UMTS In Vitro Exposure System and Test Signal for Health

Chapter 8

UMTS In VitroExposure System andTest Signal for HealthRisk Research

107

108 CHAPTER 8. UMTS IN VITRO SETUP

SUMMARY

A setup for the nonthermal exposure of cell monolayers toelectromagnetic fields with UMTS signal schemes has beenrealized. The waveguide-based and computer-controlled ex-posure system has a power efficiency for SAR of up to 17W/kg/W, a nonuniformity of the SAR distribution of lessthan 26% and provides a temperature load of less than 0.03Cper W/kg average SAR. In order to compose a UMTS testsignal for health risk assessment, the UMTS signal specifica-tions are analyzed with respect to low frequency power varia-tions. A test signal for the frequency division duplex mode ofUMTS is proposed, representing a scenario with maximizedextremely low frequency spectral components. The signalis compliant with the 3GPP WCDMA modulation specifi-cations and covers power controlled channel fading as wellas pulsed structures due to compressed mode and physicalrandom access channel.

8.1 Introduction

A large majority of studies focussing on the health risk assessmentof electromagnetic field exposure from mobile phones were performedin order to test current second generation 2G technologies such asthe GSM system (Global System for Mobile Communications). Thirdgeneration (3G) systems are designed for multimedia communication.Among them, WCDMA (Wideband Code Division Multiple Access)technology has emerged as the most widely adopted 3G air interface.The IMT-2000 standard (International Mobile Telecommunications)for Europe is named UMTS (Universal Mobile TelecommunicationSystem). Its specifications have been created by the 3rd GenerationPartnership Project (3GPP), who have drafted a UMTS standard inthe year 2000 (3GPP Release 99, www.3gpp.org). Although accep-tance of first data services such as the mobile internet based on inter-mediate (so called 2.5G) standards like GPRS (General Packet RadioService) has been well below expectations, UMTS will have a consid-erable market penetration in the near future and therefore needs tobe assessed with respect to its safety.

8.2. DESIGN OF THE EXPOSURE SYSTEM 109

Although similarities between 2G and 3G with respect to carrierfrequency and exposure strength are large, the low frequency spectralcontent differs significantly due to the distinct modulation techniques:2G TDMA (Time Division Multiple Access) signal schemes provide(1) a high coherence of extreme low frequency amplitude modulation(ELF-AM) spectra (main frequency components GSM: 2, 8, 217 Hz;US NADC: 50 Hz), (2) high crest factors (pulsed signals), and (3) apower regulation with an update in the order of seconds [122]. In con-trast, 3G WCDMA signals have (1) low coherence, (2) a broad-bandELF-AM spectrum resulting from the fast and dynamic power control(e.g., UMTS update rate: 1500 Hz) and (3) amplitude variations inthe MHz range due to spreading. Since the low frequency amplitudevariations are considered to have a potential for evoking biological re-sponses, research with 3G signals is required in order to complementthe data already gained with 2G.

The objective of this study is the realization of an in vitro expo-sure system for the assessment of possible nonthermal biological effectsfrom UMTS exposures. The requirements for such exposure setups aresummarized in [68], [119]. The requirement which is especially impor-tant for UMTS is the application of a signal scheme optimized withrespect to maximum significance for health risk assessment. Assum-ing the ELF spectral content to be important, highest significance isachieved with maximized spectral power of AM modulation compo-nents. Therefore, in addition to the development and dosimetry ofthe exposure system, the definition of a 3GPP compliant UMTS testsignal with maximized ELF components is an objective of this article.

8.2 Design of the Exposure System

8.2.1 Mechanical Design

The exposure setup design is an adaption of the sXc1800 system asreported in [109]. It is based on two short-circuited R18 waveguidesoperating at the frequency of 1950 MHz (Figure 8.1). The setup hasthe dimensions h x w x d = 450 x 250 x 580 mm3 and is placed inside anincubator, which provides the necessary environmental conditions forthe cell cultures (temperature, humidity, CO2/O2). Due to the 5 MHz


bandwidth of the WCDMA signal, it is not possible to use a resonatingcavity structure as for sXc1800 (3 dB resonance bandwidth: 2 MHz).Therefore, a commercial broadband coax-to-waveguide coupler wasused to excite the waveguides. Reflected power is terminated in a 50Ω load. Six 35 mm diameter Petri dishes are exposed per waveguidechamber. They are arranged on in three pairs and positioned in the H-field maxima of the standing waves (Figure 8.1). A dish holder ensuresthe correct position. The distances of the dish centers to the shortare 95 mm, 188 mm and 280 mm. As for sXc1800, tight exposure andenvironmental control is realized by field sensors (2.5 mm monopoleantennas), temperature sensors for the air environment, and by anoptimized air-flow system based on ventilators (Papst, 612NGHH: airflow 56m3/h) with a common inlet for both waveguides.

