In multiple channel applications of the next generation communication systems, it is often desirable to develop an antenna system to combine many of these antennas into fewer antennas by using modern wide-band antennas and power amplifiers for multiple transmitters. This can be extremely helpful since in many locations, the deck of a vessel for instance, the real estate is so limited that it is not possible to mount every antenna that is required for each possible frequency range and application. Through the use of wide-band antennas and power amplifiers (PA), the power, weight, and the amount of real estate consumed by both antennas and electronics can be significantly reduced. This would allow smaller vessels to have the same capability as larger vessels and increase their tactical advantage.
In order to maximize the efficiency, modern wide-band power amplifiers must operate in a non-linear region. However, this creates large variations in the instantaneous output power, a condition described as high peak-to-average ratio (PAR). As a result, signals are distorted. In order to compensate for this distortion, linearization techniques must be applied to minimize spectral re-growth or intermediation products created by the non-linearity of wide-band amplifiers. A method called digital pre-distortion is used to distort the signal prior to the input of the power amplifier. The signal is distorted such that the composite output of the power amplifier appears to have linear amplification over the desired frequency range, without distortion. Figure 1-1 shows the digital pre-distortion method which is used to linearize power amplifiers over very broad bandwidths. Figure 1-2 shows an example of the effects of power amplifier nonlinearities on a transmitted spectrum and the potential spectral benefits of applying digital pre-distortion.
The effectiveness of digital adaptive pre-distortion is that it enables high power amplifier linearization of spectrum to combine multiple transmitters across entire VHF and UHF ranges. Harmonic or intermediation products are reduced by more than 15dBm. Additionally, cancellation improves receiver dynamic range.
In the proposed antenna combining, this pre-distortion is performed by digital signal processing techniques utilizing open architectures which enable reduced cost and increased flexibility of the system. System level control, switch control for communication equipment selection, oscillator control, and antenna selection, is supplied by a standard single board computer utilizing a Versa Module European (VME) bus based chassis. Also, populated in this VME chassis is a software defined radio (SDR) board, a wideband transceiver board consisting of field-programmable gate arrays (FPGAs), analog to digital converter (ADC), and digital to analog converter (DAC). The digital signal processing for the pre-distorter 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 which is then 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-distorter is large enough, then multiple communication systems within the same frequency range can be combined to utilize the same amplifier and antenna set. Some precautions need to be taken to ensure that the communication systems are not utilizing the same frequencies, and have ample channel separation. Since the pre-distorter is implemented in reconfigurable hardware FPGA, modifications to the pre-distorter are possible even after the system has been produced.
Benefits
Antenna combining utilizing digital linearization techniques, digital pre-distortion (DPD), offers many benefits for multicarrier RF applications.
- Less real estate required for mounting multiple antennas
- Reduced interference between antennas
- Reduced size and weight
- Fewer communications electronics, low power consumption, and low cost
- High efficiency and high reliability
- Improved functionality of the communication systems
The EMPF engineering staff is able to leverage this experience of developing antenna combining and software defined radio solutions for many of today’s radio applications from military communications to embedded control software for commercial products. For additional information regarding these engineering services, please contact Lead Engineer John Doyle at 610.362.1200, extension 210.
