Optical Channelizer Evaluation Using Empirical Data and Simulation

Westinghouse Electric Corporation Communication Division under NASA contract NAS3-25865 developed a proof-of-concept (POC) multichannel demultiplexer implemented as an acousto-optic radiofrequency (RF) spectrum analyzer. A detailed analysis of the experimental results indicate that the expected degradation caused by the acousto-optical channelizer is approximately 2.0-dB degradation at 10-5 bit-error-rate (BER) and 3.0-dB degradation at 10-8 BER. This degradation may be quite acceptable when considering the excellent volume, mass, and power characteristics of acousto-optical channelizing relative to other technologies. In addition, system performance can be greatly improved by using digital pulse shaping in the modem and increasing the channel spacing from 40 to 45 kHz for 64-kbps quadrature phaseshift keying (QPSK) modulation. Westinghouse Acousto-Optic Channelizer Westinghouse Electric Corporation Communication Division under NASA contract NAS3-25865 developed a proof-of-concept (POC) multichannel demultiplexer implemented as an acousto-optic radiofrequency (RF) spectrum analyzer that demonstrated the capability of demultiplexing 1000 low data rate frequency-division, multiple-access (FDMA) uplinks.1 The multichannel demultiplexer was implemented as an acousto-optic RF spectrum analyzer utilizing heterodyne detection with a modulated reference. Demodulation was performed using a commercial demodulator. A photo-detector was placed at the focal point of the channel of interest and the signal fed into the commercial demodulator to fully characterize the effect that the optical demultiplexer has on a modulated signal such as quadrature phase-shift keying (QPSK). The acousto-optic spectrum analyzer (Fig. 1) is based on the Bragg interaction between light and sound in a crystal material. An ultrasonic acoustic wave is impressed on the crystal. A portion of a laser beam passing through the Bragg cell is diffracted at an angle proportional to the RF applied to the acoustic transducer. The diffracted beam is amplitude modulated and frequency and phase shifted by the Bragg interaction with the intensity proportional to the power of the applied RF signal. For heterodyne detection with a modulate reference, the output of the signal at the photo-detector is at a common intermediate frequency (IF) and proportional to the amplitude of the communication signal. Thus, the multichannel demultiplexer (MCD) performs both the demultiplexing and downconversion of the composite RF signal. The acousto-optic channelizer was designed to allow 64 kbps of offset quadrature phase-shift keying (OQPSK) modulated information to be frequency stacked with 40-kHz channel spacing. The modem filters that were initially specified and simulated by Westinghouse were 6-pole, 16-kHz Butterworth filters. These modem filters are far from an optimal choice for bandwidth and power efficient modulation. In addition, because of technical difficulties at the onset of the contract that strained available funds, NASA and Westinghouse decided to allow testing with a government-supplied modem, Comstream CM421. This modem is capable of generating uncoded 64-kbps QPSK modulation signals at a 70-MHz IF within a 50-kHz bandwidth. Both NASA and Westinghouse were unable to obtain detailed information on the internal data filters used in the modem. However, NASA subsequently learned that the data filter for the 64-kbps QPSK mode is a 16-kHz equalized 6-pole, Butterworth filter (6-pole, Butterworth filter at one-half the symbol rate). Thus, the overall Westinghouse testing utilized suboptimal filtering and mismatch between the commercially available modem and the optical channelizer. Figure 2 shows the basic test