MXPA05011870A - Apparatus and method for decoding in a hierarchical modulation system - Google Patents

Abstract

A satellite receiver receives a hierarchical modulation based signal, which has at least an upperlayer (UL) and a lower layer (LL), and simultaneously or independently recovers therefrom data conveyed in the UL signal and data conveyed in the LL signal.

Description

APPARATUS AND METHOD FOR DECODING IN A HIERRAIC MODULATION SYSTEM BACKGROUND OF THE INVENTION The present invention relates, in general, to communication systems and, more particularly, to satellite-based communication systems. As described in the U.S. Patent. No. 5,966,412, issued on October 12, 1999 to Ramaswamy, hierarchical modulation can be used in a satellite system as a way to continue the support of existing legacy receivers, still providing a growth path to offer new services. In other words, a satellite system based on hierarchical modulation compatible with the previous ones, allows additional features, or services, to be added to the system without requiring existing users to buy new satellite receivers. In a communication system based on hierarchical modulation, at least two signals, for example, an upper stratum signal (UL) and a lower stratum signal (LL). They are added together to generate a satellite signal modulated synchronously for transmission. In the context of a satellite-based communications system that provides backward compatibility, the LL signal provides additional services, while the UL signal provides legacy services, that is, the UL signal is, in fact, the same signal that was transmitted before - therefore, the satellite transmission signal can continue in order not to develop impact on the users with legacy receivers. Therefore, a user who already has a legacy receiver can continue to use the legacy receiver until such time that the user decides to upgrade to a receiver, or box, which can retrieve the LL signal in order to provide additional services . In a hierarchical modulation receiver, the received signal is encoded sequentially, that is, the received signal is processed first to recover the data transported in the UL signal, which is then used to recover the data conveyed in the signal of LL. In particular, the received signal is demodulated first and the upper layer is decoded from it - this provides the data transported in the UL. This data, that is, the decoded UL signal, is then re-encoded to provide a re-encoded UL signal. The re-encoded UL signal is then subtracted from the remodulated received signal to discover the LL signal, which is then decoded to recover the data carried therein. However, the demodulation and decoding of the LL signal depends on the UL signal.

BRIEF DESCRIPTION OF THE INVENTION We have observed that decoding in a hierarchical modulation receiver not only adds complexity to the receiver, but can also degrade receiver performance if the recovered upper stratum signal (UL) does not match the UL signal originally. transmitted due to errors in the UL decoding process - thus introducing errors in the recovered LL signal. Accordingly, and in accordance with the principles of the invention, a receiver receives a received signal based on hierarchical modulation, which comprises at least a first signal layer and a second signal layer, and simultaneously retrieves the data from it. transported in the first signal layer and the data transported in the second signal layer. In one embodiment of the invention, a satellite communications system comprises a transmitter, a satellite transponder and a receiver. The transmitter transmits a signal based on hierarchical uplink modulation to the satellite transponder, which broadcasts the signal downlink based on hierarchical modulation to a receiver. The receiver processes the signal based on hierarchical modulation received in such a way that the data transported in the UL and the data transported in the LL are recovered simultaneously from it. In another embodiment of the invention, a satellite communication system comprises a transmitter, a satellite transponder and a receiver. The transmitter transmits a signal based on hierarchical uplink modulation to the satellite transponder, which broadcasts the signal downlink based on hierarchical modulation to a receiver. The receiver processes the signal based on hierarchical modulation received in such a way that the UL and the LL are processed independently of each other. In another embodiment of the invention, a receiver for receiving a signal based on hierarchical modulation, comprising at least a first signal layer and a second signal layer constructs a search table of soft metric values. In particular, the receiver receives a training signal from an endpoint and calculates soft metric values as a function of a combined signal space and the received training signal, where the combined signal space is a combination of a space signal of the first signal layer and a signal space of the second signal layer. The receiver then stores the soft metric values calculated in the search table.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an illustrative satellite communications system that incorporates the principles of the invention; FIG. 2 shows a block diagram illustrative of a transmission path through satellite 15 of FIG. 1; FIG. 3 shows an illustrative embodiment for implementing the hierarchical modulation in the transmitter 5 of FIG. 1; FIG. 4 shows an illustrative symbol constellation for use in the upper stratum and the lower stratum; FIG. 5 shows an illustrative symbol constellation for a signal based on hierarchical modulation; FIG. 6 shows another illustrative embodiment for implementing the hierarchical modulation in the transmitter 5 of Fig. 1; FIG. 7 shows a block diagram illustrative of a receiver according to the principles of the invention; FIG. 8 shows an illustrative block diagram of the simultaneous demodulator / decoder 320 of FIG. 7, in accordance with the principles of the invention; FIG. 9 shows an illustrative block diagram of the demodulator 330 of FIG. 8; FIG. 10 shows an illustrative signal space; FIG. 1 1 shows a logarithmic, illustrative probability search table, according to the principles of the invention; FIG. 12 shows an illustrative symbol constellation; FIGs. 13 and 14 illustrate logarithmic probability calculations; FIG. 15 shows an illustrative flow chart for use in the receiver 30 of FIG. 1; and FIG. 16 shows another illustrative embodiment according to the principles of the invention.

