Wireless Systems Design

Superconductors Do More With Less

While Carriers Struggle With Financial Constraints, Alternative Technologies Offer Network Enhancements.

In recent years, wireless operators have invested heavily in their network infrastructure. To anyone who has been around the telecommunications industry over the past decade, this fact is no surprise. The expansion of infrastructure—particularly by adding new base stations—has been the most convenient way to meet carriers' ever-expanding capacity needs. In some ways, it also has been the most cost-effective method.

Today, more than 140,000 wireless base stations are deployed across the U.S. Collectively, they provide service to more than 147 million people. As carriers have built out their footprints, wireless users have continued to gobble up capacity. Roughly half of the U.S. population now subscribes to mobile services. Intense competition and huge bundles encourage users to make more and more use of their wireless devices. As a result, minutes of use (MOUs) have skyrocketed.

At the end of 2002, U.S. wireless customers averaged 488 monthly minutes of use per person. According to The Yankee Group, that number rose to more than 500 at the end of the first quarter of 2003. In the two years prior, the average minutes of monthly use were 397 per user in 2001 and 282 in 2000. In 2002, U.S. wireless customers used an estimated 600 billion MOUs—a staggering amount by any measure (FIG. 1).

Now, customers expect and even demand that their mobile phones have performance that is on par with landline devices. This attitude was enough to give carriers heartburn. Then, the carriers realized that greater wireless traffic has led to an associated rise in radio-frequency (RF) interference. RF interference directly contributes to a greater percentage of dropped calls, blocked calls, and origination failures. All of these outcomes negatively affect customer satisfaction.

In the past, wireless carriers would address these network strains by building base stations and bringing them online to expand their capacity. CAPEX budgets have been trimmed, however, and communities have pushed back on carriers. Site selection and approval is now more difficult and costly.

In response to these problems, many carriers are beginning to adopt a "do-more-with-less" approach. To keep up with capacity demands, they must find, test, and eventually implement cost-effective alternatives to the somewhat dated solution of building base stations. The alternatives include a mixture of hardware and technology. They range from such well-publicized, cleverly named devices as smart antennas to various iterations of amplifiers, front ends that integrate cryogenic cooling, and superconducting technology.

Although the technology advances daily, here is a laundry list of the most accepted solutions on today's market:

  • Smart antennas provide a very powerful performance enhancement for networks. All forms of smart antennas provide more intelligence in the directionality of the RF energy from the antenna. In all cases, the energy is focused toward the users. This can vary from simple narrower antenna patterns to complex beam forming, which tracks individual mobile users.

    Results from published tests suggest that smart antennas can increase capacity on an individual site by as much as 8X. Yet these devices can be expensive and difficult to integrate into a network. Moreover, complex integration and subsequent re-optimization require significant expertise and resources.

  • Repeaters provide local RF "points of presence." To fill RF coverage holes, these points act as a means of remote coverage or capacity away from the base station. Compared to a base station, repeaters are small and relatively inexpensive. They enhance coverage in inaccessible areas, which makes them particularly attractive for tunnels, buildings, and hilly or "shadowed" terrain. But repeaters also reduce base-station capacity—sometimes substantially due to interference effects. Moreover, added costs and siting headaches can be associated with this solution.
  • High-selectivity conventional filters are passive devices. They are comprised of metal cavity or dielectric resonators, which protect against out-of-band interference. The filter's size, complexity, and rejection all vary depending upon the prospective interference environment.

    These filters are inexpensive and easy to integrate. As the selectivity requirements increase, however, receiver sensitivity degrades. When high-selectivity conventional filters are deployed, receiver sensitivity is reduced. The result is coverage shrinkage and/or a further reduction in capacity utilization.

  • Tower-mounted (TMAs) or mast-head amplifiers (MHAs) consist of weatherized low-noise amplifiers (LNAs). These LNAs are placed close to the antenna on the antenna tower. The tower-mounted amplifier overcomes the losses associated with cable runs to the base station. These losses can be substantial for tall towers.

    TMAs enhance receiver sensitivity—a requirement for link balance when the uplink is the limiting link. Yet reliability and associated operational issues are associated with any tower components. Plus, any interference problems will be exacerbated when using a TMA.

