Method for rapid signal acquisition in a satellite communications system

The signal acquisition process of the present invention enables a radio to rapidly acquire a pilot signal from a satellite after the radio has been out of contact with the satellite for a length of time. The radio first acquires a satellite pilot signal and determines its position in relation to the earth. If the radio is now shut off or loses track of the pilot signal for some other reason, the radio keeps track of the length of time that it has been out of contact with the satellite. This length of time enables the radio to estimate the extent of possible position changes and, therefore, which satellites might be in view and their relative positions to the radio. This allows the radio to make assumptions about variables such as the Doppler shift of the signal that reduces the uncertainty inherent in the acquisition process and the size of the window that must be searched.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to satellite communications. More 
particularly, the present invention relates to acquiring satellite 
signals. 
2. Description of Related Art 
When a radiotelephone is turned on in a cellular or satellite 
radiotelephone environment, it typically must search for and acquire the 
forward link signal transmitted by a base station. This forward link 
signal, referred to as the pilot signal in a CDMA system, is continuously 
transmitted by the base station. The pilot signal is used by the 
radiotelephone to obtain initial system synchronization and to provide 
robust time, frequency, and phase tracking of the signals from the base 
station. 
In a CDMA radiotelephone system, the radiotelephone cannot transmit until 
the pilot signal is acquired. This signal, therefore, must be acquired 
rapidly after the radiotelephone is turned on since a radiotelephone user 
typically does not want to wait to make a telephone call after turning on 
the radiotelephone. 
In a ground based CDMA cellular system, pilot signal acquisition typically 
takes only a few seconds. This is largely due to the known proximity of 
the cell sites to the radiotelephone and the fact that the cell is 
stationary. The pilot signal, therefore, has a relatively short delay to 
reach the radiotelephone and the Doppler shift of the frequency of the 
pilot signal is slight. 
Acquiring a pilot signal in a satellite based communication system, 
however, may take significantly longer since there is a greater frequency 
uncertainty that must be searched. The frequency uncertainty may be due to 
the range of possible Doppler shift caused by the rapidly moving low earth 
orbit (LEO) satellites. Other sources of uncertainty, such as the larger 
possible round trip time delay of the signal through the satellite can 
magnify the acquisition problem. There is a resulting need for a method 
for rapidly acquiring a signal transmitted from a satellite communication 
system. 
SUMMARY OF THE INVENTION 
The process of the present invention enables a mobile radio to rapidly 
acquire a signal from a satellite or a number of satellites that are part 
of a constellation of satellites. The radio has a real-time clock and 
memory containing the ephemeris of each of the satellites. The process 
begins by the radio first determining its spatial position in relation to 
the earth. The time that this position was determined is also noted. Both 
the position and the time are stored for future use. If the radio is 
turned off and back on or the signal is lost for some other reason, a 
second time from the real-time clock is determined. The radio then 
searches for the signal in response to a difference between the first and 
the second times. Knowing the position of the user allows assumptions to 
be made that simplify acquisition. Knowing the time since the last 
position fix allows varying degrees of confidence in these assumptions. 
The smaller the difference, the smaller the frequency or time uncertainty 
window to be searched, thus reducing acquisition time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A typical satellite communication system is illustrated in FIG. 1. In the 
preferred embodiment, this communication system is comprised of 48 
satellites (101) that act as repeaters for the signals transmitted to and 
from mobile radios (102) located on the earth or in an airplane at some 
altitude above the earth. The satellite bounces the signal back to a 
ground receiving station (103) connected with the public switched 
telephone network (PSTN). The PSTN switches the call to another cellular 
network or to a land-line telephone. 
In the preferred embodiment, the radios are CDMA-type radiotelephones. U.S. 
Pat. No. 4,901,307 to Gilhousen et al. and assigned to Qualcomm 
Incorporated discloses the operation of such a CDMA-type radiotelephone in 
a terrestrial CDMA system. Alternate embodiments use time division 
multiple access (TDMA) or frequency division multiple access (FDMA) type 
radiotelephones. 
The satellite communications system of the present invention operates by or 
through the radio first locating a pilot carrier signal transmitted by the 
satellites. In the preferred embodiment, this pilot signal is an 
unmodulated PN sequence. However, alternate embodiments have a pilot 
signal comprised of a preamble sequence, a preamble sequence mixed with 
data, a sufficiently strong information-bearing signal, or other signal 
types. 
In the preferred embodiment, the pilot signal is transmitted by each 
satellite using a known frequency and pseudo noise (PN) code, possibly 
with different spread spectrum code generators or phase offsets, thus 
allowing the radio to distinguish the different satellites. Since the 
satellites use a known PN code and code phase, the radiotelephone's 
internal real-time clock synchronizes to the system timing by a search 
through all code generators and phases. The strongest signal found by the 
radio corresponds to the best satellite Signal. 
The satellite's rapid movement across the sky causes the frequency of the 
pilot signals from the various satellites to be changed due to Doppler 
shift. If the radio searches the frequency spectrum at the pilot signal's 
transmitted frequency, the signal will only be found if the satellite is 
directly overhead. This is also true of the PN code phase. The radio, 
therefore, must search a time uncertainty for the PN code phase and the 
frequency uncertainty to acquire the satellite. The process of the present 
invention reduces this uncertainty by using knowledge of satellite and 
radio position. 
A typical radio (200) of the present invention is illustrated in FIG. 2. 
