U.S. patent application number 09/766433 was filed with the patent office on 2001-05-24 for methods for enhancing service and reducing service cost in mobile satellite systems.
This patent application is currently assigned to Hughes Electronics Corporation. Invention is credited to Chen, Shou, Davarian, Faramaz.
Application Number | 20010001764 09/766433 |
Document ID | / |
Family ID | 22325335 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001764 |
Kind Code |
A1 |
Davarian, Faramaz ; et
al. |
May 24, 2001 |
Methods for enhancing service and reducing service cost in mobile
satellite systems
Abstract
Methods are provided for determining and enhancing the service
efficiency of mobile communications. To assess communication
efficiency in each communication region of interest, a
signal-fading record is generated for that region and this record
is then analyzed to find availability and energy cost for each of a
plurality of power-control parameter sets. The results facilitate
the selection of power-control parameter sets that enhance system
efficiency in each communication region.
Inventors: |
Davarian, Faramaz; (Los
Angeles, CA) ; Chen, Shou; (West Covina, CA) |
Correspondence
Address: |
HUGHES ELECTRONICS CORPORATION
PATENT DOCKET ADMINISTRATION
BLDG 001 M/S A109
P O BOX 956
EL SEGUNDO
CA
902450956
|
Assignee: |
Hughes Electronics
Corporation
|
Family ID: |
22325335 |
Appl. No.: |
09/766433 |
Filed: |
January 19, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09766433 |
Jan 19, 2001 |
|
|
|
09109012 |
Jul 1, 1998 |
|
|
|
Current U.S.
Class: |
455/522 ; 455/65;
455/67.16; 455/69 |
Current CPC
Class: |
H04B 7/18543 20130101;
Y02D 70/446 20180101; Y02D 30/70 20200801 |
Class at
Publication: |
455/522 ; 455/65;
455/67.6; 455/69 |
International
Class: |
H04B 001/10; H04Q
007/20 |
Claims
We claim:
1. A method of enhancing service and service cost in a
communication region served by a mobile communication system in
which reception during the absence of power fades requires that
transmitted power exceed a power threshold P.sub.th, the method
comprising the steps of: in said communication region, determining
the availability and energy cost of power-control communication
signals for each of a plurality of power-control parameter sets
wherein availability is the percentage of transmitting time in
which signals are received; choosing a minimum availability
A.sub.min to be provided in said communication region; selecting,
from said power-control parameter sets, that power-control
parameter set that realizes said minimum availability for the
lowest energy cost wherein energy cost represents the average
energy needed to transmit a received bit of information; and
transmitting communication signals to said communication region
with said selected power-control parameter set.
2. The method of claim 1, wherein said determining step includes
the steps of: receiving transmitted signals in said communication
region to obtain a signal-fading record; and analyzing said
signal-fading record to find said availability and said energy cost
for each of said power-control parameter sets.
3. The method of claim 1, wherein an i.sup.th power-control
parameter set S.sub.i includes a selected static power margin
M.sub.i, a selected power boost B.sub.i and a selected hold time
T.sub.i, and wherein said transmitting step includes, for a
power-control parameter set S.sub.i, the steps of: transmitting
signals with power equal to the sum of said power threshold
P.sub.th plus said static power margin M.sub.i; in response to the
start of each power fade F that exceeds said static power margin
M.sub.i, boosting the transmitted power of said signals by said
power boost B.sub.i; and in response to the end of each power fade
F, terminating said boosting step after said hold time Ti.
4. The method of claim 3, wherein said i.sup.th power-control
parameter set S.sub.i also includes a selected larger power boost
B.sub.i.sub..sub.lgr that is larger than said power boost B.sub.i;
and wherein said boosting step includes the step of boosting the
transmitted power of said signals by said larger power boost
B.sub.i.sub..sub.lgr in response to the start of each power fade F
that exceeds the sum of said static power margin M.sub.i and said
power boost B.sub.i.
