INVESTIGATION OF THE INTERACTION BETWEEN CDMA WIRELESS PHONES AND HEARING AIDS
Executive Summary
Background
Interference between digital wireless phones and
hearing aids occurs when the RF bursts from the phone transmission are
demodulated by the hearing aid amplifier. The amplified interference
signal is heard as noise by the hearing aid wearer. CDMA phones
characteristically produce a "static" noise due to the random pulse
structure of their transmission
signal. This noise is not only annoying, but can seriously affect the
intelligibility of the transmitted speech.
Depending upon speech activity, CDMA phones transmit data at a variety of
rates. These transmission (or puncture) rates are categorized as full,
half, quarter, eighth or variable. Typically, CDMA phones transmit at
full rate when speech (or data) activity is continuous, and drop to
eighth rate during idle speech or data periods.
The CDMA Development Group (CDG) commissioned this study to investigate
the effects of various phone puncture rates and transmission power levels
with the objective of determining the power level at which interference
becomes unacceptable. This threshold could be used to trigger the phone
to switch to a full puncture rate, thereby reducing the interference and
increasing the phones usability by hearing aid wearers. Data from the
study provide evidence that this switch would be effective in improving
speech intelligibility and reducing annoyance for all aids tested.
Furthermore, the amount of improvement at a given power level can be
quantified.
Two CDMA phones (800 MHz and 1900 MHz) from each of two phone
manufacturers were tested with nine hearing aids (3 BTE aids and 6 ITE
aids) from four different manufacturers. The aids were selected to
provide a conservative estimate of the maximum acceptable interference
threshold. The response of each aid was tested at all combinations of
six levels of phone transmission power and four levels of puncture rate.
During testing, a Brüel & Kjær Type 2144 frequency analyzer was used to
measure the output of the hearing aid (while in the vicinity of a
transmitting phone) over a broad range of frequencies (0 to 5.6 kHz).
The frequency response of the hearing aid (gain curve) was subtracted
from this output to obtain the Input Referenced Interference Spectrum
(IRIS), a spectral representation of the interference.
The Overall Input Referenced Interference Level (OIRIL) was computed as
the total power (unweighted summation) of the IRIS across its frequency
range. This single value, computed in dB SPL as a function of the RMS
power in the pulse, depicts the interference that occurs for a specific
phone power at a particular puncture rate. The OIRIL for each of the
four puncture rates was plotted against the "Full Rate" phone power
levels. This "Full Rate" power was used rather than average power to
more appropriately represent a change in puncture rate with no change in
RF carrier amplitude. Relating the OIRIL values to the findings of the
EMC Centers Phase II-B Study, the respective speech intelligibility and
annoyance ratings were added to the plots to aid in locating the power at
which the interference becomes unacceptable. The mean response of all
nine hearing aids at the various puncture rates was graphed for each of
the four phones. Further analysis was conducted based on a subjective
grouping of the aids according to the immunity (low, medium, and high)
exhibited in this study.
Note that, for this study, full puncture rate power levels were pushed to
30 dBm under laboratory conditions. This is not typical power operation
for the CDMA phones and is above the specification for the minimum
Effective Isotropic Radiated Power (EIRP) at maximum output (but is at or
below the maximum EIRP level at maximum output). The minimum EIRP at
maximum output specifications are 25 dBm and 23 dBm for the cellular and
PCS bands, respectively.
The ambient noise level across all nine hearing aids for the 800 MHz
phone from Company A was approximately 48 dB SPL and ranged from 47 dB to
50 dB SPL. Typically, the mean OIRIL for the full puncture rate was
below this ambient noise floor up to a power level of 18 dBm. For
hearing aids with higher (better) immunity, the full-rate interference
remained below the ambient noise floor all the way to 30 dBm (the highest
power tested). For other puncture rates, the interference typically
became noticeable when power reached 12 dBm. As the interference
increased substantially above the noise floor, a linear response region
was observed in which a specified increase in power output resulted in a
proportional increase in interference. The theoretical slope of the
response curve is 2:1 in this region due to the square-law property of
the interference. That is, a 5 dB increase in power output resulted in a
10 dB increase in the interference level. As power was increased beyond
the linear region, the hearing aid typically entered a saturation region
where an additional power increase resulted in little or no increase in
interference.
Moyer (1998) proposed that the theoretical difference in interference
between puncture rates was 2.4 dB between eighth and quarter rates, 1.2
dB between quarter and half rates, and 3.6 dB between eighth and half
rates. Within the repeatability limits of the measurement procedure,
these differences were found to be correct.
