Abstract:
A base station for mobile telecommunications comprises a main body and an antenna assembly mounted in a position at a distance from the main body. The antenna assembly comprises at least one antenna for transmission to and reception of signals from mobile user terminals. The antenna assembly further comprises a receiver for determining position from satellite signals so as to determine the location of the antenna assembly.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a base station for mobile telecommunications comprising a main body and an antenna assembly mounted in a position at a distance from the main body. The present invention also relates to an antenna assembly for a base station. The present invention also relates to a method of determining the location of an antenna assembly.  
         BACKGROUND OF THE INVENTION  
         [0002]    Accurate knowledge of both antenna location and Radio Frequency (RF) feeder cable length between a base station and its antenna in a cellular radio system are necessary for the implementation of location based services.  
           [0003]    The length of the feeder cable must be known because of the timing delay that it introduces and also to estimate the insertion loss of the feeder cable for the purposes of calculating appropriate transmit and receive power. It is possible to determine feeder cable length manually during cell site installation or upgrade, but this is time consuming, expensive, and prone to inaccuracy and human error.  
           [0004]    Antenna location may be determined from an accurate map but this is unlikely to be accurate enough and information on height is likely to be poor. A stand-alone Global Positioning System (GPS) receiver can be used to give an accurate determination of location in three dimensions, but requires a long time to minimise errors. Alternatively, some known base stations incorporate a GPS receiver in the main body (e.g. cabinet) of the base station. An example of such a known cellular base station is shown in FIG. 1. The base station contains a high-accuracy (high-cost) GPS receiver  108 , which is used as a reference for the base station&#39;s master oscillator  10  (which is itself used as a high precision timing reference for the base station  104 ). The length of the RF co-axial feeder cable  114  between the GPS antenna  112  and the GPS receiver  108  can be considered to be negligible. The main RF co-axial feeder cable  122  for cellular transmit and receive signals is much longer and is of a length that is not known accurately before installation.  
           [0005]    After installation, feeder cable length is known to be measured using Time Domain Reflectometry (TDR), but the test equipment is expensive and requires a trained operator. Feeder cable length is also difficult to measure using other known methods, once the feeder cable is installed. Inaccuracy due to human error is also a serious concern.  
         SUMMARY OF THE INVENTION  
         [0006]    A base station, an antenna assembly and a method according to the present invention are defined in the independent claims to which the reader should now refer. Preferred features are laid out in the dependent claims.  
           [0007]    An example of the present invention is a base station for mobile telecommunications comprising a main body and an antenna assembly mounted in a position at a distance from the main body. The antenna assembly comprises at least one antenna for transmission to and reception of signals from mobile user terminals. The antenna assembly further comprises a receiver for determining position from satellite signals so as to determine the location of the antenna assembly.  
           [0008]    The receiver for determining position from satellite signals is preferably a Global Positioning System (GPS) receiver. GPS receivers in the form of chipsets are cheap and easy to install.  
           [0009]    The main body of the base station preferably comprises a second receiver of satellite signals. The base station then further comprises a comparison unit so as to compare the positions of said two receivers, or timings of output signals from the two receivers corresponding to a pulsed satellite signal. This is so as to estimate the length of, or time delay in conducting signals caused by, the cable connecting the main body and antenna assembly. Accurate knowledge of both (a) the time delay caused by the feeder cable, which is related to the feeder cable length by a known factor, and (b) antenna location enables location based services to be provided with a high degree of accuracy. Location based services are such as “E-911” which is emergency location of a mobile user terminal (as used in the USA).  
           [0010]    The present invention also provides an antenna assembly. An example is an antenna assembly for a base station for mobile telecommunications in use mounted in a position at a distance from the main body of the base station. The antenna assembly comprises at least one antenna for transmission to and reception of signals from mobile user terminals. The antenna assembly further comprises a receiver for determining position from satellite signals so as to determine the location of the antenna assembly.  
