WO2001054314A1

WO2001054314A1 - Diversity system method in a satellite telecommunication network

Info

Publication number: WO2001054314A1
Application number: PCT/US2001/001772
Authority: WO
Inventors: Peter D. Karabinis; Wail M. Refai
Original assignee: Ericsson Inc
Priority date: 2000-01-19
Filing date: 2001-01-18
Publication date: 2001-07-26
Also published as: KR20020067933A; AU2001234488A1; EP1250769A1

Abstract

A single uplink signal burst from a user terminal in an uplink region of a satellite radiotelephone system is received during an uplink signal frame at two or more visible satellites in the uplink region. The single received signal burst from the user terminal that is received at the two or more satellites then is diversity combined. The two or more satellites preferably receive a single uplink signal burst from a plurality of, and preferably all of, the user terminals in the uplink region without time overlap, so that diversity combining may be performed. Most preferably, all of the satellites receive signal bursts from all of the user terminals in the uplink region during the uplink signal frame, without time overlap, so that return link diversity combining using all of the visible satellites may be obtained. In order to allow reception by two or more visible satellites in the return link, of signal bursts from multiple user terminals in the uplink region, a guard time is established between adjacent uplink signal bursts that are transmitted from the user terminals in the uplink region. The guard time is based upon a time of arrival difference for the adjacent uplink signal bursts to one of the satellites, for example having lowest elevation angle or at the horizon. Fixed or variable guard times may be provided between adjacent uplink signal bursts that are transmitted from user terminals in the uplink region. Thus, the satellite radiotelephone system preferably can utilize all of the transmitted signals from the user terminals, to allow improved reception by the satellite and/or reduced power consumption by the user terminals.

Description

DIVERSITY SYSTEM METHOD IN A SATELLITE TELECOMMUNICATION NETWORK

CROSS REFERENCE TO RELATED APPLICATION

This invention is related to co-filed application Serial No. to the present inventors, entitled Timing Systems and Methods for Forward Link Diversity in Satellite Radiotelephone Systems and assigned to the assignee of the present invention (Attorney Docket 8194-379), the disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION This invention relates to radiotelephone systems and methods, and more particularly to satellite radiotelephone systems and methods.

BACKGROUND OF THE INVENTION Satellite radiotelephone systems are being developed and deployed at many locations around the world. As is known to those having skill in the art, a satellite radiotelephone system generally includes at least one satellite and at least one gateway that interfaces the satellite radiotelephone system to other telephone systems, such as wire telephone systems and/or cellular radiotelephone systems. A plurality of user terminals communicate with the at least one satellite, to provide satellite communications. The user terminals may be mobile or fixed. It will be understood that the user terminal may be a satellite radiotelephone, a combined cellular and satellite radiotelephone, a high functionality terminal including Personal

Communications Systems (PCS) terminals and/or a portable computer with a satellite radiotelephone modem. The basic principles regarding a satellite radiotelephone system are described in the publication entitled Dual-Mode Cellular/Satellite Hand- Held Phone Technology by coinventor Karabinis et al, WESCON/96, pp. 206-222, October 22, 1996, and need not be described in further detail herein.

In geostationary systems, or non-geostationary systems such as Low Earth Orbit (LEO) or Medium Earth Orbit (MEO) systems, a user terminal can communicate with more than one satellite Accordingly, satellite diversity may be provided so that the user terminal communicates with two satellites, to thereby allow reduced shadowing and/or blockage problems In particular, uplink or return link signal bursts such as uplink Time Division Multiple Access (TDMA) bursts, are received by at least two satellites over at least two corresponding earner frequencies and then diversity combined, for example at a ground station that connects the satellite radiotelephone system with the wire, cellular and/or other satellite radiotelephone systems A gam of 3dB in signal to noise ratio is obtainable when two satellites receive the same burst of equal quality and relay it to the ground station When the burst is fading m an uncorrelated fashion with respect to the two satellites, significant statistical benefits in link quality may be obtained Improved reception of the uplink signal bursts by the satellites and/or reduced power consumption by the transmitter of the user terminals thereby may be obtained

Notwithstanding the above-descnbed improvements, there continues to be a desire to provide improved reception of uplmk signal bursts by satellites and/or reduced power consumption by the transmitters of satellite radiotelephone system user terminals Accordingly, there continues to be a need for improved uplink diversity systems and methods for satellite radiotelephone systems.

