Patent Publication Number: US-7215713-B2

Title: Method to minimize compatibility error in hierarchical modulation

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/525,616 filed on Nov. 26, 2003. 

   TECHNICAL BACKGROUND 
   The present invention generally relates to the transmission of digital data, and more particularly, to the transmission of digital data in a satellite digital audio radio (“SDAR”) system. 
   BACKGROUND OF THE INVENTION 
   In October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. In doing so, the FCC allocated twenty-five (25) megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting, twelve and one-half (12.5) MHz of which are owned by XM Satellite Radio, Inc. of Washington, D.C. (XM), and 12.5 MHz of which are owned by Sirius Satellite Radio, Inc. of New York City, N.Y. (Sirius). Both companies provide subscription-based digital audio that is transmitted from communication satellites, and the services provided by these and other SDAR companies are capable of being transmitted to both mobile and fixed receivers on the ground. 
   In the XM satellite system, two (2) communication satellites are present in a geostationary orbit—one satellite is positioned at longitude 115 degrees (west) and the other at longitude eighty-five (85) degrees (east). Accordingly, the satellites are always positioned above the same spot on the earth. In the Sirius satellite system, however, three (3) communication satellites are present that all travel on the same orbital path, spaced approximately eight (8) hours from each other. Consequently, two (2) of the three (3) satellites are “visible” to receivers in the United States at all times. Since both satellite systems have difficulty providing data to mobile receivers in urban canyons and other high population density areas with limited line-of-sight satellite coverage, both systems utilize terrestrial repeaters as gap fillers to receive and re-broadcast the same data that is transmitted in the respective satellite systems. 
   In order to improve satellite coverage reliability and performance, SDAR systems currently use three (3) techniques that represent different kinds of redundancy known as diversity. The techniques include spatial diversity, time diversity and frequency diversity. Spatial diversity refers to the use of two (2) satellites transmitting near-identical data from two (2) widely-spaced locations. Time diversity is implemented by introducing a time delay between otherwise identical data, and frequency diversity includes the transmission of data in different frequency bands. SDAR systems may utilize one (1), two (2) or all of the techniques. 
   The limited allocation of twenty-five (25) megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting has created a need in the art for an apparatus and method for increasing the amount of data that may be transmitted from the communication satellites to the receivers in SDAR systems. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for increasing the amount of digital data that may be transmitted from communication satellites to receivers in SDAR systems. In doing so, the present invention provides an advantage over the prior art. While hierarchical modulation schemes have been previously used in other data transmission applications (e.g., Digital Video-Broadcasting-Terrestrial [DVB-T] and DVB-Satellite [DVB-S] systems), until now, such hierarchical modulation schemes have not been envisioned for use in SDAR systems. By introducing the use of hierarchical modulation in SDAR systems, the present invention increases the amount of data that may be transmitted in SDAR systems and enables the enhanced performance of the receivers that receive the satellite-transmitted signals in SDAR systems. 
   In one form of the present invention, the method for transmitting two levels of data in a hierarchical transmission system comprises the steps of: generating, superimposing, and transmitting. Generating creates a first modulated signal based on first input data. The second phase shift keying modulation is based on the second input data. Superimposing involves a second phase shift modulation overlayed on the first modulated signal. The second phase shift modulation is time synchronized to the first modulated signal. The second input data is transmitted at known instances in time relative to the first modulated signal. The modified signal is then transmitted. 
   In another form of the present invention, a receiver for receiving two levels of data in a hierarchical transmission system is provided. The receiver comprises an antenna for receiving RF signals, a demodulator for downconverting received RF signals; and signal detectors. The first detector coupled has a first output and is capable of providing digital information based on a first level of data on the first output. The second detector is coupled to the demodulator and has a second output. The second detector is capable of providing digital information based on a second level of data on said second output, and is adapted to know when said second level of data is present and relative to the first level of data. 
