Dr. Mohammed Nadder Hamdy, Director, Wireless Network Engineering
Base station antennas are like closed boxes that we rarely get a chance to look inside. They can’t be opened without getting damaged, and their size and weight makes them difficult to display inside offices. Despite significant technological advancements on the inside of the box, we still see only the same shaped box from the outside. This could be one of the reasons behind many misperceptions surrounding their function, which have turned into myths over time. Let’s explore some of the most common myths related to these mysterious boxes, revealing a few of the industry’s best-kept secrets.
While there are many industry myths, the five more prevalent are:
- Antenna parameters are invariable
- A panel antenna has the same pattern as its radiating elements
- Multiple input ports mean multiple arrays inside
- Multibeam antennas have multiple arrays inside
- Beam steering requires active antennas
Myth 1: All Antenna Parameters are Invariable
Most radio planners will look to select their antenna base stations by gain, beamwidth, supported bands, and input ports. When consulting a datasheet listing these variables for different antennas, many planners will only see these values in their headers and summaries. However, what is often missed is the wide operating bandwidth and electrical tilt, both of which are key factors in affecting performance. To showcase this, we will examine two different examples – horizontal beamwidth (HPBW), and a front-to-back ratio example.
The graph below examines three different half power horizontal beamwidth antennas, deployed at 65 degrees. Each colored line represents a different frequency at a specific e-tilt.
As you can see, the HPBW varies quite widely across the variety of supported bands and tilts of each antenna. If HPBW falls below expectations, this can lead to coverage holes – while conversely, increasing can lead to interferences. This is why tighter deviations are ideal, as it provides the most consistency of service.
Front to Back Ratio
In examining the front-to-back ratios, with once again each colored line representing a different frequency, we can see there is still a level of variance in each of the three models.
As we can see, there is an enormous variance between each different antenna, which is not accounted for in the datasheet.
While most engineers, when comparing the antenna datasheets of different antennas, might observe identical parameter values – it is important to bear in mind that the antenna which has the fewest dispersions will always be the better on in the long run.
Myth 2: A Panel Antenna has the Same Pattern as its Radiation Elements
Many operators are familiar with panel antenna patterns and electrical tilting in the horizontal and vertical planes. However, there is a misperception about how the panel’s overall pattern is shaped.
A panel antenna comprises a number of radiating antenna elements’ arrays, for high and low bands. Low band elements dipoles are larger in size, and require more isolation distances in between each other than high-band ones. It is thus, logical that more of the high-band arrays can fit into the same antenna radome.
However, the panel antenna pattern is quite dissimilar from its radiating antenna element pattern. This is the result of multiplying the antenna elements and the array factor patterns. The latter’s shape is based on its antenna element’s spacing, phase shifts and amplitude variation – with the graph below depicting this concept. It should be noted that changing the AE’s phase shifts and amplitudes results in an electrical beamforming of the array factor – and, consequently the final overall beam shape.
Myth 3 – Multiple Input Ports Mean Multiple Arrays Inside
One of the most important RF planning parameters is the number of input ports and independent electrical tilt controls. Unfortunately, there is a perception held by most RF planners that buying an antenna with different connectors and separate tilt controls means more integrated antenna elements arrays inside.
Low-band elements dipoles are both larger in size, and also require more isolation distances in between. In this case, if an RF planner needs to run GSM 900MHz and LTE 800MHz on separate antenna parts, this would impose doubling of the antenna width – a practice which is highly discouraged.
There are several solutions that have been used in the past:
Common Tilt Control
One solution is to diplex the G900 and L800 signals before the antenna ports, which should be followed by a normal single-array antenna of normal width. In order to separate input ports from the outside, the diplexer can even be embedded inside the antenna radome. However, optimizers often complain about the difficulty involved in tuning both the LTE and GSM tilts separately. This lead to a better approach being developed.
Separate Tilt Control
The most common solution deployed by most antenna manufacturers today involves using diplexers behind each antenna element. This will cause each signal to have its own path, until it is diplexed upon reaching an AE.
The technique works well for the most part, although it does involve an awareness of the PIM risks involved when diplex’ing bands on the same antenna elements. Not all bands combinations’ can be safely diplexed over the same antenna elements without risking PIM. There are free resources online that allow you to calculate this more effectively.
