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Fast website downloads, smooth video playback, compelling social networking, unlimited roaming, long battery life—these are some of the key elements that define the quality of consumers’ experiences when using a smartphone. Critical to meeting user expectations in the phones of today and tomorrow is the 4G next-generation wireless technology known as Long Term Evolution (LTE). Because of this, it’s essential for the suppliers of the chipsets that constitute the heart of smartphones to deliver products that support LTE’s capabilities.

However, support for basic LTE functionality isn’t enough. To enable a truly compelling user experience, chipset providers must include modems that can handle the advanced aspects of LTE.

“Basic” support for LTE is not that basicBefore assessing more advanced capabilities such as carrier aggregation and coexistence optimization, the challenges that chip suppliers face in supporting LTE at its most basic functionality must first be evaluated—namely, mobile broadband data connectivity on one contiguous channel.

Smartphone chipset suppliers must assemble the right

ingredients in a crowded, lightning-fast game

Modems in Smartphones Hold the Key to Optimal 4G LTE Delivery

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Standards certification and interoperability testing require the modem to perform specific steps in order to acquire the base station, authenticate, transmit/receive data, perform handovers from one site to the next, and manage power levels—all done within network parameters. But an optimized modem goes beyond these “must-have” capabilities and also addresses important questions such as:•How quickly can the modem perform

these steps?•How well does the modem operate in

low-signal-strength environments?•How efficiently can the modem deliver optimum

performance while maintaining battery life?

At their core, these questions really concern two modem design elements: the protocol stack and peripheral algorithms; and the radio-frequency (RF) architecture.

The protocol stack and the peripheral algorithms are essentially the “programs” that run on the baseband of the modem chipset. These programs in turn dictate how and when the modem should communicate with the network as defined by the supported air interface technology—in this case, LTE—to perform functions like base-station acquisition, authentication, radio-resource requests and traffic-channel acquisition.

Teardown image of a Samsung Galaxy smartphone, showing modem components for 4G LTE

Modems in Smartphones Hold the Key to Optimal 4G LTE Delivery

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When designed properly, the protocol stack allows not only for the rapid processing of control elements—called Layer 2 and 3 messages—that enable the aforementioned functions, but also mitigates the effects that poor signaling algorithms could impose on the consumer’s experience of the data call.

Smooth handoffFor instance, one element that all LTE modems are required to support is handing over calls between the LTE network and a legacy 3G network. A modem might demonstrate it could do this in the lab or in ideal conditions that would allow the device to pass a certification test. In the field, however, where the LTE signal in cell-boundary conditions is potentially just as bad as the lower-generation 3G signal, conditions are less than ideal. If the algorithm for this function is not optimized, the modem could simply bounce back and forth between technology modes and never establish a traffic channel, causing the consumer experience to degrade.

Similarly, should the algorithm for requesting the new 3G channel be called upon too late and the signal with the LTE channel is lost prior to establishing a new traffic channel, the effect on the consumer experience would be akin to someone disconnecting the Ethernet cable while in the middle of trying to stream the end of a championship football match on a PC—an unpleasant experience, to say the least.

The RF front-end architecture, meanwhile, is the section of the core chipset that handles the conversion of the information sent or received with mobile devices between digital and analog states, and is responsible for the physical transmission and reception of the RF signals that carry voice or data packets over the air. RF architecture design utilizes components such as RF transceivers, power amplifiers, switches, duplexes, low-noise amplifiers, filters, antenna tuners and envelope trackers.

When executed properly, the RF architecture design will have the ideal objective of using the least amount of power during transmission and reception, while still ensuring the best speed and latency possible for current channel conditions. This is because transmitting data is the most power-hungry function that a modem can execute; and by minimizing the amount of time a modem is actively transmitting, the RF architecture

can help deliver improved consumer experience at a fraction of battery life.

But to accomplish this, the RF architecture must minimize any self-inflicted noise and interference generated by its own components, while also dealing with—or otherwise offsetting—the effects of external noise and interference inherently present in a lossy transmission medium such as air. System-level design and the inclusion of capabilities such as antenna tuning and envelope tracking also help in achieving this ideal.

A brief history of LTEThe first commercially available LTE devices that generated production volumes hit the market in 2009, marked by Telia Sonera’s LTE launch in Europe’s Nordic countries. Currently the majority of LTE smartphones available in the market utilize a chipset made by Qualcomm, which controlled 97 percent of mobile handset LTE baseband shipments as of the first half of 2013.

Other suppliers that also have solutions are MediaTek, Samsung, Intel, GCT Semiconductor, Nvidia and Broadcom. However, not all these modem suppliers have design wins that have ramped up to production volumes.

Advanced functionality via carrier aggregationAs challenging and nuanced as the designs might be for basic modem functionality, building on them to include advanced LTE features adds another few layers of complexity. Furthermore, the successful implementation of the advanced features depends on how well the basic functionality was implemented to begin with.

The primary benefits of LTE are increased bandwidth, decreased latency and improved spectrum efficiency. But these benefits are not fully realized until channel bandwidths of greater than 10 megahertz (MHz) are used. To optimize consumer experience as well as the operator’s return on investment for building out an LTE network, there is a demand for finding ways of using 15-, 20- and even 40-MHz and above channels. Unfortunately, due to existing usage of licensed spectrum, most countries’ spectrum plans do not allow for contiguous 20-MHz channels, much less 40.

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Here is where an advanced functionality like carrier aggregation comes in. Carrier aggregation, in its simplest form, allows an enabled device to combine two smaller, non-contiguous channels into a larger channel, yielding the same benefits that a contiguous channel of the same larger size would provide. But designs enabling carrier aggregation will need to take into account many different types of channel combinations including, but not limited to:• Non-contiguous channels in the same frequency

band—i.e., all within the 700-MHz band• Channels from two different frequency bands but

from the same end of the spectrum and in different bands—i.e., one channel from the 1.9GHz band, and another from the 2.1-gigahertz (GHz) band

• Channels from two different frequency bands but from different ends of the spectrum—i.e., one channel from the 700-MHz band, and one from the 2.1-GHZ band

Achieving these different combinations might appear as simple as adding one channel with the other. But because of RF propagation characteristics, fundamental physics and the limits imposed by physical dimensions, attempting such a feat increases design complexity exponentially.

For example, in the case of the third type above, aggregating channels from different ends of the spectrum requires that the differing propagation characteristics of a signal transmitted at 700 MHz, versus that of a signal transmitted at 2.1GHz, be taken into account in the overall system design. Signals transmitted at lower frequencies have longer wavelengths, which in turn allow for the signal to travel further given similar power and channel conditions. So, if a mobile device was aggregating two carriers from each of those bands and they were coming from the same base station, the signal strength and/or the channel quality could be significantly different. This would need to be addressed by not only the receive chain of the device, but also the baseband in how it processes—and ultimately—combines the two channels.

Another consideration when dealing with multiple active channels is the RF phenomenon known as intermodulation, and the harmonics caused by non-linear behavior of the required signal processing. This phenomenon causes spurious, unwanted signals to appear in other parts of the spectrum different from the frequency of the main carrier. Depending on the

frequency of the main carrier, these spurious signals could lie in the same frequency as the aggregated carrier, and if the RF architecture enabling carrier aggregation is not designed properly, it could cause interference that materially degrades signal conditions and, consequently, consumer experience.

A third—but by no means final—complexity is simple physical size. Inherent in supporting carrier aggregation is the need to support multiple bands. Supporting multiple bands requires more components on the front end—at least on the receive chain because for now, carrier aggregation is only supported on the downlink. Even with the trend toward larger form factors in mobile devices, the increases in size are not enough to offset the increase in the number of components that advanced LTE designs require.

Therefore, the ability of the LTE modem supplier to integrate while maintaining—or even surpassing—capability delivered by more discrete solutions will be critical in enabling advanced LTE capabilities, such as carrier aggregation, within the limitations of contemporary mobile device form factors.

Francis Sideco is Senior Director for Consumer &

Communications at IHS Technology.

Connect with Francis on LinkedIn:

http://bit.ly/Fsideco

For more information visit ihs.com/telecominnovation

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