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Duplex (telecommunications)
Duplex (telecommunications)
Duplex (telecommunications)
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A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow for simultaneous communication in both directions between two connected parties or to provide a reverse path for the monitoring and remote adjustment of equipment in the field. There are two types of duplex communication systems: full-duplex (FDX) and half-duplex (HDX).

In a full-duplex system, both parties can communicate with each other simultaneously. An example of a full-duplex device is plain old telephone service; the parties at both ends of a call can speak and be heard by the other party simultaneously. The earphone reproduces the speech of the remote party as the microphone transmits the speech of the local party. There is a two-way communication channel between them, or more strictly speaking, there are two communication channels between them.

In a half-duplex or semiduplex system, both parties can communicate with each other, but not simultaneously; the communication is one direction at a time. An example of a half-duplex device is a walkie-talkie, a two-way radio that has a push-to-talk button. When the local user wants to speak to the remote person, they push this button, which turns on the transmitter and turns off the receiver, preventing them from hearing the remote person while talking. To listen to the remote person, they release the button, which turns on the receiver and turns off the transmitter. This terminology is not completely standardized, and some sources define this mode as simplex.[1][2]

Systems that do not need duplex capability may instead use simplex communication, in which one device transmits and the others can only listen. Examples are broadcast radio and television, garage door openers, baby monitors, wireless microphones, and surveillance cameras. In these devices, the communication is only in one direction.

Simplex

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Simplex wireless communication

Simplex communication is a communication channel that sends information in one direction only.[3]

The International Telecommunication Union definition is a communications channel that operates in one direction at a time, but that may be reversible; this is termed half duplex in other contexts.

For example, in TV and radio broadcasting, information flows only from the transmitter site to multiple receivers. A pair of walkie-talkie two-way radios provide a simplex circuit in the ITU sense; only one party at a time can talk, while the other listens until it can hear an opportunity to transmit. The transmission medium (the radio signal over the air) can carry information in only one direction.

The Western Union company used the term simplex when describing the half-duplex and simplex capacity of their new transatlantic telegraph cable completed between Newfoundland and the Azores in 1928.[4] The same definition for a simplex radio channel was used by the National Fire Protection Association in 2002.[5]

Half duplex

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A simple illustration of a half-duplex communication system

A half-duplex (HDX) system provides communication in both directions, but only one direction at a time, not simultaneously in both directions.[6] [7][8] This terminology is not completely standardized between defining organizations, and in radio communication some sources classify this mode as simplex.[2] [1][9] Typically, once one party begins a transmission, the other party on the channel must wait for the transmission to complete, before replying.[10]

An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must say "over" or another previously designated keyword to indicate the end of transmission, to ensure that only one party transmits at a time. A good analogy for a half-duplex system would be a one-lane road that allows two-way traffic; traffic can only flow in one direction at a time.

Half-duplex systems are usually used to conserve bandwidth, at the cost of reducing the overall bidirectional throughput, since only a single communication channel is needed and is shared alternately between the two directions. For example, a walkie-talkie or a DECT phone or so-called TDD 4G or 5G phones requires only a single frequency for bidirectional communication, while a cell phone in the so-called FDD mode is a full-duplex device, and generally requires two frequencies to carry the two simultaneous voice channels, one in each direction.

In automatic communications systems such as two-way data-links, time-division multiplexing can be used for time allocations for communications in a half-duplex system. For example, station A on one end of the data link could be allowed to transmit for exactly one second, then station B on the other end could be allowed to transmit for exactly one second, and then the cycle repeats. In this scheme, the channel is never left idle.

In half-duplex systems, if more than one party transmits at the same time, a collision occurs, resulting in lost or distorted messages.

Full duplex

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A simple illustration of a full-duplex communication system. Full-duplex is not common in handheld radios as shown here due to the cost and complexity of common duplexing methods, but is used in telephones, cellphones and cordless phones.

A full-duplex (FDX) system allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously.[6][7][8] Land-line telephone networks are full-duplex since they allow both callers to speak and be heard at the same time. Full-duplex operation is achieved on a two-wire circuit through the use of a hybrid coil in a telephone hybrid. Modern cell phones are also full-duplex.[11]

There is a technical distinction between full-duplex communication, which uses a single physical communication channel for both directions simultaneously, and dual-simplex communication which uses two distinct channels, one for each direction. From the user perspective, the technical difference does not matter and both variants are commonly referred to as full duplex.

