The present invention relates to a digital communication system and method that sends and receives data to and from stations in a satellite communication system, digital radio and cellular telephony and refers particularly, though not exclusively, to such a system and method that enables the use of relatively small remote antennas.
In a digital communication system, the minimum useable antenna size normally depends on the antenna's required transmit and receive gain, required receive carrier-to-noise (“C/N”) level, transmit and receive beam width, and the maximum allowed EIRP flux density signal energy for off-axis antenna patterns. A larger antenna has advantages of a better antenna gain, smaller beam width and less antenna noise, compared to a smaller antenna, if all other parameters remain the same. However, a big antenna is expensive, and is not conveniently transportable. A smaller antenna allows portability, but has lower transmit gain and lower receive gain, compared to a larger antenna.
A lower receive gain may lead to a poor quality link. A lower transmit gain may mean the transmitter must increase its transmit power to compensate for the shortfall in the transmit gain. By increasing its transmission power, the transmitter may exceed the defined EIRP flux density signal energy requirements for off-axis antenna patterns. It may also introduce interference to adjacent receivers of the same or different types of communication systems, and may even introduce a health hazard. A smaller antenna also has a wider beam width, which makes it more susceptible to interference from surrounding units. This further reduces the quality of the link.
In one aspect the present invention provides a digital communication system for communication between a first terminal and a second terminal, the first terminal comprising a spread spectrum modulator for spreading a signal, the signal being spread by a spread factor.
The spread factor may be in the range 1 to 999, preferably the spread factor is in the range 10 to 50, and more preferably the spread factor is 31. The spread spectrum modulator may be a direct sequence spread spectrum modulator or a frequency hopping speed spectrum modulator.
The second terminal may have a spread spectrum demodulator, which can be a frequency hopping spread spectrum modulator, or a direct sequence spread spectrum demodulator, but is preferably a direct sequence spread spectrum demodulator The direct sequence spread spectrum demodulator may form part of a second terminal modem, the second terminal modem further comprising at least one of: a block converter, a down converter, a forward error correction decoder, a microcontroller, and an interface.
The direct sequence spread system spectrum modulator may form part of a first terminal modem, the first terminal modem also comprising at least one of an interface, a microprocessor, a forward error correction encoder, a further modulator, an up converter, a block up converter, and an amplifier.
The first terminal modem may be part of a first terminal equipment the first terminal equipment further comprising at least one of a transmit reject filter, a low noise block filter, a block up converter, an up converter, and an amplifier.
The second terminal modem may be part of a second terminal equipment, the second terminal equipment further comprising at least one of a transmit reject filter, a block down converter, a microcontroller
The first terminal may be a remote terminal and the second terminal may be a hub terminal.
In another aspect of the invention, there is provided a method for the reduction of noise relative to a signal, the method including the steps:
The spread signal may be received by a second terminal; the second terminal using a demodulator to de-spread the spread signal and any received signal noise.
In a further aspect of the invention there is provided a method for the reduction of noise relative to a signal, the method including the steps a second terminal receiving a spread signal; and the second terminal using a demodulator to de-spread the spread signal and any received signal noise to form the signal and to reduce the received signal noise. The spread signal may be transmitted by a first terminal, the first terminal modulating a signal to spread the transmitted signal to form the spread signal prior to transmitting the spread signal.
The present invention also provides a computer usable medium comprising a computer program code that is configured to cause at least one processor to execute one or more functions to enable an apparatus to perform the method described above.
For both forms, the first terminal may comprise a spread spectrum modulator for spreading the signal, the signal being spread by a spread factor.
In order that the invention may be better understood and readily put into practical effect, there shall now be described by way of nonlimitative example only. preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings in which:
a is an illustration and graph of an antenna situation with negligible interference;
b is an illustration and graph of an antenna situation with interference;
c is an illustration and graph of the antenna situation of
In communication systems, whether it is satellite, digital radio or cellular (or a hybrid of two or more of them) by using accessing techniques that require a lower power density, such as spread spectrum (for example DSSS, FHSS, and so forth), a smaller antenna can be used. Despite using a smaller antenna it is possible to still meet the defined EIRP flux density signal energy requirements for off-axis antenna patterns, and minimum adjacent interference, whether it is from the same type of communication system (for example, satellite-to-satellite) or different type of communication systems (satellite, digital radio, cellular, UHF, and so forth).
Various modulation schemes such as, for example, BPSK, QPSK, OQPSK and so forth; network topologies such as, for example Star, Mesh, Hybrid and so forth; communication systems such as for example, satellite, digital radio, cellular & so forth; and frequency band such as, for example, L-band, C-band, Ku-band, S-band, and so forth, may be used.
The accessing technique may be Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS), or otherwise, and may be used in satellite communications. By using DSSS, after spreading the signal will have a lower power density. Thus will meet regulatory requirements, while requiring very low energy bits/noise to achieve a good link quality. The low power transmission should introduce only a small amount of adjacent interfering-signal power, which typically is below the interfered system's noise level. When the received signal is being de-spread to receive the desired signal, the same process will spread the adjacent interfering signal (or signals) to a relatively low level, and thus cause reduced influence on the received link quality.
This invention can be used with different network topologies as long as the transmit and receive antenna sizes are suitable for the required link quality.
Throughout the description, like components have like reference numerals.
a shows a satellite communication, situation where the beam 6 of a 2.4 m antenna 1 communicating with hub antenna 2 via a satellite 4. Antenna 1 has a beam width that causes negligible interference with adjacent satellites 3 and 5. The graph shows the signal 10 received by the remote antenna 1, with the desired frequency carrier of the hub antenna 2 being at the peak of the curve 10.
b shows a smaller antenna 7 that has a beam 8 of a wider beam width. The beam 8 will experience interference from and to adjacent satellites 3 and 5. This is shown in the graph where the interference is shown as 9.
c shows the same smaller antenna 7 as
The present invention links remote stations or terminals 1 with a relatively small antenna to the hub station 2 that has a large antenna. The remote stations or terminals 1 may be mobile, if desired or required. The hub station 2 preferably uses a similar set-up as the remote, terminals 1, The main differences in the equipment may be in the antenna size, transceiver type, intermediate frequency, and the addition of any required routing capability to suit the network topology used.
Modem 204 is preferably an L-Band modem with Direct Sequence Spread Spectrum (DSSS) accessing or Frequency Hopping Spread Spectrum (FHSS), equipped with Turbo Coding (TPC) and Forward Error Correction (FEC) technique.
The output of the encoder 303 is then spread by a spread spectrum modulator 305, which may be, for example, a DSSS or FHSS modulator. Different spread factors can be selected for the modulator 305 according to factors such as, for example, the available uplink power, available bandwidth, and so forth.. The spread factor may be selected depending on the allowable overheads in the system. The spread factor may be in the range from 1 to 999. In a typical application, a spread factor between 10 to 50 may be used. Preferably, a spread factor of 31 may be used. The data that has been spread will then pass to the Digital-to-Analog Converter (“DAC”) 306, and the analog signal output from DAC 306 will pass through a low pass filter 307. The filtered analog signal output from filter 307 will then modulated by a modulator 308.
The data from modulator 305 needs to be converted to analog by DAC 306 because the modulator 308 may be an analog signal modulator. If a digital modulator is used in place of analog modulator 305, the output of modulator 305 may be connect directly to the modulator 308.
Any modulation scheme can be used in the modulators depending on the requirements such as, for example, available bandwidth, spread factor required, interference level, and so forth. The modulators may be selected according to the modulation schemes needed. Preferably, a modulator of capable BPSK and QPSK modulation is selected. More preferably, the modulation scheme selected is QPSK.
The Intermediate Frequency (IF) of the modulation signal may be of any frequency. Preferably a frequency in the range of 900 to 1900 MHz is used, more preferably the frequency is between 950 to 1450 MHz.
The signal is then converted to the desired transmission frequency and amplified by a bock up converter 205 (
The receiving hub antenna 2 picks up the transmitted signal. The hub antenna 2 may be of any size, depending on the application, design and other requirements. Preferably, the hub antenna 2 is a 32 m antenna. As disclosed above, the equipment used with hub antenna 2 may be similar to, or different from, the equipment used with the remote terminals 1. Preferably, the network is a Star network and thus the hub terminal 2 will do the processing and routing of the data to all remote terminals 1.
Referring to
The Global Positioning System 615 and 315 is required to receive GPS signals, for providing the remote terminal location, and timing for the both hub and remote terminals. The high stability clock 616 and 316 output is fed to the receiver 610 and 310, I/Q modulator 608 and 308, up converter 407 for hub 2, or block up converter 205 for remote terminals 1.
The Beacon signal of the system is received, down converted in the same manner as the normal signal, and further processed by the beacon receive modules 612, 312 for antenna pointing purpose. With this beacon, the antenna 400 is able to be accurately pointed at the correct satellite 4 at
Communication from hub station 2 to the remote terminal 1 is the same procedure as from the remote terminals 1 to the hub station 2 and thus a signal received at remote terminal 1 is processed in the same manner as the signal received as hub station 2; and signals transmitted by hub station 2 are processed in the same manner as those transmitted by remote terminal 1.
In
RF Connector 501 may have a transmit reject filter and therefore may be able to isolate the transmitted signal and the received signal. RF connector 501 may also have RF switches to allow the transmission to be switched to transmit or receive.
Additionally or aternatively, down converter 502 may be in accordance with (a), (b) or (c) of
Up converter 503 may be in accordance with (a) or (b) of
The invention is applicable to any digital communication systems and any network topology. The set-up of the terminals in the network may be identical or different, depending on the topology used and application requirements.
The mobile communication system is designed to provide portability to users, while maintaining efficient terminal operation. The techniques described enable the mobile communication system to be implemented using relatively small, and preferably relatively portable, remote antennas.
The present invention also provides a computer usable medium comprising a computer program code that is configured to cause at least one processor to execute one or more functions to enable an apparatus to perform the method described above.
Whilst there has been described in the foregoing description preferred embodiment of the present invention, it will be understood by those skilled in the technology that many variations or modifications in details of design, construction and operation may be made without departing from the present invention.