LTE Unlicensed (LTE-U)

Introduction

Long-Term Evolution (LTE) is the latest high-speed data standard of wireless communication. It is usually known as 4G LTE in cellphone or mobile devices.

LTE in Unlicensed (LTE-U) operates in open/unlicensed spectrum, such as in the 5GHz band. It aggregates with the licensed LTE in order to increase the data rate of the Advanced LTE system (unlicensed and licensed LTE system).

With the massive growth in data traffic and mobile devices, it is necessary to expand the mobile network system. While the licensed spectrum is limited in amount, the possible solution should be using the available unlicensed spectrum, even it might be conflict with the exist network, such as Wi-Fi.

It is recently proposed by Qualcomm, Ericsson, and Verizon that LTE-U should be utilize in unlicensed spectrum on the 5725-5850 MHz band so that it could be a "nice neighbor" with Wi-Fi.

Objectives

• Research on wireless communications, LTE, LTE-U and Wi-Fi in advance
• Simulate the LTE-U base station in ORBIT Lab using OMF commands, WiMax, and OpenAirInterface
• Transmit and Receive LTE Signal
• Apply analyzing tools, such as spectrum analyzer, GNU Radio, SNR
• Modify OAI source code and report the result

Theory

OpenAirInterface (OAI) is open-source based experimental research. It allows to simulate the digital communication environments, such as LTE. ​OAI Documentation

Real-world testbed:

• OAI SW + OAI HW or USRP B210/X300
• OAI EPC + OAI eNB <–> COTS UE
• Commercial/3rd party EPC + OAI eNB <–>COTS UE
• OAI eNB <–>OAI UE
• OAI + Signal generator/spectrum analyzer

The source code ​OpenAir4G is organized into 6 main repositories for different use cases: OpenAir1, OpenAir2, OpenAir3, OpenAir0, OpenAirCN, Targets.

Each repository focuses on a different data communication layer or focus of 3GPP implementation.

OpenAir1 ​Code

• Open-source real-time and offline Software
• Baseband DSP SIMD-x86 routines for implementing LTE UE’s and eNB’s
• Simulation TestBenches for all LTE PHY/transport channels
  • Sensing Localization Techniques
  • PHY modeling tools
  • Propagating Measurement and Modeling
  • Basic DSP Routines for implementing LTE specifications
• Sounding Software, Physical (PHY) Abstraction Software, and Channel Simulation
  • Sound Software - Eurecom’s Multi-Input Multi-Output (MIMO) OpenAir Sounder (EMOS) allows multiuser MIMO channel measurements in real time and store measurements from both user equipment(UE) and evolved NodeB(eNB)
  • PHY Abstraction Software contains all real-time/simulation signal processing, unitary TestBenches for the different channel coders and decoders, and modulation/demodulation (includes FFT/SC-FDMA front-end processing)
  • Channel Simulation Software contains simulation routines to test PHY, and TestBenches for unitary simulation of physical channels, and full system simulation, such as PHY, MAC (medium access control), RLC (radio link control), RRC (radio resource control), PDCP (packet data convergence protocol)

OpenAir2 ​Code

• Open-source real-time and offline Software
• Contains LTE MAC (36-213), RLC (36-322), PDCP (36-323), and two RRC implementations
  • LLC establishes/controls logical links between local devices on a network
  • MAC controls access to the network medium to avoid conflicts
  • Data Framing is responsible for final encapsulation of messages into frames
  • Addressing labels information with a particular destination location
  • Error Detection and Handling
• S1 interfaces for user and control planes of the eNB, X2 Application Protocol (X2AP), and OAI network driver

OpenAir3 ​Code

• Open-source Linux Software suite for cellular and MESH networks
• Provides scripts and adaptations for the Linux networking suite
• Contains OAI-MME (Mobility Management Entity), which is responsible for authentication of the mobile devices
  • Network Access Control manages authorization for UEs, allow to gain IP connectivity
  • Radio Resource Management decides radio resource management strategy (RRM)
  • Mobility Management provides seamless inter-working with multiple use cases such as Inter-eNB
  • Roaming Management supports outbound/inbound roaming subscriber
  • UE Reach-ability manages communication with the UE and HSS
  • Tracking Area Management allocates tracking area identity list to UE
  • Lawful Intercept
  • Load Balancing Between S-GWs