Many Ethernet connections achieve full-duplex operation by making simultaneous use of two physical twisted pairs inside the same jacket, or two optical fibers which are directly connected to each networked device: one pair or fiber is for receiving packets, while the other is for sending packets. Other Ethernet variants, such as 1000BASE-T use the same channels in each direction simultaneously. In any case, with full-duplex operation, the cable itself becomes a collision-free environment and doubles the maximum total transmission capacity supported by each Ethernet connection.

Full-duplex has also several benefits over the use of half-duplex. Since there is only one transmitter on each twisted pair there is no contention and no collisions so time is not wasted by having to wait or retransmit frames. Full transmission capacity is available in both directions because the send and receive functions are separate.

Some computer-based systems of the 1960s and 1970s required full-duplex facilities, even for half-duplex operation, since their poll-and-response schemes could not tolerate the slight delays in reversing the direction of transmission in a half-duplex line.[citation needed]

Echo cancellation

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Full-duplex audio systems like telephones can create echo, which is distracting to users and impedes the performance of modems. Echo occurs when the sound originating from the far end comes out of the speaker at the near end and re-enters the microphone[a] there and is then sent back to the far end. The sound then reappears at the original source end but delayed.

Echo cancellation is a signal-processing operation that subtracts the far-end signal from the microphone signal before it is sent back over the network. Echo cancellation is important technology allowing modems to achieve good full-duplex performance. The V.32, V.34, V.56, and V.90 modem standards require echo cancellation.[12] Echo cancelers are available as both software and hardware implementations. They can be independent components in a communications system or integrated into the communication system's central processing unit.

Full-duplex emulation

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Where channel access methods are used in point-to-multipoint networks (such as cellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods.[13]

Time-division duplexing

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Time-division duplexing (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full-duplex communication over a half-duplex communication link.

Time-division duplexing is flexible in the case where there is asymmetry of the uplink and downlink data rates or utilization. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction.

The transmit/receive transition gap (TTG) is the gap (time) between a downlink burst and the subsequent uplink burst. Similarly, the receive/transmit transition gap (RTG) is the gap between an uplink burst and the subsequent downlink burst.[14]

Examples of time-division duplexing systems include:

Frequency-division duplexing

[edit]
Not to be confused with Frequency-division multiplexing.

Frequency-division duplexing (FDD) means that the transmitter and receiver operate using different carrier frequencies.

The method is frequently used in ham radio operation, where an operator is attempting to use a repeater station. The repeater station must be able to send and receive a transmission at the same time and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as duplex mode or offset mode. Uplink and downlink sub-bands are said to be separated by the frequency offset.

Frequency-division duplex systems can extend their range by using sets of simple repeater stations because the communications transmitted on any single frequency always travel in the same direction.

Frequency-division duplexing can be efficient in the case of symmetric traffic. In this case, time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving, has greater inherent latency, and may require more complex circuitry.

Another advantage of frequency-division duplexing is that it makes radio planning easier and more efficient since base stations do not hear each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere with each other. Conversely, with time-division duplexing systems, care must be taken to keep guard times between neighboring base stations (which decreases spectral efficiency) or to synchronize base stations, so that they will transmit and receive at the same time (which increases network complexity and therefore cost, and reduces bandwidth allocation flexibility as all base stations and sectors will be forced to use the same uplink/downlink ratio).

Examples of frequency-division duplexing systems include:

See also

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Notes

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Duplex (telecommunications)

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In , duplex refers to a point-to-point communication system architecture in which two connected parties or devices are capable of sending and receiving signals or to and from one another simultaneously or alternately. This bidirectional capability contrasts with communication, which allows transmission in only one direction, such as in broadcast radio or television signals. The two primary types of duplex systems are half-duplex and full-duplex. In half-duplex mode, communication occurs in both directions, but only one direction at a time, requiring devices to alternate between transmitting and receiving, much like a conversation where users take turns speaking. This mode is commonly implemented using time-division duplexing (TDD), where the same frequency band is divided into time slots for uplink and downlink transmissions, making it suitable for applications with asymmetric traffic or where spectrum efficiency is prioritized over simultaneity. In contrast, full-duplex mode enables simultaneous transmission and reception in both directions, akin to a standard where both parties can speak and listen at the same time without interruption. Full-duplex systems often employ frequency-division duplexing (FDD), which uses separate frequency bands for uplink and downlink to avoid interference, providing higher throughput and eliminating collisions in network environments like Ethernet or cellular networks. This mode is essential for modern high-speed applications, such as and wireless communications, where low latency and efficient spectrum use are critical.

Communication Modes

Simplex Communication

Simplex communication is the simplest mode of transmission in telecommunications, characterized by unidirectional data flow from a transmitter to one or more receivers, with no provision for a return path or feedback from the receiver. This setup ensures that signals travel solely in one direction, akin to a one-way street, where the sender operates without needing to synchronize or alternate with the recipient./05:_RF_Systems/5.02:_Broadcast_Simplex_Duplex_Diplex_and_Multiplex_Operations) Key characteristics of simplex communication include its lack of coordination requirements between and receiver, rendering it the most straightforward and economical implementation among transmission modes. Common examples encompass broadcast radio and television systems, where a transmits signals to numerous passive receivers, as well as unidirectional networks that stream data to a monitoring hub without reciprocal signaling. The primary advantages of simplex communication stem from its inherent low complexity and efficient use of bandwidth, as it demands only a single channel without additional resources for reversal. Conversely, its disadvantages include the absence of receipt acknowledgment or response mechanisms, which restricts applicability to non-interactive scenarios and precludes correction through feedback. Historically, simplex principles underpinned early telegraph systems, such as those using simplex transmissions for one-directional message across cables, and later one-way pagers that disseminated alerts without user reply capability. This foundational approach laid the groundwork for more advanced bidirectional modes in .

Half-Duplex Communication

Half-duplex communication enables bidirectional transmission over a shared medium, allowing devices to send and receive signals in both directions but not simultaneously, with transmission alternating between parties. This mode requires devices to take turns accessing the channel, often coordinated through protocols or control signals to prevent overlap. Unlike communication, which permits flow in only one direction, half-duplex provides the flexibility of two-way interaction while utilizing a single or channel, thereby enhancing for applications where full simultaneity is unnecessary. Key characteristics of half-duplex systems include the use of mechanisms like Request to Send/Clear to Send (RTS/CTS) handshaking in networking protocols, where a device signals its intent to transmit and awaits acknowledgment before proceeding, reserving the medium for its turn. Common examples encompass walkie-talkies, in which users alternate speaking by pressing a push-to-talk button to transmit and releasing it to receive, as well as legacy Ethernet local area networks (LANs) employing CSMA/CD and certain Internet of Things (IoT) devices that switch modes to conserve power and bandwidth. These systems demand simpler hardware compared to full-duplex setups, as they rely on a shared transceiver rather than separate transmit and receive paths, reducing complexity and cost. The primary advantages of half-duplex include improved spectrum utilization over by enabling reciprocal communication on one channel, avoiding the need for dedicated paths per direction, and lower implementation costs due to minimal hardware requirements. However, it introduces disadvantages such as propagation latency from directional switching and vulnerability to collisions when multiple devices attempt transmission concurrently, which can degrade under heavy load. To mitigate collisions, protocols incorporate detection and recovery mechanisms, balancing efficiency with reliability. A prominent protocol exemplifying half-duplex operation is Carrier Sense Multiple Access with Collision Detection (CSMA/CD), standardized in for early Ethernet LANs. In CSMA/CD, stations monitor the shared medium for carrier activity before transmitting; if idle, they send data while continuously checking for collisions by comparing transmitted and received signals. Upon detecting a collision—indicated by signal — the station aborts transmission, sends a jam signal to alert others, and reschedules retry after a randomized backoff period to minimize repeated conflicts. This access method ensures orderly medium sharing in half-duplex environments but becomes inefficient at high utilization rates, prompting the shift to full-duplex Ethernet in modern networks.