DETAILED DESCRIPTION Outside the inventive concept, the elements shown in the figures are well known and will not be described in detail. Also, familiarity with satellite-based systems is assumed and is not described in detail here. For example, outside the inventive concept, satellite transponders, downlink signals, symbol constellations, a radio-frequency front end (rf) or receiver section, such as a low noise block subverter, formatting methods and source coding (such as the Systems Standard of the Moving Image Expert Group (MPEG) -2 (ISO / IEC 13818-1)) to generate transport bit streams and decoding methods such as logarithmic probability proportions, soft-exit-soft-decoder (SISO) decoders, Viterbi decoders, are well known and are not described herein. In addition, the inventive concept can be implemented through the use of conventional programming techniques, which, as such, will not be described herein. Finally, similar numbers in the figures represent similar elements. An illustrative communication system 50, in accordance with the principles of the invention, is shown in FIG. 1 . The communication system 50 includes the transmitter 5, satellite channel 25, receiver 30 and television (TV) 35. Although described in more detail below, the following is a brief overview of the communication system 50. The transmitter 5 receives a number of data streams, as represented by the signals 4-1 to 4-K and provides a signal based on hierarchical modulation 6 to the satellite transmission channel 25. Illustratively, these data streams represent control signaling, content (eg, video), etc., of a satellite TV system and may be independent of each other or related to each other, or a combination thereof. The signal based on hierarchical modulation 6 has strata K, where K > 2. It should be noted that the words "stratum" and "level" are used interchangeably herein. The satellite channel 25 includes a transmission antenna 10, a satellite 15 and a reception antenna 20. The transmission antenna 10 (representative of a terrestrial transmission station) provides a signal based on hierarchical modulation 6 as an uplink signal 1 1 to the satellite 15. Referring briefly to FIG. 2, an illustrative block diagram of the transmission path through satellite 15 for a signal is shown. The satellite 15 includes an input filter 155, a path wave tube amplifier (TWTA) 165 and an output filter 175. The TWTA uplink signal 165 is then filtered by the output filter 175 to provide the downlink signal 16 (which is typically at a different frequency from the uplink signal). Therefore, the satellite 15 provides retransmission of the uplink signal received through the downlink signal 16 to a broadcast area. This diffusion area typically covers a predefined geographical region, for example, a portion of the United States. Returning to FIG. 1, the downlink signal 16 is received by the receiving antenna 20, which provides a received signal 29 to the receiver 30, which, in accordance with the principles of the invention, remodulates and simultaneously decodes the received signal 29, a In order to provide, for example, the content to the TV 35 to see it in it. It should be noted that, although not described herein, the transmitter 5 can also predistort the signal before transmission in order to compensate nonlinearities in the channel. An illustrative block diagram of a hierarchical modulator for use in the transmitter 5 is shown in FIG. 3. Hierarchical modulation is simply described as a synchronous modulation system where a lower stratum signal is embedded synchronously in a higher stratum signal in order to create a higher order modulation alphabet. In the rest of this description, it is illustratively assumed that there are two data streams, ie, K = 2. It should be noted that the invention is not limited to K = 2 and, in fact, a particular data stream, such as the signal 4-1 which may represent an addition of other data streams (not shown). In FIG. 3, the hierarchical modulation transmitter comprises the UL 105 encoder, the UL 1 modulator 15, the LL 1 encoder 10, the LL 120 modulator, multipliers (or amplifiers) 125 and 130, combiner (or addr) 135 and up converter 140. The upper stratum path (UL) is represented by the UL 105 encoder, the UL 1 15 modulator and the 125 amplifier; while the lower stratum path (LL) is represented by the LL 1 1 0 encoder, the LL modulator 120 and the amplifier 130. As used herein, the term "UL signal" refers to any signal in the path of UL and will be apparent from the context. For example, in the context of FIG. 3, this is one or more of the signals 4-1, 1 06 and 1 16. Similarly, the term "LL signal" refers to any signal in the LL path. Again, in the context of FIG. 3, this is one or more of the signals 4-2, 1 1 1 and 121. In addition, each of the encoders implements known error detection / correction codes (for example, rotary or grid codes, concatenated forward error correction scheme (FEC), where a speed