OpenAir0 ​Code

• Open-source real-time Hardware & Software for different Xilinx targets

OpenAirCN ​Code

• 3GPP-EPC implementation
• Small-scale 3GPP-EPC implementation
• Includes MME, P and S-Gateway, and HSS components

Targets ​Code

• Top-level target designs for use with and without Hardware in emulated or real-time modes

Analyzing Tools

Spectrum Analyzer

Category	Hardware Spectrum Analyzer – Tektronix SA2600	Software Spectrum Analyzer – RTLSDR Scanner
Characteristics	Wire Measurement must be connect with 30dB attenuator to avoid saturation	PROS user-friendly, convenient, remote access, software-based spectrum analyzer
	Wireless Measurement requires much higher gain than using wire	CONS Realtek 2832 EZCap can’t tune to frequencies higher than 1.8GHz
Graphs

GNU Radio

GNU Radio is an important software-based tool, being used to obtain the FFT Plot, the Constellation Plot, and the Waterfall Plot. ​How to use GNU Radio

Signal-to-Noise Ratio (SNR)

There are 2 methods to approximate the SNR:

• Method 1 Estimate the noise power by measuring the received signal power when transmitter is turned off.

• Method 2 Estimate the noise power by calculating the received signal variance.

SNR Estimation MATLAB Program:

Experiment 1: Transmit and Receive LTE Signal

The experiment is simulated in Sandbox 1 with USRP Hardware Driver (UHD) using OpenAirInterface (OAI) software.

Figure 1a. Received LTE Signal ASCII Plot

The left figure shows the overview of the received LTE signal: the carrier and the sidebands. The right figure indicates clearly that there are noises in both the carrier and the sideband.

The SNR can be quickly estimated by taking the average of the carrier or (around -50dB) and subtract by the average of the sideband (around -90dB). Thus, the SNR should be approximately 40dB.

The SNR calculated by the MATLAB Program gives a very close result: 38.8 dB

Figure 1b. I/Q Samples

Note:

• PSS Primary Synchronization Signal
  • Transmitted twice per 10 ms radio frame
  • Uses sequence known as ​Zadoff-Chu
  • Sequence of length 62
• SSS Secondary Synchronization Signal
  • Uses sequence known as ​M-sequence
  • 2 Binary sequences of length 31
• CFI Control Format Indicator
• CSRS Cell Specific Reference Signal

Experiment 2: The Waterfall Plot

The experiment is simulated in GNU Radio and Spectrum Analyzer (Tektronix SA2600) after activating the LTE eNB Base Station to transmit signal.

Figure 2a. GNU Radio Block Diagram

Figure 2b. The Constellation Plot

Figure 2c. The FFT obtained from GNU Radio (left) and Tektronix SA2600 (right)

Figure 2d. The Waterfall Plot obtained from GNU Radio (left) and Tektronix SA2600 (right)

Experiment 3: eNB and UE GUI

Experiment 4: Varying Bandwidths

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Experiment 5: Working with TDD or FDD

By varying the E-UTRA band number in the oai.conf file to 41(band compatible with TDD transmissions at 2.66GHz) we were able to observe the difference in the transmission from TDD and the previous experiment of FDD.

Other E-UTRA bands for future experiments can be viewed ​here

Experiment 6: TDD with Varying Bandwidths

The purpose of this experiment was to view the effects of varying the LTE bandwidth of the transmitted signal. To observe the the effects of increased throughput.

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Experiment 7: TDD Waterfall Plot

Members

Cat Le Electrical and Computer Engineering Rutgers University	Demetrios Lambropoulos Electrical and Computer Engineering Rutgers University	Steven Cheng Electrical and Computer Engineering Rutgers University

*Led by Dola Saha and Ivan Seskar

Materials

​Presentation: Week 1

​Presentation: Week 2

​Presentation: Week 3

​Presentation: Week 4

​Presentation: Week 5

​Presentation: Week 6

​Presentation: Week 7 ​OpenAirInterface

​Presentation: Week 8

​Presentation: Week 9

​Presentation: Week 10

​Presentation: Week 11

​Presentation: Week 12 ​OpenAir1 ​OpenAir2

Resources

​LTE Unlicensed Augmenting Mobile Data Capacity But Coexistence Needs Consideration

​U-LTE: Unlicensed Spectrum Utilization of LTE

​Extending LTE Advanced to Unlicensed Spectrum

​The Prospect Of LTE And Wi-Fi Sharing Unlicensed Spectrum

​Coexistence of LTE and WiFi Heterogeneous Networks via Inter Network Coordination