The radio (200) is comprised of memory (201) that stores the ephemeris of 
each satellite. In the preferred embodiment, this memory (201) is 
nonvolatile random access memory (RAM). This allows the radio (200) to 
update it's ephemeris data when a satellite is out of service or changes 
orbit. Alternate embodiments use programmable read only memory in which 
each satellite's ephemeris is permanently stored. Still another alternate 
embodiment uses battery backed-up RAM. 
The radio (200) also consists of a real-time clock (205) that is discussed 
later, a processor (210) to control the radio, a vocoder (215) to encode 
and decode the user's voice, a modulation/demodulation circuit (220), and 
the radio frequency electronics (225) that convert the modulated signals 
to higher frequencies for transmission and received signals to lower 
frequencies. 
The process of the present invention, illustrated in FIG. 4, begins with 
the radio synchronizing its internal real-time clock to the satellite 
system time (401), kept by the satellites' or ground stations' internal 
clocks. In the preferred embodiment, the satellites simply reflect the 
clock signals from the ground station. Alternate embodiments use a clock 
in each satellite that is synchronized to the other clocks in the 
satellite system. Since the radio now knows the time and the ephemeris of 
each satellite, it knows the location of the satellites in relation to 
Earth. 
The radio, however, has to determine its position in relation to Earth in 
order to know its position in relation to the satellites. The 
radiotelephone's position is determined (405) by triangulation with 
different satellites or a single satellite in different positions over 
time. Since the radio knows the satellites' positions, it measures the 
time required for a signal to reach and return from each satellite to 
determine its position in relation to Earth. In an alternate embodiment, 
it is possible to use an alternate positioning system such as the global 
positioning system (GPS) or a user entered position. 
The radio's spatial position is stored in memory. If the radio is turned 
off, the time at turn-off is stored with the present spatial position 
(410). After the radio is turned back on, this turn-on time is determined 
from the a real-time clock (415). The difference between the time stored 
at power-down and the time at power-up allows an estimate of how far the 
user may have moved from the previous known position. 
When the estimate of the radiotelephone's possible new position is combined 
with known satellite positions, a list of possible and likely satellites 
in view can be determined along with the possible and likely relative 
position of each. The radio then determines the Doppler shift and PN 
sequence that will be experienced from a signal being transmitted through 
each satellite. The Doppler shift gives the new frequency to find and the 
length of time the radio was off determines the width of the search window 
(420) about this frequency. 
If the radio was only turned off a short time, it could not have traveled 
very far. In this case, the search window is going to be relatively small. 
In the preferred embodiment, the search window starts at 5 kHz for times 
on the order of an hour. If the radio has been off for so long time, such 
as greater than 24 hours, there is still a good chance that the radio did 
not move from its previous power on location. The user may have just 
turned the radio off and left it sitting. The search window, in this case, 
is again relatively small. In the preferred embodiment, this window is 5 
kHz. 
As the time that the radio is off increases, the search window increases. 
This is true up to the preferred embodiment's 24 hour point as discussed 
above. In the preferred embodiment, the search window increases by 5 kHz 
for every hour that the radio was turned off. 
If the process of the present invention does not find the frequency of the 
satellite's pilot signal within the initial search window, the window is 
widened and the search done again. In the preferred embodiment, the window 
is widened 10 kHz. This widening and searching is repeated until the 
frequency of the signal is found or a decision is made that the user 
position is in error or something else is wrong with the ability to 
acquire the signal. 
An example of a search of the frequency spectrum is illustrated in FIG. 3, 
showing the frequency of the pilot signal as f. The expected Doppler shift 
is denoted as .DELTA.. The frequency band to the fight of f is f+.DELTA. 
while the frequency band to the left of f is f-.DELTA.. The -.DELTA. side 
of the spectrum is due to the satellite moving away from the radio while 
the +.DELTA. side is due to the satellite moving towards the radio. 
If the radio has determined that the pilot frequency will be shifted by 
+.DELTA..sub.1 due to the satellite's relative position, a window (301) 
around +.DELTA..sub.1 is searched to find the pilot signal frequency. This 
window increases as the time that the radio is turned off increases. This 
is due to the increasing inaccuracy of the frequency determination the 
longer the radio is turned off. Therefore, if the radio has been turned 
off only a few minutes, the window to be searched will only be 5 kHz wide 
in the preferred embodiment. If the radio is off for a couple of hours, 
the width of this window increases. Alternate embodiments search different 
size frequency windows. 
The process of the present invention also reduces uncertainties produced by 
mechanisms other than Doppler shift of the received signal. In alternate 
embodiments, different satellites use different frequencies, PN codes, 
and/or other differentiating characteristics. The radio's knowledge of the 
location of the satellites relative to the radio enables it to determine 
which frequencies and PN codes are available and should be searched to 
acquire the signal. 
The signal acquisition process of the present invention also enables a 
radio to search a PN code phase that has been affected by a Doppler shift. 
In the same manner as the above described preferred embodiment, the 
process searches a window in the PN code phase, by estimating the Doppler 
affects on the PN code phase, to locate a particular PN code. 
Alternate embodiments of the process of the present invention searches 
different size windows depending on the time that the radio is turned off. 
Alternate embodiments also continue to increase the size of the search 
window to other maximum times, such as 48 hours, before assuming the radio 
has not left its previous position and reverting back to the initially 
small search window. 
The process of the present invention greatly reduces the time required for 
a radio to find a signal in a satellite based communication system. By 
estimating the position of the radio after the radio has been shut down 
for a period of time, the Doppler shift can be estimated and, therefore, 
the window of the frequency band and other uncertainties that must be 
searched is substantially reduced.