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