Abstract:
A method and apparatus for adaptively transmitting an uplink signal comprising information from a ground station to a satellite is disclosed. The method comprises the steps of receiving a transmitted downlink signal at a ground station from the satellite; measuring the quality of the transmitted signal; computing a prediction of a degradation of the uplink signal using the quality of the received signal, and transmitting the uplink signal according to the predicted degradation of the uplink signal. The apparatus comprises a tuner for receiving a transmitted downlink signal from the satellite; a signal analyzer, communicatively coupled to the tuner, for measuring a characteristic of the received downlink signal; an uplink degradation estimation module, communicatively coupled to the signal analyzer, for estimating the degradation of the uplink signal from the received downlink signal characteristic; and a controller, communicatively coupled to the uplink degradation module, for controlling the transmission of the uplink signal according to the estimated degradation of the uplink signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to systems and methods for improving a power and bandwidth constrained transmission system, and in particular to a system and method for adaptively varying an uplink signal from a terrestrially-based user terminal to a satellite according to a predicted degradation of the uplink signal. 
     2. Description of the Related Art 
     A typical communications system, whether terrestrial or satellite-based, is constrained in its transmission power and signal bandwidth. The communications system may also be required to deliver reliable communications to one point, as in a terrestrial microwave link, or to many points in a satellite broadcast system serving the United States. 
     A typical satellite communications system is typically designed such that all signals transmitted to and received from terrestrially based receivers have a minimum carrier to noise ratio (CNR). The CNR of the received signal is a function of a number of parameters, including the power of the signal transmitted and signal propagation characteristics such as atmospheric attenuation. Unfortunately, at the wavelengths typically used in satellite-home communications systems, rain and other sources of atmospheric attenuation have a significant effect upon the CNR. Hence, during heavy rain, subscribers can experience degraded signal quality in both transmitted and received signals. 
     In the past, system designers have assured a minimum signal quality is provided by designing transmitters, power systems, and related components to produce sufficient power to assure a minimum CNR in all conditions, even when compromised by unfavorable atmospheric attenuation characteristics. Unfortunately, the amount of attenuation from rain and similar atmospheric parameters can be substantial and widely varying. Hence, to achieve the desired CNR at all times (including intense rain periods) this solution requires that the ground and satellite transmitters, power system and related components be designed for high performance levels that are not needed most of the time. 
     Setting the transmitter power to a level that results in a sufficient CNR for all conditions (or nearly all conditions) may be economically impractical, and is typically infeasible in existing field units. What is needed is a system and method for adaptively controlling transmissions to account for signal propagation characteristics. The present invention satisfies that need. 
     SUMMARY OF THE INVENTION 
     To address the requirements described above, the present invention discloses a method, apparatus, and article of manufacture for adaptively transmitting an uplink signal comprising information from a ground station to a satellite. The method comprises the steps of receiving a transmitted downlink signal at a ground station from the satellite; measuring the quality of the transmitted signal; computing a prediction of a degradation of the uplink signal using the quality of the received signal, and transmitting the uplink signal according to the predicted degradation of the uplink signal. 
     The apparatus comprises a tuner for receiving a transmitted downlink signal from the satellite; a signal analyzer, communicatively coupled to the tuner, for measuring a characteristic of the received downlink signal; an uplink degradation estimation module, communicatively coupled to the signal analyzer, for estimating the degradation of the uplink signal from the received downlink signal characteristic; and a controller, communicatively coupled to the uplink degradation module, for controlling the transmission of the uplink signal according to the estimated degradation of the uplink signal. 
