exposure aspects of new and evolving wireless systems

Exposure Aspects of New and Evolving Wireless Systems

A Report for the GSM Association

J. Bach Andersen, P. E. Mogensen, G. Frølund Pedersen Department of Electronic Systems Aalborg University, Denmark

August 2007

Disclaimer: It is your responsibility to research the accuracy, completeness, and usefulness of all opinions and other information found in this paper. The GSM Association assumes no responsibility or liability for any consequence resulting directly or indirectly for any action or inaction you take based on or made in reliance on the information or material contained herein. Independent medical, technical and/or legal advice should be taken.

Foreword Health aspects of the use of mobile phones and other radiating systems continue to be a public concern globally. This report is a strictly technical document, collecting what is presently available information related to exposures, or dosimetry, and does not consider whether these exposures have any adverse biological or health related effect. The report has been produced for the GSMA at the request of Dr Jack Rowley, Director Research & Sustainability. Aalborg, August 2007

J. Bach Andersen, P. E. Mogensen, G. Frølund Pedersen

GSM Association Exposure Aspects of New and Evolving Wireless Systems i

GSM Association Exposure Aspects of New and Evolving Wireless Systems ii

Executive Summary

Exposure Aspects of New and Evolving Wireless Systems A Report for the GSM Association J. Bach Andersen, P. E. Mogensen, G. Frølund Pedersen Department of Electronic Systems Aalborg University, Denmark

This report is a strictly technical document, collecting presently available information related to exposures, or dosimetry, and does not consider whether these exposures have any adverse biological or health related effect. Experimental results are included where possible; especially the spectrum of the power fluctuations is seen as a convenient and compact way of describing very complex system behavior.

The most important parameter is obviously the power density of the incident field or the SAR (Specific Absorption Rate) values in the case of a mobile phone. These parameters are independent of the particular communication system used and the exposure from the handset is orders of magnitude larger than the typical exposure from the antenna masts.

From a spectral point of view GSM is unique since it uses a so‐called constant envelope modulation. Power control regulates the output power of the handset to the minimum needed value for effective communications and this is the main source of power fluctuations when the mobile is moving around in a fading environment. It means that the actual mean power level may be much lower than the maximum and has been reported as 62 mW.

In contrast GPRS is a packet switched system which stops transmitting when there are no packets to send. This gives an overall increase in capacity and allows for multi‐user access on a random basis. From an exposure point of view there are several changes. The transmissions are more bursty and intermittent with possible time periods with no power.

A further step for enhancing the data rates is taken by introducing EDGE (Enhanced Data rates for GSM Evolution). EDGE achieves higher performance by adapting to the channel conditions: if the signal is strong higher order modulation schemes and coding are introduced, and when the signal is weak it falls back to the basic GSM data rates. The higher order modulations are no longer constant envelope, so the power will now fluctuate with the basic bit rate around 270 kb/s.

A major change in technology came about with the switch to 3rd generation WCDMA systems. The main features are: multiple access is controlled by code division; rapid power control at 1500 Hz; non‐constant envelope modulation and generally higher carrier frequencies. Since fast power control of the handset by the base station tends to compensate for channel variations due to movement, the actual power variations depend on the velocity of the user and the environment. For a small cell radius (667 m) only a very small fraction, less than 1 %, of the terminals transmit

GSM Association Exposure Aspects of New and Evolving Wireless Systems iii

with the maximum power of 125 mW, the mean value being around 0.25 mW. For a larger cell radius (2000 m) about 20% of the users reach the upper transmit power limit, and the average power level is around 10% of the maximum (12.5 mW). In contrast to GSM there are also high frequency variations of the power due to the modulation and the rapid code modulation. The power level at the base station transmitters is expected to be limited to 20 W per carrier with a target power level of 10 ‐15 W as typical for full traffic. This includes a constant control (pilot) channel of 1W. The total power to many simultaneous users will be slowly varying, but it will have a peaky nature. The WCDMA system is characterized by both handling speech and data traffic and the distribution of the power is different for the two cases. The target value is higher (14 W) but the mean level is much lower for the data traffic than for speech.

HSPA (High Speed Packet Access) pushes the data rate to 1‐2 Mb/s in practice and even beyond 3 Mb/s under good conditions. This is done by using adaptive modulation and coding and shorter frame time. Downlink HSPA (HSDPA) relies on a shared channel concept, where user specific power control is substituted with fast adaptive modulation and coding. Thus, HSDPA introduces a pseudo‐TDMA mode into the WCDMA system to make the system more efficient for data. For a typical macro cellular scenario with only HSDPA enabled the total output transmit power for each cell is fairly constant and close to the maximum available power if there are data to transmit. However, with no data to transmit the data channels are switched off and in the worst case there could be large power fluctuations of approximately 15 W magnitudes at about 500 Hz. HSDPA can also be used in co‐existence with WCDMA dedicated channels, with the available power and codes dynamically shared implying that the output power is constantly close to the maximum. Uplink HSPA (HSUPA) enhances the peak data rates to 5.76 Mbps, where the peak data rate of most uplink WCDMA connections is limited to 384 kbps. The higher bit rates are obtained by having multiple codes with low spreading factors. Simulations of HSUPA with small and large cells sizes at different traffic loads show that the cell size has a larger impact on the device output power than the traffic. For a small cell radius (500 m) around 4% of devices transmit with maximum power, with a larger cell radius (2500 m) up to 40 % transmit at full power.

