Basic information

TUD coordinates the test activities that are related with the EASY-C testbed. Cellular use cases and CR-related field trials are provided by the Dresden test bed, which is supervised by TUD.

The TUD contribution is based on the EASY-C campus infrastructure, i.e, the EASY-C outdoor lab test bed which is directly operated by the Vodafone Chair research team. An LTE-like cellular infrastructure is used where relevant network parameters are measured such as frame error rates, outage events, throughput or latency. One base station at rooftop level will be used which serves multiple UEs. This BS resides at the faculty of electrical engineering and information technology, TUD. Stationary and mobile user equipment are used. Below are depicted, from left to right: mobile test UE, base station equipment, UE lab equipment.

  

EASY-C test equipment.

External users of the TUD test bed need to install their own equipment at the TUD test site. A predefined test setup is used which provides well defined and reproducible EASY-C LTE traffic – good for the CREW cognitive radio benchmarking initiative. The LTE network parameters are constantly monitored and recorded. The CR transceivers are then activated where the LTE performance parameters are compared for the non-CR and CR case. Hence, it will be possible to benchmark the impact of various CR schemes on a cellular infrastructure through a well-defined set of reproducible test setups.

Another possibility for external users is to connect via Remote Desktop to the TUD indoor test bed to perform experiments with a fixed setup of one eNB, one UE, National Instruments USRP 2920 and one Signalion HaLo device.

Please click on the thumbnail below to get an overview picture of the hardware available in LTE advanced testbed.

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Usage of the testbed

Two experimentation setups are available: the indoor lab and the outdoor lab.

In order to conduct experiments in the LTE+ testbed, participants are required to bring their spectrum sensing and/or secondary system hardware to Dresden, if the experiment cannot be performed by a USRP or HaLo device. In order to pre-evaluate certain theories and algorithms, a testbed reference signal in the form of baseband I/Q samples can be provided. Further, during the experiments it is possible to dump transmitted and received signals in the same format. This allows for offline post-processing of the signals, evaluation of the signals and replay in other testbeds.

It is important to distinguish if a downlink (DL) or an uplink (UL) experiment is desired.

In uplink experiments, it is possible to serve up to 4 UEs. The UEs use 1 antenna for transmission, while the eNBs can receive with 1 or 2 antennas. The resolution for scheduling a transmission is 1 ms, which corresponds to 1 TTI (transmission time interval). Scheduling can be done for a total duration of several minutes. The number of occupied PRBs is either 10, 20, 30 or 40 (cf. Table 1). QPSK, 16QAM and 64QAM modulation are supported.

In downlink, up to 4 UEs and up to 4 eNBs can be used simultaneously. The eNBs can transmit with up to 2 antennas and the UEs can receive with up to 2 antennas, thus up to 2 streams per UE can be sent. Time resolution is 1 ms corresponding to 1 TTI (same as UL). The number of occupied PRBs can be 12, 24, 36 or 48 (cf. Table 2).

The evaluation of an experiment happens via dumps of the received signals at the UEs / eNBs. While in the UL, signal dumps can be recorded for all eNBs in synch, the dumping process needs to be initiated manually and out of synch in the DL.

The signal dumps contain the received time samples as well as additional control information. Further processing in Matlab allows derivation of indicators like SINR, BLE, etc. in semi-realtime/offline.

The performance evaluation of experiments can be performed in real-time as well as semi real-time and offline. Real-time measures include

• Received Signal Strength Indicator (RSSI),
• Reference Signal Receive Power (RSRP),
• Path loss, and
• Channel Quality Indicator (CQI; derived from SINR).

In semi real-time, additionally QAM constellations and block error rate (BLER) can be monitored via file dump of I/Q samples and Matlab post processing. Further performance measures could be obtained in offline processing from those file dumps.

Please click on the thumbnail below to get an overview picture of the usage overview in LTE advanced testbed. 

Deviations from LTE release 8

As the eNB and UE provide only minimal LTE release 8 (Rel 8) PHY/MAC functionality, it is particularly important to note that the DL frame structure and control channels differ slightly. Differences include:

• PDCCH is always in the second OFDM-symbol (position is variable according to Rel. 8),
• PHICH (HARQ Indicator Channel) is not in the first OFDM symbol and has a different structure/content,
• PCFICH (Control Format Indicator Channel) is not supported, and
• PBCH (Broadcast Channel) is not supported.

Further, the OFDM scheme is used in the uplink. Also, 5 MHz and 10 MHz mode are not supported, thus the testbed operates in 20 MHz mode only.

