In LTE (Long Term Evolution) networks, antenna parameters are crucial for optimizing signal transmission and reception, which in turn affects the coverage, capacity, and quality of the network.
What are Antenna Parameters?
Here are some antenna parameters and their roles in LTE:
1. Antenna Gain.
Antenna gain is one of antenna parameter and it is the ratio of the maximum radiation in a given direction to that of a reference antenna in the same direction for equal power input. Usually this gain is referenced to either an isotropic antenna or a half wave dipole in free space at 0° elevation.
An isotropic reference (dBi) generally pertains to a theoretical antenna having a spherical radiation pattern with equal gain in all directions. When used as a gain reference, the isotropic antenna has a power of 0 dBi. The halfwave dipole (dBd) is an antenna which is center fed as to have equal current distribution in both halves.
When used as a theoretical reference antenna it has a power gain of 0 dBd, which equates to a 2.14 dB difference compared to an isotropic antenna. The following figure provides a graphical representation of the different antenna patterns.
The antenna gain is considered passive, as the power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has positive gain in some directions, it must have negative gain in other directions, as energy is conserved by the antenna.
The gain that can be achieved by an antenna is therefore a trade-off between the range of directions that must be covered and the gain of the antenna. For example, a dish antenna for a microwave point to point connection will have a large gain, but only over a very narrow vertical and horizontal beamwidth, as it must be accurately pointed to each end point. An antenna used for a LTE site will have a smaller gain, compared to the microwave dish, as it is required to radiate in a wider path to provide coverage to many points (i.e. subscribers).
The gain of the antenna will impact other antenna characteristics such as: size, weight, horizontal beamwidth, vertical beamwidth, and cost. The RF Engineer will need to select the appropriate antenna for the particular situation. A trade-off will need to be made by the RF Engineer as to whether a higher gain or lower gain antenna should be chosen.
The higher gain antenna typically is physically larger, more expensive and has a narrower vertical beamwidth than would a lower gain antenna. The gain of an antenna has a direct interaction with other antenna parameters.
2. Physical Size.
Physical Size is also one of antenna parameters. The size (e.g. length) of an antenna will generally be larger as the antenna gain increases but keeping other parameters relatively fixed. This is due to the greater number of dipole array and electrical elements required to reach the desired gain. The size of the antenna is also influenced by the spectrum it is designed to operate in. As the frequency increases, the size of antennas will typically be smaller for a common gain.
3. Voltage Standing Wave Ratio (VSWR).
Voltage Standing Wave Ratio (VSWR) is another antenna parameter characteristic. It deals with the impedance match of the antenna feed point to the feed or transmission line. The antenna input impedance establishes a load on the transmission line as well as on the radio link transmitter and receiver.
To have RF energy produced by the transmitter radiated with minimum loss or the energy picked up by the antenna passed to the receiver with minimum loss, the input or base impedance of the antenna must be matched to the characteristics of the transmission line. The VSWR of a base station antenna should be less than 1.5:1.
Return Loss (RL) is the decibel difference between the power incident upon a mismatched continuity and the power reflected from that discontinuity. Return loss is related to the reflection coefficient (p) and VSWR as follows;
In other words, the return loss of an antenna can be considered as the difference in power in the forward and reverse directions due to impedance mismatches in the antenna design. All other things being equal, the higher the antenna return loss, the better the antenna performance. The system engineer should choose an antenna with a return loss of 14 dB or better. Note that 14 dB corresponds to a VSWR of 1.5:1 as per the following example;
4. Power Rating.
The Power Rating of an antenna is the maximum power, normally expressed in Watts that the antenna will pass without degraded performance. Typical values for the power rating of an antenna are between 300 and 500 Watts.
LTE following a 1x3x1 frequency reuse will employ a single carrier and with the possibility of having losses associated with combining the LTE carrier with existing co-located legacy systems (GSM, CDMA, UMTS, etc.), the power rating of an antenna is not expected to be a limiting factor for antenna choice.
Even so, when choosing an antenna, the system engineer should consider system expansion and the theoretical maximum configuration of carriers that could be placed onto a single antenna.
5. Antenna Polarization.
Antenna Polarization is one of most important antenna parameter. Antennas being used for LTE and other wireless communication systems are either vertically polarized or cross polarized. The polarization of an antenna is defined by the orientation of the electric field emitted by the antenna (E- plane). A cross polarized antenna is actually two antennas housed in the same antenna panel (i.e. housing).
Each of the antennas is polarized at a 45 degree angle between horizontal and vertical and they are polarized 90 degrees apart from each other. The reason for this is to provide un-correlated separation between the two antennas so they may be used for diversity reception and for MIMO. Cross polarized antennas can offer up to 25 dB isolation between the two antennas, though real world scattering conditions seldom present sufficient polarization change to take full advantage of the antenna’s designed isolation.
6. Antenna Beamwidth.
Antenna beamwidth is measured in degrees between the half power points (-3 dB) compared to the major lobe axis of the antenna. The -3 dB points are referenced to power (dBm) and not voltage (dBv). Beamwidth is expressed in terms of horizontal (azimuth) and vertical (elevation) degrees. All antennas (vertically polarized or cross polarized) have vertical and horizontal beamwidth patterns.
These patterns express relative power measurements at specified angles away from the major lobe axis along one of two planes (horizontal plane or vertical plane). The measurements do not represent either the E – field (electrical) or H – field (magnetic) of the antenna.
The gain of an antenna defines the antenna’s best performance along the major lobe axis compared to a reference antenna (isotropic or dipole – See Antenna Gain Section). The value contained within an antenna beamwidth pattern for the major lobe axis is normalized to 0 dB. All other measurements at other angles removed from the major lobe axis are expressed in negative dB lower than the 0 dB value of the major lobe axis.
For example, if an antenna has a gain of 15 dBi and a horizontal beamwidth of 85º, then the horizontal antenna pattern for the antenna will have the value of 0 dB at the major lobe point (not 15 dB) and a value of -3 dB 43º to either side of the major lobe point (not 12 dB).
The following two figures illustrate example horizontal and vertical beamwidth patterns for a directional antenna.
For a three sector site, a sector would cover 1/3rd of 360° or 120°. Results from simulations indicate that the use of 120° antennas provide too much overlap. As the coverage of any sector within a LTE system is directly affected by the noise generated by its neighboring sectors and traffic within those sectors, the use of 120° can lead to reduced performance through the rise in system noise and interference.
The recommendation is to use an antenna with a horizontal beamwidth of approximately 65 to 70 degrees. This narrow horizontal beamwidth antenna requires extra attention to ensure adequate coverage is provided. The coverage gaps between adjacent sectors needs to be filled in by neighboring sites. It is best if the sites are deployed on a hexagonal grid pattern.
Generally, the greater the gain of the antenna, the narrower the vertical beamwidth becomes. The vertical beam can be used to focus coverage in some circumstances, but the engineer should ensure that the optimum vertical beamwidth is used to prevent the creation of “nulls” or coverage holes near to the site.
7. Antenna Tilt.
- Definition: Antenna tilt refers to the angle at which the antenna is tilted either upwards (uptilt) or downwards (downtilt).
- Types: Mechanical tilt (physically adjusting the antenna angle) and electrical tilt (adjusting the angle using software).
- Importance: Proper tilt adjustment helps in focusing the signal on the desired coverage area, reducing interference, and improving overall network performance.
Get more in detail and depth technical knowledge, See What is Downtilt of Antenna? with scenarios.
8. Front to Back Ratio.
The front to back ratio of an antenna is an important characteristic of a directional antenna. It is the ratio of the power radiated from the angle with the maximum radiation to the level of radiation at 180 degrees (the back lobe) from that point. Front to back ratio is normally expressed in terms of dB, i.e. a signal at the back of the antenna should be X dB down from a signal at a mirror angle in front of the antenna.