     The invention allows a predetermined successful communications link to a satellite using the subscriber&#39;s ODU. This is accomplished using an existing or modified power meter in the IRD to measure the downlink attenuation (typically due to weather), and to use the downlink attenuation to compute the expected uplink (from the subscriber to the satellite) degradation. If the degradation of the uplink signal is minimal, but enough to preclude normal transmission at acceptable error rates, the IRD transmits the data with a different transmission characteristic (e.g. lower transmission rate, different modulation scheme, higher power, or different error correction) to ensure successful transmission. If the degradation is sufficient to essentially preclude transmission, and the uplink cannot be established in the near future, the IRD stores the information, informs the subscriber that the uplink information will not be transmitted, continues to predict the uplink degradation from subsequent downlinks (e.g. every 5 minutes), and informs the subscriber when the uplink can commence. If the predicted degradation is between these two extremes, the IRD stores the information, informs the subscriber that the uplink will be delayed, and continues to predict the uplink degradation from subsequent downlinks. When the predicted degradation improves sufficiently to allow transmission, the information is uplinked to the satellite. 
     This allows the IRD to pre-determine the success of a message transmitted from the subscriber to the satellite. The signal degradations measured by the IRD can include degradations caused by weather, misalignment of the ODU, or reduced ODU or IRD performance from hardware or software malfunctions. 
     The present invention applies to a variety of terrestrial and satellite applications, but is especially advantageous in satellite distribution systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  is a block diagram illustrating an overview of a video distribution system; 
         FIG. 2  is a block diagram showing a typical uplink configuration showing how video program material is uplinked to a satellite for transmission to subscribers using a single transponder; 
         FIG. 3A  is a diagram of a representative data stream received from a satellite; 
         FIG. 3B  is a diagram illustrating the structure of a data packet; 
         FIG. 4  is a simplified block diagram of one embodiment of a terrestrial receiver; 
         FIG. 5  is a flow chart depicting exemplary method steps used to practice one embodiment of the present invention; 
         FIG. 6  is a diagram illustrating the application of different predicted degradation values to the present invention; 
         FIG. 7  is a flow chart showing exemplary process steps used to transmit the uplink signal according to the predicted degradation value; and 
         FIG. 8  is flow chart presenting exemplary process steps that are applied when the predicted uplink degradation is sufficient to at least temporarily prevent the transmission of the uplink signal. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
       FIG. 1  is a diagram illustrating an overview of a multi-channel video distribution system  100 . The video distribution system  100  comprises a control center  102 , an uplink center  104 , a satellite  108 , and a ground station  111  having a terrestrial receiver  112  and a subscriber  110 . 
     The uplink center  104  receives program material from external sources or the control center  102  via a communications link  114 . In accordance with instructions from the control center  102 , the uplink center  104  transmits the program material and program control information to the satellite  108  via uplink  116  using the uplink antenna  106 . The satellite  108  receives and transmits the video programs and control information to the subscriber via downlink transmission signal  118 . The subscriber  110  receives this information using the terrestrial receiver  112 . The ground station  111  also comprises device having a transmitter and a transmit antenna for sending data to the satellite  108 . The satellite  108  communicates information received from the ground station to the uplink center  104  via downlink  120 . Typically, the signal transmitted by the transmitter to the satellite has characteristics that are pre-determined (e.g. modulation technique, power). 
     The video distribution system  100  can comprise a plurality of satellites  108  in order to provide wider terrestrial coverage, to provide additional channels, or to provide additional bandwidth per channel. In one embodiment of the invention, each satellite comprises 16 Ku band transponders to receive and transmit program material and other control data from the uplink center  104  and provide it to the subscribers  110 . However, by using data compression and multiplexing techniques the television channel capabilities are far greater. For example, two satellites  108  working together can receive and broadcast over 200 conventional (non-HDTV) television channels via 32 satellite transponders. 
     While the invention disclosed herein is described with reference to a satellite based digital video distribution system  100 , the present invention may also be practiced with terrestrial-based transmission of program information, whether by broadcast, unicast, cable, the Internet, or other means. Further, the different functions collectively allocated among the control center  102  and the uplink center  104  as described above can be reallocated between these facilities as desired without departing from the intended scope of the present invention. 
