Mobile device chips coming next year from Qualcomm will be able to use wide spectrum bands that carriers are beginning to patch together with new technology, but its lofty performance claims need to be taken with a grain of salt.

Qualcomm says its new modem and transceiver chips can take advantage of spectrum-bundling capability for theoretical speeds as high as 300Mbps. Several mobile operators are already using or preparing for those techniques, but the actual speed a subscriber could get with all that spectrum will depend on a lot of factors.

The silicon heavy-hitter announced the two chips on Wednesday. They're due to ship in sample quantities early next year and go into the company's RF360 Front End Solution. The new chips, called the Gobi 9x35 modem and the WTR3925 RF (radio frequency) transceiver, are designed to work with LTE networks that combine multiple bands of spectrum for higher speed.

Qualcomm has supported that network technique, called carrier aggregation, in previous chips. But the latest silicon can support aggregation of as much as 40MHz of spectrum, which could deliver the theoretical top speed of 300Mbps, according to Qualcomm.

"Consumers can expect operators to market tremendous bandwidth improvements. However, these improvements will be relatively short-lived as subscriber usage network capacity demands increase."

Carrier aggregation is part of LTE-Advanced, a set of upgrades to the 4G standard coming over the next few years. By combining multiple bands of spectrum into one virtual band, mobile operators can achieve higher speeds and take advantage of all the frequencies they control. Qualcomm says its new silicon supports any method of combining 5MHz, 10MHz, 15MHz and 20MHz chunks of spectrum that is approved by the 3GPP, which oversees LTE.

Each advance in mobile networks requires a corresponding improvement in the devices that run over them, or the gains will mean nothing. In that sense, Qualcomm's new chips are well timed. Carrier aggregation is just beginning to emerge.

Verizon Wireless is already combining two 20MHz channels in some markets using carrier aggregation, according to spokesman Tom Pica. Sprint expects to do the same starting late next year using spectrum it acquired through its Clearwire purchase. By late 2015, it plans to pull together three 20MHz bands of that spectrum. AT&T and T-Mobile USA could not immediately respond to questions about carrier aggregation, but both are expected to take advantage of it.

"Pretty much everyone has talked about it and is in the process of rolling it out or going to do so shortly," meaning by 2014, analyst Chetan Sharma, of Chetan Sharma Consulting, said via email.

Earlier this year, SK Telecom in South Korea claimed it had the first publicly available LTE-Advanced network with carrier aggregation. The carrier is combining two 10MHz bands and says that gives a theoretical top speed of 150Mbps.

Chips that can use a 40MHz band could give device makers the headroom to use better networks as they roll out. But there are other factors that come into play with mobile performance, Ovum analyst Daryl Schoolar said. For one thing, a wireless network with that much bandwidth would need a fat wireline pipe at the base station, probably using fiber, to complete the connection, he said.

"You could see carrier aggregation being used in some areas where the backhaul doesn't support it," Schoolar said. Where that's the case, the maximum speed a subscriber could get for viewing a video or downloading a file would be limited by the wired network. But even so, the additional spectrum could make the user experience more consistent from the center to the edge of a cell's coverage area, he added.

Such performance quotes harken back to lessons consumers learned in the early days of Wi-Fi, said analyst Peter Jarich of Current Analysis.

"Yes, in theory you're going to be able to get 300Mbps with some of this stuff, if you're right next to the base station, and there's no one else using it, and you've got fiber backhaul, and/or there's content right at the base station," Jarich said.

How to estimate real-world performance from such a network? Verizon still quotes the speed of its LTE network at 6Mbps to 12Mbps where it has just 10MHz of spectrum. That would indicate a conservative guess of just 48Mbps with four times as much bandwidth. But the gains from carrier aggregation won't just be linear, Jarich said. Subscribers can get an extra boost because at higher speeds, each user on the network can finish their downloads and uploads sooner and free up the network for the next user, he said.

No matter how fast these networks may be, the game never really ends, according to analyst Phil Marshall of Tolaga Research.

"Consumers can expect operators to market tremendous bandwidth improvements," Marshall said via email. "However, these improvements will be relatively short-lived as subscriber usage network capacity demands increase."