5. A method of enhancing service and service cost in a
communication region served by a mobile communication system in
which reception during the absence of power fades requires that
transmitted power exceed a power threshold P.sub.th, the method
comprising the steps of: in said communication region, determining
the availability and energy cost of power-control communication
signals for each of a plurality of power-control parameter sets
wherein availability is the percentage of transmitting time in
which signals are received and energy cost represents the average
energy needed to transmit a received bit of information; choosing a
maximum energy cost C.sub.max for said communication region; from
those power-control parameter sets whose energy cost does not
exceed said maximum energy cost C.sub.max, selecting that
power-control parameter set which has the greatest availability;
and transmitting communication signals to said communication region
with said selected power-control parameter set.
6. The method of claim 5, wherein said determining step includes
the steps of: receiving transmitted signals in said communication
region to obtain a signal-fading record; and analyzing said
signal-fading record to find said availability and said energy cost
for each of said power-control parameter sets.
7. The method of claim 5, wherein an i.sup.th power-control
parameter set S.sub.i includes a selected static power margin
M.sub.i, a selected power boost B.sub.i and a selected hold time
T.sub.i, and wherein said transmitting step includes, for a
power-control parameter set S.sub.i, the steps of: transmitting
signals with power equal to the sum of said power threshold
P.sub.th plus said static power margin M.sub.i; in response to the
start of each power fade F that exceeds said static power margin
M.sub.i, boosting the transmitted power of said signals by said
power boost B.sub.i; and in response to the end of each power fade
F, terminating said boosting step after said hold time Ti.
8. The method of claim 7, wherein said i.sup.th power-control
parameter set S.sub.i also includes a selected larger power boost
B.sub.i.sub..sub.lgr that is larger than said power boost B.sub.i;
and wherein said boosting step includes the step of boosting the
transmitted power of said signals by said larger power boost
B.sub.i.sub..sub.lgr in response to the start of each power fade F
that exceeds the sum of said static power margin M.sub.i and said
power boost B.sub.i.
Description
[0001] This application is a continuation of pending application
Ser. No. 09/109,012 filed Jul. 1, 1998.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to mobile
communications systems.
[0004] 2. Description of the Related Art
[0005] In modern communication systems, the term "mobile" typically
refers to a radio terminal that is attached or carried in a
high-speed mobile platform (e.g., a cellular telephone in a moving
vehicle). This is in contrast to the term "portable" which
typically refers to a hand-held radio that is used by a person at
walking speed (e.g., a cordless telephone inside a home).
[0006] Subscribers generally pay a fee to use a mobile
communication system such as the exemplary system 20 of FIG. 1. The
system 20 has one or more satellites 22 in orbits (e.g.,
geostationary) about the earth 24. The satellites 22 have
transmit-receive communication links 26 (e.g., at L-band
frequencies) with various mobile units 28. The satellites 22 also
have communications (e.g., at optical wavelengths) through links 30
to other satellites and links 32 with various system ports that
form "gateways" into independent communication systems.
[0007] Exemplary gateways are a public-switched telephone network
(PSTN) 34 and an access port 36 of a cellular telephone system. The
PSTN 34 allows any of the mobile units 28 to communicate over
telephone lines 38 with telephone system customers 40. The access
port 36 of the cellular system typically has a plurality of
transceivers arranged in a cellular network so that communications
from the satellites 22 can be transmitted over cellular links 42 to
mobile users 44 of the cellular system.
[0008] Communications with the mobile units 28 are subject to
fading which is a temporary random decrease in the received signal
level. The principal types of fading are multipath fading and power
fading. Multipath fading occurs in areas where multiple reflected
signals (e.g., from nearby buildings) arrive at the mobile unit.
The combined power of these signals can vary widely because it is
dependent upon the phasing of the signals.
