Abstract:
A communications signal repeater apparatus ( 106, 110, 114, 118 ) is disclosed which is configured to receive a communications signal from a cable network  105.  The communications apparatus is further configured to delay the communications signal by a delay period relative to a delay parameter and configure the delayed communications signal for transmission over a radio communications channel. The delayed communications signal is converted to a radio signal and output to antenna ( 108, 112, 116, 120 ) for transmission over the air. A communications system incorporating communications apparatus is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of United Kingdom Application No. GB1110292.8, filed Jun. 17, 2011; U.S. Provisional Application No. 61/502,825, filed Jun. 29, 2011; United Kingdom Application No. GB1119764.7, filed Nov. 16, 2011; U.S. Provisional Application No. 61/568,088, filed Dec. 7, 2011, all of which are incorporated by reference in their entirety. In addition, this application is related to U.S. patent application Ser. No. ______ (attorney docket 28793-20954), titled “Communication System, Apparatus, and Method” with inventor Euros Davies, filed Jun. 13, 2012, and U.S. patent application Ser. No. ______ (attorney docket 28793-20955), titled “Communication System, Apparatus, and Method” with inventor Euros Davies, filed Jun. 13, 2012. 
     
    
     BACKGROUND 
       [0002]    1. Field of Art 
         [0003]    The present disclosure relates to communications systems, apparatus and methods therefore. In particular, but not exclusively, the present disclosure relates to private mobile radio (PMR) communications systems such as, without limitation, the Terrestrial Trunked Radio (TETRA) system and the P25 or APCO-25 Land Mobile Radio system. 
         [0004]    2. Description of the Related Art 
         [0005]    PMR communications systems, and TETRA in particular, are suitable for use by emergency services, government agencies, public safety networks and the military where security and reliability of communications is of paramount importance. PMR systems are also used in commercial enterprises, for example in distributed or wide area locations such as large industrial sites, mine environments and the like. 
         [0006]    A PMR system often comprises a single main site over which radio communications signals are transmitted from a Base Transceiver Station (BTS). Such a site may be termed a “cell” or “main site”. Mobile transceiver units, termed “Mobile Stations (MS)” in the TETRA standard lexicon, receive and transmit radio communications from and to the BTS when in the site/cell coverage area. In common with many radio communication systems, PMR radio systems such as TETRA can suffer from gaps in coverage due to the terrain, intervening structures such as buildings and within buildings or tunnels for example. To overcome the poor signal conditions repeater stations known as Trunked Mode Operation (TMO) repeaters are used to extend coverage into the affected area to fill gaps in the outdoor coverage or to extend coverage into buildings and tunnels. Without limitation to a particular system or communications protocol, WCDMA systems may also require repeaters to extend coverage into buildings, tunnels or the like and to mitigate obstruction caused by terrain features. 
         [0007]    Poor signal conditions are a particular problem in urban areas and within buildings and tunnels since radio propagation is obstructed by the building materials such as bricks and concrete and also earth formations when seeking to propagate radio into tunnels. In such environments, the radio signal is propagated by a cable or fiber to a repeater station which re-transmits the radio signal over its local environment within the building or tunnel for example. 
         [0008]    Aspects and embodiments of the present disclosure were devised with the foregoing in mind. 
       SUMMARY 
       [0009]    Viewed from a first aspect the present disclosure provides a communications signal repeater apparatus, configured to receive a communications signal from a cable; delay the communications signal by a delay period relative to a delay parameter; and configure a delayed communications signal for transmission over a radio communications channel. 
         [0010]    Viewed from a second aspect the present invention provides a communications system, comprising: a cable network for distributing a communications signal; a communications signal distribution module operative to receive a communications signal for distribution over the cable network and configurable to couple to the cable network; and first and second repeater communications apparatus as set out above respectively coupled to the cable network for receiving the communications signal from the distribution module. 
         [0011]    Viewed from a third aspect the present invention provides a method of synchronizing signals transmitted from two or more repeater communications apparatus coupled to receive a communications signals distributed over a cable network, the method comprising: introducing a first delay in the communications signal relative to a delay parameter in a first communications apparatus; introducing a second delay in the communications signal relative to the delay parameter in a second communications apparatus; wherein the first and second delay are configured to delay the communications signal at the first and second communications apparatus such that they are synchronized to within a delay spread parameter for the cable network. 
