Abstract:
The present invention relates to a method and apparatus for selectively enabling and disabling transmit and receive functions of a radio, and more particularly to a system and method utilizing an out-of-band signal over a time division duplex channel to control transmit and receive functions of a radio so as to synchronize with the functions of a remote modem.  
     The invention relates to a system and method for synchronizing the transmit and receive modes of a radio via an out of band signal. The out of band signal is preferably a control tone at a predetermined frequency and is substantially coextensive with the communication signal to be transmitted. The radio filters a multiplexed signal and detects if the control tone is present. If the control tone is not detected, the radio preferably remains in the default receive mode. If the control tone is detected, the radio operates in the transmit mode for the duration of the control signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Wireless radio links have become increasingly important to provide data communication links for a variety of applications. For example, Internet Service Providers have begun to utilize wireless radio links within urban settings to avoid the installation expense of traditional wired connections or optical fiber. The typical architecture of such wireless systems involves establishing an outdoor unit at a node of the wireless system. The outdoor unit contains an antenna and a transceiver. Additionally, the radio unit is connected via a cable to an indoor unit. The indoor unit contains a modem to recover the digital data contained within the received signal and to add digital data to a carrier for ultimate transmission by the radio unit. The separation of functionality between indoor units and outdoor units has been maintained for several reasons. Among these reasons, maintenance of an indoor unit facilitates greater thermal stability of the modem unit and further increases reliability of the communication link. Additionally, the communication channel between the indoor unit and the outdoor unit is often allocated utilizing a Time Division Duplex (TDD) scheme, i.e. the communication over the cable successively alternates between receive and transmit modes.  
           [0002]    By maintaining separate modem and radio functionality and utilizing a TDD scheme, these systems require some method of synchronizing the modem and radio units. The synchronization mechanism ensures that both units will be in receive and transmit modes at the proper times. Known systems utilize a unique word (a series of pre-defined digital symbols for detection by the radio unit) to derive a frame pulse. Each frame pulse results in a counter being reset. The radio unit is preprogrammed to define transmit and receive time slots based upon this counter. Accordingly, the modem is capable of causing the radio unit to synchronize to the modem by signaling the unique word. Additionally, dynamic variations in transmit and receive slots may be implemented by signaling time slot parameters via a control channel.  
           [0003]    The use of a unique word and time slot parameters suffers several limitations. First, the use of a unique word and time slot parameters prevents rapid variation in time slot duration. Specifically, the bandwidth utilized to signal the unique word as well as the time slot parameters is not sufficient to permit dynamic allocation of the communication channel on a millisecond by millisecond basis. It is anticipated that more efficient channel access methods will be required for point-to-multipoint systems as these systems provide services to a greater number of users. Accordingly, dynamic and rapid variation of time slots is a valuable mechanism for maximizing the efficiency of such access methods. Additionally, the use of a unique word is problematic, since the timing of the system relies upon the frame pulse. If a unique word is not detected and a frame pulse is consequently missed, the timing information may become skewed which could cause the communication link to be broken. Accordingly, unique word synchronization involves a degree of inherent unreliability.  
           [0004]    The present invention is directed to a system and method which selectively enable and disable the transmit and receive functions of a radio. In the preferred embodiment, a modem and a radio are remotely located. The modem and radio preferably communicate via an IF signal over a communication channel in a Time Division Duplex manner, i.e. the same communication channel is alternatively allocated as a transmit path and a receive path. Additionally, the modem preferably creates an unique frequency as a discrete out-of-band tone. The transmit path of the radio preferably filters the tone from the received signal. The radio preferably contains a separate mechanism to detect the out-of-band tone. This mechanism converts the out-of-band tone to a logical signal. If the logical signal is present, the radio operates through the transmit path. Preferably, the radio defaults to the receive path if the logical signal is not present.  
           [0005]    According, certain embodiments of the present invention provide a more robust mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules. Certain embodiments the present invention provide a more easily detected signaling mechanism for dynamically controlling the transmit and receive functions between separate modem and radio modules. Also, certain embodiments of the invention provide a mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules that requires minimal switching times.  
           [0006]    One embodiment of the present invention avoids the problems of the prior art by providing a control signal from the modem to the transceiver where said control signal is substantially coextensive with the communication signal being sent by the modem to the transceiver for transmission. The control signal and the communication signal may be sent over the same cable connecting the modem and the transceiver. In the situation where the control signal and the communication signal are sent over the same cable, the control signal preferably is at a frequency different than the frequency of the communication signal so as to avoid material interference between the two signals. The control signal may be a tone.  
           [0007]    In a preferred embodiment, the transceiver is normally in receive mode and is in transmit mode for the duration of the control signal. The control signal is not sent from the modem as a part of the communication signal to set the transmit and receive slots via a counter thereby taking up valuable bits in the communication signal and thereby delaying the switching of the transceiver between modes. The control signal, unlike a unique word in the prior art, does not set a counter which defines the transmit slot of the transceiver.  
           [0008]    Accordingly, it is an object of the present invention to provide a more robust mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules.  
           [0009]    It is another object of the present invention to provide a more easily detected signaling mechanism dynamically controlling the transmit and receive functions between separate modem and radio modules.  
           [0010]    It is yet another object of the present invention to provide a mechanism for dynamically controlling transmit and receive functions between separate modem and radio modules that require switching times.  
           [0011]    It is still another object of the present invention to provide a control signal from a modem to a transceiver where the control signal is substantially coextensive with, and at a different frequency than, a communication signal to be transmitted.  
           [0012]    It is a further object of the present invention to provide a control signal from a modem to a transceiver wherein the transceiver remains in the transmit mode only for the duration of the control signal.  
           [0013]    It is yet a further object of the present invention to provide a control signal and a communication signal from a modem to a transceiver over the same cable at different frequencies.  
           [0014]    These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.  
           [0015]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0017]    [0017]FIGS. 1A and 1B depict an exemplary embodiment of a RF Subsection Diagram illustrating receive and transmit paths controlled by the present invention.  
         [0018]    [0018]FIG. 2 is a functional block diagram depiction of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0019]    Turning now to FIGS. 1A and 1B, millimeter wave transceiver  10  implements a preferred embodiment of the present invention. The remotely located modem (not shown) communicates a multiplexed signal via cable connection  100  to transceiver  10 , which contains transmit section  11  and receive section  12 . In this preferred embodiment, the modem is an on/off keying modem associated with indoor equipment with transceiver  10  and the antenna being associated with an outdoor unit. Cable connection  100  provides a bi-directional link. The bi-directional link is alternatively allocated between transmit and receive signaling in a Time Division Duplex (TDD) scheme. Additionally, transceiver  10  receives commands from indoor equipment. Similarly, transceiver  10  reports status and alarm information to the indoor equipment.  
         [0020]    In a preferred embodiment, cable connection  100  is coupled to pentaplexer  13 . Pentaplexer  13  implements functionality to allow multiplexing of five signals via the single communication link between the modem assembly and transceiver  10 . The five signals include one DC power signal to power transceiver  10 , a high stability frequency source, a reference signal to synchronize the local oscillators  17  and  18  for the up-conversion and down-conversion, a communication signal, and a tone control signal. As previously noted, this preferred embodiment remotely locates the modem from transceiver  10 . In practice, the modem may be located at any distance including co-location utilizing a single commercially available cable.  
         [0021]    Transceiver  10  contains switch  14 . Switch  14  comprises three sub-components  14 - 1 ,  14 - 2 , and  14 - 3 . When the switch sub-components ( 14 - 1 ,  14 - 2 ,  14 - 3 ) are in the upper state, transceiver  10  is in transmit mode. Conversely, when the switch subcomponents are in the lower state, transceiver  10  is in receive mode. In transmit mode, the IF signal is received via cable connection  100 , processed through transmit section  11  (as will be detailed later), and outputted via antenna connection  101 . In receive mode, the RF receive signal is received from antenna connection  101 , processed through receive section  12  (as will be detailed later), and outputted via cable connection  100 . Oscillators  17  and  18  are preferably designed to be common to both the receive and transmit sections.  
         [0022]    In transmit mode, the modem generates a RF signal communicated to transceiver  10  via cable connection  100 . The RF signal contains an IF modulated data signal that will be up-converted for transmission via the antenna. In a preferred embodiment, the IF signal is a 140 MHz signal. In other embodiments, the IF signal is one of the following frequencies: 70, 280, 480 MHz. Additionally, the RF signal contains the tone control signal, which preferably possesses a frequency within the range of 5-50 MHz. In an embodiment, the control tone possesses a frequency of 35 MHz. As previously noted, certain embodiments may utilize a high stability frequency source (i.e., a timing reference signal) to synchronize the local oscillators  17  and  18 . In such embodiments, the control tone may be implemented as a convenient harmonic of the frequency associated with the timing reference signal. Preferably, the tone control is sufficiently separated from the 140 MHz IF signal to avoid interference. The IF signal passes through filter  1301  and, in a preferred embodiment, is band-limited filtered to a spectrum width of 50 MHz. It shall be appreciated that other bandwidths for the filter are contemplated so long as the control signal can be distinguished from the communication signal. Detector  19  receives the band-limited signal to determine whether the modem has communicated the tone control signal. Detector  19  produces a logical signal. If detector  19  detects the tone control signal, detector  19  cause the logical signal to represent a true state. If the logical signal is true, switch  14  causes sub-components  14 - 1 ,  14 - 2 , and  14 - 3  to remain in or transition to the upper state. If the logical signal is false, switch  14  causes sub-components  14 - 1 ,  14 - 2 , and  14 - 3  to remain in or transition to the lower state. Preferably, the detector  19  causes the logical signal to default to the false state. Thus, transceiver  10  defaults to receive mode.  
         [0023]    It shall be appreciated by those skilled in the art that the present method and system employing a detector and an out of band control tone provides significantly lower switching times. Use of a control tone in lieu of a unique word considerably simplifies the detection mechanism. Additionally, the use of a control tone is more robust than the unique word approach. Systems employing the unique word approach are more vulnerable to timing errors due to noise. Since the unique word is signaled by a predetermined series of symbols, timing errors may occur if one or a few symbols of the unique word are improperly decoded due to noise. However, the present invention does not involve this limitation. The present invention does not detect the control tone in a dependent manner based upon prior states. Accordingly, the timing functionality of the present invention is more resistant to thermal noise and interference. Moreover, utilizing the control tone to dynamically schedule transmit and receive modes requires far less bandwidth than communicating receive and transmit frame parameters via a control channel. Accordingly, the control tone facilitates dynamic alteration of the receive and transmit timing in a more efficient manner.  
         [0024]    When the transceiver is in receive mode, switch sub-components  14 - 1 ,  14 - 2 , and  14 - 3  remain in the lower state. This allows RF signals received by the antenna from cable  101  to pass through switch sub-component  14 - 2  to linear amplifier  1218 . After amplification, the RF signal passes through band-pass filter  1217  and down conversion mixer  1215 . Down conversion mixer  1215  is coupled to local oscillator  18  via amplifier  1216 . The signal passes through a second amplifier  1214 , a second band-pass filter  1213 , and a second buffer  1212 . The signal passes through down-conversion mixer  1210  and IF amplifier  1209 . Down-conversion mixer  1210  is coupled to local oscillator  17  via amplifier  1211 . Filter component  1208  selectively filters the signal by selecting from a plurality of predetermined bandwidths. The signal is processed by slope attenuator  1207  to provide slope compensation. The signal is additionally amplified by linear amplifier  1204 . Additional slope compensation is provided by slope attenuator  1204 . Additional slope compensation is provided by slope attenuator  1203 . The signal is path is continued by switch sub-component  14 - 3 , which provides further path isolation. The signal is further processed by voltage variable attenuator  1202 . The signal passes through buffer amplifier  1201  and switch sub-component  14 - 1  to pentaplexer  13  via filter  1302 . Pentaplexer  13  multiplexes the processed IF signal for communication over cable connection  100  to the modem.  
         [0025]    When the tone control signal is detected and the logical signal state is true, the transceiver operates in transmit mode. Switch sub-components  14 - 1 ,  14 - 2 , and  14 - 3  are maintained in the upper state. In transmit mode, the modem produces and multiplexes a pay-load IF signal via cable connection  100 . The IF signal is received by pentaplexer  13 . The signal is filtered by filter  1302  and proceeds via switch sub-component  14 - 1  to voltage variable attenuator  1101  for cable and gain compensation. The signal passes through IF amplifier  1104  and voltage variable attenuator  1105  that provides gain adjustment due to temperature variation. The signal is up-converted by mixer  1106 . Mixer  1106  is coupled to local oscillator  17  via amplifier  1108 . The signal is successively filtered by bandpass filters  1109  and  1110 . The signal is further amplified by amplifier  1111  before up-conversion to millimeter wave range by mixer  1112 . Mixer  1112  is coupled to local oscillator  18  via amplifier  1115 . The millimeter wave signal is filter by bandpass filter  1113  and amplified by amplifier  1114 . Since the control tone has been detected and the logical signal is true, switch  14 - 2  completes the circuit path allowing the processed signal to proceed to the antenna via antenna connection  101 .  
         [0026]    The modem also generates an auxiliary modem signal communicated via cable connection  100 . The auxiliary modem signal is separated from the main IF signal and is received by on/off keying transceiver item  15 . Microprocessor  16  controls the synthesizers and switches internal to transceiver  10  assembly via the data stream from the modem communicated by the auxiliary signal. Microprocessor  16  also communicates status information in the reverse direction to the modem. Additionally, the microprocessor may control detector  19  to operate the logical signal independently of the out of band signal. In this manner, the transceiver may be made compatible with modems that do not employ the present invention.  
         [0027]    With reference now to FIG. 2, where like components have like numbers and function in a like manner to the similar numbered components in FIGS. 1A and 1B, the modem  40  is connected to the transceiver  10  via the cable  100 . The transceiver  10  includes the tone decoder  50  which includes the filter  1301  and the detector  19 . The tone decoder is connected to the switch  14 - 1 , which operates in conjunction with switch  14 - 2  and switch  14 - 3  (not shown), all of which operate to allow the transceiver to function in either the transmit or receive modes as previously described. The switch  14 - 2  is connected to the antenna  30  which transmits outgoing communication signals and receives incoming communication signals.  
         [0028]    It shall be appreciated that the preceding circuitry and signal path descriptions are merely exemplary. The transmit and receive sections are shown solely as a representative circuit. Any transmit and receive circuitry may be utilized. Additionally, any frequency ranges may be implemented with the present invention. Moreover, the control tone is not limited to a 35 MHz signal. The present invention may employ any other control signal provided that sufficient separation (at either end of the working frequency, or within the working frequency) is utilized to allow detection of the control signal. Also, the control tone signal need not be continuous. The control tone may be transmitted from time to time, depending upon system requirements.  
         [0029]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.