1、Rec. ITU-R F.1101 1RECOMMENDATION ITU-R F.1101CHARACTERISTICS OF DIGITAL RADIO-RELAY SYSTEMSBELOW ABOUT 17 GHz(Question ITU-R 135/9)(1994)Rec. ITU-R F.1101The ITU Radiocommunication Assembly,consideringa) that it is preferable to define certain aspects of the characteristics of digital radio-relay s
2、ystems (DRRSs)operating below about 17 GHz in order to facilitate system design;b) that digital radio-relay system characteristics are determined by the gross bit rate, modulation method,spectrum shaping, interference susceptibility and other relevant factors;c) that adaptive techniques offer effect
3、ive countermeasures to adverse propagation conditions and a means ofreducing interference in certain circumstances. These techniques are particularly suitable for large bandwidth systemsand for systems using complex modulation schemes;d) that multi-state modulation is an effective method for increas
4、ing spectrum utilization efficiency;e) that when spectrum efficiency is not a major issue, more simple modulation schemes (up to four states) arealso suitable for low and medium capacity systems,recommends1. that the factors contained in Annex 1 should be taken into consideration in the design of di
5、gital radio-relaysystems operating below about 17 GHz.Note 1 The material contained in this Recommendation is for guidance only. DRRS are not required to have all thefeatures listed herein but can make use of one or several of them, depending on the application for which they aredesigned.ANNEX 1Fact
6、ors to be considered in the design of digital systems operating below 17 GHz1. Categorization of digital radio-relay systemsIt seems advisable to subdivide digital radio-relay systems into the following categories: low capacity radio-relay systems for the transmission of digital signals with gross b
7、it rates up to andincluding 10 Mbit/s; medium capacity radio-relay systems for the transmission of digital signals with gross bit rates rangingfrom 10 Mbit/s up to about 100 Mbit/s; high capacity radio-relay systems for the transmission of digital signals with gross bit rates greater than100 Mbit/s.
8、2 Rec. ITU-R F.11012. Predominant propagation factorError performance and availability are representative parameters featuring digital radio-relay systems. As far asthe propagation path characteristics are concerned, rain attenuation predominates at frequencies above about 17 GHz,while multipath dis
9、tortion predominates at frequencies below about 10 GHz.For this reason, digital radio-relay systems should be mainly designed in terms of unavailability at frequenciesabove 17 GHz and error performance at frequencies below about 10 GHz, while in the range 10-17 GHz both objectivesshould be considere
10、d.3. Modulation and coding techniquesCoding and modulation techniques used are of particular concern to the radio-relay system. Coding consists ofa transformation of the format of the signals in the alphabet to take account of the methods of synchronization,introduction of redundancy in accordance w
11、ith the error-control and/or correction system (forward error correction),spectrum shaping and to meet the requirements of interfacing with the transmission medium or channel. Modulationconsists of transferring information in the baseband signal onto an RF carrier. In general, this is achieved by a
12、singlechange or combined changes in the phase, frequency or amplitude of the RF carrier for radio-frequency transmission.3.1 Comparison of some methods of modulationDifferent modulation techniques may be compared theoretically on the basis of their Nyquist bandwidth andthe normalized carrier-to-nois
13、e ratio. The real carrier-to-noise ratio (allowing for all imperfections) needs to beconsidered in order to define real systems.Detailed information on this subject is given in Appendix 1.3.2 Modulation methodsA suitable modulation method is selected by taking into account the system requirements. F
14、or instance, ifspectrum efficiency is not a major issue and/or high interference tolerance is important, a simple modulation methodshould be used. The features of simple modulation methods are: easy implementation in all frequency bands, robustness against propagation effects, high tolerance against
15、 all kinds of interferences, high system gain characteristics.On the other hand, a multi-state modulation method improves spectral efficiency on a route. Typicalapplications for multi-state modulation methods are high capacity trunk, junction and access networks.Careful design of the multi-state con
16、stellation for QAM schemes might achieve some system gain againstnon-linear distortion while retaining a fairly simple implementation.Consideration of the carrier-to-noise requirement for the same BER when changing from 16-state to 512-statemodulation, for example, shows the need for significant inc
17、reases in peak power, average power and peak-to-averagepower ratio. This places more stringent requirements on the high power amplifier, and will in many cases require the useof linearization measures, such as pre-distortion.3.3 Data coding and error correctionIn order to improve the tolerance of th
