08 Oct 2013
Antenna Technology in the LTE Era
Lou Meyer and Junaid Syed of CommScope look at the technical issues with antennas to support 4G LTE and its advanced technologies.
Wireless networks are increasingly complex. Cell site architectures and infrastructure have evolved over four generations of technology.
The amount of traffic they support is staggering. Base station and microwave antenna technologies have evolved to match the increased usage demands. According to one report, the most important thing wireless operators can do to keep their users happy is to ensure superior network performance.
Deploying the right antenna technology in the right way is an important part of accomplishing that task.
Base station antennas are one of the most important components in the radio access network. They radiate the RF signals that communicate with mobile devices. To support the enormous amount of traffic today, wireless networks repeatedly re-use frequencies or codes throughout the network to increase capacity and throughput. Operators employ base station antenna pattern control techniques to decrease interference between cells operating on the same frequencies or codes. As wireless networks become denser with more sites, users and traffic, the coverage area of each cell site must diminish to limit interference with adjacent sites.
One way to decrease a cell site’s coverage area is to lower the height of the antenna radiation center. However, this is often a poor option since it could position the antenna below nearby obstacles such as buildings or foliage that interfere with the signals. A second way of reducing the coverage area is to tilt the vertical pattern of the antenna downward, thus shrinking the coverage on the horizon where interference to adjacent cell sites occurs.
Mechanical vs. Electrical Down-tilt
The easiest yet less favourable way of beam tilting is to mechanically tilt the entire sector antenna using adjustable brackets, which are available from most antenna vendors. This technique has a significant downside since it does not reduce coverage consistently across the horizon over the entire sector. Mechanical tilting reduces coverage more in the bore sight direction and less at other angles away from the bore sight. The result is an inconsistent decrease in the cell coverage area a phenomenon often referred to as pattern blooming, which can be quantified.
We think the acceptable amount of pattern blooming should not exceed 10% of the antenna’s azimuth beamwidth. More blooming than that often generates interference levels that can cause network inefficiencies, diminishing quality of service for subscribers and increasing customer churn.
A more preferred method for tilting the vertical pattern of a sector antenna is by using electrical down-tilt instead of mechanical. This technique accomplishes the beam tilting by using phase shifters to manipulate the electrical phase delivered to each antenna element. The antenna itself remains mounted upright while the RF signal shifts. The resulting elevation pattern is tilted consistently over the entire 360°, reliably shrinking the coverage area. Pattern blooming does not increase regardless of the amount of electrical down-tilt.
Another advantage of electrical down-tilting is that it can be done remotely by connecting a motor to the phase shifter mechanism. This benefit is becoming even more important as next generation air interface technologies such as LTE mature. One concept linked closely to LTE is the self-organizing network (SON), wherein the network re-optimizes itself routinely based on demand levels. Dynamic, flexible coverage adjustments such as electrical down-tilt are required for the implementation of the full SON concept. This Radio-Electronics article includes more information about these different aspects to SON.
Add Some Class to Your Microwave Antennas
Not everyone realises that capacity is just as important on the backhaul side of the network as the access side. A bottleneck in the backhaul will slow down traffic, shrink capacity and hurt the quality of service. Like base station antennas, microwave antennas have evolved in capabilities to better and more cost effectively handle increased network traffic. Microwave networks today are moving to Class 4 of the ETSI (European Telecommunications Standards Institute) standards. Class 4 microwave antennas create a tighter signal pattern that offers significant benefits to operators, including cost and capacity improvements.
The theoretical maximum capacity of a microwave link is defined by Shannon’s Law, which specifies that capacity depends on two things: channel bandwidth and carrier signal-to-interference ratio. Imagine the channel bandwidth as being the size of the tube carrying the backhaul signal—if you increase the diameter of that tube, more signal can travel through it. But, of course, radio spectrum is a limited and often an expensive resource. Trying to expand the channel bandwidth is not an ideal fix. Broadening the bandwidth on one channel reduces the available spectrum on another channel. It might not even be an option depending on licensing and spectrum rights. These limitations make this an undesirable option.
The other path to increasing the backhaul capacity available for increasing traffic is to improve the carrier signal-to-interference ratio. In general terms, this ratio is a measure of the amount of the carrier’s intended radio signal that is received at a certain point compared against how much interference distorts it. Wireless operators can realise significant availability improvements of higher capacities by deploying ETSI Class 4 microwave antennas. Class 4 antennas envelope the radiation pattern more tightly than lower classes.
This feature enables operators to mitigate interference with a smaller sized antenna (e.g. a 2-foot instead of a 4-foot), which decreases shipping and tower leasing costs. The tighter radiation pattern also improves carrier signal-to-interference ratio, increasing the availability of throughput capacity.
