Abstract:
A first synchronization signal is used by a master base station and a remote base station in a digital cordless communication system. The master base station generates a second synchronization signal. The second synchronization signal is used to transfer information over a transmission medium between the master base station and the remote base station. The master base station generates the first synchronization signal from the second synchronization signal. Information is transferred over the transmission medium between the master base station and the remote base station using the second synchronization signal. The remote base station generates the first synchronization signal from the second synchronization signal. The first synchronization signal is utilized by the master base station and the remote base station for digital cordless communication.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 08/525,890, filed Sep. 8, 1995, now U.S. Pat. No. 5,978,369. 
    
    
     BACKGROUND 
     The present invention concerns the field of digital communication and pertains particularly to efficient generation within a remote base station of a synchronization signal for a cordless communication system. 
     A cordless telephone system generally includes a fixed part, which connects a telephone network to one or more base stations. Base stations are generally connected to a fixed part by wires, or may be integrated directly with the fixed part. For example, a system may include a master base station and several remote base stations connected by a wire highway to the master base station. Each base station communicates with one or more portable parts, typically a handset, via a radio frequency (RF) signal link. For example, the cordless system may be included in a private branch exchange (PBX) and utilize a cordless telephone system in accordance with the Digital European Cordless Telecommunications (DECT) Common Interface. 
     For example, the base stations may be connected together using a data highway operating in accordance with a voice coding algorithm such as an Adaptive Pulse Code Modulation (ADPCM) highway. An ADPCM highway utilizes four wires to connect the base stations. For each base station, one wire carries a synchronization signal (ADPCM.FS), one wire carries a clock signal (ADPCM.CLK), one wire carries data in to the base station (ADPCM.DIP) and one wire carries data out from the base station (ADPCM.DOP). 
     When a system includes more than one base station which utilize the DECT common interface, each base station must transmit messages in synchronization with a system wide DECT frame synchronization signal. In order to assure proper synchronization, the DECT frame synchronization signal is forwarded by a master source to each remote base station over a dedicated wire. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention, a first synchronization signal is used by a master base station and a remote base station in a digital cordless communication system. The master base station generates a second synchronization signal. The second synchronization signal is used to transfer information over a transmission medium between the master base station and the remote base station. The master base station generates the first synchronization signal from the second synchronization signal. Information is transferred over the transmission medium between the master base station and the remote base station using the second synchronization signal. The remote base station generates the first synchronization signal from the second synchronization signal. The first synchronization signal is utilized by the master base station and the remote base station for digital cordless communication. 
     For example, in the preferred embodiment the first synchronization signal is a DECT frame synchronization signal and the second synchronization signal is an ADPCM synchronization signal. Also in the preferred embodiment, the first synchronization signal within the remote base station and the master base station is synchronized using the synchronization information sent from the master base station to the remote base station. For example, this synchronization information includes a current count from a counter used by the master base station to generate the first synchronization signal from the second synchronization signal. 
     In the preferred embodiment, the remote base station uses a lock window signal to filter out parasitic pulses within the second synchronization signal. When a pulse from the second synchronization signal is absent from a lock window, the absent pulse is reconstructed by the remote base station when the remote base station generates a reconstructed second synchronization signal. 
     The present invention allows for the generation of a DECT frame synchronization signal from an ADPCM frame synchronization signal. This eliminates the need for a master base station to transfer to remote base stations the DECT frame synchronization signal over a dedicated wire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a private branch exchange (PBX) which includes a master base station and is connected to three remote base stations. 
     FIG. 2 is a block diagram of synchronization circuitry within each base station in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a timing diagram which shows relative timing of signals within a master base station in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a timing diagram which shows relative timing of signals within a remote base station in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a timing diagram which illustrates use of a lock window in accordance with a preferred embodiment of the present invention. 
     FIG. 6 is a timing diagram which illustrates use of a lock window during initialization in accordance with a preferred embodiment of the present invention. 