[0009] Power fading is caused by blockage of the transmitted signal
by fixed structures (e.g., trees and buildings). The received power
of a mobile unit can drop dramatically (e.g., on the order of 10 or
20 dB) as it moves into the transmission shadow of such structures.
Accordingly, this type of fading is also referred to as
shadowing.
[0010] FIG. 2 illustrates a typical scenario 50 of power fading. A
mobile unit 52 receives signals from a satellite 54 as the unit
moves along a road 56 which borders a line of trees 58. Initially,
the mobile unit receives signals along a transmission path 60 which
is not blocked by the trees. At a subsequent point along the road
56, the mobile unit receives signals along a transmission path 62
which must pass through the canopy of the trees 58. As a result,
the signal power at the mobile unit fades. Typically, fading
increases with decrease in the elevation angle 64 of the
satellite's transmission path.
[0011] Extensive studies of shadowing (e.g., see Goldhirsh, Julius,
et al., Propagation Effects for Land Mobile Satellite Systems,
National Aeronautics and Space Administration Reference Publication
1274, February, 1992) have documented the degree of tree shadowing
in different scenarios. As might be expected, the shadowing depends
upon a variety of factors such as the tree density, the type of
trees, the season and the elevation angle of the transmission
path.
[0012] Availability is a term often used to define a communication
system's reliability. In particular, availability is the fraction
of transmission time that communication signals are successfully
received. Although an availability of 100% is seldom achieved, this
is a goal of communication system design.
[0013] In a conventional passive process for improving
availability, transmitted signal power is increased by a link
margin L.sub.mg above a predetermined power threshold P.sub.th that
is necessary for successful reception (e.g., see Robert G. Winch.,
Telecommunication Transmission Systems, McGraw-Hill, Inc, New York,
1993, pp. 182-186). Although the use of a significant link margin
can improve availability in areas subject to heavy shadowing, it
also increases the transmission energy and, hence, the transmission
cost of the communication system. The cost of satellite-based
communication systems are especially sensitive to increases in link
margin.
[0014] A more efficient approach is an active system that is
conventionally referred to as power control. In power control,
transmitted power is increased over the power threshold P.sub.th by
a static power margin M that is reduced from the link margin
L.sub.mg. When a fade in received power exceeds the static power
margin M, the transmitted power is temporarily increased by a boost
B. The boost B is then removed in response to the fade's
termination. Typically, boost removal is delayed from fade
termination by a hold time T.
[0015] In comparison to passive processes, active power control
allows transmitted power to be reduced generally by L.sub.mg-M with
temporary power increases of B applied in response to excessive
fading. In this process, additional transmitted power is directed
only to those system users that are experiencing fading. Because
the number of users experiencing fades is generally smaller than
the total number of users, power control systems can improve the
ratio of availability to cost.
[0016] Although conventional power control techniques facilitate an
increase in system efficiency, they are generally applied without
any means for assessing their efficiency nor any means for
determining parameter selections that would further enhance that
efficiency.
SUMMARY OF THE INVENTION
[0017] The present invention is directed to power control methods
that include processes for determining and enhancing the efficiency
of mobile communications in different service regions.
[0018] These goals are achieved with processes that receive
transmitted signals in communication regions to obtain a
signal-fading record for each region. Subsequently, each
signal-fading record is analyzed to find availability and energy
cost for each of a plurality of power-control parameter sets. An
exemplary parameter set S.sub.i includes a selected static power
margin M.sub.i, a selected power boost B.sub.i and a selected hold
time T.sub.i.
[0019] In one process of the invention, a minimum availability
A.sub.min is chosen for each region and, from the parameter sets of
that region whose availability is not less than A.sub.min, the
parameter set with the least energy cost is selected. Finally,
communication signals are transmitted to that region with the
selected parameter set.
[0020] The teachings of the invention thus allow a communication
provider to reduce energy costs while being certain of providing
communication users with an availability that is not less than a
predetermined minimum.
