WO2000031901A1 - Transmit timing control messages rate reduction in a communication system by using the rate of change of a timing information - Google Patents

Abstract

A method, and related apparatus, for reducing transmit timing control messages in a communications system where there is relative motion between a first communications terminal and a second communications terminal. The method consists of reducing transmit timing control messaging sent by a first communications terminal to a second communications terminal by establishing a communications link between the first communications terminal and the second communications terminal. Then transmitting a transmit timing control message from the first communications terminal to the second communications terminal. Then periodically setting a transmit timing of the second communications terminal in response to the transmit timing control message. The transmit timing control message defines a timing rate-of-change due to the relative motion between the first communications terminal and the second communications terminal.

Description

TRANSMIT TIMING CONTROL MESSAGES RATE REDUCTION IN A COMMUNICATION SYSTEM BY USING THE RATE OF CHANGE OF A TIMING INFORMATION

TIMING ADJUSTMENT RATE REDUCTION USING RANGE RATE INFORMATION

This patent document claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Serial No. 60/109,749 of Parr, et al.; entitled TIMING ADJUSTMENT RATE REDUCTION USING RANGE RATE INFORMATION; filed November 24, 1998, which U.S. Provisional Patent Application is incorporated herein by reference.

BACKGROUND OP THE INVENTION

The present invention relates to mobile communications systems, and more particularly to mobile satellite communications systems. Even more particularly, the present invention relates to accounting for delay seen by the mobile satellite communications system due to relative motion between a satellite and a terminal. In mobile communications systems, and in particular mobile satellite communications systems, it is common to experience relative motion between the satellite/s and the terminals. The motion may be due to the velocity of the satellite as it orbits the Earth and the terminal moving at a velocity, such as when the terminal is operated in a moving car. As this relative motion occurs, delay experienced by signals traveling between the satellite and the terminal varies as the satellite follows a pattern, e.g. approximately a sinusoidal pattern, due to satellite motion in relation to a stationary terminal. This variation in relative movement must be accounted for by the mobile communications system in some way in order to avoid degradation in the performance and to prevent collision of signals.

By way of example, if a satellite and a terminal are moving closer to each other, then a signal transmitted from the terminal to the satellite will arrive earlier and earlier at the satellite as the signal is compressed by this relative movement. Soon the signal will arrive outside an acceptable timing window, resulting in poor system performance and/or possible collisions with other signals sent to the satellite.

One way to compensate for this problem is to send a control message, or a transmit timing control message, from the satellite to the terminal to adjust the timing of the terminal by a set amount. Then the terminal is able to transmit a signal within an acceptable timing window of the satellite.

However, such control messages have to be sent at regular intervals to continuously put the timing of the terminal within an acceptable window for the satellite. Thus, such control messages have to be sent before the timing of the terminal degrades beyond the acceptable window of the satellite. Unfortunately, such control messages occupy bandwidth during communication and thus, consume valuable communication resources. Additionally, such control messages have the potential to disrupt a communication from the terminal if the control message is sent during a "talk-spurt". (This results typically in degradation of the quality of the sound from the user terminal's handset). The greater the relative motion between the satellite and the terminal, the more control messages will have to be sent within a given amount of time, increasing the bandwidth used by the messages and increasing the likelihood of disrupting a communication.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as well as other needs by providing a method, and related apparatus, for reducing transmit timing control messages sent in a mobile satellite communications system where relative motion exists between a satellite and a terminal. In one embodiment, the present invention can be characterized as a method of reducing transmit timing control messaging sent by a first communications terminal to a second communications terminal by establishing a communications link between the first communications terminal and the second communications terminal . Then transmitting a transmit timing control message from the first communications terminal to the second communications terminal. Then periodically setting a transmit timing of the second communications terminal in response to the transmit timing control message.

In another embodiment, the present invention can be characterized as a method of reducing transmit timing control messages in a communication system by establishing a communications link between a first communications terminal and a second communications terminal, then deriving a timing rate-of-change from a received transmit timing control message from the first communications terminal to the second communications terminal . Once the timing rate-of-change is derived, periodically setting the transmit timing of the second communications terminal in response to the timing rate-of-change. In a further embodiment, the invention can be characterized as a method of adjusting a transmit timing of a communications terminal by receiving a transmit timing control message containing a timing rate-of-change. Then periodically adjusting the transmit timing of a transmitter of the communications terminal in response to the timing rate-of-change.

In an additional embodiment, the invention can be characterized as a communications system that reduces transmit timing control messages sent between communications terminals. The communications system having a first communications terminal including a transmitter for transmitting a transmit timing control message, a second communications terminal including a receiver for receiving the transmit timing control message. A timing delay compensator is coupled to the receiver and a transmitter of the second communications terminal is responsive to the transmit timing control message. The timing delay compensator periodically sets a transmit timing of the transmitter in response to the transmit timing control message. And a communications link is established between the first communications terminal and the second communications terminal.

It is an additional feature of the invention to describe a communications terminal that reduces transmit timing control messages sent in a communications system. The communications system contains a receiver located in the communications terminal for receiving a transmit timing control message, a timing delay compensator coupled to the receiver and a transmitter for deriving a timing rate-of-change in response to the transmit timing control message. The timing delay compensator periodically sets a transmit timing of the transmitter in response to the timing rate-of-change.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 is a schematic view of a satellite-based mobile communications system in accordance with the present invention;

FIG. 2 is a block diagram of a communication terminal as shown in FIG. 1 in accordance with the present invention; FIG. 3 is a block diagram of a portion of the time delay compensator show in FIG. 2 that determines the transmit start time in accordance with the present invention;

FIG. 4 is flowchart of the steps for a gateway station of the mobile communications system of FIG. 1 to define the timing rate of change of another communications terminal ; FIG. 5 is a flow chart of steps traversed by the communications terminal of FIG. 2, to effect the communications terminal ' s transmit timing in response to a transmit timing control message containing a frequency offset parameter;

FIG. 6 is a flow chart of the steps for frequency offset estimation performed by a digital signal processor at the gateway station of the mobile communications system of FIG. 1; and FIG. 7 is a block diagram of a frequency tracking loop used in a digital signal processor when estimating a frequency offset.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

Referring first to FIG. 1, a schematic view of a satellite-based mobile communications system 100 is shown.

A communications terminal or gateway station 104, having an antenna 106 is located on the Earth 102. A communications terminal or access terminal 110, having an antenna 112 is connected to a satellite 108 through a communications link 114. The satellite 108 is connected to the gateway station 104 having an antenna 106 through a communications link 116. The access terminal 126 having an antenna 128 is shown at time t later. The satellite 130 is also shown at time t later. The access terminal 126 is connected to the satellite 130 through communications link 132. The satellite 130 is connected to the gateway station 104 through communications link 134. Access terminal 110 has a velocity vector 118 and the component of the velocity vector 120 in the direction of the satellite 108 while the access terminal 126 at time t later has a velocity vector 136 and the component of the velocity vector 138 in the direction of the satellite 130. Additionally, the satellite 108 has a velocity vector 122 and the component of the velocity vector 124 in the direction of the access terminal 110 and the satellite 130 at time t later has a velocity vector 140 and the component of the velocity vector 142 in the direction of the access terminal 126.