         [0012]    Embodiments in accordance with the first, second and third aspects provide for the synchronization of signals distributed over a cable network. The delay parameter may be selected to ensure that the radio signals transmitted from each repeater apparatus are synchronized thereby reducing the likelihood of a mobile terminal operative to receive the radio signals experiencing unacceptable inter-symbol interference. 
         [0013]    The repeater communications apparatus may further comprise an output port configured to couple the delayed communications signal to radio frequency transmission apparatus. The output port may be configured to couple an optical signal to the radio frequency transmission apparatus where it is converted to a radio signal or may communicate a radio frequency signal to the radio communications apparatus. 
         [0014]    Typically, the repeater communications apparatus is further configured to receive control signals for setting a length of the delay period. Thus, the apparatus may be user programmed in accordance with the delay necessary for a particular cable network/system arrangement. Suitably, the communications apparatus further comprises a user interface operative to receive user input and generate the control signals responsive to the user input. 
         [0015]    The repeater communications apparatus may be further configured to provide remote access to a user for receiving the control signals. Such remote access allows a user to configure the delay in an apparatus without having to visit the location of the apparatus. This may be particularly advantageous if new repeater communications apparatus is being added to an existing network necessitating reconfiguration of the delays in existing repeater communications apparatus. 
         [0016]    Typically, the length of the delay period is based upon the difference between a delay due to transmission of the signal over the cable from a communications signal source to the repeater communications apparatus and the delay parameter. 
         [0017]    In an embodiment the communications apparatus further comprises communications signal delay path apparatus which is user configurable to determine the delay period. 
         [0018]    The repeater communications apparatus may further comprise a transmitter station arranged to transmit the delayed communications signal over the radio communications channel. The transmitter station may be integrally formed with or housed with the communications apparatus thereby providing a unitary repeater module. 
         [0019]    Typically, the transmitter station is included as a part of a transceiver station thereby providing both downlink and uplink communications. 
         [0020]    In an embodiment of the communications system a first cable distance between the first repeater communications apparatus and the distribution module is greater than a second cable distance between the second repeater communications apparatus and the distribution module such that there is a difference between a first time taken for the communications signal to travel between the distribution module and the first communications apparatus and a second time taken for the communications signal to travel between the distribution module and the second communications apparatus. In such a system, the first communications apparatus is configured to introduce a first delay in the communications signal relative to the delay parameter and the second communications apparatus is configured to introduce a second delay in the communications signal relative to the delay parameter such that the delayed communications signal at the first and second communications apparatus are synchronized to within a delay spread parameter for the system. The delay spread parameter defines the maximum delay between received signals that a mobile terminal can experience without experiencing unacceptable levels of inter-symbol interference. 
         [0021]    The delay parameter is at least as long as the first time taken for the communications signal to travel between the distribution module and the first communications apparatus. Generally, it may be made longer to provide for timing tolerances and allow minor deviations in timing. 
         [0022]    For a TETRA system utilizing Class A mobile terminals, the delay spread parameter is about 15 μs. However, the delay spread parameter may be greater or lesser than 15 μs depending on the tolerance of the communications signal protocol to inter-symbol interference. 
         [0023]    The cable network may comprise a ring topology and/or a direct connection (star) topology. 
         [0024]    The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the disclosed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]    The disclosed embodiments have other advantages and features which will be more readily apparent from the detailed description, the appended claims, and the accompanying figures (or drawings). A brief introduction of the figures is below. 
           [0026]    One or more embodiments in accordance with aspects disclosed herein will now be described, by way of example only, with reference to the accompanying drawings: 
           [0027]      FIG. 1  is a schematic illustration of an embodiment disclosed herein; 
           [0028]      FIG. 2  is a graphical representation of the time delay that may be caused by a signal travelling over a length of cable; and 
           [0029]      FIG. 3  is schematic illustration of a second embodiment in accordance with embodiments as disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The Figures (FIGS.) and the following description relate to preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of what is claimed. 
