Abstract:
In a DOCSIS based satellite gateway data is transmitted over a single downstream channel, at different throughput rates. Data destined for each subscriber/receiver is assigned a throughput rate depending upon the downstream signal quality of that subscriber/receiver. To accomplish this, the downstream DOCSIS MAC data is parsed to extract DOCSIS packets. The DOCSIS packets are then loaded into packet queues based on an identifier within such packets such as the MAC destination address or SID. Each of the queues represents a bandwidth efficiency or throughput rate that can be currently tolerated by specific subscribers based on the current signal quality being experienced at the subscriber location. A PHY-MAP describing the downstream data structure to be transmitted and inserted into the downstream data. Data is extracted from the packet queues in queue blocks as defined by the PHY-MAP. The queue blocks are modulated with transmission parameters appropriate for each queue block and transmitted to the DOCSIS based satellite modems. The satellite modems extract the PHY-MAP from the downstream data and use the information contained in it to demodulate and decode the queue for which they have sufficient downstream signal quality. Satellite modems measure and transmit downstream signal quality to the satellite gateway to be used to assigned traffic to the appropriate queues.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Application Ser. No. 60/424,205, filed Nov. 6, 2002, which is incorporated herein by reference in its entirety for all purposes. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to broadband communication systems, and more particularly those that use the Data Over Cable Service Interface Specification (DOCSIS) media access protocol or its derivatives. 
   2. Description of the Related Art 
   Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems, to the Internet, to cable systems, to local area networks (LANs), to wide area networks (WANs) to in-home wireless networks and the like. Often, these systems are comprised of numerous different forms of transmission media. 
   In the example of two-way data communications via satellite, data may be transmitted using time division multiplexing (TDM) over a single carrier (i.e. channel). A gateway receives data from a network such as the Internet, performs forward error correction (FEC) then modulates the data. The data is transmitted up to a satellite and back down from the satellite to one or more receivers. 
   For each communication device to participate in communications of whatever type, it includes or is coupled to a transceiver (i.e., for terrestrial wireless, a radio receiver and transmitter; and for two-way satellite, a satellite receiver and transmitter). As is known, the transmitter of radio and satellite transceivers include a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless or satellite communication standard. The one or more intermediate frequency stages mix the baseband signals with the signal generated by one or more local oscillators to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna or satellite dish. 
   As is also known, the receiver of a transceiver is also coupled to the antenna or satellite dish and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery (i.e. demodulation) stage. The low noise amplifier receives an inbound RF signal via the antenna or satellite dish and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with the signal generated by one or more local oscillators to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. This is typically referred to as frequency down-conversion. The filtering stage filters the down-converted baseband or IF signal to eliminate unwanted out of band signals to produce a filtered signal that is only that which falls within the bandwidth of the selected channel. Thus, this filter is sometimes referred to as a channel select filter. The data recovery or demodulation stage recovers raw data from the filtered signal in accordance with the particular wireless or satellite communication standard. 
   One common data network architecture, specified as the Data Over Cable Service Interface Specification) DOCSIS 1 , originated with cable operators interested in deploying high-speed packet-based communications systems on cable television systems. These include IP based Internet data, packet telephony service, video conferencing service, and many others. The goal of DOCSIS is to define a data service that will allow transparent bi-directional transfer of Internet Protocol (IP) traffic between a cable system headend or Cable Modem Termination System (CMTS) and customer locations using a cable modem (CM), over an all-coaxial or hybrid-fiber/coax (HFC) cable network.  1 The DOCSIS Radio Frequency Interface Specification SP-RFIv1.1-I09-020830 is publicly available from and is hereby incorporated herein by this reference for all purposes. 