18、e modem to various sources of C/N impairment, data coding and errorcorrection techniques may be used for radio systems employing multi-state modulation schemes.The introduction of forward error correction coding is also useful for reducing the residual bit errors. Varioustypes of codes are employed
19、in multi-state modulation schemes. It should be noted that code efficiency is required forband-limited digital radio applications.Rec. ITU-R F.1101 33.3.1 Forward error correctionThere are several types of error correction techniques. One involves the use of error correction codes such asblock codes
20、 and convolutional codes, where redundant parity bits are inserted into the time domain.In the conventional method of forward error correction, the incoming data are passed through an encoderwhich adds redundant parity bits. The combined set of information and parity bits are then modulated and tran
21、smitted.Upon reception, the demodulated data are subjected to a symbol-by-symbol hard decision on each demodulated symbol.The demodulated symbols are then decoded to extract the information bits with appropriate corrections as governed byparity bits.3.3.2 Coded modulationThis method is a technique t
22、hat combines coding and modulation which would have been done independentlyin the conventional method. Redundant bits are inserted in multi-state numbers of transmitted signal constellations (seeAppendix 1). This is known as coded modulation. Representative examples of coded modulation are block cod
23、edmodulation (BCM), trellis coded modulation (TCM) and multi-level coded modulation (MLC or MLCM). In BCM,levels are coded by block codes whereas TCM uses only convolutional codes. On the other hand, different codes can beused for each coded level in MLCM, so MLCM can be seen as a general concept th
24、at includes BCM and to some extentTCM. These schemes require added receiver complexity in the form of a maximum likelihood decoder with softdecision. Tables 1a and 1b provide indications of expected performances.A technique similar to TCM is the partial response, sometimes called a duo-binary or cor
25、relative signallingsystem. A controlled amount of intersymbol interference, or redundancy is introduced into the channel. Hence, the signalconstellation is expanded without increasing the transmitted data bandwidth. There are various methods utilizing thisredundancy to detect and then correct errors
26、 to improve performance. This process is called ambiguity zone detectionor AZD.Additional information on BCM, TCM, MLCM and partial response with AZD is given in Appendix 2.4. Radio spectrum utilization efficiencyRadio spectrum utilization efficiency is an important factor to be considered in the de
27、sign of digital radio-relaysystems and it is determined by the “information transferred over a distance” in relation to the frequency bandwidthutilization, the geometric (geographic) spatial utilization and the time denied to other potential users. The measure ofutilization (U) is given by the produ
28、ct:U = BRF S Twhere:BRF:radio-frequency bandwidth occupiedS : geometric space occupied (usually area for radio-relay systems)T : time.The utilization efficiency E for a system operating continuously in time is expressed by:E = 2 N B BRF Swhere:N : total number of “go” and “return” channels in the ra
29、dio band BRFB : channel gross bit rate.4 Rec. ITU-R F.1101In the case of single high or medium capacity multi-channel long-haul routes, in which spurious emissions areadequately controlled and the dimensions of space and time can be ignored, the radio-frequency spectrum utilizationefficiency reduces
30、 to a bandwidth utilization efficiency EB, given by:EB= 2 N B BRFwhere N includes both polarizations on the route.It can be seen that the use of multi-state modulation methods increases the bandwidth utilization efficiency inthis case by an increase of B.Detailed information on this subject is given
31、 in Recommendation ITU-R SM.1046.5. Technical basis of co-channel and alternated arrangements for digital radio-relay systems5.1 Parameters affecting co-channel and alternated radio-frequency arrangementThis subject is described in recommends 2 of Recommendation ITU-R F.746.5.2 Shaping filter requir
32、ementsChannel shaping filtering may, in principle, be performed at baseband, intermediate or radio frequencies. Itmust be designed so as to control the overlap of adjacent spectra.Generally, transmitter and receiver filters, used to control adjacent frequency channel interference and torestrict the
33、receiver noise bandwidth, are designed to have a Nyquist raised cosine roll-off shaping, which theoreticallydoes not give intersymbol interference.The roll-off factor, , of such a Nyquist filter may be chosen taking into account that, for a theoreticalno-interference condition, the following relatio
