[4G&5G Topic-29]: Physical Layer-Overview of Internal Architecture, Function Protocols and Main Interfaces

The NR physical layer must support a wide frequency range (from below 1 GHz to as high as 100 GHz) and various deployment (picocell, microcell, macrocell) scenarios.

Human-centered and machine-centered use cases coexist. Some use cases have extreme requirements, and even the requirements of different use cases contradict each other. There may be new demands for new applications in the future.

In order to be able to meet these challenges smoothly, 3GPP has designed a flexible physical layer for NR. Based on an accurate understanding of radio wave propagation and the non-idealities of network and terminal hardware, these flexible components can be appropriately optimized.

NR is the first mobile wireless access technology applied in the millimeter wave frequency range (supporting frequencies up to 100 GHz), the channel bandwidth is expected to reach the GHz range, and large-scale multi-antenna technology is used.

1.2 The position of the LTE&NR physical layer in the air interface protocol stack

For the idle protocol stacks of 5G and 4G, from the perspective of protocol layering, except for the SDAP protocol on the data plane, other layer protocols are consistent with 4G LTE, as shown in the figure above:

(1) RF (Radio Frequency radio frequency unit) : IF processing and mixing.

(2) L1 PHY (Physical physical layer) : physical layer encoding and decoding, OFDM baseband modulation and demodulation, Fourier transform and inverse transform, etc.

(3) L2 MAC (Media Access Control Layer) : Physical layer frame scheduling and channel mapping.

The channel structure involved in MAC includes three aspects: logical channels, transmission channels, and the mapping between logical channels and transmission channels. The transport channel is the service access point of the MAC layer and the physical layer, and the logical channel is the service access point of the MAC layer and the RLC layer.

(3) L2 RLC (Radio Link Control Radio Link Layer Control Protocol) : Define three wireless link transmission modes, transparent mode (TM), unconfirmed mode (UM), and confirmed mode (AM).

(4) L2 PDCP (Packet Data Convergence Protocol Packet Data Convergence Protocol) : is responsible for the IP header compression and decompression, transfer of user data and maintenance of lossless radio network service subsystem ( the SRNS SEQ ID NO radio bearer setup).

(5) L3 RRC (Radio Resource Control) : Radio resource management, control and scheduling are carried out through certain strategies and means, while satisfying the requirements of service quality, the limited radio network resources are fully utilized as much as possible to ensure arrival The planned coverage area is to improve service capacity and resource utilization as much as possible.

(6) SDAP: Service Data Adaptation Protocol, service data adaptation protocol.

1.3 LTE&NR physical layer technology overview

The physical layer is mainly addressed are: how certain bandwidth of the "baseband" radio frequency electromagnetic signals, a plurality of different users, a series of transmitted modulated binary bit coded data.
This sentence points out the 4 types of technologies used in the physical layer:

(1) Wireless resources:

A certain bandwidth "baseband" wireless electromagnetic wave signal refers to the resources available at the physical layer, through which data is transmitted for users.

In 4G and 5G, the radio resources are further refined as follows:

Cell frequency bandwidth resources: For example, 4G LTE supports 5M/10M/15M/20M, and 5G LTE can support larger bandwidths, such as 50M/100M/200M/400M.
Asymmetric and variable bandwidth BWP resources: This is a new technology introduced by 5G, which makes the terminal signal bandwidth different from the carrier bandwidth of the base station.
Frequency sub-carrier resources : 4G LTE is 15K sub-carriers, 5G sub-carriers can be 15K, 30K, 60K, 120K sub-carriers.
Time resources : different users share the same subcarrier in different time division multiplexing methods
Time-frequency resources : frequency and time resources are collectively referred to as time-frequency resources
Power resources : The transmission and reception of any signal requires energy. Energy is also a very important resource. Each base station has the maximum power, and each UE will be allocated a part of the power.
Spatial resources : Spatial resources are the "layers" of MIMO. In LTE, all "layers" of MIMO of the same RE can only belong to the same user, which is used to increase the user's bandwidth. In 5G, combined beamforming, MIMO The layer can be assigned to different users, called multi-user MIMO
Wireless channel : Organize various wireless resources in a functional manner to provide services to the MAC layer.

These wireless resources are organized organically and structured in a certain way, which is called the frame structure of the physical layer.

requires attention:

The modifier "baseband" here refers to the wireless electromagnetic wave signal at the physical layer, which is a baseband signal near zero frequency, not a high-frequency radio frequency signal!

(2) Multiple access technology

"For multiple different users" means that wireless resources are shared for different users.

Therefore, a certain technology is required. When sending, multiplex the data of different users into wireless resources for transmission. When receiving, demultiplex, separate and distinguish the data of different users from the wireless resources. Technology, called multiple access technology, multiple access technology includes:

Frequency Division Duplex FDD: By the same time the different carrier frequencies carrying uplink data and downlink data, to achieve simultaneous bidirectional data transmission.
Time Division Duplex TDD: Carrying uplink data and downlink data through the same carrier frequency at different times to achieve simultaneous two-way data transmission.
Frequency division multiple access FDMA: pass through at the same time the different carrier frequencies carrying data of different users, multiple users share a radio band resource.
Time Division Multiple Access TDMA: Carrying data of different users through the same carrier frequency at different times , so that multiple users can share wireless resources in one band.
Orthogonal frequency division multiple access OFDM: pass through at the same time in different, orthogonal, sub-carriers carrying data of different users, multiple users share a radio band resource. And the conversion of OFDM frequency domain to time domain is completed by fast Fourier transform.
Code Division Multiple Access CDMA: pass through at the same time in different codewords carrying data of different users, multiple users share a radio band resource. In 4G and 5G, it is mainly reflected in the scrambling and descrambling of the cell.
Space division multiple access SDMA: pass through at the same time the beam different spatial carrying data of different users, multiple users share a radio band resources, including MIMO and beamforming.