   The DOCSIS Media Access Control (MAC) sublayer specifies that the CMTS provide a single carrier transmitter for each downstream (i.e. from head-end to subscriber) channel. All CMs at subscriber locations listen to all frames transmitted on the downstream channel upon which they are registered and accept those frames where the destinations match the CM itself or CPEs (customer premises equipment). CMs can communicate with other CMs only through the CMTS. 
   The upstream channel is thus characterized by many transmitters (CMs) and one receiver (the CMTS). Time in the upstream channel is slotted, providing for Time Division Multiple Access at regulated time ticks. The CMTS provides the time reference and controls the allowed usage for each interval. Intervals may be granted for transmissions by particular CMs, or for contention by all CMs. CMs may contend to request transmission time. To a limited extent, CMs may also contend to transmit actual data. In both cases, collisions can occur and retries are then used. 
   The DOCSIS protocol has been adapted to other types of media, including terrestrial fixed wireless and two way satellite. For these applications, as well as the original data over cable service, data is transferred between a central location and many remote subscribers. The term for the centrally located equipment for broadband terrestrial fixed wireless systems is a Wireless Access Termination System (WATS). The subscriber equipment is called a wireless modem. With respect to two way satellite, the centrally located equipment is a satellite gateway (SG), while the subscriber equipment is a satellite modem (SM). Those of average skill in the art will recognize that in each of these types of service, the DOCSIS architecture is substantially maintained, even if some of the implementation details are adapted to the type of media used for transmission. 
   In standard DOCSIS based systems such as those described above, the downstream transmission is defined to be a time division multiplexed (TDM) signal with a fixed modulation type as well as a fixed forward error correction (FEC) coding rate. Thus, by nature the downstream signal has a fixed spectral efficiency in bits per second/Hertz [bps/Hz]. Signal parameters such as the modulation type, FEC coding type, and FEC coding rate determine the minimum signal to noise ratio (SNR) that must be present for the SM to have error-free or quasi error-free operation in a given channel having those parametric limitations. Thus, there is an inherent trade-off between the values of receiver parameters that yield a high level of throughput (e.g. high-order modulation and high FEC code rates) and those values (e.g. low-order modulation and more robust but lower FEC code rates) that ensure that the signal can be reliably received under conditions of low SNR but with lower throughput. 
   In many real world environments, subscribers of such systems experience a wide range of path losses and channel degradations. One example is a satellite based system where a downstream spot beam broadcasts to SMs that are located over a wide geographic area. Various conditions such as localized rainfall, partial obstructions, antenna misalignments, etc. can significantly degrade the signal power levels (and thus SNRs) received by individual subscribers. Those of average skill in the art will recognize that similar channel degradation may be experienced for subscribers of terrestrial fixed wireless and even data over cable, although the causes may be different. 
     FIG. 1  illustrates the basic elements of a two-way satellite system. A satellite gateway (SG) with a baseband modulator/demodulator  100  receives data from a network, such as the Internet. The data is assembled into an appropriate format in accordance with, for example the DOCSIS architecture previously described, and is then provided to transceiver  102 . The transceiver performs certain functions necessary for transmitting the data using the satellite dish  104 , up to the satellite  106  and down to a plurality of SMs  112  over downstream channel  114 . The downstream signal is received by the dish  108 , processed by the transceiver  110 , and demodulated by SM  112 . The SMs  112  transmit data, generated by the customer premise equipment (CPE) (not shown) back to the SG  100  over the upstream channel  116  uses the format recognized by the SG  100 . 
     FIG. 2  is a block diagram illustrating the processing blocks of a known SG  100   a , along with the processing blocks of the transceiver  102 . Data from a network  250 , such as the Internet, is transmitted between the network and the gateway DOCSIS MAC  204   a . The data is formatted in accordance with the DOCSIS protocol. This protocol uses an MPEG format in the “downstream transmission convergence sublayer” that serves as the interface between the MAC and physical layer (PHY). MPEG specifications are publicly available and are incorporated herein by this reference for all purposes. 
   The downstream MPEG data stream that is output at  240   a  from the MAC  204   a  is encoded and modulated using a single type of modulation and a single set of FEC parameters by the fixed encoding and modulation stage  206   a . The modulated and encoded MPEG stream is then up converted and filtered, and then fed into a high power amplifier by transceiver  214 . This signal is transmitted continuously in a single frequency band, through a satellite ( 106 ,  FIG. 1 ) and received by subscriber SMs ( 112 ,  FIG. 2 ). 
