Patent Publication Number: US-9894687-B2

Title: Methods and apparatuses for providing random access communication

Description:
FIELD OF THE INVENTION 
     The invention relates to apparatuses and methods that provide random access to a communication medium for transmissions, without diversity, using low rate forward error correction encoding. In particular, the invention relates to apparatuses and methods for use in a communication system in which information is transmitted asynchronously, without diversity, using low rate forward error correction (FEC) encoding with a code rate no higher than ½. 
     BACKGROUND 
       FIG. 1  illustrates an exemplary communication network  100 , which includes a number of user terminals  102  competing for access to a gateway terminal  106  via a communication satellite  104 . A number of access methods have been developed to permit multiple user terminals  102  to access gateway terminal  106 . Fixed Frequency Division Multiple Access (FDMA) is an access method in which each user terminal  102  is assigned a respective fixed frequency and may transmit simultaneously with one or more other user terminals  102 , each terminal transmitting on a different frequency. Time Division Multiple Access (TDMA) is an access method in which multiple user terminals  102  are assigned respective time slots at a same frequency and may transmit in rapid succession using their respective time slots, each terminal using a different time slot. However, in FDMA and TDMA, respectively, the user terminals may use the assigned frequency or the assigned time slots intermittently. Therefore FDMA and TDMA access methods are inefficient. 
     In the early 1970s, a random access method called Aloha was introduced. Aloha allows each user terminal  102  to transmit at will in a same frequency. When multiple user terminals  102  transmit simultaneously, their respective transmissions become corrupted, each multiple user terminal  102  selects a random, and most probably different respective delay, and retransmits after expiration of the respective delay. 
     Sometime after the introduction of Aloha, a revised access method known as Slotted Aloha, or S-Aloha was introduced. Using S-Aloha, user terminals  102  begin their transmission on a common time marker and have a same transmission duration. S-Aloha reduced, with respect to Aloha, a probability of two user terminals  102  transmitting simultaneously by about 50% due to elimination of partially overlapping transmissions by separate terminals. It is well known that maximum channel utilization for S-Aloha is e −1 , or about 37%, whereas maximum channel utilization for Aloha is 0.5 e −1 , or 18.5%. However, both Aloha and S-Aloha become unstable when operating close to capacity because retransmission of previously unsuccessful transmissions tie up the channel. To keep the channel stable, actual channel utilization is kept much lower than the maximum channel utilization stated above. 
     To improve throughput and minimize delay, it became clear that a user terminal could transmit multiple copies of information in different time slots and have a receiving device sort out duplicate information and request retransmission only when none of the multiple copies of the information are correctly received. Devices in most modern wireless communication systems establish initial communications in this way, which is known as Diversity S-Aloha. In systems using Diversity S-Aloha, typically two or three copies of the information are transmitted. 
     In the early 2000&#39;s, it was observed that, if a receiving device knows locations of duplicated transmissions within received information, and one of the duplicated transmissions is received without corruption, then the receiving device may use the one of the duplicated transmissions to cancel another copy of the duplicated transmission that corrupts a received transmission from a second user terminal, thereby increasing a probability of clear reception. If cancellation is performed iteratively, a probability of receiving uncorrupted transmissions is increased and channel capacity is improved beyond that of Diversity S-Aloha. Such a technique is known as Contention Resolution Diversity Slotted Aloha (CRDSA). 
