WO1995034138A1 - Communications system - Google Patents

Communications system Download PDF

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Publication number
WO1995034138A1
WO1995034138A1 PCT/US1995/007036 US9507036W WO9534138A1 WO 1995034138 A1 WO1995034138 A1 WO 1995034138A1 US 9507036 W US9507036 W US 9507036W WO 9534138 A1 WO9534138 A1 WO 9534138A1
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WO
WIPO (PCT)
Prior art keywords
satellite
signals
user
nodes
frequency
Prior art date
Application number
PCT/US1995/007036
Other languages
French (fr)
Inventor
Robert E. Wieblen
Albert J. Mallinckrodt
Original Assignee
Celsat America, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to US25534194A priority Critical
Priority to US08/255,341 priority
Application filed by Celsat America, Inc. filed Critical Celsat America, Inc.
Publication of WO1995034138A1 publication Critical patent/WO1995034138A1/en

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W52/00Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04TPC
    • H04W52/18TPC being performed according to specific parameters
    • H04W52/22TPC being performed according to specific parameters taking into account previous information or commands
    • H04W52/228TPC being performed according to specific parameters taking into account previous information or commands using past power values or information
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/14Relay systems
    • H04B7/15Active relay systems
    • H04B7/185Space-based or airborne stations; Stations for satellite systems
    • H04B7/1853Satellite systems for providing telephony service to a mobile station, i.e. mobile satellite service
    • H04B7/18532Arrangements for managing transmission, i.e. for transporting data or a signalling message
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/14Relay systems
    • H04B7/15Active relay systems
    • H04B7/185Space-based or airborne stations; Stations for satellite systems
    • H04B7/1853Satellite systems for providing telephony service to a mobile station, i.e. mobile satellite service
    • H04B7/18558Arrangements for managing communications, i.e. for setting up, maintaining or releasing a call between stations
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/14Relay systems
    • H04B7/15Active relay systems
    • H04B7/204Multiple access
    • H04B7/2041Spot beam multiple access
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/14Relay systems
    • H04B7/15Active relay systems
    • H04B7/204Multiple access
    • H04B7/216Code division or spread-spectrum multiple access
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W36/00Hand-off or reselection arrangements
    • H04W36/16Performing reselection for specific purposes
    • H04W36/18Performing reselection for specific purposes for allowing seamless reselection, e.g. soft reselection
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W52/00Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04TPC
    • H04W52/18TPC being performed according to specific parameters
    • H04W52/22TPC being performed according to specific parameters taking into account previous information or commands
    • H04W52/225Calculation of statistics, e.g. average, variance
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B7/00Radio transmission systems, i.e. using radiation field
    • H04B7/14Relay systems
    • H04B7/15Active relay systems
    • H04B7/185Space-based or airborne stations; Stations for satellite systems
    • H04B7/1853Satellite systems for providing telephony service to a mobile station, i.e. mobile satellite service
    • H04B7/18532Arrangements for managing transmission, i.e. for transporting data or a signalling message
    • H04B7/18534Arrangements for managing transmission, i.e. for transporting data or a signalling message for enhancing link reliablility, e.g. satellites diversity

Abstract

Passive intermodulation interference of signals transmitted and received by nodes of a multi-satellite node system (10) is eliminated by using certain of the satellite nodes (300) for transmit-only operation and other of the satellite nodes (301) for receive-only operation. In another embodiment PIM interference is eliminated by time duplexing the signals transmitted and received by a satellite's antenna. PIM interference is also reduced by assigning unique portions of each transmitter subband and each receiver subband to each of the satellite nodes.

Description

COMMUNICATIONS SYSTEM

BACKGROUND

This invention relates to improvements in mobile wireless communication systems.

More particularly, the invention relates to communication systems such as a cellular mobile communications system having integrated satellite and ground nodes.

According to another aspect the invention concerns methods and apparatus for minimizing interference due to passive intermodulation (PIM) of radiated energy in a satellite using a single large antenna for communication to and from a user's transceiver.

