Chapter 8. UMTS In Vitro Exposure System and Test Signal for Health Risk Research

Transcription

1 Chapter 8 UMTS In Vitro Exposure System and Test Signal for Health Risk Research 107

2 108 CHAPTER 8. UMTS IN VITRO SETUP SUMMARY A setup for the nonthermal exposure of cell monolayers to electromagnetic fields with UMTS signal schemes has been realized. The waveguide-based and computer-controlled exposure system has a power efficiency for SAR of up to 17 W/kg/W, a nonuniformity of the SAR distribution of less than 26% and provides a temperature load of less than 0.03 C per W/kg average SAR. In order to compose a UMTS test signal for health risk assessment, the UMTS signal specifications are analyzed with respect to low frequency power variations. A test signal for the frequency division duplex mode of UMTS is proposed, representing a scenario with maximized extremely low frequency spectral components. The signal is compliant with the 3GPP WCDMA modulation specifications and covers power controlled channel fading as well as pulsed structures due to compressed mode and physical random access channel. 8.1 Introduction A large majority of studies focussing on the health risk assessment of electromagnetic field exposure from mobile phones were performed in order to test current second generation 2G technologies such as the GSM system (Global System for Mobile Communications). Third generation (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 Telecommunication System). Its specifications have been created by the 3rd Generation Partnership Project (3GPP), who have drafted a UMTS standard in the year 2000 (3GPP Release 99, Although acceptance of first data services such as the mobile internet based on intermediate (so called 2.5G) standards like GPRS (General Packet Radio Service) has been well below expectations, UMTS will have a considerable market penetration in the near future and therefore needs to be assessed with respect to its safety.

3 8.2. DESIGN OF THE EXPOSURE SYSTEM 109 Although similarities between 2G and 3G with respect to carrier frequency and exposure strength are large, the low frequency spectral content 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) a power regulation with an update in the order of seconds [122]. In contrast, 3G WCDMA signals have (1) low coherence, (2) a broad-band ELF-AM spectrum resulting from the fast and dynamic power control (e.g., UMTS update rate: 1500 Hz) and (3) amplitude variations in the MHz range due to spreading. Since the low frequency amplitude variations are considered to have a potential for evoking biological responses, research with 3G signals is required in order to complement the data already gained with 2G. The objective of this study is the realization of an in vitro exposure system for the assessment of possible nonthermal biological effects from UMTS exposures. The requirements for such exposure setups are summarized in [68], [119]. The requirement which is especially important for UMTS is the application of a signal scheme optimized with respect to maximum significance for health risk assessment. Assuming the ELF spectral content to be important, highest significance is achieved with maximized spectral power of AM modulation components. Therefore, in addition to the development and dosimetry of the exposure system, the definition of a 3GPP compliant UMTS test signal with maximized ELF components is an objective of this article. 8.2 Design of the Exposure System Mechanical Design The exposure setup design is an adaption of the sxc1800 system as reported in [109]. It is based on two short-circuited R18 waveguides operating at the frequency of 1950 MHz (Figure 8.1). The setup has the dimensions h x w x d = 450 x 250 x 580 mm 3 and is placed inside an incubator, which provides the necessary environmental conditions for the cell cultures (temperature, humidity, CO 2 /O 2 ). Due to the 5 MHz

4 110 CHAPTER 8. UMTS IN VITRO SETUP bandwidth of the WCDMA signal, it is not possible to use a resonating cavity structure as for sxc1800 (3 db resonance bandwidth: 2 MHz). Therefore, a commercial broadband coax-to-waveguide coupler was used to excite the waveguides. Reflected power is terminated in a 50 Ω load. Six 35 mm diameter Petri dishes are exposed per waveguide chamber. They are arranged on in three pairs and positioned in the H- field maxima of the standing waves (Figure 8.1). A dish holder ensures the correct position. The distances of the dish centers to the short are 95 mm, 188 mm and 280 mm. As for sxc1800, tight exposure and environmental control is realized by field sensors (2.5 mm monopole antennas), temperature sensors for the air environment, and by an optimized air-flow system based on ventilators (Papst, 612NGHH: air flow 56m 3 /h) with a common inlet for both waveguides. The voltage output of the sensor was calibrated to the square of the H-field inside the cavity. Since no field probes for WCDMA were available, calibration of the monopole sensor was performed via true RMS power measurements: First, the H-field as a function of input power was determined for an unmodulated exposure (CW) using an H3DV3 field probe (SPEAG). Then the sensor voltage as a function of WCDMA input power was recorded and finally identified with the CW H-field at the same average power level Signal Unit The objective for the signal unit is the generation of 3GPP compliant UMTS 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 environment and (3) application of blinded protocols. For that purpose, the sxc1800 signal unit [109] was upgraded with an UMTS signal generator (Rhode & Schwarz, SMIQ02), enabling a full quadrature-phaseshift-keying (QPSK) modulated WCDMA signal. To simulate power control of the handset, amplitude modulation is applied according to a transmit-power-control waveform stored on the arbitrary function generator (Agilent, 33120A). Any waveform with a length of points with a 12 bit resolution can applied. Similar to the sxc1800 system, the data logger (Agilent, 34970A) continuously monitors all sensor signals (field values, air flow temperature, fan driving currents)