   The SG  100   a  also receives (i.e. at antenna dish  104 ,  FIG. 1 ) the upstream signal transmitted by subscriber satellite modems SM  112  of the system. The signal is filtered and down converted back to baseband at stage  212  using a fixed set of demodulation and decoding parameters at stage  208   a  to recover the upstream data stream. 
     FIG. 3  is a block diagram illustrating the processing blocks of a known SM  112   a , along with the processing blocks of the transceiver  110 . The SM  112   a  receives the signal through dish  108  and down converts the signal using a fixed set of demodulation and decoding parameters to recover the MPEG stream at processing box  540 . The information is processed by the local DOCSIS MAC  504   a  in conjunction with the host processor  500   a . The data is then passed on to the CPE of the subscriber at line  250 . 
   Note that the modulation type and the FEC encoding parameters are fixed for all data transmitted by the SG  100   a  and received by each SM  112   a  over the downstream channel of the system of  FIGS. 2 and 3 . Indeed, to ensure that customers do not experience total loss of service under conditions of low SNR, current DOCSIS based systems must operate with channel parameters (and therefore fixed modulation and FEC encoding parameters) that ensure that even the subscriber situated the worst in terms of signal degradation (as manifested by bit error rate or SNR) is able to obtain service with a high probability of success. As a result, the majority of subscribers that could otherwise receive data at a higher rate are penalized by the presence of the relatively fewer environmentally disadvantaged subscribers. 
   BRIEF SUMMARY OF THE INVENTION 
   Thus, in order to overcome the above-described limitations of the prior devices, among other limitations, a satellite communication system constructed according to the present invention implements downstream adaptive modulation that allows subscribers receiving the downstream channel with a higher SNR and/or operating in less degraded channels to achieve higher bandwidth efficiency. This results in a combination of improved channel capacity, increased throughput, and improved coverage. 
   An embodiment of the method of the invention independently and adaptively controls the throughput rate of data traffic destined for each of a plurality of receivers. First, a number of packet queues are defined. Each packet queue is associated with a unique set of transmission parameters. For example, the most robust queue could be defined as using QPSK modulation with a FEC code rate of ½. This queue has relatively low throughput but also requires a very low SNR received at the SM. The least robust queue could be defined with 16 QAM modulation and a FEC code rate of ¾ for example. This queue has a much higher throughput, but also requires higher received SNR to achieve low error rate performance. Using this technique, a number of packet queues are defined, each of which meets respective downstream signal quality requirements. The plurality of packet queues is spaced rationally across a range that corresponds to an expected operating range of the SMs in the system. 
   The data traffic is assigned to a queue based on the downstream signal quality information for each of the subscribers. This information is measured by each SM and transmitted back to the SG in the upstream channel. Downstream signal quality is periodically updated to reflect the possibility of changing channel conditions for each subscriber. Knowledge of the downstream signal quality for each SM allows the SG to assign the traffic for each SM to the proper packet queue. Overall efficiency is maximized when traffic for each subscriber is placed in the queue that has the highest throughput that can be supported by the downstream signal quality of the SM in question. However, if necessary traffic can be placed in or moved to a more robust packet queue and still be received by the SM. 
   Data is extracted from each packet queue in queue blocks (QBs) that have a known size and duration. Each of these QBs must be modulated with the transmission parameters associated with the originating packet queue. Each SM configures its demodulation parameters to receive the QBs that its downstream signal quality allows it to receive. The SG must communicate the data structure (i.e., the type, order and number of each QB) to both the downstream modulator (at the SG) and the SM. 
   The SG communicates this information to the SM via a PHY-MAP control message. This message must be receivable by all SMs, and must specify the parameters required to demodulate and decode the downstream data. At the SG, the modulator can use the PHY-MAP to set the required transmission parameters for each QB, or other control information can be generated and used. 