The cellular communications industry has grown at a fast pace in the United States and even faster in some other countries. It has become an important service of substantial utility and because of the growth rate, saturation of the existing service is of concern. High density regions having high use rates, such as Los Angeles, New York and Chicago are of most immediate concern. Contributing to this concern is the congestion of the electromagnetic frequency spectrum which is becoming increasingly severe as the communication needs of society expand. This congestion is caused not only by cellular communications systems but also by other communications systems. However, in the cellular communications industry alone, it is estimated that the number of mobile subscribers will increase on a world¬ wide level by an order of magnitude within the next ten years. The radio frequency spectrum is limited and in view of this increasing demand for its use, means to more efficiently use it are continually being explored.

Mobile communications system such as Specialized Mobile Radio (SMR) , the planned Personal Communications Service (PCS) and existing cellular radio are primarily aimed at providing mobile telephone service to automotive users in developed metropolitan areas. For remote area users, airborne users, and marine users, AIRFONE and INMARSAT services exist but coverage is incomplete and/or service is relatively expensive. Mobile radio satellite systems in an advanced planning stage will probably provide improved direct-broadcast voice channels to mobile subscribers in remote areas but still at significantly higher cost in comparison to existing ground cellular service. The ground cellular and planned satellite technologies complement one another in geographical coverage in that the ground cellular communications service provides voice and data telephone service in relatively developed urban and suburban areas but not in sparsely populated areas, while the planned earth orbiting satellites will serve the sparsely populated areas.

In the case where one band of frequencies is preferable over others and that one band alone is to be used for mobile communications, efficient communications systems are necessary to assure that the number of users desiring to use the band can be accommodated. For example, there is presently widespread agreement on the choice of L-band as the technically preferred frequency band for the satellite-to-mobile link in mobile communications systems.

In the case where this single band is chosen to contain all mobile communications users, improvements in spectral utilization in the area of interference protection and in the ability to function without imposing intolerable interference on other services will be of paramount importance in the considerations of optimal use of the scarce spectrum.

The spread spectrum communications technique is a technology that has found widespread use in military applications which must meet requirements for security, minimized likelihood of signal detection, and minimum susceptibility to external interference or jamming. In a spread spectrum system, the data modulated carrier signal is further modulated by a relatively wide-band, pseudo¬ random "spreading" signal so that the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. Commonly the "spreading" signal is generated by a pseudo-random deterministic digital logic algorithm which is duplicated at the receiver.

By further modulating the received signal by the same spreading waveform, the received signal is remapped into the original information bandwidth to reproduce the desired signal. Because a receiver is responsive only to a signal that was spread using the same unique spreading code, a uniquely addressable channel is possible. Also, the power spectral density is low and without the unique spreading code, the signal is very difficult to detect, much less decode, so privacy is enhanced and interference with the signals of other services is reduced. The spread spectrum signal has strong immunity to multipath fading, interference from other users of the same system, and interference from other systems.

Accompanying the problems of congestion of the electromagnetic frequency spectrum is intermodulation interference of a transmitted signal. It is often desired to operate satellite communication systems in a full duplex mode, which means that different signals are going in both directions, e.g. from forward from base- station-to-user and reverse from user-to-base-station simultaneously. A fundamental design consideration in such cases is that the transmitted signal and any of its spurious sideband signals must be prevented from entering the receiver and thus desensitizing it. Careful attention is paid to isolation and filtering so that if the frequency separation is more than five or ten percent it is usually straightforward to attenuate the transmit frequency sufficiently so that it has no direct adverse effect on the receiver sensitivity.

