Fundamental of the LTE (long-term evolution) system, which is my note for a related video on Youtube.

Prologue

In order to understand how Narrowband Internet-of-things (NB-IOT) works, I first need to understand how LTE works, because they share many designs in the physical architecture.

Then I learn from a Youtube channel whose name is exactly “LTE”. They post about 12 videos regarding the fundamental of LTE (can be found here). This is my note on it. However, I started taking note starting from the 5th video. So I don’t have anything to post for the previous series.

LTE Logical Channel

Responsibility for LTE logical channel

• Implement the segmentation logic
• Transport mode
  • Dedicated transport group
• Deal with delivery failure
  • Cancel
  • or go into a different mode
• Handle transport priority
  • Record of arrangement: improve efficiency

User Data in Application Layer

Two types of planes:

• User plane
• Control plane

In data link layer (logical channel), Radio link control layer is responsible for RLC segmentation / concatenation.

Control plane

There are 5 logical channels in control plane:

• C1 - PCCH: paging control. Determine if the mobile is in the idle state.
• C2 - BCCH: broadcast message :
  • Downlink bandwidth
  • Antenna configuration
  • Reference signal power
• C3 - CCCH: (C for “connection”) mobile device’s attempt to latch to the network, establishing a connection after powered on.
• C4 - DCCH: (D for “dedicated”) transfer control information after bearer setup
• C5 - MCCH: (m for “multicast”) broadcast control channel for group user in the same cell.

User plane

There are only 2 logical channels in user plane

• U1 - DTCH (dedicated traffic channel): downlink and uplink data.
• U2 - MTCH: broadcast data to multiple users (in sport or event scenarios etc.)

MAC Layer (Transport Channel)

MAC layer segments the logical channel to transport channel.

There are 4 channels in the Mac layer:

• PCH: C1
• BCH: C2
• DL-SCH: C2, C3, C4, C5, U1, U2
• MCH: C5, U2

They receive data from the channel behind the comma : above.

BCCH (C2) contains two types of blocks, so it has two destinations in the MAC layer

• MIB (master information block): critical information to acquiring cell. This will be forwarded to BCH.
• SIB (system information block): dynamic information to access the cell. This is forwarded to DL-SCH

For the uplink, there is only one transport channel, which is UL-SCH. The data from CCCH, DCCH, DTCH will be forwarded to UL-SCH.

Besides the 4 channels mentioned above, there is one more:

• RACH (random access channel)

Function of MAC layer

1. Mux/Demux of logical channels.
2. Logical channel prioritization (prioritize them and assign resources).
3. HARQ retransmission control.
   • At the first place, Tx MAC layer sends HARQ to Rx MAC. If Rx cannot decode or it is corrupted, Rx will respond an NACK. Then Tx MAC is responsible for retransmission.
4. Random access control.
5. Decide on the amount of data from each logical channel to be included in the MAC PDUs.

Physical Channel Starvation and Prioritization

Usually, the PDU size is fixed. Suppose we have 4 channels with priority A > B > C > D. They want to transmit 20 bits, 15 bits, 10 bits and 3 bits respectively. If the length of the physical channel is 20 bits. After we put A into the channel, starvation occurs, i.e. there is no room for other information.

We use PBR to schedule physical channel in this case. PBRs are set to 5 bits, 4 bits, 3 bits and 2 bits respectively for A, B, C, D channels (higher priority channel has more bits to sent each time). In the first round, only a partition of the total data from each channel will be transmitted. we put the size of data equal to the PBRs size into the physical channel.

Then there are 14=5+4+3+2 bits in the channel, and 6 bits of empty slots.

In the second round, no PBR consideration is in the process. The physical channel will be filled by the rest of the data from channel A. Up till now, 11 bits in A are transmitted.

MAC Scheduler and Physical Channel

Besides the function mentioned above, MAC layer also has MAC-controller that performs two important task.

MAC scheduler

1. DRX operation

In order to save power in the mobile device, it is not connected to the cell all the time. Instead, it is in idle mode for most of the time. After a certain interval, the device will ping the cell and the cell respond a paging alert message.

This is called Discontinuous reception (DRX operation).

Paging signals are retransmitted on fixed intervals, based on discontinuous reception or DRX inactivity timer. It does not depend on HARQ response.

1. Alignment of uplink timing and power headroom reporting

The most important function of eNB is to allocate resources dynamically

The mobile device sent channel quality indicator (CQI) aka channel state information messages, to the cell. In response, the cell sends modulation scheme to the mobile device. When the cell sending user data, it decides to use whether QPSK or 16-64QAM depending on the channel quality. However, signaling traffic is always transferred by QPSK. QPSK is the most robust modulation scheme.

MAC scheduler module takes most of the decision, the rest just follow accordingly. It is the “brain of LTE”.

Physical layer

Physical layer implements the three function of the MAC layer:

1. Decide which are the terminals to transmit.
2. Transmit set of resource block for DL data to different terminals.
3. The selection of transport block size.

Channels in PHY layer:

• Downlink
  • PMCH: from MCH.
  • PBCH: from BCH.
  • PDSCH: from PCH and DL-SCH.
  • PCFICH: PDCCH info.
  • PDCCH: scheduled TF DL, scheduled grant UL, power control command, HARQ info.
  • PHICH: for HARQ ACK/NACK, to check the successful delivery.

The mobile device does not know what modulation scheme it is, so we need PCFICH, providing the info.

• Uplink
  • PUCCH: CQI, ACK/NACK, scheduled request.
  • PUSCH: forward to UL-SCH.
  • PRACH: forward to RACH.

The data flow in uplink is opposite to downlink.

Detail of PDCCH

• Short for Physical Downlink Control Channel.
• It provides the following function:
  • Control information about the data being transmitted on the current subframe.
  • Information about the SIB needed by the user to transmit uplink data. It is mandatory for the user to decode to successfully transmit and receive data.
  • Uplink power control information.
  • HARQ information.

Detail of PCHICH

• Short for Physical Control Format Indicator Channel.
• It informs the user about the format of the signal being received.
• It indicates the number of OFDM symbols used for the PDCCH, whether 1, 2, or 3.
• Information within this channel is essential to decode PDCCH.

Physical Signal

The physical signal is different from physical channels. It is before the mixing RF part.

Workflow of physical signal before the electromegnetic wave is transmitted in the air:

• CRC attachment

Transport block (or MAC PDF) is 10 kB size. When we add a CRC block, it becomes too large to fit into the block. To address this, we divide the 10 kB into two 5 kB block and append CRC to each of them. This is called Code Block Segmentation.

• Channel coding

This is where turbo code applied. The coding rate is 1/3. Check turbo code on the Internet.

• Rate matching
• Scrambler

Scramble the package, changing the order

• Mapper

Add modulation indicator to the transport block as scheduled by the MAC scheduler.

• Layer mapping
• Pre-coding
• RE (resource element) mapper
• OFDM signal generation

In the uplink, the final one is replaced by SC-FDMA.

After the above processes, the signal goes to mixing RF and is broadcasted.

Conclusion

In a nutshell, we can conclude that:

• We cannot transmit the data without knowing where to transmit and how it should be transmitted.
• We cannot decide how a data should be transmitted without knowing what is the data.
• Logical channel tells what, transport layer tells how and physical layer tells where.