Diversity reception system

A diversity reception system for use in narrow-band radio frequency communication systems includes a plurality of receivers that each generate a detected signal from a received radio frequency signal. The diversity reception system switches between receivers to output a high quality audio signal. The system selects a receiver having the highest quality reception at any given time by using a detected signal having the highest relative signal-to-noise ratio when the detected signals have less than a preselected threshold signal-to-noise ratio and a detected signal having the highest relative signal strength when one or more of the receivers is generating a detected signal having a signal-to-noise ratio at or above the preselected threshold signal-to-noise ratio.

TECHNICAL FIELD 
The present invention relates to narrow-band radio frequency communication 
systems, and more particularly to diversity reception systems that reduce 
the damaging effects of multipath fading in mobile communication 
receivers. 
BACKGROUND OF THE INVENTION 
Mobile radio communication systems rely upon radio frequency signals to 
transmit data. The quality of a received signal in such a system depends 
upon the strength of a carrier signal relative to any noise signal that is 
introduced during transmission or by the circuitry of the communication 
system. The relative strength of the received signal is affected by the 
strength of the transmitted signal and by the distance between the 
receiver and the transmitter. As the distance increases, received signal 
strength tends to deteriorate. 
In addition, the signal does not usually travel solely along a direct path 
from the transmitter to the receiver. Although the direct path is one 
potential propagation path, other possible paths exist. For example, the 
signal may reflect off of objects that are large with respect to the 
signal wavelength, such as the side of a building, thereby causing the 
signal to travel along a reflected path. The signal may be refracted by a 
knife-edge surface, such as the corner of a building, thereby causing the 
signal to travel along a refracted path. Finally, an object that is small 
relative to the wavelength, such as a traffic light, may cause the signal 
to scatter, thereby causing the signal to travel to the receiver along a 
scattered path. 
Thus, the signal may travel from the transmitter to the receiver along a 
direct path, a reflected path, a refracted path, a scattered path, or some 
combination thereof. When the signal travels along multiple paths, each of 
the multiple coherent signals travels a different distance between the 
transmitter and receiver. As a result, each signal has a somewhat random 
phase and amplitude. The phase and amplitude of the overall received 
signal results from the vector addition of the multiple coherent signals. 
In some instances, this combination results in an improvement in the 
strength of the received signal (constructive interference). In other 
cases, the received signal strength is degraded (destructive interference) 
by the multiple path (multipath) propagation. 
In the mobile environment, multipath fading occurs as the receiver moves 
from a zone of constructive interference to a zone of destructive 
interference. As the vector sum of the multiple coherent signals varies 
over time, there are significant variations in the strength of the overall 
carrier signal with respect to the strength of the noise signal. Although 
these variations may exist when the transmitter-to-receiver distance is 
static, multipath fading tends to increase with increases in the relative 
velocity of the transmitter and receiver as the receiver travels between 
zones of constructive and destructive interference. Because of the 
significant variations in received signal strength, multipath fading will 
sometimes cause the strength of the noise signal to instantaneously exceed 
that of the carrier signal. When this occurs, the received signal may 
experience a 360 degree phase rotation, which will cause clicks or pops in 
the received audio signal. 
One way to diminish the effects of multipath fading is to employ a 
diversity reception system. A diversity reception system uses a plurality 
of receivers and selects between receivers to generate an improved overall 
signal. The receivers in a diversity reception system are individually 
coupled to antennas that are spatially separated from one another. When 
one receiver is experiencing a fade caused by the multipath propagation of 
the carrier signal, another receiver may have better reception because of 
the spatial separation of the receivers. By selecting the receiver with 
the best reception, the overall audio signal produced by the diversity 
reception system can be improved. 
In one type of diversity reception system, the system selects the active 
receiver according to the RF input signal level of each receiver. Use of 
receiver RF input signal level as the selection criterion, however, is not 
always effective because the receiver RF input signal level does not 
provide an accurate indication of signal quality at low RF input signal 
levels. A diversity reception system may also rely on the signal-to-noise 
ratio of the receiver output signals to select the active receiver. Use of 
the signal-to-noise ratio of the output signal as the selection criterion, 
however, is also problematic because the signal-to-noise ratio tends to 
become saturated at high RF signal levels. 
Therefore, a diversity reception system is needed that provides for 
improved receiver selection at both high and low RF signal input levels. 