More pernicious, however, are intermodulation products of the transmitted signal generating new frequencies not present in the original spectrum, which may fall exactly within the frequency band of the signal the system is designed to receive, so that no amount of filtering can help keep such intermodulation products out of the receiver input once they are generated. Such intermodulation products are generated whenever a multi- frequency transmit signal encounters a non-linear transmission element such as a non-linear receiver front end, or the back end ("back door") of another channel power amplifier, magnetic iron, nickel, chrome, or other such alloys, or other certain common ferric ceramics or oxidized electric contacts ("rusty bolts") . Most of these can be avoided by careful design and ground testing.

However, the one source that has, historically caused the most difficult and costly problems in communication satellite programs, is Passive Intermodulation, "PIM", arising from the transmit fields impinging on the reflector antenna used for both transmitting and simultaneous receiving. Structurally these antennas commonly consist of deployable frame and mesh structures, with literally thousands of opportunities for contact oxidation, and that cannot be fully tested in the ground gravity environment. At least half a dozen major communications satellites have suffered major program time setbacks and cost overruns when, in spite of presumed careful engineering and best possible testing on the ground, serious PIM problems were first discovered on orbit.

In typical present day systems, mobile units transmit and receive communication signals in the frequency range of 1.90 GHz - 2.18 GHz. In contrast, communication signals between a satellite and a nodal control center, connected to a public telephone switching network (PSTN) , are typically transmitted and received at 11 GHz - 14 GHz. In general, the frequency separation between the two systems is sufficient that PIM interference is not significantly created by both frequency bands operating on a single satellite spacecraft. However, present day satellite systems typically include satellite nodes that both transmit and receive in the 1.90 GHz - 2.18 GHz frequency range and the 11 GHz - 14 GHz frequency range. The transmission by a satellite of signals in the 1.90 GHz - 2.18 GHz frequency range may create significant intermodulation products which fall exactly within the 1.90 GHz - 2.18 GHz frequency range that the satellite is designed to receive. Likewise, the transmission of signals in the 11 GHz - 14 GHz frequency range may create significant intermodulation products which fall exactly within the 11 GHz - 14 GHz frequency range that the satellite also is designed to receive. These interference problems are further exacerbated by the high power by which signals are transmitted from the satellite node to a mobile unit in comparison to the signals which are received by the satellite node from the mobile unit. A mobile unit, with its small omnidirectional antenna, requires a comparatively strong signal in order to receive communication signals compared to the signal typically received by the large directional antenna of a satellite system. Accordingly, any passive intermodulation products that could desensitize the receive antenna of the satellite are thereby amplified further interfering with communication system performance.

Accordingly, there is a need to for a communication system that minimizes interference due to Passive Inter Modulation of radiated energy in a satellite system.

SUMMARY OF THE INVENTION

The invention provides improvements in wireless communications systems. While various aspects of the invention will be explained by reference, for example, to a cellular communications system using code division multiple access (CDMA) and spread spectrum waveforms, it will be apparent to those skilled in the art that these techniques are applicable to similar forms of wireless communications systems, such as, for example, Specialized Mobile Radio (SMR) , the planned Personal Communications Service (PCS) and existing cellular radio systems.

The invention provides improvements in such wireless communications systems, for example, a cellular communications system using code division multiple access (CDMA) and spread spectrum waveforms. The CDMA/spread spectrum system makes possible the use of very low rate, highly redundant coding without loss of capacity to accommodate a large number of users within the allocated bandwidth.

According to still another aspect of the invention, in the operation of a satellite communications system, which includes at least two satellite nodes for separately receiving and transmitting signals to and from user units, the improvement is provided for eliminating passive intermodulation interference of the transmitted and received signals, comprising transmitting all node- transmitted signals from the antenna of a first one of said satellite nodes and receiving all of said node- received signals by a second one of said satellite nodes, In another embodiment, the passive intermodulation interference is eliminated by time-duplexing the signals transmitted to and received from each satellite's antenna. In yet another embodiment, passive intermodulation interference is reduced by assigning unique transmit and receive subbands to each of the satellites.

