Smart Antenna Technology—Beamforming

There is an ever-increasing demand on mobile wireless operators to provide voice and high-speed data services. At the same time, these operators want to support more users per basestation to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities are a pressing need.

Interference Limited Systems

Unfortunately, because the available broadcast spectrum is limited, attempts to increase traffic within a fixed bandwidth create more interference in the system and degrade the signal quality.

Smart Antenna Technology—Beamforming

Smart antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity. With this technology, each user’s signal is transmitted and received by the basestation only in the direction of that particular user. This drastically reduces the overall interference in the system. A smart antenna system, as shown in Figure bellow, consists of an array of antennas that together direct different transmission/reception beams toward each user in the system. This method of transmission and reception is called beamforming and is made possible through smart (advanced) signal processing at the baseband.

Smart Antenna System—Beamforming

In beamforming, each user’s signal is multiplied with complex weights that adjust the magnitude and phase of the signal to and from each antenna. This causes the output from the array of antennas to form a transmit/receive beam in the desired direction and minimizes the output in other directions.

Switched and Adaptive Beamforming

If the complex weights are selected from a library of weights that form beams in specific, predetermined directions, the process is called switched beamforming. Here, the basestation basically switches between the different beams based on the received signal strength measurements. On the other hand, if the weights are computed and adaptively updated in real time, the process is called adaptive beamforming. Through adaptive beamforming, the basestation can form narrower beams towards the desired user and nulls towards interfering users, considerably improving the signal-to-interference-plus-noise ratio.

• http://www.pafkiet.edu.pk/LinkClick.aspx?fileticket=141bbnhm%2Fi0%3D&tabid=579&mid=2014

Transmit Beamforming

Transmit beamforming is a method of transmitting two or more phase-shifted signals so that they will be in-phase at particular points in space where the transmitter believes the receiver to be, thereby increasing SNR.

• ftp://ftp.dlink.fr/Windows_Seven/White%20Paper%20by%20Ralink%20on%20802.11n%20Radio%20Beamforming%20technology.pdf
• http://www.spectrumsignal.com/publications/beamform_primer.pdf

Various Beamformer Structures Used In Smart Antennas

• http://omidi.iut.ac.ir/SDR/2007/WebPages/07_SmartAntenna/pages/sdr.html

Fixed Beam Forming Techniques

Fixed Beam Forming Techniques
Some array applications require several fixed beams that cover an angular sector.
Several beam forming techniques exist that provide these fixed beams. Three examples
are given here.
 Butler matrix The Butler matrix is a beam forming network that uses a
combination of 90° hybrids and phase shifters. An 8x8 Butler matrix is shown in Fig 3-5.
The Butler matrix performs a spatial fast Fourier transform and provides 2n orthogonal
beams. These beams are linearly independent combinations of the array element patterns



When used with a linear array the Butler matrix produces beams that overlap at
about 3.9 dB below the beam maxima. A Butler matrix-fed array can cover a sector of up
to 360° depending on element patterns and spacing. Each beam can be used by a
dedicated transmitter and/or receiver, or a single transmitter and/or receiver can be used,
and the appropriate beam can be selected using an RF switch. A Butler matrix can also
be used to steer the beam of a circular array by exciting the Butler matrix beam ports with
amplitude and phase weighted inputs followed by a variable uniform phase taper.
http://scholar.lib.vt.edu/theses/available/etd-04262000-15330030/unrestricted/ch3.pdf

Principles of Transmit Beamforming
The basic principle of transmit beamforming is to maximize the received Signal to
Noise Ratio (SNR) at the intended destination. It is a general signal processing technique
used to control the directionality of the transmission of a signal on a transducer array. A
transmit beamformer itself is a spatial filter that operates on the output of an array of
sensors in order to enhance the amplitude of a coherent wavefront relative to background
noise and directional interference.


Energy Gains of Transmit Beamforming
The question of how is directional transmission achieved is of particular importance. In
beamforming, each user’s signal is multiplied with complex weights that adjust the
magnitude and phase of the signal to and from each antenna. This causes the output from
the array of antennas to form a transmit beam in the desired direction and minimizes the
output in other directions. This directionality provides a strong energy gain observed at
the destination.

BEAMFORMING

 
Beamforming is a general signal processing technique used to control the directionality of the reception or transmission of a signal on a transducer array.

Using beamforming you can direct the majority of signal energy you transmit from a group of transducers (like audio speakers or radio antennae) in a chosen angular direction. Or you can calibrate your group of transducers when receiving signals such that you predominently receive from a chosen angular direction. The physics and math are essentially the same for both the transmitting and receiving cases.

  Follow the links for Beamforming & Smart Antennas:

• http://www.altera.com/end-markets/wireless/advanced-dsp/beamforming/wir-beamforming
• http://www.waset.org/journals/waset/v19/v19-33.pdf

Beamforming Techniques for OFDM

• http://www.mprg.org/research/OFDM/ofdm_beamforming.html
• http://www.ece.uci.edu/~ayanoglu/ICC05.pdf
• http://userspages.uob.edu.bh/mangoud/paper3.pdf

Beamforming Boosts the Range and Capacity of WiMAX Networks

• http://www.fujitsu.com/downloads/MICRO/fma/formpdf/WiMAXbeamform.pdf
• http://www.sequans.com/pdf/mimo_whitepaper.pdf
• http://www.commscope.com/andrew/eng/product/antennas/bsa/bsa_tech_info/Performance_Evaluation_%20MIMO_Base_Station_Antenna_Designs.pdf

Beam Forming vs Diversity

 http://users.soe.ucsc.edu/~friedlan/TXDIVBF.PDF

The only difference between transmit and receive diversity
is that, when Channel State Information (CSI) is not available at
the transmitter, then array gain is not possible to obtain, whereas
array gain is possible to obtain in the receive diversity system, as we
assume that perfect CSI is always available at the receiver .