by Dermot O’shea, President, Taoglas Antenna Solutions
Cellular signal amplifiers help bring reliable communications to remote locales, but there are subtleties to their deployment that can trip up the uninitiated.
As we all know, cellular radio networks use base stations to cover a specific area. Rural areas have fewer base stations. So cellular coverage in these geographies can be weak or drop out completely between cell sites. Urban areas have different problems: There are often too many users on a base station, which slows data connections and leads to call failures.
Radio waves are absorbed by many substances—examples include earth, vegetation and most building materials. Thus, it is not uncommon to experience signal loss while surrounded by areas where RF signals come through just fine. Signals often drop out, for example, in ravines, canyons and valleys, as well as on flood plains and other areas shadowed from the nearest base station by hills, buildings or both.
The key point is that these network details are all managed on the base station side of the radio link. The mobile station (handset, tablet, data router and so on) is assumed to have a fixed level of performance that is determined by the combination of its radio and antenna performance.
The amount of received signal strength determines whether the connection is stable. Signal strength is easier to assess with data routers than with handsets because routers often provide a more granular measurement than the typical three or five-bar signal-strength graph on a cell phone. But it can be challenging to assess signal strength accurately without a calibrated instrument like a spectrum analyzer. The problem is that readings on uncalibrated instruments can be affected by small variations in component tolerances, which can make different copies of the same device yield different measurement values.
Temperature variations can have the same effect. At different temperatures, an instrument can report varying measurement values though the signal level is identical. Errors due to device offset, unit-to-unit variations and temperature can easily add up to 30 dB. That is the difference between no connection and a solid one. It is advisable to make measurements with the same device, kept at the same temperature, and to assume that a reading could be ±10 dB. Nevertheless, with proper care, one can still make relative measurements of signal strength.
There are various ways to handle poor coverage locations. Clearly, the antenna height and transmit power on the network side are fixed. In rare cases, it may be possible to move obstacles out of the link path. But the equipment at the end of the link is usually the only part of the system one can control. To limit the scope of this discussion, we’ll assume the performance details of the radio device are fixed. All cellular devices typically have the same transmit power and similar receive sensitivity metrics. This leaves the selection of antenna, the location of the antenna and the connection from the antenna to the radio as avenues for improving link performance.
The antenna choice usually boils down to either omni-directional or directional radiation patterns. Omni-directional antennas are best for mobile applications or those involving more than one base station. Fixed applications may be able to use a directional antenna to improve the link.
There is a practical limit to variations in antenna performance. An omni-directional antenna might have as little as 2 dBi gain at the band of interest, while a reasonably sized directional antenna would likely have a maximum of 8 dBi. Unfortunately, hills in the way could drop the signal level by 20 dB, while trees could have an impact of 10 dB or more.
Luckily, antenna height is controllable. A 40-ft telephone pole or 60-ft small tower will overcome local obstacles, especially trees. Antenna height is typically the easiest thing to adjust that makes the biggest difference for link quality.
Ditto for mobile applications. An antenna buried under the dash or in a metal trunk has a poorer link path than that of an antenna mounted outside the vehicle. A high-quality external antenna can make a difference.
Sometimes, improvement of the radiation path involves moving the antenna away from the radio device, usually with a coaxial transmission line. In general, the thinner the transmission line, the more loss per unit length. The greater the length, the greater the loss. The coaxial loss, in extreme cases, can be enough to cancel out the advantages of a better antenna location. A thicker, heavier, more expensive coaxial line can minimize this loss unless, of course, it’s impractical for installation reasons.
An in-line, bi-directional amplifier can help overcome coaxial losses. In the context of cellular communications, this is commonly referred to as a cellular booster and is a device that contains an amplifier for transmit and/or receive. The amplifier resides on the antenna side of the coaxial link back to the radio. A properly designed device will have similar gain in both the transmit and receive directions because both paths experience the same coaxial loss. RF connections to booster amps always have a 50-Ω characteristic impedance and normally use standard coaxial connectors.
For the receive path, it is best that the incoming signal be amplified before it sees the extra coaxial loss between the antenna and the amplifier and/or between the amplifier and the radio receiver. In contrast, there is no value in having the amplifier at the receiver end of the coaxial line. Adding an amplifier after the coaxial line boosts not only the signal, but also the noise associated with the cable link. The actual signal-to-noise ratio (S/N) gets a little worse because the amplifier itself adds a small amount of noise to whatever it amplifies.
In the transmit path, the signal coming from the radio’s transmitter is much higher than the receive signal. As such, the small amount of added noise from the coaxial loss has little impact on the transmit signal’s S/N ratio. The amplitude of the transmit signal dwarfs any added noise. Thus, on the transmit path, the goal is to compensate for the actual losses and get the transmit signal back up to optimal strength when it goes into the antenna.
