U.S. patent application number 09/967479 was filed with the patent office on 2002-08-01 for dynamic adaptive modulation negotiation for point-to-point terrestrial links.
Invention is credited to Ho, Shin-Fang, Masters, Jeffrey Tony, Molina, Victor Hugo, Yurtkuran, Erol Kenneth.
Application Number | 20020101913 09/967479 |
Document ID | / |
Family ID | 23424459 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020101913 |
Kind Code |
A1 |
Masters, Jeffrey Tony ; et
al. |
August 1, 2002 |
Dynamic adaptive modulation negotiation for point-to-point
terrestrial links
Abstract
A transmission device that transmits a signal along a link and
method therefor that includes a forward error correction encoder
unit to insert error correction information into the signal
transmitted along a link and output a corresponding encoded signal.
A modulation unit variably modulates the encoded signal and outputs
a modulated signal, having a corresponding quadrature amplitude
modulation index, to the receiving device. A control unit variably
controls the inserted forward error correction information and the
quadrature amplitude modulation index based on link quality
information with respect to substantially the entire link to
increase throughput during periods of reduced environmental
degradation.
Inventors: |
Masters, Jeffrey Tony; (Glen
Allen, VA) ; Yurtkuran, Erol Kenneth; (Richmond,
VA) ; Molina, Victor Hugo; (Richmond, VA) ;
Ho, Shin-Fang; (Glen Allen, VA) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2211
US
|
Family ID: |
23424459 |
Appl. No.: |
09/967479 |
Filed: |
September 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09967479 |
Sep 28, 2001 |
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09362043 |
Jul 28, 1999 |
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6330278 |
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Current U.S.
Class: |
375/219 |
Current CPC
Class: |
H04L 1/0009 20130101;
H04B 17/309 20150115; H04L 1/0025 20130101; H04L 1/0003 20130101;
H04L 5/16 20130101; H04L 1/0015 20130101 |
Class at
Publication: |
375/219 |
International
Class: |
H04B 001/38 |
Claims
What is claimed is:
1. An apparatus to communicate with at least one remote device, the
apparatus transmitting information over a first channel and
receiving information over a second channel, the apparatus
comprising: a controller to process link quality information
regarding the second channel and first link control information
regarding the first channel, the controller adjusting at least one
parameter of the first channel based on the first link control
information and generating second link control information
regarding the second channel based on the link quality information.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a wireless modem, and more
particularly, the present invention relates to a wireless modem
that improves link characteristics between modems during periods of
reduced environmental degradation of the link, and method
therefor.
[0003] 2. Description of the Related Art
[0004] Although there are several link dependencies that must
necessarily be taken into consideration during the transmission of
a carrier signal from a wireless transmitter to a wireless
receiver, channel capacity is primarily dependent upon
signal-to-noise ratio, or "SNR". Typically, as the SNR decreases,
the channel capacity decreases, causing a link formed between the
transmitter and the receiver to be degraded, corrupting the
transfer rate of the corresponding carrier signal. On the other
hand, as the SNR increases, the channel capacity increases,
resulting in improved transfer rate of the carrier signal.
[0005] At the same time, while there are several factors that have
a tendency to cause the SNR to decrease, environmental degradation,
such as rain, snow, fog, and other non-transient man-made
interference sources tend to be major factors causing a decrease in
SNR. For example, individual raindrops absorb/scatter energy from
radio waves and a certain amount of energy in the waves is
scattered away from the propagation path. Rain attenuation and
depolarization of a transmitted carrier signal particularly occurs
during periods of intense rainfall, causing the SNR to degrade.
[0006] The level of the effects of these interactions between the
carrier signal and the rainfall depend on both the number of
raindrops encountered by the carrier signal, and the distribution
of the size and shapes of the raindrops, both of which depend on
the rate of the rainfall. In a wireless modem operating over a
carrier at millimeter wave frequencies, where the wavelength of the
carrier is close to the size of a raindrop, or on the order of a
couple of millimeters, a raindrop is substantial enough in size to
degrade the link during periods of moderate to intense rainfall.
When the wireless broadband link is a terrestrial link, the entire
link may be covered in rain, depending on the size of the
associated storm, and therefore substantially the entire link is
degraded.
[0007] As a result, in order to insure successful data transmission
along a link when implementing wireless broadband links in the
wireless modem, it is important that the links be engineered to
operate during the period of the year in which the rainfall is the
most intense. Therefore, since the most intense rainfall occurs
typically during less than one percent of a given year, additional
capacity of the carrier is available for more than ninety-nine
percent of the time and cannot be used.
