U.S. patent application number 09/738824 was filed with the patent office on 2001-09-06 for system and method for dynamic amplitude adjustment of modulating signal in frequency modulated transceivers.
Invention is credited to Castagna, Peter, Lee, Richard, McDonald, Patric, McMillen, Mark, Read, Dick.
Application Number | 20010019580 09/738824 |
Document ID | / |
Family ID | 25406625 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019580 |
Kind Code |
A1 |
McDonald, Patric ; et
al. |
September 6, 2001 |
System and method for dynamic amplitude adjustment of modulating
signal in frequency modulated transceivers
Abstract
A system and method for dynamically optimizing occupied
bandwidth of a direct modulated FM radio is disclosed, in which a
remote receiver periodically monitors the received output of a
local transmitter and compares the received output occupied
bandwidth to a reference level. If the monitored level of the
demodulated signal unacceptably exceeds the reference level an
error message is developed and sent back to the local transmitter
to cause the output of the local transmitter to be adjusted to more
closely conform to the desired spectrum output mask. An alternative
embodiment is also disclosed in which temperature and operating
frequency are monitored at the local transmitter and a correction
factor appropriate for that operating temperature and frequency are
applied.
Inventors: |
McDonald, Patric; (Granite
Bay, CA) ; Lee, Richard; (Bellevue, WA) ;
Castagna, Peter; (Renton, WA) ; McMillen, Mark;
(Seattle, WA) ; Read, Dick; (Bremerton,
WA) |
Correspondence
Address: |
GRAHAM & JAMES LLP
600 Hansen Way
Palo Alto
CA
94304-1043
US
|
Family ID: |
25406625 |
Appl. No.: |
09/738824 |
Filed: |
December 15, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09738824 |
Dec 15, 2000 |
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08896682 |
Jul 18, 1997 |
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6208686 |
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Current U.S.
Class: |
375/219 |
Current CPC
Class: |
H04B 1/1027 20130101;
H04B 17/18 20150115 |
Class at
Publication: |
375/219 |
International
Class: |
H04B 001/38 |
Claims
What is claimed is:
1. A method for optimizing transmitter output of a digital
microwave radio including the steps of monitoring the output of the
transmitter, generating a correction signal in response to the
monitored output, summing the correction signal with a conventional
signal to conform the transmitter output to a predetermined mask,
and repeating the monitoring, generating and adding steps
periodically.
2. A method for optimizing transmitter output of a digital
microwave radio including the steps of monitoring a characteristic
of the transmitter, generating a correction signal in response to
the monitored characteristic, summing the correction signal with a
conventional signal to conform the transmitter output to a
predetermined mask, and repeating the monitoring, generating and
adding steps periodically.
3. The method of claim 2 in which the monitored characteristic is
temperature.
4. The method of claim 2 in which the monitored characteristic is
the transmitter output occupied bandwidth.
5. A method for optimizing transmitter output of a digital
microwave radio including the steps of monitoring at a remote
receiver the output of the transmitter, generating an error message
in response to the monitoring step, developing a correction signal
in accordance with the error message, and adding the correction
signal to the transmitter FM modulator occupied bandwidth to
conform the transmitter occupied bandwidth to a predetermined mask.
repeating the monitoring, generating, developing and adding steps
periodically.
6. In a digital microwave radio having an outdoor unit and
operating at a preselected frequency band, a method for optimizing
transmitter output including the steps of monitoring the
temperature of the outdoor unit, selecting, from a lookup table, a
correction factor corresponding to the monitored temperature and
operating frequency band, generating a correction signal in
response to the correction factor, and adding the correction signal
into the transmitter to optimize transmitter output occupied
bandwidth.
7. A digital microwave radio having transmitter and receiver
portions comprising a bandwidth monitoring circuit for detecting
the bandwidth transmitted from a remote radio, error logic for
developing a digital error message representative of the difference
between the monitored bandwidth and a reference value, a feedback
circuit for providing the digital error message to the remote
radio, and amplifier means for adjusting the output bandwidth in
response to the digital error message.
