GB2448510A

GB2448510A - Alignment of directional antenna beams to form a high gain communication link

Info

Publication number: GB2448510A
Application number: GB0707404A
Authority: GB
Inventors: David Thomas
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-04-17
Filing date: 2007-04-17
Publication date: 2008-10-22
Also published as: GB0707404D0

Abstract

A radio frequency communication method, apparatus or system comprises a first antenna which transmits information regarding its location to a second antenna which receives the said information and uses it to align a directional radiation beam from the second antenna toward the location of the first antenna. Transmitted location information may be provided by the first antenna and second antenna, via a low data rate signal in a directional antenna beam, to the second antenna and first antenna, respectively. The respective antenna location information may be used, via automatic antenna beam direction control means, to accurately form and maintain the alignment of a high gain, high data rate directional antenna beams forming a low noise communication link between the antennas. The antennas may be phased array and/or sectored antenna element arrangements. The communication system may be used to provide a reliable high data rate communication link between an antenna on a vessel and another antenna in a communication network.

Description

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ALIGNMENT OF HIGH GAIN ANTENNAS

FIELD

The present invention relates to wireless apparatus, wireless communication systems and methods of operation thereof. In particular, but not exclusively, the present invention relates to the method of controlling the beam of maximum receiver gain afforded by high gain antennas, such as phased array or sectored beam antenna systems. The system and method directs beams of maximum gain of a receiver and a transmitter source, such that an optimum signal to noise ratio is obtained.

BACKGROUND

Radio communications systems are well known. Examples of such are radio communication systems that utilize signals that are transmitted to and from geostationary satellites, or Low Earth Orbiting (LEO) satellites or from ground based antennas.

Geostationary satellites orbit at a distance of the order of 22,300 miles above the Earth and the radiated energy from the satellite transmitter covers a large footprint, thus at any point the received signal is of a low level. Unless particular care is taken in the design of systems that interact with the geostationary satellites, the information intended for the receiver is degraded beyond use, due to the low signal quality

relative to the background noise at the receiver.

Geostationary satellites can be utilized for communication of data to maritime vessels or portable receivers as they afford reasonable speed of data transfer over wide geographic areas. Maritime vessels often use Very Small Aperture Terminals (VSAT) fed by parabolic antennas at the receiver. The parabolic antenna provides gain in the specific direction of the geostationary satellite. Such a parabolic dish can be re-orientated, or can change direction relative to the vessel's axis as the vessel moves, thus maintaining the axis of the parabolic dish aligned to the satellite.

In order to maintain the optimum orientation of the parabolic antenna towards a satellite, a mechanical movement of the entire reflector dish is usually necessary. The speed at which the beam can be steered is limited by the mechanical inertia of the dish, its mounting, and other mechanical parts. The speed of re-orientation of the dish, and thus its ability to compensate for the movement of a vessel, is limited by the mechanical structure and accuracy of the control system. Rapid re-orientation of the direction of maximum gain of the dish is limited. Thus for a single parabolic dish one can only receive a signal from one general direction.

The use of parabolic VSAT dishes enables an improvement in the signal strength at the receiver, but only in the direction of the axis of the dish. Any signal received from a direction that is orthogonal to the axis of the dish is highly attenuated. Thus a single parabolic dish is unable to track two independent signal sources. Thus one could not use a single parabolic receiver to facilitate roaming, and seamless handover, afforded by modern communication networks (e.g. the GSM, Wide Band CDMA or Wimax).

These communication systems provide extended coverage and seamless handover using a network of geographically distributed base stations, where at the point of handover the antenna needs to receive a signal from two independent base stations.

A further disadvantage of conventional communications via geostationary satellites is the fact that the group delay associated with the transfer of data is of the order of seconds, due to the time taken for transmission to and from the satellite. Such a group delay is barely acceptable for streaming data downlink and is orders of magnitude slower than terrestrial broadband systems in use today. The large group delay associated with the transmission path has a malor detrimental effect on the ability to execute interactive searching of data bases, such as those provided via the World Wide Web. The high group delay also has a detrimental impact on the quality of real time bi-directional services. As a result of the large group delay, services such as voice telephony, or Voice over Packet (V0IP), or video telephony, suffer from the effects of echoing. With group delays in excess of 1 second one can experience a breakdown of interactive speech.

In an attempt to reduce the effects of group delay, satellite systems such as Iridium utilize a constellation of Low Earth Orbiting (LEO) satellites to provide bi-directional voice and data comml.mications over a wide area. Such systems are expensive to install and maintain due to the number of LEO satellites necessary to provide a wide coverage and the cost of maintaining an orbit which is constantly being degraded. Thus the bandwidth and the data rate afforded by such systems is limited, rendering the systems inefficient for communication of data rich services such as Internet browsing.

In recent years high bandwidth systems supporting high quality voice and data services have been introduced based on the GSM, or GPRS, or 3G, or Wideband CDMA, or WIF or WiMax standards. Due to limitations in the placement of land-based transmitters, and restrictions on the maximum transmitted power, these services have tended to be limited to areas close to shore.

A phased array antenna, comprising a number of sub antenna elements, is an alternative method to parabolic dishes used to increase the gain of an antenna in a given direction. Since the beam can be directed or steered electronically by varying phase and gain of individual transceiver elements, the speed of beam motion is considerably faster than for a parabolic reflector antenna. Beam shaping enables the gain of the antennas to be optimized in a number of directions. Conversely it is possible to create a null in a specific direction (for example the direction of a source of interference) whilst maintaining gain in the direction of wanted signal. With the introduction of more complex tracking algorithms it is possible to track two independent sources of transmission. The direction of the beam is more easily controlled, and due to the lack of moving mechanical parts, a more or less instantaneous change in direction can be achieved. Thus with an antenna array one can facilitate the possibility of roaming and seamless handover within a geographically distributed network.

There is a relationship between the number of active elements in the array (i.e. beam width) and the accuracy of the direction of the beam, relative to the transmitting source being acquired. There is also a direct relationship between the beam width and the signal to noise S/N ratio at the receiver, the beam width being defined by the number of active elements within the array. Thus the performance of the phased array antenna is typically driven by the number of array elements and the accuracy of the control of the gain, and the phase associated with each array element. If the beam width is narrow, the S/N ratio at the receiver combiner is increased and the Bit Error Rate (BER) of the communication system improved or the range of the communication system can be increased. However, as the beam width reduces, the sensitivity of the communication system to errors in the direction of the beam relative to the transmitting source becomes more pronounced. At the extreme the signal, and thus the communication link, can be totally lost if a narrow beam is not correctly aligned to the source.

Sectored antennas are a further example of a method for improving the gain of an antenna system. Unlike conventional omni directional antennas that provide gain in all directions equally, the sectored antenna provides gain in a specific direction. By utilizing different carrier frequencies with each of the adjacent sectors it is possible to increase the gain of the antenna within a given direction without unduly interfering with other sectors. One disadvantage of such sectored antennas is that the transmission of data can only take place if the receiver is tuned to the carrier frequency that is associated with the sector within which it is located. If the carrier frequency associated with an adjacent sector is utilized by the receiver, then communication is not established.

Thus for a communication system that utilizes beam forming to increase gain (such as phased array antennas or sectored antennas) to operate, one needs to ensure that the transmit beam and the receiver beam are correctly aligned. For example in a system where the receiver is stationary over time one can correlate the carrier frequencies of the transmitter to receiver, and ensure that the direction of the point of maximum gain of the antennas is optimally aligned.

However if the geographic position of the receiver is unknown, or constantly changing, or subject to varying directions of interference, one requires a procedure to set up the direction of the beam of maximum gain relative to the transmitter prior to the transmission of data. In the worst case of a dynamic movement of vessels relative to a network of transmitters, one first needs a means of identifying the exact location and orientation of the vessel, prior to entering the procedure for initiating a high speed communication link.

Embodiments of the present invention seek to address one or more of the drawbacks or limitations of known wireless apparatus and systems outlined above and to preferably improve the performance thereof.

SUMMARY

Embodiments of the present invention seek to address one or more of the drawbacks or limitations of known high gain antenna apparatus and systems and to preferably improve the performance thereof.

Particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Combinations of features of the independent claims as appropriate are not explicitly set out in the claims.

In accordance with a first aspect of the present invention, there is provided a first wireless apparatus for a wireless communication system comprising: -A phased array antenna or a sectored antenna that can direct the beam of maximum gain from a transmitting source, or that can select one of a number of sectors for communication.

A receiving antenna controller for directing a directional beam antenna in an optimal signal quality direction relative to the source.

A separate communication path that can communicate the geographic latitude and longitude co-ordinates of a moving object on which the receiving antenna is located, such that the transmitting source can select the direction for the transmitting beam of maximum gain or the sector that is optimum for transmission.

In accordance with a second aspect of the present invention, there is provided a method for operating said wireless apparatus for a wireless communication system comprising, the method comprising:-Communicating via a secondary communication path to a fixed transmitter/receiver, the geographic location of a moving receiver/transmitter, such that a broad band communication path can be established via the alignment of antennas that provide directional gain at said transmitter and said receiver.

In accordance with a third aspect of the present invention, there is provided a method and apparatus for improving the accuracy of said wireless communication systems over time, said method comprising: -Tracking the geographic co-ordinates of a moving transmitter/receiver relative to a fixed transmitter/receiver on an ongoing basis via a secondary communication path, such that at all times, even in the presence of an interfering source, the optimum direction of the beam of maximum gain, or the optimum sector is used for communications.

In accordance with a fourth aspect of the present invention, there is provided a method and apparatus for utilizing a secondary communication path for tracking the location of a moving transmitter/ receiver relative to a stationary or moving source of interference, such that an optimum signal path can be selected from a number of available transmission sources, antenna sectors or transmitting directions.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will now be described, by way if example only, and with reference to the accompanying drawings, in which Fig 1 is a diagram of the pattern of radiated power from an omni directional antenna system.

Fig 2 is a diagram showing a typical pattern of radiation from a sectored antenna comprising three sectors.

Fig 3 shows a typical pattern of radiated power from a directional antenna e.g. a phased array.

Fig 4 shows a typical scenario where a transmitter utilizing a directional antenna and a receiver using a similar directional antenna cannot communicate with each other due to the fact that the direction of the beams are not aligned.

Fig 5 shows a scenario where the transmitting and receiving beams of maximum gain are optimally aligned.

Fig 6 shows a typical coast line with two transmitters Txl and Tx2 together with vessel A that is entering into the area of theoretical range of either transmitter.

Fig 7 shows a typical scenario where communications carmot be established due to an interfering source B degrading the signal strength from transmitter Txl, and non aligned beams prohibiting communications between the vessel A and transmitter Tx2.

Fig 8 shows a scenario where the vessel A communicates its geographic location to enable transmitter Tx2 via a secondary communication path thus enabling transmitter Tx2 and vessel A to align their beam or select an optimum sector to enable high speed communications.

DETAILED DESCRIPTION OF THE DRAWINGS

The typical radiation pattern from an omni directional transmitter 101 is shown in figure 1. The concentric rings show points of similar power. The power level reduces as one travels away from the transmitter. In practice the locations of points of similar power do not form concentric circles, but will be irregular due to reflections from buildings or physical features of the terrain. For a given receiver and a given transmitted power, there is a distance 102 (range) where the signal strength reduces to a level which is below the sensitivity of the receiver and communications fail. Conversely if a transmitter is moved towards a receiver 101 then one must achieve a minimum distance 102 prior to communications being established.

In practice the range of the communication system can be enhanced by use of specialized antennas that provide gain in a specific direction. For example as shown in figure 2 a sectored antenna utilizes a number of separate antennas 201a, 201b, and 201c designed to increase the transmitted power or the sensitivity in a given direction or sector. To avoid interference between the sectors, different carrier frequencies are often utilized for communication within the sectors 201a, 201b and 201c. Thus communication using a carrier frequency c would not be able to communicate with the transmitter/receiver when located in sectors 201a or 201b or when beyond the range of sector 201 c.

The advantage of such a sectored configuration is that the range over which communication can be established can be increased, relative to an omni directional system, for a given average transmitted power.

Alternatively a directional phased array antenna can be utilized to further increase the performance of a communication system by focusing a narrow beam of maximum gain from a transmitter! receiver 301, to a specific communication system located at a position 302. In practice one can further enhance the performance of the system in the presence of interferers such as 303, by using complex beam patterns to direct a point of maximum gain towards the communication equipment of choice whilst directing a point of low gain toward the interfering source.

One can further enhance the range of communication utilizing either a combination of a sectored transmitter and a directional receiver, or both a directional transmitter and receiver. However, if the direction of the transmitted power beam 402 associated with the transmitter 40,1 and the direction of maximum gain 404 associated with the receiver 403, are not correctly aligned then communication cannot be established. Once the beam of transmitted power and the direction of maximum gain are aligned as in figure then the communication link can be established. The communication link can be maintained provided that the transmitter and receiver maintain alignment over time.

In a typical practical scenario, such as that shown in figure 6, one may have two (or more) transceiver base stations operating with different carrier frequencies, and either sectored or directional antennas or a combination of both, to which a moving vessel A wishes to establish broad band communication. Whilst out of range, the transceiver base station is unaware of the presence of the vessel A, its location or its coarse over ground (COG), and thus is not able to assess the direction in which an optimized communication path would be established. Further the vessel A is unaware of any interferers (e.g. vessel B with an active radar) that may be located in the direction of the nearest geographic transceiver base station. Thus as vessel A enters into range of transceiver Txl, it is not able to establish communication or may establish a low quality communication path due to the interference from vessel B. In the scenario shown in figure 7 it is also not able to establish contact with transceiver base station Tx2 due to a misalignment of antenna Tx2 and the receiving antenna on vessel A. Worst case in the scenario shown in Fig 7 vessel A cannot establish connection using the high speed communication path.

In scenario 8 the vessel A which enters into the theoretical range for high speed communications with either transceiver base station Txl and/or Tx2, uses an secondary communication path to initialize the communication by transmitting information relating to the geographic location of the vessel (latitude/longitude and coarse over ground) . The initialization geographic location could be for example be transmitted via VHF Digital Subscriber Communications DSC or via the VHF control channel of a Automatic Identification System AIS or via SSB radio or via an Iridium satellite link or another form of medium range highly protected low data rate communication system. The secondary communication link operates independent of the main high speed data communication channel, may utilize other forms of antennas (for example omni directional VHF DSC) and is selected to not be subject to the alignment problems associated with the main communication channels. )

Once the base stations Txl and or Tx2 are made aware of the presence of vessel A requesting high speed data communications, a suitable base station can be selected, suitable sector or beam direction can be set up and the vessel A can be advised via the secondary communication channel of the associated information.

Thus the main high speed communication channel can be established, as shown for example in Fig 8. )

Claims

Claims: 1. A high data rate wireless communications system, comprising:

a transmitter module associated with a first antenna, wherein the transmitter module is operative to transmit wirelessly at a relatively low data rate location information corresponding to the first antenna a second directional beam antenna configured f or high data rate wireless communications; a receiver module for receiving over a relatively low data rate wireless communications channel location information corresponding to the first antenna; and a beam direction module operative to provide beam direction control signals for the second directional beam antenna to direct a relatively high data rate antenna beam toward the location determined by the location information received by the receiver module.
2. A communications system according to claim 1 wherein the first antenna is a directional beam antenna, the communications system further comprising: a transmitter module associated with the second directional beam antenna, wherein the transmitter module is operative to transmit wirelessly at a relatively low data rate second location information corresponding to the second directional beam antenna; a second receiver module configured to receive over a relatively low data rate wireless communications channel the second location; and a second beam direction module operative to provide beam direction control signals for the first antenna to direct a relatively high data rate antenna beam toward the second location determined by the second location information received by the receiver module.
3. Apparatus for automatically controlling a directional beam antenna configured for high data rate wireless transmission, comprising: a receiver module for receiving over a relative low data rate wireless communications channel location information corresponding to a first antenna; and a beam direction module operative to provide beam direction control signals for a second directional beam antenna to direct a relatively high data rate antenna beam toward the location determined by the location information received by the receiver module.
4. Apparatus according to claim 3, comprising: a transmitter module associated with the first antenna configured for high data rate wireless transmission, wherein the transmitter module is operative to transmit wirelessly at a relatively low data rate location information corresponding to the second directional beam antenna.
5. A system according to claim 1 or apparatus according to claim 3, wherein the first antenna is a directional beam antenna.
6. A communications system according to claim 1 or 2 or apparatus according to claim 3, 4 or 5, wherein the second directional beam antenna is a steerable beam antenna.
7. A communications system according to claim 6 or apparatus according to claim 6, wherein the second directional beam antenna is a phased array antenna.
8. A communications system according to claim 5 or apparatus according to claim 5, wherein the first antenna is a steerable beam antenna.
9. A communications system according to claim 8 or apparatus according to claim 8, wherein the first antenna is a phased array antenna.
10. A method for high data rate wireless communications, comprising: wirelessly transmitting at a relatively low data rate location information corresponding to a first antenna; receiving over a relatively low data rate wireless communications channel the location information corresponding to the first antenna; and controlling a second directional beam antenna to direct a relatively high data rate antenna beam toward the location determined by the location information.
11. A method according to claim 10, further comprising: wirelessly transmitting at a relatively low data rate second location information corresponding to the second directional beam antenna; receiving over a relatively low data rate wireless communications channel the second location information corresponding to the second directional beam antenna; and controlling the second directional beam antenna to direct a relatively high data rate antenna beam toward the second location determined by the second location information.