The Role of Small Cells in Boosting Capacity in 4G Networks

by Vladan Jevremovic, PhD, Director of Engineering Solutions, iBwave

Mobile networks worldwide are carrying increasing quantities of data traffic, reflecting customer demand for new services including broadband internet access via 3G modems, smart phones and other 3G enabled devices. The growth in data traffic over the last 2-3 years has been particularly rapid, and this growth is expected to continue over the next 3 years. Data growth rate estimate vary among different sources, but is generally in the compound annual growth rate (CAGR) range of 25-100%. Here we will explain three different ways to satisfy the demand for data in the long run using technology, spectrum and network topology.

One potential avenue for expanding the capacity of mobile networks is the evolution from 3G to 4G. Among the key differentiators for 4G systems is increased spectrum efficiency relative to 3G. The capacity gain brought on by implementing new technology is so called “technology” gain. In the figure below, we compare spectrum efficiency of 4G vs. 3G technologies over the next 10 years, assuming dense urban topology [1]:

Source: Real Wireless (www.realwireless.biz)

The question then becomes: is the combined growth of 3G and 4G spectrum efficiency enough to cover the data demand growth over the same time period? Analysts vary in the assessment of the projected 10 year data growth, but in general 28% data CAGR (compound annual growth rate) is considered modest. If we start with Year 2010 as the benchmark year for data demand and spectrum efficiency, we can normalize both data and spectrum efficiency growth and show them as multipliers of Year 2010. The comparison is shown in the following figure [1].

It is clear that by Year 2020, spectral efficiency growth will be only about half of the projected data growth. Therefore, the switch to 4G technology alone will not be able to close the gap between the market demand and available technology resources. That leaves us with the question: how are we going close the gap?

One option is for the telecom governing body to make available additional spectrum for wireless broadband. In both UK and US, the telecom governing bodies announced plans to release at least 500 MHz of public sector spectrum over the next 10 years for mobile communication uses. The capacity brought on by releasing additional spectrum is called the “spectrum” gain. However, in some countries freeing up such massive amount of spectrum is not feasible, so an alternative solutions needs to be found out.

Yet another option is to implement small cells (pico and femtocells) in the areas where high concentration of users is coupled with very high usage demands. Examples of such locations are train stations, business high rise towers and university student dorms. Small cells increase capacity by the virtue of improved signal to interference and noise ratio (SINR) in their immediate vicinity, which in turn allows higher order modulation to be transmitted. An example of such modulation is 64 QAM, which also has high spectrum efficiency which we seek out. This is so called the “topology” gain.

While the actual spectrum efficiency per cell improves slightly, the actual “topology” gain over the area of a macro cell is improved many times, because it takes many small cells to cover the same area that a macro cell can cover. In the following figure we show comparison between data demand and small cell spectrum efficiency for selected deployment scenarios [1].

In the above deployment scenarios, the train station is covered with an equal mix of pico cells and femtocells, while the other two are covered with femtocells only. The train station scenario shows that data demand growth is equal to the “topology” spectral efficiency gain. For the business high rise and university student dorm scenarios, gain in “topology” spectral efficiency is far bigger than increase in data growth.

In these scenarios, we see that “topology” gain brought on by small cell deployment may offset data growth demand. Therefore, adding spectrum to satisfy data growth might not be necessary. Here we implicitly assume that the cost of small cell deployment is favorable, and that small cell interference can be managed. While we have seen positive industry trends in both areas (the cost per femtocell has been steadily declining, the inclusion of HetNet and SON in LTE), these issues are crucial in determining the role of “topology” gain in boosting capacity and saving spectrum in 4G networks.

Source: [1] “4G capacity gains final report v1.5”, Real Wireless, 27th January 2011

Maximizing Network Capacity by Minimizing Passive Intermodulation (PIM)

Modern wireless high-speed data networks use tightly grouped channels and complex modulation schemes to enable transmitting vast amounts of data. This in association with ultra-sensitive receivers may face unanticipated but serious capacity losses if the network is disturbed by Passive Intermodulation or PIM for short. Generally, modulating RF signals is necessary to transport information, but arbitrary passive intermodulation can significantly impact RF signal performance. Unfortunately, PIM can happen whenever more than one signal is channeled through one RF path. As a result, we may see unwanted non-linear frequency responses of passive components including connectors and cable feeds. These components start acting like mixers, modulators and frequency multipliers creating unwanted spurious products.

PIM (Passive Intermodulation distortion)

PIM may become a major problem when Tx and Rx signals share one RF path. Typically, VSWR measurements are standard procedure after network installation. Such measurements determine how much RF energy the antenna emits, and how much unwanted energy is reflected back into the transmitter. VSWR meters are, however, are incapable of detecting non-linearity in system components. Validating PIM network quality requires special PIM test systems. The preferred scenario for best network quality is – preventing PIM in the first place. To achieve this, it is paramount to utilize only high quality, low PIM components, apply proper installation procedures, and ensure excellent grounding of the RF system.

Why is it critical to eliminate PIM?

