FEATURE ARTICLE

WWW.ANTENNASONLINE.COM FALL 2012 ANTENNA SYSTEMS & TECHNOLOGY 9

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 that more cellular/wireless spectrum EDQGV�EHFRPH�DYDLODEOH��7KH�86��8.�and India for example have all indi-FDWHG� WKDW�DQRWKHU�����0+]�RI�8+)�spectrum will made available by the end of the decade.

2. � 6SHFWUXP� HI¿FLHQF\� JDLQV� FDQ� EH�exploited as operators move from �*� WR� �*� DFFHVV� WHFKQRORJLHV�� /7(�Advanced offers up to 8x8 MIMO schemes and advanced Interference Mitigation features.

3. Small cell and Het-Net network to-pologies are very much at the heart of LTE and LTE-Advanced. Cell split-WLQJ�� DQG�0DFUR� FHOO� RIÀRDG� WR� /7(�small cells ensures spectrum can be re-used cost-effectively. Also, Wi-Fi is evolving, and while already a small cell, it too is becoming smarter to help take demand off the cellular layers as part of the Het-Net.

In this article, we provide a discus-VLRQ�RQ�WKH�¿UVW�WZR�DUHDV�DV�VXPPDUL]HG�above, with a focus being on the base sta-tion antenna.

Radio Spectrum: The Past, Present And Future Figure 1 depicts where we have been and perhaps where we are headed in terms of spectrum bands, access technologies DQG� VSHFWUDO� HI¿FLHQF\� WHFKQLTXHV�� IRU� D�European context.

Two decades ago, there was only GSM technology in the 900 MHz band in Eu-rope. The base station antenna feature of WKH�GD\�ZDV�8SOLQN� VSDFH�GLYHUVLW\�� DV� D�means of extending coverage. The mid 1990s saw 1,800 MHz band being in-troduced for additional GSM capacity, WKHQ�PLG�����V� �*� LQ� WKH�8076� ÀDYRU�emerged in the 2,100 MHz band. The base station antenna innovation of this era was perhaps triple band X-Polar with variable/remote electrical tilt features. Europe has now introduced two further bands; 800 0+]�DQG�������0+]�ZKHUH�/7(�ZLOO�EH�deployed. Bands such as 700 MHz, 1,500 MHz and 2,300 MHz being exploited in other areas of the world and are expected to become available in Europe too, lever-aging economies of scale globally. For example, the Digital Dividend two band at 700 MHz is an agenda item at WRC-���� :KLWH� 6SDFH� VSHFWUXP� DW� WKH� ORZHU�8+)� IUHTXHQFLHV� RSHQV� XS� LQQRYDWLRQ�in cognitive and co-operative radio ac-cess techniques too. LTE-Advanced sup-ports carrier aggregation for spectrum re-farming in legacy bands, and a wide variety of MIMO techniques at the base VWDWLRQ� DQG�8(��7KLV� LV� LOOXVWUDWHG� DW� WKH�right aiming to illustrate in-band spectral HI¿FLHQF\�HYROXWLRQ��2QH�FRXOG�FRQFOXGH�that at the end of the decade, we may need Macro base station antennas supporting 10 bands, legacy technologies and offer high order MIMO including spatial mul-tiplexing and beamforming.