     Also in the early 2000&#39;s, an access scheme was developed based on low-rate forward error correction (FEC) coding and scrambling codes with iterative interference cancellation performed at a receiving device such that a large number of transmissions, within a time slot, from different terminals can be correctly separated and decoded. The access scheme is known as Scrambled Code Multiple Access (SCMA) and provides much greater capacity than S-Aloha, Diversity S-Aloha, and CRDSA. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that is further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     One aspect of the invention is a random access method for communicating in a communication system. Asynchronous communications are communications transmitted without any time slot synchronization. In the communication system, a gateway terminal receives and buffers the asynchronous communications. The asynchronous communications include multiple overlapping bursts, transmitted without diversity, from multiple devices in the communication system. Each of the bursts includes respective data encoded using a low rate forward error correction code having a code rate no higher than ½. The gateway terminal may detect bursts in a sliding window of W burst times, may demodulate and decode the detected bursts, may select incorrectly decoded bursts (as may be determined by a bad cyclic redundancy check (CRC), a bad parity check, or by other methods) in the sliding window of W burst times, and may perform an iteration of an iterative interference cancellation process with respect to the detected bursts. When at least one burst in the sliding window remains incorrectly decoded and a number of repeat iterations of the iterative interference cancellation process is not equal to a maximum number of repeat iterations, another iteration of the iterative interference cancellation process is performed with respect to the bursts of the sliding window that remain incorrectly decoded. When all of the bursts in the sliding window of W burst times are correctly decoded, the sliding window may be stepped by T, where T is an amount of burst times. Correctly decoded bursts may be forwarded to a next process or a next device at any time after being determined as correctly decoded bursts. 
     Another aspect of the invention is a terminal in a communication system. The terminal includes an analog-to-digital converter, a channel filter and at least one burst processing engine. The analog-to-digital converter converts received analog signals, representing bursts, including multiple overlapping bursts from multiple devices in the communication system, to digital signals. The multiple devices transmitted the analog signals using a random access method, without diversity, and low rate encoding of data at a code rate no higher than ½. The channel filter filters out all but a subset of the communication channel from the digital signals to produce filtered digital signals from the received multiple overlapping bursts. The at least one burst processing engine receives the filtered digital signals from the channel filter, detects bursts of a sliding window of W burst times within the filtered digital signals, demodulates and decodes the detected bursts, determines whether each of the detected bursts is or is not correctly decoded, selects incorrectly decoded bursts in the sliding window (as may be determined by a bad CRC, a bad parity check, or by other methods), and performs an iteration of an iterative interference cancellation process with respect to the incorrectly decoded bursts in the sliding window of W burst times. After completing each iteration, when a number of performed repeat iterations of the iterative interference cancellation process is not equal to a maximum number of iterations and at least one burst in the sliding window of W burst times remains incorrectly decoded, the terminal performs another iteration of the iterative interference cancellation process with respect to the incorrectly decoded bursts of the sliding window of W burst times. When all of the bursts in the sliding window of W burst times are correctly decoded, the sliding window may be stepped by T, where T is an amount of burst times. Correctly decoded bursts may be forwarded to a next process or a next device at any time after being determined as correctly decoded bursts. 
     A third aspect of the invention is a device in a satellite communication system. The device includes an encoder to forward error correction encode, at a code rate no higher than ½, information to be transmitted to produce encoded information, a modulator to modulate the encoded information, including an added unique word (UW), to produce modulated encoded information, and a transmitter to asynchronously transmit the modulated encoded information without diversity, as a burst, to a second device within the satellite communication system. 
    
    
     
       DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description is provided below and will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments and are not therefore to be considered to be limiting of its scope, implementations will be described and explained with additional specificity and detail through the use of the accompanying drawings. 
         FIG. 1  illustrates an exemplary environment in which embodiments may operate. 
         FIG. 2  shows a portion of an exemplary terminal that receives and processes bursts according to one embodiment. 
         FIG. 3  is a functional block diagram of an exemplary processing device that may process received data from bursts. 
         FIG. 4  is a functional block diagram of an exemplary device that asynchronously transmits data in some embodiments. 
         FIG. 5  illustrates an exemplary format of a burst in some embodiments. 
         FIG. 6  shows exemplary bursts, including overlapping bursts, that may be received from multiple transmitting devices. 
         FIGS. 7-12  are flowcharts that explain exemplary processing performed in various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the subject matter of this disclosure. 
     Overview 
     In communication systems, including but not limited to satellite communication systems, retransmissions can greatly increase communication delays. Various embodiments described herein provide a random access method with improvements beyond those of SCMA. Unlike SCMA, S-Aloha, Diversity S-Aloha, and CRDSA, the improved random access method, which we call Asynchronous Scrambled Coded Multiple Access (A-SCMA), assumes that transmissions from terminals are not synchronized on a time-slot basis. As previously described, time slot synchronization reduces a probability of collisions of transmissions among multiple terminals, and improves capacity for S-Aloha and Diversity S-Aloha, but adds to complexity of the terminal. Without time slot synchronization, one-way autonomous applications, including, but not limited to remote sensing, can be enabled. 
     In various embodiments, a gateway terminal may receive and buffer asynchronous communications transmitted, without diversity, by multiple devices, which encode information to be transmitted using low rate forward error correction coding at a code rate that is no higher than ½. In some embodiments, each of the multiple devices may modulate a respective entire code block, including a UW, by using Binary Phase Shifted Keying (BPSK), Quadrature Phase Shifted Keying (QPSK), or Offset QPSK and may transmit the modulated code block as a burst. The gateway terminal may receive the respective bursts, which may include multiple overlapping bursts that cause interference with other overlapping bursts. Individual received bursts may be detected based on detecting any one Unique Word (UW) from among a group of possible UWs. The gateway terminal may demodulate bursts within a sliding window of W burst times, where W is a window size, to produce demodulated bursts. The gateway terminal may then decode the bursts of the sliding window to produce decoded demodulated bursts and may determine whether the decoded demodulated bursts in the sliding window of W burst times are correctly decoded. 
     The gateway terminal may then perform interference cancellation using the decoded demodulated bursts. 
     When the gateway terminal determines that at least one of the decoded demodulated bursts in the sliding window of W burst times is not correctly decoded, then the gateway terminal may perform demodulation, decoding, and interference cancellation iteratively. When the gateway terminal determines that the iteratively performed interference cancellation resulted in all of the decoded demodulated bursts of the sliding window of W burst times being correctly decoded, the sliding window of W bursts may be advanced by an amount of burst time T (i.e., advancing the sliding window by some positive integer, S, of symbols). Copies of the correctly decoded demodulated bursts are provided for further processing to a next process or a next device via a computer interface. 
     When the gateway terminal determines that the iteratively performed interference cancellation did not result in all of the bursts of the sliding window of W burst times being correctly decoded, and a maximum number of repeats of the iteratively performed interference cancellation was performed, then the sliding window of W burst times is advanced by the amount of burst time T. In a variation of the embodiment, each of the UWs corresponds to a respective scrambling code and each demodulated burst is descrambled based on the corresponding respective scrambling code before decoding each of the demodulated bursts. 
     Description of Embodiments 
       FIG. 2  illustrates a portion of an exemplary gateway terminal  200  that receives and processes bursts according to one embodiment. Gateway terminal  200  may include a number of burst processing engines  202 , a timing generator  204 , an analog-to-digital converter (ADC)  206 , a channel filter  208 , temporary storage  210 , a programming bus  212 , a received sample bus  214 , and a computer interface bus  216 . In  FIG. 2 , each burst processing engine  202  includes a respective Field Programmable Gate Array (FPGA) for processing received bursts. In other embodiments, each burst processing engine  202  may include, but not be limited to, a respective Application Specific Integrated Circuit (ASIC). Timing generator  204  may provide sliding window timing signals to burst processing engines  202  and ADC  206 . Received analog signals may be down converted from Radio Frequency (RF) (e.g., L-band, Ka-band, etc.) to Intermediate Frequency (IF) and presented to ADC  206 , which converts the received analog signals to digital signals and provides the digital signals to channel filter  208  via received sample bus  214 . 
     Channel filter  208  may include a FPGA or an ASIC for filtering out all but a subset of channels of the digital signals. Channel filter  208  separates distinct input frequencies and may store filtered samples in temporary storage  210  until one of burst processing engines  202  is ready to process the stored filtered samples. Temporary storage  210  includes a non-transitory storage medium suitable for storing the filtered samples. Channel filter  208  may provide the filtered samples of the digital signals to respective burst processing engines  202  via received sample bus  214 . Burst processing engines  202  may forward correctly decoded data, via computer interface bus  216 , to one or more processing devices for processing the correctly decoded data. Programming bus  212  may load programming information into FPGAs of burst processing engines  102 . 
     Although  FIG. 2  shows an embodiment including three burst processing engines  202 , other embodiments may have fewer or more burst processing engines  202 . 
