Abstract:
Method and apparatus for maintaining synchronization in a CDMA communication system by operating closed loop timing control between a radio base unit and subscriber units, operating open loop timing control between the radio base unit and the subscriber units, and using timing information derived from the open loop to correct the closed loop. Generally the open loop is faster in response than the closed loop. The open loop timing control may be generated by using GPS data. A radio base station and a subscriber unit operating in accordance with these principles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to communications systems using CDMA (Code Division Multiple Access). More particularly it relates to the use of open-loop timing control for the reverse link of a synchronous CDMA system. 
     2. Background Art 
     Synchronous CDMA is an efficient multiple access scheme wherein users are allocated PN codes which are optimized to have a minimal cross correlation when time-aligned with each other. In order to benefit from these minimal cross correlations, these PN codes must be time aligned at the receiver. In the forward channel (radio base unit, RBU, to subscriber unit, SU, direction) it is easy to time align the codes because they are all modulated at the RBU. In the reverse direction, this is more difficult because the SUs are distributed throughout the cell and their transmissions must be coordinated so that their signals all arrive synchronously at the RBU. To accomplish this reverse channel synchronization, an initial coarse synchronization is performed (See U.S. Pat. No. 5,825,835). Next, a fine synchronization is performed using a timing control loop (see U.S. Pat. No. 5,867,525) which is a closed loop. 
     The closed-loop timing control loop operates using feedback from the RBU to the SU. The time-of-arrival (TOA) of the reverse channel signal from an SU is estimated and compared to the RBU&#39;s desired TOA. The timing error is fed back to the SU via the forward channel and the SU responds by adjusting its transmitter&#39;s timing in an attempt to drive the TOA error at the RBU to zero. Closed-loop timing control is adequate for a non-mobile synchronous CDMA system such as the Prime Wave 2000 system. However, each iteration of the closed-loop timing control loop takes a significant amount of time. As a result, if it is desirable to make the system capable of supporting mobile SUs, then the closed-loop timing control loop may be too slow to keep the users adequately synchronized. 
     In a mobile system, the range, and thus the propagation time between the RBU and SU will vary. As a result, the SU will need to adjust its transmitter clock more frequently than the closed loop is able to iterate. The Prime Wave 2000 system offers timing control updates about once every quarter of a second. Since the chip rate of Prime Wave 2000 is 2.72 Megachips/sec., the wavelength of a chip is 110 meters. This implies that a vehicle moving at a velocity of greater than 79 km/hour with respect to the RBU would see a 1/20 chip range change every quarter of a second. This would result in a change in TOA at the RBU of 1/10 chip every quarter of a second. This is probably the extreme limit of what the closed-loop timing control loop can track, and even this relative velocity is probably too great. If the SUs move at a higher rate, one possible approach is to speed up the closed-loop timing control loop. However, circuitry for achieving this result may not be available, or may be prohibitively expensive, especially for use in SUs, where cost can be a major issue. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to use information from the forward channel TOA (time of arrival) to determine the appropriate reverse channel transmitter adjustments. 
     It is another object of the invention to use a GPS receiver to provide a stable clock reference against which the received chip clock from the forward channel may be compared. 
     It is yet another object of the invention to provide a reference frame in which to measure forward channel TOA by using a GPS receiver to provide clocks to the RBU and SU. 
     In accordance with the invention, generally, an open-loop timing control loop may be used to augment the closed-loop approach. Thus, open-loop timing control may use information available at the SU to make corrections to the SU&#39;s transmitter clock. Since open-loop updates do not require feedback from the RBU, they can typically be made much more often than a closed-loop update. As a result, if open-loop corrections are used along with the standard closed-loop corrections, then an SU will be capable of remaining synchronous in a much more dynamic environment. 
     Generally, according to the invention, there is provided a method for maintaining synchronization in a CDMA communication system comprising operating closed loop timing control between a radio base unit and the subscriber unit, operating open loop timing control between the radio base unit and the subscriber units; and using timing information derived from the open loop to correct the closed loop. The open loop is faster in response than the closed loop. Information from a short time constant control loop and information from a long time constant control loop is combined to derive the open loop timing information. The open loop timing information is combined with a closed loop timing control update to provide information to correct the closed loop. The information used to correct the closed loop is provided as a time offset signal to adjust phase of a transmitter in a subscriber unit. 
