Highlights of LTE problems (4)

Question 16: Time Synchronization

1.Wireless Link Quality Detection

To ensure the correct transmission of downstream signaling and data, the UE side needs to check the downlink quality after the cell search is complete to ensure that the downlink signaling and data are correctly received, the UE implements uplink synchronization with the base station through the random access process. After that, the base station continuously sends timed adjustment commands to the UE to maintain uplink synchronization.

Therefore, the UE must detect the downlink wireless channel quality of the service area before connecting to the LTE system, and want to report the status of Synchronization Based on the detection results, that is, the in-sync status) or the out-of-sync status ).

Wireless Channel quality detection is divided into the following two situations:

1) in non-drx mode, each wireless frame in the UE-side physical layer must evaluate the quality of the wireless link (relative to the threshold qout and Qin values in the detection );

2) In drx mode, the quality of wireless links should be evaluated at least once per drx cycle in the UE-side physical layer (relative to the thresholds qout and Qin in the relevant detection );

Conclusion:

UE compares the detected link quality with the decision threshold (qout and Qin) to determine that it is in the synchronous/out-of-step state.

• When the wireless link quality is lower than the threshold qout, the UE-side physical layer reports the out-of-sync status to the upper layer.
• When the wireless link quality is better than the threshold qout, the UE-side physical layer reports the in-sync status to the upper layer.

2.Transmission Time modulation

Signal Transmission is delayed in space. If the UE moves farther away from the base station during the call, the signal sent from the base station will arrive at the UE more and more late, the UE signal will also arrive at the base station "More and more late". If the delay is too long, the UE signal received by the base station on this time slot will overlap with the time slot for the base station to receive other ue signals, cause inter-code interference.

In LTE, when the upstream signals of different ue reach enodeb, the time must be aligned to ensure the orthogonal uplink signals between ue, which helps to eliminate interference in the residential area.

To ensure the synchronization between the UE and the base station, you need to maintain the uplink synchronization of the UE, that is, you need to adjust the scheduled time of the UE.

• Enodeb checks the UE's reference signal (RS) to determine whether the UE is synchronized with the base station. If there is a synchronization deviation, enodeb sends a timed Adjustment Command (also calledTime advance,Timing advance,Ta,TaThe time range is:0 ~ 0.67 Ms, Granularity is0.52us, That is16 * TS) To indicate that the UE needs to adjust the timing synchronization point.
• Once the UE receives the TA from enodeb, the UE will adjust its uplink transmission time (adjusted to an integer multiple of 16 TS) for pucch, pusch, and SRS ).
• In the random access response, enodeb uses the 11bit scheduled command TA (TA =, 2 ,..., 1282, in the unit of 16 TS ). After receiving the scheduled command Ta, ue calculates the time advance, which is expressed by NTA, where NTA = TA * 16.
• In other cases, enodeb uses the 6-bit scheduled command TA (TA =, 2 ,..., 63 ). After receiving the timer command, ue calculates the new timer prefix NTA based on the current timer NTA and old. For new, NTA, new = NTA, old + (Ta-31) * 16. Here, the adjustment amount can be positive or negative, indicating that the scheduled ue needs to be advanced or delayed.

1) whenNAfter the sub-frame receives the time advance command, the corresponding timing is adjusted inN + 6The start of the sub-frame. During the scheduled adjustment process, if the Sub-FrameNUplink pucch/pusch/SRs transmission andN+ If one sub-frame overlaps, the UE will transmitNSub-frame, andN+ The overlapping part of a sub-frame will not be transmitted.

2) If the received downstream time changes and no corresponding compensation is obtained, or if the command does not have time adjustment, the UE adjusts NTA accordingly.

Question 17: LTE peak rate calculation method

Assume that the normal cycle prefix normal CP is used (that is, each time slot has 7 symbols), and the modulation encoding method uses 64QAM (that is, each symbol contains 6 bits ), therefore, the following equivalence relationship exists:

1 Wireless Frame (10 ms) = 10 sub-frames = 20 time slots

1 time slot = 0.5 ms

1 time slot = 7 symbol = 7 * 6bit = 42bit

Single Carrier peak rate = number of bits per time slot/time occupied by each time slot = 42bit/0.5 ms = 84bit/MS = 84000bit/S = 84 kbps

20 MHz is equivalent to 100 RB = 1200 sub-carriers

The peak LTE rate at 20 MHz bandwidth can be calculated as: 1200*84 kbps = 100.8 Mbps

Note:One RB contains 12 sub-carriers, each of which is separated by 15 kHz. Therefore, 100 Rb is equivalent to 100*12*15 kHz = 18000 kHz = 18 MHz, so why does 20 MHz bandwidth mean 100 RB ??? At this time, because the 20 MHz bandwidth cannot be fully used, the two ends must reserve a protection band.

Question 18: LTE Data Transmission Mechanism

In the LTE system, the data communication mechanism is shown in. the uplink data transmission channel is pusch, the downlink data transmission channel is pdsch, and the wireless resource carrying data transmission is RB, physical uplink control channel (pucch) transmits upstream control information, and physical downlink control channel (pdcch) transmits DCI. During Enode B scheduling, allocate the data transmission resource location for the UE and notify the UE through DCI. In the middle, at the time of N, the UE first obtained the pdcch blind solution, and obtained any of the DCI formats 1 to 2a, so that we can know the location of the UE downstream data and the modulation encoding method, then, the downstream data is received at the corresponding position in pdsch at the time of N. When the DCI format 0 is obtained by blind decoding, the UE can upload data in pusch at the N + 4 time point and the modulation encoding method.

 

Highlights of LTE problems (4)