3GPP specifications allow for the deployment of LTE in a wide variety of spectrum bands, globally. It is deployable in any of the existing 2G and 3G spectrum bands, as well as several new frequency bands that may be identified [Aru11], in both licensed and unlicensed spectrum. Operating in different frequency bands has limited or no impact on a radio access technology perspective. What may differ between them are mainly the more specific RF requirements, such as the allowed maximum transmit power, noise on the air interface, requirements/limits on Out-of-Band (OOB) emission and so on.
The frequency bands are divided into paired bands (FDD), where separated frequency ranges are assigned for uplink and downlink, and unpaired bands (TDD), with a single shared frequency range, as was previously explained. Release 14 includes a total of 37 frequency bands for FDD and 16 for TDD. Table 2.4 specifies some of the frequency bands used in LTE.
E-UTRA Operating Band UL Range [MHz] DL Range [MHz] Duplex Mode 1 1920 - 1980 2110 - 2170 FDD 3 1710 - 1785 1805 - 1880 FDD 5 824 - 849 869 - 894 FDD 7 2500 - 2570 2620 - 2690 FDD 8 880 - 915 925 - 960 FDD 20 832 - 862 791 - 821 FDD 22 3410 - 3490 3510 - 3590 FDD 31 452.5 - 457.5 462.5 - 467.5 FDD 38 2570 - 2620 2570 - 2620 TDD 40 2300 - 2400 2300 - 2400 TDD
Table 2.4: Some of the frequency bands used in LTE (adapted from [3GP17a]). Due to the propagation properties, lower-frequency bands (e.g. Band 31) are good for wide-area coverage deployments, both in urban, suburban and rural environments. Higher- frequency bands, on the contrary, are more difficult to use for wide-area coverage, having therefore been used for boosting capacity in dense deployments. With new services requiring higher data rates and higher capacity in dense deployments, frequency bands above 6 GHz are being looked at as a complement to the frequency bands below 6 GHz. With the upcoming fifth generation of telecommunications requirements for extreme data rates and localized areas with very high area traffic capacity demands, deployments using much higher frequencies (even above 60 GHz) is considered [Eri16].
This chapter contains introductory concepts regarding the LTE standard, providing a basis for the dissertation work. First, the network architecture was described, specifying the functionalities of the UE, E-UTRAN and EPC, along with their respective nodes and identifiers. Then, the LTE protocol stack was presented, followed by a deeper description of the physical layer, where aspects such as the frame structures used for the TDD and FDD duplex modes, LTE physical channels, multiple access and modulation schemes were approached. To finalize, a selection of LTE frequency bands available for transmission were presented.
The next chapter will provide an overview of several experimentation frameworks that implement the LTE standard by means of software, instead of the traditional hardware im- plementations. The main objective is to pick the most adequate platform to be used for experimentation on this dissertation. To be run along with the chosen software framework, several software radio hardware platforms will be compared.
LTE Experimentation FrameworksThis chapter begins with an introduction, in section 3.1, followed by an overview and comparison of several experimentation frameworks that implement the LTE standard, in section 3.2. One of the analysed software platforms is selected as a point of departure for the implementation of the proposed dissertation work. That choice is detailed in section 3.3.
Testbeds are essential to experimentally evaluate new technologies. Their architecture should be flexible, so it is possible to easily update the system for certain configurations. Scalability is an important factor, meaning that various configurations with different hardware capability requirements are supported. Furthermore, the testbed should support logging, so the user can analyse the system’s output and performance.
An LTE-specific testbed is usually composed by one or more UEs, an eNB and an EPC. A wide variety of COTS hardware products available nowadays already provide a solution for each of these components. Albeit providing an implementation with very few to no errors, these commercial products are too complex (or sometimes impossible) to modify and to add new features. As doing so could prove costly, open-source LTE SDR software is preferred for research and prototype developing purposes. Before going into more details, the concept of software-defined radio will be explained.
Software-Defined Radio (SDR) is a concept in which radio communication systems’ com- ponents are implemented by means of software, instead of the traditional hardware implemen- tation. As most communication modules are implemented by software (e.g. frequency band selection, modulation/demodulation, filtering, encoding/decoding, signal enhancement), the technology upgrading process is more flexible and less expensive, because the user only has to upgrade the software and can keep the hardware as is. This kind of flexibility allows for oper- ation on multi-band and multi-modulation [Tor10]. SDR systems can be implemented on var- ious reconfigurable hardware platforms, such as Field-Programmable Gate Arrays (FPGAs), Application-Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs) or GPPs. When compared to the others, GPP-based SDR systems have advantages in reconfigurability and flexibility because high-level programming languages are used, which are cross-platform, instead of the hardware description languages used on the former - e.g. Very High Speed Integrated Circuit Hardware Description Language (VHDL).
GPP; the first being responsible for the frequency conversion and digitization and the latter for baseband signal processing [Hen16]. Several of these peripheral equipments will be discussed through the course of this dissertation’s work.
Several experimentation frameworks that implement the LTE elements - UE, eNB and EPC - are evaluated in the next section.