SUMMARY OF THE INVENTION

The present invention can provide systems, methods, user terminals and satellites in which a single uplink or return link signal burst from a user terminal in an uplink region of a satellite radiotelephone system is received dunng an uplink signal frame at two or more visible satellites in the uplink region The single received signal burst from the user terminal that is received at the two or more visible satellites is diversity combined The two or more satellites preferably receive a single uplink signal burst from a plurality of, and preferably all of, the user terminals in the uplink region dunng the uplink signal frame without time overlap, so that diversity combining can be performed Most preferably, all of the visible satellites in the uplink region receive a single uplink signal burst from each of the user terminals in the uplink region during the uphnk signal frame without time overlap, so that diversity combining using all of the visible satellites in the uplink region may be obtained Improved reception of uplink signal bursts and/or reduced power consumption in the user terminals thereby may be obtained In order to allow reception by two or more visible satellites in an uplink region, of signal bursts from multiple user terminals in the uplink region, a guard time is established between adjacent in time uplink signal bursts that are transmitted from the user terminals in the uplink region The guard time is based upon a time of arnval difference for the adjacent uplink signal bursts to one of the visible satellites, for example to one of the visible satellites having lowest elevation angle Alternatively, time of arnval differences to a real or fictitious satellite at the honzon may be used Fixed or vanable guard times may be provided between adjacent uplink signal bursts that are transmitted from two user terminals m the uplink region The fixed guard time preferably coπesponds to a maximum time of arnval difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region, to one of the plurality of satellites, preferably to one of the plurality of satellites having lowest elevation angle or a real or fictitious satellite at the honzon By taking into account the maximum time of arnval difference from user terminals that are a maximum distance apart in the uplmk region, it can be assured that all uplink signal bursts from all user terminals in the uplink region will be received at two or more visible satellites, and preferably all of the visible satellites in the uplink region, without time overlap between adjacent bursts Since the fixed guard time is used by all user terminals, the fixed guard time can be programmed into the user terminals or otherwise provided without the need to factor the actual position of the user terminal and/or the visible satellites However, the fixed guard time may be excessive for adjacent uplink signal bursts by pairs of user terminals that are closer together than the maximum distance in the uplink region Accordingly, reduced complexity may be obtained at the possible expense of decreased capacity

Vanable guard times may be incorporated, according to the present invention, between adjacent uplink signal bursts that are transmitted from the user terminals located m the same uplmk region and using the same uphnk earner frequency The vanable guard time preferably corresponds to a time of arnval difference between adjacent in time uplink signal bursts from the corresponding pair of user terminals to one of the visible satellites, preferably the visible satellite having lowest elevation angle or a real or fictitious satellite at the honzon Thus, a guard time may be computed between each pair of adjacent uplink signal bursts based on the actual position in the uplink region ot the corresponding pair of user terminals Improved efficiency thereby may be obtained compared to a fixed guard time, because the guard time between each pair of adjacent signal bursts may be based on the actual position of the user terminal rather than a worst case separation. Increased complexity may be produced however, because the guard time may need to be computed and separately transmitted to each user terminal and may change as the position of the user terminal and/or satellite changes within the uplink region.

In the above embodiments, signal bursts from user terminals in an uplink region, and preferably signal bursts from all of the user terminals in the uplink region using the same carrier frequency, may be received at two or more, preferably all, of the visible satellites in the uplink region. Thus, the satellite radiotelephone system preferably can utilize all of the transmitted signal bursts from the user terminals, to allow improved reception by the radiotelephone system and/or reduced power consumption by the user terminals.

Satellite user terminals according to the present invention include a transmitter that transmits an uplink signal burst for reception by a plurality of satellites that receive transmissions from an uplink region in which the user terminal is located. The uplink signal is transmitted a guard time after an immediately preceding uplink signal burst from another user terminal in the uplink region. The guard time is based upon a time of arrival difference for the adjacent uplink signal bursts to one of the plurality of visible satellites, for example having lowest elevation angle or a real or fictitious satellite at the horizon. The satellite user terminals also may include a user interface that receives user inputs including voice and/or data and provides the user inputs to the transmitter for transmission.

Satellite user terminals according to the invention may use a fixed guard time that corresponds to maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region, to one of the plurality of visible satellites, preferably the visible satellite having lowest elevation angle or a real or fictitious satellite at the horizon. Alternatively, satellite user terminals may use a variable guard time that corresponds to a time of arrival difference between adjacent uplink signal bursts from the user terminal and another user terminal, to one of the plurality of visible satellites, preferably a visible satellite having lowest elevation angle or a real or fictitious satellite at the horizon. Finally, in the above-described embodiments, the uplink or return signal bursts and downlink or forward signal bursts are communicated in a plurality of repeating uplink and downlink burst frames. Preferably, two downlink signal bursts are received from two satellites in a downlink burst frame. More preferably, the two downlink signal bursts arrive at the center of the downlink region one half a downlink burst frame apart. In one embodiment, a first downlink signal burst is received from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst is received from a second one of the visible satellites at a second carrier frequency, during a single downlink signal burst frame. In another embodiment, a first downlink signal burst is received from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst is received from a second one of the visible satellites at the first carrier frequency, during a single downlink signal burst frame. By providing two forward link bursts in the frame, forward link diversity also may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is an overall block diagram of satellite radiotelephone systems and methods according to the present invention.

