1、CCIR RECMN*734 92 m 4855212 0518753 TA5 m Rec. 734 131 RECOMMENDATION 734 THE APPLICATION OF INTERFERENCE CANCELLERS IN THE FIXED-SATELLITE SERVICE (Question 58/4) (1992) The CCR, considering a) that one of the principal limitations in achieving greater utilization of the RF spectrum and geostationa
2、ry orbit for the fixed-satellite service (FSS) is mutual interference among satellite networks operating in common frequency bands and serving the same or adjacent geographical areas; b) that inter-satellite network interference protection is primarily achieved by earth-station antenna discriminatio
3、n; Cl lower noise receivers may be used to increase communication capacity; d) that other techniques such as frequency re-use, improved modulation, higher power satellite transmitters and that some of these techniques increase the susceptibility of sateliite networks to mutual interference; e interf
4、erence canceiling devices; f) amplitude and phase, and to phase invert the replica and add it to the wanted signal plus interference, that the deleterious effects of some of these techniques may be reduced through the employment of that the basic principle of interference cancellers is to construct
5、a replica of an interfering signal in both recommends 1. reduce interference or the effects of interference. that interference cancellers should be used, where cost effective, in satellite communication systems to Note 1 - Some examples of interference cancelling techniques are discussed in Annex 1.
6、 ANNEX 1 A survey of interference cancellers for use in the fixed-satellite service 1. Examples of interference cancellers A few examples of these techniques reported in the technical literature are described below. The results obtained by the various investigators were in the absence of any signifi
7、cant thermal noise. 1.1 Baseband interference cancellers For angle-modulated carriers, a method of interference cancellation at baseband has been achieved by mixing wanted and interfering RF signals into the experimental system depicted in Fig. 1. It should be noted that this approach requires that
8、the interference be received directly as the input for the interfering channel. This can be achieved by having a separate antenna directly facing the interference source. The interference signal is mixed with the wanted carrier at RF or IF. The low frequency components resulting from this process we
9、re shown to be the replica of the CCIR RECMN*734 92 W 4855232 0538754 731 132 Rec. 734 baseband interference. Cancellation was achieved by subtracting this replica from the demodulated baseband signal plus interference. Laboratory tests showed this method reduced interference by about 15 dB for a wi
10、de range of operating parameters. FSGURE 1 Block diagram of test Set-up Wanted channel Splitter Demodulated signal NPR 1 and interference channel FM demod , generator GI + I - - - - - - - - - - - - - - - I I I Mixer I I Interference channel l I I I ,I l- + I L J Interfering channel FM, NPR 2 generat
11、or Gp _* mod I . Re- ceiver amp Cancellation device In cases where a separate interfering signal is not available for purposes of control or reference, another technique has been proposed for angle modulation signals. In this approach, the information contained in the envelope waveform of the combin
12、ed wanted and interfering carriers is used to reconstruct the baseband interference. For effective cancellation, the interference source should be separated from the wanted carrier by the sum of the highest modulating frequencies. Analysis and experiments revealed that the detected envelope of the d
13、esired carrier plus the interfering carrier is equal to the demodulated interference component shifted by plus or minus 90“. Once identified, the interference ccan be removed by subtraction. As much as 15 dB of interference reduction was achieved for a range of tested parameters (frequency separatio
14、n, carrier-to-interference ratios, and modulation indices). The best results occurred at the lower modulation indices and larger carrier frequency separations. For frequency modulation (FM) signals where the interfering signal is small and cannot be received separately, it is possible to detect the
15、interference at baseband with a crystal detector. A pure FM signal has a constant envelope and the addition of interference results in amplitude modulation which can be detected, and thus removed by subtraction. However, in laboratory tests, this technique was successful in cancelling only those int
16、erference components which had not experienced spectral foldover. Since only the first order component can be assured of cancellation, the interference must be sinall compared to the wanted FM signal. CCIR RECMN*34 92 W 4855212 0518755 858 = Rec. 734 133 The higher order terms become troublesome if
17、the interference power is significant. They hnit the degree of interference cancellation which can be realized in a baseband cancellation technique since the phasing required to cancel the first order term is not the same as that required to cancel the higher order terms. A preferable approach in th
18、ese circumstances is to cancel interference at RF or IF prior to demodulation, where all orders of the baseband interference spectrum are suppressed. 1.2 Cancellation at IF Another technique investigated for FM systems was to subtract the IF or RF spectra of an interfering signal prior to the receiv