   The system and method may involve one or more satellite transmitters. The second phase shift modulation may have a data rate that is a fraction of the data rate of the first modulated signal. The signal may be generated using phase shift keying (PSK) modulation. Decoding the modified signal may be accomplished by performing a first demodulation of the first modulated signal to obtain the first input data then a second demodulation that occurs at the known instance when second input data is present. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is an illustrative view of a constellation chart for 64-quadrature amplitude modulation (QAM) with an embedded quadrature phase shift keying (QPSK) stream; 
       FIG. 2  is a diagrammatic view of a SDAR system implementing a method of the present invention; 
       FIG. 3  is a block diagram of a SDAR communication system adapted to enable a method of the present invention; 
       FIG. 4  is a diagrammatic view of a QPSK constellation; 
       FIG. 5  is a diagrammatic view of a binary phase shift keying (BPSK) constellation; 
       FIG. 6  is a diagrammatic view of a hierarchical 8-PSK constellation; 
       FIG. 7  is a flow chart illustrating a method of the present invention as utilized in a SDAR receiver; and 
       FIG. 8  is a diagrammatic view of a hierarchical 8-PSK constellation according to another embodiment of the invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplifications set out herein illustrate embodiments of the invention in several forms and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
   DESCRIPTION OF INVENTION 
   The embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings. 
   For the purposes of the present invention, certain terms shall be interpreted in accordance with the following definitions. 
   Baseband: A signal whose frequency content is in the vicinity of direct current (DC). 
   Carrier: A single frequency electromagnetic wave the modulations of which are used as communications signals. 
   Channel: A propagation medium for communication such as a path along which information in the form of an electrical signal passes (e.g., wire, air, water). 
   Data rate: The amount of data, or number of symbols, which may be transmitted on a signal per a unit of time. 
   Detector: A circuit that is capable of determining the content of a signal. 
   Downconvert: To convert a radio frequency signal from a higher to a lower frequency signal for processing (i.e., to baseband). 
   Downlink: To transmit data from a satellite to a receiver on earth. 
   Feed Forward Correction (FFC): Methods of improving secondary data detection. By knowing the relative “I” (in-phase) and “Q” (quadrature) components of a constellation quadrant, the secondary detector may be enhanced to perform better by having a priori knowledge from the first detector to assist detection. 
   First Level Data and/or Primary Data: Existing data that may be interpreted by current (i.e., “legacy”) SDAR receivers. Because the first level data can be interpreted by the legacy receivers, the first level data may also be considered to have backwards compatibility. 
   Hierarchical Modulation: A method in which two separate data or bit streams are modulated onto a single data stream by superimposing an additional data stream upon, mapped on, or embedded within the primary data transmission. The additional data stream may have a different data rate than the primary data stream. As such, the primary data is more susceptible to noise than it would be in a non-hierarchical modulation scheme. The usable data of the additional stream may be transmitted with a different level of error protection than the primary data stream. Broadcasters of SDAR services may use the additional and primary data streams to target different types of receivers, as will be explained below. 
   Legacy receiver: A current or existing SDAR receiver that is capable of interpreting first level data. Legacy receivers typically interpret second level data as noise. 
   Preamble: A known symbol or symbols in a transmission packet (typically used for synchronization). 
   Quadrature: A method of coding information that groups data bits and transmits two separate signals on a carrier by summing the cosine and sine of the separate signals to produce a composite signal which may be later demodulated to recover both signals. 
   Second Generation Receiver: A SDAR receiver that contains hardware and/or software enabling the receiver to interpret second level data (e.g., demodulator enhancements). Second generation receivers may also interpret first level data. 
   Second Level Data, Secondary Data and/or Hierarchical Data: The additional data that is superimposed on the first level data to create a hierarchically modulated data stream. Second level data may be interpreted by SDAR receivers containing the appropriate hardware and/or software to enable such interpretation (i.e., “second generation” receivers). Second level, or secondary, data may perform differently from first level, or primary, data. 
   Signal: A detectable physical quantity or impulse by which information can be transmitted. 
   Symbol: A unit of data (byte, floating point number, spoken word, etc.) that is treated independently. 
   Unitary Signal: A signal on a single channel or path. 
   Upconvert: To convert from a lower frequency signal (i.e., baseband) to a higher radio frequency signal for broadcasting. 
   Uplink: A communications channel or facility on earth for transmission to a satellite, or the communications themselves. 
   Upmix: To combine multiple electrical signals to a radio frequency signal for broadcasting. 
   Waveform: A representation of the shape of a wave that indicates its characteristics (frequency and amplitude). 
   Quadrature Amplitude Modulation (QAM) is one form of multilevel amplitude and phase modulation that is often employed in digital data communication systems. Using a two-dimensional symbol modulation composed of a quadrature (orthogonal) combination of two (2) pulse amplitude modulated signals, a QAM system modulates a source signal into an output waveform with varying amplitude and phase. Data to be transmitted is mapped to a two-dimensional, four-quadrant signal space, or constellation. The QAM constellation employs “I” and “Q” components to signify the in-phase and quadrature components, respectively. The constellation also has a plurality of phasor points, each of which represent a possible data transmission level. Each phasor point is commonly called a “symbol,” represents both I and Q components and defines a unique binary code. An increase in the number of phasor points within the QAM constellation permits a QAM signal to carry more information. 
   Many existing systems utilize QPSK modulation systems. In such QPSK systems, a synchronous data stream is modulated onto a carrier frequency before transmission over the satellite channel, and the carrier can have four (4) phase states, e.g., 45 degrees, 135 degrees, 225 degrees or 315 degrees. Thus, similar to QAM, QPSK employs quadrature modulation where the phasor points can be uniquely described using the I and Q axes. In contrast to QAM, however, the pair of coordinate axes in QPSK can be associated with a pair of quadrature carriers with a constant amplitude, thereby creating a four (4) level constellation, i.e., four (4) phasor points having a phase rotation of 90 degrees. Differential quadrature phase shift keying (D-QPSK) refers to the procedure of generating the transmitted QPSK symbol by calculating the phase difference of the current and the preceding QPSK symbol. Therefore, a non-coherent detector can be used for D-QPSK because it does not require a reference in phase with the received carrier. 
   Hierarchical modulation, used in DVB-T systems as an alternative to conventional QPSK, 16-QAM and 64-QAM modulation methods, may better be explained with reference to  FIG. 1 .  FIG. 1  illustrates 64-QAM constellation  100 . Each permissible digital state is represented by phasors  110  in the I/Q plane. Since eight (8) by eight (8) different states are defined, sixty-four (64) possible values of six (6) bits may be transmitted in 64-QAM constellation  100 .  FIG. 1  shows the assignment of binary data values to the permissible states. In a 16-QAM constellation, there are four (4) by four (4) different states and four (4) transmitted bits, in a 4-PSK constellation, there are two (2) by two (2) states and two (2) transmitted bits, and in a BPSK constellation, there is one (1) state and one (1) transmitted bit. 
   In systems employing hierarchical modulation schemes, the possible states are interpreted differently than in systems using conventional modulation techniques (e.g., QPSK, 16-QAM and 64-QAM). By treating the location of a state within its quadrant and the number of the quadrant in which the state is located as a priori information, two separate data streams may be transmitted over a single transmission channel. While 64-QAM constellation  100  is still being utilized to map the data to be transmitted, it may be interpreted as the combination of a 16-QAM and a 4-PSK modulation.  FIG. 1  shows how 64-QAM constellation  100 , upon which is mapped data transmitted at six (6) bits/symbol  116 , may be interpreted as including QPSK constellation  112  (which includes mapped data transmitted at two (2) bits/symbol) combined with 16-QAM constellation  114  (which includes mapped data transmitted at four (4) bits/symbol). The combined bit rates of QPSK and the 16-QAM data steams is equal to the bit rate of the 64-QAM data stream. 
