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Real-time LTE/Wi-Fi Coexistence Testbed

Publish Date: Feb 16, 2016 | 1 Ratings | 5.00 out of 5 | Print

Overview

The exponential rise in usage of mobile devices have not shown any signs of slowing down. Efforts are underway in the standardization bodies to define new ways to increase data rates and network capacity by enhancing 4G technologies and by introducing new technologies in 5G [1]. This white paper focuses on the efforts to utilize unlicensed spectrum to improve capacity of cellular networks. The unlicensed spectrum, specifically in the 5 GHz band, can be leveraged either using existing technologies in that band (Wi-Fi) or by modifying cellular technologies to aggregate those channels directly into the cellular PHY and MAC layer. The latter approach has lead to disagreement between cellular and Wi-Fi ecosystems regarding its impact on existing and future Wi-Fi networks. As with any disruptive 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 of such 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.

Table of Contents

  1. Overview
  2. Design Targets Defined by LTE-U/LAA
  3. PHY Enhancements to Meet Design Targets
  4. 802.11 and LTE Application Frameworks
  5. Implemented PHY Enhancements

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

This white paper focuses on the efforts to utilize unlicensed spectrum to improve capacity of cellular networks. The unlicensed spectrum, specifically in the 5 GHz band, can be leveraged either using existing technologies in that band (Wi-Fi) or by modifying cellular technologies to aggregate those channels directly into the cellular PHY and MAC layer. The latter approach has lead to disagreement between cellular and Wi-Fi ecosystems regarding its impact on existing and future Wi-Fi networks. As with any disruptive 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 of such 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.  (View the parts list for the Real-time LTE/Wi-Fi Coexistence Testbed)

The rest of the paper is organized as follows. Section 2 starts with an overview of a variety of approaches proposed or in use for cellular networks to take advantage of unlicensed bands. The section provides a detailed look into the design targets and PHY enhancements 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 Design Suite and the included 802.11/LTE Application Frameworks to create a prototyping platform for this use case. Section 4 gives a selection of results obtained using the platform regarding the performance of LTE-U and LAA and its implications on Wi-Fi.

LTE/Wi-Fi coexistence landscape

1. Overview

Cellular 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 one step 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 interfaces in unlicensed bands.

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 directly integrated into the LTE lower layers. Key members behind this proprietary technology include Qualcomm, Verizon, Ericsson, Samsung etc.

In LTE-U, the unlicensed band is used as a secondary cell (SCell) within the LTE carrier aggregation framework. There will be a licensed anchor that will serve as the primary cell (PCell). In the current specifications, the unlicensed band is used only for downlink (DL) traffic opportunistically. 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 implements algorithms 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 can interpret channel type (primary/secondary), packet type, packet length, etc. This information is used to evaluate channel activity, and consequently for dynamic channel selection and adaptive duty cycling. The online algorithm used by LTE-U that adapts the duty cycle is called carrier sense adaptive transmission (CSAT) [5]. The duty cycle could be modified by changing the TON and TOFF values appropriately or by skipping some TON periods creating longer TOFF times periodically. The resolution of duty cycling is at LTE sub-frame (1ms) boundaries.

Figure 1: LTE-U waveform example

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 of unlicensed channels. Some regulatory regimes such as those in Europe and Japan require LBT in unlicensed bands, and LTE-U cannot be deployed in those regions. Due to the controversial nature of LTE-U, regulators such as FCC in the US, where LTE-U can be deployed without LBT, is reviewing input from the ecosystem and is evaluating if further regulation is needed [8].

Licensed Assisted Access (LAA)

LAA is the standards based approach to LTE in unlicensed bands. 3GPP is currently creating specifications for LAA with the initial version 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 always anchored 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. Various options 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 they expect LBT design to be critical to achieve good coexistence performance.

Figure 2: LAA waveform example

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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 assessing the feasibility of meeting the following design targets:

  • A global solution should ensure region-independent compliance, e.g. implementing LBT would ensure compliance in regions with regulatory LBT requirements
  • Ensure effective and fair co-existence between LAA and Wi-Fi
  • Ensure effective and fair co-existence between different LAA operator nodes.

The technical report [11] concluded that co-existence is feasible and suggested a number of PHY enhancements that should be studied in detail. A selection of the important enhancements are described in Sec. 2.3. The detailed specification of those PHY enhancements 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 required for unlicensed channel access. Hence, the design targets for LTE-U are the last two in the above list for LAA.

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3. PHY Enhancements to Meet Design Targets

In 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)

In the case of LTE-U, duty cycling and puncturing is imposed on the LTE frame structure at the resolution of LTE sub-frames. The implementation of discontinuous transmission can be done to include support for LTE-U patterns.

Discontinuous Transmission

In unlicensed spectrum, some regulatory regimes such as Japan do not allow continuous transmission and limit the maximum duration of a transmission burst. Moreover, as several users might share the medium at the same time, continuous access to channel might not be possible all the time and is not preferable for the sake of fair medium sharing. Since LTE and other cellular standards have so far functioned in licensed spectrum where continuous transmission is possible, the addition of discontinuous transmission feature generates several design challenges.

