Evaluation of LTE implementation platforms

In document Terminal LTE flexível (Page 58-61)

This dissertation’s goal is to build a fully LTE standard-compliant real-time platform testbed, with special attention to the UE. There are several open-source software platforms available, each with different characteristics and implementations in either simulation or real- time mode. For the purpose of this dissertation’s work, simulation is not relevant, so, to fulfil its goal, this platform must be able to run with the aid of commercial SDR hardware like the previously mentioned USRP. The framework should offer realism regarding the internal structure of the EPS, including the authentication and key agreement and an UE running in SDR. Logging is required as well, to allow for easier debugging and understanding of the system. The following sections are dedicated to each analysed platform.

3.2.1 gr-LTE

gr-LTE [gr-13] is an open-source software package which aims to provide a GNU Radio LTE Receiver to receive, synchronize and decode LTE signals. It can basically be considered as an UE. While gr-LTE is nice for familiarizing with the LTE protocol, it isn’t fully developed and doesn’t provide an eNB or EPC implementation. Thus, it cannot be used for the purpose of this dissertation.

3.2.2 OpenEPC

OpenEPC [Ope17b] provides a full implementation of the LTE Release 12 core network and includes an emulation of the eNB and UE. The name is deceiving though, as the source code is only available under paid licensing models. Therefore, this platform was not chosen.

3.2.3 Open-Source LTE Deployment (OSLD)

The mission of the Open-Source LTE Deployment (OSLD) project [Ope13] is to promote open-source SDRs and shared development of software for wireless communications systems. It provides a basic library of LTE PHY layer DSP blocks, implemented in a general-purpose way for multi-platform use. It relies on Abstraction Layer and Open Operating Environ- ment++ (ALOE++) [ALO12], an open-source middleware and execution environment for distributed signal processing, and provides two main waveform description files, one for the LTE DL and one for the UL.

It can be ran in an USRP although a Real-Time Operating System (RTOS) is necessary for latency reduction, otherwise it won’t work. As no EPC implementation is provided and there isn’t an active community anymore, this platform was not chosen.

3.2.4 ns-3 with the LTE Module

ns-3 [ns-17] is an open-source discrete-event network simulator targeted primarily for research and educational use, written in C++. Its LTE Module contains software libraries that allow the simulation of LTE networks and the EPC. It implements the entire LTE radio protocol stack, whose entities reside within the eNB and UE nodes, and a partial core network with S-GW, P-GW and MME nodes (missing several HSS functionalities).

While the code is extensively documented and the community is active, it only simulates the radio communication. As using physical radio hardware is required in this dissertation, this platform was not chosen.

3.2.5 OpenLTE

OpenLTE [ope17c] is an open-source implementation of the 3GPP LTE specifications, written in Octave, Python and C++. The project’s current focus is on adding capabilities to its simple eNodeB application and extending the capabilities of the GNU Radio applications. These include LTE In-phase/Quadrature (I/Q) file recording or transmission and reception of downlink communications, using either of the following peripheral SDRs: rtl-sdr [RTL17], HackRF One [Hac17] or USRP B2X0 [Ett17].

OpenLTE partly implements the MME and HSS, including part of the authentication procedure. There’s no UE implementation, however, and many other features are still unstable or under development. It also requires a great amount of processing power, as well as very low latency. If there is any delay in the processing, the system won’t be able to respond in time and will lose samples.

The project’s community isn’t very active nowadays. Despite of not providing an UE implementation, OpenLTE won’t be discarded as an option, for now.

3.2.6 srsLTE, srsUE and srsENB

srsLTE is an open-source LTE library for the PHY layer, written in C, which includes srsUE and srsENB [SRS17], a complete software radio LTE UE and eNB, respectively, written in C++.

srsLTE, developed by Software Radio Systems, provides a LTE Release 8 compliant imple- mentation, for FDD, including all DL and UL channels/signals. The code is well-structured and divided in modules, allowing for easier customization or replacement of components with- out affecting the existing parts.

srsUE covers all UE layers from PHY to IP and builds upon the srsLTE library which provides the PHY layer processing [Ism16]. srsENB also builds upon the srsLTE library but requires a commercial license, so it won’t be used. The full EPS protocol stack is supported and some functions are imported from the OpenLTE project.

Regarding hardware, srsLTE is able to work with any RF front-end and currently gives support to the Ettus USRP B210/X300 and Nuand’s bladeRF [Nua17]. srsUE has been tested and validated with several commercial eNBs and also works with other open-source platforms, so this option won’t be discarded.

3.2.7 AMARI LTE 100

Amarisoft’s AMARI LTE 100 [Ama17] requires a paid license to use, but it is arguably the best and most complete LTE SDR implementation available. It’s a software suite which provides an LTE Release 13 compliant eNB, EPC, Evolved Multimedia Broadcast Multicast Services (eMBMS) gateway and IMS server, allowing end-to-end communication with up to 1000 commercial UEs. There’s also AMARI UE 100, a UE simulator software that simulates up to 1000 UEs.

It can be built in a SDR PCIe board [Ama17], USRP N-series or LimeSDR [Lim17]. As it is not an open-source platform, it was not picked.

3.2.8 OpenAirInterface

OpenAirInterface (OAI) wireless technology platform is a flexible platform towards an open LTE ecosystem, written in C. It offers an open-source software-based implementation of the LTE system, spanning the full protocol stack of the 3GPP standard both in E-UTRAN and EPC. It can be used to build and customize a LTE base station, a user equipment and a core network on a Linux-based computer (Intel x86 processors).

The eNB can be connected either to commercial UEs or OAI UEs to test and monitor different configurations and network setups in real-time [Ope17a]. It works with several hardware RF front-ends, such as the USRP B210, as well as in simulation mode, where the radio interface is simulated by Ethernet. Several built-in tools are included, such as a protocol analyzer, a configurable logging system for all layers and channels plus debugging and soft monitoring tools.

While OAI is a very complete implementation, it is also very complex. The code is at times confusing and not well organized and documented, making it hard to understand or customize for a new user. It is in constant development, so it might also prove to be unstable. The community is very active, which can be helpful for problem solving. This was the last platform to be analysed and it will also be considered, leaving us with three open-source platforms to choose from.

3.2.9 Final decision

Eight software radio platforms were analysed, in which three stood out: OpenLTE, srsLTE and OAI. As srsLTE can only be used to implement a user equipment (srsUE), the first decision to be made is between the OpenLTE and OAI platforms. OpenLTE’s code is well structured and documented, however it is missing many features. OAI, on the other hand, is an almost complete implementation and offers great performance when well configured. Therefore, OpenLTE is discarded and OAI is picked as the main platform.

As srsUE is still in play, the second decision relies on choosing which SDR UE is going to be implemented, srsUE or OAI UE. Over the past year OAI UE had serious improvements and its characteristics are identical to the srsUE’s, with both achieving the same Internet speeds in similar circumstances. Therefore, in other to avoid compatibility issues and deep multi-platform knowledge, srsLTE is discarded.

Regarding the other platforms, either they weren’t open-source or didn’t have enough capabilities for the purpose of this dissertation’s work, so they weren’t considered. As the selected platform, the following section will be entirely dedicated to OAI.

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