Analysis and Solution of Multipath Effect on Wireless Communications of Launch Vehicle

ZHANG Qian, ZHANG Shuo

1 Beijing Institute of Astronautical Systems Engineering, Beijing 100076

2 China Aerospace Academy of Systems Science and Engineering, Beijing 100048

Abstract: The electrical system in a launch vehicle contains multiple wireless communications systems. A large number of prelaunch tests are needed to verify the correctness and reliability of electric systems at the launch site.The complex electromagnetic environment at a launch site will inevitably cause multipath effect which interfere with prelaunch tests. Theory analysis and simulation of multipath effect, combined with engineering practice, can effective identify and quickly correct abnormal situations, and identify methods and measures to eliminate the influence of multipath interference .

Key words: launch vehicle, multipath effect, wireless communications

1 INTRODUCTION

The main feature with a wireless communications channel in a complex electromagnetic environment is multipath propagation. It will cause energy penetration and absorption, as well as reflection, scattering and diffraction when in contact with buildings, objects or persons, during the process of electromagnetic wave propagation which generates a propagation environment filled with reflected waves. The signal of the receiving antenna is the synthesis of reflected waves from different paths[1]. The influence on the multipath wave on the direct wave is related to their relative phase, delay and amplitude.The energy will be superimposed when the multipath wave is in the same phase as the direct wave, and the energy will be cancelled, when the phase is opposite, resulting in a severe fluctuation of the signal to noise ratio. Worst case it will cause the signal to lose lock.

During the ground tests and the flight, wireless communications such as telemetry signal transmission, external ballistic measurement, and the safety control system are used for measurement, communications and information transmission[2].Before the launch, there will be a comprehensive prelaunch test at the launch site to verify the correctness of the rocket's technical state. These tests are carried out with an all-metal structure launch tower which has a complex electromagnetic environment, and the multipath effect can affect tests and the launch when the interference is severe. The multipath effect cannot be completely eliminated, but the abnormal phenomena caused by the multipath effect can be effectively recognized, and the analysis and correction can be quickly performed after the interference occurs, thus effectively ensuring the progress of the test and the mission smoothly.

2 BASIC MECHANISM FOR MULTIPATH EFFECT

The multipath channel between the receiver and the transmitter in wireless communications is likely to occur, as shown in Figure 1, where the path includes a line-of-sight path and a non-line-of-sight path. Line-of-sight (LOS): the direct path between the receiver and the transmitter (solid line in the figure),non-line-of-sight (NLOS): the path that is formed by reflection(dashed line in the Figure).

Figure 1 Schematic diagram of multipath propagation(solid line is direct wave, dotted line is reflected wave)

The mature channel model has been studied for different wireless channels, and the signal energy loss formula under large-scale fading is given[3], as shown in the following equation (1):

where λ is the wavelength, λ=c/f. c is the speed of light, and f is the frequency. n is the path loss index which depends on a specific propagation environment. In a complex electromagnetic environment, n is taken between 1.6 and 6.

Due to the existence of various reflectors when the launching tower is closed, there are multiple transmit paths between the transmitting antenna and the receiving antenna, and due to the spatial variation of the surrounding reflectors, including the personnel movement and tower opening, this will change the propagation path. If the receiving antenna is just at the critical point of the receiving amplitude, the received signal will be weakened by the multipath effect.

3 BASIC THEORETICAL ANALYSIS

Under normal circumstances, the uplink and downlink waves can be received normally. Taking the Doppler speed measuring device as an example. The signal spreads through the signal source, photoelectric conversion device, transmitting antennas, receiving antennas and Doppler speed measuring device. The signal transmission path is shown in Figure 2. The Doppler speed measuring device receives the uplink signal and then locks the coherent and forward transmission.

In the tests, the telemetry parameters of the Doppler speed measuring device showed occasional lock-up fluctuations, which indicated that the uplink signal was out of lock. This phenomenon occurred frequently in the ground tests at the launch site,which seriously interfered with the test progress. The following is a theoretical analysis for this phenomenon.