More Spectrum: But it May Come with More Interference As more spectrum will be consumed in more spectrum bands, as using higher power PA’s then this creates more inter-system and intra-system RF interference issues that need to be addressed. A recent example of inter-system interference is-sues include LightSquared’s proposed use of the MSS 1,500 MHz band as a ter-restrial cellular network being spectrally close to GPS and the fact GPS receivers GR� QRW� KDYH� VXI¿FLHQW�$GMDFHQW� &KDQQHO�Selectivity (ACS). Another example in Europe is the Digital TV band only has a 1 MHz guard band between it and the LTE800 band which can create, similar to LightSquared’s case, adjacent channel in-WHUIHUHQFH��7KH�86�DOVR�KDV�D�VLPLODU�LVVXH�concerning 700 MHz band lower A-Block being proximate to Channel 51 Digital TV broadcast. There are also numerous FDD/TDD and TDD/TDD proximate spectrum interference issues. Intra-system interference issues include Passive Intermodulation (PIM) interfer-ence. PIM has always been a concern, but as more spectrum is being used and at higher powers, PIM is becoming increas-LQJ� GLI¿FXOW� WR� PLWLJDWH� RU� FRQWURO�� $Q-tenna and antenna line component (cables, FRQQHFWRUV�� HWF��� 3,0� VSHFL¿FDWLRQV� DUH�bound by laws of physics and cannot be made perfectly linear. By way of an exam-SOH��)LJXUH���GHSLFWV�DQ�RSHUDWRU�ZLWK��[���MHz of 1,800 MHz band and 2x35 MHz

Base Station Antenna Evolution and Revolution %\�'DYLG�%DUNHU��&72���4XLQWHO�7HFKQRORJ\�/WG�

)LJXUH����8VH�RI�5DGLR�6SHFWUXP�3DVW��3UHVHQW�DQG�)XWXUH�IRU�(XURSH�

FEATURE ARTICLE

WWW.ANTENNASONLINE.COMANTENNA SYSTEMS & TECHNOLOGY FALL 201210

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 SRZHU�VSHFWUDO�GHQVLW\��DQG�WKH�$QWHQQD�3,0�VSHFL¿FDWLRQ�LV������dBc then there exists a complex convolution of third order PIM products; many exceeding an acceptable level at the receiver, as shown in the PIM Power Spectral Density graphs in Figure 2. One very simple solution, shown in Figure 2, is to use two an-tennas at a Macro base station cell site sector; one carrying all the Tx sub-bands and the other carrying all the Rx sub-bands. PIM SURGXFWV�IDOOLQJ�LQWR�WKH�5[�EDQG�DUH�VLJQL¿FDQWO\�UHGXFHG�VLQFH�Tx and Rx paths are physically separated in space. This technique is being considered by some operators, and can be viewed as a “Back to the Future” technique since many in the industry will recall the use of separate Tx, Rx and even RxD antennas at cel-lular sites before duplexing became popular (in order to reduce the antenna count at sites). Similar PIM combinations can exist in DQG�EHWZHHQ�RWKHU�EDQGV�VXFK�DV�ORZHU�����8SSHU��������������DQG������$:6�LQ�WKH�86��IRU�H[DPSOH�

6SHFWUXP�(I¿FLHQF\��0XOWLSOH�$QWHQQD�7HFKQLTXHV� 7KH�+DUWH\�6KDQQRQ� WKHRUHP�SURPLVHG�XV�VSHFWUDO�HI¿FLHQ-cies when information theory was developed mid last century. They didn’t however show exactly how we could achieve such HI¿FLHQFLHV��,Q�WKH�ODVW�WZR�GHFDGHV��VLJQL¿FDQW�DGYDQFHV�KDYH�been made in modulation and coding research, so much so that we are today virtually at the Shannon limit, again helped of

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 DQWHQQDV�DW�VLWHV�DQG�WHUPLQDOV��8S�WR��[�EDVH�VWDWLRQ�DQWHQQDV�DQG� �[�8(� DQWHQQDV� DUH� VSHFL¿HG� IRU�/7(�5HOHDVH� �� �WHUPHG��[��0,02��DQG�XS�WR��[��0,02�IRU�/7(�$GYDQFHG�5HOHDVH�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-dynamic adaptation between MIMO modes during a connected call state, where each active connection can have its own inde-pendent MIMO mode.� 7KH� FKRLFH� RI� RSWLPDO� EDVH� VWDWLRQ� DQWHQQD� FRQ¿JXUDWLRQ� LV�complex as performance is highly dependent upon the radio channel characteristics (cell geometry, spatial dispersion, etc.), PRELOLW\�VSHHGV��DQG�VSDWLDO�GLVWULEXWLRQ�RI�WUDI¿F��'LIIHUHQW�EDVH�VWDWLRQ�DQWHQQD�FRQ¿JXUDWLRQV�PD\�SHUIRUP�PRUH�RSWLPDOO\�WKDQ�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. � 7KHUH�DUH�DW�OHDVW�D�QXPEHU�RI�GHIDXOW�DQWHQQD�FRQ¿JXUDWLRQV�emerging as preferred practical solutions for LTE Release 8. One VXFK�DQWHQQD�FRQ¿JXUDWLRQ�EHLQJ�WKH�GXDO�FURVV�SRODU�DUUD\�DQ-WHQQD��ZKLFK�KDV��[�SRUWV�ZKHUH�WKH�FURVV�SRODU�DUUD\V�DUH�KRUL-]RQWDOO\�VHSDUDWHG�E\�a�������O termed CLA-2X (Clustered Lin-ear Array – 2 X-Polar columns) with the intention that 2x pairs

