LTE learning path (5)-Physical Layer

Frame Structure

LTETwo supported wireless frames

Type 1: applied to FDD

Type 2: Used In TDD

FDDWireless Frame Structure

 

The FDD wireless frame length is 10 ms, as shown in. Each frame is divided into 10 sub-frames of the same size. Each sub-frame is divided into two time slots of the same size, that is, each FDD wireless frame contains 20 time slots of the same size, each time slot is 0.5 ms. In a common CP configuration, a time slot contains seven consecutive OFDM symbols (Symbol ).

TDDWireless Frame Structure

 

In the TDD frame structure, a 10 ms wireless frame consists of two half frames with a length of 5 ms. Each half frame consists of five child frames with a length of 1 ms, it includes four normal sub-frames and one special sub-frame. A normal sub-frame consists of two Ms time slots, and a special sub-frame consists of three special time slots (dwpts, GP, and uppts.

【Note:]

• Sub-frame 0 and sub-frame 5 can only be used for downlink Transmission
• When the 5 ms switching period is configured, the sub-frame 1 and sub-frame 6 are used as special sub-frames.
• Sub-frame 1 is used as a special sub-frame during the 10 ms switching cycle Configuration
• The first regular sub-frame after uppts can only be used for uplink transmission.

As a feature of the TDD system, time resources are allocated in the upstream and downstream directions. The TDD frame structure supports seven different time ratios for uplink and downlink allocation (0 ~ 6), which can be configured based on the characteristics of the system's business volume to support asymmetric businesses. These seven configurations include 4 5 ms cycles and 3 10 ms cycles.

 

? "D" indicates that this sub-frame is used for downlink transmission, "U" indicates that this sub-frame is used for uplink transmission, and "S" is a special sub-frame consisting of dwpts, GP, and uppts. ?

In special sub-frames, the length of dwpts and uppts is configurable. The total length of dwpts, GP, and uppts is 1 ms.

For a 5 ms upstream/downstream switching period, the sub-frames 0, 5, and dwpts must go downstream. For a 10 ms upstream/downstream switching cycle, each half frame has dwpts, with only GP and uppts within 1st half frames, and the dwpts length of 2nd half frames is 1 ms. Uppts and sub-frame 2 are used as the uplink, and sub-Frames 7 and 9 are used as the downlink.

Physical resources

 Basic time unit

 

Antenna Port

• LTE uses antenna ports to differentiate space resources. The definition of an antenna port is defined from the receiver's perspective. That is, if the receiver needs to distinguish the difference in space between resources, multiple antenna ports need to be defined. There is no one-to-one relationship between the antenna port and the actual physical antenna port.
• At present, LTE uplink only supports single-RF link transmission and does not need to distinguish space resources. Therefore, the concept of antenna port is not introduced in uplink.
• At present, three types of antenna ports are defined for LTE downlink, corresponding to the antenna port numbers 0 ~ 5.

Residential dedicated reference signal transmission antenna port: Antenna port 0 ~ 3

Mbsfn reference signal transmission antenna port: Antenna port 4

Terminal reference signal transmission antenna port: Antenna Port 5

Resource Unit(Re,Resource element)

For each antenna port, a unit corresponding to a sub-carrier on an OFDM or SC-FDMA symbol is called a resource unit;

Resource particle set(REG,Resource Element Group)

Reg = 4re

Resource Block(Rb,Resource block)

In a time slot, a physical resource with a consecutive width of kHz in the frequency domain is called a resource block;

The most common scheduling unit in the LTE system. upstream and downstream business channels are all scheduled in units of Rb. RB = 84re

Resource Grid(Resource grid)

All the resource units occupied by signals transmitted in a time slot constitute a resource grid, which contains an integer pRb, which can also be expressed by the number of subcarriers and the number of OFDM or SC-FDMA symbols.

Channel

Air Interface (uu port)

An air interface is an interface between a terminal and an access network. It is also called a wireless interface. In LTE, the air interface is the interface between the terminal UE and enodeb.

The air interface protocol is mainly used to establish, reconfigure, and release various wireless bearer services. An air interface is a fully open interface. devices produced by different manufacturers can communicate with each other as long as they comply with the interface specifications.

The air interface protocol stack is divided into three layers: the physical layer, the data link layer, and the network layer, and the control plane and the user plane. SlaveUser planeIt mainly includes the physical layer, MAC layer, RLC Layer, and PDCP layer.Control PlaneLook, in addition to the above layers, it also includes the RRC layer and the NAS layer. The RRC protocol entity is located in the UE and ENB network entities and is mainly responsible for the control and management of the access layer. The NAS control protocol is located in the UE and mobile management entity Mme and is mainly responsible for the control and management of non-access layers. Shows the structure of the air interface protocol stack.

Air Interface user plane protocol stack structure air interface control plane protocol stack structure

Channel Definition and functions

ChannelIt can be considered as a Service Access Point (SAP) between different protocol layers. It is a service provided by the next layer to its upper layer.

LTE follows three UMTS channels:Logical Channel,Transmission ChannelAndPhysical channel.

From the perspective of the protocol stack, the physical channel is the physical layer, the transmission channel is between the physical layer and the MAC layer, and the logical channel is between the MAC layer and the RLC Layer.

• Logical Channel: What content is transmitted, such as the Broadcast Channel (BCCH), which is used to transmit broadcast messages;
• Transmission Channel: how to transmit, such as downstream shared channel DL-SCH, that is, the business and even some control messages are transmitted by sharing air resources, it will specify MCS, space reuse, and so on, that is to say, it tells the physical layer how to transmit this information;
• Physical channels: transmission of signals in the air, such as pbch, that is, transmission of broadcast messages by specially modulated encoding in actual physical locations.