34、n applies: x 1where x is the width of the radio-frequency channel normalized with respect to the symbol frequency.The balance between allowable interference level for the chosen modulation format, the increase of peak-to-average carrier power requirements (as reported in Appendix 1) and the reductio
35、n of the timing margin for a nointersymbol interference condition, governs practical implementations.6. Technical bases of performance and availability objectives for digital radio-relay systems employingmulti-state modulationIn order to efficiently utilize the limited resource of radio-frequency sp
36、ectrum, multi-state modulation schemesmay be employed for high capacity digital radio-relay systems. As the number of modulation states increases, theperformance of the modem and the radio system is impaired by several factors. These are: the stability of carrier and clock synchronization circuits,
37、amplitude distortion due to the transmission path, distortion caused by saturation of the high power amplifier, interference from other systems (adjacent channels, satellite, adjacent route, etc.), multipath fading.It is therefore necessary to consider the mechanisms causing the impairments and devi
38、se the appropriatecountermeasures. The following topics should be studied. Fuller details may be found in Recommenda-tion ITU-R F.1093.Rec. ITU-R F.1101 56.1 Factors affecting error performanceWaveform distortion and interference during multipath fading are dominant factors in determining the severe
39、lyerrored seconds in digital radio-relay systems operating below about 10 GHz. Both factors have to be taken into accountwhen designing digital radio systems.The following gives a brief introduction to many methods used to estimate the effects of propagation on theperformance of digital radio.6.2 Su
40、sceptibility to multipath fading6.2.1 System signatureDigital radio-relay systems are particularly affected by the frequency selective nature of multipath fading.Fading effects on a radio may be characterized by a “system signature”. Numerical methods with computer simulationcould be useful for sign
41、ature computation. A “system signature” is basically a static measure of the susceptibility of aparticular equipment to a two ray model of the multipath channel in both minimum phase and non-minimum phaseconditions. It is being used as a basis for equipment comparisons.An important aspect of multipa
42、th fading is its dynamic nature. Therefore dynamic tests should be performed toassure satisfactory equipment performance. Dynamic multipath fading may be simulated in the laboratory by a dynamicsimulator capable of simulating the time sequences of multipath fading. These tests will allow the optimiz
43、ation ofsynchronization and equalizer coefficient adaptation circuits.Further studies are needed to define optimum test sequences for dynamic testing.6.2.2 Normalized signatureAnother approach to the use of waveform factor is the use of a normalized signature constant. This constantcan be derived fr
44、om theoretical signature and/or measurements and can be used to compare modulation methods andcountermeasures such as equalizers and diversity.6.3 Countermeasures to multipath fadingIn order to mitigate the propagation impairments, various countermeasures such as space diversity, frequencydiversity,
45、 angle diversity and adaptive equalizers in the time and frequency domains are described in Recommenda-tion ITU-R F.752.6.3.1 EqualizersTechnological advances in equalization techniques have been very effective in minimizing the effects ofanomalous propagation on digital radio performance. As the nu
46、mber of modulation states increases, the radio systembecomes more vulnerable to multipath fading. Various types of adaptive equalizers are currently in use for digital radio-relay systems and are implemented at intermediate and/or at baseband frequencies. Among these equalizers, decision-feedback eq
47、ualizers and linear transversal equalizers are in common use. A powerful solution is represented by the so-called fractionally-spaced linear transversal equalizer (FSLTE), in which the signal is sampled several (at least two)times within one symbol period. Such a solution shows a better performance/
48、complexity ratio, together with a lowersensitivity to timing phase (when the central tap is not fixed). Moreover, FSLTEs, implemented with a twosamples/symbol sampling, combine “naturally” with receiver over-sampling digital filtering (shaping filters), and withsimplified minimum mean square error (
49、MMSE) algorithms for timing recovery.For optimum performance, the dynamic convergence of FSLTEs should be accomplished by means of “blind”algorithms (as with synchronous LTE), combined with the recursive updating of tap coefficients, reference-tap positionand timing phase, for both initial convergence and channel tracking.6 Rec. ITU-R F.11016.3.2 Multi-carrier transmissionA multi-carrier transmission method in which the symbol rate can be decreased in proportion to the number ofcarriers of the multi-carrier system is also an effective technique to mitigate induc