The voltage output of the sensor was calibrated to the square ofthe H-field inside the cavity. Since no field probes for WCDMA wereavailable, calibration of the monopole sensor was performed via trueRMS power measurements: First, the H-field as a function of inputpower was determined for an unmodulated exposure (CW) using anH3DV3 field probe (SPEAG). Then the sensor voltage as a functionof WCDMA input power was recorded and finally identified with theCW H-field at the same average power level.

8.2.2 Signal Unit

The objective for the signal unit is the generation of 3GPP compliantUMTS signal schemes. Additionally, the signal unit must provide (1)flexibility with respect to AM and WCDMA modulation schemes, (2)quality control by continuously monitoring the exposure and environ-ment and (3) application of blinded protocols. For that purpose, thesXc1800 signal unit [109] was upgraded with an UMTS signal gener-ator (Rhode & Schwarz, SMIQ02), enabling a full quadrature-phase-shift-keying (QPSK) modulated WCDMA signal. To simulate powercontrol of the handset, amplitude modulation is applied according toa transmit-power-control waveform stored on the arbitrary functiongenerator (Agilent, 33120A). Any waveform with a length of 16000points with a 12 bit resolution can applied. Similar to the sXc1800system, the data logger (Agilent, 34970A) continuously monitors allsensor signals (field values, air flow temperature, fan driving currents)

8.2. DESIGN OF THE EXPOSURE SYSTEM 111

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Figure 8.1: Mechanical and electronic design of the UMTS exposuresystem (all values in mm). Inner dimensions of the R18 waveguides:64.8 x 129.6 x 425 mm3 (height x width x length).

with a sampling rate of 0.1 Hz. A MS-Windows based user softwarecommunicates via GPIB connection with all devices and is able toself-detect malfunctions. Sensor data is stored within a file to providequality control. Blinded protocols are realized by a random decision


prior to exposure, which defines the state of the RF switch to exciteone of the two waveguides.

8.3 Dosimetry

Identical methods and tools as described in [109] have been applied tocharacterize the exposure of the UMTS setup: A 3D FDTD analysiswith high resolution numerical models (minimal voxel size: 0.3 mm)including details such as the meniscus at solid/liquid boundaries wasused to characterize average SAR levels and the uniformity of the SARdistribution. Four different models with cell medium volumes between2.1 - 4.9 ml (2, 3, 4, 5 mm liquid height) of DMEM medium (Dul-becco’s Modified Eagle’s Medium) were investigated. The simulationswere verified by field measurements with free space and dosimetricfield probes using the DASY3 near-field scanning system (SPEAG).SAR values along a vertical line in the center of the Petri dishes wereevaluated for verification of the simulations. Temperature measure-ments inside the media were carried out during exposure in orderto assess the thermal load experimentally. Measurements were per-formed at SAR values of about 50 W/kg resulting in a temperatureincrease of approximately 1C. The values were linearly scaled downwith the SAR and normalized to 1 W/kg average SAR.

The results for the dosimetric parameters are summarized in Table8.1. Data for SAR is normalized to the incident H2-field and to theinput power. High power efficiency of up to 17 W/kg for 1 W ofinput power is achieved. The nonuniformity of the SAR distributionfor cell monolayers was quantified by the ratio between the standarddeviation and the average SAR value of the lowest FDTD voxel layersof all six cell media. High uniformity with less than 26% deviationfrom the average was found.

The analysis for the uncertainty of the SAR assessment covers allparameters as reported for the sXc1800 system [109]. Numerical sim-ulation and SAR measurements show good agreement. The averagedeviation of 8.3% is well inside the range of the uncertainty of theSAR assessment of 18%. Additionally, less than 2% variability ofSAR during the experiments is present (Table 8.1).

The data of the measured temperature load indicate that experi-

8.3. DOSIMETRY 113

ments up to 4 W/kg cell monolayer averaged SAR can be performedwithout significant temperature load (< 0.1C). No temperature “hotspots” are generated within the medium as was numerically shown at1800 MHz [109]. This result can be transferred to 1950 MHz withhigh probability, because a similar geometry, SAR distribution, andfan cooling is present.


