RFUnderstanding UMTS Channels: Logical, Transport, and Physical
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UMTS channels form the backbone of data transfer, categorizing information into logical, transport, and physical channels. Each channel type has distinct roles in data flow, facilitating smooth communication and reducing interference.
Understanding these channels provides clarity on how data is processed, transmitted, and received in a UMTS network. As shown in the figure, there are three types of UMTS channels viz. logical channels (RLC layer), transport channels (MAC layer) and physical channels (PHY layer).
As they traverse between layers, they map to other layer frames. For example, logical channels are mapped to transport channels, and transport channels are mapped to physical channels.
UMTS Logical Channels
Logical channels define what type of information is transmitted over the network. They are categorized into two groups: control channels and traffic channels.
- Control Channels: Carry signaling and control information.
- Traffic Channels: Carry user data such as voice and internet traffic.
As the name suggests, traffic channels carry information (voice), and control channels carry signaling information useful to establish and maintain a connection between the UE and the network (NodeB).
Following are the functions of each of these logical channels. Note that DL stands for Downlink from network to UE and UL stands for Uplink from UE to Network.
- DTCH (DL and UL, point to point): Dedicated Traffic channel, carries user information.
- CTCH (unidirectional point to multi-point): Common traffic channel.
- BCCH (DL broadcast): Broadcast Control Channel, carries SYSTEM INFORMATION.
- CCCH (Bidirectional): Common Control Channel.
- DCCH (Dedicated Control Channel, point to point bidirectional):
- PCCH (Paging Control Channel, DL): Carries paging information.
- SHCCH (Shared Channel Control Channel, Bidirectional):
| UMTS/WCDMA Logical Channel | Function | Direction | Duplex Mode |
|---|---|---|---|
| Dedicated traffic channel (DTCH) | Carry user plane data (speech/packet data) | Uplink (UL)/Downlink (DL) | FDD/TDD |
| Dedicated control channel (DCCH) | Carry signaling messages used for control operations | Uplink/Downlink | FDD/TDD |
| Shared control channel (SHCCH) | Carry control messages | Uplink/Downlink | TDD |
| Common control channel (CCCH) | It is used to carry control messages | Uplink/Downlink | FDD/TDD |
| Common traffic channel (CTCH) | It is used to carry user traffic (e.g., cell broadcast SMS) | Downlink | FDD/TDD |
| Paging control channel (PCCH) | It is used to carry paging messages directed towards UE | Downlink | FDD/TDD |
| Broadcast control channel (BCCH) | It is used to carry broadcast messages towards UE | Downlink | FDD/TDD |
UMTS Transport Channels
Transport channels define how data is transmitted over the air interface. They determine characteristics such as data rate, error protection, and transmission timing.
Following are the transport types of UMTS channels:
- BCH (DL broadcast): SYSTEM/Cell related INFORMATION
- FACH (DL/Forward Access Channel): Carries control information from network to UE
- PCH (DL): Paging Channel
- DCH (UL or DL): Dedicated Channel
- DSCH (DL): Downlink Shared Channel
- USCH (UL): Uplink Shared Channel
- RACH (UL): Random Access Channel
| UMTS/WCDMA Transport Channel | Function | Direction | Duplex Mode |
|---|---|---|---|
| Random access channel (RACH) | It is used for initial access of the network by UE | Uplink | FDD/TDD |
| Common packet channel (CPCH) | It is used as contention channel for bursty data | Uplink | FDD/ |
| Forward access channel (FACH) | It is used to transfer user data/control signaling messages | Downlink | FDD/TDD |
| Downlink shared channel (DSCH) | It is used as shared channel carrying dedicated user data/control messages in the downlink | Downlink | FDD/TDD |
| Uplink shared channel (USCH) | It is used as shared channel carrying dedicated user data/control messages in the uplink | Uplink | TDD |
| Broadcast channel (BCH) | Broadcast channel to all UEs in a cell | Downlink | FDD/TDD |
| Paging channel (PCH) | Broadcast of paging & notification messages, also allows UE to make use of sleep Mode | Downlink | FDD/TDD |
| Dedicated Channel (DCH) | It is used as dedicated channel to transfer traffic/control messages in both the direction | Uplink/Downlink | FDD/TDD |
UMTS Physical Channels
Physical channels are responsible for transmitting the actual data over the air using specific modulation, coding, and scrambling techniques.
Following are physical types of UMTS channels for the 3.84 Mcps case:
- P-CCPCH: Primary Common Control Physical Channel
- SCH: Synchronization Channel
- S-CCPCH: Secondary Common Control Physical Channel
- PICH: Paging Indicator Channel
- DPCH (DL): Dedicated Physical Channel in the downlink
- PDSCH: Physical Downlink Shared Channel
- DPCH (UL): Dedicated Physical Channel in the uplink
- PUSCH: Physical Uplink Shared Channel
- PRACH: Physical Random Access Channel
- PNBSCH: Physical Node B Synchronization Channel
Mapping between channels
Conclusion
A clear understanding of UMTS channels helps optimize network performance, minimize data loss, and improve service quality. Familiarizing yourself with the channel structures enables efficient management of resources in UMTS networks.
