Abstract:
A method of routing a frequency hopping signal to one of two transmitting antennas in a multiple carrier communication system is disclosed. The method comprises storing known a-priori information regarding frequency hopping signal assignments, scanning a transmitter frequency and antenna path assignment table to determine the closest existing transmitting frequency and its antenna to the frequency hopping signal to be assigned, assigning the frequency hopping signal to an available path on the other antenna, and updating the transmitter frequency and antenna path assignment table. In addition the method includes determining whether the assigned path on the other antenna conflicts with existing frequency assignments, and preempting the antenna assignment when a conflict exists.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio frequency communications systems, and more particularly to a method for routing frequency hopping signals to antennas in a multiple carrier communications system. 
     2. Description of the Prior Art 
     Communication systems must be able to simultaneously broadcast and receive multiple radio frequency (RF) signals that share the same frequency spectrum while being susceptible to very little system degradation. This entails isolating the transmit and receive functions of the communication system, as well as, in the case of multiple transmit antennas, isolating each transmit antenna from the other transmit antennas when more than two channels are simultaneously transmitting. Isolation may be accomplished through techniques such as phase cancellation, frequency separation, and spatial separation. Spatial separation mandates that not only the transmit and the receive antennas be separated, but also that the multiple transmit antennas be separated from each other. 
     Achieving isolation between the antennas in a communication system is extremely difficult since there always is a signal path between the antennas, commonly referred to as a “back door path”. The existence of this path makes the communication system susceptible to the deleterious effects caused by intermodulation products that may be within the transmit filter bandwidth. Intermodulation products are radiated frequencies produced by coupling of two or more carriers between the multiple transmitter amplifier outputs and also by two different frequencies combining at proximate non-linear junctions. To improve system performance these intermodulation products must be reduced, in essence by reducing the frequency mixing products that fall within adjacent receiving or transmitting channels of the system. This enables the receiving antenna to receive the relevant transmitted signals and also enables each separate transmitting coupler and antenna combination to radiate relatively clean frequencies. 
     What is needed, therefore, is a method of routing frequency hopping signals to one of two transmitting antennas in a multiple carrier communication system so as to minimize the production of intermodulation products and to improve system performance. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings are addressed and overcome by the present invention which provides generally, a method of routing a frequency hopping signal to one of two transmitting antennas in a multiple carrier communication system. Briefly, the method comprises storing known a-priori information regarding frequency hopping signal assignments, scanning a transmitter frequency and antenna path assignment table to determine the closest existing transmitting frequency and its antenna to the frequency hopping signal to be assigned, assigning the frequency hopping signal to an available path on the other antenna, and updating the transmitter frequency and antenna path assignment table. In addition the method includes determining whether the assigned path on the other antenna conflicts with existing frequency assignments, and preempting the antenna assignment when a conflict exists. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description. 
     FIG. 1 is a flow chart illustrating a method of routing a frequency hopping signal to one of two transmitting antennas in a multiple carrier communication system in accordance with the present invention; 
     FIG. 2 is a block diagram of a hardware router embodiment of the multiple transmitter two-transmitting antenna communication system using the present invention; 
     FIG. 3 is a block diagram of a software router embodiment of the communication system using the present invention; 
     FIG.  4 A and FIG. 4B are two tables illustrating the assignment of a transmitter frequency to an antenna path in accordance with the present invention; and 
     FIG. 5 is a graph of a simulation output showing the raw bit error rate versus range for 16 SINCGARS VHF radios comprising a communication system utilizing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the flow chart of FIG. 1 a method of routing a frequency hopping signal to one of two transmitting antennas in a multiple carrier communication system  10  in accordance with the present invention is shown. 
     The invention is described with reference to FIG.  2  and FIG. 3 which are hardware and software implementations of the algorithm shown in FIG.  1 . In the FIGS. the symbols or letters indicated below are defined to have the meanings as follows: Rx means receive, Tx means transmit, BPF means bandpass filter, LNA means low noise amplifier, T/C means timing and control, ICS means interference cancellation system, EX or Ex means exciter, R/T means receive/transmit, Modem means modulator/demodulator, N means the number of channels, FAA means frequency arbitration algorithm, and Assy means assembly. 
     With reference to FIG. 2 the hardware implementation of the algorithm is shown in a multiple carrier communication system  20 . The communication system  20  is a conventional radio system, generally used in an aircraft, that comprises a transceiver, a multiple signal receiving function and a multiple signal transmitting function. A radio baseband backbone  22  routes and switches digital information received from the signal receiving function  24  and transmitted by the signal transmitting function  26  throughout the radio system. A plurality of N modems  28  when acting in the receive mode demodulates the baseband signal from the Rx channel into digital or analog information that is routed and switched by the backbone  22 . When acting in the transmit mode, the corresponding modem  28  modulates a radio frequency (RF) carrier using digital information. A receive transmit switch determines the state of the modem  28 . The backbone  22  and the modems  28  form part of the transceiver as will be subsequently described. 
