GB2459131A

GB2459131A - Locating nodes of a telecommunications system

Info

Publication number: GB2459131A
Application number: GB0806604A
Authority: GB
Inventors: Max Gasparroni; Ralf Irmer; Trevor Gill
Original assignee: Vodafone Group Services Ltd
Current assignee: Vodafone Group Services Ltd
Priority date: 2008-04-11
Filing date: 2008-04-11
Publication date: 2009-10-14
Also published as: GB0806604D0

Abstract

An arrangement is described for adding a joining communication node 5 to a point to point microwave network comprising at least one active communication node 23. Respective ones of the nodes 5,23 communicate with each other wirelessly by means of a directional narrow-beam communication antenna 30. The node 5 is operated in a search mode to search for a beacon or pilot signal transmitted by the node 23 using a search antenna 34 which has a beam width greater than the beam width of the narrow beam antenna 30. Information obtained by the search antenna may then be used to steer the communication antenna of the node 5 in order to align the communication antenna 30 of the node 5 with the communication antenna 30 of the node 23 to allow data communication between the nodes.

Description

LOCATING NODES OF A TELECOMMUNICATIONS SYSTEM

The present invention relates to a method of enabling a joining communication node to communicate with an active communication node of a network, where respective ones of the nodes are operable to communicate with each other wirelessly in a communication mode by means of a directional narrow-beam antenna. The invention also relates to a node for use in a network comprising a plurality of nodes, and to a network including a plurality of nodes.

Networks such as cellular telecommunications networks include base stations for communicating with mobile terminals. The base stations form part of the radio access network of the cellular telecommunications network. The radio access network also includes a radio network controller (RNC), which serves a multiplicity of base stations. A backhaul network connects the base stations to the radio network controller. Given the typically high number of base stations served by a radio network controller, the backhaul network is normally organized in a hierarchical way. The nodes directly connected to the RNC constitute primary hubs. These primary hubs connect nodes (which could be hubs as well) forming a second level of hierarchy, and so on, until all the base stations are included. The hubs are typically base stations themselves. On top of sending the traffic generated by the mobile terminals attached to them towards the RNC, these hubs also relay the traffic coming from nodes of lower level in the hierarchy towards the RNC. The hubs do not interfere with the traffic coming from other base stations (which could be hubs themselves). To limit the overall delay introduced by the backhaul network, the backhaul network normally specifies a maximum of 3 or 4 levels of hierarchy. The connection between a base station and the hub next level up in the hierarchy is commonly referred to as the "last mile" backhaul, although typically the average distance between base stations is in fact around 5-6 kilometres at present.

Given the increasing data rates offered by the latest cellular technologies like 3GPP HSxPA, 3GPP2 EvDo RevA and Mobile WiMAX, in the recent years, microwave technology has gained popularity for providing last mile backhaul connectivity, given its price advantage over the traditionally used leased lines.

The ever increasing capacity required by the backhaul links, has resulted in highly directional point-to-point microwave-based products being currently the prime choice of most mobile network operators to provide last mile connectivity. These products operate at high frequencies (above 10GHz) to have availability of broad radio channels, consequently they require the line of sight.

Despite the falling price of the hardware and the increasing data rates delivered, the current microwave products still suffer from the need of manual link alignment configuration at each end of the link. Conventionally this is a manual task requiring visual alignment to high accuracy, which inevitably increases the installation and maintenance (due to periodic re-alignment) cost.

Although this drawback can be tolerable for the cell density typical of 2G and 3G deployments, it becomes a major burden for the cell density required by future "Beyond 3G" cellular systems, which could be up to a tenfold increase compared to the current typical cell density.

In a radio telecommunication system, the term link budget is used to determine if a signal from a transmitter can reach a receiver, taking into account all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication system. A node with a 10cm dish antenna operating at 40GHz may have a gain of the order of 35dB and a beam-width of less than 3 degrees. Even if enough link budget to achieve a connection with another node at the desired data rate is ensured, a random search in all directions near the horizon from both ends of a link would take a very long time before a signal was found.

WO-A-02/06 1956 describes a joining process for admitting a node to a wireless mesh network. In this network each node has a plurality of spatial coverage sub-sectors which together cover a larger sector angle of 1200. Each sub-sector covers 750 When a new node is to be admitted to the network, that node sequentially scans each of its sub-sectors to detect a signal received from an active network node. When the joining node determines which of its sub-sectors receives the signal from the active network node, that sub-sector is selected to establish a communication link. Similarly, the active network node scans its sub-sectors sequentially to identify the sub-sector of the joining node which transmits signals from the joining node. When the aligned sub-sectors of the joining node and the active network node are known, the link between them can be established. The joining node listens only one sub-sector at one specific frequency at a time. This frequency will be varied over a period of time in order to detect the frequency at which the active network node is transmitting.

A global positioning system (GPS) may be used to assist in the joining process by identifying the location of the active network node and/or the joining node.

It is an object of embodiments of the invention to avoid the known manual cumbersome alignment process, and to provide an improved or alternative mechanism able to automatically discover adjacent nodes and to automatically align the narrow beam of the microwave highly directional antenna to the direction of maximum gain.

In one aspect the present invention provides a method of enabling a joining communication node to communicate with an active communication node of a network, respective ones of the nodes being operable to communicate with each other wirelessly in a communication mode by means of a directional narrow-beam communication antenna, the method including operating at least one of the nodes in a search mode to search for another of the nodes using a search antenna with a beam width greater than that of the narrow beam antenna.

In the search mode, a beacon or pilot signal may be transmitted with a bandwidth substantially less than the signal transmitted in the communication mode by the communication antenna.

The "active" communication node in the embodiment is a node already connected to other nodes of the network. The "joining" node in the embodiment is a node wishing to join that network.

The communication antenna and/or the search antenna may operate in the microwave frequency band. The frequency band may comprise frequencies above 10GHz. Other frequencies can also be used.

The communication antenna may have a substantially high gain (around 35dB for example), whereas the search antenna may have a significantly lower gain.

The communication antenna may have a beam width of substantially 3° or less to provide it with the desired high gain and bandwidth. In order to facilitate alignment of the communication antennae of the respective nodes, one or other of the communication antennae may be steerable. The steering may be performed electrically or mechanically.

In an embodiment the active node may be operated in the search mode to transmit a beacon signal on the search antenna of the active node, and the joining node may be operated in the search mode to search for this transmitted signal using its search antenna. The communication antenna of the active node and/or the joining node may be steered in dependence upon information obtained in the search mode in order to allow the communication antennae of the active node and the joining node to be aligned for performing data communication therebetween.

The present invention also provides a node for use in the network as defined in the claims, and a network including a plurality of nodes, as defined in the claims.

Embodiments of the present invention concern radio based mechanisms able to automatically discover adjacent nodes and to automatically align the narrow-beam of microwave highly directional antenna to the direction of maximum gain.

A method, network and node will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a network in which the invention may be used; Figure 2 shows schematically functional elements of a communications node in accordance with an embodiment of the invention; Figure 3 shows schematically an alternative arrangement to the functional elements of a communications node in accordance with an embodiment of the invention; Figure 4 shows schematically beams generated by the antennae of the communications node of Figure 3; Figure 5 shows an intermediate node which includes two sets of antennae in accordance with an embodiment of the invention; and Figures 6 to 11 show the steps performed when a joining node wishes to connect to an active node.

In the drawings like elements are designated with the same reference signs.

Elements of a conventional mobile or cellular network will now be briefly described with reference to Figure 1.

Figure 1 shows schematically a network in which the invention may be used.

The figure shows a cellular network. However, it should be appreciated that the invention is applicable to any type of network, having a plurality of nodes which communicate wirelessly.