5 8.2. DESIGN OF THE EXPOSURE SYSTEM 111 Ω Figure 8.1: Mechanical and electronic design of the UMTS exposure system (all values in mm). Inner dimensions of the R18 waveguides: 64.8 x x 425 mm 3 (height x width x length). with a sampling rate of 0.1 Hz. A MS-Windows based user software communicates via GPIB connection with all devices and is able to self-detect malfunctions. Sensor data is stored within a file to provide quality control. Blinded protocols are realized by a random decision

6 112 CHAPTER 8. UMTS IN VITRO SETUP prior to exposure, which defines the state of the RF switch to excite one of the two waveguides. 8.3 Dosimetry Identical methods and tools as described in [109] have been applied to characterize the exposure of the UMTS setup: A 3D FDTD analysis with high resolution numerical models (minimal voxel size: 0.3 mm) including details such as the meniscus at solid/liquid boundaries was used to characterize average SAR levels and the uniformity of the SAR distribution. Four different models with cell medium volumes between ml (2, 3, 4, 5 mm liquid height) of DMEM medium (Dulbecco s Modified Eagle s Medium) were investigated. The simulations were verified by field measurements with free space and dosimetric field probes using the DASY3 near-field scanning system (SPEAG). SAR values along a vertical line in the center of the Petri dishes were evaluated for verification of the simulations. Temperature measurements inside the media were carried out during exposure in order to assess the thermal load experimentally. Measurements were performed at SAR values of about 50 W/kg resulting in a temperature increase of approximately 1 C. The values were linearly scaled down with the SAR and normalized to 1 W/kg average SAR. The results for the dosimetric parameters are summarized in Table 8.1. Data for SAR is normalized to the incident H 2 -field and to the input power. High power efficiency of up to 17 W/kg for 1 W of input power is achieved. The nonuniformity of the SAR distribution for cell monolayers was quantified by the ratio between the standard deviation and the average SAR value of the lowest FDTD voxel layers of all six cell media. High uniformity with less than 26% deviation from the average was found. The analysis for the uncertainty of the SAR assessment covers all parameters as reported for the sxc1800 system [109]. Numerical simulation and SAR measurements show good agreement. The average deviation of 8.3% is well inside the range of the uncertainty of the SAR assessment of 18%. Additionally, less than 2% variability of SAR during the experiments is present (Table 8.1). The data of the measured temperature load indicate that experi-

7 8.3. DOSIMETRY 113 ments up to 4 W/kg cell monolayer averaged SAR can be performed without significant temperature load (< 0.1 C). No temperature hot spots are generated within the medium as was numerically shown at 1800 MHz [109]. This result can be transferred to 1950 MHz with high probability, because a similar geometry, SAR distribution, and fan cooling is present.

8 114 CHAPTER 8. UMTS IN VITRO SETUP Medium volume of numerical model a 2.1 ml 3.1 ml 3.8 ml 4.9 ml Average SAR [W/kg/(A 2 /m 2 )ref ] b Average SAR per W input power [W/kg/W ] Average impedance [Ω] Nonuniformity of SAR [%] Combined relative uncertainty of SAR assessment [%] c 18 Deviation between measurement and simulation [%] 8.3 Combined relative variability of SAR [%] d 1.9 Temperature load [ C/(W/kg)] a A relative permittivity of εr = 76 and conductivity of σ = 2.3 S/m for cell medium was used in the simulation. For all plastic parts εr = 3, σ = 0 S/m was applied. b Data is normalized to the H-field at the reference position (center of the short). c The following sources of uncertainty are considered: fit for extrapolation to monolayer, fit for varying medium height, vertical location of cells (< 0.1 mm from bottom), numerical discretization (0.1 mm 3 reference), determination of medium volume (± 5µl), dielectric parameters, E-field probe, probe positioning, incident field assessment. d The following sources of variability are considered: determination of medium volume (± 5µl), dish holder misplacement (± 2 mm), incident field assessment, drift Table 8.1: Dosimetric data for the UMTS exposure system.

9 8.4. UMTS SIGNAL UMTS Signal UMTS Handset Parameters Relevant for Bioexperiments The discussion in this section concentrates on parameters for the uplink from the UMTS handset to the base station that are of particular relevance to bioexperiments. UMTS Bands. The UMTS frequency allocations assigned by the International Telecommunication Union define a paired frequency band for the Frequency Division Duplex (FDD) mode at MHz for the uplink and MHz for the downlink as well as two unpaired bands for the Time Division Duplex (TDD) mode at MHz and MHz. At least in the beginning the FDD mode will dominate usage because the TDD mode will only be used to complement FDD in hot spots [59]. Spreading and Modulation. Details about the modulation and duplexing are specified in the 3GPP TS 25.2xx series of specifications. Up to six dedicated physical data channels (bit rates kbit/s) and one dedicated physical control channel (bit rate 15kbit/s) are I/Q multiplexed and independently spread to the chip rate of 3.84 Mchip/s with orthogonal channelization codes. Power weighting, I/Q branch summing and π/2 phase shifting yields to a complexly valued chip sequence that is additionally spread with either a long bit or a short 256 bit scrambling code. Afterwards, the complex chip sequence is split into real and imaginary parts, pulse shaped and finally modulated to the high frequency carrier. The short-term (i.e., high frequency) variations in the UMTS signal envelope are mainly determined by the chip rate and the QPSK modulation, which does not produce a constant-envelope signal like in GSM, but power variations of approximately 3-10 db. With respect to the envisaged task for a signal with maximized power variations, different configurations for data rates and channel numbers were analyzed, leading to maximum amplitude 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 into 15 slots with a duration of 0.67 ms. Each slot consists of 2560 spread symbols (chips).

10 116 CHAPTER 8. UMTS IN VITRO SETUP Power Control in the Handset. Power control is the main source of low frequency variations in UMTS. Two basic power control mechanisms 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 power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink. The handset is capable of changing the output power with a step size of 1, 2 and 3 db, in the slot immediately after the TPC command. The inner loop power control update rate is 1500 Hz, i.e., the output power of the handset can be changed every 0.67 ms. Open loop power control is the ability of the handset to set its output power to a specific value. It is only used for the physical random access channel to set initial uplink transmission powers when the handset is accessing the network. Its tolerance is ± 9 db (normal conditions) or ± 12 db (extreme conditions). The open loop power control works on a per-frame basis, i.e., the maximum update rate is 100 Hz. Handovers and Compressed Mode. In contrast to GSM, a handover itself does not have a large influence on the output power; however, it is one of the reasons for the so-called compressed mode that is normally part of an UMTS system. The compressed mode is used to achieve transmission gaps in the downlink, uplink or both. In these gaps the handset can search other frequencies or modes (e.g., a GSM network). How often the compressed mode will be used depends on the overall system implementation (e.g., combined UMTS/GSM network) and individual handset design (e.g., whether the handset is capable of simultaneous reception of GSM during an UMTS transmission). For bioelectromagnetic experiments it is important to note that the power is switched off completely for a minimum of 3 and a maximum of 7 slots when the handset enters compressed mode, whereas in a discontinuous data transmission, the control channel is always transmitted. PRACH/PCPCH Channels. Physical random access (PRACH) and physical common packet (PCPCH) channels occur only temporarily but lead to pulsed signal transmissions. PRACH is used to set the initial connection between the handset and base station and PCPCH for packet-oriented services (e.g., SMS, MMS). PRACH and PCPCH procedures are initialized by a repeated transmission of preambles with a length of 4096 chips. When acknowledged from the base sta-