Description
SPECIFICATION
[0001] 1. Field of the Invention
[0002] The invention relates to digital microwave radios, and in
particular relates to methods and systems for stabilizing FM
deviation or occupied bandwidth of the transmit output signal of
digital FM microwave radios.
[0003] 2. Background of the Invention
[0004] Digital radio terminals have become particularly
advantageous in a number of key types of communication. High
frequency point to point communications are used by, among others,
cellular operators, telecommunications operators, private network
operators, governments, and large telecommunications
operations.
[0005] While many modulation techniques are available for use on
microwave digital radios, such as QPSK, QAM and so on, cost and
other issues have militated in favor of the use of direct modulated
oscillators, or direct modulated FSK systems.
[0006] Operators of such microwave digital radios are typically
assigned to specific frequencies, or channels, for their
communications. Each channel is characterized by a center frequency
and a spectrum emission mask or template which permits a higher
energy level at the center frequency and decreasing energy levels
as the transmitted frequency diverges (in either direction) from
the center frequency. The spectrum emission mask, sometimes
referred to simply as the "mask," is defined by the federal
government, and transmissions outside the mask can interfere with
transmissions on adjacent channels as well as resulting in serious
adverse consequences to the system operator. Such interference with
neighboring channels is referred to "stepping on" those
channels.
[0007] Superficially, it would seem to be straightforward to avoid
stepping on adjacent channels simply by setting the direct
modulated FSK occupied bandwidth of the system. However, this has
been proven not to be correct. Most importantly, it is now
recognized that the transmit occupied bandwidth--and therefore
radio performance--can vary significantly with temperature or
frequency in a direct modulated FM system. While temperature varies
relatively slowly, it can vary over a significant range. This can
cause a carefully tuned output spectrum to exceed the mask.
[0008] The historical approach to compensate for temperature
variations and avoid stepping on adjacent channels has been to
reduce the maximum bandwidth; however, this has the unacceptable
effect of reducing the FM demodulated signal-to-noise ratio. This
reduction in the demodulated signal amplitude can result in
significantly poorer performance for the radio network. A common
approach to representing such degradation is to perform a
conversion of the occupied bandwidth from the frequency domain to
the time domain. Where multiple digital modulation levels are used,
the result of the time conversion is a plurality of random
time-variant waveforms of different levels which are generally
arcuate and, plotted together, take the general shape of an eye.
This is frequently referred to as "the eye", and such terminology
will be used from time to time hereinafter. A reduction in the
demodulated signal amplitude--and the corresponding reduction in
the occupied bandwidth--basically is depicted in the eye by the
arcuate waveforms which form the eye becoming less arcuate (i.e.,
flatter) and moving closer to one another, such that the overall
impression is that the eye opening becomes smaller. An enhancement
in the demodulated signal amplitude--and the corresponding increase
in occupied bandwidth--is depicted by the waveforms becoming more
arcuate and moving further apart. This is commonly referred to as
the eye becoming larger. A larger eye is generally more
desirable.
[0009] Variations in frequency, even with constant temperature, can
also lead to significant variation in occupied bandwidth. Thus, for
tunable systems which can be operated at any of a wide range of
frequencies, undesirable occupied bandwidth changes can result from
changes in selected channel. For many operators of microwave radio
systems, the frequency of operation is chosen on-site. Thus, the
occupied bandwidth of the system must be readily configurable
outside of the manufacturing facility, and must take into account
the variations in occupied bandwidth which can result from changes
in frequency at even a stable temperature.
[0010] As a result, there has been a long-felt need for a system
which dynamically maintains optimized occupied bandwidth over a
significant range of operating temperatures and frequencies.
SUMMARY OF THE INVENTION
[0011] The present invention provides a system and method for
dynamically optimizing the system transmitted occupied bandwidth of
a radio system and maintaining that energy at a level closely
approximating the mask. More specifically, in a first embodiment of
the invention, in a system comprising a transmitter and a receiver,
the energy received from the transmitter at the receiver is
monitored periodically. The monitoring cycle is matched to the
reasonable period for meaningful variation in the occupied
bandwidth. The measured FM demodulator output voltage level (the
recovered eye) is then compared to acceptable voltage levels and,
if the difference exceeds a predetermined threshold, a control
packet is fed back to the transmitter. The control packet is then
supplied to the microprocessor on the transmitter side, and the
amplitude of the transmitter waveform is adjusted either upward or
downward to optimize occupied bandwidth, which typically involves
maximizing occupied bandwidth while remaining within the mask. It
can thus be seen that this embodiment of the invention comprises a
form of digital feedback from the remote receiver side of one of a
pair of transceivers to send a control message to the transmitter
portion of the other one of the pair of transceivers to modify the
output. It will be appreciated that, within a network of such
transceivers, each transmitter portion will receive individualized
feedback transmitted by each associated receiver portion, with the
feedback path including the remaining transmitter/receiver portions
of the transceiver pair.
[0012] In an alternative embodiment, the need for feedback from the
receiver portion of the pair is not required. Instead, a method of
predictive adjustment is implemented. In this embodiment, a data
table is established reflecting the correlation between temperature
and/or frequency and occupied bandwidth for the output waveform.
Then, by monitoring both frequency and temperature at the
transmitter, rather than relying on the outboard unit of the
transceiver pair, the appropriate correction can be looked up in
the data table and the correction made locally. As with the first
embodiment, the correction is provided to the microprocessor of the
transmitter to cause appropriate correction of the occupied
bandwidth.
[0013] The invention may be better appreciated from the following
Figures, taken together with the accompanying Detailed Description
of the Invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates in flow diagram form the operation of a
first embodiment of the invention used in a microwave digital
radio.
[0015] FIG. 2 illustrates in block diagram form a hardware
representation of the embodiment of the invention shown in FIG.
1.
[0016] FIG. 3 illustrates in block diagram form a more detailed
view of a portion of the embodiment shown in FIGS. 1 and 2.
[0017] FIG. 4 illustrates in block diagram form a second embodiment
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Referring first to FIG. 1, the operation of a first
embodiment of the present invention is shown in a greatly
simplified manner. A digital microwave radio network comprises a
plurality of identical transceivers, shown for simplicity as local
transceiver 10 and remote transceiver 20. In a presently preferred,
but not required, arrangement, each transceiver is of the type
described in U.S. patent application Ser. No. ______, entitled
Digital Cable System And Method for Microwave Communications, filed
Jun. 13, 1997, commonly assigned herewith and incorporated herein
by reference. Typically, such transceivers comprise an indoor unit
(IDU) 10A and 20A, respectively, and an outdoor unit (ODU) 10B and
20B, respectively, with each unit incorporating some transmitter
functions and some receiver functions. In typical arrangements, the
indoor unit and outdoor unit are connected by a single coaxial
cable 11 which may be, for example several hundred meters in
length. Such transceivers typically transmit in the gigahertz
region, and the wireless link between them is shown at 25. In
addition, in a typical arrangement both the indoor unit and the
outdoor unit have microprocessors incorporated therein, as will be
better appreciated hereinafter.
[0019] The process of the present invention begins at step A with
the transmission of an output signal from the transmitter (TX)
portion 12 of the local outdoor unit 10B. The occupied bandwidth of
the signal is intended to conform to the spectrum mask assigned for
the particular channel, but may vary due to frequency or
temperature changes.
