Abstract:
Time alignment of a signal from a remote unit at a hub station of a multiple access system is achieved based upon an initial time indication received from the hub station at the remote unit via a satellite. The remote unit transmits a signal to the satellite, monitors for a retransmission of the signal from the satellite and measures the time difference between the outgoing and incoming signals. The remote unit, then, uses the time difference to more finely adjust the time alignment. Alternatively, the remote unit transmits a first signal advanced with respect to an on-time estimate to the hub station and receives a responsive energy indication from the hub station. The remote unit transmits a second signal delayed with respect to the on-time estimate to the hub station and receives a responsive energy indication from the hub station. The remote unit compares the two energy indications and adjusts the on-time estimate. In another embodiment, the remote unit receives a compensated time indication from the hub station. The remote unit compares the time indication with the time at which the indication was actually received based upon a local, accurate time reference. The remote unit sets a transmission timer equal to the current time indicated by the local reference advanced by the difference between the time indication and the time which the time indication was received.

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 60/095,341, entitled METHOD AND APPARATUS FOR TIME SYNCHRONIZATION IN A COMMUNICATION SYSTEM, filed Aug. 4, 1998, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to communications. More specifically, the invention relates to synchronization in a wireless communication system. 
     II. Description of the Related Art 
     The wireless medium is becoming one of the predominate means by which voice information and digital data are transferred in modern society. In wireless communication systems, it is common that a plurality of remote units communicate with a single hub station. In order to combat the harsh wireless environment while providing multiple access to a plurality of remote units, modulation schemes which provide efficient data transfer, as well as user channelization, have been developed. In general, these modulation schemes operate most efficiently when the signal from each remote unit arrives at the hub station with a synchronized timing. For example, in a time division multiple access (TDMA) system, each remote unit is assigned a time slot in which to send its data. If the timing of one remote unit is misaligned with respect to the timing from another remote unit, the transmission from one remote unit may overlap in time the transmission of another, thereby, yielding both transmissions unusable. In a typical code division multiple access (CDMA) system, the remote unit signals are distinguished through the use of nearly orthogonal pseudo random codes. If the transmission from one remote unit becomes misaligned with the transmission from another remote unit, the orthogonal nature of the pseudo random codes is degraded and the signals may significantly interfere with one another. Therefore, in both of these types of systems, it is necessary that the remote units operate synchronously with one another. 
     Therefore, it is desirable to provide an effective method for synchronizing plurality of remote units communicating with a common hub station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objectives, and advantages of the invention will become apparent from the detailed description set forth below when taken in conjunction with the drawings wherein like parts are identified with like reference numeral throughout, and wherein: 
     FIG. 1 is a block diagram showing a satellite-based communication system which operates in accordance several embodiments of the invention. 
     FIG. 2 is a flowchart illustrating time synchronization using a satellite feedback signal. 
     FIG. 3A is a graph showing the energy received at the hub station as a function of the time offset used by the hub station to demodulate a remote unit signal. 
     FIG. 3B is a graph showing the energy received at the hub station as a function of the time offset used by the hub station to demodulate a remote unit signal when the on-time alignment is skewed to be slightly late of the ideal timing. 
     FIG. 4 is a flowchart showing remote unit operation in a system using transmissions offset in time to determine time alignment of a remote unit. 
     FIG. 5 is a block diagram illustrating a wireless system incorporating time synchronization using a global positioning system (GPS) reference. 
     FIG. 6 is a flow chart showing exemplary hub station operation using GPS to achieve synchronization. 
     FIG. 7 is a flowchart showing remote unit operation in a system using GPS to achieve synchronization. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a system in which a plurality of remote units communicate with a common hub station over a satellite link, the delay incurred by the remote unit-to-hub station signals as they traverse the satellite link is a function of the position of the remote unit. Because the remote units may be located throughout a large geographic region, the delay incurred by the remote unit-to-hub station signals is different for the various remote units. Most modulation and multiple access schemes operate most efficiently when the remote unit signals arrive at the hub station with a common time alignment. The invention provides a means and method for aligning the timing of a remote unit-to-hub station signal. 
