The goal of a wireless network
The convergence of voice, video and data services is the ultimate goal of many providers of communications services. To achieve this goal, technology dominates the traditional fixed voice communication connected with new technologies that will accommodate the bandwidth requirements of today's consumers replaced. Access systems such as Frequency Division Multiple Access (FDMA) and Frequency Division Duplex (FDD), was considered innovative whenapplied first to the needs of the traditional voice network. Today, however, there are other technologies on the market, necessary to implement the demands of high bandwidth and dynamic nature of the current network, voice, video, to provide Internet services and data can actually meet.
FDD and TDD
Frequency Division Duplex (FDD) and Time Division Duplex (TDD) are two most common systems used in duplex broadband fixed wirelessNetworks. FDD, which has historically been used in voice-only applications, it supports two-way radio communications using two different radio stations. TDD uses a single frequency, alternatively, to transmit signals in both downstream and upstream directions.
In point to point fixed wireless systems that use FDD frequency channel is transmitted by radio to a valley on the radio as a second frequency in upstream direction and supports the transmission of radio-BA. Thanks to the pair of radio frequencies is the simultaneous transmission in both directions. To mitigate the interference between transmission and downstream, must be a minimum frequency separation between the pair of frequencies to be taken.
In point to point fixed wireless systems that use TDD, frequency of single channel is a direction to the signals in both downstream and upstream.
Symmetry data
FDD systems use channelThe plans, which includes the frequencies with the same bandwidth. Since each channel has a fixed bandwidth, channel capacity of each frequency is fixed and equal to all other channels in the frequency band. This makes it ideal for communications applications in which the FDD symmetrical identical or similar information flows in both directions, such as voice communications.
TDD works by switching directions of the antennas in a time interval. The routing is done very quicklyand is imperceptible to the user. TDD can support voice and other communications services, symmetric and asymmetric data services. TDD can also handle a dynamic mix of both types of traffic. The relative performance of the upstream and downstream links can be changed in favor of one direction over another. This is done by a greater allocation of time slots obtained in time intervals for transmitting downstream and upstream. This asymmetry is useful for communication processesflow characterized by asymmetric information. An obvious question for this technology is Internet access where a user receives a short message in front of and downstream of large payloads information.
FDD can be used for asymmetric traffic. However, spectrally efficient, the bandwidth of the downstream and upstream channel can be matched precisely to the asymmetry. Since Internet traffic is bursty by nature and the asymmetry is always changing, it is not possible bandwidth of the channelbe set precisely in FDD. In this respect, TDD is more efficient. In addition, the channel bandwidth typically set by the FCC or limited by the capabilities of available devices. As a result, users of the FDD does not have the ability to channel bandwidth varies dynamically in the upstream and downstream.
Spectrum Efficiency
Radio spectrum is an increasingly scarce resource. This scarcity drives the need for optimum use of available bandwidth. FDD systemswork on the principle of paired frequencies. A channel plan is designed, made up of downstream and upstream channels, typically defined by the body, FCC, ITU, or other government. FDD channel set to maintain a guard band between upstream and downstream channels. The guard band is required to avoid self-interference, and because it is not used, essentially wasted spectrum.
In contrast, TDD systems require a guard time (instead of a guard band) between the sending and receiving streams.The Rx / Tx Transition Gap (TTG) is a gap between the downstream and upstream transmission system operator transmission. This gap allows time for switch-mode base station transmission mode and subscribers switching from receive to transmit mode. During this gap, the base station and subscriber transmitting modulated data but are simply the carrier base station transmitter to ramp down, press the TX / RX antenna to pass, and the base station receiver section. Enable
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 old system that is best for applications such as voice, symmetric traffic, which was suitable to generate, while TDD is especially for burst, asymmetric traffic, such as the Internet or other suitable services to the data.
In TDD, both the transmitter and receiver operate at the same frequency, but differentTimes. Therefore, TDD systems reuse the filters, mixers, frequency sources and synthesizers, making the complexity and cost of isolating the electromagnetic field and the receiving antenna connected. An FDD system uses a duplexer and / or two antennas, the spatial separation and thus require the resources can not be reused. The result is the most expensive hardware.
TDD uses the spectrum more efficiently than FDD. FDD can not be used in environments where the service is notnot enough bandwidth to the guard band required between transmit and receive channels of supply.
FDD TDD is more flexible in meeting the needs of dynamically allocated bandwidth upstream and downstream reconfigure in response to customer needs.
TDD allows interference mitigation through the proper frequency planning. TDD only requires a clear channel with FDD, the two channels without problems requires respect.
In summary, a desirable TDD duplexTechnology that enables network operators to get the most from their investment in equipment and spectrum of telecommunications and the needs of each client.
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