80 GHz antenna measurement
Microwave Class 4 antennas, due to low side lobe levels, give higher spatial efficiencies and allow the use of higher spectral efficiency techniques (for example, higher modulation schemes), using spectrum more efficiently in mobile backhaul networks. Class 4 antennas are among the most recent evolutions in microwave antenna technology being deployed today.
Optical Fibre in the Air
Another advanced method for boosting backhaul capacity is by moving into high-capacity spectrum, which is referred to as millimetre wave backhaul, the E band, or even optical fibre in the air. In this spectrum, the 80 GHz frequency range offers benefits due to its unique propagation characteristics. It can potentially deliver fibre equivalent throughput with signals transmitted in very narrow beams—often called pencil beams—rather than the wider beams in lower bands. Pencil beams make 80 GHz more spatially efficient, meaning more paths can be used in the same channel. These benefits come with increased risks, however.
Along with pencil beams comes side and back lobes. The primary source of interference from a 70/80 GHz microwave link narrow beam is line-of-sight power directed into the main lobe or side lobes of a victim receiver antenna. A high density environment increases the potential for interference as well as the impact of that interference. Interference can not only distort the signal but cause complete signal drop—risking a total loss of the wider pipe! Signal interference can be catastrophic in millimetre wave frequency ranges and needs to be defended against.
The E band is also more sensitive to the environment than the lower frequencies. Rain begins to attenuate wireless signals above 10 GHz. When you reach the 80 GHz range, attenuation due to 25 mm of rain per hour occurs at 10 dB/km, while rain rates of 100 mm/hr can cause attenuation of 30 dB/km. That level of degradation will significantly impact network performance and can also lead to dropped calls. The worst case scenario is when the desired signal is fully rain faded and the interference signal has no rain fading. If this level of interference and performance degradation happens, you lose all the benefits of deploying in the 80 GHz band in the first place. To protect against this threat, operators should ensure that their microwave and millimetre wave antennas are compliant and performance verified. A small mechanical defect can result in a major RF problem. Quality is key. Ask your antenna manufacturer for test reports to verify performance. If your antennas cannot contain interference properly, you are risking major network disruptions.
The bottom line is that base station and microwave antenna technologies continue to evolve to support capacity demands. Maximum performance requires proper product selection and deployment. The more complex and sensitive wireless networks become—on both the access and backhaul sides—the more important interference containment becomes.
To realise the benefits of a truly self-organizing network, base station antennas need remote electrical tilt capabilities. Electrical tilt in general is preferential to mechanical tilt, which does not consistently decrease cell coverage areas. Deploying microwave antennas in millimetre wavelengths for “optical fibre in the air” can be better accomplished using the newer Class 4 antennas. Wireless operators can achieve cost benefits from them while defending against catastrophic link failures. The right RF path equipment can make a big difference in network performance.
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About the author
Lou Meyer, P.E., is CommScope’s director of technical marketing—RF Path. Lou has spent a lifetime advancing RF technology, taking it from the drawing board to practical use. Over the years in various roles with Allen Telecom, Andrew Ltd. and CommScope, Lou holds five patents and has been active as a chair and vice-chair of the TIA’s TR-8.11 Antenna Standards subcommittee. He earned his Bachelor of Science degree in electrical engineering from Marquette University in Milwaukee, Wisconsin and is currently a registered professional engineer in the state of Texas.
Dr. Junaid Syed works for product line management of the Microwave Systems team at Commscope. Junaid covers Middle East, Asia, and Africa in the areas of microwave and millimetre wave antenna systems, flexi waveguides and waveguide components that support mobile backhaul systems. He holds 10 patents, has penned a number of published articles and is a current member of SE Scotland IET and Technical committees. He also represents CommScope as a technical committee member with ETSI and FWCC. Junaid earned his B.S. from Punjab University with Silver Medal honours, and a Bachelor of Engineering degree in electronics/avionics from NED University of Engineering and Technology with Gold Medal honours, both in Pakistan. He earned his Ph.D. in microwave and millimetre wave from the University of London and conducted his post-doctoral research on reflect array antenna design at Queen's University Belfast, both in the United Kingdom.
CommScope has played a role in virtually all the world’s best communication networks. We create the infrastructure that connects people and technologies through every evolution. Our portfolio of end-to-end solutions includes critical infrastructure our customers need to build high-performing wired and wireless networks. As much as technology changes, our goal remains the same: to help our customers create, innovate, design, and build faster and better. We’ll never stop connecting and evolving networks for the business of life at home, at work, and on the go.
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