Patent Publication Number: US-2005129093-A1

Title: Digital communication system and method

Description:
FIELD OF THE INVENTION  
      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.  
     BACKGROUND OF THE INVENTION  
      In a digital communication system, the minimum useable antenna size normally depends on the antenna&#39;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.  
     SUMMARY OF THE INVENTION  
      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: 
          (a) at a first terminal, generating a signal to be transmitted;     (b) at the first terminal modulating the signal to spread the signal to form a spread signal; and     (c) the first terminal transmitting the spread signal.        

      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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      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:  
       FIG. 1   a  is an illustration and graph of an antenna situation with negligible interference;  
       FIG. 1   b  is an illustration and graph of an antenna situation with interference;  
       FIG. 1   c  is an illustration and graph of the antenna situation of  FIG. 1   b  after use of a preferred aspect of the present invention;  
       FIG. 2  is a block diagram of a preferred system for use in a remote terminal;  
       FIG. 3  is a block diagram of the modem of  FIG. 2 ;  
       FIG. 4  is a block diagram for a preferred system for use in a hub station;  
       FIG. 5  is a block diagram of the modem of  FIG. 4 ;  
       FIG. 6  is a block diagram of an alternative system for use in a remote terminal;  
       FIG. 7  is a block diagram of three different forms of down converters for use in the embodiment of  FIG. 6 ; and  
       FIG. 8  is a block diagram of two different forms of up converters for use in the embodiment of  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      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 &amp; 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&#39;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.  
       FIG. 1   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 .  
       FIG. 1   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 .  
       FIG. 1   c  shows the same smaller antenna  7  as  FIG. 1   b.  The received interference  9  is the same as in the graph of  FIG. 1   b.  After spreading, the desired signal transmitted is  11  on Graph  1 . At the receiver, after dispreading, the desired signal will be de-spread from the spread signal  11  of Graph  1  to the de-spread signal  10  of Graph  2 ; while the interference signal will be spread from the un-spread signal  9  of Graph  1  to the spread signal  11  of Graph  2 .  
      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.  
       FIG. 2  shows the station block diagram of the remote station. An antenna  200 , such as the antenna  7  of  FIGS. 1   c,  receives desired signals  9  and interference  11  as shown in Graph  1  in C-band. Both signals  9  and  11  pass through a transmit reject filter  201  and a low noise block  202  that process the C-band signal to L-band. The L-band signal  203  is then processed by a modem  204 . The transmission L-band signal  206  is sent to block up converter  205  by modem  204 . The block up converter  205  includes an up converter  207 . The C-band signal is then amplified in amplifier  208  and transmitted by antenna  200 .  
      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.  
       FIG. 3  shows the modem  204  block diagram. The modem  204  receives digital data via physical interface ( 301 ). The digital data is processed by micro controller  302 . The data can be at any rate and be according to any protocol, depending on S the application and available resources. For example, the data may have been sent from an SIP phone. The data may be multiplexed and may operate at aggregate data rate of 64 kps via an RJ45 10 Base T interface. The micro controller  302  can pass the data directly to forward error correction encoder  303 , or process the data by, for example, protocol translation or adding extra information, before passing the data to the encoder  303 .  
      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  ( FIG. 2 ). Different IF frequencies may be used. The selection of IF frequency  203  or  206  will be dependent on application requirement and availability of equipment  202 ,  207  and  204 . The desired transmit frequency conversion of the converter  205  depends on the application (C-band, Ku-band, S-band, and so forth). Preferably, C-band frequency is used. The output of converter  205 , which is in C-band, is fed to the antenna  200  and is transmitted. The antenna  200  can be of any size and category, depending on the application, need, and available technology.  
      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  FIG. 4 , it shows the equipment of hub antenna  2  of  FIG. 1   c.  The received signal is received by the receiving antenna  400 , filtered by the transmit reject filter  401 , amplified and down converted to second Intermediate Frequency  403  by a low noise block converter  402 , and then received by the modem  406 . The frequency selection of the IF frequencies  403  &amp;  405  depends on the equipment  402 ,  407  and  406 , and on application requirement. The signal received at antenna  400  is the frequency corresponding to the transmitted signal, like C-Band, Ku-Band, S-Band or otherwise, preferably a standard C-band satellite downlink frequency. The modem used  406  is the matching modem to the modem  204  used at the remote terminal  1 . For example, a DSSS or FHSS modem may be used.  
       FIG. 5  is a block diagram of the modem  406  of the hub station  2 . The down converted signal from low noise block converter  402  ( FIG. 4 ) is received by the receiver  610  and then converted to digital by an analog-to-digital converter  611 , and send to a demodulator  613 , preferably, a DSSS or FHSS demodulator, which must be identical to the modulating techniques used in remote station, for example modulate and demodulate using DSSS. The demodulator  613  will demodulate the signal and un-spread the data before passing it to the forward error correction decoder  614 , which may be a turbo coding decoder. The decoded data is then transferred to micro controller  602  for further processing, such as, for example, protocol translation, extracting the extra control data added by micro controller  302  of the transmit station  1 , or otherwise as required. The output of the micro controller  602  is then passed to the user via the physical interface  601 , which may be an RJ45 10 BaseT interface.  
      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  FIG. 1   c,  and for verification to take place.  
      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 .  
       FIG. 6  shows that by varying  501 ,  502  and  503  using different combination as shown in Combination  1 ,  2  and  3  in  FIG. 7  and Combination  1  and  2  in  FIG. 8 , another version of the system can be applied to any digital communication systems and any network topology.  
      In  FIG. 6 , antenna  500  can be a transmit only, receive only, transmit and receive or otherwise, depending on the application requirement. If antenna  500  is for transmit only, down converter  502  would not be required. If Antenna  500  is for receive only, up converter  503  would not be required for the system.  
      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  FIG. 7 , depending on the incoming frequency RF 2  down in  FIG. 6 . RF 2  may be C-Band, Ku-Band, S-Band or otherwise. Down converter  502  converts the signal according to the application requirements with the signal output being indicated as IF down in  FIG. 6 . IF down may be L-Band, 70 Mhz or otherwise. For example,  FIG. 7 ( a ) uses a low noise block (LNB) to convert RF 2  to L-Band;  FIG. 7 ( b ) uses a low noise amplifier (LNA) with a 1-stage down converter to convert RF 2  to L-Band; and  FIG. 7 ( c ) uses a low noise amplifier (LNA) with a 2-stage down converter to obtain a lower frequency, such as, for example, 70 Mhz, 140 MHz or otherwise.  
      Up converter  503  may be in accordance with ( a ) or ( b ) of  FIG. 8 , depending on the desired signal output RF 2  up in  FIG. 6 , RF 2  up may be C-Band, Ku-Band, S Band, or otherwise. Up converter  503  will convert the signal according to the application requirements. For example, in  FIG. 8 ( a ), two mixers are used to up convert a low frequency, such as, for example, 70 MHz, 140 MHz or otherwise, to a desired signal output.  FIG. 8 ( b ), which uses BUC  503  to up convert an L-Band signal to a desired signal output.  
      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.