3G HSPA for Broadband Communications with High Speed Vehicles

Santiago Tenorio#1, Paul Spence*2, Beatriz Garriga#3, Javier López#4, Aitor García#5, Miguel Arranz #6

#Vodafone Technology Networks, Vodafone Spain

Isabel Colbrand, 22, Madrid, Spain 1 Santiago.Tenorio@vodafone.com

3 beatriz.garriga@vodafone.com 4 javier.lopez3@vodafone.com

5 aitor.garcia@vodafone.com 6 miguel.arranz@vodafone.com

*McLaren Electronic Systems

Woking, Surrey, GU21 4YH, United Kingdom 2 Paul.Spence@mclarenelectronics.com

Keywords: HSPA, Doppler, Mobility, High Speed, Telemetry, Formula 1

Abstract— This paper presents a proof of concept for a continuous superior quality Broadband Vehicular communication system enabled through 3G HSPA in very high speed mobility scenarios (beyond 300 km/h), suitable for telemetry applications in trains, emergency vehicles and motor sport events. The system is quite unique as radio transmission for telemetry services under extreme speed conditions requires not only superior Quality of Service guarantees but must also be able to satisfy these performance requirements under extreme and arbitrarily demanding environments as are typical during any e.g. Formula 1 racing event.

Issues related to the Doppler Effect and abrupt changes of the serving HSPA channel are analyzed and addressed here. Conclusions show how a special 3G network design can help to mitigate Doppler Effect impacts. The processes carried out by both the UE and the network to cope with this high speed environment has proven essential to sustain the service in these conditions, as it has the use of suitable receiver Types in the UE. Using derived guidelines and conclusions, a unique system has been developed, built and tested in a Formula 1 environment with very promising results.

I. BACKGROUND 3rd Generation Partnership Project (3GPP) has standardized WCDMA-based packet-switched air interfaces for both downlink and uplink called High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA) respectively [1]. Under conditions where the signal strength on the source cell is rapidly deteriorating (as in high speed scenario) it can occur that the UE may not be able to reliably decode the necessary mobility information, the Service Cell Change (SCC) and Radio Resource Control (RRC) message(s) from the source cell leading to a call drop. An attempt to re-establish the call as defined in the standard [2] under these extreme speed conditions is also a challenge as verified in live testing environment.

Other known standard mobile communication systems encounter equal or worse technical challenges as for instance for any Orthogonal Frequency Division Multiplex (OFDM) based system, frequency offsets, phase noise, and Doppler in a time varying channel quickly result in a significant degraded performance. In addition, this is coupled with impact from inter-carrier interference (ICI) between the OFDM sub-carriers and increasing complexity in the carrier estimation. This paper addresses the challenge of utilising and optimising an existing commercial 3G HSPA Broadband system for communication to very high speed vehicles. As the main immediate objective, this activity sought to establish a working baseline for a new generation of telemetry systems suitable for high speed applications and delivers a Proof of Concept in a Formula 1 environment. In this paper, we introduce the setup utilised and the results obtained from several tests performed in different high speed scenarios covering controlled environments.

II. EQUIPMENT DESCRIPTION The UTRAN network was provided by a 3G equipment manufacturer using products and features commercially available. Five types of devices were tested [3]

Type 1 Receiver, cat 8 HSDPA and cat 5 HSUPA Type 2 Receiver, cat 8 HSDPA and cat 5 HSUPA Type 3 Receiver, cat 8 HSDPA and cat 5 HSUPA Type 3 Receiver, cat 10 HSDPA and cat 6 HSUPA Type 3 Receiver, cat 14 HSDPA and cat 6 HSUPA

III. SCENARIO DESCRIPTION All testing took place in a high speed circuit - IDIADA - in the north east of Spain. A dedicated 3G network was installed in the 2.1GHz band using four Nodes B covering the entire circuit, and in addition a compact RNC and CORE network

978-1-4244-2519-8/10/$26.00 ©2010 IEEE

was also installed on-site. Two of the sites covered the curves in the circuit and were located in a strategic position in order to avoid Doppler Effect impact. The remaining two sites were installed close to the straight sections of the track - “straights” - to fully analyse the Doppler Effect. For coverage purposes, 20m masts were used. Sites in the curves and straights had tri-sector and bi-sector design with 65º and 33 º cross-polar antenna beam width respectively.

