Patent Publication Number: US-9838152-B2

Title: Modulation indication method for communication systems

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/US2006/47912, filed on Dec. 14, 2006, which was published in accordance with PCT Article 21(2) on Jun. 19, 2008, in English. 
     BACKGROUND 
     Field of the Technology 
     The present principles relate to communication systems. 
     Description of Related Art 
     Layered coding systems are commonly known and used throughout the communication industry. An example of such layered coding system is a MIMO (Multiple Input, Multiple Output) architecture. These multi-antenna systems have increased spectral efficiency through the use of spatial multiplexing. 
     A MIMO system is a system in which multiple transmit antennas and multiple receive antennas are employed. MIMO systems can generally achieve higher capacity in a rich-scattering environment compared to SISO (single input single output) systems. Different approaches can be used to achieve the MIMO capacity: a space-time code can be applied to multiple transmit antennas over multiple channel uses; a layered structure can also be applied where only a one-dimensional code is applied to each layer. Examples of layered structures are V-BLAST and D-BLAST structures proposed by Bell Labs. In V-BLAST, independently encoded data streams are sent through different transmit antennas. Hence, a layer represents one antenna in V-BLAST. In D-BLAST, the data-stream/antenna association is periodically cycled. 
     In general, a layered architecture means any interleaving method such that, at any time, different antennas belong to different layers and each antenna index belongs to one and only one layer at any time. A layer is the indexes of the antenna as a function of time. For purpose of illustration, a layered structure is shown in  FIG. 1 , where layer  1  from time index  1  to  7  is represented by antenna index  3 ,  2 ,  1 ,  3 ,  2 ,  1 ,  3 . 
     SUMMARY 
     In accordance with one general aspect of the present principles, information is accessed that indicates quality of communications over at least one communication channel. Based on the accessed information, a modulation scheme is indicated for sending data over the channel encoded with a rateless code. 
     According to another general aspect, a controller is configured to access information relating to quality of communications over at least one communication channel. A processor is configured to indicate, in response to the accessed communication quality information, a modified modulation scheme for sending data encoded with a rateless code over the at least one communication channel. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings wherein like reference numerals denote similar components throughout the views: 
         FIG. 1  is diagram of a layered communication structure showing the indexes of an antenna as a function of time; 
         FIG. 2  is a block diagram of a communication system according to an aspect of the present principles; 
         FIG. 3  is a more detailed block diagram of an exemplary MIMO communication system into which the present principles can be integrated; 
         FIG. 4  is a flow diagram of a method according to an aspect of the present principles; 
         FIG. 5 a    is flow diagram of a method according to another aspect of the present principles; 
         FIG. 5 b    is flow diagram of a method according to another aspect of the present principles; 
         FIG. 5 c    is a flow diagram of a method according to another aspect of the present principals; 
         FIG. 6  is block diagram of an apparatus according to an aspect of the present principles; 
         FIG. 7  is a flow diagram of a method according to another aspect of the present principles; 
         FIG. 8  is block diagram of an apparatus according to a further aspect of the present principles; 
         FIG. 9  is a block diagram of an apparatus according to yet a further aspect of the present principles; 
         FIG. 10  is a flow diagram of a method according to yet a further aspect of the present principles; 
         FIG. 11  is a flow diagram of a method according to another aspect of the present principles; 
         FIG. 11 a    is a flow diagram of a method according to another aspect of the present principles; 
         FIG. 12 a    is flow diagram of a method according to one aspect of the present principles; 
         FIG. 12 b    is a flow diagram of a method according to another aspect of the present principles; 
         FIG. 13  is flow diagram of a method according to another aspect of the present principles; 
         FIG. 14  is a block diagram of a decoder according to an aspect of the present principles; 
         FIG. 15 a    is a diagram of an exemplary transmission using a concatenated coding according to a further aspect of the present principles; 
         FIG. 15 b    is a diagram of exemplary receiving using a concatenated coding according to another aspect of the present principles; 
         FIG. 16  is a flow diagram of a method using concatenated codes on a transmitter side of a communication system according to an implementation of the present principles; 
         FIG. 17  is a flow diagram of a method using concatenated codes on a transmitter side of a communication system according to a further implementation of the present principles; 
         FIG. 18  is a flow diagram of a method of using concatenated cades on a receive side of a communication system according to an implementation of the present principles; 
         FIG. 19  is a flow diagram of a method of using concatenated codes on a receiver side of a communication system according to a further implementation of the present principles; 
         FIG. 20  is a block diagram of an apparatus according to an aspect of the present principles; 
         FIG. 21  is a flow diagram of a method for modifying a modulation scheme according to an aspect of the present principles; 
         FIG. 22  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 23  is a block diagram of an apparatus for modifying a modulation scheme according to an aspect of the present principles; 
         FIG. 24  is a flow diagram of a method for modifying a modulation scheme according to a further aspect of the present principles; 
         FIG. 25  is a flow diagram of a method for modifying a modulation scheme according to yet another aspect of the present principles; 
         FIG. 26  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 27  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 28  is a flow diagram of a method for modifying a modulation scheme according to a further aspect of the present principles; 
         FIG. 29  is a flow diagram of a method for modifying-a modulation scheme according to another aspect of the present principles; 
         FIG. 30  is a block diagram of an apparatus for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 31  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 32  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; 
         FIG. 33  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles; and 
         FIG. 34  is a flow diagram of a method for modifying a modulation scheme according to another aspect of the present principles. 
     
    
    
     DETAILED DESCRIPTION 
     In a layered MIMO system, such as, for example, V-BLAST or D-BLAST architectures, rateless codes can be used across sub-channels to provide error correction. In such a system, codewords can be decoded when enough information has been received. In order to determine when enough information has been received, the channel conditions, for example, the instantaneous signal-to-noise ratio (SNR), are monitored. The mutual information is a function of the SNR. By averaging the mutual information over time, the maximum transmission rate can be determined. One problem is that, under poor conditions, the time required to accumulate enough mutual information in order to decode a codeword may be long, and systems having real-time constraints, for example, streaming video, will suffer undue delay and error. 
     According to an implementation of the present principles, the sending of subsequent codewords encoded with a rateless code can be based on timing, or alternatively, can use the SNR information and correspondingly stored look up tables to obtain capacity values of the communication layer to avoid over-estimation of the received mutual information. 
       FIGS. 2 and 3  show a transmitter  102  and a receiver  104  of a communication system implementing various concepts of the present principles. By way of example,  FIG. 3  shows a V-BLAST communication system showing the transmitter (i.e., encoder) having an input multiplexer  106  and a plurality of channel encoders  108  each having its own antenna  109 . The receiver  104  includes a decoder  110  and an output multiplexer  112 . Those of ordinary skill in the art will recognize that the communication system of  FIGS. 2 and 3  is disclosed herein for exemplary purposes only and the present principles can be applied to any layered communication system utilizing rateless codes (e.g., all MIMO systems). For example, the transmitter and receiver can each be replaced with a transceiver depending on the particular application. Those of skill in the art will recognize that the modulators contained within the transmitter  102  and the demodulators contained within the receiver  104  are not shown, for simplification purposes, in  FIG. 3 . 
     Referring to  FIG. 4 , and in accordance with one implementation of some of the present principles, a codeword is encoded with a rateless code and transmitted  402 . Upon transmission, or at a time substantially equal to the transmission, a time interval is monitored  404 . The time interval is generally predetermined and has a length that enables the transmitter to assume that the transmitted codeword has been successfully received by a receiver. Those of skill in the art will recognize that the time interval can be established and monitored in many different ways without departing from the spirit of the present principles. For example, the time interval can be established and monitored through the use of setting interrupts using a processor or other computing medium, using clocks with timing flags or other indicators, and possibly maintaining a separate timer for each predetermined time interval. 
     During the time interval, the transmitter determines whether an indication signal in the form of an acknowledgement signal (ACK) has been received ( 406 ). When the indication signal ACK has been received during the time interval, the next codeword is transmitted  410 . 
     If the indication signal ACK is not received during the time interval, the system proceeds as if an erasure flag has been received  408  and forces the subsequent transmission of the next codeword  410 . Although an erasure flag may not necessarily be received by the transmitter, by forcing the transmitter to comply with the time intervals for codeword transmission, the real time constraints or requirements of the system can still be met. 
       FIG. 5 a    shows another implementation of the present principles where the erasure flag is embodied in the form of a negative acknowledgment NACK. Thus, when the time interval has expired  504 , the next codeword is transmitted  510  regardless of whether an indication signal has been received. If during the time interval an indication signal is received, a determination is made  506  as to whether the indication is a positive acknowledgement ACK or a negative acknowledgement NACK. 
     According to various implementations, the positive acknowledgement ACK can be an indication that the transmitted codeword was received successfully, or an indication of successful receipt and successful decoding. The negative acknowledgement NACK can be an indication that the transmitted codeword may be unreliable or unable to be fully decoded. When a NACK is identified, the transmitter modifies a subsequent modulation scheme  508  used to transmit the next codeword  510 . In one implementation, the NACK functions to notify the transmitter that the current modulation scheme is not working on the decoder side and the transmitter responds by modifying the modulation scheme for subsequent transmissions. 
     In accordance with another implementation shown in  FIG. 5 b   , the transmitter can monitor or count the number of NACKs ( 514 ) received for a predetermined period of time  505 . This predetermined period of time  505  is generally longer than the predetermined time period  504  and is selected to confirm whether the channel is really bad (in the case of NACKS), or really good (in the case of ACKs—See  FIG. 5 c   ). If the number of received NACKs is greater than or equal to the predetermined number of NACKs received  516  during the predetermined time period  505 , the transmitter can respond by lowering the order of modulation  518  (e.g., from 16-QAM to QPSK) and transmit the next codeword accordingly, without requiring any further information from the receiver (for example, without requiring a modulation modification index, etc.) 
     Conversely, as shown in  FIG. 5 c    when the number of received ACKs is greater than or equal to the predetermined number of ACKs received  522  during the predetermined time period  505 , the transmitter responds by increasing the order of modulation  520  (e.g., from QPSK to 16-QAM) prior to the transmission of the next codeword  510 . Implementations of  FIGS. 5 b  and 5 c    also may continue to transmit additional codewords while accumulating and counting ACKs and/or NACKs. 
       FIG. 6  shows a diagram of a transmitter  102  according to one implementation of the present principles. The transmitter  102  includes a controller  600 , a processor  602  and at least one memory/data storage device  608 . Processor  602  includes an onboard clock or timer  604  that, as described above, can be used in many different ways to provide the timer functions of various present principles. According to various implementations, controller  600  can be an application specific integrated circuit (ASIC), a processor programmed to function according to one or more of the present principles, or any other combination of logic or integrated circuit designs necessary to function according to one or more of the present principles. The memory  608  can be embodied in many different forms without departing from the spirit of the present principles. For example, memory  608  can be a ROM, RAM, removable disk media, hard drive, FLASH memory, or any other suitable storage device. 
       FIG. 7  shows the flow diagram of a method  700  for receiving codewords encoded with a rateless code. As indicated a codeword encoded with a rateless code is received  702 . The operation of receiving the rateless encoded codeword  702  is an ongoing operation. During receipt, a time interval is monitored  704  to determine if the time interval has expired. Before the expiration of the time interval, it is determined (repeatedly, if needed) whether the codeword has been received and decoded successfully  706 . If so, an indication signal is sent  708  providing a positive indication that the codeword has been successfully received and decoded, and a subsequent codeword is received. 
     When the time interval at step  704  expires and the successful receipt and decoding of the codeword is not confirmed (step  706 ), the receiver forces the sending of the indication signal  708  to enable the receipt of the subsequent codeword  710 . As described above, the indication signal in this instance may include an erasure flag or have the same embodied in a negative acknowledgement (NACK) that is used to inform the source of the received data to modify the modulation scheme used for subsequent transmitting. 
     In accordance with one implementation, the indication signal sent  708 , can include a modulation modification index or other modulation scheme modification instruction to the source of the encoded codewords. This is particularly applicable when the encoded codeword is not successfully received and/or cannot be successfully decoded (e.g., when the NACK signal is generated and returned to the source of the received data). As mentioned above, the modulation modification index or other modulation scheme modification instruction can be a positive increase in the modulation order when successful receipt and/or decoding is confirmed multiple times within a predetermined time interval. 
       FIG. 8  shows a block diagram of the decoder  110  and some of the constituent parts of the decoder  110 . The decoder  110  includes a receiver  800 , a controller  802 , a processor  804 , at least one form of memory/data storage  806 , and a clock  808 . As described above with reference to  FIG. 7 , the decoder  110  receives the encoded codewords and provides an indication signal to the source of the encoded codewords in response to either a positive or negative receipt condition. The positive receipt condition, in one implementation, includes successful receipt of the encoded codeword, and in another implementation, includes both successful receipt and decoding of the received encoded codeword. The negative receipt condition, in one implementation, indicates that the data received is considered unreliable and may not be decodable, and in another implementation the negative receipt condition indicates that the data received is identified as not decodable. 
     As mentioned above, it is possible that during communication in a layered MIMO system, the computed mutual information (using an unconstrained channel capacity formula) may be much higher than the actual mutual information obtained in the receiver, especially when common modulation schemes such as, for example QPSK or 16-QAM are used in the system. This is an over-estimation of the mutual information which has an adverse effect on subsequent transmissions. 
     In order to overcome this problem and avoid over-estimation of the mutual information acquired in the receiver the actual capacity formula for the modulation is used in each layer. For example, when the SNR=5 db, the capacity for QPSK modulation is 1.7 bits/symbol. In the event there is no closed form capacity formula (for example, there is no close-form capacity formula for 16-QAM modulation) or the capacity computation is complicated (for example, would take too much processing time), a look-up table (LUT) can be used to obtain the received mutual information based on a determined quality metric for the layer/channel in the communication system and the type of modulation being used. In accordance with one implementation, the quality metric is the signal-to-noise ratio (SNR) in the layer. Furthermore, look-up tables (LUTs) can also be used to obtain the optimum modulation format supported by the communication channels based on the determined quality metric for the layer/channel in the communication system. 
     Referring to  FIGS. 9 and 10 , there is shown the decoder  110  according to a further implementation of the present principles. The controller  900  of the decoder is configured to receive codewords encoded with a rateless code ( 1002 ). In a rateless code, the codewords may be defined as having an infinite length, although only a finite length is transmitted or received. Throughout this application, the term codeword is frequently used to refer to the portion of an infinite length “codeword” that is actually transmitted or received. The processor  902  is configured to compute a quality metric of the communication medium on which the codewords are transmitted. The memory  904  stores look-up table (LUT) data that can be used in determining the optimum modulation format supported by the channel. The type of LUT data would include various LUTs for each of the respective modulation schemes known to be used in the particular communication system. Some examples of such modulation schemes include, but are not limited to, BPSK, QPSK, and 16-QAM. 
     The computed quality metric is used to determine, using for example the processor  902 , the optimum modulation format supported by the communication medium  1006 . Once determined, the controller of the decoder is configured to receive subsequent codewords based on the modified modulation scheme. The modified modulation scheme was derived from the determined optimum modulation format capable of being supported by, and used for sending data over, the communication medium  1008 . 
     In accordance with a further implementation  1100 , shown in  FIG. 11 , after receiving the codeword  1102 , the decoder identifies the modulation scheme  1104  being used by the source of received data. A quality metric for the communication channel is then computed  1106 , using for example a processor. The computed quality metric (for example, the SNR of the channel or layer) and the known modulation scheme are used to access memory  904  and obtain a LUT value corresponding to the known modulation scheme. The optimum modulation format supported by the communication medium is determined  1108 . The optimum modulation format may be determined, for example, by a processor determining which modulation format&#39;s LUT provides the highest expected capacity at the computed quality metric. If the determined optimum modulation format is different from the current modulation scheme (or perhaps, having a difference that is large enough to justify a switch), the optimum modulation scheme is identified as a new modulation scheme. A “hysteresis” value can be used to avoid a ping-pong switching effect. The new modulation scheme is fed back  1110  to a source of the transmitted data. The source is presumed to send subsequent codewords using the fed-back modified modulation scheme. Subsequent encoded codewords are received  1112  using the fed-back modified modulation scheme. Of course, the “optimum” format need not be globally optimum, and many implementations determine a format that improves (but need not optimize) performance. 
     Referring to  FIG. 11 a   , a method  1115  is shown. In various implementations, LUTs are also used to determine the amount of mutual information that is being received. The method  1115  is an example of such an implementation. The method  1115  includes receiving data encoded with a rateless code  1002 , determining a quality metric for the channel (for example, SNR)  1004 . The quality metric may be determined by, for example, computing the metric, receiving the metric from another device, or accessing the metric from storage. A modulation scheme used in transmitting the received encoded data is identified  1104 . 
     The method  1115  further includes determining an estimate, based on the identified modulation scheme/format and the determined quality metric, an estimate of an amount of mutual information being received per unit of received encoded data. Determining the estimate may be performed in various ways, such as, by example using a closed-form capacity equation. The method  1115  illustrates another implementation that may provide increased speed, and that will also accommodate capacity equations that are not closed-form. 
     The method  1115  further includes accessing a particular LUT based on the identified modulation scheme/format and the determined quality metric  1120 , and accessing an entry in the particular LUT that provides an estimate of an amount of mutual information being received per unit of received encoded data  1130 . The method  1115  further includes determining an amount of mutual information received based on the accessed entry  1140 . In one implementation, the LUT is a one-dimensional table (for example, a list) including mutual information for a given modulation format, with each entry corresponding to a different SNR. In another implementation, the LUT is a two-dimensional table (for example, a matrix), with rows corresponding to modulation format and columns corresponding to SNR, and entries corresponding to mutual information indicators for a particular row (modulation format) and column (SNR). The mutual information indicators (the entries in the LUTs) may be determined, for example, based on a capacity formula corresponding to the identified modulation scheme and SNR. This use of LUTs may be performed without using other concepts and aspects described in this application, or may be used in conjunction with one or more other concepts and aspects. 
     In MIMO and other communication systems, although the capacity calculation is performed to compute the received mutual information, unfortunately, it is typically only an approximation and accurate in the limit over time. Thus, even when the overall received mutual information is declared to be sufficient for decoding by the receiver, it may, in fact, still be insufficient to decode a codeword. 
     According to one aspect of the present principles, this insufficiency is addressed by enabling the accumulation of additional mutual information in the receiver beyond that which is considered an ordinary amount to enable accurate decoding of the received codeword. The accumulation of additional mutual information provides a higher probability of successful decoding. 
     Thus, referring to  FIGS. 12 a  and 12 b   , there is shown a method  1200  according to one implementation of the present principles. Initially, data for a data block encoded with a rateless code is received  1202 . A determination is then made that an initial predetermined amount of mutual information (MI) for the data block has been received  1203  by a receiver. This “initial predetermined amount” is an amount that is believed to be sufficient to enable the receiver to successfully decode a received codeword. Those of skill in the art will recognize that this “initial predetermined amount” may be different for each communication system, and takes into consideration any known communication medium variables such as noise, etc. 
     Once the “initial predetermined amount” of mutual information has been received, additional encoded data for the data block is received  1204 , and another determination is made as to whether an extra predetermined amount of mutual information for the data block has been received beyond the initial predetermined amount  1205 . One the additional amount or “extra predetermined amount” of mutual information has been accumulated, the receiver decodes the received codeword ( 1206 ) and then continues to receive MI and encoded codewords for the next received transmission. The decoding of the codeword  1206 , in this implementation is performed using only the initial predetermined amount of encoded data. As shown in  FIG. 12 b   , in another implementation, the decoding  1206  can be performed using both the initially received encoded data and the additionally received encoded data  1208 . By using the extra accumulated MI associated with the extra/additional encoded data for the data block ( 1204 ), the probability of successful decoding by the receiver is increased. 
     Those of skill in the art will recognize that the actual amount of the “extra predetermined amount” can vary from communication system to communication system without departing from the spirit of the present principles. 
     In accordance with another implementation, the amount of initial mutual information and extra mutual information that is accumulated can be based on timing. For example, and referring to  FIG. 13 , there is shown a method  1300  according to another implementation of the present principles. As shown, encoded data for a data block encoded with a rateless code is received  1302 . A determination is then made as to whether or not an initial predetermined amount of MI associated with the encoded data has been received  1304 . This determination  1304  can be based, for example, on a time interval or based on the capacity formula of the communication medium. Further, look-up tables may be used that contain estimates of the mutual information per unit of received data based on a capacity formula for a given modulation and signal-to-noise ratio. When the decoder determines that the initial predetermined amount of MI has been received (either by the expiration of a time interval, or otherwise), the receiver begins to accumulate (receive) additional encoded data for the data block  1306 . The receiver accumulates the additional or extra encoded data and corresponding MI until an extra predetermined amount of MI has been received  1308 . The determination as to the extra predetermined amount of MI can be based on time, bit length, or any other known method for determining an expected amount of received data. When it has been determined that the extra predetermined amount of MI has been received, the codeword is decoded  1310  and the receiver/decoder can return to the beginning and start receiving a next transmission of encoded codewords. 
     As mentioned above, the first predetermined time interval is of a length that is considered sufficient to enable or allow the decoder to successfully receive and decode the encoded codeword. This first predetermined time interval may be different for different communications systems and different modulation techniques employed by such systems. Implementations may combine the two timers into a single timer. 
       FIG. 14  shows a decoder  1400  according to an implementation of the present principles. The decoder includes a controller  1402  configured to receive the mutual information (the amount of mutual information is calculated based on the received encoded codewords or other received data). Through the application of a processor  1404  and a memory  1406 , the controller is further configured to receive both the initial predetermined amount of mutual information, and the extra or additional predetermined amount of mutual information. In one implementation, the decoder  1400  may include a receiver  1410  adapted to receive the mutual information. 
     In accordance with the present principles, the accumulation of extra or additional mutual information may provide increased reliability, however this typically comes with a trade-off of slower data rates due to the added information. In addition, the accumulation of additional or extra mutual information leads to longer codewords for the decoder to decode and therefore generally results in a higher decoding complexity. These trade-offs are acceptable for many applications. However, in view of the increased complexity in decoding and/or the slower data rates resulting from the accumulation of additional mutual information, some implementations use concatenated coding for the codewords, in which the outer code is, for example, a block code, such as a Reed Solomon or BCH code, and the inner code is a rateless code. 
     In one such implementation, K information bits are first encoded into a codeword of length N bits using an (N, K) outer block code. Each codeword is broken up into sub-blocks of smaller size. For example, an N-bit codeword can be broken into four sub-blocks, each having a length of N/4 bits. An inner rateless code is applied to each sub-block of the codeword. By breaking up the block codeword into sub-blocks prior to encoding with an inner rateless code, the decoding complexity of the rateless codewords can be reduced because the size of the rateless codeword required for successful decoding is expected to be smaller. Alternatively, by breaking up the codeword into sub-blocks prior to encoding with the rateless code, the outer block codes can be larger and may thereby provide more error correction for a given number of parity bits than would be achieved with several smaller block codes used serially. Additionally, larger block codes provide better burst error correction, compared to smaller block codes, by, for example, correcting bursts that a smaller block code would not be able to correct. Further, at least some of the advantages of a larger block code and a smaller rateless code may be achieved together in the same implementation. 
       FIGS. 15 a  and 15 b    show diagrams of the configuration for sending and receiving of codewords encoded with an inner rateless code, respectively, according to an implementation of the present principles. As shown in  FIG. 15 a   , the transmitter/encoder sequence is input into outer encoder  1502  for encoding blocks of the input data sequence into codewords provided at the output of the encoder  1502 . A divider  1503  divides the data block output into sub-blocks, and an inner encoder  1504  encodes the sub-blocks with a rateless code. The encoded transmit sequence is input to a modulator  1506  where it is modulated and transmitted via the antenna.  FIG. 15 b    shows the receiver/decoder side, where a demodulator  1508  receives and demodulates the received modulated signal. An inner rateless code decoder  1510  decodes the rateless codes of the sub-blocks first to determine the sub-blocks, and then a combiner  1511  combines the sub-blocks into outer codewords, which are passed into the outer decoder  1512  to further remove the residual errors and clean up the decoding. The combiner  1511  and the divider  1503  may be implemented, for example, in software and/or in hardware. One software implementation uses registers and appropriate instructions, and one hardware implementation uses a shift register and appropriate logic. 
       FIG. 16  shows a method  1600  for sending encoded data according to another implementation of the present principles. Data is accessed  1602 , an outer block code is generated for the data  1604 , and the outer block codeword is broken into sub-blocks  1606 . Once sub-divided, the Sub-block codewords are encoded using a rateless code  1608 . Once encoded with a rateless code, the rateless codewords for the sub-blocks are sent  1610  to a receiver. 
     According to a further implementation, a method  1700  generates an outer block codeword for an input block of data  1710 , and an inner rateless codeword is determined for a sub-block of the outer block codeword  1720 . The method  1700  then begins sending a predetermined amount of the rateless codeword  1730 . After a first time interval expires  1740 , the predetermined amount is presumed to have been sent, and the method  1700  begins sending a second predetermined amount of the rateless codeword  1750 . After a second time interval expires  1750 , the second predetermined amount is presumed to have been sent. The method  1700  can be repeated for sending a rateless codeword for each sub-block. Further, the timers may be combined in an implementation. 
       FIGS. 18-20  show another implementation of the present principles on the side of the receiver. Referring to  FIG. 18 , a receiver starts by receiving a predetermined amount of mutual information for a codeword encoded with a concatenated code  1802 . Once the predetermined amount is received, the receiver continues to accumulate or receive an extra amount of mutual information beyond the predetermined amount  1804 . Once the extra mutual information is received, the receiver is configured to decode the concatenated encoded codeword by first decoding the inner rateless codes to determine the sub-blocks  1806 . Once the inner rateless code is decoded, the sub-blocks are combined to resolve the block codeword  1808 . Once the block codeword is resolved, the outer code is decoded  1810  to clean up residual errors. 
       FIG. 19  shows another implementation where timers are established by the receiver in order to determine that enough information has been received. In the present example, the receiver starts to receive the predetermined amount of mutual information for a codeword encoded with a concatenated code  1902 . A first predetermined time period  1903  is monitored for its expiration. The receiver continues to receive the predetermined amount of mutual information until the first predetermined time period expires. Upon expiration of the first predetermined time period, the “extra” or additional mutual information is received (accumulated) at the receiver  1904 . This accumulation of extra mutual information is performed for a second predetermined time period  1905 . Upon expiration of this second time period, the codeword is decoded by first decoding the inner rateless codes to determine the sub-blocks of the block codeword  1906 . The sub-blocks are then combined to resolve the block codeword  1907 . Once combined, the outer block code is decoded  1908  to clean up residual errors. 
       FIG. 20  shows a block diagram of a decoder  2000  according to one implementation of the present principles. As shown, the decoder  2000  includes a controller  2002 , a processor  2004 , and a memory  2006 . The controller  2002  is configured to receive the encoded codewords (which provide the mutual information) through receiver  2010 . The processor  2004  includes a clock  2008  and can be programmed to establish all time intervals required for receiving mutual information. The memory  2006  is part of the decoder  2000  and can include information, such as, for example, the requisite time intervals or periods for mutual information receipt depending on things such as, for example, the modulation scheme being used to transmit the encoded data. The decoder  2000  is further configured to decode the inner rateless code of the respective sub-blocks such that they can be combined to resolve the codeword. The decoding of the outer codes is performed after the codeword has been resolved by the decoding of the inner rateless codes and the combination of the sub-blocks. 
     Those of skill in the art will recognize that the controller and processor can be configured separately to function together, or alternatively could be embodied in a single device having corresponding program and logic to function as described herein. 
     The use of acknowledgement signals in layered communication systems employing rateless codes has its advantages, as discussed above. Particularly, an acknowledgement can be sent as soon as enough mutual information has been deemed received. However, although valuable, the sending of such acknowledgements may not take full advantage of the communication channel (for example, may not utilize the full capacity of the channel to increase data rates). 
     By way of example, when the modulation on each channel is BPSK or QPSK, the channel may support a higher order of modulation (e.g., 16-QAM), which will, in turn, support higher data rates. 
     There are several ways to determine whether such modulation scheme modifications are possible. The implementations shown and described with reference to  FIG. 21-34  are for exemplary purposes and show the use of a communication quality determination. Those of skill in the art will recognize that other methods and/or implementations of the same concepts can be made without departing from the spirit of the present principles. 
     Referring to  FIG. 21 , there is shown a method  2100  according to an implementation of the present principles. Initially, information is accessed  2102  relating to the quality of communications over the at least one communication channel. Based on that accessed information, an indication is provided  2104  for a modulation scheme to be used to send data over the channel encoded with a rateless code. In this example, the information relating to the quality of the communication channel can be the frequency of occurrences of indication signals within a pre-designated or predetermined time period. 
       FIG. 22  shows a modified implementation of the method of  FIG. 21 . In this method  2200 , information relating to the quality of the communications over the at least one communication channel is accessed  2202 , and indication signals are identified. Up on initiation of the accessing, a time interval is established  2208 . During the time interval, the number or frequency of occurrences of identified indications signals  2206  is counted. Upon expiration of the time interval, a modulation scheme for transmitting subsequent data is indicated  2204  in response to and based upon the accessed information relating to quality. The length of the predetermined time interval  2208  can be any suitable length of time based on, for example, a current modulation scheme being used, an estimated time period based on expected results, etc. 
       FIG. 23  shows an apparatus  2300  according to an implementation of the present principles. The apparatus includes a controller  2302 , a processor  2304  having a clock  2306 , a memory  2308 , and connection to a bi-directional communication channel  2310 . As discussed above, the quality of communications over the communication channel  2310  is determined and used to indicate a modulation scheme to be used for the channel. In this manner, controller  2302  accesses information relating to the quality of the communication channel  2310 . Processor  2304  is configured to provide the indication for a modified modulation scheme for the subsequent sending of data encoded with a rateless code over the communication channel  2310  using the accessed quality information. 
     By way of example, processor  2304  has been shown with a clock  2306 . Those of skill in the art will recognize that the clock  2306  can be implemented in many different ways without departing from the spirit of the present principles. For purposes of this example, clock  2306  can be used to establish a predetermined time interval, during which, the controller  2302  accesses the information relating to the quality of the communication channel  2310 . In this manner, the monitoring of the communication quality over the channel  2310  can be limited to a pre-set or predetermined time period, which assists in meeting any of the real time constraints of the communication system. 
       FIG. 24  shows the method according to another implementation of the present principles. In this example, a decoder monitors the frequency of occurrences at which indications signals are received over the at least one communication channel  2402 . These indication signals are indicative of the quality of the communications over the at least one communication channel. Based on the monitored frequency of occurrences of such indication signals, the modulation scheme being used to transmit data on the communication channel can be varied or changed  2404  for subsequent data transmissions. This variation or change can be, for example, an instruction to increase the modulation order for higher data rates, or alternatively, an instruction to decrease the modulation order, which results in lower bit rates, but increased reliability of the data when it has been determined that the communication channel can better support the lower order of modulation. 
     According to several exemplary implementations, the form of the indication signals can be acknowledgement signals (e.g., ACK), negative acknowledgement signals (NACK), or any other designated signal whose generation or receipt can be used to determine the communication channel quality. Examples of the use of ACKs and NACKs have been described earlier. 
       FIG. 25  shows a further modified method of the implementation of  FIG. 24 , where a time interval is imposed on the monitoring of the frequency of occurrences of the received indication signals. As shown, the frequency of occurrences of received indication signals is monitored  2502  during a predetermined time interval  2504 . Upon expiration of the predetermined time interval  2504 , the modulation scheme used to transmit subsequent data over the at least one communication channel is varied  2506  to maximize the modulation scheme and increase data rates to the highest the channel can support. However, as mentioned above, the variation of the modulation scheme could be a reduction or request for lower order modulation, as the communication channel may not be capable of supporting higher orders of modulation. Implementations may vary in how they determine whether a channel is capable of supporting higher orders of modulation. For example, one implementation declares that a channel can support a particular modulation format (order) only if there is a high expectation that all errors will be corrected (using a typical forward-error-correction code) in data sent over the channel. 
       FIG. 26  shows a method  2600  according to a further implementation of the present principles. Initially, the frequency of occurrences of the received indication signals is monitored  2602 . At this point, a determination is made  2604  of a modulation format that is expected to improve, or perhaps maximize, the “good” throughput (the amount of data per unit time that is correctly received and decoded). The determined format may be an optimal modulation format that can be supported by the at least one communication channel using the frequency of occurrence of indication signals data. LUTs may be developed, for example, to correlate ACK frequency (for a given modulation format) with the highest modulation format that can be supported. The subsequent varying of the modulation scheme  2606  is performed using the determined modulation format supported by the channel. 
       FIG. 27  shows a method  2700  according to another implementation of the present principles. In this implementation, the receiver of transmitted data generates and transmits indication signals to the transmitter indicating safe receipt and decoding of data  2702  received over the at least one communication channel. The frequency of occurrences of the generating indication signals is monitored  2704 . Based on the monitored frequency of occurrences of the indication signals, an indication of a modulation scheme for sending subsequent data over the channel ( 2706 ) is provided to the source of the received data. 
       FIG. 28  shows a modified method  2800  of the method shown in  FIG. 27 . In this implementation, a predetermined time interval is established  2805  for timing the monitoring of the frequency of occurrences of the generated indication signals  2804 . Upon expiration of the predetermined time interval, the receiver indicates a modulation scheme for subsequent data transmitted over the at least one communication channel  2806 . 
       FIG. 29  shows another implementation of the method  2900  according to the present principles. As shown, the receiver generates and transmits indication signals  2902  to the source of the received data, and the frequency of occurrences of the same is monitored  2904 . As before, in this implementation, the monitoring is performed for a predetermined time interval  2906 . When the time interval expires, the receiver transmits a modulation index  2908  to the source of incoming data to modify the modulation scheme used to transmit subsequent data over the at lease one communication channel. 
     The modulation index can be any type of signal that the transmitter/encoder can recognize and act upon. For example, the modulation index can be a control signal provided in a header of other data being fed back from the receiver to the transmitter. In other implementations, it can be part of the indication signal (either ACK or NACK). Those of skill in the art will recognize that the form of the modulation index can be any suitable form. 
       FIG. 30  shows a decoder  3000  according to one implementation of the present principles. The decoder  3000  includes a receiver  3002  configured to receive incoming data via the at least one communication channel  3012 . The incoming data may be encoded with a rateless code. A controller  3004  is configured to compute a quality metric for the communication channel  3012 . The quality metric represents the capacity of the at least one communication channel. Once the quality metric is computed, the controller, in conjunction with a processor  3006 , causes the decoder to generate a modulation index using the computed quality metric. As discussed above, the modulation index is a control signal that is fed back to the source of transmitted data to provide said source with instructions to modify the modulation scheme being used to transmit subsequent data over the communication channel  3012 . The decoder  3000  also includes a memory/data unit  3008  and a clock  3010 . 
     In accordance with this implementation, the quality metric is a tangible determination as to the level of quality that can be obtained on the communication channel  3012 . One example of such quality metric would be the Signal to Noise (SNR) ratio of the channel  3012 . Using the SNR of the channel, along with other known information, such as, for example, the current modulation scheme, a new or modified modulation scheme can be identified for maximizing the use of the channel. In systems, such as a layered communication system where there are multiple communication channels for transmitting the modulated data, an average SNR for all the channels can be used as the quality metric or an average SNR for each channel can be used as the quality metric of each channel. The average could be an average over the layers, an average over time, etc. In other alternative implementations, a block wise SNR may also be used without departing from the spirit of the present principles. Other implementations may include a peak SNR determination for use as the quality metric. 
     Referring to  FIG. 31 , there is a shown an exemplary method  3100  implemented by the decoder  3000  according to an aspect of the present principles. Initially, the decoder accesses information relating to the quality of the communication over the at least one communication channel  3102 . A determination is then made as to the modulation format currently being used  3104  in sending signals over the at least one communication channel. Once the current modulation format is determined, the decoder can use that information, in conjunction with the accessed quality information and provide an indication  3106  to a source of received data with instructions to modify the modulation format used for sending subsequent data. 
     According to another implementation, shown in  FIG. 32 , a method  3200  provides for accessing information relating to the quality of communications  3202 , and for determining the current modulation format  3204  being used on the at least one communication channel. The quality metric (for example, SNR or frequency of ACKs) is then computed  3206  from the accessed information relating to the quality of the communication channel. The computed quality metric is then used, in conjunction with the known or current modulation format, to indicate a modulation scheme to be used for sending subsequent data over the channel  3208 . 
       FIG. 33  shows another implementation of the method  3300  according to the present principles. In this implementation, the decoder receives data encoded with a rateless code  3302 . Using this data, the decoder computes a quality metric  3304  which provides information relating to the optimum modulation format supported by the at least one communication channel. 
     Using the computed quality metric, the decoder instructs the source of the received data to modify the modulation scheme used to transmit subsequent data over the at least one communication channel  3306 . 
       FIG. 34  shows a modified method implementation  3400  of the method  3300  shown in  FIG. 33 . As before, the decoder receives data encoded with a rateless code  3402 , and computes the quality metric  3404  indicating the optimum modulation format supported by the at least one communication channel. The decoder then generates a modulation index signal in response to the computed quality metric for indicating a modulation scheme to be used by the source of the received data for sending subsequent data over the at least one communication channel. 
     As should be clear, many implementations described in this application may be performed by a receiver, a transmitter, or both. 
     The various aspects, implementations, and features may be implemented in one or more of a variety of manners, even if described above without reference to a particular manner or using only one manner. For example, the various aspects, implementations, and features may be implemented using, for example, one or more of a method, an apparatus, an apparatus or processing device for performing a method, a program or other set of instructions, an apparatus that includes a program or a set of instructions, and a computer readable medium. 
     An apparatus may include, for example, discrete or integrated hardware, firmware, and software. As an example, an apparatus may include, for example, a processor, which refers to processing devices in general, including, for example, a microprocessor, an integrated circuit, or a programmable logic device. As another example, an apparatus may include one or more computer readable media having instructions for carrying out one or more processes. 
     A computer readable medium may include, for example, a software carrier or other storage device such as, for example, a hard disk, a compact diskette, a random access memory (“RAM”), or a read-only memory (“ROM”). A computer readable medium also may include, for example, formatted electromagnetic waves encoding or transmitting instructions. Instructions may be, for example, in hardware, firmware, software, or in an electromagnetic wave. Instructions may be found in, for example, an operating system, a separate application, or a combination of the two. A processor may be characterized, therefore, as, for example, both a device configured to carry out a process and a device that includes a computer readable medium having instructions for carrying out a process. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, elements of different implementations may be combined, supplemented, modified, or removed to produce other implementations. Accordingly, other implementations are within the scope of the following claims.