Abstract:
Controlling a tower-top low noise amplifier (TTLNA) without transmit mode or receive mode timing control signals from a base station. The TTLNA system and associated components autonomously determine the proper mode of operation (transmit/receive), and automatically control the operation of a low noise amplifier (LNA) to prevent signal distortion and/or damage to the wireless system hardware. A preferred method comprises: at a TTLNA, measuring a transmit time period based on detecting radio frequency (RF) transmit signal energy; determining a receive time duration based on the measured time period and a predetermined frame time; and, configuring the TTLNA to a receive mode by placing a low noise amplifier into a receive signal path during the determined receive time duration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to wireless communications and, more particularly, to a method and system of controlling tower top low noise amplifiers. 
     BACKGROUND 
     Wireless communication systems typically include a base transceiver station (BTS) that provides service to one or more mobile stations within the coverage area of the BTS. The BTS may include a radio antenna system having one or more antennas mounted on a tower. The antennas may transmit downlink signals to and/or receive uplink signals from one or more mobile stations serviced by the BTS. Further, the radio antenna system may also include a tower-top low-noise amplifier assembly (TTLNA) and a feedline system (collectively, a receive signal path), which connects the base station located at the bottom of the tower to various components located at the top of the tower, such as the TTLNA. 
     It is standard practice to initialize the TTLNA in receive mode. Once the TTLNA has been initialized, the TTLNA will typically operate in either receive mode or in transmit mode. In receive mode, the TTLNA operates to receive signals from the antenna. The TTLNA passes the received signals through a sensitive microwave low noise amplifier (LNA) to amplify the signals. The amplified signals are then sent to the base station via the feedline system. In transmit mode, the TTLNA receives powerful transmit signals from the base station. In this mode, the LNA is bypassed and the powerful transmit signals from the base station are sent to the antenna. 
     According to current practice, the TTLNA relies on a relatively simple on-off signaling scheme to switch between receive mode and transmit mode. In this signaling scheme, the presence of a control signal causes the TTLNA to switch into transmit mode. And the absence of a signal causes the TTLNA to switch to its default resting state (i.e., receive mode). 
     SUMMARY 
     Disclosed herein is an improved method of switching a low noise amplifier into or out of the receive signal path. In particular, a TTLNA system and associated components are disclosed that autonomously determine the proper mode of operation (transmit/receive), and automatically controls the operation of a low noise amplifier (LNA) to prevent signal distortion and/or damage to the wireless system hardware. 
     In one embodiment, a preferred method of controlling a tower-top low noise amplifier (TTLNA) comprises, at a TTLNA, measuring a transmit time period based on detecting radio frequency (RF) transmit signal energy; determining a receive time duration based on the measured time period and a predetermined frame time; and, configuring the TTLNA to a receive mode by placing a low noise amplifier into a receive signal path during the determined receive time duration. 
     In a further preferred embodiment, a method of controlling a tower-top low noise amplifier (TTLNA) comprises configuring a TTLNA to the transmit mode by removing a low noise amplifier (LNA) from a receive signal path; at a TTLNA receive duration timing circuit, detecting a start of a radio frequency (RF) transmit signal and an end of an RF transmit signal and responsively generating a receive mode control signal; configuring a TTLNA to the receive mode by placing the LNA into the receive signal path in response to detecting the end of the RF transmit signal; and returning the TTLNA to the transmit mode in response to the receive mode control signal generated at the TTLNA in response to the receive duration timing circuit. 
     Preferably a power detection circuit is used to detect the start and end of the RF transmit signal. The LNA may be removed from a receive path by the use of RF switches and/or grounding an input of the LNA. In some embodiments, the receive duration timing circuit comprises a counter or timer circuit. The output from the counter circuit is preferably used to generate the receive mode control signal to initiate or terminate the receive period. Alternatively, the output from the counter is used to interrupt a processor or microcontroller, which generates the receive mode control signal to terminate a receive period. 
     In some embodiments a latch circuit or flip-flop may be used for generating the receive mode control signal. The receive mode control signal also preferably provides a time delay before placing the LNA in the receive signal path after the detection of the end of the RF transmit signal during a transmit-to-receive gap. Similarly, the receive mode control signal switches the TTLNA back to transmit mode during the receive-to-transmit gap. In further embodiments, the receive mode control signal may be used to control a second LNA that services an additional receive signal path. 
     Control signaling is also provided for generating an alarm signal when the start of an RF transmit signal is not detected during a predetermined time. Additionally, the method may use a command signal that overrides the receive mode control signal and responsively places the TTLNA into either the transmit mode or the receive mode. 
     In still further embodiments, a tower-top low noise amplifier assembly (TTLNA) is provided. The TTLNA preferably includes a low noise amplifier (LNA) that is switchably coupled to a receive signal path; a power detection circuit coupled to a transmit signal path for generating a transmit power signal indicative of transmit energy being present; a receive timing circuit coupled to the power detection circuit for generating a receive mode control signal in response to the transmit power signal; and, wherein the receive timing circuit is coupled to the LNA and the receive mode control signal is used to couple and decouple the LNA from the receive signal path. 
     The receive timing circuit may be a microprocessor and a counter or timer for generating a microprocessor interrupt signal indicating an end-of-frame time. Alternatively, a counter and a logic circuit may be used to provide the receive slot timing reference. The counter or timer is preferably started in response to the transmit power signal, and provides an output indicative of the end of the receive mode substantially coincident with an end of a frame time. The counter may be configurable to provide for desired time durations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are described herein with reference to the drawings, in which: 
         FIG. 1  is a block diagram depicting a radio antenna system, in accordance with exemplary embodiments; 
         FIG. 2  is a block diagram of preferred TTLNA assembly; 
         FIG. 3  is one embodiment of a timing circuit for generating a receive mode control signal; 
         FIG. 4  is a timing diagram associated with the circuit of  FIG. 3 ; 
         FIG. 5  is an alternative embodiment of a timing circuit for generating a receive mode control signal; 
         FIG. 6  is a timing diagram associated with the circuit of  FIG. 5 ; 
         FIGS. 7 and 8  are flow diagrams associate it with preferred methods described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified block diagram depicting a radio antenna system  100  that may be used in accordance with exemplary embodiments. As illustrated, the radio antenna system  100  includes an antenna  102 , a cable  104 , a tower-top low noise amplifier (TTLNA) system  106 , a tower  108 , a feedline  110 , and a base transceiver station (BTS)  112 . The system illustrated in  FIG. 1  preferably operates in a Worldwide Interoperability for Microwave Access (WiMAX) system (i.e., the IEEE 802.16 standard). 
     It should be understood that the arrangements described herein are for purposes of example only. For example, antenna  102  may include a plurality of antennas in which different antennas are dedicated to either receiving uplink signals or transmitting downlink signals. As another example, antenna  102  may be situated on a structure other than tower  108 . For instance, antenna  102  may be situated on a building. As such, those skilled in the art will appreciate that other arrangements and other elements (e.g. machines, interfaces, functions, orders, and groupings of functions, etc.) can be used instead. Further, many of the elements described herein are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination or location. 
     In normal operation, antenna  102  receives uplink signals from devices in communication with the radio antenna system  100 . The uplink signals received by antenna  102  are transmitted to the TTLNA system  106  via cable  104 . TTLNA system  106  amplifies the received uplink signals and then sends the amplified uplink signals to BTS  112  via the feedline  110 . In addition, antenna  102  operates to transmit downlink signals to devices in communication with the radio antenna system  100 . The power amplifier in BTS  112  sends the downlink signals to TTLNA system  106  via the feedline  110 . The TTLNA system  106  then passes the downlink signals via cable  104  to antenna  102 . The downlink signal is then transmitted to device in communication with radio antenna system  100 , via antenna  102 . 
     Radio antenna system  100  separates uplink and downlink signals by using a time division duplex (TDD) transmission scheme. Using the TDD transmission scheme, BTS  112  assigns a common frequency channel to both the uplink and downlink communication signals. BTS  112  toggles back and forth between sending communications signals to the antenna  102  (i.e., downlink signals) and receiving communication signals from the antenna  102  (i.e., uplink signals). In a preferred transmission scheme such as WiMAX, BTS  112  may switch between receiving uplink signals and sending downlink signals up to 200 times a second. 
     In a TTD transmission scheme, feedline  110  carries amplified uplink signals from the TTLNA system  106  to the BTS  112  during a first time period. And in a second time period, the feedline  110  carries downlink signals from the BTS  112  to the antenna  102  via TTLNA system  106 . In this way, feedline  110  alternates between carrying uplink and downlink communication signals. 
     As noted above, radio antenna system  100  operates in a TDD transmission scheme. As the feedline  110  alternates between receiving uplink and sending downlink signals, the TTLNA system  106  must also alternate between transmit mode and receive mode. When the feedline  110  is carrying downlink signals to the antenna  102 , it is preferred that the LNA  106  is not configured to receive mode. Should the LNA  106  be in receive mode when feedline  110  is carrying downlink signals to the antenna  102  (because of a failure to properly switch modes), the downlink signals may damage the sensitive LNA  106  resulting in a possible sector outage. In such a scenario, a technician may need to physically climb the tower to replace and/or repair the damaged LNA  106 . 
     Along with carrying uplink signal and/or downlink signals, feedline  110  also carries other signals to and from the antenna  102 . As examples, feedline  110  carries DC power up from the BTS  112  to power the TTLNA system  106  and other components located at the top of the tower  108 . The feedline  110  may also carry an Antenna Interface Standards Group (AISG) signal, which is used to monitor the system status and provide control over non-time critical functions. For example, the AISG signal may be used for positioning the antenna  102 . 
     With reference to  FIG. 2 , a TTLNA assembly  200  is provided. The TTLNA preferably includes a low noise amplifier (LNA)  218  that is switchably coupled to a receive signal path, or uplink path. Signals received from the antenna are filtered by filter  202  and passed through the circulator  204 . The received signals are then passed through the assembly  208  to the isolator  228  and the circulator  230  and finally provided to the base transceiver station (BTS). Additionally a bypass switch  216  is provided to bypass the assembly  208  in the event of a power failure or in response to a control command. Transmit signals from the BTS are passed through circulator  230  to circulator  204  for transmission by the antenna. 
     A power detection circuit  210  in the form of a transmit radio frequency (RF) sensor is coupled to the transmit signal path, or downlink path. The power detection circuit  210  generates a transmit power signal indicative of transmit energy being present. The TTLNA controller  214  receives the transmit power signal and responsively controls the LNA  218 . The receive mode control signal is provided to the assembly  208  and is used to couple and decouple the LNA from the receive signal path. The LNA  218  may be provided in an assembly  208 , which includes switches  206 ,  226  that operate in response to the receive mode control signal to place LNA  218  in the received path. Also included are switches  212 ,  220 , that are responsive to the receive mode control signal for grounding the input to LNA  218 , or placing a terminating load  222  on the output of LNA  218 , respectively. Switches  206 ,  226  may be analog RF switches, magnetic circulators, microwave PIN diode switches, or the like. Switches  212 ,  220  may be analog RF switches, microwave PIN diode switches, or the like. The receive mode control signal may also be provided out control port  224  for controlling additional LNAs. 
     The position of switches  206 ,  226  determines whether LNA  218  is operating in transmit mode (T x ) or receive mode (Rx). According to the position of switches  206 ,  226  as illustrated in  FIG. 2 , LNA  218  is currently operating in receive mode. In this mode, the LNA  218  is connected to the uplink path associated with feedline  110 . 
     The position of switches  212 ,  220  determines whether LNA  218  is operating in transmit mode (T x ) or receive mode (Rx). According to the position of switches  212 ,  220  as illustrated in  FIG. 2 , LNA  218  is configured to operate in receive mode. In this mode, the LNA  218  has no termination on its input and no terminating load attached to its output. 
     In transmit/bypass mode, switches  212 ,  220  are positioned such that the LNA  218  has a ground on its input and a load  222  on its output. In this mode, LNA  218  is prevented from amplifying any extraneous signals that may be present in the associated circuitry, avoiding the possibility of creating localized interference while maintaining power to the amplifier and maintaining its stability. 
     The TTLNA controller  214  preferably includes a receive timing circuit, as shown in  FIG. 3 , for generating a receive mode control signal in response to the transmit power signal. A logic circuit preferably includes an inverter  306 , a one-shot flip-flop  310 , and RS flip-flop  312 . Waveforms associated with the elements of  FIG. 3  are depicted in  FIG. 4 . The TX detector  302  is preferably an RF sensor used for detecting the presence of RF energy. The output on line  304  is a logic signal as shown in  FIG. 4 , depicting the presence of RF transmit energy by a high logic value, and the absence of transmit energy by a low logic value. The rising edge transition  400  indicates the start of RF transmit signal energy, while the transition  402  indicates the end of RF transmit signal energy. The rising edge transition  400  initiates the timer  316 , causing it to begin counting. 
     The output of TX detector  302  is also provided to inverter  306 , which delays and inverts the power detection signal from TX detector  302  to generate the signal  TX DETECTOR  as shown in  FIG. 4 . The output of inverter  306  is applied over a line  308  to the one-shot flip-flop  310 . The rising edge  404  causes the one-shot  310  to emit a pulse  406  on the ONE SHOT signal shown in  FIG. 4 . The rising edge of pulse  406  sets the flip-flop  312  causing the output on line  314  to go to a logic high, as shown by rising edge  408 . The RX MODE signal is applied to assembly  208  and the rising edge of the signal appropriately configures the switches  206 ,  226 ,  212 ,  220 , to place LNA  218  into the received signal path. Note that the inverter  306  preferably provides a short delay with respect to the end of the transmit power detection. The amount of this delay is preferably configurable, and may be adjusted by providing additional buffer circuits, by altering the threshold input of one-shot  310 , or by any other suitable means. The delay is desirable to ensure that the transmit energy level has sufficiently subsided prior to placing the LNA in the received signal path. 
     Timer  316  initiates counting at the beginning of the transmit cycle in response to the TX DETECTOR signal, and is configured to count an entire frame duration, or some predetermined amount less than an entire frame duration. At the end of the frame duration the timer/counter output goes from a logic low to a logic high on line  318 , as shown by rising edge  412  in  FIG. 4 . The rising edge  412  is applied to the flip-flop  312  to reset the flip-flop output  314  to a logic low value, as shown by falling edge  416  of the RX MODE signal. The low value of the RX MODE signal is applied to the assembly  208 , and the respective switches  206 ,  226 ,  212 ,  222  thereby remove the LNA  218  from the received signal path. 
     As shown in  FIG. 5 , the receive timing circuit may take the form of a microprocessor  506  and a counter or timer  512  for generating a microprocessor interrupt signal on line  510  indicating an end-of-frame time. The microprocessor  506  receives the TX DETECTOR signal from TX detector  502  over line  504 . Line  504  may also be an interrupt line to the microprocessor, or may be an input port monitored by the microprocessor  506 . Alternatively, the microprocessor  506  may utilize a scheduling algorithm to determine the beginning and ending of the receive period in response to the transmit signal power detect signal. 
     As shown in  FIG. 6 , the falling edge  602  of the TX DETECTOR signal is detected by microprocessor  506  whereupon the microprocessor  506  generates an RX mode signal on line  508 , having a rising edge  604 . The TX DETECTOR signal is also provided to the interrupt timer  512 , wherein a rising edge causes the interrupt timer  512  to begin counting a frame time duration. At the end of the frame time or substantially near the end of the frame time, the interrupt timer  512  initiates an interrupt signal  606  to the microprocessor  506  on line  510 . The microprocessor  506  responsively terminates the RX mode signal, returning it to a low logic value shown by transition  608 . 
     AISG control signaling may also be utilized. In one aspect, the TTLNA assembly may generate an alarm signal when the start of an RF transmit signal is not detected during a predetermined time, such as 10 frames or 50 ms. The alarm may be transmitted to the BTS via AISG signaling. Additionally, the system is preferably responsive to a command signal (such as a command conveyed by AISG signaling) that overrides the receive mode control signal and responsively places the TTLNA into either the transmit mode or the receive mode, in accordance with the command. The commands may be in accordance with a Simple Networking Management Protocol (SNMP). 
     In one preferred embodiment, a method  700  of controlling a tower-top low noise amplifier (TTLNA) is shown in  FIG. 7 . At step  702 , the TTLNA is configured to the transmit mode by removing a low noise amplifier (LNA) from a receive signal path. As described above, the LNA  218  may be removed from a receive path by the use of RF switches and/or grounding an input of the LNA  218 . At step  704 , the start of a radio frequency (RF) transmit signal and an end of an RF transmit signal is detected. Preferably a power detection circuit is used to detect the start and end of the RF transmit signal. The power detection circuit is used in conjunction with the receive duration timing circuit, which responsively generates a receive mode control signal. At step  706  the TTLNA is configured to the receive mode by placing the LNA into the receive signal path in response to detecting the end of the RF transmit signal. At step  708  the TTLNA is returned to the transmit mode in response to the receive mode control signal generated at the TTLNA in response to the receive duration timing circuit. 
     An alternative embodiment a preferred method  800  of controlling a tower-top low noise amplifier (TTLNA) is shown in  FIG. 8 . At step  802  a transmit time period is measured at a TTLNA. The measurement is based on detecting RF transmit signal energy from the BTS. At step  804  a receive time duration is determined based on the measured transmit time period and a predetermined frame time. At step  806  the TTLNA is configured to a receive mode by placing a low noise amplifier into a receive signal path during the determined receive time duration. 
     Exemplary embodiments of the present invention have been described above. Those skilled in the art will understand, however, that changes and modifications may be made to the embodiments described without departing from the true scope and spirit of the present invention, which is defined by the claims.