Figure 2. Example of Intra-System Cross Band 3rd Order PIM power spec-WUDO�GHQVLW\�DQG�QRYHO�3,0�PLWLJDWLRQ�FRQ¿JXUDWLRQ�

Getting Beyond Fear, Uncertainty and Doubt

'HFHPEHU�������������,UYLQJ��7;ZZZ�*ULG�,QWHURS�FRP

Driving to Grid 2020

$����\HDU�VWUDWHJLF�SODQ�IRU�JULG�LQWHURSHUDELOLW\��$�FHOHEUDWLRQ�RI�LQWHURSHUDELOLW\�VXFFHVVHV��$�YLVLRQ�IRU�WZR�RUJDQL]DWLRQV�GULYLQJ�LQWHURSHUDELOLW\�DV�WKH\�VKLIW�WKHLU�IRFXV�LQ�������$�JDWKHULQJ�RI�WKH�LQWHURSHUDELOLW\�FRPPXQLW\��$QG�D�YLVXDO�GLVSOD\�RI�LQWHURSHUDELOLW\�LQ�DFWLRQ�

,W¶V�DOO�KDSSHQLQJ�DW�WKH�VL[WK�DQQXDO�*ULG�,QWHURS�

FEATURE ARTICLE

WWW.ANTENNASONLINE.COM FALL 2012 ANTENNA SYSTEMS & TECHNOLOGY 11

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-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 SHU�8(�RU�8(�JURXS� UHVROXWLRQ�� XVLQJ�/7(� FRGHERRN� RU� QRQ�FRGHERRN� SUH�FRGLQJ��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-WLRQ�SODQH�%HDPVWHHULQJ�SURPLVHV�HQKDQFHG�VSHFWUDO�HI¿FLHQFLHV�RYHU�$]LPXWKDO�SODQH�Beamsteering. Of course a composite of Elevation and Azimuth plane Beamsteering will promise even further gains. Quintel technology has been researching this area and has developed a novel antenna GHVLJQ�WKDW�HIIHFWLYHO\�LQWURGXFHV�D�QHZ��[�SRUW�%DVH�6WDWLRQ�DQWHQQD�RSWLRQ�IRU�FRQVLG-HUDWLRQ��,W�LV�ORJLFDOO\�WKH�VDPH�DV�WKH�&/$��;�DQWHQQD�FRQ¿JXUDWLRQ�EXW�SHUIRUPV�FR-KHUHQW�%HDPVWHHULQJ�RQ�D�SHU�8(�RU�8(�JURXS�EDVLV�LQ�WKH�(OHYDWLRQ�SODQH��UDWKHU�WKDQ�Azimuth plane, and is based on a completely passive traditional column array, rather than an Active phased Array solution.

)LJXUH����([DPSOH�RI�8SOLQN�WKURXJKSXW�HI¿FLHQF\�IRU�$]LPXWK��&/$��;��DQG�(OHYDWLRQ��4XLQWHO��SODQH�DGDSWLYH�%HDPVWHHULQJ�WHFKQLTXHV�IRU��*33�Macro A radio channel.

- 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.