   In systems employing hierarchical modulation schemes, one (1) data stream is used as a secondary data stream while the other is used as a primary data stream. The secondary data stream typically has a lower data rate than the primary stream. Again referring to  FIG. 1 , using this hierarchical modulation scheme, the two (2) most significant bits  118  may be used to transmit the secondary data to second generation receivers while the remaining four (4) bits  119  may be used to code the primary data for transmission to the legacy receivers. 
   The present invention contemplates the use of hierarchical modulation in a SDAR system, while maintaining backward compatibility for legacy receivers. Shown in  FIG. 2  is a diagrammatic view of a SDAR system in which a hierarchical modulation scheme is employed. SDAR system  210  includes first and second communication satellites  212 ,  214 , which transmit line-of-sight signals to SDAR receivers  216 ,  217  located on the earth&#39;s surface. A third satellite may be included in other SDAR systems. Satellites  212 ,  214 , as indicated above, may provide for spatial, frequency and time diversity. As shown, receiver  216  is a portable receiver such as a handheld radio or wireless device. Receiver  217  is a mobile receiver for use in vehicle  215 . SDAR receivers  216 ,  217  may also be stationary receivers for use in a home, office or other non-mobile environment. 
   SDAR system  210  further includes a plurality of terrestrial repeaters  218 ,  219 . Terrestrial repeaters  218 ,  219  receive and retransmit the satellite signals to facilitate reliable reception in geographic areas where the satellite signals are obscured from the view of receivers  216 ,  217  by obstructions such as buildings, mountains, canyons, hills, tunnels, etc. The signals transmitted by satellites  212 ,  214  and terrestrial repeaters  218 ,  219  are received by receivers  216 ,  217 , which either combine or select one of the signals as receiver&#39;s  216 ,  217  output. 
     FIG. 3  illustrates a block diagram of a SDAR communication system in which hierarchical modulation is utilized. In an exemplary embodiment of the present invention, SDAR communication system  300  includes SDAR transmitter  310 , SDAR receiver  340  and terrestrial repeater  350 . As in conventional SDAR communication systems, SDAR communication system  300  will input data content  302 ,  304  and perform processing and frequency translation within transmitter  310 . The digital data is transmitted over transmission channel  330  to receiver  340  or terrestrial repeater  350 . Generally, receiver  340  performs the converse operations of transmitter  310  to recover data  302 ,  304 . Repeater  350  generally re-transmits data  302 ,  304  to receiver  340 . Unlike conventional SDAR communication systems, however, transmitter  310 , receiver  340  and repeater  350  of the present invention provide hardware enabling SDAR communication system  300  to utilize a hierarchical modulation scheme to transmit and receive more digital data than conventional systems. 
   SDAR transmitter  310  includes encoders  312 ,  322 . The audio, video, or other form of digital content to be transmitted comprises primary input signal  302  and secondary input signal  304 , which are typically arranged as series of k-bit symbols. Primary input signal  302  contains primary, or first level, data and secondary input signal  304  contains secondary, or second level, data. Encoders  312 ,  322  encode the k bits of each symbol as well as blocks of the k-bit symbols. In other embodiments of the present invention, separate encoders may be used to encode the blocks of k-bit symbols, for example, outer and inner encoders. In an exemplary embodiment of the present invention, encoder  312  may encode primary data stream  302  using a block or a convolutional forward error correction (FEC) algorithm, and encoder  322  may encode secondary data stream  304  using a turbo coding algorithm or a low density parity check FEC algorithm. It is contemplated that other FEC encoding methods may be utilized to encode primary and secondary data streams  302 ,  204 , including, for example, Hamming codes, cyclic codes and Reed-Solomon (RS) codes. 
   Again referring to  FIG. 3 , inner interleaver  316  multiplexes encoded secondary content data stream  304  with encoded primary content data stream  302  to form a transmit data stream. This transmit data stream is passed to mapper  317 , which maps the data stream into symbols composed of I and Q signals. Mapper  317  may be implemented as a look-up table where sets of bits from the transmit signal are translated into I and Q components representing constellation points or symbols.  FIG. 6  is representative of an exemplary embodiment of the present invention, in which a hierarchical modulation scheme is employed and the constellation points are in accordance with either a uniform or non-uniform 8-PSK constellation  600 , where each phasor is represented by a three (3) bit symbol composed of I and Q signals. 