The satellite 108 experiences motion with a certain velocity, as indicated by velocity vector 122. The satellite 108 also experiences motion relative to the access terminal 110, indicated by the component of the velocity vector 124 in the direction of the access terminal 110. The access terminal 110 may also experience motion, as indicated by the velocity vector 118. The access terminal's motion relative to the satellite 108 is indicated by the component of the velocity vector 120 in the direction of the satellite 108.

The velocity vector 140 of the satellite 130 at time t later and the component of the velocity vector 142 in the direction of the access terminal 126 may be different. The velocity vector 136 of the access terminal

126 at time t later and the component of the velocity vector 138 in the direction of the satellite 130 at time t later may also be different due the change of motion of the access terminal 126. This may be due to the access terminal 126 being located within a car that changes direction or stops.

FIG. 1 illustrates the relative motion of the satellite 108 and the access terminal 110. Due to the relative motion between the satellite 108 and the access terminal 110, the signals sent from the access terminal 110 through the communications link 114 take longer and longer to reach the satellite 108. The satellite 108 expects the signals to be received within a certain timing window. The relative motion may cause the signal to arrive outside of the timing window, potentially disrupting the communication or interfering with other signals.

Consider a satellite 108 in geosynchronous orbit, with some north-south motion as seen from the Earth 102. For a stationary observer on the Earth 102, the speed of the satellite 108 could reach about 200 km/hr. This corresponds to about 0.2 μs/sec change in the timing delay, or timing rate-of-change, between the satellite 108 and the access terminal 110. Assume that the access terminal 110 uses the signaling from the satellite 108 to provide a basis for its own timing and frequency reference. The access terminal 110 will then transmit to the satellite 108 incurring a further 0.2 μs/sec timing rate-of-change, yielding a 0.4 μs/sec timing slew as seen by the satellite 108. If the access terminal 110 is moving, then the total timing rate-of-change can approach 0.8 μs/sec for vehicular environments.

A typical satellite-based communications system might have a symbol duration of 50 μs. For such systems, adjustments to transmit timing, sent in conventional transmit timing control messages, would typically attempt to adjust transmit timing at least 10 times per symbol, or once every 5 μs change in delay. Thus, the acceptable timing window is 5 μs in length. For a stationary target, this adjustment rate would imply an adjustment every 12.5 seconds (i.e. 5 μs/0.4 μs/sec), and as often as about every 6 seconds (i.e. 5 μs/0.8 μs/sec) for a moving access terminal 110.

In order to conserve bandwidth, the timing adjustment messages (i.e. conventional transmit timing control messages) are sent using "in-band" signaling. Thus, the timing adjustment messages have the potential to disrupt the payload traffic, e.g., voice. This may result in degradation of the quality of the sound during the communication. Interruptions of this type should be minimized. Once every 12.5 seconds or less is unreasonable. Furthermore, it is desirable to transmit the timing control messages after a talk spurt, but it is difficult to send such a message after a talk spurt within such a short period of time. The present invention as described overcomes these problems by defining a timing rate-of-change.

The timing rate-of-change represents the rate at which the timing error or timing delay is changing due to the relative motion of the satellite 108 and the access terminal 110. The timing rate-of-change is defined by a transmit timing control message sent from the gateway station 104 to the access terminal 110. Once the gateway station 104 estimates the timing rate-of-change between the satellite 108 and the access terminal 110, the information is sent to the access terminal 110.

In response to the received timing rate-of- change, the access terminal 110 is able to periodically adjust it's own transmit timing at the rate specified by the timing rate-of-change. Thus, the transmit timing of the access terminal 110 will be adjusted by the access terminal itself to correct for the relative motion between the satellite 108 and the access terminal 110. If the estimate of the timing rate-of-change is accurate and does not change, then the timing at the satellite 108 will always be within the acceptable timing window. If the estimate is not accurate or the estimate changes, then further transmit timing control messages defining an updated timing rate-of-change will need to be sent; however, less frequently than a conventional transmit timing control message that defines only a timing adjustment.

The timing rate-of-change information is proportional to the frequency offset as seen at the satellite 108. The frequency offset is primarily due to Doppler in a system with relative motion present. For example, assume a carrier frequency of a signal sent through the communications link 114 in the downlink (from the satellite 108 to the access terminal 110) is 1.5 GHz. Further assume the carrier frequency of a return signal in the uplink (from the access terminal 110 to the satellite 108) is 1.6 GHz. With a total 0.4 μs/sec rate of change of timing as described above, the Doppler frequency offset in the downlink will be 300 Hz (1.5 GHz * 0.2 μs/sec) and the Doppler frequency offset in the uplink will be 320 Hz (1.6 GHz * 0.2 μs/sec), yielding a total Doppler frequency offset as seen by the satellite 108 of 620 Hz. The timing rate-of-change is proportional to the Doppler frequency offset by using the formula:

where dT_d/dt is the timing rate-of-change between the satellite 108 and the access terminal 110 in seconds/second, f_d is the frequency offset due to Doppler in Hertz between the satellite 108 and the access terminal 110, and f_cup is the uplink carrier frequency in Hertz. Thus, with knowledge of the Doppler frequency offset as seen by the satellite 108, the uplink carrier frequency from the access terminal 110 back to the satellite 108, a timing rate-of-change between the satellite 108 and the access terminal 110 can be determined.

As an example, if the Doppler frequency offset is estimated as 620 Hz (see above) , and the carrier frequency in the uplink is 1.6 GHz, then the timing rate- of-change between the satellite 108 and the access terminal 110 is approximately 387.6 ns/sec (620 Hz/1.6 GHz) .

In deriving the above formula, the relationship between the timing changes and frequency offsets depends on the architecture of a particular satellite communications system. The above stated relationship in Eq. (1) holds true in a satellite-based communications system having the following characteristics. First, all communications between the satellite 108 and the access terminal 110 are synchronized at the satellite 110. This means that the carrier frequencies of signals received from the satellite 108 and the signals transmitted to the satellite 108 are the defined carriers for the system, i.e. these carrier frequencies are not adjusted to compensate for Doppler or other frequency offsets at the satellite 108 input or output. Furthermore, the timing of the transmitters and receivers of the satellite are fixed in that transmissions from the access terminals 110 and the gateway station 104 must adjust their timing to ensure synchronization at the satellite 108.

Second, the access terminal 110 uses the received carrier frequency of signals received from the satellite 108 as its reference. Thus, any Doppler frequency offset or other frequency offset will be reflected in the received carrier signal, and thus, the reference frequency of the access terminal. And, third, the transmit frequency of the access terminal 110 is defined by (a) a multiplicative relationship to the received carrier frequency of signals from the satellite 108 (i.e. the received carrier frequency at the access terminal 110 is used to derive the transmit frequency of the access terminal 110) , and (b) optionally, a transmit timing offset conventionally provided to the access terminal 110 from the gateway station 104 in a transmit timing control message. Note that this transmit timing offset is a one time adjustment to the transmit timing of the access terminal 110, whereas the timing rate-of-change as described herein is intended to minimize such transmit timing offsets by enabling the access terminal 110 to periodically adjust its own transmit timing.

