Multicarrier continuous signal

To create the scenario in Table 1, we first start by identifying the symbol duration period of the hopping signal (#6). We select a multicarrier continuous signal and a minimum waveform length defined by the symbol period of the hopping signal. The compiled settings shown in Figure 2 are created as follows:

• Select multicarrier option.

• Select 128 × 3 = 384 symbols as wave-form length from compile settings. This will match the symbol rate to the duration of three frequency hops as shown in Figure 2.

• Select oversampling to be 6.

• Other settings like wrap around can be set to default.

• Select set up menu as show in Figure 3. Assign frequency, modulation scheme, received power, symbol rate, filter and window function for carriers 1, 2, 3 and 4 as per Table1.

Add a burst signal

For carrier 5, which is in burst mode, set the parameters (as per Table 1) and select power ramping from the menu. Set the power to 0 dB for duration of 32 symbols, then select the power to a low value (-60 dB) for the remaining symbols.

Add a frequency-hopping signal

The setup in Figure 4 shows details of how to create the hopping signal. In addition, a power ramp is added to each signal to define the turn-on characteristic of this signal.

1. For transmission 6, which has three hopping frequencies with BPSK modulation, select the first hopping frequency 648 MHz. Then set the parameters for this signal. Set 0 dB for the first 128 symbols through power ramping and -60 dB for the remaining symbols. This will enable frequency 1 (648 MHz) for the first 128 symbols. Then select the next hopping frequency and set power to 0 dB from 129 to 256 symbols and -60dB at the remaining symbols. This will enable frequency 2 (673 MHz) from symbol 129 to 256. Similarly carry out the settings for the third frequency (623 MHz).
2. Power ramping can be added by selecting the power ramp tab. The rise time can be selected for 10 nanoseconds. Power ramping, when combined with the symbol amplitudes above, create a user-defined control of the frequency- hopping radio.
3. Compile and generate the waveform. The spectrum of the combined waveform is shown in Figure 5.

At this point, the steps are completed and the waveform is ready to be generated through the AWG. This underscores how software significantly benefits the designer in validating the capabilities of a SIGINT receiver to identify the various transmissions it is likely to encounter in the field.

Test signal verification

The signal designed with using the software described above is generated through an AWG. This signal is captured in a real-time spectrum analyzer (RTSA) and analyzed for each frequency. The frequency-hopping pattern of carrier 6, consisting of three frequencies (648 MHz, 673 MHz and 623 MHz) is captured on an RTSA as a spectrogram, which is an advanced form of a time vs. frequency plot. The plot is obtained by suitable selection of time window and frequency span. The RTSA screen shot is shown in Figure 6. The frequency-hopping pattern can be seen as a function of time.

Ensuring margin

The modern combat environment rapidly changes. Technological advances in microprocessors, signal processing and waveforms are leading to the development of new types of radios, radar and tactical communications systems. Existing RF intelligence gathering and jamming platforms have difficulty countering current frequency-hopping and combat networked radios, and they will find it more difficult to counter the new generation of software-defined, low-power packet networks and radar systems. Hence, these SIGINT receivers have to be fully tested so that they have enough margin to successfully perform the desired functions when deployed.

The issue is further magnified in the case of a SIGINT receiver. Unlike other traditional receivers where the signal can be easily predicted and tested thoroughly in the lab, a SIGINT receiver is required to receive and analyze unknown modulation at unknown carrier frequency. For example, two emitters closely spaced in carrier frequency with different modulation should be analyzed correctly. Another example could be to test the receiver for a scenario having a frequency-hopping emitter coupled with normal transmission at one of the hopping frequency. The SIGINT receiver should be able to analyze such scenarios accurately. The user also might want to create a SIGINT waveform with low signal-to-noise ratio depicting a far-off and/or low power emitter. Hence, it is imperative that these receivers are tested against all real-world impairments and interferences so that the receiver design is robust and can take enough margin under these unknown conditions that it may have to overcome.

Software such as RFXpress allows the designer to create such scenarios and also add a variety of impairments to the SIGINT waveform which includes IQ impairments like carrier leakage, quadrature error and IQ imbalance. Interference can be added in the form of either sinusoidal or as an offset to the carrier frequency. The user can define up to 10 different paths to simulate the multipath effects on the waveform. The paths can be defined for individual carriers in terms of delay in symbols, amplitude and phase.

Conclusion

The complex, fast-changing environments that SIGINT receivers must operate in create significant test challenges, particularly around simulating the real-world environment in the lab. Software-based tools combined with high-performance AWG instruments provide an easy, effective way to ensure that SIGINT receivers are able to handle these environments. Moreover, the designer can add impairments to ensure that the receiver is robust and can operate effectively in an unknown environment.

Techniques for creating various SIGINT signals have been demonstrated, which include the ability to create various digital and analog modulation including frequency-hopping waveforms using RFXpress software. It also covered the direct digital synthesis of these waveforms using an AWG and verifying it by analyzing it using an RTSA.

ABOUT THE AUTHOR

Iqbal Bawa is a technical lead in Tektronix' Signal Sources Product Line. He has more than nine years of experience in digital signal processing, embedded systems and communication.

Sampath Desai is an algorithm specialist at Tektronix. He is involved in development of signal-generation algorithms for RF and high-speed serial data signals. He has more than 20 years of experience in radar and communication signal processing.

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