Usually, when work begins on defining a new system, certain key use cases are identified that need to be supported in an optimized manner and will provide novel functionalities beyond the capabilities of legacy technology. As discussed in previous chapters, for 2G this was voice, and then 3G was designed to support Web browsing in addition to voice, first in Release 99 and from Release 5 on with High Speed Packet Access (HSPA). But although there was hype about video telephony, it never reached the mass-market. LTE was a natural extension of HSPA to allow higher throughput.
It should be noted, however, that many popular use cases arose during the operation of those systems that were not anticipated during the specification phase: SMS was unexpectedly one of the main revenue sources for 2G operators; the introduction of smartphones and social networking together with Fast Dormancy brought many 3G networks close to collapse; and LTE development started in 2005—before the introduction of smartphones. Hence, use case discussions about 5G will have some aspect of the crystal ball.
Even though it is generally acknowledged that 5G will be the next cellular standard, it is still not described or defined in any particular specification or any official document published by an official telecommunication standardization body. 3GPP has started work on 5G technologies and defined a 5G timeline [5], however it does not explicitly use the term “5G” [5].
Representing major mobile network operators, Next Generation Mobile Networks (NGMN) has published a white paper on 5G [6]. The definition of 5G given there is more a prediction of future mobile networks beyond 2020 than a specific vision of the underlying
technologies that will enable 5G. However, it is clear that 5G will introduce completely new and revolutionary concepts. Those technologies and solutions are currently being discussed among network operators, semiconductor manufacturers, standardization bodies, and research institutions. Therefore, the definition of 5G use cases, requirements, and enabling technologies will extend over several years.
It is expected that 5G will be unlike previous generations. The major differences will not be merely combinations of old and new radio access technologies; 5G will also enable new use cases and requirements of mobile communication beyond 4G systems. It will be an integration of existing cellular standards and technologies, including new disruptive technologies like mmWave and spectrum sharing.
The evolution of LTE in Release 14 is expected to offer a first step toward 5G by enabling wireless access for frequency bands below 6 GHz. Hence, LTE Advanced Pro might be considered a special case of 5G in those frequency bands. For higher bands, a new radio-access technology (RAT) and inherent supporting and integration solutions will be introduced. Therefore, the 5G architecture will be an integration of Multi-RAT, supporting the simultaneous operation of multiple heterogeneous technologies. Next to mobile broadband radio access, 5G will incorporate systems that enable massive machine-type communications (MTC). Within Release 13, 3GPP already specifies NB-IoT (Narrowband Internet of Things) to operate within a 200 kHz bandwidth. In the work on 5G specifications, this is expected to be further optimized toward a high number of supported devices, low device cost, and ultra-low power consumption.
To provide the required capacity and features, 5G will build on the following three main pillars, as illustrated in Figure 3-3:
• More spectrum: Access to a new spectrum in the upper mmWave bands and new spectrum usage paradigms such as spectrum sharing
• More spectrum efficiency: Not necessarily in terms of link capacity as described in Shannon’s Theorem, but through better exploitation of the entire heterogeneous environment
• denser deployment: To be enabled by the use of small cells
Figure 3-3. Pillars enabling 5G
43 While all three of the pillars in Figure 3-3 are essential parts of 5G, a specific
challenge is the provision of additional spectrum. In traditional wireless systems, carrier frequencies up to about 6 GHz have proven to be suitable for mobile usage.
Unfortunately, this part of the spectrum is fully allocated to a variety of incumbents, and opportunities for repurposing spectrum to commercial cellular usage are becoming scarce. To address this challenge, regulation administrations and standardization bodies such as ETSI in its RRS (Reconfigurable Radio Systems) Technical Committee and 3GPP have recently developed spectrum-sharing solutions, which allow secondary usage of cellular applications while incumbents leave the band unused in a specific geographic area and during a specific time period. In Europe, the technology is called Licensed Shared Access (LSA) and addresses sharing in the 2.3-2.4 GHz band. In the US, an even more innovative approach has been introduced, called Spectrum Access System (SAS), targeting the 3.55-3.7 GHz band. In both cases, a key challenge lies in a system definition that enables a viable business model. This requires all of the following: long-term investment certainty (guaranteeing access to spectrum on a multiple-year basis through suitable sharing agreements), and guaranteed QoS, and exclusive access to spectrum (during the absence of incumbent users, the access needs to be prioritized for the cellular Mobile Network Operators (MNO) under a suitable sharing agreement), and protection of confidential information (avoiding any public sharing of detailed configuration information by spectrum licensees).
In parallel to spectrum-sharing technology, a further complementary technology is being developed. In cmWave and mmWave bands above approximately 10 GHz, available frequency bands can still be identified and allocated to commercial cellular usage. However, the viability of corresponding technology for various use cases (static indoor, nomadic, pedestrian, high speed, and so on) is currently under study [15] and needs to prove its suitability. The outcome of this evaluation obviously impacts the (potential) need for sharing technologies below 6 GHz dramatically. Key challenges to be studied and addressed are the increased propagation losses at higher frequency ranges, the need for 3-dimensional propagation models to take specific multi-antenna patterns into account, system behavior in the context of user and environment mobility, and the suitable configuration of highly directional transmissions.
As illustrated in Figure 3-3, the second pillar in the three-pillar model relates to an increase in spectral efficiency. Although link-level capacity purely on a physical layer and for a given available bandwith is unlikely to see a substantial increase in efficiency, the potential resides in layer two (Medium Access Control) and higher layers.
Indeed, the amount of overhead (signaling, management, and so on) in recent wireless communication systems has reached an intolerable level. More intelligent management of overhead, possibly in a cross-layer approach, is a key direction to be studied in future 5G systems. Furthermore, it is obvious that in cellular modem architecture, new design breakthroughs and changes are necessary to meet these 5G goals. A solution with feasible implementation complexity including resource sharing across different RATs of individual building blocks like working memory and data path logic will be needed.
Figure 3-4 shows an example. Naturally, the cellular modem architecture explained in Chapter 2 will undergo significant changes at the system and component level.
Finally, the third pillar in the model shown in Figure 3-3 relates to densification. The benefits of moving from macro base stations to densely deployed small-cell setups are twofold: First, the smaller coverage area provided by a small-cell compared to a macro base-station typically means a smaller number of users, assuming that the distribution over space is comparable. Consequently, the available resources can be distributed across a reduced number of users and thus more capacity is allocated per user. Second, thanks to the denser deployment of small cells, users are typically closer to the base-station antenna, and thus the signal propagation conditions are more favorable. Indeed, propagation losses and similar effects are reduced, and thus a more spectrally efficient transmission mode (such as denser modulation constellations, a less redundant channel code, and so on) can be employed. In consequence, the observed QoS of concerned users is substantially improved.
It is important to note that the three pillars are mutually independent. Consequently, the observed effects are multiplicative. For achieving targeted capacity gains in the order of 1000 to 10,000 times, as it is typically requested for 5G systems over 4G technology, a straightforward way is to assign a factor of 1000 103 = ¼31000 22» to each of the pillars in Figure 3-3. The final choice may deviate from this initial guess, but these numbers give a good first idea of the engineering challenges for 5G.
Figure 3-4. 5G Multi-RAT architecture
45