LTE physical Transmission Resources (1)-Frame structure

After writing the previous blog, "LTE cell search-Physical cell ID and synchronization signal PSS, SSS," I would like to continue to write system information related content, but found to write the time necessary to involve PDCCH, Phich and other content, and these content has not been systematically written. So the next few posts will write some of the LTE background knowledge that needs to be mastered.

This article describes the frame structure-related content of LTE.

About the frame structure, the previous it scattered mentioned some, such as the blog "LTE-TDD Random Access Process (2)-Preamble Code preamble format and time-frequency location", in the preamble format, referring to the length of each sub-frame is 30720Ts, and different up and down sub-frame configuration , downlink, special sub-frame, upstream of the ratio. This article is a comprehensive collation of the content.

1. Basic Time Unit

In LTE, both FDD and TDD, its time base unit is the sampling period Ts, the value is fixed equal to:

Where15000 indicates that the sub-carrier interval is 15KHz,2048 represents the number of sample points , so the sample rate equals 30.72MHz (15000*2048=30.72mhz). In addition to the sub-carrier interval of 15KHz, the 3GPP protocol actually defines a 7.5KHz carrier interval. This reduced sub-carrier interval is specifically for multicast/broadcast transmissions of the MBSFN (multimedia Broadcast Multicast Service single Frequency Network) and is only partially implemented in the R9 protocol. So unless specifically stated in this blog, the default sub-carrier interval is 15KHz.

2.FDD Frame Structure

The frame structure of the LTE-FDD is generally referred to as frame structure Type 1, in order to refer to it explicitly, or to call it the FDD frame structure.

In FDD, the length of each wireless system frame is Tf=307200*ts=10ms, which consists of a time slot (slot) with each timeslot length tslot=15360*ts= 0.5MS, cycle number is numbered from 0 to 19. Each sub-frame consists of 2 consecutive slots, numbered 0 to 9 cycles, so 1 wireless system frames consist of 10 sub-frames, and the period of the wireless frame is 1024.

In FDD, the 10 sub-frames of each system frame can be transmitted downstream, also can be transmitted upstream, the upper and lower lines in different frequency domain respectively. In the half-duplex FDD mode, the UE cannot send and receive data in the same frame, while in the full- duplex FDD mode, the UE does not have this limitation, and the data can be sent and received simultaneously in the same frame.

The following is the frame structure of the FDD format.

3.TDD Frame Structure

The frame structure of the LTE-TDD is generally referred to as frame structure Type 2, in order to refer to it explicitly, or to call it the TDD frame structure.

In TDD, the length of each wireless system frame is Tf=307200*ts=10ms, consisting of 2 "half frames", each "half frame" of length equal to 5ms, consisting of 5 consecutive sub-frames, each sub-frame length equal to 1ms. In addition to special sub-frames, each sub-frame consists of 2 consecutive slots. Special sub-frames are fixed at 1, 6th sub-frames, consisting of dwpts (downlink pilot time slot), GP, uppts (uplink pilot time slot). Similarly, 1 wireless system frames are made up of 10 sub-frames, and the period of the wireless frame is 1024.

The following is the frame structure of the TDD format.

When the same sub-frame is configured on a different top-down line (Uplink-downlink configuration), data may be sent in different directions. is the direction in which all sub-frames send data under various upper and lower sub-frame configurations. D indicates that the sub-frame can only send the downstream data,U means that the sub-frame can only send upstream data,S represents a special sub-frame, generally used for downlink data transmission. The ul/dl configuration parameter is derived from the SIB1 message (36331 protocol) of the RRC layer, and the specific parameter path is:systeminformationblocktype1->tdd-config-> Tdd-config->subframeassignment, see the LTE-TDD Random access process (2)-The format and time-frequency position of the preamble preamble.

Downlink-Upstream switching period is related to the number of special sub-frames in 10ms, the calculation method reference.

Essentially , the dwpts can be used as a regular downlink sub-frame, except that the effective RB is only 0.75 times times the normal downlink sub-frame when scheduling , so the amount of data transmitted is small. In general, when the upper and lower sub-frames are matched, special sub-frames are considered as downlink sub-frames. is shown in the TDD format, the seed frame up and down line ratio relationship. and Uppts because the time is too short, not for data transmission, can be used as a random access PRACH (remember the random access DCI Format 4?). Please look again at the article "LTE-TDD Random Access Process (2)-Preamble Code preamble format and Time frequency position").

The length of the special sub-frame is related to the special subframe configuration, as shown in. For special sub-frame configurations (special subframe configuration Parameters ), refer to the LTE-TDD random access process (2)-format and time-frequency position of the preamble preamble. This table is then combined with the length of the OFDM symbol.