An overview of the uplink and downlink processing chains can be seen below:

Uplink and Downlink Processing Chain

 

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Instructions for external participants

Before the experiment:

• Contact the testbed staff, make sure the hardware is compatible (frequencies) and the testbed supports all features necessary for the intended experiment
• If necessary for the experiment and after the inital contact with the research staff: Get an account to access the network and computers. Remote desktop access over internet is also possible.
• Make sure there will be enough testbed hardware available (indoor/outdoor?)
• If reasonable, ask for reference signal files to check compatibility with external hardware
• Get familiar using the spectrum analyzer R&S FSQ
• Prepare UE/eNB configuration files or ask testbed staff to do it

During the experiment:

• Carefully check the setup
• Use a terminator when there is no antenna/cable plugged
• Check if all cables are ok
• Make sure you are using the latest version of the config tool to configure the hardware
• Keep a record of the config files you use as you are changing parameters
• Check the signal on the spectrum analyzer, several things can be validated that way
• If in nothing else seems to work reboot and reconfigure the UE/eNB prototype hardware
• When in doubt, ask the testbed staff

After the experiment:          

• Put everything back where you took it from

Example experiment

Detection of occupied frequency bands is the foundation for applications of dynamic spectrum access (DSA). In order to convince network operators that DSA is feasible in cellular frequencies, it has to be shown that a reliable detection of their primary signals is possible. In this section, we present an experimental validation of an algorithm and hardware, which can detect the presence of a Long Term Evolution (LTE) signal. In contrast to the classical mono antenna approach, an array of antennas is used, which allows to enhance the detection capabilities, particularly when besides the useful signal there is also interference.

Objectives of experiments

• Reliable sensing in real environment
• Performance gain of multi-antenna vs mono-antenna
• Effectiveness of primary and secondary synchonisation criteria
• Parametrization of sensing algorithm (detection threshhold)

Setup

The multi-antenna LTE sensing platform allows to acquire LTE (I, Q) data and to process them using advanced antenna processing algorithms. As described in the pictures below, the multi-antenna demonstrator is made of:

• A set of 5 antennas,
• A 4-channel receiver,
• A 4-channel acquisition board,
• A GPS system for positioning,
• A laptop for data storage and off-line multi-antenna signal processing.

Multi-antenna sensing platform block diagram

Filtering and gain control are applied to the signal in the multi-channel receiver unit, the multichannel acquisition board is used to convert the signals to digital domain and a control computer handles processing and evaluation of the digital samples. The multi-antenna LTE sensing platform is validated with lab tests by measuring sensitivity and co-channel interference rejection with real LTE eNBs. A hardware array simulator consisting of splitters, coupling modules and a set of cables of particular lengths is employed to virtually create a multiantenna, mono-path propagation channel with two directions of arrival.

Multi-antenna sensing platform

The main characteristics of the multi-channel receiver are: frequency bands - 1920-1980 MHz / 2110-2170 MHz); bandwidth - 5 MHz; Output intermediate frequency - 19.2 MHz; Noise factor (at maximal gain) <7 dB; Rx gain - 0 to 30dB (1 dB step); Frequency step - 200 kHz;  number of Rx channels – 4; Gain dispersion<1 dB; Phase dispersion<6°; Frequency stability <10^-7; Selectivity at ±5 MHz >50dB.

The main characteristics of the multi-channel acquisition are: Resolution - 12 bit; internal quartz clock - 15.36 MHz; Number of channels - 4; -3 dB bandwidth >25 MHz; Memory - 8MSamples (i.e. 2MSamples per channel).

Sensitivity tests

For sensitivity measurements, the platform depicted below is used. The level of the BTS is gradually lowered in order to estimate the sensitivity level when using one or four channels for detection processing.

Lab test platform for sensitivity measurements

 

Interference rejection tests

For interference rejection measurements, the platform described in the picture below is used. The second BTS is considered as the interfering BTS in the following. Its bandwidth was set to 20MHz in order to be able to highly load the OFDM sent symbols. The level of the first BTS is gradually lowered while the level of the second one does not change. The 80% detection limit level is searched when using one or four channels for detection processing.

Lab test platform for interference rejection measurements

Results

LTE detection sensitivity level for an 80% detection rate

	1 antenna	4 antennas	Multi-antenna gain
Sensitivity level	-121 dBm	-129 dBm	8 dB

The LTE detection sensitivity performance is summarized in the table above. It is slightly higher to what can be expected (6 dB with 4 antennas). This might be due to a lower sensitivity of the first channel compared to the other three.

LTE rejection capacity for an 80% detection rate

	1 antenna	4 antennas	Multi-antenna gain
Sensitivity level of the first BTS	-92 dBm	-124 dBm	32 dB
Rejection capacity of the second BTS	11 dB	43 dB	32 dB

We can see that, with four antennas, the rejection gain is equal to 32 dB. 

