From Dipole to Active Antenna Systems and 4×4 MIMO

Recently, T-Mobile US has announced that they have switched on 4×4 MIMO support in their network and first smartphones are now supporting 4 simultaneous downlink streams. Quite an interesting announcement as most antennas and base station sites currently deployed by most operators do not yet support this. So let’s have a look at what has changed in recent years in base station technology.

Going back 20 years to the simple days of GSM, the base station world was relatively simple compared to today. There was a single band, e.g. the 900 MHz band, that had to be supported. Especially in rural areas it was not uncommon to see flagpole antennas (dipoles) being deployed. In cities, 3-sector panel antennas were used to increase capacity. From a design point of view, the digital baseband and the radio part of a base station were in a voluminous cabinet usually at ground level or indoors and coax cables fed the RF signal to the passive antennas on a rooftop or tower.

Things became a bit more difficult with UMTS, as suddenly two frequency bands needed to be supported (e.g. 900 MHz for GSM and 2100 MHz for UMTS). Most network operators at the time installed dual band antennas so a single antenna panel had two antennas inside. The number of coaxial feeder cables increased but it was still manageable.

With LTE, yet another frequency band had to be added to a base station site. Technology had advanced at that point so many network operators used the upgrade cycle to install a remote radio head close to the antenna which is connected by an optical cable to the baseband unit further away. This saved a lot of cost because coaxial cable is expensive. Also, this improved performance as there was no RF loss in the coaxial cable anymore as the distance between the remote radio head and the passive antenna was very short. Also, the size of the digital baseband unit and backhaul equipment significantly reduced in size. This meant that the baseband unit could also move closer to the antenna mast or even onto the antenna mast itself. In addition, air conditioned cabinets for cooling in summer and heating in winter are no longer necessary.

This setup is still used today by many network operators even though in the meantime a typical urban base station uses even more frequency bands: For LTE, 800, 1800 and 2600 MHz are often deployed simultaneously and in addition 900 MHz for GSM and 2100 MHz for UMTS. In the future even more bands will be added. In the US, a different set of bands is used but the number of bands is comparable. Fortunately, antenna technology has advanced as well and a single flat antenna casing now contains 3 antennas inside, one for 700-900 MHz, one for 1800 to 2100 MHz and one for 2600 MHz. There are also antenna casings that include a single antenna for the 1800 – 2600 MHz range as shown in a picture in this article.

The next step in base station and antenna design are active antennas where the remote radio heads that were so far separate now move to the backside of the antennas and become an integrated part. In addition, a single antenna has been split into several smaller parts which are individually controlled by different integrated active radio modules. This way it’s possible to control tilt electronically and add 4×4 MIMO capability via 2 separate cross polarized antenna strings for 4 paths. The antenna casing width becomes somewhat larger because of the second cross polarized antenna string to upgrade from 2×2 MIMO to 4×4 MIMO and a bit thicker because of the integrated remote radio heads. In return there’s only a single fiber cable between the baseband unit and a second cable for power. Gone is all the cable mess.

In the case of T-Mobile US, the Ericsson AIR 32 active antenna system seems to be used and Ericsson even put a promotional video about it on Youtube. NSN calls a similar system the Flexi Multiradio 10. Huawei and others have similar systems but I couldn’t find good links to include here.

For more details I recommend this presentation by Kathrein about antenna evolution from 4G to 5G.