Why Not a Personal Base Station?

If you had a 3G base station in your own home or office, wouldn't life be wonderful? You could surf the Web at warp speed with high signal strength, watch streaming video all day long, and make unlimited phone calls all from your 3G phone. With a target price of only $100 or so, a femtocell will provide these capabilities and more just for you.

Actually, a femtocell, also termed a 3GPP Home Node B for WCDMA/HSPA and enhanced Home Node B for long-term evaluation (LTE), really does have a lot to offer. However, the benefits for consumers and carriers differ greatly, and it's still early days for this new technology: Few femtocells are being used at this time.

Carriers have heavily invested in 3G technology, buying expensive spectrum, building cell towers, and developing the applications they hoped that consumers would adopt. Unfortunately, the 3G market hasn't grown as fast as many business plans assumed, partly because consumers have found the performance of the higher frequency 3G phones uninspiring inside buildings. This isn't surprising given that concrete walls can reduce signal strength by 40:1.

Indoor use of 3G mobile phones accounts for about 30% of calls as well as a lot of the video that's viewed. Nevertheless, the quality of these user experiences often is inadequate.

Low signal strength from the macrocell slows down throughput and requires high signal strength from the customer premises equipment (CPE), draining the battery. Building more cell towers might solve the problem in urban areas, but the cost of doing so and the difficulty in obtaining permission makes this solution impractical. In rural areas, it's not even an option.

A femtocell inside the customer's home obviously is close to any CPE, so only low-power transmissions are required to achieve good quality of service (QoS). As shown in Figure 1, the femtocell connects to the telephone system and Internet through the customer's broadband service, whether Ethernet, DSL, or cable. This means that backhaul bandwidth generally will support whatever services are running, especially if Ethernet or cable is used.

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Figure 1. Femtocell Relationship to Internet and Wireless Network
Courtesy of Femto Forum

Handoff from Sprint's AIRAVE™ or Verizon's Wireless Extender femtocells to a nearby macrocell is supported, but not all femtocells work in the opposite direction: You have to re-establish the call when entering the femtocell area. Both these CDMA products are made by Samsung and similar to that company's Ubicell™. AT&T's MicroCell is based on GSM technology and supports 3G phone handoffs both to and from a macrocell.

Offloading indoor services from the macrocell has a large benefit. Not only does the femtocell give a customer better reception, but because the macrocell does not need to support that customer at maximum power, it can handle several others at lower power. And, carriers hope that by providing good QoS, femtocells will prompt customers to use more services.

With a femtocell, your 3G phone can access the Web, download bandwidth-intensive applications such as YouTube video, and even make phone calls. Some carriers promote the triple-screen advantage; that is, the same applications can run on your 3G phone, PC, or TV.

Customers basically want their phones to continue working properly wherever they are, indoors or out. It's especially frustrating that higher frequency 3G phones, perhaps bought for new features, may not perform as well indoors on basic calls as older 2G models that operated at lower frequencies.

Femtocells can solve this problem, but carriers can't be too greedy too quickly. In a number of plans, the customer buys and installs the femtocell, continuing to use contract minutes for any 3G calls made in his home. In addition, he continues to pay for the broadband connection.

Several industry pundits have commented on what seems a one-sided advantage for the carriers. Not only does the carrier get paid for the use of the femtocell, the macrocell capacity increases without requiring further base station construction. There would appear to be an argument for at least partially subsidizing the femtocell cost as is assumed in some European business models. If a subsidy increased the rate of femtocell adoption, it could be a very sound approach to improving network QoS as well as revenue.1

Wireless Alternatives

If a consumer wants TV, Internet access, and phone service, the combinations of available choices and technologies can become confusing. From a cable-network provider's point of view, the broadband connection is the key. If you are happy using an IP phone, your home services could be very Web-centric and have little to do with traditional phone carriers. Of course, it's not wireless, you can't use your 3G phone, and you're stuck using the service where it's wired.

For wireless operation, T-Mobile, Verizon, and others provide a combination of 3G services and Wi-Fi through HotSpots. The high speeds and mobility offered by Wi-Fi are attractive, but your CPE including phones need to have Wi-Fi and Web browsing capabilities. If you like coffee, the Starbucks HotSpots may be convenient, but this approach doesn't help home 3G reception problems very much. One solution is to install Wi-Fi access points at home and the office and simply use that capability in the phone.

Taking a WiMax-based approach, major cable network companies together with Sprint, Intel, and Google have formed the Clearwire Consortium. Clearwire offers wireless broadband but via WiMAX as a 4G technology. Recently, Clearwire launched a Wi-Fi router for Mobile WiMAX that promises faster throughput and larger files than directly available on Wi-Fi. Currently, only two cities have this service although it's scheduled to be rolled out in several others.

Of course, in addition to Wi-Fi and WiMAX, there are the well-entrenched 3G wireless networks. The wireless phone companies have a licensed band of spectrum, good cell coverage, and the advantage of very strong user attachment to personal 3G phones. These companies are motivated to encourage more use of the existing spectrum and phones, and femtocells achieve this in the home.

