Technical Analysis of Wireless Communication Duplex Modes

In communication systems, duplex technology stands as one of the most crucial technologies enabling bidirectional communication between devices. It determines how to efficiently utilize bandwidth and spectrum resources within a single device or network, coordinates uplink (from user equipment to base station) and downlink (from base station to user equipment) data transmission, and directly impacts spectrum efficiency, system capacity, and communication real-time performance.

Duplex modes in communication are generally categorized into three types: simplex, half-duplex, and full-duplex, and the same classification applies to wireless communication. Simplex mode supports only unidirectional communication, such as early broadcast systems. Half-duplex mode allows devices to transmit data in both directions but not simultaneously. Full-duplex mode breaks through this limitation, enabling bidirectional communication at the same time and on the same frequency. In practical applications, full-duplex technology has further evolved into various specific implementation methods, such as Co-frequency Co-time Full Duplex (FD), Hybrid Duplex, and Subband Full Duplex (SBFD).

Translation of the Text–Ethernet, the representative of wired networks, has two operating modes at the physical layer: half-duplex and full-duplex. The half-duplex mode allows data to be either received or transmitted at any given time; it adopts the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) mechanism and is subject to limitations on maximum transmission distance. Hubs operate in half-duplex mode.

After Layer 2 (L2) switches replaced hubs in Ethernet construction, Ethernet evolved from a shared network to a switched network. Furthermore, full-duplex mode replaced half-duplex mode, significantly improving the efficiency of data frame transmission, with the maximum throughput reaching double the rate. Full-duplex fundamentally resolves the collision issue in Ethernet, marking the end of CSMA/CD for Ethernet.

The operating characteristics of Ethernet full-duplex mode are as follows: it enables simultaneous data reception and transmission; the maximum throughput achieves double the rate; and it eliminates the physical distance limitation of half-duplex. Currently manufactured network interface cards (NICs), Layer 2 devices, and Layer 3 devices all support full-duplex mode, with the exception of hubs. To implement full-duplex, the hardware requires NIC chips that support full-duplex, physical media with completely separated transmit and receive lines, and a point-to-point connection.

Compared with wired networks, wireless communication relies primarily on air interfaces due to the limited availability of spectrum resources. It is more susceptible to the impacts of physical conditions such as interference and obstruction. Therefore, the duplex technologies used in wireless communication pose greater technical challenges than those in wired communication. This article will focus on the duplex modes of wireless communication, systematically sort out the mainstream duplex modes, analyze their respective technical characteristics, advantages, disadvantages, application scenarios, and development prospects, and provide a professional perspective for understanding the underlying logic of wireless communication.

1. Basic Concepts of Duplex

Two-way transmission refers to a mode of communication where both parties can transmit information to each other, including two types: half-duplex (alternating transmission) and full-duplex (simultaneous transmission). Duplex technology in optical communication requires channel multiplexing to achieve bidirectional data interaction. Common multiplexing methods include Space Division Multiplexing (SDM), Time Compression Multiplexing (TCM), Wavelength Division Multiplexing (WDM), and Subcarrier Multiplexing (SCM). Among them, TCM transmits signals alternately through a single fiber; WDM separates uplink and downlink optical signals using different wavelengths; and SCM distinguishes bidirectional signals by different frequency bands.

Duplex refers to the capability of both communicating parties to conduct bidirectional communication, that is, a device can both send and receive information. The duplex mode describes the rules for coordinating transmission and reception in such bidirectional communication. The core goal of the duplex mode is to solve the problem of resource allocation when communication devices receive and send data, while minimizing interference as much as possible. Especially in wireless communication, spectrum resources are extremely valuable, and how to improve spectrum utilization is a primary issue that all wireless product and technology manufacturers need to address. For the limited nature of spectrum resources, please refer to Basics of Radio Frequencies and Frequency Allocation.

At present, the mainstream duplex modes in wireless communication are Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD).

Frequency Division Duplexing (FDD)

It realizes bidirectional communication by allocating uplink and downlink data to different frequencies. Both communicating parties send and receive data simultaneously without the need to wait for time slot switching. Its core advantages are that the uplink and downlink frequency bands are separated, which naturally avoids signal conflicts within the same frequency band, making it suitable for scenarios with wide coverage and scattered user distribution (such as traditional cellular networks); there is no need for time slot switching, so both communicating parties can send and receive data simultaneously, meeting the low-latency requirements of services such as voice calls and real-time videos. Its disadvantages are low spectrum utilization, as it needs to occupy double the frequency band resources; in addition, its frequency band configuration flexibility is poor, and the spectrum needs to be planned in advance, making it difficult to dynamically adjust according to service traffic (for example, it cannot be flexibly expanded when the demand for uplink data surges). For instance, in 4G LTE-FDD, the uplink may use the 2.1GHz frequency band, and the downlink may use the 2.6GHz frequency band. A guard interval is set between the two frequency bands to avoid adjacent channel interference.

Time Division Duplexing (TDD)

It realizes bidirectional communication on the same frequency by dividing different time slices (tiny time units). Due to the characteristic of sharing the same frequency band, it is naturally suitable for scenarios with tight spectrum resources (such as the Internet of Things and Device-to-Device (D2D) communication). Its core advantages are saving frequency resources, as the uplink and downlink share the same frequency band without the need for additional allocation, making it suitable for scenarios with tight spectrum resources (such as the Internet of Things and D2D communication); it has flexible dynamic adjustment, and the ratio of uplink and downlink time slots can be dynamically adjusted according to service requirements (for example, more uplink data uploads than downlink downloads, such as burst uplink data transmission in the Internet of Vehicles), thereby improving resource utilization. Therefore, TDD is widely used in scenarios with symmetric data services and tight spectrum resources. Its disadvantage is that it requires strict clock synchronization, such as relying on Beidou or GPS, as well as inter-base station time synchronization protocols; otherwise, the overlap of uplink and downlink gaps will cause interference and increase system complexity. For example, in 5G New Radio (NR) TDD, the base station and the terminal share the same frequency band: the first half of the time is used to send downlink data, and the second half of the time is used to receive uplink data. Both communicating parties must strictly synchronize time to ensure that the uplink and downlink time slots do not overlap.

As shown in the following conceptual diagram of frequency domain and time domain: the complex waveform on the left side of the diagram represents the change of the signal over time, which is formed by superimposing multiple sine waves of different colors, indicating the amplitude change of the signal on the time axis; the spectrum diagram on the right side of the diagram represents the distribution of the signal in the frequency domain, and the vertical bars in the spectrum diagram represent the intensity of different frequency components, reflecting the frequency composition relationship of the signal.

In the 3G era, among the three major international standards, WCDMA and CDMA2000 adopted the FDD mode, while TD-SCDMA adopted the TDD mode. In the 4G and 5G eras, the converged networking of FDD and TDD is a wireless communication networking solution formed based on the complementarity of these two duplex modes.

II. Duplex Modes in Different Wireless Systems

The duplex mode has a significant impact on communication networks. However, different application scenarios have different requirements for networks, so the duplex technologies adopted also vary. Currently, for 5G macro base stations and wireless private network technologies, the duplex modes are dominated by FDD/TDD, while WiFi has both similarities and differences.

1. Duplex Modes of WiFi

WiFi mainly involves two duplex modes: half-duplex and full-duplex, with specific implementation methods depending on device support.

Half-duplex mode: Traditional WiFi devices (such as early routers and terminals) usually adopt the half-duplex mode, meaning that transmission and reception must be carried out in separate time slots on the same channel. For example, when a device is sending data, it cannot receive data; and when it is receiving data, it cannot send data. This mode is implemented through the CSMA/CA protocol (Collision Avoidance Mechanism) to ensure that data transmission is only performed when the channel is idle.
Full-duplex mode: In the WiFi 7 standard, the MLO (Multi-Link Operation) mechanism supports the STR (Simultaneous Transmit and Receive) mode. This mechanism allows multiple RF links (Radio Frequency Links) to work simultaneously, theoretically achieving full-duplex communication. In other words, WiFi 7 APs (Access Points) and STAs (Stations) that support MLO can achieve bidirectional data transmission through multiple links. Most previous WiFi versions were half-duplex. This is why WiFi networks have relatively high latency, and services requiring high real-time performance (such as high-definition video surveillance) occasionally experience “freezing”.

For basic knowledge of WiFi technology, please refer to the article How Powerful is WiFi 7? The Past and Present of WiFi.

2. Duplex Modes of Wireless Private Networks

Wireless private networks mainly use TDD/FDD as their duplex modes, and can realize multi-frequency converged TDD communication through multiple links.

In wireless private network communication systems, the main duplex mode is TDD. For example, the iMAX wireless metropolitan area network system. Due to its fundamental differences from traditional WiFi systems, the iMAX wireless metropolitan area network private network system has significant technical advantages over WiFi (or bridges) in indicators such as frequency utilization, bandwidth utilization, video bandwidth capacity, and network real-time performance.

The iMAX wireless metropolitan area network system mainly realizes duplex based on the TDD mode. It improves the utilization rate of uplink and downlink resources through algorithms and configurations, and especially achieves millisecond-level delay control in industrial Internet of Things (IoT) scenarios, meeting the demand for efficient bearing of asymmetric traffic in intelligent manufacturing.

In some special scenarios, the iMAX wireless metropolitan area network uses the “Multi-Link Data Mirroring (MLDM) technology”. Its underlying logic is to use different frequencies to construct more than 2 independent wireless links (each of which adopts the TDD mode), enabling the same data to be sent simultaneously through different frequencies. Therefore, from a macro perspective, this technology actually realizes FDD+TDD hybrid networking at the network architecture level, thus significantly enhancing system reliability.

Guoxin Longxin also has some wireless communication systems (such as FibeAir E_BAND microwave), which realize duplex based on FDD. The uplink and downlink can be set to different working frequencies respectively, thereby obtaining larger bandwidth and higher capacity. With a maximum bandwidth of up to 20 Gbps, it is known as “wireless optical fiber”.

3. Duplex Modes of 5G

5G mainly adopts TDD/FDD as its duplex modes.

Currently, 5G macro base stations are the backbone of current wireless communication networks, responsible for wide-area coverage and large-scale user access. In this scenario, Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) occupy a dominant position due to their mature technical foundation and good deployment capabilities.

The FDD mode is mainly used for symmetric services in 5G macro base stations, such as voice calls and real-time video transmission. Its advantage lies in that the uplink and downlink signals do not interfere with each other, ensuring high communication stability. However, FDD requires paired frequency bands, which becomes a limiting factor in environments where spectrum resources are scarce.

The TDD mode, on the other hand, is more suitable for asymmetric services (such as Internet services where data download is much higher than upload) due to its flexible time slot configuration, and is widely used in data-intensive scenarios in 5G macro base stations. By dynamically adjusting the ratio of uplink and downlink time slots, TDD can utilize spectrum resources more efficiently. For example, in China Mobile’s 5G network, the subframe configuration of the TDD mode can be flexibly adjusted to have more downlink time slots than uplink time slots, thereby meeting users’ demand for high download speeds.

Overall, the FDD and TDD modes remain the mainstream choices in 5G macro base stations, mainly due to their mature standardization process, low equipment costs, and strong interference control capabilities.

III. Technological Development of Duplex Modes

1. Frequency Division Duplexing (FDD)

The core advantages of Frequency Division Duplexing (FDD) lie in its strong anti-interference capability and high coverage stability, which enable it to be used for a long time in scenarios such as wide-area mobile communication (e.g., 5G RAN) and satellite communication. However, its inherent drawback of “occupying double spectrum resources” makes it gradually replaced by more efficient modes in the context of scarce spectrum resources in high-frequency bands (such as millimeter waves and terahertz). In the future, FDD may be more used in low-frequency wide-area coverage (e.g., 700 MHz frequency band) or asymmetric traffic scenarios (e.g., downlink-dominated streaming media transmission), while optimizing resource utilization through Dynamic Frequency Sharing (DFS) technology.

2. Time Division Duplexing (TDD)

Due to its characteristic of sharing the same frequency band, Time Division Duplexing (TDD) is naturally suitable for scenarios with tight spectrum resources (such as IoT and D2D communication). With the advancement of 5G-Advanced and 6G, TDD will evolve towards “more flexible time slot configuration”:

Ultra-dynamic time slot allocation: Through AI-driven network scheduling algorithms, the system can sense uplink and downlink service traffic in real time (such as burst uploads in smart meter reading or low-latency requirements in the Internet of Vehicles) and dynamically adjust the time slot ratio, avoiding “resource waste caused by fixed time slots” in traditional TDD.
In-depth integration with MIMO technology: By combining with Massive MIMO, TDD can further improve capacity through beamforming and spatial multiplexing. Especially in Ultra-Dense Networks (UDN), it is expected to become a key means to solve the “capacity bottleneck”.
Adaptation to millimeter wave and terahertz frequency bands: In high-frequency communication, FDD is limited by the need for double spectrum resources, while TDD, relying on its single-frequency band advantage, may become the mainstream choice for millimeter wave/terahertz frequency bands (e.g., 6G terahertz communication).

In addition to TDD and FDD, several other duplex modes have also attracted much attention.

3. Co-Frequency Co-Time Full Duplex (CCFD)

Co-Frequency Co-Time Full Duplex (CCFD) is a wireless communication technology that enables simultaneous transmission and reception of signals at the same time and frequency, theoretically doubling the spectrum efficiency. This technology needs to solve the problem of self-interference between transmitted and received signals, and adopts a three-level joint elimination mechanism including the spatial domain (antenna isolation, beamforming), radio frequency domain (analog cancellation circuit), and digital domain (baseband processing). In 2019, a prototype developed by the University of Electronic Science and Technology of Chengdu achieved a self-interference suppression capability of 122.1 dB. In 2022, 3GPP launched the Rel-18 Sub-Band Full Duplex (SBFD) research project, with China Mobile serving as the reporter and releasing a technical report.

By transmitting and receiving signals at the same frequency and time, the CCFD mode can theoretically double the spectrum efficiency. For example, in LTE FD experiments, its spectrum efficiency can reach more than 70%, far exceeding that of FDD and TDD modes. However, the self-interference cancellation technology of the CCFD mode is complex and the hardware cost is high, so it is currently mainly used in experimental scenarios and specific high-value services.

In Ultra-Reliable Low-Latency Communication (URLLC) services, such as industrial automation, remote surgery, and the Internet of Vehicles, the CCFD mode is a key choice. The CCFD mode can perform transmission and reception operations at the same time and frequency, significantly reducing communication latency and meeting the high requirements of URLLC for real-time performance and reliability. However, the self-interference cancellation technology of the FD mode and its hardware cost are relatively high, so in specific deployments, delay optimization design needs to be carried out according to service characteristics.

Co-Frequency Co-Time Full Duplex (CCFD) is an emerging full-duplex mode that allows devices to transmit and receive simultaneously at the same time and frequency. Theoretically, CCFD technology can double the spectrum efficiency, thereby alleviating the problem of scarce spectrum resources. However, this mode faces a key issue: Self-Interference (SI). Self-interference refers to the interference caused by a device’s transmitted signal to its local received signal, and the intensity of this interference is much higher than that of the target signal. To solve this problem, CCFD technology needs to combine multiple methods such as antenna interference cancellation, radio frequency interference cancellation, and digital interference cancellation.

By enabling simultaneous transmission and reception at the same frequency and time, CCFD can theoretically double the spectrum efficiency of traditional duplex modes (FDD/TDD). Its development prospects focus on two directions:

In-depth application in D2D communication: FD can also be used in 5G D2D (such as vehicle-to-vehicle direct communication in the Internet of Vehicles). In the future, with the development of “Cell-Free Networks” and “Device Clustering” in 6G, CCFD will become a core supporting technology for efficient collaboration between devices.
Maturity of self-interference suppression technology: The core challenge of FD is Self-Interference Cancellation (SIC). Current SIC technology relies on high-isolation antennas (analog domain suppression) and digital domain signal processing (such as self-interference reconstruction and cancellation based on transmitted signals). In the future, with breakthroughs in AI-driven SIC algorithms (such as deep learning to predict interference waveforms) and intelligent antenna technologies (such as Reconfigurable Intelligent Surfaces, RIS), the practicality of CCFD will be significantly improved, and it may be extended to scenarios such as UAV communication and edge computing device interconnection.

4. Hybrid Duplex (HD)

Hybrid Duplex (HD) falls into the category of “multi-mode dynamic adaptation”, and its core is to switch duplex modes according to service requirements. Its future development directions include:

Intelligent network decision-making system: By introducing AI/ML technologies, the network can analyze service types in real time (such as the low-latency requirements of URLLC or the large-connection requirements of mMTC), and automatically select the optimal duplex mode (e.g., switching to FDD in high-real-time scenarios, and switching to TDD or FD in spectrum-scarce scenarios) to reduce handover overhead.
Adaptation to 6G “Ultra-Reliable Low-Latency Communication” (URLLC): In URLLC scenarios (such as industrial automation and remote surgery), hybrid duplex can reduce handover latency and improve reliability through a “pre-handover” strategy (e.g., sensing service traffic in advance and adjusting modes).
Expansion of spectrum sharing: The “dynamic frequency band adjustment” capability of HD can be combined with Dynamic Frequency Sharing (DFS) technology to support different operators or devices in flexibly allocating uplink and downlink resources in shared spectrum, promoting efficient reuse of spectrum resources.

5. Sub-Band Full Duplex (SBFD)

Another evolution direction worthy of attention is Sub-Band Full Duplex (SBFD). SBFD is an intermediate form of CCFD technology. It configures uplink sub-bands in the TDD frame structure, allowing specific devices to perform full-duplex communication within the sub-bands, while traditional terminals can still operate in the TDD half-duplex mode. This technology not only improves spectrum utilization but also takes into account the compatibility of traditional devices, providing a feasible path for large-scale full-duplex deployment in 6G networks.

The SBFD mode provides a compromise solution. It introduces uplink sub-bands into the TDD frame structure, enabling some devices to perform full-duplex communication within the sub-bands, while traditional devices can still operate in the TDD duplex mode. This design not only improves spectrum efficiency but also retains compatibility with traditional devices. In high-density hotspot areas, the SBFD mode can significantly increase uplink capacity, meeting the needs of applications requiring high uplink rates such as the Internet of Things (IoT), Augmented Reality (AR), and high-definition video streaming.

In addition, the frequency-domain resource allocation modulo algorithm of SBFD also enables it to perform well in multi-user scenarios. For example, in a hotspot area with multiple user devices, the system can dynamically allocate sub-band resources according to the real-time needs of users based on algorithms or mechanisms, thereby optimizing overall performance.

For Enhanced Mobile Broadband (eMBB) services, such as high-definition video streaming and VR/AR, Sub-Band Full Duplex (SBFD) and Time Division Duplex (TDD) are preferred solutions. Through frequency-domain sub-band division, SBFD realizes full-duplex communication for some devices while taking into account the compatibility of traditional terminals; TDD, with its flexible time slot configuration, can adapt to the needs of asymmetric services and improve the overall throughput of the system.

Summary

The evolution of wireless communication duplex technology, from FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing) to Co-Frequency Co-Time Full Duplex (FD) and Sub-Band Full Duplex (SBFD), reflects the continuous improvement of spectrum resource utilization efficiency and communication performance. In the evolution of 5G and 6G, CCFD and SBFD have become research hotspots due to their significant advantages in spectrum efficiency; while FDD and TDD, as traditional duplex modes, still play an irreplaceable role in wide-area coverage and symmetric service scenarios.

Guoxin Longxin has been researching and following up on duplex technologies in the communication industry (especially the wireless communication industry). Its iMAX series products mainly realize duplex based on the TDD mode, improving the utilization rate of uplink and downlink resources through algorithms and configurations, and especially achieving millisecond-level delay control in industrial IoT scenarios, meeting the demand for efficient bearing of asymmetric traffic in intelligent manufacturing. However, the Multi-Link Data Mirroring (MLDM) technology uses different frequencies to construct more than 2 independent wireless links (each adopting the TDD mode), enabling the same data to be sent simultaneously through different frequencies. Therefore, from a macro perspective, this technology actually realizes FDD+TDD hybrid networking at the network architecture level.

In general, in the evolution from 5G to 6G, duplex modes may show the following trends:

Multi-mode dynamic collaboration: Networks will no longer be a “choice” between a single FDD/TDD/CCFD, but will realize “multi-mode parallelism” through AI scheduling (e.g., base stations supporting FDD and TDD simultaneously, and terminals switching according to service requirements) to maximize resource utilization.
Scenario-customized duplex: Different scenarios (such as wide-area coverage, the Internet of Vehicles, and industrial Internet) will have exclusive duplex strategies (e.g., retaining FDD for wide-area coverage and mandating CCFD for the Internet of Vehicles), and improve efficiency through “scenario sensing – mode adaptation”.
Integration with integrated communication and sensing technology: The “communication + sensing” function of 6G (such as integrated radar and communication) requires duplex modes to support simultaneous transmission and reception (e.g., transmitting detection signals while receiving reflected signals), and CCFD may become a key technology.

Continuous research and innovation in technology are the fundamental reasons for Guoxin Longxin to stand out and remain prosperous in the wireless communication industry. We will continue to follow the progress of duplex-related technologies and launch more advanced technologies in a timely manner to solve practical problems faced by users in network construction.

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