A device may be usable with several antenna options, battery options and other accessories, and the number of possible combinations can be very large. Two traditional methods of experimentation are given below. These methods have drawbacks compared to the design of experiments (DOE) approach described in L.4.2.
L.4.2 Test reductions based on a design of experiments (DOE) L.4.2.1 General
Analysis of SAR data, e.g. statistical analysis based on a design of experiments (DOE) approach, may be used to develop scientific or engineering rationales for the test reduction of certain SAR tests. For example, if devices are available with optional faceplates with paint coatings of varying metal content, statistical analysis of SAR data may be used to justify excluding the testing of faceplates with less than a certain amount of metal content. The application of the test reduction should be limited to products that are sufficiently similar to the original product for which the test reduction was determined.
L.4.2.2 Search for highest SAR test conditions
A device may operate in different transmission modes and may be usable with several antenna options, battery options and other accessories, and the number of possible combinations can be very large. Methods are therefore needed to streamline the measurement process, so that the highest SAR test conditions can be quickly identified. For example, a device with two antenna configurations (antenna extended and retracted) and four battery types testing all possible combinations would result in a considerable number of tests.
It is unnecessary to test all possible combinations; statistical techniques can be used to show trends from a smaller set of data and determine which device–accessory combinations result in higher SAR values. Using a design of experiments (DOE) is the preferred statistical method of achieving this. A DOE is a structured, organized method for analysing the influence of factors and the interactions between factors on the output of a process. The DOE approach is extensively covered in the literature [115].
L.4.2.3 One factor at a time (OFAT) search
With this method, the experimenter starts with a baseline test condition and successively varies one factor at a time while holding all other factors constant. For example, this could be achieved by first varying antenna configurations, then battery types, then carry accessory types, then audio accessory types. At the end of each step, the factor giving the highest SAR is selected for the next steps. The main drawback of this approach is that it does not consider any interactions between the different accessory types (e.g. the interaction of the battery and the antenna on SAR that is not explained by the influences of each factor independently). If interactions exist, the OFAT approach may not find the optimum (i.e. highest SAR) solution.
L.4.3 Analysis of unstructured data
A common source of unstructured data is historical data. This data typically was collected without any specific objective in mind, or it may have been collected for different purposes than the current experimental objective. This data may be useful in spotting trends, but it may be very difficult to have high confidence in the findings.
For this reason, any findings from the analysis of unstructured data should be verified (e.g.
using a DOE).
Annex M (informative)
Applying the head SAR test procedures
Annex M illustrates an example of how to apply the SAR test protocol in 6.4. A mobile handset supporting GSM mode in 850/900/1800/1900 MHz bands is used for the illustration.
The device has a sliding keypad, which can be used for voice calls with the slide in open and closed positions. The test procedures in 6.4.2 are applied according to the following.
The test procedure as given in 6.3 is used as follows for the example device.
Step 1: SAR measurements are performed at the channel closest to the centre of each transmit frequency band according to procedures in 6.4. The corresponding channels are 190 at 836,6 MHz for GSM 850, 38 at 897,6 MHz for GSM 900, 699 at 1 747,6 MHz for GSM 1800, and 661 at 1 880 MHz for GSM 1900. All device test conditions described in 6.2.4.2 and 6.2.4.3 for cheek and tilt positions on the left and right sides of the SAM phantom are tested with the sliding keypad open and closed. The device supports only GSM voice mode for head use. Therefore other GSM modes are not applicable for this example.
Step 2: SAR measurements are performed for additional frequency channels in each frequency band and mode as required by 6.2.5, for the highest peak spatial-average SAR test configuration in Step 1 among the device test positons. In addition, SAR is also measured at other frequency channels required by 6.2.5 for all configurations tested in Step 1 where the peak spatial-average SAR is greater than or equal to half of the applicable SAR limit.
Step 3: The SAR measurement results are shown in Tables M.1 to M.4. The highest peak spatial-average SAR among all GSM frequency bands is 1 205 W/kg on channel 251 at 848,8 MHz for GSM 850.
Table M.1 – SAR results tables for example test results – GSM 850
Mode and band
Device configuration (slide position)
SAM phantom test position
SAR, averaged over 10 g [W/kg]a Ch. 128b
824,2 MHz
Ch. 190b 836,6 MHz
Ch. 251b 848,8 MHz
GSM 850
closed
Left Cheek 0,776 0,653c 0,552
Tilt - 0,492 -
Right Cheek - 0,626 -
Tilt - 0,448 -
open
Left Cheek 1,011 1,192d,e 1,195
Tilt - 0,430 -
Right Cheek 0,892 1,120e 1,205f
Tilt - 0,418 -
a The regulatory SAR limit is assumed to be 2,0 W/kg averaged over a 10 g tissue volume.
b According to 6.2.5, the number of channels to be tested is Nc = 3 for all of the modes (see Step 2).
c This is the hghest SAR measured for the mid-band channel with slide closed, the two other channels in this frequency band also require SAR measurement (see Step 2).
d This is the highest SAR measured for the mid-band channel with slide open, the two other channels in this frequency band also require SAR measurement (see Step 2).
e The measured SAR is within 3 dB of the applicable SAR limit, the two other channels in this frequency band also require SAR measurement (see Step 2).
f This is the highest peak spatial-average SAR measured among all GSM frequency bands and test configurations (see Step 3).
Table M.2 – SAR results table for example test results – GSM 900
Mode and
band Device
configuration Device position
SAR, averaged over 10 g [W/kg]a Ch. 975b
880,2 MHz
Ch. 38b 897,6 MHz
Ch. 124b 914,8 MHz
GSM 900
closed
Left Cheek 0,766 0,730c 0,652
Tilt - 0,522 -
Right Cheek - 0,631 -
Tilt - 0,482 -
open
Left Cheek 0,618 0,723d 0,833
Tilt - 0,443 -
Right Cheek - 0,620 -
Tilt - 0,406 -
See footnotes in Table M.1.
Table M.3 – SAR results table for example test results – GSM 1800
Mode and
band Device
configuration Device position
SAR, averaged over 10 g [W/kg]a Ch. 512b
1 710,2 MHz
Ch. 699b 1 747,6 MHz
Ch. 885b 1 784,8 MHz
GSM 1800
closed
Left Cheek - 0,254 -
Tilt - 0,230 -
Right Cheek 0,336 0,343c 0,466
Tilt - 0,215 -
open
Left Cheek - 0,154 -
Tilt - 0,162 -
Right Cheek - 0,174 -
Tilt 0,188 0,192d 0,211
See footnotes in Table M.1.
Table M.4 – SAR results table for example test results – GSM 1900
Mode and
band Device
configuration Device position
SAR, averaged over 10 g [W/kg]a Ch. 512b
1 850,2 MHz
Ch. 661b 1 880,0 MHz
Ch. 810b 1 909,8 MHz
GSM 1900
closed
Left Cheek - 0,240 -
Tilt - 0,233 -
Right Cheek 0,246 0,353c 0,387
Tilt - 0,221 -
open
Left Cheek - 0,158 -
Tilt - 0,178 -
Right Cheek - 0,169 -
Tilt 0,188 0,195d 0,301
See footnotes in Table M.1.
Annex N (informative)
Studies for potential hand effects on head SAR