     Although the foregoing has been described with respect to an embodiment in which the program material delivered to the subscriber is video (and audio) program material such as a movie, the foregoing method can be used to deliver program material comprising purely audio or purely data, or any combination of information as well. 
       FIG. 2  is a block diagram showing a typical uplink configuration for a single satellite  108  transponder, showing how video program material is uplinked to the satellite  108  by the control center  102  and the uplink center  104 .  FIG. 2  shows three video channels (which could be augmented respectively with one or more audio channels for high fidelity music, soundtrack information, or a secondary audio program for transmitting foreign languages), and a data channel from a computer data source  206 . 
     The video channels are provided by a program source of video material  200 A- 200 C (collectively referred to hereinafter as video source(s)  200 ). The data from each video program source  200  is provided to an encoder  202 A- 202 C (collectively referred to hereinafter as encoder(s)  202 ). Each of the encoders accepts a program time stamp (PTS) from the controller  216 . The PTS is a wrap-around binary time stamp that is used to assure that the video information is properly synchronized with the audio information after encoding and decoding. A PTS time stamp is sent with each I-frame of the MPEG encoded data. 
     In one embodiment of the present invention, each encoder  202  is a second generation Motion Picture Experts Group (MPEG-2) encoder, but other decoders implementing other coding techniques can be used as well. The data channel can be subjected to a similar compression scheme by an encoder (not shown), but such compression is usually either unnecessary, or performed by computer programs in the computer data source (for example, photographic data is typically compressed into *.TIF files or *.JPG files before transmission). After encoding by the encoders  202 , the signals are converted into data packets by a packetizer  204 A- 204 F (collectively referred to hereinafter as packetizer(s)  204 ) associated with each source  200 . 
     The data packets are assembled using a reference from the system clock  214  (SCR), a control word (CW) generated by the conditional access manager  208 , and a system channel identifier (SCID)  210  that associates each of the data packets that are broadcast to the subscriber with a program channel. This information is transmitted to the packetizers  204  for use in generating the data packets. These data packets are then multiplexed into serial data, encoded, modulated, and transmitted. A special packet known as a control word packet (CWP) (which comprises control data including the control word (CW) and other control data used in support of providing conditional access to the program material) is also encrypted and transmitted. 
       FIG. 3A  is a diagram of a representative data stream. The first packet segment  302  comprises information from video channel  1  (data coming from, for example, the first video program source  200 A). The next packet segment  304  comprises computer data information that was obtained, for example from the computer data source  206 . The next packet segment  306  comprises information from video channel  5  (from one of the video program sources  200 ), and the next packet segment  308  includes information from video channel  1  (again, coming from the first video program source  200 A). The data stream therefore comprises a series of packets from any one of the data sources in an order determined by the controller  216 . The data stream is encrypted by the encryption module  218 , modulated by the modulator  220  (typically using a QPSK modulation scheme), and provided to the transmitter  222 , which broadcasts the modulated data stream on a frequency bandwidth to the satellite via the antenna  106 . 
     Subscribers  110  receive media programs via a subscriber receiver or integrated receiver/decoder (IRD)  400 . Using the SCID, the IRD  400  reassembles the packets to regenerate the program material for each of the channels. As shown in  FIG. 3A , null packets  310  created by the null packet module  312  may be inserted into the data stream as desired. 
       FIG. 3B  is a diagram of a data packet. Each data packet (e.g.  302 - 316 ) is 147 bytes long, and comprises a number of packet segments. The first packet segment  320  comprises two bytes of information containing the SCID and flags. The SCID is a unique 12-bit number that uniquely identifies the data packet&#39;s data channel. The flags include 4 bits that are used to control whether the packet is encrypted, and what key must be used to decrypt the packet. The second packet segment  322  is made up of a 4-bit packet type indicator and a 4-bit continuity counter. The packet type identifies the packet as one of the four data types (video, audio, data, or null). When combined with the SCID, the packet type determines how the data packet will be used. The continuity counter increments once for each packet type and SCID. The next packet segment  324  comprises 127 bytes of payload data, which is a portion of the video program provided by the video program source  200 . The final packet segment  326  is data required to perform forward error correction. 
