Despite widespread calls for more spectrum to carry mobile data, there is a wide range of technologies already being used or explored that could help to speed up networks or put off the day when more frequencies need to be cleared.

Spectrum is the lifeblood of mobile services. The planned purchase by Verizon Wireless of 20MHz of spectrum from a group of cable operators, which the U.S. Department of Justice approved on Thursday, is the latest sign of how important this invisible resource is to mobile operators.

Any service on the airwaves needs frequencies it can use without being overwhelmed by interference, whether the frequency it uses comes from an exclusive license or from a sharing arrangement. The more packets of data are being exchanged over a network, the more spectrum will be needed to carry them -- unless something else is done.

Mobile operators, technology vendors and governments have been sounding alarms about mobile networks nearing capacity for years, and those alarms are getting louder. A study released last year by investment bank Credit Suisse said mobile networks worldwide were filled to 65 percent of capacity on average, while North American networks were running at 80 percent. The U.S. Federal Communications Commission said in 2009 that it expected mobile data traffic to grow by 35 times in the next five years. Equipment vendor Ericsson predicts 10x growth by 2016. Moreover, the growth in demand is unpredictable, because new applications arrive all the time. And an overloaded spectrum band can slow down users' mobile experience.

To meet that demand, regulators and carriers are trying to make more spectrum available for mobile data services. In the U.S., the FCC pledged in 2009 to make 500MHz of additional spectrum available for mobile broadband in the coming years. CTIA, the U.S. mobile industry group, had called for 800MHz.

However, if the predictions about traffic growth come true, there won't be enough "new" spectrum available to keep up with it. Supply and demand are on different orders of magnitude. For example, if the FCC auctioned off 300MHz tomorrow and the two biggest U.S. carriers split it, that will increase their spectrum in the average market by not much more than double, according to independent industry analyst Andrew Seybold. (Then, it would take three or four years to get that spectrum online, he said.)

To fill the gap between those growth rates, governments and industry agree that what's needed is at least a two-pronged approach, with more spectrum as well as strategies to make better use of the spectrum that's already available. Carriers also are taking steps that should help to dampen the growth in demand.

Here is an overview of some of the techniques that may help to stave off a mobile crunch. They fall into three main categories: getting more out of service providers' current mobile data spectrum, making better use of all possible mobile frequencies, and reducing the demand on spectrum assigned to mobile services.

Making better use of existing spectrum

1. Small cells

Mobile operators can serve more subscribers and give them better performance using the very same spectrum they already have. One way to do this is by installing smaller cells to supplement the traditional "macro" cells that cover an entire neighborhood. As long as the two types of cells don't interfere with each other, subscribers' mobile devices can connect to a small cell serving a street corner or an office and share it only with the other subscribers nearby. If there are enough small cells in the area, users can be handed off from one to the other without ever taxing the macro cell, which remains available for subscribers who aren't near to an area of small cells.

The Small Cell Forum industry group says setting up just four small cells within the area of a macro cell can offload 56 percent of the data traffic from the nearest tower. All this requires exactly the same amount of spectrum as the carrier used to operate the original cell tower, and it lets more subscribers use the network simultaneously.

2. LTE-Advanced

The next generation of LTE is actually a variety of new features coming in version 10 of the standard, which is now complete and is expected to be deployed starting next year. Though it draws attention for its theoretical maximum throughput of 100Mbps (bits per second), the enhancements in LTE-Advanced are mainly for efficient use of spectrum, according to Arne Schaelicke, a global LTE product marketing executive at Nokia Siemens Networks. The advances include the ability to use more antennas per cell and a mechanism to manage interference between macro and small cells.

LTE cells have been limited to just two antennas, but with the new standard they can be configured with as many as eight, to create eight separate streams of data for higher throughput. To allow macro and small cells to work in the same area, LTE-Advanced provides a way for the two types of cells to back off from each other once every millisecond so they can use the same frequency and not interfere with each other, Schaelicke said.

3. Carrier aggregation

However, the biggest benefit of LTE-Advanced is expected to be carrier aggregation. This part of the standard lets service providers combine multiple small blocks of spectrum into one block that's large enough to deliver a strong LTE service. They'll even be able to tie together frequencies that are in widely different bands, taking advantage of already licensed spectrum that otherwise might have gone unused or underutilized.

Clearwire, the WiMax wholesale operator that plans to launch an LTE-Advanced network next year, plans to use carrier aggregation to combine two 20MHz spectrum blocks into a single block with 40MHz, said John Saw, Clearwire's chief technology officer. That will outstrip the 20MHz that both Verizon and AT&T are using for LTE in most markets. However, any carrier aggregation move has to use spectrum combinations that are certified by the 3GPP so device makers will have a specification to work with, Saw said.

4. TDD-LTE

Another technology that can help service providers use their spectrum more efficiently is TDD-LTE (time-division duplexing LTE). Most operators are building their networks with FDD (frequency-division duplexing) systems, which use two paired bands of spectrum, one for upstream and one for downstream traffic. Some are required to do so by regulators. TDD-LTE, on the other hand, uses one block of spectrum for traffic in both directions and segregates them by the amount of time they can take on the network. That better suits real traffic patterns and gives the operator more flexibility in how to use its spectrum, according to Clearwire, which uses TDD-LTE.

