In the previous article of this series, IHS explored the intricacies of designing and commercializing (Long-Term Evolution) LTE modem solutions. Even supporting what some might call “basic” LTE presented technical challenges as well as opportunities for optimization that go beyond simply meeting standards certification requirements. As challenging and nuanced as those designs might be, building on them to include advanced LTE features add another few layers of complexity – the successful implementation of which is dependent on how well the basic functionality was implemented to begin with. For example, in order to evolve current LTE chipset solutions to include advanced features such as carrier aggregation involves not only solving design challenges specific to carrier aggregation (CA) but also relies on the optimized design of the base RF architecture. If this basic architecture did not foresee and account for the interference and co-existence issues inherent in CA designs, the solution will need to be re-designed from the ground up and the chipset supplier’s customers will find it harder to upgrade to the new designs. In this second edition of the LTE Modem Insights series, IHS explores not only the design aspects that affect LTE advanced features but also the impact that the original LTE solution designs have in the implementation of those features.
Carrier aggregation design complexity: 1+1 Doesn’t just equal 2
The primary benefits of LTE are increased bandwidth, decreased latency and improved spectrum efficiency. However, these benefits, when compared to HSPA+ are not fully realized until channel bandwidths of greater than 10MHz are used. At 10MHz channel widths, LTE performance, while still better than HSPA+, are arguably, only marginally so. As such, in order 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 plan do not allow for a contiguous 20MHz channels, much less 40. This is where carrier aggregation, introduced in Release 10 of the LTE standard, comes in. In its simplest form, carrier aggregation allows an enabled device to combine two, smaller, non-contiguous channels into a larger channel yielding the same benefits as a contiguous channel of the same larger size would provide. However, as in most things about LTE, implementing carrier aggregation will not take its simplest form. 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 2 different frequency bands but from the same end of the spectrum but in different bands (i.e. one channel from the 1.9GHz band and another from the 2.1GHz band)
- Channels from 2 different frequency bands but from different ends of the spectrum (i.e. one channel from the 700MHz band and one from the 2.1GHZ band)
Achieving these different combinations might appear as simple as adding one channel with the other. However, due to RF propagation characteristics, fundamental physics and just plain physical dimensions, attempting to do so 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 700MHz 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 the signal to travel further given similar power and channel conditions. This means that if a mobile device were aggregating two carriers from each of those bands and they are coming from the same base station, the signal strength and/or the channel quality could be significantly different which in turn 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 such as is the case with carrier aggregation 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 that is 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. Alternatively, these spurious emissions could also lie outside of the operators licensed spectrum and interfere with other systems such as is the case with harmonics from LTE Band 13 showing up in the legacy DTV spectrum in the United States – a situation which would render the modem design unusable on that operator’s network. These design challenges become exacerbated as solutions come into the market that are able to push carrier aggregation enablement to 3 20MHz channels for full category 6 support.
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 since for now; carrier aggregation is only supported on the downlink. Even with the trend towards 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. This may sound straight forward, however, due to the complexities mentioned above and need for design and commercialized maturity, some solutions still require multiple major components such as transceivers to support advanced features like carrier aggregation while others do not.
Monitoring the “evolution” in Long-Term Evolution
As expected, the recently completed 2014 Mobile World Congress in Barcelona, Spain yielded numerous LTE modem solution announcements as the competitive landscape heats up and challengers look to take a piece of Qualcomm’s market share dominance. On the one hand, Qualcomm has the advantage of scale and maturity as they continue to push the envelope with advanced LTE designs such as a newly announced solution, which can handle aggregation of 3 20MHz channels enabling 60MHz category 6 support. However, device OEMs, as is their strategy are always looking to diversify their supplier base and maintain competition giving challengers such as MediaTek, Intel, Broadcom and Marvell an opening. It will be incumbent upon these competitors to prove that they can field viable solutions that not only address basic capability but closes the gap in enabling these advanced feature sets in order to have a chance at realizing some of the market share potential that LTE and its continuing evolution is generating.