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Wireless Technology Assessment with Radio Channel Emulator

Hiro Onishi (Alpine Electronics Research of America, Inc. honishi@alpine-la.com) Fanny Mlinarsky (octoScope, Inc. fm@octoscope.com)

ABSTRACT

Mission-critical automotive safety applications are now being introduced to market, thanks to advances in wireless technologies. However, radio channel conditions on the road present many challenges to connected car technologies, including motion of the radio modules located in vehicles, multipath caused by proximity of moving reflectors, such as cars and trucks, and Doppler fading of the radio signal due to the motion of both radios and reflectors. To ensure success of the connected vehicle Dedicated Short-Range Communications (DSRC) technology, we have to measure and analyze the impact of real-world challenges to radio communications on the road. The industry has used three approaches to assess feasibility of DSRC: computer simulations, field testing and radio channel emulation. In this paper we will examine the benefits and trade-offs of these three approaches with the focus on radio channel emulation, which involves placing real radio modules into a controlled laboratory-based wireless testbed.

KEYWORDS Wireless, DSRC (Dedicated Short-Range Communication), IEEE 802.11p, crash avoidance applications, radio channel emulator, radio propagation, radio performance 1. INTRODUCTION Testing performance and functionality of wireless devices and systems is challenging due to the dynamic conditions in the radio channel, including:

- Motion of the radios located inside the vehicles - Severe multipath produced by urban canyons and tunnels - Motion of reflectors (e.g. other vehicles) - Noise and interference - Fast-changing conditions

These variables are difficult to control in an outdoors setting, making test results difficult to reproduce from measurement to measurement. Reflectors in the wireless channel on the road, for example buildings or other vehicles, create multiple versions of the transmitted signal that all enter the radio receiver simultaneously but with some delay spread. This phenomenon causes multipath fading – fading that’s variable vs. time and vs. frequency. Motion of the radios and of the reflectors in the channel causes

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frequency spread of the signal, which results in Doppler fading. Doppler fading and multipath fading are both time-variable and statistical in nature. The amount of Doppler are spread as a function of the speed of the radios and reflectors. Noise and interference are also time-variable and unpredictable. Modern radios employ digital signal processing (DSP) to adapt in real-time to the changing conditions in the radio channel. For example, data rate can be lowered in hostile channel conditions to achieve lower packet error rate. To validate the performance of the radio DSP algorithms and RF front end, a controlled RF environment testbed can be useful. Such testbed enables engineers to vary channel conditions in a defined manner, adding a controlled amount of fading, noise and interference [1]. A laboratory testbed also makes it possible to emulate data traffic from multiple radios to help assess the performance and behavior of DSRC devices and systems under heavy data traffic. Time-sensitive mission-critical crash avoidance applications using DSRC wireless technology based on the IEEE 802.11p standard [2] [3] [4] should be thoroughly evaluated under both real-world conditions on the roads and in a controlled RF environment in the laboratory using radio channel emulation. Automotive radios used in critical safety applications must perform well under challenging and dynamic road conditions and they should be tested more thoroughly than radios used for Infotainment (Information and Entertainment) applications, such as traffic or weather information reception. A radio channel emulation testbed is able to synthesize or replay many real-world conditions, such as radio interference, multipath fading, effect of Doppler and other signal impairments. A crash avoidance application is expected to operate at crossings under non-line-of-sight (NLOS) conditions, i.e. with signal obstructions in the communication path. In Japan, to enhance radio performance in such hostile conditions, 700MHz instead of 5.9GHz wireless spectrum is being considered for crash avoidance applications [5]. A radio channel emulation testbed can be used to test and compare radio performance in 700MHz and 5.9MHz bands under hostile NLOS radio propagation conditions. Another challenge for DSRC is that a high density of radios on the road can put a strain on the Carrier Sense Multiple Access with Collision-Avoidance (CSMA/CA) protocol [6] employed for medium (airlink) access by 802.11p radios [7]. Unlike 2G/3G/Long Term Evolution (LTE) radio networks in which scheduling of transmissions is tightly controlled by the base station [8], the Wi-Fi CSMA/CA based Medium Access Control (MAC) layer is non-deterministic with stations sensing the medium prior to transmitting and then being required to detect simultaneous transmissions (data collisions) and retransmit the potentially corrupted packets involved in collisions. The efficiency of CSMA/CA networks can degrade