PIM can be generated whenever base stations transmit RF signals. The resulting intermodulation frequency products are often found within the receiving bands of a network. Since RX signals are, by nature, of very low power, interference with regular voice and data traffic occurs. Unwanted PIM interference may desensitize one or more receiving channels to such a degree that it not only creates very high BER that reduces network bandwidth, but it may even drop calls altogether. In the worst case scenario, it can even lead to permanently unusable receiver channels. Loss of already sparse network capacity caused by PIM is unacceptable for high volume, high speed wireless data networks.

What causes Passive Intermodulation?

Ferromagnetic metals, like iron, nickel and steel, show hysteresis effects with applied energy. The resulting signal levels are altered and the signal response is no longer linear.
Dissimilar metal plating on connectors constitutes potential voltaic elements that act like a diode, causing unwanted random modulation effects.
Corroded surfaces cause PIM. Corrosion may happen on unprotected component surfaces or by human influence (e.g. touching a connector pin with bare fingers)
Irregular contact surfaces, even on a microscopic scale, can cause an inconsistent flow of charge carriers and generate inhomogeneous electromagnetic fields. Causes can be of mechanical or electrical nature: low quality components, shearing by forced connections and disconnections of components, spark craters caused when “hot” connections are disconnected.
Wind load and dissimilar expansion coefficients of tower and feed lines stress both, connectors and cables, and will cause deteriorating connection quality.

The wireless network of a large Australian operator presented serious bandwidth problems when extended with modern 3G and 4G technologies. Despite installing latest technologies, the overall network performance was not even close to the expected levels. It turned out that the network itself generated extraordinarily strong interference during signal transmissions. Culprits were inferior diplexers that generated strong PIM. Once these components were replaced with high quality, low PIM types, the network performed flawlessly.

Another operator in the US faced high rates of dropped calls in a certain market. Many sites were tested and swept by service crews but revealed to be in great condition. Basic screening, however, did not detect that the newly installed wideband antennas generated too much PIM due to a manufacturing issue. Once the problem had been pinpointed with PIM measurement equipment, action could be taken. The faulty antennas were replaced and dropped calls virtually disappeared.

Conclusion

Minimizing Passive Intermodulation (PIM) is critical for achieving maximal system capacity and efficiency of wireless high-speed networks. PIM awareness is paramount for PIM prevention. Installers need to be trained properly to ensure their familiarity with the causes of PIMs and their expertise on how to prevent it. RF and DAS equipment manufacturers must deliver products that possess low PIM characteristics, but guarantee sustained specifications over time under environmentally harsh conditions. System designers must account for PIM in their DAS designs, consider low PIM products where appropriate, and pay special attention to material and plating of mating component surfaces. Finally, wireless operators need to maintain the performance level of their network, ensuring that PIM behavior does not deteriorate the operation of the system.

Insight from DAS in Action – DAS, Small Cells, Wi-Fi, Dolphins and More!

It’s time for Wi-Fi and DAS co-existence – that was the popular consensus during the two-day DAS in Action forum that took place from April 10 to 11 in Atlanta, Georgia. The DAS Forum organizers say this year’s attendance increased by 85 percent, a result attributed to the growing role distributed antenna systems play in North American network deployments.

Day 1 of the event began at the Marriott Marquis, where iBwave’s Engineering Solutions Director, Vladan Jevremovic, took part in a morning panel discussion, Indoor RF Design and Planning: Optimizing the Ideal Network. Dr. Jevremovic discussed the importance of having a centralized information depository system for network projects. “When technicians receive a network trouble ticket, they need access to all relevant and current documentation to understand the core of the problem. Having quick and easy access to this information helps technicians resolve the problem much faster,” he said.

Panel participants – Indoor RF Design and Planning: Optimizing the Ideal Network

Many noted that 2012 is a big year for the in-building wireless industry. While no one expected to play games on their wireless devices just five years ago, now it has come to be expected. Data traffic is both growing and changing at the same time. These days upload traffic at large venues has caught up with the number of downloads. During the Wi-Fi and DAS panel, AT&T’s Hany Fahmy explained that while deploying small cell solutions were necessary, Wi-Fi is just as important in this data-hungry age. He added both have their own sweet spot and are needed to manage increased data traffic.

After some great discussions, a case study on the Georgia Aquarium, the world’s largest aquarium, was presented by AT&T, Connectivity Wireless Solutions and Commscope. In 2011, the team took on the challenge of upgrading the existing DAS network to 4G, also expanding the network to encompass the new Dolphin Tail show area. It was a whale of a feat to accomplish – the site’s unique marine environment made for an equally diverse RF environment. Managing the reflections coming off 10 million gallons of water, which were impacting propagation results, was one thing. But installers also worked the night shifts so as not to disturb the sea creatures (dolphins are light sleepers). And all work came to a halt when penguins and seals were escorted by making their way to vet appointments.

The entire event was an informative one. During the Wi-Fi and DAS panel, CCI Systems’ Peter Murray observed that iPads have become the new desktops, and smartphones, the new laptops. Indeed a new paradigm shift has taken place. Convergence was the buzz word – many agreed working with heterogeneous networks more efficiently will be the best way to deal with the high data traffic.