Figure IB is a timing diagram of transmitted and received signal bursts for satellite radiotelephone systems and methods of Figure 1 A.

Figures 2A and 2B conceptually illustrate calculation of guard times according to the present invention.

Figure 3 is a timing diagram that illustrates integration of downlink and uplink timing according to the present invention. Figures 4A and 4B are timing diagrams that illustrate alternate embodiments of forward link burst timing according to the present invention.

Figure 5 illustrates preferred embodiments of the present invention for an eighth rate GSM mode satellite radiotelephone system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which prefeπed embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

As will be appreciated by one of skill in the art, the present invention may be embodied as methods and/or devices. The present invention may take the form of an entirely hardware embodiment or an embodiment combining software and hardware aspects.

The present invention is described herein with reference to block diagrams. It will be understood that a block, and combinations of blocks, can be implemented by computer program instructions. These program instructions may be provided to a processor to produce a machine, such that the instructions which execute on the processor create means for implementing the functions specified in the block or blocks. The computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions specified in the block or blocks.

Accordingly, blocks of the drawings support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and computer program instruction means for performing the specified functions. It will also be understood that each block, and combinations of blocks, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or by combinations of special purpose hardware and computer instructions.

Referring now to Figures 1A and IB, a conceptual overview of the present invention will be provided. As shown in Figure 1 A, a satellite radiotelephone communications system 100, such as a LEO or MEO satellite radiotelephone communications system, includes a plurality of visible satellites HOa-l lOn that receive transmissions from a plurality of user terminals 120a-120m in an uplink region 130. It will be understood that although only three visible satellites 110, three user terminals 120 and a single uplink region 130 is shown, a satellite radiotelephone communications system may include many more satellites, user terminals and uplink regions. The user terminals may be mobile or fixed.

As shown in Figure 1 A, uplink or return link signal bursts, such as TDMA bursts, from a user terminal 120 and preferably from all of the user terminals 120a- 120m in the uplink region 130 are received at two or more, preferably all of the visible satellites l lOa-HOn in the uplink region 130 More specifically, as shown in Figure 1 A, an uplink signal burst 140a from user terminal 120a is received at each of the visible satellites 1 lOa-l 1 On An uplink signal burst 140b from user terminal 120b is received at all of the visible satellites HOa-llOn An uplink signal burst 140m from user terminal 120m is received at all of the visible satellites llOa-HOn

The uplink signal bursts that are received at each of the visible satellites 110a- 1 lOn preferably are received without time overlap Thus, each visible satellite 110 transmits to a ground station 150 a sequence of received uplink signal bursts without time overlap In particular, as shown in Figure 1A, satellite 110a transmits to the ground station 150 a sequence 160a of received uplink signals bursts without time overlap Satellite 110b transmits to the ground station 150 a sequence 160b of received uplink signal bursts without time overlap Finally, satellite 11 On transmits to the ground station 150 a sequence 160n of received signal bursts without time overlap A diversity combiner 170 combines the received signals from the satellites 1 lOa-l lOn It will be understood by those having skill in the art that more than one ground station 150 may be provided, and the diversity combiner 170 need not be included in the ground station 150 Moreover, contπbutions from a plurality of ground stations 150 may be provided to a single diversity combiner 170 In order to allow the satellites 110 to receive the uplink signal bursts without time overlap, a guard time is provided between adjacent uplink signal bursts that are transmitted from the user terminals 120 The guard time is established based upon a time of arnval difference for the adjacent uplink signal bursts to one of the satellites 110, and preferably the one of the satellites 11 On having a lowest elevation angle Moreover, the guard time can be calculated assuming that the satellite with the lowest elevation angle is at the horizon Timing diagram (1) of Figure IB graphically illustrates a time sequence of uplink signal bursts that are transmitted by the plurality of user terminals 120a-120m with a fixed guard time G_f therebetween The fixed guard time G_f preferably conesponds to a maximum time of arnval difference between adjacent uplink signal bursts from a pair of user terminals, for example user terminals 120a and 120m, that are a maximum distance apart in the uphnk region 130, to one of the satellites 110, preferably the satellite l lOn having lowest elevation angle Thus, as shown in timing diagram (4) of Figure IB, the uplink signal bursts that are received at any of the satellites l lOa-HOn will be received without time overlap. In the worst case, shown in timing diagram (3) of Figure IB, the received uplink signal bursts at the satellite 1 lOn having lowest elevation angle (or at the horizon) also will have no time overlap, although adjacent signals may abut one another. Burst collisions therefore may be avoided. Returning to timing diagram (1) of Figure IB, since the fixed guard time G_f is fixed for every user terminal 120, it may be preprogrammed into the user terminal or provided to the user terminal upon initialization or at other times. Accordingly, simplified user terminals may be obtained. However, since the worst case guard time G_f is used regardless of the position of the user terminal, the number of user terminals that may be accommodated within an uplink region may be reduced.