Other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, illustrating by way of example the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) -(c) are diagrams showing an overview of the principal elements of typical communications systems which embody the principles of the invention;

FIG. 2 is a diagram of the frequency sub-bands of the frequency band allocation for a mobile system, e.g., a cellular system; FIG. 3 is a diagram showing the interrelationship of the cellular hierarchial structure of the ground and satellite nodes in a typical section and presents a cluster comprising more than one satellite cell;

FIG. 4 explains the term Code Delay;

FIG. 5 depicts transmit and receive time slots when the satellite is time duplexed and the user is frequency duplexed;

FIG. 6 depicts transmit and receive time slots when the satellite and the user unit are both time duplexed;

FIG. 7 is a block diagram showing an overview of the principal elements of a communications system in accordance with the principles of the invention wherein one satellite is used to transmit and a second satellite is used to receive the signals from the mobile user; and

FIG. 8 is a diagram of the frequency sub-bands of the frequency band allocation as modified in one embodiment to minimize the PIM with a multiple satellite system; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is shown in the exemplary drawings, the invention is embodied in a mobile system, e.g., a cellular communications system utilizing integrated satellite and ground nodes both of which use the same modulation, coding, and spreading structure and both responding to an identical user unit.

With reference to FIG. 1, in many present systems, the satellite 20 both receives signals (IS) and transmits (OS) signals to the user through a single large antenna 62. Unfortunately, only if the transmit and receive frequencies are sufficiently far apart, Passive Inter Modulation (PIM) of the transmit signals will not cause perceptible distortion of the received signals. If, however, the transmit and receive frequencies which are assigned are too close together, PIM of the transmit signals may cause distortion and/or loss of capacity or even render unintelligible the signals received by the satellite. In such cases, the solution in the past has been to use two antennas on each satellite, one for transmission and one for reception of the signals. In the case of a very large spacecraft antenna, which permits very high capacity and very low user power, it may not be practical to provide a two-antenna satellite. An object of the present invention is to reduce this PIM interference.

Referring now to FIG. 1(a), an overview of a typical communications system 10 is presented showing the functional inter-relationships of the major elements. The system network control center 12 directs the top level allocation of calls to satellite and ground regional resources throughout the system. It also is used to coordinate system-wide operations, to keep track of user locations, to perform optimum allocation of system resources to each call, dispatch facility command codes, and monitor and supervise overall system health. The regional node control centers 14, one of which is shown, are connected to the system network control center 12 and direct the allocation of calls to ground nodes within a major metropolitan region. The regional node control center 14 provides access to and from fixed land communication lines, such as commercial telephone systems known as the public switched telephone network (PSTN) . The ground nodes 16 under direction of the respective regional node control center 14 receive calls over the fixed land line network, encode them, spread them according to a unique spreading code assigned to each designated user, combine them into a composite signal, modulate that composite signal onto the transmission carrier, and broadcast them over the cellular region covered.

Satellite node control centers 18 are also connected to the system network control center 12 via status and control land lines and similarly handle calls designated for satellite links such as from PSTN, encode them, spread them according to the unique spreading codes assigned to the designated users, and multiplex them with other similarly directed calls into an uplink trunk, which is beamed up to the designated satellite 20.

Satellite nodes 20 receive the uplink trunks, frequency demultiplex the calls intended for different satellite cells, frequency translate and direct each to its appropriate cell transmitter and cell beam, and broadcast the composite of all such similarly directed calls down to the intended satellite cellular area. As used herein, "backhaul" means the link between a satellite 20 and a satellite node control center 18. In one embodiment, it is a K-band frequency while the link between the satellite 20 and the user unit 22 uses an L-band or an S- band frequency. As used herein, a "node" is a communication site cr a communication relay site capable of direct one or two- way radio communication with users. Nodes may include moving or stationary surface sites or airborne or satellite sites.

