1、 Rec. ITU-R F.752-2 1 RECOMMENDATION ITU-R F.752-2*Diversity techniques for point-to-point fixed wireless systems (1992-1994-2006) Scope This Recommendation provides diversity techniques for point-to-point fixed wireless systems. The diversity techniques include those considered in the domain of spa
2、ce, angle, frequency or their combinations. Basic methods for choice of diversity, obtaining or processing the diversity signals are presented in the Annex, which also gives practical diversity effects based on propagation data. Diversity techniques using alternative transmission media or route/site
3、 diversity which may be applied to improve system availability are not handled in this Recommendation. The ITU Radiocommunication Assembly, considering a) that frequency-selective fading may distort and reduce the strength of received signals on line-of-sight and trans-horizon paths and thereby impa
4、ir the performance of a fixed wireless system; b) that the application of diversity techniques is useful in reducing the effects of fading on system performance; c) that the provision of diversity reception is necessary to achieve satisfactory performance on diffraction and trans-horizon paths; d) t
5、hat various techniques for implementing diversity on line-of-sight, diffraction and trans-horizon paths have been studied and are in use; e) that the application of diversity techniques can provide high performance in line-of-sight systems that make efficient use of the radio spectrum; f) that furth
6、er information regarding the use and application of diversity techniques can be found in Recommendations ITU-R P.530 and ITU-R F.1093, recommends 1 that the information presented in Annex 1 should be used when applying diversity techniques to fixed wireless systems (see Note 1). NOTE 1 The ITU-R Han
7、dbook Digital radio-relay systems also contains relevant material on the application of diversity techniques to fixed wireless systems. *This Recommendation should be brought to the attention of Radiocommunication Study Group 3. 2 Rec. ITU-R F.752-2 Annex 1 Diversity techniques for point-to-point fi
8、xed wireless systems 1 Methods of obtaining diversity signals The most common methods are generically described as frequency diversity and space diversity. In frequency diversity, the same information is transmitted over more than one radio channel. In space diversity, the signal reaches the receive
9、r through more than one transmit/receive antenna path. A description of diversity implementation in different systems operating in distinctive propagation regimes requires a more detailed description of space diversity methods. Trans-horizon systems have used diversity on both transmit and receive.
10、They use multiple diversity with full, three-dimensional, flexibility in the placement of antennas, and sometimes use angle diversity in which multiple beams or directivity patterns are formed with a single antenna. Angle diversity provides relatively uncorrelated signals by taking advantage of the
11、variations in the angle of arrival of the scattered energy at the receiver. On line-of-sight paths, space diversity is usually implemented using two antennas at the receiver with a large enough vertical separation to provide two signals in which the impairments due to multipath fading are sufficient
12、ly decorrelated, where the impairments are signal distortion and loss of signal power. Concerns for the performance of digital radios, for which signal distortion is the dominant propagation impairment, have led to the introduction of diversity methods that rely on the non-uniform structure of the i
13、ncident electromagnetic field near the main receiving antenna, instead of large spatial separations, to decorrelate the signal impairments. In these methods, which are called pattern diversity or angle diversity, the diversity signal is derived from a second antenna or beam that has a different dire
14、ctivity pattern or angular beamwidth in the vertical dimension and/or a different boresight elevation angle. These diversity methods, which can be implemented with antennas that are at the same, or nearly the same, height or with multiple feeds within a single antenna, allow diversity to be added to
15、 an existing hop without requiring tower height extensions to obtain path clearance for diversity reception. Although some studies attempt to distinguish between angle and pattern diversity, others use the terms interchangeably. Since the effectiveness of any diversity system depends on the correlat
16、ions of the impairments in the signals, the displacements in space, pointing angles, and frequency are fundamental in determining system performance. 2 Methods of processing signals 2.1 Arrangements Figures 1a) and 1b) show the basic arrangements in which two or four signals are used to derive a com
17、mon output or decision, in the case of some digital systems. Although systems in line-of-sight paths often use frequency diversity to implement 1 + 1 protection switching with the arrangement of Fig. 1a), it is more common to use one or sometimes two protection channels for several working channels.
18、 Figure 1c) shows the arrangement for a 1 + 4 operation. Such protection is often applied in tandem with space diversity operation on the individual channels. Under extreme conditions, such as those sometimes encountered on reflecting line-of-sight paths, space diversity is used in conjunction with
19、1 + 1 frequency protection. Difficult long or over-water paths have also used quadruple diversity, either in the form of quadruple space diversity or in a combination of dual space and dual frequency diversity. Rec. ITU-R F.752-2 3 2.2 Implementation considerations Current systems implement diversit
20、y operation by using combiners at radio, intermediate, or baseband frequencies, or by using switches at intermediate or baseband frequencies. These combiners may employ equal gain, maximum power, or minimum dispersion control algorithms. Because the differences in the transmission performance are sm
21、all, the equipment design choice between these alternatives is usually based on convenience or simplicity. Whether diversity is needed on a given link is determined by the expected severity of propagation conditions and by the characteristics of the transmitted signal. The actual improvement which c
22、an be obtained from diversity depends upon many parameters, which include the influence of in-band amplitude dispersion and inter-system interference on channel performance, and the algorithms which are used in the implementation of diversity. In high speed digital fixed wireless systems, use has be
23、en made of minimum dispersion combining devices (MID) and also of dual reception receivers in which the diversity signals are processed by maximum power combining (MAP) and the aggregate signal by adaptive equalizing (EQ). The use of MAP + EQ produces a synergistic effect, i.e. a considerable decrea
24、se in the bit error rate. The methods used for combining space diversity signals on the basis of the maximum power principle which are used in receivers with MAP + EQ can be classified into the following three categories: MAPEG or maximum power equal gain combining. This method which uses linear add
25、ition has the following drawback: in the case of complete fading of one of the diversity signals, the signal/noise ratio at the combiner output is 3 dB worse than with an ordinary receiver. MAPEG/SW or maximum power equal gain/switch combining. This method uses linear addition and there is no signal
26、/noise ratio loss at the combiner output when the input signal from the first receiver is switched off when it becomes ( 2 + 1) times weaker than the input signal from the second receiver. This method has the following drawback: when switching takes place, a transient process occurs which gives rise
27、 to an increase in the error ratio in high speed digital systems. MAPOPT or maximum power optimized combining. This method does not suffer from the drawbacks inherent in the MAPEG and MAPEG/SW systems. In the MAPOPT system, the diversity branches in front of the combiner contain electronic attenuato
28、rs, the attenuation of which varies automatically in proportion to the ratio of the powers of the space diversity signals received. In a MAPOPT combiner, linear addition occurs when there is a 4 Rec. ITU-R F.752-2 strong/weak signal power ratio of 25) Rec. ITU-R F.752-2 13 4.5 Dispersion statistics
29、Published works have been developed that provide statistical descriptions of the dispersive effects of multipath fading. A theoretical evaluation of the non-diversity and diversity distributions of linear amplitude dispersion, assuming Rayleigh distributions for the individual amplitudes, has been d
30、eveloped in the context of trans-horizon systems. A more recent study developed predictions of the probability that linear amplitude dispersion exceeded a critical value at the output of both maximum power and minimum dispersion combiners. Simulations of frequency diversity with a wideband three-ray
31、 model have been used to provide a simultaneous description of the selective fading in all the channels of a radio band. For a 1 + 1 arrangement this model predicts that, contrary to analogue radio-relay applications, within certain limits, the performance improvement of digital radio tends to increase as the frequency separation between the two channels decreases.
