4. PRACTICAL EXPERIMENTS
4.1 Experimental Equipment
4.1.3 Half-Duplex Tactical Communication Link
The word 2 is used for binding purposes, and it is either hexadecimal value of 0x55 or value of 0xAA depending on a binding button’s state in the transmitter. When the binding button is pressed, the transmitter starts transmission at its lowest carrier frequency without FH. Thus, the receiver listens to this transmission and marks up the IDs transmitted. The following words 4–11, also called as payload, are 16-bit channels 1–8 for transmitting tactical information. The transmitter and the receiver used here utilize only six channels so the channels seven (word 10) and eight (word 11) remain unused.
The word 12, according to datasheet [1], is cyclic redundancy check (CRC) bit string which is automatically transmitted after payload. The CRC is cyclic code that is used for detecting errors in transmitted data. The transmitter’s circuit calculates the CRC of the payload by calculating checksum of polynomial and the payload. The payload does not include the preamble or ID codes. Also, the chip A7105 utilizes a CCITT-16 CRC polynomial standard (𝑥16 + 𝑥15 + 𝑥2 + 1). However, according to a source [57], the CCITT-16 polynomial standard uses the polynomial𝑥16+ 𝑥12+ 𝑥5+ 1for an error detection which differs from the datasheet’s polynomial. Measurements with the Matlab showed that the CRC changes depending on the sent frame, but a dependence to these polynomials was not observed.
Finally, last two bits (word 13) are so-called ”stop bits” which importance is difficult to evaluate. It is likely that these two bits give additional margin before the frame ends. Also, these stop bits facilitate decoding when few bits at the end of the frame are known.
To make the MSK output power spectrum more compact, the NRZ data sequence is passed through the Gaussian low-pass filter [41]. This outgoing signal can be called as the GMSK.
Width of the Gaussian filter is determined by the𝐵𝑇value that was already discussed earlier.
The global system for mobile communications (GSM) [41,69,75] utilizes the𝐵𝑇value of 0.3 where as the SRW [9, 28, 37], which is also used in the experiments, utilizes the𝐵𝑇 value of 0.1. Comparison of such spectrums are illustrated in following Fig. 4.5.
0 0.5 1 1.5 2 2.5 3
frequency (MHz) -50
-40 -30 -20 -10 0 10 20 30 40 50 60
power spectral density (dB)
GMSK, BT = 0.1 GMSK, BT = 0.3 MSK, BT =
Figure 4.5. The power spectral density of a traditional MSK, the GSM (𝐵𝑇 = 0.3) and the SRW (𝐵𝑇 = 0.1).
For smoothening the spectrums for an illustration, ten GMSK signals are generated consist- ing of 1750 randomly generated symbols. By converting the GMSK signal to frequency domain and by averaging each of ten GMSK signals at each sample, smoothened spectrums can be obtained. From this can be seen that lower 𝐵𝑇 value reduces the width of the spectrum. Besides, when frequency increases, with the lower𝐵𝑇values, signal’s power spectral density decreases faster.
As concluded earlier, the SRW-like signal is generated which phase𝜃(𝑡)is:
𝜃(𝑡) = ∑
𝑖
𝑠𝑖𝜋ℎ ∫𝑡−𝑖𝑇
−∞ 𝑔(𝑡 − 𝑖𝑇𝑠𝑦𝑚𝑏)𝑑𝑡 (4.6)
where the𝑠𝑖is the NRZ sequence andℎ = 0.5. The𝑔(𝑡)is the same as in (4.2). Modulation index 0.5 results in maximum phase change of𝜋/2radians per data interval [41]. Thus, final GMSK signal𝑠𝐺𝑀𝑆𝐾(𝑡)can be presented as
𝑠𝐺𝑀𝑆𝐾(𝑡) = 𝐴 cos(2𝜋𝑓𝑐𝑡 + 𝜃(𝑡)) (4.7) where𝐴is a signal’s amplitude, that remains constant, and𝑓𝑐is a center frequency of the transmission.
Signals Used In the Measurements
Signal that is used in experiments for tactical communication is randomly generated, cyclic, and transmitted repeatedly with continuous feed. This makes, for example, channel estimation much easier because the transmitted bits are known exactly. However, an implementation of the cyclic signal is here more difficult because the Gaussian filter affects four previous and subsequent symbols. Here, this is solved by connecting two identical NRZ sequence vectors to each other and thus driving this NRZ sequence through Gaussian filter. After this, suitable length of the sequence is selected in the center of this vector.
After Gaussian filtering, the signal is again filtered with finite impulse response (FIR) type of low pass filter to attenuate frequencies beyond an eligible band.
2436 2437 2438 2439 2440 2441 2442 2443 2444 frequency (MHz)
-140 -120 -100 -80 -60 -40 -20 0 20 40 60
power spectral density (dB)
single carrier GMSK four carrier GMSK
Figure 4.6. The finite impulse response filtered GMSK signals.
In the Case OHD, only single carrier is used resulting in 𝐵 = 1.2 MHz, where the 𝐵 denotes bandwidth. In the CaseD, wideband tactical radio link is formed consisting of four adjoining carriers as the result of𝐵 = 4.8MHz. For the single carrier scenario, the
100th-order FIR filter with passband of 1.0 MHz and stopband of 1.4 MHz is implemented.
For the wideband tactical radio signal, the 200th-order FIR filtering is done to each carrier separately with the passband of 0.6 MHz and with the stopband of 0.8 MHz. The final FIR filtered signals’ spectra can be seen in Fig. 4.6.
For Nyquist-Shannon sampling theorem, the𝐹𝑠 > 2𝐵where sampling frequency is𝐹𝑠. This means that the Nyquist sampling criterion requires setting the sampling frequency at least twice the maximum frequency of interest. For the single carrier scenario, oversam- pling factorsixis used when generating the signal resulting𝐹𝑠 of10.5MHz with symbol frequency𝐹𝑠𝑦𝑚𝑏of1.75MHz. Thus, for the four adjoining carriers, the oversampling factor eight is used and it results𝐹𝑠 = 14.0MHz with the same𝐹𝑠𝑦𝑚𝑏. Thus, it can be concluded that for both scenarios the Nyquist-Shannon criterion is easily satisfied.
2400 2410 2420 2430 2440 2450 2460 2470 2480
frequency (MHz) -10
0 10 20 30 40 50
power spectral density (dBm / 100 kHz)
tune jammer barrage jammer adaptive jammer
Figure 4.7. Jamming strategies used in the experiments.
From a jamming point of view, several different jamming techniques are used and these can be seen in Fig. 4.7. Tune jammer is used for jamming the opponent’s receivers with bandwidth of 3 MHz in CaseOHD and with the bandwidth of 5 MHz in the Case D. For an illustration and clarification purposes, only the 5 MHz tune jammer can be seen in Fig.
4.7. The barrage jammer with the bandwidth of 80 MHz is utilized in all three cases, and adaptive jammer is examined in the Case D and CaseORCWith the adaptive jammer, an improvised RC system can be jammed efficiently where all subbands get constantly jammed.
Next, we move on to laboratory experiments where also these jammers and especially, jamming powers are further discussed.