What are the differences between 4G and 5G networks?
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1. Frame structure comparison
What 4G and 5G have in common
- The frame and subframe lengths are both 10ms and 1ms.
- Minimum scheduling unit resource: RB
Differences between 4G and 5G
1); Subcarrier width
- 4G : Fixed to 15kHz.
- 5G : Multiple options: 15kHz, 30kHz, 60kHz, 120kHz, 240kHz, and multiple subcarrier bandwidths can be transmitted simultaneously in one 5G frame.
2) Minimum scheduling unit time
- 4G : TTI, 1 ms;
- 5G : slot, 1/32 ms ~ 1 ms, depending on the subcarrier bandwidth.
- In addition, 5G adds a mini-slot, which only occupies at least 2 symbols.
3); Number of time slots per subframe (number of symbols)
- 4G : 2 time slots per subframe, normal CP, 7 symbols per time slot.
- 5G : Depends on the subcarrier bandwidth, 1-32 time slots per subframe, and 14 symbols per time slot for normal CP.
- The scheduling unit of 4G is subframe (normal CP contains 14 symbols); the scheduling unit of 5G is time slot (normal CP contains 14 symbols).
Analysis of 5G design concepts
1); Time-frequency relationship
- Basic principle: There is an inverse relationship between subcarrier width and symbol length, wide subcarriers have short symbols, and narrow subcarriers have long symbols;
- Performance: When the total bandwidth is fixed, the number of RE resources in the time-frequency two-dimensional structure is fixed and does not change with the subcarrier bandwidth, and the throughput is also the same.
2); Reduce latency
- By selecting wide subcarriers, the symbol length becomes shorter, while 5G scheduling is fixed to 1 time slot (12/14 symbols), and the scheduling delay becomes shorter.
- When the maximum subcarrier bandwidth is selected, single scheduling is reduced from 1 millisecond (15kHz) to 1/32 millisecond (480kHz), which is more conducive to URLLC services.
5G subcarrier bandwidth comparison
1); Coverage: Narrow subcarrier is good
- Service and public channels: small subcarrier bandwidth, long symbol length, and short CP length, which have strong resistance to inter-symbol interference caused by multipath.
- Common channels: For example, PUCCH and PRACH need to be uploaded in one RB. The bandwidth of each RB of small subcarriers is also small, and the uplink power density is high.
2) Overhead: Narrow subcarriers are better
- Scheduling overhead: For large carrier bandwidth, more slot units need to be scheduled in each frame, which increases the scheduling overhead.
3) Delay: Wide subcarrier is better
- Minimum scheduling delay: large subcarrier bandwidth, short symbol length, short occupancy time of the minimum scheduling unit slot, the shortest being 1/32 milliseconds.
4) Mobility: Wide subcarrier is good
- Doppler shift tolerance: Under certain frequency shift conditions, the impact of large bandwidth is small and the interference between subcarriers is small.
5); Processing complexity: wide subcarrier is better
- FFT processing complexity: For example, at 15kHz, it is much better than FFT, and the device can only support up to 275 RBs (50MKz).
5G Common Subcarrier Bandwidth
1); C-Band
- eMBB : 30kHz is currently recommended.
- URLLC : Wide subcarrier bandwidth.
Self-contained
- 4G : A single subframe is either only downlink or only uplink (except for special subframes). The uplink subframe is transmitted after the downlink subframe is transmitted. At a ratio of 3:1, the uplink feedback is not sent until 3ms after the downlink transmission starts, and the delay is relatively large.
- 5G : Introducing a control channel in the opposite direction of data transmission in each time slot can achieve rapid feedback reduction (downlink feedback delay and uplink scheduling delay). For example, at 30kHz, feedback can be achieved in 0.5ms units. Other large subcarrier bandwidths can achieve even shorter delays.
2. TDD uplink and downlink ratio
1.TDD analysis
1) Advantages
- Resource adaptation: Adjust the uplink and downlink resource ratio according to network requirements.
- Better support for BF: Uplink and downlink frequencies are the same and different, which better supports BF.
2) Disadvantages
- GPS synchronization required: Strict time synchronization is required.
- Overhead: Uplink and downlink switching requires a GAP, which wastes resources.
- Interference: Inter-station interference is likely to occur, such as TDD ratio misalignment, long-distance interference, etc.
2. 5G from the perspective of TDD-LTE
- No innovation in TDD ratio: LTE and 5G are similar in TDD ratio design, and the uplink and downlink ratios are adjustable.