Thus, a satellite-based communication system 100 having the above characteristics will enable the specific use of Eq. (1) . The following shows how to calculate the timing rate-of-change. The satellite 108 sends signals (e.g. sent on the BCCH or TTCH of the communications link 114) using f_cdn/ or the carrier frequency in the downlink (i.e. satellite 108 to access terminal 110), to the access terminal 110. The actual carrier frequency received at the access terminal 110 will have experienced a Doppler frequency offset; thus, the carrier frequency actually received (f '_cdn) at the access terminal 110 is:

where v is the relative velocity between the satellite 108 and the access terminal 110, and c is the speed of light. The received frequency, f ' _{cdn l} becomes the reference frequency of the access terminal 110. For simplicity in the following analysis, the effect of Doppler is defined as follows:

x = (1 + v/c) Eq. (3)

Substituting Eq. (3) in to Eq. (2) , the received carrier frequency at the access terminal 110 including the effect of Doppler is:

Corresponding return signals (e.g. sent on the RACH or TCH of the communications link 114) from the access terminal 110 to the satellite 108 are sent at an uplink frequency that has a preset multiplicative relationship to the downlink carrier frequency. For example, the uplink carrier frequency (i.e. from the access terminal 110 back to the satellite 108) is defined as:

K = f_cup/ f_cdn Eq . ( 5 ) where f_cup is the uplink carrier frequency, and K is the multiplicative relationship between the downlink carrier frequency and the uplink carrier frequency. Thus, the received frequency at the access terminal 110 is multiplied by K to create the uplink carrier frequency (f_cup) . However, the actual uplink carrier frequency will be slightly different than the uplink carrier frequency transmitted from the access terminal 110 since the carrier frequency received at the access terminal 110 has been effected by Doppler (i.e. see Eq. (4)). Thus, the actual uplink carrier frequency or the transmit frequency (f_t) generated at the access terminal 110 is determined by multiplying the received carrier frequency (f'_cdn) by K as follows:

ft = f 'cdn (fcup/fcdn) Eq. (6)

= _CdnX ( f_CUp/ cd_n) ( substituting Eq . ( 4 ) )

= f_cupx Eq. ( 7 )

In order to compensate for the Doppler in the downlink, the access terminal 110 will adjust the transmit frequency back to the satellite 108 (i.e. f_t) by the doppler frequency offset, f_d, as measured by the gateway station 104 (and described below with reference to FIGS. 6 and 7) . Thus, if the satellite 108 and the access terminal 110 are approaching each other, the adjusted actual transmit carrier frequency (f'_t) from the access terminal 110 will be defined as follows:

f \ = f_cupx - f_d Eq. (8) .

However, the Doppler frequency adjustment (i.e. -f_d) is a function of the received downlink carrier frequency (f'_cdn) from the satellite 108 (i.e. the reference frequency of the access terminal 110) , which has been affected by Doppler itself. Therefore, the amount of the Doppler adjustment is really f_dx, and Eq. (8) becomes

f \ = f_cupx - f_dx Eq. (9) .

Additionally, the return signal (at f'_t) back to the satellite 108 will also experience a Doppler frequency offset, thus, the received frequency at the satellite 108 (i.e. f'_CUp) will be:

f'cup = (fcupX - f_dx)x Eq. (10)

= x²(f_cup - f_d) Eq. (11)

With the appropriate Doppler adjustments, it is desirable that f '_cup = f_cup so that the uplink carrier frequency received back at the satellite 108 matches the expected uplink carrier frequency, and thus, the signal is received within the appropriate timing window at the satellite 108. Thus, substituting f'_cup = f_cup into Eq. (11),

x²(f_cup - f_d) = f_cup Eq. (12)

Thus, x = [f_cup /(f_cup - f_d) Eq. (13)

Manipulating Eq. (13) ,

1/x = l/[f_cup /(f_cup - f_d)T Eq. (14)

= [(fcup - fd)/ cupf Eq. (15)

= [1 - fd/f_cupr Eq. (16)

Using a Taylor series approximation as known in the art wherein typically (1+y)^ ~ 1+^y, then Eq. (16) becomes: 1/x = [1 - f_d/2f_cup] Eq. (17)

Thus, X = 1/[1 - f_d/2f_cup] . Eq. (18)

Furthermore, the timing rate-of-change (i.e. dT_d/dt) between the satellite 108 and the access terminal 110 is equal to the fractional Doppler (i.e. v/c) for both the uplink and the downlink over communications link 114. Thus, since there is a fractional Doppler effect in both the uplink and the downlink, the timing rate of change is as follows:

d _d/dt = 2 (v/c) Eq. (19)

Substituting x = 1 + v/c (Eq. (3)) into Eq. (19),

dT_d/dt = 2(x-l) Eq. (20)

Substituting Eq. (18) for x into Eq. (20) , then

dT_d/dt = 2{1/[1 - f_d/2f_cup] -1} Eq. (21)

Manipulating Eq. (21) ,

dT_d/dt= 2 {[1 -1(1 - f_d/2f_cup)]/[l - f_d/2f_cup]}

= (f_d/fcup)/[l - fd/2f_cup] Eq. (22)

Furthermore, since typically, the uplink carrier frequency is much larger than the Doppler frequency offset (i.e. f_cup >> f_d) , for example the uplink carrier frequency is 1.6 GHz and the Doppler frequency offset is 600 Hz, then the term [1 - f_d/2f_cup] in the denominator approximates 1. Therefore, the timing rate-of-change is approximated using the formula:

where dT_d/dt is the timing rate-of-change in seconds per second. Note that the timing rate-of-change is independent of the downlink carrier frequency, f_cdn, from the satellite 108 to the access terminal 110. The timing rate-of-change is also independent of the multiplicative constant, K. This scenario may be used to match any set of receive and transmit signals, e.g. BCCH/RACH signals or TTCH/TCH signals.

In the event that the satellite 108 and the access terminal 110 are moving further apart relative to each other, the equations above are slightly altered, but yield the same results. For example, Eq. (3) becomes x = (1 - v/c) and Eq. (9) becomes f'_t = f_cupx + f_dx. The same analysis yields the same resulting Eq. (1) .

Thus, the timing rate-of-change may be determined knowing the frequency offset (due to Doppler and other offsets) and the desired uplink carrier frequency from the access terminal 110 to the satellite 108. Note that this is the timing as "seen" by the satellite 108, i.e. from the satellite's point of view. The Doppler frequency offset between the gateway station 104 and the satellite 108 is not required since the communications link 116 is a Doppler compensated link.