Myth 4: Multibeam Antennas Have Multiple Arrays Inside
Often when asked about twin beam or multibeam antennas, the first picture that jumps to many installers’ minds is a combination of two or more arrays, which are positioned to radiate in different directions. However, it isn’t necessarily true that all multibeam antennas have multiple arrays inside
It’s important to keep in mind that while common, this is not the only deployment technique that can be used. One example of this is a technique commonly deployed by CommScope, called the Butler Matrix.
As we already discussed, the array factor is dependent on its antenna elements’ spacing, phase shifts, and amplitude variation. Considering that AE’s can’t be moved, the only method to change pattern shapes revolves around phase and amplitude variations.
Ralph Lowe and Jesse Butler (for whom the matrix is named) first described this matrix in 1961. It is essentially constructed out of phase shifters and hybrid combiners, such that each antenna element sees different shift and amplitude combinations of the input ports. Please see figure 3 for a more detailed look at how feeding the input signal from a different input port results in a different beam direction.
From here, there are two key uses:
Switched Single Port Feed
An application in which only one input signal is switched across all input ports – so that only one port is connected at a time. This will result in the beam being steered towards traffic areas, but required some intelligence to be added to the switch position decision.
Multi-Port Simultaneous Feed
If instead, two ports are fed with different signals simultaneously, the antenna will radiate dual beams, superimposed on each other. This is the same technique that is used in commercial twin beam antennas.
It’s important to keep in mind, however, that electrically oversteering beams can result in undesired grating lobes, as shown in figure 3 – something to keep an eye on while comparing twin beam antennas.
Myth 5: Beam Steering Requires Active Antennas
Even though techniques highlighted in the Butler Matrix are labelled as “electrical”, these types of techniques might still need some mechanical actions to operate. One example includes e-tilt that gets adjusted by mechanically pulling tilt control sticks. Many operators believe that the only solution lies in active antennas – antennas with integrated radios. However, these are still being debated in the industry – despite reducing the footprint of a site, they restrict future upgrades and harden maintenance. Yet, one of the main long-term benefits is fully electrical beam steering and massive MIMO.
However, in reality, we are actually able to do full electrical beam steering over regular antennas with the support of the base station. Let’s take the TTTT65AP-1XR as an example:
- Planar array antenna, 2496-2690 MHz, 65 degree horizontal beamwidth, single internal RET
- Four columns of x-polarized arrays
- Tested for 0 and 30 degree beam steering
The TTTT has four x-polarized arrays with two input ports each. The antenna is designed for TDD-LTE with 0 degree and 30 degree beam steering directions. The BTS radios are connected to the four column arrays. The radios apply specific amplitude and phase difference as per the antenna manufacturer-provided data.
Below is a table that showcases the values provided to the BTS vendor for application at the antenna ports. There will mainly be two beams that can be formed:
- Broadcast beam: Changing phase and amplitude across ports
- Service (steered) beam: Changing phase only across ports
While this all sounds reasonable, it does pose a challenge.
For multi column antennas, such as the TTTT, the array factor is determined by the column spacing, phase, and amplitude distribution. The column spacing is the most commonly debated factor here.
- If set to 0.5 (narrow) we get the best beam steering with degraded MIMO.
- If set to 1 (wide) we get the best MIMO but with larger steering grating lobes.
It’s important to strike a balance between the resulting grating lobe and MIMO performance.
We rarely get to truly examine base station antennas, and as such, misconceptions about the technology have grown into industry-wide myths. However, I hope this article has help cleared some from your mind, and helped you to understand some of the antenna industry’s best kept secrets.
Dr. Mohamed Nadder Hamdy is director of wireless network engineering for CommScope, based in Dubai. Prior to joining CommScope, Mohamed worked for Etisalat, starting in 1997, in Abu Dhabi, Cairo and Lagos. His last role there was the director of Mobile Networks Technology Strategy, outlining future strategies for small cells, spectrum bands, antennas, LTE CA, VoLTE and others. Mohamed holds doctorate, master of science and bachelor of science degrees in electrical communications engineering from Alexandria University in Egypt.
For more information please visit www.commscope.com.