Interview questions of LTE

Summary of LTE interview questions

1 Which indicators are concerned? LTE test?

Answer: The LTE test mainly focuses on PCI, RSRP (received power), SINR (signal quality), PUSCH power (UE transmit power), transmission mode (TM3 is dual-stream mode), uplink and downlink rate, dropped call rate, connection success rate and handover success rate. ........................

2. What is the transmission power of 2.UE?

A: The transmission power of UE in LTE is measured by PUSCH power, and the maximum transmission power is 23dBm.

3. What are the similarities and differences between 3.LTE and CDMA?

Answer: 1, the network architecture is different, LTE has no base station controller, that is, 2G BSC and 3G RNC；;

2.CDMA technology is adopted for CDMA and OFDM technology for LTE;

3.CDMA has CS and PS domains, while LTE only has PS domains;

4. What is the scheduling effect of 4.LTE parameters?

There are 1 00 RBs in the bandwidth of1and 20M, and the peak rate can only be reached if it is fully scheduled. The fewer RBs are scheduled, the lower the rate is.

2. In the F\D\E frequency band, the downlink of PDCCCH DL Grant Count is fully scheduled for 600 times per second, and the peak rate can only be reached when it is fully scheduled, and the less scheduling times, the lower the rate; The complete uplink scheduling of the PDC UL grant count is 200 times/sec in the F band and 400 times/sec in the D\E band. Only full scheduling can reach the peak rate, and the less scheduling times, the lower the rate.

5. Related steps of 5.LTE background operation, including adding neighbor cells and adjusting parameters?

6. What are the key technologies of 6.LTE? LTE uses OFDM technology.

4. Downlink OFDM: Orthogonal frequency division multiplexing technology, a kind of multi-carrier modulation. Divide a broadband channel into several orthogonal sub-channels, convert high-speed data signals into parallel low-speed sub-data streams, and modulate them for transmission on each sub-channel; Uplink SC-FDMA multi-antenna technology; MIMO HARQ: In order to obtain correct data transmission, LTE still adopts forward error correction coding (FEC) and automatic repeat request (ARQ).

Combined error control, namely hybrid ARQ(HARQ). HARQ applies the retransmission strategy of incremental redundancy (IR), while chase combines it.

(CC) is actually a special case of IR. For easy implementation and avoiding wasting time waiting for feedback messages, LTE still chooses.

N-process parallel waiting protocol (SAW), which sorts the data received by multiple processes through reordering function at the receiving end. HARQ

The retransmission time can be divided into synchronous HARQ and asynchronous HARQ. Synchronous HARQ means that UE must know the retransmitted data.

It is sent immediately, so there is no need to attach the HARQ processing sequence number, such as the subframe number. Asynchronous HARQ can be used at any time.

Retransmission data block. According to whether the transmission characteristics change, HARQ can be divided into adaptive and non-adaptive. At present,

LTE tends to adopt adaptive asynchronous HARQ scheme.

5.64QAM high-order demodulation;

7. What is the specific information of the control channel?

A: Physical downlink control channel (PDC).

1. Notifies UE of PCH and DL-SCH resource allocation and hybrid HARQ information related to DL-SCH.

2. Carrying uplink scheduling permission information

3. Multichannel PDCCH can be transmitted in one subframe.

4. OFDM symbols used for the PDCCH in the subframe are set as the first n OFDM symbols, where n < 3.

8. What does 8.RB mean? What is the bandwidth of Shenzhen? What is the Rb of 20 megabits bandwidth?

Answer: RB (resource block) is the smallest resource allocation unit in frequency domain for physical layer data transmission, with 1 slot in time domain and 12 continuous subcarriers in frequency domain;

At present, the bandwidth of Shenzhen is 20M, and the 20m bandwidth is 100 Rb.

9. Switch signaling process

Pci planning?

A: Principles of PCI planning:

? Other co-frequency cells with strong interference to the main cell cannot use the same PCI as the main cell (adjacent cells of different frequency cells can use phase.

The same PCI level, but it will still interfere with the reception of UE, so can these cells adopt the same PCI (multiplexed with PCI) as the main cell?

? V-shift interleaving optimization principle of pilot symbols in adjacent cells;

? Based on the goal of simplicity, clarity and easy expansion, the current planning principle is that the PCI of the same site is allocated in the same place.

In a PCI group, the PCI of adjacent stations is in different PCI groups.

? When there is an indoor coverage scene, it is necessary to consider whether to plan it separately.

? Adjacent cells cannot share the same PCI, and adjacent cells of adjacent cells cannot share the same PCI;

There are 504 PCI***, and PCI planning mainly needs to avoid PCI mode 3 interference as much as possible;

11.What's the difference between LTE and TD? What's your understanding of LTE?

1, the network architecture is different, LTE has no base station controller, that is, 2G BSC and 3G RNC；;

2.TD uses time division duplex code division multiple access (TD-SCDMA), while LTE uses orthogonal frequency division multiple access (OFDM).