Basic on the parameters, the total gain of the photoelectric conversion device was about 30 dB, the loss of the fiber was about 1 dB, and the cable loss was about 15 dB, so the total gain of the ground antenna is about 14 dB, giving the ground antenna transmit EIRP value:

In the above formula, Pzis the output power of the ground signal source, Gtis the gain of the ground transmitting antenna.The receiving power at the launch vehicle Pris:

In the above formula, Gris the gain of receiving antenna, Ldis the cable loss from receiving antenna to Doppler speed measuring device, Gr-Ld=0dB, Lsis space loss, under ideal free space conditions, where:

f (MHz) is the signal frequency, d (km) is the distance between transmitter and receiver antenna. Combining (2), (3),and (4) we can see:

The maximum value of gain of the ground antenna Gtis about 0 dB, so we can set the appropriate output power of signal source to match the sensitivity of Doppler speed measuring device according to the formula (5). Compared with the sensitivity, there is a theoretical ～20 dB margin to ensure no signal loss.

Figure 2 Doppler speed measuring device signal path

In an ideal environment, wireless signals will not be affected by the multipath effect, but in complex electromagnetic environments, spatial attenuation will change dramatically[4]. According to the theoretical analysis, when the distance between transmitting antenna and receiving antenna is determined, the above formula (1) is reduced to :

The path loss is highest 46 dB at n =1.6, and least 41.8 dB at n = 6.

Small-scale fading (fast fading) caused by the movement of personnel or objects can cause dramatic changes in signal power. The electric field strength at the receiving antenna is shown in the formula (7).where the antenna gain of the direct signal angle φdis Gφd,and the antenna gain of the reflected signal angle φris Gφr, the reflection coefficient is a function of the incident residual angle φ and the operating frequency w0, and the phase caused by the path difference is θ. It can be seen from (7) that the electric field strength at the receiving antenna changes drastically with the result in parentheses in (7).

According to the theoretical analysis, System View simulation software is used to build a multipath effect simulation model, as shown in Figure 3.

Figure 3 Multipath effect transmission simulation model diagram

In the model, dual-frequencies f1and f2are combined and phase-modulated to the carrier wave fc. The effect of the multipath effect can be judged by observing the spectrum change at the receiver. The number of the paths and the maximum delay time which represent the characteristics of multipath effect can be set by software. These parameters determine the severity of the multipath effect.

Figure 4 shows the power spectrum density without multipath effect model. The power is strongest at carrier wave fc.The spectrum type with the lateral tones f1and f2at upper and lower side bands are orderly and symmetrical and the power of other modulation frequency is weaker than useful frequency.This is a very standard phase-modulated signal power spectrum.

Figure 5 is the power spectrum density with the multipath effect modeled, with the number of paths N Path=1000, and the maximum delay time T delay (max) =1×10-3s. The power is strongest at carrier wave fc, but the power spectrum of lateral tones f1and f2has been changed and the signal power distribu-tion has become asymmetric, the power of the upper band has become stronger and lower band has become weaker.

Figure 6 shows the power spectrum density with the multipath effect modeled, with the number of paths N Path=2000,and the maximum delay time T Delay (max) =1×10-3s. The power is weakened at the carrier wavefc, but the power of lateral tonesf1andf2in the lower band is greater than the carrier wave which will cause a serious impact.

Figure 4 Power spectrum density without multipath effect model

Figure 5 Power spectrum density with multipath effect model

Figure 6 Power spectrum density with multipath effect model

When the number of paths remains unchanged, but the maximum path delay is set to 1×10-2s, the power at the carrier frequencyfc, becomes weaker, and the power in the upper and lower sidebands of the side tonesf1andf2become stronger,completely exceeding the carrier power, and the multipath effect is more influential.

According to simulation results, the multipath effect can cause the amplitude at different frequencies to change. The influence of the multipath effect varies with the number of paths and the maximum delay time. In the worst conditions, the useful signals can been suppressed in unwanted ones. There are many reflectors in the tower and personnel movements, hence abnormal phenomena will be frequent inducing multipath effect.

4 ENGINEERING VERIFICATION

Verification tests were carried out at the launch site. The first state is that the launch tower is closed, the electrical system of rocket is powered on, and the receiver antenna of the Doppler speed measuring device is disconnected. The receiver is connected to a spectrum analyzer to monitor the signal strength, and a person is weaving between the receiver and the transmitting to simulate interference. It can be observed that signal at the spectrum analyzer is nearly the same as the receive sensitivity. It means that the movement has a great influence on the signal strength of the wireless communication. Table 1 data shows that movement by personnel can cause fluctuations of the power strength up to a maximum of 20 dB.

