Abstract:
A power reduction device ( 20 ) for reducing power in a transmission signal ( 25 ) received from a mobile telecommunications base station ( 30 ) and its use as part of a distributed antenna system ( 5 ) is disclosed. The power reduction device ( 20 ) comprises a transmission line ( 40 ) for transmission of the transmission signal ( 25 ), a coupler ( 70 ) attached to the transmission line ( 40 ) for extraction of at least a small portion of the transmission signal ( 25′ ) and an energy conversion unit ( 80 ) attached to the transmission line ( 40 ) for the conversion of a majority portion of the transmission signal ( 25″ ) to electrical energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and benefit of Great Britain Patent Application No. 1502819.4 filed on 19 Feb. 2015. This application is also related to UK patent applications numbers GB 1414280.6, GB 1414288.9 and GB 1414283.0 all filed on Aug. 12, 2014. The entire disclosure of each of the above-listed applications is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a device and a method for reducing power in a transmission signal received from a mobile telecommunications base station. 
       BACKGROUND TO THE INVENTION 
       [0003]    The use of mobile communications networks has increased substantially over the past two decades. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network need to reduce the running costs of the base station as well as improve the coverage of the base station. One option to do this is to implement systems for relaying the telecommunications signals of the mobile communication network as a distributed antenna system (DAS). 
         [0004]    The constantly increasing capacity demand in wireless communications and the fact that about 80% of the traffic on the mobile communication system is generated indoors requires new methods to provide flexible signal relaying systems to enable efficient spectrum usage. When indoor traffic is handled with a pure outdoor macro coverage solution, the signal penetration and the signal quality is poor in the indoor environment. Indoor coverage solutions with distributed antenna systems help overcome this issue, but the increasing capacity demand require more advanced indoor solutions beyond pure coverage systems 
         [0005]    These indoor DAS have the capability of dynamic traffic and cell switching. The radio frequency (RF) signals in the DAS are digitalised and communicated between a central hub and a plurality of remote units to which antenna elements are connected. The central hub is connected to one or more of the base stations, which can be provided by different network operators. In the indoor DAS, the coverage of a single cell is not necessarily provided by a single one of the remote units. The plurality of the remote units relays the same telecommunication signal of the cell throughout the coverage area of the cell. The total coverage area of the cell is therefore the sum of the individual coverage areas of each ones of the remote units, which are assigned to the cell. In the case of a plurality of antenna elements being connected to one remote unit, the coverage area of the cell is the sum of the individual antenna element coverage areas connected to the remote unit, which is assigned to the cell. Such systems are described, for example in U.S. Pat. No. 7,761,093 (ADC Wireless). 
         [0006]    The DAS may be used to provide coverage and capacity inside a building, as well as coverage and capacity in metropolitan or campus areas. 
         [0007]    The applicant&#39;s co-pending patent applications No. GB 1414280.6, GB 1414288.9 and GB 1414283.0 describe a distributed antenna system in which the remote units and the antenna elements are distributed inside a building. The use of the distributed antenna system in the building means that attenuation of an uplink path between a mobile station, such as a handset or smartphone, is significantly lower than to a macro base station, which is located outside of the building. The DAS enables connection quality to be maintained, even when the user of the mobile station is moving within the building. On the other hand, the lower attenuation in the uplink path can cause high interference levels on the receiver inputs and block the uplink path completely. 
         [0008]    Current analogue active indoor distributed antenna systems use conventional base stations with remote (radio) units (also called remote radio heads) that are interconnected through the analogue ports (RF or Radio over Fibre) of the remote units and the base stations. The remote units are typically also connected to antennas. Passive distributed antenna systems known in the art use the relatively high power of the base stations to distribute the signals through splitters and tappers to connected antenna elements for transmission. This is possible because each one of the antenna elements requires only a small amount of RF power for the transmission of the signals. Such prior art systems are often used in large buildings and thus the capacity requirements for the complete distributed antenna system are usually of the same order of magnitude as required for macro cells located outdoors. Thus, the prior art DAS currently installed in large buildings require a level of power for indoor coverage, which can be provided by existing base stations used in the macro cells. 
