6.2 Hierarchical Channel Structure of LTE
To efficiently support various QoS classes of services, LTE adopts a hierarchical channel structure. There are three different channel types defined in LTE—logical channels, transport channels, and physical channels, each associated with a service access point (SAP) between different layers. These channels are used by the lower layers of the protocol stack to provide services to the higher layers. The radio interface protocol architecture and the SAPs between different layers are shown in Figure 6.5. Logical channels provide services at the SAP between MAC and RLC layers, while transport channels provide services at the SAP between MAC and PHY layers. Physical channels are the actual implementation of transport channels over the radio interface.
Figure 6.5 The radio interface protocol architecture and the SAPs between different layers.
The channels defined in LTE follow a similar hierarchical structure to UTRA/HSPA. However, in the case of LTE, the transport and logical channel structures are much more simplified and fewer in number compared to UTRA/HSPA. Unlike UTRA/HSPA, LTE is based entirely on shared and broadcast channels and contains no dedicated channels carrying data to specific UEs. This improves the efficiency of the radio interface and can support dynamic resource allocation between different UEs depending on their traffic/QoS requirements and their respective channel conditions. In this section, we describe in detail the various logical, transport, and physical channels that are defined in LTE. The description of different channel types and the channel mapping between different protocol layers provides an intuitive manner to understand the data flow of different services in LTE, which builds the foundation to understand the detail processing procedures in later chapters.
6.2.1 Logical Channels: What to Transmit
Logical channels are used by the MAC to provide services to the RLC. Each logical channel is defined based on the type of information it carries. In LTE, there are two categories of logical channels depending on the service they provide: logical control channels and logical traffic channels.
The logical control channels, which are used to transfer control plane information, include the following types:
- Broadcast Control Channel (BCCH): A downlink common channel used to broadcast system control information to the mobile terminals in the cell, including downlink system bandwidth, antenna configuration, and reference signal power. Due to the large amount of information carried on the BCCH, it is mapped to two different transport channels: the Broadcast Channel (BCH) and the Downlink Shared Channel (DL-SCH).
- Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting control information to UEs in the cell. It is only used by UEs that receive multicast/broadcast services.
- Paging Control Channel (PCCH): A downlink channel that transfers paging information to registered UEs in the cell, for example, in case of a mobile-terminated communication session. The paging process is discussed in Chapter 10.
- Common Control Channel (CCCH): A bi-directional channel for transmitting control information between the network and UEs when no RRC connection is available, implying the UE is not attached to the network such as in the idle state. Most commonly the CCCH is used during the random access procedure.
- Dedicated Control Channel (DCCH): A point-to-point, bi-directional channel that transmits dedicated control information between a UE and the network. This channel is used when the RRC connection is available, that is, the UE is attached to the network.
The logical traffic channels, which are to transfer user plane information, include:
- Dedicated Traffic Channel (DTCH): A point-to-point, bi-directional channel used between a given UE and the network. It can exist in both uplink and downlink.
- Multicast Traffic Channel (MTCH): A unidirectional, point-to-multipoint data channel that transmits traffic data from the network to UEs. It is associated with the multicast/broadcast service.
6.2.2 Transport Channels: How to Transmit
The transport channels are used by the PHY to offer services to the MAC. A transport channel is basically characterized by how and with what characteristics data is transferred over the radio interface, that is, the channel coding scheme, the modulation scheme, and antenna mapping. Compared to UTRA/HSPA, the number of transport channels in LTE is reduced since no dedicated channels are present.
LTE defines two MAC entities: one in the UE and one in the E-UTRAN, which handle the following downlink/uplink transport channels.
Downlink Transport Channels
- Downlink Shared Channel (DL-SCH): Used for transmitting the downlink data, including both control and traffic data, and thus it is associated with both logical control and logical traffic channels. It supports H-ARQ, dynamic link adaption, dynamic and semi-persistent resource allocation, UE discontinuous reception, and multicast/broadcast transmission. The concept of shared channel transmission originates from HSDPA, which uses the High-Speed Downlink Shared Channel (HS-DSCH) to multiplex traffic and control information among different UEs. By sharing the radio resource among different UEs the DL-SCH is able to maximize the throughput by allocating the resources to the optimum UEs. The processing of the DL-SCH is described in Section 7.2.
