Definition of microwave
Microwave is a kind of electromagnetic wave. The frequency range of microwave is 300 MHz to 300 GHz. But in microwave communication, the frequency range is generally from 3 GHz to 30 GHz. accordingly, the wavelength is between 1 decimeter and 1 centimeter, so microwave is also called “centimeter wave”. 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.
Digital microwave communication is a way of transmitting digital information in atmosphere on microwave or radio frequency (RF). Digital microwave communication refers to the microwave communication that adopts the digital modulation scheme. The baseband signal is processed in the Intermediate Frequency (IF) unit. Then the signal is converted into the microwave frequency band through frequency conversion. Modulation can be done within the microwave frequency band directly, but only for the Phase Shift Keying (PSK) modulation scheme.
Applications of Digital Microwave
- Used for Complementary networks to optical networks
- BTS backhaul transmission
- Redundancy backup of important links
- VIP customer access
- Emergency communications
- Special transmission conditions
Generally-used frequency bands in digital microwave transmission include:
7GHz / 8GHz / 11GHz / 13GHz / 15GHz / 18GHz / 23GHz /26GHz / 32GHz / 38GHz
Every frequency band is identified by its central frequency, for example, the central frequency of the 7 GHz frequency band is 7575 MHz.
the rough principle of microwave band application is that low-band microwave is usually used for large-capacity. But the network sometimes uses the lowest frequency band. The lower frequency band has the longer wavelength and the stronger diffraction ability. There are several rules on the use of frequency bands based on practical application experience,
- For long-haul PDH microwave links (distance between stations is generally longer than 15 km), 8 GHz frequency band is recommended. If the distance between stations is not longer than 25 km, 11 GHz frequency band can also be used. The specific frequency band shall be determined based on the local climate conditions and microwave transmission cross-section.
- For short-haul PDH microwave links (generally used in the access layer and the distance between stations is shorter than 10 km), 11G / 13G / 14G / 15G / 18G frequency bands are recommended.
- For long-haul SDH microwave links (distance between stations is generally longer than 15 km), 5G / 6G / 7G / 8G frequency bands are recommended. If the distance between stations is not longer than 20 km, 11 GHz frequency band can also be used. The specific frequency band shall be determined based on the local climate conditions and microwave transmission cross-section.
RF channel configuration
There are some important parameters, which are considered in common RF channel configuration.
- Frequency Range
- Central Frequency
- Transmit / Receive Spacing
- Channel Spacing
The central frequency indicates the specific frequency band. A frequency band is divided by the central frequency into low bands and high bands. There is protection spacing between two frequency bands. It prevents interference between frequency bands. After selecting the microwave frequency band, RF channels should be configured. In this scheme frequency band are divided into several smaller sub-bands to provide the spectrum required by the transmitter. These sub-bands are called as “Channels”. These channels are usually indicated by their center frequencies and sequence number. To avoid interference between the transmit frequency and the receive frequency; there must be a certain spacing between them. This spacing is called “Transmit/receive spacing”. The transmit frequency and the receive frequency always come in pairs, one in a low band and the other in a high band.
As an example, let’s take a common RF channel configuration scheme for the 7 GHz. In this configuration scheme, the frequency range is from 7425 MHz to 7725 MHz and the central frequency is 7575 MHz. The central frequency of each wave channel varies with the configuration of the specific transmit/receive spacing and channel spacing. When these parameters are fixed, the central frequency of each channel is determined. It is calculated based on a certain formula.
Modulation schemes used in digital microwave communication
The baseband signal cannot be directly transmitted over microwave radio channels but must be converted into frequency band signal in order to implement microwave transmission. Generally, the digital baseband signal is the service signal to be transmitted. After the digital baseband signal is modulated into the IF signal, it still cannot be directly transmitted over the air link but must be converted once more into the signal of a higher frequency.
By the parameters used to modulate the carrier signal with the digital baseband signal, there can be various modulation methods
1. ASK (Amplitude Shift Keying): Use the digital baseband signal to change the carrier amplitude.
2. FSK (Frequency Shift Keying): Use the digital baseband signal to change the carrier frequency.
3. PSK (Phase Shift Keying): Use the digital baseband signal to change the carrier phase.
4. QAM (Quadrature Amplitude Modulation): Use the digital baseband signal to change the carrier phase and amplitude.
At present, PSK and QAM are commonly used in digital microwave communication.
More Topics;
- Digital Microwave Communication Equipment
- Microwave Antenna
- Digital Microwave Networking
- Microwave Propagation
- Diversity Technology
- Designing Microwave Transmission Link