- Dynamic TDD is unlikely in the short term: the same network can only have one TDD ratio, otherwise there will be serious interference between base stations.
- The TDD ratio will converge: From the perspective of LTE, many TDD ratios were defined in the early stage, but they all converged to a ratio of 3:1 (the ratio of downlink and uplink resources), and 5G should be the same.
- Synchronization: synchronization between 5G operators, and synchronization between NR and TDD-LTE.
3. Channel: Transmitting high-level information
1. Public Channel
1) ; Downward
a) PCFICH, PHICH
- 4G : This channel is available.
- 5G : This channel is removed, reducing latency requirements.
b) PDCCH
- 4G : No dedicated demodulation pilot, no support for BF, no support for multi-user multiplexing, poor coverage and capacity; PDCCH is hashed in the frequency domain and has frequency selective gain, but forward compatibility is poor, such as GL dynamic sharing, and it is necessary to consider how to avoid PDCCH.
- 5G : It has a dedicated demodulation pilot (DMR), supports BF, supports multi-user multiplexing, and has good coverage (9db gain) and capacity; the PDCCH is set in a specific location, has strong forward compatibility, and it is very easy to take out some of the frequency bands.
c) Broadcast Channel
- 4G : The frequency domain position is fixed and placed in the center of the bandwidth. BF is not supported.
- 5G : Flexible location configuration, strong forward compatibility, support for BF, and coverage improvement of 9db.
2) Uplink
a) PUCCH
- 4G : The minimum scheduling unit is RB.
- 5G : The smallest unit symbol for scheduling can be placed in a special subframe.
2. Business co-channel
1) Downlink PDSCH
- 4G : No dedicated pilot except LTE MM, and the highest modulation is 64QAM.
- 5G : It has a dedicated pilot, the highest modulation is 256QAM, and the efficiency is improved by 33%.
2) Uplink PUSCH
- 4G : Maximum modulation: 64QAM.
- 5G : Maximum modulation 256QAM, efficiency improved by 33%.
4. Signal: auxiliary transmission, no high-level information
1. Signal type
- 4G : Both measurement and demodulation use a shared CRS (measure RSRP PMI RI.CQI and measure phase for demodulation). Of course, LTE MM ( MM: Massive Mimo, multi-antenna technology, the same below ) has a dedicated pilot shared with CRS.
- 5G : Remove CRS. Add CRI-RS (measure RSRP PMI RI CQI) and support BF; add DMRS for DMRS demodulation (measure phase demodulation) and support BF. All channels have dedicated DMRS. 12 ports of DMRS plus spatial multiplexing support a maximum of 32 streams.
2. Contrast
1); Coverage
- 4G : CRS has no BF and RSRP is poor.
- 5G : CRI-RS has BF ( BF: Beam Forming, the same below ), which has a 9db coverage gain (10*log(8-column array)) compared to LTE RSRP.
2); Light load interference
- 4G : Large interference under light load. Without BF, the interference is larger; it sends all the time, even if it is not loaded, it must be sent in the entire cell, which interferes with the neighboring cells; it sends staggered data between cells, even if it is not loaded, it will interfere with the data of the neighboring cells.
- 5G : With BF and narrowband scanning, the interference is smaller; only a certain subband can be sent, the interference to the neighboring cells is small, and countless transmitted subbands will not interfere with the neighboring cells; the positions between neighboring cells are not misaligned, and there is no data RE interference to the neighboring cells.
3) Capacity
a); Pilot overhead: about the same
- 4G : CRS in each RB occupies 16 REs, and if MM is used, there are 12 dedicated pilot REs.
- 5G : CSI-RS 2~4 REs and DMRS 12~24 REs in each RB.
b); Single user capacity
- 4G : The protocol defines DMRS for 2 ports, so a single user can have a maximum of 2 streams in MM.
- 5G : 12 ports of DMRS are defined. A single user can support up to 8 streams as specified in the protocol. Of course, considering the size limitation of the terminal, the maximum number of streams is estimated to be 4.
5. Multiple access
1. Peak value increased by 9%
- 4G : OFDM bandwidth utilization rate is 90%, and 5% of the bandwidth is reserved on each side as protection band.
- 5G : F-OFDM bandwidth utilization 98.3% (filter reduces guard band).
2. Upward average increase of 30%
- 4G : Single-carrier technology is used for uplink. Advantages: low PAPR, high transmission power, and good edge coverage; Disadvantages: single-carrier, single-user data must be transmitted on continuous RBs, which may cause waste if the number of RBs is insufficient to transmit one user's data; user pairing is 1:1, and if the resources required by two users are different, waste will occur.
- 5G : Use single-carrier and multi-carrier adaptation. Edge users use a single carrier with good coverage; mid-range users use multiple carriers, and users can be paired one to many, with high user pairing efficiency and high resource utilization; user resource allocation can use discontinuous RB resources, with frequency selection gain, and can fully utilize scattered RB resources.
6. Channel Coding
- 4G: Traffic channel Turbo, control channel convolutional coding, block coding and repetition coding.
- 5G: LDPC code-service channel, high data block transmission rate, good demodulation performance, and low power consumption; Polar code-control channel, small data block transmission, good demodulation performance, and coverage improvement of 1dB.
7. BF weight generation
- 4G: TM7/8 terminals: The base station calculates the weight based on the SRS transmitted by the terminal; TM9 terminals (R10 and above): The base station calculates the weight (mid-near point) when the terminal transmits SRS and the terminal calculates the PMI (far point) based on the CRS adaptively.
- 5G: When the terminal transmits SRS, the base station calculates the weight (mid-near point) and the terminal calculates the PMI (far point) based on the CRS, which is adaptive. SRS requires full-bandwidth transmission. Due to limited collection power at the edge, it may be unrecognizable when it reaches the base station. The PMI standard has an index and only 1 to 2 RBs are needed to send it to the base station, which has good coverage.
8. Uplink and Downlink Conversion
- 4G: Uplink and downlink conversion is performed once for each frame (5ms/10ms), resulting in high latency.
- 5G: Larger carrier bandwidth and self-contained time slots enable fast feedback and low latency.
9. Large bandwidth
- 4G: maximum support 20MHZ;
- 5G: supports up to 100MHZ (C-band) and 400MHZ (millimeter wave).
10. Carrier Aggregation
11. 5G has enhanced capacity compared to 4G
1. Downward
1); MM: No change
- The most critical technology of 5G is to significantly improve spectrum efficiency. LTE also has MM. Based on LTE experience, the spectrum efficiency of MM is about 5 times that of 2T2R.
2) F-OFDM: 9% improvement
- 5G bandwidth utilization increased by 9%;
3); 1024QAM: <5%
- The peak value is increased by 25%. However, considering that it is difficult to enter 1024QAM in the existing network, the average throughput gain is estimated to be less than 5%.
4); LDPC: Unclear
5) More accurate feedback: 20%~30%
- The terminal SRS is transmitted in turn by the four antennas of the terminal, and the base station obtains the information of all four channels of the terminal, which makes the MIMO scheduling and coordination between single-user multi-stream and multiple users better; SRS and PMI are adaptive. When the edge SRS is inaccurate, the use of PMI makes the BF effect better than LTE.
6) Expenses: basically the same
- While 5G reduces CRS, it actually increases CRI-RS and DMRS. The reduction and increase in overhead are consistent. It cannot be said that after CRS is free, the overhead is reduced compared to LTE. CRS free is actually to reduce interference when the load is light.
7) ; Slot aggregation: 10%
- 4G : DCI Grant information must be sent every two slots.
- 5G : Multiple slots are aggregated and only one DCI Grant message is sent, resulting in low overhead.
2. Upward
1); MM: No change
2); Single and multi-carrier adaptation: 30%
- One-to-many user pairing is not aligned, and RBs are not allocated continuously;
3); LDPC: unknown
12. 5G has better coverage than 4G
1. Downward
1) LDPC: unknown
2) Power: 2dB
- LTE power is 120w, 5G power is 200W.
2. Upward
1) LDPC: unknown
2) Uplink and downlink decoupling: 11dB+
13. 5G has better latency than 4G
1. Short TTI
- The shortest scheduling time of 5G is shortened from 1ms of LTE to a minimum of 1/32 millisecond.
2. Self-contained
- Shorten the uplink and downlink feedback intervals to a single slot, with a minimum of 1/32 milliseconds.
3. Uplink authorization-free
- Unauthorized uplink access reduces latency.
4. Preempting transmission
- URLLC preempts resources.
5. Pilot pre-position
- It takes a certain amount of time for the terminal to process DMRS.
6. Mini-slots
- Several symbols are selected as transmission scheduling units to further compress the scheduling delay.
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