[0020] The transmitted signal is received at step B by the receiver
(RX) portion 13 of the remote outdoor unit 20B. The FM demodulated
received signal is compared to stored acceptable values, and an
error value is ascertained. The error value is transmitted at C to
the RX portion 14 of the remote IDU 20A, where it is processed into
a message packet on the overhead channel by a microprocessor within
the IDU 20A. The message packet is forwarded at step D from the RX
portion 14 to the TX portion 15, where it is further processed and
muxed with the data channel, then scheduled to be sent up the cable
11 to the TX portion 16 of the ODU 20B at step E.
[0021] The message is then transmitted at step F from the TX
portion 16 of the remote ODU 20B. It is received at step G by the
RX portion 17 of the local ODU 10B, and at step H forwarded down
the cable 11 to be separated from the data channel by the RX
portion 18 of the IDU 10A. The message packet is reformatted as an
error signal and then provided at step I to a microprocessor in the
local IDU 10A. The microprocessor in the local IDU 10A processes
the error signal to develop a correction signal, which is forwarded
up the cable 11 to the transmitter portion 12. The correction
signal is combined with the conventional transmitter signal to
yield a dynamically-adjusted output spectrum conforming to the
required spectrum mask.
[0022] From the general description of FIG. 1, it can be seen that
the present invention, in a first embodiment, involves remotely
monitoring the signal transmitted from a local transceiver. The
remotely monitored signal is used to develop an error message,
which is then fed back from the remote transceiver to the local
transceiver. Based on a correction signal developed from the error
message, the output of the local transmitter is adjusted, as
necessary, to maximize occupied bandwidth and not exceed the limits
of the spectrum mask.
[0023] Referring next to FIG. 2, the automatic correction scheme of
the present invention may be understood in greater detail. As with
FIG. 1, a pair of microwave transceivers 10 and 20 communicate over
the airwaves indicated at 25. The information transmitted in
accordance with the present invention is typically carried on the
overhead channel, while user data is carried on the user
channel.
[0024] An "eye" 30 represents an intermediate signal internal to
the transceiver 10 and ready to be transmitted. The "eye" 30
provides the input signal to an amplifier having automatic gain
control, or AGC amp 35. The gain of the AGC amp 35 is controlled by
a microprocessor 40 resident in the outdoor unit, or ODU, portion
of the transceiver 10 and, for at least the present operation,
associated with the transmit function. The output of the AGC amp 35
is supplied through a resistor 45 to a transmitter 50, which in
simplest terms may be thought of as a varactor diode and an FM
modulated microwave oscillator feeding an antenna (not shown).
[0025] The transmitter 50 outputs a particular distribution of
energy, which is detected at a receiver portion of the transceiver
20. The receiver process begins by demodulating the signal in the
FM demodulator 60. The FM demodulator 60 converts the transmitted
frequency domain signal back to the time domain "eye"
configuration, shown at 65, after which the signal is passed
through an AGC amp 70 and converted to constant amplitude, as shown
at node 75, through the use of a peak detection circuit as
discussed below. The output is then passed through a dibit
converter 80 and converted to a conventional digital signal, which
is provided to the customer as the system output at 85. One
suitable device for the AGC amp 70 is, for example, a Comlinear
CLC522 amplifier.
[0026] In addition, the AGC amp 70 is capable of correcting the
variance between the "eye" signal at 65 and a predetermined
required eye amplitude at 75. This variance is supplied
periodically to a microprocessor 90 in the remote transmitter
portion of the transceiver 20 and also to the AGC amp 70 by virtue
of a diode 95 and capacitor 100 which together serve as a peak
detector circuit. The peak detector circuit measures the amplitude
of the eye 65 and provide a DC level proportional to that
amplitude, shown as V.sub.c in FIG. 2. The variance, or correction
voltage V.sub.c is monitored by being sampled on the order of once
per minute, although a range of acceptable sample rates can vary
from a few hertz to one cycle every few minutes. Too high a sample
rate, for example in the kilohertz range, may lead to instability,
while too slow a sample rate, say one every few hours, may lead to
insufficient responsiveness to variations, particularly variations
in temperature.