rotary code 1/2, 2 / 3, 4/5 or 6/7 is used as an internal code, and a Reed Solomon code is used as an external code, LDPC codes (low density parity verification codes), etc.). For example, typically the UL 105 encoder uses a rotary code or a short block code; while the LL 1 10 encoder uses a turbo code or LDPC code. For the purposes of this description, it is assumed that the LL 1 10 encoder uses an LDPC code. In addition, a rotating interposer (not shown) can also be used. As can be seen from FIG. 3, the signal 4-2 is applied to the LL 1 encoder 10, which provides a coded signal 1 1 1 to the LL 120 modulator. Similarly, the signal 4-1 is applied to the UL 105 encoder, which provides a coded signal 106 to the UL 1 modulator 15. The coded signal 106 represents N bits per symbol interval T, while the coded signal 1 1 1 represents M bits per symbol interval T, where N may or may not be equal to M. Modulators 1 15 and 120 modulate their respective coded signals in order to provide modulated signals 16 and 121, respectively. It should also be noted that since there are two modulators, 1 15 and 120, the modulation may be different in the UL path and the LL path. Again, for the purposes of this description, it is assumed that the number of coded data bits of UL is two, i.e.? / = 2, and that the UL 1 modulator 15 generates a modulated signal 1 16 that lies in one of four quadrants of a signal space. That is, the UL 1 modulator 15 graphically represents data bits coded for one of four symbols. Similarly, the number of encoded data of LL is also assumed to be two, that is, M = 2 and that the LL modulator 120 also generates a modulated signal 121 that lies in one of four quadrants of the signal space. It should be noted that the signal space 89 is merely illustrative and that constellations of symbols of other sizes and shapes can be used. However, the output signals of the UL 1 modulator 15 and the LL modulator 120 are further adjusted in amplitude by a predefined UL gain and a predefined LL gain through amplifiers 125, 130, respectively. It should be noted that the gains of the lower and upper stratum signals determine the final placement of the points in the signal space. For example, the UL gain can be set in the unit, ie, 1, while the LL gain can be set to 5. The UL signal and the LL signal are then combined through a combiner or adder 135, which provides the combined signal 136. In this way, the modulator of FIG. 3, for example, the amplifiers 125 and 130, together with the combiner 135, effectively reinstall and further divide the signal space, such that the UL signal specifies one of a number of sub quadrants of a quadrant in particular of the signal space, as illustrated in FIG. 5 by the signal space 79. In fact, the resulting signal space 79, also referred to herein as the combined signal space 79, comprises 16 symbols, each symbol located at a particular signal point in the signal space and associated with four bits in particular. For example, the lower portion of two bits 81 is associated with the UL and specifies a quadrant of signal space 79; while the two-bit upper portion 82 is associated with the LL and specifies a sub-quadrant of the quadrant specified by the two-bit portion 81. It should be noted that, since the UL signal identifies the quadrant, the LL signal efficiently searches for similar noise in the UL signal. In this aspect, the combined signal space 79 is representative of the concept and the distance between the symbols therein is not to scale. Returning to FIG. 3, the combined signal 136 is applied to the upconverting 140, which provides a multi-level modulated signal 6 at the appropriate transmission frequency. Returning briefly to FIG. 6, another illustrative embodiment is shown to implement the hierarchical modulation in the transmitter 5. FIG. 6 is similar to FIG. 3 except that the hierarchical modulator 180 is shown to carry out the graphic representation of the lower stratum and upper stratum bits in the combined signal space. For example, the upper layer can be a signal space of QPSK (quadrature phase shift key), while the lower layer is a signal space of BPSK (binary phase shift key). As noted above, after reception of the downlink signal 16 upon receiving the antenna 20, the receiver 30 demodulates and decodes the received signal 29 in order to provide, for example, the content to the TV 35 to be seen in the same An illustrative portion of the receiver 30 according to the principles of the invention is shown in FIG. 7. The receiver 30 includes front end filter 305, analog-to-digital converter 310 and simultaneous demodulator / decoder 320. The front end filter 305 sub-converts and filters the received signal 29 to provide an almost baseband signal to the A / D 310, it displays the sub-converted signal to convert the signal to the digital domain and provides a sequence of samples 31 1 (also referred to as hierarchical signal 31 1) to the simultaneous demodulator / decoder 320.