Mobile terminal 1 is registered with GSM/GPRS or UMTS (3G) mobile telecommunications network 3. The mobile terminal 1 may be a handheld mobile telephone, a personal digital assistant (PDA) or a laptop computer (either having an embedded mobile telecomunications module or equipped with a mobile telecommunications datacard' or modem). The mobile terminal 1 communicates wirelessly with and is registered with base station (Node B) 5.

Communications between the mobile terminal 1 and the mobile telecommunications network 3 are via the radio access network (RAN) of the mobile telecommunications network 3, comprising, in the case of a UMTS network, the base station (Node B) 5, and radio network controller (RNC) 7.

Communications between the mobile terminal 1 and the mobile telecommunications network 3 are routed from the radio access network via GPRS support nodes (SGSN) 9, which may be connected by a fixed (cable) link to the mobile telecommunications network 3.

In the conventional manner, a multiplicity of other mobile terminals are registered with the mobile telecommunications network 3. These mobile terminals include mobile terminal 11. Terminal 11 communicates with the mobile telecommunications network 3 in a similar manner to the terminal 1 that is via an appropriate Node B 5A, RNC 7A and SGSN 9A.

The mobile telecommunications network 3 includes a gateway GPRS support node (GGSN) 17 which enables IP-based communications with other networks, such as the Internet or other IP network 19 via an appropriate link 22.

Each of the mobile terminals 1 and 11 is provided with a respective subscriber identity module (SIM) 15. During the manufacturing process of each SIM, authentication information is stored thereon under the control of the mobile telecommunications network 3. The mobile telecommunications network 3 itself stores details of each of the SIMs issued under its control. In operation of the mobile telecommunications network 3, a terminal 1 and 11 is authenticated (for example, when the user activates the terminal in the network with a view to making or receiving calls) by the network sending a challenge to the terminal 1 and 11 incorporating a SIM 15, in response to which the SIM 15 calculates a reply (dependent on the predetermined information held on the SIM -typically an authentication algorithm and a unique key Ki) and transmits it back to the mobile telecommunications network 3. The mobile telecommunications network 3 includes an authentication processor 21 which generates the challenge and which receives the reply from the terminal 1, 11.

Using information pre-stored concerning the content of the relevant SIM 15, the authentication processor calculates the expected value of the reply from the mobile terminal 1, 11. If the reply received matches the expected calculated reply, the SIM 15 and the associated mobile terminal are considered to be authenticated.

The node B 5 with which the mobile terminal 1 is registered will typically be the nearest node B of the network 3 to the terminal 1. The node B 5 is selected in accordance with known handover and cell reselection procedures.

Communications between the mobile terminal 1 and the node B 5 are wireless communications based on the UMTS Standards.

Often, communications originating from the mobile terminal 1 and destined for the network 3 are not routed directly from the node B 5 to the RNC 7 for onward transmission to the network 3 via the SGSN 9. To provide a fixed (cable) connection between each node B (including node B 5) and the RNC 7 serving the node B's would usually be prohibitively expensive. Therefore, the node B's associated with a particular RNC will relay communications from one node B to another to pass communications to the RNC 7. For example, communications between the mobile terminal 1 and the node B 5 are routed to the RNC 7 in Figure 1 via intermediate node B's 23, 24 and 25.

Communication between node B 5 and node B 23, between node B 23 and node B 24, between node B 24 and node B 25, and between node B 25 and RNC 7 is by a microwave point-to-point directional link operating at high frequency (above 10GHz). These microwave communications are transmitted by a dish antenna having a beam-width around 3° or less.

What has been described thus far is a conventional arrangement. As discussed above, conventionally, the procedure for aligning the microwave beams between the node B's and RNC was a process performed manually. The embodiment of the present invention provides an improved arrangement for automatically aligning the microwave antennae of node B's and RNC's.

However, it should be understood that the invention is applicable to the alignment process for any type of communications nodes.

