MXPA01001851A - Method and apparatus for a cdma random access communication system - Google Patents

Abstract

In a system in which multiple remote units compete (104) for limited communication resources, a remote unit (104A) accesses the system by randomly selecting a first sequence from a set of predetermined sequences. The remote unit (140A) transmits a data bit stream modulated with the first sequence beginning at one of a set of recurring admission boundaries. If the remote unit (104A) determines that the data bit stream has not been successfully received at a hub station (100), the remote unit (104A) randomly selects a second sequence from the set of predetermined sequences. The remote unit (104A) transmits the data bit stream modulated with the second sequence beginning at one of the set of recurring admission boundaries.

Description

METHOD AND APPARATUS FOR A COMMUNICATION SYSTEM WITH RANDOM CDMA ACCESS.

BACKGROUND OF THE INVENTION. I. FIELD OF THE INVENTION This invention relates generally to communication systems. More specifically, the invention relates to random access communication systems. II. DESCRIPTION OF THE RELATED TECHNIQUE. The use of wireless communication systems for the transmission of digital data becomes increasingly widespread. In a wireless system, the most valuable resource in terms of cost and availability is typically the wireless link itself. Therefore, a main design objective when designing a communication system comprising a wireless link is to efficiently use the available capacity of the wireless link. In addition, it is also important to minimize the delay associated with data transmissions. In a system in which multiple units compete for the limited resources of the system, a means must be developed to regulate access to such resources. In a digital system, remote units tend to generate data in bursts. Burst data is characterized by having a high proportion of peak-to-average traffic, which means that large blocks of data are transferred for short periods of time, interposed between significantly long periods of inactivity. Dedicating an individual communication channel to each active unit does not result in an efficient use of the system's capacity in a system in which the units generate data in bursts, because during those times in which the remote unit is not using the system, the assigned channel remains inactive. The use of dedicated channels can also impose a severe limit on the number of remote units that the system can use simultaneously, regardless of the usage patterns of the remote units. In addition, the use of dedicated channels can cause an unacceptable delay if the portion of capacity allocated to each remote unit is so small that the transfer rates are greatly compromised. The characteristics of the incoming and outgoing traffic tend to differ significantly in a digital data system. For example, in a system that provides wireless Internet services, a typical input transmission from a remote unit is relatively short, such as the requirement of a web page. However, a typical output data transfer to a remote unit tends to be rather large. For example, in response to a request from a web page, the system can transfer a significant amount of data. Because the characteristics of the input and output channels are very different, the efficiency of the system can be increased through the development of two different protocols for the input and output links. An ALOHA random access protocol was developed to be used in the input link from a remote unit in a digital data system. The basic idea behind ALOHA is quite simple: remote units transmit whenever they have data to send. If the remote units are using a communication resource that can only be accessed by one remote unit at a time, the information from each of the remote units is destroyed if two units transmit at the same time, causing a collision. In a system in which the remote unit can monitor the random access channel, the remote unit can pay attention to the channel to determine if its transmission is the victim of a collision. In a system in which the remote unit does not monitor or can not monitor the random access channel, the remote unit can detect a collision, based on the absence of an acknowledgment received from a central station in response to the transmission. According to the standard operation of ALOHA, every time a collision occurs, the remote unit waits a random amount of time and retransmits the data. The waiting time must be random or the remote units that collide generate repetitive closed row collisions. Figure 1 is a timing diagram showing the operation of a pure ALOHA multiple random access system. As shown in Figure 1, five remote units designated A, B, C, D, and E, are transmitting data packets within a common communication channel. Each time two remote units transmit at the same time, a collision occurs and both transmissions are lost. In a pure ALOHA system, if the first bit of a new transmission is superimposed only with the last bit of a transmission already in progress, both transmissions are completely destroyed and both have to be retransmitted at another time. For example, in the frequency modulated channel (FM) shown in Figure 1, where two packets can not be transmitted at the same time, a packet 12 transmitted by the remote unit B collides with the packet 10 transmitted by the remote unit A and a packet 14 transmitted by the remote unit C. remote unit A must retransmit the information in packet 10, remote unit B must retransmit information in packet 12 and remote unit C must retransmit information in packet 14. Figure 1 shows remote unit C retransmitting packet 14 as package 14R.

