LTE: The roadmap ahead to Gigabit Speeds
In the cellular industry LTE-enabled smartphones made up 1 in 4 new devices shipped back in 2013. By 2015, that percentage share rose to over half of all new smartphones shipped globally. LTE is arguably the most successful generational wireless technology having just been commercialized in late 2009 and evolved to capture the majority of the market for new handsets today. Previous 3G technologies took over a decade to achieve what LTE has achieved in just 6 years. Gigabit capacities are expected and planned.
The surge in LTE adoption – both expected and planned – parallels the growth of the smartphone creating a symbiotic relationship that propelled massive adoption of wireless broadband and smartphone use. The order of magnitude improvement in network latency provided by LTE wireless connectivity coupled with the rapid growth in digital content and the readily available computing power within everyone’s reach created a rich tapestry of mobile opportunities.
Global LTE-Advanced Deployments
Global LTE network deployments enter a new phase of network enhancements as the industry is now turning to enhanced wireless technologies to evolve the speed and capacity to keep up with consumer demand for ever faster downloads, video streams and mobile applications. The first stage of LTE network improvement revolved around the use of carrier aggregation which is a method of combining disparate spectrum holdings to create a larger data pipe. This development tracked with the evolution of LTE from single carrier Cat-3 devices to dual carrier Cat-4 and Cat-6 devices. Further development of carrier aggregation extended the concept to include 3 carrier aggregation specified by Cat-9 LTE standards which brought the maximum throughput speed to 450mbps in the downlink.
However, in order for the industry to evolve further and keep up with the insatiable demand for mobile broadband, LTE Advanced will require further improvements. The next step in the evolution of LTE relies on LTE Advanced. This new set of technologies is destined to improve LTE speed to and past the gigabit-per-second barrier. To this end, IHS will be delivering a series of LTE Advanced Insights to further explore the key enabling technologies to get us to that gigabit per second barrier. This article is the first of this series.
Critical Areas of Exploration
Operators typically ask critical questions including but not limited to:
- What is higher order modulation and how does radio signaling enhancement lead to faster wireless broadband?
- How can advanced antenna designs be incorporated into existing smartphone form factors and what are the physical challenges involved in doing so?
- What are the opportunities to leverage additional spectrum use especially in the unlicensed portions of 3.5GHz and 5GHz? What are the advantages as well as the challenges of doing so?
- How can the industry take learnings from the 3G to 4G transition and build on the foundations of LTE moving into 5G?
QAM: Higher Order Modulation to Break Through Gigabit per Second Barrier
Higher order modulation schemes have been used throughout 3G technologies and now enabling the increased bandwidth coming into 4G LTE Advanced. As WCDMA evolved into HSPA and HSPA+ in the 3G era, higher order modulations of 16QAM and 64QAM replaced the older QPSK modulation schemes to improve throughput data rates that enable mobile broadband services to take off. Fundamentally, sophisticated signal processing such as 64QAM are used in wireless networks to improve the spectral efficiency of communications by packing in as many bits as possible into each transmission. The bits-per-symbol carried by 16QAM modulation scheme is 4 bits while higher order 64QAM yields 6 bits per symbol, a 50% improvement. Extending this concept, LTE Advanced will use 256QAM modulation from Category 11 onwards which is expected to provide a 33% improvement in spectral efficient over that of 64QAM over the same stream of LTE by increasing the bits-per-symbol to 8 from 6.
|Modulation Level (QAM)||Bits per Symbol||Incremental Efficiency Gain|
Table 1 – Modulation Levels and Corresponding Efficiency Gains
While higher order modulations equate greater spectral efficiencies, within the framework of wireless networks, achieving higher order signaling remains a significant challenge. Real world applications of higher order modulations are difficult to implement network wide as the more sophisticated signaling schemes are inherently less resilient to noise and interference. In normal deployments of macro cellular coverage, network operators employ adaptive modulation techniques to detect signal channel conditions and adjust modulation schemes accordingly. For example, if the wireless user is closer to the center of the macro cell area, the network will negotiate the signaling scheme to best take advantage of the wireless fidelity and communicate using the most efficient modulation scheme available. However, if the conditions are deemed inadequate, for example, at a cell site coverage edge, the network may resort to lower orders of modulation signaling in order to achieve higher reliability of connections.