     In accordance with the invention the open loop timing control may be generated by using GPS data. The data may include subscriber unit position data from a GPS receiver in the subscriber unit. The data may further include subscriber unit velocity data. Projected position data of the subscriber unit may be calculated based on the position data and the velocity data. The projected position data and the closed loop timing control update data may be used to derive a time offset signal to adjust phase of a transmitter in the subscriber unit. 
     In accordance with the invention, the radio base unit may receive GPS data. This data may be received by the radio base unit from a GPS receiver located in the radio base unit. The GPS data received by the radio base unit is synchronous with GPS data received by the subscriber unit. If the radio base unit is fixed in position, the GPS data may be stored in a memory in the radio base unit. If the radio base station is moved to a new location, new GPS data is loaded into the memory. 
     The invention is also directed to a radio base station for use in a CDMA communications system, comprising a receiver for receiving signals from subscriber units; a transmitter for transmitting signals to the subscriber units; and a source of position data associated with the radio base station for providing data concerning the position of the radio base station, the position data being transmitted by the transmitter to the subscriber units. The source of position data may be a GPS receiver or a position data memory. 
     The invention is further directed to a subscriber unit for a CDMA communications system, comprising a receiver for receiving signals from a radio base station; a transmitter for transmitting signals to the radio base station; a closed loop timing control circuit for adjusting timing of signals transmitted by the transmitter; an open loop timing control circuit for adjusting timing of signals transmitted by the transmitter; and a circuit for correcting the closed loop timing control based on information from the open loop timing control. The open loop timing control is faster in response than the closed loop timing control. 
     The subscriber unit further comprising a short time constant control loop; a long time constant control loop; and a first means for combining information from the short time constant loop and from the long time constant loop to derive the open loop timing information. 
     The subscriber unit may further comprise means for supplying a timing control update for the closed loop; and a second means for combining the timing control update with the open loop timing information to provide information to correct the closed loop. The second means may be a microprocessor. The second means may provide a time offset signal to correct the closed loop. 
     The subscriber unit may further comprise a source of GPS data; the GPS data being used to generate the open loop timing control. The source of GPS data may be a GPS receiver. It may provide GPS position data of the subscriber unit. The data may further include subscriber unit velocity data. 
     The subscriber unit may further comprise means for calculating projected position data of the subscriber unit based on the position data and the velocity data; and phase adjusting means for using the projected position data and closed loop timing control update data to derive a time offset signal to adjust phase of the transmitter in the subscriber unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of an idealized forward channel time-of-arrival augmented timing control loop in accordance with the concept of the invention. 
     FIG. 2 is a first embodiment of a forward channel time-of-arrival augmented timing control loop in accordance with the invention. 
     FIG. 3 is a second embodiment of a forward channel time-of-arrival augmented timing control loop in accordance with the invention. 
     FIG. 4 is a third embodiment of a forward channel time-of-arrival augmented timing control loop in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, each of a plurality of subscriber units  10  communicates with an RBU  12  via a communication channel  14  having a propagation delay which, as is generally the case, is dependent on range between the subscriber unit  10  and RBU  12 . The RBU modulator  16  obtains a clock signal from the RBU master oscillator  18 , which provides chip, symbol and frame clocks, and uses the clock signal to create the forward channel timing signal. 
     In the receiver of the subscriber unit  10  the forward channel timing signal is converted from analog form to digital form by an analog to digital converter  19 , which is associated with an early—late gate chip—tracking loop  20  used to track the incoming signal (assuming that an initial signal acquisition has already occurred). The above-mentioned Prime Wave 2000 systems use two direct digital synthesizers (DDS&#39;s) to create the receiver&#39;s chip clock and the transmitter&#39;s chip clock. In the receiver, the DDS function is implemented in FIG. 1 as a clock frequency-to-phase accumulator  22 A, which provides an input to a clock phase-to-amplitude converter  24 A. Chip-tracking loop  20  applies a chip frequency command to frequency-to-phase accumulator  22 A to maintain a cumulative phase register. The phase register forms an input to a look-up table in clock phase to amplitude converter  24 A which provides a phase to amplitude conversion. The amplitude command is then applied to a digital-to-analog converter (not shown), and then filtered by a low pass filter in low pass filter and comparator  26 A, where it is smoothed to provide a sine wave function which can be squared using a comparator in low pass filter and comparator  26 A. The result is a square-wave clock that is applied to the receiver&#39;s analog to digital converter  19  and to chip tracking loop  20 . 