Listen Before Talk

Since 3GPP envisages a global solution for LAA and LBT is mandatory for unlicensed channel usage in some regulatory regimes, the need of LBT feature is mandatory for LAA. Different LBT schemes have been proposed in Sec. 7.2.1.6 and Sec. 8.2 of [11] and extensive performance evaluations have been conducted, see Sec. 8.3 and related Annex B of [11]. The LBT Category 4 (Cat 4) scheme has been identified as the preferred scheme for most of the use cases. Throughout this paper, the LBT Cat 4 procedure will be treated as shown in Figure 3, which corresponds to the flowchart of DL LAA SCell Cat 4 LBT procedure as in Fig. 7.2.1.6-1 of [11].

The LBT Cat 4 procedure starts when the device has a packet to transmit. The device then conducts an initial clear channel assessment (iCCA), where it is checked if the channel is idle for a defined period of time. If the channel is determined to be free, the transmission can proceed. If not, the device conducts a slotted random back-off procedure, where a random number is selected from a specified interval called the contention window. A back-off countdown is done whenever the channel is determined to be free as shown in the extended CCA (eCCA) portion of the flow chart and the transmission is initiated when the back-off counter goes to zero.

Figure 3: LBT Cat4 procedure from [14]

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 fair sharing of unlicensed spectrum. However, there are still some points of discussion among the participants of 3GPP. A couple of crucial 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 used by 802.11 devices to detect if the channel is idle or in use. LAA/LTE-U nodes can use this method to optimize detection performance. The alternate approach is to use energy detection-based sensing, which is simpler to implement, but has lower performance.
  • Contention window update based on unsuccessful transmission:- 802.11 CSMA employs exponential back-off, while 3GPP is evaluating less aggressive schemes as well.

Prototyping LTE-U/LAA solutions

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4. 802.11 and LTE Application Frameworks

The 802.11 and LTE application frameworks, see [12] and [13], respectively, build the base for this LTE/Wi-Fi coexistence prototyping example. Detailed information regarding the architecture and implementation of the frameworks can be found in the white papers for 802.11 application framework under [14] and for the LTE application framework under [15]. The rest of this paper assumes knowledge of 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 channel access related modules within the LTE application framework is one of the basic ideas towards the implementation of the LAA LBT capabilities.

For the extension of the existing LTE application framework towards implementing discontinuous transmission and Cat 4 LBT, the architectural 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

Figure 5: Architectural changes on RX

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5. Implemented PHY Enhancements

This section explains how discontinuous transmission and LBT category 4 have been implemented.

Discontinuous Transmission

Discontinuous transmission is a major change in the entire LTE scheduling scheme. In the LTE application framework, this functionality can be implemented via leveraging the OFDM-symbol-wise resource block allocation capabilities and the embedded TX trigger mechanisms. 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 in two steps:

  1. One radio frame on a per sub-frame-basis as shown in Figure 6 and
  2. 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 extended towards 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 like what is needed for traditional LTE and LTE-U to add the ability to transmit on a trigger from LBT functionality that has sensed the 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 configurable number TXOP of sub-frames as shown in Figure 9. Based on this additional parameter TXOP, several counters are controlled in the bit processing as well I/Q base-band processing to transmit only TXOP consecutive sub-frames instead of an entire radio frame. Appropriate changes have been done on the UE RX side as well as shown in Figure 10.

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

 

Listen-Before-Talk Category 4

Figure 11 shows the configuration capabilities of the provided LBT Cat 4 implementation. If “Fixed Backoff” is disabled, the random backoff procedure is applied. As the current version does not yet contain HARQ feedback, the backoff is always picked from the interval [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 application framework as shown in Figure 12. The purpose of this state machine is to, based on the LBT Cat 4 procedure, control the power measurement unit to do clear channel assessment and to trigger the discontinuous transmission at the appropriate time. The state machine 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 framework has 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 Results

In order to gain a better understanding of the basic underlying principles, a simple setup has been used consisting of one USRP RIO representing the LTE eNB as well as the UE and commercially available Wi-Fi components off the shelf as shown in Figure 14 and Figure 15. The NI 802.11 application framework together with USRP can also be used for emulating the Wi-Fi network.

 

Figure 14: Basic LTE/Wi-Fi coexistence setup

 

Figure 15: Basic LTE/Wi-Fi coexistence setup in lab environment

 

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 ratio between the number of consecutive DL sub-frames and one radio frame. The plot shows the corresponding change in throughput of the Wi-Fi and the LTE-U link. The results match the expectations and show higher Wi-Fi throughput as we lower the LTE-U duty 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 has been 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].

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

 

 

Conclusions

The NI 802.11 and LTE application frameworks have been used to build an application in order to serve the emerging need for a neutral platform that addresses research on coexistence between LTE and Wi-Fi.  All such methods either already proposed or envisioned in the 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 on PHY 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 this feature allows easy plug-in of algorithms to control the LTE-U duty cycle like CSAT algorithm. Discontinuous transmission in combination 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-U as 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 how to optimize the parameters in different use cases. The open architecture allows easy modifications or extensions towards more sophisticated coexistence schemes.

The presented LTE/Wi-Fi coexistence prototyping example based on NI 802.11 and LTE application frameworks is a ready-to-go solution 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]

Next Steps

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 on Current 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," National Instruments, 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, November 2015. [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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