5 Technical basis of co-channel and alternated arrangements for digital fixed wireless systems 5.1 Parameters affecting co-channel and alternated radio-frequency arrangement
This subject is described in recommends 2 of Recommendation ITU-R F.746.
5.2 Shaping filter requirements
Channel shaping filtering may, in principle, be performed at baseband, intermediate or radio frequencies. It must be designed so as to control the overlap of adjacent spectra.
Generally, transmitter and receiver filters, used to control adjacent frequency channel interference and to restrict the receiver noise bandwidth, are designed to have a Nyquist raised cosine roll-off shaping, which theoretically does not give intersymbol interference.
The roll-off factor, , of such a Nyquist filter may be chosen taking into account that, for a theoretical no‑interference condition, the following relation applies:
where 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 reduction of the timing margin for a no intersymbol interference condition, governs practical implementations.
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