[0008] FIG. 1 is a graphical representation of a relationship
between the SNR and rainfall over time. Environmental degradation
of a signal that occurs, for example, during an intense snowfall in
January is indicated by a downward extending spike 20a. In
addition, environmental degradation of the signal that occurs
during intense rainfall in June and July is indicated by downward
extending spikes 20b-d, and environmental degradation of the signal
that occurs during an intense snow storm in December is indicated
by a downward extending spike 20e. Although degradation of a
carrier signal due to intense snow or rainfall might only occur
less than one percent of the time in a given year, for a link to be
reliable it must be engineered to always operate throughout the
year at an SNR corresponding to the periods of intense snow or
rainfall. Accordingly, the link must be engineered to always
operate at the lowest SNR, indicated by a horizontal line 22.
[0009] During the remaining ninety-nine percent of the year, when
environmental degradation is no longer a factor, and therefore when
the SNR that can be tolerated is greatest, indicated by a line 24,
additional or excess capacity is available that cannot be used.
This excess capacity is illustrated by a hashed region located
between where the SNR can be tolerated, line 24, and the engineered
level of the SNR, line 20. Therefore, the excess capacity is wasted
ninety-nine percent of the time during the year, resulting in
reduced throughput.
BRIEF SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide a
wireless modem and method therefor that transmits a signal having a
greater throughput during periods when there are no effects on the
carrier signal resulting from environmental degradation of a
terrestrial link.
[0011] It is a further object of the present invention to provide a
wireless modem and method therefor that negotiates modulation of a
carrier signal along a link in response to changes in effects of
environmental degradation on the link.
[0012] It is a still further object of the present invention to
provide a wireless modem and method therefor that negotiates
modulation of a carrier signal along a link in response to changes
in effects of environmental degradation on the link, while
minimizing the impact of the negotiation on data transmitted along
the carrier signal.
[0013] Objects of the invention are achieved by a wireless modem
that includes a controller that samples a number of parameters of a
wireless terrestrial signal and a data adjusting unit that adjusts
data throughput responsive to the parameters.
[0014] Further objects of the invention are achieved by a device
for transmitting a signal to a remote device and receiving a signal
transmitted from the remote device that includes a transmitting
unit that generates the signal transmitted to the remote device,
and a receiving unit that receives the signal transmitted from the
remote device and outputs remote modulation variation information
included in the received signal. The receiving device also
generates link quality information corresponding to the received
signal, and a control unit generates a modulation change command
packet instructing the remote device to change to a quadrature
amplitude modulation index corresponding to the link quality
information. The control unit variably controls the generation of
the signal by the transmitter according to the remote modulation
variation information output from the receiving unit.
[0015] According to the present invention, the link quality
information corresponds to environmental degradation of the signal.
The quadrature amplitude modulation index is increased in response
to the link quality information indicating reduced degradation of
the link, and decreased in response to the link quality information
indicating degradation of the link.
[0016] Further objects of the invention are achieved by a wireless
modem for transmitting a signal to a remote device and receiving a
signal transmitted by the remote device that includes a forward
error correction encoder unit that inserts error correction
information to the signal transmitted by the wireless modem and
outputs a corresponding encoded signal. A modulation unit variably
modulates the encoded signal and outputs a modulated signal having
a corresponding quadrature amplitude modulation index to the remote
device. A demodulating unit demodulates the signal transmitted by
the remote device and outputs a corresponding demodulated signal,
and a control unit generates a modulation change command packet
instructing the remote device to change quadrature amplitude
modulation index corresponding to link quality information
generated by the demodulating unit and the forward error correction
decoder unit, and variably controls the inserted forward error
correction information and the quadrature amplitude modulation
index based on remote modulation variation information included in
the signal received from the remote device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] These and other objects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the preferred embodiments, taken in conjunction with
the accompanying drawings of which:
[0018] FIG. 1 is a graphical representation of a relationship
between signal-to-noise ratio of a signal and rainfall over
time.
[0019] FIG. 2 is a schematic diagram illustrating interconnections
for dynamic adaptive modulation negotiations between wireless
modems according to the present invention.
[0020] FIG. 3 is a block diagram of a transmitter according to the
present invention that is included in the wireless modems of FIG.
2.
[0021] FIG. 4 is a block diagram of a receiver according to the
present invention that is included in the wireless modems of FIG.
2.
[0022] FIG. 5 is a receiver state transition diagram illustrating
negotiation for point-to-point links that occurs on a receiver side
according to the present invention.
[0023] FIG. 6 is a transmitter state transition diagram
illustrating negotiation for point-to-point links that occurs on a
transmitter side according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings, wherein like reference
numerals refer to like elements throughout.
