real time lte wi fi coexistence testbed

8/15/2019 Real Time LTE Wi Fi Coexistence Testbed

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Real-time LTE/Wi-Fi Coexistence Testbed

Contents

1 Introduction ........................................................................................................................................... 2 2 LTE/Wi-Fi coexistence landscape .......................................................................................................... 2

2.1 Overview ....................................................................................................................................... 2 2.1.1 LTE-Unlicensed (LTE-U) ......................................................................................................... 2 2.1.2 Licensed Assisted Access (LAA) ........................................................................................... 3

2.2 Design targets defined by LTE-U/LAA ........................................................................................... 3 2.3 PHY Enhancements to Meet Design Targets ........... ............. ............ ............ ............. ............ ....... 3

2.3.1 Discontinuous Transmission .................................................................................................. 4 2.3.2 Listen Before Talk .................................................................................................................. 4

3 Prototyping LTE-U/LAA solutions .......................................................................................................... 5 3.1 802.11 and LTE Application Frameworks ...................................................................................... 5 3.2 Implemented PHY Enhancements ................................................................................................ 6

3.2.1 Discontinuous Transmission .................................................................................................. 6

3.2.2 Listen-Before-Talk Category 4 ............................................................................................... 9 4 Preliminary Results .............................................................................................................................. 10 5 Conclusions ......................................................................................................................................... 13 6 Bibliography ......................................................................................................................................... 14

Send questions to [email protected]



1 IntroductionThe exponential rise in usage of mobile devices have not shown any signs of slowing down. Efforts are underway in the standardizationbodies to define new ways to increase data rates and network capacity by enhancing 4G technologies and by introducing newtechnologies in 5G [1].

This white paper focuses on the efforts to utilize unlicensed spectrum to improve capacity of cellular networks. The unlicensedspectrum, specifically in the 5 GHz band, can be leveraged either using existing technologies in that band (Wi-Fi) or by modifyingcellular technologies to aggregate those channels directly into the cellular PHY and MAC layer. The latter approach has lead todisagreement between cellular and Wi-Fi ecosystems regarding its impact on existing and future Wi-Fi networks. As with anydisruptive technology, prototyping using a realistic test-bed is the best way to truly understand the performance trade-offs. Hence,neutral modifiable prototyping platforms that enable engineers and researchers to evaluate and compare performance trade-offs ofsuch algorithms in realistic environments are valuable. This white paper describes such a platform based on National Instruments (NI)USRP RIO hardware and NI LabVIEW Communications System Design Suite.

The rest of the paper is organized as follows. Section 2 starts with an overview of a variety of approaches proposed or in use forcellular networks to take advantage of unlicensed bands. The section provides a detailed look into the design targets and PHYenhancements required for the two main approaches under detailed discussion in the wireless communications ecosystem; LTE-Unlicensed (LTE-U) and License Assisted Access (LAA). Section 3 describes how to use NI LabVIEW Communications System DesignSuite and the included 802.11/LTE Application Frameworks to create a prototyping platform for this use case. Section 4 gives aselection of results obtained using the platform regarding the performance of LTE-U and LAA and its implications on Wi-Fi.

2 LTE/Wi-Fi coexistence landscape

2.1

OverviewCellular network providers already take advantage of unlicensed spectrum via opportunistic offloading of traffic to Wi-Fi networks,especially in demand hotspots [2]. The LTE/Wi-Fi aggregation (LWA) work item [3] in 3GPP targeting LTE Release-13 takes this onestep further and enables aggregation of LTE and Wi-Fi at the packet data convergence protocol (PDCP) layer. In both of these cases,the Wi-Fi air interface is used in unlicensed bands. In the following sections, we explore technologies that use LTE-based air interfacesin unlicensed bands.

2.1.1 LTE-Unlicensed (LTE-U)LTE-U created by LTE-U forum [4] is expected to be the first technology to be deployed where the unlicensed band is directlyintegrated into the LTE lower layers. Key members behind this proprietary technology include Qualcomm, Verizon, Ericsson, Samsungetc.