Because there are so many paths through the communications maze, it's not at all certain how femtocells will evolve. As well as having to meet stringent spectral regulations and cost targets, femtocells must be capable of remote upgrades and management. In addition, designs should be universal to the degree that little is required to change from one carrier's network to another, or even from 3G to 4G WiMAX or LTE.

Testing

These considerations have significant test implications at several levels. A femtocell should appear to be part of the carrier's network so it must interface to its macrocell neighbors and seamlessly handoff as well as accept calls. If a large percentage of customers adopt femtocells, especially in dense population areas such as apartment buildings, will the macrocell's list of neighboring cells accommodate all the femtocells?

Local operation is much easier to specify. The femtocell has to provide the air interface environment the CPE is designed to work with. This includes power monitoring, which reduces a local phone's transmission power and minimizes interference among neighboring femtocells. Users should have the capability to restrict femtocell access to only certain phone numbers.

End-to-End, Including Backhaul
The third part of the equation, the customer's broadband backhaul, is taken for granted in many femtocell discussions although its performance is critical to throughput. Nevertheless, as Iain Wood, marketing manager at Epitiro, explained, broadband service and femtocell voice quality can be tested without impacting operating costs or end users.

“We test the actual customer experience of the femtocell call. With our FemtoLite™ product, software agents download to a customer PC and qualify the connection prior to installing a femtocell. ISP-I™ Femto provides operators with in-life monitoring and service-quality metrics of any national or international femtocell delivery network,” he continued. “Our unique software delivers real-time insight into femtocell performance whether on a carrier's own or an unmanaged third-party broadband network.”

Before deploying a new femtocell design, it's critical that the capability to handle simultaneous voice, video, and data traffic is verified. Here, Epitiro's FemtoLab™ generates complex traffic profiles within an automated lab environment. You can create virtually any required test scenario, specifically including functional testing of voice, high-speed packet access (HSPA), and short message service/multimedia message service (SMS/MMS) and scalability. The detailed analysis of each call includes perceptual evaluation of speech quality (PESQ), voice quality ratings, network quality, signaling, and voice latency.

As a result of actual tests run with Epitiro tools, it is clear that voice quality may suffer unless the customer service level can be assured by the ISP providing backhaul. This means that the 3G carrier can only guarantee femtocell voice quality if a service level agreement has been established.

Also concerned about the effect of backhaul performance on call quality, Spirent Communications has launch-ed the Landslide™ CDMA and 3GPP Femtocell Solution. James Brunson, the company's director of multimedia applications services, explained, “Testing a femtocell network requires a test solution that supports highly scalable IPSec and multigigabits of data throughput. All control and bearing plan traffic is encrypted in a femtocell network.

“The Landslide focuses on testing the femtocell core network that includes the security gateway, wireless Softswitch, and media gateways,” he explained. “The testing of the femtocell is not supported on Landslide. The Landslide emulates millions of femtocells and clients to test the core networks deployed by the carriers.”

ZK Celltest already provides the Model ZK-SAMp Portable Monitor used to benchmark indoor wireless performance. A floorplan feature allows collected data to be correlated with an interior location. Up to five phones can be simultaneously connected, and for femtocell-monitoring purposes, different brands of the type corresponding to the femtocell technology could be used.

John Sleet, the company's CEO, confirmed that the ZK-SAMp can measure and record parameters such as RSSI, RxQual, and Ec/Io and present them for real-time viewing on the built-in LCD panel. The data can be logged to a standard PC flash card for later review or post-processing as needed. The ZK-MPSp is identical to the ZK-SAMp, except the ZK-MPSp also provides integrated high-speed multiband multiprotocol scanning. All of the ZK Celltest product line is non-PC based, which allows lower equipment cost and eliminates the time taken to boot a PC-based instrument.

RF Production Testing
An area where RF instrument manufacturers have been concentrating their efforts is measurement speed. And, this is precisely the test system capability needed for economic femtocell production. Base stations are expensive and complex, but a carrier has a limited number of them. If a simplified, low-power version of a macrocell is to be manufactured in quantities of millions and sell for around $100, the amount of test time must be minimal. Yet, a femtocell must operate within the bounds of the carrier's licensed spectrum and correctly implement the protocol.

Keithley Instruments' Series 2800 Vector Signal Analyzers and Series 2900 Vector Signal Generators are recent designs based on a software-defined radio architecture. This means that most functions are achieved digitally, directed by the appropriate signal personality option.

As an example of measurement speed, six signal measurements can be completed on a GSM signal in 24 ms and seven measurements can be completed on a WCDMA signal in 60 ms. Switching between measurement types takes from 8 ms to 29 ms according to the Model 2820 datasheet.

Figure 2 shows how the WCDMA uplink signal required for 3GPP TS25.141 base station conformance testing is formed. An Arb file is being generated that will drive the company's Model 2910 VSG. Signal generation uses Keithley's SignalMeister™ application and starts from a standard 3GPP reference measurement channel template. In this example, the template waveform has only been resampled, but SignalMeister also allows drag-and-drop creation of nonstandard channel configurations.