Timing diagram (2) of Figure IB illustrates another embodiment of the invention, wherein a variable guard time Gi-G_m-i is used that corresponds to a time of arrival difference between adjacent uplink signal bursts from a conesponding pair of user terminals to one of the satellites HOa-l lOn, preferably the satellite HOn having lowest elevation angle or a real or fictitious satellite at the horizon. Again, as shown in timing diagrams (3) and (4) of Figure IB, two, preferably three, and most preferably all of the satellites 110 may receive the sequence of uplink signal bursts without time overlap. However, since the variable guard time Gι-G_m-ι is individually selected based on the positional relationship in the uplink region between corresponding pairs of user terminals that transmit adjacent uplink signal bursts, an increased number of signal bursts may be accommodated compared to the fixed guard time of timing diagram ( 1) of Figure IB. Unfortunately, since each user terminal 120a-120m may need to use a different guard time Gi-G_m-i, the guard times may need to be computed and supplied to the individual terminals 120a-120m upon initialization, and may need to change as the position of a user terminal 120 changes. Accordingly, complexity of the user terminals and/or the satellite radiotelephone communications system 100 may increase. It also will be understood that a system may use both fixed and variable guard times for different user terminals 120 in the system. Figures 2A and 2B conceptually illustrate calculation of guard times according to the present invention. Figures 2A and 2B assume that a GSM TDMA architecture is used in a satellite radiotelephone system. In Figure 2A, it is assumed that the uplink region 130 is 250 km by 250 km in size. Four user terminals, designated User #1-User #4 are shown in the uplink region 130. As shown in Figure 2 A, User #1 and User #2 are very close to each other and a guard time need not be provided. User #2 is separated from User #3, and User #3 is separated from User #4 by a distance of about 173 km. As shown, the guard time Δt may be computed by the distance between the users Δd divided by the speed of light c (3 x 10 m/s). In Figure 2A, the guard time between User #2 and User #3, and the guard time between User #3 and User #4 is about one forward link slot in the GSM TDMA architecture or about 0.577 msec.

Figure 2B illustrates an example where User #1 and User #2 are adjacent one another and User #3 and User #4 are adjacent one another, but User #2 and User #3 are separated by a maximum amount in an uplink region 130 of about 700 km. In this scenario, the guard time G between User #2 and User #3 is about 4 GSM forward link slots or about 2.3 msec. It also will be understood by those having skill in the art that the guard time G of Figure 2B may be used as a fixed guard time between each adjacent uplink signal burst, because it represents the worst case scenario of maximum separation between users corresponding to adjacent uplink signal bursts. Figure 3 illustrates integration of downlink (forward) and uplink (return) timing according to the present invention. For simplicity in depicting the timing relationships, Figure 3 assumes that all user terminals are co-located within uplink region 130. As shown in timing diagrams (a) and (b) of Figure 3, first and second downlink bursts may be provided from first and second satellites at first and second carrier frequencies fl and f2. Each burst may occupy one slot of 64 slots in an eighth rate GSM TDMA forward link frame architecture. As shown in timing diagrams (c) - (f) of Figure 3, the uplink bursts may be provided for each user using four separate frequencies and received by all satellites that are in view. Figures 4A and 4B illustrate alternate embodiments of forward link burst timing. Figure 4A corresponds to timing diagrams (a) and (b) of Figure 3, wherein two downlink bursts are provided per frame using two separate carrier frequencies from two separate satellites. In contrast, referring to Figure 4B, one carrier frequency may be used by two satellites. In an embodiment of Figure 4B, a guard time should be provided between the last burst in the first part of the TDMA frame and the first burst in the second part of the TDMA frame. In a GSM TDMA architecture, this may reduce the capacity by about 18.75%, assuming one forward link carrier is serving a 1000 km area. The forward link utilizes two carriers to serve each user in a diversity mode. In one embodiment, the forward link carrier frame contains 64 slots or signal bursts. This may be referred to as eighth rate GSM mode, wherein full rate GSM denotes 8 slots per frame, half rate GSM denotes 16 slots per frame, quarter rate GSM denotes 32 slots per frame and eighth rate GSM denotes 64 slots per frame.

For each forward link carrier of 200 kHz, four corresponding 50 kHz return link carriers are provided. For the eighth rate GSM embodiment, each forward link carrier can support 64 users, while each return link carrier can support 16 users.

According to a prefeπed embodiment, two forward link carriers are used to serve 64 users in diversity mode. Corresponding to these two forward link carriers are eight return link carriers. The return link can achieve diversity with only one transmitted burst. Figure 5 illustrates the use of eight return link carriers for each set of two forward link carriers. A guard time of one return link slot or four forward link slots, corresponding to about 2.3 msec also is shown. The analysis of timing diagrams has revealed that as long as users who are being served by the same return link carrier, and are using adjacent in time return link slots, are within a physical distance of about 345 km, then a fixed guard time of 2.3 msec (one return link slot, or four forward link slots, or 4τ) between adjacent user bursts can suffice to preclude any burst collisions at any one of the visible satellites, including those that may be at the horizon.

Each return link carrier can support eight users subject to the maximum fixed guard time of 2.3 msec between adjacent user bursts. A single forward link region can conespond to four return link regions, i.e. a region of 1000 km x 1000 km, and can be served by two 200 kHz forward carriers, one from each of two visible satellites. Thus, 64 users can be served in diversity mode over a 1000 km x 1000 km region via two forward link carriers when the system is operating in eighth rate GSM mode, while the eight return link carriers (as shown in Figure 5) are providing return link diversity for the same 64 users.

The transmission time for each return link burst for the variable guard time case may be calculated as follows. For user 1 , the transmission time may be given as follows:

where

is the transmission time for user 1 ; t_R is the receive time for user 1 ; τ is a forward link slot time; and

is the receive time for the user's downlink burst at a reference point in the region.

Similarly, the transmit time for user 2 may be calculated as follows:

where ΔDι_₂ denotes the physical distance between users 1 and 2. The transmit time for user 3 may be calculated as: ,3 _{= 3 + 14r +} Δ _{bL +} Δ ₂, _{+ (},₃ a _{} > (3)} c c where ΔD?_-3 denotes the physical distance between users 2 and 3.

In general, the transmission time for any arbitrary user n corresponds to:

fom = l, tj_r = t_R ^] + 14τ + (t^l _R - t^) (6)

Constraint : [(t^ + 4r) - t ≤ 60r. (7) where the quantity expressed by the third and fourth terms on the right hand side of Equation (4) preferably is calculated by the Gateway and relayed to the user terminal at call set up. The same holds for the third quantity on the right hand side of Equation (1 ) and Equation (6).

For the fixed guard time case, the transmission times for each return link burst may be calculated as: t_T" = t + l4r + (n - l)(4r) n = 1,2,3,...$ where each return link carrier supports eight users (assuming eighth rate GSM mode of system operation) as is illustrated in Figure 5. It will be recognized by those skilled in the art that t_R" - t_R ^~] - 4r for n=2, 3, ...8. This is clearly illustrated in Figure 3. It will be understood that the definition of N above, for example in Equation (5) and in the constraint statement of Equation (7), assumes eighth rate GSM as the mode of operation. Furthermore, it is also assumed that the forward link earner contains 64 slot frames, whereas the return link earner utilizes 16 slot frames, with four return link earners used to support the capacity provided by one forward link earner. Further details on the pnnciple of using non-symmetncal forward and return link earners can be found in the aforementioned publication Dual Mode Cellular Satellite Hand-Held Phone Technology by coinventor Karabinis et al, WESCON/96, October 22, 1996, pp. 206-222. It will be understood by those skilled in the art that the quantity 14τ in the above expressions is chosen as specified so that the return link pulse occurs at the terminal substantially midway between the two forward link pulses (see Figure 3) However, other values ranging from about 2τ to about 28τ may be used instead of 14τ in Equations (1 ), (2), (3), (4), (6) and (8) It will also be understood by those skilled in the art that the system mode of operation may be one other than eighth rate GSM.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descnptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims

WHAT IS CLAIMED IS

1 A method of transmitting uplink signal bursts from a plurality of user terminals in an uphnk region for reception by a plurality of visible satellites that receive transmissions from the uplink region, the method compnsmg the steps of establishing a guard time between adjacent uplink signal bursts that are transmitted from two user terminals that are spaced apart from one another m the uplink region based upon a time of arnval difference for the adjacent uplink signal bursts to one of the plurality of visible satellites, and transmitting the adjacent uplink signal bursts from the two user terminals with the guard time therebetween

2 A method according to Claim 1 wherein the steps of establishing and transmitting are repeatedly performed for all pairs of adjacent uplink signal bursts for the plurality of user terminals

3 A method according to Claim 1 wherein the one of the plurality of visible satellites corresponds to one of the plurality of visible satellites having lowest elevation angle