It is recommended that a directional antenna utilized in the system should have a front to back ratio greater than 25 dB. Figure above (Example Horizontal Pattern (~85º).) illustrates a front to back ratio (FB Ratio) of ~ 35 dB. Using directional antennas in a system design with an adequate front to back ratio helps to reduce the amount of interference that will be produced in a system.
Field measurements have shown a common phenomenon where back scatter originating on co-located/co-channel adjacent sectors is reflecting off of ground clutter in front of these adjacent sectors and propagates back into the coverage areas of the best serving sector close to the site. This effect is only observed close in to a best serving sector (within 100m or less). The interfering energy limits the best serving sector ability to achieve CINR values greater than 25 dB in these areas.
Every sector at a site effectively produces this form of back scatter interference to all the other sectors of the same site. This interfering energy is not eminating off the back of the adjacent sector antennas and therefore does not produce additional interference further from the site at the coverage edge of the best serving sector.
Though the interference is not from the back of the antenna, but caused by a reflection, in modeling the impact in an RF prediction tool (one not based on ray tracing), the front to back ratio can be reduced (i.e. not having the back lobe as attenuated) as a way to model the increased signal level that is observed from the reflections.
9. Side and Back Lobes.
Side and back lobes are regions within the pattern where the chosen “directional” antenna may present gain, other than in the maximum lobe. The system engineer should pay attention to these characteristics when downtilting an antenna, as a side lobe (lobe that appears above the horizon on the vertical pattern) may cause interference. The mechanical downtilting of an antenna will directly affect the radiation of both side and back lobes. The mounting of panel antennas on buildings or using of an antenna with electronic down/up tilt are two possible ways to limit back lobe interference.
The system engineer should choose the optimum directivity and gain of an antenna while paying attention to the strength of the side lobes and the back lobe (refer to previous sub-section – Front to Back Ratio).
The previous figure shows a null in the pattern between the main lobe and the secondary lobe. For vertical patterns, the nulls that appear in the pattern below the horizon could potentially cause degraded signal in the area of the site’s footprint where this null would fall. Some antennas are designed by the antenna vendor to minimize the amount of null in the first several nulls. This is sometime referred to a “null fill”. Refer to Downtilting article for further discussion on downtilting and locating where the nulls of the vertical pattern will fall.
10. CPE Antenna Variations.
A couple differences may exist between the specifications of a given CPE device and the perceived performance in the field. The differences are primarily due to two factors which are not typically considered in the RF design process.
- Operating in a non-LOS scattering environment.
- Non optimal positioning of the CPE.
When creating an RF design it is important to understand the antenna characteristics of the CPE and where the CPE will be located. If the CPE is using directional antennas and is being installed in a non-LOS environment, then including adjustment to the antenna gain, orientation, and log-normal margin may be necessary.
The Antenna Gain Correction Factor (AGCF) is a reduction of the antenna gain due to the installation of the antenna in a non line of sight (NLOS) scattering environment. The paper “Gain reductions due to scatter on wireless paths with directional antennas” provides equations which can be used to approximate the antenna gain correction mean and standard deviation values.
The antenna Orientation Loss is a reduction in the performance of the antenna due to the antenna not being oriented in the optimal direction. An orientation loss is typically not applicable in a design that uses devices with one highly directional antenna, as it is assumed that the antenna will be optimally positioned towards the best serving site. Orientation Loss should be included in the RF design if:
- Devices are being randomly installed (i.e. not optimally placed to insure orientation for maximum performance).
- Devices are not fixed to the installed location (i.e. the device can be rotated or moved).
- RF environment is likely to change.
- New sites added which could change optimal orientation
- New construction or changes in foliage which may change optimal orientation.
- Changes in nearby obstructions (e.g. people, furniture, etc.)