     FIG. 7 is a timing diagram that illustrates when a frame synchronization signal is out of synchronization with a lock window. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a public branch exchange (PBX)  11 . A common central fixed part (CCFP)  27  within PBX  11  includes a central processing unit (CPU)  21 , a Digital European Cordless Telecommunications (DECT) station  22 , echo cancel circuitry  23 , line circuitry  24  and integrated services digital network (ISDN) circuitry  25 . DECT station  22  functions as a master base station. 
     CPU  21  is for example an 80386 processor available from Intel Corporation, having a business address of 2200 Mission College Boulevard, Santa Clara, Calif. 95050, or a Model Number 68000 processor available from Motorola, Inc., having a business address of P.O. Box 20512, Phoenix Ariz. 85036. Line circuitry  24  connects to a public switching telephone network (PSTN) line  16 . Echo cancel circuitry  23  is used to cancel the echo from the far side of PSTN line  16 . This is required since DECT introduces a 10 millisecond round trip delay for voice. ISDN circuitry  25  connects CCFP  27  to an ISDN network  17 . 
     A four wire ADPCM highway  12  connects an ADPCM interface  26  within CCFP  27  to a remote base station  13 , a remote base station  14  and a remote base station  15 . 
     FIG. 2 shows synchronization circuitry  31  which is used for base stations which are operating either as a master base station or a remote base station. Through an input/output  34 , synchronization circuitry  31  receives in or transmits out an ADPCM frame synchronization signal (ADPCM.FS), through a tri-state gate  38  and a tri-state gate  37 , respectively. Through an input/output  35 , synchronization circuitry  31  receives in or transmits out an ADPCM clock signal (ADPCM.CLK), through a tri-state gate  40  and a tri-state gate  39 , respectively. Through an input/output  36 , synchronization circuitry  31  receives in or transmits out an DECT frame synchronization signal (DECT.FSYNC), through a tri-state gate  41  and a tri-state gate  42 , respectively. 
     A multiplexor  45  selects an ADPCM clock signal either from input  35  or from a ADPCM clock signal output  48  of an ADPCM clock control block  43  to forward to an ADPCM clock signal input  50  of ADPCM circuitry  32 . A multiplexor  46  selects an ADPCM frame synchronization signal either from input  35  or from an ADPCM frame synchronization signal output  49  of ADPCM clock control block  43  to forward to an ADPCM frame synchronization signal input  52  of ADPCM circuitry  32 . A multiplexor  47  select an DECT frame synchronization signal either from input  36  or from DECT synchronization circuitry  44  to forward to an DECT frame synchronization signal input  54  to DECT core circuitry  33 . DECT core circuitry  33  sends a DECT frame synchronization signal from a DECT frame synchronization signal output  53  to a DECT frame synchronization signal input  51  of ADPCM circuitry  32  and through tri-state gate  42  to input/output  36 . DECT synchronization circuitry includes a counter  56 . ADPCM block  32  processes data to and from the ADPCM highway. DECT core circuitry  33  processes data to and from the DECT highway. ADPCM block  32  and DECT core circuitry  33  exchange data through a shared memory. 
     When used in a base station which is acting as a master base station, ADPCM clock control block  48  generates the ADPCM clock signal which is placed on input/output  35  and generates the ADPCM frame synchronization signal which is placed on input/output  34 . The DECT frame synchronization signal is generated by the DECT synchronization circuitry  44  from the ADPCM frame synchronization signal received from the output of multiplexor  46 . 
     When used in a base station which is acting as a remote base station, the ADPCM clock signal is received on input/output  35  and the ADPCM frame synchronization signal is received on input/output  34 . The DECT frame synchronization signal is generated by the DECT synchronization circuitry  44  from the ADPCM frame synchronization signal received from the output of multiplexor  46 . Alternately, DECT frame synchronization signal may be received from input/output  36 , for example over a dedicated wire connected to a master base station. 