Overall, the interference appeared to be more severe with the 1900 MHz
phone, although individual aids yielded varying results. On average, the
ambient noise floor still occurred at 46 to 47 dB SPL, but the
interference at less than the full puncture rate was apparent even at the
lowest power level (0 dBm). For two aids, the interference at 24 dBm was
10 to 25 dB SPL lower at 1900 MHz. One of these aids used electrostatic
shielding in the shell to improve the immunity. This shielding was
evidently more effective at 1900 MHz than at 800 MHz. Of the remaining
aids, three demonstrated 5 to 10 dB SPL higher interference at 1900 MHz
and four demonstrated 15 to 20 dB SPL higher interference at 1900
MHz.
The general pattern of results for the phones from Company B was very
similar to that for the phones from Company A. However, the power levels
employed were 6 dB lower than the levels used for Company A, which
removed the opportunity to observe a saturation effect (or even
interference) in some aids. In addition, most aids showed a reversal in
the interference levels for the quarter and half puncture rates. This is
believed to be due to potential errors in setting the puncture rate for
the Company B phones. Individual differences between the Company A and
Company B phones might have resulted from minor differences in the
separation distance and relative orientation of the hearing aid and the
phone.
On the average, the summary data for the various categories of hearing
aid immunity were remarkably similar for the 800 MHz phones and for the
1900 MHz phones between the two companies. However, the interference
levels for the 1900 MHz phone from Company B were typically 10 dB lower
at the 22 dBm power level.
This study was undertaken to evaluate the effectiveness of switching to
full puncture rate above a specific phone output power level in order to
reduce the audible interference in hearing aids. Data from the study
provide evidence that this switch would be effective in improving speech
intelligibility and reducing annoyance for all aids tested. Furthermore,
the amount of improvement at a given power level can be quantified. This
improvement is substantial for hearing aids with poor immunity. For aids
with medium or high immunity, the degree of improvement depends upon
specific characteristics of the aid such as the type of EMC treatment
applied during design and manufacture.
This report includes summary charts that provide the power switchover
point to full puncture rate necessary to achieve various hearing aid
interference levels. Hearing aid performance may be chosen from three
levels: "Not Annoying", "Annoying", and "Very Annoying". The charted
curves reflect the percentage of hearing aids that exhibited interference
at or below each interference level. For example, one may set the
performance criterion that 90% of the hearing aids will exhibit
interference at or below the "Not Annoying" level. The charts may then
be used to determine the power level at which the switch to full puncture
rate is required to meet this criterion.
Figure 1 illustrates this procedure for all phones combined (assuming a
50/50 mix of cellular and PCS phones). For a given level of interference
(55 dB SPL - "Mildly Annoying", 65 dB SPL - "Annoying", or 75 dB SPL -
"Very Annoying"), the figure shows the percentage of aids below the
specified interference level at the indicated phone power switching
point. The dashed vertical line in the figure identifies the power level
(8 dBm) at which the phone should be switched to full puncture rate to
alleviate "Annoying" (65 dB SPL) interference in approximately 90% of the
hearing aids tested. In other words, switching to full puncture rate at
this power level would result in "Annoying" interference being
experienced in 10% of the hearing aid tests. Switching at this power
level would also result in interference below 55 dB SPL ("Mildly
Annoying") for approximately 75% of the hearing aids tested, and
interference below 75 dB SPL ("Very Annoying") for 95% of the aids
tested.
Experimental Design
Results and Conclusions
Company A
Company B
Minimizing the
Interference
Figure 1: Power Level to Switch to Full Puncture Rate Such That the Percentages of Tested Hearing Aids Perform Below Specified Interference Levels for All Phones Combined.
The effectiveness of switching puncture rates increases with increases in power level. However, the highest power levels, where the approach would be most effective may be substantially higher than typical urban and suburban CDMA mobile transmit power levels. This may reduce the effectiveness for all but the poorest immunity aids used in these environments.
The hearing aids tested in this study were chosen to represent the entire
range of immunity levels, from very sensitive to highly immune. While
the distribution of the immunity levels of hearing aids in actual use is
unknown, it is probably not evenly distributed among the population.
Hence, although the range of immunity levels was covered by the hearing
aids tested in this study, the distribution of those levels may not
accurately represent that of the hearing aids in actual use. Therefore,
caution should be taken in applying the results of this study to the
general population.
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