Market & Technology Drivers Effecting Modern Spectral Analysis
The spectrum analyser is the foundation of any RF test scenario. Through it engineers are able to quantify the amplitude of signals present against frequency so that the occupied bandwidth can be determined and sources of interference identified.
Thanks to modern analyser models inter-modulation, harmonics, noise and any
other unwanted artefacts that may arise can be identified and dealt with.
However, the ongoing progression of the wireless communication systems now being
used, driven by the need for ever greater bandwidth, means that further headway
is being called for.
As new communications standards emerge, considerably higher frequencies are now
being required - up to 40 GHz for applications such as point-to-point microwave
links. Other broadband wireless technologies are likely to start seeing greater
uptake, which will also necessitate higher frequency analysis being undertaken.
In addition, there are frequency modulation issues arising from deployment of
next generation 3G/4G mobile network infrastructure and WLAN hubs supporting the
proposed 802.11ac standard.
Frequency Modulation
The more complex modulation techniques now being used for various forms of
wireless communications will allow far greater quantities of data to be
transferred. This is particularly underlined by the advent of the 802.11ac WLAN
standard - the draft specification for which was just published by the IEEE
earlier this year. Located in the 5 GHz frequency band, this will be able to
support far higher data rates than the previous 802.11n standard, with up to 1
Gigabit/s total throughput being possible (effectively doubling the capacity of
802.11n). It is fully back compatible with the both 802.11a and 802.11n
technologies and is capable of unhindered coexistence with them. However it does
need a much wider RF bandwidth than earlier WLAN generations, with a larger
number of multiple-input/multiple-output (MIMO) streams (up to 8x8 will
eventually be supported). As the 802.11ac standard relies on high-density
modulation (BPSK, QPSK, 16-QAM, 64-QAM, or 256-QAM), the measuring of modulation
quality of these wireless signals is certain to be a challenge.
Operators’ labs may start to stock multi-standard radio analysers for studying
data streams in real-time, to see where signals hop to and from – as more of the
spectrum begins to be opened up to frequency hopping. This will allow analysis
of different standards at different frequencies in parallel. With 256-QAM
modulation, there is a clear need for higher specification instruments to
analyse the output from WLAN hubs. Error vector magnitude (EVM) gives a
convenient performance metric for the modulated signal produced by a WLAN
transmitter. It can be visualised in terms of the difference between the
measured vector and an ideal (reference) vector. The test system being used has
an effect on how well the signal is acquired. A spectrum analyser which exhibits
poor EVM performance characteristics will increase the error magnitude when
higher order modulation is present and will produce results that are not of
sufficient quality.
Congestion Problems
As mobile base stations supporting LTE technology begin to be rolled out and
LTE-Advanced follows on from this in the coming years, there will be exacting
demands placed on signal analysis equipment. The infrastructure being utilised
by network operators will still need to support UMTS/WCDMA/GSM technologies – so
that all of the subscribers are served effectively. The problem is how operators
ensure that signals from these different wireless technologies do not start
interfering with one another and impinge on communication clarity. LTE-Advanced
will rely on a greater degree of multi-path technology (with 4x4 MIMO streams to
begin with, scaling up to 8x8 over time) and will also employ multi-carrier
frequency division multiplexing. Once again this means there will be a need for
more sophisticated modulation analysis to be embarked upon. Furthermore digital
pre-distortion techniques will help to get rid of mobile base station power
amplifiers’ intrinsic distortion characteristics, but when the power amplifier
works at 20 MHz, the spectrum analyser that is being used with it will need to
support 100 MHz of instantaneous bandwidth or above with high spurious free
dynamic range if it is going to be able to accurately assess the situation and
give results with real value.
Other Considerations
Real-time analysis is proving of particular importance when it comes to checking
system compliance with RFID standards, with wider frequency ranges now becoming
involved in order to tackle today’s dynamic, overcrowded RF environment. It is
necessary to undertake analysis across an expanding frequency hopping range for
both the RFID tag and RFID reader. Also the dynamic range envelope is being
stretched – as often the tag will transmit at same frequency as the reader,
making it’s considerably lower power signal difficult to distinguish without
high performance analysis tools.
Increasingly radar systems are making use of intricate techniques and algorithms
to enhance their resolution, so they can accurately gauge the speed of moving
objects. This also has implications for the spectrum analysis hardware that
tests such systems, with strong RF performance becoming mandatory.
The growing number of wind farms being implemented also has an effect on the
integrity of a variety of wireless signals. The rotating blades of a wind
turbine can cause disruption -physically breaking the transmission path of
microwave links, creating anomalies on radar equipment, or causing multi-path
interference for television broadcasts. Once again there is a need to employ
modern spectral analysis tools to deal with such problems.
Test equipment manufacturers have responded to the challenges now being set by
the industry. Among the new breed of signal/spectrum analysis solutions now
entering the market is the FSW series from Rohde & Schwarz. Covering a frequency
range from 2 Hz to 26.5 GHz, the FSW is able to support a 160 MHz demodulation
bandwidth for analysis of wideband and frequency hopping signals and can quickly
detect spurious signals given off by the wireless transmitter being tested. It
has a phase noise of –137 dBc at 10 kHz offset and a -88 dB dynamic range which
are key technical requirements for accurate measurement of the signals
discussed.
Financial Pressures
There are cost issues associated with the higher performance pieces of test
equipment now desired by engineers (with price tags of £40K to £100K now
becoming commonplace), and this is leading to a change of attitude when
considering how such items should be sourced. Direct purchase of equipment is
now beginning to lose its edge and as a result a significant proportion of
operators, and the contractors they engage with for network deployment projects,
are now of the opinion that rental is a better option. The need for greater
flexibility, as well as reduced capital expenditure, is attracting them towards
equipment rental partnerships.
The ever rising demand for greater wireless transmission capacity is leading to
increasingly congested airwaves. The multi-standard base stations and WLAN hubs
of the future will require spectrum analysers that can handle higher speeds and
have the ability to measure different parallel signals, as well as having pin
point accuracy, strong sensitivity and wide dynamic range. These advances in
analysis technology need to be matched by an evolution in current business
models, so that engineering and economical factors are both taken into account.
Figure 1: Rohde & Schwarz FSW Spectrum Analyser
Figure 2: Multi-Standard Radio Analysis
By Alain Mignot , Livingston & Darren Tipton ,
Rohde & Schwarz
20120503016.5.2012Мерящая техника-