  • High-power, multi-carrier power amplifiers (MCPAs) amplify the transmit signal from the base station to the mobile device. By increasing power on the base-station downlink, it's possible to increase coverage. Under certain circumstances, doing so will allow for increased capacity utilization.

    Different technologies, such as TDMA, GSM, CDMA, and AMPS, can be combined onto one antenna through a single, high-power MCPA system. But high-power MCPAs are expensive. They also are hard to integrate from a regulatory and a hardware/software perspective.

  • Newly existing network-optimization tools use drive test and switch data to optimize coverage patterns and capacity distribution for a network. While there is relatively little investment needed for new hardware, this solution requires detailed network analysis.

Obviously, each of these devices or technologies offers ways to increase or enhance network performance. Yet a tradeoff accompanies each solution. Carriers must weigh the performance gains against these tradeoffs. They then have to decide which solution, if any, is right for their situations.

Over the past decade, devices that employ high-temperature-superconducting (HTS) technology have been developed and tested. With increasing frequency, they've also been deployed into wireless-network infrastructures in domestic and international marketplaces. The results show major improvements in network performance without the drawbacks exhibited by other alternatives.

Though some people once described HTS technology as a "science-fair project," it has come of age in the past year. The wireless signals passing through a super-cooled environment at a temperature of 77 Kelvin (−321° F) experience virtually no interference or loss in strength. Moreover, unwanted interference can be easily eliminated as the signal is boosted. The result is a clearer, stronger signal in the frequency band of interest.

This combined technology and hardware has become known as a cryogenic receiver front end (CRFE). These systems are relatively simple to describe. But given the temperature and performance requirements, they are intricate to construct. CRFE systems are comprised of two key elements: a superconducting receiver filter and cryo-cooled LNAs.

CRFE systems reject unwanted out-of-band interference, thereby preventing it from entering the receiver chain. At the same time, they add amplification to enhance signals in the frequency band of interest. These front-end systems replace conventional cavity filters and room-temperature LNAs as the first elements in a base station's receive chain. Consequently, they provide very high sensitivity to the signals of interest from the customer (FIG. 2).

Base-station receiver sensitivity increases in all interference environments. Consequently, CRFE systems promise to better system performance by improving capacity utilization, extending range, and providing higher data rates. Unlike some alternative solutions, CRFE systems are easily installed. This convenience is derived from their location between the base station's electronics and the base-station antenna on the receive path. In fact, these "plug-and-play" solutions can be up and running in a couple of hours. CRFE systems also are independent of the air interface. Only the frequency of the customer's spectrum is specific to a particular CRFE system.

In wireless environments, HTS solutions offer two great advantages over other alternatives: low operating temperature and loss-less signal propagation at radio frequencies. These qualities permit the highest frequency selectivity together with the lowest noise.

The noise figure of a receiver is dominated by its front-end unit, as shown in the cascaded noise factor equation to below. Here, Fi and Gi are the noise factors and gains, respectively, of each receiver-chain element. The low operating temperature, 77 Kelvin, ensures that the front end adds only a minimal amount of thermal noise to the signal. Meanwhile, the out-of-band interference suppression allows more gain to be added to the early stages of the receiver chain without compromising dynamic range. For example, Superconductor Technologies' SuperLink Rx products have noise figures as low as 0.5 dB. They also flaunt band-specific suppression of unwanted near-frequency interference (FIG. 3).

Superconductors make possible the sharp filtering benefit that is provided by the CRFE. They enable the high-Q resonators that are needed for any filter system requiring a large number of poles and low insertion loss. The total receiver noise figure for the whole receiver chain is the one that determines the base-station and network performance.

When using a CRFE system, the improvement in the total receiver noise figure is substantially more than the front-end noise-figure improvement alone. The receive elements after the front end comprise several nonlinear elements. These elements generate intermodulation distortion products, which act as additional noise components. In a demanding electromagnetic environment operating without a CRFE, these components will generate additional intermodulation distortion products. Such products will add to the noise. Granted, this effect is usually ignored in conventional considerations. It can, however, be an important factor in determining the overall system performance.