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under heavy data traffic from too many stations. Furthermore, the CSMA/CA protocol can become inefficient in the networks where some radios cannot detect transmissions from other radios, a condition known as a ‘hidden node’ scenario [9]. 2. WIRELESS TEST AND CERTIFICATION STANDARDS Every industry defines its own performance, interoperability and certification testing standards. A sampling of test standards and organizations defining these standards is shown in Table I.

Table I Major test standards and organizations for wireless technologies For the cellular industry, interoperability, backwards compatibility and protocol compliance are important requirements and operators mandate extensive certification testing prior to allowing new devices onto their radio networks. Cellular 2G/3G and Long Term Evolution (LTE) certification standards are defined by organizations such as 3GPP [10] and ETSI [11] and administered by PTCRB [12] and Global Certification Forum (GCF) [13] to numerous test laboratories around the world. Test standards ensure that devices are tested in a consistent manner under controlled and repeatable RF conditions at test laboratories located throughout the world. Controlled RF environment testing ensures accurate and reproducible test results and enables engineers to quickly and efficiently diagnose issues using monitors and other diagnostic equipment in the presence of repeatable RF conditions, traffic emulation and other test stimulus [14] [15] [16].

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Fig. I Sample of radio anechoic room [16] DSRC’s physical layer, 802.11p, has been standardized by the IEEE 802.11 Working Group [17]. IEEE 802.11 interoperability certification testing is defined and performed by the Wi-Fi Alliance (WFA) [18]. 802.11 performance test methods and metrics are defined in the IEEE 802.11.2 document [19] created by Task Group T (TGT). More than half the test methods specified by the WFA and the TGT document require controlled RF environment testing. TGT defines numerous performance tests such as, packet loss, latency, throughput vs. range, throughput vs. receive power and throughput vs. Adjacent Channel Interference (ACI). 3. WIRELESS TECHNOLOGY ASSESSMENT APPROACHES As we discussed in section 1, proper functionality and performance of wireless technology is critical for automotive safety applications, requiring extensive assessment of radio technologies prior to widespread DSRC deployment. The industry has taken three main approaches to ensure that DSRC radios meet their stringent performance requirements: computer simulations, field measurements and radio channel emulation. Computer simulations include simulation of the radio architecture, data traffic and wireless channel conditions. Techniques, such as ray-tracing [20] and statistical channel modelling [21] are used to simulate multipath and Doppler in the radio channel. Packet error rate can be simulated by computer models for a variety of radio signalling techniques and radio channel conditions. Sometimes real-world road models and building shapes are used in ray tracing based channel simulation (Fig. II-b) [22] [23]. Computer models can also simulate 802.11p data traffic based on statistical behavior models of vehicle traffic (Fig. II-a) [22] [23]. In short, software simulation can model data traffic, vehicle traffic, radio architectures and radio channel conditions, including noise, multipath and motion of radios. Using software simulation, these parameters can be controlled easily, but real radio modules cannot be evaluated.

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Fig. II-a

Concept of wireless simulation with geographical data and vehicle traffic simulator [23]

Fig. II-b Signal reception simulation with geographic data [23]

The DSRC industry has tested the performance of actual radio modules using field measurements and collected data on automotive safety application usability in the real-world [4] [24] [25] [26] [27]. US Department of Transportation assessed 5.9GHz DSRC technology performance in various environments (Fig. III a-b) [24]. The Japanese government evaluated DSRC performance for crash avoidance applications in two frequency bands: 5.8GHz and 700MHz (Fig. IV a-d) [25]. The disadvantage of field testing is that the real-world test environment (for example, speed of radio modules or noise level) is impossible to control, thus rendering measurements become non-repeatable. During field testing, device behavior and any implementation issues with the radio or protocol layers can be difficult to monitor. Another challenge with field testing is the requirement to use radios with regulatory certification, for example, FCC (Federal Communications Commission) in the US makes it

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impossible to evaluate early stage innovations using prototype technology. In addition, of course, field testing can be costly as it requires vehicles, drivers and test courses.

Fig. III-a Fig. III-b Tall urban canyon Hilly location test environment [24] test environment [24]

Fig. IV-a

Test environment [25]

Fig. IV-b Test geometry pattern [25]

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Fig. IV-c Field measurement result - 5.9 GHz [25]

Fig. IV-d

Field measurement result - 700 MHz [25] The 3rd method, radio channel emulation, involves placing actual radio modules into a controlled RF environment testbed with small radio chambers (Fig. V-a) used to isolate radios under test from noise and interference and from one another. A channel emulator or RF attenuators can be used to connect radios as a system, while manipulating radio signals, multipath configuration, noise and interference in a controlled manner (Fig. V-b) [28]. We can easily monitor and analyze performance and behavior of DSRC devices and systems under a variety of radio channel and data traffic conditions, using such a testbed. We can add data

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traffic emulation to observe the device and system behavior in the presence of high network load that can be encountered in dense DSRC networks on busy roads and highway interchanges.

Fig. V-a Fig. V-b Controlled RF testbed [1] [28] Radio channel emulator (concept) [1] [28] Table II shows the pros and cons of 3 individual assessment approaches. The radio channel emulation approach offers the best of both worlds – realistic conditions and controlled RF environment. It enables testing of actual radio module while easily controlling radio channel parameters in an isolated interference-free environment. It also enables testing of early DSRC prototypes without the burden of obtaining regulatory certification.

Table II Comparison of wireless assessment methods

4. PACKET ERROR RATE ASSESSMENT Radio module performance under LOS (line-of-sight) and NLOS (non-line-of-sight) conditions is critical for safety applications. As a first step, we selected several critical use cases from 6 DSRC safety applications at ITS World Congress 2011 (Orlando, FL) (Refer to Table III) [2]. Fig. VI-a ~ d demonstrate selected critical use cases, such as, EEBL, LTA and IMA, under challenging road conditions with the radio signal being blocked by a large truck or buildings around corners of city intersections obstructing RF propagation [29].

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Table III DSRC safety applications

at ITS World Congress 2011 (Orlando, FL) [2]

Fig. VI-a Critical use case - 1 Fig. VI-b Critical use case - 2 EEBL with a large truck obstruction LTA with a large truck obstruction

Fig. VI-c Critical use case - 3 Fig. VI-d Critical use case - 4 IMA with signal blocked by 4 corners IMA with signal blocked by 1 corner As a second step, we examined radio environments that are challenging for safety applications. These challenging environments include Tall Urban Canyon (Fig. VII-a), which has been

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noted by the US Department of Transportation [24], and Urban Tunnel (Fig. VII-b). Both Urban Canyon and Urban Tunnel radio channels are particularly challenging for DSRC radios because 802.11 OFDM (Orthogonal Frequency-Division Multiplexing) signaling [30] has been originally designed for indoor environments where signal reflections exhibit a much shorter delay spread than in these outdoor radio channels. In a tunnel, the signal strength of multipath reflections can build over distance as in a waveguide producing strong reflections with long delays that an indoor wireless technology such as 802.11p may not be optimized to process [31].

Fig. VII-a Critical environment - 1. Fig. VII-b Critical environment - 2. Tall Urban Canyon [24] Urban Tunnel We plan to test some of the critical use cases and by simulating the environment mentioned above in a controlled radio testbed where we can emulate real-world radio channel conditions in order to measure the parameters in Table III.

Table III Measurement parameters with channel emulator

5. MAC LAYER PERFORMANCE ASSESSMENT As we mentioned in section 1, the 802.11 Medium Access Control (MAC) layer Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol [6] used by DSRC is an unscheduled access protocol that relies on transmitting stations to avoid, detect and recover from data packet collisions with other stations. CSMA/CA is a contention-based protocol

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which requires all stations to sense the medium before transmitting in order to avoid collisions that result in corrupted packets. The 802.11 CSMA/CA protocol incorporates the following mechanisms: +. Back-off following a collision, with the back-off time being randomly selected within a back-off window (refer to Fig. VIII) [7] +. Announcements by transmitting stations in the packet data fields specifying the anticipated duration of the transmissions When two or more packet transmissions collide in the air, each station involved in the collision has to back off its next transmission for a randomly selected period of time within a backoff window. Upon repeated collisions, the back-off window is set to double the duration of the previous window (Fig. VIII). For example, after the first collision, the backoff window is increased from 15 slots to 31 slots. With each consecutive collision, the backoff window is doubled until it reaches CWmax (1023 slots for 802.11a). During periods of heavy traffic load, multiple successive collisions are not uncommon and the 802.11 CSMA/CA back-off protocol significantly decreases the capacity of the airlink. In dispersed networks, where some stations are out of range of other stations and therefore cannot detect the other stations’ transmission. Stations that are out of range create ‘hidden node’ scenarios. A hidden node scenario is depicted in Fig. IX, which shows that terminal A and terminal B are within range of each other, while terminal C is out of range of terminal A [9]. If terminal A cannot receive terminal C’s transmissions, terminal A cannot ‘hear’ terminal C’s announcement about how long terminal C plans to transmit and hence the collision avoidance protocol breaks down. Also, collisions between terminals A and C can go undetected and hence packets will not be retransmitted and therefore will be lost, increasing packet error rate (PER) [7]. In traditional Access Point (AP) based 802.11 networks hidden node scenarios are handled by using the RTS/CTS (request to send/clear to send) protocol with the stations requesting a permission to send from the AP and with the AP then announcing to all other stations via CTS transmission that the medium is about to be busy [32]. This works to address hidden node issues for AP based networks, since APs are never ‘hidden’ from their associated stations. However, such protocol is not available in a peer-to-peer DSRC network topology, making DSRC networks vulnerable to hidden node scenarios.

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Fig. VIII 802.11 transmit cycle [7] Fig. IX ‘hidden node’ scenario [9] Traffic generators that emulate traffic from multiple stations can be used to evaluate the performance of the CSMA/CA behavior of DSRC networks. Some 802.11 traffic generators are available on the market [33], but they may not yet properly implement back-off behavior of the emulated stations and may not yet support 802.11p. 6. NEXT STEPS We started the assessment of DSRC 5.9GHz terminals for the US market using a controlled environment testbed (Fig. X). We will be working towards emulating a variety of RF propagation and network motion scenarios in this testbed, especially focusing on the above mentioned critical use cases and environments. We plan to report on our findings in future papers.

Fig. X Network performance assessment with multiple radio modules

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REFERENCES and FOOTNOTES

[1] F. Mlinarsky, ITS-America web seminar “DSRC Evaluation under Controlled Environment”, 2013

February. Available: http://itsa.org/images/Francis/its-america-webinar-30208.pdf

[2] US Department of Transportation and CAMP (Crash Avoidance Metrics Partnership) VSC3 (Vehicle Safety Communication 3) consortium, “Connected Vehicle Technology Demonstration”, in

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ITS World Congress, Orland FL, 2011 October, [Demonstration]. Available: www.itsworldcongress.org/techshowcase_usdot.html

[3] “Demonstration Guide”, ITS World Congress, Vienna Austria, 2012 October. Available:

http://www.itsworldcongress.at/uploads/media/Demoguide_final_03102012_LowRes_01.pdf

[4] “Safety Pilot Program Overview” by US Department of Transportation. Available:

http://www.its.dot.gov/safety_pilot/index.htm [5] Japan National Police Agency, “Start of Driving Safety Support System (DSSS)”, 2011 July,

Japanese, [Internet]. Available: http://www.npa.go.jp/koutsuu/kisei/DSSS/20110701_DSSS.pdf

[6] CSMA: Carrier Sense Multiple Access. One kind of MAC (Media Access Control) protocols.

Abailable: http://en.wikipedia.org/wiki/Carrier_sense_multiple_access

[7] F. Mlinarsky et al., “Colliding Views on Call capacity Measurement”, 2005 September. Available: http://www.octoscope.com/English/Collaterals/Whitepapers/octoScope_WP_CollidingViews_20050823.pdf

[8] http://www.3gpp.org/specifications provides links to 2G/3G and LTE standards, all of which use

scheduled medium access for mobile devices.

[9] http://en.wikipedia.org/wiki/Hidden_terminal

[10] 3GPP: http://www.3gpp.org/

3GPP-RAN5: http://www.3gpp.org/RAN5-Mobile-terminal-conformance

[11] ETSI(European Telecommunications Standards Institute): http://www.etsi.org/

[12] http://www.ptcrb.org/

[13] http://www.globalcertificationforum.org/

[14] http://www.satimo.com/content/solutions-automotive

[15] http://www.thehowlandcompany.com/wireless/wireless-test.htm

[16] Owned by Alpine Electronics, Inc. (in Japan). http://www.alpine.com/e/research/estimate/index07.html

[17] IEEE 802.11 Working Group: http://ieee802.org/11/

[18] Wi-Fi Alliance: www.wi-fi.org

[19] IEEE P802.11.2TM/D1.01 document, “Draft Recommended Practice for the Evaluation of 802.11 Wireless Performance”, issued 2008 February.

[20] S. Knorzer et al., Wave Propagation in Communication, Microwave Systems and Navigation

2007, Chemnitz, Germany, 2007. Available: http://www.qucosa.de/fileadmin/data/qucosa/documents/5521/data/WFMN07_I_B3.pdf

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[21] M. Garg, “Statistical Wireless Channel Model”, [Internet]. Available: http://www.mohrahit.in/find/statistical-wireless-channel-model.pdf

[22] T. Hikita et al., “Integrated Simulator Platform for Evaluation of Vehicular Communication

Applications” in IEEE Conference on Vehicular Electronics and Safety, Columbus, OH, 2008.

[23] Japan Automotive Research Institute “Prototype integration of the ITS simulator” Japanese, 2010 June. Available: www.ieee-jp.org/japancouncil/chapter/VT-06/vt.files/VTS-ITS20100623-5.pdf

[24] The VII Consortium, “Final Report: Vehicle Infrastructure Integration Proof of Concept Results and Findings Summary - Vehicle”, 2009 May. Available: http://ntl.bts.gov/lib/31000/31100/31135/14477.htm

[25] “Radio Transmission for Safety Driving Assist” by Japanese government, Japanese, 2008 October. Available: www.soumu.go.jp/main_content/000025424.pdf

[26] J. B. Kenney, et al., “Comparing Communication Performance of DSRC OBEs from Multiple Suppliers”, ITS World Congress, Vienna Austria, 2012 October.

[27] S. Bai et al., “DSRC Performance Comparison with and without Antenna Diversity Using Different Transmission Power” (SAE 2012-01-0491), SAE World Congress, Detroit MI, 2012 April.

[28] octoBoxTM small anechoic chamber channel emulation solution by octoScope:

http://www.octoscope.com/English/Products/octoBox_Stackable/octoBox_Stackable.html and octoFade channel emulation, http://www.octoscope.com/English/Products/octoFade/octoFade.html

[29] According to [26], Fig. VI-d is more critical for PER than Fig. VI-c, because of no reflection signals.

[30] http://en.wikipedia.org/wiki/OFDM

[31] 3GPP TR 25.996, "3rd Generation Partnership Project; technical specification group radio access networks; Spatial channel model for MIMO simulations“

[32] http://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS

[33] For example, IxVeriWaveTM data traffic emulators by IXIA. http://www.ixiacom.com/products/ixveriwave/