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8.4. UMTS SIGNAL 115

8.4 UMTS Signal

8.4.1 UMTS Handset Parameters Relevant for Bio-experiments

The discussion in this section concentrates on parameters for the up-link from the UMTS handset to the base station that are of particularrelevance to bioexperiments.UMTS Bands. The UMTS frequency allocations assigned by the In-ternational Telecommunication Union define a paired frequency bandfor the Frequency Division Duplex (FDD) mode at 1920 - 1980 MHzfor the uplink and 2110 - 2170 MHz for the downlink as well as twounpaired bands for the Time Division Duplex (TDD) mode at 1900 -1920 MHz and 2010 - 2025 MHz. At least in the beginning the FDDmode will dominate usage because the TDD mode will only be usedto complement FDD in hot spots [59].Spreading and Modulation. Details about the modulation and du-plexing are specified in the 3GPP TS 25.2xx series of specifications.Up to six dedicated physical data channels (bit rates 15 - 960 kbit/s)and one dedicated physical control channel (bit rate 15kbit/s) are I/Qmultiplexed and independently spread to the chip rate of 3.84 Mchip/swith orthogonal channelization codes. Power weighting, I/Q branchsumming and π/2 phase shifting yields to a complexly valued chipsequence that is additionally spread with either a long 38400 bit ora short 256 bit scrambling code. Afterwards, the complex chip se-quence is split into real and imaginary parts, pulse shaped and finallymodulated to the high frequency carrier. The short-term (i.e., highfrequency) variations in the UMTS signal envelope are mainly deter-mined by the chip rate and the QPSK modulation, which does notproduce a constant-envelope signal like in GSM, but power variationsof approximately 3 - 10 dB. With respect to the envisaged task fora signal with maximized power variations, different configurations fordata rates and channel numbers were analyzed, leading to maximumamplitude variations for six 960 kbit/s data channels.UMTS Frame Structure. The UMTS superframe (duration 720 ms)is divided into frames of 10 ms. Each frame is again subdivided into15 slots with a duration of 0.67 ms. Each slot consists of 2560 spreadsymbols (chips).


Power Control in the Handset. Power control is the main sourceof low frequency variations in UMTS. Two basic power control mecha-nisms are used, inner loop power control and open loop power control.Inner loop power control (also called fast closed loop power control)in the uplink is the ability of the handset to adjust its output powerin accordance with one or more Transmit Power Control (TPC) com-mands received in the downlink. The handset is capable of changingthe output power with a step size of 1, 2 and 3 dB, in the slot immedi-ately after the TPC command. The inner loop power control updaterate is 1500 Hz, i.e., the output power of the handset can be changedevery 0.67 ms. Open loop power control is the ability of the handset toset its output power to a specific value. It is only used for the physicalrandom access channel to set initial uplink transmission powers whenthe handset is accessing the network. Its tolerance is ± 9 dB (normalconditions) or ± 12 dB (extreme conditions). The open loop powercontrol works on a per-frame basis, i.e., the maximum update rate is100 Hz.Handovers and Compressed Mode. In contrast to GSM, a han-dover itself does not have a large influence on the output power; how-ever, it is one of the reasons for the so-called compressed mode that isnormally part of an UMTS system. The compressed mode is used toachieve transmission gaps in the downlink, uplink or both. In thesegaps the handset can search other frequencies or modes (e.g., a GSMnetwork). How often the compressed mode will be used depends on theoverall system implementation (e.g., combined UMTS/GSM network)and individual handset design (e.g., whether the handset is capable ofsimultaneous reception of GSM during an UMTS transmission). Forbioelectromagnetic experiments it is important to note that the poweris switched off completely for a minimum of 3 and a maximum of 7slots when the handset enters compressed mode, whereas in a discon-tinuous data transmission, the control channel is always transmitted.PRACH/PCPCH Channels. Physical random access (PRACH)and physical common packet (PCPCH) channels occur only temporar-ily but lead to pulsed signal transmissions. PRACH is used to set theinitial connection between the handset and base station and PCPCHfor packet-oriented services (e.g., SMS, MMS). PRACH and PCPCHprocedures are initialized by a repeated transmission of preambleswith a length of 4096 chips. When acknowledged from the base sta-

8.4. UMTS SIGNAL 117

tion, the data part of the message is transmitted, which has the lengthof some few frames.