- In a pure ALOHA system, if the average packet transfer rate is low, most packets are transmitted without collisions. As the average packet transfer rate begins to increase, the number of collisions increases, and therefore, the number of retransmissions also increases. As the average packet transfer rate increases linearly, the probability of retransmissions and multiple retransmissions increases exponentially. To a certain extent, as the average packet transfer rate increases, the probability of successful transmissions falls below a reasonable number and the system becomes practically inoperable. In a pure ALOHA system, the best utilization of a channel that can be achieved is approximately 18%. Below 18% the system is being wasted. Over 18% the number of collisions increases in such a way that the total effective performance of the system begins to fall. The introduction of a satellite link within a digital communications system complicates the multiple access dilemma. The use of a geosynchronous satellite typically introduces a delay of 270 milliseconds (msec) between the transmission of a signal from a remote unit and the reception of the same signal at a central station. Due to the delay introduced by the satellite link, scheduled access schemes that require the remote unit to request system resources before initiating any transmission are impractical for many applications. Therefore, a satellite link that serves a large number of remote units that transmit data in bursts, is the appropriate environment in which to implement an ALOHA system. If an ALOHA system is implemented in a satellite system in which the remote units do not monitor or can not monitor the random access channel, in the event of a collision, the remote unit does not know of the collision by at least 540 msec. In addition to the notification delay, the remote unit typically must wait some random amount of time before retransmitting the data (to avoid retransmissions in a repeating closed row). The retransmitted signal is once again subjected to the time delay of 270 msec. The cumulative delay of such transmission can easily exceed one second. In a fully loaded system, the delay can be significantly longer due to the increased probability of repeated collisions. Therefore, when using a satellite link, it is advantageous to limit the number of retransmissions attributable to collisions as well as other causes. The number of retransmissions due to collisions can be reduced by simply reducing the allowable load of the system. The satellite link also introduces oppositions concerning the successful transmission of data about the link. Due to the high level of interference and the high path loss that characterizes a satellite link, typically a relatively robust physical interface should be used. A physical interface commonly used over satellite links is the direct sequence propagation spectrum (DSSS). In a DSSS system of the prior art, the communication channel is defined by a binary propagation sequence of maximum length. Each discrete binary value that forms a propagation sequence is referred to as a "chip". The propagation sequence is selected in such a way that the self-correlation of the sequence with itself is almost zero for all shifts aligned on non-zero chips. A sequence of pseudo noise (PN) of maximum length, of length n, has the property that its correlation 1 circular with itself (self correlation) is 1 or - for n any displacement aligned in chips. The correlation of the propagation sequence chosen with itself is the same 1 a - for all shifts aligned on n chips other than zero offset. The correlation is 1 also - between the sequence and its inversion for any n displacement aligned in chips different from zero displacement. The correlation of the sequence and the sequence itself in the displacement at zero time is equal to 1. Therefore, as the length n of the propagation sequence increases, the orthogonality and therefore the isolation between the corresponding channels, also increase. The means by which sequences of maximum length can be identified and generated are well known in the art.

In a typical system, each data bit generated by the remote unit is modulated with one or more chips before transmission over the wireless link. In this way, narrowband digital data propagates through a wider bandwidth of transmission. In the receiver, the received data is multiplied by the same propagation sequence that was used to create the signal, in order to extract the corresponding digital data. Because the waveform of the DSSS signal is robust in the presence of interference, the number of retransmissions due to interference other than collisions can be decreased through the incorporation of DSSS signaling.

In a DSSS system, the remote units transmit in a common frequency band, thus causing some level of interference to other system users. The efficiency of the system in a DSSS system is increased if the power received at the central station from each remote unit is controlled in such a way that each signal reaches the receiver of the central unit at approximately the same level. If a signal transmitted by a remote unit reaches the receiver of the central station at a relatively low power level, the quality of the signal may fall below an acceptable level. If, on the other hand, the signal from the remote unit reaches a relatively high power level, the high power signal acts as unnecessary interference to the other remote units. The power transmitted by the remote units is typically controlled by the central station. The central station sends power adjustment commands to the remote unit in order to equalize the power received at the central station. Figure 2 is a timing diagram illustrating the random access operation of the ALOHA propagation spectrum of the prior art for five example remote units A ', B', C, D 'and E' using an identical code. For purposes of illustration, the system of Figure 2 uses a propagation code having only seven chips (cx-c7) in length. In current systems it is common for the length of the propagation code to be as long as 255 chips in length. In the time slot ti the remote unit A 'begins to transmit a series of data bits, each modulated with the entire seven-bit propagation sequence. In systems that use longer propagation sequences, it is common for each data bit to propagate with only a subset of the entire propagation sequence. At time t3 the remote unit E 'begins to transmit a series of data bits modulated by the same seven-chip propagation code. Similarly at time t5, remote unit C begins to transmit a series of data bits modulated by the same seven-chip propagation code. As mentioned above, the propagation sequence is selected in such a way that it is almost orthogonal to itself in all shifts aligned in chips other than zero offset. Therefore, under the scenario depicted in Figure 2, the remote units A ', C and E' produce a low level of interference with each other because their respective transmissions are displaced one from the other. Nevertheless, in case two remote units start transmitting during the same time slot (resulting in a zero offset), a collision occurs and both transmissions are lost. Because each remote unit propagates its signal with the same propagation