   In an adaptive modulator controller constructed according to the present invention, a DOCSIS MAC receives network data such as IP data and produces a number of DOCSIS packets. These packets are placed directly into the packet queues, or framed into the traditional MPEG stream specified by the DOCSIS downstream transmission convergence layer specification. If the MPEG stream is used, a parser extracts the DOCSIS packets and places them in a number of packet queues. Each packet queue represents particular throughput rate or bandwidth efficiency. The more bandwidth efficient, the less tolerant the transmission is of degraded signal at the downstream receiver. The traffic is assigned to each of the receivers based on its current signal quality. Data is formed into queue blocks and each queue block is transmitted with the transmission parameters assigned to the given queue. A PHY-MAP is transmitted in the downstream data to give the SMs knowledge of the downstream data structure. Each SM decodes the PHY-MAP and demodulates the queue blocks with transmission parameters appropriate for that queue block. In general, some SMs will be unable to decode certain queue blocks due to downstream signal quality requirements that are higher than that be experienced by the given SM. The SG endeavors not to place traffic for a given SM in a queue block that it cannot receive. It accomplishes this task with knowledge of the downstream signal quality for each SM that is reported to it via the upstream channel. 
   Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The Downstream Adaptive Modulation (DS-AM) method and apparatus of the invention may be better understood, and its numerous objectives, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element. 
       FIG. 1  illustrates the basic elements of a two way satellite data system; 
       FIG. 2  is a block diagram illustrating the processing blocks of a known satellite gateway (SG); 
       FIG. 3  is a block diagram illustrating the processing blocks of a known satellite modem (SM); 
       FIG. 4  is a block diagram illustrating the processing blocks of an embodiment of a satellite gateway (SG)  100   b , that incorporates the Downstream Adaptive Modulation (DS-AM) method and apparatus of the invention; 
       FIG. 5  illustrates a block diagram of the adaptive modulation formatter &amp; controller (AMFC) of  FIG. 4 ; 
       FIG. 6  is a block diagram illustrating the processing blocks of an embodiment of a satellite modem (SM), which incorporates the DS-AM method and apparatus of the invention. 
       FIG. 7  illustrates a queing example given a MPEG data stream input to the AMFC. 
       FIG. 8  illustrates an example output the AMFC based on the profile established for the packet queue in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4  is a block diagram illustrating the processing blocks of an embodiment of a satellite gateway (SG)  100   b  that incorporates the Downstream Adaptive Modulation (DS-AM) method and apparatus of the invention. The SG  100   b  includes an adaptive modulation and formatting and control (AMFC) stage  406  that receives and processes the DOCSIS encapsulated traffic received from the MAC processing block  204   b . Interface  240   b  could be a MPEG data stream compliant with the DOCSIS downstream transmission convergence sublayer, or it could have an alternative format. The SG  100   b  also includes a variable encoding &amp; modulation stage  408 , which is a modulator that is capable of having its modulation type and FEC encoding parameters dynamically controlled on a QB by QB basis. 
     FIG. 5  illustrates a block diagram of the AMFC block  406  of  FIG. 3 . The host controller  200   b  receives downstream signal quality information for each of the particular SMs  112   b  over the upstream channel. The SM  112   b  is an embodiment of a satellite modem that is operable to receive and decode downstream transmissions from the SG  100   b  that have been adaptively modulated and encoded using the DS-AM method and apparatus of the invention. The SM  112   b  will be discussed in more detail below. 
   The downstream signal quality for each SM  112   b  can be based on, for example, the SNR, packet or code word error rate, or other parameters defining signal quality. The SM  112   b  can be made to monitor that information continuously, so that any changes in the signal quality may be dynamically reflected at the SG  100   b . Different sets of transmission parameter profiles (including values for modulation type, FEC type and FEC rate) are defined spanning the range of expected signal quality. An example of a set of four profiles based on SNR as a signal quality measurement are shown below in Table 1. As Table 1 illustrates, the profiles trade-off higher throughput (shown as higher bandwidth efficiency in Table 1) for higher required signal quality. Clearly, Table 1 is only a hypothetical example. Other profiles with different performance characteristics and different transmission parameters could be specified while staying within the scope of the invention. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Example Transmission Parameter Profiles for DS-AM 
             
