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.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     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. 
     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. 
     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. 
     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. 
     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  20   a . In addition, environmental degradation of the signal that occurs during intense rainfall in June and July is indicated by downward extending spikes  20   b-d , and environmental degradation of the signal that occurs during an intense snow storm in December is indicated by a downward extending spike  20   e . 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 . 
     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 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     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: 
     FIG. 1 is a graphical representation of a relationship between signal-to-noise ratio of a signal and rainfall over time. 
     FIG. 2 is a schematic diagram illustrating interconnections for dynamic adaptive modulation negotiations between wireless modems according to the present invention. 
     FIG. 3 is a block diagram of a transmitter according to the present invention that is included in the wireless modems of FIG.  2 . 
     FIG. 4 is a block diagram of a receiver according to the present invention that is included in the wireless modems of FIG.  2 . 
     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. 
     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 
     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. 
     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 . 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. 
     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. 
     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 . 
     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 . 
     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. 
     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. 
     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 . 
     The controller  34 ,  40  outputs a modulation index change command packet  78  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. 
     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 . 
     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 . 
     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  78  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  78  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. 
     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  78  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  78  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 . 
     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. 
     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. 
     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. 
     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 transmitter 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. 
     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 . 
     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. 
     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. 
     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. 
     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  78  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 affected by the transmission of the modulation index change command packet  78 . 
     The controller  40  outputs the modulation index change command packet  78  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  78  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  78  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  78  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 its 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. 
     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. 
     If the sync loss event does not occur while in the upgrade state  66 , 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, the receiver  38  of the second wireless modem  28  moves from the upgrade state  66  to the stable state  64  after preferably a one second timeout event. The receiver  38  then resumes sampling the line quality. During this time, neither the first wireless modem nor the second wireless modem  28  stop receiving or transmitting data, and therefore no loss in data transmission has resulted. 
     When the receiver  38  is in the upgrade state  66  and achieves the sync loss event, the receiver moves to the upgrade wait state  68  to wait for the 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. 
     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  78  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  78  to the forward error correction encoder  44  of the transmitter  36  of the second wireless modem  28  and the modulation index change command packet  78  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  78  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  78  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 its modulation index accordingly, and then transmits data including the empty frames as described above. 
     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. 
     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  78  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. 
     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. 
     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. 
     If, on the other hand, while sampling the line quality in the stable state  64  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  78  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  78  to the forward error correction encoder  44  of the transmitter  36  and the modulation index change command packet  78  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  78  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  78 . 
     In the same way as in the upgrade state  66  described above, when the modulation index change command packet  78  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 its modulation index to correspond to the decreased QAM index, and then resumes transmitting data, beginning with empty frames. 
     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  78 , 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. 
     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 downgraded 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 downgrade state  72  to the stable state  64  and resumes sampling the line quality. During this time, neither the first wireless modem nor the second wireless modem  28  stop receiveing or transmitting data, and therefore no loss in data transmission has resulted. 
     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 case 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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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.