Microwave Radio Communication Electromagnetic waves with Frequency range from approximately 300MHz to 300 GHz. High frequency > Short wavelengths > “Microwave” Wavelengths of microwaves that are used for communication fall between 1 cm to 60 cm. For full-duplex microwave communication the frequency band is divided into two halves. Lower half is called low band. Upper half is called high band. If Tx operates in low band Rx has to operate in high band and vice versa. Microwave frequency band: Frequency Band Range (GHz) Application L Band 1-2 Military, Mobile, Satellite S Band 2-4 Television, Mobile, Satellite C Band 4-8 Military, Satellite X Band 8-12 Military, Satellite Ku Band 12-18 Cable TV, Satellite K Band 18-27 Satellite Ka Band 27-40 Military , Satellite Millimeter 40-100 Satellite Submillimeter 100-300 Not used Modulation and multiplexing Modulation Multiplexing Analog Microwave radio relay systems Frequency Modulation Frequency Division Multiplexing Digital Microwave radio relay systems Phase Shift Keying or Quadrature Amplitude Modulation Pulse Coded Modulation Time Division Multiplexing Based on the distance Microwave systems can be classified as: Short haul Intrastate or feeder service applications Long haul Interstate or backbone route applications Microwave radio link: Side view: Top view: Information originates and terminates at the terminal station. Repeaters relay or conveys the information to the next downlink microwave station. Microwave stations must be geographically placed in such a way that natural or man-made barriers should not interfere with transmissions between stations. Microwave stations are placed about 15 to 30 miles apart . Microwave radio systems has a capacity to carry thousands of information channels without the need of any physical facilities such as coaxial cables or optical fibers. Advantages: 1. Right-of –way acquisition between stations is not needed 2. Each station requires a purchase or lease of only a small area of land. 3. Requires relatively small antennas (shorter wavelength) 4. Because of high operating frequencies it can carry large quantity of information 5. Propagation is easy around physical obstacles such as water and high mountains. 6. Fewer repeaters are necessary for amplification 7. Underground facilities are minimized 8. Minimum delay times are introduced 9. Minimal crosstalk exists between voice channels 10. Increased reliability and less maintenance Disadvantages: 1. More difficult to analyze and design circuits at microwave frequencies. 2. Measuring techniques are more difficult to perfect and implement at microwave frequencies. 3. It is difficult to implement conventional circuit components(resisters, capacitors, inductors etc) at microwave frequencies. 4. Transient time more critical at microwave frequencies. 5. It is often necessary to use specialized components for microwave frequencies. 6. Microwave frequencies propagate in straight line, which limits their use to line-of-sight applications. Frequency modulated microwave radio system: Provide flexible, reliable and economical point-topoint communication. It can simultaneously carry thousands of voice and data channels. FM microwave radio transmitter: Baseband signal is a composite signal consists of, FDM voice band channels TDM voice band channels Broadcast quality composite video Wideband data Pre-emphasis network provides an artificial boost in amplitude to the higher baseband frequencies. This will provide a uniform signal to noise ratio. FM modulator provides modulation of the IF carrier(60-70Mhz) (that eventually becomes the microwave carrier). The IF and its associated sidebands are up-converted to the microwave region by the mixer, microwave oscillator, and bandpass filter. Mixing, rather than multiplying, is used to translate the IF frequencies to RF frequencies. Microwave generators consists of crystal oscillators and frequency multipliers. FM microwave radio receiver: Channel separation network provides the isolation and filtering necessary separate individual microwave channels and direct them to their respective receivers. The bandpass filter, mixer and microwave oscillator down-convert the RF microwave frequency to IF frequency . FM demodulator is a non-coherent FM detector (PLL demodulator) De-emphasis restores the baseband signal to its original amplitude-versus-frequency characteristics. FM Microwave radio repeaters: The permissible distance between FM microwave Tx and its associated microwave receiver depends on Tx output power Receiver noise threshold Terrain Atmospheric conditions System capacity Performance expectations Typical distance is 15 miles to 40 miles. A microwave repeater is a receiver and a transmitter placed back to back or in tandem with the system. A repeater station receives a signal, amplifies and reshapes it and then transmits the signal to the next repeater or terminal station down line from it. There are 3 types of repeaters: IF, Baseband and RF. IF repeater: (Heterodyne repeater) Baseband intelligence is unmodified here. Baseband repeater: Here the baseband signal, which is FDM voice band channels is further demodulated to master group, super group, group or even channel level. This allows the baseband signal to be reconfigured to meet the routing needs of the overall communication network. Modified baseband repeater: Here there is no reconfiguration of baseband signals. The operation is same as IF repeaters. The only difference is that the amplifier and filters acts only on the baseband frequencies rather than IF frequencies. So the design of filters and amplifiers becomes simple and less expensive. RF repeater: Here the RF frequency is up-converted or down- converted by mixing with a local oscillator frequency (shift oscillator). The RF signal is simply converted and then re-amplified and transmitted to the next down line repeater or terminal station. Here reconfiguration and reshaping are not possible. LINE-OF-SIGHT PATH CHARACTERISTICS: The free-space path is the line-of-sight path directly between the transmit and receive antennas (this is also called the direct wave). The ground-reflected wave is the portion of the transmit signal that is reflected off Earth’s surface and captured by the receive antenna. The surface wave consists of the electric and magnetic fields associated with the currents induced in Earth's surface. The magnitude of the surface wave depends on the characteristics of Earth's surface and the electromagnetic polarization of the wave. The sum of these three paths (taking into account their amplitude and phase) is called the ground wave. The sky wave is the portion of the transmit signal that is returned (reflected) back to Earth's surface by the ionized layers of Earth's atmosphere.