Point to Point Microwave Links
Point to Point (PTP) Microwave technology provides dedicated, point to point connectivity using directional antennas. A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few meters to several kilometers apart. PTP links typically require clear Line of Sight (LOS) between the transmitting antennas.
A simple one-way microwave link includes four major elements: a transmitter, a receiver, transmission lines, and antennas.
- Transmitter: The transmitter produces a microwave signal that carries the information to be communicated. The transmitter has two fundamental functions: generating microwave energy at the required frequency and power level, and modulating it with the input signal so that it conveys meaningful information. Modulation is accomplished by varying some characteristic of the energy in response to the transmitter’s input.
- Transmission line: The transmission line carries the signal from the transmitter to the antenna and, at the receiving end of the link, from the antenna to the receiver. At microwave frequencies, coaxial cables and, especially, hollow pipes called waveguides are used as transmission lines.
- Recently due to the growth of IP based networks, the demand for IP or Ethernet based microwave systems has increased dramatically. With this development, the equipment vendors have come up with a unique solution called Power over Ethernet (PoE): the data and power for the radio is carried over a single UTP/STP Ethernet cable. This has further reduced the cost of microwave systems since co-axial transmission lines and waveguides are costly and expensive to replace.
- Antenna: The third part of the microwave system is the antennas. On the transmitting end, the antenna emits the microwave signal from the transmission line into free space. At the receiver site, an antenna pointed toward the transmitting station collects the signal energy and feeds it into the transmission line for processing by the receiver. Antennas used in microwave links are highly directional, which means they tightly focus the transmitted energy, and receive energy mainly from one specific direction. By concentrating the received signal, this characteristic of microwave antennas allows communication over long distances using small amounts of power.
- Receiver: The receiver extracts information from the microwave signal and makes it available in its original form. To accomplish this, the receiver must demodulate the signal to separate the information from the microwave energy that carries it. The receiver must be capable of detecting very small amounts of microwave energy, because the signal loses much of its strength on its journey.
The following are some of the typical technical characteristics of microwave point to point transmission links.
- Range: microwave point to point links can support range of up to 100 km. Microwave radio communication requires clear line-of-sight (LOS) between the respective endpoints. Increase in range typically requires higher antenna heights to account for the “earth bulge”.
- Data Rate: Microwave PTP systems can support up to Gigabit speeds.
- Frequency Bands: Microwave PTP links operate in Licensed and Unlicensed frequency bands
- Licensed frequency bands require expensive license. However there are multiple benefits associated with using licensed frequencies including
- High system availability
- Secure as compared to unlicensed band
- Optimum system performance due to lack of interference
- Lower long term operational cost
- Considerations with licensed band
Unlicensed frequency bands offer the benefits of no licensing cost and lower equipment cost. However the following need to be considered when using unlicensed frequencies
- High probability of interference
- Transmit power and therefore distance is limited
- Potentially less secure as compared to licensed band
Duplexing: Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) systems are available
- FDD: Transmission and reception occurs on independent frequencies. FDD systems are suited to applications with symmetric traffic for e.g. voice.
- TDD: Transmission and reception occurs during separate time slots. TDD systems are suited to applications with asymmetric traffic for e.g. bursty data.
Interfaces: Support for multiple interfaces is available on microwave systems including serial, Ethernet and T1/E1 interface
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Microwave systems are typically deployed in the following topologies
- Point to Point topology is used for connectivity between two sites with clear LOS.
- Repeater: this configuration is used when the LOS between two end-points is not clear due to the presence of an obstruction such as a high rise building
- Ring topology is used to provide fault tolerance: the unavailability of a single link will not affect traffic flow between any two sites on the ring.
- Star topology is used to provide connectivity between a central site and multiple peripheral sites
- Chain topology is used when the distance between the end points is much larger than that supported by a single link.
Following are some of the features of point to point microwave links
- Low Latency and high reliability
- Rapid deployment and commissioning
- Low total cost of ownership
- Not susceptible to “backhoe fade”
Microwave PTP can be used for a very wide range of applications.
- Currently mostly PTP Microwave systems are used for cellular backhaul application. Cellular companies utilize PTP microwave links for their equipment connectivity.
- Enterprise networks utilize microwave PTP system for Internet/Intranet access, Corporate voice , File transfers, Video-conferencing etc
- Last mile: PTP systems are commonly used for last mile access to the PSTN and other voice and data networks.
- PTP microwave system are widely used for increasing rural telephony and network extensions
- SCADA (Supervisory Control and Data Acquisition)