In the previous two articles of this series, IHS introduced key LTE Advanced (LTE-A) technologies vital to the evolution of 4G LTE. The first article focused on the use of higher order modulation in the signaling protocol in order to fit more bits into each LTE broadcast signal while the second article focused on the scalability of LTE-A by exploiting spatial multiplexing through 4x4 MIMO. Combined with the use of multiple carrier aggregation fundamental to LTE-A, these evolved LTE technologies are leveraged to reach the near gigabit-per-second speed threshold. However, given the limited licensed spectrum holdings today, LTE network operators will be challenged to extract further broadband speed gains beyond that magical one gigabit-per-second barrier. While the 5G wireless standard and promise of order-of-magnitude improvement in data throughput are still at least 4 to 5 years away from deployment, the industry will need to rely on highly evolved LTE-A networks to fill the gap in wireless broadband capacity. That means LTE-A networks needs to scale beyond the gigabit-per-second limit, offering a performance on-ramp to the next generation 5G networks. In order to do so, significantly more spectrum will be required to continue LTE-A evolution and network enhancements. While additional licensed spectrum are being planned for auction, the total amount of licensed spectrum available will still come up short relative to supporting LTE-A networks beyond the gigabit-per-second performance realm. This begs the question, where can the industry go to find additional spectrum to make next generation performance a reality? The answer? Use of LTE-U and LAA over shared and unlicensed spectrum.
Figure 1 – Licensed and unlicensed LTE spectral holdings and the correlation of bandwidth with speeds
Today, all global LTE networks run exclusively over licensed spectrum. In order to keep up with rising demand for 4G LTE services, wireless carriers have relied on a strategy of re-farming existing 2G and 3G spectrum for use in LTE-A deployments. However, this move to build up additional licensed spectrum can realistically yield only 2 to 3 times additional capacity meanwhile the growing demands for LTE-A services will likely out-stripe the rate of additional LTE spectrum by more than an order of magnitude. Currently, vast majority of global wireless carriers have about 20 to 30MHz of licensed LTE spectrum in deployment while a handful of leading markets may have upwards of 50 to 60MHz at their disposal. Given the current global LTE spectrum holding portfolio, only one in four wireless carriers is capable of deploying LTE Advanced Pro1 service capable of supporting high bandwidth applications such as 4K video streaming or real-time VR broadcast. In order for the remainder of global LTE carriers to catch up to the gigabit-per-second level of service, the industry needs to tap into additional spectrum and the most promising arena of easily accessible and usable spectrum lies in the unlicensed realm. This seems straight forward enough. However, as with most things in the wireless world, things are not always as they seem.
LTE over shared and unlicensed spectrum; Current developments and challenges
Unlicensed spectrum is assigned by national governments for the open use by all its citizenry. This common use model works most effectively when the spectrum is shared equitably and everyone given an equal opportunity to use the appropriated air waves. Globally, there are 2 blocks of viable unlicensed spectrum wide enough to address the bandwidth requirements of LTE-A beyond the gigabit-per-second speed barrier.
I. 5GHz – A large swath nearly 500MHz wide unlicensed global available spectrum. A portion of this frequency is currently used for WiFi access, cordless phones and other consumer electronic devices.
II. 3.5GHz – in the US, this is referred to as the Citizen’s Broadband Radio Service (CBRS) which the FCC has set aside 150MHz as unlicensed secondary use2 or shared spectrum.
Given the available unlicensed spectrum, extending LTE connectivity into these unlicensed frequencies will not be a trivial exercise. Firstly, standards will be required to ensure interoperability between network equipment and devices. Secondly, hardware ecosystems has to be developed to support both infrastructure and user devices such as smartphones. Currently, there are 2 generational standards for deployment of LTE over unlicensed spectrum (LTE-U) with a corresponding licensed anchor LTE frequency.
1. LTE-U; 3GPP Rel. 10/11/12 implementation of LTE in unlicensed spectrum, specifically unlicensed NII-1 and unlicensed NII-3 (UNII 1 and 3) with a combined downlink only bandwidth of 225MHz.
