Investigation On Fading Of High Frequency Radio Signals-Books Pdf

Investigation on Fading of High Frequency Radio Signals
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paths can interfere constructively or destructively when Workstation. they arrive at the receiver depending on the phase differ Crossed dipole Anti aliasing. Multi channel, Digital receiver, ence The phase difference between the different waves will Active antenna filter. vary in time due to the dynamic nature of the ionosphere. and therefore causes signal fading at the receiver Multi. path fading will produce a rapid variation in the signal. amplitude with the fading period in the order of several. seconds because it is highly sensitive to slight phase differ Matlab interface. ent between the paths Conversely focussing effects due. to the movement of large scale irregularities will produce. a slower type of fading, In the subsequent sections descriptions of the fading ex Figure 1 The compact Channel Probe. periment along with the experimental results will be pre. sented ers ADC and several digital down converters the full. HF band can be digitised To ensure that no signal alias. ing was present after the ADC stage a high performance. 2 Experimental campaign low pass filter with 80dB attenuation at the Nyquist fre. quency of 32MHz was used Configuration of the channel. probe can be done in software via the MATLAB interface. The experimental campaign consisted of observing the. fading of HF signals propagating through the ionosphere. The main objectives of the experiment were 1 to char 2 2 Experimental details. acterise the fading of HF radio signals propagating in the. ionosphere and 2 to understand the different mecha. Data were collected over the three days 29th to 31st. nisms that cause the different types of fading In this pa. of March 2005 With the assistance of personnel from. per results on the characterisation of the different types. Defence Science and Technology Organisation dedicated. of fading are presented Ionosonde data recordings for. transmissions of HF radio signal for the sole purpose of. the experimental period have recently become available. this experiment were obtained from the Jindalee Over the. so objective 2 will also be achieved shortly and reported. Horizon OTH radar transmitter located in Alice Springs. The signal was received in Darwin which resulted in an. The main experimental apparatus is the compact chan ionospheric path 1220 km aligned almost in the north. nel probe which was developed specifically for this experi south direction as shown in Figure 2. ment Details on the channel probe and the suitable trans. mitter source will be given in the following section. 2 1 Compact channel probe, The compact channel probe was designed to include a. number of important features it is portable and easily. reconfigurable and is able to monitor horizontal and ver. tical polarisations simultaneously A block diagram of the. channel probe is shown in Figure 1, To achieve portability short dipole active antennas were. used An active circuit performs the impedance match Figure 2 The 1220 km propagation path between Alice Springs. ing 5 which enables good performance over the full HF and Darwin. region Two dipole active antennas are arranged in the. cross dipole configuration to allow the monitoring of both Each day during the experiment HF radio signals were. vertical and horizontal polarisations transmitted from 15 00 until 21 00 local time LT The. centre frequency and bandwidth of the signals were based. The antennas were connected to a two channel digital re on the spectral availability at the time of transmission. ceiver Software radio techniques 2 were used to develop determined by the frequency management system FMS. a fully configurable channel probe with tunable centre of the Jindalee OTH radar 3 On the 30th of March two. frequency and variable bandwidth As the digital receiver bands of frequency were available from around 16 00 until. card has two high performance analog to digital convert 21 00 LT viz 10 677 MHz and 10 858 MHz with band. widths of 46 kHz and 48 kHz respectively The transmit. Time fading at 10 6667 MHz 30 Mar 2005 18 42 11 Frequency fading at 55 5633 s 30 Mar 2005 18 42 11. vertical vertical, ted signals were Frequency Modulated Continuous Wave.
25 horizontal 25 horizontal, FMCW with a full bandwidth sweep repetition rate of 35 35. Signal Power dB, Signal Power dB, Unfortunately the radar was not set up optimally The 55 55. geographic footprint of the transmit beam cycled between 65 65. adjacent regions every 14 seconds Thus the signal level 70. 10 655 10 66 10 665 10 67 10 675 10 68 10 685 10 69 10 695. Frequency MHz, was observed to vary in a step fashion with a 14 second. period To overcome this problem the data were power Time fading at 10 654 MHz 30 Mar 2005 18 42 11 Mode Index 132. Freq fading at 60 744 s 30 Mar 2005 18 42 11 Mode Index 132. compensated with an empirically calculated factor 10. horizontal 10, horizontal, Signal Power dB, Signal Power dB. 3 Results 40, Our analysis concentrated on the data collected on the 80.