That is, the gateway station 104 knows the positioning of the satellite 108 in orbit and is able to determine any frequency offsets and compensate for such frequency offsets due to the satellite's motion in orbit relative to the gateway station 104. The ability of the gateway station 104 to calculate the frequency offset as seen by a satellite 108 is known to one of ordinary skill in the art. However, as an illustration, one technique is shown for determining such a frequency offset (i.e. the Doppler frequency offset) as seen at the satellite with reference to FIGS. 6 and 7. Furthermore, a similar timing/frequency offset relationship may be established between other satellite- based communications systems that do not have the exact configuration as described, in order to calculate a timing rate-of-change. Thus, the invention described is not limited to the specifically configured system, but is intended to cover other types of satellite-based systems (e.g. that are not "synchronized at the satellite") . Such other systems would require slight manipulations of the calculations as given (see FIG. 5 for further discussion) . Using the above relationship in Eq. (1) , the rate at which transmit timing control messages need to be sent will be significantly reduced. Assuming that the gateway station 104 can estimate the Doppler frequency offset as seen at the satellite 108 with an accuracy of 10 Hz RMS, then the timing error in the estimate of the timing rate-of-change will be approximately 6 ns/sec (10 Hz/1.6 GHz, see Eq. (1)). Assuming as above that the system would try to adjust the timing once every 5 μs change in delay, the transmit timing control messages would only need to be sent once every 800 seconds (5 μs/6 ns/sec) due to the 10 Hz error in the frequency estimate alone, compared to every 6 to 12.5 seconds or less as described earlier. However, the relative motion of the satellite 108 and the access terminal 110 will reduce this to some extent, if the timing rate-of-change changes. In general, though, the rate at which to send transmit timing control messages will be reduced by at least an order of magnitude, typically no less than 60 seconds. Thus, the gateway station 104 has sufficient time to transmit a transmit timing control message to adjust the transmit timing of the access terminal 110 before the timing as seen at the satellite 108 is outside the acceptable timing window. This allows enough time to ensure that the transmit timing control message is sent by the gateway station after an appropriate talk spurt. Several embodiments are possible to accomplish the present invention. In one embodiment, the timing rate-of-change may be generated at the gateway station 104 from the estimated Doppler frequency offset as seen by the satellite 108 using Eq. (1) . A transmit timing control message containing the Doppler frequency offset and the timing rate-of-change is sent to the access terminal 110. The access terminal 110 adjusts it's uplink carrier frequency by the Doppler frequency offset and adjusts the transmit timing periodically in response to the timing rate-of-change so that the timing at the satellite 108 will continue to be within the acceptable window. Note that this embodiment, as well as the following embodiments, assume that the gateway station 104 can estimate the frequency offset due to Doppler as seen by the satellite 108. The details of this process are described further with reference to FIGS. 6 and 7 below. In another embodiment, the gateway station 104 sends a transmit timing control message containing a timing adjustment parameter and a timing rate-of-change to the access terminal 110. The access terminal 110 makes an adjustment (+ a specific time) to the transmit time of the transmitter in response to the timing adjustment parameter. Then the access terminal 110 periodically adjusts (i.e. either advances or retards) the transmit timing of the transmitter in response to the timing rate- of-change.

In another embodiment, the gateway station 104 sends a transmit timing control message containing only the Doppler frequency offset. The access terminal 110 receives the transmit timing control message and calculates a timing rate-of-change from the Doppler frequency offset using Eq. (1) , for example. The access terminal 110 then sets the transmit timing of it's transmitter in response to the timing rate-of-change. The access terminal 110 also adjusts it's carrier frequency in the uplink by the amount of the Doppler frequency offset. The details are these processes are discussed with regard to FIG. 2 and FIG. 3.

In another embodiment, the gateway station 104 sends a transmit timing control message containing a default timing rate-of-change to the access terminal 110 at the beginning of the communication. The default timing rate-of-change represents the timing rate-of-change between a satellite 108 in geosynchronous orbit relative to a stationary terrestrial access terminal. This default timing rate-of-change may typically be approximately + 200 ns/sec. Note that the default timing rate-of-change will vary for differently configured systems with different satellite orbits and as a function of time.

The gateway station 104 then observes the timing error and the Doppler frequency offset as seen by the satellite 108. If a timing error develops, the gateway station 104 determines how long before the timing error puts the timing out of the acceptable timing window at the satellite 108. The gateway station 104 also estimates the rate at which the error is increasing, as shown by the frequency offset seen at the satellite 108. Before the error becomes unacceptable, the gateway station 104 sends another transmit timing control message to the access terminal 110 containing a Doppler frequency offset and a new timing rate-of-change. The access terminal 110 then adjusts its uplink carrier frequency by the Doppler frequency offset and adjusts its transmit timing in response to the timing rate-of-change. Instead of sending a Doppler frequency offset, the gateway station 104 could send a timing adjustment parameter that would advance or retard the transmit timing by the desired amount; however, the timing rate-of-change is still needed to periodically adjust the transmit timing due to the relative motion between the satellite 108 and the access terminal 110. Furthermore, the new timing rate-of-change could be determined with the Doppler frequency offset sent in the second transmit timing control message. In the above embodiments, the timing rate-of- change may be defined absolutely or relatively. For example, the current transmit timing rate-of-change for the access terminal 110 may be adjusted absolutely to a new timing rate-of-change of 250 ns/sec (from the default timing rate-of-change of 200 ns/sec) . Alternatively, the timing rate-of-change of the access terminal 110 may be adjusted relative to the current timing rate-of-change. This message would adjust the current timing rate-of- change of the access terminal (in this case, the default timing rate-of-change) by 50 ns/sec, so that the overall timing rate-of-change at the access terminal 110 would be 250 ns/sec.

The present invention is designed to work in any satellite communications system where the timing delay varies reasonably linearly, or approximately close to linearly, with frequency offset. It is applicable for satellites in low earth orbit (LEO) and middle earth orbit (MEO) . Generally, timing delay and Doppler effects are not a constraint in a satellite in geostationary orbit (GEO) ; however, the preferred embodiment of the present invention is designed to handle a particular type of satellite system. The satellite 108 is in geosynchronous orbit, not geostationary, so that the satellite 108 moves in a sinusoidal pattern north and south as much as 6 degrees in either direction. This motion simulates a greater relative motion than would normally be experienced with a satellite in geostationary orbit. Referring next to FIG. 2, a block diagram of a communication terminal in accordance with the present embodiment is shown. A communications terminal or access terminal 200 having an antenna 202 is shown. The antenna 202 is coupled to a first multiplier 208. The first multiplier 208 is coupled to a receiver 212 and a local oscillator 210. The local oscillator 210 is coupled to a timing delay compensator 218 and a transmit to receive offset 228. The receiver 212 sends a control signal 216 to the timing delay compensator 218 and a locking control signal 214 to the local oscillator 210 to lock the local oscillator 210 to the received carrier frequency. The timing delay compensator 218 sends a control signal 220 to a transmitter 224 and another control signal 222 to a frequency rotator 226. A second multiplier 230 is coupled to the output of the transmitter 246 and the output of the frequency rotator 248. A third multiplier 232 is coupled to the output of the second multiplier 250 and the output of the transmit to receive offset 240. The output of the third multiplier 244 is coupled back to the antenna 202 of the access terminal 200.

