  Massive MIMO is a promising air interface technique for 5G wireless
communications. Such antennas offers capabilities to utilize channel
correlations to create beams suitable for user specific transmissions. Most of
the prior research is devoted to user-specific transmission in such systems,
where a narrow beam is formed towards a user to improve its reception.
Meanwhile, public-channel transmission to multiple users at once by means of
broad beams has been understudied. In this paper, we introduce the concept of
array-size invariant (ASI) beamforming, enabling generation of broad beams from
very large antenna arrays where all elements are transmitting at maximum power.
The ASI technique offers the possibility to achieve a perfectly flat array
factor by exploiting the additional degree of freedom coming from polarization
diversity. The technique is applicable to uniform linear and rectangular arrays
and is characterized by low-complexity beam synthesis. The usefulness of the
ASI beamforming is illustrated by means of a numerical example showing the
design of a cell-specific broad beam for publicchannel transmission in a
realistic network deployment.