[0025] FIG. 2 is a schematic diagram illustrating interconnections
for dynamic adaptive modulation negotiations between wireless
modems, according to the present invention. As illustrated in FIG.
2, a first wireless modem 26 transmits a signal to a second
wireless modem 28, and receives a signal transmitted from the
second wireless modem 28, and the second wireless modem 28
correspondingly transmits a signal to the first wireless modem 26,
and receives a signal transmitted from the first wireless modem
26.
[0026] The first wireless modem 26 includes a transmitter 30 that
transmits the signal to the second wireless modem 28, a receiver 32
that receives the signal transmitted from the second wireless modem
28, and a microprocessor, microcontroller or controller 34 that
receives parameters from the receiver 32 related to link quality of
the signal, such as SNR, bit error rate, etc., which will be
described below. The transmitter 30 includes an output buffer 31
that buffers data to be transmitted just prior to the
transmission.
[0027] Similarly, the second wireless modem 28 includes a
transmitter 36 that transmits a signal to the first wireless modem
26, a receiver 38 that receives the signal transmitted from the
first wireless modem 26, and a microprocessor, microcontroller or
controller 40 that receives parameters from the receiver 38 related
to link quality of the signal, such as SNR, bit error rate, and so
forth, as described below. The transmitter 36 includes an output
buffer 31 that buffers data to be transmitted just prior to the
transmission.
[0028] The transmitter 36, receiver 38, and controller 40 of the
second wireless modem 28 are the same as the transmitter 30,
receiver 32, and controller 34 of the first wireless modem 26.
[0029] As illustrated in FIG. 2, a signal transmitted from the
transmitter 30 of the first wireless modem 26 is received by the
receiver 38 of the second wireless modem 28. Information about the
signal is output by the receiver 38 to the controller 40 and is
processed by the controller 40 to control the transmitter 36 and
receiver 38 of the second wireless modem 28. In the same way, a
signal transmitted by the transmitter 36 of the second wireless
modem 28 is received by the receiver 32 of the first wireless modem
26. Information about the signal is output by the receiver 32 to
the controller 34 and is processed by the controller 34 to control
the transmitter 30 and receiver 32 of the first wireless modem 26.
In this way, a feedback loop is formed between the controller 34 of
the first wireless modem 26 and the controller 40 of the second
wireless modem 28.
[0030] FIG. 3 is a block diagram of the transmitters 30, 36
according to the present invention. As illustrated in FIG. 3, in
each respective transmitter 30, 36 of the wireless modems 26, 28 of
the present invention, a signal from a data source network 42 of
any type, such as a LAN, the Internet, telephony, video, etc., is
received by a forward error correction unit 44, such as described,
for example, in the 1960 article entitled "Polynomial Codes Over
Certain Finite Fields", by I. S. Reed and G. Solomon incorporated
by reference herein. The forward error correction unit 44, under
control of the controller 34, 40, inserts error correction
information into the signal and outputs a corresponding encoded
signal. The amount of error correction can be variably controlled
by the controller 34, 40. For example, the amount of channel
capacity typically used for error correction is varied from one to
ten percent.
[0031] The encoded signal output by the forward error correction
unit 44 is input to a variable level quadrature amplitude
modulation, or QAM modulation unit 46, such as, for example, a
BCM3033 available from Broadcom Corporation. The QAM modulation
unit 46, under control of the controller 34, 40 varies a modulation
index of the signal and outputs a modulated signal. The modulated
signal output by the QAM modulation unit 46 is received by a
conventional RF millimeter wave upconverter and transmitter 48.
Both the amount of error correction performed by the forward error
correction unit 44 and the variation in the modulation index
performed by the QAM modulation unit 46 is controlled based on
feedback FB received, respectively, by the controllers 34, 40 from
the receivers 32, 38 of the corresponding wireless modems 26 and
28, as will be described in detail below.
[0032] The modulated signal received by the upconverter and
transmitter 48 of the first wireless modem 26, for example, is then
transmitted through an antenna 50 of the transmitter 30 and
received by the receiver 38 of the second wireless modem 28. In the
same way, the modulated signal received by the upconverter and
transmitter 48 of the second wireless modem 28 is transmitted
through an antenna 50 of the transmitter 36 of the second wireless
modem 28 and received by the receiver 32 of the first wireless
modem 26.
[0033] The controller 34,40 outputs a modulation index change
command packet requesting the modulation index to be changed to the
forward error correction encoder 44 of the respective receiver 32,
38, as will be described below.