         [0009]    On the other hand, the increasing demand for mobile communications capacity for indoor mobile communications means that more flexible distributed antenna systems are required, which are able to adapt themselves to changes in the capacity requirements within the building without requiring costly changes to the cabling or system infrastructure. Such solutions are known, for example the Kathrein K-bow system, which is a digital and analogue (also called hybrid) distributed antenna system. This hybrid DAS, unlike the passive distributed antenna systems, does not require a higher input signal power from the base station and indeed the components, such as an analog-to-digital converter, used in such digital or hybrid DAS are not able to cope with such high signal input powers. The DAS are therefore provided with an attenuator, which can attenuate the high signal input power (typically in a range of 40-80 Watts) of the signals received from the base station to ensure that the components are not damaged. 
         [0010]    In addition to the digital to analog converters, other components used in the DAS system can be at risk from a high input signal power. Examples of such components include optical transceivers that are used, for example in state of the art analog active DAS systems. Such analog active DAS systems are characterized by an amplification of the distributed base station RF signal within the remote units of the DAS system. The analog active DAS systems do not digitize the base station signal for transmission to the remote units along a distribution network. In this case, the analogue signals are converted to light beams and distributed between the base station of the analog active DAS system and the remote units by using optical transceivers along fibre optic cables. It has been found that, a similar degree of attenuation as required for the output signals from the base station of the digital or hybrid DAS system is needed to ensure that the high power RF output signal generated from the base station and passed to a first optical transceiver of the analogue DAS system does not overdrive and damage this first optical transceiver. 
         [0011]    Alternatively, it is possible to reduce the output power of the signals generated from the base station. This power reduction would be easy and only requires a small change to one of the configuration parameters of the base stations. However, from the safety point of view, it is necessary that the whole of the DAS can at least tolerate for a short period the high input power. In addition, the network operators generally prefer to power the base stations with a maximum amount of power. The base stations used are base stations for outdoor macro cells, as explained above, and therefore can provide a high output power. There is therefore a danger that even those base stations, which are operating at reduced output power, could erroneously be driven at the higher output power and overload the antennas and other components in the DAS. 
         [0012]    The use of attenuators is also not very efficient because the attenuators turn the unwanted electrical power from the attenuated signals into heat. Solutions are known in which this unwanted power could be used, for example, to heat buildings. This requires, however, the construction of additional infrastructure in order to use profitably this generated heat for heating or the production of warm water. 
       SUMMARY OF THE INVENTION 
       [0013]    The disclosure therefore teaches a distributed antenna system including a power reduction device for reducing power in a transmission signal generated from a base station. The distributed antenna system has at least one front end unit (or central hub) which is connected to at least one base station and receives a transmission signal from one or more of the connected base stations. A plurality of remote radio units is connected through a distribution network to the front end unit and a plurality of antenna elements are connected to the plurality of remote radio units. The power reduction device includes a coupler, attached to the transmission line, which is able to extract at least a small portion of the transmission signal, and an energy conversion unit attached to the transmission line for the conversion of the majority portion of the transmission signal into electrical energy. The power reduction device is therefore able to produce electrical energy from the ‘surplus’ energy in the transmission signal. 
         [0014]    The distributed antenna system is typically a digital or hybrid system and the front end unit of the typically used central hub includes an analog-to-digital converter which can be destroyed if too much power is received on an input port. 
         [0015]    In one aspect of the invention, the energy conversion unit can include a rechargeable battery or a rectifier. The energy can also be supplied from the rectifier to a power line or even to the base station or remote units. 
         [0016]    A circulator can be attached between the coupler and the energy conversion unit in order to ensure that there are no unwanted reflections back along the transmission line. 
         [0017]    The disclosure also teaches a method for reducing power in the transmission signal, which comprises coupling at least a small portion of the transmission signal and forwarding the small portion of the transmission signal to a transmission port of at least one remote radio head. The majority portion of the transmission signal is then forwarded to the energy conversion unit. 
         [0018]    The device and method can find application in a distributed antenna system in which a plurality of remote radio heads is connected to one more base stations. The device is used to reduce the amount of power in the transmission signal forwarded to the remote radio heads from the base station. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1A  shows a first simplified aspect of a distributed antenna system. 
