Recommendation itu-r f. 1101 Characteristics of digital fixed wireless systems below about 17 ghz
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ĐỒ ÁN MÔN TRUYỀN THÔNG DI ĐỘNG K2018, Ho ng Th Hu , Chuong 2 SV, Bai-4, VNU, document tailieudaihoc
6.6 Repeater spacingRain attenuation progressively restricts the repeater spacings in digital fixed wireless systems operating above 10 GHz. The degree of restriction depends on frequency and rain rate. Under such circumstances the multipath fading occurrence probability becomes less significant (see Recommendation ITU‑R P.530). APPENDIX 1 TO ANNEX 1
Different modulation techniques may be compared on the basis of their Nyquist bandwidth and BER versus carrier-to-noise ratio behaviours. Since noise power is limited by the actual channel bandwidth used, it is wise to define a “normalized noise bandwidth” in order to compare different modulation and different implementations of the same modulation. Also the carrier power levels may have different definitions, mainly for multi-state multi-level modulation formats, because of the various levels reached by the carrier during its phase/amplitude transition from one state to the next, as defined in Fig. 1. From Fig. 1, three different concepts of power level may be defined: Win : received maximum steady-state signal power (the value of the carrier mean level related to the highest state of the modulation format) Wav : received average signal power Wp : received absolute peak of the mean power (peak of the signal envelope). The ratio between Win and Wav for multi-level QAM-like modulation formats is reported in Fig. 2. The ratio between Wp and Wav values of the carrier depends upon the pulse shaping and, for an ideal modulation format with 50% TX/RX equally split raised cosine shaping, may be represented versus the roll-off factor as shown in Fig. 3. Practical shaping implementation may give actual results that show a small discrepancy from this Figure. Various normalized carrier-to-noise ratio concepts have been introduced, the most common of which are: (1)
(2) where:
Wav, Win : as defined in Fig. 1 Wn : noise power density at the receiver input n : a bandwidth numerically equal to the bit rate (B) of a binary signal before the modulation process bn fn / n : a bandwidth numerically equal to the symbol rate bandwidth of a binary signal before a modulation process of 2n states. The bit rate B is the gross bit rate along the radio system and takes into account the redundancy possibly introduced for transmission of service or supervisory channels, error control, etc., but not the redundancy introduced by coded modulations (see Note 1). The above carrier-to-noise definitions are related through simple scaling factors: (3) where Win / Wav may be derived from Fig. 2 for QAM-like formats. By taking values of carrier-to-noise ratio corresponding to an error probability of 106, Table 1 is constructed for the most common modulation formats. The parameters listed in Table 1 are, in practical terms, idealistic because the only source of errors is taken to be thermal noise in the receiver. It is recognized that some important factors are not considered, in particular: – signal non-linear distortion introduced by non-ideal transmission characteristics, – the effects of intersymbol interference due to imperfect pulse shaping and I and Q channel interference, – modem imperfection (e.g. timing errors), – phase noise of IF and RF carrier generation. TABLE 1a tải về 179 Kb. Chia sẻ với bạn bè của bạn:
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