User units 22 respond to signals of either satellite or ground node origin, receive the outbound composite signal, separate out the signal intended for that user by despreading using the user's assigned unique spreading code, de-modulate, and decode the information and deliver the call to the user. Such user units 22 may be mobile or may be fixed in position. Gateways 24 provide direct trunks that is, groups of channels, between satellite and the ground public switched telephone system or private trunk users. For example, a gateway may comprise a dedicated satellite terminal for use by a large company or other entity. In the embodiment of FIG. 1, the gateway 24 is also connected to that system network controller 12.

All of the above-discussed centers, nodes, units and gateways are full duplex transmit/receive performing the corresponding inbound (user to system) link functions as well in the inverse manner to the outbound (system to user) link functions just described. FIGs. 1(b) and 1(c) represent systems with space only and ground only nodes. Certain aspects of this invention relate to these two systems as well as the "hybrid" system previously described.

Referring now to FIG. 2, the allocated frequency band 26 of a communications system is shown. The allocated frequency band 26 is divided into 2 main sub- bands, an outgoing sub-band 25 and an incoming sub-band 27. Additionally, the main sub-bands are themselves divided into further sub-bands which are designated as follows:

OG Outbound Ground 28 (ground node to user) OS Outbound Satellite 30 (satellite node to user) OC Outbound Calling and Command 32 (node to user) IG Inbound Ground 34 (user to ground node) IS Inbound Satellite 36 (user to satellite node) IC Inbound Calling and Tracking 38 (user to node)

All users in all cells use the entire designated sub-band for the described function. Unlike existing ground or satellite mobile systems, there is no necessity for frequency division by cells; all cells may use these same basic six sub-bands. This arrangement results in a higher frequency reuse factor as is discussed in more detail below. A preferred communication system includes the use of spread spectrum multiple access so that adjacent cells are not required to use different frequency bands. All ground-user links utilize the same two frequency sub- bands (OG 28, IG 34) and all satellite-user links use the same two frequency sub-bands (OS 30, IS 36) . This obviates an otherwise complex and restrictive frequency coordination problem of ensuring that frequencies are not reused within cells closer than some minimum distance to one another (as in the FM approach) , and yet provides for a hierarchial set of cell sizes to accommodate areas of significantly different subscriber densities.

The economic feasibility of a mobile telephone system is related to the number of users that can be supported. Two significant limits on the number of users supported are bandwidth utilization efficiency and power efficiency. In regard to bandwidth utilization efficiency, in either the ground based cellular or mobile satellite elements, radio frequency spectrum allocation is a severely limited commodity. Measures incorporated in the invention to maximize bandwidth utilization efficiency include the use of code division multiple access (CDMA) technology which provides an important spectral utilization efficiency gain and higher spatial frequency reuse factor made possible by the user of smaller satellite antenna beams. In regard to power efficiency, which is a major factor for the satellite- mobile links, the satellite transmitter source power per user is minimized by the use of forward-error-correcting coding, which in turn is enabled by the above use of spread spectrum code division multiple access (SS/CDMA) technology and by the use of relatively high antenna gain on the satellite. CDMA and forward-error-correction coding are known to those skilled in the art and no further details are given here.

In FIGs. 1(a) and 1(b) the potential passive intermodulation (PIM) problem may be said to have its roots in the use of the single large antenna 62 in both directions, that is to transmit and receive simultaneously. FIG. 7 shows an embodiment of the invention which completely eliminates the PIM interference problem. Of importance, the satellites shown in FIG. 7 as including two antennas uses only a single large antenna on each satellite for communication to and from the user in the 2 GHz region and a smaller antenna for completing the communication links to the ground station, typically in the 11-14 GHz region. To eliminate PIM interference, one satellite 300 is used to transmit to the mobile user and a second satellite 301 is used to receive the signals from the mobile user. Such satellites could be paired together at one orbital location, or they could be separated, but both must be within the field of view of the user.