"TDD allows me to tailor my spectrum resources to where my customers' behavior is," Clearwire's Saw said. This system can also make it easier to combine multiple chunks of spectrum, because each chunk doesn't have to be paired with another, he said.

Making better use of all spectrum

5. Spectrum-sharing

Some experts say it's time to throw out the whole notion of allocating certain frequencies exclusively to commercial mobile services, or to any exclusive use. Instead, they advocate mobile operators sharing spectrum with current users, such as government agencies.

In a report issued earlier this year, the U.S. President's Council of Advisors on Science and Technology recommended that the secretary of commerce identify 1,000MHz of frequencies where commercial and federal users could coexist. "This study finds that today's apparent shortage of spectrum is in fact an illusion brought about because of the way spectrum is managed," the report said.

Advances in technology, including small cells and radio performance improvements, help make it possible for mobile networks to use the same frequencies as other services, as long as potentially interfering signals don't rise above a certain level, the report said. It cited the success of Wi-Fi in unlicensed spectrum as an example. Sharing could multiply the effective capacity of spectrum by a factor of 1,000, the group said.

In a response to the PCAST report, CTIA, the main industry group for U.S. mobile operators, cast doubt on its conclusions. CTIA said cleared, exclusive spectrum is the "gold standard" for mobile service, and some technologies that PCAST cited aren't available yet.

6. Spectrum refarming

As carriers adopt LTE, which uses spectrum much more efficiently than earlier technologies, they plan to gradually migrate users off of their oldest networks and reuse those frequencies. Those moves ultimately will make a significant amount of additional spectrum available for high-speed data services, but it's not an overnight solution.

Earlier this month, AT&T announced it would phase out its 2G GSM and EDGE networks by the beginning of 2017, shifting customers to 3G and 4G services market by market. It estimated 12 percent of its subscribers use 2G phones. In May, Sprint Nextel won FCC approval to use spectrum in the 800MHz band, which is now used for its aging iDEN network and has been limited to narrowband technology, for LTE services that use wider bands.

7. Wi-Fi

When mobile users shift over from a cellular network to Wi-Fi, their traffic stops using the carrier's spectrum altogether. Wi-Fi is a big part of carriers' so-called offload strategies because it offers two fat bands of unlicensed spectrum and is built into nearly all smartphones and tablets. Many carriers have already built or bought extensive networks of Wi-Fi hotspots, and all encourage users to move their devices from the cellular network to their own Wi-Fi at home.

Continual advances in Wi-Fi technology, including the IEEE 802.11ac gear expected to be widely available next year, make wireless LANs even better for handling large numbers of users. Going from cellular to Wi-Fi may also become easier with new systems such as the Wi-Fi Alliance's Passpoint standard. Efforts are now under way to let users roam automatically, among hotspots operated by different carriers and by aggregators such as Boingo.

The experience would be like roaming between cell networks and it could send a lot more mobile traffic off the scarce cellular frequencies, helping mobile operators to get by longer with the spectrum they already own. But carriers are expected to adopt the new roaming technology slowly. Consumers should note that unlike the access offered now on carrier-owned hotspots, Wi-Fi roaming may not always be free for subscribers.

Reducing the amount of data on the network

8. Compression

No matter what frequencies are used to carry mobile data, less traffic means less spectrum is needed. Data compression is a time-honored way of cutting files down, but it has its limits as a solution to the wireless crunch, according to Rajat Roy, a senior product line manager at Broadcom. To start with, the biggest files, such as video, audio and images, are already compressed using standard protocols such as JPEG and MPEG. For other types of files, the industry is still trying to settle on a common standard so mobile devices will have the software to decompress what's been compressed in the network.

However, Broadcom has targeted one type of traffic that doesn't make for huge files but is often inefficient. The company builds technology into its wireless base-station chips that can compress the header fields of VOIP (voice over Internet Protocol) packets. Though voice doesn't take up much bandwidth, the packet headers containing routing and other information are sometimes twice the size of the payload itself, Roy said. Compressing the headers reduces the load on the network.

9. Caching

There are at least two ways in which caching could help reduce the need for spectrum. One is time-shifting traffic to reduce peak demand. The growing amount of storage capacity on devices and in removable flash cards could allow users to download large files such as video automatically during off hours, said Tolaga Research analyst Phil Marshall. Users who signed up to have that content sent could then watch it later on the device. Security and digital rights management are possible barriers to adopting this technique, Marshall said.

Broadcom has another caching idea: The company equips its cellular base station processors to identify and cache multimedia content while sending it out to client devices. The idea is to keep filling the "time slots" on a wireless pipe that the network allocates for the file transfer. Letting those slots go unfilled wastes network capacity. Broadcom's chips can store enough packets of a file or a multimedia stream in memory so that the base station can pack as much data as possible into every time slot devoted to that application, Roy said.

10. Reducing signaling traffic

In some cases, it isn't mobile video or big email attachments that are consuming an operator's spectrum, but small signals sent between devices and networks. With busy applications such as push email, social networking and even Web browsers, these small signals can add up.

"We've seen cases where carriers had lots of data capacity ... available in their network and congestion being defined by signaling capacity limits," said Peter Carson, a senior director of marketing at Qualcomm. He expects the problem to get worse.