The present invention allows for receiver selection at high and low RF 
input signal levels by selecting the receiver with the highest output 
signal-to-noise ratio when the receiver is delivering less than a 
threshold signal-to-noise ratio and selecting the receiver with the 
highest RF input signal level otherwise. 
SUMMARY OF THE INVENTION 
The present invention comprises a multi-branch diversity reception system 
for reducing the damaging effects of multipath fading in a mobile radio 
environment. Each branch of the system includes a receiver for receiving 
radio frequency (RF) input signals and for generating a detected audio 
signal in response to an RF input signal. The system of the present 
invention also includes a receiver selector that selects between the 
plurality of receivers. The receiver selector selects the receiver having 
a detected audio output signal with the highest signal-to-noise ratio when 
the receivers are delivering less than a maximum achievable 
signal-to-noise ratio. When the receivers are delivering a detected audio 
output signal having a signal-to-noise ratio that is at the maximum 
achievable signal-to-noise ratio, the receiver selector selects the 
receiver having the highest RF input signal level. 
Each receiver includes a Received Signal Strength Indicator (RSSI) that 
generates a voltage signal indicative of the received signal strength. A 
diversity selection controller in each branch of the system equalizes the 
RSSI voltage signal in each branch with respect to the RSSI voltage 
signals in the other branches of the system to adjust for variations in 
RSSI and receiver performance. In accordance with the present invention, 
each diversity reception controller is calibrated for adjusting RSSI 
voltage signals to accurately indicate the signal-to-noise ratio of the 
receiver output when the receiver is delivering less than the maximum 
achievable signal-to-noise ratio. Each diversity reception controller is 
further calibrated for adjusting RSSI voltages to accurately indicate the 
RF signal input level when the receiver is delivering the maximum 
achievable signal-to-noise ratio. Once the diversity reception controllers 
are calibrated, a receiver selector compares the adjusted RSSI voltages of 
all receivers to select the receiver with the best reception at any given 
time. The receiver selector outputs the detected signal of the selected 
receiver as determined by the diversity selection process. 
In accordance with another feature of the invention, each branch of the 
diversity reception system includes a detected signal input stage for 
adjusting the amplitude of the detected audio signals to compensate for 
variations in detector output levels and to remove any DC bias present. In 
addition, each receiver in the diversity reception system includes a low 
noise amplifier for amplifying the received signals. A circuit for 
disabling the low noise amplifier at high RF input signal levels is also 
provided. By disabling the low noise amplifier at high RF input signal 
levels, the system prevents saturation of the RSSI voltages and allows the 
diversity selection process to continue at higher input signal levels than 
would otherwise be possible.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is now made to the Drawings wherein like reference characters 
denote like or similar parts throughout the various Figures. The diversity 
reception system of the present invention functions as a post-detection 
system by receiving transmitted signals that have already been detected by 
a discriminator or demodulator. The system has utility in many narrow band 
radio systems and has application in both voice and data communication 
systems. 
A diversity reception system selects between multiple receivers to generate 
a higher quality audio signal than can be generated by a system having a 
single receiver. The quality of the audio signal generated by a diversity 
reception system depends upon the ability of the system to select the 
receiver with the best signal reception at any given time. The improvement 
in the overall audio signal output produced by a diversity reception 
system is referred to as the diversity improvement factor. A diversity 
reception system avoids the "click" noise phenomenon caused by multipath 
fading by changing between receivers to produce an overall audio output 
signal that does not include the deep signal fades that occur in single 
receiver systems. 
At low RF signal input levels, the quality of reception by a receiver at a 
particular instant is determined by the signal-to-noise ratio of the 
receiver output signal. Generally, the signal-to-noise ratio of a receiver 
output signal increases with input signal strength. At some level, 
however, a receiver reaches a maximum achievable signal-to-noise ratio, 
and further increases in the RF input signal level cease to have any 
noticeable effect on the signal-to-noise ratio of the receiver output 
signal. 
One type of diversity reception system measures the signal-to-noise ratio 
of output signals of the system receivers to select among the multiple 
receivers. A diversity reception system that uses only the measured 
signal-to-noise ratio as the selection criterion will cease to function 
properly in areas where the RF input signal level is at or above the level 
required to produce the maximum signal-to-noise ratio. Because the 
signal-to-noise ratio stops changing once the maximum level is reached, 
receiver selection in the diversity reception system is not changing 
either. This type of system is prone to the high-level "click" noise 
phenomenon that results from multipath fading. 
Another type of diversity reception system uses a Received Signal Strength 
Indicator (RSSI) to measure the RF input signal strength of each receiver. 
At relatively high RF signal levels, the strength of the received signal 
provides a fairly accurate indication of the quality of the received 
signal. The RSSI generates a voltage that is proportional to the input 
signal strength. A diversity system controller selects the receiver with 
the highest RSSI voltage, and the system outputs the audio signal from the 
selected receiver. 
A diversity reception system that uses the RSSI voltage as the sole 
selection criterion exhibits two operational flaws that act to reduce the 
effectiveness of the system. The first flaw stems from a lack of accuracy 
in the quantification of a receiver's RF input signal level. The highest 
quality RSSI circuitry available will indicate received signal strengths 
with a +/-1.5 dB accuracy over the RSSI's linear range. RSSI performance 
is linear from approximately -124 dB to about -30 dB. The RSSI's accuracy 
further degenerates at both extremes of the RSSI's range. Referring to 
FIGS. 1A and 1B, there is illustrated how a +/-1.5 dB inaccuracy in the 
RSSI voltage affects the process of selecting between receivers 102. Two 
assumptions are made for purposes of this illustration. First, the output 
signal-to-noise ratio of the receivers 102 is assumed to be identical when 
the RF input signal level for each of the two receivers 102 is the same; 
normally, receivers do not have such identical performance. Second, the 
RSSI's accuracy is assumed to be +/-1.5 dB. At the lower extreme of the 
RSSI's range, however, an RSSI is not normally this accurate. These two 
assumptions are made for ease of explanation; in an actual application, 
reliance on RSSI voltage as the selection criterion is more problematic 
than this illustration indicates. 
The performance of two receivers 102 is measured by connecting an RF signal 
generator 104 to an input terminal 106 of each receiver 102 and an audio 
analyzer 112 to an audio output terminal 110 of each receiver. The RF 
signal generator 104 outputs a controlled input signal, and the audio 
analyzer 112 measures the signal-to-noise ratio (SNR) of the audio signal 
output from each receiver 102. The receivers 102 also include an RSSI 
output terminal 108. An RSSI circuit contained within each receiver 102 
outputs a voltage proportional to the received signal strength. 
In this example, the RF signal generator 104 connected to a first receiver 
102.sub.1 transmits a signal with a level of -116.0 decibels above 1 
milliwatt (dBm) to the first receiver 102.sub.1. With this input signal 
level, the first receiver 102.sub.1 produces an audio output signal with a 
12.0 dB signal-to-noise ratio, as measured by the audio analyzer 112. The 
RSSI output on the line 108 of the first receiver 102.sub.1 indicates that 
the input signal has a level of -114.5 dBm. Thus, the RSSI output has a 
+1.5 dB error. Another signal generator 104 connected to a second receiver 
102.sub.2 transmits a signal with a level of -113.1 dBm to the second 
receiver 102.sub.2. In this case, the audio analyzer 112 indicates that 
the audio output signal from the second receiver 102.sub.2 has an 18.0 dB 
SNR. The higher SNR generated by the second receiver 102.sub.2 as compared 
to the first receiver 102.sub.1 results from the higher input signal level 
transmitted to the second receiver 102.sub.2. The RSSI voltage of the 
second receiver 102.sub.2, however, indicates a signal level of -114.6 
dBm. Thus, the RSSI voltage output for the second receiver 102.sub.2 has a 
-1.5 dB error. If the active receiver 102 in a diversity reception system 
is selected based upon RSSI voltage, the first receiver 102.sub.1 would be 
selected because it has a higher RSSI voltage. Selecting the first 
receiver 102.sub.1, however, is inappropriate because the second receiver 
102.sub.2 has a higher signal quality as indicated by the higher 
signal-to-noise ratio reading on the audio analyzer 112. Use of the RSSI 
voltage in this example, therefore, would result in inefficient receiver 
selection. 
A second operational flaw caused by using the RSSI voltage as the selection 
criterion stems from a lack of correlation between a receiver's RF input 
signal level and the signal-to-noise ratio of the receiver's output. 
Receiver performance varies from receiver to receiver even when the 
receivers 102 have an identical design. One receiver 102 may generate an 
audio output signal with a high signal-to-noise ratio at a lower relative 
input signal level than another receiver 102. If received signal strength, 
as indicated by the RSSI voltage, is the criterion for selecting a 
particular receiver 102 and the receivers do not have identical 
performance regarding the relationship between receiver RF input signal 
level and receiver output signal-to-noise ratio, then the improvement in 
the overall audio signal will be reduced from a system that uses output 
signal-to-noise ratio as the selection criterion. 
Referring to FIGS. 2A and 2B, there is illustrated how variations in 
receiver performance between different receivers 102 reduces the diversity 
improvement factor. In this illustration the RSSI voltages are assumed to 
be accurate. As discussed in connection with FIGS. 1A and 1B, however, 
RSSI voltages normally have no more than a +/-1.5 dB degree of accuracy. 
In this example, an RF signal generator 104 transmits a signal with a level 
of -116.0 dBm to the first receiver 102.sub.1. The RSSI voltage output on 
line 108.sub.1 accurately indicates that the received signal has a level 
of -116.0 dBm. The first receiver 102.sub.1 produces an audio output 
signal with a 12.0 dB signal-to-noise ratio with the -116.0 dBm input 
signal level. An RF signal generator 104 coupled to the input 106.sub.2 of 
a second receiver 102.sub.2 transmits a signal with a level of -116.5 dBm. 
The RSSI voltage output on line 108.sub.2 accurately indicates an input 
level of -116.5 dBm. In this example, the second receiver 102.sub.2 has a 
higher efficiency than the first receiver 102.sub.1 and produces an audio 
output signal with a 18.0 dB signal-to-noise ratio corresponding to the 
-116.5 dBm input level. If a diversity reception system selects receivers 
based upon the RSSI voltage, the first receiver 102.sub.1 would be 
selected because it has a higher input signal strength. Selecting the 
first receiver 102.sub.1 is inappropriate in this case because the second 
receiver 102.sub.2 generates an audio output signal having a higher 
signal-to-noise ratio. Use of the RSSI voltage in this example, therefore, 
would result in inefficient receiver selection. 
Referring now to FIG. 3, there is illustrated a block diagram of a 
two-branch diversity reception system 100. The diversity reception system 
100 selects from the receivers 102 in the two branches by transmitting an 
audio output on line 110 of a receiver 102 having the highest 
signal-to-noise ratio when the receivers 102 are generating output signals 
having less than the maximum achievable signal-to-noise ratio, and by 
transmitting the output of a receiver 102 with the highest RF input signal 
level when the receivers 102 are generating the maximum achievable 
signal-to-noise ratio. The selection of the active receiver 102 based on 
receiver signal-to-noise ratio as the selection criterion at low RF input 
levels and on the RF input signal strength of the receivers 102 as the 
selection criterion at high RF input levels enables the system to generate 
the best possible overall output signal. 
The diversity reception system 100 includes two branches each having an 
antenna 120, a receiver 102, a detected signal input stage 140, and a 
diversity reception controller 160. An RF input signal is received by the 
antenna 120 and transmitted to the receiver 102 through an input signal 
terminal 106. The receiver 102 includes a detected signal terminal 110 and 
an RSSI output line 108. The detected signal terminal 110 is coupled to a 
detected signal input stage 140. The detected signal input stage 140 
buffers the detected signals from the receiver 102 and provides a means to 
adjust the amplitude of the signals to compensate for variations in 
detector output levels by equalizing the detected signal levels. The 
equalization of the detected signals minimizes any transients that would 
be caused by switching between receivers 102 that generate detected 
signals having unequal levels. In addition, the detected signals may 
include a DC bias that differs between receivers 102. The detected signal 
input stage 140 high-pass filters the signals to remove any DC bias 
present. The RSSI output line 108 connects to a diversity reception 
controller 160 that adjusts the RSSI voltage to compensate for differences 
in RSSI performance between the receivers 102. 
A receiver selector 180 selects an active receiver 102 by comparing the 
different diversity reception controller output signals transmitted along 
lines 162. The receiver selector 180 also receives the adjusted audio 
output from each receiver 102 along lines 142. To select the receiver 102 
having the highest quality reception, the receiver selector 180 outputs 
the adjusted RSSI voltage of the selected receiver 102 on an RSSI line 182 
and outputs the audio signal of the selected receiver 102 on an audio line 
184. Audio line 184 is coupled to an output filter 200 for removing any 
high speed transients that may be induced by high-speed switching between 
receivers 102. The output filter 200 generates a diversity system audio 
output on an audio output line 202. A frequency-shift keying (FSK) data 
slicer 204 is coupled to the audio output line 202. The frequency-shift 
keying data slicer 204 generates diversity system data outputs at a data 
output terminal 206 by decoding data messages that may be encoded in the 
audio signal. 
The diversity reception system 100 also includes a gain reducing circuit 
190 that responds to the adjusted RSSI output voltage of the selected 
receiver 102 to control the operation of amplifiers included in each 
receiver 102. By disabling the amplifiers in the receivers 102 at high RF 
input signal levels, the gain reducing circuit 190 enables the diversity 
selection process to continue at higher input levels than would otherwise 
be possible. 
Increasing the number of branches in the system 100 increases the diversity 
improvement factor of the system if the antennas 120 are placed in an 
efficient configuration. The antenna spacing required for efficient system 
operation is primarily a function of the wavelength of the transmitted 
signals. 
The degree of overall correlation between receivers 102 is reduced by 
increasing the number of receivers 102. The incremental improvement that 
results from adding receivers, however, decreases with each additional 
receiver. For example, the addition of a third receiver provides a greater 
increase in the diversity improvement factor than the further addition of 
a fourth receiver; the addition of a fourth receiver provides a greater 
increase in the diversity improvement factor than the further addition of 
a fifth receiver; and so on. In one preferred embodiment, the diversity 
reception system 100 includes three receivers 102. Each receiver 102 is 
individually coupled to a respective antenna 120. By placing each antenna 
120 at a corner of an imaginary equilateral triangle, there is a low 
likelihood of correlation between the three received signals if a properly 
sized triangle is used. Thus, use of a triple antenna configuration 
reduces the occurrence of fading to a greater extent than does a 
two-branch diversity reception system. 
Referring now to FIG. 4, there is illustrated a more detailed block diagram 
of a receiver 102 in the diversity reception system 100. The receiver 102 
includes a low noise amplifier 114 for amplifying the RF input signals. 
The low noise amplifier 114 produces approximately 15 dB of gain in the 
receiver 102. The amplified signal is output along line 118 and is 
detected by a detector 130. The detector 130 includes a demodulator and/or 
a discriminator for generating an RF detected signal from the signal 
received by the antenna 120. The detected signal is output at an audio 
output terminal 110. The receiver 102 also includes an LNA DISABLE line 
192 for receiving signals from the gain reducing circuit 190 (FIG. 3). A 
logic high output from the gain reducing circuit 190 disables the 
amplifier 114 and interrupts the 15 dB gain of the amplifier. When the low 
noise amplifier 114 is disabled, the amplifier generates approximately 7 
dB of loss in the RF signal, thereby reducing the audio output by a total 
amount of about 22 dB. A Received Signal Strength Indicator 116 (RSSI) 
measures the signal strength of the signal produced along line 118 from 
the amplifier 114 and outputs a voltage indicative of the signal strength 
at an RSSI output port 108. 
Referring now to FIG. 5, there is illustrated a circuit for calibrating a 
diversity reception controller 160 in a diversity reception system 100. 
The diversity reception controller 160 includes a low pass filter 164 
coupled to the RSSI output line 108. The filter output is coupled to an 
RSSI processor 166. The low-pass filter 140 removes any high frequency 
components of the RSSI voltage signals while preserving the 
lower-frequency amplitude fluctuations produced by relative motion-induced 
multipath fading. Although the low-pass filter 140 improves the response 
of the receiver selection system in general, the filter is particularly 
useful for improving system response at low RF input levels. 
The RSSI processor 166 contains level adjusting circuitry that enables 
tuning of the processor output level with respect to the level of the RSSI 
voltage signal generated by the receiver 102. Each RSSI processor 166 in 
the diversity reception system 100 is individually calibrated to produce a 
voltage indicative of the signal-to-noise ratio when the receiver 102 is 
generating audio output signals having less than a selected threshold 
signal-to-noise ratio. Further calibration of the RSSI processor 166 also 
enables the RSSI processor to produce a signal indicative of the RF input 
signal level once the audio output signal of the receiver reaches the 
selected threshold signal-to-noise ratio. In the preferred embodiment, a 
signal-to-noise ratio value that is at or near the maximum achievable 
signal-to-noise ratio is selected as the threshold signal-to-noise ratio. 
The calibration procedure is performed before the receiver 102 is installed 
in the diversity reception system 100. In the calibration procedure, an RF 
signal generator 104 is coupled to the input signal line 106 of the 
receiver 102 for generating controlled RF input signals. An audio analyzer 
112 is coupled to the audio output line 110 for measuring the 
signal-to-noise ratio of the audio signal from the receiver 102, and a 
digital voltage meter (DVM) 164 is coupled to the output of RSSI processor 
166 for measuring the processed, low-pass filtered RSSI voltage signal. 
The signal generator 104 is first adjusted to generate a receiver input 
signal having sufficient amplitude to produce a first specified 
signal-to-noise ratio (normally 12.0 dB) as measured by the audio analyzer 
112. The level adjusting circuitry in the RSSI processor 166 is then tuned 
to output a preselected voltage, as measured by the DVM 164, 
representative of the first specified signal-to-noise ratio. 
The calibration process continues by increasing the signal generator 104 
output until the receiver 102 produces an audio output signal with a 
second signal-to-noise ratio (22.0 dB, for instance). The level adjusting 
circuitry in the RSSI processor 166 is adjusted to further tune the 
processor output to produce a second preselected voltage, as measured by 
the DVM 164. The second preselected voltage is representative of the 
second signal-to-noise ratio. The calibration process is repeated at a 
plurality of different signal-to-noise ratio values until the maximum 
signal-to-noise ratio (typically around 40 dB) is reached. At this point, 
further increasing of the signal generator's output will have no effect on 
the signal-to-noise ratio of the recovered signal. By repeating the 
calibration process at a plurality of different signal generator 104 
levels for each of the receivers 102, differences in the performance of 
each receiver 102 can effectively be removed by properly adjusting the 
level adjusting circuitry of each RSSI processor 166. This equalization of 
receiver performance permits the receiver selector 180 to compare the RSSI 
processor outputs to accurately determine which receiver 102 is producing 
the best signal-to-noise ratio when operating at less than the maximum 
achievable signal-to-noise ratio. 
Once the RSSI processor 166 is calibrated across the dynamic range of 
signal-to-noise ratios, the calibration procedure is continued by further 
increasing the signal generator output for the receiver 102 above the 
level required to produce the maximum achievable signal-to-noise ratio. In 
the preferred embodiment, the level of the RF signal input to the receiver 
102 is increased by 50 dB. The level adjusting circuitry is tuned to 
output a voltage representative of this higher input signal level. By 
following this procedure for each RSSI processor 166, the RSSI output of 
each receiver 102 is calibrated to allow for accurate comparisons between 
the RSSI voltages. Once the RSSI processors 166 are fully calibrated, the 
diversity reception system 100 is set to select the receiver 102 producing 
the highest signal-to-noise ratio when operating below the maximum 
achievable signal-to-noise ratio and adjusted to select the receiver 102 
having the highest RF signal level otherwise. 
With some receivers 102 it may be possible to use an abbreviated 
calibration procedure in which the receivers are calibrated at just two 
receiver RF input signal levels. In an abbreviated calibration procedure, 
the signal generator 104 is first adjusted to generate an input signal 
that produces an audio signal having less than the maximum achievable 
signal-to-noise ratio. The level adjusting circuitry is calibrated to 
output a specified voltage at the selected signal-to-noise ratio. The 
signal generator 104 is then adjusted to produce a second RF signal input 
level that is above the level necessary to produce the maximum achievable 
signal-to-noise ratio, and the level adjusting circuitry is calibrated to 
output a higher specified voltage for the second specific input level. 
Using either the full or the abbreviated calibration procedure in each 
branch of the diversity reception system permits efficient selection among 
receivers by effectively removing the effects of measurement inaccuracies 
in RSSI circuitry. The calibration procedure also adjusts for differences 
among receivers in the degree of correlation between receiver RF input 
signal levels and receiver output signal-to-noise ratios. Thus, the 
diversity reception system of the present invention represents an 
improvement over systems that solely rely on RSSI voltages as the 
selection criterion. In addition, unlike systems that use only the 
signal-to-noise ratio as the selection criterion, the calibration 
procedure described above also allows for diversity selection to continue 
even after a receiver reaches its maximum achievable signal-to-noise 
ratio. 
Referring now to FIG. 6, there is shown a block diagram of the receiver 
selector 180. The receiver selector 180 includes a comparator 186 for 
comparing the adjusted RSSI voltages that are output from the RSSI 
processors 166 in the various branches of the diversity reception system 
100. The comparator 186 generates a signal indicating the branch of the 
system having the highest adjusted RSSI voltage level at any given time. 
The comparator signal output is coupled to an RSSI selector 189 and an 
audio selector 188. In response to the comparator signal output, the RSSI 
selector 189 outputs the RSSI voltage (RSSI SELECT) of the selected 
receiver 102, and the audio selector 188 outputs the audio signal output 
(AUDIO SELECT) of the selected receiver 102. Thus, the receiver selector 
180 outputs only the signals from the receiver 102 that has the highest 
quality audio output at any given time. 
The receiver selector 180 shown is FIG. 6 includes two branches. As 
discussed above, a diversity reception system 100 may include additional 
branches for increasing the diversity improvement factor. The receiver 
selector 180 in such a system 100 may select among three or more branches 
by using a comparator 186 that compares the three or more RSSI voltages. 
The RSSI selector 189 and the audio selector 188 select among the RSSI and 
audio signals from the three or more branches in response to the 
comparator output. 
In another embodiment, the receiver selector 180 includes additional 
comparator/selector stages identical to the single stage shown in FIG. 6. 
For example, in a three-branch system, a second-stage comparator 186 
compares the RSSI output from the first comparator/selector stage with the 
adjusted RSSI output from a third branch of the system 100. The RSSI 
selector 189 and the audio selector 188 in the second stage then output 
the RSSI and audio signals from the selected receiver 102 in accordance 
with the second-stage comparator output signal. 
Referring again to FIG. 3, the diversity reception system includes a gain 
reducing circuit for controlling the operation of the low noise amplifier 
114 (shown in FIG. 4) in each receiver 102. RSSI voltages tend to become 
saturated at relatively high RF input levels. Once the RF input reaches a 
certain level, the RSSI 116 in the receiver 102 is limited and will not 
generate voltage outputs that are proportional to further increases in the 
RF input level. When the RSSI voltage becomes saturated, the RSSI 
processor 166 will not produce a voltage output that accurately indicates 
the high quality of the RF input signal. The damaging effects of multipath 
fading occur, however, even at high RF signal levels. To alleviate this 
situation, the diversity reception system 100 includes gain reducing 
circuitry 190 that reduces the signal gain introduced by the low-noise 
amplifier 114 in the receiver 102. 
The gain reducing circuit 190 detects the level of the RSSI signal received 
from the receiver selector 180 along line 182. Upon detecting an RSSI 
signal of sufficient amplitude, the gain reducing circuit 190 produces a 
logic level output (LNA DISABLE). This logic level output turns off the 
low-noise amplifiers 114 in the receivers 102. Disabling the low-noise 
amplifier 114 reduces the RF input to the receiver 102 by about 22 dB, 
essentially adding another 22 dB of dynamic range at the upper end of the 
signal strength range. Normally, the receiver selection process would not 
continue when the RSSI becomes saturated because the RSSI voltages are not 
changing. If the RSSI voltages are not changing, the selected receiver 102 
will not change either. By reducing the gain introduced in the receiver 
102, the gain reducing circuitry 190 allows the diversity selection 
process to continue at higher signal levels and ensures that diversity 
improvement will extend into areas with even the highest signal levels. 
The diversity reception system 100 also supplies a voltage proportional to 
signal quality, as generated by the diversity selection process, to an 
external signal quality device 196. The external device 196 displays an 
indication of signal strength to a user of the communication system. 
Turning off the low-noise amplifiers 114 in response to an LNA DISABLE 
signal, however, produces an abrupt change in the RSSI voltage generated 
by the receivers 102. This abrupt change causes a discontinuity in the 
signal quality voltage and will result in an inaccurate reading of the 
signal quality by the external signal quality device 196. To avoid the 
discontinuity in the signal quality voltage, the diversity reception 
system 100 includes a voltage adder 194. The voltage adder 194 responds to 
the LNA DISABLE signal by adding to the signal quality voltage a voltage 
equal in magnitude to the abrupt change in the RSSI voltage. Thus, the 
voltage adder 194 minimizes the discontinuity in the voltage supplied to 
the external signal quality device 196 allowing the device 196 to indicate 
a high signal quality even when the internal gain of the system is 
reduced. 
Although a preferred embodiment of the invention has been illustrated in 
the accompanying drawings and described in the foregoing Detailed 
Description, it will be understood that the invention is not limited to 
the embodiment disclosed, but is capable of numerous rearrangements and 
modifications of parts and elements without departing from the spirit of 
the invention.