Abstract:
Disclosed is a wireless backhaul network for a communications system. The network comprises a congregate node connected to the communications system; a plurality of access points, each access point having associated amounts of incident bidirectional traffic to be conveyed to and from the congregate node; and a plurality of bidirectional wireless links adapted to convey the traffic between the access points and the congregate node. The congregate node is configured to allocate spectrum to each directional component of each link within a predetermined available spectrum for the conveyance of the traffic, wherein the allocation is dependent on the amounts of traffic at the respective access points.

Description:
BACKGROUND 
       FIG. 1  is a block diagram of an exemplary wireless backhaul network  100  for a communication system. The wireless backhaul network  100  has a tree topology connecting one or more access points, represented by the “leaf” nodes  120 - 2 ,  120 - 3 ,  120 - 5 , and  120 - 6 , to a “congregate” node  110 , which is in turn connected to a core network of a communication system (not shown). Intermediate between the access points  120 - 2  and  120 - 3  and the congregate node  110  is the “relay” node  120 - 1 . Likewise, intermediate between the access points  120 - 5  and  120 - 6  and the congregate node  110  is the relay node  120 - 4 . Connecting the nodes  120 - i  and the congregate node  110  are 6 bidirectional wireless communication links  130 - i  (i=1, . . . , 6). 
     Access traffic from surrounding adjacent user devices can be incident at any node  120 - i  in the backhaul network  100 , including the relay nodes  120 - 1 ,  120 - 4 . The traffic is bidirectional, and can be divided into “uplink” traffic (to be conveyed from the node  120 - i  to the congregate node  110 ) and “downlink” traffic (to be conveyed from the congregate node  110  to the node  120 - i ). Each bidirectional link, e.g.  130 - 1 , therefore comprises two directional link “components”, an uplink  130   u - 1  and a downlink  130   d - 1 . The traffic is converted to signals on the links  130 - i  for conveyance through the network  100 . The capacity of the wireless backhaul network  100  for conveying this traffic has a strong impact on the capacity of the communication system of which the wireless backhaul network  100  forms part. 
     The problem of frequency allocation within a backhaul network is how to allocate spectrum within a predetermined frequency range to each directional link component so that as much as possible of the incident traffic at the nodes served by the link may be conveyed through the network. A complication is that links can interfere with one another, e.g. the uplink and downlink components of a single link, or two link components transmitting to the same node, so the allocation must take this potential for interference into account. 
     In conventional wireless backhaul networks, manual efforts are used to statistically allocate frequencies “optimally” within the network, and then the statistically “optimal” frequency allocations are fixed for months or years. However, the performance of such manual frequency allocation for general tree-structured multiple-hop wireless backhaul networks is extremely low. Hence, to improve access data rates in multi-user communication systems employing backhaul networks, more efficient techniques for frequency allocation are desirable. 
     SUMMARY 
     Disclosed are arrangements which seek to address or ameliorate or more of the above problems by dynamically allocating spectrum to links in a wireless backhaul network based on incident traffic amounts at a given time, taking into account interference constraints imposed by the network topology. The allocation may be performed periodically, so that the disclosed arrangements adapt to changing traffic amounts. 
     According to a first aspect of the present disclosure there is provided a wireless backhaul network for a communications system. The network comprises a congregate node connected to the communications system; a plurality of access points, each access point having associated amounts of incident bidirectional traffic to be conveyed to and from the congregate node; and a plurality of bidirectional wireless links adapted to convey the traffic between the access points and the congregate node. The congregate node is configured to allocate spectrum to each directional component of each link within a predetermined available spectrum for the conveyance of the traffic, wherein the allocation is dependent on the amounts of traffic at the respective access points. 
     According to a second aspect of the present disclosure, there is provided a method of dynamically configuring a wireless backhaul network for a communications system, the network comprising a congregate node connected to the communications system, a plurality of access points, each access point having associated amounts of incident bidirectional traffic to be conveyed to and from the congregate node, and a plurality of bidirectional wireless links adapted to convey the traffic between the access points and the congregate node. The method comprises computing a bandwidth request associated with each link from the traffic amounts associated with each node connected by the link; and allocating bandwidth within a predetermined available spectrum to each link based on the computed bandwidth requests. 
     According to a third aspect of the present disclosure, there is provided a congregate node in a wireless backhaul network for a communications system, the network comprising a congregate node connected to the communications system, a plurality of access points, each access point having associated amounts of incident bidirectional traffic to be conveyed to and from the congregate node, and a plurality of bidirectional wireless links adapted to convey the traffic between the access points and the congregate node. The congregate node is adapted to compute a bandwidth request associated with each link from the traffic amounts associated with each node connected by the link; and allocate bandwidth within a predetermined available spectrum to each link based on the computed bandwidth requests. 
     An advantage of the disclosed arrangements is that less bandwidth is allocated to handle a given amount of traffic than is the case for conventional, manually allocated backhaul networks. In other words, utilisation of allocated spectrum is higher. In addition, the disclosed arrangements require reduced manual efforts in maintaining backhaul networks. No manual effort is required to adjust resource allocation when traffic distribution changes, for example, when more users move into the coverage of an access point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments will now be described with reference to the drawings, in which: 
         FIG. 1  is a block diagram of an exemplary bidirectional tree-structured wireless backhaul network for a communication system, within which the embodiments of the invention may be practised; 
         FIGS. 2A and 2B  collectively form a schematic block diagram representation of an electronic device as which the congregate node of the system of  FIG. 1  may be implemented; 
         FIG. 3  illustrates the two bands in a frequency division duplex (FDD) structure; 
         FIG. 4  is a flow chart illustrating a method of frequency allocation in a wireless backhaul network according to one embodiment; 
         FIGS. 5A and 5B  illustrate the compatibility graphs for bands  1  and  2  respectively for the exemplary wireless backhaul network of  FIG. 1 ; 
         FIG. 6  is a flow chart illustrating a method of allocating bandwidth to links in a wireless backhaul network, as used in the method of  FIG. 4 ; 
         FIG. 7  is a flow chart illustrating a method of provisionally allocating bandwidth to links in a wireless backhaul network, as used in the method of  FIG. 6 ; 
         FIG. 8  illustrates an exemplary provisional bandwidth allocation to links in a sub-network of the exemplary wireless backhaul network of  FIG. 1 ; and 
         FIG. 9  is a flow chart illustrating a method of allocating further bandwidth to unsatisfied links in a wireless backhaul network, as used in the method of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears. 
     The embodiments of the invention may be practised within a bidirectional tree-structured wireless backhaul network, e.g. the network  100  of  FIG. 1 . In particular, the congregate node in the wireless backhaul network, e.g. the congregate node  110  in  FIG. 1 , is responsible for allocating spectrum to the links, e.g. the links  130 - i , in response to uplink and downlink traffic at each node, e.g. the nodes  120 - i , served by the links  130 - i  of the backhaul network  100 . 
       FIGS. 2A and 2B  collectively form a schematic block diagram of a general purpose electronic device  201  including embedded components, as which the congregate node  110  in the network  100  of  FIG. 1  may be implemented. As seen in  FIG. 2A , the electronic device  201  comprises an embedded controller  202 . Accordingly, the electronic device  201  may be referred to as an “embedded device.” In the present example, the controller  202  has a processing unit (or processor)  205  which is bi-directionally coupled to an internal storage module  209 . The storage module  209  may be formed from non-volatile semiconductor read only memory (ROM)  260  and semiconductor random access memory (RAM)  270 , as seen in  FIG. 2B . The RAM  270  may be volatile, non-volatile or a combination of volatile and non-volatile memory. 
     As seen in  FIG. 2A , the electronic device  201  also comprises a portable memory interface  206 , which is coupled to the processor  205  via a connection  219 . The portable memory interface  206  allows a complementary portable memory device  225  to be coupled to the electronic device  201  to act as a source or destination of data or to supplement the internal storage module  209 . Examples of such interfaces permit coupling with portable memory devices such as Universal Serial Bus (USB) memory devices, Secure Digital (SD) cards, Personal Computer Memory Card International Association (PCMIA) cards, optical disks and magnetic disks. 
     The electronic device  201  also has a communications interface  208  to permit coupling of the device  201  to a computer or communications network  220  via a connection  221 . The connection  221  may be wired or wireless. For example, the connection  221  may be radio frequency or optical. An example of a wired connection includes Ethernet. Further, an example of wireless connection includes Bluetooth™ type local interconnection, Wi-Fi (including protocols based on the standards of the IEEE 802.11 family), Infrared Data Association (IrDa) and the like. 
     The methods described hereinafter with reference to  FIGS. 4 to 9  may be implemented using the embedded controller  202  as one or more software application programs  233  executable within the embedded controller  202 . In particular, with reference to  FIG. 2B , the steps of the described methods are effected by instructions in the software  233  that are carried out within the controller  202 . The software instructions may be formed as one or more code modules, each for performing one or more particular tasks. 
     The software  233  of the embedded controller  202  is typically stored in the non-volatile ROM  260  of the internal storage module  209 . The software  233  stored in the ROM  260  can be updated when required from a computer readable medium. The software  233  can be loaded into and executed by the processor  205 . In some instances, the processor  205  may execute software instructions that are located in RAM  270 . Software instructions may be loaded into the RAM  270  by the processor  205  initiating a copy of one or more code modules from ROM  260  into RAM  270 . Alternatively, the software instructions of one or more code modules may be pre-installed in a non-volatile region of RAM  270  by a manufacturer. After one or more code modules have been located in RAM  270 , the processor  205  may execute software instructions of the one or more code modules. 
     The application program  233  is typically pre-installed and stored in the ROM  260  by a manufacturer, prior to distribution of the electronic device  201 . However, in some instances, the application programs  233  may be supplied to the user encoded on one or more portable computer readable storage media  225  and read via the portable memory interface  206  of  FIG. 2A  prior to storage in the internal storage module  209 . In another alternative, the software application program  233  may be read by the processor  205  from the network  220 , or loaded into the controller  202  or the portable computer readable storage medium  225  from other computer readable media. Computer readable storage media refers to any non-transitory or tangible storage medium that participates in providing instructions and/or data to the controller  202  for execution and/or processing. Examples of such storage media include floppy disks, magnetic tape, CD-ROM, a hard disk drive, a ROM or integrated circuit, USB memory, a magneto-optical disk, flash memory, or a computer readable card such as a PCMCIA card and the like, whether or not such devices are internal or external of the device  201 . Examples of transitory or non-tangible computer readable transmission media that may also participate in the provision of software, application programs, instructions and/or data to the device  201  include radio or infra-red transmission channels as well as a network connection to another computer or networked device, and the Internet or Intranets including e-mail transmissions and information recorded on Websites and the like. A computer readable medium having such software or computer program recorded on it is a computer program product. 
       FIG. 2B  illustrates in detail the embedded controller  202  having the processor  205  for executing the application programs  233  and the internal storage  209 . The internal storage  209  comprises read only memory (ROM)  260  and random access memory (RAM)  270 . The processor  205  is able to execute the application programs  233  stored in one or both of the connected memories  260  and  270 . When the electronic device  202  is initially powered up, a system program resident in the ROM  260  is executed. The application program  233  is permanently stored in the ROM  260  is sometimes referred to as “firmware”. Execution of the firmware by the processor  205  may fulfil various functions, including processor management, memory management, device management, storage management and user interface. 
     The processor  205  typically includes a number of functional modules including a control unit (CU)  251 , an arithmetic logic unit (ALU)  252  and a local or internal memory comprising a set of registers  254  which typically contain atomic data elements  256 ,  257 , along with internal buffer or cache memory  255 . One or more internal buses  259  interconnect these functional modules. The processor  205  typically also has one or more interfaces  258  for communicating with external devices via system bus  281 , using a connection  261 . 
     The application program  233  includes a sequence of instructions  262  though  263  that may include conditional branch and loop instructions. The program  233  may also include data, which is used in execution of the program  233 . This data may be stored as part of the instruction or in a separate location  264  within the ROM  260  or RAM  270 . 
     In general, the processor  205  is given a set of instructions, which are executed therein. This set of instructions may be organised into blocks, which perform specific tasks or handle specific events that occur in the electronic device  201 . Typically, the application program  233  waits for events and subsequently executes the block of code associated with that event. Events may be triggered in response to sensors and interfaces in the electronic device  201 . 
     The execution of a set of the instructions may require numeric variables to be read and modified. Such numeric variables are stored in the RAM  270 . The disclosed method uses input variables  271  that are stored in known locations  272 ,  273  in the memory  270 . The input variables  271  are processed to produce output variables  277  that are stored in known locations  278 ,  279  in the memory  270 . Intermediate variables  274  may be stored in additional memory locations in locations  275 ,  276  of the memory  270 . Alternatively, some intermediate variables may only exist in the registers  254  of the processor  205 . 
     The execution of a sequence of instructions is achieved in the processor  205  by repeated application of a fetch-execute cycle. The control unit  251  of the processor  205  maintains a register called the program counter, which contains the address in ROM  260  or RAM  270  of the next instruction to be executed. At the start of the fetch execute cycle, the contents of the memory address indexed by the program counter is loaded into the control unit  251 . The instruction thus loaded controls the subsequent operation of the processor  205 , causing for example, data to be loaded from ROM memory  260  into processor registers  254 , the contents of a register to be arithmetically combined with the contents of another register, the contents of a register to be written to the location stored in another register and so on. At the end of the fetch execute cycle the program counter is updated to point to the next instruction in the system program code. Depending on the instruction just executed this may involve incrementing the address contained in the program counter or loading the program counter with a new address in order to achieve a branch operation. 
     Each step or sub-process in the processes of the methods described below is associated with one or more segments of the application program  233 , and is performed by repeated execution of a fetch-execute cycle in the processor  205  or similar programmatic operation of other independent processor blocks in the electronic device  201 . 
     Formulation of the Problem 
     The disclosed arrangements allocate spectrum to links  130 - i  in variable-width portions or “subbands” of a predetermined available band of wireless spectrum. The following notation is used in the present disclosure: 
     L: number of non-congregate nodes ( 120 - j  in  FIG. 1 ) and (bidirectional) links ( 130 - i  in  FIG. 1 ) in the wireless backhaul network (e.g. for the network  100 , L=6). 
     l i  (i=1, . . . , L): bidirectional backhaul link 
     l i   u : uplink component of link l i    
     l i   d : downlink component of link l i    
     r j   u  (j=1, . . . , L): uplink access traffic (bandwidth request) at node j 
     r j   d  (j=1, . . . , L): downlink access traffic (bandwidth request) at node j 
     R i   u : uplink backhaul traffic (bandwidth request) at uplink l i   u    
     R i   d : downlink backhaul traffic (bandwidth request) at downlink l i   d    
     I i   u : number of subbands of available spectrum allocated to uplink l i   u    
     I i   d : number of subbands of available spectrum allocated to downlink l i   d    
     U i,j : upper-edge of the j-th subband allocated to uplink l i   u  (j=1, . . . , I i   u ) 
     u i,j : lower edge of the j-th subband allocated to uplink l i   u    
     D i,j : upper edge of the j-th subband allocated to downlink l i   d    
     d i,j : lower edge of the j-th subband allocated to downlink l i   d    
     Δu i,j =U i,j −u i,j : bandwidth of the j-th subband allocated to uplink l i   u    
     Δd i,j =D i,j −d i,j : bandwidth of the j-th subband allocated to downlink l i   d    
     The aim of the disclosed allocation method is to choose (u i,j ,d i,j ,U i,j ,D i,j ,I i   u ,I i   d ) for i=1, . . . , L so as to maximise the minimal satisfaction factor across all links: 
     
       
         
           
             
               
                 
                   
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     The allocation is subject to the following, constraints: 
     Constraint 1: To keep the data rates in consistency (in other words, to avoid congestion at any node), the allocated uplink and downlink bandwidths of a link should be the sums of the allocated bandwidths of the uplink and downlink components of the “one-hop subordinate links” of that link respectively. That is, 
                       e   i   u     ⁢       ∑     k   =   1       l   i   u       ⁢     Δ   ⁢           ⁢     u     i   ,   k             =       ∑     j   ∈     S   i         ⁢     (       e   j   u     ⁢       ∑     k   =   1       l   j   u       ⁢     Δ   ⁢           ⁢     u     j   ,   k             )               (   2   )               
for the uplinks l i   u , and
 
                       e   i   d     ⁢       ∑     k   =   1       l   i   d       ⁢     Δ   ⁢           ⁢     d     i   ,   k             =       ∑     j   ∈     S   i         ⁢     (       e   j   d     ⁢       ∑     k   =   1       l   i   d       ⁢     Δ   ⁢           ⁢     d     j   ,   k             )               (   3   )               
for the downlinks l i   d , where S i  is the set of one-hop subordinate links of link l i . For example, the one-hop subordinate links of link  130 - 1  in the network  100  are the links  130 - 3  and  130 - 4 . The quantities e i   u  and e i   d  are the achievable spectral efficiencies of the uplink component l i   u  and the downlink component l i   d , respectively. These quantities, in bits/sec/Hz, indicate the properties of the wireless channels used for the links and can be obtained through measurement.
 
     Constraint 2: the allocated spectra for the uplink and downlink components of a link should be B FDD  apart in frequency to avoid mutual interference, where B FDD  is the frequency division duplex (FDD) Separation Bandwidth. Specifically, if the l-th subband of downlink l i   d  is located higher than k-th subband of uplink l i   d  on the frequency axis,
 
 d   i,j   −U   i,k   ≧B   FDD   (4a)
 
     If the l-th subband of downlink l i   d  is located lower than k-th subband of uplink l i   u  on the frequency axis,
 
 u   i,k   −D   i,j   ≧B   FDD   (4b)
 
     Constraint 3: Two directional links transmitting to (terminating at) the same node arc termed “incompatible” links. For example, in the wireless backhaul network  100 , the uplinks of links  130 - 3  and  130 - 4  are incompatible because both transmit to the same node  120 - 1 . Incompatible links must be B G  apart in frequency to avoid adjacent-frequency interference, where B G  is the Guard Bandwidth. That is, for incompatible uplinks l i   u  and l j   u , if the k-th subband of uplink l i   u  is located higher than l-th subband of uplink l j   u  on the frequency axis,
 
 u   i,k   −U   i,j   ≧B   G .  (5a)
 
     If the k-th subband of uplink l i   u  is located lower than l-th subband of uplink l i   u  on the frequency axis,
 
 u   i,j   −U   i,k   ≧B   G .  (5b)
 
     For incompatible uplink l i   u  and downlink l j   d , if the k-th subband of uplink l i   u  is located higher than l-th subband of downlink l j   d  on the frequency axis,
 
 u   i,k   −D   i,j   ≧B   G .  (5c)
 
     If the k-th subband of uplink l i   u  is located lower than l-th subband of downlink l j   d  on the frequency axis,
 
 d   i,j   −U   i,k   ≧B   G   (5d)
 
     Constraint 4: Any two link components simultaneously transmitting to and from a single node should be at least B FDD  apart in frequency to avoid mutual interference. For example, in the wireless backhaul network  100 , the uplink components of links  130 - 3  and  130 - 1  are transmitting to and from the node  120 - 1  respectively and should therefore have allocations at least B FDD  apart in frequency. Likewise, the uplink component of link  130 - 3  and the downlink component of link  130 - 4  are transmitting to and from the node  120 - 1  respectively and should therefore have allocations at least B FDD  apart in frequency. 
     That is, if the k-th subband of uplink l i   u  is located higher than l-th subband of downlink l j   d  on the frequency axis,
 
 u   i,k   −D   j,i   ≧B   FDD .  (6a)
 
     If the k-th subband of uplink l i   u  is located lower than l-th subband of downlink l j   d  on the frequency axis,
 
 d   i,j   −U   i,k   ≧B   FDD .  (6b)
 
     If the k-th subband of uplink l i   u  is located higher than l-th subband of uplink l j   u  on the frequency axis,
 
 u   i,k   −U   j,i   ≧B   FDD .  (6c)
 
     If the k-th subband of uplink l i   u  is located lower than l-th subband of uplink l j   u  on the frequency axis,
 
 u   j,i   −U   i,k   ≧B   FDD .  (6d)
 
     If the k-th subband of downlink l i   d  is located higher than l-th subband of downlink l j   d  on the frequency axis,
 
 d   i,k   −D   j,i   ≧B   FDD .  (6e)
 
     If the k-th subband of downlink l i   d  is located lower than l-th subband of downlink l j   d  on the frequency axis,
 
 d   j,i   −D   i,k   ≧B   FDD   (6f).
 
     Constraint 5: all the allocated spectra should be within the available continuous spectrum, i.e. from f lower  to f upper . That is, 
     
       
         
           
             
               
                 
                   
                     
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                           ⁢ 
                           
                               
                           
                           , 
                           L 
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           u 
                           
                             i 
                             , 
                             k 
                           
                         
                         , 
                         
                           d 
                           
                             i 
                             , 
                             l 
                           
                         
                       
                       ) 
                     
                   
                   ≥ 
                   
                     f 
                     lower 
                   
                 
               
               
                 
                   ( 
                   
                     7 
                     ⁢ 
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     Thus formulated, the allocation problem potentially involves a large number of unknown variables, and the computational complexity of a constrained global optimisation according to equation (1) is therefore impractically high in most situations. It is also important to note that a transceiver with the capability of reconfiguring its bandwidths and carrier frequencies for transmission and reception is required, which increases the cost. 
     Solution 
     To satisfy constraints 2 and 4, a frequency division duplex (FDD) structure is imposed on the wireless backhaul network. In an FDD wireless backhaul network, two frequency bands (labelled herein as band  1 , or B 1 , and band  2 , or B 2 ) separated by at least B FDD  are defined within the range (f lower ,f upper ) for use by the links  130 - i , as illustrated in  FIG. 3 , B 1  ( 310 ) extends on the frequency axis  300  from f lower   1  to f upper   1 , and B 2  ( 320 ) extends from f lower   2  to f upper   2 , where f lower ≦f lower   1 &lt;f upper   1 &lt;f lower   2 &lt;f upper   2 ≦f upper  and f lower   2 −f upper   1 ≧B FDD .  FIG. 3  illustrates the symmetrical situation where B 1  and B 2  are of equal and maximal width 
     
       
         
           
             
               
                 
                   f 
                   upper 
                 
                 - 
                 
                   f 
                   lower 
                 
                 - 
                 
                   B 
                   FDD 
                 
               
               2 
             
             . 
           
         
       
     
     In an FDD wireless backhaul network, each node  120 - i  receives signals on one of the FDD bands and transmits signals on the other FDD band. For example, in the wireless backhaul network  100 , under the FDD structure the relay node  120 - 1  receives uplink signals from access points  120 - 2  and  120 - 3  and a downlink signal from the congregate node  110  on B 1  and transmits downlink signals to the access points  120 - 2  and  120 - 3  and an uplink signal to the congregate node  110  on B 2 . 
     A bidirectional tree-structured wireless backhaul network (e.g. the network  100 ) with FDD structure is effectively partitioned into two sub-networks (in other words, two directional trees), SN 1  and SN 2 . Each sub-network SN m  utilises only one of the two FDD bands. In  FIG. 1 , the link components utilising B 1  are represented by solid arrows and the link components utilising B 2  are represented by dashed arrows. The two sub-networks are thus represented side-by-side in  FIG. 1 . 
     The effect of an FDD structure is that every end-to-end signal alternates between B 1  and B 2  as it traverses each uplink or downlink. As a result of the alternate use of shared FDD bands, the data rates of uplink and downlink in the FDD wireless backhauling network  100  are correlated. 
     FDD has been widely implemented in transceivers. The use of separate bands for transmission and reception reduces the complexity and cost of transceiver hardware. The FDD structure also simplifies spectrum assignment and maintenance for the radio spectrum regulators. 
       FIG. 4  is a flow chart illustrating a method  400  of frequency allocation in an FDD wireless backhaul network, e.g. the wireless backhaul network  100 , according to one embodiment. The method  400  is carried out by the congregate node  110 . The method  400  may be performed periodically, at fixed or varying intervals, so that the frequency allocation in the wireless backhaul network  100  is dynamic, i.e. adaptive to changing traffic amounts. 
     The method  400  starts at the step  410 , where the congregate node  110  computes the bandwidth request R i   u  in Hertz at each uplink l i   u  from the sum of the uplink data rate requests r j   u  (in bits per second) at the nodes  120 - j  in the subordinate tree “below” that uplink in the uplink direction divided by the spectral efficiency at uplink l i   u : 
                     R   i   u     =         ∑     j   ∈     T   i         ⁢     r   j   u         e   i   u               (   8   )               
where T i  is the set of nodes in the subordinate tree below uplink l i   u  and e i   u  is the achievable spectral efficiency of the uplink l i   u . For example, the uplink bandwidth request R i   u  at the uplink component  130   u - 1  of link  130 - 1  in the wireless backhaul network  100  is equal to the sum of the uplink data rate requests r 1   u , r 2   u , and r 3   u  at the relay node  120 - 1 , access point  120 - 2 , and access point  120 - 3 , respectively, divided by the spectral efficiency at uplink component  130   u - 1 .
 
     The congregate node  110  then (still at step  410 ) computes the bandwidth request R i   d  in Hertz at each downlink l i   d  from the sum of the downlink data rate requests r j   d  (in bits per second) at the nodes in the subordinate tree “below” that downlink still in the uplink direction divided by the spectral efficiency of the downlink l i   d . 
                     R   i   d     =         ∑     j   ∈     T   i         ⁢     r   j   d         e   i   d               (   9   )               
where e i   u  is the achievable spectral efficiency of the downlink l i   d . For example, the downlink bandwidth request R i   d  at the downlink component  130   d - 1  of link  130 - 1  in the wireless backhaul network  100  is equal to the sum of the downlink bandwidth request r 1   d , r 2   d  and r 3   d  at the relay node  120 - 1 , access point  120 - 2  and access point  120 - 3 , respectively, divided by the spectral efficiency at downlink component  130   d - 1 .
 
     The computations at step  410 , together with the subsequent steps, guarantee that Constraint 1 is satisfied. 
     After step  410 , each link in the sub-network SN 1  associated with B 1  has an associated bandwidth request R 1   1 . For example, for the backhaul sub-network SN 1  (represented with solid arrows in  FIG. 1 ) of the wireless backhaul network  100 , the bandwidth request R 1   1  associated with link  130 - 1  is the downlink bandwidth request R 1   d . Likewise, each link in the sub-network SN 2  associated with B 2  has an associated bandwidth request R 1   2 . For the backhaul sub-network SN 2  (represented with dashed arrows in  FIG. 1 ) of the wireless backhaul network  100 , the bandwidth request R 1   2  associated with link  130 - 1  is the uplink bandwidth request R 1   u . 
     In the next step  420 , the congregate node  110  defines an L by L “compatibility matrix” CM m  for each sub-network SN m  based on the topology of the sub-network (m=1 or 2 indicates the current sub-network and associated FDD band). Each entry of CM m  indicates whether a guard band is required between backhaul links  130 - i  and  130 - j  in the associated sub-network SN m . If a guard band is not required, i.e. links  130 - i  and  130 - j  are compatible, CM m (i,j)=1; otherwise, CM m (i,j)=0 (note CM m (i,i)=0 for all i=1, . . . , L). As described above in Constraint 3, two links in a sub-network are compatible unless they terminate at the same node in the sub-network. 
     Based on this definition, for the exemplary backhaul network  100  of  FIG. 1 , links  130 - 3 ,  130 - 4 , and  130 - 1  are mutually incompatible in SN 1  as they all terminate at node  120 - 1 . Likewise, links  130 - 5 ,  130 - 6 , and  130 - 2  are mutually incompatible in SN 1  as they all terminate at node  120 - 4 . However, in SN 2 , only links  130 - 1  and  130 - 2  are incompatible as they both terminate at the congregate node  110 . The compatibility matrices CM 1  and CM 2  for B 1  and B 2  respectively in the exemplary backhaul network  100  are therefore defined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       CM 
                       1 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             0 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             0 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
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                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                   , 
                   
                     
                       CM 
                       2 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             0 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             0 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
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                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             1 
                           
                           
                             0 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The compatibility matrix CM m  defines a “compatibility graph” CG m  for the sub-network SN m . Each vertex in a compatibility graph CG m  represents a link in the sub-network SN m , e.g.  130 - i , and two vertices are joined by a non-directional edge if the corresponding links  130 - i  and  130 - j  are compatible, i.e. CM m (i,j)=1  FIGS. 5A and 5B  illustrate the compatibility graphs CG 1  ( 500 ) and CG 2  ( 550 ) for SN 1  and SN 2  respectively of the exemplary backhaul network  100  of  FIG. 1 . The vertices in the graphs CG 1  and CG 2  are labelled with the corresponding links  130 - i  and are joined by edges according to the matrices CM 1  and CM 2  above. It may be seen that the graph CG 2  is more “connected” to than the graph CG 1 , since the only edge missing from the graph CG 2  is between vertices corresponding to the incompatible (in SN 2 ) links  130 - 1  and  130 - 2 . 
     At the next step  430  of the method  400 , the congregate node  110  allocates bandwidth to the links in each sub-network SN m  based on the bandwidth requests R i   m  computed at step  410 . Step  430  will be described in detail below with reference to  FIG. 6 . 
       FIG. 6  is a flow chart illustrating a method  600  of allocating bandwidth to links in a backhaul sub-network, as used in step  430  of the method  400  of  FIG. 4 . The method  600  is carried out twice (independently) in step  430 , once for the sub-network SN 1  (m=1) and once for the sub-network SN 2  (m=2). (The sub- and super-scripts m are omitted from  FIG. 6  for ease of reading.) The method  600  uses a “provisional” guard band amount B g  that is initially set larger than the guard bandwidth B G , and works through the sub-network, allocating subbands of the corresponding band to the links in a way that satisfies constraint 3 above until all the bandwidth requests have been satisfied. The provisional guard band amount B g  is then adjusted based on the total amount of bandwidth allocated, and the allocation is performed again with the new provisional guard band amount. This process is repeated until the provisional guard band amount B g  converges to B G /c m , where c m  is a scaling factor to be provided by Step  640 . 
     The method  600  starts at step  610  where upper and lower limits for B g , namely B g   U  and B g   L , are initialised. The initial values of B g   U  and B g   L  should be sufficiently large and small, respectively, to ensure B G /c m  is in between those limits. Typical initial values are B g   L =0 and B g   U =C×B G , where C is a predefined constant equal to 1.0e+03. 
     Step  620  follows, at which the provisional guard band amount B g  is set to the average of the upper and lower limits B g   U  and B g   L . The method  600  then proceeds to step  630 , at which the congregate node  110  provisionally allocates bandwidth in the current band to the links  130 - i  in the associated sub-network SN m  based on the bandwidth requests R i   m  using the provisional guard band amount B g , and taking into account the compatibility constraints encapsulated in the matrix CM m . The result of step  630  is a K-vector b m  of provisional allocation bandwidths b h   m  (k=1, . . . , K), where K is the number of subbands, and a binary K-by-L “occupation matrix” C m . The i-th column c i   m  of the occupation matrix C m  is the binary “occupation vector” of link  130 - i , indicating which of the K subbands are allocated to that link. The provisional bandwidth allocation to link  130 - i  may be written in terms of the earlier defined “bandwidth” variables as 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         j 
                         = 
                         1 
                       
                       
                         l 
                         i 
                         u 
                       
                     
                     ⁢ 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         u 
                         
                           i 
                           , 
                           j 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           b 
                           m 
                         
                         ) 
                       
                       T 
                     
                     ⁢ 
                     
                       c 
                       i 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   
                     11 
                     ⁢ 
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     for uplinks in sub-network m and 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         j 
                         = 
                         1 
                       
                       
                         l 
                         i 
                         d 
                       
                     
                     ⁢ 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         d 
                         
                           i 
                           , 
                           j 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           b 
                           m 
                         
                         ) 
                       
                       T 
                     
                     ⁢ 
                     
                       c 
                       i 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   
                     11 
                     ⁢ 
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     for downlinks in sub-network m. 
     Step  630  will be described in detail below with reference to  FIG. 7 . 
     After step  630 , the bandwidth request associated with each link has been satisfied by the provisional allocation. However, the total available bandwidth BW m  in the current band, defined as
 
BW m   =f   upper   m   −f   lower   m   (12)
 
     may have been exceeded by the total provisionally allocated bandwidth. At the next step  640 , the congregate node  110  therefore computes the “utilisation ratio” U m  of the total to provisionally allocated bandwidth to the available bandwidth in the current band: 
     
       
         
           
             
               
                 
                   
                     U 
                     m 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         K 
                       
                       ⁢ 
                       
                         b 
                         k 
                         m 
                       
                     
                     
                       BW 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     If the utilisation ratio U m  is greater than one, the provisional allocation from step  630  is downscaled to precisely fit the current band. In step  640 , the congregate node  110  computes the scaling factor c m ≦1 that would make the total provisionally allocated bandwidth from step  630  equal to the available bandwidth BW m  in the current band: 
     
       
         
           
             
               
                 
                   
                     c 
                     m 
                   
                   = 
                   
                     min 
                     ( 
                     
                       
                         
                           BW 
                           m 
                         
                         
                           
                             ∑ 
                             
                               k 
                               = 
                               1 
                             
                             K 
                           
                           ⁢ 
                           
                             b 
                             k 
                             m 
                           
                         
                       
                       , 
                       1 
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     Note that the computed scaling factor c m  is the reciprocal of the utilisation ratio U m , if U m &gt;1; otherwise, the scaling factor c m  is one since no downscaling is required. 
     Step  650  follows, at which the congregate node  110  determines whether the scaled provisional guard band amount c m B g  is greater than the absolute lower limit B G . If so, the method  600  proceeds to step  660 ; if not, the method  600  proceeds to step  670 . At step  660 , the provisional guard band amount B g  may be reduced, so the congregate node  110  decreases the upper limit B g   U  to B g . At step  670 , the provisional guard band amount B g  is too small, so the congregate node  110  increases the lower limit B g   L  to B g . After both step  660  and  670 , the method  600  proceeds to step  680 , at which the congregate node  110  determines whether the upper limit B g   U  and the lower limit B g   L  have converged within a small predetermined separation ε (typically set to 1e-6), If not, the method  600  returns to step  620  for another pass through the provisional bandwidth allocation with an adjusted value of the provisional guard band B g . If the upper limit B g   U  and the lower limit B g   L  have converged sufficiently, no further adjustment may be made to the provisional guard band amount B g . The method  600  then proceeds to step  690 , where the congregate node  110  obtains the final provisionally allocated bandwidth amounts by scaling the provisional allocated bandwidth vector b m  obtained in the last execution of step  630  by the final scaling factor c m  computed in the last execution of step  640 . The method  600  then concludes. It may be shown that, after step  690 , the final scaled provisional guard band amount c m B g  is equal to the minimum guard band amount B G . 
     The described method  600  uses the “bisection” method to adjust the value of B g  for each iteration, because each adjustment of B g  is half the size of the previous adjustment. In alternative implementations of the step  430 , there are no limits B g   U  and B g   L ; instead B g  is adjusted in step  620  by some other means, and step  680  tests whether c m B g  has converged sufficiently closely to B G . 
     The “satisfaction factor” of a link  130 - i  is defined as the ratio of total bandwidth allocated to the link to the bandwidth request associated with the link: 
     
       
         
           
             
               
                 
                   
                     SF 
                     i 
                     m 
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             b 
                             m 
                           
                           ) 
                         
                         T 
                       
                       ⁢ 
                       
                         c 
                         i 
                         m 
                       
                     
                     
                       R 
                       i 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     As mentioned above, after step  630 , the satisfaction factor of each link  130 - i  is equal to one. Because of the final scaling (step  690 ) of the provisionally allocated bandwidth vector b m  by the final scaling factor c m , step  430  leaves the final satisfaction factor SF i   m  equal to c m  for all links  130 - i  (i=1, . . . L). 
       FIG. 7  is a flow chart illustrating a method  700  of provisionally allocating bandwidth to links  130 - i  in the backhaul sub-network SN m  associated with the current FDD band, as used in step  630  of the method  600  of  FIG. 6 . (The sub- and super-scripts m are omitted in the following description and from  FIG. 7  for ease of reading). The method  700  starts allocating spectrum from the lower limit f lower  of the current band. 
     The method  700  starts at step  705  where the congregate node  110  initialises to zero a “guard band vector” g of length L, each entry g i  of which indicates the amount of bandwidth required to be reserved in the corresponding link  130 - i  from the end of the subband allocated in the previous iteration before any further spectrum can be allocated to the link  130 - i . Also, a subband counter k is initialised to one. 
     At the next step  710 , the method defines a compatibility graph CG k  for the current iteration k from the compatibility matrix CM for the current sub-network, excluding each row and column of CM corresponding to a link  130 - i  that has a non-zero value of g i  in the guard band vector g. 
     Step  720  follows, at which the congregate node  110  computes the cliques of the current compatibility graph CG k . (A clique of a graph is defined as a subset of the nodes of the graph, each pair of which is connected by a graph edge. A clique can be of size one.) The congregate node  110  chooses the clique C k  with the largest cardinality (number of nodes). The congregate node  110  then (at step  730 ) computes the width b k  of the subband to be allocated to the links belonging to the chosen clique C k  as follows: 
     
       
         
           
             
               
                 
                   
                     b 
                     k 
                   
                   = 
                   
                     min 
                     ( 
                     
                       
                         
                           min 
                           
                             i 
                             ∈ 
                             
                               C 
                               k 
                             
                           
                         
                         ⁢ 
                         
                           ( 
                           
                             R 
                             i 
                           
                           ) 
                         
                       
                       , 
                       
                         
                           min 
                           
                             j 
                             : 
                             
                               
                                 g 
                                 j 
                               
                               &gt; 
                               0 
                             
                           
                         
                         ⁢ 
                         
                           ( 
                           
                             g 
                             j 
                           
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     The allocation is also performed at step  730  by setting the entries in the k-th row of the occupation matrix C corresponding to the chosen clique C k  equal to one. 
     At the next step  740 , the congregate node  110  updates the bandwidth request values of R i  for the links in the chosen clique C k  by subtracting the allocation amount b k :
 
 R   i   →R   i   −b   k   ,i∈C   k   (17)
 
     Also at step  740 , the congregate node  110  updates the non-zero guard band vector to entries g i  by subtracting the allocation amount b k :
 
 g   i   →g   i   −b   k   ,i:g   i &gt;0  (18)
 
     As a consequence, either one or more of the links in clique C k  is fully satisfied (R i  goes to 0) or at least one of the guard band-requiring links no longer requires a guard band (g i  goes to 0). 
     Finally at step  740 , the congregate node  110  ensures that each guard band vector entry g j  corresponding to a link  130 - j  that is incompatible with the links  130 - i  in the chosen clique C k  (as determined from the compatibility matrix CM) have value at least equal to the provisional guard band amount B g . 
     At step  750 , the congregate node  110  removes from CM the row and column corresponding to any link  130 - i  that is fully satisfied, i.e. whose value of R i  has gone to 0. Step  760  follows, at which it is determined whether CM is null, i.e. whether all links are fully satisfied. If not, the method  700  increments k (step  780 ) and returns to step  710  for the next iteration. Otherwise, all links  130 - i  are fully satisfied and the method  700  concludes (step  770 ). 
       FIG. 8  illustrates an exemplary provisional bandwidth allocation  800  to links  130 - i  in a sub-network of the exemplary wireless backhaul network  100  of  FIG. 1 . The provisional bandwidth allocation  800  results from the application of the method  700  to the sub-network SN 1  associated with FDD band  1  for some exemplary values of R i   1  and B g . Each row of the allocation  800  represents a link  130 - i  in the sub-network SN 1 , numbered i=1 to 6 from bottom to top. Each column represents a subband k, of which there are K=10. The solid blocks in each row represent spectrum allocated to the corresponding link in the corresponding subband for conveying signals. The diagonally hatched blocks represent reserved spectrum not to be used for conveying signals in the corresponding subband. 
     It may be seen in the bandwidth allocation  800  that incompatible links  1 ,  3 , and  4  do not share any allocated subbands, and their respective allocated subband blocks are separated by at least the provisional guard band amount B g , as required by constraint 3. Likewise, incompatible links  2 ,  5 , and  6  do not share any allocated subbands, and their respective allocated Subband blocks are separated by at least the provisional guard band amount B g , as required by constraint 3. Also, compatible links  1  and  5 ,  2  and  4 , and  3  and  6  share allocated subbands  1 ,  5 , and  9  respectively. In particular, links  2  and  4  share subband  5  which is also reserved as a guard band of link  6  (which is incompatible with link  2 ). The ability of the method  700  to share spectrum between compatible links and to allocate spectrum to links within subbands reserved for guard bands by other, incompatible links makes the bandwidth allocation  800  efficient in terms of total allocated bandwidth. 
     After step  430 , the final satisfaction factors of the links in each sub-network are not in general equal, since in general c 1 ≠c 2 . To ensure consistency of data rates, at step  440  the congregate node  110  equalises the satisfaction factor across both sub-networks. To do this, the congregate node  110  chooses the lower of the two final scaling factors (c 1 , c 2 ) from the two FDD bands:
 
 c   min =min( c   1   ,c   2 )  (19)
 
     The bandwidth allocation for the band m min  corresponding to c min  is kept unchanged. The allocated subband widths of the other band m max  are scaled down by c min /c max , while the reserved guard band portions are not scaled (and therefore remain of width B G ). After step  440 , the satisfaction factor of both sub-networks is equal to c min . 
     If c min &lt;1 after step  440 , the bandwidth requests remain unsatisfied. In step  450 , to which is only carried out if c min  is less than 1, the congregate node  110  therefore allocates further bandwidth to “unsatisfied” links in the wireless backhaul network  100 , while still observing constraints 1 to 5 above. Step  450  will be described in more detail below with reference to  FIG. 9 . The method  400  then concludes. 
       FIG. 9  is a flow chart illustrating a method  900  of allocating further bandwidth to unsatisfied links in a wireless backhaul network, as used in step  450  of the method  400 . The method  900  starts at step  910 , where the congregate node  110  checks all nodes  120 - i  in the backhaul network  100 , including itself, to find those nodes whose receiving subbands plus reserved guard bands fill a complete FDD band. Such nodes are “saturated”, and it is impossible to allocate more bandwidth to the directional link components terminating at such nodes. 
     If the congregate node is saturated (tested at step  920 ), the method  900  concludes, since no more bandwidth can be allocated to any links. Otherwise, the method  900  proceeds to step  930 , at which the congregate node  110  constructs new topologies for the two sub-networks by removing the saturated nodes and their subordinate trees. The links connecting the unsaturated nodes to the saturated nodes are maintained as “disconnected links” in the new topologies. In the remaining steps of the method  900 , the congregate node  110  allocates bandwidth to the remaining links exclusive of the disconnected links in the new topologies, under the constraint that the subbands allocated to the “disconnected links” should remain unchanged. 
     To do this, in step  940  the congregate node  110  carries out the method  600 , as used previously in step  430 , once for each sub-network, with the following alterations:
         After the step  630 , the allocated subbands of the disconnected links and their reserved intervals on the frequency axis are scaled up by B g /B G .   The guard band vector g is initialised in step  705  and updated in step  740  only for the non-disconnected links.   The scaled subbands allocated to the disconnected links are taken into account when setting up the reserved bandwidths for the non-disconnected links. For example, the subband allocated in the previous pass through step  730  is overlapping some of the subbands originally allocated to the disconnected links. Then each element corresponding to a link incompatible with any of the disconnected links should indicate that the next subband possibly allocated to the link must be B g  away from the farthest end of the subbands allocated to these disconnected links.   If the width from the end of the subband allocated in the previous pass through step  730  to the fixed subband of a disconnected link is less than B g , in step  710  the compatibility graph CC k  is defined from the compatibility matrix CM without the rows and columns corresponding to the links that arc incompatible with the disconnected link.       

     After step  940 , step  950  follows, at which the congregate node  110  equalises the satisfaction factor across both sub-networks, as previously described with reference to step  440 . 
     Step  450  is carried out iteratively until the congregate node  110  is saturated or no connected links remain in the backhaul network  100 . 
     The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.