     Since it is desired to have the transmitter of subscriber unit  10  frequency locked to the receiver with only a phase offset, a second DDS chain including clock frequency-to-phase accumulator  22 B, clock phase-to-amplitude converter  24 B and low pass filter and comparator  26 B is used. However, this transmit chain is given a phase offset command from the subscriber unit microprocessor  28 , which is obtained from a combination of the open loop and closed loop information supplied to and useable by the microprocessor. Microprocessor  28  has an input  35  for receiving a closed loop timing control update from RBU  12 . The output of an oscillator  34  (more fully discussed below) and the chip frequency command from chip-tracking loop  20  are applied to a junction  36  where they are subtracted. The difference output of junction  36  is applied to a second input  37  of microprocessor  28 . Microprocessor  28  reads the closed loop timing control update on first input  35  and the open loop timing control update on second input  37  and sums them, under the control of appropriate programming code, to produce a correlation offset timing signal which is supplied to clock frequency-to-phase accumulator  22 B. In other words the offset is the sum of old offset data and new input minus a command received from the RBU as represented by oscillator  34 . 
     Clock pulses from low pass filter and comparator  26 B are used by a modulator  30  to provide digital signals to be transmitted from SU  10  to RBU  12 . The signals are converted from digital to analog form by a digital-to-analog converter  32 . These signals, when received by RBU  12 , are demodulated by a demodulator  33 . 
     It will be understood that in FIG. 1, the channel propagation delay is a function of the transmission path length from the transmitter to the receiver. If this delay becomes larger in the forward channel (RBU to SU) then it will become larger in the reverse link (SU to RBU) as well. As a result, it is possible to measure a relative change in the time-of-arrival (TOA) of the forward channel signal and then make a correction to the reverse link transmission time based on the observed change in the forward link. As an example, if the SU receiver measures the received signal arrival time to be 1 millisecond earlier than a previous measurement, then it is reasonable to assume that the SU is closer to the RBU by a distance that corresponds to a 1 millisecond propagation time. A reasonable reaction is to then adjust the SU transmitter to transmit its signal 1 millisecond later in an attempt to maintain the same TOA at the RBU that existed at the previous measurement time. 
     Since the closed-loop portion of the timing control loop is relatively slow, the received TOA information can be used to speed up the overall timing control process. The received TOA is available as often as the SU circuitry can measure it. This is much more often than a closed loop offset can be reported to the SU. 
     The crux of making this method work is the TOA change measurement. As described above the SU has chip and carrier tracking loops that keep the SU receiver locked to the incoming signal from the RBU. To recognize a change in TOA, the SU must have a stable reference to compare the tracked signal TOA against. If it were possible to employ an ideal reference oscillator  34  that had exactly the same frequency as the RBU&#39;s transmitted signal and no phase noise, then the received signal phase could be compared against this stable reference to determine the phase offset between the two. 
     It is noted that the received signal phase could be any one of a number of clock phases. For example, the most common method would be to use the chip phase extracted from the early-late gate chip-tracking loop  20 . Since the symbol clock is usually derived from the chip clock by dividing the chip clock by the processing gain, this symbol clock can also be used to compare against the reference oscillator. Furthermore, the frame and superframe clocks are derived from the symbol clock, so these clocks may also be used as the basis of comparison with the reference oscillator. For the purposes of discussion, it is assumed that the chip clock is used to determine the TOA. 
     When the received signal/reference oscillator relative phase relationship changes, the SU determines that either the range from the SU to the RBU has changed, or phase/frequency noise between the RBU oscillator and SU reference oscillator has caused the relative phase shift. If it is assumed, as in FIG. 1, that both the reference oscillator in the SU and the RBU master oscillators are perfect, in the sense that they have no phase noise, then the operation of this idealized loop becomes easy to visualize. Due to the lack of phase noise and frequency errors, the TOA estimated with the chip-tracking loop can be compared directly with the reference oscillator. Any change in the forward channel TOA will be illustrated by a phase change between the received signal and the reference oscillator. This will imply directly that the propagation time through the channel  14  has changed and that a corresponding and opposite change is required in the SU transmitter to keep the TOA at the RBU the same as it was before the propagation delay changed. 
     One subtle issue is that the open-loop timing control must be significantly faster than the closed-loop timing control so that an open-loop adjustment does not destabilize the closed-loop timing control. Typically this open-loop adaptation will be several times faster than the closed-loop adaptation. However, it will be understood that the long time constant loop will begin changing its timing due to the relative phase change caused by a change in the short time constant loop, which by comparison will appear to be a step function change in timing. It is necessary to take this slow change into account in making subsequent corrections. 
     The approach illustrated in FIG. 1 is not realistic because any real oscillator will have a frequency offset from the RBU and phase noise. FIG. 2 illustrates a method of approximating the ideal system of FIG.  1 . 
     In FIG. 2 the ideal reference oscillator ( 34  of FIG. 1) is replaced with a chip-tracking loop  39 A with a long time constant. There is also a chip tracking loop  39 B with a short time constant. The output of chip tracking loop  39 A and the output of chip tracking loop  39 B are summed in junction  36 A. The goal of any tracking loop is to remain either frequency locked or frequency and phase locked to an incoming signal. The time constant of the loop determines its ability to follow short-term fluctuations of the incoming signal. A chip phase tracking loop with a long time constant will track the average frequency and phase of the received signal, but will be slow to respond to short term changes in phase. As a result, this long-time-constant loop provides a reference that only follows the long-term variations of the incoming signal. If this loop&#39;s time constant is longer than the time constant of the closed-loop timing control loop, then the variations of the long-time-constant loop will be tracked by the closed-loop timing control loop. Any changes in the range between the SU  10  and the RBU  12  which are too fast for the closed-loop timing control loop to track create relative phase offsets between the long-time-constant loop and the short-time-constant loop. These relative phase changes provide information to the SU microprocessor  28  that is used to make open-loop corrections to the SU transmitter&#39;s phase. 
     FIG. 3 illustrates another system and method of achieving open loop timing control. A GPS receiver  40  is used to produce a clock that serves as a reference against which the forward channel clock is compared. The GPS clock must be frequency locked to the forward clock to permit the SU  10  to be able to recognize phase shifts. The RBU clocks must also be derived from the same GPS reference to insure that the RBU reference and SU reference clocks are frequency locked. Thus, the RBU has a GPS receiver  42  which produces the reference for the forward channel clock, and the GPS receiver  40  in SU  10  provides the reference to compare the received clock against. Thus, any drift in these clocks will be due to either short-term phase jitter of the GPS clocks, the synthesizers that use the GPS references or a range change between RBU  12  and SU  10 . For some applications, the GPS phase jitter will be too large to allow the relative phase changes between the received forward channel clock and the SU reference clock to be attributed to a range change. In contrast, if the GPS clocks are averaged sufficiently (in other words tracked with a phase lock loop with a large time constant), then the presence of the same reference at both ends of the link will permit reasonable operation. 
     The system and method of FIG. 3 can be viewed as an alternative way to achieve the ideal system illustrated in FIG.  1 . Since it is not possible to use a stand-alone oscillator that is frequency locked to the RBU stand-alone oscillator, instead the same clock reference is supplied to both sides of the link using GPS. This method has the disadvantage of requiring the addition of a GPS receiver to the SU. However, it has the advantage of being able to provide a perfectly frequency locked pair of references to each side of the link. 
     Referring to FIG. 4, in yet another embodiment of the invention, the position and velocity estimates provided by the GPS unit in the SU is used to predictively estimate the change in the transmission time that is required to maintain synchronization at the base station. To do this, the SU must also have information concerning the location of the RBU. The RBU can transmit its latitude/longitude coordinates to any SU establishing communications with that RBU. Thus, a range estimation algorithm is used to calculate the path length change for the forward and reverse link. This algorithm may be implemented in software within the SU microprocessor  28 . This algorithm calculates the current position of the SU relative to the RBU, or predictively calculates where the SU is likely be at some future time using the velocity and direction information from the GPS receiver. 
     This embodiment has the advantage of not requiring a GPS receiver to be permanently in place in the RBU, provided that its position remains fixed, as is almost always the case. Instead, the GPS coordinate data may simply be loaded into a position data memory  44  so that it is available for transmission to the SU&#39;s in the cell. Loading can be accomplished using a mobile GPS receiver or another source of position data, at the time of installation of the RBU. New data is necessary, if the RBU is relocated.