Microwave Equipment and RF Detail

Trunk Microwave Equipment

  • High cost, large transmission capacity, more stable performance, applicable to long haul and trunk transmission
  • RF, IF, signal processing, and MUX/DEMUX units are all indoor. Only the antenna system is outdoor.

All Outdoor Microwave Equipment:







Split-Mount Microwave Equipment












The RF unit is an outdoor unit (ODU). The IF, signal processing, and MUX/DEMUX units are integrated in the indoor unit (IDU). The ODU and IDU are connected through an IF cable.

Split-Mount Microwave Equipment (2)

  • Unit Functions
    • Antenna: Focuses the RF signals transmitted by ODUs and increases the signal gain.
    • ODU: RF processing, conversion of IF/RF signals.
    • IF cable: Transmitting of IF signal, management signal and power supply of ODU.
    • IDU: Performs access, dispatch, multiplex/demultiplex, and modulation/demodulation for services.



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Microwave Antenna:


  • Antennas are used to send and receive microwave signals.
  • Parabolic antennas and cassegrainian antennas are two common types of microwave antennas.
  • Microwave antenna diameters includes: 0.3m, 0.6m, 1.2m, 1.8m,2.0m, 2.4m, 3.0m, 3.2metc.

Different frequency channels in same frequency band can share one antenna.

Antenna Adjustment (2)

  • During antenna adjustment, change the direction vertically or horizontally. Meanwhile, use a multimeter to test the RSSI at the receiving end. Usually, the voltage wave will be displayed as shown in the lower right corner. The peak point of the voltage wave indicates the main lobe position in the vertical or horizontal direction. Large-scope adjustment is unnecessary. Perform fine adjustment on the antenna to the peak voltage point.
  • When antennas are poorly aligned, a small voltage may be detected in one direction. In this case, perform coarse adjustment on the antennas at both ends, so that the antennas are roughly aligned.
  • The antennas at both ends that are well aligned face a little bit upward. Though 1–2 dB is lost, reflection interference will be avoided.



Antenna Adjustment (3)

During antenna adjustment, the two wrong adjustment cases are show here. One antenna is aligned to another antenna through the side lobe. As a result, the RSSI cannot meet the requirements.

Split-Mount Microwave Equipment – Antenna (1)

Antenna gain

Definition: Ratio of the input power of an isotropic antenna Pio to the input power of a parabolic antenna Pi when the electric field at a point is the same for the isotropic antenna and the parabolic antenna.

Calculating formula of antenna gain:

G = Pio/Pi = (πD/λ)2 * η

Half-power angle

Usually, the given antenna specifications contain the gain in the largest radiation (main lobe) direction, denoted by dBi. The half-power point, or the –3 dB point is the point which is deviated from the central line of the main lobe and where the power is decreased by half. The angle between the two half-power points is called the half-power angle.
‡
Calculating formula of half-power angle:
θ0.5 = (65o ~ 70o) λ/D

Split-Mount Microwave Equipment – Antenna (2)

Cross polarization discrimination

Suppression ratio of the antenna receiving heteropolarizing waves, usually, larger than 30 dB.
‡ XdB＝10lgPo/Px
‡ Po: Receiving power of normal polarized wave
‡ Px: Receiving power of abnormal polarized wave

Antenna protection ratio

‡Attenuation degree of the receiving capability in a direction of an antenna compared with that in the main lobe direction. An antenna protection ratio of 180° is called front-to-back ratio.

Split-Mount Microwave Equipment –ODU (1)




Split-Mount Microwave Equipment –ODU (2)

  • Specifications of Transmitter
    • Working frequency band
      Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13 GHz and
      higher frequency bands are used in the access layer (e.g. BTS access).
    • Output power
      The power at the output port of a transmitter. Generally, the output power is 15 to 30 dBm      .

Split-Mount Microwave Equipment –ODU (3)

  • Local frequency stability
    If the working frequency of the transmitter is unstable, the demodulated effectived signal ratio will be decreased and the bit error ratio will be increased. The value range of the local frequency stability is 3 to 10 ppm.
  • Transmit Frequency Spectrum Frame
    The frequency spectrum of the transmitted signal must meet specified requirements, to avoid occupying too much bandwidth and thus causing too much interference to adjacent channels. The limitations to frequency spectrum iscalled transmit frequency spectrum frame    .

Split-Mount Microwave Equipment –ODU (4)

  • Specifications of Receiver
    • Working frequency band
      Receivers work together with transmitters. The receiving frequency on the local station is the transmitting frequency of the same channel on the opposite station.
    • Local frequency stability
      The same as that of transmitters: 3 to 10 ppm
    • Noise figure
      The noise figure of digital microwave receivers is 2.5 dB to 5 dB

Split-Mount Microwave Equipment –ODU (5)

  • ‡Passband
    To effectively suppress interference and achieve the best transmission quality, the passband and amplitude frequency characteristics should be properly chosen. The receiver passband characteristics depend on the IF filter.
  • Selectivity
    Ability of receivers of suppressing the various interferences outside the passband, especially the interference from adjacent channels, image interference and the interference between transmitted and received signals.
  • ‡ Automatic gain control (AGC) range
    Automatic control of receiver gain. With this function, input RF signals change within a certain range and the IF signal level remains unchanges.

Split-Mount Microwave Equipment –ODU (6)








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