Dear community friends, today I want to share with you information about the LTE physical channel.
LTE Channel Type
Logical Channel
Indicate the type of information transferred.
Transmission Channel
Describe what typical configuration the physical layer uses to provide transport services on the air interface.
Physical Channel
Describe the physical features of signals, such as coding and modulation.
Figure 1 Location of LTE Physical Channels
Both GSM and UMTS defined various channel categories, and the terminology of channels for LTE is closer to UMTS. Broadly there are four categories of channels, which are logical channels, transport channels, physical channels, and radio channels.
RLC layer passes data to MAC layers as logical channels. Logical channels define the type of information to be transferred.
MAC layer formats and sends the logical channel data to PHY layer as transport channel. Transport channels define how and in which format the information is transferred.
Physical layer encodes the transport channel data to physical channels. Physical channels define the physical resource for information to be transferred between air interfaces.
The term “radio channel” is typically used to describe the overall channel, i.e. the downlink and uplink carriers for FDD or TDD operation.
Function Overview of Physical Channels
Physical channels are divided into uplink and downlink physical channels.
Downlink physical channels include:
PBCH: broadcasts system information.
PCFICH: indicates the number of symbols PDCCH resources occupy in the time domain.
PHICH: indicates the demodulation status on the PUSCH (ACK/NACK).
PDCCH: indicates user scheduling information (uplink and downlink).
PDSCH: transmits user DL data.
Uplink physical channels include:
PRACH: transmits uplink random access preambles
PUSCH: transmits user UL data.
PUCCH: transmits information about downlink data demodulation performance(ACK/NACK), channel quality measurement results, and scheduling requests.
The system broadcast message includes the MIB transmitted on the PBCH and SIB1 to SIB13 transmitted on the PDSCH.
PBCH (physical broadcast channel)
1. The PBCH transmits only MIB messages.
2. The PBCH is transmitted only on the center frequency band and occupies 72 subcarriers.
3. The PBCH is mapped to four sub tones within 40 ms.
Subframe #0 of each radio frame occupies four consecutive OFDM symbols.
The start position of the 40 ms period is obtained through blind detection and is not indicated by signaling.
Data mapped to each subframe can be self-decoded.
PDSCH (physical downlink shared channel）
1. Transmits all system broadcast messages except the MIB.
2. Mapping to DL-SCH and finally to PDSCH
3. SIB1 is independently transmitted at a fixed interval of 80 ms and is transmitted in even radio frames.
4. Other SIBs are dynamically scheduled and sent.
PCFICH (physical control format indicator channel)
1. The PCFICH indicates the number of symbols occupied by the PDCCH.
2. The length of the PCFICH information is 2 bits.
3. The bits carried by the PCFICH are mapped to a specific sequence with length 32 (equivalent to coding).
4. PCFICH transmission
The PCFICH is transmitted in the first symbol of each downlink subframe, occupying four REGs (16 REs).
Four REGs are evenly distributed in the entire system bandwidth.
Different PHY cell IDs correspond to different REG frequency shifts.
PHICH (physical HARQ indicator channel)
1. The PHICH channel is used to transmit the ACK/NACK response to the uplink data transmission.
2. The PHICH is triggered by the uplink HARQ mechanism at the physical layer.
3. Each PHICH group includes eight PHICHs, a quantity of PHICH groups is configurable, and the configuration indication information is carried in a PBCH.
4. PHICH resource mapping
One PHICH is mapped to three REGs.
Three REGs are distributed in the entire system bandwidth.
PHICHs are mapped to 1, 2, or 3 OFDM symbols, and each REG is cyclically mapped to all OFDM symbols.
Different PHY cell IDs correspond to different REG frequency shifts.
PDCCH (physical downlink control channel)
1. Main Features of the PDCCH
The PDCCH may occupy the first one, two, or three OFDM symbols in each subframe. The specific number of symbols is indicated by the PCFICH.
Control signaling of different UEs is sent independently and maybe adaptively transmitted based on channel conditions of different UEs.
2. Types of PDCCH Control Signaling
Scheduling and authorization information for uplink data transmission
Scheduling information about downlink data transmission
Scheduling information for Paging Message Transmission
Scheduling information about random access response uplink transmission
Uplink power control signaling
PRACH (physical random access channel)
1. PRACH is used for UE capture, uplink delay estimation, and frequency offset estimation.
2. PRACH includes five formats.
Different PRACH formats are configured for different cell coverage radiuses.
Formats 0 to 3 correspond to the cell radius of 14, 77, 29, and 100 km respectively.
Format 4 applies only to TDD uplink PTS, with a cell radius of 1 km.
3. PRACH Sequence Generation Mode
Each cell has 64 preamble sequences.
The 64 sequences are obtained by a cyclic shift of Zadolff-Chu root sequences. There are 838 root sequences in total.
If a root sequence cannot obtain 64 preamble sequences, a new one is selected according to the sequence of logical root sequences.
NOTE:
Preamble format 0 applies to normal cell coverage areas.
Preamble format 1 applies to large-coverage cells.
Preamble format 2: Preamble signals are repeated once. This mode applies to large-coverage cells and scenarios where UEs move at a high speed.
Preamble format 3: The preamble is repeated once, which is applicable to large coverage areas and high-speed moving UEs.
Preamble format 4, which is transmitted only on the UpPTS and applies to hotspot areas with a small coverage.
PUCCH (physical uplink control channel)
1. The PUCCH carries UCI information.
2. UCI includes:
ACK/NACK
CQl
SR (Scheduling Request)
3. The PUCCH and PUSCH are not transmitted at the same time.
PUSCH (physical uplink shared channel)
1. The PUSCH can carry data information and UCI signaling information. When UCI signaling information needs to be transmitted while transmitting uplink data, the UCI is placed on the PUSCH and transmitted together with the data.
2. PUSCH resource allocation modes include:
Centralized allocation (Localized)
Frequency Hopping
Figure 2 Mapping Relationship Between Physical Channels and Other Channels
Downlink physical channels PCFICH, PHICH, and PDCCH and uplink physical channel PUCCH carry physical layer and MAC layer control information. Higher layer information is not carried by these channels. No transport channel is mapped to these physical channels.
Use of Physical Channels
Physical channel used for cell search
SCH-> PBCH-> PCFICH-> PDCCH-> PDSCH
Physical channel used for random access
PRACH-> PCFICH-> PDCCH-> PDSCH-> PUSCH
Physical channel used for downlink data transmission
PCFICH-> PDCCH-> PDSCH-> PUCCH
Physical channel used for uplink data transmission
PCFICH-> PDCCH-> PUSCH-> PHICH
Here's what I've shared about physical channels. This information is very important. If you want to take the LTE certification exam, you must read it carefully. Because you're gonna need it.
I'll share uplink and downlink reference signals in my next post.