       FIG. 3  is a functional block diagram of an exemplary processing device  300  for processing received data. Processing device  300  may include one or more processors  302  connected to a bus  310 . Memory  304  may store instructions for one or more processors  302  as well as data. Input device  306  may receive the data provided via computer interface bus  216  and may store the received data in memory  304  until one or more processors  302  is ready to process the received data. One or more processors  302  may process the received data and may provide the received data to an output device  308 , which may output the received data. 
       FIG. 4  is a functional block diagram of an exemplary device  400  for transmitting data according to various embodiments. Exemplary device  400  may be included in a terminal or may be included in a separate device that transmits data, but does not receive data. Exemplary device  400  may include an encoder  404 , a scrambler  406 , a modulator  408 , and a transmitter  410 . In some embodiments, device  400  may be a sensor, which may include a measuring component (not shown) to measure an aspect of a surrounding environment. The sensor may transmit information to other devices in a network such as measurement data, but may not receive information from other devices in the network. In some embodiments, encoder  404  and scrambler  406  may be implemented by a FPGA or an ASIC. 
     Encoder  404  may encode information to be transmitted using a low rate FEC code having a code rate that is no higher than ½. Scrambler  406  may receive the encoded data from encoder  404  and may exclusive- or a scrambling code, which is a random sequence, with the encoded data. In some embodiments, each transmitting device is assigned a respective Unique Word (UW) from a group of UWs. Each respective UW may be associated with a corresponding scrambling code. In such embodiments, scrambler  406  may scramble the encoded data using a scrambling code that corresponds to the respective UW. Modulator  408  may receive and modulate the scrambled encoded data, and may provide the scrambled encoded data to a transmitter  410 , which may include the respective assigned UW with the scrambled encoded data when transmitting. In some embodiments, a signal may be modulated onto QPSK symbols. Although other alphabets are possible in other embodiments. 
     In some embodiments, exemplary device  400  may not include scrambler  406 . In such embodiments, modulator  408  may receive and modulate the encoded data and may provide the encoded data to transmitter  410 . 
       FIG. 5  illustrates an exemplary burst format, which may be used in various embodiments. UW  502  may be at a beginning of a burst followed by segments of coded information pilot symbols  506  inserted periodically, or otherwise, to provide channel estimation at a receiver. I 0    504  and I 1    508  may be portions of scrambled encoded information (in embodiments in which device  400  includes scrambler  406 ) or I 0    504  and I 1    508  may be portions of encoded information (in embodiments in which device  400  does not include scrambler  400 ). Although  FIG. 5  shows UW  502  at the beginning of the burst, in other embodiments, UW  502  may appear in a middle or an end of the burst. 
       FIG. 6  illustrates exemplary overlapping asynchronous bursts, which may be received at any time from multiple transmitting devices in various embodiments. The overlapping bursts are not aligned on time slots. Each of the overlapping bursts include data, transmitted without diversity, encoded using a low rate FEC code having a code rate that is no higher than ½. In this example, the code rate may be 1/9. In embodiments in which device  400  includes scrambler  406 , each of the overlapping bursts may include encoded data that has been scrambled according to a scrambling code, or sequence, corresponding to a respective UW assigned to respective transmitting devices. Burst  602  overlaps and interferes with bursts  618  and  604 , which also interfere with burst  602 . A portion of burst  604  and burst  616  interfere with each other. Portions of bursts  606 ,  608  and  616  interfere with each other. Burst  610  is transmitted without interference from other bursts. Portions of bursts  612  and  614  interfere with each other. 
       FIG. 7  is a flowchart that explains exemplary processing performed in various embodiments. The process may begin with asynchronous bursts being received by a gateway terminal at any time from multiple user terminals or other transmitting devices. The bursts may be buffered and provided to one or more burst processing engines  202 , which may store the received bursts for processing in an input buffer (act  702 ). The use of the input buffer will be described subsequently. Act  702  continues to be performed while processing described in the remaining acts of  FIG. 7  are performed. The asynchronous bursts were transmitted without diversity and were encoded using a low rate FEC code with a code rate no higher than ½. In some embodiments, the low rate FEC code has a code rate of 1/9, although other code rates that are no higher than ½ may be used in other embodiments. 