The latter carries out the demodulation of the hierarchical signal 31 1 and, according to the principles of the invention, the simultaneous or independent decoding of the resulting demodulated signal in order to provide a number of output signals, 321 -1 to 321 -K, representing data conveyed by the hierarchical signal 31 1 in the strata K. Data from one or more of these output signals are provided to the TV set 35 through the signal 31. (In this aspect, the receiver 30 may further process the data before the application to the TV set 35 and / or directly provide the data to the TV set 35). In the following example, the number of levels is two, that is, K = 2, but the inventive concept is not limited as such. For example, the simultaneous demodulator / decoder 320 provides a signal of UL 321 -1 and a signal of LL 321 -2. The first ideally represents what was transmitted in the upper layer, that is, the signal 4-1 of FIG. 3; while the latter ideally represents what was transmitted in the lower layer, ie the signal 4-2 of FIG. 3. Returning now to FIG. 8, an illustrative block diagram of the simultaneous demodulator / decoder 320 is shown. The simultaneous demodulator / decoder 320 comprises the UL 330 demodulator, the UL 335 decoder, the logarithmic probability ratio (LRR) search table (LUT). 570 and the LL 340 decoder. Hierarchical signal 31 1 is applied to the UL demodulator 330, which demodulates this signal and provides a demodulated UL signal as represented by the UL signal point current. demodulated 333. Referring now to FIG. 9, an illustrative block diagram of the UL demodulator 330 is shown. The UL demodulator 330 includes the repeated sampler 415, coupled filter 420, des-rotator 425, synchronization recovery element 435 and vehicle recovery element 440. The hierarchical signal 31 1 is applied to the digital repeating sampler 415, it repeatedly samples the hierarchical signal 31 1 by use of the UL synchronization signal 436, which is provided by the synchronization recovery element 435 in order to provide the re-sampled hierarchical signal 316. The re-sampled hierarchical signal 316 is applied to the coupled filter 420. The latter is a bandpass filter for filtering the re-sampled hierarchical signal 316 around the UL vehicle frequency in order to provide a filtered signal to both the de-rotator 425 as well as the aforementioned synchronization recovery element 435, which generates from it the synchronization signal UL 436. The derailor 425 de-rotates, ie, removes the vehicle from the filtered signal in order to provide a demodulated UL signal point current 333. The vehicle recovery element 440 utilizes the signal point current of Demodulated UL 333 for recovering therefrom the vehicle signal of IL 332, which is applied to des-rotator 425. Referring again to FIG. 8, the UL 335 decoder acts in a complementary manner to correspond to the UL encoder 105 of the transmitter 5 and decodes the demodulated UL signal point current 333 in order to provide the UL 321-1 signal. As noted above, the UL signal 321 -1 represents the data transported in the upper layer, for example, as represented by the signal 4-1 of FIG. 3. It should be noted that the UL 321-1 decoder recovers the data conveyed in the UL, by, in fact, processing the LL signal as noise in the UL signal. In other words, the UL decoder 321 -1 operates as if the UL signal 321 -1 represented the symbols selected from the signal space 89 of FIG. 4. Returning now to the LL signal, and in accordance with the principles of the invention, the signal point current of UL 333 is a stream of signal points received, each received signal point having an in-phase component (IREC) (572) and a quadrature component (QREC) (571) in a signal space. This is further illustrated in FIG. 10 for a received signal point z, where: Z = Irec + jQrec (1) The lREc and QREC components of each received signal point are applied to LUT of LLR 570. The latter stores a LUT 599 of precalculated LLR values as illustrated in FIG. 1 1. In particular, each row of LUT 599 is associated with a particular component I value (a row value I), while each column of LUT 599 is associated with a particular component Q value (a column value) Q). LUT 599 has L rows and J columns. LR of LLR 570 quantizes the values of component lREc and QREC of a received signal point to form an input address, which is used as an index to LUT 599 to select a respective precalculated LLR therefrom. At each symbol interval, T, selected LLR is provided through the signal 396 to the LL 340 decoder. For example, if the value of the lREc component is quantized for the first row and the value of the QREC component is quantified for the First column, then LLR 598 would be selected and would provide through signal 396 to the LL decoder 340 of FIG. 8. Out of the inventive concept, and as is known in the art, for a graphical representation of bits-to-symbols given M (b, -), where M are the objective symbols and b, = 0, 1, ... B -1, are the bits to be represented where B is the number of bits in each symbol (for example, B can be two bits for QPSK, three bits for 8-PSK, etc.), the