Specification
SIMULATED DEGRADATION OF SNR IN DECODED DIGITAL AUDIO
CORRELATED TO WIRELESS LINK BIT-ERROR RATE
The invention concerns radio communication method, apparatus, and
system. More particularly, the invention concerns a method for a digital radio to
provide an audible indicator of communication link quality.
Analog radios provide a communication link for a baseband signal,
such as an audio signal, by use of an analog radio transmitter and an analog radio
receiver. An analog radio transmitter operates by amplifying the baseband signal,
modulating the baseband signal by use of analog modulation techniques that are
known in the art, frequency upconverting the modulated signal to radio frequencies
(RF), and transmitting the RF signal to an analog radio receiver. The analog radio
receiver recovers the baseband signal by downconverting and demodulating the
received RF signal. Radio operators such as public safety personnel are very familiar
with the operation of analog radios. Analog radios are simple, but a disadvantage of
analog radio is that the quality of the received radio transmission, after demodulation
in order to provide a received baseband signal, is prone to be poor (e.g., noisy) in
situations such as low RF received power level, low signal-to-noise (SNR) ratio, and
the presence of interference.
Receiver performance of analog radios gracefully degrades, such that
the radio operator can hear increased noise on the demodulated baseband signal as the
received RF signal weakens or the SNR degrades. The increased noise provides an
aural cue to the radio operator, who may then use the aural cue to move to an area of
better coverage. Furthermore, if the analog radio operator is able to hear
conversations of other radio operators, the radio operator may be able to get additional
audio cues of quality by listening to the quality of those other conversations on the
analog radio.
In contrast, digital radios employ digital modulation techniques that are
known in the art in order to provide a digitized communication signal from a
transmitter to a receiver. The digitized communication signal may include a digitized
baseband voice signal, or other baseband audible signals (e.g., music), or IP-based
data traffic. Compared to analog radios, digital radios provide a relatively noise-free
received demodulated baseband signal under typical operating conditions. Digital
radios improve the received signal quality delivered to a radio operator over a wide
range of received signal conditions by using error detection and correction techniques.
Digital radios also provide other benefits compared to analog radios, e.g., more
efficient spectral usage. The usable error detection and correction techniques may
vary depending upon the type of communication and the latency, and may be
implemented at different levels of a protocol stack.
At a link layer, error detection and correction techniques may include
an error-correcting code (ECC). An example of an ECC is a forward error correction
(FEC) code. The transmitter encodes the data with an error-correcting code (ECC)
and sends the coded message. The receiver receives a noise-corrupted signal, and
makes a maximum-likelihood estimation of the original transmitted message. ECC
decoders are often located close to the front end of the digital radio receiver, e.g., in
the first stage of digital processing after a signal has been received. ECC coders may
also generate a bit-error rate (BER) signal or error count signal, which can be used as
a feedback to gauge the quality of the received signal. The BER may be an uncoded
BER, which is the bit error rate prior to ECC correction, or a coded BER which is the
BER after ECC decoding and which is what is delivered to the listener. The uncoded
BER is more useful than a coded BER for the purpose of monitoring RF link
degradations, because the uncoded BER is more sensitive to such degradations.
An example of a digital radio is the Harris' OpenSky® family of
products, which offers digital audio and packet data communications using a high
performance IP backbone network. OpenSky uses a continuously transmitting base
station with separate error correction schemes for control channel and data.
Continuous monitoring of base station traffic can provide a received signal strength
indicator ("RSSI") and error numbers provided by an ECC decoder.
The error detection and correction capabilities of digital radio provide
high-quality baseband analog audio transmission capability, as long as the digital
radio is operating within the error detection and correction limits of the decoder. This
is generally seen as an advantage because it increases the useful range within which
digital radios can operate compared to analog radios.
However, beyond the error correction capabilities of the error detection
and correction code, performance rapidly degrades. This presents a human-factors
problem for operators of digital radio because the communication link appears to fail
unexpectedly, without adequate warning to a radio operator. Furthermore, digital
radios are frequently trunked — i.e., operated by packet transmission in order to
deliver a communication only to an intended recipient — so that the radio operator is
not able to receive an aural cue of transmission link quality by listening to other radio
operators' communications.
Some radio operators (e.g., firefighters) object to the absence of an
intuitive awareness that the signal is degrading and that interruption of
communications is imminent. Some radio operators find this shortcoming
objectionable enough to decide to revert to their familiar analog systems.