- Broadcast Channel (BCH): A downlink channel associated with the BCCH logical channel and is used to broadcast system information over the entire coverage area of the cell. It has a fixed transport format defined by the specifications. The processing of the BCH will be described in Section 7.4.
- Multicast Channel (MCH): Associated with MCCH and MTCH logical channels for the multicast/broadcast service. It supports Multicast/Broadcast Single Frequency Network (MBSFN) transmission, which transmits the same information on the same radio resource from multiple synchronized base stations to multiple UEs. The processing of the MCH is described in Section 7.5.
- Paging Channel (PCH): Associated with the PCCH logical channel. It is mapped to dynamically allocated physical resources, and is required for broadcast over the entire cell coverage area. It is transmitted on the Physical Downlink Shared Channel (PDSCH), and supports UE discontinuous reception.
Uplink Transport Channels
- Uplink Shared Channel (UL-SCH): The uplink counterpart of the DL-SCH. It can be associated to CCCH, DCCH, and DTCH logical channels. It supports H-ARQ, dynamic link adaption, and dynamic and semi-persistent resource allocation. The processing of the UL-SCH is described in Section 8.2.
- Random Access Channel (RACH): A specific transport channel that is not mapped to any logical channel. It transmits relatively small amounts of data for initial access or, in the case of RRC, state changes. The processing of the RACH is described in Section 8.5, while the random access procedure is described in Section 9.9.
The data on each transport channel is organized into transport blocks, and the transmission time of each transport block, also called Transmission Time Interval (TTI), is 1 ms in LTE. TTI is also the minimum interval for link adaptation and scheduling decision. Without spatial multiplexing, at most one transport block is transmitted to a UE in each TTI; with spatial multiplexing, up to two transport blocks can be transmitted in each TTI to a UE.
Besides transport channels, there are different types of control information defined in the MAC layer, which are important for various physical layer procedures. The defined control information includes
Downlink Control Information (DCI): It carries information related to downlink/uplink scheduling assignment, modulation and coding scheme, and Transmit Power Control (TPC) command, and is sent over the Physical Downlink Control Channel (PDCCH). The DCI supports 10 different formats, listed in Table 6.1. Among them, Format 0 is for signaling uplink transmission allocation, Format 3 and 3A are for TPC, and the remaining formats are for signaling downlink transmission allocation. The detail content of each format can be found in , some of which is discussed in Section 7.3.
Table 6.1. DCI Formats
Uplink scheduling assignment
Downlink scheduling for one codeword
Compact downlink scheduling for one codeword and random access procedure
Compact downlink scheduling for one codeword with precoding information
Very compact downlink scheduling for one codeword
Compact downlink scheduling for one codeword with precoding and power offset information
Downlink scheduling for UEs configured in closed-loop spatial multiplexing mode
Downlink scheduling for UEs configured in open-loop spatial multiplexing mode
TPC commands for PUCCH and PUSCH with 2-bit power adjustments
TPC commands for PUCCH and PUSCH with 1-bit power adjustments
- Control Format Indicator (CFI): It indicates how many symbols the DCI spans in that subframe. It takes values CFI = 1, 2, or 3, and is sent over the Physical Control Format Indicator Channel (PCFICH).
- H-ARQ Indicator (HI): It carries H-ARQ acknowledgment in response to uplink transmissions, and is sent over the Physical Hybrid ARQ Indicator Channel (PHICH). HI = 1 for a positive acknowledgment (ACK) and HI = 0 for a negative acknowledgment (NAK).
- Uplink Control Information (UCI): It is for measurement indication on the downlink transmission, scheduling request of uplink, and the H-ARQ acknowledgment of downlink transmissions. The UCI can be transmitted either on the Physical Uplink Control Channel (PUCCH) or the Physical Uplink Shared Channel (PUSCH). The detail transmission format is discussed in Section 8.3.
6.2.3 Physical Channels: Actual Transmission
Each physical channel corresponds to a set of resource elements in the time-frequency grid that carry information from higher layers. The basic entities that make a physical channel are resource elements and resource blocks. A resource element is a single subcarrier over one OFDM symbol, and typically this could carry one (or two with spatial multiplexing) modulated symbol(s). A resource block is a collection of resource elements and in the frequency domain this represents the smallest quanta of resources that can be allocated. The details of the time-frequency resource structures for downlink and uplink are described in Section 6.3 and Section 6.4, respectively.
Downlink Physical Channels
- Physical Downlink Control Channel (PDCCH): It carries information about the transport format and resource allocation related to the DL-SCH and PCH transport channels, and the H-ARQ information related to the DL-SCH. It also informs the UE about the transport format, resource allocation, and H-ARQ information related to UL-SCH. It is mapped from the DCI transport channel.
- Physical Downlink Shared Channel (PDSCH): This channel carries user data and higher-layer signaling. It is associated to DL-SCH and PCH.
- Physical Broadcast Channel (PBCH): It corresponds to the BCH transport channel and carries system information.
- Physical Multicast Channel (PMCH): It carriers multicast/broadcast information for the MBMS service.
- Physical Hybrid-ARQ Indicator Channel (PHICH): This channel carries H-ARQ ACK/NAKs associated with uplink data transmissions. It is mapped from the HI transport channel.
- Physical Control Format Indicator Channel (PCFICH): It informs the UE about the number of OFDM symbols used for the PDCCH. It is mapped from the CFI transport channel.
Uplink Physical Channels
- Physical Uplink Control Channel (PUCCH): It carries uplink control information including Channel Quality Indicators (CQI), ACK/NAKs for H-ARQ in response to downlink transmission, and uplink scheduling requests.
- Physical Uplink Shared Channel (PUSCH): It carries user data and higherlayer signaling. It corresponds to the UL-SCH transport channel.
- Physical Random Access Channel (PRACH): This channel carries the random access preamble sent by UEs.
Besides physical channels, there are signals embedded in the downlink and uplink physical layer, which do not carry information from higher layers. The physical signals defined in the LTE specifications are
Reference signal: It is defined in both downlink and uplink for channel estimation that enables coherent demodulation and for channel quality measurement to assist user scheduling. There are three different reference signals in the downlink:
- - Cell-specific reference signals, associated with non-MBSFN transmission
- - MBSFN reference signals, associated with MBSFN transmission
- - UE-specific reference signals
There are two types of uplink reference signals:
- - Demodulation reference signal, associated with transmission of PUSCH or PUCCH
- - Sounding reference signal, to support uplink channel-dependent scheduling
The processing of reference signals in the downlink and uplink are treated in Section 7.6.1 and Section 8.4, respectively.
- Synchronization signal: It is split into a primary and a secondary synchronization signal, and is only defined in the downlink to enable acquisition of symbol timing and the precise frequency of the downlink signal. It is discussed further in Section 7.6.2.
6.2.4 Channel Mapping
From the description of different channel types, we see that there exists a good correlation based on the purpose and the content between channels in different layers. This requires a mapping between the logical channels and transport channels at the MAC SAP and a mapping between transport channels and physical channels at the PHY SAP. Such channel mapping is not arbitrary, and the allowed mapping between different channel types is shown in Figure 6.6,4 while the mapping between control information and physical channels is shown in Figure 6.7. It is possible for multiple channels mapped to a single channel, for example, different logical control channels and logical traffic channels are mapped to the DL-SCH transport channel. The channel mapping in Figures 6.6 and 6.7 will reappear in different sections in Chapters 7 and 8 when we discuss downlink and uplink transport channel processing.
Figure 6.6 Mapping between different channel types.
Figure 6.7 Mapping of control information to physical channels.