     FIG. 4  shows QPSK constellation  400  for primary data having two (2) transmitted bits/symbol. Phasors “00”, “10”, “11”, “01” correlate to a phase of 45 degrees, a phase of 135 degrees, a phase of 225 degrees and a phase of 315 degrees, respectively.  FIG. 5  shows BPSK constellation  500  for secondary data having one (1) transmitted bit/symbol. Phasors “0” and “1” correlate to a phase of zero (0) and 180 degrees, respectively. When a secondary data symbol is added onto a primary data symbol, constellation  600  of  FIG. 6  is illustrative of the resulting hierarchical modulation. 
   Constellation  600  may be perceived as two (2) sets of superimposed modulations—QPSK constellation  400  transmitting two (2) bits/symbol  620  combined with BPSK constellation  500  comprising one (1) bit/symbol. The first modulation is the primary QPSK data, which is represented by “x” marks  620 ,  622 ,  624 ,  626 . In order to superimpose the secondary data onto the primary data, the primary QPSK data is phase offset by the additional, secondary data, which is represented by any of data points  601 ,  602 ,  603 ,  604 ,  605 ,  606 ,  607 ,  608  depending on the phase offset. Positive phase offsets include phasors  602 ,  604 ,  606  and  608 , and negative phase offsets include  601 ,  603 ,  605  and  607 . 
   Shown in  FIG. 6 , phase offset  610  is the offset angle relative to the QPSK symbol. As explained above, a typical QPSK constellation contains 45 degree, 135 degree, 225 degree and 315 degree points. The hierarchical data is represented by a phase offset relative to those four (4) degree points, and the phase offsets with the four (4) degree points represent a hierarchical (8-PSK) constellation. A uniform 8-PSK constellation is created when offset angle  610  is 22.5 degrees. Every other offset angle  610  creates a non-uniform 8-PSK constellation. For example, as shown in  FIG. 6 , a 15 degree phase offset relative to primary data phasors  620 ,  622 ,  624 ,  626  produces a phase offset correlative to phasors  601  (“000”) or  602  (“001”),  603  (“101”) or  604  (“100”),  605  (“110”) or  606  (“111”), and  607  (“011”) or  608  (“010”), respectively. Gray coding is a method which may be used to make the bit assignments for the hierarchical constellation. For example, reference is made to the secondary data bit (b 2 ). Instead of making b 2 =0 a negative offset and b 2 =1 a positive outset, the hierarchical constellation may be configured so as to increase the bit error rate (BER) performance (e.g., b 2 =1 can be made a negative offset). 
   The amount of the phase offset is equal to the amount of power in the secondary signal. The amount of energy in the secondary signal may not be equal to the amount of energy in the primary signal. As phase offset  610  is increased, the energy in the secondary data signal is also increased. The performance degradation to the primary data signal is minimized by the perceived coding gain improvement as phase offset  610  is increased. The application of the hierarchical phase modulation on top of an existing QPSK signal containing primary data causes phase offset  610  to adjust either positively or negatively relative to the hierarchical data. 
   In general, a secondary data bit causes either a larger Q magnitude and smaller I magnitude or a larger I magnitude and smaller Q magnitude. With FEC techniques utilized in encoders  312 ,  322 , the I and Q signals are used in conjunction with each other over a block of data. These techniques give the appearance that the primary data bits are spread over time, enabling the secondary data to appear somewhat orthogonal to the primary data bits. Indeed, it has been shown in simulations that the secondary data&#39;s impact on the primary data is somewhat orthogonal. For example, for a twenty (20) degree phase offset for secondary data, the primary data has a one (1) decibel (dB) degradation when using a rate 1/3 convolutional code with a constraint length of seven (7), followed by a ( 255 ,  223 ) RS block code (8 bits/symbol). However, when the primary data has no FEC coding, the impact of the twenty (20) degree phase offset is 4.1 dB. This data demonstrates a perceived coding improvement of 3.1 dB in the case where phase offset  610  is set to twenty (20) degrees. 