     FIG. 1 is a block diagram showing a communication system comprising a wireless satellite link. A hub station  20  transmits hub station-to-remote unit signals to a geosynchronous satellite  30  over a forward uplink channel  26 . The satellite  30  receives the hub station-to-remote unit signals on the forward uplink channel  26  and amplifies and retransmits the signals over a forward downlink channel  32 , typically at a different frequency than at which they were received. 
     The hub station-to-remote unit signals transmitted over the forward downlink channel  32  are received by a remote unit  40 . A receiver  42  within the remote unit  40  receives and demodulates one or more of the signals on the forward downlink channel  32 . The receiver  42  produces digital data bits for a processor  46 . The processor  46  also generates reverse link information bits and passes a corresponding data signal to a transmitter  44 . The transmitter  44  transmits a corresponding remote unit-to-hub station signal on a reverse uplink channel  34  to the satellite  30  using a time sensitive modulation or channelization technique. The transmitter  44  transmits the signal with reference to a transmission clock. In one embodiment, the transmission clock is generated by a time alignment module  48 . The satellite  30  typically amplifies the remote unit-to-hub station signals and retransmits the remote unit-to-hub station signals at a different frequency over a reverse downlink channel  24  to the hub station  20 . 
     The remote unit  40  shares the satellite and hub station resources with a plurality of other remote units  42 A- 42 N. The multiple access scheme by which the remote unit  40  and the remote units  42 A- 42 N share the reverse link channels  24  and  34  operates most efficiently if the remote unit-to-hub station signals arrive at the hub station  20  with a common timing. When the remote unit  40  transmits a signal to the hub station  20  via the satellite  30 , the signal experiences a delay of approximately 270 milliseconds. The cumulative forward link and reverse link transmission delay is different for each remote unit and varies as a function of time. Specifically, the transmission delay is a function of the location of the remote unit in relation to the satellite. Typically those remote units which are closest to the satellite experience the lowest transmission delay and those remote units that are furthest from the satellite experience the longest transmission delay. However, other factors may influence the delay. For example, if the remote unit is located within a structure, additional delay may be added into the transmission path. In addition, the satellite is not perfectly stationary relative to the surface of the earth. The transmission delay, thus, varies due to the relative movement of the satellite. For these reasons, even if an initial synchronization between the remote unit  40  and the remote units  42 A- 42 N is achieved, the synchronization may be lost over time if a correction mechanism is not available. 
     Notice that the delay of the reverse downlink channel  24  is the same for all remote units  40  and  42 . Therefore, if the remote unit-to-hub station signals arrive in time synchronization at the satellite  30 , they also arrive at the hub station  20  in time synchronization. 
     In one embodiment of the invention, synchronization is initially achieved and periodically updated based upon the remote unit  40  monitoring a feedback signal retransmitted by the satellite  30 . The feedback signal is the remote unit&#39;s  40  signal transmitted by the satellite  30  over the reverse downlink channel  24 . FIG. 2 is a flow chart showing an exemplary embodiment of such a system. Flow begins in block  50  when the remote unit  40  transmits a reverse link signal over the reverse uplink channel  34 . Initially, the remote unit  40  uses a time indication received from the hub station  20  over the satellite  30  to determine a first time alignment estimation. The remote unit  40  monitors the reverse downlink channel  24  in order to detect its own transmitted signal in block  52 . The reverse downlink channel  24  may operate at a different frequency than the reverse uplink channel  34  or the forward downlink channel  32 . For this reason, the remote unit may comprise two receivers. Alternatively, the remote unit may comprise a single receiver which is time multiplexed between the different frequencies, such as the receiver  42  within the remote unit  40  of FIG.  1 . 