In accordance with the present embodiment, the transmit timing control message contained within a carrier signal sent by the gateway station through the satellite is received into the access terminal 200 at the antenna 202 and passed to the first multiplier 208.

It is important to note that the transmit timing control message is a message that contains one or more of the following depending on the embodiment of the system: Doppler frequency offset, transmit timing offset, and timing rate-of-change. Typically, the transmit timing control message is contained within the regular signaling (e.g. "in-band" signaling) between the satellite 108 and the access terminal 110, such as a signal that carries voice communications, control information, etc. Thus, the transmit timing control message is simply a message contained within the carrier signals between the satellite 108 and the access terminal 110.

Alternatively, the transmit timing control message may be a separate signal, independent of the regular signaling from the satellite 108 to the access terminal 110, sent from the gateway station 104 to the access terminal 110 via the satellite 108. Thus, the transmit timing control message is actually a transmit timing control signal having a message embedded within. The message would comprise one or more of the Doppler frequency offset, timing adjustment, and timing rate-of- change. Furthermore, the specific message contained within such a transmit timing control signal may be conveyed in the waveform of the signal itself, without the use of embedded messaging. Thus, the waveform of the transmit timing control signal itself may provide the necessary Doppler frequency offset or timing rate-of- change. Consequently, the transmit timing control message may contain a Doppler frequency offset or a timing adjustment. It may also contain a timing rate-of-change depending on the embodiment of the invention. The access terminal's local oscillator 210 responds to the timing of the received signal 206. The transmit timing control message is relayed to the receiver 212 where the respective messages, i.e. Doppler frequency offset, the timing adjustment, and timing rate-of-change, are extracted. The receiver 212 then sends a control signal 216 containing the Doppler frequency offset or the timing adjustment of the transmit timing control message to the timing delay compensator 218. The timing delay compensator 218 uses the information supplied to correct the transmit timing of the transmitter 224. The timing delay compensator 218 is an application specific integrated circuit (ASIC) that has been designed to perform the following tasks.

If the transmit timing control message contains only the Doppler frequency offset, the timing delay compensator 218 will send a control signal 222 to the frequency rotator 226 to make the necessary frequency adjustment to the uplink carrier frequency; thus compensating for the Doppler frequency offset. This Doppler frequency adjustment is reflected in Eqs. (8) and (9), i.e. -f_dx described above. This adjustment does not compensate for the fact that relative motion exists between the satellite and the access terminal 200. Another transmit timing control message, containing another Doppler frequency offset, would need to be sent within a short period of time. However, since the Doppler frequency offset is proportional to the timing rate-of-change, the timing delay compensator 218 converts the Doppler frequency offset directly to a timing rate-of- change by dividing the frequency offset by the uplink carrier frequency used. The relationship between the Doppler frequency offset and the timing rate-of-change is governed by the formula:

dT dt = f_d / f_cup Eq. (1)

where dT_d/dt is the timing rate-of-change measured in seconds per second, f_d is the Doppler frequency offset measured in Hertz, and f_cup is the uplink carrier frequency measured in Hertz. If the Doppler frequency offset is 200 Hz, the downlink carrier frequency is 1.5 GHz, and the uplink carrier frequency is 1.6 GHz, then the timing rate if change is 125 ns/sec (i.e. 200 Hz/1.6 GHz). The timing delay compensator 218 then sends a control signal 220 to the transmitter 224 to begin transmission at the desired time according to the determined timing rate-of-change. Thus, the transmit timing of the transmitter 224 is periodically set in response to the timing rate-of-change determined from the received Doppler frequency offset.

In another embodiment, the transmit timing control message already contains the timing rate-of- change; thus, there is no need to calculate it at the timing delay compensator 218. However, whichever embodiment is implemented, the timing delay compensator 218 must tell the transmitter 224 when to start transmission of a signal to the satellite. The details of how this is done is discussed with reference to FIG. 3. Once the transmitter 224 begins to transmit (which is a function of the timing rate-of-change) , the uplink carrier frequency of the outgoing signal 246 is adjusted by the amount of the Doppler frequency offset if a Doppler frequency offset is supplied in the transmit timing control message. The timing delay compensator 218 sends a control signal 222 to the frequency rotator 226 in order to accomplish this function. The adjustment is done digitally by altering the Is (in phase component) and Qs (quadraphase component) of the vector generated for the outgoing signal. In essence, the vector of the baseband signal is rotated resulting in a frequency adjustment of the uplink carrier frequency. The frequency rotator 226 is well known in the art and may typically be a part of the same ASIC containing the timing delay compensator 218. The signal 250 then goes to the third multiplier 232 which outputs the return signal 244 (f'_t of Eq. (8)) to the antenna 202 for transmission to the satellite and gateway station. The transmit to receive offset 228 adjusts the frequency from the local oscillator 210 (i.e. the received downlink carrier frequency, f'_cdn) by the multiplicative factor K (i.e. f_c__P/f_cdn) to create the uplink carrier frequency, so that the output 240 of the transmit to receive offset 228 will be the transmit frequency (i.e. f_t in Eqs. (6) and (7)). The transmit frequency (f_t) is multiplicative of the frequency of the local oscillator 210, which is sourced by the received carrier signal from the satellite. Thus, the locking control signal 214 locks the frequency of the local oscillator 210 to the received carrier frequency. In other words, the transmit frequency is proportional to the frequency of the local oscillator 210, e.g. by the determined constant K in Eq. (5) . Note that due to the relative motion of the access terminal 200 and the satellite, the output 240 of the transmit to receive offset 228 will actually reflect a delay due to the Doppler offset of the signal in the downlink direction

(see Eqs. (6) and (7)). Once the output 240 is multiplied at the third multiplier 232 with the signal 250, the resulting signal (i.e. f'_t in Eqs. (8) and (9)) reflects the adjusted transmit carrier frequency adjusted for the Doppler frequency offset (see Eq. (9)). Furthermore, the return signal 244 (at f'_t) is sent at the appropriate time as determined by the timing rate-of-change so that the return signal will arrive at the satellite 108 within the acceptable timing window.

In sum, once provided a Doppler frequency offset by the gateway station, the access terminal 200 can autonomously calculate a timing rate-of-change, using Eq. (1) for example. Thus, the access terminal 200 can periodically adjust the transmit time of the transmitter 224 so that the signals 244 sent to the satellite will be within a desired timing window. If the relative motion between the satellite and the access terminal 200 continues at the same rate, the gateway station should not have to send any more transmit timing control messages. If the relative motion changes, then the gateway station will see further error at the satellite in the form of a timing error and/or a frequency offset. Thus, by enabling the access terminal 200 to adjust the transmit timing of the transmitter 224 periodically, the number of times a gateway station would need to send a transmit timing control message to the access terminal 110 is significantly reduced. The components and devices used in the access terminal, as well as the ability to design the functions of the components and devices used, are known and understood to those skilled in the art.