In LTE-U, the unlicensed band is used as a secondary cell (SCell) within the LTE carrier aggregation framework. There will be a licensedanchor that will serve as the primary cell (PCell). In the current specifications, the unlicensed band is used only for downlink (DL) trafficopportunistically. In the future, uplink (UL) will also be considered.

LTE-U uses a duty cycled version of LTE waveform to access the unlicensed channel as shown in Figure 1. LTE-U implementsalgorithms to improve coexistence between LTE-U networks and Wi-Fi networks. The LTE-U access point (AP) listens actively to Wi-Fi and other LTE-U transmissions to estimate the network usage patterns. Active reception of Wi-Fi transmission implies that it caninterpret channel type (primary/secondary), packet type, packet length, etc. This information is used to evaluate channel activity, andconsequently for dynamic channel selection and adaptive duty cycling. The online algorithm used by LTE-U that adapts the duty cycleis called carrier sense adaptive transmission (CSAT) [5]. The duty cycle could be modified by changing the T ON and T OFF valuesappropriately or by skipping some T ON periods creating longer T OFF times periodically. The resolution of duty cycling is at LTE sub-frame (1ms) boundaries.

Figure 1: LTE-U waveform example

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The proponents of LTE-U have showed results that indicate that the deployment of these networks do not significantly degrade Wi-Fi network performance [6]. However, papers such as [7] disagree since LTE-U does not implement listen before talk (LBT)mechanisms (such as CSMA/CA with defer period and exponential back-off in Wi-Fi) that they believe is critical to fair sharing ofunlicensed channels. Some regulatory regimes such as those in Europe and Japan require LBT in unlicensed bands, and LTE-U cannotbe deployed in those regions. Due to the controversial nature of LTE-U, regulators such as FCC in the US, where LTE-U can bedeployed without LBT, is reviewing input from the ecosystem and is evaluating if further regulation is needed [8].

2.1.2 Licensed Assisted Access (LAA)LAA is the standards based approach to LTE in unlicensed bands. 3GPP is currently creating specifications for LAA with the initialversion to be released as part of LTE Release-13 [9]. Like in LTE-U, the unlicensed carrier is used as an SCell for DL only and is alwaysanchored by a licensed PCell. UL operations will be considered in future revisions.

The key difference from LTE-U is that LAA is designed for worldwide operations and hence includes an LBT framework. Variousoptions are under discussion, and a Wi-Fi like system with an initial defer period and exponential back-off is expected to be included.An example waveform is shown in Figure 2, where the LBT procedure is used to sense channel and get transmit opportunity (TXOP)for up to 10 LTE sub-frames.

Wi-Fi providers have indicated a preference for LAA over LTE-U since they can participate in the open standards process and theyexpect LBT design to be critical to achieve good coexistence performance.

Figure 2: LAA waveform example

2.2 Design targets defined by LTE-U/LAA

3GPP started to work on LAA with a study item about the feasibility of LTE/Wi-Fi coexistence [10] – specifically assess ing the feasibilityof meeting the following design targets:

• A global solution should ensure region-independent compliance, e.g. implementing LBT would ensure compliance in regionswith regulatory LBT requirements

• Ensure effective and fair co-existence between LAA and Wi-Fi• Ensure effective and fair co-exis tence between different LAA operator nodes.

The technical report [11] concluded that co-existence is feasible and suggested a number of PHY enhancements that should bestudied in detail. A selection of the important enhancements are described in Sec. 2.3. The detailed specification of those PHYenhancements will be part of the work item [9].

LTE-U, on the other hand, is targeted primarily at regulatory regimes such as USA, Korea, China, India etc. where LBT is not requiredfor unlicensed channel access. Hence, the design targets for LTE-U are the last two in the above list for LAA.

2.3

PHY Enhancements to Meet Design TargetsIn order to meet the design targets mentioned in Sec. 2.2, it has been suggested to provide the following functionality to enable LAA:

• Channel access framework including clear channel assessment,• Discontinuous transmission with limited maximum transmission duration,• UE support for carrier selection and• AGC, coarse and fine time and frequency synchronization.