     Although the signal level received by the remote unit  40  from the reverse downlink channel  24  may be extremely small, the remote unit  40  can detect it because the remote unit  40  knows the data values which were transmitted which greatly facilitates the detection process according to well known techniques of signal detection based on reception of a signal with known characteristics. In addition, the remote unit  40  can integrate the received power over a relatively long period of time in order to increase the cumulative power received at the remote unit  40 . In block  54 , the remote unit  40  determines the transmission delay between the remote unit  40  and the satellite  30  by comparing time at which the remote unit  40  transmitted the signal on the reverse uplink channel  34  to the time at which the remote unit  40  received its signal on the reverse downlink channel  24 . For example, in FIG. 1, the time alignment module  48  performs these functions. 
     In order to synchronize the remote units  40  and  42  to a common time, the hub station  20  periodically sends a timing indication according to any one of a myriad of well known techniques. For example, in one embodiment, the hub station  20  transmits a pilot signal. In another embodiment, the hub station  20  transmits a first signal which indicates that an up-coming transmission will signify the occurrence of a pre-determined absolute time. For example, the signal transmitted by the hub station  20  may indicate that at the next forward link data boundary, the time will be exactly 5:02:00.00000 a.m. 
     In block  56 , the remote unit  40  receives the time indication from the hub station  20 . When the remote unit  40  receives the indication signal from the hub station  20 , the signal has been delayed by the sum of the transmission delay over the forward uplink channel  26  and the forward downlink channel  32 . In block  58 , the remote unit  40  sets the transmission clock equal to the time indication advanced by the determined transmission delay. In one embodiment, these functions are performed by the time alignment module  48  within the remote unit  40 . In block  59 , the remote unit  40  uses the time indication adjusted according to the determined transmission delay to transmit its signal to the satellite  30 . In this way, each remote unit signal arrives at the satellite  30  having a common timing. The satellite  30  retransmits the time synchronized signals to the hub station  20  where they can be demodulated. Because the delay of the reverse uplink channel  24  is the same for all remote units  40  and  42 , the remote unit signals are also synchronized upon arrival at the hub station  20 . The embodiment shown in FIG. 2 has the advantage that no changes need to be made to the hub station  20  or the satellite  30  in order to implement such a system if the hub station  20  is already supplying an indication of time 
     In an alternative embodiment, synchronization may be achieved by the transmission of a series of time offset signals from the remote unit  40 . FIG. 3A is a graph showing the energy detected at the hub station  20  as a function of the time offset used by the hub station  20  to demodulate a remote unit signal. In FIG. 3A, the vertical axis represents the energy detected by the hub station  20  and the horizontal axis represents the time offset used by the hub station  20  to demodulate the signal. When the hub station  20  demodulates the signal with ideal synchronization at an ideal on-time alignment t o , the hub station  20  detects the maximum energy available from the remote unit signal as shown by data point  60  on FIG.  3 A. If the hub station  20  demodulates the remote unit signal using a timing which is delayed by a time offset δ t  from the ideal on-time alignment t o  to a late time alignment t l , the hub station  20  detects less energy as shown by data point  64  on FIG.  3 A. In a like manner, if the hub station  20  demodulates the remote unit signal using a timing which is advanced by the time offset δ t  from the ideal on-time alignment t o  to an early time alignment t e , the hub station  20  detects less energy as shown by data point  62  in FIG.  3 A. So long as the early and late alignments are offset from the on-time alignment by the same amount of time and the on-time alignment is ideal, the energy detected at the early and late alignments is the same. 
     FIG. 3B is a similar diagram to FIG. 3A except that an on-time alignment t o ′ has been skewed to be slightly late of the ideal timing. Notice that due to the offset, the amount of energy detected at data point  66  is less than that detected in the ideal case at data point  60 . If the hub station  20  demodulates the remote unit signal at time offset δ t  earlier than the on-time alignment t o ′ at the early time alignment t e ′ in FIG. 3B as shown by data point  68 , the hub station  20  detects more energy than at data points  62  and  64  of FIG.  3 A. Likewise, if the hub station  20  demodulates the remote unit signal at an offset delayed by time offset δ t  from the on-time alignment t o ′ at the late time alignment t l ′ as shown by data point  70 , the hub station  20  detects less energy than at data points  62  and  64  in FIG.  3 A and also data point  68  in FIG.  3 B. By comparing the energy detected by the hub station  20  at an early time alignment and a late time alignment, it is possible to determine whether the on-time alignment is ideally aligned. If the early and late time alignments yield the same energy level, the hub station  20  is likely to be detecting the signal with an accurate time alignment. If an energy level detected at the early alignment is significantly higher than the energy level detected at the late alignment, the hub station  20  is likely to be detecting the signal with an alignment delayed from the ideal. If an energy level detected at the late alignment is significantly higher than the energy level detected at the early alignment, the hub station  20  is likely to be detecting the signal with an alignment advanced from the ideal. 