There are many types of WLANs under the IEEE 802.11 umbrella and we consider only the most common 802.11b (WiFi). The carrier is mostly in the unlicensed ISM band at 2.4 GHz and the modulation is spread spectrum with output power about 30 mW, max. 100 mW. Considering duty cycle and other factors, measurements have shown typical exposure levels around 105 times below international exposure guidelines. Power fluctuations measured near a laptop while uploading files show a somewhat random frame access with a local broad peak around 500 Hz.

WIMAX stands for Worldwide Interoperability for Microwave Access. The technology is described in IEEE standard 802.16 and the intention is to fill the gap between very high data rate WLANs and very high mobility cellular systems. The carrier frequencies include both licensed and unlicensed bands; typical frequencies are in the 2‐4 GHz region. The standard allows both TDD with burst transmissions and normal FDD. The modulation may also vary depending on the system, but is typically OFDM (orthogonal frequency division modulation). OFDM is characterized by a rather high peak to mean value. For a downlink carrier at 3.5 GHz and the spectrum shows a widespread signal in the MHz region with a peak near 4MHz.

Concluding, we should emphasize that the time variations may not have a significant biological effect, but they are part of many experimental studies, so it is important to know the details.

(August 2007)

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Exposure Aspects of New and Evolving Wireless Systems

Page

Abstract 1 1. Introduction 3 2. Relevant signal features for uplink and downlink 4

2.1 Power density and SAR 42.2 Modulation and access techniques 52.3 Measurements and signal analysis 5

3. Digital communications systems 73.1 GSM, Global System for Mobile Communications 73.2 GSM evolutions, GPRS and EDGE 83.3 WCDMA (UMTS) 93.4 HSPA 173.5 WLAN 233.6 WIMAX 26

Discussion 28 Acknowledgements 29 References 29 Appendix – List of Acronyms 30

GSM Association Exposure Aspects of New and Evolving Wireless Systems v

GSM Association Exposure Aspects of New and Evolving Wireless Systems vi

Abstract The use of modern digital communication devices like GSM, WCDMA, HSPA, WIMAX and WiFi changes the exposure of electromagnetic waves towards the users, be it from handsets or base station antennas. Assuming that the power level and its variation on a slow and fast time scale are the important parameters, these new systems are discussed. Experimental results are included where possible; especially the spectrum of the power fluctuations is seen as a convenient and compact way of describing very complex system behavior. The results should be of interest for scientific studies in epidemiology and biological effects.

GSM Association Exposure Aspects of New and Evolving Wireless Systems 1

1 - Introduction Widespread public concern about possible health risks related to radiofrequency (RF) energy emanating from portable devices or base stations still motivates scientific studies. These studies may be related to human experimental exposure effects, epidemiological studies, animal studies or in‐vitro cell studies. Many studies use aspects of the true wireless systems, simplified for practical reasons to contain as few parameters as possible. A dominant parameter for mobile phone use is the SAR value, related to the energy content of the radiation, or in cases where this is impractical the incident power density. It is still an open question whether the energy (time‐averaged) is the only active ingredient in the signal or whether there could be other signal dependent parameters which might have a biological significance, so‐called athermal effects [1]. It is assumed here that mean power and time variations of the power are the only parameters of interest. Some biological mechanisms have been suggested by Balzano [2]. At the same time technology keeps evolving with new advanced wireless systems mainly with the purpose of increasing the data rates to the user. The evolution is away from simple telephony towards transport of data files with hand‐held devices, which creates quite a different local exposure. The purpose of this paper is to give the possible exposure aspects of some modern, digital wireless systems, where the parameters include the average power, and its time variations. The time variations might also include intermittent use, which decreases the average exposure. The results should be relevant for experimental biological studies and epidemiology. The paper is an updated version of a report by the authors from 2001 [3]. The paper is organized as follows. In section 2 there is a more detailed discussion of the relevant technical parameters, followed by an analysis in section 3 of the most important digital technologies, starting with GSM (a list of acronyms is given as an appendix) and other 2nd generation systems. Third generation systems are now in widespread use and apart from WCDMA (also known as UMTS in Europe) its follow‐up HSPA (high speed packet access) is also discussed. To a lesser extent also WiFi and WIMAX are treated.


