Abstract:
Systems and methods for providing an improved multiradio system. An exemplary system includes first and second antennas and a first receiver that receives a signal from the first antenna, filters the received signal based on bandwidths associated with a traffic collision-avoidance system (TCAS), a transponder, and a universal access transceiver (UAT). The system digitizes the filtered signal and digitally downconverts the digitized signal. A second receiver receives a signal from the second antenna, filters the signal received from the second antenna based on the TCAS, the transponder, the UAT, and distance-measuring equipment (DME), separates the filtered signal into a first signal having a bandwidth associated with the TCAS, the transponder, the UAT and the lower half of the DME RF band, and into a second signal having a bandwidth associated with the upper half of the DME RF band, digitizes the first and second signal, and digitally downconverts the digitized first and second signals.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Currently, up to four radio systems are required to implement traffic collision avoidance system (TCAS), air traffic control (ATC) transponder, distance-measuring equipment (DME), and universal access transceiver (UAT) avionics functions on an aircraft. This would require four different transceivers. This implementation is not optimal in terms of weight, cost, volume, and power consumption. 
         [0002]    Some systems have attempted to resolve this problem. In one current embodiment of an L-band receiver system, a DME receiver is included along with a receiver for the TCAS, transponder, and UAT. This prior-art design still includes all the analog circuitry for generating three separate narrowband signals for the TCAS, transponder, and UAT intermediate frequency (IF) outputs. The DME IF output is separately generated. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention combines transmit and receive functions of all four radios (traffic collision advisory system (TCAS), air traffic control (ATC) transponder, distance-measuring equipment (DME), and universal access transceiver (UAT)) into a single radio. This reduces the number of transmitters from four to one. This also reduces the number of receivers, power supplies, and digital modules from four to one. This reduces weight, cost, volume, and power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Preferred and alternative embodiments of the present invention are described in detail below, with reference to the following drawings: 
           [0005]      FIG. 1  illustrates a schematic view of components included in an aircraft in accordance with an embodiment of the present invention; 
           [0006]      FIG. 2  is a flowchart of an exemplary process performed by the system components shown in  FIG. 1 ; and 
           [0007]      FIGS. 3 ,  4 - 1 ,  4 - 2  and  4 - 3  illustrate components of an exemplary system formed in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    The invention incorporates a wideband receiver capable of receiving multiple signals within the frequency range of 962 MHz to 1213 MHz. It simultaneously decodes four different formats—traffic collision-avoidance system (TCAS), at 1090 MHz, air traffic control (ATC) transponder at 1030 MHz, universal access transceiver (UAT) at 978 MHz, and distance-measuring equipment (DME) in 1-MHz channels within a 962 MHz to 1213 MHz DME band. 
         [0009]    This invention also incorporates a wideband transmitter capable of transmitting multiple signal formats. That is: TCAS at 1030 MHz, ATC transponder at 1090 MHz, and DME in 1-MHz channels within 1025 MHz to 1150 MHz. 
         [0010]    Limitations of analog-to-digital conversion (ADC) over wide bandwidth, while simultaneously meeting required signal-to-noise ratio, prevented a direct conversion (sampling at LB and frequency directly) realization of wideband multichannel radio as an integrated solution. This invention uses a subbanding technique and block-down conversion to cover the entire L-band avionics radio spectrum, thereby making wideband ADC practical. Another practical limitation of wideband radio is the linearity at high input signal levels of the analog front end. This limitation is overcome by limiting the gain of the analog front end and judicious implementation of the block downconversion to place the harmonics of down converted signals outside of the sampled and digitally filtered bandwidth. The transmitter covers multiple L-band functions by using either a single multimode modulator or switchable mode-specific modulators. The entire L-band frequency spectrum is amplified using wideband power amplifiers. Higher power level and efficiency for the DME function are achieved by using mode-select information and adaptively changing power amplifier bias. 
         [0011]    Traditionally a first Nyquist sample region is used in A to D conversion. For example, when a sampling clock frequency of F c  (example 320 MHz) is used for the ADC, the analog frequency to be digitized is selected to be less than 1/2  the sampling clock frequency (160 MHz). If this method is employed in wideband radio, harmonics of received down converted signals produce interference for other desired channels. 
         [0012]    For example, to block down convert 962 MHz to 1100 MHz to the first Nyquist zone using F c  of 320 MHz, LO frequency of 1112 MHz could be used. This block down conversion produces signals from 12 MHz to 150 MHz. However when the receiver receives a strong TCAS signal at 1090 MHZ, this is down converted to 22 MHz and this strong signal generates harmonics at 44 MHz, 66 MHz . . . If there is a weak DME signal is present at 1068 MHz this will be down converted to 44 MHz using the same LO at 1112 MHz. Because of the harmonics generated by TCAS reception, the DME channel will be polluted rendering the wideband radio inoperable. 
         [0013]    However, if LO frequency is chosen such that the block down converted signal&#39;s harmonics are placed outside the A to D sampling bandwidth using second Nyquist region, this problem is avoided. For example if LO frequency is chosen to be 1265 MHz, then the TCAS signal at  1090  will be down converted to 175 MHz. Harmonics of this fall outside the maximum frequency for any channel used in this wideband radio implementation. 
         [0014]      FIG. 1  illustrates an aircraft  20  that includes a multichannel, multimode, multifunction L-band radio transceiver system. The transceiver system includes top and bottom antennas  30 ,  32  that are in signal communication with respective receivers  34 ,  36  and transmitters (not shown). The receivers  34 ,  36  block downconvert the radio frequency (RF) signals received by the antennas  30 ,  32 , digitize the resulting wideband intermediate frequency (IF) spectrum with a high speed ADC, and sends the digital signals to digital down converters (DDCs)  38 . The DDCs  38  apply digital signal processing, including but not limited to filtering and decimation, to the multichannel digital signals which convert them into single function data streams that are then sent to the four radio signal processors: a TCAS  40 , a transponder (XPDR)  42 , a UAT  44 , and a DME  46 . Exemplary contents of the receivers  34 ,  36  are shown in  FIG. 3 . 
         [0015]      FIG. 2  illustrates an exemplary process  80  performed by the transceiver system shown in  FIG. 1 . First, at a block  82 , the top and bottom antennas  30 ,  32  receive first and second radio signals. Next, at a block  84 , the first received signal is filtered according to a first bandwidth that is associated with the TCAS  40 , the transponder  42 , and the UAT  44 . At a block  86 , the filtered first signal is converted to a first digital signal. At a block  88 , the second received signal is filtered and split into low and high band signals. The low band signal is associated with the TCAS  40 , the transponder  42 , and the UAT  44  and the lower half of the DME RF band. The high band signal is associated with the upper half of the DME  46  RF band. At block  90 , the low and high band signals are converted to second and third digital signals. The steps performed at blocks  88  and  90  may be performed concurrently with the steps performed at blocks  84  and  86 . Next, at a block  94 , the TCAS, transponder, and UAT decoder input signals are generated, based on the first and second digital signals. At a block  96 , a DME decoder input signal is generated, based on the second and third digital signals. Finally, at a block  98 , the generated decoder input signals are sent to the respective decoders (the TCAS  40 , the transponder  42 , the UAT  44 , and the DME  46 ). 
         [0016]      FIGS. 3 ,  4 - 1  and  4 - 2  illustrate analog and digital components of the receiver portion of an exemplary transceiver system  130 . The transceiver system  130  includes top and bottom antennas  160 ,  162 , first and second bandpass filters (BPF)  166 ,  168 , first and second circulators  150 ,  152 , a transmit/receive switch  154 , a transmitter  140 , a top antenna analog receiver component  144 , a bottom antenna analog receiver component  146 , and a plurality of digital down-converters (DDC)  300 . 
         [0017]    The first BPF  166  is in signal communication with the top antenna  160  and the first circulator  150 . The first circulator  150  is also in signal communication with the transmitter switch  154  and the top antenna&#39;s analog receiver component  144 . The second BPF  168  is in signal communication with the bottom antenna  162  and the second circulator  152 . The circulator  152  is in signal communication with the transmitter switch  154  and the bottom antenna&#39;s analog receiver component  146 . In this embodiment, the BPFs  166 ,  168  filter signals in the 962 to 1213 MHz bandwidth. The bandwidth that is filtered by the BPFs  166 ,  168  encompasses only those radio signals associated with the TCAS  40 , transponder  42 , the UAT  44 , and the DME  46 . The circulators  150 ,  152  provide signal directionality such that signals generated by the transmitter  140  are passed to the respective antennas  160 ,  162  and signals received by the respective antennas  160 ,  162  are passed to the respective receiver components  144 ,  146 . 
         [0018]    The top receiver component  144  includes a T/R switch (limiter)  172  that receives the bandwidth-limited signal from the first circulator  150 . A first low-noise amplifier (LNA)  174  receives the output of the T/R switch (limiter)  172  to produce a first amplified signal. The T/R switch (limiter)  172  prevents overdriving the LNA  174  when high power signals are present at the antennas, including the transmitter output. An image-filtering and second LNA component  176  receives the output of the first LNA  174  to produce a radio frequency (RF) signal with a bandwidth of 962 to 1100 MHz. At a mixer  180 , the output of the image-filtering and second LNA component  176  is combined with a local oscillator (LO) signal  182 . A BPF  186  and an amplifier  188  receive the output of the mixer  180  to produce an intermediate frequency (IF) with a bandwidth of 165 to 303 MHz. The signal outputted by the amplifier  188  is then sent to an analog-to-digital converter (ADC)  190 . 
         [0019]    The bottom antenna&#39;s analog receiver component  146  includes a T/R switch (limiter)  200  that receives the signal received by the bottom antenna  162 , via the circulator  152  and BPF  168 . The output of the T/R switch (limiter)  200  is received by a first LNA  202 , which produces an amplified signal that is sent to a demultiplexer/splitter  204 . Diplexer/splitter  204  splits the amplified signal received from the first LNA  202  into a low band (RF 962 to 1100 MHz) and a high band (RF 1101 to 1213 MHz). The low band RF is sent to a first image-filtering and second LNA component  206 , which generates a signal that is combined with an LO signal  210  at a mixer  208 . The output of the mixer  208  is sent to a BPF  214  and then to an amplifier  216 , thus producing an IF signal with a bandwidth of 165 to 303 MHz. The output of the amplifier  216  is sent to an ADC  218 . 
         [0020]    The high band RF outputted from the diplexer/splitter  204  is sent to an image-filtering and LNA  222 , which outputs a signal to a mixer  224 , which is combined with an LO signal  226 . The output of the mixer  224  is filtered by a BPF  230 , then amplified by an amplifier  232  to produce an IF with a bandwidth of 176 to 288 MHz. The output of the amplifier  232  is sent to an ADC  234 . 
         [0021]    The outputs of the ADCs  190 ,  218 , and  234  are sent to the DDCs  300 , as shown in  FIGS. 4-1  and  4 - 2 . The low band digital signals produced by the ADCs  190  and  218  are sent to two TCAS DDCs  310 , two transponder (XPDR) DDCs  320 , and two UAT DDCs  330 .  FIG. 4-1  shows only one each of the TCAS, transponder, and UAT DDCs. The high and low band outputs from the ADCs  234  and  218  are sent to a DME DDC  340 . 
         [0022]    The TCAS DDC  310  receives the output of one of the ADCs  190 ,  218  at two mixers. The first mixer mixes the received digital IF signal with a zero-phase complex LO signal and the second mixer combines the received digital IF signal with a 90° phase-shifted complex LO signal. The complex LO produces two outputs at the same frequency and amplitude with 90° phase difference between them (i.e., in-phase (I) and quadrature (Q) representing real and imaginary components). The frequency value for the complex LO of the TCAS DDC is 175 MHz. Next, the outputs of the mixers are sent to respective CIC-decimating LPFs, which is a Cascade Integrator Comb filter which is a decimating filter structure with decimation factor R, M# of differential stages, and N# of stages. Decimation reduces the input rate by the decimation factor. If the input is clocked at F s =320 Mhz and R=4 then the output is clocked at F S2 =80 Mhz. The output of the CIC-decimating LPFs are sent to respective Finite Impulse Response filter (FIR)-decimating LPFs with cutoff frequency F C  input clock frequency F S2 . Next, a component receives the outputs of the FIR-decimating LPFs to determine a magnitude and phase value. 
         [0023]    The components of the transponder DDC  320  are identical to those of the TCAS DDC  310 , except that the complex LO operates at a frequency of 235 MHz. The data buffer, up and down samplers and interpolating FIR provide a resampler circuit for changing the output data rate to match the decoder&#39;s input data rate requirements. A delay/multiplier exists after the phase output to demodulate the DPSK (Differential Phase Shift Keying) data. 
         [0024]    The UAT DDC  330  includes all the components of the other DDCs  310 ,  320 , except that it does not include the component for generating the magnitude and phase of the signals produced by the FIR-decimating LPFs. Also, the UAT DDC  330  includes a complex LO that operates at 287 MHz and a variety of other components configured to produce an in-phase signal (I) and a quadrature phase signal (Q). The data buffers, up and down samplers and interpolating FIRs provide resampler circuits for changing the output data rate to match the decoder&#39;s input data rate requirements. 
         [0025]    The high and low band AGC (automatic gain control) circuits control the gain of external variable gain amplifiers driving the ADC inputs. This ensures that the inputs to the ADC do not exceed their maximum linear range. 
         [0026]    The DME DDC includes similar circuit components as that of the TCAS DDC  310 , except that the frequency of the complex LO is 155 to 303 MHz. 
         [0027]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.