2. LAA (License Assisted Access); 3GPP Rel. 13 and beyond standard deployed in TD-LTE. LAA in 5GHz is designated as Band 46.
LTE-U represents a mature, ready to deploy solution due to the fact that UNII1 and 3 resides in the same frequencies as today’s 802.11ac WiFi networks. LTE-U is slated for mobile operator deployment in markets such as the US3 and Korea as early as 2017. Carriers are incentivized to take advantage of opportunistic use of unlicensed spectrum to off load data traffic and improve customer experience. License Assisted Access (LAA) incorporates “listen-before-talk” algorithms to ensure more harmonious co-existence with existing WiFi signaling. The ecosystem for LAA is less developed but makes available significantly more bandwidth for use as an additional LTE TDD aggregated unlicensed spectrum (figure 1).
What LTE over shared and unlicensed spectrum means for smartphone electronics?
While benefits of LTE over unlicensed spectrum are self-evident, the engineering challenge it imposes on smartphone electronic design should not be overlooked. Beyond the current evolution of the LTE RF front end to support key LTE Advanced technologies such as carrier aggregation, 256QAM and 4x4 MIMO, the addition of LTE-U and eventually LAA adds more nuances to an already complex RF design. Not only will LTE-U capable smartphones require additional RF components such as frequency specific power amplifiers, filters and switches but major downstream componentry such as the RF transceiver will need to be redesigned to support the new unlicensed spectrum and all the various permutations of carrier aggregation with the anchor licensed LTE frequencies. Further, as antenna design becomes already complicated by 4x4 MIMO, it will now need to address a wider gamut of radio spectrum from low frequencies (~700MHz) to mid frequencies (~2GHz) to high frequencies (3.5-5GHz). A potential solution to this physical antenna design challenge is to leverage the existing 5GHz WiFi antenna in order to share a physical antenna between the WLAN and WWAN portions of the smartphone. This antenna sharing scheme has potential to relieve some but not all of the added complexity of LTE-U and LAA in the 5GHz unlicensed spectrum.
In terms of hardware ecosystem readiness, currently, only industry leader Qualcomm has announced chipsets that support LTE-U and/or LAA with their Snapdragon X12 and X16 LTE modem. The availability of these LTE-U/LAA capable chipsets will make possible early smartphone designs that can support anticipated carrier deployments next year4. Pending the successful roll out of the initial LTE-U networks in the US and Korea, IHS anticipates other competitive LTE chipset makers to jump on the bandwagon in supporting both LTE-U and LAA in their upcoming flagship chipsets.
LTE Advanced Pro is the future of 4G LTE
As with the historical evolution of 3G networks that saw increasing bandwidth and capacity through technological enhancements such as EvDO and HSPA+, LTE is similarly undergoing that transition to LTE Advanced and now with LTE Advanced Pro. While LTE Advanced Pro is defined as the next evolution of LTE-A with broadband speeds starting at one gigabit-per-second, wireless carriers can technically achieve this performance threshold within the licensed spectrum holdings but will be hard pressed to offer additional service improvements beyond that benchmark. However, LTE-A along with unlicensed spectrum via LTE-U and LAA is critical for the industry to evolve LTE Advanced past the gigabit per second barrier and provide a performance “on ramp” to the upcoming 5G networks. Further, LTE-A with unlicensed will be built on a network topography that leverages denser small cell architecture that will map very well to the anticipated millimeter wave 5G network deployments. All of the key LTE Advanced technologies introduced thus far in this series are additive and scalable. For example, not only will LTE-U and LAA rely on aggregation of the anchor licensed LTE spectrum with the larger unlicensed portion but can leverage LTE-A enhancements such as higher order modulation and 4x4 MIMO spatial multiplexing on all channels, compounding the benefits of the evolved LTE network. Thus future of 4G LTE is LTE Advanced Pro and the path to gigabit-per-second broadband performance is paved with key LTE Advanced technologies and LTE over unlicensed spectrum.