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

Transmission Method in Current Communication Networks

Development of Microwave Comunication

Development of Microwave Communication

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)

Microwave Frequency Band Selection & RF Channel Configuration

Microwave Frequency Band Selection and RF Channel Configuration (2)

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

Microwave Frequency Band Selection & RF Channel Configuration

Microwave Frequency Band Selection and RF Channel Configuration (3)

Microwave Frequency Band Selection & RF Channel Configuration
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 7  
7110-7750 7275 196 28  
  7597 196 28  
7250-7550 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

Digital Microwave Communication Modulation (2)

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

Digital Microwave Communication Modulation

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

Digital Microwave Frame Structure

RFCOH: Radio Frame Complementary Overhead
RSC: Radio Service Channel
MLCM: Multi-Level Coding Modulation
INI: N:1 switching command
DMY: Dummy
ID: Identifier
XPIC: Cross-polarization Interference Cancellation
FA: Frame Alignment
ATPC: 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.

Microwave Frame Structure

I: STM-1 information bit
C1/C2: Two-level correction coding monitoring bits
FS: Frame synchronization
a/b: Other complementary overheads