[0034] FIG. 4 is a block diagram of the receivers 32, 38 according
to the present invention. The receiver 32 of the first wireless
modem 26 receives the signal transmitted from the transmitter 36 of
the second wireless modem 28, and the receiver 38 of the second
wireless modem 28 receives the signal transmitted from the
transmitter 30 of the first wireless modem 26, and each receiver
32, 38 essentially performs the reverse process described
previously with respect to the transmitters 30, 36.
[0035] In particular, as illustrated in FIG. 4, the signal
transmitted from the antenna 50 of the corresponding transmitter
30, 36 is received by a conventional RF millimeter wave down
converter 54 of the respective receiver 32, 38 through a
corresponding antenna 52. The RF millimeter wave down converter 54
down converts the received signal and outputs a corresponding down
converted signal to a variable level QAM demodulator unit 56, such
as, for example, a BCM 3118 available from Broadcom Corporation.
The QAM demodulator unit 56 demodulates the down converted signal
received from the RF millimeter wave down converter 54 and outputs
a corresponding demodulated signal to a forward error correction
decoder 58, as described by I. S. Reed and G. Solomon in the 1960
article entitled "Polynomial Codes Over Certain Finite Fields". The
forward error correction decoder 58 decodes the demodulated signal
received from the QAM demodulator unit 56 and outputs a decoded
signal to a data destination 60.
[0036] The QAM demodulator unit 56 also provides parameters related
to the link quality of the signal, such as SNR, etc., to the
controller 34. In addition, the forward error correction decoder 58
provides link quality parameters of the signal related to bit error
rate (BER) to the controller 34. Based upon the value of these link
quality related parameters, the controller 40 of the second
wireless modem 28, for example, determines an appropriate
modulation index and encoding of the signal. This determined level
of modulation and encoding is then included as a modulation index
change command packet in the signal transmitted by the transmitter
36 of the second wireless modem 28 to the receiver 32 of the first
wireless modem 26. The controller 34 of the first wireless modem 26
receives the modulation index change command packet from the output
of the forward error correction decoder 58 of the receiver 32 of
the first wireless modem 26. As a result, the determined modulation
index and encoding is fed back to the controller 34 of the first
wireless modem 26, which then controls the QAM modulator 46 of the
transmitter 30 of the first wireless modem 26 to change the level
of modulation and/or encoding in the transmitter 30,
accordingly.
[0037] In the same way, the controller 34 of the first wireless
modem 26 determines an appropriate modulation index and encoding of
the signal. This determined level of modulation and encoding is
then included as a modulation index change command packet in the
signal transmitted by the transmitter 30 of the first wireless
modem 26 to the receiver 38 of the second wireless modem 28. The
controller 40 of the second wireless modem 28 receives the
modulation index change command packet from the output of the
forward error correction decoder 58 of the receiver 38 of the
second wireless modem 28. As a result, the determined modulation
index and encoding is fed back to the controller 40 of the second
wireless modem 28, which then controls the QAM modulator 46 of the
transmitter 36 of the second wireless modem 28 the data to change
the level of modulation and/or encoding in the transmitter 36,
accordingly. The command packet can also be used to change the
demodulation and decoding in the receiver, although such is not
necessary as will be understood from the discussion of FIG. 5.
[0038] In this way, the wireless modem according to the present
invention actively detects SNR and BER, and variably increases or
decreases the modulation index and/or level of encoding based on
the detected SNR and BER. For example, depending on whether
environmental degradation is a factor, the modulation index can be
increased from QPSK, which gives 2 bits per symbol, to 16 QAM that
gives 4 bits per symbol, 32 QAM that gives 5 bits per symbol, or 64
QAM that gives 6 bits per symbol, or decreased from any one
modulation index to another modulation index. Likewise, the
percentage of the link used for error correction can be increased
as the link degrades and decreased as the link improves.
[0039] There are discrete steps between the modulation indexes, and
therefore the forward encoding of the forward error correction unit
44 is used to enable the transition between the decreases or
increases in the modulation index to take place in a more
controlled, smooth manner. For example, when the signal is
degrading and a switch to a lower modulation index should be
considered, rather than making the switch to a lower modulation
index, the level of encoding can be increased, for example, from
two to three bits of encoding used for error correction. This
maintains the modulation index but decreases the bit throughput
rate because of the increased encoding while maintaining link
availability. When the maximum level of encoding is reached and
further improvement in signal quality is necessary, the modulation
index can be switched (lowered) and the encoding level
decreased.
[0040] The reverse can also be accomplished where the encoding is
decreased as the signal quality improves until the minimum encoding
is used, at which time the modulation index can be increased,
thereby increasing the throughput in a smoother fashion. As a
result, the present invention achieves a more robust link having a
variable lower throughput during periods of increased environmental
degradation, and a variable greater throughput during periods of
less environmental degradation. As a result, the present invention
provides improved link characteristics between modems by varying
the modulation index and/or encoding in response to changes in link
quality as a result of environmental degradation or other
non-transient man-made interference sources of the link.
[0041] FIG. 5 is a receiver state transition diagram illustrating
the negotiation for point-to-point links that occurs on a receiver
side of the signal when the modulation index and/or encoding is
adjusted, according to the present invention. FIG. 6 is a receiver
state transition diagram illustrating the negotiation for
point-to-point links that occurs on a transmitter side of the
signal, according to the present invention.
[0042] In the description of the quadrature amplitude modulation
negotiation for point-to-point links, according to the present
invention, described in reference to FIGS. 5 and 6 below, it is
assumed for the sake of simplicity that the receiver state
transition diagram illustrated in FIG. 5 corresponds to the
receiver 38 of the second wireless modem 28, and the transmitter
state transition diagram illustrated in FIG. 6 corresponds to the
transmitter 30 of the first wireless modem 26. At the same time, it
is understood that the respective transmitters 30, 36 of the first
and second wireless modems 26 and 28 each include the negotiation
for point-to-point links illustrated in FIG. 6, and the respective
receivers 32, 38 of the first and second wireless modems 26 and 28
each include the negotiation for point-to-point links illustrated
in FIG. 5.
[0043] According to the present invention, the receiver 38 of the
second wireless modem 28 receives a signal from the transmitter 30
of the first wireless modem 26 through the antenna 52. The QAM
demodulator unit 56 receives and demodulates the signal and outputs
the SNR, etc. to the controller 40, and the forward error
correction decoder 58 receives and decodes the demodulated signal
and outputs the corresponding bit error rate to the controller 40
to enable the controller 40 to determine whether the modulation
index, or QAM index, should be increased or decreased. The
determination of whether to increase or decrease the QAM index is
dependent upon and varies according to field tests corresponding to
a particular application. For example, typical SNR threshold values
associated with each QAM index for determining upgrade QAM index
eligibility have been determined to be a minimum SNR of 12.0 for
QPSK, 18.0 for 16 QAM, 24.0 for 32 QAM, and 26.0 for 64 QAM.
Minimum SNR values for 128 QAM and 256 QAM have been determined to
be 27.0 and 28.0, respectively.
[0044] According to a preferred embodiment of the present
invention, when the wireless modems 26 and 28 are initially turned
on, synchronization ("sync") has not been achieved. Packets
advertising the ability of each of the wireless modems to support a
certain version of a protocol, which is typically automatic for
standard link establishment, are transmitted at the lowest
modulation index between the wireless modems 26 and 28. Therefore,
when initially powered on, the second wireless modem 28 is in a
recovery state 62, as illustrated in FIG. 5, and the state machine
of the receiver 38 is not automatically initialized until sync is
acquired and the packet is received from the first wireless modem
26 specifying the version of protocol that the first wireless modem
26 supports. In the same way, when initially powered on, the first
wireless modem 26 is in the recovery state 62 and the state machine
of the receiver 32 is not automatically initialized until sync is
acquired and the packet is received from the second wireless modem
28 specifying the version of protocol that the second wireless
modem 28 supports. Once this information is exchanged, the state
machines in the wireless modems 26 and 28 are initialized and the
corresponding transmitters 30, 36 are transmitting using the same
QAM index.
[0045] As illustrated in FIG. 5, once the wireless modems 26 and 28
are initialized and have achieved sync, the respective receivers
32, 38 move from the recovery state 62 to a stable state 64. When
in the stable state 64, the receivers 32, 38 continuously sample
the line quality of the signal to determine whether to upgrade or
downgrade the modulation index and/or encoding. For example, the
controller 40 of the second wireless modem 28 receives the SNR
parametric output by the QAM demodulator unit 56 of the receiver 38
in addition to the bit error rate parametric output by the forward
error correction decoder 58 of the receiver 38 and, on the basis of
the received parameters, determines that the line quality has not
decreased as a result of environmental degradation. The controller
40 makes such a determination by comparing the difference in the
current SNR to a previous SNR average, or to a SNR threshold, and
determining that the link quality is clean, i.e. that there is a +q
event.