2 - Relevant signal features for uplink and downlink 2.1 Power density and SAR The most important parameter is obviously the power density of the incident field or the SAR (Specific Absorption Rate) values in the case of a mobile phone. ICNIRP [4] states reference levels for the power density for general public exposure for frequencies above 2GHz as 10 W/m2, and basic restrictions for localized SAR for the head as 2W/kg. In the ICNIRP guidelines the SAR values are averaged over a 6 minute period and over any 10 g of contiguous tissue. These parameters are independent of the particular communication system used, so some general conclusions can be made. Figure 1a gives an overview picture of power density versus distance from a typical base station antenna mast with one transmitter with 20 W transmit power. This is the downlink situation in contrast to the uplink situation, where the transmitter is at the user. The curve is for the situation where the receiver is in the main beam of the antenna, assumed to have a gain of 17 dB. In practice the antenna is often tilted downwards and the presence of buildings and other obstructions gives an additional attenuation of the power. After 10 meter we are in the far field where the power falls off as the square of the distance (in free space). Closer than 10 meter the power fluctuates, an effect which is ignored here. It is noted that at a distance of 200 meters the power density is one thousand times lower than the recommended limit of 10 W/m2, and a person must be a few meters from the antenna before the level reaches significant levels. The curve is for one transmitter, and in case of multiple transmitters at the same site, the powers should be added. For a given frequency it is possible to transform the incident power density to SAR values in the head ignoring that the values depend on the anatomy and direction of incidence. This is done in Figure 1b where it is then possible to compare with the uplink situation with SAR values for a range of mobile phones (upper left corner). The figure illustrates clearly the well‐known fact that the exposure from the telephone is orders of magnitude larger than the typical exposure from the antenna masts. The lower curve in both figures is from a mobile phone in free space approximated as a dipole. At a distance of 2 meters the exposure is the same as at a distance of 200 meters from a base station antenna. The physical significance of the SAR concept is that it is the source of heating. This may be of significance in the mobile phone case, but is insignificant in the base station case. Thus it is relevant to study other possible aspects of the radiation, and theoretically there is a possibility of nonlinear detection such that low frequency power fluctuations generate low frequency currents in the tissue. This is the reason for the interest in power fluctuations in the following discussions, where the spectrum of the power is shown for the various communication systems. It is appropriate to note that in the communications community the power spectrum is the usual measure of the occupied spectrum around the carrier, but the two measures are different (technically speaking the power spectrum is the Fourier transform of the autocorrelation of the signal, while the spectrum of the power is the Fourier transform of the square of the signal). Considering the complexity of modern systems it is recommended that future studies include the spectrum of the power variations.


2.2 Modulation and access techniques A few technical terms are needed for a better understanding of the following. The digital signals, be they speech or data, are modifying the high frequency carrier by the process of modulation. Typically the phase of the signal is carrying the information, like in the common BPSK or QPSK (binary or quadrature phase shift keying) methods, although recently also combinations of phase and magnitude are used as in QAM (quadrature amplitude modulation). A novel modulation/access method is OFDM, which is a multi‐carrier technique, where the spectrum is divided into a large number of narrowband channels, which may be distributed among the symbols or users. Note that the process of modulation does not introduce low frequency components of the signal, only a broadening of the spectrum around the carrier frequency. The low frequency components appear only after a nonlinear process as discussed above. The access techniques are the means of having multiple users at the same time without disturbing each other. Examples are TDMA (time division multiple access) where the different users have different time slots, FDMA (frequency division multiple access) for different frequency bands, and CDMA (code division multiple access) with separate codes for the different users. A code is an overlay modulation with a user specific set of digits. Modern systems are often combinations of these techniques. Finally, it is worth mentioning how the systems differentiate between uplink (from the user to the base station) and downlink (from the base station to the user). In the TDD (time division duplex) case part of the time is used for uplink, part of the time for downlink, and in the FDD (frequency division duplex) case different frequency bands are used. 2.3 Measurements and signal analysis For some systems it has been possible to measure the power fluctuations for later processing. The power is measured with a Spectrum Analyzer, in zero span, and center frequency at the wanted frequency. The bandwidth and sweep time is set for the wanted system. The antenna is placed close by the Device Under Test (DUT), to make sure it is the only one measured. The channel is kept as steady as possible, i.e. no movement nearby during the test, except where this is wanted. The 32000 samples are then used in a Fourier transform for generation of the spectra of the power variations. A suitable anti‐aliasing filter is applied.


Figure 1. a) Intensity from base station antenna and from mobile phone as a function of distance. b) Corresponding SAR values for 2100 MHz.


3 - Digital communications systems 3.1 GSM, Global System for Mobile Communications The GSM system is an FDD system operating in the 900 and 1800 MHz bands (850 and 1900 MHz in US). It is a TDMA system with each user occupying 1 out of 8 time slots, each time slot having a length of 4.615/8 = 0.58 ms. This means it is a periodic power burst mode with a line spectrum of 217 Hz (1000/4.615) and its harmonics (measurements in Figure 2. There is a further source of periodicity due to the management of the system. A frame is 8 time slots, and a multiframe is 26 frames; for each multiframe one frame is missing. This creates a frequency of 8 Hz, which is also seen as the ‘grass’ in Figure 2a (uplink). A random sample of the downlink case with multiple users is shown in Figure 2b, the frequency components are the same, but the relative strengths are different. The peak power from a handset may take on several values; the typical case is 2 W, leading to a mean power of 250 mW. Power control however regulates the output power to the needed minimum value for effective communications, and this is the main source of power fluctuations, when the mobile is moving around in a fading environment. Another mechanism reducing the output power is DTX (discontinuous transmission) which turns the power off and adds noise in period when the user is not speaking. The DTX mode introduces a 2 Hz periodicity. Overall, it means that the actual mean power level may be much lower than the maximum. Wiart et al [5] find that on average under a mixture of situations the mean power is 62 mW. From a spectral point of view GSM is unique since it uses a so‐called constant envelope modulation, the power is constant when transmitting. This is in contrast to all the later systems to be discussed. Figure 2a. Spectrum of power for speech measured from a mobile phone over a 1 sec time span.


