The difference between FDD and TDD in Microwave Transmission
Microwave links typically use Frequency-division duplexing (FDD) which is a method for establishing a full-duplex communications link that uses two different radio frequencies for transmitter and receiver operation. The transmit direction and receive direction frequencies are separated by a defined frequency offset.
Advantages of FDD
In the microwave realm, the primary advantages of this approach are:
The full data capacity is always available in each direction because the send and receive functions are separated;
It offers very low latency since transmit and receive functions operate simultaneously and continuously;
It can be used in licensed and license-exempt bands;
Most licensed bands worldwide are based on FDD; and
Due to regulatory restrictions, FDD radios used in licensed bands are coordinated and protected from interference, though not immune to it.
Disadvantages to FDD
The primary disadvantages of the FDD approach to microwave communication are:
Complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, then it may not be possible to deploy the system in that band;
Radios require pre-configured channel pairs, making sparing complex;
Any traffic allocation other than a 50:50 split between transmit and receive yields inefficient use of one of the two paired frequencies, lowering spectral efficiency; and
Collocation of multiple radios is difficult.
TDD compared with FDD
Time-division duplexing (TDD) is a method for emulating full-duplex communication over a half-duplex communication link. The transmitter and receiver both use the same frequency but transmit and receive traffic is switched in time. The primary advantages of this approach as it applies to microwave communication are:
It is more spectrum friendly, allowing the use of only a single frequency for operation and dramatically increasing spectrum utilization, especially in license-exempt or narrow-bandwidth frequency bands ;
It allows for the variable allocation of throughput between the transmit and receive directions, making it well suited to applications with asymmetric traffic requirements, such as video surveillance, broadcast and Internet browsing;
Radios can be tuned for operation anywhere in a band and can be used at either end of the link. As a consequence, only a single spare is required to serve both ends of a link.
Disadvantages of TDD
The primary disadvantages of traditional TDD approaches to microwave communications are:
The switch from transmit to receive incurs a delay that causes traditional TDD systems to have greater inherent latency than FDD systems;
Traditional TDD approaches yield poor TDM performance due to latency;
For symmetric traffic (50:50), TDD is less spectrally efficient than FDD, due to the switching time between transmit and receive; and
Multiple co-located radios may interfere with one another unless they are synchronized.
Rain fade refers primarily to the absorption of a microwave radio frequency (RF) signal by atmospheric rain, snow or ice, and losses which are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by precipitation at the uplink or downlink location. However, it does not need to be raining at a location for it to be affected by rain fade, as the signal may pass through precipitation many miles away, especially if the satellite dish has a low look angle. From 5 to 20 percent of rain fade or satellite signal attenuation may also be caused by rain, snow or ice on the uplink or downlink antenna reflector, radome or feed horn. Rain fade is not limited to satellite uplinks or downlinks, it also can affect terrestrial point to point microwave links (those on the earth’s surface).
Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, receiving antennas larger (i.e. higher gain) than the required size for normal weather conditions, and hydrophobic coatings.
Two models are generally used for Rain modelling: Crane and ITU. The ITU model is generally preferred by microwave planners. A global map of Rain distribution according to the ITU model is shown below:
Used in conjunction with appropriate planning tools, this data can be used to predict the expected Operational Availability (in %) of a microwave link. Useful Operational Availability figures typically vary from 99.9% (“three nines”) to 99.999% (“five nines”), and are a function of the overall link budget including frequency band, antenna sizes, modulation, receiver sensitivity and other factors.
Another useful Rain Fade map is shown here, showing the 0.01% annual rainfall exceedance rate:
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Boost capacity and reliability with advanced networking
With a modern microwave network, you should expect advanced Carrier Ethernet networking capabilities that can double network capacity while delivering high availability. These capabilities include:
Unique ring and mesh topology configurations that can double network capacity, improve reliability and reduce network costs
Integrated IP-microwave solutions that reduce space and power consumption
The ability to support TDM, Ethernet and IP services on a single packet-based network
Simplify operations with an end-to-end approach
Expect to see: a complete family of microwave solutions that addresses all network sizes and locations including tail, hub and backbone. With an approach that uses common equipment and software across all sites, vendors should help you streamline management processes and reduce TCO. Features offered:
Common radio transceivers that reduce the need for spares across all applications
A flexible range of Indoor Units (IDUs) and Outdoor Units (ODUs) to reduce space and power consumption
Common software and network management that simplify operations across the network
Find out information on technology, deployment and applications for modern Digital Microwave Links
Microwave links are widely used for connectivity in modern digital IP networks. With capacities up to 3Gbps and beyond, a modern Microwave Link network can deliver bandwidth in a reliable, cost-effective and flexible manner – without need for disruption and delay caused by digging up streets and avoiding costly leased-line or leased fibre optic alternatives.
On this website you can find more information about radio link deployment and technology. Also we invite you to contact our experts with any questions by sending a message to us on our contact page.
Microwave links are used extensively in 4G/LTE backhaul networks, 2G (GSM) and 3G (UMTS) mobile operators, wireless metropolitan area networks (Wi-MAN) and corporate networks where high performance, flexibility, speed of deployment and low operating costs are required. Key features of links include high spectral efficiency (256QAM, 1024QAM, 2048QAM and 4096QAM), Automatic Transmit Power Control (ATPC) and Adaptive Coding and Modulation (ACM).
Globally, MW radio links are used for around 60% of all mobile backhaul connections due to the compelling technical and commercial arguments in favour of MW radio compared to leased line and trenched fibre alternatives. Speed of deployment and flexibility – the ability to move sites or provision rapidly – are greatly in favour of MW radio over fibre and cabled alternatives.
A link typically features a radio unit and a parabolic antenna, which may vary in size from 30cm up to 4m diameter depending on required distance and capacity. The radio unit is generally either a “Full Outdoor”, “Split Mount” or “Full Indoor” design depending on operator preference, deployment, features and available indoor space for specific sites and installation.
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