4.OFDM symbols

Each timeslot of LTE consists of a certain number of OFDM symbols, including the cyclic prefix (CP). In addition to the CP, the OFDM symbol time is known as the useful OFDM symbol time, which is tu=2048*ts=66.7us. If the system is the normal CP type (normal CP type), then each timeslot includes 7 OFDM symbols, if the Extended CP type (extended CP type), then each timeslot includes 6 An OFDM symbol. For the Normal CP Type, the CP length at the front of the first OFDM symbol per timeslot is 160*ts, and the other CP lengths are 144*ts, The reason for the difference in the length of the first symbol is simply to fill the 0.5ms timeslot. For the Extended CP Type, the length of each CP is 512*ts. As shown in.

In my blog, "LTE cell search-Physical cell ID and synchronization signal PSS, SSS," The end of 2 sheets of PSS and SSS, you can see that the downstream CP type is the normal CP type. From this blog post, you can also know that after the detection of the PSS/SSS synchronization signal, the UE learned the downstream CP type, and the uplink CP type is RRC ul-cyclicprefixlength Field sent to the UE, as shown in.

5. Number of OFDM symbols occupied by special sub-frames

Combined with the length of the dwpts, uppts and the length of each OFDM symbol in the special sub-frame, the number of OFDM symbols occupied by each part of the special sub-frame can be obtained, as shown (partial configuration is listed, other configurations can be drawn).

calculation of the number of uppts occupied OFDM symbols in special sub-frames :

For the upper and lower lines are normal CP, because the uppts is certainly not the first sign of the time slot, so for uppts, each OFDM symbol occupies a length of 2192Ts(2048+144). So: For the duration is 2192Ts uppts, then only need 1 OFDM symbols can be transmitted, for the time is 4384Ts uppts, then need 2 OFDM symbols can be transmitted. Similarly, for the upper and lower lines are extended CP, for the duration is 2560Ts uppts, then only 1 OFDM symbols can be transmitted; for the duration is 5120Ts Uppts, then 2 OFDM symbols are required for transmission.

The position of the OFDM symbol at 6.PSS

Protocol 36213 mentions that if the special subframe configuration is 0, 5, and the downstream CP type is normal, or if the special subframe configuration is 0, 4, and the downstream CP type is extended, the Pdsch data cannot be sent in that special sub-frame .

For the special subframe configurations 0 and 5 with normal downlink CP or configurations 0 and 4 with extended downlink CP, there shall is no Pdsch transmission in dwpts of the special sub Frame.

Here's an analysis of why this conclusion is, and why.

is a position of the synchronous signal PSS and SSS. As you can see, this is a TDD format, and the downstream CP type is the normal CP type. SSS is at the last OFDM symbol in sub-frame 0, 5, regardless of the upper and lower sub-frame configuration, the 0, 5 subframe is always the downstream sub-frame. PSS is located in the third symbol of sub-frame 1, 6, and 1, 6 sub-frames are always special sub-frames, you need to ensure that PSS does not fall into GP or even uppts.

As you can see in the "Special subframe time Length" table from the previous article, the length of the dwpts is determined by the special sub-frame configuration (special Subframeconfig), ranging from 6592Ts to 26336Ts Range. for normal CP, the total length of the first three OFDM symbols per timeslot (including the cyclic prefix cp) = (160+2048+144+2048+144+2048) ts=6592Ts. That is, PSS, which is located on the third symbol of a special sub-frame, does not fall into GP or even uppts, regardless of the special subframe configuration.

So the problem is, if the special sub-frame configuration is 0 or 5, and the downstream is the normal CP type, the length of the dwpts is 6592Ts, then there is no way to send the downstream data in the special sub-frame . Therefore, sometimes changing the special sub-frame configuration will also affect the downstream traffic (remember the gap configuration will affect downstream traffic?). Refer to "LTE-TDD Resource scheduling (3)-measuring gap").

Take a look at the expansion of the CP case. The next line of CP is the extended CP, which ranges from 7680Ts to 25600Ts for the length of the dwpts. The total length of the first three OFDM symbols per timeslot (with cyclic prefix cp) = (512+2048) *3ts=7680Ts. Similarly, at this time, no matter what special sub-frame configuration, the third symbol on the special sub-frame PSS, will always fall in the dwpts, and not fall into the GP or even uppts. Similarly, if the special sub-frame configuration is 0 or 4, and the downstream is the extension of the CP type, the length of the dwpts is 7680Ts, this time there is no way to send the downstream data in a special sub-frame .

Reference documents:

(1) 3GPP TS 36.211 V9.1.0 (2010-03) physical Channels and modulation

(2) "4G lte/lte-advanced for Mobile Broadband"

(3) http://www.sharetechnote.com/

(4) 3GPP TS 36.213 V9.3.0 (2010-09) Physical layer procedures

LTE physical Transmission Resources (1)-Frame structure