     In one embodiment, the remote units and the hub station collectively implement a time alignment adjustment process using those principals illustrated in FIGS. 3A and 3B. According to the embodiment, the remote unit  40  sends a delayed and an advanced signal to the hub station  20 . The hub station  20  detects the two offset signals using a common time alignment. The hub station  20  measures the energy, power level or other signal quality indication of the signal received at each offset and notifies the remote unit  70 . Based upon the value of the detected signal quality, the remote unit  40  determines whether the current on-time alignment should be advanced or retarded. Alternatively, the hub station  20  may perform the comparison of the signal quality and return an advance or retard command. 
     FIG. 4 is a flow chart illustrating the steps performed by the remote unit  40  to implement the time alignment adjustment process just described. In block  80 , the remote unit  40  sends a signal to the hub station  20  advanced from a current on-time indication by a time offset δ t ′. In response to receipt of the signal, the hub station  20  sends the remote unit  40  an indication of the signal quality at which the signal was received. For example, the hub station  20  may send a power adjustment command, a relative power or energy measurement, or an absolute power or energy measurement value. In block  82 , the remote unit  40  receives the signal quality indication from the hub station  20 . In block  84 , the remote unit  40  sends a signal retarded from a current on-time indication by time offset δ t ′ to the hub station  20 . Once again, the hub station  20  responds by sending the remote unit  40  an indication of the signal quality at which the signal is received. In block  86 , the remote unit  40  receives the signal quality indication from the hub station  20 . In block  88 , if the signal quality received by the hub station  20  corresponding to the early time offset is better than the signal quality received by the hub station  20  at the late time offset, the remote unit  40  advances the current on-time indication. In block  90 , if the signal quality received by the hub station  20  at the late time offset is better than the signal quality received at the hub station  20  at the early time offset, the remote unit  40  retards the current on-time indication. 
     The signal transmitted by the remote unit  40  at the early and late time offsets may be a signal that carries user data to the hub station  20 . Alternatively, the signal may be a special overhead or dummy signal which carries no user data. In yet another embodiment, the early and late signals may be transmitted on a different channel than the user data. For example, in a TDMA system, the time alignment may be tested using a different time slot than the one used for the data. In a CDMA system, the timing signals may be transmitted using a different code or offset than the user data signals. 
     FIG. 5 is a block diagram illustrating time synchronization using a global positioning system (GPS) reference. In the system shown in FIG. 5, the hub station  20 ′ and each remote unit  40 ′ and  42 ′ has access to a GPS reference (such as generated by a commercially available GPS receiver) which supplies an extremely accurate indication of current time. The hub station  20 ′ periodically sends a time indication signal based upon the timing signal produced by the GPS reference. In one embodiment, for example, the hub station  20 ′ transmits a first signal which indicates that a future time tag transmission will signify an occurrence of a predetermined absolute time T tag . The hub station  20 ′ sends the time tag transmission in advance of the actual occurrence of the tag time in an attempt to compensate for forward link transmission delays. The amount by which the time tag transmission is advanced is the current forward link delay estimate. 
     The hub station  20 ′ is coupled to a local remote unit  42 ″. The remote unit  42 ″ receives the time tag transmission at time t cal  over the forward downlink channel  32 . The remote unit  42 ″ uses a GPS reference to determine the value of time t cal . The remote unit  42 ″ may also receive a priori knowledge from the hub station  20 ′ regarding the data within or other known characteristic of the transmission in order to more efficiently detect the signal. 
