Highlights of LTE problems (3)

Question 11: special sub-frame ratio of LTE sub-frames (Supplement to question 7)

To solve this problem, I didn't quite understand the protocol at the beginning. For example, I often encounter this problem:LTENeutron frame ratio is1, Special sub-frame Ratio7What does this mean?

Search for related questions online. The answers collected online are summarized as follows:

First, in the TDD frame structure, the frame length is 10 ms and is divided into 10 child frames with a length of 1 ms. Upstream and downstream data are transmitted on different sub-frames within the same frame. TDD supports different upstream/downstream time ratios. You can adjust the upstream/downstream time ratios based on different business types to meet asymmetric upstream/downstream business needs. The configurations of different upstream and downstream sub-frames within the same frame are shown in:

We can draw the following conclusions:

Conclusion 1:In the upstream/downstream sequence configuration, the switching period from 5 ms to 10 ms is supported;

Conclusion 2:For a 5 ms downlink to uplink switching period, a special sub-frame is configured in each half frame of 5 ms;

Conclusion 3:For 10 ms downstream to uplink switching period, configure a special sub-frame in the first 5 ms sub-frame;

Conclusion 4:In a 5 ms period, the sub-frame 1 and sub-frame 6 are fixed as special sub-frames;

Conclusion 5:In a 10 ms period, the sub-frame 1 is fixed as a special sub-frame, and each special sub-frame is composed of three special time slots: dwpts, GP, and uppts;

Conclusion 6:Sub-frames 0, 5, and dwpts time slots are always used for downlink data transmission, and uppts and their first sub-frames are always used for uplink transmission.

　

The special sub-frame consists of three parts: dwpts (downlink pilot time slot), GP (guard period), and uppts (uplink pilot time slot ). Dwpts transmits downstream reference signals and some control information. Uppts can transmit short Rach and SRS information. GP is the Protection time between upper and lower rows.

Similar to other sub-frames, the length of a special sub-frame is also 1 ms, but the length of each part is different and can be configured through high-level signaling.

Relatively speaking, the uppts length is relatively fixed, only one symbol and two symbols are supported, to avoid too many options, simplify the system design, GP and dwpts have great flexibility, this is mainly to achieve variable GP length and GP position to support cell radius of various sizes.

Shows the configuration of special sub-frames:

At this point, we can answer the following questions:

Sub-frame ratio 1It indicates the ratio of downlink to uplink sub-frames: 2: 2

Special sub-frame ratio 7Dwpts: GP: uppts: 10: 2: 2

Question 12: several transmission modes in LTE

In the LTE network, there are eight downlink transmission modes:

TM1: single antenna port (SISO or Simo) single antenna;

TM2: transmit diversity (2 or 4 antennas, transmit diversity );

Tm3: Open-Loop Spatial multiplexing (2 or 4 antennas, Open-Loop Space Division Multiplexing );

TM4: Closed-Loop Spatial multiplexing (2 or 4 antennas, closed-loop space division multiplexing );

TM5: multi-user MIMO (2 or 4 antennas, multi-user MIMO );

TM6: Closed-Loop rank-1 Spatial multiplexing (2 or 4 antennas, single-layer Closed-Loop Space Division Multiplexing );

Tm7: single antenna port beamforming (single antenna or multiple antennas, repeat );

Tm8: dual-layer beamforming4 (dual-layer repeat)

Note:The transmission modes of tm3/4/5/6 are pre-encoded Based on the code book, And tm7/8 adopts the non-code-based Beam Based on the special guide.

Question 13: what is the role of PDCP Header Compression?

Simply put, the IP address header is compressed to reduce the overhead of the empty port. Especially for small data packets, such as VoIP, the voice packet is very small, usually 20 bytes, And the IP packet header is about 40 bytes. That is to say, a packet contains a total of 60 bytes, however, if 2/3 of them are headers, the efficiency is very low. If header compression is enabled, the header can be compressed to 2 or 3 bytes. In this way, the header only occupies one tenth and the efficiency is high.

Specific compression types include: compressing the RTP/udp/IP header, compressing the UDP/IP header, or only pressing the IP address header.

Question 14: LTE modulation and encoding scheme

The physical channels defined by LTE can be divided into uplink physical channels and downlink physical channels. Both uplink and downlink channels support the following modulation modes. Different modulation modes correspond to different modulation ing modes. modulation ing uses 0 and 1 as input to generate a complex modulation symbol x = I + JQ as the output.

Two-bit B (I), B (I1) ing is used to form a complex modulation symbol x = I + JQ.

During 16QAM modulation, four-bit B (I), B (I1), B (I2), and B (I3) are mapped to the complex value modulation symbol x = I + JQ.

When 64QAM is modulated, six bits (I), B (I1), B (I2), B (I3), B (I4), B (I5) are used) maps to the complex value modulation symbol x = I + JQ.

 

Highlights of LTE problems (3)