High-temperature superconductivity leads to a number of advantages that are difficult to obtain by other means. Its low operating temperature guarantees that only a small amount of thermal noise is added. The losses in passive components are minimal, reducing the noise figure. In addition, filters can be made very sharp. They can eliminate intermodulation products in the later, nonlinear elements (amplifier and mixer). Lastly, low-noise amplifiers can have large gain because of sharp filtering and power control. This aspect reduces the total noise figure of the system. It makes the noise figure for the receive chain's later components insignificant. Such advantages spawn several key performance enhancements for cellular networks, including increased reverse-link capacity utilization, coverage, quality of service, and reduced handset transmit power.

The effect of a CRFE system on network performance will vary by both air interface (CDMA, TDMA, GSM, and TD-SCDMA) and base-station architecture. To show the value of cryogenic-receiver front-end systems in a network, it is best to perform a carefully monitored deployment or field trial. In those scenarios, network statistics and radio-frequency performance are measured before and after deployment. The measurements use switch statistics for the base stations affected by the deployment. They also drive test to show the changes in user experience.

In the following example, a wireless carrier identified five base stations in a suburban environment. The base stations were operating two CDMA carriers on each sector. Because of the high traffic load on this core network, the sites were experiencing a high incidence of dropped and blocked calls. Two alternatives existed: More CDMA carriers could be added to increase the sites' capacity. Or, it would be possible to "cell split"—adding extra sites to reduce some of the traffic from the sites that were most capacity constrained.

To address this problem in the field trial, the targeted base stations were outfitted with CRFE systems. The switch statistics were collected for three weeks prior to and three weeks after installation. All comparisons were performed using 24-hr. and "busy-hour" data collected on Thursdays—the most stable traffic day for that market. The CRFE systems were the only optimization performed on the sites.

Here are the results gained by comparing and analyzing data from all five sites: The average improvement in dropped-call rate for each sector and every CDMA carrier on each site was 40%. Improvements ranged from 20% to 60%. Ineffective attempts to connect to the network decreased by an average of 20%.

The data collected during the field trial showed that more than 5000 dropped calls and nearly 2500 ineffective attempts were eliminated each week for this five-site cluster. These improvements were directly attributable to receiver sensitivity, which increased after the CRFE-system installations. The systems enabled the mobile handsets to power down. Because they could then "hear" signals more clearly, they could use less power for the same quality signal at the base station.

This observation was spawned from the drive test data. The data tracked the power of a sample mobile over a prescribed route around the five sites involved in the field trial. Drive testing provided the best quantitative method to evaluate the user experience. The drive tests were performed at the same time of day, over the exact same route, while traveling at identical speeds. The drive route's color coding shows the mobile transmit power for the sample mobile (FIG. 4).

The sites that have a CRFE system installed are depicted in blue. The lower power shown on the right is after CRFE installation. It should result in longer battery life and improved quality. In addition, more users may now be accommodated on the same spectrum. Specifically, the drive test results showed the following:

  • The average improvement in mobile transmit power for this cluster was 6.5 dB following the installation of CRFE systems.
  • Look at the points shown in red (handset transmitting above 5 dBm) in the left figure (prior to CRFE). They show lower transmit power in the right figure (after CRFE) to yellow (−5 to 0 dBm) and green (below −5 dBm).
  • The reduction in transmit power from the mobile leads to fewer dropped calls. Fewer mobiles reach the maximum power of 23 dBm.

Obviously, the telecommunications landscape continues to evolve amid a turbulent economy. Carriers are working to keep up with demand by reviewing, testing, and deploying alternative technologies and hardware into their network infrastructure. Many of these solutions provide positive results. They usually involve some tradeoff, however, in performance, maintenance, or cost benefits.

Superconducting solutions for network enhancement have matured from curious R&D applications to field-proven solutions. Aside from being easy to install, they provide demonstrable benefits in short time periods for any network. They also have reliability metrics that exceed many—if not all—competitive solutions.

In the past 12 to 18 months, the need to do more with less to improve network performance has been increasingly important to carriers. If the issue of local number portability (LNP) for carriers becomes a reality, that urgency will accelerate dramatically before year's end. CTIA estimates that one-third of all wireless users switch carriers annually. Industry observers suggest the rate of consumer "churn" could explode after LNP takes hold. If consumers can select their preferred wireless provider without fear of losing their current phone numbers, the carriers with superior networks should benefit from customer acquisition and retention. Even though carriers are feeling the pinch of capital restraints, this possibility magnifies the need for outstanding network performance. The time may have come for solutions that offer more for less, such as high-temperature superconducting front ends.

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