Figure 2b. Spectrum of power measured near base station over a 0.5 s time span. 3.2 GSM evolutions, GPRS and EDGE The basic data rate for a user is only 9.6 kb/s, but GSM was designed originally as a speech system. It became clear though that there was a need for data transmission, internet access et cetera, which has been driving all further developments. The first modification of GSM, generation 2.5, was GPRS (General Packet Radio Service), which is fundamentally different from the basic GSM setting up of a fixed connection between two users. In contrast GPRS is a packet switched system which stops transmitting when there are no packets to send. This gives an overall increase in capacity and allows for multi‐user access on a random basis. The way the data rate is increased is by using multiple slots, possibly in an asymmetric way, different in uplink and downlink. GPRS offers typically data rates up to approximately 40‐50 kb/s in downlink. From an exposure point of view there are several changes. The transmissions are more bursty and intermittent with possible time periods with no power. When the transmission is on the power increases proportional to the number of time slots, so with 4 slots active, the mean power reaches 1 W. It should also be realized that the user terminal will not necessarily be close to the head when in the GPRS mode, more likely some distance from the body which reduces the SAR values significantly. The basic timing structure with the 217 Hz is unchanged. A further step for enhancing the data rates is taken by introducing EDGE (Enhanced Data rates for GSM Evolution). EDGE achieves its higher performance by adapting to the channel conditions; if the signal is strong higher order modulation schemes and coding are introduced, and when the signal is weak it falls back to the basic GSM data rates. The higher order modulations (8 PSK) are no longer constant envelope, so the power will now fluctuate with the basic bit rate around 270 kb/s, which is a new feature. Applying multiple slots like GPRS EDGE can achieve data rates around 200 kb/s. The standard bodies have defined further improvements in EDGE Evolution by using even higher modulation schemes like 16QAM and 32 QAM. Like the ordinary EDGE good signal strengths are required.


3.3 WCDMA (UMTS) A major change in technology came about with the switch to 3rd generation systems. The main features are:

• multiple access is controlled by code division • rapid power control at 1500 Hz • non‐constant envelope modulation • higher carrier frequencies with some exceptions

In the following section these features will be addressed in greater detail. In theory, both an FDD and a TDD mode is a possibility, but only the FDD mode will be treated here. The FDD mode uses the following frequencies in Europe:

• Uplink: 1920‐1980 MHz • Downlink: 2110‐2170 MHz

Recently, lower frequency licensed bands have been allowed in different countries; These are in the 450, 800 and 900 MHz bands, and refarming from analogue television channels may also occur. The 60 MHz uplink band is divided into 5 MHz bands, where the power is in principle on all the time. The handset power is controlled by the base station in a closed loop power control, ensuring that the received power is at the correct and constant level. The steps of the power control is 1, 2 or 3 dB, and the frequency is 1500 Hz. Since this fast power control tends to compensate for the channel variations due to movement, the actual power variations in the handset depend on the velocity of the user and the environment. In general there will be a 1500 Hz component in the spectrum of the power variations, and a wider continuous spectrum at lower frequencies. The CDMA access is performed by multiplying the bit sequence with a higher frequency code sequence, in this case with a so‐called chip frequency of 3.84 MHz, and the modulation is QPSK. The frequency spreading is advantageous from an exposure point of view, since the related gain reduces the handset power significantly, as shown later. The final user bit rate may reach 2 Mb/s, but more typical values are in the 200‐300 kb/s range. Uplink The nominal maximum output power for a power class 4 terminal (or UE, user equipment in technical language) is 125 mW. The true output power depends on the requested service, the network load and the range of the cell. Two simulated examples are given in Figure 3a and 3b. The figures show the distribution of mobile transmission power for a cell radius of 667 and 2000 m, respectively. Other simulation parameters are: VehicularA channel model, a 64 kb/s link, NRtarget1 of 4 dB, and a mobile speed of 50 km/h. For the small cell range only a very small 1 NRtarget , Noise Raise, an uplink network planning parameter.


fraction, less than 1 %, of the terminals transmit with the maximum power of 125 mW (21 dBm), the mean value being around –6.5 dBm (~1/4 mW). For the larger cell range about 20% of the users reach the upper transmit power limit, and the average power level being around 10% of the maximum. This shows that over the population of users in the network the transmit power level is far below the maximum value for a majority of the users, but there will always be a few users at the edge of the cells where the terminal is operating at maximum transmit power level. The mean power level is also much lower than the GSM value of 62 mW. Figure 3a. Distribution of mobile transmit power for a cell size of 667 m. For other parameters, see text. Figure 3b. Distribution of mobile transmit power for a cell size of 2000 m. For other parameters, see text.


In contrast to GSM there are now high frequency variations of the power due to the modulation and the rapid code modulation. This is seen in the experimental results of Figure 4, where a terminal is fixed (non moving) while browsing on the internet. The frequency of 3.84 MHz is clearly seen, but there is also a more random broad spectrum with a maximum around 2 MHz.

Figure 4. Spectrum of the power from a handset while browsing. Time span 0.5 ms. WCDMA.

The lower frequency spectrum is shown in Figure 5 with no special features. Note the absence of the 1.5 kHz control signal, probably due to the stationarity of the handset.

Figure 5. Spectrum of the power from a handset while browsing. Time span 64 ms. WCDMA.