For further details refer to: Nicola Michailow, David Depierre and Gerhard Fettweis: “Multi-Antenna Detection for Dynamic Spectrum Access: A Proof of Concept”,  QoSMOS Workshop at IEICE 2012

https://mns.ifn.et.tu-dresden.de/Lists/nPublications/Attachments/895/main.pdf

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Basic tutorial

This tutorial explains how to set up a basic transmission.
Download this Zip archive with all necessary default configuration files.

Setup the eNB

• Power the hardware
  • Sorbas eNB Simulator
  • Radio Unit
  • eNB control computer
• Configure the eNB
  • Open the SimpleProxy 1.3.1 tool
  • Click Load Settings and select CREW_DL_config_default_1eNB_2UEs.xml or CREW_UL_config_default_1eNB_2UEs.xml
  • Click Reset to reset the eNB
  • Wait for eNB broadcast message to appear in the logging box below
  • Click Config to send the configuration to the eNB
  • Check logging box for errors

Setup the UE

• Power the hardware
  • Sorbas Test UE
  • Radio Unit
  • UE control computer
• Configure the UE
  • Open the TestUE Config tool
  • Select the System Config tab
  • Click Load settings and select CREW_DL_config_default_UE_id=0 or CREW_UL_config_default_UE_id=0
  • Click DL config and wait for UE response
  • Click UL config and wait for UE response
• Trace
  • Open the TestUE Trace tool
  • Select the Display (4) tab
  • Click Enable
  • Click Run

The system is now running. Check spectrum on R&S FSQ.

Record IQ data dumps

• Dump at the eNB
  • Open the SimpleProxy 1.3.1 tool
  • Click Freq && Dump tab
  • Click Start Dump
• Dump at the UE
  • Browse into the directory of the UE_dump_tool
  • Click start.bat
• Process IQ dumps
  • Extract IQ samples and AGC values with dumpDemux.m script
  • Perform further processing in Matlab

Advanced tutorial

Besides the manual, gui-based control for test bed related programs and devices, script controlled measurements are also possible. This approach developed at the Vodafone chair is called TestMan. The basic idea is to provide a common interface to exchange data, commands and status messages between different application, running on the same or distributed systems and written in different languages.

TestMan is based on the Microsofts .NET framework, which similar to java, is platform independent. So even a Linux computer can make use of .NET programs if the Mono project is used. In three important languages Matlab, Labview and Python it is possible to use dynamic link libraries (DLL). Therefore TestMan is a DLL which can be loaded into the specific application or script and it makes sure that data is transferred over the network from one Application to another, respectively to a group of applications.

TestMan uses two different network techniques to exchange information’s: For SNMP like status messages and commands UDP multicast is used whereas TCP peer to peer connection comes into place for transferring bigger data.

To distinguish between different applications a type and an ID are introduced for every application. So it is possible to group similar applications together and the TestMan DLL filters out messages which are intended for other applications.

UDP packets can be thrown away by network elements like routers and switches. For a status messages this is not always a problem, but definitely when a command is send. To mitigate this problem TestMan uses 4-way handshaking for commands as stated in the following figure.

The following picture shows an example how an OFDM-Transmitter can be implemented using TestMan.

Are more detailed example code will be published soon.

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Configuration

Dresden’s LTE/LTE+ like testbed was set up in 2008 as part of the Easy-C project (www.easy-c.com).

The signal processing hardware includes:

• Sorbas602 eNodeB Simulators with ZF Interface to a Sorbas Radio Unit ("ZF Interconnect”)
• Sorbas202 Test UEs  with ZF Interface to a Sorbas Radio Unit ("ZF Interconnect”)
• Sorbas472 Radio Units, by Signalion: EUTRAN band VII (2.5 - 2.57 GHz and 2.62 - 2.69 GHz) as well as close to band I (1.98-2 GHz and 2.17-2.19 GHz), 20MHz bandwidth, Tx power approx. 15dBm (indoor) and approx. 30dBm (outdoor), supports up to two Tx and two Rx channels.

They were supplied by Signalion (www.signalion.com). The eNBs and UEs are connected through IF interconnects at 70MHz with the radio unit frontend. The hardware supports up to two Tx and two Rx channels for MIMO capability. The testbed operates in EUTRAN band VII (DL @ 2670-2690 MHz / UL @ 2550-2570 MHz) with fixed bandwidth of 20MHz and in FDD mode.