Referring next to FIG. 3, a block diagram of a portion of a communications terminal show in FIG. 2 that determines the transmit start time in accordance with one embodiment is shown. A portion of the timing delay compensator 218 called the start time compensator 300 is shown. The start time compensator 300 contains a counter 308 coupled to the local oscillator 310 and to a comparing element 312. A timing rate-of-change 302 (supplied by a transmit timing control message or derived from a Doppler frequency offset) is received in to block 304 which is coupled to a register 306. The register 306 is coupled to the comparing element 312. A control signal 316 is sent from the comparing element 312 to the transmitter 318. Block 304 receives the timing rate-of-change 302 and determines a value for the start time based upon the timing rate-of-change 302, which is stored in the register 306. The value in the register 306 is compared to the counter 308 by the comparing element 312. The counter 308 counts at a rate responsive to the local oscillator 310. Once the value of the counter 308 equals the value in the register 306, a control signal 316 (i.e. control signal 220 in FIG. 2) is sent to the transmitter 318 to begin transmission. The start time compensator 300 is similar to existing parts of similar satellite access terminals, except that in accordance with the present invention, the start time compensator 300 calculates a start time that changes periodically in response to a timing rate-of- change 302 to reflect the relative motion present between the satellite and the access terminal; thus, the start time of the transmitter is advanced or retarded periodically according to the timing rate-of-change. The start time compensator 300 can also calculate the start time when a conventional timing adjustment message is sent. The components used and the ability to design such components used in the start time compensator 300 are known and understood by the those skilled in the art.

Referring next to FIG. 4, a flowchart of the steps for a gateway station to define the timing rate-of- change of another communications terminal is shown. Block

402 sends a transmit timing control message defining a timing rate-of-change to the communications terminal (e.g. access terminal 110) . Block 404 monitors the timing and the frequency offset "seen" at the satellite. Block 406 estimates the timing error and the frequency offset. In the event of a timing error, block 408 determines the maximum time before the timing error is unacceptable. Block 410 sends another transmit timing control message defining a new timing rate-of-change to the communications terminal, whenever possible after a talk spurt. In the event there is no timing error seen at the satellite in block 406, the step in block 404 is performed again.

The first step is for the gateway station to send a transmit timing control message "defining" the timing rate-of-change (block 402) . By "defining" the timing rate-of-change, the transmit timing control message either provides the timing rate-of-change directly or indirectly. The transmit timing control message directly defines the timing rate-of-change simply by including the timing rate-of-change within the message. In contrast, the transmit timing control message indirectly defines the timing rate-of-change by supplying a frequency offset

(e.g. Doppler frequency offset) which enables the access terminal to determine the timing rate-of-change, e.g. using Eq. (1) . Additionally, there may be other ways (beside providing a frequency offset) in which the timing rate-of-change may be indirectly defined.

In one embodiment, the gateway station sends a transmit timing control message defining the timing rate- of-change (block 402) at the beginning of the call. This transmit timing control message includes a default timing rate-of-change to simulate the amount of relative motion to between a stationary communications terminal (e.g. access terminal 110) and a satellite 108 in orbit. For a satellite in geosynchronous orbit, the default timing rate-of-change may be within a range of + 150 ns/sec to + 250 ns/sec, realistically about + 200 ns/sec. Sending such a signal at the beginning of the call will immediately allow an access terminal to periodically adjust it's own transmit timing as discussed with reference to FIG. 2 and FIG. 3; and thus, continually account for the relative motion present.

The timing and frequency are monitored as seen by the satellite in Block 404. The details of the frequency estimation process are shown in reference to FIG. 6 and FIG. 7. Next, block 406 determines if a timing error is present. The timing error is evidenced by the time delay in the expected signal within the timing window. This error is typically measured in μs. The process of determining the timing error in seconds is known and understood by those skilled in the art. If no error is present, the gateway station will continue to monitor the timing until the end of the usage of the channel.

If a timing error is detected, the gateway station must determine how long before the timing error becomes unacceptable, shown in block 408. For example, a typical system may have a symbol duration of 50 μs and a first (e.g. default) transmit timing control message sent to the access terminal includes a timing rate-of-change of 200 ns/sec. The timing seen by the satellite is 2 μs from the desired point while the timing error is increasing at a rate of 20 ns/sec. Assume that the maximum acceptable error or the timing window is 5 μs. Then the maximum time before the error is unacceptable is (5 μs - 2 μs) / 20 ns/sec = 150 seconds. Thus, another transmit timing control message defining a new timing rate-of-change must be sent within 150 seconds to adjust the timing of the access terminal. This provides plenty of time to send a message to the access terminal after a talk spurt. This is a vast improvement over a conventional system that is required to send transmit timing control messages containing timing adjustments as often as 6 to 12 seconds or less.

As shown in block 410, a transmit timing control message defining a new timing rate-of-change is sent to the access terminal within the time frame determined in block 408. The transmit timing control message could contain a timing adjustment of 2 μs, as in the previous example, and also a timing rate-of-change. Furthermore, the timing rate-of-change may be defined as an absolute value or a relative value. An absolute value would be a timing rate-of-change of 220 ns/sec, since the access terminal is currently using a timing rate-of-change of 200 ns/sec and the error is increasing at a rate of 20 ns/sec. Alternatively, the timing rate-of-change could be relative. The transmit timing control message could send a message to adjust the transmit timing 20 ns/sec relative to the current timing rate-of-change.

Whenever possible, the gateway will attempt to send the transmit timing control message after a talk spurt (block 410) . Thus, a transmit timing control message won't disrupt the voice traffic. With the present invention in use, the transmit timing control messages need to sent less often (once very 150 seconds compared to once every 6-12 seconds or less without the use of the timing rate-of-change as described earlier, for example) . In another embodiment, the step in block 402 is not done and the process starts with block 404 which monitors the timing and the frequency seen at the satellite. The remaining steps are then followed.

In another embodiment, the transmit timing control message could send a frequency offset to adjust the uplink carrier frequency. This has the same result as making a conventional timing adjustment as shown in the previous example. Additionally, the access terminal can derive the timing rate-of-change from the frequency offset. The system designer will want to conserve bandwidth used; thus, sending a transmit timing control message containing a frequency offset may be the most economical method to save bandwidth since the access terminal can derive the necessary timing rate-of-change from the frequency offset. Sending a transmit timing control message containing a timing adjustment and a timing rate-of-change uses more bandwidth than sending the (Doppler) frequency offset alone.

Referring next to FIG. 5, a flow chart of the steps a communications terminal (e.g. access terminal) completes to change it ' s transmit timing in response to a transmit timing control message containing a Doppler frequency offset in order to reduce the number of transmit timing control messages is shown 500. Block 502 calculates a timing rate-of-change from a frequency offset (due to Doppler and other frequency offsets) of a received transmit timing control message. Then a transmit start time is calculated in block 504 based on the timing rate- of-change. Block 506 stores the transmit start time in register. The transmit start time is compared to a counter that is responsive to a local oscillator shown in block 508. Block 510 compares the register to the counter. Block 512 sends a transmit signal to begin transmission and sends a frequency offset signal to offset the transmission frequency when the counter equals the transmit start time in Block 510.

