Abstract:
Restoration of a link in fixed wireless transmission network, in the case of failure, is achieved by redirecting a directional antenna of a first network element from a second failed network element to a third network element, whereby a new microwave link in the network is established. A network element suited to allow fast and effective restoration in a fixed wireless transmission network has a directional antenna and corresponding control circuitry which are controlled by a control device to redirect transmission direction in the case of a failure. Preferably, the directional antenna is a phased array antenna. The control device can either be a central network management system or a local control system of the network node.

Description:
[0001]    The invention is based on a priority application EP 02 360 071.1 which is hereby incorporated by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to fixed wireless transmission networks and more specifically to link restoration in such networks.  
         BACKGROUND OF THE INVENTION  
         [0003]    Today&#39;s transmission networks such as for example an SDH (Synchronous Digital Hierarchy) network, consist of a number of interconnected nodes called network elements such as line multiplexers, add/drop multiplexers or cross-connects. The interconnections between the network elements use either optical fiber links, copper cables or wireless microwave links. Microwave links are often used in environmental situations where the deployment of cables are not feasible for economical or technical reasons.  
           [0004]    Transmission networks are required to have a high reliability and availability, which means that if a failure occurs in a network due to a fiber breakage or a damage in a network element, the network needs to be restored within short time to maintain availability. Typically, some resources in the network are reserved for restoration purpose so as to take over operation of failed resources. In terms of interconnections between network elements, this means that spare links are provided in the network over which traffic is rerouted in the case of a failure. Obviously, this is costly and reserved network resources are unused for most of the time.  
           [0005]    It is therefore an object of the present invention, to provide a method of restoring a transmission network using microwave links which uses network resources in a more economic way. Moreover, it is an object of the present invention, to provide a network element suited to allow fast and effective restoration.  
         SUMMARY OF THE INVENTION  
         [0006]    These and other objects that appear below are achieved by a method of restoring a link in fixed wireless transmission network by redirecting, in the case of failure, a directional antenna of a first network element from a second failed network element to a third network element, thereby establishing a new microwave link in the network.  
           [0007]    A network element suited to allow fast and effective restoration in a fixed wireless transmission network has a directional antenna and corresponding control circuitry which are controlled by a control device to redirect transmission direction in the case of a failure. Preferably, the directional antenna is a phased array antenna. The control device can either be a central network management system or a local control system of the network node.  
           [0008]    Advantages of the present invention include that it allows the establishment of a network with switching and protection capabilities exploiting the environmental and geographic properties of typical microwave deployment. Additionally, the invention is best suited to protect against catastrophic events which destroy an entire network node. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which  
         [0010]    [0010]FIG. 1 shows a microwave link;  
         [0011]    [0011]FIG. 2 directional properties of a phased array antenna;  
         [0012]    [0012]FIG. 3 the principle of redirecting radio transmission direction;  
         [0013]    [0013]FIGS. 4 a,    4   b  protection switching in a fixed wireless transmission network;  
         [0014]    [0014]FIG. 5 the use of a smart antenna for space multiplexing in a network; and  
         [0015]    [0015]FIG. 6 a radiation diagram of a phased array antenna. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Typically the application of microwave technology is the wireless bridging of certain distances basically under ‘line of sight’ conditions. Microwave links are a basic technology—together with optical fibres or copper cables—constituting transmission networks.  
         [0017]    [0017]FIG. 1 shows schematically a microwave link MW between a first node N 1  an a second node N 2 . Each node has an antenna A 1 , A 2  for transmitting and receiving microwave signals, respectively. In typical microwave network applications, these antennas are directed antennas, i.e., antennas which exclusively transmit into a limited number of directions, typically into only one direction.  
         [0018]    A special type of directed antennas are so called phased array antennas, also known as “smart” antennas. Such an adaptive antenna is described for example in Simon C. Swales et al., “The Performance Enhancement of Multibeam Adaptive Base-Station Antennas for Cellular Land Mobile Radio Systems”, IEEE Transactions on Vehicular Technology, vol. 39, no. 1, February 1990, which is incorporated by reference herein. Phased array antennas allow to point the transmission power of the antenna along beams into one or more directions. FIG. 2 shows the antenna characteristic of a circular array of 12 dipoles having only one main direction at 60° (left) or 3 main directions at 0°, 120°, 240° (right).  
         [0019]    When all dipoles of the phased array antenna are arranged parallel the z-axis of the co-ordinate system, the antenna characteristic f(φ, θ) of a phased array of M dipoles serving T beams is given by  
         f        (     φ   ,   θ     )       =       sin   2        θ          ∑     n   =   1     M            ∑     m   =   1     M            ∑     g   =   1     T            ∑     h   =   1     T                       [       k        (       s   n     -     s   m       )       -     (       a   gn     -     a   hm       )       ]                                         
 
         [0020]    where  
         [0021]    s n =x n  cos φ sin θ+y n  sin φ sin θ+z n  cos θ 
         [0022]    x n , y n , z n  are the co-ordinates of the n-th dipole in the plane,  
         [0023]    φ is the angular co-ordinate in het x-y-plane, 0&lt;φ&lt;2π 
         [0024]    θ is the angular co-ordinate off the z-axis, 0&lt;θ&lt;π 
         [0025]    a gn  is the phase of the current feeding the n-th dipole for the g-th beam, i.e.  
         [0026]    a gn =k(x n  cos φ g  sin θ g +y n  sin φ g  sin θ g +z n  cos θ g )  
         [0027]    φ g , θ g  are the angular co-ordinates of the beam k  
       k                 is                     2      π     λ                           
 
         [0028]     with λ the wave length of the carrier  
         [0029]    Optimal reception at a mobile station located at a point with angular co-ordinates (R, φ g , θ g ) is achieved when the n-th dipole is fed with a current  
           i   n          (   t   )       =       ∑     g   =   1     T            I   g                       [       ω                 t     -     a   gn       ]                                   
 
         [0030]    where l g  is the amplitude of the feeding current.  
         [0031]    Correspondingly the reception of a first network station from a second network station at a point with angular co-ordinates (R, φ g , θ g ) results in a current i n (t) at the n-th dipole.  
         [0032]    A phased array antenna system hence produces a beam of radio frequency energy and direct such beam along a selected direction by controlling the phase of the energy passing between a transmitter/receiver and an array of antenna elements through a plurality of phase shifter sections. This direction is provided by sending a control word i.e., data representative of the desired phase shift, as well as attenuation and other control data such as a strobe signal, to each of the phase shifter sections.  
         [0033]    As presented above the basic principle of phased array antenna technology is the possibility to point the maximum power of the antenna into one or more directions by suitably choosing the phases of the feeding currents. Thus, simultaneously changing the phases of the feeding currents results in an immediate switch of the beam from one defined direction to another as shown in FIG. 3.  
         [0034]    A basic idea of the present invention is to use in a fixed microwave network adaptive antennas to redirect the direction of transmission in the case of a failure of one transmission node to another transmission node to restore the network. In a preferred embodiment, use is made of the phased array antenna technology. The invention thus uses the mechanism of controlling the transmission direction for protection switching.  
         [0035]    In FIG. 4 a,  microwave network is shown. It has six transmission nodes N 41 -N 46  interconnected by microwave links. Links are established for example between network node N 41  and network node N 42  and between network node N 42  and network node N 44 . If network node  42  is down for some reason the signal from node N 41  to node N 44  is lost. This loss of signal can be detected at the sending node and an immediate switch of the beam to node  43  is initiated, restoring the link almost immediately. This is shown in FIG. 4 b.  Network node N 42  is affected by a failure and node N 41  redirects the direction of transmission to node N 43 , which in turn redirects its direction of reception to node N 41 , too, to receive the microwave signal from node N 41 . Preferably, the restored microwave link between node N 41  and node N 43  uses a reserved protection carrier frequency.  
         [0036]    Detection of a node failure can either be done autonomously be the transmitting node N 41 , for example by detecting the loss of signal in the backward direction from node N 42  to node N 41 , or via a central network management, which informs the transmitting node of the failure and initiates restoration.  
         [0037]    The use of adaptive antenna technology allows to have more than one signal processed at a time due to the principle of superposition. Hence, it is possible to point simultaneously with the same antenna a multitude of signals to different receiving stations as presented in FIG. 5. Exploiting this property even further a sort of space multiplexing is possible, which may be called adaptive SDMA (space division multiple access). If the signals arrive in packets they can even be switched packet-wise.  
         [0038]    [0038]FIG. 5 shows a network node  51  receiving microwave signal from network nodes N 52  and N 53 . The signal from node N 52  contains three time slots  1 ,  2 , and  3  and the signal from node N 53  contains three time slots a, b, and c. Node N 51  now uses adaptive antenna technology and space multiplexing to transmit the signals to nodes N 54 , N 55  and N 56 . For example, time slot received from node N 53  and timeslot  3  received from node N 52  are re-transmitted to node N 56 . Conversely, timeslots  1  and b are re-transmitted to node N 55  and timeslots  2  and c are re-transmitted to node  56 .  
         [0039]    Use of the technology of redirecting transmission direction in microwave systems hence allows the establishment of a network with switching and protection capabilities exploiting the environmental and geographic properties of typical microwave deployment. It should be noted that the adaptive SMDFA technology can also be used without the protection switching capabilities explained above.  
         [0040]    In switchable microwave networks the accuracy of the direction of the transmitting beam is crucial to the power requirements needed to set-up a functioning communication connection. In the following a simple algorithm is described as a preferred development of the invention that is based on a protocol between the communicating nodes and GPS information (GPS: Global Positioning System). This avoids the necessity of deploying Direction Of Arrival (DOA) algorithms (ESPRIT, MUSIC) which require substantial computing power.  
         [0041]    A typical diagrams of directional properties of a microwave station with a “smart” antenna had been presented in FIG. 2.  
         [0042]    To ensure minimum power requirement it is important that microwave stations have optimally directed beams. The location of a microwave station can easily be determined using the GPS position. This position however, does not provide the accuracy necessary for optimal main beam positioning. However, once a connection is established (here the GPS accuracy is sufficient) the following simple algorithm for fine-tuning beam positioning is proposed.  
         [0043]    [0043]FIG. 6 shows a radiation diagram of a smart antenna. It is evident that within the range of interest (shown gray-shaded) the radiation function is uni-modal. Each microwave station determines the direction into which the beam has to be re-positioned to achieve optimum.  
         [0044]    The following algorithm serves to find the direction of the maximum transmission power in a microwave link between a station 1 and a station 2:  
         [0045]    Loop:  
         [0046]    Station 2: Determines the receiving power and communicates this to Station 1.  
         [0047]    Station 1: Shifts its beam clockwise while maintaining the transmission power by some angle α 0 .  
         [0048]    Station 2: Determines again the receiving power and communicates this to Station 1.  
         [0049]    Station 1: Evaluates the results:  
         [0050]    If the power has become smaller it shifts the beam counter-clockwise by the same angle α 0  with respect to the initial position.  
         [0051]    If the power has become bigger it shifts the beam further clockwise by the same angle α 0 . Repeat until the power becomes smaller.  
         [0052]    Station 1: Adjust α n+1 =β*α n , 0&lt;β&lt;1. Repeat until the desired accuracy is achieved.  
         [0053]    In other words, a transmitting node adjusts its direction of transmission stepwise until a receiving node reports substantially maximum reception power. With this type of algorithm a desired accuracy of the beam is achieved using only minimum computing resources.  
         [0054]    Having now described preferred embodiments of the invention in detail, it should be noted that several modifications are possible and apparent to those skilled in the art. For example, the antenna must not necessarily be an phased array antenna, but any kind of directional antenna with adjustable direction can be used. The antenna may for example be rotated to redirect it into a new transmission/reception direction, however, this would be more difficult in terms of required accuracy than using smart antenna technology. Another modification concerns the re-directing of antenna to a third node in the case of a failure. Obviously, it would be necessary for the transmitting node to know the direction where to re-direct the antenna. As described above, this information could be determined in a first step by the use of GPS information and subsequent fine tuning. However, the information can also be pre-determined or configured by an operator or a central network management system.