In particular, a signal transmitted from a mobile user 22 and arriving at receive antenna dish 62 of satellite 301 is relayed to satellite node control 18. The signal is then processed through ground gateways to local landlines and to a public switched telephone network (PSTN) where it is ultimately routed to the desired recipient of the communications signal . The recipient of this signal, however, does not relay a return signal through satellite 301 but instead relays a transmitted signal through receive antenna dish 70 and transmit dish 62 of satellite 300 by way of satellite control center 18. In this manner PIM interference is greatly reduced or eliminated due to the wide spatial separation of the satellites 300 and 301. For example, in a typical system, satellite 301 only receives signals at approximately 2.0 GHz, while only transmitting at approximately 11.0 GHz. Likewise, satellite 300 only receives signals at approximately 11.0 GHz, while only transmitting at approximately 2.0 GHz. The frequency separation is sufficient to eliminate any significant PIM interference in the received signals. The listed frequency ranges are for illustration purposes only and are not intended to limit the scope of the invention in any manner.

If specific system design considerations indicate that a third satellite should also be used, then two satellites could be used to transmit, and one could be used to receive (or vice versa) , again resulting in no problems with PIM products. This aspect of the invention can be extended to any practical number of satellites.

Yet another aspect of the invention, which reduces but does not completely eliminate PIM effects with a frequency duplexed system, is a multiple satellite system in which the total transmission bandwidth is divided into n non-overlapping portions allocated to the different satellites, as shown in FIG. 8. This has the result of increasing the lowest order of interfering intermodulation product and therefore decreasing the amplitude of intermodulation interference. This comes about as follows:

Consider just two frequency components of the complex transmitted signal at the upper, u, and lower, v, frequency edges of the transmit band. The sum of these two signals is x = sin (2 i u t ) + sin ( 2 i v t ) ( 1 )

Now assume that the signal, x, encounters a "third order" transmission or reflection nonlinearity of the form

y(x) - x + A3x3 (2)

Substituting equation (1) into equation (2) and carrying out elementary trigonometric simplications shows that in addition to the original frequencies u and v, y now contains new spurious frequency components at frequencies 3 u, 3 v, 2 u-v, 2 v-u, 2 v+u, and 2 u + v.

In general the frequency products of first concern for receiver interference are those of the form [m v - (m-l)u] or [m u-(m-l)v] where "m" is any positive integer greater than 2. These frequency products are called 2m-1 order products and generated only by 2m-1 or greater odd order non-linearities, i.e., transmission function terms involving x2"1"1. From this it may be derived that given transmit bandwidth B=u-v, and transmit-receive band separation, Ftx - Frx, the minimum possible order of intermodulation (IM) product which can fall in the receive band is given by

Lowest order= [2*integer( |Ftx-Frx|/B)+1 This is important because the amplitude of the various intermodulation products diminishes generally with increasing order. Thus one approach to minimizing PIM problems is to maximize the order of the lowest order of interfering intermodulation products.

If more than one (n>l) satellite is to be launched, a unique portion (most likely 1/n) of the satellite sub- bands OS and IS is assigned to each of the n satellites. This increases the lowest order of the intermodulation product by a factor of n, and hence greatly reduces the magnitude of the intermodulation power fed into the satellite receiver.

For example only, one frequency band under consideration for mobile satellite services is 1970-1990 MHz (satellite receive, Frx) and 2160-2180 MHz (satellite transmit, Ftx) . In this case the lowest order IM product is equal to 17, which would be considered too low for the design of this satellite service. In the case where a three satellite system is used, if, instead of assigning the entire band to all three satellites, one third of the band is assigned to each of the three satellites, the lowest IM product is 51, which would be acceptable. The duplexing technique discussed throughout this application is, obviously, frequency duplexing wherein signals from the user are in a different band from the signals to the user. This is the situation shown in FIG. 2. An alternate duplexing method which will completely eliminate any PIM problem is to use time duplexing of the signals to and from the satellite instead of frequency duplexing the signals. PIM is eliminated since the satellite transmits and receives at different times. When time duplexing at the satellite is used to resolve a potential satellite IM problem, the user's unit can be either time duplexed or frequency duplexed. In both cases, the satellite transmit and receive signals must not overlap in time.