           [0011]    The present invention also provides a method of determining the location of an antenna assembly. An example is a method of determining the location of an antenna assembly where a base station for mobile telecommunications comprises a main body and the antenna assembly. The antenna assembly is mounted in a position at a distance from the main body. The antenna assembly comprises at least one antenna for transmission to and reception of signals from mobile user terminals. The antenna assembly further comprises a receiver which determines position from satellite signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Preferred embodiments of the present invention will now be described by way of example and with reference to the drawings, in which:  
         [0013]    [0013]FIG. 1 is a diagram illustrating a known cellular-base station (Prior Art),  
         [0014]    [0014]FIG. 2 is a diagram illustrating a preferred base station with location determination of its antenna assembly,  
         [0015]    [0015]FIG. 3 is a diagram illustrating a further preferred base station with determination of antenna assembly location and feeder length,  
         [0016]    [0016]FIG. 4 is a diagram illustrating a further preferred base station in which tower top equipment includes a low cost GPS receiver and its associated antenna,  
         [0017]    [0017]FIG. 5 is a diagram illustrating a further preferred base station in which tower top equipment includes a low cost GPS receiver having an associated antenna, that antenna being mounted in the base station antenna assembly, and  
         [0018]    [0018]FIG. 6 is a diagram illustrating a further preferred base station in which tower top equipment contains a low cost GPS receiver with its associated antenna mounted close to the base station antenna assembly. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 2 shows a preferred base station  202  enabling location determination of its antenna assembly  206 . The base station  202  consists of a main body  204  positioned at the base of a tower (not shown) and a tower-mounted antenna assembly  206 . The main body contains a high-accuracy (high-cost) GPS receiver  208 , which is used as a timing reference for the base station&#39;s master oscillator  210  (used as a high precision frequency reference for the base station  202 ) also within the main body  204 . The high-quality GPS receiver  208  has its own associated antenna  212  connected by a co-axial cable  214 . The main body  204  also includes transmitter circuitry  216  and receiver circuitry  218  connected to a transmit/receive filter  220 , which is connected via a co-axial feeder cable  222  to the tower-mounted antenna assembly  206 . The antenna assembly  206  includes a remote antenna electrical tilt (RAET) unit  224  having a data/power connection cable  228  back to its control unit  226  in the main body  204  of the base station  222 .  
         [0020]    Determination of Location of Antenna Assembly  
         [0021]    As also shown in FIG. 2, a low cost GPS receiver  230  including an associated antenna is incorporated into the antenna assembly  206 . The low cost GPS receiver  230  provides antenna location information which is transferred to the main body  204  of the base station  202  via a RAET Data link  228 , the control unit  226  acting also as an antenna location information interface unit. (In systems with remote antenna electrical tilt (RAET), also known as remote antenna electrical down tilt (RAEDT) and remote electrical tilt (RET), it is already necessary to have a data/power connection to the antenna, and this is used here to also transfer the antenna location information.)  
         [0022]    The control unit  226  of the base station  202  sends the antenna location information to a remotely located Operations &amp; Maintenance Centre (OMC)  231 . If the antenna location information is only monitored for a short period of time, it is liable to inaccuracies, so accordingly long term averaging of the antenna location information allows a more accurate determination of the antenna&#39;s exact position This averaging is performed in the low cost GPS Receiver  230 .  
         [0023]    As an added bonus, monitoring for sudden changes in antenna assembly position at the OMC  231  may help detect storm damage or vandalism.  
         [0024]    The data link  228  for Remote Antenna Electrical Tilt (RAET), between the base station&#39;s main body  204  and the antenna assembly  206 , may be implemented in a number of ways, such as using Serial Data (RS232, RS422, RS485 etc.) cables, ethernet cable or an optical fibre link.  
         [0025]    In an alternative embodiment (not shown) a dedicated link for the antenna location information is employed to transfer the antenna location information to the main body of the base station rather than using the RAET Data/power link as in FIG. 2.  
         [0026]    In an alternative embodiment (not shown) the dedicated RAET data link is done away with, the RAET data signals and power supply signal being superimposed on to the coaxial feeder cable between the transmit/receive filter of the main body and the antenna assembly, i.e as a modulated RF sub-carrier signal.  
         [0027]    In alternative embodiments (not shown), the long term averaging of antenna location information is undertaken in the main body of the base station, or in the Operations &amp; Maintenance Centre (OMC).  
         [0028]    Feeder Cable Length Determination  
         [0029]    By placing a second satellite positioning receiver in the base station namely in the main body (e.g. cabinet), antenna feeder length may be accurately determined by comparing timing delay and location information of the output signals between the two GPS receivers (i.e. antenna vs. cabinet) in a timing and location comparison unit.  
         [0030]    [0030]FIG. 3 shows a further base station  302  which is basically the same as the base station shown in FIG. 2 but involves determination not only of antenna assembly location, but also of the length of the feeder cable  322 .  
         [0031]    To determine feeder cable  322  length, the output signals from the high precision GPS receiver  308  and the low cost GPS receiver  330  are compared in a timing and location comparison unit  332 . An estimate of feeder cable  322  length is calculated by comparing timing difference between the output signals of the two GPS receivers  330  and  308  corresponding to a pulsed GPS input signal. This is possible since electrical or optical signals are known to travel more slowly in the cable  322  than through the air (free space). For cable  322  that is co-axial cable, this ratio (velocity in cable/velocity in free space) is called the velocity factor and is known. For a similar embodiment (not shown) where the cable  322  is an optical fibre, the inverse ratio (velocity of light in free space/velocity of light in glass fibre) is, of course, the refractive index, and is known.  
         [0032]    A rough estimate of length of the feeder cable  322  is also determined by comparing the distance between the locations (in  3  spatial dimensions, X, Y &amp; Z) of the two GPS antennas  330 , 308 , after long term averaging of the location data from each. Provided the estimate from the timing difference agrees with this rough estimate, the estimate from the timing difference is taken as correct.  
         [0033]    For accuracy, the length of cable  314  that connects the high precision GPS receiver  308  to its own antenna  312  should be as short as possible, and ideally, should be of known length so as to be compensated for.  
         [0034]    In an alternative embodiment (not shown) similar to the FIG. 3 embodiment, the dedicated RAET data link is done away with, the RAET data signals and power supply signal being superimposed on to the coaxial feeder cable between the transmit/receive filter of the main body and the antenna assembly, i.e as a modulated RF sub-carrier signal.  
         [0035]    Base Stations with More Processing Circuitry Being Tower-Mounted  
         [0036]    [0036]FIGS. 4, 5 &amp;  6  show three further base stations (denoted  402 , 502 , 602  respectively) where the low cost GPS receiver  430 , 530 , 630  is located in an antenna assembly  406 , 506 , 606  consisting of a tower mounted assembly  434 , 534 , 634  (often referred to as Tower Top Equipment (TTE) and containing some control and processing circuitry) connected to an antenna unit  407 , 507 , 607 . In these examples, the low cost GPS receiver is located in the TTE. In particular, the low cost GPS receiver is co-located with a tower mounted (receive) amplifier (TMA)  436 , 536 , 636  in the TTE. (Incidentally, the TMA provides a path for both transmitted and received signals, but only amplifies the received signals).  
         [0037]    The three base stations shown in FIG. 4 to  6  are basically similar, however the position of antenna of the low Cost GPS Receiver varies. As shown in FIG. 4, that antenna  438  may be mounted in the Tower Top Equipment  434 . Alternatively, as shown in FIG. 5, the Tower Top Equipment  534  may contain the (main part of the) low cost GPS receiver  530 , but its associated antenna  538  may be mounted in the antenna unit  507 . Another option, shown in FIG. 6 is that the antenna  638  is separate from, but close to, the antenna unit  607 .  
         [0038]    In these three examples shown, some control &amp; interface electronics for the RAET is in the Tower Top Equipment (TTE), leaving only the RAET Actuators  442 , 542 , 642  in the antenna unit  407 , 507 , 607 .  
         [0039]    Referring again to FIG. 4 where the Tower Top Equipment  434  contains the Low Cost GPS Receiver  430  including its associated Antenna  438 , the RF Co-axial Feeder cable  422  length between the antenna unit  407  and the Tower Top Equipment  434  must be of known length in order to be compensated for. This “offset” cannot be calculated remotely, and must thus be measured on site.  
         [0040]    Referring again to FIG. 5 where the Tower Top Equipment  534  contains the Low Cost GPS Receiver  530 , but its associated antenna  538  is mounted in the antenna unit  507 , the RF Co-axial Feeder cable (“Jumper”)  544  for the GPS Receive Signals of the low cost GPS receiver  530  should optimally be of the same length as the RF Co-axial Feeder cable  522  for the cellular transmit &amp; receive signals.  
         [0041]    Referring again to FIG. 6 where the Tower Top Equipment  634  contains the low cost GPS receiver  630 , but its associated antenna  638  is mounted close to the antenna unit  607 , the RF Co-axial Feeder cable  644  (Jumper) for the GPS Receive Signals (for the Low Cost GPS Receiver) should be of known length. The RF Co-axial Feeder cable  622  for the cellular transmit and receive signals should also be of known length.  
         [0042]    In the base stations of FIGS.  3  to  6 , the RAET data and power connection is by a dedicated cable  428 , 528 , 628 . In some alternative embodiments (not shown) this dedicated cable is done away with and the RAET signals are sent via the RF co-axial feeder cable  422 , 522 , 622 .  
         [0043]    General  
         [0044]    The base stations described above can be in accordance with one or more of various cellular radio standards such as Time Domain Multiple Access (TDMA), Global System for Mobiles (GSM), Code Division Multiple Access (CDMA), Wideband-Code Division Multiple Access (WCDMA), or similar standards such as Wireless Local Area Network (LAN). A Global Navigation Satellite System (GLONASS) receiver or similar can be used in the antenna assembly in place of a GPS receiver.