Downlink The downlink case of transmission to many users is complicated by the unknown traffic distribution. Seen as a total transmission towards the general public the issue is also complicated by co‐siting by different operators. A worst case and an average case may be estimated. Since a user may be in contact with several base stations simultaneously, this must also be taken into account in estimating the total exposure.

The power level at the base station transmitters is expected to be limited to 43 dBm, 20 W per carrier. There may likely be 1‐2 carriers per operator, and maybe several operators on one site. A target power level of 10 ‐15 W is typical for full traffic, and this includes a constant control (pilot) channel of 10%, i.e. 1W. The power control to the individual user is similar to the one above for the handsets with a frequency of 1500 Hz. The total power to many simultaneous users will be slowly varying, but it will have a peaky nature. The situation is illustrated in Figures 6a and b. Figure 6a shows the total mean power from one carrier as a function of time, averaged over 100 ms, as 64kbit/s circuit users are added and removed randomly. The mean power fluctuates around 10 W with a standard deviation of 3 W. The upper curve in the figure is the number of users. Figure 6b shows the instantaneous transmit power (without time averaging of 100ms). It can be observed that the instantaneous variations are much higher than shown in Figure 6a and for high circuit switched bitrates traffic of 64kbit/s the instantaneous power hits the upper limit of 20 W.

Figure 6a. Time variation of the total mean power in W, the standard deviation, and the number of users (all averaged over 100ms) . 64 kb/s circuit switched user.


Figure 6b. Instantaneous power variation plotted over time in slots. 64 kb/s circuit switched user. The WCDMA system is characterized by both handling speech and data traffic. Figure 7 shows that the distribution of the power is different for the two cases. The target value is 14 W, but the mean level is much lower for the data traffic than for speech. For high data rates the variance in transmitted power becomes higher (less channel multiplexing gain), hence the mean transmit level has to be reduced in order to ensure the same outage probability (saturation of the power amplifier).

Figure 7. The downlink power distribution for speech user scenario and 64 kb/s circuit switched user. Means over 100 ms. Upper figures are total power, lower figures standard deviations.


In the downlink normal QPSK modulation is used with time multiplexed data and control channels. This creates large power variations during DTX etc., but this is smoothed out due to the many parallel codes to the multiple users. Figure 8a and b shows an experimental result of measuring the relative power near an active base station at one carrier frequency (f = 2162 MHz) in the time domain and the frequency domain, respectively. In the time domain the frame length (10 ms) and slot length (2/3 ms) are clearly seen, and in the frequency domain the slot frequency equal to the power control frequency of 1500 Hz is dominant. A different resolution is shown in Figure 8c and 8d, where now a 15 kHz signal is the new feature. The repetitive control channels form happens to form a signal pattern, giving raise to strong increase in transmit power with a rate of 15 kHz. The relative strength of the 15 kHz is expected to reduce when a base station is loaded with multiple active users. Hence the 15 kHz signal component will be mostly observed during low traffic load periods, i.e. off peak hours.


Figure 8a. Measured downlink power from an active WCDMA base station. Time period 1.6 s.

Figure 8b. Spectrum corresponding to Figure 8a.


Figure 8c. Measured power from base station Time period 6.4 ms.

Figure 8d. Spectrum corresponding to Figure 8c.


3.4 HSPA HSPA (High Speed Packet Access) pushes the data rate up to 1‐2 Mb/s in practice and even beyond 3 Mb/s under good conditions. This is done by using adaptive modulation and coding and shorter frame time. The basic features like chip rate and channel codes are unchanged from basic WCDMA, but there are also some new features justifying a separate treatment. Downlink (HSDPA) HSDPA was introduced to further improve the data rate and the average spectral efficiency of WCDMA. While WCDMA relies on dedicated channels to each user with fast power control and fixed modulation and coding, HSDPA relies on a shared channel concept, where user specific power control is substituted with fast adaptive modulation and coding (AMC). HSDPA supports 16QAM for improved peak data rates and cell capacity. Hence, users on HSDPA are primarily time‐multiplexed on a shared channel in each cell with a 2 ms resolution – also known as the transmission time interval (TTI). HSDPA is introducing a pseudo‐TDMA mode into the WCDMA system to make the system more efficient for highly bursty data traffic. From a transmission pattern perspective, this imposes some differences compared to basic dedicated channel transmission on WCDMA. Some cell‐level aspects that impact the actual transmission pattern include:

• Specific channels that are added to support the HSDPA concept. • Traffic mix and nature; e.g. burstiness of the service, ratio of WCDMA and HSDPA

traffic, etc. • Nature of packet scheduling strategy; e.g. its impacts on burstiness for the same traffic

load. • Power allocation strategy; e.g. whether dynamic or fixed HSDPA power control is

employed in the network.

HS-PDSCH (SF=16)

HS-SCCH

HS-DSCH {

Slot(0.67ms)

TTI(2ms)

Figure 9. Illustration of the basic channel structure.