sequence, if a collision occurs, each remote unit waits a random amount of time before starting to transmit again in order to avoid repeated collisions in a closed row. Additional information concerning the use of identical propagation codes can be found in a CDMA ALOHA communication system in the US Patent. Do not. ,537,397 entitled "SPREAD ALOHA CDMA DATA COMMUNICATIONS" (CDMA ALOHA PROPAGATION DATA COMMUNICATIONS) issued July 16, 1996. The chip data shown in FIG. 2 is modulated with the data bits that carry the wireless link information, such as the message transmitted. Figure 3 is a timing diagram showing the printing of the bit data in the system shown in Figure 2. In Figure 3 it is assumed that if a logical "1" is transmitted, the propagation code is transmitted in the form not increased. If a logical "0" is transmitted, the inverse of the propagation code is transmitted. The transmission of a logical "0" and the inversion of the corresponding chip data is indicated in FIG. 3 by a top bar. In Figure 3, the remote unit A 'transmits a logical "1" followed by a logical "0", the remote unit C transmits a logical "1" followed by a logical "0" and the remote unit E' transmits two bits of consecutive logical "0" data. In order to examine the effect of the bit data on the inter-channel interference, let us examine the first data bit transmitted by the remote unit C in the time slots t5 to you. By examining the interference from the remote unit E 'to the remote unit C during the time periods t5 to you, we can conclude that the interference is 1 equal to - or a seventh of the energy transmitted by the remote unit E during this period due to the DSSS properties described above. A more problematic case is illustrated by examining the interference from the remote unit A 'to the remote unit C during the same period of time. Note that the data transmitted by the remote unit A 'during period t5 to you, transits from one logical value to another. The transition interrupts the propagation sequence and reduces the orthogonality between the signal of the remote unit C and the signal from the remote unit A 'during that period. For this reason, the interference from the remote unit A 'to the remote unit C during the period 1 t5 to you is probably greater than this. This phenomenon is referred to as partial sequence interference and can significantly reduce the signal-interference ratio experienced by a active system. For example, in a system in which n is equal to 255 operating in a fully loaded ALOHA situation, the resulting signal-to-interference ratio for each transmission on average is 5.5 decibels (dB) due to partial sequence interference resulting from transitions of data rather than 1/255 or approximately 24 dB. The relatively low signal-to-interference ratio decreases system performance, thus increasing the likelihood of retransmissions caused by sources other than collisions. The incidence of retransmission caused by other sources can increase the delay introduced by the system to an intolerably high level.

Therefore, there is a need for a random access system that provides an advantageous use of spectral resources as well as a tolerable delay. SUMMARY OF THE INVENTION. In a random sequence direct sequence (DSSS) propagation system, each time a remote unit has a message to transmit, it randomly selects a sequence from a set of predetermined sequences with which to modulate the available data. When the next admission limit is presented, the remote unit transmits the modulated data. If a collision occurs, the remote unit may retransmit the message using another randomly selected sequence of predetermined sequences, starting at an admission limit. When using a second randomly selected sequence, the remote unit does not need a delay retransmission by an arbitrary amount to avoid a collision in a repetitive closed row with another remote unit signal. Therefore, by eliminating the need to postpone the transmission attempt randomly, the average delay associated with the random access process is reduced. The use of recurring admission limits allows remote units to align their data bit limits, which results in a significant decrease in inter-channel interference. In a system in which multiple remote units compete for limited communication resources, a remote unit accesses the system by randomly selecting a first sequence from a set of predetermined sequences, and then transmitting a stream of data bits modulated with the first sequence, starting in one of a set of recurring limits of admission. If the remote unit determines that the data bitstream has not been successfully received at the central station, it randomly selects a second sequence from the set of predetermined sequences and transmits the modulated data bit stream with the second sequence, beginning at one of a set of recurring admission limits. The remote unit can receive a message from the central station by ordering it to remove a reserved sequence from the set of predetermined sequences. In response, the remote unit temporarily abstains from transmitting using the sequence reserved for random access communications. In one embodiment, the set of predetermined sequences consists of a maximum length sequence and a plurality of permuted versions of the maximum length sequence. In another embodiment, the recurring admission limits coincide with boundaries between data bits within the data bit stream. In one embodiment, the remote unit determines when the data bitstream has been successfully received by monitoring a communication channel carrying the data bit stream modulated with the first sequence to detect a collision with another remote unit signal. In another embodiment, the remote unit determines that the data bit stream has not been received successfully, by determining whether an acknowledgment of the data bit stream is received from the central station. In one embodiment, the remote unit randomly selects a frequency channel on which the data bit stream modulated with the second sequence transmits. In yet another embodiment, the remote unit randomly delays the transmission of the data bit stream modulated with the second sequence. The set of predetermined sequences can be Walsh codes or alsh codes that have been concealed by a second code. The second code can determine spectral properties of the set of predetermined sequences. The set of predetermined sequences may comprise a maximum length sequence and a plurality of permuted versions of the maximum length sequence. The stream of data bits modulated with the first sequence can be transmitted using a higher order modulation scheme. For example, the higher order modulation scheme may be a quadrature amplitude modulation comprising at least 16 values, or 8-ary or 16-ary phase shift manipulation. The remote unit can receive a power control command from a central station in order to determine a power level used to transmit within approximately 1 dB of accuracy. The transmission path can consist of a satellite link. The remote unit can receive a command from the central station to use the reserved one from the set of predetermined sequences. In such a case, the remote unit transmits a stream of data bits modulated with the reserved sequence of the set of predetermined sequences beginning at one of the set of recurring admission limits. The central station correlates one or more of the set of predetermined sequences with the set of incoming data samples, beginning at a limit of the set of recurring admission limits. If the central station detects a signal level above the detection threshold corresponding to a sequence detected from the set of predetermined sequences, demodulates the signal of a remote unit using the detected sequence of the set of predetermined sequences. In one aspect, the central station detects the length of a message from a remote unit that exceeds a threshold. The central station sends a message to other remote units by temporarily removing the detected sequence from the set of predetermined sequences in such a way that the other units refrain from using the detected sequence from the set of predetermined sequences for random access communications. The central station can transmit an indication of the set of recurring admission limits to a set of remote units. In one embodiment, the central station demodulates a series of non-containment channels which share a common frequency spectrum with the set of incoming signal samples that can consume 10% to 25% of the common frequency spectrum capacity and they are aligned in bits with the set of - recurring admission limits. The central station can send a power control command to the remote unit in order to determine a power level at which the remote unit transmits within approximately 1 dB of accuracy. The central station can monitor a plurality of random access direct sequence propagation spectrum communication channels for random access communication signals. The central station may temporarily allocate a channel chosen from the plurality of random sequence direct sequence propagation spectrum communication channels to a particular remote unit. The central station can command other remote units using the plurality of random sequence direct sequence propagation spectrum communication channels to temporarily refrain from using the channel chosen for random access communication. A remote unit may comprise of a process that randomly selects the first sequence from a set of predetermined sequences, a process in which it transmits a stream of modulated data bits with the first sequence starting at one of a set of recurring admission limits, a process that determines that the data bitstream has not been successfully received at the central station, a process that randomly selects a second sequence from the set of predetermined sequences, and a process that transmits the data bit stream modulated with the second sequence starting at one of a set of recurring admission limits. The remote unit may further comprise a process that randomly selects a frequency channel on which to transmit the stream of data bits modulated with the first and second sequences. The remote unit may comprise a process that randomly delays the transmission of the data bit stream modulated with the second sequence. In addition, the remote unit may comprise a process that receives a command from the central station to use the reserved from the set of predetermined sequences, and a process that transmits a stream of data bits modulated with the reserved code of a set of predetermined codes beginning in one of the set of recurrent limits of admission. BRIEF DESCRIPTION OF THE DRAWINGS. The characteristics, objectives and advantages of the invention will be more apparent from the detailed description set forth below, when taken in conjunction with the drawings in which corresponding parts are identified with corresponding reference numbers, through the document, and in where: Figure 1 is a timing diagram showing the operation of a pure ALOHA multiple random access system; Fig. 2 is a timing diagram illustrating a random access ALOHA of propagation spectrum, of the prior art; Figure 3 is a timing diagram showing the printing of bit data in the system shown in Figure 2; Figure 4 is a block diagram illustrating a system according to the invention; Figure 5 is a timing diagram showing the illustrative operation according to the invention; Figure 6 is a flow diagram showing the exemplary operation of a remote unit; and Figure 7 is a flow chart showing the exemplary operation of a central station. DETAILED DESCRIPTION OF THE INVENTION. A direct sequence direct sequence propagation spectrum (DSSS) system, according to the present invention, operates based on the use of a set of predetermined unique sequences rather than a single sequence. Each time the remote unit has a message to transmit, it randomly selects one of the predetermined sequences with which it modulates the available data. When the next admission limit occurs, the remote unit transmits the modulated data. If a collision occurs, the remote unit may retransmit the message using another randomly selected sequence of the predetermined sequences upon noticing the transmission failure, starting at an admission limit. When using a second randomly selected sequence, the remote unit does not need a delay retransmission in an arbitrary amount to avoid a collision in closed rows with another remote unit signal. Therefore, by eliminating the need to postpone retransmission attempts at random, the average delay associated with the random access process is reduced. The use of recurring admission limits allows remote units to align their bit data limits, resulting, therefore, in a significant decrease in inter-channel interference. Admission limits are a set of recurring instances of time, in which the remote unit is allowed to begin the transmission of a new message. Admission limits can be presented as frequently as each data bit limit. In order to decrease the delay associated with waiting for the occurrence of the next admission limit, it is advantageous that the admission limits occur at relatively high proportions. In this way, the delay associated with waiting for the next admission limit is much less than the delay associated with the random postponement of the transmission according to the prior art. Starting at each admission limit, the central station searches for transmissions of remote units by correlating the incoming signal samples with one or more of the set of predetermined sequences. In one embodiment, the central station can collect the incoming signal samples and serially correlate them with the set of predetermined sequences. In another embodiment, the central station may correlate the incoming signal samples with two or more of the predetermined sequences in parallel. The central station can perform the correlation using any of a variety of known techniques to receive a signal having a proportion of data that is much smaller than the modulation ratio. The correlation can be made in a analog base band, digitally, ~~ at a radio frequency or intermediate frequency, or using the replica of the sequence as well as using other techniques. In one embodiment, if a remote unit is currently transmitting a signal using one of the predetermined sequences, the central station does not need to correlate the incoming signal with that sequence in order to detect a new transmission, because any new transmission would result in a collision in which all data would be destroyed.

- If a correlation with one of the predetermined sequences produces an energy value greater than the detection threshold, the central station begins to demodulate that channel in order to recover the signal from the remote unit. If two or more remote units transmit using the same predetermined sequence at the same time, the central station can detect the presence of signals, but not be able to demodulate any signal. In one embodiment, when the central station successfully demodulates the signal from a remote unit, it sends an acknowledgment to the remote unit. Various means are known in the art for the central station to correlate an incoming signal with a coding sequence. Figure 4 is a block diagram illustrating a system according to the invention. In Figure 4, a central station 100 provides communication resources to a plurality of remote units 104A-104N. The link between the central station 100 and the remote units 104 comprises a satellite 102. The input signals from the remote units 104 are transmitted to the satellite 102 where they are retransmitted to the central station 100. In the same way the signals coming from the central station 100 are transmitted to satellite 102 where they are retransmitted to remote units 104A-104N. The central station 100 can transmit information to the remote units, which allows them to predict the occurrence of admission limits such as by the use of a pilot signal and a synchronization process or other well-known technique. The remote units 104 may comprise a series of one or more processes that enable them to carry out the functions of the invention. In the same way, the central station 100 may comprise a series of one or more processes that enable it to carry out the functions of the invention. The processes can be incorporated, for example, into one or more integrated circuits, such as the specific application integrated circuit (ASIC), or they can be incorporated into software or firmware routines that are executed by a microcontroller. A large number of unique predetermined sequence sets can be developed for use with the present invention. The set of predetermined sequences may depend on other criteria of system operation. A convenient means of generating sequences is to select a maximum length pseudo noise (PN) sequence and generate a sequence family based on the sequence. For example, a set of n predetermined maximum length pseudo noise (PN) sequences can be generated from a PN sequence of maximum length of length n by permuting the base sequence as follows: Sequence 1 C? C2C3C4 ... cn Sequence 2 c2c3c4 ... CnC? Sequence 3 C3C4 ... CnC? C2 Sequence n cnC? C2c3c4 ... In addition, the unique sequences can be Walsh codes generated using well-known techniques. Walsh codes have the advantage of being completely orthogonal if they are aligned in time. The use of signals that are completely orthogonal to one another also reduces channel interference. The use of Walsh codes under the prior art is impractical because the Walsh codes produce high correlation values for many non-zero chip shifts. A potential problem with Walsh codes is that they typically do not provide good propagation properties. To overcome this limitation, new orthogonal sets of sequences can be determined by covering each code with a family of Walsh codes with an arbitrary binary sequence. For example, Walsh codes can be concealed with a sequence of maximum length that provides the desired spectral properties to the resulting signal. In addition, both Walsh codes and maximum length sequences can be overlaid with a long code that provides encryption (that is, an added impediment to intentional intersection) to transmissions. In a modality where the spectral resources allow the establishment of two or more parallel frequency channels, the remote unit can also choose from among the available channels on which to transmit its signal. If a collision occurs, the remote unit can automatically change the frequency channels or can randomly re-elect from the available frequency channels. Figure 5 is a timing diagram showing the illustrative operation of five remote units designated A '', B '', C ', D' 'and E' 'according to the invention. In Figure 5, seven different sequences have been generated based on a PN propagation sequence of maximum length of seven chips. Each bit of information transmitted by the remote unit is modulated by the entire seven-bit sequence in this example. In Figure 5, the admission limits occur before the time segments ti, t8, t15 and t22 as indicated by the broken marks in the corresponding vertical time limit indications in Figure 5. As long as no other remote unit transmits using the same sequence at the same admission limit, the central station can distinguish between signals from the remote unit using well-known propagation and demodulation spectrum acquisition techniques. However, if two remote units transmit using the same sequence at the same admission limit, a collision occurs and the information from each remote unit can not be detected properly by the central station 100. An advantage of the operation according to FIG. Comparison with the operation according to the prior art is that the bit data transition limits are aligned for each remote unit. Therefore, in Figure 5, the interference from the remote unit E '' to the signal of the remote unit C 'during the period of t8 to ti4 is equal to - n for all the values of the data. In this way the interference from the remote unit A '' to the signal transmitted by the remote unit C 'is - for all n data values. Said operation greatly increases the quality of the signal with which the signals can be demodulated by the central station 100. For example, in a system where a PN sequence of length 255 is used, a fully loaded system exhibits a signal-to-interference ratio of approximately 24 dB indicating an improvement of about 80 times in inter-channel interference over the prior art.

The elimination of the partial sequence auto correlation interference is achieved by placing the limits of admission on the data bit limits. Admission limits can be placed on each bit limit or on a subset of all bit limits. The slight delay associated with the transmission delay until the occurrence of the intake limit does not significantly add delay if the admission limits follow one another in rapid succession. The delay introduced into the system when waiting for the limits of admission is negligible and that introduced by the random delay that must be inserted according to the operation of the classic ALOHA is much smaller. As mentioned above, the average load of a random access system can not exceed a predefined percentage of the total capacity without risking a decrease in the total effective yield of the current system based on the increasing incidence of collisions and retransmissions. In a system using the present invention, the loading of the random access channel must be limited in order to avoid such phenomena. However, the increased performance gained through the use of the present invention provides many advantages to the system. As mentioned above, a DSSS system operates more efficiently when the signal from each remote unit reaches the central station at approximately the same level. If the signals do not reach the central station at the same level, the average of the signal-interference ratio of the system decreases and the performance of the system is negatively affected. Due to the increased immunity of the interference through the use of bit alignment, the accuracy requirements of the power balance may decrease. For example, in the prior art system described above, where n equals 255 and where the resulting signal-to-interference ratio is 5.5 dB for a fully loaded system, in order to preserve the signal-interference ratio of 5.5. dB, the power received from each remote unit at the central station should be controlled within 1/4 dB. In order to maintain a precise power level, the central station must send commands often power control commands to the remote unit, thus consuming significant system resources. Due to the increased performance, a system that incorporates bit alignment and has a value n = 255 that results in a signal-to-interference ratio of 24 dB for a fully loaded system, the power point determination requirement can relax within 1 dB without significantly affecting the signal-to-interference ratio resulting from a fully loaded system. This reduction in the need for precision in the power swing process reduces the amount of power control information that needs to be sent from the central unit to the remote unit by approximately three bits of resolution, thereby decreasing the resulting load on the system resources. In addition to the random signaling of the remote unit, the random access channel can be overridden with one or more non-containment connections. Non-containment connections can be scheduled or pre-assigned communications in which the system resource is only assigned to a specific remote unit. In order to reduce interference, the non-containment channels are aligned in bits with the random access channels. For example, non-containment connections may be limited to starting transmissions only at an admission limit. The properties of the propagation spectrum that allow the central station to discriminate between the random access channels also allows the central station to discriminate between the random access channels and one or more non-containment channels. The propagation sequences used for the non-containment connections should be selected to be closely orthogonal to one another, as well as the propagation sequences of the random access channels. Due to the increased immunity of the interference resulting from the incorporation of bit alignment, additional non-containment connections can be superimposed within the same spectrum as the random access channels without appreciably degrading the performance of the system's bit-error rate. . For example, 10%, 15%, 20%, and up to 25% or more of the channel can be assigned to non-containment connections in addition to the random use of the channel without significantly decreasing the performance of the bit error rate of the channel. Random access due to increased performance according to the invention. Another advantage of the increased performance of the system is that higher order modulation techniques can be used to print the data on the non-containment or random access channels. Higher-order modulation schemes, such as quadrature amplitude modulation 16-ary (QAM) or QAM 64 -aria or 64-ary phase shift manipulation (PSK), increase capacity by two or three or more times of random access or non-containment channels. In one embodiment, the remote unit operates using a higher order modulation scheme comprising at least 16 different data values. Due to the increased performance, these higher order modulation schemes can be incorporated into the system without significantly impacting the performance of the bit error rate of the system. If the higher-order modulation techniques are applied to the random access channel, the average time during which the remote unit uses one of the random access channels for a given data transmission is reduced, thereby releasing the access channel randomized to be used for another one sooner and decreasing the number of collisions In addition, the increased performance of the system eliminates the need for additional means to reduce interference. For example, the U.S. Patent. No. 5,537,397 to which reference was made above, suggests the use of an interference storage device that stores possible interference sequences. The information in the interference storage device can be used to increase the efficiency of the system. Such elements are not necessary in a system embodying the present invention. A multiple random access system may also incorporate a reservation scheme without significantly decreasing the bit error rate of the system. In a general mode, the remote unit randomly selects one of the random access channels each time it has a message to be sent. In some cases, a remote unit may have an abnormally long message to send. In such a case, the central station may reserve a sequence of the set of predetermined sequences for use only by the remote unit for the duration of the message. The central station can simply allow the remote unit to continue using a channel on which it initiated the transmission or can assign another remote channel to the remote unit. In this way, the remote unit is not interrupted during the transmission of the long message by other remote units that try to access the system. In addition, the incidence of collisions by other remote units is reduced if they refrain from transmitting through the reserved channel. The central station can transmit in general or specifically a message to