Fig. 1 Test scenario, IDIADA circuit

IV. DOPPLER SHIFT AND MOVILITY ANALYSIS Several tests were carried out to measure and quantify the impact of both Doppler spread and mobility management limitations in extreme conditions i.e. a high speed vehicle. Particular attention was given to the evaluation of data service performance itself, in particular measuring the throughput, the RTT (Round Trip Time), and the effect of cell change when the vehicle was traveling across two or more cells at speeds of around 250km/hr and beyond.

A) DOPPLER SHIFT IMPACT The main affection of high speed scenario is Doppler shift, also known as Doppler Effect. This effect is the change in frequency in of a wave for an observer moving relative to the source of the waves. For waves that propagate in a medium, such as radio waves, the velocities of the observer and of the source are relative to the medium in which the waves are transmitted. Doppler shift follows the next formula, also represented in the figure 2:

θcos××= vCffd

θ: angle between UE mobility and signal propagation directions v: vehicle rate C: radio spread rate f: carrier frequency

Fig. 2 Doppler shift components

The main outcomes regarding Doppler Effect on HSPA performance are:

In downlink, in neither RSCP nor Ec/No no significant degradation was detected. The HSDPA throughput loss was 16% in the worst case

In uplink, the BLER and retransmission rate critically increases at speeds beyond 180km/h where throughput drops to almost 0kbps if no Doppler compensation functionality is activated in the node B

Figure 3 shows the effect of Doppler Speed on HSUPA UL data traffic. As speed increases the number of retransmissions increases impacting on throughput.

Fig. 3 CAT6 HSUPA UL throughput degradation above 180km/h

Figure 4 shows the effect of Doppler Speed on HSDPA DL data throughput depending on the UE receiver Type. Type 3 receiver achieved the best performance under different speeds , and Type 2 showed the biggest throughput degradation of around 16% respect to 30km/h. Regarding HSDPA coverage loss measured by CQI, all devices showed similar degradation trend at high speeds i.e. approximately 1.2dB less compared to low speeds.

UE Receiver Type: --- Type 3 ---Type 2 ---Type 1

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Fig. 4 CAT8 HSDPA DL throughput and CQI vs. receiver type and

Doppler Speed

To mitigate the effect of Doppler shift in user performance, the following algorithms were implemented in the network side:

1) Frequency offset estimation 2) Frequency offset compensation

There are different methods to perform frequency offset estimation and compensation - although most of them are vendor proprietary algorithms. Figure 5 shows the HSUPA throughput gain with a Doppler shift compensation algorithm on. At 250Km/h, there is no

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throughput degradation comparing to 80 and 150km/h cases. Regarding UE power, 17dB less is required to get 3.7Mbps more if Doppler shift compensation is put in place.

Fig. 5 CAT6 HSUPA Throughput and Remaining available power in the

UE vs. Doppler speed

B) MOBILITY IMPACT Current 3G network mobility processes are optimized to operate at medium-low speed mobility (<150km/h) so under high speed scenarios a different parameterization and design is needed.

• Soft handover: New target cell addition time is about 400~800ms (from new cell detection by UE till active set cell update complete message). Additionally, HSDPA DL service requires cell change when a new target cell becomes x-dB’s better than the actual cell level in terms of EcNo. This process takes usually a longer period of time since physical channel reconfigurations must be performed in the UE. The average time required to perform such action is about 1.2~3.3s with 1.75s the average and 0.71s the standard deviation (from new cell change condition detection by UE till physical channel reconfiguration complete message). This broad range is due to the fact that many 3G vendors implement specific timers to avoid ping pong effect during HSDPA cell change. A full HSDPA cell change procedure requires soft handover plus cell change actions. Figure 6 shows the average distance traverse during the cell addition, cell deletion and HSDPA Reference Cell Change event depending on the speed. The minimum total overlap distance required between 2 cells would be around 260m at 300Km/h, 345m if being more conservative.