   Again referring to  FIG. 3 , the FEC coding technique implemented by encoders  312 ,  322  spreads the primary and secondary data over many QPSK symbols, which essentially spreads the energy over time and the I and Q bits. To overcome the unequal signal-to-noise ratio (“Eb/No”) between primary data bits and secondary data bits, the amount of phase offset  610  may be increased until the performance of the primary data is equal to the performance of the secondary data. However, as phase offset  610  is increased, legacy receivers may have a difficult time acquiring and tracking the desired primary data signal. By spreading the second level bits over multiple symbols, spread spectrum coding techniques may be used to increase the amount of energy in the secondary bits. This allows phase offset  610  to be adjusted and made more compatible with legacy receivers. Additionally, the use of second level data spreading reduces overall second level data throughput. Overall, several techniques may be utilized to maximize the performance of the secondary data. These techniques include: increasing phase offset  610  to maximize the secondary data energy per symbol; using multiple symbols per secondary data bit; using more complex FEC algorithms, and using a beam steering antenna to improve the performance of the secondary data (e.g., a higher gain directional antenna for stationary reception and a pointing/steering antenna for mobile reception). 
   Referring back to  FIG. 3 , after mapper  317  translates encoded and interleaved primary and secondary data streams  302 ,  304 , respectively, into I and Q components, the I and Q components are modulated by modulator  318 . Modulation enables both primary data stream  302  and secondary data stream  304  to be transmitted as a single transmission signal via antenna  326  over single transmission channel  330 . Primary data stream  302  is modulated with secondary data stream  304  using one of a number of modulation techniques, including amplitude or phase (e.g., BPSK, QPSK, differential Q-PSK (D-QPSK) or pi/4 differential QPSK (pi/4 D-QPSK)) and may include differential or coherent modulation. According to the technique that modulator  318  employs, modulator  318  may be any amplitude or phase modulator. Each modulation technique is a different way of transmitting the data across channel  330 . The data bits are grouped into pairs, and each pair is represented by a symbol, which is then transmitted across channel  330  after the carrier is modulated. 
   An increase in the capacity of the transmitted signal would not cause backwards compatibility problems with legacy receivers as long as the legacy receivers may interpret the first level data. Second generation receivers, however, are capable of interpreting both first and second level data. Techniques may be employed to minimize the degradation in the legacy receiver, including decreasing phase offset  610  to limit the amount of the second level data energy per symbol, limiting the amount of time over which the second level data is transmitted, and making the second level data energy appear as phase noise to the legacy receiver. 
   Referring back to  FIG. 2 , after modulator  318  modulates first data stream  302  and second level data stream  304  ( FIG. 3 ) to create a transmission signal, transmitter  213  uplinks the transmission signal to communication satellites  212 ,  214 . Satellites  212 ,  214 , having a “bent pipe” design, receive the transmitted hierarchically modulated signal, performs frequency translation on the signal, and re-transmits, or broadcasts, the signal to either one or more of plurality of terrestrial repeaters  218 ,  219 , receivers  216 ,  217 , or both. 
   As shown in  FIG. 3 , terrestrial repeater  350  includes terrestrial receiving antenna  352 , tuner  353 , demodulator  354 , de-interleaver  357 , modulator  358  and frequency translator and amplifier  359 . Demodulator  354  is capable of down-converting the hierarchically modulated downlinked signal to a time-division multiplexed bit stream, and de-interleaver  357  re-encodes the bit-stream in an orthogonal frequency division multiplexing (OFDM) format for terrestrial transmission. OFDM modulation divides the bit stream between a large number of adjacent subcarriers, each of which is modulated with a portion of the bit stream using one of the M-PSK, differential M-PSK (D-MPSK) or differential pi/4 M-PSK (pi/4 D-MPSK) modulation techniques. Accordingly, if a hierarchically modulated signal is transmitted to one or both terrestrial repeaters  218 ,  219  ( FIG. 2 ), terrestrial repeaters  218 ,  219  receive the signal, decode the signal, re-encode the signal using OFDM modulation and transmit the signal to one or more receivers  216 ,  217 . Because the signal contains both the first and second level data, the terrestrial signal maintains second level data bit spreading over multiple symbols. 
   Also shown in  FIG. 3 , SDAR receiver  340  contains hardware (e.g., a chipset) and/or software to process any received hierarchically modulated signals as well. Receiver  340  includes one or more antennas  342  for receiving signals transmitted from either communication satellites  212 ,  214 , terrestrial repeaters  218 ,  219 , or both ( FIG. 2 ). Receiver  340  also includes tuner  343  to translate the received signals to baseband. Separate tuners may be used to downmix the signals received from communication satellites  212 ,  214  and the signals received from terrestrial repeaters  218 ,  219 . It is also envisioned that one tuner may be used to downmix both the signals transmitted from communication satellites  212 ,  214  and the signals transmitted from repeaters  218 ,  219 . 
   Once the received signal is translated to baseband, the signal is demodulated by demodulator  344  to produce the l I and Q components. De-mapper  346  translates the I and Q components into encoded primary and secondary data streams. These encoded bit streams, which were interleaved by interleaver  316 , are recovered by de-interleaver  347  and passed to decoder  348 . Decoder  348  employs known bit and block decoding methods to decode the primary and secondary bit streams to produce the original input signals containing the primary and secondary data  302 ,  304 . In other embodiments of the present invention, multiple decoders may be used, e.g., outer and inner decoders. Receiver  340  may also use a feed forward correction technique to improve its detection of the secondary data. By knowing the relative I/Q quadrant, receiver  340  may be enhanced to perform better by having such a priori knowledge, which assists in the detection of the transmitted signal. For example, referring to  FIG. 6 , if it is known from a priori first level data knowledge that symbol  602  or  601  was transmitted at some point in time, and the received symbol lands at  604 , it can be inferred by minimum distance that the received second level data bit is a weak one (1) by utilizing feed forward correction. However, without feed forward correction the second level data bit would have been detected as a strong zero (0). Therefore, feed forward detection utilizes the decoded symbol with the detected offset (either positive or negative) to determine the secondary data bit. 
   In another embodiment of the present invention, a method of enabling extra data bits from a hierarchical modulation scheme to be used to transmit additional data for each channel in a SDAR system is contemplated. A flow chart illustrating this embodiment of the present invention as utilized in an SDAR communication system is shown in  FIG. 7 . It is contemplated that the inventive method would be carried out by a receiver adapted to be used in a SDAR system. The receiver may concurrently process the receipt of first data stream  710  and second data stream  730 . If first data stream  710  is valid as determined by error checking at step  712 , first data stream  710  is passed to a channel data select at step  750 . If first data stream  710  is selected and second data stream  730  is either independent or not valid, only first data stream  710  is decoded at step  720  at its original rate, e.g., forty-eight (48) kbps. The decoded data from first data stream  710  is then passed to an output unit at step  724 . 
   If second data stream  730  is valid as determined by error checking at step  732 , then second data stream  730  is passed to the channel data select at step  750 . If second data stream  730  is selected and is independent from first data stream  710 , only second data stream  730  is decoded at step  740  at its original rate, e.g., sixteen (16) kbps. The decoded data from second data stream  730  is then passed to an output unit at step  744 . 