The LTE testbed at TUD has been upgraded with a new spectrum license in the 2.1 GHz band (1980 MHz to 2000 MHz and 2170 MHz to 2190 MHz). This step was necessary to ensure the continuous operation of the LTE testbed when the spectrum license for 2.6 GHz is withdrawn due to commercial use of the corresponding frequencies in Germany. Along with the license, several nodes have been equipped with 2.1 GHz frontends. Note that only the RF parts have been replaced, while LTE eNB and UE baseband processing remains unaffected due to the modular structure of the equipment. Further note that as long as the 2.6 GHz license is not withdrawn, those frequencies are still available for experimentation.

The operation of the new equipment has been successfully tested. The internal US5 experiment “LTE Multi-Antenna Sensing” has been conducted in the 2.1 GHz frequency range.

Base station (eNB) and mobile terminal (UE) nodes each are connected to a host PC and configured with text files in XML format. The host computer also manages measurements of the received signals and stores them in dumps. At the eNBs, a GPS unit is used for synchronization, while the UEs employ GPS for position tracking. Additionally, UEs can be powered by a mobile power supply if necessary. 

Configuration of a baste station node (eNB)

 

Configuration of a mobile terminal node (UE)

 

All UEs in the testbed, as well as the indoor eNBs are equipped with Kathrein 800 10431 omnidirectional antennas. The antennas of the outdoor eNBs are sectorized and of type Kathrein 800 10551. You can find detailed information about these antennas here:

http://www.kathrein-scala.com/catalog/80010431.pdf
http://www.kathrein.de/en/mcs/catalogues/download/99811214.pdf

Other testbed equipment includes six batteries that can power an individual UE for around 2-4 hours, GPS receivers for time synchronization, various cables, attenuators and splitters. Measurement equipment includes spectrum analyzers Rohde & Schwarz FSH4, Rohde & Schwarz FSQ8 and Rohde & Schwarz TSMW. For more details click on following links:

R&S FSH4 data sheet:

http://www.test-italy.com/occasioni/2013/rs_fsh8-18/FSH_dat-sw_en.pdf

R&S FSQ8 data sheet:

http://cdn.rohde-schwarz.com/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/FSQ_dat-sw_en.pdf

R&S TSMW operating manual:

http://www.rohde-schwarz.de/file_9446/TSMW_Operating_Manual.pdf

R&S TSMW software manual: 

http://www.rohde-schwarz.de/file_11255/TSMW_Interface_and_Programming_Manual.pdf

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Indoor and outdoor setups

For the CREW project, two experimentation setups are available.

The indoor lab features 5 eNBs and 4 UEs. While the hardware itself is stationary, the Tx and Rx antennas can be positioned anywhere in the lab room. Further, four additional UEs are mounted on studio racks/carts and can be moved within the building. The approximate transmit power is 15 dBm.

The outdoor lab consists of two base station sectors that are fixed on two opposing corners of the faculty building, approximately 150 m apart. In addition to the mobile indoor UEs from setup 1, three rickshaw UEs are available for outdoor experiments in the vicinity of the building. There are also 6 batteries which can supply an UE for around 2-4 hours. The transmit power is approximately 30 dBm.

Outdoor setup

Indoor setup

 

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Secondary System

The Signalion Hardware-in-the-Loop (HaLo) is a platform designated to simplifying the transition from simulation to implementation. To support cognitive radio setups that consider a primary/secondary user configuration, the LTE testbed has been extended by a HaLo node that can take the role of the secondary user. On the HaLo device, a novel modulation scheme called Generalized Frequency Division Multiplexing (GFDM) is now available.

This enables experimenters to consider experiment setups in LTE testbed, where the LTE system can act as a monitored primary system, while the GFDM system can run as an interfering secondary system.

HaLo Concept

The HaLo consists of a wireless transceiver that can operate in the 2.6 GHz frequency band. The concept is such, that complex valued data samples are transmitted to the device’s memory via USB from a control computer. The samples can be either read from a previously recorded file or generated on the fly e.g. by a Matlab script. The signal is transmitted over the air and received in a similar way. The device digitizes the signal and stores complex valued samples to an internal memory before they are fed back via USB to the control computer.

The HaLo setup

Note that due to limitations in the HaLo’s internal memory real-time operation is not possible.

 

GFDM Theory

The transmission scheme chosen to be implemented on the HaLo device to act as a secondary system in the testbed is a novel, non-orthogonal and flexible modulation scheme called GFDM. The concept is such that a multicarrier signal is transmitted, quite similar to the well know and established OFDM scheme, however one of the differences is in the pulse shaping of the individual subcarriers. This step allows shaping of transmissions and produces a signal with particularly low out-of-band radiation, which is a very desirable property in cognitive radio. For further details please refer to:

 https://mns.ifn.et.tu-dresden.de/Lists/nPublications/Attachments/826/Michailov_N_VTCfall12.pdf

GFDM transmitter and receiver block diagram

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