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Figure 2. 3GPP Transmitter Test Signal Generation

Courtesy of Keithley Instruments

The VSA and VSG instruments are used by U.K. companies ip.access and Ubiquisys. In separate Keithley press releases, the vice presidents for operations at the companies cited the combination of high speed and accuracy as important for femtocell production test. Both also mentioned Keithley's knowledge of the application and support during test development.

Keithley and Agilent Technologies have independently adopted a technique Agilent terms single acquisition combined measurements (SACM). The Keithley 2820 datasheet states, “The Model 2820 captures signal data and stores it in its waveform memory, allowing the DSP to make measurements on one common set of data.”

According to Frank Palmer, wireless marketing program manager at Agilent, “The SACM application allows manufacturers to make measurements much faster than with traditional instruments. A single acquisition of data is used for multiple measurements, saving valuable time in comparison to traditional measurements that recapture data for each individual measurement.”

Agilent has a long track record of providing base station test equipment and a wide range of instruments suited to all product stages from R&D through manufacturing, deployment, and service support. “For femtocell chipset design, development, and verification test,” Mr. Palmer continued, “the ADS and SystemVue EDA software and design libraries; ESG and MXG VGAs; PSA, MXA, and EXA Signal Analyzers; and technology-specific software are most suitable. Although designed to address the 3GPP Home node B requirements, much of this same equipment is leveraged into the design and test of the femtocell modules.

“For manufacturing, the system of MXG Signal Generators and EXA Signal Analyzers along with the appropriate technology software is ideal for low-cost, high-volume production test,” he concluded. “Agilent also provides solutions for the network with the J6801A Distributed Network Analyzer, the J7830A Signaling Analyzer Real Time (SART), and the E6474A Drive Test Optimization System, enabling call tracing, data measurements, and layer 3 protocol messaging as well as indoor mapping and data collection.”

A version of the J7830A, SART for Femto, has been released with femtocell-specific test capabilities. With SART for Femto, users can interpret, correlate, and analyze both wireless and backhaul signals.

Mr. Palmer also mentioned another requirement to test a femtocell's GPS or A-GPS capabilities. A femtocell is licensed to operate at a fixed location, and GPS is one way to check that it has remained there. In addition, system timing and the need to satisfy E911 emergency location assistance requirements are facilitated by GPS and A-GPS.

Spirent recently announced that ETS-Lindgren's EMQuest™ Antenna Measurement Software now supports Spirent's UMTS Location Test System. This means that wireless device manufacturers can quickly and easily add Spirent's A-GPS instrumentation to ETS-Lindgren's OTA Wireless Test Solutions, streamlining efforts to meet emerging standards for wireless device A-GPS performance.

Summary

Although femtocells are in their infancy, the business case for their deployment seems sound. To help accelerate femtocell adoption, the Femto Forum was formed in July 2007 and has more than 40 members from the operator and vendor community worldwide. In addition to supporting and driving industry-wide femtocell standards and common architectures, the forum also will direct and implement a multifaceted marketing campaign to raise the profile of femtocell solutions.

A major accomplishment has been development of the Iu-h standard that defines how femtocells communicate via the Internet with a carrier's phone network gateways. According to the Femto Forum, “A femtocell standard resolves key operator concerns. Crucially, it allows operators to multisource both femtocells and their network gateways from multiple vendors. This means operators aren't tied into a single vendor and also creates a healthier, more competitive femtocell marketplace—which should lead to lower costs.

“At a 3GPP meeting convened in December 2008, the basis of the femtocell standard that will be built into Release 8 was agreed upon. The crucial interface between femtocells and the gateways in the network core has been determined and will be known as Iu-h…. Elsewhere, the 3GPP2 and WiMAX Forum have formed clear femtocell requirements and are moving to create detailed technical solutions.”2 3GPP officially published Release 8 on April 7, 2009.

With many of the standardization issues settled, femtocells appear ready to burst forth on the scene as yet another part of the ongoing fixed-mobile convergence. Because the frequencies and protocols largely are the same or similar to existing ones, test instruments and methods already exist. On the other hand, measurement speed and automation will be key to rapid production of the millions of low-cost femtocells needed to satisfy the anticipated market.

References

  1. “Femtocell Briefing Paper: Introducing 3G Femtocells Successfully to the Home,” ip.access.com, 2008.
  2. “A Look Back and Forth—2008 and 2009 in a Nutshell” Femto Forum Newsletter, Issue 4, February 2009.
FOR MORE INFORMATION   Click below
Agilent Technologies EXA Signal Analyzer Click here
Epitiro FemtoLab Click here
ETS-Lindgren EMQuest Antenna Measurement Software Click here
Femto Forum Femtocell Information Click here
ip.access Femtocell White Papers Click here
Keithley Instruments Series 2800 VSA Click here
Spirent Communications Landslide CDMA and 3GPP Femtocell Solution Click here
ZK Celltest ZK-SAMp Portable Monitor Click here
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