Figure 2 shows schematically selected functional elements of a node 5. The node 5 includes a dish communication antenna 30 that produces a narrow beam of microwave radiation typically with a beam width of 3° or less, for example, in the conventional manner. In accordance with the embodiment of the invention, the node 5 is provided with a search antenna 34 with a lower gain and a wider beam width to produce a beam with a beam width of 30°, for

example.

The search antenna 34, if operated at 40GHz, could be very small with an aperture of only of the order of 1 square cm, but would have a gain of 20dB less than the communication antenna 30. Even if the main link between the communication antennae 30 of respective nodes is planned needing an signal to noise ratio (SNR) of 15dB with a margin of 15dB (margin considered to account for fading -distortion that a carrier-modulated telecommunication signal experiences over certain propagation media), the signal received between respective search antennae 34 of the nodes would still be 10dB below the noise and would not be detectable. To overcome this problem, as no user data is transmitted in search mode, the transceiver bandwidth of the search antennae 34 is reduced to a small fraction of the bandwidth used by the transceiver of the communication antennae, and a low data rate beacon or pilot signal is transmitted from one end of the link between the nodes.

The Node 5 includes a communications transceiver 26 "TX RX" and a control unit 27. As shown in Figure 2, the single transceiver 26 may be used to operate both the search antenna 34 and the communication antenna 30 (at different times). Alternatively, as shown in Figure 3, each antenna 34,30 could have a separate transceiver associated with it -that is, the communication antenna 30 has a first transceiver 26C "TX-C RX-C" and the search antenna 34 has a second transceiver 26S "TX-S RX-S". The actual design depends on operational requirements and cost target. The transceiver(s) may transmit and receive at the same time (FDD mode) or at separate times (TDD mode) depending on the radio technology used.

Each node needs to have at least two antennae if it behaves as "end node" only and at least three antenna if it can behave as "intermediate node" with more than one link towards other nodes (an intermediate node will generally only do one search at a time and thus requires only one search antennae).

An "end node" would have at least two antennae; * One communications antenna (narrow-beam).

* One or more search antennae of one or more beamwidths. Each search antenna may be omnidirectional, steerable (e.g. electrically or mechanically) over 360 degrees but with a much wider beamwidth than the communications antenna, or made up of a small number of switchable beams which in total cover 360 degrees. The optimum combination of bandwidth and search antenna beam width can be determined by taking into account that the signalling waveforms need only transmit a two bit pattern as there are only four states ( "stop", "stopped", "beacon" and "locked"].

An "intermediate node" has two communication antennae and two search antennae.

Figure 4 shows an end node having a communication antenna 30 that produces a narrow beam 32 and a search antenna 34 that produces a relatively wide beam 36. The communication antenna 30 is coupled to communications transceiver 26C and the search antenna is coupled to search transceiver 26S. The communications transceiver 26C and search transceiver 26S are controlled by control unit 27.

Figure 5 shows an intermediate node. As in Figure 4 the node includes communication antenna 30 producing narrow beam 32, search antenna 34 producing wide beam 36, communication transceiver 26C, search transceiver 26S and control unit 27. However, the intermediate node further includes a second communication antenna 30A which produces narrow beam 32A and a second search antenna 34A which produces wide beam 36A. A second communications transceiver 26CA "TX-CA RX-CA" is provided which controls transmission and reception of data by the communications antenna 34A. A second search transceiver 26SA "TX-SA RX-SA" is provided which controls transmission and reception by search antenna 34A. The second transceivers 26CA and 26SA are controlled by the control unit 27.

The steps performed when a joining node J wishes to connect to an active node A will now be described with reference to Figures 6 to 11. Each of the nodes shown in Figures 6 to 11 includes all the transceiver and antenna elements shown in Figure 5, but in Figures 6 to 11 only the activated transceivers and antennae are shown to clarify the explanation.