     In the demodulation process, a signal is received in the receiver Rx by the receiving antenna  30 . The received signal is applied to a band pass filter BPF which limits the RF front end Gaussian noise. For example when VHF-FM signals are received, the BPF is set for 30-90 MHz. The filtered signal is further processed through an interference cancellor system ICS and amplified by a low noise amplifier LNA that sets the cascaded noise figure of the RF front end and provides gain to offset noise introduced by the splitter or the N receive-tuned preselectors. The splitter divides the wideband RF signal to feed the N receive-tuned preselectors. The receive-tuned preselectors are each a narrow bandwidth filter having about a 3% bandwidth to further reduce the Gaussian noise and reduce the signal power of adjacent channels to prevent desensitization of the receiver front end. The ICS also includes N RF samplers  40  which feed forward part of the transmit signal that will be transmitted by the transmitting antennas  44  or  46 . In the preferred embodiment, the ICS is an ANC-10932 interference cancellor system. 
     The transceiver comprises the R/T function, the Rx, the Tx, the modems  28  and the backbone  22 . The R/T function recognizes that conventional receivers either transmit or receive, but not both simultaneously. The R/T is a switch that isolates the two functional subsystems of the transceiver, the receiver thread and the exciter thread. The Rx is the receiver that converts the modulated received RF signal to baseband to be processed by the modems  28 . 
     A master controller  32  comprises an embedded processor, memory and computer software that is programmed to respond to timing and control T/C signals and serves to control all aspects of the radio system by generating other T/C signals as shown in FIGS. 2 and 3. 
     The transmitting portion Tx of the transceiver includes the exciter, Ex or  34 , the router  36 , the ICS  38  that includes an RF sampler portion  40 , two hopping filter multicoupler power amplifier (PA) assemblies which combination is designated by the numeral  42 , two bandpass filters  45  and two transmitting antennas  44  and  46 , respectively. The exciter  34  includes N channels, each of which produces the RF carrier and has means for impressing digital or analog information on it. The router  36  includes an N×N switch matrix and appropriately and in accordance with a routing algorithm  50  and the present invention routes signals from the Tx output of each exciter to either of the two hopping filter/multicoupler/power amplifier assys  42 . The dynamic router  36  under command and control of the routing algorithm  50  uses a different exciter for every hop in frequency. The RF samplers  40  of the ICS samples the RF output from the router  36  and feeds this signal to the ICS  38  to be processed and inverted and then coupled to the Rx thread. The hopping filter/multicoupler/power amp assy  42  includes a multicoupler, a power amplifier assembly and a transmit follower filter to reduce the intermodulation products caused by the non-linear properties of the power amplifiers. The multicoupler couples a plurality of carrier signals from N radios to one antenna  44  or  46 , as appropriate. The power amplifier increases the amount of radiated power to be sent to the Tx antenna  44  or  46 . The BPF  45  is a bandpass filter. A post power amplifier RF sampler  56  is similar to the RF samplers  40  at the output of the router  36 . It should be understood that the post power amplifier transmitted energy will contain more intermodulation products but may be used to further reduce the cosite interference in the Rx thread. The two transmitting Tx antennas,  44  and  46 , serve to convert the RF current supplied by the assemblies  42  to an electromagnetic signal to radiate out to free space to its associated receivers. As will be described the invention concerns a method of routing the frequency hopping transmitted signal to one of the two transmitting antennas  44  and  46 . 
     Referring also to FIG. 3 the software implementation of the router/algorithm  50  in a conventional RF radio is shown. Many of the elements illustrated are similar to the elements comprising the hardware implementation of the router shown in FIG.  2  and described previously. More particularly, the routing algorithm  50  is provided to the master controller  32 , which provides signals to the radio baseband backbone  22  and the digital N to N router  36 . Feedback signals from the backbone  22  and the router  36  are also fed back to the master controller  32  as are timing and control T/C signals from other elements in the system. The exciter  34  receives baseband waveform information modulates and upconverts it to provide digital information for RF carrier conversion. Generally, this is a 16K bit per second voice data stream. It provides outputs to its output ports which are fixed channels. The channels implement the frequency hopping antenna selection in accordance with the routing algorithm. 
     Now, refer back to FIG.  1  and to FIG.  4 . FIGS. 4A and 4B are two tables illustrating the assignment of a transmitter frequency after it has hopped from a frequency of 33 MHz shown in FIG. 4A to a new frequency of 76.5 MHz and thus new antenna path (Path  1 A to  2 B) shown in FIG. 4B using the routing algorithm shown in FIG. 1 to help understand the operation of the present invention. FIG. 4 includes four columns depicting the antenna path, the radio channel number, the frequency in MHz, and whether preemption is to occur. The antenna path means the antenna and the path, i.e. antenna  1 , path A. 
     As shown in FIG. 1, the method comprises a first step  100  of dynamically monitoring the frequency hopping of the two transmitter paths  26 . This requires a-priori information of frequency hopping assignments. This information is well known and is supplied to the decision block  100  by the master controller  32 . Following the monitoring, a determination is made  102  as to whether there has been a change in frequency assignment. If no change has occurred, then the dynamically monitoring step  100  is again queried. If a change has occurred then a determination is made  104  as to whether the radio channel transmitter has terminated its transmission via a particular antenna path. 
     If the transmitter has terminated transmission as shown by block  106  it is removed from its antenna path assignment in accordance with the table shown in FIG.  4 . This information is provided to the dynamically monitoring block  100  by path  108 . If the transmission of a radio channel is ending, which means that the channel is going into its receive mode then the transmit antenna path is de-assigned from the carrier and removed from the antenna path column in FIG.  4 . 
     If the radio channel transmitting is not terminating then a new radio channel frequency is needed, which determination occurs at block  110 . If a new frequency is not needed then the algorithm returns to the dynamically monitoring frequency hopping block  100 . 
     If a new radio channel frequency is required then at block  112  the radio channel is assigned to an optimal antenna path. This requires dynamically scanning or searching the frequency column in the table of FIG. 4 to find the closest existing transmitter frequency (from column  2 ) and its antenna path (from column  3 ). The radio channel is assigned to an available path on the opposite antenna from the nearest frequency neighbor. If the radio channel is to be assigned to the same antenna, then no change in the antenna path is made. This assumes that preemption is barred as will be subsequently explained. When the new radio channel is assigned and the optimal antenna path is provided then the determination is made in block  114  whether this assignment conflicts with any existing transmitter-antenna-path assignment. This determination utilizes a frequency arbitration algorithm (FAA) that is programmed into the memory of the master controller  32  and which controls the co-channel and the preempted signal. Advantageously, this determination enhances the performance of the antenna multicouplers. 
     The frequency arbitration algorithm determines if interference and system performance degradation will occur if a radio channel is assigned to a given transmitting antenna path. If interference and degradation would occur then any transmitting signal assignment can be preempted to allow for graceful degradation. 
     If the conflict is deemed to occur then the radio channel for the transmitter is preempted at block  116  and the preemption column of the tables of FIG. 4 are updated. This information regarding preemption is supplied via path  118  to the dynamic monitoring block  100 . 
     If there is no conflict then a determination is made  120  to route the radio channel to the assigned antenna path and to also update the assignment table of FIG.  4 . The radio channel is routed to a designated transmit antenna path via router  36 . The routing and updated assignment information are supplied via a path  122  to the dynamic monitoring block  100 . 
     In operation, there are certain characteristics built into the algorithm. First, it must be recognized that the router  36  can assign a radio channel to either antenna path  44  or  46  as long as there is capacity to do this in the system. However, if the transmitting antenna  44  is at full capacity then no decision is required to be made about routing the channel or frequency. Hence, the channel shall not be routed to the full capacity transmitting antenna  44  and it must be routed to the antenna  46 . Moreover, it is assumed that the radios are transmitting on average fifty percent of the time. The probability of there being an open slot on an antenna is equal to 1−M/2 N, where M is the number of antennas and N is the number of radios. Assuming a random distribution of signals between the antennas, for example, for eight signals being provided to the two antennas, there is an open slot on each antenna 93.7 percent of the time. 
     Transmitter routing works in accordance with the following. The distribution of signals to each antenna is known. The hopping frequencies change constantly at a given hop rate. When a carrier hops it will be assigned to the antenna that allows for the maximum spacing between frequencies. Also, hopping radios are not necessarily synchronized with respect to actual hop times. Therefore, coherency between alternative exciter/up conversion paths may be required to allow the assignment of any radio in real time to the appropriate antenna path. This can be provided by the appropriate timing and control (T/C) signal. 
     When two frequencies have overlapping filters, the third order intermodulation products that are generated will be attenuated by the isolation between the two transmitting antennas. Also, it should be recognized that if the amplifier is turned off during a hop, bit errors occur. Should this happen the message may not arrive at a remote receiver or it is unintelligible. However, interleaving allows for graceful degradation even if an entire hop&#39;s worth of data is lost. 
     Assuming that the two frequencies are within the transmit filter pass band, they can be separated or routed through a switch matrix to the two isolated transmitting antennas  44  and  46 . Alternatively, if a software radio implementation is used the baseband data can be routed through the software radio backbone  22  to the appropriate hardware exciter/transmitter path driving the appropriate antennas. This will reduce the intermodulation product power by between 10 and 15 dB. 
     Now, with reference to FIG. 4, antenna  1  means transmitting antenna  44 , antenna  2  means transmitting antenna  46  and there are eight paths, namely A-H associated with each antenna. As shown in FIG. 4A radio channels  7 ,  3 ,  15 ,  11 ,  8 ,  1  and  12  are assigned initially to the transmitting antenna  44 . No channel is assigned to antenna path  1 D. Radio channels  2 ,  5 ,  14 ,  16 ,  9  and  13  are assigned to the transmitting antenna  46  (i.e. antenna  2 ). No radio channels are Initially assigned to antenna paths  2 B and  2 F. In the system, radio channel  7  transmitting at 33 MHz has completed transmission and is hopping to transmit frequency 76.5 MHz. With reference to FIG. 4A, note that the closest frequency to the radio channel  7  at new frequency 76.5 MHz is transmitting frequency 76 MHz on radio channel  11  on antenna path  1 E. Thus, to minimize intermodulation products the algorithm monitors the frequency hopping in block  100 , recognizes in block  102  that a frequency assignment change is to be made, recognizes in block  104  that channel  7  at 33 MHz has terminated transmitting, removes, in block  106 , the antenna path from the table in FIG. 4A, recognizes, in block  100 , that the channel  7  has hopped to 76.5 MHz, passes through the blocks  102  and  104  and assigns, inblock  110  76.5 MHz as the new frequency for radio channel  7 . Since antenna paths  1 A (now open),  2 B and  2 F are available, the algorithm causes the router  36  to route radio channel  7  to the transmitting antenna  46  and more particularly to antenna path  2 B. This is because the closer frequencies to 76.5 MHz are at antenna path  2 E at 87.8 MHz. No preemption occurs. This assignment is reflected in FIG.  4 B. 
     FIG. 5 illustrates a graph showing the raw bit error rate (BER) versus range in nautical miles nm for 16 single channel ground and airborne radio system (SINCGARS) VHF radios. This shows a sample Monte-Carlo simulation to gauge the effectiveness of transmitter signal routing in accordance with the present invention. In making the calculation it was assumed that 16 radios were synchronous; 15 synchronous transmit radios are turned on 100 percent of the time on a common aircraft and the signals are sorted in frequency space and alternatively assigned to transmit antenna  44  and then to transmit antenna  46 . There was one synchronous receiver. The BPF filters were three percent band width Rx/Tx filters that has 3 dB bandwidths that were approximately 1 MHz wide, and there were 30 dB of separation between the transmitter and the receiver and 15 dB separation between the transmitter to transmitter. There were 40 dB of transmitter ICS and 30 dB of receiver ICS. Co-channel and adjacent channel transmitter carriers were preempted if they could not be routed to separate transmit antennas. Graph  70  shows system performance without routing of the transmit antennas, i.e., random assignment of carriers. Graph  72  shows the raw bit error rate when the transmitters were routed in accordance with the present invention, i.e., using best case frequency sorting and alternate antenna assignments. It can be seen that for a 60 nm range between the receiver and the transmitter on the ground, the BER decreased by a factor of two using the dynamic routing algorithm invention. 
     In a preferred embodiment the transmitting antennas  44  and  46  each includes eight channels. The effectiveness of routing the radios is dependent upon the actual system configuration. Since the decrease in intermodulation product power is in proportion to the transmitting antenna isolation, maximizing the isolation between the transmitting antennas without increasing the coupling between the transmitter and the associated receiver is desirable. The spatial isolation due to the separation of the antennas provides 15 dB of isolation. Back door third order intermodulation products between transmitting power amplifiers are reduced by 15 dB and 30 dB due to isolation. The eight radios associated with each antenna are not hard wired and are not within the same filter band width. The power amplifier is one manufactured by Collins Radio and designated by them as Model Q and has wideband characteristics. The bandpass filters are digitally tunable filters. The multicoupler is similar to the FHMUX (frequency hopping multiplexer) manufactured by Xetron and utilized for VHF or UHF frequencies. Alternatively it may be a switch multicoupler. Also the effectiveness in transmitter signal routing as a co-site interference mitigation technique is achieved because the back door intermodulation products become significant as co-site interference mechanisms once the standard phase cancellor/preselector/preemptive transmitter scheduling co-site measures are applied to minimize the interference effects of the primary carriers. 
     Systems of the type shown could be used for an airborne communications node (ACN), for Army C 2 I Shelters, Army RAH-66 Commanche Helicopters and Navy DD21 Surface Combat Ships. In general this dynamic routing invention can be used for any platform that receives or transmits a plurality of RF signals sharing a common electromagnetic spectrum whether it be airborne, terrestrial-based or sea-based platforms. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been shown and described hereinabove. The scope of the invention is limited solely by the claims which follow.