[0021] In another process of the invention, a maximum energy cost
C.sub.max is chosen for each region and, from the parameter sets of
that region whose energy costs do not exceed C.sub.max, the
parameter set with the greatest availability is selected.
Communication signals are then transmitted to that region with the
selected parameter set. A communication provider can thus provide
the greatest possible availability while being certain of not
exceeding a predetermined maximum energy cost.
[0022] The teachings of the invention can be applied to any
transmitter of of the communication system 20 of FIG. 1, but they
are especially advantageous for the satellites 22 because their
energy sources are limited and their income-generating ability is
related to the number of communication users they can serve. This
number can be increased by increasing the operating efficiency of
each satellite.
[0023] The novel features of the invention are set forth with
particularity in the appended claims. The invention will be best
understood from the following description when read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematized view of an exemplary mobile
satellite communications system;
[0025] FIG. 2 is a side elevation view of a portion of the
satellite communications system of FIG. 1 which illustrates mobile
unit power fading that is addressed by the present invention;
[0026] FIG. 3 is a timing diagram which illustrates power control
processes, timing and nomenclature as applied to the present
invention;
[0027] FIGS. 4A and 4B are graphs which illustrate prototype tests
of the present invention with a mobile unit in two exemplary field
locations;
[0028] FIG. 5 is a flow chart which shows exemplary methods of the
present invention; and
[0029] FIG. 6 is a flow chart which shows other exemplary methods
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Methods of the present invention provide processes for
determining and enhancing the service efficiency of mobile
communications. An understanding of a detailed description of these
methods will be facilitated by preceding this description with the
following introduction into power control processes, timing and
nomenclature.
[0031] FIG. 3 illustrates a timing graph 70 of power control
processes in one of the transmission links of FIG. 1 (e.g,, the
down-links 26 to mobile units 28). The graph 70 includes a plot 72
of the power of transmitted signals, a predetermined power
threshold (P.sub.th) 74 that is necessary for successful reception
in the absence of fading and a plot 76 of the power of received
signal power. The transmitted power 72 has a nominal level 78 that
exceeds the threshold 74 by a static power margin M.
[0032] In the absence of fading, the received power has a nominal
level 80. In an exemplary fading, the received power fades to a
level 82. Because this power fade exceeds the static power margin M
(as indicated by the broken line 84), power control activates a
boost B of the transmitted power to a new level 86 (static power
margin M and boost B are typically expressed in decibels (dB)). At
this time the fade margin of the transmitted power 72 has increased
from the static fade margin M to a boosted fade margin of M+B. In
this case, the fade is captured by power control, i.e., successful
reception is achieved.
[0033] The power boost B is delayed from initiation of the power
fade by a boost delay time .tau.. Because of fading, the signals
transmitted from the mobile unit exhibit the same power fade as the
signals received by the mobile unit. Thus, detectors in the
communication satellite can be used to detect the fade in the
satellite's received power. In response, the satellite's
transmitted power is then boosted by the power boost B. In this
exemplary boost activation process, detection begins one signal
transit time following initiation of the power fade. After a
processing time, power is boosted and the boosted power level
arrives at the mobile unit following another signal transit
time.
[0034] Typical signal transit times from satellites in
geostationary orbits are on the order of 120 milliseconds. When
this transit time is doubled and increased by the processing time,
the total boost delay time .tau. has a nominal value that is
somewhat less than one second. The delay time .tau. is reduced if
the orbit altitude is less than that of a geostationary orbit. For
example, the signal transit time is less than 1/2second for a
satellite in a low earth orbit (LEO).
[0035] In another exemplary boost activation process, a system
gateway (e.g., the PSTN 34 of FIG. 1) detects power fades. In
response to a detected fade, the gateway sends a fade signal to the
satellite which then boosts the transmitted signal. Accordingly,
gateway detection systems typically add an additional pair of
transit times to the delay time of satellite detection systems.
[0036] In all power control systems, it is apparent from FIG. 3
that signal reception is lost for a boost delay time .tau. for each
occurrence in which a received power fade exceeds the static power
margin M.
[0037] FIG. 3 indicates a termination 90 of the received power
fade. Following a hold time T, the boost B is removed and
transmitted power 72 falls back to its nominal level 78. It has
been found that the introduction of the hold time T often improves
the effectiveness of power control because it reduces excessive
toggling of the boost.
[0038] For example, FIG. 3 indicates another fade 92 of received
power which exceeds static power margin M. Depending upon the
duration of the face 92, it might or might not initiate a power
boost on its own. Because of the hold time T, however, the boosted
transmitted power is still present and prevents a loss of reception
without the need for initiation of another boost cycle.
[0039] In contrast, momentary fades (e.g., the fade 94) whose fade
time t.sub.1 is less than the boost delay time .tau., and which
occur after the hold time T, will not be benefited by power
control. Even if the transmitted power is boosted, this boost
arrives too late to prevent the loss of reception during the
fade.
[0040] The timing diagram of FIG. 3 indicates that the timing of
the hold time T is uninterrupted once it has been initiated by the
termination 90 of a power fade. In other embodiments of the
invention, the hold time T can continue to be initiated by each
power fade that occurs prior to the end of the hold time. In this
embodiment, for example, the hold time T would be reinitiated by
the power fade 92 if its duration exceeds the delay time .tau..
This method for initiating hold times may be more effective for
certain mobile communication locales.
[0041] Following the fade 94, the timing diagram 70 indicates that
received power 76 again fades to a fade level 96 that exceeds the
static power margin M. In response, the transmitted power 72 is
boosted by boost B to the level 86 after a boost delay time .tau..
Subsequently, the received power fades further to a level 98 which
exceeds a fade of M+B (as indicated by the broken line 99). Fades
of this intensity cannot be captured by power control, i.e.,
reception is lost during the time t.sub.2 of a fade that exceeds
M+B.
[0042] However, various power control modifications may be made to
further improve availability. For example, the power control system
may provide multilevel boosts such as a selected first boost
B.sub.1 and a selected second power boost B.sub.2 that is larger
than the boost B.sub.1. Activation of these power boosts would
depend on the magnitude of the received fade level 82 in FIG. 3
(e.g., a fade greater than M+B.sub.1 would initiate a boost of
B.sub.2).
[0043] In contrast to an active power control system, a static
communication system typically transmits signals with a static
level that exceeds the power threshold 74 by a fixed link margin
L.sub.mg. The fixed link margin L.sub.mg generally exceeds the
static power margin M. Although it is shown by a broken line 100 in
FIG. 3 to be somewhat less than M+B, this relationship is a
function of each specific communication system. It is apparent that
the power savings of a power control system is L.sub.mg-M during
periods in which power fades are less than M. Although this savings
is reduced because of the boost B during power fades that exceed M,
power control still increases system efficiency except in cases
where such fades are extremely numerous.
[0044] As previously stated, teachings of the present invention
provide processes for determining and enhancing the service
efficiency of mobile communications. In these teachings, the
results of various possible power control settings are assessed
over a measured time T.sub.mes. Without power control but with a
power margin M, availability is given by 1 V = 1 - D tot T mes = T
mes - D tot T mes ( 1 )
[0045] in which D.sub.tot is the total time in which signal fades
exceed M. In a power control communications system, availability is
2 V pc = 1 - D tot - [ ( D i ) - D ] T mes ( 2 )
[0046] In equation (2), D.sub.i is the duration of the i.sup.th
fade that is captured by power control so that .SIGMA.D.sub.i is
the total time in which fades exceed M but do not exceed M+B.
D.sub..tau.is the total time that reception is lost due to the
first portion of faded signals that have been captured by power
control, i.e., D.sub..tau. equals .tau. times the total number of
fades captured by power control.