For the time duplexing option, the user response signal time is accurately locked to the time of receipt of the polling or timing signal, to a fraction of a PRN chip width. Measurement of the round trip poll/response time from one or more nodes or time differences of arrival at several nodes provides the basic measurement that enable solution and provision of precise user position. Accordingly, the transit time for signals can be computed. The user unit's transmissions are then timed to arrive at the satellite within designated receive time slots, with no loss of communications capacity. The satellite power is doubled, but it transmits only half the time. FIG. 4 shows that the signal is buffered 306 into a frame which has a fixed number of bits for transmission and requires a time Ti/o 302. The signal is encoded with a time frame of te. Buffering in the transmit buffer requires a time Ttx. FIG. 4 depicts a system with a zero transmission time delay. There is, then, a final time delay Td before the signal fills the out buffer 307.

With a geosynchronous satellite FIG. 5 there is a round trip transit time of from 238.6 to 277.8 ms. In this aspect of this invention the satellite is time- duplexed with 50% duty cycle. The user unit operates on 50% transmit and 50% receive duty cycle as well, but since the two periods overlap at the user, separate uplink and downlink frequencies are required and the user is frequency duplexed, but with closer frequency separation than could be tolerated in the single large satellite.

In yet another aspect of the invention, both the satellite and the user units can be time duplexed FIG. 6. In this case, both satellite and user units are time duplexed and guard bands must be provided to make certain that transmit and receive signals do not overlap at either the satellite or the user unit. The example shown relates to a geosynchronous satellite. In this aspect of this invention, the satellite can transmit on a 41% duty cycle.

In one embodiment, a mobile user's unit 22 will send an occasional burst of an identification signal in the IC sub-band either in response to a poll or autonomously.

This may occur when the unit 22 is in standby mode. This identification signal is tracked by the regional node control center 14 as long as the unit is within that respective region, otherwise the signal will be tracked by the satellite node or nodes. In another embodiment, this identification signal is tracked by all ground and satellite nodes capable of receiving it. This information is forwarded to the network control center 12 via status and command lines. By this means, the applicable regional node control center 14 and the system network control center 12 remain constantly aware of the cellular location and link options for each active user 22. An intra-regional call to or from a mobile user 22 will generally be handled solely by the respective regional node control center 14. Inter-regional calls are assigned to satellite or ground regional system resources by the system network control center 12 based on the location of the parties to the call, signal quality on the various link options, resource availability and best utilization of resources. A user 22 in standby mode constantly monitors the common outbound calling frequency sub-band OC 32 for calling signals addressed to him by means of his unique spreading code. Such calls may be originated from either ground or satellite nodes. Recognition of his unique call code initiates the user unit 22 ring function. When the user goes "off-hook", e.g., by lifting the handset from its cradle, a return signal is broadcast from the user unit 22 to any receiving node in the user calling frequency sub-band IC 38. This initiates a handshaking sequence between the calling node and the user unit which instructs the user unit whether to transition to either satellite, or ground frequency sub-bands, OS 30 and IS 36 or OG 28 and IG 34.

A mobile user wishing to place a call simply dials the number of the desired party, confirms the number and "sends" the call. Thereby an incoming call sequence is initiated in the IC sub-band 38. This call is generally heard by several ground and satellite nodes which forward call and signal quality reports to the appropriate system network control center 12 which in turn designates the call handling to a particular satellite node 20 or regional node control center 14. The call handling element then initiates a handshaking function with the calling unit over the OC 32 and IC 38 sub-bands, leading finally to transition to the appropriate satellite or ground sub-bands for communication.