19、er demodulator as shown in the test Set-up of Fig. 2. It was assumed in this case that the interfering signal was available separately. It should be emphasized that the auxiliary channel (interference only) required “virtually identical“ down conversion and filtering as the main channel (wanted sign
20、al plus interference). Also, the electric lengths for the interference signal in the two channels had to be nearly identical in order that the modulation on the two signals at the cancellation point was coherent. Phasing of the cancellation signal was refined with a variable phase shifter. Test resu
21、lts showed that the IF interference spectrum was reduced by approximately 25 to 30 dB across the IF filter bandwidth. The advantages of this technique were: - simplicity, - first order and higher order terms are c,ancelled simultaneously, - cancellation was possible with any value of CU, - lower imp
22、ulse noise was experienced compared to baseband cancellation methods. 1.3 Interference suppression bridge Another passive technique, demonstrated in laboratory tests, uses a bridge network to suppress the interfering signal at the receiver with an auxiliary signai derived from an interference channe
23、l. A simplified block diagram of this Set-up is shown in Fig. 3. Ideally, the bridge circuit output phase angle must remain at 180“ and the power ,amplitude must remain equal to the interference across the desired frequency band and continuously with time. The objective of the test was to determine
24、the performance of the system for various phase angle and power amplitude errors generated by the bridge. Tests were conducted using a signal tone at approximately 3807 MHz and interfering signals at 3 809 zk 4.5 MHz and 3 950 MHz. Suppression of the interfering signals from 15 dB to 50 dB was achie
25、ved with this technique, the latter associated with relatively narrow band interference signals under clear weather conditions. 134 CCIR RECMN*734 92 = 4855232 0538756 794 Rec. 734 4 ! - I CCIR RECMN*73i.I 92 W 4855212 0518757 620 Rec. 734 135 FIGURE 3 Bridge interference suppression system Undesire
26、d signal Bridge circuit Desired I signal Y .( L + r- _* LNA b To receiver coupler (a) System diagram Interference channel (B) phase Amp. Spect. Wanted satellite system for interference cancellation from the ?Moskva? satellite system. The additional feed (a pyramidal horn) was connected to the main o
27、ne via a directional coupler and an electrically controllable microwave phase shifter and attenuator. The angular separation between the wanted and interfering satellites was 3?. This adaptive cancellation system used the distinction between the wanted and unwanted dispersal signals and secured addi
28、tional interference suppression up to 20 dB. 1.5 Adaptivefiltering of narrow-band interference A technique for suppressing narrow-band interference where the frequency of the interferer is unknown (and may even vary in a slow manner) has been applied to a wideband digital communication system. Imple
29、mentation of this technique is depicted in the block diagram of Fig. 7. The system tracks the centre frequency of an interferer and centres a notch filter around that frequency. It functions by making use of the real-time Fourier transformation properties of surface acoustic wave filters. The condit
30、ions required are: - the interferer bandwidth is less than the bandwidth of the desired signal; and - in the Fourier domain, the interferer amplitude is greater than that of the desired signal. An automatic gain control (AGC) feature allows the system to handle a large dynamic range of input signals
31、. Substantial reductions in interference were obtained during system tests. FIGURE 7 Block diagram of adaptive system Thresh. -mP h i- + F; I I I I - AGC: automatic gain control FT : Fourier transform 140 CCIR RECMNx734 72 H 48552112 05118762 T78 m Rece 734 2. Conclusions The examples of interferenc
32、e cancellers described in this Annex are only a sample of the current literature on this subject. However, interference cancellers, as a means of reducing satellite inter-system interference, are still in an early stage of development. Up to the present, the method pursued by the ITU and recommended
33、 by the CCIR has been to impose limits on antenna side-lobe patterns and radiated power flux-densities in order to avoid excessive interference between systems. Interference cancellers have been used in relatively isolated situations where an existing or newly constructed earth station experienced u
34、nexpected interference from a nearby source. The need for additional antennas and signai processing equipment is a burden that a communication network planner would prefer to avoid. More development is required to reduce equipment complexity and costs before interference cancellers are likely to hav
35、e wide application in FSS systems. The results of the experiments carried out in the USSR show that it is possible to use an additional feed in the primary antenna for the cancellation of interference from a neighbouring satellite when the direction to the interfering source is known. This method se
36、ems to be in some cases more cost-efficient compared to the use of an auxiliary antenna. On the other hand, a great deal of interest has been evidenced in the development of interference cancellers for iiiuasystem applications associated with cross-polarization techniques. Since the interfering signal can be characterized and defined internally, the developments in this field are likely to result in practical, commercial equipment in the near future. The products of this type of interference canceller will likely benefit the development of inter-system applications.
