Can Antennas Rise to the 4G Challenge?
By Dermot O’Shea, Co-Founder and Joint Managing Director • Taoglas

According to the Telecommunications Industry Association (TIA), overall wireless equipment and infrastructure spending will total $23.3 billion in 2013, up from $17.3 billion in 2009. Buoyed by this optimism, developers and OEMs are forging ahead with a host of WiMAX and LTE applications and devices. The surge in bandwidth usage, the TIA claims, requires more antennas for a given area, creating demand for additional cell towers. There were 127,540 cell towers in 2001 and by 2013, the TIA claims there will be more than 438,000 cell towers in the US [1]. The focus now turns to the RF engineers responsible for optimizing networks for the new 4G data transmission rates.

But putting up more cell towers is not the only solution. At Taoglas, we believe that the wireless devices in the field must reach a certain level of transmission and reception sensitivity in order to ensure that they can send back and receive data when they are within base station reach. In short, for existing networks, the goal should be to maximize the amount of users that use networks for high-speed data transmission and, at the same time, provide consistent quality of service (QoS).

But, is this mission impossible for antennas? We don’t think so; there are two areas where good quality antenna design can make a difference: device antenna optimization and base station antenna optimization.

Optimizing Your Device Antenna
As the wireless market grows, designers of cellular devices face more challenges; multiple bands are required for different operators in different countries, there are lower frequency band requirements (e.g. 700 MHz) and now multiple antennas giving multiple-input, multiple-output (MIMO) are needed for devices to meet targeted 4G data throughput rates. Individual device antenna customization becomes a necessity in order to reach the highest possible device sensitivity and efficiency. If not, at best, products will only be able to receive data but won’t send data back seamlessly. In the worst scenarios, antennas won’t send any data back at all. This will also make already compromised device power consumption worse.

Lowering Device Sensitivity Requirements
Improved antenna efficiency gives greater device sensitivity and vastly improves the ability of devices to send data back to base stations. On the receiver side, device performance is measured by the Total Isotropic Sensitivity (TIS), a measure of how weak a signal a device can receive. Today, traditional 2G and 3G modules are designed to receive signal levels of -109 dBm to reach all global network approval requirements. An 8 dBm loss is allowed as loss that cannot be avoided via transmission lines, cables or matching circuits and the loss in the antenna itself. This means devices should be designed to receive a signal as weak as -101 dBm. At Taoglas, we believe that all network operators should set a requirement of -101 dBm for device sensitivity knowing that this will lessen their requirements for base stations. This is a huge challenge when going to 700 MHz. Consequently, we are likely to see the requirements relaxed and reduced for efficiency and TIS sensitivity targets at this band in mobile phones, unless ID designers accept physically bigger phones. Coupled with the ideal scenario of two antennas in a device, this raises question marks over how today’s small phones can possibly achieve the published target data rates for LTE.

Using the Right Antenna in the Right Location
Improving the rate of transmitting information back to the base station is also key to meet with new 4G networks requirements. This is really where antenna efficiency and device Total Radiated Power (TRP) comes into play. If these figures are low, networks have to compensate with more base stations so that they are close enough to receive the weak signals coming from the devices. With the high data throughput needed for 4G applications to succeed, it will be too costly to just add base stations alone so careful antenna selection and integration will be required to keep efficiency and TRP high in the devices. A mobile device transmits on less power than the base station and can not accommodate a large directional base station antenna. The solution is an omni-directional antenna with more than 50 percent efficiency.

Two such antennas fit the bill. The FXP100 designed for WiMAX and the FXP400 for LTE are high efficiency flexible polymer embedded antennas. They are ultra wideband antennas and offer the best market efficiency of more than 70 percent for WiMAX (2,100 to 4,320 MHz) and LTE (690 to 940 MHz and 1,720 to 3,130 MHz).

Investigating Base Station Antennas, Network Interference
In a typical cellular environment, without optimized radiation patterns at the base station end, interference will reduce capacity and quality. Minimizing interference with innovative antenna design is crucial. Here are three steps below to overcome this challenge.