That would imply ability to incorporate the following PHY enhancements:

• Discontinuous transmission (DTX)• Listen before talk (LBT)

1-10ms TXOP

Sensing & LBT Procedure



This procedure is similar to the carrier sense multiple access (CSMA) employed by the 802.11 distributed co-ordination function (DCF)MAC. In fact, the algorithm was developed based on feedback from those in the 802.11/Wi-Fi community with experience in fairsharing of unlicensed spectrum. However, there are still some points of discussion among the participants of 3GPP. A couple ofcrucial differences to keep in mind are listed below.

• The 802.11 legacy training field, located at the beginning of all 802.11 packets is a fixed known sequence, and this is usedby 802.11 devices to detect if the channel is idle or in use. LAA/LTE-U nodes can use this method to optimize detectionperformance. The alternate approach is to use energy detection-based sensing, which is simpler to implement, but haslower performance.

• Contention window update based on unsuccessful transmission:- 802.11 CSMA employs exponential back-off, while 3GPPis evaluating less aggressive schemes as well.

3 Prototyping LTE-U/LAA solutions

3.1 802.11 and LTE Application FrameworksThe 802.11 and LTE application frameworks, see [12] and [13], respectively, build the base for this LTE/Wi-Fi coexistence prototypingexample. Detailed information regarding the architecture and implementation of the frameworks can be found in the white papers for802.11 application framework under [14] and for the LTE application framework under [15]. The rest of this paper assumes knowledgeof the LTE application framework architecture and implementation.

As the LAA channel access framework tends to be very close to the 802.11 channel access scheme, the re-use of the 802.11 channelaccess related modules within the LTE application framework is one of the basic ideas towards the implementation of the LAA LBTcapabilities.

For the extension of the existing LTE application framework towards implementing discontinuous transmission and Cat 4 LBT, thearchitectural changes as shown in Figure 4 and Figure 5 are necessary on the following modules/sub-systems:

• Control of resource mapper, TX trigger mechanism and synchronization unit for discontinuous transmission and reception.• Integration of a new channel sensing unit and TX trigger mechanism for LBT.

Figure 4: Architectural changes on TX



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3.2 Implemented PHY EnhancementsThis section explains how discontinuous transmission and LBT category 4 have been implemented.

3.2.1 Discontinuous TransmissionDiscontinuous transmission is a major change in the entire LTE scheduling scheme. In the LTE application framework, this functionalitycan be implemented via leveraging the OFDM-symbol-wise resource block allocation capabilities and the embedded TX triggermechanisms. Both are explained in the following.

The resource block allocation can be defined for each single OFDM symbol within a radio frame. This allocation is done on the host intwo steps:

1. One radio frame on a per sub-frame-basis as shown in Figure 6 and2. Per sub-frame on a PRB and per OFDM-symbol-basis as shown in Figure 7.

This approach gives enough flexibility to support configurable maximum transmit operation time for LAA and configurable cycle time,puncturing and master cycle for LTE-U.



Figure 6: Radio frame definition on a per sub-frame-basis

Figure 7: Sub-frame definition on a PRB and per OFDM-symbol-basis

The LTE application framework already has a wide range of capabilities to trigger transmissions. These are re-used and extendedtowards the needs of discontinuous transmission. Two changes are incorporated:

1. The functionality to trigger the transmission has been extended as shown in Figure 8 from a radio frame-based trigger likewhat is needed for traditional LTE and LTE-U to add the ability to transmit on a trigger from LBT functionality that has sensedthe channel and assessed it as idle and ready for transmission.

2. Instead of transmitting for a full radio frame, additional functionality has been added to allow transmission for a configurablenumber TXOP of sub-frames as shown in Figure 9. Based on this additional parameter TXOP, several counters are controlledin the bit processing as well I/Q base-band processing to transmit only TXOP consecutive sub-frames instead of an entireradio frame. Appropriate changes have been done on the UE RX side as well as shown in Figure 10.



