The purpose 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, the technologies associated with the traditional voice-dominated network are replaced with 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 considered innovative technologies when they were first applied to traditional voice network requirements. Today, however, there are other technologies on the market that enable the performance required to meet the high-bandwidth demands and dynamic nature of today’s network that must efficiently deliver voice, video, Internet, and data services.

FDD and TDD

Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are the two most commonly used duplex schemes in fixed broadband wireless networks. FDD, which has historically been used in voice-only applications, supports two-way radio communication by using two different radio channels. Alternatively, TDD uses a single frequency to transmit signals both downstream and upstream.

In fixed wireless point-to-point systems using FDD, one frequency channel is transmitted downstream from radio A to radio B. A second frequency is used in the upstream direction and supports transmission from radio B to radio A. Due to frequency pairing, simultaneous transmission in both directions is possible. To mitigate self-interference between the upstream and downstream transmissions, a minimum amount of frequency separation must be maintained between the frequency pair.

In fixed wireless point-to-point systems using TDD, a single-frequency channel is used to transmit signals in the downstream and upstream directions.

data symmetry

FDD systems use channel plans that comprise frequencies with the same bandwidth. Since each channel has a fixed bandwidth, the channel capacity of each frequency is also fixed and equal to that of all other channels in the frequency band. This makes FDD ideal for symmetric communication applications where the same or similar information flows in both directions, such as voice communications.

TDD operates by alternating transmission directions during a time interval. This change occurs very quickly and is imperceptible to the user. Therefore, TDD can support voice and other symmetric communication services as well as asymmetric data services. TDD can also handle a dynamic mix of both types of traffic. The relative capacity of upstream and downstream links can shift in favor of one direction over the other. This is achieved by giving a greater allocation of time across time slots to downstream transmission slots than upstream. This asymmetry is useful for communication processes characterized by an unbalanced flow of information. An obvious application for this technique is Internet access where a user enters a short message upstream and receives large amounts of useful information downstream.

FDD can be used for asymmetric traffic. However, to be spectrally efficient, the downstream and upstream channel bandwidths must precisely match the asymmetry. Since Internet traffic is bursty in nature and the asymmetry is always changing, the channel bandwidth cannot be accurately set in FDD. In this sense, TDD is more efficient. In addition, channel bandwidths are generally set by the FCC or limited by available equipment functionality. As a consequence, users of FDD systems do not have the option of dynamically varying channel bandwidths in the upstream and downstream directions.

Spectrum Efficiency

The frequency spectrum is an increasingly scarce commodity. This scarcity drives the need to optimize the use of available bandwidth. FDD systems work on the principle of paired frequencies. A channel plan is designed that includes upstream and downstream channels, typically defined by the FCC, ITU, or other governing body. FDD channel plans maintain a guard band between downstream and upstream channels. The guardband is necessary to avoid self-interference and since it is not used, it is essentially wasted spectrum.

In contrast, TDD systems require a guard time (rather than a guard band) between the transmit and receive streams. The TX/RX transition gap (TTG) is a gap between the downlink transmission and the uplink transmission. This gap gives time for the base station to switch from transmit mode to receive mode and for subscribers to switch from receive mode to transmit mode. During this interval, the base station and subscriber do not transmit modulated data, but simply allow the base station’s transmitter carrier to drop, the TX/RX antenna switch to activate, and the receiver section of the antenna to turn on. the base station.

recommendations

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 better suited to applications, such as voice, that generate symmetric traffic, while TDD is more suitable for bursty asymmetric traffic, such as Internet or other data-centric services.

In TDD, both the transmitter and the receiver operate on the same frequency but at different times. Therefore, TDD systems reuse filters, mixers, frequency sources, and synthesizers, thereby eliminating the complexity and costs associated with isolating the transmitting and receiving antennas. An FDD system uses a duplexer and/or two antennas that require spatial separation and therefore cannot reuse resources. The result is more expensive hardware.

TDD uses spectrum more efficiently than FDD. FDD cannot be used in environments where the service provider does not have enough bandwidth to provide the necessary guard band between the transmit and receive channels.

TDD is more flexible than FDD in meeting the need to dynamically reconfigure allocated upstream and downstream bandwidth in response to customer needs.

TDD enables interference mitigation through proper frequency planning. TDD requires only one channel without interference compared to FDD which requires two channels without interference.

In short, TDD is a more desirable duplex technology that allows system operators to get the most out of their investment in spectrum and telecommunications equipment, while meeting the needs of each individual customer.