1. Using Suppressed Upper Sidelobes to Improve Performance
Interference sources can steal capacity from 4G networks. One way to combat the problem is to deploy antennas with reduced upper sidelobe levels. Usually the sidelobe above the horizon and the nearest main beam is referred to as the first upper sidelobe. The next one away from the main beam is called the second upper sidelobe, it has lower intensity. The third sidelobe is even lower in intensity and so on. In an ideal world, antennas would have no upper sidelobes but in real life this is unavoidable. The theoretical upper side lobe level of an antenna with equal amplitude and phase distribution is -13 dB. The first upper sidelobe creates the most interference problems because it has the highest intensity and points just above the horizon. However as more downtilt is added to the antenna then the second and third upper sidelobes become an issue. In order to fully optimize the antenna radiation pattern, the upper sidelobes should be maintained below -18 db. This should be the case for all downtilt angles.

Taking Advantage of Innovative Market Solutions
Suppressing upper side lobes can even be done in all frequencies and now even in frequencies such as 4.9 to 5.9GHz. Taoglas recently launched a 5 GHz WiMAX/WiBro base station antenna, which was the first 4.9 to 5.9 GHz antenna to have suppressed upper sidelobes. It is used in the field for point-to-point or point-to-multipoint applications.

2. Achieving More Exact Tilt Accuracy
Electrical downtilt is achieved by adjusting the antenna phase to each radiating element in the array. The accuracy of this downtilt is dependent on the type of feed network used in the antenna, which is typically formed using cable harnesses or printed circuit board networks or a combination of both. Antennas that use special low loss printed circuit board technology, and eliminate cable and FR4, in their design yield superior amplitude and phase control as well as being more repeatable during manufacturing. This ensures greater control of the radiation pattern.

Varying the Electrical Downtilt
Variable electrical downtilt in antennas allows network operators to adjust sector coverage. It enables operators to optimize sector coverage based on end-user traffic and use patterns. Redirecting the antenna beams however, alters the effects of other pattern characteristics and when the electrical tilt of the antenna is adjusted, interference with the upper sidelobes becomes an issue. Taoglas designs antennas to display, not only suppressed upper sidelobes across the whole frequency band, but also suppressed upper sidelobes over the entire tilt range.

Efficient variable tilt antennas should maintain a consistent elevation pattern across the tilt range. The desired variable tilt elevation pattern at T0° and T10° are compared to a typical current market antenna in the schematic below. The aim is to maintain sidelobe suppression and null fill over the entire tilt range, and not show unwanted high upper sidelobes at minimum and maximum tilt.

3. Minimizing Front-to-Back Ratio Interference
Front-to-back (F/B) radio is important in a low-interference antenna design. Interfering signals like the ones transmitted or received through the upper sidelobes can also seep through the pattern’s backlobes. F/B is a measure of an antenna’s ability to withstand rearward interference. In a data-driven 4G world, back lobe interference takes on much bigger implications. To ensure that interference is minimized, back lobe interference should be specified at 180° ±30° from boresight. Careful design consideration should also be given to the radiator and ground plane geometry to ensure that the F/B ratio is >30 dB.

A good example of this is Taoglas’ SA.panel21 antenna where there is >25 F/B ratio, which displays excellent field performance.

Realizing the 4G Goal
If 4G is to realize its full potential in delivering high throughput rates and super-fast data transfer then antenna manufacturers must continue to push for base station and device antenna systems that can deliver greater performance. In 2015, ABI estimates more than 12.6 million M2M cellular devices will ship worldwide [2]. Techniques such as suppressing upper sidelobes, altering antenna tilt and adjusting front-to-back ratio will all contribute to helping manufacturers and antenna makers realize their goal.

References
[1] Source: 2010: TIA Industry Association Webinar: How Will Smart Devices Drive Mobile Growth in 2010?
[2] Source: M2M Premier: 4G Hype or Hero. May 14, 2010.

Dermot O’Shea is co-founder and joint managing director of Taoglas, a leading M2M antenna provider. Having founded Taoglas with co-director Ronan Quinlan in Taiwan in 2004, he is currently responsible for sales, finance and marketing. Prior to founding Taoglas, Mr. O’Shea worked for Network International. Today, he advises healthcare, automotive, tracking, telemedical and utility companies worldwide on remote antenna solutions and also provides high-level counsel on device noise debug, testing services, device certification and approval management. He can be reached at doshea@taoglas.com.

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