11 8.4. UMTS SIGNAL 117 tion, the data part of the message is transmitted, which has the length of some few frames UMTS Test Signal While using an UMTS handset, an infinite number of exposure time courses are possible. The rationale for the proposed UMTS test signal is to provide a signal waveform which is highly relevant for health risk assessment. For that purpose it is not the objective to derive the statistically most probable exposure scenario, but to define a signal scheme which leads to maximized ELF spectral components (assuming ELF to be relevant). This can be achieved with signals with a high peak-to-average ratio. Additionally, absolute AM variations should be maximized, because the higher the power gradients, the more spectrum is involved. On the other hand the signal must be in compliance with the 3GPP specifications. Table 8.2 summarizes the settings for the proposed UMTS FDD mode test signal. The center frequency of the uplink of 1950 MHz is used as the transmit carrier frequency. Six 960 kbit/s data channels are applied, because this configuration results to maximized shortterm AM variations. The channel data is represented as 32 kbit (8.5 ms) pseudo noise sequences. The scrambling code has only a low effect on the AM variations. A long code was chosen, because short codes will only be applied for multiuser detector base stations. The TPC waveform is composed of deep fades, compressed mode and a PRACH/PCPCH procedure (Figure 8.2). The TPC function has a periodicity of 1 s. Inner loop power controlled fades with a dynamic range of 24 db are composed as zigzag functions with a step size of 3 db (maximum step size for open loop power control). The fades are arranged within a 1 1/2 n seconds scheme, n = 0,.., 6, leading to strong 1 Hz components (signal periodicity) but also to various higher components defined by the temporal distances between the individual fades. A compressed mode of three slots occurs in every fade, providing a sharp power gradient. PRACH/PCPCH channel was simulated with a sequence consisting of two preambles together with an open loop power controlled two frame data segment. The resulting ELF and RF spectra are shown in Figure 8.3. The ELF spectrum was computed with an FFT analysis of the TPC envelope

12 118 CHAPTER 8. UMTS IN VITRO SETUP High frequency parameters Transmit frequency 1950 MHz Signal bandwidth 5 MHz WCDMA parameters Duplex mode FDD Chip rate 3.84 MHz Number of data channels 6 Data channel bit rate 960 kbit/s Data source Pseudonoise, 2 15 bit Data channel spreading codes C 4,0 C 4,3 Data channel weighting factor 1 Control channel structure Pilot, TFCI, FBI, TPC bits TFCI, FBI, TPC all bits 1 Control channel spreading code C 256,0 Control channel weighting factor 1 Scrambling code C long,64000 Pulse shaping filter RRC, roll off 0.22 Low frequency parameters Signal periodicity 1 s Inner loop power control Step size 3 db Open loop power control Step size 12 db Fading dynamic range 24 db Fading zigzag functions every 1 1/2 n s (n = 0,.., 6) Envelope ratio peak/average 27.7 Pulsed components Compressed mode sequence 3 slots idle, every fade PRACH/PCPCH sequence 2 preambles, 2 frames data part Table 8.2: Specifications for the proposed UMTS test signal.

13 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 and is composed as a sum of 1 Hz spectral components (Fig. 8.3a). The RF spectrum was measured with a signal analyzer. As specified by 3GPP, a spectral bandwidth of 5 MHz is present (Fig. 8.3b). 8.5 Conclusions An exposure setup allowing the blinded exposure of cell monolayers to UMTS signal schemes was developed and dosimetrically analyzed. Cells can be exposed to up to 17 W/kg/W with less than 26% nonuni-

14 120 CHAPTER 8. UMTS IN VITRO SETUP Figure 8.3: (a) Spectrum of the TPC envelope function, spectral power is present in every 1 Hz component (peak amplitude of the envelope corresponds to 1); (b) Signal bandwidth as measured with a spectrum analyzer. formity of SAR. The temperature load for the exposed cells is less than 0.03 C/(W/kg). The UMTS specifications have been analyzed in order to identify ELF spectral components in the signal. They

15 8.5. CONCLUSIONS 121 mainly result from inner loop power control; however, pulsed signal structures due to compressed mode and PRACH/PCPCH procedures also contribute to the ELF components. A test signal is proposed which is compliant to the 3GPP FDD modulation specifications and is optimized for maximized ELF spectral power (1 Hz harmonics).