3.TD has CS and PS domains, while LTE only has PS domains;

4. Different frame structures;

12. What do RSRP and SINR mean?

Reference signal reception power Reference signal reception power.

SINR: Signal interference plus noise ratio means that the signal interference plus noise ratio (SINR) is the ratio of the strength of the received useful signal to the strength of the received interference signal (noise and interference); It can be simply understood as "signal to noise ratio".

How many scrambling codes does 13 have? Is there LTE?

LTE uses PCI (Physical Cell ID) to distinguish cells instead of scrambling codes. LTE has the concept of scrambling code, and LTE*** has 504 PCI.

What are the main interferences of 14? LTE？

A: Interference can be divided into internal interference and external interference: internal interference is the interference within the system. Because at present, there are three kinds of interference in the same frequency networking: the same frequency neighbor interference and PCI mode interference; External interference is interference outside the system. At present, it is mainly caused by DCS interference and wireless signals from other external wireless devices and devices.

Interference caused by frequencies falling on the frequency band used by LTE;

15.LTE networking structure, which network elements are included in EPC, and EPC is spelled in English?

The core network EPC of LTE is composed of MME, S-GW and P-GW, and the evolved packet core is the packet core network.

16.LTE radio frame structure, subframe, etc. , uplink and downlink ratio, what special subframes are there and how to configure them?

At present, the ratio of uplink and downlink time slots in Shenzhen F-band is 1:3, and the dedicated time slot is 3: 9: 2.

The ratio of uplink and downlink time slots in D\E band is 2:2, and the dedicated time slot is10: 2: 2;

17. What are the reasons why the single measurement rate can't meet the standard?

At present, the single inspection rate requirements of macro stations in Shenzhen are: the average downlink rate is greater than 40M, and the statistical time is 30 seconds; The average ascending speed is greater than 6M, and the statistical time is 30s.

Ventricular rate: the average descending speed is more than 50M, and the statistical time is 60s; the average ascending speed is more than 15M, and the statistical time is 60s;

18. What is the relationship between SINR value and quality?

Downlink SINR calculation: the power on RB is evenly distributed to each RE;

RS of downlink RS = RS received power/(interference power+noise power) = s/(I+n);

It can be seen from the formula that the SINR value is related to RSRP, interference power and noise power received by ue, specifically: external interference and internal interference (co-frequency neighbor interference, mode 3 interference).

19. What is the content of LTE network planning?

1, planned by TA and TAL;

2.PRACH planning;

3.PCI planning;

20. Have you ever been to the front desk for testing? What are the requirements for coverage and quality?

21.What's the difference between TD-LTE and GSM?

1, the network architecture is different, LTE has no base station controller, that is, 2G BSC and 3G RNC；;

22.TD-LTE coding method?

The modulation of downlink data mainly adopts QPSK, 16QAM and 64QAM. π/2 shift BPSK, QPSK, 8PSK and 16QAM are mainly used for uplink modulation. Like the downlink, the uplink channel coding still follows the Turbo coding of R6.

23. How are the Chinese names of PCI and 504 numbers calculated?

PCI consists of primary synchronization sequence and secondary synchronization sequence. The main synchronization signal is three different values of frequency domain Zadoff-Chu sequence with length 62, and the sequence orthogonality of the main synchronization signal is good. The auxiliary synchronization signal is taken from two auxiliary synchronization slots (0 and 5) within 10 millisecond.

With different sequences and combinations of 168, the orthogonality of the secondary synchronization signal is worse than that of the primary synchronization signal, and the primary synchronization signal and the secondary synchronization signal * * * form 504 PHY _ Cell _ID codes;

PCI distinguishes the downlink cells and the uplink cells according to the root sequence.

24. What is the identification of single stream and double stream in LTE foreground test?

In the parameter window of radio station: from the transmission mode, it is TM3 mode (only TM3 mode supports dual stream, while TM2 and TM7 only support single stream), and the rank indicator is Rank 2, indicating that the terminal is in dual stream mode (bottom left);

Due to the slow response speed of the probe software, we can still judge it in the MCS window at ordinary times: as shown in the MCS diagram at the lower right, there are several columns of numbers, both of which are not zero, indicating that it is in double-flow mode. For example, if the number in the left column is not zero and the number in the right column is all zero, it means that it occupies a single flow;

25. What is the tracking area in LTE?

26. The tracking area in LTE is also called TA, and the tracking area code is called TAC (Tracking Area Code).

The tracking area is used for paging and location update. Similar to the concept of location area (LAC) in UMTS network. The planning of the tracking area should ensure that the paging channel capacity is not limited, and the location update cost of the area boundary is the lowest, and it is required to be easy to manage. As a part of LTE network planning, tracking area planning is closely related to network paging performance. Reasonable planning of tracking area can balance paging load and TA location update signaling flow, and effectively control the system signaling load.

When designing a tracking area in an LTE/SAE system, the following requirements need to be met:

LTE access network and core network keep the same concept of tracking area.

When the UE is idle, the core network can know the tracking area where the UE is located.

When an idle UE needs to be paged, it must be paged in all cells in the tracking area where the UE is registered.

In LTE system, location update signaling caused by location change should be minimized.

The paging load determines the maximum range of the tracking area, and correspondingly, the location update load of the edge cell determines the tracking area.

The minimum range and the most important constraint is the maximum paging capacity of the mme.

28. the difference between b and RB

RB is 7 OFDM symbols in time domain, but the actual scheduling is 1 subframe and 14 OFDM symbols in time domain.

So the scheduling is based on PRB, PRB=2RB.

RB has two concepts: VRB and PRB. VRB is a virtual RB, which is under pressure when the mac layer allocates resources.

The VRB is allocated, and then the VRB is mapped to the PRB. There are also two ways to map VRB to PRB: distributed and centralized. Centralized VRB and PRB are in one-to-one correspondence, and the mapping from distributed VRB to PRB needs to be interwoven first, and then mapped to the actual PRB position according to certain rules.

29. What are the uplink and downlink physical channels, physical channels and physical signals of LTE?

The difference between

The answer to the question:

Physical channel: a set corresponding to a series of RES, which is called a physical channel that needs to carry high-level information; Such as PDC, PDC, etc.

Physical signal: It corresponds to a series of REs used by the physical layer, but these REs do not transmit any information from higher layers, such as reference signal (RS) and synchronization signal.

Downlink physical channel:

Physical downlink * * * shared channel). Mainly used for transmission services.

Data,

Signaling can also be transmitted. UE is scheduled by frequency division,

Physical downlink control channel). Carrying pilot calls and user data

endowment

Resource allocation information and HARQ information related to user data.

Physical broadcast channel). Carrying cell ID and other system information, used for small-sized

Regional search

Cable technology.

Channel (physical HARP indication channel), used for bearer.

Harp

ACK/NACK feedback.

Physical control format indication channel) is used for

azimuth control

Location information of OFDM symbols where the control information is located.

Physical multicast channel) is used to carry multicast information.

Downlink physical signal:

A reference signal, also commonly referred to as a pilot signal;

: synchronization signal, which is divided into primary synchronization signal and secondary synchronization signal;

Uplink physical channel:

Physical random access channel) carries a random access preamble.

Physical uplink shared channel) carries uplink user data.

Physical uplink * * * shared channel) carries HARQ.

ACK/NACK，

Scheduling requests, channel quality indications and other information.

Uplink physical signal:

: reference signal;

30. What is the optimization of Swiss coverage in cluster optimization?

A: Increase the number of stations; B: Increase its transmitting power, adjust the azimuth and inclination of the antenna feed, and enhance the coverage.

3 1. Cross-regional coverage reduces the transmission power, and the height of the dip angle reduction station is configured with the corresponding adjacent cell relationship.

32. What are the three types of transfer?

Hard handover is a handover between base stations or coverage cells with different frequencies. The handover process is that the mobile phone first disconnects the communication with the source cell and then establishes the connection with the target cell; Soft handover is a handover between two different base stations with the same frequency. The handover process is to disconnect the communication with the source cell after the mobile phone establishes the connection with the target cell; Relay switching is a characteristic technology of TD system. In the handover process, the uplink and downlink communication links are successively transferred to the target cell by using the mobile phone uplink pre-synchronization technology.

33.8. What are the transmission modes currently used by LTE, and what are their differences and functions?

Nine transmission modes of LTE:

1.TM 1, single antenna port transmission: mainly used for single antenna transmission.

2.TM2, Open-loop transmit diversity: No feedback PMI is needed, which is suitable for the case of complex cell edge channels and large interference, and sometimes it is also used for high-speed situations, and diversity can provide diversity gain.

3.TM3, open-loop spatial multiplexing: PMI feedback is not needed, which is suitable for the case of high-speed mobile terminal (UE).

4.TM4, closed-loop spatial multiplexing: feedback PMI is needed, which is suitable for occasions with good channel conditions and is used to provide high data rate transmission.

5.TM5, MU-MIMO transmission mode (downlink multi-user MIMO): mainly used to improve cell capacity.

6.TM6, closed-loop transmit diversity, closed-loop Rank 1 precoding transmission: PMI that needs feedback is mainly suitable for cell edge.

7. Single beam forming mode of 7.TM7 and Port5: mainly at the edge of the cell, which can effectively resist interference.

8.TM8, dual-stream beamforming mode: It can be used in scenes such as cell edge.

9.TM9, transmission mode 9 is a newly added mode in LTE-A, which supports at most 8 layers of transmission, mainly to improve the data transmission rate.

34. What will mode 3 interference cause? A: SINR is getting worse and worse, which affects the normal handover and the download rate is low.

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