Physical channel

Defined by LTEDownstreamThere are six main types of physical channels:

(1) physical downlink shared channel (pdsch): used to carry downstream user information and high-level signaling.

(2) Physical Broadcast Channel (pbch): used to carry information block information of the main system and transmit parameters used for initial access.

(3) Physical multicast channel (pmch): used to carry multimedia/multicast information.

(4) physical control format indication channel (pcfich): information used to hold the control area size of the sub-frame.

(5) Physical downlink control channel (pdcch): used to carry downlink control information, such as upstream scheduling instructions, downstream data transmission, and public control information.

(6) Physical Haro indication channel (phich): it is used to carry the ACK/Nack feedback information for the upstream data of the terminal, which is related to the Haro mechanism.

Defined by LTEUpstreamThere are three main types of physical channels:

(1) physical uplink shared channel (pusch): used to carry upstream user information and high-level signaling.

(2) Physical uplink control channel (pucch): used to carry uplink control information.

(3) Physical Random Access Channel (prach): it is used to carry the transmission of random access to the first track sequence. The base station establishes uplink synchronization by detecting the sequence and subsequent signaling communication.

Transmission Channel

The physical layer provides data transmission services to the Mac sublayer or higher layer through the transmission channel. The transmission channel features are defined by the transmission format. The transmission channel describes how data is transmitted on wireless interfaces and the data features transmitted. For example, how data is protected to prevent transmission errors, channel encoding types, CRC protection or interweaving, and packet size.

Defined by LTEDownstreamThere are four main types of transmission channels:

(1) Broadcast Channel (BCH): used for broadcasting system information and specific information of cells. The fixed predefined format can be used to broadcast in the entire residential coverage area.

(2) downstream shared channel (DL-SCH): used to transmit downstream user control information or business data. Able to use harq; able to implement link adaptation through various modulation modes, encoding and transmission power; able to send in the entire community; able to use beam-shaped; Support for dynamic or semi-continuous resource allocation; supports non-continuous receiving of terminals for power-saving purposes; supports mbms service transmission.

(3) Paging channel (PCH): the control information sent to the UE when the network does not know the location of the UE's cell. It can support non-continuous receiving of terminals for power-saving purposes; it can be sent in the coverage area of the entire community; it is mapped to physical resources used for business or other dynamic control channels.

(4) multicast channel (MCH): used for mbms user control information transmission. It can be sent across the coverage area of the entire residential area. It supports mbms transmission in multiple residential areas for a single-frequency network, and uses semi-continuous resource allocation.

Defined by LTEUpstreamThere are two main types of transmission channels:

(1) uplink shared channel (UL-SCH): used to transmit downstream user control information or business data. Able to use beam shaping; able to adapt to link condition changes by adjusting transmit power, encoding and potential modulation modes; able to use harq; dynamic or semi-continuous resource allocation.

(2) Random Access Channel (Rach): it can carry limited control information, such as when an early connection is established or when the RRC status changes.

Logical Channel

The Logical Channel defines the transmitted content. The MAC Sub-layer uses a logical channel to communicate with the upper layer.

Logical channels are generally divided into two types:Control CHannelAnd used to transmit user Plane InformationBusiness Channel. The types of transmission information can be divided into multiple logical channel types, and different transmission services can be provided based on different data types.

Defined by LTEControl CHannelThere are 5 types:

(1) broadcast control channel (BCCH): This channel is a downstream channel used to transmit control information of the broadcast system.

(2) Paging control channel (pcch): This channel is a downstream channel used to transmit paging information and system information for changing notification messages. When the network side does not have the information of the residential area where the user terminal is located, use this channel to call the terminal.

(3) Public control channel (ccch): This channel includes uplink and downlink. When there is no RRC connection between the terminal and the network, transmission of terminal-level control information uses this channel.

(4) multicast control channel (mcch): This channel is a downstream channel from point to multiple points for UE to receive mbms services.

(5) Dedicated Control Channel (dcch): This channel is a point-to-point bidirectional channel, used to transmit the dedicated control information when the terminal side and the network side have a RRC connection.

Defined by LTEBusiness ChannelThere are two main types:

(1) dedicated Business Channel (dtch): This channel can be unidirectional or bidirectional and provides point-to-point Service Transmission for a single user.

(2) multicast service channel (mtch): This channel is a downstream channel from point to multiple points. The user only uses this channel to receive mbms services.

Channel ing

Uplink to downlink

 

Ing between transmission channels and physical Channels

Ing between transmission channels and logical Channels

Basic downlink/uplink transmission mechanism based on OFDM

• The downlink transmission mechanism is based on traditional OFDM (with the cyclic prefix and CP;
• Normal sub-carrier interval of OFDM;
• The 12 consecutive subcarriers in a time interval are equivalent to a downstream Rb (180 kHz );
• In the frequency domain, the range of Rb numbers in each carrier or cell can be expressed :;
• There is also a reduced sub-carrier interval, which is only applicable to mbms-dedicated cells;
• At the sub-carrier interval, the CP length is divided into two types (conventional/extended), corresponding to 7 OFDM symbols and 6 OFDM symbols under each time slot;

Normal CP:

Extended CP:

Where,

• When the sub-carrier interval is, the CP length is only one, that is, it corresponds to three OFDM symbols under each time slot.

The normal CP configuration () of upstream and downstream time slots is as follows:

The upper and lower lines expand the CP configuration (). The lower time slot structure is as follows:

The time slot structure of the downstream CP configuration () is as follows: