Physical Channel

Description

In 3GPP, a Physical Channel is a fundamental concept in the physical layer of the radio interface. It is defined as a path through a communication space defined in time, frequency, and code, established for a given period of time. The specific parameters that define a physical channel depend on the duplex mode. In Frequency Division Duplex (FDD) mode, a physical channel is defined by code, frequency, and, in the uplink, relative phase (I/Q). In Time Division Duplex (TDD) mode, a physical channel is defined by code, frequency, and time-slot. This definition is provided in specification 21.905 and further detailed in 23.050.

The Physical Channel is the lowest layer in the 3GPP protocol stack, responsible for the actual transmission of bits over the air interface. It is distinguished from transport channels and logical channels, which are higher-layer concepts. Transport channels define how data is transferred over the radio interface, while logical channels define the type of information being transferred. Physical channels are the actual implementation of these channels in the physical layer, mapping transport channels to specific physical resources.

In FDD mode, the uplink and downlink use different frequency bands, so a physical channel is defined by a specific code (spreading code) and frequency. In the uplink, the relative phase (I/Q) is also used to distinguish channels, as multiple channels can share the same frequency and code but have different phase. In TDD mode, the same frequency band is used for both uplink and downlink, but they are separated in time by time-slots. Therefore, a physical channel in TDD is defined by code, frequency, and time-slot. This allows multiple users to share the same frequency and code by using different time-slots.

The Physical Channel is responsible for various functions, including modulation, spreading, scrambling, and power control. It carries both user data and control information. Examples of physical channels in 3GPP include the Physical Random Access Channel (PRACH), Physical Downlink Shared Channel (PDSCH), Physical Uplink Shared Channel (PUSCH), and Physical Control Format Indicator Channel (PCFICH). Each of these channels has a specific role in the radio interface, such as random access, data transmission, or control signaling.

The definition of Physical Channel has evolved over releases. In R99, it was defined as a path through a communication space defined in time, frequency, and code. In Rel-4, the definition was revised to explicitly mention FDD and TDD modes and the specific parameters for each. This change reflected the maturation of the 3GPP specifications and the need for a more precise definition. Since Rel-4, the definition has remained largely unchanged, although the specific physical channels have evolved with new technologies like HSPA, LTE, and 5G NR.

In summary, the Physical Channel is a core concept in the 3GPP physical layer, defining how radio resources are used for transmission. Its definition varies by duplex mode and has been stable since Rel-4. Understanding physical channels is essential for engineers working on radio access network design and optimization.

Purpose & Motivation

The Physical Channel concept exists to provide a clear and standardized way to define the physical layer transmission resources in 3GPP networks. It solves the problem of how to allocate and distinguish multiple simultaneous transmissions over the radio interface. By defining a physical channel in terms of code, frequency, and time-slot (and phase in FDD uplink), it allows multiple users and services to share the same spectrum efficiently.

Before the formal definition, the physical layer was described in more abstract terms, which made it difficult to specify exact transmission parameters and to ensure interoperability between different vendors' equipment. The introduction of the Physical Channel concept in R99 and its refinement in Rel-4 provided a precise framework for physical layer design. It enabled the development of complex multiple access schemes like WCDMA and TD-SCDMA, and later OFDMA for LTE and 5G.

The Physical Channel also serves as a foundation for higher-layer channel mapping. Transport channels are mapped to physical channels, and logical channels are mapped to transport channels. This layered approach allows for flexibility and separation of concerns. The Physical Channel definition has been maintained across releases because it remains a valid abstraction for new radio technologies, even as the specific parameters (e.g., subcarrier spacing, bandwidth parts) have evolved. It is a testament to the enduring relevance of the concept.