4 A method according to Claim 1 wherein the one of the plurality of visible satellites corresponds to a satellite at the honzon

5 A method according to Claim 1 further compnsing the steps of receiving the adjacent signal bursts from the two user terminals at each of the plurality of visible satellites, and diversity combining the received signals from each of the plurality of visible satellites for each of the two user terminals

6 A method according to Claim 1 wherein the uplmk signal bursts and downlink signal bursts are communicated in a plurality of repeating uplink and downlink burst frames, the method further compnsing the steps of receiving at the user terminals, two downlink signal bursts from two of the plurality of visible satellites in a downlink burst frame

7 A method according to Claim 6 wherein the receiving step compnses the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first earner frequency, and a second downlink signal burst from a second one of the visible satellites at a second earner frequency, dunng a downlink signal burst frame

8 A method according to Claim 6 wherein the receiving step compnses the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at the first earner frequency, dunng a downlink signal burst frame

9 A method of transmitting uplink signal bursts from a plurality of user terminals in an uplink region for reception by a plurality of visible satellites that receive transmissions from the uplink region, the method compnsing the steps of establishing a guard time between the uplink signal bursts that are transmitted from the plurality of user terminals, the guard time conespondmg to a maximum time of arnval difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region to one of the plurality of visible satellites, and transmitting the adjacent uplink signal bursts from the plurality of user terminals with the guard time therebetween

10 A method according to Claim 9 wherein the step of establishing compnses the step of establishing a guard time between the uplmk signal bursts that are transmitted from the plurality of user terminals, the guard time conespondmg to a maximum time of arnval difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region, to one of the plurality of visible satellites having lowest elevation angle

1 1 A method according to Claim 9 wherein the step of establishing compnses the step of establishing a guard time between the uplink signal bursts that are transmitted from the plurality of user terminals, the guard time corresponding to a maximum time of arnval difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region, to a satellite at the horizon.

12. A method according to Claim 9 wherein the plurality of user terminals comprises at least two user terminals, wherein the plurality of visible satellites comprises at least two visible satellites and wherein the transmitting step comprises the step of transmitting the adjacent uplink signal bursts from the at least two user terminals to the at least two visible satellites with the guard time between all adjacent signal bursts.

13. A method according to Claim 12 further comprising the steps of: receiving the adjacent signal bursts from the plurality of user terminals at each of the plurality of visible satellites; and diversity combining the received signals from each of the plurality of visible satellites for each of the user terminals.

14. A method according to Claim 12 further comprising the steps of: receiving the adjacent signal bursts from the at least two user terminals at each of the at least two visible satellites; and diversity combining the received signals from each of the at least two visible satellites for each of the at least two user terminals.

15. A method according to Claim 9 wherein the uplink signal bursts and downlink signal bursts are communicated in a plurality of repeating uplink and downlink burst frames, the method further comprising the steps of: receiving at the user terminals, two downlink signal bursts from two of the plurality of visible satellites in a downlink burst frame.

16. A method according to Claim 15 wherein the receiving step comprises the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at a second carrier frequency, during a downlink signal burst frame.

17 A method according to Claim 15 wherein the receiving step compnses the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first earner frequency, and a second downlink signal burst from a second one of the visible satellites at the first earner frequency, dunng a downlink signal burst frame

18 A method of transmitting uplink signal bursts from a plurality of user terminals in an uplink region for reception by a plurality of visible satellites that receive transmissions from the uplink region, the method compnsing the steps of establishing a vanable guard time between adjacent uplink signal bursts that are transmitted from the plurality of user terminals, the vanable guard time conespondmg to a time of arnval difference between adjacent uplink signal bursts from a conespondmg pair of user terminals to one of the plurality of visible satellites, and transmitting the adjacent uplink signal bursts from the plurality of user terminals with the vanable guard times therebetween

19 A method according to Claim 18 wherein the step of establishing compnses the step of establishing a vanable guard time between adjacent uplink signal bursts that are transmitted from the plurality of user terminals, the vanable guard time corresponding to a time of arnval difference between adjacent uplink signal bursts from a conespondmg pair of user terminals to one of the plurality of visible satellites having lowest elevation angle

20 A method according to Claim 18 wherein the step of establishing compnses the step of establishing a variable guard time between adjacent uplmk signal bursts that are transmitted from the plurality of user terminals, the vanable guard time corresponding to a time of arnval difference between adjacent uplink signal bursts from a conespondmg pair of user terminals to a satellite at the honzon

21 A method according to Claim 18 wherein the plurality of user terminals compnses at least two user terminals, wherein the plurality of visible satellites compnses at least two visible satelhtes and wherein the transmitting step comprises the step of transmitting the adjacent uplink signal bursts from the at least two user terminals to the at least two visible satellites with the variable guard time between all adjacent signal bursts.