[0046] Once a +q event is achieved when the receiver 38 is in the
stable state 64, the controller 40 generates corresponding feedback
information in the form of a modulation index change command packet
specifying an increased QAM index to which the receiver 38 intends
to move. The receiver 38 of the second wireless modem 28 then moves
from the stable state 64 to an upgrade state 66. While in the
upgrade state 66, the second wireless modem 28 continues to receive
and transmit data at the initial QAM index so that data
transmission is not effected by the transmission of the modulation
index change command packet.
[0047] The controller 40 outputs the modulation index change
command packet to the forward error correction encoder 44 of the
transmitter 36 of the second wireless modem 28 which then transmits
the modulation index change command packet to the receiver 32 of
the first wireless modem 26. The controller 34 of the first
wireless modem 26 receives the modulation index change command
packet after it is forward error correction decoded by the forward
error correction decoder 58 of the first wireless modem 26.
[0048] If the modulation index change command packet is received by
the receiver 32 of the first wireless modem 26, the transmitter 30
of the first wireless modem 26 stops placing data in the output
buffer 31, and data that remains to be transmitted in the output
buffer 31 of the transmitter 30 is transmitted. The first wireless
modem 26 then flushes out the output buffer 31 of the transmitter
30 and as soon as the last data element is sent, the controller 34
controls the QAM modulator 46 of the first wireless modem 26 to
upgrade it's modulation index to correspond to the increased QAM
index, and then resumes transmitting data. When data transmission
by the first wireless modem 26 is resumed, the resumed data
transmission initially includes empty frames for a certain period
of time, such as 20 ms, for example.
[0049] When the modulation index is upgraded by the QAM modulator
46 of the first wireless modem 26, the link between the first
wireless modem 26 and the second wireless modem 28 is momentarily
lost, resulting in a sync loss event. As soon as the sync loss
event occurs, the controller 40 controls the QAM demodulator of the
receiver 38 to upgrade the modulation index to correspond to the
upgraded QAM index requested in the modulation index change command
packet, and the receiver 38 of the second wireless modem 28 moves
to an upgrade wait state 68. Since the first wireless modem 26 is
already transmitting empty frames at the upgraded QAM index, a sync
event occurs. Once this sync event occurs, the receiver 38 moves
from the upgrade wait state 68 to the stable state 64 and continues
sampling the line quality.
[0050] If, while in the upgrade state 66, the sync loss event does
not occur, i.e., the modulation index is not upgraded by the QAM
demodulator 56 of the second wireless modem 28 and therefore the
link between the first wireless modem 26 and the second wireless
modem 28 is not momentarily lost, after a preferably one second
timeout event occurs, and the receiver 38 of the second wireless
modem 28 moves from the upgrade state 66 to the stable state 64 and
resumes sampling the line quality. During this time, neither the
first wireless modem or the second wireless modem 28 stop receiving
or transmitting data, and therefore no loss in data transmission
has resulted.
[0051] When the receiver 38 is in the upgrade state 66, achieves
the sync loss event and moves to the upgrade wait state 68 to wait
for receipt of the empty frames from the transmitter 30 of the
first wireless modem 26, if the receiver 38 does not receive the
empty frames while in the upgrade wait state 68, or the empty
frames are received in a degraded condition, a no sync event
occurs.
[0052] In response to the no sync event that occurs while the
receiver 38 is in the upgrade wait state 68, the receiver 38
instructs the controller 40 of the second wireless modem 28 to
generate a modulation index change command packet specifying the
previous modulation index, and the receiver 38 moves from the
upgrade wait state 68 to a downgrade wait state 70 after changing
to the previous modulation index. As described above, the
controller 40 generates and outputs the modulation index change
command packet to the forward error correction encoder 44 of the
transmitter 36 of the second wireless modem 28 and the modulation
index change command packet is transmitted to the receiver 32 of
the first wireless modem 26. The controller 34 of the first
wireless modem 26 receives the modulation index change command
packet after it is forward error correction decoded by the forward
error correction decoder 58 of the first wireless modem 26. If the
modulation index change command packet is received by the receiver
32 of the first wireless modem 26, the transmitter 30 of the first
wireless modem 26 ensures that the output buffer 31 is flushed out,
changes it's modulation index accordingly, and then transmits data
including the empty frames as described above.
[0053] If a sync event occurs after the receiver 38 of the second
wireless modem 28 moves to the downgrade wait state 70, meaning
that the transmitter 30 of the first wireless modem 26 is now
transmitting at the previous modulation index, the receiver 38
moves to the stable state 64 and continues sampling the line
quality.
[0054] If the receiver 38 of the second wireless modem 28 moves
from the upgrade wait state 68 to the downgrade wait state 70 as
described above, and a sync event does not occur, meaning that the
transmitter 30 of the first wireless modem 26 is not transmitting
at the previous modulation index after preferably one second, a no
sync event occurs and the receiver 38 of the second wireless modem
28 moves to the recovery state 62. In the recovery state 62, the
receiver 38 of the second wireless modem 28 instructs the
controller 40 of the second wireless modem 28 to generate a
modulation index change command packet specifying the lowest QAM
index and to immediately change the receiver 38 to the lowest QAM
index without waiting for a response from the first wireless
modem.
[0055] If a sync event occurs after the receiver 38 of the second
wireless modem 28 moves from the downgrade wait state 70 to the
recovery state 62, meaning that the transmitter 30 of the first
wireless modem 26 is transmitting at the lowest QAM index, the
receiver 38 moves from the recovery state 62 to the stable state 64
and continues sampling the line quality. On the other hand, if a no
sync event occurs after the receiver 38 of the second wireless
modem 28 moves from the downgrade wait state 70 to the recovery
state 62, meaning that the transmitter 30 of the first wireless
modem 26 is not transmitting at the lowest QAM index, both the
transmitter 36 and the receiver 38 of the second wireless modem 28
are reset or initialized.
[0056] When the transmitter 36 of the second wireless modem 28 is
reset, the receiver 32 of the first wireless modem 26, which is in
the stable state 64, experiences a sync loss event, since the line
goes down momentarily, and therefore immediately goes to the
recovery state 62 and the transmitter 30 and receiver 32 of the
first wireless modem 26 are reset, as described above. As a result,
the effect of resetting the transmitter 36 and receiver 38 of the
second wireless modem 28 when a no sync event occurs while in the
recovery state 62 is that the transmitter 30 and receiver 32 of the
first wireless modem 26 are reset as well, so that both wireless
modems 26, 28 are at the lowest available QAM index and attempting
to attain sync, similar to when the wireless modems 26, 28 are
powered on, as described above.
[0057] If, on the other hand, while sampling the line quality in
the stable state 62 by comparing the difference in the current SNR
output by the QAM demodulator unit 56 of the receiver 38 to a
previous SNR average, or to a SNR threshold, the controller 40 of
the second wireless modem 28 determines that the line quality is
degrading as a result of environmental degradation of the link, a
-q event occurs. In response to the -q event that occurs while the
receiver 38 is in the stable state 64, the controller 40 generates
corresponding feedback information in the form of a modulation
index change command packet specifying a decreased QAM index to
which the receiver 38 intends to move and the receiver 38 moves
from the stable state 64 to a downgrade state 72. The controller 40
outputs the modulation index change command packet to the forward
error correction encoder 44 of the transmitter 36 and the
modulation index change command packet is transmitted to the
receiver 32 of the first wireless modem 26. The controller 34 of
the first wireless modem 26 receives the modulation index change
command packet after it is forward error correction decoded by the
forward error correction decoder 58 of the first wireless modem 26.
While in the downgrade state 72, the second wireless modem 28
continues to receive and transmit data at the initial QAM index so
that data transmission is not effected by the transmission of the
modulation index change command packet.
[0058] In the same way as in the upgrade state 66 described above,
when the modulation index change command packet is received by the
receiver 32 of the first wireless modem 26 after a -q event occurs
while the receiver 38 is in the stable state 64, the transmitter 30
of the first wireless modem 26 stops sending data, flushes out the
output buffer 31 of the transmitter 30, and as soon as the last
data element is sent, the controller 34 controls the QAM modulator
46 of the first wireless modem 26 to downgrade it's modulation
index to correspond to the decreased QAM index, and then resumes
transmitting data, beginning with empty frames.
[0059] When the modulation index is downgraded by the QAM modulator
46 of the first wireless modem 26, the link between the first
wireless modem 26 and the second wireless modem 28 is momentarily
lost, resulting in a sync loss event. As soon as the sync loss
event occurs, the controller 40 controls the QAM demodulator of the
receiver 38 to downgrade the modulation index of the receiver 38 to
correspond to the downgrade QAM index requested in the modulation
index change command packet, and the receiver 38 of the second
wireless modem 28 moves from the downgrade state 72 to the
downgrade wait state 70. Since the first wireless modem 26 is
already transmitting empty frames at the downgraded QAM index, a
sync event occurs. Once this sync event occurs, the receiver 38
moves from the downgrade wait state 70 to the stable state 64 and
continues sampling the line quality.
[0060] While the receiver 38 is in the downgrade state 72, if the
sync loss event does not occur, i.e., the modulation index is not
upgraded by the QAM modulator 46 of the first wireless modem 26 and
therefore the link between the first wireless modem 26 and the
second wireless modem 28 is not momentarily lost, a timeout event
occurs, and the receiver 38 of the second wireless modem 28 moves
from the upgrade state 66 to the stable state 64 and resumes
sampling the line quality. During this time, neither the first
wireless modem or the second wireless modem 28 stop receiving or
transmitting data, and therefore no loss in data transmission has
resulted.
[0061] Once the sync loss event occurs and the receiver 38 of the
second wireless modem 28 moves from the downgrade state 72 to the
downgrade wait state 70, the state transition of the receiver 38 is
the same as in the ease when the receiver 38 moves from the upgrade
wait state 68 to the downgrade wait state 70 after a no sync event
occurs while in the upgrade wait state 68, described above, and
therefore the repeated description will be omitted.
[0062] Finally, while sampling the line quality in the stable state
64, if the receiver 38 no longer detects the signal, or loses sync,
without initiating the loss of sync, such as during a loss of sync
resulting from an event outside protocol, a sync loss event occurs.
Once this sync loss event occurs, the receiver 38 moves directly
from the stable state 64 to the recovery state 62. While such an
occurrence would be rare, once the receiver 38 is in the recovery
state 62 as a result of an event outside protocol, the receiver 38
commands the controller 40 to reset both the receiver 38 and the
transmitter 36. This action forces the link between the transmitter
and the receiver to momentarily go down, which communicates the
controller to reconfigure the transmitter and receiver to be at the
minimum QAM index, as described above.
[0063] If at recovery state the no sync event is received, the
receiver 38 takes the same action as described above; that is, the
receiver 38 and transmitter 36 are reset (reconfigured) which in
turn resets the remote modem 26 to do the same. This way, in the
recovery state both modems constantly try to achieve sync at the
lowest QAM level.
[0064] As illustrated in FIG. 6, the state diagram for the
transmitter 30 includes a stable state 74 and a recovery state 76.
Once initialization of the wireless modems is performed as
described above, and the transmitter 30 is therefore in the stable
state 74 and continuously transmits data at the particular QAM
index determined by the protocol during initialization. The
transmitter 30 remains at that particular QAM index until the
above-described feedback information is received from the second
wireless modem 28 requesting the first wireless modem 26 to change
QAM index. Upon receipt of the feedback information, the
transmitter 30 changes to a QAM index corresponding to the feedback
information that in turn is related to whether the line quality is
determined to be degrading -q, or clean +q, as described above. The
transmitter 30 after changing the QAM index simply returns to the
stable state where is ready to receive feedback information again.
When the transmitter 30 of the first wireless modem 26 is reset and
configured to the lowest QAM index as a result of the receiver 38
of the second wireless modem 28 losing sync, the transmitter 30
moves to the recovery state 76. Once the transmitter 30 receives
feedback information requesting some QAM index, the transmitter
moves to the stable state 74 and transmits data.
[0065] By using the dynamic adaptive modulation negotiation
according to the present invention described above, the present
invention enables wireless modems to make use of the increased SNR
corresponding to the hashed region of the graph of FIG. 1 located
between where the SNR can be tolerated, line 24, and the engineered
level of the SNR, line 20. As a result, the modems of the present
invention are able to successfully maintain a link in periods of
environmental degradation, such as during periods of intense
rainfall, while allowing the modems to operate with greater
throughput during the time of the year when environmental
degradation does not occur. In addition, since the wireless modems
26, 28 continue to transmit and receive data while the modulation
negotiation takes place, negotiation of the signal received is
performed transparent to the payload, thereby minimizing the impact
of the negotiation on data transmitted along the carrier
signal.
[0066] It will be understood that while the embodiment of the
present invention is described in association with modems operating
at millimeter wave frequencies, the present invention could also
apply to modems operating at lower frequencies.
[0067] While the negotiation for point-to-point links of the
present invention are described in terms of negotiating a
modulation index, it is understood that the negotiation is not
limited to modulation index, but could also involve other features
of the transmitted data, such as bandwidth, Reed-Solomon correction
bytes, carrier frequency, convolution code rate, antenna beam
focus, and excess bandwidth, etc. In addition, while the modulation
negotiation has been described in relation to wireless
communications, the modulation negotiation of the present invention
could also applied in cable communications, and stratospheric links
provided by high altitude aircraft and satellites.
[0068] Although a few preferred embodiments of the present
invention have been shown and described, it will be appreciated by
those skilled in the art that changes may be made in these
embodiments without departing from the principles and spirit of the
invention, the scope of which is defined in the claims and their
equivalents.
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