Table 1 Strength test result when person movement

The second state is the free-space field environment when the launch tower is completely open and there is no personnel movement. Table 2 data shows that the power strength is stable at 1-3 dB. Then when gradually reducing the output strength of signal source, it can be observed that the wireless channel becomes unlocked when the power strength is turned down to about 20 dBW.

Table 2 Strength test result when tower is open

Based on the verification tests, we can see personnel movement can cause fluctuation of the wireless power strength. At the same time, the position and angle of the ground transceiver antenna will change slightly after the opening and closing of the tower, as the wireless signal wavelength is in the order of cm.As long as the antenna has centimeter-level movement, a rapid fading phenomenon maybe caused, and large fluctuations in the amplitude above 15 dB maybe seen. These two factors both cause a drastic change in the amplitude at the receiving antenna.The multipath effect in the complex electromagnetic environment contributes significantly to the signal transmission anomaly.

5 METHODS TO SUPPRESS THE INFLUENCE

Considering the problems of the signal loss, the influences of multipath effect can be suppressed from three aspects: the source, the transmission path and the receiving device[5].

5.1 The Source

(1) Select a high performance antenna. For the transmitting antenna and the receiving antenna we can select an anti-fading antenna or an anti-transmitting antenna array, and at the same time, a directional antenna array can be selected, which has strong directivity and can achieve high gain in the direction of receiving signals.

(2) Increase the output level of the source or shorten the distance between the transmit antenna and the receive antenna.Improve the level of the receive signal, ensure that the signal power reaches the sensitivity of the receiving device.

5.2 The Transmission Path

(1) Minimize or restrict the objects in the signal transmission path. By properly arranging the objects in the transmission path,raising the antenna, and adjusting the direction of the antenna,the transmission antenna and the receiving antenna should be kept as close as possible to line of sight. Within the communication range, try to avoid the generation of regular multipath fields and reduce the possibility of superposition.

(2) In the closed space of the launch tower, absorbing material can be properly laid to reduce the generation of reflection signals, which can effectively reduce the influence of the multipath effect[6].

(3) Reasonably set a wireless signal monitoring point in the transmission path, monitor the electric field strength as much as possible, and comprehensively evaluate the influence of the multipath effect on signal fading, and take corrective measures in time.

5.3 The Receiving Device

(1) Diversity reception. The signals are dispersed, and multiple receive signals are simultaneously obtained at the receiver. These signals are appropriately combined to form a total received signal, which enhances the signal strength and avoids multipath interference. Diversity reception is divided into three classes: spatial diversity, frequency diversity, and time diversity[7].

(2) Using spread spectrum technology[8]. The spread spectrum technology uses a high-frequency spread spectrum sequence to spread the spectrum range of the signal at the transmitter, and then restores the signal at the receiver[9]. After restoring, the signals are aligned and superimposed in phase,thereby improving anti-multipath performance[10].

(3) Mount with a diameter reducing plate. Applicable to the case where the transmitting antenna and the receiving antenna are fixed in position and the transmission path is fixed. A metal suppression plate is installed in some directions of the receiving antenna to shield the antenna from reflected signals.

5.4 Application of Solution

In the case of abnormal signal acquisition, after the following additional measures are taken, the abnormal signal phenomenon caused by multipath effect disappears and the signal transmission returns to normal:

(1) Replace the high-frequency cable to reduce the distance between the antenna of the signal source and the receiver, en-hance the useful signal power, and reduce the influence of the reflector on the signal at the same time;

(2) Adjust the antenna direction and monitor the spectrum signal at the uplink channel end until the spectrum at the receiving antenna is within the normal range;

(3) Improve that intensity of the output signal of signal source, so that the intensity of the uplink signal is more than 20 dB higher than the sensitivity of the receiver.

6 CONCLUSIONS

In the ground tests at the space launch site, there are many types of rocket wireless systems. The wireless electromagnetic environment is relatively complex, and multipath effect cannot be completely eliminated. We verified the influence of multipath effect with both theoretical analysis and engineering experiments and proposed various methods to reduce the influence,thus ensuring normal operation of the wireless system improving the reliability and quality of the ground tests.