At the physical layer of 5G, all the above-mentioned multiple access technologies are applied! ! !

(3) Modulation technology:

"Send a series of modulated and encoded binary bit data", the meaning of this sentence is how to carry binary numbers on the baseband wireless signal.

Common digital modulation techniques are:

Frequency shift keying FSK: distinguish 0 and 1 through different frequencies of the baseband signal
Phase shift keying PSK, such as 8PSK, 16PSK, distinguishes 0 and 1 through the different phases of the baseband signal
Amplitude shift keying ASK, distinguish 0 and 1 through the different amplitude of the baseband signal
Quadrature amplitude modulation QAM, distinguish 0 and 1 through the amplitude + phase of the baseband signal
IQ modulation, distinguish between 0 and 1 through the amplitude + phase of the complex exponential signal

In 4G and 5G, in addition to not using FSK modulation technology, start-up debugging technology has been applied.

(4) Coding technology

In order to ensure reliable communication through the wireless channel, some additional auxiliary coding techniques are required.

Error-reduction technology: interweaving
Error detection technology: CRC, parity check
Error correction technology:
~~Retransmission technology: The physical layer does not support retransmission, and the retransmission is done by the MAC layer.~~

Chapter 2 The two sublayers and three major interfaces of the physical layer

2.1 Two sub-layers

In the 5G system, in order to support large-scale antenna arrays, the physical layer is further divided into PHY_High and PHY_LOW.

(1) PHY_High: The non-real-time part of the physical layer. The function of PHY_High resides in the DU network element.

(2) PHY_Low: The real-time part of the physical layer. The function of PHY_Low is sunk into the RRU network element, and the network element entity of the RRU becomes the AAU network element entity. The main functions of PHY_LOW mainly involve the functions related to the large-scale antenna matrix, including xMIMO mapping,

2.2 Three major interfaces

(1) nFAPI: The interface between the physical layer PHY_High and the MAC, and provides services to the MAC layer in the form of a physical channel.

(2) eCPRI interface: the forward interface of PHY_High and PHY_Low.

(3) CPRI interface: the forward interface between the physical layer PHY_Low and RF.

Chapter 3 Internal Function Protocol Stack of the Physical Layer

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nFAP interface: services based on physical channels

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PHY_HIGH

(1) Coding and decoding of the physical layer: Responsible for error prevention, error detection, and error correction.
(2) Rate matching Rate Matching: In 5G NR, there are two main channel coding schemes adopted by 3GPP, Polar and LDPC, among which the control channel and broadcast channel adopt Polar coding, and the data channel adopts LDPC. Rate matching is to do different processing according to different code stream lengths after channel coding, so that the code stream length matches the actual transmission capacity. The rate matching scheme is strongly related to the encoding method.
(3) Scrambling and DeScrambling: Scrambling and DeScrambling: use the cell-specific scrambling code sequence at the sender to scramble, and then descrambling at the receiver. Only the UE in the cell can form a cell-specific scrambling code based on the ID of the cell. The sequence descrambles the received information in the cell.
(4) MAC layer data modulation and demodulation Modulation and DeModulation: including QAM modulation, PSK modulation and other modulation techniques happen here.
(5) Physical layer signal generation and detection: multiple access technologies such as physical layer frame definition, physical channel definition, multiple access technology, etc., occur here.
(6) MIMO "layer" mapping Layer mapping: In the case of MIMO, the modulated user data is allocated to a specific MIMO layer to support a larger bandwidth.

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eCPRI interface: 7-2 option

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PHY_LOW：

(7) Precoding matrix, MIMO "layer" to logical antenna port mapping: precoding the data mapped to each layer of MIMO, and complete the mapping of logical antenna ports, the number of logical antenna ports can be greater than or equal to the "layer of MIMO" number".
(8) Channel estimation and channel equalization: MIMO technology adopts multi-antenna transmission and reception. Due to the physical location difference between the antennas, the transmitter and receiver have produced a variety of different transceiver path effects through the positional relationship between the multiple antennas. If these multiple path effects can be effectively combined to make up for each other's shortcomings, it can be achieved Better transmission quality.
(9) RE time-frequency resource mapping RE mapping: Map data to the time-frequency resources of each sub-carrier of a carrier with a certain bandwidth.
(10) Digital Beamforming: By controlling the phase weight of each subcarrier, the beamforming of each subcarrier can be controlled automatically.
(11) Fast Fourier transform FFT and inverse transform IFFT and cyclic prefix processing: OFDM multiple access multiplexing.
(12) CPRI physical port mapping: Map the time domain OFDM symbols in IQ AxC format to different CPRI physical ports. This mapping is very important when there are multiple CPRI connections between the BBU and RRU.