A picocell primer

Picocells help wireless operators improve coverage and boosting voice capacity and data throughput where it’s needed most.

Indoor coverage is just as important for consumers because it accelerate the trend toward wireline displacement. In October 2005, 17 percent of consumers who signed up for wireless service over the previous 90 days said that they stopped using their wired phone, according to a survey by the Consumer Electronics Association.

By 2009, between 23 percent and 37 percent of consumer will have switched, according to In-Stat. Even if the actual amount comes in at the low end of In-Stat’s forecast, the number of wireless-only users still represents a huge market of more than 30 million people, assuming that the number of U.S. subscribers keeps growing at its current rate.

The wireless-only market is simply too big for any cellular carrier to ignore. Those that do, risk losing revenue to other service providers, particularly operators of public Wi-Fi hot spots, which give consumers and business users another option. That alternative also is driven by the growing selection of handsets and PC card modems that combine 3G and 802.11, making it easy for users to switch to Wi-Fi when cellular isn’t available.

Picocells are an ideal solution for improving coverage indoors and in outdoor areas such as a city’s downtown.

What are picocells?
Picocells are small versions of base stations, ranging in size from a laptop computer to a suitcase. Although picocells historically have been used primarily in cellular networks, the design also will be used in WiMAX networks, and for the same reason: Regardless of the wireless standard used, even the best full-sized base stations can’t penetrate every nook and cranny of a building or outdoor area.

Repeaters and distributed antenna systems are two alternatives to picocells, but it’s important to understand why they’re not an ideal solution for all situations. One reason is capacity: Besides plugging coverage holes, picocells also are frequently used to add voice and data capacity—something that repeaters and distributed antenna systems can’t do.

Picocells’ compact size makes them a good fit for the places that need all the capacity thy can get. Take New York City, which had about 3 million wireless users in 1999, according to the city’s Department of Information Technology and Telecommunications.

By late 2003, it had 10.5 million mobile users. But splitting cells to add capacity is expensive, time-consuming and occasionally impossible in dense-urban environments such as Manhattan, where room for a full-size base station often is expensive or unavailable.

By ensuring that users can always make calls or get a data connection, picocells improve the chances that users will choose cellular rather than alternatives such as Wi-Fi. That’s one example of how picocells can improve a cellular operator’s competitive position.

The backhaul factor
Regardless of whether it’s used in cellular or WiMAX, the ideal picocell should support multiple wired and wireless backhaul technologies in order to give the service provider more choices. For example, some wireless carriers are owned by RBOCs, so a picocell that supports Gigabit Ethernet (Gig-E) for backhaul can leverage their parent’s metro backbone.

Today’s common backhaul-interface options include ATM, E1/T1 and Ethernet. The latter is likely to be an increasingly popular option, one that backhaul providers have already begun to address. For example, in Houston, Time Warner Cable recently began providing Ethernet backhaul services for a major wireless carrier. By using a picocell that supports multiple backhaul interfaces, a wireless carrier can take advantage of emerging wired and wireless backhaul players—and their competitively priced alternative technologies.

Multiple backhaul choices also can speed deployment. For example, when Omnipoint was building its GSM network in New York City in the late 1990s, months-long delays in getting T1s hamstrung the fledgling service’s QoS and market potential because many cell sites couldn’t be turned on.

This is a cautionary tale for cellular carriers launching 3G technologies such as CDMA2000 1xEV-DO and High-Speed Downlink Packet Access (HSDPA), as well as for service providers building WiMAX networks. Picocells that can use multiple backhaul technologies give carriers a way to maintain a tight buildout schedule because they can use what’s available.

Finally, backhaul flexibility also gives service providers a way to reduce overhead costs. A prime example is cellular: Regardless of whether they use CDMA2000 or GSM/GPRS, today’s base stations typically require only one or two T-1 lines.

But as those base stations are upgraded to broadband technologies such as EV-DO and HSDPA, backhaul must be expanded to keep up with the high-speed services that those technologies deliver. That’s why over the next few years, base stations with a dozen T-1 lines are likely to become the rule rather than the exception.

Because they deliver 3G services, picocells will have to follow that trend. By supporting multiple technology interfaces, a picocell can give the carrier the flexibility to choose the backhaul network that improves its competitive position.

By some estimates, between 30 percent and 40 percent of a carrier’s operating expenses (opex) go toward backhaul. With the freedom to choose the most cost-effective backhaul technology, a carrier can add bandwidth to each picocell as demand rises, yet keep opex in check, improving the ability to price services competitively and still turn a profit.

Power plays
When choosing a picocell, service providers also look at power requirements, which affect their operating expenses. High-efficiency components help reduce power requirements. For example, Texas Instruments’ TCI6482 DSP uses a process called Software Pipelined Loop (SPLOOP), where the loop pipeline is copied to the buffer in the CPU core.

The loop pipeline then is executed out of this buffer, a design that saves power – each TCI6482 DSP draws only 3 watts – by reducing the amount of times that the memory has to be accessed. By pairing SPLOOP with Compact ISA, which reduces code size, memory also is freed up for other tasks, such as additional channels.

Picocells designed for the three most widely used 3G wireless standards—CDMA2000, TD-SCDMA and UMTS—also should use a crest-factor-reduction processor, which sits between the digital upconverter and the digital-to-analog (D/A) converter and selectively reduces the peak-to-average ratios (PARs) of the wideband digital signals used in 3G systems.

Reducing PAR improves power amplifier efficiency, which in turn significantly reduces the cost of power amplifiers, as well as the amount of electricity used to power and cool them.

Easing 3G’s growing pains
Picocells can help flatten the learning curve that cellular carriers face as they migrate from 2G to 3G. For example, CDMA-based systems are susceptible to "cell breathing," where a base station’s coverage begins to shrink as the number of connected users increases, creating holes where calls drop and data connections are lost. GSM, GPRS and EDGE networks don’t experience this phenomenon, so it’s one example of the RF-engineering issues that operators face when rolling out W-CDMA.

Another example is that a UMTS Node B typically has a smaller coverage area than a GSM/GPRS base station, usually because UMTS is deployed in a higher frequency band. A picocell can fill those coverage gaps much more quickly and cost-effectively than additional UMTS macrocells, whose expense may not be justifiable in areas where 3G adoption is still low.

What’s wrong with macrocells?
The price of a cellular base station has fallen by more than 60 percent over the past two years, according to analysts such as Arete Research. That trend begs an obvious question: At the rate that base station prices are falling, why not just use macrocells instead of picocells?

One common reason is size. A suitcase-sized picocell can be installed in more places than a refrigerator-sized BTS or Node B. That flexibility is key in urban areas, where site leases carry a premium because space is limited. For example, in New York City, space on atop street signs, lampposts and traffic signals leases for $6,000 per month. That’s steep—but a macrocell would require even more space, triggering even higher fees.

Compact size also means that picocells can be installed in locations where their signals aren’t wasted. A prime example is a downtown business district, where picocells installed at street level can provide highly focused coverage and capacity. By comparison, the signal from a rooftop-mounted macrocell signal is likely to sneak into other areas, causing interference that saps network capacity.

Installation costs also typically are lower with picocells. A macrocell usually requires a forklift or crane to install, but most modern picocells are small enough that they can be installed without machinery.

They’re also often designed to be nearly “plug and play,” reducing overhead costs even further because installation doesn’t require an army of highly trained technicians. One example is Nortel Networks’ Univity eCell picocell, which the company says "can be installed by low-skilled staff in just 15 minutes."

Table 1 illustrates the difference in size, weight, installation requirements, and location options between macrocells and picocells.

Click here for Table 1

Small solutions
Picocells are an ideal way to address five major issues in wireless:

• License requirements—Many operators have to hit coverage targets under the terms of their 3G licenses. For example, in the United Kingdom, 3G licensees must cover at least 80 percent of the country by 2007. Using a mix of macrocells and picocells can be a faster and more cost-effective way to expand coverage than a build-out that relies entirely on macrocells.
• Spectrum—In most countries outside of North America, UMTS is relegated to the 2.1 GHz band. Signals travel farther at the lower frequencies used by GSM/GPRS/EDGE, so when UMTS equipment is co-located at existing cell sites, the 3G service usually has coverage holes. Compared to full-size Node Bs, picocells are a convenient, cost-effective way to fill in those gaps.
• Analog’s phase-out—Coverage holes often emerge when an operator begins phasing out analog in favor of CDMA2000 or GSM/GPRS. When customers begin encountering black holes after switching to digital, their complaints can drive up the operator’s overhead costs due to remedies such service credits and adding customer service staff. If the market has established digital operators, the former analog operator’s service must be able to match rivals’ quality of service in order to attract and retain customers. These challenges aren’t going away anytime soon in areas such as Latin and South America, where analog’s phase-out is still in its early stages.
• Wi-Fi competition—When a cellular operator has spotty coverage, particularly indoors, it’s at a competitive disadvantage in areas with public or private wireless LANs. With the growing availability of PC card modems that support cellular and 802.11, users who can’t connect via CDMA2000 or GPRS/EDGE/UMTS can easily try Wi-Fi. As voice-over-802.11 devices become more common, a strong wireless LAN signal can siphon off voice revenue, too.
• Accommodating growth—Coverage isn’t the same thing as capacity. (If it were, repeaters and distributed antenna systems would work just as well as picocells.) In dense-urban areas such as New York City, splitting a cell to add capacity might be cost-prohibitive or impossible with a full-size base station.

The bottom line is that for both cellular and WiMAX networks, picocells are an increasingly popular choice because they solve real-world problems. The catch is that not all picocells are created alike. Component choices and backhaul options determine whether a picocell’s design provides the right mix of flexibility and performance that today’s hypercompetitive telecom market demands.

Coming soon: The technology behind picocells.

About the author
Ramesh Kumar is the worldwide DSP Manager for Communications Infrastructure of Texas Instruments. He is responsible for product and business development worldwide including Asia and Japan. Previously, Ramesh served as a chip design engineer for Motorola. Ramesh holds an MBA from North Eastern and an MSEE from Purdue. He can be reached at ramesh.kumar@ti.com.