[0047] A communication system user can therefore use a figure of
merit given by 3 r = V pc V ( 3 )
[0048] to assess the value of power control.
[0049] If w denotes satellite power in watts that is allotted to
one communication channel, this power is increased, during power
control capture, to wb in which b=10.sup.0.1B. Without power
control, the satellite energy per communication channel is
E=wT.sub.mes (4)
[0050] and with power control, the satellite energy per
communication channel is 4 E pc = wT mes { 1 + ( b - 1 ) D pc T mes
} ( 5 ) or E pc = E + D pc ( b - 1 ) w ( 6 )
[0051] in which D.sub.pc is the total time duration that power
control is activated and (b-1)w is the additional power
transmitted.
[0052] When power control is used, a useful service time can be
defined to be
S.sub.pc=.beta..sub.v.sub..sub.pcV.sub.pcT.sub.mes (7)
[0053] and a useful service time without power control is
S=.beta..sub.vVT.sub.mes (8)
[0054] in which .beta..sub.v.sub..sub.pc and .beta..sub.v are
coefficients less than one. The coefficients account for the data
lost or communication links dropped during deep fades and is a
function of availability. Lost data must generally be retransmitted
while lost links must be re-established with consequent call setup
overhead (i.e., increased cost).
[0055] Operational cost function C is defined as satellite energy
used for each second of useful service time. An equivalent
definition is the average energy needed to transmit a useful bit of
information. When power control is used, the cost function is 5 C
pc = E pc S pc ( 9 )
[0056] and without power control, the cost function is 6 C = E S
.
[0057] The teachings of the invention facilitate decreases in the
operational cost function C.sub.pc by proper selection of values of
various parameters such as hold time T and power boost B. Static
power margin M can also be a variable parameter. In this more
general case, the operational cost function of equations (9) and
(10) are changed to 7 C pc = m E pc S pc = m E + D pc ( b - 1 ) w v
pc V pc T mes ( 11 ) and C = m E S ( 12 )
[0058] in which an adjustment m=10.sup.M/10. This adjustment is
needed because larger fade margins (M) increase the satellite power
provided to each unit. Thus, the cost function must be
appropriately penalized.
[0059] Among possible sets of variable communication parameters,
the processes of the invention facilitate the selection of a
parameter set that minimizes the cost functions C.sub.pc of
equations (9) and (11).
[0060] Prototype tests of the teachings of the present invention
were conducted under real field conditions. These tests were
carried out with a mobile unit at various locations in the United
States with communication signals received from a geostationary
satellite (MARECS-B at orbital position of 304.5.degree. E) at a
carrier frequency of 1.542 GHz. The mobile unit's sampling rate was
1000 samples/second, the data resolution was 0.01 dB and the
receiver's dynamic range was 25 dB.
[0061] The mobile unit's received power in each region formed a
record of actual fading for that region. For each of a plurality of
parameter sets, this fading record was then analyzed (with a
genetic algorithm) to determine the availability and the energy
cost that would result in that region when signals were transmitted
with that parameter set.
[0062] The test results for two exemplary locations are shown in
the graphs 120 and 130 of FIGS. 4A and 4B respectively. Each graph
plots availability as a function of the average energy needed to
transmit a useful bit (i.e., operational cost function C.sub.pc).
The mobile unit location for the tests summarized in FIG. 4A was in
the western portion of the state of Washington. This location was
wooded and the satellite elevation angle (angle 64 in FIG. 2) was
.about.7 degrees. The mobile unit location for FIG. 4B was near
Schaumburg, Ill. This latter location was rural, not wooded and the
satellite elevation angle was .about.30 degrees.
[0063] Each parameter set was made up of selected values of static
power margin M, power boost B and hold time T. As indicated in the
legend of graph 120, the analyzed results for a first group of
parameter sets is represented by solid squares. In this group, the
static power margin M was held at 1 dB with power boost variations
of 1, 3 and 5 dB and with hold time variations of 0, 2 and 4
seconds. The results of this group of parameter settings is shown
in the plot group 122 where data points with common power boosts
are connected by solid lines. For clarity of illustration,
different hold times are not noted but are distinguishable because
increased hold time increases availability.
