Frequency Division Duplexing (FDD)

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Description	A duplexing technique used in licensed solutions that uses a pair of spectrum channels, one for the uplink and another for the downlink.
Advantages	Proven technology for voice, designed for symmetrical traffic, does not require guard time.
Disadvantages	Cannot be deployed where spectrum is unpaired, spectrum is usually licensed, higher cost associated with spectrum purchase.
Usage	Environments with predictable traffic patterns, where equipment costs are more important than RF efficiency.

Challenges to Overcome in WiMax Deployment

RF Interference: Disrupts a transmission and decreases performance. Common forms are multi-path interference and attenuation. Overlapping interference generate random noise.
Infrastructure Placement: The physical structure that houses or supports the base station must be RF friendly. A metal farm silo, for example, may distort signals, or a tree swaying in the wind may change signal strength. Obstacles such as trees and buildings frequently block signal paths. High RF activity in the area can cause interference.

Proper network design and infrastructure placement are critical for solving the challenges.
- Subscriber Site Survey, Statistics Gathering, coordination of RF use with neighbouring providers.
- Antennas (Type, Tilt Angles, Array Gain, Diversity Gain)
- Proper design and deployment of the provider’s NOC.
- Well deployed base station or cells with 24/7 access, RF friendly structure, and shielding from weather elements.

All profiles currently defined by the WiMax Forum specify the 256-carrier OFDM air interface.
Allows digital signal to be transmitted simultaneously on multiple RF carrier waves. Adaptable to NLOS schemes.
Resistant to multi-path effects.
Spectrally efficient technique to transmit wireless digital data.
Able to deliver higher bandwidth efficiency.
There are some obstacles in using OFDM in transmission system in contrast to its advantages. A major obstacle is that the OFDM signal exhibits a very high Peak to Average Power Ratio (PAPR).

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