[0027] The AGC amp 70 receives the correction signal V.sub.c and
internally compares it to a predetermined, fixed voltage V.sub.ref.
Based on that comparison an internal gain control signal is
generated, which causes the output of the AGC amp 70 to remain a
fixed amplitude as shown at 75.
[0028] The microprocessor 90 generates a digital error correction
signal in much the same way as the AGC generates an analog
correction signal. The microprocessor 90 receives the DC signal
V.sub.c from the peak detector circuit and, in response to the
variance between the digitally represented Reference Value
representative of the maximum permissible value and the signal
V.sub.c (also converted to digital form by the microprocessor 90),
provides an error message packet to a FPGA (Field Programmable Gate
Array) Mux 105. The FPGA Mux 105 multiplexes the customer data
channel DATA with the overhead channel to permit transmission of
the combined signal from the transceiver 20 back to the transceiver
10. The output of the FPGA mux 105 is supplied to a transmitter
portion 110 of the transceiver 20, where it is sent back to the
transceiver 10.
[0029] The input 115 at the receiver portion of the transceiver 10
thus includes the error message packet to adjust, as necessary, any
variance in the transmitted occupied bandwidth of transmitter 50.
The signal is decoded by the receiver front end 120, and then
supplied to the FPGA demultiplexer, or demux 125 of the transceiver
10. The FPGA mux 125 separates out the overhead channel data from
the user channel, and supplies the error packet on the overhead
channel to the receiver microprocessor 130. The microprocessor then
supplies a control signal through a resistor 135 to the
microprocessor 40, which generates a control signal to the AGC amp
35. The control signal modifies the AGC output signal, which is
then supplied to the transmitter 50 to increase or decrease the
output power as appropriate. It will be appreciated by those
skilled in the art that the overall objective is to cause the eye
65 to remain constant, which represents an optimum occupied
bandwidth.
[0030] Referring next to FIG. 3, the development of the error
message and correction signal can be better appreciated. From the
foregoing discussion, it can be appreciated that an error signal is
developed, in an exemplary embodiment, in the receiver portion of
the remote transceiver, and then fed back to the transmitter
portion of the local transceiver. To aid in clarity, FIG. 3
illustrates in greater detail the relevant portion of the remote
receiver, for developing the error signal, together with the
relevant portion of the local transmitter, where the modulation
deviation signal is developed. The intermediate elements, by which
the error signal from the remote receiver is fed back to the local
transmitter to cause a correction signal to be generated, have been
omitted.
[0031] Although most of the operations shown in FIG. 3 are
performed in software, the block diagram of FIG. 3 is believed to
be the simplest way to provide a clear understanding of the
invention. As discussed previously in connection with FIG. 2, the
receiver demodulator 60 in the remote transceiver sends to the AGC
amplifier 70 the eye 65. As discussed previously, the amplitude of
the eye provides a representation of the occupied bandwidth of the
transmitted signal. The measurement of the occupied bandwidth is
provided from the peak detector as the voltage V.sub.c and
converted to a digital word, nominally ten bits, as discussed in
connection with MPU 90 in FIG. 2. The digital word is then provided
(terminal "A") to an averaging circuit shown at 320. In the
exemplary embodiment described herein, the averaging is actually
performed by the microprocessor, and the hardware illustration is
used for clarity only. The averaging circuit 320 samples the output
V.sub.c of the peak detector circuit on the order of once per
minute, although a fairly broad range of timing is acceptable as
previously discussed. In an exemplary embodiment, the averaging
circuit 320 averages the last 64 values.
[0032] The output of the averaging circuit 320 is supplied as the
negative input to a comparator 325. The positive input to the
comparator 325 is provided by a reference input 330. To
accommodate, for example, capacities of both four T1 and eight T1
operation of the radio, a selector 335 may be provided; as
previously noted, although this functionality is shown as hardware,
in a presently preferred embodiment the selection is made in
software and selects among look-up tables in which the reference
data has been stored.