[0064] Test results with the static power margin M increased to 3
dB and further increased to 5 dB are respectively shown with solid
triangles and solid circles and respectively labeled as plot groups
124 and 126. In each plot group, availability was improved as the
hold time T was increased.
[0065] The analyzed test results of FIG. 4A enable a communication
system designer to determine performance of various parameter sets
with respect to cost function and availability. As a first example,
it is apparent that increasing the static power margin from 1 dB to
3 dB in the test region caused a significant improvement in
availability whereas availability increase was less dramatic when
the static power margin was further increased to 5 dB. In addition,
the availability improvement with a static power margin of 3 dB was
generally gained with a decrease in the cost function C.sub.pc. In
contrast, the availability improvement with a static power margin
of 5 dB required an appreciable increase in the cost function
C.sub.pc.
[0066] Second, FIG. 4A shows that increasing the power boost
generally increased availability when the static power margin was 1
dB. However, this increase was realized with a substantial increase
in cost function. When the static power margin was 5 dB, increased
power boost again required a substantial increase in cost function
but with very little gain in availability. The results when the
static power margin was 3 dB fell between those for static power
margins of 1 and 5 dB.
[0067] Thirdly, it is seen that large availability increases were
realized with increased hold times when the static power margin was
1 dB (plot group 122). These improvements required very little
increase in the cost function (even a decrease in cost function in
some cases). With a static power margin of 5 dB, increasing the
hold time required significant increases in the cost function with
only moderate increase in availability. The results with a static
power margin of 3 dB fell between those with static power margins
of 1 and 5 dB.
[0068] It can be further observed that availability is not
appreciably increased by increased power boost unless this is
accompanied with significant hold times. This follows because many
fades have a short time duration. Thus, power boosts do not occur
soon enough to be beneficial. Availability is enhanced mainly by
catching the fades which follow the one that triggered activation
of power control.
[0069] As stated above, the first location was wooded and had a low
satellite elevation angle. At this location, increased power boost
and increased hold time gave large efficiency improvements when the
static power margin was low. Increasing these two parameters was
less beneficial as the power margin increased.
[0070] The teachings of the invention facilitate efficient choices
of communication parameters which enhance mobile communications
service. In a first example, assume the designer of a communication
system for the region of FIG. 4A wished to provide service with the
lowest possible cost. The test results show that an appropriate
parameter set would be the set of M=3 dB, B=1 dB and T=0 seconds.
For this selection, the cost function would be on the order of 6.7
and availability would be on the order of 0.66.
[0071] In a second example, assume that the communication system
designer wished to provide mobile customers with an availability of
at least 70% as indicated by the broken line 127. FIG. 4A shows
that this availability can be achieved most efficiently (i.e., with
the least cost function) with a parameter set of M=3 dB, B=3 dB and
T=2 seconds.
[0072] In a third example, assume that the communication system
designer wished to provide service to his mobile customers without
exceeding a cost represented by the broken line 128. With this
limitation on cost, FIG. 4A shows the greatest availability can be
provided with a parameter set of M=3 dB, B=1 dB and T=4
seconds.
[0073] Finally, assume that the communication system designer was
willing to raise his cost to the broken line 129. With this revised
limitation on cost, FIG. 4A shows the greatest availability can be
provided with a parameter set of M=5 dB, B=1 dB and T=4
seconds.
[0074] In the service region associated with FIG. 4A, communication
signals have a low satellite elevation angle and must often
penetrate thick tree cover to reach mobile units. In contrast,
communication signals in the region of FIG. 4B have a high
satellite elevation angle and seldom encounter a tree cover. In
this region, the test results for each setting of static power
margin lie very close to each other. Accordingly, they are not
clearly differentiated and are only shown in general groups 132,
134 and 136. Similar to FIG. 4A, test results with static power
margins of 1 dB, 3 dB and 5 dB are respectively indicated with
solid squares, triangles and circles.