22. A method according to Claim 18 further comprising the steps of: receiving the adjacent signal bursts from the plurality of user terminals at each of the plurality of visible satellites; and diversity combining the received signals from each of the plurality of visible satellites for each of the user terminals.

23. A method according to Claim 18 further comprising the steps of: receiving the adjacent signal bursts from the at least two user terminals at each of the at least two visible satellites; and diversity combining the received signals from each of the at least two visible satellites for each of the at least two user terminals.

24. A method according to Claim 18 wherein the uplink signal bursts and downlink signal bursts are communicated in a plurality of repeating uplink and downlink burst frames, the method further comprising the steps of: receiving at the user terminals, two downlink signal bursts from two of the plurality of visible satellites in a downlink burst frame.

25. A method according to Claim 24 wherein the receiving step comprises the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at a second carrier frequency, during a downlink signal burst frame.

26. A method according to Claim 24 wherein the receiving step comprises the step of receiving at the user terminals, a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at the first carrier frequency, during a downlink signal burst frame.

27 A method of transmitting uplink signal bursts from at least two user terminals in an uplink region for reception by at least two visible satellites that receive transmissions from the uplmk region, the method compnsing the step of transmitting a single uplmk signal burst from each of the at least two user terminals dunng a single one of a plurality of repeating uplmk signal frames, with timing therebetween such that each of the at least two visible satellites receives the single uplmk signal burst from each of the at least two user terminals without time overlap

28 A method according to Claim 27 wherein the step of transmitting compnses the step of transmitting the single uplink signal burst from each of the at least two user terminals with a fixed guard time therebetween such that each of the at least two visible satellites receives the single uplmk signal burst from each of the at least two user terminals without time overlap

29 A method according to Claim 27 wherein the step of transmitting compnses the step of transmitting the single uplink signal burst from each of the at least two user terminals with a vanable guard time therebetween such that each of the at least two visible satellites receives the single uphnk signal burst from each of the at least two user terminals without time overlap

30 A method according to Claim 27 further compnsing the steps of receiving the adjacent signal bursts from the at least two user terminals at each of the at least two visible satellites, and diversity combining the received signals from each of the at least two visible satellites for each of the at least two user terminals

31 A method of receiving uplink signal bursts from a plurality of user terminals in an uplink region of a satellite radiotelephone system compnsing at least two visible satellites that receive transmissions from the uplink region, the method compnsing the steps of receiving a single uplink signal burst from each of the user terminals at the at least two visible satellites that receive transmissions from the uplink region dunng a single one of a plurahtv of repeating uplink signal frames, and diversity combining the single received signal burst per frame for each of the user terminals that are received at the at least two visible satellites that receive transmissions from the uplink region.

32. A method according to Claim 31 wherein the receiving step comprises the step of receiving a single signal burst per frame from all of the user terminals at each of the at least two visible satellites that receive transmissions from the uplink region without time overlap.

33. A method of transmitting an uplink signal burst from a user terminal for reception by a plurality of visible satellites that receive transmissions from an uplink region in which the user terminal is located, the method comprising the step of: transmitting the uplink signal burst a guard time after an immediately preceding uplink signal burst from another user terminal in the uplink region, the guard time being based upon a time of arrival difference for the adjacent uplink signal bursts to one of the plurality of visible satellites.

34. A method according to claim 33 wherein the one of the plurality of visible satellites conesponds to one of the plurality of visible satellites having lowest elevation angle.

35. A method according to Claim 33 wherein the one of the plurality of visible satellites coπesponds to a satellite at the horizon.

36. A method according to Claim 33 further comprising the step of: receiving two downlink signal bursts at the user terminal from two of the plurality of visible satellites in a downlink burst frame.

37. A method according to Claim 36 wherein the receiving step comprises the step of: receiving a first downlink signal burst from a first one of the visible satellites at a first carrier frequency and a second downlink signal burst from a second one of the visible satellites at a second carrier frequency, during the downlink signal burst frame.

38. A method according to Claim 36 wherein the receiving step comprises the step of: receiving a first downlink signal burst from a first one of the visible satellites at a first carrier frequency and a second downlink signal burst from a second one of the visible satellites at the first carrier frequency, during the downlink signal burst frame.

39. A method of transmitting an uplink signal burst from a user terminal for reception by a plurality of visible satellites that receive transmissions from an uplink region in which the user terminal is located, the method comprising the step of: transmitting the uplink signal burst a guard time after an immediately preceding uplink signal burst from another user terminal in the uplink region, the guard time corresponding to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance part in the uplink region, to one of the plurality of visible satellites.