8.4.2 UMTS Test Signal

While using an UMTS handset, an infinite number of exposure timecourses are possible. The rationale for the proposed UMTS test sig-nal is to provide a signal waveform which is highly relevant for healthrisk assessment. For that purpose it is not the objective to derive thestatistically most probable exposure scenario, but to define a signalscheme which leads to maximized ELF spectral components (assum-ing ELF to be relevant). This can be achieved with signals with a highpeak-to-average ratio. Additionally, absolute AM variations should bemaximized, because the higher the power gradients, the more spec-trum is involved. On the other hand the signal must be in compliancewith the 3GPP specifications.

Table 8.2 summarizes the settings for the proposed UMTS FDDmode test signal. The center frequency of the uplink of 1950 MHz isused as the transmit carrier frequency. Six 960 kbit/s data channelsare applied, because this configuration results to maximized short-term AM variations. The channel data is represented as 32 kbit (8.5ms) pseudo noise sequences. The scrambling code has only a low effecton the AM variations. A long code was chosen, because short codeswill only be applied for multiuser detector base stations.

The TPC waveform is composed of deep fades, compressed modeand a PRACH/PCPCH procedure (Figure 8.2). The TPC functionhas a periodicity of 1 s. Inner loop power controlled fades with adynamic range of 24 dB are composed as zigzag functions with a stepsize of 3 dB (maximum step size for open loop power control). Thefades are arranged within a 1 − 1/2n seconds scheme, n = 0, .., 6,leading to strong 1 Hz components (signal periodicity) but also tovarious higher components defined by the temporal distances betweenthe individual fades. A compressed mode of three slots occurs in everyfade, providing a sharp power gradient. PRACH/PCPCH channelwas simulated with a sequence consisting of two preambles togetherwith an open loop power controlled two frame data segment. Theresulting ELF and RF spectra are shown in Figure 8.3. The ELFspectrum was computed with an FFT analysis of the TPC envelope


High frequency parametersTransmit frequency 1950 MHzSignal bandwidth 5 MHzWCDMA parametersDuplex mode FDDChip rate 3.84 MHzNumber of data channels 6Data channel bit rate 960 kbit/sData source Pseudonoise, 215 bitData channel spreading codes C4,0 − C4,3

Data channel weighting factor 1Control channel structure Pilot, TFCI, FBI, TPC bitsTFCI, FBI, TPC all bits 1Control channel spreading code C256,0

Control channel weighting factor 1Scrambling code Clong,64000

Pulse shaping filter RRC, roll off 0.22Low frequency parametersSignal periodicity 1 sInner loop power control Step size 3 dBOpen loop power control Step size 12 dBFading dynamic range 24 dBFading zigzag functions every 1− 1/2n s (n = 0, .., 6)Envelope ratio peak/average 27.7Pulsed componentsCompressed mode sequence 3 slots idle, every fadePRACH/PCPCH sequence 2 preambles, 2 frames data part

Table 8.2: Specifications for the proposed UMTS test signal.

8.5. CONCLUSIONS 119

Figure 8.2: Transmit power control envelope in the time domain.

function. Spectral power is distributed over the entire ELF region andis composed as a sum of 1 Hz spectral components (Fig. 8.3a). TheRF spectrum was measured with a signal analyzer. As specified by3GPP, a spectral bandwidth of 5 MHz is present (Fig. 8.3b).

8.5 Conclusions

An exposure setup allowing the blinded exposure of cell monolayersto UMTS signal schemes was developed and dosimetrically analyzed.Cells can be exposed to up to 17 W/kg/W with less than 26% nonuni-


Figure 8.3: (a) Spectrum of the TPC envelope function, spectralpower is present in every 1 Hz component (peak amplitude of theenvelope corresponds to 1); (b) Signal bandwidth as measured with aspectrum analyzer.

formity of SAR. The temperature load for the exposed cells is lessthan 0.03C/(W/kg). The UMTS specifications have been analyzedin order to identify ELF spectral components in the signal. They

8.5. CONCLUSIONS 121

mainly result from inner loop power control; however, pulsed signalstructures due to compressed mode and PRACH/PCPCH proceduresalso contribute to the ELF components. A test signal is proposedwhich is compliant to the 3GPP FDD modulation specifications andis optimized for maximized ELF spectral power (1 Hz harmonics).

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UMTS In Vitro Exposure System and Test Signal for Health