Embodiments of the present invention add a controlled amount of
noise back into the error corrected audio output of a digital radio, in order to produce
a composite audio output that simulates the operation of analog radios. The amount
of noise is controlled by the detected quality of the signal received by the digital
radio. The radio operator can interpret the noise as a warning that his communication
is in danger of failing, and as an aural guide in finding a better area of coverage.
One or more embodiments of the invention may provide method or
apparatus to provide an audible indicator of a quality of a received digital radio
transmission, including receiving a digital radio transmission in a digital radio
receiver, detecting the quality of the received digital radio transmission, decoding an
audible communication from the received digital radio transmission, superimposing
an audible indicator onto the audible communication, to form a composite audible
signal, and dynamically adjusting an amplitude of the audible indicator relative to an
amplitude of the audible communication responsive to a quality of the received digital
audio transmission.
One or more embodiments of the invention may provide software
stored in a memory that is coupled to a microprocessor, wherein, after reception of a
digital radio transmission in a digital radio receiver, the microprocessor is
programmed by the software to provide an audible indicator of a quality of the
received digital radio transmission by detecting the quality of the received digital
radio transmission, decoding an audible communication from the received digital
radio transmission, superimposing an audible indicator onto the audible
communication, to form a composite audible signal, and dynamically adjusting an
amplitude of the audible indicator relative to an amplitude of the audible
communication responsive to a quality of the received digital audio transmission. The
microprocessor may be in the form of a digital signal processor.
Optional variations of the foregoing embodiments may include:
detecting the quality of the received digital radio transmission by detecting a received
signal strength indicator of the received digital radio transmission; detecting the
quality of the received digital radio transmission by detecting a coded bit error rate of
the received digital radio transmission; and detecting the quality of the received
digital radio transmission comprises detecting a signal to noise ratio of the received
digital radio transmission.
Optional variations of the foregoing embodiments may further include:
comparing the quality of the received digital radio transmission to a first
predetermined threshold, wherein the audible indicator is superimposed onto the
audible communication only if the quality of the received digital radio transmission is
below the first predetermined threshold; comparing the quality of the received digital
radio transmission to a second predetermined threshold, wherein the audible indicator
is superimposed onto the audible communication only if the quality of the received
digital radio transmission is above the second predetermined threshold; and detecting
whether the received digital radio transmission includes a communication from a
remote radio operator, wherein the audible indicator is suppressed if a communication
from the remote radio operator is detected.
Optional variations of the foregoing embodiments may further include
selectively disabling the audible indicator responsive to a radio operator input to the
digital radio receiver.
Optional variations of the foregoing embodiments may further include
the audible indicator being one or more of a broadband noise signal, a simple tone, a
complex tone, and a buzz.
Embodiments will be described with reference to the following
drawings figures, in which like numerals represent like items throughout the figures,
and in which:
FIG. 1 is a comparison of audio intelligibility versus link quality for
analog radio, digital radio without embodiments of the invention, and digital radio
incorporating embodiments of the invention.
FIG. 2 is a simplified block diagram of a first embodiment of a portion
of a digital receiver that is useful for understanding the present invention.
FIG. 3 is a simplified block diagram of a second embodiment of a
portion of a digital receiver that is useful for understanding the present invention.
FIG. 4 is a simplified block diagram of a third embodiment of a portion
of a digital receiver that is useful for understanding the present invention.
FIG. 5 is a flow chart of a method for simulating the degradation of
SNR in decoded digital audio, correlated to wireless link BER, according to an
embodiment of the invention.
Analog radios receive an RF signal modulated by analog methods, and
provide a demodulated baseband signal. The demodulated baseband signal includes
noise that increases under increasingly degraded RF link conditions. Although
generally the noise is unwanted, it beneficially provides an audible cue to degraded
RF link conditions and an early warning to the possibility of loss of the
communications link.
Digital radios use digital modulation techniques to provide, via a data
channel, a demodulated baseband signal that is less susceptible to degraded RF link
conditions. Digital radios may provide a control channel in addition to the data
channel. The control channel can be routed to the same recipients as the data channel,
and/or can be routed to other recipients such as a base station controller. The control
channel can provide a way to control settings within the digital radio, or to report back
the status of the digital radio, or to provide information about connectivity and/or link
quality between various digital radios that are grouped to form a network. The
control channel can also be used to set up and control trunking (i.e., connectivity)
between digital radios.
An exemplary network may include a base station and one or more
subscribers. The subscribers typically are mobile and are more likely to experience
degraded RF link conditions. The base station typically is less mobile and may be in
a fixed location, and therefore is less likely to experience degraded RF link
conditions. The base station may be in charge of the network. Optionally, a
dispatcher may be used to help control the network and assign network resources.
The demodulated baseband data channel signal may include digitized
audio (e.g., voice) and/or non-audio packet data. The non-audio packet data may
include, for instance, web pages, file transmissions, datalinks, etc., that may be
intended for visual display on a screen or terminal. As long as the received signal is
of sufficient quality to reliably demodulate the received signal, the digital radio
should be able to determine whether an individual packet of data contains audio data
or non-audio data, for instance by monitoring the contents of the packet itself (e.g., a
header portion), or by information obtained via the control channel.
The demodulated baseband audio signal includes little or no
perceptible noise for RF link conditions within design limits, but provides little
warning to the radio operator if RF link conditions degrade toward exceeding design
limits. Embodiments of the present invention selectively add a controlled amount of
noise back into the error corrected audio output of the demodulated baseband data
channel of a digital radio, in order to produce a composite audio output which
simulates degraded RF link conditions.
For a non-audio packet data component of the demodulated baseband
data channel, the packet data will not be audibly monitored by a human user. Adding
noise to the data channel when it is carrying non-audio data would be ineffective to
warn a user of degraded link conditions, and would further degrade the non-audio
packet data. Therefore, embodiments of the present invention can inhibit the addition
of a controlled amount of noise back into the error corrected output of the
demodulated baseband data channel of a digital radio, on a packet-by-packet basis, if
it is determined that a particular packet contains non-audio data.
Optionally, for non-audio packet data, it may be desirable to provide to
a non-audio packet data subscriber and/or base station an alternate indication (e.g., an
annunciator) of degraded link conditions, rather than adding the controlled amount of
noise as is used for audio data. The annunciator can take one or more forms such as a
pop-up window on a terminal in order to alert a user of degraded link conditions, or a
chart/bars/bar-graph of RF link quality, or some change in attribute of at least a
portion of a terminal display (e.g., changing text or screen color, or making text be
bolder, bigger, blinking, displaying a status bar, etc.), or an audio indication that is
separate from and not added to the received non-audio packet data (e.g., a chirp, alarm
sound, recorded voice alert, tone, buzz, etc.).
There may be additional circumstances in which it may be desirable to
selectively inhibit the controlled amount of noise. For instance, noise may be
inhibited if a digital radio is monitoring the control channel at a base station without
receiving the data channel. On the other hand, if an audio communication is trunked
to more than one recipient, it may be desirable allow (i.e., not inhibit) the controlled
amount of noise at a first digital radio (e.g., the base station) based on the
transmission link quality at a second digital radio (e.g., a subscriber unit). This would
inform the base station user of the poor transmission link quality to the subscriber
unit.
There are at least two measurement metrics that can be used as an
indication of degraded communications for use in controlling the amount of noise to
add to demodulated baseband audio signals: First, the RSSI value can be used as a
control to inject a specified level of noise into the audio output of the radio. The noise
level can be made inversely proportional to the signal strength so that as the signal
strength decreases, noise increases. The radio operator can then move to a position of
better signal strength without needing to look at his radio.
Second, the error correction algorithm implemented in the receiver can
report how many errors the algorithm found, and how many errors the algorithm
corrected. As performance degrades, the number of both kinds of errors will rise.
The number of errors of either kind, or both together, can therefore be used to control
the amplitude of added noise to the composite audio output. This measurement metric
is suitable for both low signal powers, and for conditions that cause errors without
necessarily causing a loss of signal power. An example of the latter is errors that may
be induced by the presence of strong adjacent channel interference.
A combined approach can also be used, in which both the RSSI and the
count of detected and/or corrected errors can be used to determine the amount of
injected noise. This approach has the advantage of indicating failing communications
both in the presence of strong interference, as well as in weak RF signal strength
regions. If desired, a separate indicator such as a simple tone, complex tone, buzz or
the like could also be used to indicate interference, wherein the volume of the separate
indicator depends upon the error count or BER. A tone, buzz, or the like has the
advantage in that such an audio indicator may be familiar to operators of legacy
analog radios as indicating the presence of an adjacent channel interference.
Figure 1 is a qualitative comparison of analog radio performance to
digital radio, with and without additive noise, as the link quality is varied. The
abscissa is link quality, and the ordinate is a subjective intelligibility perception rating
(i.e., received signal merit). Curve 101 represents performance of analog radio. At
very good link qualities, analog radio does not have certain degradations such as
quantization noise which are inherent to digital modulation. As link quality degrades,
the perception rating gradually degrades. Curve 102 represents performance of digital
radios. The curve 102 perception rating is maintained at a high level for a large range
of link quality, but rapidly degrades beyond a threshold of link quality. Difference
104 between curves 101 and 102 represents the improvement afforded by use of
digital radio. Curve 103 represents performance of digital radios with additive noise
according to one or more embodiments of the invention. The curve 103 perception
rating is maintained at a level similar to the curve 102 perception rating for the better
link qualities. As more noise is added for poorer link qualities, the curve 103
perception rating approaches that of curve 101 for analog radio.
Referring now to FIG. 2, there is provided a simplified functional
block diagram of an output section 210 of a digital receiver that is useful for
understanding the present invention. For simplicity, output section 210 of a digital
receiver may be referred herein simply as the output section 210 when the
surrounding context is clear that the reference is to the output section 210 of the
digital receiver. Not shown is the input section of the digital receiver, including
amplifiers, filters, and other components known in the art to digital radio designers.
As shown in FIG. 2, the output section 210 is configured to accept an error-corrected,
demodulated and downconverted baseband signal 211, generated by a front end (not
shown) of the digital receiver.
Output section 210 is further configured to accept an RSSI value 215.
RSSI value 215 is generated in a front end (not shown) of the digital receiver, by
circuits and methods known to persons skilled in the art of RF radio receiver design.
RSSI value 215 is an indicator of the power of the RF energy received by the digital
receiver. Higher RSSI value 215 corresponds to higher received RF power. The RF
energy includes both a desired digital radio signal, and noise energy within a
predetermined bandwidth, the noise energy corrupting the desired digital radio signal.
The noise energy may include a broadband noise arising from, e.g., the noise floor of
the receiver. The noise energy may further include energy from one or more nonbroadband
noise sources, such as an unwanted interfering signal (e.g., an adjacent
channel transmission) received by the digital receiver. The predetermined bandwidth
for detecting RSSI may be determined by, e.g., a channel bandwidth of the receiver,
or a detection bandwidth of the demodulator circuit.
Output section 210 is further configured to accept an error count 216 of
the number of errors. Error count 216 is generated in a front end (not shown) of the
digital receiver, by the error detection and correction circuit (e.g., the ECC decoder).
The error count 216 may indicate the number of digital errors from the input RF
digital radio signal that were detected and/or corrected by the error detection and
correction circuit. Alternatively, error count 216 may indicate a bit error rate, rather
than a count of errors, so that the method is adaptable to different data rates of the RF
digital radio signal, or to changes in the data rate.
The error count 216 may also represent an error count based upon a
portion of the input RF digital radio signal, rather than upon the entirety of the input
RF digital radio signal. For instance, when a control channel and a data channel are
transmitted together but have separate ECC schemes, then separate error counts may
be available for each portion of the input RF digital radio signal. In this situation,
because the data channel is processed to form the demodulated baseband signal
presented to the radio operator, then if error counts are being used to control the
additive noise, it would be preferable to control the additive noise based upon an error
count of the data channel. However, an error count of the control channel could also
be used as long as the control channel error count is correlated with the data channel
error count.
Output section 210 includes a noise source 212 which may be a
broadband noise source such as a white noise source; or other kind of noise such as a
simple tone, a complex (i.e., multispectral) tone, a buzzing noise, or similar. The
output of noise source 212 is provided to a variable gain amplifier 213, which
produces an amplified noise. The gain of variable gain amplifier 213 is controlled by
a control signal 219 produced by combining circuit 217.
Combining circuit 217, included in output section 210, is configured to
accept the RSSI value 215 and the error count 216 as inputs, and is configured to
produce the control signal 219 that is used to control the gain of variable gain
amplifier 213.
In one embodiment, combining circuit 217 is configured to control
noise source 212 such that the noise level is inversely proportional to the RSSI value
215 within at least a predetermined range of RSSI values.
In another embodiment, combining circuit 217 is configured to control
noise source 212 such that the noise level is dependent on the error count 216, such
that a larger error count 216 produces a larger gain in variable gain amplifier 213
within at least a predetermined range of error count values.
In another embodiment, combining circuit 217 is configured such that
both the RSSI value 215 and error count 216 are used in order to control the noise
level according to the noise levels indicated by a combination of RSSI value 215 and
error count 216. Furthermore, the type of noise may be controlled by the relative
sizes of the RSSI value 215 and error count 216. For instance, if RSSI value 215 is
relatively high, then noise source 212 produces a broadband noise, regardless of the
size of error count 216. However, if RSSI value 215 is relatively high, but error count
216 is also relatively high, then noise source 212 may produce another kind of noise
such as a simple tone, a complex (i.e., multispectral) tone, a buzzing noise, or similar.
In another embodiment, the gain of variable gain amplifier 213 may be
controlled such that a duty cycle is imparted upon the noise signal to form bursts of
noise, and the duty cycle and/or repetition rate of the bursts is controlled by the
quality of the received radio transmission.
It is not desirable to have the digital radio outputting noise
continuously, from both battery life and radio operator fatigue considerations.
Therefore, embodiments of the present invention can gate (i.e., enable or disable) the
audio noise output upon: detection of incoming transmission (e.g., another radio
operator's communication) from a transmitting digital radio in communication with
the digital radio receiver and/or operation at an RSSI and/or error rate curve beyond
one or more thresholds. For example, no noise need be output when the radio is in a
strong signal strength area with low error correction rate, because there is unlikely to
be imminent link loss. Noise might be occasionally output in an increasingly marginal
coverage area, with more frequent or larger amplitude noise presented to a radio
operator when the digital radio communication is close to failing.
To this end, output section 210 further includes a thresholding circuit
218 in communication with the combining circuit 217 via interface 226.
Thresholding circuit 218 is configured to enable or disable addition of noise based on
the RSSI value 215 and error count 216. If RSSI value 215 is relatively high and
error count 216 is relatively low, then a very good received RF signal is indicated, and
addition of noise to the error corrected audio 211 is suppressed. Noise suppression,
when determined to be appropriate by thresholding circuit 218, is by use of gate 214.
Gate 214 produces gated noise, which is provided on interface 222. Addition of noise
is suppressed in this situation because there is little risk of imminent communication
loss, and therefore little need to inform the radio operator of the link condition.
Suppressing noise in this situation may conserve power usage by the radio, and will
improve intelligibility of the communication because of the elimination of
unnecessary noise, if the noise has not already been suppressed on account of the
detection of a radio operator's communication.
Conversely, if the RSSI value 215 is relatively low, then a very poor
link condition is indicated. If the RSSI value is beyond the ability of the ECC to
correct, then a signal may already be lost on the error corrected audio 211 input line,
and there would be no need to add noise in order to warn the radio operator of
impending signal loss. Suppressing noise in this situation may conserve power usage
by the radio.
The noise may be further gated by radio operator control, e.g., a
squelch button, such that the radio operator can selectively disable or enable the
addition of noise.
Figure 3 presents an alternative embodiment, in which gate 214 gates
the noise source 212 before the noise is applied to variable gain amplifier 213. A
disadvantage of this configuration is that noise generated within variable gain
amplifier 213, as quantified by the noise figure of variable gain amplifier 213, is not
suppressed before being added to the audio stream sent to the speaker.
Figure 4 presents an alternative embodiment, in which noise source
212 is configured to accept an enable signal 30 from threshold circuit 218. An
advantage of this configuration is that the on/off status of noise source 212 can be
positively controlled, thereby allowing for reduced power consumption by output
section 210 if the noise source 212 is turned off when not needed.
Returning again to FIG. 2, a combiner 223 is configured to accept the
error-corrected, demodulated and downconverted baseband signal 211 and the gated
noise from gate 214 via interface 222, in order to produce a composite audible signal
that is presented to a speaker (not shown) via interface 224. Similarly, referring to
FIGs 3-4, the combiner 223 is configured to accept the amplified noise signal from
variable gain amplifier 213, via interface 225, in order to produce a composite audible
signal that is presented to a speaker (not shown) via interface 224.
Referring now to FIG. 5, there is provided a flow chart of a method
according to an embodiment of the invention. The method begins with step 501, the
step of receiving a digital radio transmission in a digital radio receiver. This step
includes steps known to those skilled in the art of radio design.
The method continues with step 502, the step of detecting the quality
of the received digital radio transmission. The step of detecting the quality may be
accomplished by way of detecting the RSSI, and/or by detecting one or more error
counts provided by an ECC circuit. In one or more embodiments, step 502 may be
interchanged with step 503 described below.
The method continues with step 503, the step of decoding an audible
communication from the received digital radio transmission. This step involves
producing an audible baseband signal from the received digital RF signal. In one or
more embodiments, step 503 may be interchanged with step 502 described above.
The method continues with step 504, the step of superimposing an
audible indicator onto the audible communication, to form a composite audible signal.
The type of audible indicator may include broadband noise, a single tone, a complex
tone, and/or a buzz or the like.
The method continues with step 505, the step of dynamically adjusting
an amplitude of the audible indicator relative to an amplitude of the audible
communication responsive to a quality of the received digital audio transmission. In
one or more embodiments of the invention, the amplitude of the audible indicator may
be adjusted inversely proportionally to the quality of the received digital audio
transmission within at least a portion of the range in variation of quality - i.e., as the
quality increases, the amplitude of the audible indicator decreases relative to the
amplitude of the audible communication. In one or more embodiments, the audible
indicator may be turned off or adjusted to a substantially inaudible level if the quality
of the received digital audio transmission exceeds a first predetermined level. In this
situation, the quality of the received digital audio transmission is such that there is
little risk of imminent link loss, and therefore little need to inform the radio operator
of the link quality. In another embodiment, the audible indicator may be turned off or
adjusted to a substantially inaudible level if the quality of the received digital audio
transmission does not exceed a second predetermined level. In this situation, the
quality of the received digital audio transmission is so poor that the audible baseband
signal of step 503 cannot be produced (i.e., the signal is lost) or is already corrupted
or distorted because the capability of the ECC code has been exceeded. In another
embodiment, the audible indicator may be turned off or adjusted to a substantially
inaudible level if it is detected that the transmitting digital radio is actively
transmitting a radio operator's conversation.
CLAIMS
1. A method to provide an audible indicator of a quality of a received digital
radio transmission, comprising the steps of:
receiving a digital radio transmission in a digital radio receiver;
detecting the quality of the received digital radio transmission;
decoding an audible communication from the received digital radio
transmission;
superimposing an audible indicator onto the audible communication, to form a
composite audible signal; and
dynamically adjusting an amplitude of said audible indicator relative to an
amplitude of said audible communication responsive to a quality of said received
digital radio transmission.
2. The method of claim 1, wherein the step of detecting the quality comprises
detecting a received signal strength indicator of the received digital radio
transmission.
3. The method of claim 1, wherein the step of detecting the quality comprises
detecting a coded bit error rate of the received digital radio transmission.
4. The method of claim 1, wherein the step of detecting the quality comprises
detecting a signal to noise ratio of the received digital radio transmission.
5. The method of claim 1, further comprising selectively disabling the audible
indicator responsive to a user input to said digital radio receiver.
6. An apparatus to provide an audible indicator of a quality of a received digital
radio transmission, comprising:
a detector of the quality of the received digital radio transmission;
a decoder configured to decode an audible communication from the received
digital radio transmission;
a signal source configured to form an audible indicator signal for indicating
said received digital radio transmission has a predetermined signal quality; and
a combiner to superimpose the audible indicator onto the audible
communication, in order to form a composite audible signal.
7. The apparatus of claim 6, further comprising a first comparator to compare the
quality of the received digital radio transmission to a first predetermined threshold,
wherein the audible indicator is superimposed onto the audible communication only if
the quality of the received digital radio transmission is below the first predetermined
threshold.
8. The apparatus of claim 6, further comprising a second comparator to compare
the quality of the received digital radio transmission to a second predetermined
threshold, wherein the audible indicator is superimposed onto the audible
communication only if the quality of the received digital radio transmission is above
the second predetermined threshold.
9. A method to detect a narrowband interference of a received digital radio
transmission, comprising the steps of:
receiving a digital radio transmission in a digital radio receiver;
detecting a received signal strength indicator of the received digital radio
transmission;
detecting a coded bit error rate of the received digital radio transmission; and
detecting a narrowband interference if the received signal strength indicator
exceeds a first predetermined threshold, and if the coded bit error rate exceeds a
second predetermined threshold.
10. The method of claim 1, further comprising the step of:
presenting an alert signal to a user of the digital radio receiver, wherein the
alert signal is separate from the audible communication.