     In the preferred embodiment, an ADPCM synchronization pulse is sent at the beginning of every ADPCM frame. Transmission time for an ADPCM frame is 125 microseconds (8 kHz). Each ADPCM frame includes a number of slots of data. These are received or sent using the ADPCM data input signal or the ADPCM.DOP signal, respectively. For example, when ADPCM highway  12  is configured to transfer 2.048 mega bits per second (Mbps), each ADPCM frame includes 64 four-bit slots. When ADPCM highway  12  is configured to transfer 1.536 mega bits per second (Mbps), each ADPCM frame includes 48 four-bit slots. When ADPCM highway  12  is configured to transfer 768 kilo bits per second (Kbps), each ADPCM frame includes 24 four-bit slots. When ADPCM highway  12  is configured to transfer 128 Kbps, each ADPCM frame includes 4 four-bit slots. 
     Also, in the preferred embodiment, the DECT frame synchronization signal occurs at the beginning of each DECT frame. Each DECT frame synchronization signal is 125 microseconds in duration. In the preferred embodiment, there are twenty-four DECT slots in each DECT frame. Transmission time for the twenty-four DECT slots in a DECT frame is 10 milliseconds, or the equivalent of 80 ADPCM frames. 
     In order to alleviate the necessity of utilizing an additional line to transmit a DECT frame synchronization signal between each base station, in the preferred embodiment of the present invention, the DECT frame synchronization signal is derived from the ADPCM frame synchronization signal. This is done by use of counter  56  within DECT synchronization circuitry  44 . Counter  56  counts eighty ADPCM synchronization pulse signals between issuing a DECT frame synchronization signal. 
     The count of counter  56  in each base station needs to be synchronized in order to assure that the DECT frame synchronization signal generated by each base station is in synchronization. Synchronization typically occurs whenever the system is powered up. The DECT frame synchronization signal can also be re-synchronized whenever synchronization is lost. 
     In order to perform synchronization of DECT frame synchronization signal, a master base station sends to each remote base station a synchronization message within a special ADPCM time slot. The message includes a current count of the counter within the DECT synchronization circuitry within the master base station. The remote base station uses this current count to synchronize the count of the counter within the DECT synchronization circuitry within the remote base station. 
     For example, FIG. 3 is a timing diagram which shows relative timing of DECT frame slots  61 , ADPCM frame synchronization signal  62 , ADPCM data input signal  63 , and DECT frame synchronization signal  64  within a master base station. During ADPCM synchronization frame slot  70 , a synchronization message (M)  65  within ADPCM data input signals  63  is sent to the remote base station. During ADPCM synchronization frame slot  70 , the counter within the master base station has a current count of  70 . The synchronization message (M)  65  includes the current count ( 70 ). As also can be seen from FIG. 3, in this embodiment, the DECT frame synchronization signal is asserted during ADPCM synchronization frame slot  77 , which corresponds to the end of DECT frame slot  23  and the beginning of DECT frame slot  0 . 
     FIG. 4 is a timing diagram which shows relative timing of DECT frame slots  81 , ADPCM frame synchronization signal  82 , ADPCM data input signals  83 , and DECT frame synchronization signal  84  within a remote base station. During ADPCM synchronization frame slot  74 , the slave station receives the synchronization message (M)  85  within ADPCM data input signals  83  that was sent from the master base station. During ADPCM synchronization frame slot  74 , the counter within the remote base station has a current count of 74. The synchronization message (M)  85  includes the current count (70) of the master base station. The synchronization message  85  is used to synchronize the count in the remote base station to the count in the master base station. 
     In order to do this, after a current DECT frame synchronization signal, the remote base station sets a timer. Upon the expiration of the timer a synchronization window is started. When the remote base station receives a ADPCM frame synchronization pulse signal within the synchronization window, the DECT frame synchronization signal is again triggered and counter  56  is restarted at an appropriate value. 
     This is illustrated by FIG.  4 . There, after receiving synchronization message (M)  85 , the remote base station calculates a value for the timer. The value for the timer is equal to the count (74) within the remote base station when the remote base station receives the synchronization message minus the current count (70) of the master base station placed within the synchronization message and further minus 1. In the example shown illustrated in FIG. 4, the timer is equal to the duration of three (74−70−1) ADPCM frames. 
     As shown in FIG. 4, a DECT frame synchronization signal occurs at ADPCM synchronization slot  77 . Upon completion of this first DECT frame synchronization signal an interrupt signal  86  within the remote base station is issued starting the timer  88 . Timer  88  waits a period equal to the duration of three ADPCM frame slots (375 microseconds) and then triggers the opening of a synchronization window  87 . Upon the remote base station receiving a next ADPCM frame synchronization pulse, the remote base station issues another DECT frame synchronization signal, resets the DECT frame slots  81  and the count of ADPCM frame synchronization signal  82 . 
     In a large system, there may be long distances between master base station  11  and remote base stations  13 ,  14  and  15 , shown in FIG.  1 . Due to the long distance, and other factors, transmissions over ADPCM highway  12  can be noisy. This can be a significant problem where, as in the present invention, the DECT synchronization signal is derived from the ADPCM synchronization signal. Therefore a lock window is used to filter out parasitic pulses outside the lock window. In addition, missing ADPCM synchronization pulses are reconstructed. 
     FIG. 5 illustrates the use of a lock window. An ADPCM frame synchronization signal  91  is received by a remote base station from a master base station. Because of noise on the system two ADPCM frame synchronization signal pulses  95  are missing. In addition a parasitic pulse  96  was generated in the transmission. 
     A lock window signal  92  is used to filter out parasitic pulse  96 . In addition, even though two ADPCM frame synchronization signal pulses  95  are missing, these pulses are reconstructed when no pulse is received within the corresponding lock window. The result is reconstructed ADPCM frame synchronization signal  93 . A synchronization lock status signal  94  indicates when ADPCM frame pulses are received within lock windows. Synchronization lock status signal  94  retreats to a value of logic zero when expected ADPCM frame synchronization signal pulses  95  are missing. Upon receipt of additional ADPCM frame synchronization signal pulses within each lock window, synchronization lock status signal  94  returns to a value of logic one. The value of synchronization lock status signal  94  may be read by a microprocessor within the remote base station and indicates whether the system is locked in synchronization. 
     For example, for each ADPCM frame synchronization signal pulse an associated lock window has a duration of 1.6 microseconds, and is centered near the rising edge of the expected ADPCM frame synchronization signal pulse. 
     Upon power-up, the lock window is wide open to insure a quick synchronization. In typical operation, the lock window is wide open for a period greater than 125 microseconds to be sure to get at least one ADPCM synchronization pulse during the wide open lock window. In practice, a typical value for a wide open lock window is 500 microseconds. 
     This is illustrated by FIG. 6 where lock window  102  is shown to be open for the first three pulses of ADPCM frame synchronization signal  101 . Once the signal is synchronized, lock window signal  102  is composed of short filtering windows (lock windows). The result is reconstructed ADPCM frame synchronization signal  103 . Upon receipt of ADPCM frame synchronization signal pulses, synchronization lock status signal  104  switches to a value of logic one indicating the system is locked in synchronization. When the microprocessor within the remote base station which reads synchronization lock status signal  104  determines that synchronization lock status signal  104  has been at logic one for a sufficient length of time, the microprocessor causes lock window signal  102  to be composed of short filtering windows. 
     Upon the synchronization lock status signal being at logic zero for a predetermined length of time, for example three or four times the duration of am ADPCM frames (i.e., 3×125 μs=375 μs, or 4×125 μs=500 μs), the lock window is opened to allow re-synchronization. 
     This is illustrated by FIG. 7 where ADPCM frame synchronization signal  111  is out of synchronization with lock window  112 . This is reflected by synchronization lock status signal  114  being at logic zero. As discussed above, even though there is no ADPCM frame synchronization signal pulses detected in the lock windows during this period, pulses are nevertheless generated for reconstructed ADPCM frame synchronization signal  113 . When lock window  112  is opened up, reconstructed ADPCM frame synchronization signal  113  is re-synchronized with ADPCM frame synchronization signal  111 . 
     The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.