         [0031]    Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the disclosed system (or method) for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
         [0032]      FIG. 1  schematically illustrates an example embodiment  100  comprising a base transceiver station  102  coupled to an optical fiber distribution module  104  which feeds into optical fiber distribution network  105  having a ring topology at network node  128 . The optical fiber distribution network  105  couples together a group of repeater antennas  108 ,  112 ,  116  and  120 , for propagating radio frequency (RF) signals corresponding to the signals sent over the optical fiber distribution network  105 . A base transceiver station  102  transmits RF signals in a frequency band F 1 . As will be well-known to persons or the skilled in the art, a frequency band comprises a number of individual carrier frequencies each providing a respective communications channel. 
         [0033]    The antennas  108 ,  112 ,  116  and  120  are distributed throughout an area in which radio propagation is difficult and interrupted, for example a high-rise urban environment, an in-building environment or an underground environment such as a railway tunnel (for example, the London “Tube” system) or other underground facility. Each antenna  108 ,  112 , and  116  and  120  is respectively associated with a fiber enhancer unit  106 ,  110 ,  114  and  118 . Each fiber enhancement unit  106 ,  110 ,  114  and  118  is coupled to distribution network  105  at respective network nodes  150 ,  144 ,  138  and  132 . 
         [0034]    Each of the fiber enhancer and antenna pair is configured to take a signal from network  105  and up convert it to a RF signal in frequency band F 1 . Although each antenna is disposed in an environment in which radio propagation is difficult and interrupted RF signals transmitted from each antenna and also the base transceiver station  102  antenna may nevertheless interfere at a mobile terminal  160  thereby causing inter-symbol interference due to the delay between respective signals caused by the different length of cable a signal has travelled over before being transmitted from a respective antenna as well as the difference in distance between the mobile terminal  160  and respective antennas. 
         [0035]    Turning now to base transceiver station  102  and optical fiber distribution module  104 , in general principle the base transceiver station is coupled by an RF coupler to the optical fiber distribution module. An RF signal to be transmitted from base transceiver station  102  is coupled to the optical fiber distribution module  104 . In the optical fiber distribution module  104  the RF signal is down converted to an optical signal and output over cable  126  to network node  128 . An example of an optical fiber distribution module is the “Optical Master Unit” provided by Axell Wireless Ltd of Asheridge Road, Chesham, Bucks, UK and set out in datasheet OMU_revB_web. 
         [0036]    In the described embodiment the optical signal output from optical fiber distribution module  104  exits network node  128  into network segment  130  to begin a clockwise propagation through network  105 . At each node  132 ,  138 ,  144  and  150  the optical signal is tapped off to a respective fiber enhancer  118 ,  114 ,  110  and  106 . Each fiber enhancer receives the optical signal and converts it to an RF signal in frequency band F 1  and outputs the RF signal to their respective antenna  120 ,  116 ,  112  and  108 . Each fiber enhancer unit may comprise any further repeater unit such as provided by Axell Wireless Ltd with the details as set out in datasheet CSF Fiber fed repeater WCDMA_rev C_web for a WCDMA implementation optionally an Optical Master Unit such as provided by Axell Wireless Ltd. for receiving and optical signal and converting it to an RF signal output to a respective antenna. 
         [0037]    At or associated with each fiber enhancer is a delay module,  119 ,  115 ,  110  and  107  which in the embodiment illustrated in  FIG. 1  operates on an input optical signal. The delay module may be integrated with the fiber enhancer, for example it may be a software module which configures the digital signal processing circuitry of a fiber enhancer to introduce the delay. Optionally, the delay lines may be implemented as physical delay lines such as loops of cable. 
         [0038]    The delay module will typically include a user interface for configuring a delay. The user interface may be remotely accessible so that it may be configured from a central location. 
         [0039]    In the described embodiment, the network is a TETRA network as an example of a communications system which may utilize the invention. Mobile terminal  160  may be a TETRA Class A terminal which typically can tolerate inter-symbol interference caused by up to around 15 μs of delay spread between received signals. Delay between received signals of greater than around 15 μs may generate sufficient inter-symbol interference at mobile terminal  160  to cause it to be inoperable. The various antennas  120 ,  116 ,  112  and  106  may be disposed such that terminal  160  will not see a delay spread of greater than about 15 μs merely due to the distance signals transmitted from each antenna have to travel to get to mobile terminal  160 . However, the optical signal will take an increasingly longer path to get antenna depending on where it is in the network ring topology. 
         [0040]    In the described embodiment, the optical signal will travel across segment  130  to node  132  and then over cable  134  to fiber enhancer  118  where it is converted to an RF signal and transmitted from antenna  120 . The signal transmitted from antenna  116  must also travel over segment  136  to node  138  and then over cable  140  to fiber enhancer  114 . The optical signal has to travel yet further over segment  142  to node  144  and then over cable  146  to fiber enhancer  110  before being converted into an RF signal and transmitted from antenna  112 . Yet further, the optical signal travels over segment  148  to node  150  and then over cable  133  to fiber enhancer  106  before being converted into an RF signal and transmitted from antenna  108 . 
         [0041]    Typically propagation delay of an optical signal through optical fiber is about 5.48 μs per km for an average delay. The average propagation delay for illustrative distances of optical fiber is set out as follows:
   5 km has 27.4 μs average delay;   10 km has 54.8 μs average delay;   15 km has 82.2 μs average delay; and   20 km has 109.6 μs average delay.   
 
         [0046]      FIG. 2  is a schematic illustration of the above example distances superimposed on the ring network  105  illustrated in  FIG. 1 . Site A corresponds to fiber enhancer  118  and antenna  120 , site B corresponds to fiber enhancer  114  and antenna  116 , site C corresponds to fiber enhancer  110  and antenna  112 , and site D corresponds to fiber enhancer  106  and antenna  108 . In order to avoid inter-symbol interference experienced by mobile terminal  160  being due to two or more signals being received at greater than a 15 μs delay spread, the fiber enhancer sites  118 ,  114 ,  110  and  106  may have output synchronized. This may be synchronized to a value at least greater than the greatest delay experienced at a fiber enhancer site due to fiber propagation delay, i.e., that experienced at fiber enhancer  106  (site D) in the illustrated embodiment. 
         [0047]    In the illustrated embodiment the user definable value to which each of the sites may be synchronized is 137 μs which is the equivalent of the average propagation delay of an optical signal over a 25 km cable. Using the 137 μs delay in the embodiment illustrated in  FIG. 1  having the ring topology distances illustrated in  FIG. 2  the following delays may be inserted at respective sites:
   site A (5 km) is a distance 27.4 μs from the distribution module node  128  and therefore fiber enhancer  118  will need to generate a delay of 109.6 μs;   site B (10 km) is a distance 54.8 μs from the distribution module node  128  and therefore fiber enhancer  114  will need to generate a delay of 82.2 μs;   site C (15 km) is a distance 82.2 μs from the distribution module node  128  and therefore fiber enhancer  110  will need to generate a delay of 109.6 μs; and   site D (20 km) is a distance 109.6 μs from the distribution module node  128  and therefore fiber enhancer  106  will need to generate a delay of 109.6 μs.   
 
         [0052]    Respective delay modules are configured to have the delay corresponding to the site with which the delay module is associated as set out above. By introducing respective delays as set out above, the RF signal converted from the output signal received at respective fiber enhancers and output from respective antennas is in synchronization. Therefore, mobile terminal  160  will only experience delay spread between two or more signals due to the distance RF signals have to travel from respective transmitting antennas to the mobile terminal  160 . Network  105  can be arranged so that such delay spread will not exceed the 15 μs beyond which inter-symbol interference causes the mobile terminal  160  to be inoperative. 
         [0053]    An optional network topology is illustrated in  FIG. 3 . The network topology illustrated in  FIG. 3  is a direct line (sometimes called a “star”) topology and each fiber enhancer  206 ,  210 ,  214  and  218  has a direct cable connection  252 ,  250 ,  254  and  256 , to respective fiber enhancers. As with the embodiment illustrated in  FIG. 1 , each fiber enhancer is associated with a delay module configured to apply a delay to the optical signal received at the fiber enhancer. As before, it is immaterial whether the delay is applied to the optical signal, the RF signal output to respective antenna or an intermediate signal during signal processing conversion. 
         [0054]    The direct cable connections to each of the enhancers are of different lengths and for simplicity and ease of explanation the distance is corresponds to be distances of each statement in the ring topology of  FIG. 1 , namely 5 km, 10 km, 15 km and 20 km respectively. As for the embodiment of  FIG. 1 , a user definable value may be set to which each of the sites are synchronized which again can be 137 μs. The delay in each fiber enhancer may be set to be: 109 μs for fiber enhancer  218 ; 82.2 μs for fiber enhancer  214 ; 54.8 μs for fiber enhancer  210 ; and 27.4 μs fiber enhancer  206 . In this way, the RF output from respective antennas are synchronized and any delay spread experienced by mobile turn a  160  will we do to the difference in path taken by respective RF signals. 
         [0055]    In each of the embodiments illustrated in  FIG. 1  and  FIG. 2  the propagation delay in each fiber enhancer may be determined by completing a “ping” test. As will be known to persons of ordinary skill in the art a ping test is carried out by polling a device and waiting for a response, i.e., a message is sent to the unit and a response waited for. The total time includes the time the poll signal takes to reach the device and also the response sent back to the originator of the ping. Therefore, the time to the device is the total time minus the time to generate a response at the device all divided by two. 
         [0056]    In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. For example, although the delay modules are illustrated as being integral with the fiber enhancers they may be physically separate modules. Additionally, they may be remotely controllable from a central control station. 
         [0057]    Although embodiments in accordance with the present invention have been described with reference to the downlink direction of communication, similar issues arising in the uplink direction and may be solved using the same approach as described herein. Furthermore, the term base transceiver station and acronym BTS are not intended to restrict embodiments in accordance with the invention to systems, standards or protocols using such terminology but are generally intended to refer to communications equipment serving a geographic area with radio communications coverage providing downlink and/or uplink communications. 
         [0058]    The user definable delay value to which respective fiber enhancers are synchronized may be any suitable value and is not limited to the particular values used in the illustrative embodiments. 
         [0059]    Although the embodiment illustrated in  FIGS. 1 and 3  show delay modules acting on an optical signal, the delay may be introduced after the optical signal has been converted to an RF signal or in the signal processing during conversion of the optical signal to an RF signal. Whether or not the delay is introduced into the optical signal, the RF signal or some intermediate signal is not important for the purposes of introducing a delay. The delay could be introduced anywhere at the relevant fiber enhancer site. 
         [0060]    Additionally, the delayed signal need not be transmitted at the same frequency as the signal transmitted by the BTS  102 , although it is preferable to do so as such an arrangement provides for substantially continuous coverage as a mobile terminal moves from the coverage area of BTS  102  into the area served by the cable network and repeater apparatus. That is to say, the coverage area of the BTS  102  is effectively extended into the coverage area of the repeater apparatus formed by the fiber enhancer/antenna combinations since they transmit on the same frequency. 
         [0061]    Insofar as embodiments of the invention described above are implementable, at least in part, using a software-controlled programmable processing device such as a general purpose processor or special-purposes processor, digital signal processor, microprocessor, or other processing device, data processing apparatus or computer system it will be appreciated that a computer program for configuring a programmable device, apparatus or system to implement the foregoing described methods, apparatus and system is envisaged as an aspect of the present invention. The computer program may be embodied as any suitable type of code, such as source code, object code, compiled code, interpreted code, executable code, static code, dynamic code, and the like. The instructions may be implemented using any suitable high-level, low-level, object-oriented, visual, compiled and/or interpreted programming language, such as C, C++, Java, BASIC, Perl, Matlab, Pascal, Visual BASIC, JAVA, ActiveX, assembly language, machine code, and so forth. A skilled person would readily understand that term “computer” in its most general sense encompasses programmable devices such as referred to above, and data processing apparatus and computer systems. 
         [0062]    Suitably, the computer program is stored on a carrier medium in machine readable form, for example the carrier medium may comprise memory, removable or non-removable media, erasable or non-erasable media, writeable or re-writeable media, digital or analog media, hard disk, floppy disk, Compact Disk Read Only Memory (CD-ROM), Compact Disk Recordable (CD-R), Compact Disk Rewriteable (CD-RW), optical disk, magnetic media, magneto-optical media, removable memory cards or disks, various types of Digital Versatile Disk (DVD), subscriber module, tape, cassette, solid-state memory. The computer program may be supplied from a remote source embodied in the communications medium such as an electronic signal, radio frequency carrier wave or optical carrier waves. Such carrier media are also envisaged as aspects of the present invention. 
         [0063]    As used herein any reference to “one embodiment” or “an embodiment” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
         [0064]    As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
         [0065]    In addition, use of the “a” or “an” are employed to describe elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
         [0066]    The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalization thereof irrespective of whether or not it relates to the claimed invention or mitigate against any or all of the problems addressed by the present invention. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.