Time Division Duplex (TDD) vs Frequency Division
Duplex (FDD) in Wireless Backhauls
The Goal of a Wireless Network
The convergence of voice, video and data services is the ultimate
goal of many communication service providers. To achieve this goal,
technologies associated with the traditional voice dominated network
are replaced by newer technologies that accommodate the bandwidth
demands of today's consumer. Access schemes, such as Frequency Division
Multiple Access (FDMA) and Frequency Division Duplex (FDD), were
regarded as innovative technologies when first applied to the requirements
of the traditional voice network. Today, however, there are other
technologies on the market that allow for the performance required
to meet the high bandwidth demands and the dynamic nature of the
current network that must deliver voice, video, Internet and data
services efficiently.
FDD and TDD
Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are
the two most prevalent duplexing schemes used in fixed broadband
wireless networks. FDD, which historically has been used in voice-only
applications, supports two-way radio communication by using two distinct
radio channels. Alternatively, TDD uses a single frequency to transmit
signals in both the downstream and upstream directions.
In fixed wireless point-to-point systems that use FDD, one frequency
channel is transmitted downstream from a radio A to radio B. A second
frequency is used in the upstream direction and supports transmission
from radio B to radio A. Because of the pairing of frequencies, simultaneous
transmission in both directions is possible. To mitigate self-interference
between upstream and downstream transmissions, a minimum amount of
frequency separation must be maintained between the frequency pair.
In fixed wireless point-to-point systems that use TDD, a single frequency
channel is used to transmit signals in both the downstream and upstream
directions.
Data Symmetry
FDD systems utilize channel plans that are comprised of frequencies
with equal bandwidth. Since each channel has a fixed bandwidth, the
channel capacity of each frequency also is fixed and equal to that
of all other channels in the frequency band. This makes FDD ideal
for symmetrical communication applications in which the same or similar
information flows in both directions, such as voice communications.
TDD operates by toggling transmission directions over a time interval.
This toggling takes place very rapidly and is imperceptible to the
user. Thus, TDD can support voice and other symmetrical communication
services as well as asymmetric data services. TDD also can handle
a dynamic mix of both traffic types. The relative capacity of the
downstream and upstream links can be altered in favor of one direction
over the other. This is accomplished by giving a greater time allocation
through time slots to downstream transmission intervals than upstream.
This asymmetry is useful for communication processes characterized
by unbalanced information flow. An obvious application for this technique
is Internet access in which a user enters a short message upstream
and receives large information payloads downstream.
FDD can be used for asymmetric traffic. However, in order to be spectrally
efficient, the downstream and upstream channel bandwidths must be
matched precisely to the asymmetry. Since Internet traffic is bursty
by nature and the asymmetry is always changing, the channel bandwidth
cannot be precisely set in FDD. In this respect, TDD is more efficient.
Furthermore, channel bandwidths typically are set by the FCC or limited
by the functionality of available equipment. As a consequence, users
of FDD systems do not have the option to vary channel bandwidths
dynamically in the upstream and downstream directions.
Spectrum Efficiency
Frequency spectrum is an increasingly scarce commodity. This scarcity
drives the need to optimize the use of available bandwidth. FDD systems
operate on the principle of paired frequencies. A channel plan is
devised that is comprised of downstream and upstream channels, typically
defined by the FCC, ITU, or other governing body. FDD channel plans
maintain a guardband between the downstream and upstream channels.
The guardband is required to avoid self-interference and, since it
is unused, essentially is wasted spectrum.
In contrast, TDD systems require a guard time (instead of a guardband)
between transmit and receive streams. The TX/RX Transition Gap (TTG)
is a gap between downstream transmission and the upstream transmission.
This gap allows time for the base station to switch from transmit
mode to receive mode and subscribers to switch from receive mode
to transmit mode. During this gap, the base station and subscriber
are not transmitting modulated data but are simply allowing the base
station transmitter carrier to ramp down, the TX /RX antenna switch
to actuate, and the base station receiver section to activate.
Conclusions
The above discussion has highlighted the differences and some significant
advantages of TDD over FDD. These advantages can be summarized as
follows:
- FDD is an older scheme that was best suited for applications,
such as voice, that generate symmetric traffic, while TDD is
best suited for bursty, asymmetric traffic, such as Internet
or other datacentric services.
- In TDD, both the transmitter and receiver operate on the same
frequency but at different times. Therefore, TDD systems reuse
the filters, mixers, frequency sources and synthesizers, thereby
eliminating the complexity and costs associated with isolating
the transmit antenna and the receive antenna. An FDD system uses
a duplexer and/or two antennas that require spatial separation
and, therefore, cannot reuse the resources. The result is more
costly hardware.
- TDD utilizes the spectrum more efficiently than FDD. FDD cannot
be used in environments where the service provider does not have
enough bandwidth to provide the required guardband between transmit
and receive channels.
- TDD is more flexible than FDD in meeting the need to dynamically
reconfigure the allocated upstream and downstream bandwidth in
response to customer needs.
- TDD allows interference mitigation via proper frequency planning.
TDD requires only one interference-free channel compared with
FDD, which requires two interference-free channels.
- In summary, TDD is a more desirable duplexing technology that
allows system operators to receive the most from their investment
in spectrum and telecom equipment, while meeting the needs of
each individual customer.
This text is found at Raze
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