digital microwave communication overview

Digital Microwave Communication Overview

Definition of Microwave

Microwave is a kind of electromagnetic wave. In a broad sense, the microwave frequency range is from 300 MHz to 300 GHz. But In microwave communication, the frequency range is generally from 3 GHz to 30 GHz.

According to the characteristics of microwave propagation, microwave can be considered as plane wave. The plane wave has no electric field and magnetic field longitudinal components along the propagation direction. The electric field and magnetic field components are vertical to the propagation direction.

Therefore, it is called transverse electromagnetic wave and TEM wave for short.

Transmission Methods in Current Comunication Networks

Development of Microwave Comunication

Concept of Digital Microwave Communication

Digital microwave communication is a way of transmitting digital information in atmosphere through microwave or radio frequency (RF).

Microwave communication refers to the communication that use microwave as carrier.Digital microwave communication refers to the microwave communication that adopts the digital modulation.The baseband signal is modulated to intermediate frequency (IF) first . Then the intermediate frequency is converted into the microwave frequency.

The baseband signal can also be modulated directly to microwave frequency, but only phase shift keying (PSK) modulation method is applicable.

The electromagnetic field theory is the basis on which the microwave communication theory is developed.

Microwave Frequency Band Selection and RF Channel Configuration (1)

Generally-used frequency bands in digital microwave transmission:

7G/8G/11G/13G/15G/18G/23G/26G/32G/38G (defined by ITU-R Recommendations)


In each frequency band, subband frequency ranges, transmitting/receiving spacing (T/R spacing), and channel spacing are defined.


7 G Frequency Range

F 0 (MHz)

T/R Spacing (MHz)

Channel Spacing (MHz)

Primary and Non-Primary Stations

7425-7725 7575 154 28 Fn=f0-161+28n,

Fn'=f0-7+28n,(n: 1-5)

7575 161 77110-7750 7275 196 28

7597 196 287250-7550 7400 161 3.5

Digital Microwave Communication Modulation (1)

Digital baseband signal is the unmodulated digital signal. The baseband signal cannot be directly transmitted over microwave radio channels and must be converted into carrier signal for microwave transmission.

Digital Microwave Communication Modulation (2)

The following formula indicates a digital baseband signal being converted into a digital frequency band signal.

ASK: Amplitude Shift Keying. Use the digital baseband signal to change the carrier amplitude (A). Wc and φ remain unchanged.FSK: Frequency Shift Keying. Use the digital baseband signal to change the carrier frequency (Wc). A and φ remain unchanged.PSK: Phase Shift Keying. Use the digital baseband signal to change the carrier phase (φ). Wc and A remain unchanged.QAM: Quadrature Amplitude Modulation. ). Use the digital baseband signal to change the carrier phase (φ) and amplitude (A). Wc remains unchanged.

Microwave Frame Structure (1)

RFCOH

RFCOH: Radio Frame Complementary Overhead RSC: Radio Service ChannelMLCM: Multi-Level Coding Modulation INI: N:1 switching commandDMY: DummyID: IdentifierXPIC: Cross-polarization Interference CancellationFA: Frame AlignmentATPC: Automatic Transmit Power Control WS: Wayside Service

Microwave Frame Structure (2)

RFCOH is multiplexed into the STM-1 data and a block multiframe is formed. Each multiframe has six rows and each row has 3564 bits. One multiframe is composed of two basic frames. Each basic frame has 1776 bits. The remaining 12 bits are used for frame alignment.

I: STM-1 information bitC1/C2: Two-level correction coding monitoring bitsFS: Frame synchronization a/b: Other complementary overheads

Digital Microwave Communication Equipment

Microwave Equipment Category

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

• All the units are outdoor.• Installation is easy.• The equipment room can be saved.

Split-Mount Microwave Equipment (1)

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 areconnected through an IF cable.

The ODU can either be directly mounted onto the antenna or connected to the antenna through a short soft waveguide. Although the capacity is smaller than the trunk, due to the easy installation and maintenance, fast network construction, it's the most widely used microwave equipment.

Split-Mount Microwave Equipment (2)

Unit Functionso Antenna: Focuses the RF signals transmitted by ODUs and

increases the signal gain.o ODU: RF processing, conversion of IF/RF signals.o IF cable: Transmitting of IF signal, management signal and

power supply of ODU.o IDU: Performs access, dispatch, multiplex/demultiplex, and

modulation/demodulation for services.

Split-Mount Microwave Equipment – Installation

Microwave Antenna (1)

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.

Microwave Antenna (2)

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

Antenna Adjustment (1)


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.


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)


Specifications of Transmittero Working frequency band

Generally, trunk radios use 6, 7, and 8 GHz frequency bands. 11, 13 GHz andhigher frequency bands are used in the access layer (e.g. BTS access).

o Output powerThe power at the output port of a transmitter. Generally, the output power is 15 to 30 dBm.


Local frequency stabilityIf 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 FrameThe 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.


Specifications of Receivero 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.

o Local frequency stabilityThe same as that of transmitters: 3 to 10 ppm

o Noise figureThe noise figure of digital microwave receivers is 2.5 dB to 5 dB.


�PassbandTo 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.

SelectivityAbility 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) rangeAutomatic 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 –IDU