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Figure 8: Select either radioframe-based trigger or LBT-based trigger

Figure 9: Modifications on DL transmitter towards discontinuous transmission with TXOP sub-frames



Figure 10: Modifications on DL receiver towards discontinuous transmission with TXOP sub-frames

3.2.2 Listen-Before-Talk Category 4Figure 11 shows the configuration capabilities of the provided LBT Cat 4 implementation. If “Fixed Backoff” is disabled, the randombackoff procedure is applied. As the current version does not yet contain HARQ feedback, the backoff is always picked from theinterval [0, CWmax-1]. Furthermore, the energy detection threshold can be configured.

Figure 11: LBT Cat 4 configuration capabilities



The LBT Cat 4 top level module consists of a state machine and the power measurement unit from the NI 802.11 applicationframework as shown in Figure 12. The purpose of this state machine is to, based on the LBT Cat 4 procedure, control the powermeasurement unit to do clear channel assessment and to trigger the discontinuous transmission at the appropriate time. The statemachine given in Figure 13 follows the LBT procedure given in Figure 3.

The current version of the code does not implement 802.11 preamble detection for LAA LBT, but the NI 802.11 application frameworkhas the necessary blocks and can be used to extend the feature set to include preamble detection.

Figure 12: LBT Cat 4 top level module

Figure 13: LBT Cat 4 state machine

Preliminary ResultsIn order to gain a better understanding of the basic underlying principles, a simple setup has been used consisting of one USRPRIO representing the LTE eNB as well as the UE and commercially available Wi-Fi components off the shelf as shown in Figure14 and Figure 15. The NI 802.11 application framework together with USRP can also be used for emulating the Wi-Finetwork. (View the parts list for the Real-time LTE/Wi-Fi Coexistence Testbed )



In Figure 16, first results are given for the LTE-U use case. In this case, the LTE-U duty cycle is varied by changing the ratiobetween the number of consecutive DL sub-frames and one radio frame. The plot shows the corresponding change in throughputof the Wi-Fi and the LTE-U link. The results match the expectations and show higher Wi-Fi throughput as we lower the LTE-Uduty cycle and vice versa.

Figure 16: Normalized throughput for legacy Wi-Fi 802.11a and LTE-U for different duty cycle ratios.

Figure 17 shows how the throughput of a Wi-Fi 11ac VHT40 and LAA LBT Cat 4 system change, respectively, if the LAA eNB hasbeen configured with different CCA energy detection threshold values. Both the Wi-Fi 11ac VHT40 node and the LAA node are seeing

each other with an RSSI of around -67 dBm.

More experiments and results can be found in 3GPP contributions [16] and [17].

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Figure 17: Throughput for Wi-Fi 802.11ac VHT40 and LAA with LBT cat 4 for varying LAA CCA energy detection thresholds

4 ConclusionsThe NI 802.11 and LTE application frameworks have been used to build an application in order to serve the emerging need for a neutralplatform that addresses research on coexistence between LTE and Wi-Fi. All such methods either already proposed or envisioned inthe future can be implemented using this test bed.

In this white paper, the basic ideas, the architecture and the essential building blocks have been disclosed which enables research onPHY layer coexistence between LTE and Wi-Fi. Specifically, the design of the following features are discussed.

• Discontinuous transmission• Listen Before Talk

The implementation of the discontinuous transmission feature allows support of LAA as well as LTE-U. The high configurability of thisfeature allows easy plug-in of algorithms to control the LTE-U duty cycle like CSAT algorithm. Discontinuous transmission incombination with the configurable LBT Cat 4 builds the backbone of the LAA channel access framework and can be used for real-world experiments as well as for further research. Preliminary results have proven that the testbed and the tools are ready for LTE-Uas well as well LAA.

The high configurability of the prototype, namely the configurability of the duty cycle in LTE-U, the TXOP in LAA, the CCA ED threshold,the contention window size, etc. allows for a wide range of experiments for better understanding how coexistence will work and howto optimize the parameters in different use cases. The open architecture allows easy modifications or extensions towards moresophisticated coexistence schemes.