40. A method according to Claim 38 wherein the guard time conesponds to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance part in the uplink region, to one of the plurality of visible satellites having lowest elevation angle.

41. A method according to Claim 39 wherein the guard time corresponds to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance part in the uplink region, to a satellite at the horizon.

42. A satellite radiotelephone system receiving method comprising the step of: receiving uplink signal bursts from at least two user terminals in an uplink region, at at least two visible satellites without time overlap.

43. A method according to Claim 42 further comprising the step of: diversity combining the received uplink signal bursts from each of the at least two visible satellites for each of the user terminals.

44. A satellite radiotelephone system receiving method comprising the step of: receiving uplink signal bursts from at least one user terminal over a single carrier frequency at at least two visible satellites in an uplink region.

45. A method according to Claim 44 further comprising the step of: diversity combining the received uplink signal bursts from the at least one user terminal.

46. A satellite user terminal, comprising: a transmitter that transmits an uplink signal burst for reception by a plurality of visible satellites that receive transmissions from an uplink region in which the user terminal is located, the uplink signal being transmitted a guard time after an immediately preceding uplink signal burst from another user terminal in the uplink region, the guard time being based upon a time of arrival difference for the adjacent uplink signal bursts to one of the plurality of visible satellites.

47. A user terminal according to Claim 46 wherein the one of the plurality of visible satellites conesponds to one of the plurality of visible satellites having lowest elevation angle.

48. A user terminal according to claim 46 wherein the one of the plurality of visible satellites conesponds to a satellite at the horizon.

49. A user terminal according to Claim 46 wherein the uplink signal bursts and downlink signal bursts are communicated in a plurality of repeating uplink and downlink burst frames, the user terminal further comprising: a receiver that receives two downlink signal bursts from two of the plurality of visible satellites in a single downlink burst frame.

50. A user terminal according to Claim 49 wherein the receiver receives a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at a second carrier frequency, during the single downlink signal burst frame.

51. A user terminal according to Claim 49 wherein the receiver receives a first downlink signal burst from a first one of the visible satellites at a first carrier frequency, and a second downlink signal burst from a second one of the visible satellites at the first carrier frequency, during a single downlink signal burst frame.

52. A satellite user terminal, comprising: a transmitter that transmits an uplink signal burst for reception by a plurality of visible satellites that receive transmissions from an uplink region in which the user terminal is located, the uplink signal burst being transmitted a guard time after an i immediately preceding uplink signal burst from another user terminal in the uplink region, the guard time conesponding to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance part in the uplink region, to one of the plurality of visible satellites.

53. A user terminal according to Claim 52 wherein the guard time conesponds to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance part in the uplink region, to one of the plurality of visible satellites having lowest elevation angle.

54. A user terminal according to Claim 52 wherein the guard time conesponds to a maximum time of arrival difference between adjacent uplink signal bursts from a pair of user terminals that are a maximum distance apart in the uplink region to a satellite at the horizon.

55. A satellite user terminal, comprising: a transmitter that transmits an uplink signal burst for reception by a plurality of visible satellites that receive transmissions from an uplink region in which the user terminal is located, the uplink signal burst being transmitted a guard time after an immediately preceding uplink signal burst from another user terminal in the uplink region, the guard time corresponding to a time of arrival difference between adjacent uplink signal bursts from the user terminal and the other user terminal, to one of the plurality of visible satellites.

56. A user terminal according to Claim 55 wherein the guard time conesponds to a time of arrival difference between adjacent uplink signal bursts from the user terminal and the other user terminal, to one of the plurality of visible satellites having lowest elevation angle.

57. A user terminal according to Claim 55 wherein the guard time conesponds to a time of arrival difference between adjacent uplink signal bursts from the user terminal and the other user terminal, to a satellite at the horizon.

58. A satellite radiotelephone system comprising: at least two satellites that receive uplink signal bursts from at least two user terminals in an uplink region, each of the satellites receiving the uplink signal bursts from the at least two user terminals without time overlap.

59. A satellite radiotelephone system according to Claim 58 further comprising: a diversity combiner that combines the received uplink signal bursts from the user terminals.

60. A method of transmitting downlink signal bursts from a plurality of satellites to a plurality of user terminals in a downlink region, over a plurality of repeating downlink burst frames, the method comprising the step of: transmitting two downlink signal bursts from two of the plurality of satellites > for receipt by one of the user terminals in the downlink region, such that the two downlink signal bursts arrive at a reference point of the downlink region one half a downlink burst frame apart.

61. A method according to Claim 60 wherein the two downlink signal bursts are transmitted from the plurality of satellites over two respective carrier frequencies.

62. A method according to Claim 60 wherein the two downlink signal bursts are transmitted from two of the plurality of satellites over one carrier frequency.