           [0020]      FIG. 1B  shows a second aspect of a distributed antenna system. 
           [0021]      FIG. 2  shows the device for reducing power of this disclosure 
           [0022]      FIG. 3  shows the method for reducing the power in the transmission line 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The invention will now be described on the basis of the drawings. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claim in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiment of the invention. 
         [0024]      FIG. 1A  shows a first aspect of a distributed antenna system  5  incorporating the features of this disclosure. The distributed antenna system  5  has a plurality of antenna elements  10   a - c  connected to a corresponding remote radio unit  15   a - c . The plurality of antenna elements  10   a - c  is distributed, for example, throughout a building or over a campus or metropolitan area, but this is not limiting of the invention. The transmission signals at the antenna elements  10   a - c  are received from the remote radio units  15   a - c  and converted from digital domain to analog domain by digital-to-analog converters  17   a - c . The antenna elements  10   a - c  include an amplifier to amplify the transmission signals. 
         [0025]    The remote radio units  15   a - c  are connected via a distribution network  47  and through a front end unit  22  and a power reduction device  20  to a base station  30 . The base station  30  is connected to at least one fixed line network  60  operated by a network operator. It will be appreciated that there may be more than one base station  30  connected to the remote radio units  15   a - c  and that the base station  30  can be connected to more than one fixed line network  60 . The term “base station”  30  is used in this disclosure to encompass not only base transceiver stations, as known in the GSM protocol, but also a Node B known in the UMTS protocol, an eNode B in the LTE protocol, as well as similar units in other wireless protocols. The base station  30  passes transmission signals  25  to the power reduction device  20 . 
         [0026]      FIG. 1B  shows a further aspect of the invention in which two base stations  30   a  and  30   b  are connected to a central hub  100  in the distributed antenna system  5 . The central hub  100  has a first hub module  100   a  and a second hub module  100   b  as well as a first power reduction device  20   a  and a second power reduction device  20   b.  Each one of the base stations  30   a  and  30   b  is connected to one of the first hub module  100   a  or the second hub module  100   b.  As explained in the Applicant&#39;s co-pending application, GB 1414280.6, the number of the base stations  30   a,    30   b  and the hub modules  100   a,    100   b  is not limiting of the invention. Similarly, the number of the power reduction devices  20   a  and  20   b  is not limiting of the invention. 
         [0027]    The central hub  100  is connected to the distribution network  47  having a plurality of expansion units, including coverage area modules,  105   a - c . One or more of the remote radio units  115   a - e  are connected to one or more expansion units  105   a - c  to which a plurality of the antenna elements  110   a - e  are assigned. The one or more expansion units  105   a - c  are adapted to pass the same transmission signals to the connected ones of the remote radio units  115   a - e  and the corresponding antenna elements  110   a - e . The communication of the transmission signals  26  between the central hub  100  and the expansion units  105   a - c  is, in one aspect of the invention, in the digital domain and is reconfigurable. A digital to analogue conversion is provided within the expansion units  105   a - c  and the communication between the expansion units  105   a - c  and the plurality of antenna elements  110   a - e  is, in one aspect of the invention, in the analog domain. A plurality of digital to analog converters  117   a - e  are present in the remote radio units  115   a - e  to convert the signal received over the distribution network  47  to an analog signal for transmission. 
         [0028]    The connection between the central hub  100  and the plurality of expansion units  105   a - c  is reconfigurable. The central hub  100  and the expansion units  105   a - c  enable the signals from the base stations  30   a  and  30   b  to be transmitted through the distributed antenna system  5 . As described in the applicant&#39;s co-pending patent application No. GB1414280.6 the distributed antenna system  5  is highly flexible and allows a reconfiguration of the coverage area. 
         [0029]      FIG. 2  shows the power reduction device  20 ,  20   a  or  20   b  connected to the transmission line  40  between the base station  30  and the front end unit  22  of the distributed antenna system  5  (as shown in  FIG. 1A ) or the central hub  100  (as shown in  FIG. 1B ). The operation of one of the power reduction devices  20 ,  20   a  or  20   b  will now be explained in connection with the simplified embodiment shown in  FIG. 1A . It will be appreciated that the power reduction devices  20 ,  20   a,    20   b  operate in a similar manner. 
         [0030]    The power reduction device  20  receives the transmission signals  25  at an analog input port  21  from the base station  30 . A coupler  70  is inserted into the transmission line  40  and extracts an RF signal  25 ′ (a small portion) from the transmission signals  25 , which is around 30 dB less than the strength of original transmission signals  25 . The coupler  70  passes this small portion of the transmission signal  25 ′ along line  45  to an analog port  23  of the front end unit  22 . The front end unit  22  includes an analog-to-digital converter  27  for converting the received small portion of the transmission signal  25 ′ to a digital signal  26  for distribution through the distributed antenna system  5 . The digital signal  26  is passed to remote radio units  15   a - c  of the distributed antenna system  5  ( FIG. 1A ) at which point the digital signal  26  is converted by the digital-to-analog converters  17   a - c  to an RF signal for transmission by the antenna elements  10   a - c.    
         [0031]    The majority portion of the transmission signal  25 ″, i. e. the remaining part of the received input transmission signal  25 , is passed to a circulator  75 . The circulator  75  is designed to remove possible reflections and intermodulation products from a rectifier  80  and thus to avoid interferences in the uplink from the transmission signal  25 . The circulator  75  would not be required in an ideal world. 
         [0032]    The output of the circulator  75  is passed to the rectifier  80 , which converts the remaining part of the transmission signal  25 ″ to a DC voltage. The DC voltage is passed, in this aspect of the invention, to a rechargeable battery  85 . The rechargeable battery  85  can be connected over a power line  90 , for example, to one or more of the remote radio units  15   a - c  or any other unit or element in the distributed antenna system  5 . The power line  90  could be implemented using power-over-coax. 
         [0033]    In one further aspect of the invention, the rechargeable battery  85  can be omitted and the output of the rectifier  80  passed directly to one or more of the remote radio units  15   a - c  or any other unit or element in the distributed antenna system  5 . In another aspect of the invention, the rechargeable battery  85  could be used as a back-up battery in the event that the main power to the distributed antenna system  5  is cut. 
         [0034]    In another aspect of the invention, the stored electrical power can be used and managed in a central power management unit (not shown), which manages the power supply of the whole distributed antenna system  5 . 
         [0035]      FIG. 3  shows an outline of the method for reducing the power in a transmission signal. In step  300 , a radio frequency transmission signal from the base station  30  is received, originating from the fixed line network  60 . The base station  30  passes the transmission signal  25  along the transmission line  40  to the analog input port  21  of the power reduction device  20  in step  305 . The small portion of the transmission signal  25 ′ is coupled out of the transmission signal  25  in step  310  and is forwarded in step  320  along the line  45  to the front end unit  22  where the small portion of the transmission signal  25 ′ is converted to the digital domain in step  322  and thence passed in step  324  to one or more of the remote radio units  15   a - c . The digitized small portion of the transmission signal  25 ′ is converted back from the digital domain to the analog domain in step  326  by the digital to analog converters  17   a - c  and passed to the antenna elements  10   a - 10   c  for transmission in step  330 . 
         [0036]    The majority part of the transmission signal  25 ″ is forwarded to the circulator  75  in step  340  and then rectified by the rectifier in step  350 . If required, in step  360  any excess energy is stored in the battery  85 . Alternatively, the energy can be used to power a device, such as any unit in the distributed antenna system  5  or the front end unit  22 . 
       REFERENCE NUMERALS 
       [0000]    
       
           5  Distributed antenna system 
           10   a - c  Antenna elements 
           15   a - c  Remote radio units 
           17   a - c  Digital to analog converters 
           20  Power reduction device 
           21  Analog port 
           22  Front end unit 
           23  Analog port 
           25  Transmission signal 
           26  Digital signal 
           27  Analog to digital converter 
           30  Base station 
           40  Transmission line 
           45  Line 
           47  Distribution network 
           60  Fixed line network 
           70  Coupler 
           75  Circulator 
           80  Rectifier 
           85  Rechargeable battery 
           90  Power line