|LTE Category||Carrier Aggregation||MIMO||Spatial Streams||Modulation||Max. Throughput|
Table 2 –LTE Categories and Corresponding Throughput Gains
The previous paragraph described limitation of higher order modulation was a hallmark of 3G networks. However, as LTE deployments begin to rely a more 5G- like heterogeneous network architecture leveraging augmented network equipment such as small cells, the use of higher order modulation becomes more practical as the distance from LTE antenna and the mobile device is reduced. Yet again, with a challenging transmission medium such as over the air wireless, obstacles still exist. Even with the most optimized heterogeneous networks, issues such as site to site signal interference can negate much of the benefits of small cells. Therefore, network operators, with help from their equipment vendors, are working on network optimization software to accommodate these interferences. At the end, any network, even ones designed for Cat-11 LTE and above, will not be able to cover all their mobile subscribers with the highest efficiency signaling. In actual deployment, only a portion of the devices within a coverage area will be at 256QAM while the majority other devices will fall back to a lower modulation scheme such as 64QAM or 16QAM. Also, additional challenges exist in carrier aggregated LTE connections whereby 2 or 3 carriers can be aggregated to form a wider virtual channel, here, depending on the frequencies used and the placement of cell towers associated with those specific spectrum, not all of the aggregated radio signals can be adapted to signal in higher order modulation. Therefore, reaching the theoretical maximum throughput data rates using higher order modulation will be particularly difficult in actual network deployments.
With these real-world deployment limitations on the handset side in mind, higher order modulation schemes have been shown to be a net benefit for LTE networks. Under trial tests, it has been shown that even a small fraction of users in a coverage cell using 256 QAM create improvements in network capacity performance. As devices with 256QAM enter and exit a network faster and more efficiently, it frees up wireless capacity to serve non-256QAM signaling devices on the network. Overall, enabling higher order modulation on an LTE network present a cost advantageous proposition to network carriers as the upgrade is primarily a software based solution. Going to 256QAM gives network carriers immediate benefits without significant hardware changes that are typically associated with other LTE Advanced features such as adding additional carriers or scaling MIMO antennas.
Evolving LTE Advanced to Gigabit Speeds
Putting the Pieces Together:
In order to achieve gigabit speeds in LTE Advanced, higher order modulation is one tool in a vast tool box of technologies the industry can use to propel 4G LTE further as the market waits for the consensus around the next generation 5G networks. By building on top of carrier aggregation technology discussed earlier in this series, the implementation of higher order modulation in Cat-11 LTE increased the maximum theoretical throughput to 600mbps using the same 3-carrier aggregated spectrum as dictated by LTE Cat-9. This 33% improvement from 450mbps is directly accredited to the improved bits-per-symbol efficiency described in this paper.
QAM Modulation, MIMO, CA and Spectrum
So what else is required to take LTE Advanced to gigabit speeds? Well, if we look at the 3GPP Release 12, Cat-16 LTE can get to a theoretical gigabit per second speed using the following combination of the following technologies in concert:
- 256QAM modulation
- 4×4 MIMO with 10 spatial streams (2 high frequency carriers with 4 layers each and 1 low frequency carrier at 2 layers)
- Multi-carrier aggregation (3x20MHz or greater)
- Use of additional spectrum such as LTE over unlicensed frequencies
What’s needed? Chipset advances
Currently, while the bulk of LTE smartphones sold today is still on Cat-6 modems, modem manufacturers are fast working to prepare the electronic component ecosystem with very capable LTE modems that can take advantage of the huge potential and headroom of evolved LTE. Qualcomm in particular has recently announced their X16 modem chipset which has been designed to take advantage of LTE Cat-16. The company claims that volume shipment of X16 modem devices will begin in H2 2016. Other modem makers have not yet announced a CAT-16 capable LTE modem yet but the next iteration of LTE Advanced will clearly be on their roadmap. Meanwhile, wireless infrastructure equipment manufacturers such as Ericsson are lining up technologies to achieve CAT-16 network deployments. Therefore, technically, commercial gigabit speed LTE Advanced Pro networks and devices can be realized in early 2017.
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