Full-Duplex Communication

Full-duplex communication in refers to a bidirectional transmission mode where data flows simultaneously in both directions between two points, either over the same channel or separate channels. This mode relies on two channels operating in opposite directions to enable concurrent sending and receiving, distinguishing it from sequential alternatives. Key characteristics of full-duplex include its ability to double the effective bandwidth utilization compared to half-duplex systems by allowing uninterrupted simultaneous operation, though it necessitates mechanisms to isolate incoming and outgoing signals to prevent overlap. Representative examples encompass modern calls, where users converse naturally without pausing, and fiber-optic links, which support high-speed bidirectional data transfer over paired or single strands using . The primary advantages of full-duplex are reduced latency for real-time interactions, higher overall throughput that enhances in bandwidth-constrained environments, and support for natural conversation flows in applications like voice calls. However, it introduces disadvantages such as increased system complexity in signal management and susceptibility to interference, including acoustic in audio systems. In terms of throughput, the effective data rate in full-duplex can be expressed as the sum of the uplink and downlink rates, avoiding the resource-sharing penalty of half-duplex: Effective data rate=Ruplink+Rdownlink\text{Effective data rate} = R_{\text{uplink}} + R_{\text{downlink}} where RuplinkR_{\text{uplink}} and RdownlinkR_{\text{downlink}} represent the respective transmission capacities, potentially doubling under ideal conditions.

Traditional Duplexing Techniques

Frequency-Division Duplexing

Frequency-division duplexing (FDD) is a duplexing technique in that employs separate frequency bands for the uplink (transmit) and downlink (receive) directions, allowing simultaneous full-duplex operation without temporal coordination. This separation ensures that the transmitter and receiver can operate concurrently on disjoint portions, a core feature in many standards. Key characteristics of FDD include the necessity of a between the allocated frequency ranges to mitigate interference between uplink and downlink signals, as well as support for fixed or adaptive bandwidth allocation based on . In practice, FDD systems pair spectrum blocks symmetrically, such as in LTE cellular networks where Band 3 assigns 1710–1785 MHz for uplink and 1805–1880 MHz for downlink. The duplex spacing—the frequency offset between these bands—typically ranges from 45 to 190 MHz in mobile systems to prevent overlap and ensure isolation; for example, LTE Band 8 uses a 45 MHz spacing (uplink 880–915 MHz, downlink 925–960 MHz), while Band 1 employs 190 MHz (uplink 1920–1980 MHz, downlink 2110–2170 MHz). FDD offers advantages such as low latency for real-time applications like voice calls, owing to its continuous bidirectional transmission capability. However, it suffers from spectrum inefficiency, as it demands paired allocations plus guard spaces, potentially underutilizing bandwidth in asymmetric traffic scenarios.

Time-Division Duplexing

Time-Division Duplexing (TDD) is a duplexing method in that enables bidirectional communication over a single channel by allocating distinct time slots for uplink and downlink transmissions. This approach divides the transmission timeline into alternating periods where the base station transmits to the user equipment during downlink slots and receives from it during uplink slots, ensuring no simultaneous overlap to prevent interference. TDD's time-based separation contrasts with frequency-based methods by leveraging temporal , making it suitable for unpaired allocations. Key characteristics of TDD include dynamic allocation of time slots based on traffic demands, allowing systems to adapt to asymmetric data flows such as downlink-heavy browsing or uplink-intensive applications like video uploading. Strict among devices and base stations is essential to avoid slot overlaps, which could cause signal interference, and this often relies on precise timing protocols to maintain guard periods between transmissions. Common examples of TDD implementation include , which uses time-slotted master-slave exchanges for low-power personal area networks; standards, employing time-division access for half-duplex contention-based medium sharing; and certain modes in New Radio (NR), where TDD supports flexible subframe configurations in mid-band deployments. TDD offers advantages such as improved efficiency in unpaired bands, where a single allocation suffices for both directions, and greater flexibility to handle varying uplink-downlink ratios without fixed frequency pairing. However, it introduces disadvantages including potential latency increases from scheduling and slot switching, as well as heightened sensitivity to timing errors that can degrade performance in large cells or unsynchronized networks. These trade-offs make TDD particularly valuable in resource-constrained or asymmetric environments, though it demands robust mechanisms. In protocols like TDD-LTE, the frame structure incorporates special subframes to facilitate smooth transitions between downlink and uplink, consisting of a Downlink Pilot Time Slot (DwPTS) for control and data transmission, a Guard Period (GP) to account for propagation delays and switching transients, and an Uplink Pilot Time Slot (UpPTS) for initial uplink signaling such as sounding reference signals. The lengths of DwPTS, GP, and UpPTS are configurable via special subframe configurations defined in standards, ensuring the total duration equals one while adapting to cell size and interference conditions; for instance, longer GPs support larger coverage areas by accommodating round-trip delays. This structured approach enables efficient duplex switching in LTE TDD deployments.

Echo Cancellation

Echo cancellation is a signal processing technique employed in full-duplex wired telecommunications systems to mitigate echo caused by imperfections in hybrid circuits, which separate transmit and receive paths in two-to-four-wire conversions. It achieves this through adaptive filtering that estimates and subtracts replicas of the transmitted signal from the received signal, preventing the far-end talker from hearing their own voice echoed back. This method is essential for enabling clear simultaneous bidirectional communication in telephony networks. The technique typically utilizes digital signal processors (DSPs) to model the echo path impulse response and generate an anti-phase signal for subtraction from the incoming audio. Key performance characteristics include convergence time, which determines how quickly the adaptive filter adapts to the echo path, and tracking ability, allowing it to adjust to variations in the channel due to changing line conditions or noise. These attributes ensure effective echo suppression without introducing significant latency or in real-time voice applications. Mathematically, the echo estimate y^(n)\hat{y}(n) is computed as the convolution of the far-end transmitted signal x(n)x(n) with the estimated echo path impulse response h(k)h(k): y^(n)=∑k=0M−1h(k) x(n−k)\hat{y}(n) = \sum_{k=0}^{M-1} h(k) \, x(n - k) where MM is the filter length. The cancellation e(n)e(n), which represents the near-end signal after echo removal, is then: e(n)=y(n)−y^(n)e(n) = y(n) - \hat{y}(n) with y(n)y(n) being the received signal containing both the near-end speech and the . Adaptive algorithms iteratively update the coefficients h(k)h(k) to minimize the error, often using the normalized least squares (NLMS) method for its balance of simplicity and robustness in echo paths with varying characteristics. Echo cancellation techniques were developed in the by Bell Laboratories to address in long-distance , particularly for communications that facilitated transatlantic voice links. Modern implementations build on this foundation with advanced adaptive algorithms like NLMS, which improve convergence in dynamic environments compared to earlier fixed-filter approaches. In practice, echo cancellation is applied in (VoIP) systems through acoustic echo cancellers in devices like speakerphones, where it subtracts output echoes captured by the to enable hands-free full-duplex operation. Similarly, in traditional (PSTN) hybrids, it compensates for impedance mismatches that cause line echo, ensuring high-quality bidirectional calls over twisted-pair lines.

Advanced Full-Duplex Systems

In-Band Full-Duplex

In-band full-duplex (IBFD) refers to a communication mode that enables simultaneous transmission and reception of signals within the identical band, fundamentally overcoming the challenge of self-interference to achieve near-doubling of compared to traditional half-duplex systems. This approach represents a significant beyond frequency-division duplexing (FDD) and time-division duplexing (TDD), which separate uplink and downlink resources in or time, respectively. By allowing true simultaneity, IBFD can potentially double the capacity of wireless links without requiring additional spectrum allocation. A core characteristic of IBFD systems is the necessity for multi-stage self-interference cancellation across analog, radio-frequency (RF), and digital domains to suppress the transmitted signal's leakage into the receiver. Self-interference in these setups can be 100-120 dB stronger than the desired received signal, demanding cancellation levels exceeding 110 dB to ensure reliable operation. Experimental prototypes for networks, such as real-time full-duplex transceivers developed for cellular base stations, have demonstrated this capability through integrated hardware that achieves over 100 dB of total suppression. A notable commercial example is 's 2024 full-duplex transport solution for fixed point-to-point links, which operates in the D-band spectrum and delivers 10 Gbps bidirectional throughput over a single channel by leveraging advanced RF cancellation techniques. In 2025, further advanced this technology by integrating dual polarization, enabling a fourfold capacity increase to up to 25 Gbps per direction over a single 2 GHz channel. The primary advantages of IBFD include a theoretical 100% gain in and reduced end-to-end latency, as data flows bidirectionally without scheduling delays inherent in half-duplex modes. These benefits make IBFD particularly suitable for high-demand scenarios like dense urban deployments. However, implementation drawbacks arise from the high complexity of cancellation hardware, which increases power consumption and design costs while requiring precise to handle dynamic channel variations. Recent advancements have focused on integrating IBFD with massive multiple-input multiple-output () architectures, where spatial separation via antenna arrays further aids interference mitigation, as outlined in a 2024 IEEE review of over a decade of progress in this area. Looking ahead, IBFD holds substantial potential for networks, especially in supporting ultra-reliable low-latency communication (URLLC) applications such as industrial , by enabling instantaneous bidirectional exchanges that minimize delay in time-critical services.

Self-Interference Management

Self-interference in in-band full-duplex (IBFD) systems refers to the leakage of the transmit signal into the collocated receiver , which overwhelms the desired incoming signal and serves as the primary technical barrier to practical . The sources of self-interference include antenna between transmit and receive elements, hardware non-linearities in amplifiers and mixers, and multipath reflections in the surrounding environment that create additional leakage paths. To mitigate this, multi-domain cancellation approaches are employed, encompassing passive suppression methods such as circulators and isolators to physically isolate transmit and receive paths, analog-domain pre-cancellation circuits that subtract interference before downconversion, and digital-domain post-FFT that models and removes residual components after signal . Specific techniques for self-interference management include advanced antenna designs that exploit orthogonal polarization to decouple transmit and receive signals, RF cancellers that generate anti-phase replicas of the interference in the analog domain, and machine learning-based adaptive filtering algorithms that dynamically estimate and subtract non-linear interference components in real-time. The effectiveness of these methods is often quantified by the residual interference power, given by Pres=PtxGcP_{\text{res}} = \frac{P_{\text{tx}}}{G_c}, where PtxP_{\text{tx}} is the transmit power and GcG_c is the total cancellation gain, typically targeting over 100 dB to ensure viable operation; this residual then impacts the signal-to-interference-plus-noise ratio as SINR=PrxPres+N\text{SINR} = \frac{P_{\text{rx}}}{P_{\text{res}} + N}, with PrxP_{\text{rx}} as the received signal power and NN as thermal noise. In 2024, advancements in full-duplex massive systems have demonstrated self-interference suppression levels of 90-110 dB through the use of multi-antenna arrays that enable spatial and enhanced passive isolation. These techniques are particularly driven by the needs of IBFD deployments in next-generation wireless networks.

Applications and Examples

In and Wired Networks

In , duplex communication enables natural, simultaneous two-way conversation in (POTS) by converting bidirectional 2-wire local loops to unidirectional 4-wire paths using hybrid circuits at central offices. These hybrids separate transmit and receive signals, preventing the speaker's voice from looping back directly while allowing full-duplex operation over shared copper pairs. This setup supports the core requirement of voice calls, where both parties can speak and listen without interruption, mimicking face-to-face dialogue. In analog telephone lines, reduction plays a key role in full-duplex performance by feeding back a controlled portion of the user's own voice to the earpiece, confirming transmission without overwhelming . This technique, implemented via anti-sidetone circuits in handsets, balances feedback to avoid discomfort while ensuring the line remains active. For digital systems, Integrated Services Digital Network (ISDN) and (DSL) technologies achieve full-duplex transmission over twisted-pair copper, enabling simultaneous upload and download of voice and data at rates up to 128 kbps for ISDN basic rate interfaces. DSL variants, such as asymmetric DSL, further leverage these pairs for broadband duplexing, separating upstream and downstream signals through modulation techniques. Historically, early 20th-century introduced with echo suppressors to manage reflections in long circuits, as developed by Bell Laboratories in 1925 for suppressing delayed speech replicas in transcontinental links. These devices attenuated signals in one direction during active talking, reducing but not eliminating es. By the 1980s, the transition to full echo cancellers occurred, driven by advances that subtract predicted echo paths adaptively, improving clarity in integrated circuits for switches and PBXs. Echo cancellation became the key enabler for robust duplexing in modern wired systems. Wired duplexing offers cost-effective deployment over existing copper infrastructure, avoiding the expense of new cabling while supporting gigabit speeds in advanced configurations like G.fast. However, challenges persist in long-haul links, where near-end echo arises from local hybrids reflecting the talker's voice back immediately, and far-end echo results from distant hybrids causing delayed returns, both degrading conversation quality without mitigation. These issues are pronounced in or international trunks exceeding 30 ms delay.

In Wireless and Mobile Networks

In wireless and mobile networks, frequency-division duplexing (FDD) is widely employed in LTE systems to support low-latency applications such as voice calls and video streaming, utilizing paired bands to enable simultaneous uplink and downlink transmissions without interference. This approach ensures stable connectivity in mobile environments by allocating separate frequencies for transmit and receive paths, which is particularly suited for symmetric traffic patterns common in real-time communications. In contrast, time-division duplexing (TDD) facilitates flexible data allocation in systems like TD-SCDMA (a standard) and New Radio (NR), where uplink and downlink share the same frequency but alternate in time slots, allowing dynamic adjustment to asymmetric data demands such as high downlink video streaming. TDD's adaptability enhances in bandwidth-constrained scenarios, making it ideal for evolving mobile data services in networks. Representative examples include the Global System for Mobile Communications (), which relies on FDD to provide reliable voice services across paired bands like 900 MHz and 1800 MHz, supporting global mobility through frequency separation. Similarly, Worldwide Interoperability for Microwave Access () predominantly uses TDD to handle variable data traffic, enabling efficient resource sharing in fixed and mobile deployments via time-slotted frames. Recent advancements in in-band full-duplex (IBFD) technology have introduced trials for relays and device-to-device (D2D) communications, aiming to double by allowing simultaneous transmission and reception on the same frequency band, with self-interference cancellation techniques mitigating signal overlap. These developments, explored in Release 18 (building on foundational work from Release 15), focus on enhancing coverage in relay-assisted scenarios and direct peer communications, such as in urban D2D links for offloading traffic. A notable 2024 demonstration by showcased IBFD in wireless backhaul, achieving 10 Gbps bidirectional capacity over a single 2 GHz channel in the D-band, effectively doubling throughput for fronthaul and backhaul links compared to traditional half-duplex methods. In September 2025, launched the world's first commercial full-duplex solution in the E-band for backhaul, achieving global verification for enhanced capacity. Duplex techniques in networks provide key advantages for mobility, including seamless support during user movement by maintaining continuous bidirectional links, which is essential for applications like vehicular communications. However, challenges arise, particularly in TDD systems where Doppler effects from high-speed mobility can disrupt time-slot , leading to inter-cell interference and requiring advanced timing compensation mechanisms.

In Broadband and Data Systems

In broadband and data systems, duplexing techniques enable simultaneous bidirectional data transmission over high-speed networks, addressing the growing demand for symmetric upload and download capacities in applications like and video conferencing. Full-duplex Ethernet, standardized under IEEE 802.3x since 1997, achieves this by utilizing separate twisted-pair cables for transmission and reception, eliminating collisions and doubling effective throughput compared to half-duplex modes. This approach supports speeds from 10 Mbps to multi-gigabit levels in local area networks, providing dedicated paths that enhance reliability for data-intensive tasks. In cable broadband, (DOCSIS) standards incorporate frequency-division duplexing (FDD)-like sub-bands to separate upstream and downstream signals, with DOCSIS 3.1 extending the upstream spectrum to 204 MHz for improved performance over legacy infrastructure. DOCSIS 4.0 further advances this by integrating extended spectrum DOCSIS (ESD) up to 1.8 GHz downstream and 684 MHz upstream, alongside full-duplex modes that allow overlapping frequency use for symmetric multi-gigabit services. Cable modems in these systems operate within designated sub-bands, such as 5-85 MHz for upstream in traditional FDD configurations, enabling efficient spectrum allocation without requiring full network overhauls. Fiber-to-the-home (FTTH) deployments leverage (WDM) to implement full-duplex communication over a single , assigning distinct wavelengths for upstream (e.g., 1310 nm) and downstream (e.g., 1550 nm) directions to achieve high-capacity bidirectional links. This technique supports gigabit-to-terabit speeds with minimal latency, making it ideal for residential and enterprise . Trials as early as 2022 by providers like demonstrated symmetric speeds up to 4 Gbps using full-duplex over coaxial networks. In 2024, began rolling out these technologies in select markets, offering multi-gigabit symmetrical speeds and effectively mitigating upload bottlenecks in video streaming and remote collaboration by balancing traffic loads. By September 2025, full-duplex 4.0 amplifiers were deployed across 's entire U.S. footprint, supporting widespread symmetrical multi-gigabit . The advantages of full-duplex in include significantly higher throughput for services, where symmetric speeds facilitate seamless and real-time uploads without contention delays. However, challenges persist in legacy networks, where inherent favors downstream bandwidth (often 10-20 times greater than upstream) due to historical spectrum allocations, limiting upload-intensive applications until upgrades like 4.0 are implemented.

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