each remote unit specifying the reserved channel. In response, the other remote units temporarily refrain from using the channel reserved for random access communications. The reservation scheme can be incorporated into systems that include bit alignment as well as other DSSS multiple access systems. Figure 6 is a flow chart showing an exemplary operation of a remote unit. The flow begins in the initial block 110. The block 112 determines whether data is available for transmission. If not, the process awaits the data. If the data is available the flow continues to block 114. In block 114, a predetermined sequence is chosen randomly. In block 116, in this exemplary embodiment, the remote unit also randomly chooses a frequency channel. In block 118, the remote unit pauses until an admission limit is reached. In block 120 the remote unit begins to transmit the data beginning at an admission limit. Block 122 determines whether the transmission has been received by the central station. The stage can be implemented by monitoring the channel or by waiting for an acknowledgment from the central station. The flow ends in block 124 after successful transmission of the available data. Figure 7 is a flow chart showing the exemplary operation of a central station incorporating a reservation mechanism. The flow begins in block 130. Block 132 determines whether the current time corresponds to the intake limit. If so, the central station begins to correlate a predetermined sequence with a set of incoming data samples. Block 136 determines whether the detected energy level exceeds the detection threshold. Otherwise, the flow continues back to block 132. If so, the flow continues to block 138. At block 138, the central station begins to demodulate the signal from the remote unit carried by the channel determined by the predetermined sequence . In block 140, the central station sends an acknowledgment message to the remote unit. In block 142, the central station determines whether the message length of the remote unit exceeds a threshold. If so, the central station invokes the reservation scheme. In block 144, the central station sends a message to the other remote units by removing the predetermined sequence from a set of usable predetermined sequences. In block 146 the flow ends. Many alternative modalities to the exemplary flow diagram given in figures 6 and 7 are easily apparent from the examination of their exemplary operation including simple rearrangement or parallel execution of the stages shown. In another embodiment, the received signal can be demodulated by the central station before detection. In such a case, the detection can be executed based on the results of the correction or error detection or of another quality indication signal. Various means for selecting random and pseudo-random numbers are well known in the art. The mechanisms by which these random numbers can be converted into random decisions are well known in the art. As used herein, the concept of "random selection" comprises the concepts of purely random, pseudo-random, quasi-random selection, as well as other techniques for selection such as the use of random-check functions or schedule data. , whether deterministic, statistical or based on patterns. Many alternative embodiments within the scope of the present invention will readily be discernible to one of skill in the art. For example, although the insertion of an arbitrary delay is not necessary to select a new channel in which to retransmit, after a collision, in some cases it may be advantageous to insert a delay in order to avoid an overload of the system. Obviously the invention can be implemented in many types of systems in addition to satellite systems, such as terrestrial cellular systems, terrestrial systems that incorporate repeaters, non-geosynchronous satellite systems, and even wiring systems. The invention can be incorporated into other specific forms without departing from its spirit or essential characteristics. The described modality should be considered in all aspects only as illustrative and not as restrictive and the scope of the claim of the invention is, therefore, indicated in the appended claims rather than in the preceding descriptions. All changes that come with the meaning and range of equivalence of the claims must be encompassed within its scope.

Claims (40)

1. CLAIMS 1. In a system in which multiple remote units compete for limited communication resources, a system access method for remote units comprises the steps of: randomly selecting a first sequence from a set of predetermined sequences; and transmitting a stream of data bits modulated with the first sequence starting at one of a set of recurring admission limits. The method of claim 1 further comprising the steps of: determining whether the data bit stream has been successfully received by the central station, - randomly selecting a second sequence from the set of predetermined sequences, if the data stream it has not been received successfully; and transmitting the modulated data bit stream with the second sequence starting at one of the set of recurring admission limits. The method of claim 2 wherein it further comprises the step of receiving a command from the central station to remove a reserved sequence from the set of predetermined sequences, and to temporarily refrain from transmitting using the sequence reserved for random access. The method of claim 2 wherein the set of predetermined sequences comprises a maximum length sequence and a plurality of permuted versions of the maximum length sequence. The method of Claim 2 wherein the recurring admission limits coincide with the boundaries between the data bits within the data bit stream. The method of claim 2 wherein the step of determining whether the data bit stream has been successfully received comprises the step of monitoring a communication channel carrying the data bit stream modulated with the first sequence to detect a collision with another remote unit signal. The method of claim 2 wherein the step of determining whether the data bitstream has been successfully received comprises the step of determining whether an acknowledgment of the data bit stream has been received from the central station. The method of claim 2 further comprising the step of randomly selecting a frequency channel on which to transmit the data bit stream modulated with the second sequence. The method of claim 2 further comprising the step of randomly delaying the step of transmitting the data bit stream modulated with the second sequence. 10. The method of claim 2 wherein the set of predetermined sequences are Walsh codes. The method of claim 2 wherein the set of predetermined sequences are Walsh codes that have been concealed with a second code. The method of claim 11 wherein the second code determines the spectral properties of the set of predetermined sequences. The method of claim 1 wherein the set of predetermined sequences comprises a maximum length sequence and a plurality of permuted versions of the maximum length sequence. The method of claim 1 wherein the data bit stream modulated with the first sequence is transmitted using a higher order modulation scheme. 15. The method of claim 14 wherein the higher order modulation scheme is a quadrature amplitude modulation comprising at least 16 values. The method of claim 1 further comprising the step of receiving a power control command from a central station in order to determine a power level used in the transmit stage within 1 dB of precision. The method of claim 1 wherein the step of transmitting comprises the step of transmitting a high frequency signal to a satellite for transmission to a central station. The method of claim 1 further comprising the step of: receiving a command from a central station to use a reserved sequence from the set of predetermined sequences; and transmitting a stream of data bits modulated with the reserved sequence from the set of predetermined sequences beginning at one of the set of recurring admission limits. The method of claim 1 wherein the recurring admission limits coincide with the boundaries between the data bits within the data bit stream. 20. A method for receiving random access signals from a plurality of remote units comprising the steps of: correlating one or more of a set of predetermined sequences with a set of incoming data samples beginning at one of a set of recurring limits of admission; detecting the signal level above a detection threshold corresponding to a sequence detected from the set of predetermined sequences; and demodulating the signal of a remote unit using the detected sequence of the set of predetermined sequences. The method of claim 20, further comprising the steps of: detecting the message length of a remote unit that exceeds a threshold; and sending a message to other remote units by temporarily removing the detected sequence from the set of predetermined sequences in such a way that the remote units refrain from using the detected sequence from the set of predetermined sequences, for random access communications. 22. The method of claim 20 further comprising the step of transmitting an indication of the set of recurring admission limits to a set of remote units. The method of claim 20 further comprising the steps of demodulating a series of non-containment channels that share a common frequency spectrum with the set of incoming signal samples, wherein the series of non-containment channels are aligned in bits with the set of recurring admission limits. The method of claim 23 wherein the series of non-containment channels consume from 10% to 25% of the common frequency spectrum capacity. 25. The method of claim 20 wherein the set of predetermined sequences comprises a maximum length sequence and a plurality of permuted versions of the maximum length sequence. 26. The method of claim 20 wherein the set of recurring admission limits matches the boundaries between the data bits within the signal of the remote unit. 27. The method of r20 where the set of predetermined sequences are Walsh codes. 28. The method of claim 20 wherein the set of predetermined sequences are Walsh codes that have been concealed with a second code. 29. The method of claim 28 wherein the second code determines the spectral properties of the set of predetermined sequences. 30. The method of claim 20 wherein the demodulation step comprises the step of demodulating the signal of the remote unit using a higher order modulation scheme. 31. The method of claim 30 wherein the higher order modulation scheme is a quadrature amplitude modulation comprising at least 16 values. 32. The method of claim 20 further comprising the step of sending a power control command to the remote unit in order to determine a power level at which the remote unit transmits within approximately 1 dB of precision. 33. A method of receiving random access signals from a plurality of remote units comprising the steps of: correlating one or more of a set of predetermined sequences with a set of incoming data samples beginning at one of a set of recurring boundaries of admission to generate a set of correlated data; demodulating the correlated data set to produce a set of signal quality indications; detect a valid signal when examining the set of signal quality indications; and demodulating the valid signal using the predetermined sequence corresponding to the selected indication of the signal quality indications. 34. A method for controlling access to the system comprising the steps of: monitoring a plurality of random sequence direct sequence spectrum propagation communication channels, for random access communication signals; temporarily assigning a chosen channel of the plurality of random sequence direct sequence spectrum propagation communication channels to a particular remote unit; and giving commands to other remote units using the plurality of random sequence direct sequence spectrum propagation communication channels to temporarily refrain from using the chosen channel. 35. A remote unit that is adapted to operate in a system that includes a central station and a plurality of remote units, comprising: a process that randomly selects a first sequence from a set of predetermined sequences, a process that transmits a current of data bits modulated with a first sequence, beginning with one of a set of recurring admission limits; a process that determines whether the data bitstream has been successfully received at the central station; a process that randomly selects a second sequence from the set of predetermined sequences, if the data bit stream has been successfully received; and a process that transmits the data bit stream modulated with the second sequence starting at one of the set of recurring admission limits of the data stream that was not received successfully. 36. The system of claim 35 wherein the set of predetermined sequences comprises a maximum length sequence and a plurality of permuted versions of the maximum length sequence. 37. The system of claim 35 wherein the set of recurring admission limits matches the boundaries between data bits within the data bit stream. 38. The system of claim 35 further comprising a process that randomly selects a frequency channel over which the data bit stream modulated with the first and second sequences transmits. 39. The system of claim 35 further comprising a process that randomly delays the transmission of the data bit stream modulated with the second sequence. 40. The system of claim 35 further comprising: a process that receives a command from the central station to use a reserved sequence from the set of predetermined sequences; and a process that transmits a stream of data bits modulated with a reserved code from the set of predetermined codes, starting at one of the set of recurring admission limits.