function of the logarithmic probability index for the bad bit of a bit value B is: LLR (i, z) = log [(prob (b¡ = 11 z)) / (prob (b¡ =? | z))]; (2) where bi is the iéslmo bit and z is the signal point received in the signal space. The notation "prob (b¡ = l l z)" represents the probability that the bad bit is a "1" given that the signal point z was received. Similarly, the notation "prob (b¡ =? | Z)" represents the probability that the lousy bit is a "0" given that the signal point z was received. For a two-dimensional signal space, the probabilities within equation (2) are assumed to be based on additive Gaussian white noise (AWGN) that has a probability density function (PDF) of: Therefore, the LLR for a given bit and received signal point are defined as: It can be seen from equation (4) that the LLR for a received signal point given z is a function of z, the objective symbols M, and the noise level of rms s. An LLR is also an example of a "soft metric". A pictorial illustration of the calculation of a LLR ratio is shown in FIGs. 12 and 13. FIG. 12 shows an illustrative constellation of symbols. For simplicity, a PRSA constellation (quadrature phase shift key) of 4 symbols is shown, however, it should be noted that other sizes and shapes of symbol constellations could also be used, for example, 3 bits for 8-PSK, 4 bits for 16-QAM, 16-QAM hierarchical, etc. As can be seen from FIG. 12, there are four symbols in the signal space 89, each symbol associated with a representation of two particular bits [b1, bOj ".

Returning now to FIG. 13, a received signal point z is shown in relation to the signal space symbols 89. It can be seen from FIG. 13 that the received signal point z is located at different distances dj from each of the symbols of the signal space 89. For example, the received signal point z is located at a distance d4 from the symbol associated with the graphic representation of two bits "01". Therefore, the LLR (bO) is: 1 n [(probability bO is one) / (probability bO is zero)]; or (5A) 1 n [(probability (symbol 01 or 1 1)) / (probability (symbol 00 or ))]; or (5B) 1 n [. { exp (-d42 / (2s2)) + exp (-d32 / (2s2))} /. { exp (-d22 / (2s2)) + exp (-dt 2 / (2s2))} ] (5C) while LLR (b1) is: 1 n [(probability b1 is one) / (probability b1 is zero)]; or (6A) 1 n [(probability (symbol 10 or 1 1)) / (probability (symbol 00 or ))]; or (6B) 1 n [. { exp (-d12 / (2s2)) + exp (-d32 / (2s2))} /. { exp (-d22 / (2s2)) + exp (-d42 / (2s2))} ] (6C) Returning to FIG. 8, it can be observed that LUT of LLR 570 (ie, LUT 599) is initiated in a set of hierarchical values of LLR 573. According to the principles of the invention, they are calculated a priori with respect to the combined symbol constellation such as that illustrated in FIG. 5 and the one shown again in FIG. 14. In other words, the LLRs for the LL are determined - not with respect to the LL signal space (for example, the signal space 89 of FIG 4) - but with respect to the combined signal space (for example , the signal space 79 of FIG.5). For each received signal point z, a distance between each signal space symbol 79 and the received signal point z is determined and used in the calculation of an LLR. For simplicity, only some of these distances, d, are shown in FIG. 14. Hierarchical LLR values 573 can be formed in any number of ways. For example, the receiver 30 may carry out the calculations by use, for example, of a training signal, provided by the transmitter 5 either during the start or re-start, of communications between the two endpoints (transmitter 5 and receiver 30). As is known in the art, a training signal is a predefined signal, for example, a sequence of predefined symbols that is known a priori by the receiver. A predefined "welcome" sequence can also be defined, where endpoints exchange signaling before communicating data between them. Alternatively, the calculations can be carried out remotely, for example, at the location of the transmitter 5 and sent to the receiver 30 through an in-band or out-of-band signaling channel (this could even be a through a dialing equipment (connected by cable and / or wireless) (not shown)). Referring again to FIG. 8, the decoder of LL 340 receives the sequence of LLRs (the soft input data), through the signal 396, and provides from it the signal of LL 321 -2. The LL 340 decoder operates in a manner complementary to that of the LL 1 1 0 encoder. It should also be noted that the LL 340 decoder can also be a soft-output-soft-input decoder, and provides smooth output values, which are further processed (not shown) to form the signal of LL 321 -2. In this way, and in accordance with the principles of the invention, the receiver 30 directly determines the LL signal from a signal based on received hierarchical modulation. This is referred to herein as a simultaneous decoding mode. In particular, the signal point current of UL 333 is processed to generate soft input data, for example LLRs, in order to recover the LL data therefrom. In other words, the receiver 30 processes the signal based on hierarchical modulation received in such a way that the UL and the