     The time offset between the time of receipt of the time tag transmission t cal  and the predetermined absolute time T tag  indicates the error in estimating the forward link delay associated with forward link channels  26  and  32  as perceived by the remote unit  42 ″. Based upon this difference, the hub station  20 ″ can increase or decrease the current value of the forward link delay estimate in an attempt to align time T tag  to time t cal  at the remote unit  42 ″ for future transmissions. In this way, the hub station  20 ′ transmits the time tag transmission offset in time from the actual occurrence of the indicated time T tag  by the forward link delay estimate. 
     In FIG. 5, the time tag transmission is received by the local remote unit  42 ″. In one embodiment, the remote unit  42 ″ may be a scaled-down remote unit which does not comprise all of the functionality of a standard remote unit. The advantage of using a remote unit  42 ″ is that any delay inherently associated with the remote unit  42 ″ is likely to be similar to that exhibited by the remote units  40 ′ and  42 ′, thus, increasing the accuracy of the forward link delay estimation. In another embodiment, the hub station  20 ′ may incorporate a receiver to receive the time tag transmission. 
     The remote unit  40 ′ receives the time tag transmission from the hub station  20 ′ at time t ru . The time offset between the time t ru  of receipt of the time tag transmission and the time T tag  indication in the message indicates the error in estimating the forward link delay associated with forward link channels  26  and  32  as perceived by the corresponding remote unit  40 ′. The remote unit  40 ′ compares the time t ru  at which the timing indication was received to the time T tag  indication in the message. Because the hub station  20 ′ has already compensated for the forward link delay to some degree, the remote unit  40 ′ perceives only that error due to the difference between the forward link path from the satellite  30  to the remote unit  40 ′ and the forward link path from the satellite  30  to the remote unit  42 ″. The remote unit  40 ′ sets a current time equal to the time indication received from the GPS advanced by the twice time offset between t ru  and T tag  to compensate for both the downlink and uplink delay. 
     FIG. 6 is a flow chart showing exemplary hub station  20 ′ operation, including operation of the local remote unit  42 ″, using a GPS reference. In block  100 , the hub station  20 ′ receives an indication of actual time from the GPS reference. In block  102 , the hub station  20 ′ transmits a time tag transmission. The hub station  20 ′ transmits the time tag transmission offset from the actual time by a current forward link delay estimate. In block  104 , the local remote unit  42 ″ associated with the hub station  20 ′ receives the signal at time t cal , determines the value of t cal , and passes the value of time t cal  to the hub station  20 ′. The local remote unit  42 ″ can determine the value of time t cal  based upon the GPS reference. In block  106 , based on the time t cal , the hub station  20 ′ changes the time by which the time tag transmission is advanced relative to the actual time in order to minimize the difference between time t cal  and time T tag  for future transmissions. For example, in one embodiment, the hub station  20 ′ comprises a time alignment module (such as the time alignment module  48  within the remote unit  40  in FIG. 1) which performs the functions of blocks  106 . 
     FIG. 7 is a flowchart showing remote unit operation in a system using the above-described GPS method. In block  110 , the remote unit  40 ′ receives the time tag transmission at time t ru . The remote unit  40 ′ can determine the value of time t ru  based upon the GPS reference. In block  112 , the remote unit  40 ′ compares time t ru  to the tag time T tag . In block  114 , the remote unit  40 ′ transmits a signal with a transmission time equal to the actual time received from the GPS reference advanced by the twice the time offset between time t ru  and time T tag  to compensate for both the downlink and uplink delay. 
     Although the invention was described herein with reference to a satellite system, the invention may be embodied in a variety of environments comprising a wireless link including terrestrial based systems. For example, the satellite link may be replaced with a terrestrial repeater. The embodiments disclosed above may be combined with one another. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiment is to be considered in all respects only as illustrative and not as restrictive and the scope of the claim of the invention is, therefore, indicated by the appended claims rather than by the foregoing descriptions. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.