1、 STD.1TU-R REC“ SH=85b-L-ENGL 1797 q552LZ 05287b7 978 H Rec. ITU-R SM.856-1 RECOMMENDATION ITU-R SM.856-1 NEW SPECTRALLY EFFICIENT TECHNIQUES AND SYSTEMS (Questions ITU-R 1811 and ITU-R 20311) 1 (1992- 1997) The ITU Radiocommunication Assembly, considering that the radio spectrum is a limited resour
2、ce; that there is spectrum congestion in certain frequency bands in populated areas; that when the system design parameters and geographical distribution of stations are both known, improved a b) c technical planning can enhance usage of the frequency spectrum; becoming increasingly difficult; e) se
3、paration; f, systems; 9 h) j , d) that due to the rapid development of different radio services, sharing conditions in many frequency bands are that in many cases it is difficult or impossible to ensure sharing by conventional geographic or frequency that the advance of technology has made possible
4、the introduction of new spectrally efficient techniques and that the use of interference cancellers could enable otherwise interfering links to operate simultaneously; that special screens improve antenna spatial selectivity; that adaptive antenna systems reduce interference on shared frequencies, r
5、ecommends 1 that interference cancellers, screens and adaptive antennas (see Annex 1 for an example) should be considered for use as additional methods to reduce interference and improve spectrum utilization by enabling otherwise interfering links to operate simultaneously with a minimum of interfer
6、ence. NOTE 1 - Future examples will be included with the objective of increasing spectrum utilization. ANNEX 1 Reduction of base station co-channel re-use distance using adaptive antenna techniques In conventional frequency planning the re-use distances between co-channel base stations in the land m
7、obile service have to be sufficiently separated so that interference does not occur. Studies found that if adaptive antennas are used, additional co-channel base stations can be inserted in a service area such that the resulting separation of all base stations can be reduced by a factor of about 0.6
8、25, with minimal impact on their coverage areas. This reduced frequency re-use distance effectively increases the service capacity of the frequency spectrum, by allowing channels to be used more frequently in a service area, leaving other channels free to cover other service areas. In order to show
9、that using adaptive antennas is a spectrally efficient technology, a study was conducted to demonstrate its effectiveness. I STD*ITU-R RECMN SM-85b-L-ENGL 3997 = 4855232 05287b8 309 2 Rec. ITU-R SM.856-1 1 Reduced re-use distance scenario Simplex Co-channel base stations are usually separated suffic
10、iently (by about 120 km) so that interference does not occur and each operates as a “stand-alone” system. In this study they are purposely situated closer together, at about two thirds of the normal separation distance or 80 km, so that each is “jamming” the other, effectively reducing their coverag
11、e areas. An adaptive array antenna is then deployed at each base station to restore “normal) operation at the reduced separation distance. 2 Characterization of the adaptive antennas The adaptive antenna used is a simple side lobe canceller with auxiliary (AUX) antennas combined with the conventiona
12、l (MAIN) antenna via complex adaptive coefficients (weights; see Figs. 1 and 2). Based on inputs from each element of the array of auxiliary antennas and on the sum of the main signal antenna and the sum of the array of auxiliary antennas, an algorithm determines the coefficients which are used to w
13、eigh the signal of each element of the array of auxiliary antennas. The adaptive array antenna is only activated when the interferer exceeds a certain signal strength level with respect to the desired signal, say 6 dB. The description of this activation is not in the scope of the present study, but
14、could be based on the successful demodulation of squelch tones, for example. 3 Computing coverage area In the study typical values of parameters used for the modelling of minimum re-use separation for the VHF band are summarized in Table 1. FIGURE 1 Sketch of adaptive side lobe canceller proposed fo
15、r simplex PMR re-use base stations T MAIN -0- b I I a STD-ITU-R RECHN SM-85b-L-ENGL 1797 4855232 05287b9 240 Rec. ITU-R SM.856-1 3 Transmitter power Pt =30 W FIGURE 2 Sketch of MAIN and AUX antenna patterns and locations of desired and interfering signals Minimum power received Base transmissiodrece
16、ption Mobile station antenna for the mobile, P, antenna 10 log 10 P,=-139 dBW ht=45m gt= O dB h,=2m g,=OdB AUX Local variability loss at VHF = 14 dB TABLE 1 Simplex LMR base station default parameters at VHF Line losses = O dB Line losses = O dB Using a simple low-angle, flat Earth propagation model
17、, the approximate radius of the base station coverage, R, is computed from: where: Pr : Pt : transmitter power g, g, : h, hr : minimum power received for the mobile gain of the base station antenna and mobile station antennas, respectively heights of the base station antenna and mobile station anten
18、nas, respectively. STD-ITU-R RECMN SM-85b-L-ENGL 3777 4855232 0528770 Tb2 4 Rec. ITU-R SM.856-1 The coverage area was plotted using a terrain data base and propagation modelling program which computes the interference signal strength from point-to-point calculations between two selected base station
19、s. A mobiles desired signal is stepped through all azimuth angles in the coverage area along different signal strength contours corresponding to different received carrier powers at the base receiver, to map out the various coverage areas. The stand-alone coverage area is defined by the minimum sign
20、al strength contours. The jammed coverage contour corresponds to a signal strength 6 dB above the highest interference strength from any other base station. The adaptive base station antenna would be activated only when the mobile is between these contours, This corresponds to an area in which the C
21、/Z ratio of the base station is less than 6 dF3, representing lost coverage. The adaptive antenna restores the coverage in this area, except in the angular sectors in the direction of the jamming base station. 4 Conclusions Re-use distance was reduced by an average factor of 0.625 in the example sce
22、nario. In this study, when the additional co-channel base station is introduced, area computations showed that, without an adaptive antenna, each base station would lose one third of its coverage area due to interference from the others. But with an adaptive antenna only 7.6% of the coverage is lost
23、. Substantial improvements in re-use distance may be achieved with simple suboptimal adaptive base station antennas in land mobile radio (LMR) in the VHF band. This type of adaptive array antenna has been developed into a product for cellular base stations. The promising results demonstrated in this
24、 study show that it can also be applied to land mobile radio systems. Hence reducing re-use distance between co-channel base stations can be achieved with todays technology. Simplex LMR systems were selected because an adaptive array antenna is a cost-effective near-term means of suppressing the resulting co-channel interference and is directly applicable in such systems. .
