The evolution of base station antennas for mobile communications

• During the last two decades, mobile telephony has virtually exploded around the world. Whereas fixed telephony has merely around 800 milion phone lines globally, cellular mobile telephony has today close to 4 bilion subscribers less than 20 years after the introduction of digital mobile telephony! In the centre of a mobile cell is the base station antenna whose vertical and horizontal beam pattern creates the borders. The first base station antennas deployed for cellular communications were omni-directional in the horizontal plane. These antennas are typically based on stacked circular dipole elements fed by a series feed network. However, as the need for capacity increased more radio channels were being used and the radio networks soon reconfigured to 3 sector systems. The base station antennas in these type of systems typically has half power beam widths in the horizontal plane of around 65 degrees and the elements are fed by corporate feed networks. Through the choice of antenna length and the selection of vertical element positions and excitations a broad range of antenna gains and vertical beam patterns can be achieved. We show that for a linear array the gain is mainly determined by the vertical and horizontal beam widths but reduced by losses in the feed network. The electrical down tilt is needed to provide good coverage within the desired cell and is preferred to mechanical down tilt as it is independent of horizontal angle. Remotely adjustable electrical down tilt is also available today and an important cost saver during the expansion part of a network. As we will see, system requirements on reduced channel interferencemlead to a simple expression for the necessary phase shift in such an array. Due to the rapid increase in subscribers it was soon realized by the regulators around the world that one frequency band for digital mobile communications was not enough to provide the capacity needed. Therefore, in the mid 90-ties second frequency bands were introduced in both North America and Europe. Since operators are not keen to put up bigger towers and more antennas, ´this development called for dual-band antennas. The introduction of the UMTS band in the beginning of this millennium of course then called for triple band functionality! Traditionally the diversity is achieved by using two receive antenna branches separated in space. In mobile phone networks, all base stations for macro cellular communications incorporate diversity on the up-link. Otherwise it would be virtually impossible to communicate with a low power mobile over the rapidly fading channel. Since space diversity uses two horizontally separate antennas positioned about 20 wavelengths apart it become soon of interest for the operators to incorporate polarization diversity. With polarization diversity only one dual polarized antenna is used for each sector at the base. In order to ensure good polarization diversity it is necessary and sufficient to have symmetrical patterns with equal power in horizontal and vertical polarization. In 3G it is of great interest to strengthen the up- and down-links in order to be able to increase the data rates from today’s 16kbit/s (voice) to a wireless broadband of around 10Mbits/s. However, in the Long Term Evolution of 3G, LTE, data rates of up to and around 100MBits/s are expected. In order accomplish this diversity at the base station will not be enough but complemented by multiple branch reception and transmission at both the base station and the mobile terminal (Multiple-Input-Multiple-Output, MIMO systems). For such arrays it is of interest to reduce the mutual coupling between elements. Although the mutual coupling could be compensated for by perfect channel estimation it is still of interest to reduce the coupling in practice in order to be able to handle the mismatch loss. In order to do so the use of e.g. corrugations, hard and soft surfaces as well as Electronic Band Gap, EBG materials for the inclusion into the antenna structure has been studied. In conclusion we find that the base station antenna has developed dramatically during the last two decades and base station antenna technology may become a key enabler for the Long Term Evolution of 3G.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Elektroteknik och elektronik -- Annan elektroteknik och elektronik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Electrical Engineering, Electronic Engineering, Information Engineering -- Other Electrical Engineering, Electronic Engineering, Information Engineering (hsv//eng)

Nyckelord

Electronics

Elektronik

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