Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1999-11-29
2002-08-06
Maung, Nay (Department: 2681)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C370S315000, C370S529000, C455S013100
Reexamination Certificate
active
06430391
ABSTRACT:
The invention relates to satellite communications systems for relaying signals between a first ground-based station and a second ground-based station.
BACKGROUND
In most prior art satellite communications systems, a network station transmits signals to a satellite in a first frequency band called the feeder uplink. After receiving feeder uplink signals, the satellite converts them to a second frequency band and transmits them to the mobile phones. The mobile phones reply on a third frequency band which is received at the satellite and translated to a fourth frequency band called the feeder downlink for transmission to the network station. Thus, in the prior art, four distinct frequency bands were required for a satellite communications system.
In the IRIDIUM system, the aforementioned second and third frequency bands are the same. An IRIDIUM satellite receives signals from a network station in the first frequency band and translates them to be relayed using a timeslot on a carrier frequency in a second frequency band. An IRIDIUM mobile phone receives the signal relayed by the satellite in the allocated timeslot on the allocated carrier frequency in the second frequency band. After a guard time to allow satellite transmissions that are reflected from the earth to propagate beyond the satellite orbit, the IRIDIUM phone replies in a second timeslot on the same second frequency band. The IRIDIUM satellite receives and translates the reply to the fourth frequency band and relays it to a network station. Thus, the IRIDIUM system uses three frequency bands for a given network station to communicate with a given mobile phone. The first and fourth frequency bands in the IRIDIUM system can include optical frequencies for conveying signals from satellite to satellite but ultimately radio frequency feederlinks are used for communication between the orbiting satellites and a network station. In the IRIDIUM system, the network stations do not transmit signals overlapping mobile phone transmissions in the same frequency band in the same cell or beam.
The prior art of landmobile radio systems includes the technique known as “two-frequency simplex”. In two-frequency simplex, a first mobile station desirous of communicating with a second mobile station transmits on a frequency f
1
to a base station repeater. The base or repeater station usually comprises an elevated, typically directional, antenna that can receive the weak mobile station's signal, which the second mobile at ground level cannot receive directly. The repeater station then translates the received signal from f
1
to f
2
and retransmits it at f
2
to the second mobile station. When the second mobile station wishes to reply, it transmits on f
1
to the repeater, but not at the same time as the first mobile station. If this happens, it is known as “doubling” and the signal became garbled in the prior art.
In trunked landmobile radio systems, a station desirous of transmitting transmitted first an “access request” burst to the repeater using a third frequency f
3
or calling channel, and the repeater replied on a channel f
4
with an “access grant” message indicating the channel f
1
to be used for transmitting the rest of the communication, only if another station in the same group or net was not already transmitting. Accidental “doubling” could thus be prevented in the prior art of trunked landmobile radio systems. These systems operated on a “push-totalk” basis, which is a simplex and not a duplex communications method. By using voice-operated switching or “VOX” instead of hand-operated transmit switches, the appearance of engaging in a duplex or telephone type of conversation can be provided, but VOX is not a perfect technique and occasionally fails to adapt fast enough to change of the speech direction. It imposes a discipline on the speakers to wait until the other person has completely finished talking before replying, which is not present in natural or telephone conversations.
An early military communications system called the Defense Satellite Communications System, or DSCS for short, comprised only two frequency bands for respectively transmitting to the satellite and receiving signals relayed from the satellite. The satellite merely performed a frequency translation and amplification of the received signals prior to retransmission. This type of satellite is known as a “bent pipe” transponder. These satellites had single, whole-earth coverage antenna beams. More powerful versions of DSCS type satellites known as SKYNET satellites were built and launched by Philco-Ford (now LORAL corporation) and GEC-Marconi for the British Defense Department.
In the prior art DCSC and SKYNET systems, a first station desirous of communicating with a second station transmitted a spread-spectrum signal to the satellite in a first frequency band. The second station also replied to the first station by transmitting a signal to the satellite in the first frequency band which was received overlapping the first station's signal at the satellite. The bent-pipe satellite translated the sum of the received first and second stations' signals to the second frequency band and relayed them to the first and second station. The first station despread the received signal using the second station's spread-spectrum transmission code thereby suppressing interference from other signals, including its own, to an extent limited by the spread-spectrum processing gain. The second station likewise decoded the first station's signal by despreading the received signal using the first station's spread-spectrum transmission code, thereby suppressing other interfering signals, including its own, to an extent limited by the spread-spectrum processing gain. Indeed, spread-spectrum was used in this prior art for the purpose of discriminating the desired signal from interference including own signal. Spread spectrum was also used however to obtain resistance to enemy jamming. In this prior art, no attempt was made to remove own signal interference by storing own signal in a delay memory for subtraction from the signal received later from the satellite. There was moreover no motivation to do so as own signal interference was only a small fraction of the total interference, which could include enemy jamming.
By contrast with the DSCS and SKYNET prior art, the present invention is directed to a civil communications system which does not contemplate hostile jamming. There is therefore no motivation automatically to select spread spectrum techniques. However, the use of spread-spectrum signals is one implementation of the current invention and differs from the above prior art in the subtraction of own interference at a network station so as better to decode weaker, portable-station signals transponded by the satellite.
In U.S. Pat. Nos. 5,151,919 and 5,218,619 to Applicant, methods are disclosed in CDMA systems to subtract stronger interfering signals before demodulating weaker signals. However, in the applications disclosed all signals comprised largely unknown symbols that had to be decoded first prior to subtraction. This was because the interfering signals did not originate at the same station as the receiving station, as is the case for the invention described below. The above patents incorporated herein by reference and certain of the mathematical techniques disclosed therein can be incorporated into the current invention for subtracting known signals by nulling in a transform domain, when using signals of the type contemplated therein.
SUMMARY
A satellite communications system includes at least one orbiting satellite having an antenna, or antennas, for transmitting signals in a first frequency band and receiving signals in a second frequency band. The antenna or antennas may furthermore be multi-beam antennas with each of the multiple beams covering a particular service region or cell. The present system furthermore comprises, in each cell, a first ground-based station for communicating with one or more second ground-based stations in the sam
Dent Paul W.
Ottoson Tony
Coats & Bennett P.L.L.C.
Ericsson Inc.
Gelin Jean A
Maung Nay
LandOfFree
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