DISTANCE REQUIRED FOR SHO AND HSDPA CELL CHANGE

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Note: Time to trigger 1a: 100ms; Time to trigger 1b: 640ms; Time Fig. 6 Distance per event

• Inter frequency and Inter system handover: The

process takes approximately 1.4s ~ 2s in case of inter frequency handover and 1.4s in case of intersystem handover under normal conditions.

• Cell reselection: When camped on a cell, the mobile shall regularly search for a better cell according to the cell reselection criteria. Cell reselection failure was frequently detected in high speed scenarios because UE has changed cells before the cell reselection timer expires.

Performance improvement in high speed mobility scenarios requires:

1. Avoid inter frequency and inter system handover to reduce call drop rate and zero throughput periods.

2. Make usage of single cell configurations to avoid handover and ping pong effect due to pilot pollution as much as possible. This impact increases proportionately at higher speeds. Figure 7 represents HSDPA performance when changing cell on a polluted area. The CQI, HS-SCCH success rate and HS-DSCH BLER degradation are bigger at high speed impacting on throughput

Fig. 7 Pilot Pollution effect on HSDPA performance vs. speed 3. Optimize network design maximizing handover

overlapping distance taking into account high speeds 4. Optimize parameters to accelerate UE decisions and

network reaction (e.g. accelerate 1A trigger, to make

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difficult to trigger 1B configuration…) 5. To optimize cell reselection timers (“Treselection”,

“Qoffset” and “Qhyst”) and reduce the system information update time. Additionally, call reestablishment timers should be optimized.

Figure 8 shows MAC HSDPA throughput variation during cell change by modifying 1d event reporting parameters (target cell signal level strength over serving cell level “Hys1d” and time to satisfy the threshold “Ttrig”) when triggering fast, medium or slow cell change procedures. As seen on the figure, being reactive and trying to be on the best cell always or conservative delaying cell change procedure are not the best optimization implementations to improve performance.

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Fig. 8 HSDPA Performance comparison during cell change with

different mobility optimization strategies To avoid mobility issues and increase UL capacity, a feature offered by several infra vendors called Multi-RRU has been tested. This feature permits several physical cells to work as a single cell for down link transmission and as independent cells for up link reception

The following benefits are obtained from the use of this feature in high speed scenarios: • Reduction of number of handovers controlled by the RNC • Flexibility on the coverage area of one cell to adapt the

network design to specific high speed scenarios needs • Enhancement of the uplink capacity (Throughput increase)

due to different sector RTWP management • Downlink diversity gain in the overlap coverage area of

different RRU's.

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Figure 9 shows the effect of the feature on uplink cell capacity, when both users are located in the overlap area of both sectors the traffic in UL is shared between both (4.5Mbps) but when each user is located under the coverage area of a different physical cell the throughput increase as they don’t have to share common resources (around 4.5Mbps each)

V. HIGH SPEED PERFORMANCE IN AN OPTIMIZED

NETWORK

A) HSDPA PERFORMANCE (250 KM/H) Figure 10 shows a HSPA/HSPA+ performance comparison among CAT8, CAT10 and CAT14 devices at 250Km/h. Both CAT10 and CAT14 devices benefit from 15 codes usage with QPSK modulation and the Enhanced Layer 2 3GPP feature, thus improving the HS-DSCH BLER. The benefit of using 15 codes with a new advance 3GPP Release 7 receiver type is significant with up to 88% more throughput achieved with only 5 more codes available. The reason for this is not only the number of codes available but also the improved HSDPA coverage measured by the CQI which is 3dBs better due to improved receiver type. CAT14 HSPA+ 64QAM device only gets 6% more throughput than CAT10. This is due to the fact that there was an Iub limitation and the maximum achievable throughput was 16Mbps (although Iub limitations aside, a peak of 21.8Mbps would have been possible).