   If the receiver determines at step  712  that first data stream  710  is valid and at step  732  that second data stream  730  is valid, both data streams are passed to the channel data select at step  750 . The channel data select determines if second data stream  730  is an enhancement to first data stream  710 . Audio enhancements may include audio quality enhancements, audio coding enhancements such as 5.1 audio (i.e., a Dolby® AC-3 digital audio coding technology in which 5.1 audio channels [left, center, right, left surround, right surround and a limited-bandwidth subwoofer channel] are encoded on a bit-rate reduced data stream), data/text additions, album pictures, etc. If second data stream  730  is an enhancement to first data stream  710 , the channel data select combines the two (2) data streams such that the combined signal has a data rate greater than the first data stream&#39;s  710  data rate, e.g., 64 kbps. Thus, the sixteen (16) kbps data rate of second data stream  730  acts to increase the rate of first data stream  710  from forty-eight (48) kbps to sixty-four (64) kbps. Combined data stream  758  is then decoded at step  752  and passed to an output unit at step  756 . In an exemplary embodiment, when switching from first data stream  710  to combined data stream  758 , the increase in data rate is blended so as not to enable a quick change between first data stream  710  and combined data stream  758 . If second data stream  730  is determined to be invalid, the channel data select switches to a “first data level” only implementation and sends first data stream  710  to be decoded at step  720 . The data rate of first data stream  710  remains at its original forty-eight (48) kbps. In an exemplary embodiment of this inventive method, a decrease in data rate is blended so as not to enable a quick change between first data stream  710  and combined data stream  758 . Assuming that second data stream  730  becomes or remains valid, the receiver decodes combined data stream  758  at step  752  and provides combined data stream  758  to an output unit at step  756 . 
   A further embodiment of the present invention uses a hierarchical modulation scheme as described above, and utilizes known instances where a zero phase offset is inserted as part of the second level modulation. As explained in greater detail below, this embodiment provides an advantage with legacy systems, wherein the second level of data creates a reduced level of interference or noise to the primary level compared to hierarchical systems having all non-zero phase offsets. For a legacy system, the only level of data relevant to the legacy receiver is the primary level. Thus, the amount of interference or noise created by the second level of data is reduced as the zero offsets of the secondary modulation have no effect on the perceived phase of the legacy receiver. Only in the event that a non-zero value offset is present in the secondary modulation would the signal appear irregular to a legacy receiver. Error checking step  732  in the foregoing process would know a priori of the existence of the zero phase offset and would check for validity of the secondary data accordingly. 
   In this embodiment, the secondary symbol data is typically transmitted at a rate which is only a fraction of the primary symbol rate. For example, in the special case where the secondary symbol rate is one half of the primary symbol rate, the probability of the phase offset being 0 on any primary symbol is 50%. Correspondingly, the probability that the phase offset is positive or negative is 25% for either case assuming that secondary symbol has an equal chance of being +1 or −1. A legacy receiver will perceive the secondary symbol as noise and may make error corrections to mitigate the effects of the noise. However, it is possible that such attempted corrections could cause the legacy receiver to misinterpret some symbols and thus cascade the problem. By the use of the zero offset, approximately half the time there will be no additional noise added on the primary symbol and thus the probability of the legacy receiver taking unwarranted error correction steps is reduced. 
   Constellation  800  of  FIG. 8  illustrates this zero offset phase shift technique. The quadrature signal may be perceived as two (2) sets of superimposed modulations—a QPSK constellation transmitting two (2) bits/symbol combined with a secondary data modulation comprising instances of an additional one (1) bit/symbol that imposes an additional phase shift only when this secondary data is transmitted. The first modulation is the primary QPSK data, which is represented by “X” marks  820 ,  822 ,  824 ,  826 . Unlike the “X” marks in  FIG. 6 , the “X” marks of  FIG. 8  represent symbols where the secondary data does not modify the primary signal. With a zero phase offset, in effect the secondary modulation is not apparent as the phase does not change (e.g., to symbolize a “0”). In order to superimpose secondary data onto the primary data, the primary QPSK data is phase shift modulated by the additional, secondary data phase shift, the resulting symbol being represented by any of Y (when a “1” is the secondary data bit) data points  830 ,  832 ,  834 , or  836 . The primary data will in this representation occur at the point X, exactly the value expected by the legacy receiver, when the secondary data is zero. Only in the case when secondary data is transmitted will a legacy receiver perceive interference or noise. 
   While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.