5G technology learning - 5GNR frame structure and air interface resources

4G

Time domain definition:

• Frame: 10ms, the frame number SFN is 0~1023
• Subframe: A frame is divided into 10 subframes, so a subframe is 1ms
• Time slot: One subframe is divided into two time slots, so one time slot is 0.5ms. Each slot consists of several OFDM symbols including a cyclic prefix. LET defines regular cyclic prefix: 7 OFDM symbols, extended cyclic prefix: 6 OFDM symbols
• OFDM symbols: In 5G, a subframe generally contains 14 OFDM symbols. is the smallest time-domain physical resource.

resource division

• The smallest physical resource unit RE: the smallest unit in the time domain is an OFDM symbol, and the smallest unit in the frequency domain is a subcarrier. Taking time as the horizontal axis and frequency as the vertical axis, the grid composed of one OFDM symbol and one subcarrier cross is the smallest physical resource unit, called RE
• Resource block RB: It consists of 12 consecutive subcarriers (12 15KHz=180KHz) in one time slot (0.5ms) (that is, 7/6 OFDM symbols) . Right now
• SCS (Sub Carrier Spacing) subcarrier spacing: between different subcarriers in order to avoid mutual interference between subcarriers

5G

Time Domain Frequency Domain and Space Domain Resources in NR

• The system parameter in 5G is the SCS in 4G. Compared with only one kind of SCS in 4G, the system parameters in 5G are 15KHz, 30KHz, 60KHz, 120KHz, 240KHz

• In addition to time domain and frequency domain, 5G also has air domain resources. The service data flow of the application layer becomes TB (transport block, transmission block) after being processed by the MAC layer, and then enters the physical layer. The data streams formed by different TBs after encoding and rate matching are codewords. Different codewords distinguish different data streams, and the purpose is to transmit data of different channels through MIMO to realize spatial multiplexing.
  

• In the time domain, it is still a frame of 10ms, which is divided into 10 subframes of 1ms each, but the number of time slots in a subframe is determined by system parameters. For example, there is one time slot when SCS=15KHz, and when SCS=30KHz There are two slots, but in either case there are 14 OFDM symbols in one slot

• In the frequency domain, 1RB=12SC, 1RBG=2~16RB, and then the concept of BWP (partial bandwidth), 1BWP=several RB/RBG, the highest layer is CC carrier, one or more can be configured in one CC BWP.

• The frequency domain RB in 5G is different from the RB in 4G. The RB in 4G is also a 0.5ms time slot in the time domain, while the RB in 5G is a complete frequency domain concept.

• 1REG=1PRB (physical RB, basically the same as RB); 1CCE=6REG

• Reasons for designing different system parameters (SCS) for 5G: For different applications (eMBB URLLC mMTC) with different frequency bands and other characteristics, different subcarrier spacing is required to better adapt to these applications. For example, in a high-speed mobile scenario, using a larger SCS will shorten the symbol length, thereby reducing delay; while in a large coverage scenario, a smaller SCS can obtain a more suitable CP to combat multipath delay. detailed link

Cyclic Prefix CP: Background and Principles

• The reason for the cyclic prefix: to avoid inter-channel interference ICI (Inter-Channel Interference) and inter-symbol interference ISI (Inter-Symbol Interference). When the electromagnetic wave propagates, the signals arriving at the receiver propagate through different paths. Due to the different paths passed, the arrival sequence is also different, which causes the width of the received signal to expand. The interference generated between successive different symbols is ISI; the orthogonality of the subcarriers in the OFDM system is destroyed and affects demodulation at the receiving side, which is ICI.
• Cyclic prefix: Insert a guard interval greater than the maximum delay extension between each OFDM symbol, and copy the part behind the OFDM symbol to the prefix to ensure that the number of waveform cycles contained in the delayed copy is also an integer to ensure that the subcarriers Orthogonality.
  
• Two factors affect the length of the cyclic prefix:
  1. Multipath delay extension: the longer the multipath delay extension, the longer the CP required
  2. OFDM length: For the same OFDM length, the longer the CP, the greater the system overhead. In order to control the overhead, an appropriate CP should be selected
• The basic logic of CP design: when SCS=15KHz, there are 14 symbols including CP in one time slot, and when SCS=30KHz, there are two time slots in the same time, so there are 2*14=28 symbols including CP symbol, the length of each CP becomes shorter. detailed introduction

5G frame structure: basic framework

The length of 5G wireless frame and subframe is the same as that of LTE, and the number of time slots in each subframe is determined by the subcarrier width.

• Fixed parameters: Frame=10ms, SubFrame=1ms, 1slot=14 OFDM Symbol
• Change parameters: According to different SCS, 1SubFrame=different slot, when SCS=15KHz, 1SubSymbol=1solt, when SCS=30KHz, 1SubSymbol=2solt, and so on.
• Special case: when SCS=60KHz, 1solt=12OFDMSymbol, extended cyclic prefix

5G slot classification

• In 5G, the symbols contained in the slot can be divided into uplink and downlink, and are mainly divided into four types according to the difference in the uplink and downlink allocation of symbols in a slot.

5G Frequency Domain Resources

Basic concepts of 5G frequency domain resources

• RG（Resource Grid）：
• RE：
• RB：
• RBG：
• REG：
• CCE：
  

Channel bandwidth and transmission bandwidth

• Channel bandwidth = transmission bandwidth + protection bandwidth
• Channel bandwidth supports 5MHz-100MHz in FR1 (450MHz-6000MHz)
• Channel bandwidth supports 50MHz-400MHz in FR2 (24GHz-52GHz)
• The smaller the guard bandwidth, the higher the spectrum utilization

• When SCS=30KHz, channel bandwidth=100MHz, there is the maximum spectrum utilization

BWP definition and its application scenarios

• The channel bandwidth is 5MHz-100MHz, and the flexibility in application may still be insufficient, so there is BWP (BandWidth Part), also known as adaptive bandwidth, which can be adjusted according to different application scenarios and allocated to different BWPs for different UEs to achieve Flexible bandwidth configuration on the network side and UE side; each BWP corresponds to a specific system parameter (Numerology).
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