10 66 10 67 10 68, Frequency MHz, 10 69 10 7, 30th of March 2005 because of its completeness Using c d. the FMCW processing techniques 1 and synthesised ver Time fading at 10 654 MHz 30 Mar 2005 18 42 11 Mode Index 124. Freq fading at 60 744 s 30 Mar 2005 18 42 11 Mode Index 124. sion of the transmitted FMCW waveform the received 10. horizontal 10, horizontal, chirp signal can be demodulated down to baseband The 0. Signal Power dB, Signal Power dB, various receive mode time delays are translated into dif 20 20. ferent frequency offset bins and thus can be separated. different propagation modes will have greatly different 50 50. time delays The Kaiser window was used in the signal 70 70. processing to suppress the side lobes and thus ensure the 80. 10 66 10 67 10 68, Frequency MHz, 10 69 10 7, absence of false modes 6 e f. The spectrogram of the received modes in the 10 677 MHz Figure 4 Fading behaviour of the signal commencing at 18 42 LT. band at 18 42 LT local sunset period is shown in Fig for vertical solid line and horizontal dash line polarisations a. ure 3 Multiple propagation modes are clearly present at Multi path time fading b Multi path frequency fading c Time. fading for the first received mode d Frequency fading for the first. this time The presence of the different modes indicates received mode e Time fading for the second received mode f. that multi path fading will probably dominate during this Frequency fading for the second received mode. In this paper time fading is defined as the signal strength almost periodic sinusoidal nature of time fading this has. variation in the temporal sense and frequency fading is the signature of multi path fading due to two or more dif. defined as the signal strength variation across the fre ferent propagation modes Examination of the frequency. quency bandwidth The time and frequency fading be fading behaviour shows that the fading was not flat across. haviour of the combined modes are shown in Figure 4 a the frequency bandwidth the signal level changes quite. and b respectively With the short time period and the rapidly This has important implications for modern high. bandwidth HF systems i e the assumption of flat fading. Mode diagram for 10 677 MHz channel at 30 Mar 2005 18 42 11. across the available frequency bandwidth is not valid. Utilising FMCW processing techniques it is possible to. determine the fading behaviour of individual modes Time. 2 and frequency fading behaviour of the first received mode. is shown in Figure 4 c and d respectively It can be. Time delay ms, seen that the fading between the two antennas is about.
90 degrees out of phase which indicates the fading mech. anism is polarisation fading This is an important obser. 2 vation because polarisation fading becomes the dominant. 3 80 fading mechanism for each individual mode Looking at. the frequency fading behaviour the out of phase fading. 10 655 10 66 10 665 10 67 10 675 10 68 10 685 10 69 10 695 10 7. between the two antennas is still apparent However the. Frequency MHz, signal bandwidth is not wide enough to observe the full. Figure 3 Mode spectrograms of the 10 677 MHz band commencing frequency fading bandwidth which might be due to po. at 18 42 LT on the 30th March 2005 larisation fading Similar behaviour is shown in Figure 4. e and f for the second mode This is consistent with Time fading of 10 677 MHz at 19 25 LT 1 F mode. simulation results for polarisation fading 7 vertical. 5 horizontal, Figure 5 displays the spectrogram of the received modes 0. in the 10 677 MHz band at 19 45 LT which is well after 5. Signal Power dB, sunset It is clear that only a single mode was present and 10. the time and frequency fading behaviour of this mode are 15. shown in Figure 6 The signature of polarisation fading 20. is apparent but at a slower rate compared to polarisation 25. fading during sunset This confirms that polarisation fad 30. ing is the dominant signal fading mechanism for a single 35. mode propagation path, 0 500 1000 1500 2000, We now examine the extended time fading behaviour of. the received mode in the 10 677 MHz band commencing at. Figure 7 Extended time fading behaviour of 10 677 MHz sig. 19 25 LT in Figure 7 In addition to the polarisation fading nal commencing at 19 25 LT for vertical solid line and horizontal. signature there is also a slower variation in the amplitude dash line polarisations. of the signal peaks This slower variation was observed. in both polarisations and is the signature of amplitude. VH A f t cos f t 1, Mode diagram for 10 677 MHz channel at 30 Mar 2005 19 45 30.