[0033] The output of the comparator 325 comprises an error signal
350, and provides an input to a second comparator 355. The second
input to the comparator 355 is provided by a reference threshold
signal 360, which functions to establish the minimum error which
must be detected before an error message can be generated. This can
be seen to create a digital hysteresis to avoid unnecessary cycling
of the system. As with the input 330, the input 360 may select
among, for example, a capacity for four T1's or eight T1's by
virtue of selector 365. The capacity for a specific number of T1's
is not critical to the operation of the system, and discussion of a
capacity for four T1's or eight T1's is merely for purposes of
illustration.
[0034] The output of the comparator 355 is converted, as shown in
FIG. 3, to a digital word which is packetized for transmission
across the wireless link shown at 375. The message including the
packet is provided (through various components not shown in FIG. 3)
to the transmitter of the local transceiver. The message is parsed
to segregate the error message as shown in block 380, and is
converted to a correction value. As a check to ensure adequate
signal has been received, a received signal level check is provided
at block 385. This function may be provided by hardware. Assuming
the received signal level is adequate (which may be on the order of
anything better than -60 dBm) the correction signal is summed at
summing junction 390 with the standard expected static value (which
is a function of the data rate), supplied on line 395. As with
inputs 330 and 360, the input 395 may be selected between, for
example, eight and four T1 operation. It will be appreciated that
all three inputs 330, 360 and 395 preferably always select the same
level.
[0035] The output signal from the summing junction 390 is then
provided, subject to a limiter 420, to a modulation deviation
signal circuit 425. It will be appreciated that the output of the
summing junction 390 is the corrected output signal which maximizes
the occupied bandwidth of the transmitter while remaining within
the required spectrum mask. The output of the modulation deviation
signal circuit 425 is then transmitted across the transmitter
microprocessor.
[0036] Referring next to FIG. 4, an alternative embodiment of the
present invention is shown. While the embodiment of FIGS. 1-3 uses
feedback from the remote transceiver to provide the basis for a
correction signal, there may be some instances in which such
feedback is not desired. Nevertheless, it is still important that
the output of the transmitter conform, as closely as possible, to
the spectrum mask by adjusting to fluctuations in temperature at
different operative frequencies.
[0037] To achieve these objectives, the embodiment of FIG. 4
includes, as with the earlier embodiment, an outdoor unit 450
having a transmitter portion 455 and a microprocessor 460. In
addition, the transmitter portion 455 includes a temperature sensor
465, the output of which is converted to digital form in A/D
converter 470. In an exemplary embodiment the A/D converter
provides an eight bit representation, although the exact number of
bits is not critical as long as it allows reasonable resolution of
the range of operating temperatures. The output of the A/D
converter 470 is provided as an input to the microprocessor 460,
which uses the temperature data to identify from a lookup table 475
a correction factor appropriate for the operating frequency and
operating temperature of the transmitter. In a typical arrangement,
the data in the lookup table is developed at the time of
manufacture, and includes appropriate correction factors for a
range of frequencies and a range of temperatures. It will be
appreciated that, although only one table 475 is shown in FIG. 4, a
table is required for each capacity at which the radio may operate;
i.e., if the radio can operate at a capacity for either four T1's
or eight T1's, there will typically be a data set for four T1
operation and another data set for eight T1 operation.
[0038] The correction factor from the lookup table 475 is then
provided to a D/A converter 480, which then supplies it to a
summing junction 485 to be combined with the standard expected
signal 490 in a manner otherwise identical to that described in
connection with FIG. 2.
[0039] It can therefore be appreciated that a new and novel
technique for optimizing conformance to the assigned spectrum mask
for digital microwave radios has been described. It will be
appreciated by those skilled in the art that, given the teachings
herein, numerous alternatives and equivalents will be seen to exist
which incorporate the invention disclosed hereby. As a result, the
invention is not to be limited by the foregoing exemplary
embodiments, but only by the following claims.
* * * * *