Block 502 calculates the timing rate-of-change from the Doppler frequency offset contained in the received transmit timing control message. This will enable the access terminal to periodically adjust it's own transmit timing of the transmitter to reflect the relative motion between the satellite and the access terminal. In the embodiments described above, the frequency offset, which is due to Doppler, is proportional to a timing rate- of-change using the formula:

dT_d/dt = f_d / f_cup Eq. ( 1)

where dT_d/dt is the timing rate-of-change measured in seconds per second, f_d is said frequency offset (due to Doppler, etc.) seen at the satellite and sent by the gateway station in Hertz, and f_cup is an uplink carrier frequency from the access terminal to the satellite measured in Hertz. This calculation is performed by a microprocessor within the access terminal. The process of estimating the frequency offset seen at the satellite is described with reference to FIG. 6 and FIG. 7. Note that other satellite-based communications systems (e.g. those systems not having the specific configuration as described in FIG. 1) may define and determine the timing rate-of-change. The following formulas form the basis for determining the timing rate- of-change in a generic satellite-based communications system. Of course, the skilled artist will have to vary the formulas to fit the specific configuration of the system, i.e. depending on where the "synchronization" takes place and how the carrier frequencies relate to each other. Given that: v_at = f_d * λ and λ = v_e / f_c Then:

V_at = f_d * V_β / f_c. Also given that:

T_d = D / v_e then Δ _d/Δt = (ΔD/Δt)/ v_e then dT_d/dt = v_at / v_e then substituting v_at dT_d/dt = f_d / f_c

where v_at is the relative velocity between the satellite and the access terminal, f_d is the Doppler frequency, λ is the wavelength, v_e is the speed of light, f_c is the carrier frequency, T_d is the time delay between the two terminals, D is the distance between the two terminals, and dT_d/dt is the timing rate-of-change. Note that the specific formula used by the present invention is provided in Eq. (1) .

Once a timing rate-of-change is calculated, then a transmit start time is determined in block 504 to begin transmission. The transmit start time can be determined using conventional means, except that the transmit start time is adjusted periodically, such as advanced or retarded, to reflect the timing rate-of-change due to the relative motion between the satellite and the access terminal. For example, the transmit start time may be delayed (i.e. retarded) 0.2 μs/sec. Once determined, the transmit start time is stored in a register in block 506. The transmit start time is compared to a counter that is responsive to the local oscillator in block 508. As the counter counts, it's value is compared to the transmit start time that is stored in the register. Once the counter and the register equal each other 510 a control signal 220 is sent to begin transmission in block 512. At the same time, another control signal 222 is sent to make any frequency offsets to the outgoing signal as described with reference to FIG. 2. The steps indicated are all capable of being performed by a microprocessor. The skilled artist possesses the knowledge to accomplish such steps.

In another embodiment, the step in block 502 is not done, because the gateway station has already supplied the access terminal with the timing rate-of-change contained within the transmit timing control message.

In another embodiment, the gateway station may instruct the access terminal to make an adjustment to the transmit timing in terms of time, as is conventionally done, while also providing a timing rate-of-change. For example, the gateway station may instruct the access terminal to adjust its timing by 2 μs and then instruct that the transmit timing be periodically adjusted according to a timing rate-of-change of 200 ns/sec. In block 506, the transmit start time will be calculated by adjusting itself 2 μs, then the transmit time will be periodically adjusted at 200 ns/sec according to the timing rate-of-change. Note that the timing adjustments and timing rate-of-change may instruct that the transmit start time be advanced or retarded according to the transmit timing control message. For example, a timing rate-of-change of -200 ns/sec would periodically "delay" or retard the transmit timing (i.e. the transmitter would begin transmission slightly later than otherwise) , while a timing rate-of-change of +200 ns/sec would periodically

"advance" the transmit timing of the access terminal (i.e. the transmitter would begin transmission slightly sooner than otherwise) .

Referring to FIG. 6, a flowchart is shown for the steps to perform for frequency offset estimation as seen by the satellite, which is done by a digital signal processor at a gateway station. As note above, carrier frequency estimation, as well as frequency error or frequency offset estimation, is well known in the art. Thus, the following description provides one method for estimating the frequency offset (i.e. in this case, which is due primarily to Doppler) .

As described earlier, the frequency offset as "seen" by the satellite is the Doppler frequency offset for a signal sent from the satellite to the access terminal and back to the satellite. It is the amount of frequency offset from the satellite's perspective without considering the frequency offset in the signal from the gateway station to the satellite and from the satellite back to the gateway station. The gateway station is able to estimate the frequency offset as seen by the satellite by eliminating the frequency offset between the gateway station and the satellite. The gateway station is precisely aware of the satellite's position at all times in relation to the gateway station and is able to calculate any frequency offset between itself and the satellite. Thus, the frequency that is estimated in FIG. 6 has already compensated for any frequency offset between the satellite and the gateway station. Thus, the estimated frequency offset shown below is the frequency offset between the satellite and the access terminal or the frequency offset as "seen" by the satellite.

A technique for estimating frequency offsets at the gateway station in a burst using a bisected block approach for frequency estimation is shown in FIG. 6. Symbols are received at the matched filter of the gateway station. The symbols within a moving averaging window or block of symbols are divided into a first half and a second half of the block of symbols (Block 602) . If the block size is an odd number of symbols, the Nth symbol is ignored in order to divide the block evenly into two halves and vectorially summed separately. Next, the vector sum (i.e. v_x) from the first half of the block is determined (Block 604) which has the phase offset for the middle of the first half block, which is ideally φ+4Δ» (N- 3)/4, where φ is the constant phase offset induced by the channel over the block, and Δ is the phase roll due to the frequency offset over a symbol interval, i.e. Δ = 2π«Δf«T. The factor of 4 multiplication (i.e. 4Δ) represents the effect of the phase quadrupling done on the symbols to remove the modulation phase assuming the use of a quaternary modulation, such as QPSK. A doubling (i.e. a factor of 2) would be appropriate for a binary modulation, such as BPSK. Concurrent with Block 604, the vector sum (e.g. v₂) from the second half of the block is determined (Block 606) , which similarly has the phase offset at its middle which is ideally φ+4Δ» (3N-5)/4. A vector whose phase is equal to the difference between the phases of the two vectors (v_x and v₂) is determined by finding the vector product of the v_τ and the complex conjugate of v₂ (Block 608), which, in this case, is ideally 4Δ«(N-l)/2. The phase of the resulting vector (obtained in Block 608) is proportional to the frequency offset and is used to obtain an estimate of the frequency offset. Next, the resulting vector (Block 608) is averaged over several blocks of symbols (Block 610) in order to reduce the effect of noise. Next, the phase of the vector is determined utilizing an inverse tangent operation (Block 612) as known in the art. In order to account for both positive and negative frequency offsets, the result from the phase determination operation (Block 612) is advantageously represented in the range from -π to +π radians. Then, the phase is converted to frequency (Block 614) by dividing the phase by 4π(N-l) •symbol_time yielding the estimated frequency offset 616 measured in Hertz. In order to further reduce the effects of noise, the estimated frequency offset 616 computed from each burst is filtered, usually through a first order loop (i.e. loop filter 712) , which is then used to reduce the frequency offset in successive bursts as they are received prior to the frequency offset estimation process shown in FIG. 7.