LL are processed independently from each other. Attention should now be directed to FIG. 15, which shows an illustrative flow chart in accordance with the principles of the invention of a process for use in the receiver 30 of FIG. 1 . In step 605, the receiver 30 starts (or resets) the communications with the transmitter 5 and receives a predefined hierarchical modulation training signal comprising UL symbols and predefined LL symbols. In step 610, the receiver 30 calculates hierarchical LLRs from the received training signal with respect to the combined signal space 79. In step 615, the receiver 30 stores the hierarchical LLRs calculated in the LUT of LLR 570. Finally, in step 620, the receiver 30 changes a data communication mode and begins to receive the data transmitted from the transmitter 5 of FIG. 1 . It should be noted that, although the above described modality described the LL 340 decoder as a receiver of soft metrics, the LL 340 decoder can receive signal points as represented by the signal point stream 333 and, therefore, further process the signal point data received to derive the LLRs therefrom as described above, for example, the LLR 340 decoder includes LR of LLR 570. By contrast, the UL demodulator 330 can be modified to include in the same LUT of LLR 570 to provide the soft metric to the LL 340 decoder. Another illustrative embodiment of the inventive concept is shown in FIG. 16. However, only those portions relevant to the inventive concept are shown. For example, analog-digital converters, filters, decoders, etc. , they are not shown for simplicity. In this illustrative embodiment, an integrated circuit (IC) 705 for use in a receiver (not shown) includes a simultaneous demodulator / decoder 320 and at least one register 710, which is coupled to the bus 751. The latter provides communication to and from other components of the receiver, as represented by the processor 750. The register 710 is representative of one or more IC 705 registers, where each register comprises one or more bits as represented by bit 709. The records, or portions thereof, of the IC 705 may be read-only, write-only, or read / write. In accordance with the principles of the invention, the simultaneous demodulator / decoder 320 simultaneously decodes a received, hierarchically modulated signal, and at least one bit, for example, bit 709 of the register 710, is a programmable bit that can be set by , for example, the processor 750, for the control of this operating mode. In the context of FIG. 16, the IC 705 receives an IF signal 701 for processing through an input terminal, or guide, of IC 705. A derivative of this signal, 31 1, is applied to the simultaneous demodulator / decoder 320. The latter provides signals of emission 321 -1 to 321 -K, as described above. Simultaneous demodulator / decoder 320 is coupled to register 710 via internal bus 71 1, which is representative of other signal paths and / or components of IC 705 for simultaneous demodulator / decoder 320 interface to register 710, as is known in the art. In view of the foregoing, it should be noted that although it is described in the context of a receiver coupled to a deployment device, as represented by TV 35, the inventive concept is not so limited. For example, the receiver 30 can be located upstream in a distribution system, for example, at an upper end, which then retransmits the content to other nodes and / or receivers of a network. Furthermore, although hierarchical modulation was described in the context of providing communication systems that are compatible with the above, this is not a requirement of the inventive concept. It should also be noted that the groupings of components for particular elements described and shown herein are merely illustrative. For example, either or both UL 335 decoder and LL decoder 340 may be external to the element 320, which is then essentially a demodulator that provides a demodulated signal. Therefore, the foregoing merely illustrates the principles of the invention and it will be appreciated that those skilled in the art will be able to anticipate numerous alternative facilities which, although not explicitly described herein, incorporate the principles of the invention and are within his spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be incorporated into one or more integrated circuits (ICs). Similarly, although they are shown as separate elements, any or all of the elements can be implemented in a processor controlled by stored program, for example, a digital signal processor (DSP) or microprocessor running associated software, for example, corresponding to one or more of the steps shown in FIG. 15. In addition, although they are shown as separate elements, elements in the present may be distributed in different units in any combination thereof. For example, the receiver 30 can be a part of TV 35. Accordingly, it is to be understood that numerous modifications can be made to the illustrative embodiments and that other facilities may be provided without departing from the spirit and scope of the present invention, as defined by appended claims.

Claims (14)

1. CLAIMS 1. A method for being used in a receiver, characterized in that the method comprises: receiving a signal based on hierarchical modulation, the signal comprising a hierarchical modulation comprising at least a first signal layer and a second signal layer; and simultaneously recovering the signal based on received hierarchical modulation, the data transported in the first signal layer and the data transported in the second signal layer, wherein the simultaneous recovery stage includes the steps of: decoding the signal based on hierarchical modulation to recover the data transported in the first signal layer; generate soft metrics from the signal based on hierarchical modulation as a function of a combined signal space of the signal based on hierarchical modulation; and decoding the signal based on hierarchical modulation to recover the data transported in the second signal layer as a function of the generated soft metric. The method according to claim 1, characterized in that the first signal layer is a higher signal layer and the second signal layer is a lower signal layer. 3. The method according to claim 1, characterized in that the soft metrics are logarithmic probability proportions. The method according to claim 1, characterized in that the combined signal space is a combination of a signal space of the first signal layer and a signal space of the second signal layer. The method according to claim 1, characterized in that the generation step includes the step of using the signal based on hierarchical modulation as an index in a search table of soft metrics. 6. A method for being used in a receiver to receive a signal based on hierarchical modulation, characterized in that it comprises at least a first signal layer and a second signal layer, the method comprising: receiving a training signal from an endpoint; calculating soft metric values as a function of a combined signal space and the received training signal, wherein the combined signal space is a combination of a signal space of the first signal layer and a signal space of the second layer of signal; and store the soft metric values calculated in a search table. The method according to claim 6, characterized in that it further comprises: receiving the signal based on hierarchical modulation; decoding the signal based on hierarchical modulation to recover the data transported in the first signal layer; and decoding the signal based on hierarchical modulation to recover the data transported in the second signal layer as a function of the stored metric values. The method according to claim 7, characterized in that the soft metric values are logarithmic probability proportions. A receiver, characterized in that it comprises: a demodulator for demodulating a received signal in order to provide a signal based on hierarchical modulation comprising at least two signal layers; a first decoder operative in the signal based on hierarchical modulation to decode one of at least two signal layers in order to provide data therefrom; a second decoder that provides data from the other at least two signal layers, wherein the second decoder operates independently of the first decoder; and a search table for storing soft metrics therein, where soft metrics are determined as a function of a combined signal space of the at least two signal strata and where the lookup table provides soft metrics to the second decoder to be used in the present for the proportion of the data from the other of the at least two signal layers. The receiver according to claim 9, characterized in that the at least two layers include a higher signal layer and a lower signal layer. eleven . The receiver according to claim 9, characterized in that the soft metrics are logarithmic probability proportions. 12. An apparatus, characterized in that it comprises: a television set for displaying video content; and a receiver coupled to the television apparatus for receiving a signal based on hierarchical modulation that transports the video content, wherein the receiver simultaneously decodes at least two signal layers of the signal based on hierarchical modulation received for the proportion of the video content to the television set; wherein the receiver includes a look-up table for storing soft metrics, which are determined as a function of a combined signal space of the at least two signal strata. The apparatus according to claim 12, characterized in that the signal based on received hierarchical modulation is a satellite signal. The apparatus according to claim 12, characterized in that the soft metrics are logarithmic probability proportions.