           
        
         
             
                 
                 
                 
               BW Eff 
               Required 
             
             
               Profile 
               Mod 
               FEC Rate 
               [bits/sec/Hz] 
               SNR 
             
             
                 
             
           
        
         
             
               1 
               QPSK 
               1/2 
               1.0 
               3.0 
             
             
               2 
               QPSK 
               3/4 
               1.5 
               6.0 
             
             
               3 
                8 PSK 
               2/3 
               2.0 
               9.0 
             
             
               4 
               16 QAM 
               3/4 
               3.0 
               12.0 
             
             
                 
             
           
        
       
     
   
   The host controller  200   b  assigns each of the different profiles to one or more of the queues  602 . Put another way, each packet queue  602  is associated with a unique modulation order and/or FEC code rate that defines a throughput rate in the form of bandwidth efficiency (i.e. bits per second per 1 Hz of bandwidth. Traffic for a given SM  112   b  is then assigned to a specific queue or set of the queues  602  having an assigned profile that is appropriate for the downstream quality information provided by that SM  112   b . Sufficient information is associated with each packet to allow it to be assigned to the proper queue. For example, the packets can be assigned to the different queues  602  by means of the DOCSIS Destination Address (DA), Service ID (SID), or any other unique identifier that is available with the DOCSIS protocol. 
   The adaptive modulation formatter &amp; controller block  406  receives the data stream  240   b  from the Gateway MAC  204   b . DOCSIS packets destined for individual SMs  112   b  are parsed and placed in their assigned packet queues  602 . One possible format for the data interface between the Gateway MAC  204   b  and the AMFC  406  is an MPEG format.  FIG. 7  illustrates the DOCSIS packets of the data stream  240   b  residing in a MPEG format. As shown, four DOCSIS packets are overlaid on the MPEG format. Each MPEG frame has a length of 188 bytes. The example in  FIG. 7  shows as the first packet PHY-MAP and hence as input externally to the AMFC  406 . In alternate implementations, the PHY-MAP could be generated internally in the AMFC  406 . The parser  602  is able to extract the DOCSIS packets  702 ,  704 ,  706  and  708  from the MPEG stream, and based on their DOCSIS destination addresses or other unique identifier rout them to their assigned packet queues  602   a - 602   n . In this case, it is assumed that DOCSIS packets #1,  702  and #3  706  are assigned to packet queue  602   a , while packet #2  704  is assigned to queue  602   b  and #4  708  is assigned to the last packet queue  602   n.    
   The profiles for the various queues are defined by their transmission parameters. These parameters include, but are not necessarily limited to: the modulation type, FEC type, FEC rate, FEC block size, and QB size (or equivalently number of MPEG frames per QB). The example in  FIG. 7  illustrates profiles that are defined by QPSK modulation with a rate 1/2 code (queue  602   a ), 8-PSK modulation with a rate 2/3 code (queue  602   b ) and 16 QAM modulation with a rate 3/4 code (queue  602   n ). As discussed, the profile parameters are defined to accommodate the system performance objectives and downstream signal quality requirements. They would ideally be based on traffic signal quality conditions experienced by the subscribers. In the example of  FIG. 7 , queue  602   a  has a profile that is typically used for a worst case signal to noise ratio, as the order of the modulation type is low and the error correction code rate is only one bit of two being payload, and every other bit being a parity bit. While the bandwidth efficiency is quite low for this combination, it produces a very robust transmission even in worst case signal conditions. The profile for Q #2 is somewhat better in bandwidth efficiency, and is more suitable for receivers or subscribers that are experiencing better SNR ratios. The least robust queue (queue  602   n ) has the highest bandwidth efficiency, but also requires the highest downstream signal quality. 
   As previously discussed, a worst case queue guarantees that DOCSIS MAC management type messages, or that the current configuration of the PHY-MAP has been received by all receivers. This is crucial for the proper operation of SM  112   b  receivers. They must all know how the downstream data stream has been formatted at any given time for proper demodulation and decoding of the downstream data. There is an inherent cost/benefit trade-off in selecting the size of the queue blocks. Large Q blocks tend to facilitate more efficient mapping of data packets and provides the opportunity for more effective interleaving to spread errors. Shorter packets minimize latency and facilitate a more exact match between the traffic conditions and the proportion of capacity assigned to each queue. 
   This description of the downstream data structure (i.e. the number and type of each queue block transmitted) are stored in PHY-MAPs. The PHY-MAP spans a known time period and contains the information necessary for the SMs to determine the sequence of queue blocks arriving in the downstream and hence to demodulate and decode each queue block that it is capable of receiving (i.e. the SM receives the queue blocks for which it has the required downstream signal quality). The PHY-MAP can also contain information that defines start times of bursts from each queue  602  to which each profile is assigned, and the duration in the number of QBs. PHY-MAPs span a finite period of time and are inserted into the downstream and transmitted periodically Thus, the relative time allocated for transmission of each queue  602  can be dynamically changed in the PHY-MAPs to optimize overall system throughput and maximize efficiency. For example, if a very large rain storm affects a large number of subscribers, the low throughput, more robust queue may become full more quickly than the other queues. Similarly, if it is extremely clear and sunny, the higher through rate (i.e. higher bandwidth efficient) queues may become fuller faster. In this case, the controller  604  can sense this and increase the size and/or number of the QBs that define a burst from a queue. The structure of the PHY-MAP is not critical and can be determined by the host processor  200   b , the embedded controller  604 , or other external entity that has knowledge of the traffic statistics, and signal quality distribution of the SMs  112   b.    