Referring again to FIG. 1 as well as to FIG. 3, the satellite nodes 20 make use of a large, multiple-feed antenna 62 which in one embodiment provide separate, relatively narrow beamwidth beams and associated separate transmitters for each satellite cell 56. For example, the multiple feed antenna 62 may cover an area such as the United States with, typically, about 100 satellite beams/cells and in one embodiment, with about 200 beams/cells. As used herein, "relatively narrow beamwidth" refers to a beamwidth that results in a cell of 500 km or less across. The combined satellite/ground nodes system provides a hierarchical geographical cellular structure. Thus within a dense metropolitan area, each satellite cell 56 may further contain as many as 100 or more ground cells 54, which ground cells would normally carry the bulk of the traffic originated therein. The number of users of the ground nodes 16 is anticipated to exceed the number of users of the satellite nodes 20 where ground cells exist within satellite cells. Because all of these ground node users would otherwise interfere as background noise with the intended user-satellite links, in one embodiment the frequency band allocation may be separated into separate segments for the ground element and the space element as has been discussed in connection with FIG. 2. This combined, hybrid service can be provided in a manner that is smoothly transparent to the user. Calls will be allocated among all available ground and satellite resources in the most efficient manner by the system network control center 12.

By virtue of the above discussed design factors the system in accordance with the invention provides a flexible capability of providing the following additional special services: high quality, high rate voice and data service; facsimile (the standard group 3 as well as the high speed group 4) ; two way messaging, i.e., data interchange between mobile terminals at variable rates; automatic position determination and reporting to within several hundred feet; paging rural residential telephone; and private wireless exchange.

It is anticipated that the satellite will utilize geostationary orbits but is not restricted to such. The invention permits operating in other orbits as well. The system network control center 12 is designed to normally make the choice of which satellite or ground node a user will communicate with. In another embodiment, as an option, the user can request his choice between satellite link or direct ground based link depending on which provides clearer communications at the time or request his choice based on other communication requirements.

While a satellite node has been described above, it is not intended that this be the only means of providing above-ground service. In the case where a satellite has failed or is unable to provide the desired level of service for other reasons, for example, the satellite has been jammed by a hostile entity, an aircraft or other super-surface vehicle may be commissioned to provide the satellite functions described above. The "surface" nodes described above may be located on the ground or in water bodies on the surface of the earth. Additionally, while users have been shown and described as being located in automobiles, other users may exist. For example, a satellite may be a user of the system for communicating signals, just as a ship at sea may or a user on foot.

While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except by the appended claims. Having described the invention in such terms as to enable those skilled in the art to make and use it, and having identified the presently preferred best modes thereof, we claim:

Claims

We claim:
1. In the operation of a satellite communications system, the improvement for eliminating passive intermodulation interference of the transmitted and received signals, comprising:
a) transmitting all satellite node-to-user transmitted signals from the antenna of a first satellite node; and
b) receiving all user-to-satellite node received signals by a second satellite node.
2. In the operation of a satellite communications system, the improvement for eliminating passive intermodulation interference of the transmitted and received signals, comprising:
a) transmitting all mobile-transmitted signals to the antenna of a first satellite node; and
b) receiving all mobile-received signals from a second satellite node.
3. In the operation of a satellite communications system, the improvement for eliminating passive intermodulation interference comprising:
time-duplexing the signals transmitted by and received by a satellite's antenna such that the satellite antenna's transmit and receive signals do not overlap in time.
4. In the operation of a satellite communications system having a plurality of satellites nodes transmitting and receiving signals in respective frequency subbands, the improvement for eliminating passive intermodulation interference of the transmitted and received signals, comprising:
assigning unique portions of each transmitter subband to each of the satellite nodes.
5. In the operation of a satellite communications system having a plurality of satellites nodes transmitting and receiving signals in respective frequency subbands, the improvement for eliminating passive intermodulation interference of the transmitted and received signals, comprising:
assigning unique portions of each transmitter subband and each receiver subband to each of the satellite nodes.
PCT/US1995/007036 1994-06-07 1995-06-02 Communications system WO1995034138A1 (en)

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