Beamforming vs. MIMO Antennas

MIMO (Multiple Input Multiple Output) antennas operate by breaking high-data rate signals into multiple lower data rate signals in Tx mode that are recombined at the receiver. MIMO antennas typically have narrow beamwidths with two or more columns of dipoles spaced a wavelength apart to maximize gain and minimize coupling between columns. Antennas contain multiple independent arrays, with one transmitting part of the signal.

Beamforming arrays are typically rectangular arrays and are inherently different from MIMO in that the multiple columns of dipoles work together to create a single high-gain signal. The columns need to be closely spaced (half a wavelength) together and have wide beamwidths in order to scan the beam away from the bore site while maintaining the antenna’s gain.

While both techniques work well, an antenna optimized for one method does not necessarily work well for another. Compromise geometries exist, but they sacrifice system performance in order to save money on the relatively inexpensive antenna.

Beamforming Antennas

If a beamforming solution is selected, the user will have the choice of an active or passive (switched-beam) solution. The active solution is steered and shaped by changing the power level and phases being output by the radios. Each column is fed by a dedicated radio, and a calibration port is used to guarantee the overall amplitude and phase that the antenna is seeing at its inputs. The beam can be steered to any angle within the specified range of the system, and its sidelobes are suppressed as needed.

A power divider inside the antenna controls all phasing and amplitudes in a passive solution. In the divider, also known as a Butler Matrix, the number of beams, their pointing angles, and sidelobe levels cannot be changed—similar to breaking the sector into smaller sub-sectors. The user will therefore see the benefits of higher gain and reduced interference.

This solution does not have the advantages of an active antenna, which can steer the beam directly at the user or a null in the direction of an interferer. Because each switched-beam, rather than each column, is powered by one radio, the overall EIRP (Equivalent Isotropic Radiated Power) of the passive antenna will be less than that of the active antenna that uses all the radios to form the beam.

Antenna Architecture

A base station antenna is a passive device that does not typically depend on what technology is being used in the system. It is analogous to the nozzle of a garden hose; the nozzle can be adjusted from shower to jet (low-gain to high-gain), but any type of liquid (AMPS, GSM, PCS, DCS, CDMA, etc.) that flows through the hose (RF cable) will flow equally well out of the nozzle.

Occasionally, the opposite is not true, and the success of certain systems is dependent upon the architecture of the antenna. The most obvious would be a polarization diversity system that will not work well if only vertically polarized antennas are available. Conversely, TDD systems are able to operate well with antennas that have poor PIM performance since the transmitting and receiving frequencies are never used simultaneously. That same antenna with poor PIM could cause unacceptable noise in an FDD system.

Beamforming vs. MIMO Architectures

There are significant architectural differences between an antenna that has been designed for MIMO performance and one that has been optimized for beamforming. Antenna characteristics that are desirable for MIMO (widely spaced columns with narrow beamwidths) will result in a degradation of the beamforming ability of the antenna, which requires narrowly spaced columns with wide beamwidths.

Likewise, there are differences between beamforming antennas that are used in active and passive systems. While the same radiating structure can be used for either system, an active system has a separate radio with variable amplitude and phase control of every column within the antenna. In theory, an active antenna can form a beam pointing to any angle in the sector or can steer nulls towards interferers.

A passive, or switched-beam, antenna has all of its phasing and amplitude distribution controlled by a single circuit board, which will have a number of outputs equal to the number of columns in the antenna and a number of inputs equal to the number of beams that can be created. Each beam points in a discreet angle, and the system chooses which beam to use for a given mobile.

Both MIMO and beamforming antennas are powerful tools that improve data rates and reduce the impact of noise and fading in their systems. The way they achieve these improvements is very different from one another, and they require very different antenna architectures.

The columns of a MIMO array act almost independently of each other; their narrow beamwidths are spaced far apart, and each column “carries” part of the load. The columns of a beamforming array act as a “team” to carry the whole data load simultaneously. If an operator plans on using four-column antennas in both MIMO and beamforming scenarios, two unique antennas are recommended to avoid compromising system performance.

A fully adaptive array is extremely flexible in its ability to steer the beam or the null, depending on which method provides the best result. The primary design choice for this antenna, assuming it uses the ideal half-wavelength column spacing, is the number of columns that can be supported by the radio system (and allowed by zoning authorities).

Every extra column reduces the beamwidth and adds gain, EIRP (Effective Isotropic Radiated Power), and EIRS (Equivalent Isotropic Receiver Sensitivity), but makes for a wider, more expensive antenna and more complex operating system.

The passive switched-beam antennas require the network planner to make more choices up front since they cannot be changed after installation. The first choice will still be the number of columns, which will determine beamwidth, gain, and width. After that, the number of beams the system will use needs to be decided. The maximum number of beams is limited to the number of columns within the array, but this might not be the most desirable choice for the system. By reducing the number of beams, other RF parameters (such as sidelobes or beam cross-overs) can be optimized. Once these design parameters have been determined, all beam-steering will occur within the Butler Matrix inside the antenna. Each beam of the passive antenna will act as a small sector with its own radio.

Passive antennas were used more widely in the past because of the simplicity of the system, and network planners did not need to control the phase and amplitudes of their radios because all of those settings were predetermined by the Butler Matrix. Adding a switched-beam antenna was no more complicated to the system than splitting the sector up into several sub-sectors using multiple narrow-beam antennas. This simplicity came at a cost though compared to a truly adaptive beam-steering antenna.

Today’s systems, with the radios near the antennas, are capable of varying their phase and amplitude with a good degree of accuracy, making adaptive antennas the preferred choice of network planners. The calibration boards within the antennas guarantee the antenna pattern will be pointing in the direction the system expects, the sidelobes will be suppressed, and the nulls will be as deep as possible to combat interferers. Switching from narrow service beams to a wider broadcast beam is as simple as changing the radio settings. The largest advantage the adaptive array has over the passive array is radio power sharing capabilities across the columns, significantly increasing the network’s EIRP and improving system reliability in case one or more radios fail.