How data is transmitted in
LTE
Overview
A picture below shows main stages that data packet passes in LTE base station
(eNB) to be transmitted to mobile device (User Equipment, UE), i.e. data
transmission is presented for Downlink (DL) direction. After the picture there is
short description of each step.
Stages of Data Transmission in LTE
1. LTE eNB operates with IP packets. It receives IP packets and passes them
to PDCP (Packet Data Convergence Protocol). PDCP layer supports
header compression and it's performed according to RObust Header
Compression (ROHC) defined by RFC 4995. An idea is to reduce size of
upper layers headers (not only IP) as it contains static information.
Especially it's important in case of VoIP packets transmission because of
headers can represent more than a half of packet.
2. After ROHC PDCP performs data encryption to secure user data and then
adds PDCP header. PDCP header size is between 1 and 3 bytes.
3. There is Radio Resource Scheduler that is a part of MAC (Media Access
Control) layer. It collects all information about how many data are in
buffers to be transmitted to each active UE, actual information about radio
link quality to each UE, whether there are any packets to be retransmitted,
each UE status (actually whether UE is listening to or is in power saving
mode with turned off receiver for some time), schedule for system
information transmissions (e.g. synchronization signals, Master Information
Block, System Information Blocks). And based on all of this information
scheduler each TTI (Transmission Time Interval, TTI = 1 ms) distributes
radio resources among active UE and sets transmission parameters for each
transmission. Also scheduler defines how many data can be transmitted
within a TTI for a given UE and it's called as Transport Block Size
(TBS) and set in bits.
4. Once amount of data to be transmitted is known RLC (Radio Link
Control) layer forms a packet (called as RLC PDU - RLC Packet Data
Unit). This packet includes encrypted user data, PDCP header(s) and RLC
header(s). Depending on allocation size for a given UE, RLC PDU can
include a part of PDCP packet, a whole PDU packet or several PDU packets
(as well as their parts). In case of packet segmentation and / or concatenation
additional RLC subheader is added.
5. After RLC, a packet goes to MAC layer where MAC header is added. Then
the packet is divided into Transport Blocks based on TBS set by the
scheduler. TBS depends on allocation size and MCS (Modulation and
Coding Scheme). See more about Transport Block Size, MCS and LTE
throughput calculation. Number of transport blocks is also defined by the
scheduler and can be 1 or 2. Also CRC block is added to each transport
block to distinguish between successful and unsuccessful data transmission
on receiver side. CRC size is 24 bits.
6. Then transport block is passed to Physical Layer where it's divided
into Code Blocks. Each Code Block has its CRC (unless Transport Block
size ≤ maximum Code Block size. In this case Transport Block CRC is
used).
7. Each Code Block is passed to Turbo encoder. Code rate is 1/3, i.e. there are
three times more data on encoder output comparing to input. On output there
are Systematic bits (represent 1/3 of encoder output) and Parity bits
(represent 2/3 of encoder output). Output is stored into a ring buffer. There
are as many ring buffers as a number of Code Blocks. In general the whole
ring buffer can be transmitted or just a part of it. It depends on effective code
rate (i.e. number of user bits to overall allocation size ratio) set by scheduler
for a particular transmission. If data transmission is unsuccessful there will
be retransmission scheduled and another part of ring buffer will be
transmitted.
8. Data from ring buffers are concatenated into Code Words. Number of code
words equals to a number of Transport Blocks and can be 1 or 2. Code word
size equals to Transport Block Size divided by effective code rate. Then it's
modulated (see more about code rate and modulation), mapped to MIMO
layers and radio resources, goes though IFFT (Inverse Fast Fourier
Transform) and transmitted to radio module for over the air transmission.