     Burst Processing engine  202  may set an estimated interference from unknown bursts to zero (act  704 ), may set a repeat count to 1 (act  706 ) and may detect new bursts in a sliding window of W burst times (act  708 ). We recommend that the window size, W, be greater than three burst times. Selecting a suitable window size, W, is a performance complexity trade-off. In some embodiments, the window size, W, may be 5 burst times. In other embodiments, the window size may be 3 burst times or another suitable number of burst times. 
     Burst processing engine  202  may detect bursts using correlation with each possible UW sequence. UWs may be detected by searching for correlation peaks. UW sequences may be obtained from a single sequence and may be related by a Walsh code construction. In some embodiments, if M denotes a number of different UWs and N denotes a length of each UW, rather than M correlators of length N, M correlators of length N/M may be employed, followed by a Fast Hadamard Transform (FHT). In one embodiment, there may be 64 different possible UWs, each of which may have a length of 512 symbols constructed from Walsh codes. Each transmitting device may be assigned a respective one of the UWs, which the transmitting device typically inserts at a beginning of a burst, but may insert in a middle of the burst or at an end of the burst in other embodiments. 
     Symbol times may be estimated, in one embodiment, by using a correlation output at 2 samples/symbol with inverse parabolic interpolation to approximate a sub-sample timing offset. Known pilot symbols may be used to produce a frequency estimate. 
     Burst processing engine  202  may then produce an ordered list of the bursts which have been detected, ordering may be according to the bursts&#39; signal-to-noise-plus-interference ratio (SINR), or according to any other ordering (act  710 ). 
     Over the ordered list of bursts to be processed, a number of iterations may be performed wherein in each iteration, for each burst in the ordered list, a burst is selected for processing (act  712 ). Samples from the input buffer relevant to this burst are retrieved, and any previously determined interference by this selected burst (which may have been subtracted on a previous iteration) may be added to the selected burst (act  714 ). The selected burst may then be demodulated, descrambled (if transmitters perform scrambling) and decoded (act  716 ) and then re-encoded, re-scrambled (if descrambling was performed) and re-modulated to obtain an updated estimate of the interference caused by the selected burst (act  718 ). Remodulated output is used for interference cancellation and may be an estimate of the original transmitted burst, including the effects of timing, phase, amplitude, and frequency. The estimate may be considered an expected value of the burst or an expected amount of interference, with respect to other bursts, caused by the burst. The expected amount of interference caused by the burst may be saved (act  720 ). In some embodiments, the expected amount of the interference caused by the burst may be saved in a buffer. This interference estimate is then cancelled from the input buffer (act  722 ). The estimated interference caused by the selected burst may be canceled by subtracting, from the input buffer, the estimate of the interference caused by this burst. In other embodiments, other methods may be used to cancel the estimated interference caused by the selected burst. If an estimated value of any burst was not previously determined, then a previously determined estimated value of the burst is set to zero. In this way, data in the input buffer always contains the original input data with the most current estimates of all known bursts removed. 
     Burst processing engine  202  may then determine whether the decoding of the selected burst, performed in act  716 , was performed correctly (i.e., a burst that passes parity and cyclic redundancy code (CRC) checks, or other checks) (act  724 ). If the decoding was performed correctly, then the selected burst may be sent, or forwarded, to the next device or the next process and may be removed from the ordered list of bursts (act  726 ). Processing may then continue with act  728 . If the decoding was not performed correctly, then burst processing engine  202  may determine whether any detected bursts remain to be processed (act  728 ). If detected bursts remain to be processed, burst processing engine  202  may select a next burst from the ordered list for processing (act  730 ) and control may proceed to act  714 . If, during act  728 , no detected bursts remain to be processed, then burst processing engine  202  may determine whether any bad, or incorrectly decoded bursts, remain in the window (act  732 ). If incorrectly decoded bursts are determined to remain in the window, then burst processing engine  202  may determine whether a maximum number of iterations were performed (act  734 ). If the maximum number of iterations were not performed, then burst processing engine  202  may increment a number of repeats, or iterations (act  736 ), burst processing engine  202  may prepare to begin processing of bursts at a beginning of the ordered list of bursts (act  740 ) and control may return to act  712  to select a burst from the ordered list for processing. If, during act  732 , no incorrectly decoded bursts remain in the window, then the window may be stepped (act  738 ) and control may return to act  704  to set estimated interference caused by unknown bursts to zero. The sliding window of W burst times may be stepped, or advanced, by some amount of burst times (i.e., advancing the sliding window by some positive number, S, of symbols). 
     In the above description of iteration, including the steps of selecting a burst to be processed (which includes demodulating, descrambling, decoding, re-encoding, re-scrambling, re-modulating), and interference cancellation, it is understood that the ordering of the processing steps may be varied. A particular order of selecting bursts to be processed, and interference of other bursts cancelled may be varied according to any possible schedule, reflecting tradeoffs of implementation complexity and receiver performance. 
     A variation of the process shown in  FIG. 7  may be employed in some embodiments.  FIG. 8  illustrates the variation. Acts  832  and  838 , which are shaded, illustrate differences from the process of  FIG. 7 . Since the process of  FIG. 7  was discussed previously, only the changed acts and some acts executing before and after the changed acts are discussed. In the variation of  FIG. 8 , a considered fraction (CF) of the sliding window W of burst times may include a fraction of an oldest burst time or one or more oldest burst times of the sliding window. In some embodiments, the CF may have a size of about 20% of the sliding window of W burst times. Thus, if W is 5 burst times, then CF may have a size of one burst time and may include an oldest burst time of the sliding window of W burst times. In other embodiments, CF may have a size of three burst times. 
     In this variation, the difference from the process described by  FIG. 7  is that instead of terminating the iteration and advancing the window W by some time T when no bursts remain in the ordered list (all detected bursts in the window W are correct), the iteration is terminated and the window W is advanced by some time T (i.e., the sliding window is advanced by some positive number, S, of symbols) when no bursts remain in the ordered list whose time of arrival is within the CF from the start of the window W, where CF is some fraction of the window W, or when the maximum number of iterations is reached. 
     Timestamps 
     In a variation, data packets may arrive at user terminal  102  as variable length packets. User terminal  102  may transmit each of the variable length packets as fixed-sized bursts to gateway terminal  106  on a physical channel. At gateway terminal  106 , the received fixed-sized bursts may be reassembled into a packet. A straightforward method for accomplishing this involves labeling each burst with three values: a packet ID, a sequence number and a packet length. Unfortunately, this method introduces much overhead. 
     A more efficient method is illustrated by  FIGS. 9-11 .  FIG. 9  illustrates an exemplary process performed by user terminal  102 . User terminal  102  may receive a packet (act  902 ) and may assign a packet ID to the packet (act  904 ). User terminal  102  may segment the packet into bursts of a fixed size, each of which includes the packet ID (act  906 ). User terminal  102  may then transmit the multiple fixed-sized bursts to gateway terminal  106  (act  908 ). 
       FIG. 10-11  illustrate an exemplary method that may be performed by gateway terminal  106  in this variation. In this variation, bursts are timestamped with a time at which each burst is detected by gateway terminal  106 . Also acts  717  and  724  of  FIGS. 7 and 8  are modified to provide the received bursts, which were correctly decoded, to the exemplary process illustrated by  FIG. 10 . 
     As illustrated by  FIG. 10 , packet segments may be received by gateway terminal  106  as timestamped fixed-sized bursts (act  1002 ). Gateway terminal  106  may then determine whether a packet ID in a received burst matches a previously received packet segment (act  1004 ). If the packet ID in the received burst is determined to not match the previously received packet segment, then a packet segment timer, associated with the packet ID, may be started (act  1006 ) and a first received segment of the packet may be stored (act  1008 ). 
     If, during act  1004 , the packet ID is determined to match a packet ID of a previously received packet segment, then the timestamp associated with the packet segment may be compared with the timestamp associated with the previously received packet segment to determine whether the current packet segment was received within a predefined margin of the previously received packet segment (act  1010 ). This assumes that inter-arrival times between bursts of a packet are known and fixed. In a variation of this embodiment, instead of timestamping each burst with a time at which the burst is detected, each burst may be time stamped with a time at which the burst is detected modulo an expected inter-arrival time. Act  1010  may then be performed by determining whether a current timestamp is within + or − the predefined margin. 