   The imbedded controller  604  provides control information to the MPEG Framer  608  based on the PHY-MAP profile information for each packet queue  602 , and packet data is thereby extracted from the packet queues  602  and framed according to the profile definitions in the PHY-MAP. Modulation control can be achieved by sending the PHY-MAP to the Variable Encoding &amp; Modulation block  408  of  FIG. 4 . Alternately, the modulation control could be a separate processing block that provides modulation control information (as shown as  618  in  FIG. 5 ). Regardless of the specific implementation, modulation control must be provided to the Variable Encoding &amp; Modulation block  408  such that each queue block is transmitted with the proper transmission parameters. 
     FIG. 8  illustrates an output  420  of MPEG framer  608  based on the profile established for the queues  602   a ,  602   b , and  602   c  of  FIG. 7 . The output starts out with the PHY map message  702  destined for all of the receiver/subscribers (in this example, packet  702  is interpreted to be the PHY-MAP). As shown, the PHY-MAP is included in a queue block having the most robust transmission parameters. From information in the PHY-MAP, the SMs will know the map of the content of the downstream they are receiving and therefore how to decode and demodulate it. Queue blocks are transmitted in accordance with the sequence defined by the PHY-MAP. In the example shown in  FIG. 8 , this sequence includes a QB from queue  602   a , a QB from queue  602   b , and a queue block from queue  602   n . These QBs contain the example DOCSIS packets  704 ,  706  and  708 . DOCSIS time stamps or other MAC messages would be included in queue  602   a  (the most robust queue). The QBs are provided to the variable encoding and modulation stage ( 408 ,  FIG. 4 ) over output  420 . The modulation control signals can be embedded in  420 , or implemented as a separate interface  422  (or some combination of the two approaches). 
   As previously mentioned, any data transmitted for all SMs  112   b  to receive must be processed through the packet queue  602  with the most robust profile (and therefore the lowest throughput to ensure that even the most degraded SMs  112   b  are able to receive the messages. This includes the PHY-MAP itself, which all SMs  112   b  must receive and utilize to properly decode the downstream data. The SMs must know the profiles for each frame of data coming in, so that it can adaptively apply the correct demodulation and error decoding to the received data on a QB by QB basis. Other such messages that must be transmitted through the most robust queue  602  include all DOCSIS timestamps, DOCSIS MAC management messages and all other multi-cast data. 
   As previously discussed, the SMs  112   b  must be able to decode and demodulate the adaptively modulated stream.  FIG. 6  is a block diagram illustrating the processing blocks of an embodiment of a satellite modem (SM)  112   b , which incorporates the DS-AM method and apparatus of the invention. The adaptive demodulation and decoding block  560  decodes the PHY-MAP message ( 702 ,  FIG. 8 ) sent from the Gateway (SG  100   b ). This message is used to determine the proper demodulation and decoding parameters to use during the proper time intervals (i.e. for each QB). The SM  112   b  always decodes and demodulates the most robust data packet queue to extract the timestamp and MAC management messages sent to all SMs  112   b.    
   The SMs  112   b  use the MAC management messages to set up an upstream channel to the GM  100   b . SM  112   b  uses this upstream channel to send downstream signal quality metrics to the host of the SG  100   b . This could be implemented as part of the ranging and registration process common to these systems or as separate MAC messages. Based on its signal quality, each SM  112   b  identifies the maximum downstream throughput rate that it can handle with acceptable fidelity, and decodes data received from the packet queue assigned to handle that throughput rate as well as from any queue having a more robust profile. If any packet queue cannot be demodulated by the SM  112   b  with appropriate fidelity, the SM  112   b  fills output MPEG frames corresponding to that queue with null MPEG frames, or otherwise blanks the data sent to the SM DOCSIS MAC  504   b . The decoded stream is transmitted to the SM DOCISIS MAC  540   b  over output  520   b.    
   The Gateway SG  100   b  uses the PHY-MAP for flexible and optimized assignment of QBs to the downstream. As channel conditions or traffic loading changes, the PHY-MAP can be dynamically adjusted to optimize efficiency. Decoding all possible queues by the SM  112   b  assures that all SMs  112   b  will receive PHY-MAP messages and multi-cast traffic over the packet queue having the most robust modulation and encoding. It also permits Gateway SG  100   b  the flexibility to assign traffic destined for a given SM  112   b  to the queue having the highest possible throughput, or to any of the more robust queues. 
   Those of average skill in the art will recognize that the DS-AM of the invention can be achieved in the time or frequency domain. While embodiments disclosed herein are time domain implementations, it is contemplated that the principals of the invention as disclosed may be extended to the frequency domain without exceeding the intended scope of the invention. Moreover, those of average skill in the art will recognize although the embodiments disclosed herein within the context of DOCSIS based satellite systems, the method and apparatus of the invention may easily be applied to other types of DOCSIS data systems, such as terrestrial fixed wireless systems and cable modem systems. 
   The invention disclosed herein is susceptible to various modifications and alternative forms. Specific embodiments therefore have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims.