Antenna Online article on Base Station Antenna Evolution and Revolution http://www.antennasonline.com/

1. FEATURE ARTICLE
Base Station Antenna Evolution and Revolution
Moore’s Law successfully predicted
the growth in processing power over the
last three decades and it still holds true
today and for the foreseeable future. This
growth in processing power has in turn
enabled the mobile data consumption ex-
plosion, which emerged a few years ago,
and today we are all predicting on what
logarithmic order this will grow at. The
demand is being addressing on a number
of fronts in the industry:
1. Governments are putting in place
policy and commitments to ensure Two decades ago, there was only GSM More Spectrum: But it May Come with
that more cellular/wireless spectrum technology in the 900 MHz band in Eu- More Interference
rope. The base station antenna feature of As more spectrum will be consumed
and India for example have all indi- in more spectrum bands, as using higher
means of extending coverage. The mid power PA’s then this creates more inter-
spectrum will made available by the 1990s saw 1,800 MHz band being in- system and intra-system RF interference
end of the decade. troduced for additional GSM capacity, issues that need to be addressed. A recent
2. example of inter-system interference is-
exploited as operators move from emerged in the 2,100 MHz band. The base sues include LightSquared’s proposed
station antenna innovation of this era was use of the MSS 1,500 MHz band as a ter-
Advanced offers up to 8x8 MIMO perhaps triple band X-Polar with variable/ restrial cellular network being spectrally
schemes and advanced Interference remote electrical tilt features. Europe has close to GPS and the fact GPS receivers
Mitigation features. now introduced two further bands; 800
3. Small cell and Het-Net network to- Selectivity (ACS). Another example in
pologies are very much at the heart of deployed. Bands such as 700 MHz, 1,500 Europe is the Digital TV band only has
LTE and LTE-Advanced. Cell split- MHz and 2,300 MHz being exploited in a 1 MHz guard band between it and the
other areas of the world and are expected LTE800 band which can create, similar to
small cells ensures spectrum can be to become available in Europe too, lever- LightSquared’s case, adjacent channel in-
re-used cost-effectively. Also, Wi-Fi aging economies of scale globally. For
is evolving, and while already a small example, the Digital Dividend two band concerning 700 MHz band lower A-Block
cell, it too is becoming smarter to help at 700 MHz is an agenda item at WRC- being proximate to Channel 51 Digital TV
take demand off the cellular layers as broadcast. There are also numerous FDD/
part of the Het-Net. TDD and TDD/TDD proximate spectrum
in cognitive and co-operative radio ac- interference issues.
In this article, we provide a discus- cess techniques too. LTE-Advanced sup- Intra-system interference issues include
ports carrier aggregation for spectrum Passive Intermodulation (PIM) interfer-
above, with a focus being on the base sta- re-farming in legacy bands, and a wide ence. PIM has always been a concern,
tion antenna. variety of MIMO techniques at the base but as more spectrum is being used and at
higher powers, PIM is becoming increas-
Radio Spectrum: The Past, Present right aiming to illustrate in-band spectral -
And Future tenna and antenna line component (cables,
Figure 1 depicts where we have been that at the end of the decade, we may need
and perhaps where we are headed in terms Macro base station antennas supporting bound by laws of physics and cannot be
of spectrum bands, access technologies 10 bands, legacy technologies and offer made perfectly linear. By way of an exam-
high order MIMO including spatial mul-
European context. tiplexing and beamforming. MHz of 1,800 MHz band and 2x35 MHz
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2. FEATURE ARTICLE
of 2,100 MHz band spectrum (European band example). If all
this spectrum is used, as it could be when considering LTE-Ad-
vanced Carrier Aggregation, and is used with around 2W/MHz
dBc then there exists a complex convolution of third order PIM
products; many exceeding an acceptable level at the receiver, as
shown in the PIM Power Spectral Density graphs in Figure 2.
One very simple solution, shown in Figure 2, is to use two an-
tennas at a Macro base station cell site sector; one carrying all the
Tx sub-bands and the other carrying all the Rx sub-bands. PIM
Tx and Rx paths are physically separated in space. This technique
is being considered by some operators, and can be viewed as a
“Back to the Future” technique since many in the industry will
recall the use of separate Tx, Rx and even RxD antennas at cel-
lular sites before duplexing became popular (in order to reduce
the antenna count at sites). Similar PIM combinations can exist in
-
cies when information theory was developed mid last century. Figure 2. Example of Intra-System Cross Band 3rd Order PIM power spec-
They didn’t however show exactly how we could achieve such
we are today virtually at the Shannon limit, again helped of
been made in modulation and coding research, so much so that
course by Moore’s promise of increasing processing power. To
go beyond this limit, MIMO antenna techniques allow multiple
spatial (Shannon) channels to be created and/or further improve
C/I conditions, and thus enhance what we can squeeze out of a
spectral resource, but at the expense of having to add additional
10. MIMO techniques include methods of spatial multiplexing,
Tx diversity, Rx diversity, coherent Beamforming at Tx and
coherent Beamforming at Rx. Additionally, LTE offers quasi-
Driving to Grid 2020 dynamic adaptation between MIMO modes during a connected
call state, where each active connection can have its own inde-
pendent MIMO mode.
Getting Beyond Fear,
Uncertainty and Doubt complex as performance is highly dependent upon the radio
channel characteristics (cell geometry, spatial dispersion, etc.),
others in different scenarios. Additionally, the number of anten-
nas per sector at a site, together with consideration of supporting
additional spectrum bands for existing cellular services needs to
be considered too as this impacts wind-loading, site rentals and
planning permission aspects.
emerging as preferred practical solutions for LTE Release 8. One
-
-
termed CLA-2X (Clustered Lin-
ear Array – 2 X-Polar columns) with the intention that 2x pairs
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3. FEATURE ARTICLE
of ports are uncor-
related (i.e. having
orthogonal polar-
izations, to allow
two-layer spatial
multiplexing or di-
versity) but within
each pair of ports
they are correlated
(to allow Azimuth-
al Beamforming/
Beamsteering with- Macro A radio channel.
in each spatial lay-
er). Furthermore,
such an antenna consumes a single antenna position at site, and there is a view that most
terminals can only support 2x uncorrelated antennas due to physical size and hence 2x
layers of spatial multiplexing is only possible to/from a terminal. With only two ports
per polarized array then this is really Beamsteering (rather than Beamforming) on top of
spatial multiplexing but allows C/I geometry to be enhanced, especially with later LTE
releases.
While the CLA-2X antenna in effect delivers Azimuthal plane Beamsteering on a
there has been much research recently in vertical plane Beamforming/Beamsteering.
Vertical or Elevation plane Beamforming can only be in principal delivered using a
fully Active (phase array) Antenna. In radio channels, which exhibit high angular dis-
persion in the vertical plane, full elevation Beamforming is optimal in order to equal-
ize the channel but in Macro radio channels, which exhibit weak angular dispersion in
the vertical plane simple Beamsteering can offer virtually the same gains. In certain
cellular geometries, for example, those with relatively dense Macro site grids, eleva-
Beamsteering. Of course a composite of Elevation and Azimuth plane Beamsteering
will promise even further gains.
Quintel technology has been researching this area and has developed a novel antenna
-
-
Azimuth plane, and is based on a completely passive traditional column array, rather
than an Active phased Array solution.
- See Quintel Technology at Mobile Antenna Systems 2012
David is CTO and director of Technology Applications at Quintel
Technology Limited, and has been with Quintel for more than seven
years. His role involves identifying and assessing how Quintel’s RF
technology innovations can be applied for wireless communications
markets, and in particular for the LTE and MIMO antenna world.
David started his career in RF engineering 15 years ago in radar,
and electronic warfare systems with the UK Ministry of Defense, and
Defense Research Agency. David holds a BEng (Hons) from Bradford
University and an MSc from the Royal Military College of Science.
David’s presentation abstract can be found at www.AntennasOnline.
com/conferences.
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