     Subscriber Reception of Media Programs 
       FIG. 4  is a simplified block diagram of one embodiment of a terrestrial receiver  112  known as an integrated receiver/decoder (IRD)  400 . The IRD  400  receives and decrypts the media programs broadcast by the video distribution system  100 . These media programs are streamed to the IRD  400  in real time, and may include, for example, video, audio, or data services. 
     The IRD  400  comprises a receiver/decoder  402 , a conditional access verifier (CAV)  408 , and a conditional access module (CAM)  406 . The CAV  408  and CAM  406  are typically implemented in a smart card or similar device, which is provided to the subscriber  110  to be inserted into the receiver/decoder  402 . 
     The receiver decoder  402  comprises a tuner  410 , a transport and demultiplexing module (TDM)  412 , which operates under control of a microcontroller and associated memory  414 , a source decoder  416  and communicatively coupled random access memory (RAM)  418 , and a user I/O device  420  for accepting subscriber  110  commands and for providing output information to the subscriber  110 . 
     The tuner  410  receives the data packets from the video distribution system via the outdoor unit (ODU)  422 , which includes a transmit and receive antenna, and provides the packets to the TDM  412 . Using the service channel identifiers (SCIDs) associated with each media program, the TDM  412  reassembles the data packets according to the channel selected by the subscriber  110 , and unencrypts the media programs using the CW key. The TDM  412  can be implemented by a single secure chip, and is communicatively coupled to a microcontroller and memory  414 . 
     Once the media programs are unencrypted, they are provided to the source decoder  416 , which decodes the media program data according to MPEG or JPEG standards as appropriate. The decoded media program is then provided to a D/A converter (if necessary) and provided to external interfaces  404  which can include a media program presentation device such as a television, an audio system, or a computer. The source decoder  416  makes use of communicatively coupled RAM  418  to perform these functions. Further details regarding the operation of the IRD  400  can be found in co-pending and commonly assigned U.S. patent application Ser. No. 09/491,959. 
     Transmission of Information from the Subscriber to the Satellite 
     In some circumstances, it is beneficial to permit data to be transmitted from the subscriber  110  ground station  111  to the satellite  108 . This capability is useful, for example, to provide Internet access to the subscriber  110  (via a protocol such as TCP/IP) or to transmit other data and information to the control center  102  (e.g. related to the delivery of video/audio programs to the subscriber) or to other entities. 
       FIG. 4  is a diagram depicting an IRD  400  capable of transmitting information from the subscriber  110  to the satellite  108 . This is accomplished with the use of a transmitter  426  communicatively coupled to the microcontroller  414  and the ODU  422 . To assure adequate CNR, the IRD  400  uses measurements based on signals received from the satellite  108  to command the transmitter  426  to produce an output signal with a signal characteristic (including, for example, power or modulation technique) that adapts to the measured received signal characteristic. 
       FIG. 5  is a flow chart depicting exemplary method steps used to practice one embodiment of the present invention. A downlink signal  118  is transmitted from the satellite  108 , and received by the IRD  400  at the ground station  111 , as shown in block  504 . A quality of the received signal is then measured, as shown in block  504 . The measured quality can be the power of the received signal, the frame error rate, or a measure of the received and detected signal such as the background noise level. In one embodiment, the quality of the received signal is measured with a signal analyzer  424  such as a power meter communicatively coupled to the tuner  410  or the ODU  422 . 
     A prediction of the degradation of the uplink signal is then computed by an uplink degradation prediction module or degradation computer  430 , as shown in block  506 . The prediction is computed using the measured quality of the received signal. For example, if the power level of the received signal is used as the metric to determine the quality of the received signal, the power level is used to predict or estimate the quality of the uplink signal. In one embodiment of the present invention, the downlink signal  118  may itself include parameters which describe the signal in such terms so as to provide additional data to estimate the degradation of the signal when transmitted from the satellite  108  to the ODU  422  and IRD  402 . Such parameters can include, for example, the transmitted power of the downlink signal  118 . The uplink signal is then transmitted according to the predicted degradation of the uplink signal  430 , as shown in block  508 . 