Fig. 10 CAT8, CAT10 and CAT14 HSDPA/HSPA+ performance

comparison at 250km/h

However, the main conclusion is that utilizing this Rx Type in the UE permitted 64QAM modulation to be utilized in up to 27% of samples with the BLER measured as below the BLER target of 10%. 64QAM modulation is not affected negatively with high speed

B) HSUPA PERFORMANCE (310 KM/H) Figure 11 shows a HSUPA performance comparison among CAT5 and CAT6 devices at 310Km/h. The CAT5 device achieved high and stable throughput providing 1.8Mbps on average as measured at the physical layer (10% below the

Boths user in same sector

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maximum throughput). The CAT6 device provided more unstable throughput but did achieve 4.1Mbps on average (29% below the maximum throughput). Using the CAT6 device required around 8.6dBs more power compared to the CAT5 device to achieve 127% more throughput

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Fig. 11 HSUPA performance

UL Cell capacity with multi-user was assessed at 250km/h. For this purpose, 6 CAT5 HSUPA devices were placed in the same car performing UDP uploads. For each of the users a 100kbps UL Guaranteed Bit Rate (GBR) was setup in the Node B and 2 different maximum UL RTWP increase levels relative to background noise were analysed: 30dB and 10dB. Figure 12 shows the results obtained. The UL cell throughput is slightly degraded by around 15% with a low RTWP increase but the GBR is achieved 92% of the time whilst only 45.8% was reached in the other case. Achieving 100% GBR would require an even lower RTWP increase allowance.

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C) ROUND TRIP TIME RESULTS (250 KM/H) Figure 13 shows the Round Trip Time performance comparison for CAT5 and CAT6 HSUPA devices when sending 64bytes ping packets at 250Km/h. As seen in the figure, CAT6 HSUPA device achieves 8ms lower RTT values on average (-17%) comparing to CAT5 device. Besides, both average and standard deviation RTT values increase critically when the CAT5 device was in soft handover.

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Fig. 13 Round Trip Time variation with soft handover for CAT8

HSDPA/CAT5 and CAT6 HSUPA devices The Round Trip Time of a HSUPA CAT5 device was also measured via simulating load in addition to another CAT5 device performing FTP uploads at 310Km/h. On average, the RTT value in loaded conditions increased from 59ms to 80ms (35% increase).

VI. CONCLUSIONS The trial in IDIADA demonstrated that the 3GPP HSPA technology works with only a slight degradation at speeds beyond 300km/h. A Doppler shift compensation feature is required in the node B when the UE moves at speeds over 180km/h to avoid throughput degradation. No significant Doppler effect has been seen on the UE side. HSPA performance enhances in single cell configuration with a multi RRU feature improves uplink cell capacity. Ad-hoc optimization and design solution(s) are mandatory to avoid cell change ping pong effect due to pilot pollution. Inter-cell overlapping distances over 300m are recommended to avoid HSPA performance loss due to lack of soft handover or cell change time availability. Network performance is improved using “HSPA+” CAT14/HSUPA CAT6 devices in comparison to legacy devices i.e. better downlink and uplink throughput, round trip times were achieved and adapted better to the coverage and radio environment.

VII. ACKNOWLEDGEMENTS The authors would like to acknowledge Qualcomm, Huawei and ZTE Corporation for facilitating the necessary UE, network infrastructure and related support, and in particular to the Vodafone McLaren Mercedes Racing Team for their support and access to their high-end engineering facilities.

VIII. REFERENCES [1] 3GPP Rel-7 and Rel-8 White Paper (3G Americas).

www.3gamericas.org [2] 3GPP TS 25.331 V8.9.0 Radio Resource Control (RRC); Protocol

Specification. [3] 3GPP TR 25.101 V9.0.0 (2009-05) User Equipment (UE) radio

transmission and reception (FDD).