     If the timestamp is within the predefined margin of the timestamp of the last segment received for the packet, then the packet segment timer associated with the packet ID is restarted (act  1012 ) and the current packet segment is added to the stored packet segment(s) associated with the packet ID (act  1014 ). 
     If, during act  1010 , the timestamp of the current packet segment is determined not to be within the predefined margin of the timestamp of the last received packet segment associated with the packet ID, then the packet segment(s) associated with the packet ID of the current burst may be treated as a first received packet segment associated with the packet ID and acts  1006  and  1008  may be performed to start the packet segment timer and store the first received packet segment associated with the packet ID, respectively. 
       FIG. 11  illustrates an exemplary process that may be performed in gateway terminal  106  in the just-discussed variation. In the exemplary process, gateway terminal  106  may wait for a packet segment timer to expire (act  1102 ). When a packet segment timer associated with a packet ID expires, the reassembled packet associated with the packet ID may be sent to a next process or device (act  1104 ). Acts  1102 - 1104  may be repeated. 
     Frequency Hopping 
     When burst arrivals are independent, A-SCMA works well. However, when packets are segmented, bursts are not independent. If successive bursts of the packet are transmitted continuously in time, performance at a receiving gateway terminal will be degraded. 
     Often, a communication system will have multiple available channels (frequencies). If such a communication system uses A-SCMA, successive bursts of a packet may be transmitted to a gateway terminal on randomly selected frequencies selected from a pool of known frequencies. Of course, in such a system, the receiving gateway terminal should be capable of receiving bursts sent over any of the known frequencies. 
       FIG. 12  illustrates an exemplary process performed by a transmitting device, which may include, but not be limited to, a user terminal or a sensor. The process is a variation of the process shown in  FIG. 9 . The process may begin with the transmitting device receiving a packet (act  1202 ) and assigning a packet ID to the packet (act  1204 ). The packet may be segmented into multiple bursts of a fixed size, each of which may include the assigned packet ID (act  1206 ). The transmitting device may then select a first segment of the packet (act  1208 ) and may randomly select a frequency from a pool of known frequencies, which a receiving device such as, for example, a gateway terminal, is capable of receiving (act  1210 ). The transmitting device may then transmit, to the receiving device, the segment as a fixed-sized burst on the randomly selected frequency (act  1212 ). The transmitting device may then determine whether there are additional segments of the packet to transmit (act  1214 ). If there are additional segments of the packet to transmit, the transmitting device may select a next segment of the packet (act  1216 ) and may repeat acts  1210 - 1214 . 
     If, during act  1214 , the transmitting device determines that no additional segments of the packet remain to be transmitted, then the process ends. 
     In this variation, the processes described by  FIGS. 10 and 11  would remain unchanged. However, the packets received by the receiving device as fixed-size bursts may be received on any of the known frequencies from the pool of known frequencies. 
     Advantages of the Invention 
     Embodiments of the present invention have a number of advantages over existing access methods. For example, A-SCMA was found to double utilization capacity with respect to SCMA. This is due to terminals being free to choose when to start transmissions, resulting in less mutual interference, thereby improving interference cancellation performance applied by a receiving device. Further, because time slot synchronization is not used in A-SCMA, terminals may have less complexity making possible one-way autonomous applications such as, for example, remote sensing. Other advantages include:
         1. much higher capacity than any existing random multiple access schemes;   2. negligible latency caused by retransmission due to a very high probability of a successful first transmission attempt; and   3. very simple implementation of transmit only terminals that transmit at will.       

     Conclusion 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms for implementing the claims. 
     Although the above descriptions may contain specific details, they should not be construed as limiting the claims in any way. Other configurations of the described embodiments are part of the scope of this disclosure. For example, although examples of a satellite communication system were provided above as implementing various embodiments, the various embodiments may be implemented in other types of communication systems having delays of an order of magnitude less than delays found in a typical satellite communication system. 
     Further, implementations consistent with the subject matter of this disclosure may have more or fewer acts than as described, or may implement acts in a different order than as shown. Accordingly, the appended claims and their legal equivalents should only define the invention, rather than any specific examples given.