Chapter 17

                               

      Microwave− and Radio−Based Systems

        A radio base system (1000) is multi-path-connected to a plurality of mobile terminal devices to transmit/receive signals. When a communication channel establishment request is sent from one of the plurality of mobile terminal devices, a control unit (80) detects the presence or absence of a mobile terminal device to which a communication channel is already connected for each of a plurality of slots.

           Microwave frequencies range from 300 MHz to 30 GHz, corresponding to wavelengths of 1 meter to 1 cm.  These frequencies are useful for terrestrial and satellite communication systems, both fixed and mobile.  In the case of point-to-point radio links, antennas are placed on a tower or other tall structure at sufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites. In the case of mobile radio systems, a single tower provides point-to-multipoint coverage, which may include both LOS and non-LOS paths.  LOS microwave is used for both short- and long-haul telecommunications to complement wired media such as optical transmission systems.

         Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do.

  

   Advantages:

Ø  High frequency of microwaves gives the microwave band a very large information-carrying capacity.

Ø  The microwave band has a bandwidth 30 times that of all the rest of the radio spectrum.

     Disadvantage:

Ø  Microwaves are limited to line of sight propagation.

Ø  They cannot pass around hills or mountains as lower frequency radio waves can.

  

Principles and Operation

Microwave Link Structure

        

      The basic components required for operating a radio link are the transmitter, towers, antennas, and receiver. Transmitter functions typically include multiplexing, encoding, modulation, up-conversion from baseband or intermediate frequency (IF) to radio frequency (RF), power amplification, and filtering for spectrum control. Receiver functions include RF filtering, down-conversion from RF to IF, amplification at IF, equalization, demodulation, decoding, and demultiplexing. To achieve point-to-point radio links, antennas are placed on a tower or other tall structure at sufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites.

Microwave System Design

    

     The design of microwave radio systems involves engineering of the path to evaluate the effects of propagation on performance, development of a frequency allocation plan, and proper selection of radio and link components. The frequency allocation plan is based on four elements: the local frequency regulatory authority requirements, selected radio transmitter and receiver characteristics, antenna characteristics, and potential intrasystem and intersystem RF interference.

 Microwave Propagation Characteristics

       

     The various phenomena associated with propagation, such as multipath fading and interference, affect microwave radio performance.  The modes of propagation between two radio antennas may include a direct, line-of-sight (LOS) path but also a ground or surface wave that parallels the earth's surface, a sky wave from signal components reflected off the troposphere or ionosphere, a ground reflected path, and a path diffracted from an obstacle in the terrain.

       A simple and cost-effective demultiplexing approach for a subcarrier multiplexed radio-over-fiber (RoF) system is proposed, analyzed, and experimentally demonstrated. A microwave photonic filter based on multiple optical sources is integrated into the RoF system, and by simply controlling the time delay in the remote antenna unit, the subcarrier with desirable frequency can be filtered out, and undesirable ones are suppressed. An experimental demonstration has been carried out by implementing a two-optical-source-based microwave photonic filter in an RoF downlink transmitting a 2.5-GHz subcarrier modulated with 150-Mb/s on-off keying (OOK) data, showing that it works well as a subcarrier demultiplexer. The proposed demultiplexing approach enjoys high flexibility, tunability, and cost-effectiveness and has good potential applications in the multiplexed RoF systems.

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5711628

http://www.eogogics.com/talkgogics/tutorials/microwave-line-of-sight-systems

http://www.google.com.ph/search?q=Microwave%E2%88%92+and+Radio%E2%88%92Based+Systems&hl=tl&rlz=1C1ECBA_enPH471PH471&prmd=imvns&source=lnms&tbm=isch&ei=kqs8T_S6OLCRiQfNv823BA&sa=X&oi=mode_link&ct=mode&cd=2&ved=0CBQQ_AUoAQ&biw=1366&bih=624