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.
Results and Conclusions
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.
Minimizing the Interference
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.