It should be noted that the power output of a cellular device is limited by government regulations. Thus, booster amplifiers are not allowed to amplify output power to a point where it exceeds levels proscribed by law. So, on the transmit side, the booster is there merely to overcome cable losses.
Similarly, amplifiers designed for the U.S. can work in any country that uses the same frequency bands. Canada, Mexico and some South American countries use some of the same bands as the U.S. Those countries, however, require their own government approvals. So commercial amplifiers usually target specific countries and sometimes even specific carriers.
Cellular booster amps must amplify in two directions simultaneously. Luckily, the transmit and receive signals are at different frequencies. So by using filters, the transmit and receive signals are separated and amplifiers for each mode can operate at the same time. The effectiveness of these filters is one measure of a booster’s quality. Boosters with poor filtering have problems with cellular network protocols, like CDMA, that transmit and receive at the same time.
The last part of the system is the power supply. The amplifier is an active element, so it needs power. The transmit amplifier, in particular, uses about the same amount of power as the radio transmitter that it is amplifying, so it needs a similar power source. Power can be delivered to the amplifier over the coaxial cable using bias tees. Alternatively, separate power can be supplied at the booster side of the cable.
The output of the power supply must be filtered to ensure the amplifier works properly. This filtering can take place inside the booster or using external filter elements. It is common to see this filtering done externally to lower the cost of the booster when it is used in fixed (meaning not vehicular) applications where clean power is easily available.
Cell booster considerations
If the amplifier loses power, in many cases it can block the radio signals. Thus, if the booster amplifier fails, it can actually prevent the radio link from working in a situation where it otherwise would. As an example, consider a typical cellular router mounted in a bus.
The radio mounts behind the driver with the other electronics. There are 30 ft of coaxial cable and an associated loss to the back of the bus where the antenna mounts. A booster mounts just inside the bus next to the antenna, compensating for the coaxial losses back to the radio. The bus is constantly moving, and, in many places, it receives a strong network signal.
If the amplifier fails for some reason, the bus no longer can connect, even when it’s in a good coverage area, because the amplifier blocks the signal between the antenna and the radio. There are cellular booster products designed as a fail-safe if their power or electronics go out for some reason, maintaining a path between the antenna and radio.
Another item to consider is amplifier gain. Not all commercial products provide the same amount of gain; it varies product to product. Typically you will see commercial amps providing 12 to 25 dB of gain with some vendors offering two versions of a given amp, one high gain, the other low.
The amount of gain you need is directly proportional to the loss in your coaxial system. As long as the gain is more than the coaxial loss, you’re fine. The only downside of too much gain is that it could cause problems if the system were to get close to a cell tower, as might happen in a mobile application. Some amps (like those from Taoglas) will sense when received signal levels are getting too strong. They will then switch themselves out.
Whenever there is more than 6 dB of coaxial loss, a cellular booster can add to total system performance. As long as the amplifier gain equals or exceeds the losses in the coaxial cable, the booster is sufficient. Any additional gain only hurts the ability of the amplifier to work in strong signal areas. If total coaxial loss is less than 6 dB, a booster is not likely to make a noticeable difference in performance. The greater the coaxial losses, however, the bigger the impact of an antenna-side amplifier.
Also important: Cellular amps must be designed for the specific bands of interest and, in some cases, for the specific modulation used. The distinctions here can get a little tricky. For example, one new Taoglas product handles the 850 MHz and 1900 MHz bands, and it does so with amplifiers linear enough to support OFDM as used in LTE signaling. That said, it does not support the 700, 787 or 1700/2100 MHz bands most commonly used for LTE in the U.S. by AT&T and Verizon. Sprint intends to deploy LTE on 850 and 1900 MHz bands. So a logical question is whether the device can “support LTE.” It can, but only on those bands, so only for Sprint.
Therefore, the distinctions are not really a matter of 3G versus 4G versus 5G, so much as a matter of what bands are supported, what carriers use those bands and what technology the carriers use on those bands.
There are other issues that help differentiate commercial booster amps. Competitive factors include power supply robustness, the ability of the device to fail in safe or functional mode, mounting options, robustness of the enclosure, carrier and government certifications, customer service and quality of documentation.
Finally, it is worth noting that the FCC changed the rules for cellular booster amplifiers in February of 2013. Rules now require the devices to be much more intelligent so they cannot cause any network issues. These features are mandatory on all legal cellular booster products in the U.S. It’s imperative to get a product that complies with these new rules.
References
Taoglas Antenna Solutions
www.taoglas.com