[0075] Availability of .about.80% is achieved in this region with a
static power margin of 1 dB and is only slightly changed by
increases in boost and hold time. The availability is raised to
.about.90% when the static power margin is increased to 3 dB. This
availability increase requires only a slight increase in cost
function. In contrast, availability can be raised above 90% by
increasing the static power margin to 5 dB but a significant
increase in cost function is required.
[0076] Availability in this communication region is basically a
function of static power margin and is only minimally affected by
changes in boost power and hold time. Minimum cost function would
be realized with a parameter set in which M=1 dB (plot group 132).
A minimum desired availability of .about.85% (represented by the
broken line 137) could be provided at the lowest cost with a
parameter set in which M=3 dB (plot group 134).
[0077] FIGS. 4A and 4B graphically illustrate availability and cost
function for two different mobile service regions. In practicing
the methods of the invention, analysis of test data to find
relationships of the invention (e.g., equation (11) above) can be
accomplished with various conventional data analysis tools (e.g., a
numerical-optimization method such as a genetic algorithm).
[0078] Processes of the invention are exemplified in the flow chart
140 of FIG. 5. The flow chart begins with process step 142 in which
transmitted signals are received in a communication region to
obtain a signal-fading record. This record is then analyzed in step
144 to find availability and energy cost for each of a plurality of
power-control parameter sets which have exemplary parameters of
power margin M.sub.i, boost B.sub.i and hold time T.sub.i.
[0079] A minimum availability A.sub.min is chosen for the
communication region in step 146 (i.e., an energy cost which a
communication provider is willing to incur). In step 148, the
parameter set with the least energy cost is then selected from
those parameters sets whose availability is not less than the
minimum availability A.sub.min. In step 150, communication signals
are then transmitted in the communication region with signal
parameters set to those of the selected parameter set.
[0080] As exemplified by the graphs 120 and 130 of FIGS. 4A and 4B,
the processes of FIG. 5 may lead to the selection of different
parameter sets for different communication regions and that
selection is directed by the signal-fading records of each region.
Thus, the communication provider can reduce his energy costs while
being certain of providing communication users an availability that
is not less than a predetermined minimum availability A.sub.min.
Although the teachings of the invention are applicable to any
transmitter (e.g., the mobile units 28 or the gateway 36) in the
communication system 20 of FIG. 1, they are especially advantageous
for the satellites 22 whose energy sources are limited and whose
income-generating ability is related to the number of communication
users they can serve. Accordingly, it is especially important for
the satellites to operate as efficiently as possible.
[0081] Other processes of the invention are exemplified in the flow
chart 160 of FIG. 6. The flow chart 160 is initiated by the initial
steps 142 and 144 of FIG. 5. In contrast to the processes of FIG.
5, a maximum energy cost C.sub.max is chosen for the communication
region in step 162 (i.e., an energy cost which a communication
provider is willing to incur). In step 164, the parameter set with
the greatest availability is then selected from those parameters
sets whose energy cost does not exceed the maximum energy cost
C.sub.max. In step 166, communication signals are then transmitted
in the communication region with signal parameters set to those of
the selected parameter set.
[0082] Thus, the communication provider is assured of providing the
best availability possible while not exceeding the maximum energy
cost C.sub.max which the provider can accept.
[0083] Although processes of the invention have been described
primarily with reference to downlink transmission from a satellite
to a mobile unit (e.g., along the communication links 26 of FIG.
1), these teachings also can be applied to uplink transmission from
mobile units to a satellite.
[0084] While several illustrative embodiments of the invention have
been shown and described, numerous variations and alternate
embodiments will occur to those skilled in the art. Such variations
and alternate embodiments are contemplated, and can be made without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *