TECHNICAL FIELD  
       [0001]     The invention is based on a priority application EP 05291456.1 which is hereby incorporated by reference.  
         [0002]     The present invention relates to a method for transporting a signal in a radio network, wherein the signal is transported over wires between a radio head and a radio controller. The present invention further relates to a radio head and a radio controller.  
       BACKGROUND OF THE INVENTION  
       [0003]     In the context of the development of broadband mobile infrastructure, in particular 4G (fourth generation) networks and high data rate radio enterprise networks, the installation of pico cell gets more and more important. A pico cell is a part of a cellular network providing a high bit rate and having a typical radius of 50-100 m. Per square kilometer, up to several hundreds of radio access point can be installed. This leads to the first concentration layer, i.e. the first or physical layer at a node of the network to require a high number of links.  
         [0004]     On the one hand, the usage of leased lines as in 2G (second generation) or 3G (third generation) networks is expensive, especially if the bit rate per access point is high. On the other hand, the revenue per user will almost be constant in 4G networks, as compared to 2G/3G networks. Thus, the part of revenue dedicated to transport should not significantly increase.  
         [0005]     The transport solutions providing high data rate in place are ATM (asynchronous transport mode), SDH (synchronous digital hierarchy), Ethernet etc. But currently, transport is usually not optimized in the design of radio access networks. Normally, a specific protocol is installed with the transport equipment. In case of pico cells, this approach makes it necessary to install a protocol based transport inside the radio access points. Such a radio access point is quite complex, having physical and MAC layers as well as management layers and the transport protocol. For examples, such radio access points have been implemented in WiFi solutions, where “light” and “heavy” radio access points exist according to the radio management capabilities. The transport solution in this specific case is Ethernet.  
         [0006]     The outlined approach is quite complex and in consequence quite costly. It is therefore an object of the present invention to provide a transport method with reduced complexity.  
       SUMMARY OF THE INVENTION  
       [0007]     This object is achieved by a method for transporting a signal in a radio network between a radio head and a radio controller, wherein the signal is transported over wires, said method comprising the steps of: 
        determining the useful band of each wire;     associating at least two wires;     filtering the signal into sub-bands;     transporting each sub-band over one or more wires of the associated wires;     at the receiving end rebuilding the signal from the sub-bands.        
 
         [0013]     The present method permits to concentrate almost all complexities at the radio controller side of the connection, thus avoiding a large and costly overhead of higher layers of a protocol on the radio head side. A major advantage is that the signal does not need to be demodulated for transport between radio head and radio controller. Any demodulating or modulating is done only by central points of the radio network such as radio controllers, but not by radio heads. The present method reduces the signal transport between radio head and radio controller to a transparent transmission without high level processing of the signal.  
         [0014]     Furthermore, the present method makes use of wires. Wires are almost ubiquitous, inexpensive, reliable, easily adaptive and are utilized in function of the bandwidth they actually provide. In particular, already existing wiring can be used. By processing the signal to be transported only for the transport and then restoring it, it is possible to make the transport transparent to the air interface of the radio network by choosing an appropriate signal type. Filtering the signal into sub-bands for transport over the wires permits to transport even complex signals with minimal operative expenditure.  
         [0015]     It will be noticed that the determining of the useful bands of each wire can be done in a static way, for example when installing the network with normal wires as used for conventional telephony, or also dynamically, e.g. periodically every some minutes. Particularly when using large bandwidth wires as for high data rate telecommunication, e.g. for ADSL (asymmetric digital subscriber line), it is preferred to determine the useful bands and eventually also associate the wires in groups of at least two wires dynamically, for example every time a signal or a definite group of signals is sent or a new communication is set up. How many wires are associated or grouped depends on the needed capacity, e.g. the number and width of the sub-bands.  
         [0016]     The filtering into sub-bands is done in the frequency domain, and in particular Fourier-type transformation schemes are preferred. Segmenting the signal into sub-bands and processing and transporting the sub-bands minimizes the risk of signal degradation of any kind due to the overall transport process, in particular due to linear impairments.  
         [0017]     In preferred embodiments, the method comprises the steps of: 
        creating a direct and an associated inverse spectrum of each sub-band;     transporting the direct spectra over some of the associated wires and their associated inverse spectra over the other wires of the associated wires;     at the receiving side recombining each direct spectrum and its associated inverse spectrum.        
 
         [0021]     This is particularly advantageous in case of for example outdoor, poor quality wiring, because it permits to simplify the equalization in case of a difficult channel with e.g. non-linear noise or impulsive noise.  
         [0022]     In other preferred embodiments, only a direct spectrum is created. This is less complex proceeding is preferred for example when using indoor wiring, where less noise or other disturbances are to be expected.  
         [0023]     Preferably, calibration and equalization are done digitally in case of weak distortion and attenuation, or with help of digital as well as analog tools in case of heavy distortion and attenuation. It has shown to be advantageous to equalize after recombining to palliate for low pass attenuation or dispersive attenuation that may be caused by the transport over wires. It has further shown to be advantageous to calibrate and/or fine equalize after rebuilding. Calibration reduces loss of information due to noise and fine equalization reduces loss of information due to attenuation during the transport. Fine equalization is preferably done sub-band by sub-band.  
         [0024]     In case multiplexed signals are to be transported, they should preferably be demultiplexed before further processing and transport.  
         [0025]     Any frequency-multiplexed signal is advantageously transported by the present method. In preferred embodiments of the present invention, OFDM (Orthogonal Frequency Division Multiplexing) signals are transported. OFDM is expected to be the modulation used as standard in 4G networks.  
         [0026]     Preferably, the signals are transported over copper wires. Copper wires are particularly inexpensive, as large networks of copper wires already exist. Copper wires are very well adapted for the transport of signals and can offer bandwidths of around one to several MHz, depending on the distance and the wire condition. For example, using indoor CAT5 cables allows for a range of typically up to 200 m. In case of outdoor pico cells even up to 1000 m may be attained.  
         [0027]     The method according to the present invention is especially well suited for transporting signals in a cellular network, in particular a cellular network with pico cells.  
         [0028]     In a further aspect, the present invention relates to a radio controller for transporting a signal in a radio network over wires to and from a radio head, the radio controller comprising: 
        means for determining the useful band of each wire;     means for associating at least two wires.     means for filtering signals into sub-bands;     means for rebuilding signals from sub-bands.        
 
         [0033]     In a last aspect, the present invention relates to a radio head for transporting a signal in a radio network over wires to and from a radio controller, the radio head comprising: 
        interface means to an antenna;     means for filtering signals into sub-bands;     means for rebuilding signals from sub-bands.        
 
         [0037]     The radio heads provide only the infrastructure for actually transporting signals, corresponding basically to the PHY layer of a protocol. Further means for implementing higher layers are needed only on the side of the radio controller. This significantly simplifies the design and the equipment of the radio head compared to a radio access point as commonly used, thus minimizing the cost of a radio network transporting signals over wires according to the present invention. Only the means as mentioned above are necessary for transporting any complex radio signal over wires without much operating expenditure.  
         [0038]     In preferred embodiments of the radio head or the radio controller, they further comprise signal shaping means and/or demultiplexing/multiplexing means and/or analog-digital/digital-analog conversion means and/or inversion/re-inversion means. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]     A detailed description of the invention is provided below. Said description is provided by way of a non-limiting example to be read with reference to the attached drawings in which:  
         [0040]      FIG. 1  shows schematically the basic concept of the present invention in view of affected layers;  
         [0041]      FIG. 2  is a general flowchart of a first embodiment of the method according to the present invention;  
         [0042]      FIG. 3  is a general flow chart of the second preferred embodiment;  
         [0043]      FIG. 4  illustrates exemplarily the processing of a signal for transport according to the second embodiment of the present method;  
         [0044]      FIG. 5  shows schematically an embodiment of a radio controller according to the present invention; and  
         [0045]      FIG. 6  shows schematically an embodiment of a radio head according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0046]     The present invention will be explained more in detail with respect to a cellular network with pico cell coverage. This 4G network uses OFDM signals for transmission. Other advantageous applications are for example high data rate enterprise indoor networks, where e.g. existing Ethernet wiring like CAT5 cables can be utilized. Another example would be to replace a coaxial cable in a cellular network between the antenna on the roof of a building and the processing entity in the basement of the building with twisted cables. The present invention is particularly useful for a targeted spectrum of 20 MHz to 100 MHz  
         [0047]      FIG. 1  shows schematically the basic concept of the present invention in view of affected layers of a base station controller  16  acting as a radio controller. The base station controller  16  is connected to base transceiver stations  10 ,  12  by pairs of copper wires  14  to inexpensively transport signals with a very high bit rate to the base transceiver stations  10 ,  12  acting as radio heads.  
         [0048]     As illustrated in  FIG. 1 , all higher layer functions are concentrated in a single equipment, here the base station controller  16 . Especially high level processing like modulation and demodulation is done only by the base station controller  16  and not by the by the transceiver stations  10 ,  12 . Only the radio functions—in the present example transparent and analogous—are remote and located in the base transceiver stations  10 ,  12 , thus significantly reducing the complexity and cost of the base transceiver stations.  
         [0049]     The layers describing the functions of the base station controller  16  or a network node in general are sorted by a hierarchy going from lower functions realized as hardware to more abstract functions concerning the actual applications and usually realized as software. General transport and routing of signals as well as multiplexing and demultiplexing are achieved by circuitry on the hardware level. The physical layer PHY is in charge of modulation and equalization. The media access control layer MAC and the multiple input multiple output layer MIMO are in charge of transport on a more abstract level as they manage the addresses, where to send signals (MAC), and how to conveniently regroup and process signals (MIMO). The scheduling layer is in charge of packetizing and the cache layer provides storage for the actual application, in case it is necessary for delivery, e.g. for real time services. The mobility and the gateway layer are entirely related to the actual application.  
         [0050]     In opposite to the conventional approaches for transporting signals, where the focus is on the upper layers, i.e. MAC and higher, by adding transport protocol and management layers, the present invention makes use of the possibilities provided by the lowest layers, in particular the transport, routing and multiplexing and demultiplexing layer. In some embodiments of the present invention, the PHY layer is implicated, too. This allows for efficiently utilizing wires for inexpensively transporting signals with high data rate between nodes of a network, in the present example between a base station controller  16  and base transceiver stations  10 ,  12 , and for reducing as much as possible the complexity of the respective nodes in view of necessary layers.  
         [0051]     It will be noted that in real networks not only two base transceiver stations  10 ,  12  are connected to a base station controller, but up to several hundreds, increasing considerably the potential of the method according to the present invention to reduce transport cost.  
         [0052]      FIG. 2  is a general flowchart of a first embodiment of the method according to the present invention. It is to be used in an indoor enterprise pico cell network making use of already existing CAT5 cabling for Ethernet.  
         [0053]     In a first step  20 , the wires, preferably copper wires, are probed to determine the useful band they can provide. If this is done statically, the further process is based on the minimum band they can provide. If this is done dynamically, the actual band is made use of. Depending on the result of the band determination, the wires are associated in groups (step  21 ). Preferably, wires providing approximately the same bandwidth are associated to make best use of the available resources as the narrowest band of the associated wires is determining the usable band of the associated wires.  
         [0054]     The signal is filtered into sub-bands (step  22 ), because the smaller the units to be transported are, the lower is the probability of information loss due to transport. This permits to transport even quite complex signals or signals carrying several bits of information without high expenditure. To prepare the sub-bands for transport, direct spectra are created (step  24 ), e.g. by using Discrete Fourier Transform (DFT). It is possible as well to create a direct spectrum of the signal and then split the signal&#39;s spectrum into sub-bands in order to transport the signal. The direct spectra are transported over the associated wires (step  25 ) and then the signal is rebuilt from the signal sub-bands (step  27 ) and equalized.  
         [0055]      FIG. 3  is a flowchart of a second preferred embodiment of the method according to the present invention optimized for outdoor pico cells or pico cells with bad quality wiring. This embodiment is also further illustrated in  FIG. 4 . Because of the worse transport conditions, the sub-bands are duplicated and inverse spectra are also created (step  24 ).  
         [0056]     The direct and the respective inverse spectra of signal the sub-bands are transported over the associated wires in that the direct spectra are transported over some of the associated wires and the respective inverse spectra are transported over the other wires (step  25 ). At the reception point, e.g. a base transceiver station, the direct and its respective inverse spectrum are added and thus each sub-band is restored (step  26 ), before rebuilding and equalizing the signal from the sub-bands (step  27 ). It will be noted, that providing sub-bands of the signal simplifies equalization in view of reducing the impact of linear impairments, whereas providing inverse spectra simplifies the equalization in view of reducing the impact of non-linear impairments.  
         [0057]      FIG. 4  illustrates exemplarily more in detail the processing of a signal for transport according to the present method. An analogue signal  30  is “adapted” for use in a 4G pico cell network by e.g. Fourier-transforming it and multiplexing it (step  31 ) into a OFDM signal  32 . To prepare this OFDM signal  32  for transport from e.g. a base station controller to a base transceiver station, the spectrum is filtered in order to assign a number of OFDM sub-carrier to sub-bands  300  of the OFDM signal  32 . This filtering has the function of demultiplexing and splitting of the OFDM signal  32  into sub-band spectra  300 .  
         [0058]     Each sub-band  300  is transposed or frequency-shifted, e.g. to 0 and then filtered to duplicate each sub-band  300  and inverse the spectrum of one of each pair of double sub-bands (step  301 ), resulting in a direct sub-band spectrum  304  and its associated inverse sub-band spectrum  302 .  
         [0059]     These pairs of direct and associated sub-band spectra  302 ,  304  are input to a pair of copper wires for transport (step  303 ). This changes their shapes to pairs of direct and associated sub-band spectra  302 ′,  304 ′. After transport and reception (step  305 ), the inverse sub-band spectrum  302 ′ is re-inverted into a direct sub-band spectrum  302 ″ (step  306 ). Then the re-inverted sub-band spectrum  302 ″ is associated to its direct sub-band spectrum  304 ′ and both are recombined, i.e. synchronized and added together (step  307 ). The result  300 ′ is equalized (step  309 ), especially to compensate for group delay and changes in amplitude, e.g. due to selective low pass or dispersive attenuation during transport over the wires or due to different frequency shifts resulting in different destructive and constructive interferences.  
         [0060]     Having recombined all pairs of direct and inverse sub-band spectra  302 ′,  304 ′ to single sub-band spectra  300 ′, these are utilized for rebuilding the original OFDM spectrum with sub-bands  34 ′. This is then translated and multiplexed. Besides it is calibrated from time to time to compensate the influence of variation e.g. of temperature, and fine equalized, i.e. equalized sub-band per sub-band (step  37 ). The result is the original OFDM spectrum  32 ′. Finally, the OFDM signal  32 ′ is transmitted to further processing, for example inverse Fourier-transformation resulting in the original signal  30 ′. If the signal has been transported to a radio head, it is further sent over the air interface of the radio network.  
         [0061]      FIG. 5  shows a schematic view of an embodiment of a radio controller  5  that is relied by cabling  7  containing wires with one or more radio heads. The useful band of each wire in cabling  7  is determined by band determiner  51  before the wire associator  52  groups them depending on the signal to be transported, the transport distance and/or the transport conditions, e.g. indoor or outdoor wiring or kind of wire shielding.  
         [0062]     The sub-band filter  53  splits the signal into sub-bands, whereas the signal rebuilder  54  rebuilds a signal from incoming sub-bands. Filter  53  and rebuilder  54  may also be implemented as one electronic component operating in both directions, splitting and rebuilding.  
         [0063]     If necessary, the signal or sub-bands are further processed by processing means  55 , such as multiplexing and/or demultiplexing, analog-digital and/or digital-analog conversion or signal shaping such as duplication, inversion, amplification, equalization, calibration, Fourier-type transformation etc. MAC layers, control and management of the transport, and all classical upper layer functions are implemented in tool  56 .  
         [0064]      FIG. 6  shows a schematic of a radio head  6 , relied to a radio controller by cabling  7  containing wires. It includes a sub-band filter  63  and a signal rebuilder  64 , like in the radio controller of  FIG. 5 , as well as processing means  65  for multiplexing/demultiplexing, analog-digital/digital-analog conversion, signal shaping, such as amplification, calibration, equalization, filtering, duplication, inversion, Fourier-type transformation etc.  
         [0065]     The signal is either received from an antenna via antenna interface  67  to be transported to the radio controller, or received from the radio controller to be sent over the antenna via antenna interface  67 .  
         [0066]     The present example of a radio head  6  is for a radio network with mostly downstream traffic, where the necessary band determination and wire association is done only from time to time and by the radio controller. In other radio networks, it might be necessary to implement these functions also in the radio heads.  
         [0067]     Although having described several preferred embodiments of the invention, those skilled in the art would appreciate that various changes, alterations, and substitutions can be made without departing from the spirit and concepts of the present invention. The invention is, therefore, claimed in any of its forms or modifications with the proper scope of the appended claims. For example various combinations of the features of the following dependent claims could be made with the features of the independent claim without departing from the scope of the present invention. Furthermore, any reference numerals in the claims shall not be construed as limiting scope.  
       LIST OF REFERENCE NUMBERS  
       [0068]      10  base transceiver station  
         [0069]      12  base station controller  
         [0070]      14  pair of copper wires  
         [0071]      16  layer stack  
         [0072]      20 - 27  steps  
         [0073]      30 ,  30 ′ signal  
         [0074]      31  Fourier transformation  
         [0075]      32 ,  32 ′ OFDM spectrum  
         [0076]      33  demultiplexing  
         [0077]      34 ,  34 ′ OFDM spectrum with sub-bands  
         [0078]      35  filtering and shifting  
         [0079]      36  rebuilding  
         [0080]      37  calibration and fine equalization  
         [0081]      38  inverse Fourier transformation  
         [0082]      300 ,  300 ′ sub-band spectrum  
         [0083]      301  duplicating and inverting  
         [0084]      302 ,  302 ′,  302 ″ inverse sub-band spectrum  
         [0085]      303  inputting  
         [0086]      304 ,  304 ′ direct sub-band spectrum  
         [0087]      305  outputting  
         [0088]      306  inverting  
         [0089]      307  recombining  
         [0090]      309  equalizing  
         [0091]      5  radio controller  
         [0092]      51  band determiner  
         [0093]      52  wire associator  
         [0094]      53  sub-band filter  
         [0095]      54  signal rebuilder  
         [0096]      55  processing means  
         [0097]      56  tool  
         [0098]      6  radio head  
         [0099]      63  sub-band filter  
         [0100]      64  signal rebuilder  
         [0101]      65  signal processing means  
         [0102]      67  antenna interface  
         [0103]      7  cabling
For ensuring a low cost transport of signals in a radio network, in particular with a high density of radio access points, a method is proposed, where the signals are transported over wires between a base station and radio access points, comprising the steps of: determining the useful band of each wire ( 20); associating at least two wires ( 21); filtering the signal into sub-bands ( 22); transporting each sub-band over one or more wires of the associated wires ( 25); at the receiving end rebuilding the signal from the sub-bands ( 27). Further proposed are a radio head and a radio controller both adapted to the present method.