VV kA f t sin f t 2, where A f t is the amplitude component of the received. 20 signal f t is the phase component of the received sig. nal and k is the constant amplitude offset between the. Time delay ms, 0 40 two antennas The constant amplitude offset factor k was. estimated by taking the ratio of long time average of volt. age amplitudes VH and VV k, Rearranging 1 and 2 gives the following for the phase. 10 655 10 66 10 665 10 67 10 675 10 68 10 685 10 69 10 695 10 7. 100 and amplitude components, Frequency MHz, Figure 5 Mode spectrograms of the 10 677 MHz band commencing VV. at 19 45 LT on the 30th March 2005 tan 3, Time fading at 10 654 MHz 30 Mar 2005 19 45 30 Mode Index 132.
Freq fading at 0 s 30 Mar 2005 19 45 30 Mode Index 132. A2 VH2 1 tan2 4, vertical vertical, 10 horizontal 10 horizontal. Using 3 and 4 one can determine the level of contri. Signal Power dB, Signal Power dB, 30 30 bution of polarisation and amplitude fading on the overall. fading of the signal If the amplitude fading component. 60 60 given by 4 is varying rapidly whereas the phase compo. nent given by 3 is steady then amplitude fading domi. 10 66 10 67 10 68 10 69 10 7, nates Conversely if the amplitude component is constant. 0 20 40 60 80, Time s Frequency MHz, a b while the phase varies then polarisation fading dominates. Figure 6 Fading behaviour of the signal commencing at 19 45 LT The results of fading separation for the first received mode. for vertical solid line and horizontal dash line polarisations a. Time fading behaviour of the single mode b Frequency behaviour. in the 10 677 MHz band at 18 42 LT are shown in Figure 8. of the single mode The amplitude fading has a periodicity in the order of sev. eral tens of seconds The polarisation fading factor which. is the amplitude variation due to polarisation fading also. shows a period of the same order This reasonably fast. 3 1 Fading separation, fading is most likely caused by the rapid variation in the.
electron density during the sunset period, Amplitude and polarisation fading may be separated by. simple data manipulation The received voltages at the Figure 9 shows the fading separation results for the period. two orthogonal antennas are given by after sunset Both the amplitude fading and polarisation. fading factors are much slower as expected for a more different types of fading Furthermore the ionosonde data. benign ionosphere compared to the sunset period The enables a more realistic modelling of the fading. jumps in the amplitude fading power are attributed to. the power compensation that was used to overcome the When fully analysed the data set will become an im. effects of the moving radar beam The amplitude fad portant tool for studying the types of fading that one. ing separation was able to detect the power compensation can expect for various ionospheric conditions The data. because of the over compensation applied to the received could also be used to test various mitigation techniques to. signal from both horizontal and vertical antennas combat signal fading such as polarisation diversity and. frequency diversity Also it is of interest to analyse the. Amplitude fading component at 10 654 MHz 30 Mar 2005 18 42 11. Polarisation fading component at 10 654 MHz 30 Mar 2005 18 42 11. effects of signal fading on HF Multiple Input Multiple. Output MIMO systems and whether through MIMO, horizontal. HF multi paths can be put to advantage, Polarisation fading factor dB. Amplitude fading power dB, Acknowledgements, 0 20 40 60 80 0 20 40 60 80. Time s Time s, We thank the Defence Science and Technology Organi.
sation for providing its valuable radar resources for use. Figure 8 Fading separation of first received mode commencing at during this exper. High Frequency HF radio wave propagation in the iono sphere is still widely used The ability to measure and understand the behaviour of the channel and the knowl edge of how the channel will a ect the propagating signals is imperative to ensure the reliability and maintain ade quate performance of modern wide bandwidth HF sys tems An

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