In the Figures 6 to 11 active node A is an "intermediate node". The communications antenna 30 produces narrow beam 32 that is already aligned with the narrow beam of the communications antenna of another node (not shown). Transmission and reception of data via the communications antenna 30 is controlled by the communications transceiver 26C "TX-C RX-C", which is in turn controlled by the control unit 27. The active node A is referred to as an active node because it is connected to other elements of the network. It should be appreciated that the connection to other elements of the network may be by an alternative mechanism to narrow beam communications antenna 30 -for example by a cable connection.

For an active node to be able to "search" for a possible joining node it needs to have a spare transceiver (to activate a search antenna) not currently active in communication with another node. The active node A therefore includes search antenna 34A connected to the search transceiver 26SA "TX-SA RX-SA" which is operated in the transmit mode under control of the control unit 27 and produces wide beam 36A. In this mode, the search antenna 34A and transceiver 26SA are able to detect certain received narrow bandwidth signals to facilitate the joining process. The search antenna 34A transmits a continuous narrow band "beacon" signal (or "pilot" signal). As mentioned above, the search antenna 34A may be an omni-directional antenna. However, in this example it is assumed that the search antenna 34A has a beam width of X degrees (where X<360°) and is rotated, as indicated by arrow 40, with a period T. If enough gain can be achieved, an omni-directional antenna (at least at one node) could be sufficient, and this will speed up the joining process further.

The joining node J includes the same functional elements as the active node A and these are designated with the same reference signs but preceded by "1".

When the joining node J wishes to join the network the control unit 127 operates the search transceiver 126S "RX-S" in a receiving mode. The search antenna 134 rotates as indicated by arrow 42 at a slower rate than the rotation rate of the search antenna 34A of the active node A. For example, if the beam width of this search antenna 134 of the joining node J is Y degrees, then the search antenna 34A of the joining node J will rotate with a period of slightly larger than T*360/Y to ensure that it will allow the search antenna 34A of the active node A to complete a full rotation before the wide beam 136 of the joining node J has passed through the direction of the active node A. As shown in Figure 7, as soon as the joining node J detects a signal from the search antenna 34A of the active node A, rotation of the search antenna 134 of the joining node J is stopped. The control unit 127 of the joining node J then operates the search transceiver 126S "TX-S RX-S" in a transmit mode to transmit a narrow band "stop" signal. The search antenna 34A and transceiver 265A of the active node A are operable to receive certain signals, such as the narrow bandwidth "stop" signal. When the active node A detects the "stop" signal it stops rotation of its search antenna 34A and switches the signal transmitted by the search antenna 34A from "beacon" to "stopped".

Referring now to Figure 8, the active node A then adjusts the direction of its search antenna 34A, as indicated by arrow 44 to maximise the received signal level from the search antenna 134 of the joining node J. As shown in Figure 9 the control unit 27 of the active node A then activates the communications antenna 30A via transceiver 26CA "RX-CA". The communications antenna 30A adjusts the direction of the narrow beam 32A that it produces, starting from the approximate direction indicated by the search antenna 34A, until it finds the maximum signal from the joining node J. The narrow band "stop" signal transmitted by the communications antenna 30A will be switched to a narrow band "locked" signal.

As shown in Figure 10, once the joining node J detects this "locked" signal it will follow a similar process -i.e. it activates communications transceiver 126CA "TX-C RX-C", adjusts its search antenna 134 as indicated by arrow 46 to the direction of maximum gain and aligns its narrow beam communications antenna 130 beam 132 in the same direction as the search antenna 134, and then adjusts the direction of the communications antenna 130 beam 132 to maximise the level received from the active node A. Once this process is complete the communications antenna 130 sends a narrow band "locked" signal.

As shown in Figure 11, at this point the communications antennae 30A and 130 are aligned and both the joining node J and active node A can then deactive their respective search transceivers 26SA and 126S and operate only their communications transceivers 26CA "TX-CA RX-CA" and 126C ("TX-C RX-C" to start exchange of data in a broadband communications mode. The joining node J then becomes an active node and may transmit its own "beacon" signal using its search antenna 134A via the search transceiver 126SA "TX-SA RX-SA" to enable further nodes to "join" the network via the former joining node J. Although the active node A and the joining node J are shown as having two search antennae, only a single search antenna may be provided for each node, which be used by either of the search transceivers 26S and 26SA/126S and 126SA.

Until a link between respective nodes has been established a node may not have access to an accurate frequency reference. It is advantageous for all the nodes of a network (and nodes wishing to join the network) to be synchronised to a frequency standard. The degree of synchronisation of the nodes to the frequency standard will determine how narrow the search bandwidth can be before an additional search dimension is introduced, slowing down the search and reducing the benefit of the narrowband technique. The existing active nodes of the mesh have access to an accurate network frequency standard due to their connection to the network, so the system needs only to accommodate a possible error at one end of the link for the joining node. If inexpensive Standards cannot deliver sufficient accuracy, a GPS (or other time standard) receiver may be included in the joining node to provide a frequency reference.

This GPS receiver could be operated only for the duration of the initial link establishment process. If the frequency standard in the joining nodes is not sufficiently accurate to be able to guarantee to tune the narrowband receiver 126S to the beacon frequency, the receive frequency will be stepped gradually through a tuning range guaranteed to include the beacon frequency of transmitter 265A with one step for each rotation of the search antenna 134 of the joining node. This will slow down the joining process by a factor of approximately df/B where df is the frequency uncertainty of the joining node receiver and B is the bandwidth of the receiver when operating in search mode.

Once the link has been established, the joining node could start using the synchronisation technique chosen by the network operator (e.g. GPS, adaptive clocking, IEEE 1588v2, IETF NTP v4).

If the misalignment error communicated by the wide-beam antenna is too big, the narrow-beam antenna of the joining and active nodes will take a long time to algin with each other. For this reason, if necessary, intermediate size antennae may be added to enable a multi-stage process. In practical terms, to reduce the number of antennae employed, a steerable antenna whose beam angle can be enlarged or reduced automatically (e.g. via antenna arrays) could be used instead of separate antennae of different widths.

One possible embodiment of the search antenna would comprise of several antenna elements (e.g. 4 or 8) which could be arranged for example as a linear array or as a circular array. The usage of microwave frequencies allows the accommodation of many antenna elements within a reasonable size antenna casing. The effective beam width of the search antenna could be altered using analogue or digital beamforming methods, as descrined in Frank Goss: "Smart Antennas for Wireless Communications", McGraw-Hill 2005 (fully incorporated herein by reference). Additionally, the beam direction could be steered using analogue or digital beamforming methods or Direction of Arrival (D0A) estimation algorithms. Examples of DoA estimation algorithms are ESPRIT (Estimation of Signal Parameters via Rotational Invariant Techniques) and MUSIC (Multiple Signal Classification).

If the power consumption of a transmitter is greater than that of a receiver, a variant of this embodiment could be used in which the roles of active and joining nodes are reversed, with the active node continuously operating a search receiver instead of a beacon transmitter.

One embodiment of the beacon signal is a signal which is not transmitted continuously but within a certain time interval. This would allow for a higher instantaneous power at the same average power level as a continuous signal.

For that, a time synchronization of the active node A and joining node J would be necessary, which could be obtained using GPS, the signal of an existing wireless communications signal or another common time reference.

Another embodiment of the beacon signal would be a spread spectrum sequence. This would enable detection even if the signal attenuation between the nodes measured in dB is high. The spreading gain of the spread spectrum signal has to be dimensioned in a way that receiver node processing (e.g. using a correlator or a bank of correlators) can be finished before the relative orientation between the search antennas of the nodes is changed more than the angular resolution of the beam.

Each node may have a reference signature for its unique identification. This reference signature can for example be transmitted encoded in the beacon signal, or be transmitted after a means of communication is established between two nodes. A network management system of the overall network can use this information on established links and further potential links in range to optimise the mesh network.

The automated and efficient mechanism for enabling direct communication between nodes has many applications. For example, Distributed Antenna Systems (DAS) and Network MIMO (Multiple Input Multiple Output) are network concepts which increase the performance of cellular networks substantially. However, they have the drawback that they require a large amount of backhaul traffic between neighbouring base stations. The invention is a means to provide this backhaul very cost-efficiently.

Claims

CLAIMS1. A method of enabling a joining communication node to communicate with an active communication node of a network, respective ones of the nodes being operable to communicate with each other wirelessly in a communication mode by means of a directional narrow-beam communication antenna, the method including operating at least one of the nodes in a search mode to search for another of the nodes using a search antenna with a beam width greater than that of the narrow beam antenna.
2. The method of claim 1, further including, in the search mode, transmitting a beacon or pilot signal with a bandwidth substantially less than the signal transmitted in the communication mode.
3. The method of claim 1 or 2,, wherein the communication antenna operates in a microwave frequency band.
4. The method of claim 1, 2 or 3, wherein the search antenna operates in the microwave frequency band.
5. The method of claim 3 or 4, wherein the frequency band comprises frequencies above 10GHz.
6. The method of claim 5, wherein the frequency band includes 40GHz.
7. The method of any one of claims 1 to 6, wherein the communication antenna has a gain of substantially 35dB.
8. The method of any one of claims 1 to 7, wherein the communication antenna has a beam width of substantially 3° or less.
9. The method of any one of claims 1 to 8, including steering the communication antenna.
10. The method of any one of claims 1 to 9, including steering the search antenna.
11. The method of claim 9 or 10, wherein said steering is performed electrically or mechanically.
12. The method of any one of claims ito 11, including operating said one of said nodes in the search mode to transmit beacon or pilot signal on the search antenna of that node, operating said other of said nodes in the search mode to search for this transmitted signal using its search antenna, steering the communication antenna of at least one of said nodes in dependence upon information obtained in the search mode such that the communication antennae of said one of said nodes and said other of said nodes are aligned for performing data communication therebetween.
13. The method of any one of claims 1 to 12, including synchronising the transmission frequencies of the search nodes.
14. A node for use in a network comprising a plurality of nodes, said node including a directional narrow-beam communication antenna and a search antenna with a beam width greater than that of the narrow-beam antenna, means for selectively operating the node in a communication mode in which the antenna communicates wirelessly by means of the communication antenna with other nodes of the network, and means for selectively operating the node in a search mode using the search antenna for obtaining information indicative of the position of a communication antenna of another node of the network.
15. The node of claim 14, including means, operable in the search mode, for transmitting a beacon or pilot signal with a bandwidth substantially less than the signal transmitted in the communication mode.
16. The node of claim 14 or 15, wherein the communication antenna operates in a microwave frequency band.
17. The node of claim 14, 15 or 16, wherein the search antenna operates in the microwave frequency band.
18. The node of claim 16 or 17, wherein the frequency band comprises frequencies above 10GHz.
19. The node of claim 16, 17 or 18, wherein the frequency band includes 40 GHz.
20. The node of any one of claims 14 to 19, wherein the communication antenna has a gain of substantially 35dB.
21. The node of any one of claims 14 to 20, wherein the communication antenna has a beam width of substantially 3° or less.
22. The node of any one of claims 14 to 21, including means steering the communication antenna.
23. The node of any one of claims 14 to 22, including means for steering the search antenna.
24. The node of claim 22 or 23, including means for performing the steering electrically or mechanically.
25. A network including a plurality of nodes according to any one of claims 14 to 24.
26. A method of adding a joining communication node to a network, substantially as hereinbefore described with reference to and/or substantially as illustrated in any one of or any combination of the accompanying drawings.
27. A communication node substantially as hereinbefore described with reference to and/or substantially as illustrated in any one of or any combination of the accompanying drawings.
28. A network substantially as hereinbefore described with reference to and/or substantially as illustrated in any one of or any combination of the accompanying drawings.