Because the phase determination operation (Block 612) results in a value only in the range from -π to +π radians, aliasing will arise for large frequency offsets. This limits the frequency offset which may be estimated by this technique. Ignoring the effect of noise and fading, the limit is simply governed by the following relationship :

4- Δ/-2π- 7χN- l )/2 | <π or Eq. (23)

Δ/| <0.25/T-(N- 1 )]

However, in practice the limit will be smaller than the limit indicated by the above relation because of the effects of noise and fading which may randomly contribute to frequency offset (random FM) . In such a case, frequency offsets in excess of the limit as determined by dividing the output of the integrator by 4π(Ν-l) •symbol_time, and indicated in the above relation may occur, and would need to be estimated and reduced by a different technique before the converted symbols are input to the block phase and frequency offset estimations discussed above. One technique for estimating the frequency offsets involves using a Discrete Fourier Transform (DFT) to provide the initial gross estimate of the frequency offset of the transmitted data symbols before the above technique is used for a finer estimation. Referring next to FIG. 7, the frequency offset estimator and filtering loop performed by a digital signal processor 700 at the gateway station is shown that reduces the error in of the frequency offset measured in subsequent bursts. The received signal 702 (at a carrier frequency) containing a frequency offset (or frequency error) is received at the gateway station. The received signal 702 is multiplied by an estimate of the correct frequency 706 at multiplier 704. The resulting signal 708 sent to the frequency offset estimator 710 and should then only reflect any frequency offset present. The frequency offset estimator 710 then performs the initial frequency offset estimation, i.e. the technique illustrated in FIG. 6. Then, the frequency offset estimate 616 obtained from the process shown in FIG. 6 at frequency offset estimator 710 is then smoothed through a loop filter 712, which is typically a 1st order averaging loop (e.g. a single pole IIR filter) . The filtered frequency offset estimate is delayed one burst at the delay 714 and then applied to a frequency source 716. The frequency source 716 is set to the estimate of the correct frequency 706, taking into consideration the estimated frequency offset from delay 714, which is then fed back to multiplier 704 to provide for improved frequency offset estimates (i.e. frequency error) in subsequently received bursts. It should be noted in some embodiments that the frequency source 716 does not exist in hardware in the gateway station, but it is part of the digital signal processor 700. Note that if the frequency offset estimate obtained in 710 is greater than the limit in Eq. (23) , then a Discrete Fourier Transform could be performed at the frequency offset estimator 710 prior to performing the technique illustrated in FIG. 6. The techniques describe above represent only one way in which the gateway station may estimate the amount of frequency offset, as many methods are known to those skilled in the art. While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.

Claims

CLAIMSWhat is claimed is:

1. A method of reducing transmit timing control messaging in a communications system comprising: establishing a communications link between a first communications teirminal and a second communications terminal ; transmitting a transmit timing control message from the first communications terminal to the second communications terminal; and setting periodically a transmit timing of the second communications terminal in response to the transmit timing control message.

2. The method of Claim 1 wherein said transmitting comprises transmitting said transmit timing control message including a doppler frequency offset.

3. The method of Claim 2 further comprising deriving, prior to said setting periodically, a timing rate-of-change from said doppler frequency offset.

4. The method of Claim 1 wherein said transmitting comprises transmitting said transmit timing control message including a timing rate-of-change.

5. The method of Claim 1 wherein said transmit timing control message defines a timing rate-of-change, wherein the timing rate-of-change represents a timing change due to relative motion between said first communications terminal and said second communications terminal.

6. The method as in Claim 5 further comprising: monitoring a timing error in said communications link after said setting periodically step; determining a maximum amount of time before the timing error is unacceptable to said first communications terminal ; and transmitting another transmit timing control message from said first communications terminal to said second communications terminal before the timing error is unacceptable to said second communications terminal; and setting periodically said transmit timing of said second communications terminal in response to the other transmit timing control message.

7. The method of Claim 6 wherein said transmitting said other transmit timing control message comprises transmitting said other transmit timing control message from said first communications terminal to said second communications terminal after completion of a talk spurt.

8. The method as in Claim 1 wherein said transmitting comprises transmitting said transmit timing control message relative to a current transmit timing of said second communications terminal.

9. A method of reducing transmit timing control messages in a communications system comprising: establishing a communications link between a first communications terminal and a second communications terminal ; deriving a timing rate-of-change from a received transmit timing control message from the first communications terminal into the second communications terminal; and setting periodically a transmit timing of the second communications terminal in response to the timing rate-of-change.

10. The method of Claim 9 wherein said deriving further comprises deriving said timing rate-of-change from a doppler frequency offset of said received transmit timing control message from said first communications terminal into said second communications terminal.

11. The method of Claim 10 wherein said deriving further comprises: deriving said timing rate-of-change from said doppler frequency offset of said received transmit timing control message by using a formula defined as:

dT^dt = f_d / f_c_ι

where dT_d/dt is said timing rate-of-change measured in seconds per second, f_d is said doppler frequency offset measured in Hertz , and f _o2\ is a carrier frequency from said second communications terminal to said first communications terminal measured in Hertz.

12. A method of adjusting a transmit timing of a communications terminal comprising: receiving a transmit timing control message defining a timing rate-of-change; and periodically adjusting the transmit timing of a transmitter of the communications terminal in response to the timing rate-of-change.

13. The method of Claim 12 wherein said periodically adjusting comprises: calculating a transmit start time in response to said timing rate-of-change; storing the transmit start time in a register; comparing the transmit start time to a counter responsive to a local oscillator of said communications terminal ; and beginning transmission of signals out of said transmitter when the counter is equal to the transmit start time.

14. The method of Claim 12 wherein said timing rate-of-change represents a timing change due to relative motion between said communications terminal and another communications terminal.

15. A communications system that reduces transmit timing control messages sent between communications terminals comprising: a first communications terminal for transmitting a transmit timing control message; a second communications terminal including a receiver for receiving the transmit timing control message; a timing delay compensator coupled to the receiver and a transmitter of the second communications terminal responsive to the transmit timing control message, the timing delay compensator periodically setting a transmit timing of the transmitter in response to the transmit timing control message; and a communications link established between the first communications terminal and the second communications terminal .

16. The system of Claim 15 wherein said transmit timing control message contains a doppler frequency offset.

17. The system of Claim 15 wherein said transmit timing control message contains a timing rate-of- change.

18. A communications terminal that reduces transmit timing control messages sent in a communications system comprising: a receiver located in the communications terminal for receiving a transmit timing control message; a timing delay compensator coupled to the receiver and a transmitter, the timing delay compensator for deriving a timing rate-of-change in response to the transmit timing control message and periodically setting a transmit timing of the transmitter in response to the timing rate-of-change.

19. The communications terminal in Claim 18 wherein said transmit timing control message comprises a doppler frequency offset.

20. The communications terminal of Claim 18 wherein said transmit timing control signal comprises a timing rate-of-change.