The presented LTE/Wi-Fi coexistence prototyping example based on NI 802.11 and LTE application frameworks is a ready-to-gosolution that enables

• Development engineers to run a neutral solution within a testbed• Deployment engineers to find good operating points for deployment-specific use cases and scenarios• Scientists for research towards the convergence of scheduled and ad-hoc wireless systems• Extensibility for testing and prototyping other LTE / Wi-Fi coexistence approaches such as D2D [18], LWA [3], and MulteFire

[19].

Send questions to [email protected]

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5 Bibliography[1] 3GPP, "RAN 5G Workshop," 19 September 2015. [Online]. Available: http://www.3gpp.org/news-events/3gpp-news/1734-

ran_5g.

[2] Ruckus Wireless, "Hotspot 2.0," [Online]. Available: http://www.ruckuswireless.com/technology/hotspot2.

[3] 3GPP, "RP-151114: LTE-WLAN Radio Level Integration and Interworking Enhancement," 2015.

[4] LTE-U Forum, [Online]. Available: http://www.lteuforum.org.

[5] Qualcomm Technologies, Inc., "LTE-U Technology and Coexistence," LTE-U Forum Workshop, 28 May 2015. [Online]. Available:http://www.lteuforum.org/workshop.html.

[6] Qualcomm, "On LTE-U/WiFi Coexistence," Wi-Fi LTE-U Coexistence Test Workshop, November 2015. [Online]. Available:http://www.wi-fi.org/file/wi-fi-lte-u-coexistence-test-workshop-presentations-november-2015.

[7] N. Jindal, D. Breslin and A. Norman, "LTE-U and WiFi: A Coexistence Study by Google," Wi-Fi LTE-U Coexistence Test Workshop,November 2015. [Online]. Available: http://www.wi-fi.org/file/wi-fi-lte-u-coexistence-test-workshop-presentations-november-2015.

[8] FCC, "Proceeding 15-105: Office of Engineering and Technology and Wireless Telecommunications Bureau Seek Information onCurrent Trends in LTE-U and LAA Technology," 2015. [Online]. Available: http://apps.fcc.gov/ecfs/proceeding/view?name=15-105.

[9] 3GPP, "RP-151045: New Work Item on Licensed-Assisted Access to Unlicensed Spectrum," 2015.

[10] 3GPP, "RP-141664: Study on Licensed-Assisted Access using LAA," 2014.

[11] 3GPP, "36.889, v1.0.1: Study on Licensed-Assisted Access to Unlicensed Spectrum," 2015.

[12] "LabVIEW Communications 802.11 Application Framework," National Instruments, [Online]. Available:http://sine.ni.com/nips/cds/view/p/lang/en/nid/213084.

[13] "LabVIEW Communications LTE Application Framework," National Instruments, [Online]. Available:http://sine.ni.com/nips/cds/view/p/lang/en/nid/213083.

[14] "LabVIEW Communications 802.11 Application Framework White Paper," National Instruments, [Online]. Available:http://www.ni.com/product-documentation/52533/en/.

[15] "LabVIEW Communications LTE Application Framework White Paper," National Instruments, [Online]. Available:http://www.ni.com/white-paper/52524/en/.

[16] "3GPP RAN1#82: Experimental Results on Coexistence of DL LAA and Commodity Wi-Fi Network with Cat 2 LBT," NationalInstruments, August 2015. [Online]. Available: http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_82/Docs/R1-154740.zip.

[17] "3GPP RAN1#83: Experimental Results on Impact of Energy Detection Threshold for DL LAA," National Instruments, November2015. [Online]. Available: http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_83/Docs/R1-156622.zip.

[18] A. Asadi, V. Mancuzo and R. Gupta, "An SDR-based Experimental Study of Outband D2D Communications," in IEEE INFOCOM, San Francisco, 2016.

[19] Qualcomm, Inc., "Introducing MulteFire: LTE-like Performance with WiFi-like Simplicity," June 2015. [Online]. Available:https://www.qualcomm.com/news/onq/2015/06/11/introducing-multefire-lte-performance-wi-fi-simplicity.

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