Abstract:
A method and apparatus for supplying a timing signal from a central unit to at least one radio head connected to the central unit via a communications link. The central unit transmits a downlink signal to the radio head(s) via the communications link based on a transmit clock supplied via a clock selector within the central unit. In a first mode, the clock selector supplies a clock signal derived from a network downlink signal received at the central unit, such as from a MSC, as the transmit clock. However, in response to the loss of the network downlink signal, a second mode is entered wherein the clock selector instead supplies a clock signal derived from an uplink clock signal generated by the radio head, such as by a PLL therein, as the transmit clock. Thus, the central unit may supply a reference clock signal to the radio head even when the network downlink signal is temporarily lost.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the provision of timing reference signals to one or more radio heads. 
     Many wireless communication systems include one or more radio heads connected to a central unit (e.g., a pico base station) by a wire-based communications link, such as a T 1  or E 1  line. These high speed, high bandwidth lines typically do not include a separate clock line, but may nevertheless still be used to provide timing reference signals for phase locked loops within the radio heads. The phase locked loops are in turn used to perform a wide variety of tasks, such as carrier frequency synthesis. Typically, a network downlink signal is supplied to the central unit and the relevant timing reference signals are derived from the network downlink signal by an oscillator or other means within the central unit. However, there may be times when the network downlink signal is not available, but it may be desirable for the central unit to continue to provide the timing reference signal to the radio head(s), so as to allow the radio head(s) to continue operation and/or gracefully shut down on-going RF communications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for supplying a timing signal from a central unit to at least one radio head connected to the central unit via a communications link. The central unit transmits a downlink signal to the radio head(s) via the communications link based on a transmit clock supplied via a clock selector within the central unit. In a first (normal) mode, the clock selector supplies a clock signal derived from a network downlink signal received at the central unit as the transmit clock. However, in response to the loss of the network downlink signal, a second (secondary) mode is entered, wherein the clock selector instead supplies a clock signal derived from an uplink clock signal generated by the radio head, such as by a PLL therein, as the transmit clock. Thus, the central unit may supply a reference clock signal to the radio head even when the network downlink signal is temporarily lost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wireless communication system. 
     FIG. 2 shows one configuration of a Localized Wireless Telephone System suitable for practicing the present invention. 
     FIG. 3 shows another configuration of a Localized Wireless Telephone System suitable for practicing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to provision of a timing reference to radio heads; however, an understanding of an entire communications system may be helpful in understanding the context of the present invention. While the following discussion may be couched in terms of a communication system using the TIA/EIA-136 protocol, it should be appreciated that the present invention is not limited thereto and is instead equally applicable to communications systems operating according to a wide variety protocols, including Global System for Mobile Communication (GSM) and Code Division Multiple Access (CDMA) systems. 
     Turning now to FIG. 1, a communication system  10  is illustrated. In particular, the communications system  10  includes the Public Switched Telephone Network (PSTN)  20  and the Public Land Mobile Network (PLMN)  30 , which may, in turn, be connected to one or more Localized Wireless Telephone Systems (LWTS)  60 . While not shown, satellites may be used as needed either within the PSTN  20  or the PLMN  30  to provide remote communication links, such as across oceans or the like. 
     The operation of the PSTN  20  is well established and subject to extensive documentation beyond the scope of the present invention and therefore a more detailed discussion is omitted. 
     PLMN  30  may include a plurality of proprietary mobile networks  40 , and each mobile network  40  may include a plurality of Mobile Switching Centers (MSC)  42 . Typically, at least one MSC  42  in the PLMN  30 , and more advantageously one MSC  42  in each mobile network  40 , is connected via a gateway to the PSTN  20 . Some MSCs  42  may also serve as gateways connecting the various mobile networks  40  within the PLMN  30 . Gateway functions may be all consolidated at a single MSC  42  within a mobile network  40  or dispersed amongst a plurality of MSCs  42  within a mobile network  40  as needed or desired. Typically, at least one MSC  42  within a particular mobile network  40  connects to, or includes, a Home Location Register (HLR)  44  and a Visitor Location Register (VLR)  46 , whose functions are well known in the art. Additionally, each mobile network  40  may be equipped with a message center  48  communicatively connected to an MSC  42  for handling short message service and the like. Each MSC  42  may further be communicatively connected to a plurality of base stations  50 . Each base station  50  may be communicatively connected to one or more mobile terminals  100 , typically over an RF communications channel. 
     The LWTS  60  is a wireless telecommunications system that may be public or proprietary as needed or desired, and is typically a private network installed in a building or on a campus. LWTSs  60  are typically installed to allow employees working in the building or on the campus to use a mobile terminal  100  as an office telephone. LWTS  60  typically connects with an MSC  42  in the PLMN  30  to allow subscribers of the LWTS  60  to move seamlessly between the LWTS  60  and the PLMN  30 . The MSC  42  responsible for a LWTS  60  may treat the LWTS  60  merely as another base station  50  or a plurality of base stations  50  depending on the internal structure of the LWTS  60  in question. One of many configurations of a LWTS  60  is shown in more detail in FIG.  2 . 
     The LWTS  60  of FIG. 2 includes a control and radio interface (CRI)  200  connected to one or more radio heads (RH)  300 . While only two radio heads  300  are depicted in FIG. 2, it should be understood that numerous radio heads  300  may be, and typically are, employed, with the radio heads  300  arranged in one or more chains connected to the CRI  200 . Each radio head chain may have a serial (cascaded) or parallel configuration, as appropriate. The radio heads  300  communicate with the CRI (or “central unit”)  200  via one or more communications links  400  that may each be conceptually be thought of as having downlink  410  (CRI to radio head) and uplink  420  (radio head to CRI) portions. The communications link  400  typically takes the physical form of one or more T 1  or E 1  lines, but may take other forms known in the art. It should be noted that T 1  and E 1  lines typically employ pulse code modulation, but the communications link  400  may employ other modulation techniques as is known in the art. 
     Each radio head  300  typically includes a primary framer  310 , a phase locked loop  320 , a secondary framer  330 , a transceiver  350 , and an antenna  360 . The radio head downlink signal RH d  from the CRI  200  via downlink  410  is input to the primary framer  310 . Primary framer  310  receives the radio head downlink signal RH d  and extracts the downlink data (or “payload”) therefrom. In addition, the primary framer  310  recovers the clock signal (the “radio head downlink clock signal” RH dc ) which may be advantageously embedded in the radio head downlink signal RH d . The primary framer  310  outputs RH dc  and a payload signal. The payload signal is forwarded to appropriate parts of the radio head  300  for processing in a conventional fashion. The RH dc  clock signal is forwarded to the phase locked loop  320 . 
     Within each radio head  300 , the phase locked loop (PLL)  320  generates a phase locked output signal that may be used for a variety of purposes, such as for carrier frequency reference by transceiver  350 , and as a transmit clock for uplink transmissions by primary framer  310 . For ease of reference, this output signal will be referred to as the uplink clock signal RH uc , although it should be understood that RH uc  may be used for purposes other than an uplink transmit clock. For instance, RH uc  may be used as the carrier frequency source for the RF transmissions from transceiver  350  via antenna  360 . The PLL  320  is operable in at least two modes. In the first mode, which is the normal operating mode, PLL  320  generates RH uc  based on the RH dc  clock signal. In the second mode, which may be thought of as a “holdover” mode, PLL  320  generates RH uc  independently from the current RH dc . In the holdover mode, PLL  320  freezes the frequency of its output, such as by holding constant the control voltage supplied to the voltage controlled oscillator within the PLL  320 . Thus, the output of PLL  320  is not adjusted based on RH dc  in the holdover mode. The conditions under which PLL  320  changes from normal mode to holdover mode are explained further below. 
     The transceiver  350  transmits signals via antenna  360  based on the payload information extracted by primary framer  310  and a suitable carrier frequency source, such as RH uc  from PLL  320 . 
     The secondary framer  330  outputs a signal on the downlink  410  of the communications link  400  for the next radio head  300  based on the payload signal from primary framer  310  (after any suitable processing) and a suitable transmit clock signal. This process repeats in each of the radio heads  300 . At the last radio head  300 , the secondary framer  330  transmits to the previous radio head  300  on the uplink  420 , rather than to a subsequent radio head  300  on the downlink  410 . For the uplink signal, the secondary framers  330  extract the uplink payload and pass the same towards the primary framer  310 , typically with some intervening processing to add information from the current radio head  300 . The primary framer  310  transmits the information up the chain on the uplink  420 , with the first radio head  300  in the sequence (i.e. connected most directly to the CRI  200 ) passing the information to the CRI  200 . 
     Both the primary framers  310  and the secondary framers  330  of radio head  300  use respective transmit clock signals to control their respective uplink and downlink transmissions on communications link  400 . The transmit clock signal for the secondary framer  330  may be the RH uc  signal from the PLL  320  within the same radio head  300 . However, the secondary framer  330  may advantageously use the RH dc  clock signal output from the primary framer  310  as its transmit clock signal as described in U.S. patent application Ser. No. 09/666,446, filed Sep. 21, 2000 and entitled “Cascaded Parallel Phase Locked Loops,” which is incorporated herein to the extent it does not conflict with the disclosure herein. The transmit clock signal for the primary framer  310  (for uplink data) is the RH uc  signal supplied by PLL  320 . 
     The CRI  200  serves primarily as the interface between the MSC  42  and the radio heads  300 . The CRI  200  may oversee or perform the functions of control of air channels, control of radio heads  300 , routing of data to and from the radio heads  300 , and the like. Most of these functions are well known in the art and are not discussed further herein. Relevant to the present invention, the CRI  200  provides timing reference signal(s) to the radio heads  300 , such as for air frame synchronization and carrier frequency reference. For simplicity in explaining the present invention, primarily those CRI components related to providing the timing reference are shown in FIG. 2, but the CRI  200  of FIG. 2 is to be understood to include other functional components known in the art. 
     The CRI  200  may include a network (NW) framer  210 , an oscillator  220 , one or more radio head (RH) framers  230 , a reference framer  240 , and clock selector  250 . The network framer  210  receives input, sometimes referred to herein as the “network downlink signal,” (NW d ) from the MSC  42  via downlink  510  and processes that NW d  signal in a conventional fashion to extract payload information. Because downlink  510  is typically in the form of T 1  or E 1  lines, the NW d  signal may advantageously include an embedded clock signal. The network framer  210  outputs a payload signal and a clock signal (CLK net ) based on the NW d  signal. The payload signal is directed to the appropriate portions of the CRI  200  for internal data processing, as is known in the art. The CLK net  clock signal is sent to the oscillator  220 , typically a phase locked loop, for generation of the CRI internal clock signal CLK osc . This CLK osc  internal clock signal may be used in a conventional fashion to control various processes within the CRI  200 , such as the processing of the payload information within the CRI  200  and as a transmit clock for uplink transmissions to the MSC  42  from network framer  210 . 
     The NW d  signal input from the MSC  42  via downlink  510  is also directed to the reference framer  240 . The reference framer  240  may extract the embedded clock from NW d  to produce a clock signal that will be referred to as the reference clock signal CLK ref . There is no need for the reference framer  240  to generate a payload signal for internal use within the CRI  200 , as this is handled by the network framer  210 . As such, the reference framer  240  may operate in a monitoring mode, meaning that the reference framer  240  should present high impedance to downlink  510  so as to not overload downlink  510 . The CLK ref  signal is forwarded to the clock selector  250 , as described further below. 
     The radio head framer  230  operates in a conventional fashion to send the radio head downlink signal RH d  to radio heads  300 . In addition, radio head framer  230  serves to extract the uplink clock signal RH uc  from the uplink signal forwarded by radio head  300  and forward the same to the clock selector  250 . 
     In the present invention, the RH d  signal generated by the radio head framer  230  is controlled by the transmit clock signal supplied to the radio head framer  230  by clock selector  250 . The source for that transmit clock signal is normally the reference clock signal CLK ref  generated by the reference framer  240 . That is, in normal mode, the clock selector  250  supplies clock signal CLK ref  as the transmit clock signal for radio head framer  230 . Alternatively, CLK net  may be routed to the clock selector  250 , and CLK net  may be supplied as the transmit clock signal to radio head framer  230  during normal mode operation. However, as discussed above, both CLK net  and CLK ref  are based on the network downlink signal NW d , which may not be present from time to time due to problems at MSC  42  or problems with downlink  510 . When NW d  is not present on downlink  510 , referred to as a loss of signal condition, the generation of CLK net  and CLK ref  are obviously problematic. In response to the detection of a loss of signal condition, the clock selector  250  enters another mode, sometimes referred to herein as secondary mode, where the clock selector  250  supplies the RH uc  clock signal as the transmit clock for radio head framer  230 , instead of CLK ref  (or CLK net ). Thus, the RH uc  clock signal generated by radio head  300 , supplied to the CRI  200  via uplink  420 , is used by radio head framer  230  to form the radio head downlink signal RH d , which is supplied to radio head  300  on downlink  410 . Because radio head downlink signal RH d  is now based on the RH uc  signal generated by the PLL  320 , and because RH uc  is typically does not have the long-term accuracy of the clock signal from the MSC  42 , this secondary mode should be considered a temporary mode and the clock selector  250  should return to normal mode as quickly as possible (upon return of NW d ). 
     Just for reference, most cellular communications standards require a long-term accuracy of 0.016 ppm, or Stratum  2 , for the clock signal provided to the CRI  200  from MSC  42 , in order that the proper frequency accuracy may be achieved by the transceivers  350  of the radio heads  300 . Typically, the signal from the MSC  42  meets the requirements of Stratum  2 , but may also meet the higher standard of Stratum  1  (0.01 ppb). 
     As described above, the communications assembly of the CRI  200  and the radio head(s)  300  has at least two modes. In the normal mode, the clock selector  250  supplies CLK ref  (or CLK net ) to the radio head framer  230  as the transmit clock therefor. In response to detection of a loss of NW d  signal condition, the communications assembly enters the secondary mode where the clock selector  250  supplies RH uc  to the radio head framer  230  as the transmit clock therefor. The detection of a loss of NW d  signal condition, and the switching of modes in response thereto, may be carried out in a variety of ways. Just by way of example, a microprocessor in the CRI  200  may monitor the network framer  210  and/or the reference framer  240 , looking for an indication that the NW d  signal has been lost. In response to such an indication, the microprocessor may issue appropriate commands to the clock selector  250  to change from normal mode to secondary mode. Further, the microprocessor may cause appropriate commands to be sent to radio head  300  to cause PLL  320  to enter holdover mode. The PLL  320  may then stay in holdover mode for the duration of the loss of signal condition in some realizations of the present invention, or may be commanded by the microprocessor to return to normal mode at some point earlier in other realizations. Another approach that may advantageously be employed is to have the PLL  320  be more autonomous and have the clock selector  250  be more intelligent. The PLL  320  may itself detect an interruption in the RH dc  signal caused by the loss of NW d  signal condition and automatically enter the holdover mode until RH dc  is restored. In addition, the clock selector  250  may monitor the signal from reference framer  240  and automatically switch from normal mode to secondary mode when NW d  is lost. Because the mode transition of clock selector  250  may not be seamless, there may be a short hiccup in RH d , causing PLL  320  to briefly enter holdover mode. However, once clock selector  250  has completed the transition, radio head framer  230  should resume generating RH d , causing RH dc  to be restored, thereby allowing PLL  320  to return to normal mode operation. Thus, PLL  320  may stay in holdover mode for only a short time in some realizations of the present invention. 
     FIG. 2 shows a single CRI  200  connected to one chain of radio heads  300 ; this chain may contain either only a single radio head  300  or a plurality of radio heads  300 . Further, the present invention may be used in other configurations. For instance, the CRI  200  may be connected to more than one chain of radio heads  300 , as shown in FIG.  3 . In addition, the CRI  200  may also connect to another CRI  200 , also as shown in FIG.  3 . In this latter case, it may be advantageous to have the reference framer  240  forward the relevant clock signal to the next CRI  200 . That is, both the reference framer  240  and the radio head framer  230  should use the same transmit clock, as supplied via clock selector  250 . The downstream CRI  200  may also advantageously employ the technique of the present invention to generate its radio head downlink signal RH d  for its radio heads  300  based on the clock signal from the upstream CRI  200 . 
     As used herein, the term “radio head” means an active communications station with at least RF transmit capability that is fed data to be transmitted from an upstream source. Further, as used herein, the term “mobile terminal”  100  may include a cellular radiotelephone with or without a multi-line display; a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and data communications capabilities; a Personal Digital Assistant that can include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; and a conventional laptop and/or palmtop receiver or other appliance that includes a radiotelephone transceiver. Mobile terminals  100  may also be referred to as “pervasive computing” devices. 
     U.S. patent application Ser. No. 09/705,093, entitled “Providing Reference Signal To Radio Heads,” and filed concurrently herewith, is incorporated herein to the extent it does not conflict with the present disclosure. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.