Radio Access Mode Operations

3GPP radio access for UMTS and LTE system is designed to operate in two main modes of
operation; FDD and TDD. The focus of this book is on FDD mode only.
8 Design, Deployment and Performance of 4G-LTE Networks
FDD is the common mode deployed worldwide for UMTS and LTE. Spectrum allocation
is also tied to the choice of FDD over TDD. For example, operators with WiMAX deployed
prior to LTE have utilized the WiMAX spectrum for investing in LTE TDD rather than FDD.
However, with device availabilities as well as simplicity of deployment, FDD is still the main
choice of deployment worldwide.

1.3.1 Frequency Division Duplex (FDD)
In FDD, a separate uplink and downlink channel are utilized, enabling a device to transmit
and receive data at the same time. The spacing between the uplink and downlink channel is
referred to as the duplex spacing.
The uplink channel operates on the lower frequency. This is done because higher frequencies
suffer greater attenuation than lower frequencies and, therefore, it enables the mobile to utilize
lower transmit levels.

1.3.2 Time Division Duplex (TDD)
TDD mode enables full duplex operation using a single frequency band and time division
multiplexing the uplink and downlink signals.
One advantage of TDD is its ability to provide asymmetrical uplink and downlink allocations.
Other advantages include dynamic allocation, increased spectral efficiency, and the
improved usage of beamforming techniques. This is due to having the same uplink and downlink
frequency characteristics.