Today’s Navy needs to support various communication systems for shipboard operations.  There is an ever-growing need to support more systems to maintain a tactical advantage.  Each communication system has a specific function and operation, and usually separate antenna and power amplifiers.  A potential problem with the many antennas being used on the same ship is co-site interference.  When one system is transmitting, another system operating in the same or overlapping frequency ranges, has the transmitted signal coupled into its antenna, potentially interfering with its operation.  By utilizing modern wide-band antennas and power amplifiers, the number of antennas and amount of electronics can be reduced, as well as the interference.  However, since the antenna is shared, a method for switching each communication system to the antenna is required and the capability for simultaneous communication of the systems sharing the antenna is reduced.  By using an array of antennas, multiple systems, such as those that will be placed on board the Littoral Combat Ship can communicate simultaneously, but the potential for co-site interference increases. (Figure 3-1)

By utilizing a wide-band amplifier, several amplifiers can be removed, thus reducing the power consumption, size and weight of the communication systems.  To be more energy efficient, modern wide-band power amplifiers are utilized in the non-linear region of operation, reducing the amount of power consumed for a given gain factor.  This causes the amplifier to distort the signal.  The effects of the distortion is reduced by utilizing a pre-distortion filter that distorts the input signal to the amplifier so that the output appears to be undistorted.  The savings in ship real estate, power, and weight allows the same capability of larger vessels to be installed on those which are smaller, thus increasing their tactical advantage.     

Many of the communication systems on the Littoral Combat Ship operate over the same frequency ranges and would benefit by using wide-band power amplifiers and pre-distortion filters using digital signal processing techniques and open architectures.  Open architecture is a system that conforms to open standards on all its major interfaces.  An open standard is one that is set by a collaboration of industry partners and is openly available for anyone to use such as; VME (Versa Module Europa), PCI (Periphal Component Interconnect), USB (Universal Serial Bus), and RS232.  By utilizing open standards, the number of potential sources for components is significantly increased, and component obsolescence issues can be mitigated.  It is also possible to take advantage of rapid technological advancements by utilizing open architectures.  Approximately every eighteen months, the effective processing performance of computers doubles, and there is also a regular advancement in integrated circuit densities, which can increase the capabilities of the system (Figure 3-2, COTS single board computer card and COTS SDR card). 

For the next generation of combat ships, open architecture will enable reduced cost and increased flexibility of the systems.  System level control, switch control for communication equipment selection, oscillator control, and antenna selection, is supplied by a standard single board computer utilizing a VME bus based chasis.  Also populated in this VME chassis, is a signal processing board consisting of a bank of FPGAs (Field-programmable gate arrays), analog to digital converters, and digital to analog converters.  The digital signal processing for the pre-distortion filter is performed by FPGAs utilizing a down converted and sampled signal.  The signal is mixed down to an intermediate frequency for sampling by an analog to digital converter for input to the FPGA for filtering.  Then, the filtered digital data is sent to a digital to analog converter and mixed back up to the carrier frequency for input to the power amplifier.   Thus, an undistorted signal is output to the antenna for improved signal integrity.  If the bandwidth of the pre-distortion filter is large enough, the multiple communication systems within the same frequency range, can be combined to utilize the same amplifier and antenna set.  Some precautions must be taken to ensure that the communication systems are not utilizing the same frequencies, and have ample channel separation.  Since, the pre-distortion filter is implemented in reconfigurable hardware (FPGA), modifications to the pre-distortion filter are possible even after the system has been produced. 

Through ManTech projects, the Navy can reduce the area, weight, and power consumed by antennas and communications electronics.  Also, open architectures enables expandability and rapid integration of new features with little or no impact on others systems.  On board communication systems can be expanded and adapted to changing mission requirements rapidly.  Deck real estate on small ships is at a premium, so reduction in area consumed by the antennas is a necessity.  Also, through the use of open architectures, the amount of power, weight, and area for on-board electronics can be reduced, and the functionality of the communication systems can be improved.