       FIG. 6  is a diagram illustrating the application of different predicted degradation values to the present invention. 
       FIG. 7  is a flow chart showing exemplary process steps used to transmit the uplink signal  430  according to the predicted degradation generated in block  506 . If the predicted degradation is less than a first degradation value  606  (illustrated in  FIG. 6  by the region  604  between the no degradation value  602  and the first degradation value), the uplink signal  430  is transmitted according to a first transmission characteristic, as shown in blocks  702  and  704 . If the predicted degradation is greater than the first value and less than a second value (illustrated in  FIG. 6  by the region  608  between the first degradation value  606  and the second degradation value  610 ), the uplink signal  430  is transmitted according to a second transmission characteristic, as shown in blocks  702 ,  706 , and  708 . 
     The first and second transmission characteristics can be selected from a number of characteristics that can be used to describe the transmitted uplink signal  430 . In one embodiment, the power of the transmitted uplink signal  430  is varied according to the predicted degradation value. In this embodiment, the first transmission characteristic is a first uplink transmission power, and the second transmission characteristic is a second uplink transmission power. 
     In another embodiment, the rate of the information transmitted in the uplink signal  430  is varied according to the predicted degradation value. For example, the uplink signal  430  can be modulated according to different modulation schemes, each with different data rates. For example, a phase shift keyed (PSK) modulation scheme may be utilized in the uplink signal  430 . PSK modulation schemes include signal constellations with a plurality of signal points arranged in a wide variety of constellations, including QPSK, 8-PSK, and 16-PSK. When such modulation schemes are used, the first modulation scheme can be, for example, a lower data rate modulation scheme such as QPSK, and the higher data rate modulation scheme can include 8 or 16-PSK. 
     In another embodiment, the error correcting scheme used to code the information can be varied according to the predicted degradation value. Error correcting codes can be characterized by the level of redundancy they introduce into the data stream. Typically, error correcting codes with greater redundancy (and hence permit less data throughput) are also more resistant to errors. 
       FIG. 8  is a flow chart presenting exemplary process steps that are applied (as directed by blocks  702  and  706  of  FIG. 7 ) when the predicted degradation is greater than both the first value  606 , and the second value  610  (illustrated in  FIG. 6  as region  612  and the region above the third degradation value  614 ). Typically, when the predicted degradation is greater than the second value  610 , the uplink information cannot be reliably transmitted to the satellite  108 , even at slower data rates. In this situation, the information to be transmitted is stored for subsequent transmission, as shown in block  802 . A signal subsequently transmitted from the satellite  108  to the subscriber  110  is received  804  by the ODU  422  and the IRD  400 . The quality of this signal is measured, and this measurement is used to compute an updated prediction of the degradation of the uplink signal  430 , as shown in blocks  806  and  808 . 
     In one embodiment, a comparison is made to determine whether the updated prediction of the degradation is less than the first value (thus permitting data transmission at a non-reduced transmission rate). This is illustrated in block  810 . If so, the uplink signal  430  is transmitted with the normal transmission characteristic (e.g. data rate or modulation scheme) as shown in block  812 . In another embodiment, the comparison determines whether the updated prediction of the degradation is less than the second value (thus permitting data transmission, albeit at a reduced transmission rate). If so, block  812  transmits the uplink information with the second transmission characteristic. 
     The transmission characteristic of the uplink signal can be selected as a combination of a wide number of different characteristics (e.g. selecting the uplink transmission characteristic based on error correction, data rate, and modulation) or can be selected from a continuous spectrum of choices (e.g. continuously varying uplink transmitter power according to the predicted degradation). 
     CONCLUSION 
     This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.