In Figure 9 the basic downlink channel structure for HSDPA is illustrated. The transport channel for HSDPA is the high speed downlink shared channel (HS‐DSCH). The spreading factor for the HS‐PDSCH is 16 (constant), supporting multi‐code operation from 1 to 15 codes with either QPSK or 16QAM modulation. In Figure 9 it is shown that the number of codes (number of horizontal lines) used for transmission vary from TTI to TTI depending on how many codes the active user can handle given its equipment capability and radio conditions. The use of all 15 HS‐PDSCHs yields a peak data rate of approximately 14 Mbps for transmission with 16QAM and no error coding. During each TTI, the transmit power of the HS‐PDSCH is kept constant, and in many cases the transmit power is only adjusted on a slow time‐scale. In addition to the HS‐DSCH transport channel for data, a new downlink control channel is also introduced; high speed shared control channel (HS‐SCCH). The HS‐SCCH carries control information such as indicating which user is scheduled on the HS‐DSCH in the following TTI, time‐shifted by 0.67 ms (equivalent to a slot period). The HS‐SCCH is typically power controlled every TTI. For a typical macro cellular scenario with only HSDPA enabled, the majority of the cell transmit power is allocated to HS‐PDSCH transmission. Assuming a 20 W power amplifier (PA) for a macro cell installation, it is common to allocate a constant transmit power of approximately 15 W for HS‐PDSCH transmission. The remaining 5 W are used for other WCDMA control channels. The average HS‐SCCH transmit power typically equals 0.3‐0.6 W in macro cells. Hence, the total output transmit power for each cell is fairly constant and close to the maximum available power if there are data to transmit on HSDPA in every TTI. However, during TTIs with no data to transmit the HS‐DPSCH is switched off. The latter implies that there in worst case could be large power fluctuations of approximately 15 W magnitudes on a per‐TTI scale (2 ms) if there suddenly are no data to transmit on HSDPA. The characteristics for the total Node‐B output power per slot could therefore vary from being fairly constant if there is sufficient HSDPA traffic, to having a 500 Hz component if the HS‐PDSCH is switched on and off every TTI (worst case scenario). Note that due to the time shift between the HS‐SCCH and the HS‐DSCH, there is a two‐step power function in this case; e.g. first the HS‐SCCH switches off when there is no data to schedule and then the HS‐DSCH is switched off 1 ms later upon transmission of the last data from the previous scheduling interval. HSDPA can also be used in co‐existence with transmission on WCDMA dedicated channels, so the available power and codes are dynamically shared between these two domains (see detailed studies in [6]). For such cases the base station will typically adjust the HS‐PDSCH power according to the used power for WCDMA channels (non‐HSDPA power) so the total output power is close to the maximum (see sketch in Figure 10. Thus, depending on the power fluctuations from the sum of transmitted WCDMA channels, the remaining power up to the maximum available power will in principle be used for HSDPA. This implies a high utilization of the available power as the output power is constantly close to the maximum.


Non-HSDPA power

Maximum power

Time

Carrier power

HSDPA power

Figure 10. Sketch of principle for power allocation between HSDPA and WCDMA channels. Figure 11 shows the cumulative distribution function (cdf) of the base station output power for different channel types. These results are obtained from extensive dynamic macro cellular network simulations with mixed traffic on WCDMA and HSDPA [6]. A 20 W power amplifier is assumed for cell, assuming that the network load is such that approximately 6 W is used for non‐HSDPA channels. It is observed that the total output power is fairly constant around 17 W. The HS‐SCCH power is varying from milliwatt scale up to 2.5 W. Hence, even for a scenario with mixed WCDMA and HSDPA, the total base station output power is fairly constant with only small power fluctuations compared to the average output power. Summing up for the spectral components of the power fluctuations the following main components will exist:

• 1500 Hz – coming from the synchronization channels (primary and secondary SCH), as

well as from the power control of the individual dedicated physical channels (WCDMA). • 500 Hz – coming from the channels related to the HSDPA, where we might have on/off

effects due to power control as well as bursty traffic on the HSDPA channels. • 15000 Hz – Repetitive control channels form happens to form a signal pattern, giving

raise to strong increase in transmit power with a rate of 15 kHz. The relative strength of the 15 kHz is expected to reduce when a base station is loaded with multiple active users. Hence the 15 kHz signal component, will be mostly observed during low traffic load periods, i.e. off peak hours. Similar to WCDMA.

Further, there may be other lower frequency components originating for instance from repetition of data patterns and aspects of e.g. radio resource management algorithm design. While many of the above aspects are also visible in WCDMA, there are some aspects that are specific to HSDPA. Lower frequency components are also possible provided that there are off‐times larger than 2 ms.


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Figure 11. Cumulative distribution functions of the base station output power for mixed traffic scenario with both WCDMA channels and HSDPA on the same carrier [5].

Uplink, HSUPA High Speed Uplink Packet Access (HSUPA) is the uplink equivalent to HSDPA. HSUPA enhances the peak data rates of the uplink connection up to 5.76 Mbps, where the peak data rate of most uplink WCDMA connections is limited to 384 kbps. In order to reduce the end to end delay, the HSUPA specifications facilitate the use of 2 ms TTI length besides the normal 10 ms setting. A number of features like basic power control loop (1500 Hz), variable spreading factors (SFs) and constant modulation carry over from WCDMA. The higher bit rates are obtained by having multiple codes with low spreading factors, which as before means good channel conditions for good reception. The maximal bit rates are given in the table below Table 1. Description of the essential parameters related to the terminal capability classes.

Category Codes TTI Max data rate 1 1 x SF4 10 ms 0.73 Mbps 2 2 x SF4 2 ms or 10 ms 1.46 Mbps 3 2 x SF4 10 ms 1.46 Mbps 4 2 x SF2 2 ms or 10 ms 2.9 Mbps (2 ms)

2 Mbps (10 ms) 5 2 x SF2 10 ms 2 Mbps 6 2 x SF2 + 2 x SF4 2 or 10 ms 5.76 Mbps (2 ms)

2 Mbps (10 ms)


Four simulated examples are given in Figures 15a‐d. The figures show the distribution of the terminal transmitted power for a cell radius of 500 and 2500 meter and for a low and high load situation. In the high load situation 20 terminals are present on average per cell, while in the low load situation on average 5 terminals are present. Each terminal uploads a file of 100 kB. Other simulation parameters can be seen in Table 2. Table 2. Simulation parameters Parameter Value TTI length 10 ms Channel model Veh A, 30 kmh Maximum UE bit rate 2 Mbps Maximum RTWP 6 dB BLER target 10% at the first transmission The figures show that the cell size has a larger impact on the output power than the load of the cell. For the small cell sizes only a small fraction of the terminals (around 4%) transmit with maximum power, while in case of the larger cell up to 40 % of the terminals transmit with full power, i.e., 24 dBm. The average transmission power is in all cases well below the maximum power. The doubled peak distribution for the larger cells is due to the limitation of output power, both for those near the base and for those at the edge of the cells.

-60 -50 -40 -30 -20 -10 0 10 20 300

0.005

0.01

0.015

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0.03

0.035

0.04

0.045

transmitted power (dBm)

normalised frequency

Figure 15a. Distribution of the terminal transmit power for a cell size of 500 m and low load. For other parameter see text.


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Figure 15b. Distribution of the terminal transmit power for a cell size of 2500 m and low load. For other parameters see text.

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Figure 15c. Distribution of the terminal transmit power for a cell size of 500 m and high load. For other parameters see text.


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Figure 15d. Distribution of the terminal transmit power for a cell size of 2500 m and high load. For other parameters see text. 3.4 Wireless Local Area Networks (WLAN) Most laptops today are equipped with wireless transmission capable of communication with a nearby access point (base station). Although the exposure from cards in the computer or from the access point is very low (see below) it has anyway been the source of public debate in several countries, especially with the additional exposure to children. This justifies a closer look at the exposure levels and possible power spectral frequencies as discussed above for other wireless systems. There are many types of WLANs under the IEEE 802.11 umbrella, here we shall treat only the most common 802.11b (tradename WiFi). For an overview of WLANs see [7]. The output power is of the order 30 mW, max. 100 mW, and modeling the antenna as a dipole we note that the intensity 1 meter from the terminal or access point is of the order 1 mW/m2 (Figure 1a) and if taking into account the lower duty cycle and other factors Foster [8] finds by measurements at a large number of places a factor of 10 lower, so the power intensity is around a factor of 105 below the ICNIRP limit. Heating of tissue is definitely not the case. The carrier frequency is mostly in the unlicensed ISM band at 2.4 GHz, and the modulation is spread spectrum with a spreading factor of 11. The access is time division duplex, TDD, using a so‐called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which means that the timing structure may be more random and dependent on the general interference from other users. The power fluctuations are illustrated by an example of measurement near a laptop while uploading files, Figures 16a‐b. Figure 16a‐b shows the somewhat random access for the frames in uplink with a local broad peak around 500 Hz.


Figure 16a. Time variation of the power from uploading a file from laptop. Time span 1 sec. Figure 16b. Spectrum of power for the above time variation.


Zooming in the time domain, Figure 17 a‐b, the spectrum shows peaks around 1 MHz and harmonics due to the non‐constant envelope character of the spread spectrum. Figure 17a. Time variation of the power from uploading a file from laptop. Time span 1 msec. Figure 17 b. Spectrum of power for the above time variations.


3.5 WIMAX WIMAX stands for Worldwide Interoperability for Microwave Access, and is originally designed for fixed wireless access replacing cables or optical fibers in the km range. The technology is described in IEEE standard 802.16 under the name Wireless Metropolitan Area Network, WMAN, and the intention is to fill the gap between very high data rate WLANs and very high mobility cellular systems, i.e. between WiFi and WCDMA. Originally the frequency range was in the 10‐66 GHz region, but the standard was later amended for mobility at lower frequencies, typically below 6 GHz. It is not possible to have long range and many broadband users at the same time, so some compromises must be accepted, as for other systems; 10 Mb/s at 10 km should be achievable. The carrier frequencies include both licensed and unlicensed bands; typical frequencies are in the 2‐4 GHz region. The standard has a high degree of flexibility, allowing both TDD with burst transmissions and normal FDD. The modulation may also vary depending on the system, but typically OFDM (orthogonal frequency division modulation) with QAM at the sub‐carriers. It is a so‐called Multicarrier technique where the available band is divided into a large number of sub‐bands, typically 256, where the different sub‐bands may be used for different users and different data. OFDM is characterized by a rather high peak to mean value, which means that is a non‐constant envelope system like all the other where the data rates carry over to the power variations. Figure 18 gives an example of a WIMAX downlink spectrum. The carrier is at 3.5 GHz and the spectrum shows a widespread signal in the MHz region with a peak near 4MHz.

Figure 18. An example of WIMAX downlink, carrier 3.5 GHz, OFDM modulation. Time span 20 μs.


Figures 19 a and b show the same signal over a longer time span, revealing the TDD nature of this particular transmission.

Figure 19a Time division duplex of the signal over a 53 ms time span.

Figure 19b. Frequency spectrum of the same signal as in Figure 19a. Fundamental frequency near 350 Hz.


Discussion The focus of the paper is on the power and power variation with time from a selected group of digital communication systems. The spectrum of the power variations has been chosen as a convenient compact way of presenting the result of very complex communication systems, where many types of modulation, coding, and control algorithms come into play. As clearly demonstrated, it is the mobile phone or terminal like a PDA which generates the largest exposure. The base station or access point power is under normal conditions generating very low exposure to the user. Comparing the average power levels from the user unit they range from about 50 mW for GSM to 0.25 mW for WCDMA, a very significant reduction. This is, however the average values. When the user is at the edge of a cell or in bad coverage situations the power will rise to the maximum allowed level. It should also be remembered that the local exposure of the user is different for a phone use with the antenna near the head, and for a terminal removed from the head. For the time variations there are different time scales. At the longest time scale of the order seconds the power varies due to the system power control which compensates for the variation in the radio connection. The time scale depends on the carrier frequency, since the variations follow the spatial scale of wavelengths, the higher the frequency and the faster the movement, the faster the variations. It is, however, a broad spectrum of frequencies. The next group of frequencies lies in the Hz region and these are narrow spectra, like spectral lines. Examples are the 2 and 8 Hz components of the GSM system. A significant group of frequencies lie in the kHz region, like the 217 Hz and harmonics for GSM due to the power bursting For WCDMA the relevant frequencies are 1500 Hz due to fast power control and 15 kHz control signals. For the HSPA system a new frequency of 500 Hz appears due to the choice of 2 ms time frame. The wireless LAN system studied seems to have a broad spectrum in this frequency range. For the evolving systems the data signals are not modulated with a constant envelope, as is the case with GSM. This means that the data signals, often in the microsecond range, will appear as power variations in the MHz region. A good example is the WCDMA chip frequency of 3.84 MHz, which appears as a spectral line in the transmit power. The wireless LAN systems appear also to have spectral lines in the MHz region. Concluding, we should emphasize that the time variations may not have a significant biological effect, but they are part of many experimental studies, so it is important to know the details.


Acknowledgements The authors would like to acknowledge the valuable support from Frank Frederiksen, Jeroen Wigard, Klaus I. Pedersen, Troels Kolding, and Kim Olesen.

References [1] K. Foster and M. Repacholi, Biological Effects of Radiofrequency Fields: Does Modulation Matter?, Radiation Research 162(2):219–225, August 2004] [2] Q. Balzano, “Proposed test for detection of nonlinear responses in biological preparations exposed to RF energy”, Bioelectromagnetics, vol 23, pp 278‐287, 2002 [3] J. Bach Andersen, P. E. Mogensen, G. Frølund Pedersen, „Exposure Aspects of W‐CDMA”, Report to the GSM Association, December 2001 [4] ICNIRP Guidelines for Limiting Exposure to Time‐Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz). Health Physics 74 (4): 494‐522; 1998. [5] J. Wiart, C. Dale, A. V. Bosisio, and A. Le Cornec, ”Analysis of the Influence of the Power Control and Discontinuous Transmission on RF Exposure with GSM Mobile Phones”, IEEE Trans. Electromagnetic Compatibility, vol. 42, 4, pp 376‐385, Nov. 2000 [6] K.I. Pedersen, P‐H. Michaelsen, ”Algorithms and Performance Results for Dynamic HSDPA Resource Allocation”, in IEEE Proc Vehicular Technology Conference, September 2006. [7] N. Prasad and A. Prasad (editors), WLAN Systems and Wireless IP for Next Generation Communications, Artech House, 2002 [8] K. R. Foster, “Radiofrequency exposure from wireless LANs utilizing Wi‐Fi Technology”, Health Physics, vol. 92, 3, pp 280‐289, 2007


Appendix - List of acronyms AMC Adaptive Modulation and Coding BPSK Binary Phase Shift Keying CDMA Code Division Multiple Access CSMA Carrier Sense Multiple Access DSCH Downlink Shared Channel DTX Discontinuous Transmission EDGE Enhanced Data rates for GSM Evolution FDD Frequency Division Duplex FDMA Frequency Division Multiple Access GPRS General Packet Radio Service GSM Global System for Mobile communications HSDPA High Speed Downlink Packet Access HSPA High Speed Packet Access HSUPA High Speed Uplink Packet Access ICNIRP International Commission on Non‐Ionizing Radiation Protection OFDM Orthogonal Frequency Division Modulation PSK Phase Shift Keying QAM Quadrature Amplitude Modulation QPSK Quadrature Phase Shift Keying RF Radio Frequency SAR Specific Absorption Rate SCCH Shared Control Channel TDD Time Division Duplex TDMA Time Division Multiple Access TTI Transmission Time Interval UMTS Universal Mobile Telecommunications System WCDMA Wideband Code Division Multiple Access WiFi Wireless Fidelity, a local area network system WLAN Wireless Local Area Network WIMAX Worldwide Interoperability for Microwave Access


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