ving chosen this option, old and
new figures were in unison –
and LTE measurements could
finally begin.
Important insight
For starters we gave the current
smartphone stars a going-over.
And we won an important insight: When the wireless coverage is good, an LTE phone
will transmit the maximum data rate – 100 Mbit/s or 150
Mbit/s at LTE 1800 or LTE
2600. But let the signal level
drop only about 1,5 dB below
the necessary threshold, and the
bit error rate will climb from
almost 0 to 100 per cent; the signal turns useless.
When signal quality goes
bad, LTE is able to at least
switch from the 64-QAM modulation scheme to the more
robust 16-QAM or to QPSK.
This way, data is transmitted, if
only at a quarter of the speed
possible. Therefore you should
look out for a smartphone’s
­wireless quality. connect shows
this quality – it is the only magazine in Europe to do so.
Bernd Theiss
Positioning
the smart­
phone pre­
cisely at the
artificial head
becomes
­possible
through a
­laser marker
Conclusion
Bernd Theiss,
connect editor
300 000 euros for a maximum 11
decibel difference between two
smartphones – that sounds a bit exaggerated, doesn’t it? Not so fast.
Consider this: Our LTE measurements
show that in the country you can be
three times as far away from a cell
tower with a Nokia Lumia 925 than
with a lesser device before the data
rate starts to sag. That shows how
important repeatable LTE measurements are. Especially since up to now
we have only measured the figurehead smartphones by top manufacturers – cheaper models will presumably show even bigger differences. Only measurements expose
certain weak points a phone has.
connect is proud to provide you with
this information first handedly.
A thousand details
Extensive protocol As soon as a smart­
phone’s communication qualities are ­measured –
here: the Motorola Moto G –, there’s plenty of data
for every communication standard.
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3/2014
A matter of attitude Before measuring can start, hundreds of details
have to be determined. There are bands and subcarriers, horizontal and vertical
measuring angles, equipment connection or how to determine the sensitivity.