LTE physical Transmission Resources (3)-time-frequency resources

In the blog "LTE Physical Transmission Resources (1)-frame structure and OFDM symbols" mentioned in the LTE frame structure and time domain OFDM symbols, this article continues this topic, continues to describe the sub-frame and the time slot structure of other content.

1. Granularity of resources

To improve terminal power efficiency, extend battery life, and equipment cost considerations, LTE uplinks use SC-FDMA(singleCarrier Frequency division multiple Access, Single carrier frequency division multiple access) technology. In the time domain, the smallest resource granularity is an OFDM symbol (the upstream is the SC-FDMA symbol. The following unification is called the OFDM symbol). In the frequency domain, the minimum granularity is a sub-carrier. A time-frequency resource unit consisting of a OFDM symbol and a sub-carrier called RE(resouce element). The physical layer is based on the RE as the basic unit for resource mapping. A resource block, called RB(Resource block), in which all the OFDM symbols in a time slot are made up of two sub-carriers in the frequency domain, and the LTE resource dispatch is RB as the basic unit.

The length of the cyclic prefix (CP,Cyclic Prefix) affects the number of OFDM symbols within a timeslot (slot). The total number of OFDM symbols included in a time slot is n_symb : When the CP type is normal,n_symb=7; CP type is extended type,n_symb =6. Therefore, if it is the normal CP type, then 1 RB is composed of 12*7=, and if it is extended CP, then 1 RB is composed of 12*6=.

The number of sub-carriers that the system can use is related to channel bandwidth: the larger the bandwidth, the more the number of sub-carriers included. The total number of sub-carriers included in the bandwidth is (n_rb*n_rb_sc=12*n_rb), where then_rb_sc value is fixed equal to 12 (see table above),N_ RB is associated with bandwidth and is valued as follows:

For the entire bandwidth, if the current bandwidth occupies N_rb a RB block (RB starting from 0, the identity range is: 0,1,2, ...,n_rb-1), then the number of sub-carriers occupied is n_rb*n_rb_sc=12 A *N_RB . such as 20MHz bandwidth, the number of sub-carriers capable of transmitting data =12*100=1200.

is a CP-type Normal upstream timeslot Tslot in the entire bandwidth (the structure of the lower row gap is the same as the upstream).

The horizontal axis is the time domain, with the number of SC-FDMA symbols L as the basic unit, each time slot includes 7 SC-FDMA symbols. Ordinate is the frequency domain, with the number of sub-carrier K as the basic unit. For a re with a coordinate of (k,l), the RB number to which it belongs n_prb equals (K/12) to the down-rounded value , such as k=13, then the RB number that the re belongs to IS (13/12) = 1.

2. Frequency domain Structure

Remember the concept of Carrier center frequency FC in the "LTE Physical Transmission resource (2)-band, channel bandwidth and frequency EARFCN"? In that article, there is no mention of the difference between the uplink and Downlink carrier Center frequency points, here is an attempt to explain.

In the LTE downlink, there is an unused DC sub-carrier, located at the carrier center frequency, so the number of downlink carriers is actually (12*n_rb+1). The reason is not to use this DC sub-carrier bearer user Data , is because regardless of which side, its frequency converter device has inherent local vibration leakage, but the terminal is generally used is a 0 if reception scheme (simple structure cost low), so regardless of the base station side of the radio frequency how to send, The terminal receives a strong noise at the DC sub-carrier, so it is not suitable for transmitting user data, so a sub-carrier is vacated.

In LTE uplink, because the uplink is SC-FDMA, need to use a continuous sub-carrier to host user data, not as the downstream link to skip a DC sub-carrier, which requires the base station side to receive the time can not be used when the 0 if the program, and the use of non-0 if the program. Therefore, the uplink cannot and will not add an unused DC sub-carrier, so the carrier center frequency is located between the two upstream sub-carriers, the total number of uplink carrier is (12*N_RB). As shown in.

Since the number of sub-carriers is so many, there is a code implementation problem:when the UE initial access, how to get the current carrier's central frequency ? Of course, one possible way is to let the UE blind detect all the sub-carriers currently supported in the band, then detect PSS and SSS in turn, if you can find PSS and SSS, then found the central frequency point, but the disadvantage is that the sweep time is longer. Remember the fixed relationship between the central carrier Frequency FC and the carrier frequency number EARFCN mentioned in the "LTE Physical Transmission resource (2)-band, channel bandwidth, and frequency EARFCN"? That

This formula implicitly conveys a message that the interval between each carrier frequency is 0.1MHz or 100KHz. In other words, not all sub-carriers can be used to do the carrier. The UE only needs to blind detect the sub-carriers that may exist in the central frequency point to find PSS and SSS. For example, the initial access to a possible method is the terminal in turn earfcn=0,1,2, ... Substituting the formula, we get several alternate sets of 100KHz, possibly the central carrier frequency, then detect these alternate frequencies sequentially, and then obtain the real central carrier position according to the terminal manufacturer's own algorithm. For a terminal that has been swept over the frequency point, the historical frequency point information can be preserved, followed by priority to detect these carrier frequency points, so that you can improve the frequency sweep speed.

3. Downlink Sub-frame structure

Each sub-frame of the downlink is divided into a control area and a data region . The control area is located in each downstream sub-frame the first time slot of the 1~4 OFDM symbol (note: Typically 1-3 symbols, only at the time of 1.4MHz bandwidth can appear 4 OFDM symbols) for transmission downlink L1/L2 control signaling. These l1/l2 control signaling, which are hosted in the control area, correspond to 3 different physical channel types:

(1) The physical control format indicates the channel (Pcfich,physical control format indicator channel), which indicates that the control area of the current sub-frame of the terminal occupies several OFDM symbols, the range is 1~ 4, different sub-frames this value may be different. At the same sub-frame time, all terminals in the cell get the same value.

(2) The Physical downlink control channel (PDCCH,physical downlinkcontrol channel), which transmits the signaling of the scheduling and allocation of resources on the upper and lower lines. The PDCCH channel can either give 1 terminals or send relevant signaling to multiple terminals. For example, at some sub-frame moment, ENB only assigns a resource to one UE, while the next sub-frame allocates resources to 2 UE at a time. The information obtained from the PDCCH channel may or may not be the same from different terminals. For example,crnti scrambling information, different terminal decoding to obtain different information, and with tpc-rnti scrambling or ra-rnti scrambling information, different terminals can be the same content.

(3) A physical mixed ARQ signaling channel (PHICH,physical Hybrid-arq indicator channel) for transmitting HARQ acknowledgment information for upstream data. Each terminal corresponds to a different Phich location, so the ACK reply content obtained is also different.

The control area is placed in the beginning part of the sub-frame, on the one hand, the terminal can decode the relevant scheduling information as soon as possible, so that the current sub-frame is not finished at the beginning of the downlink data decoding work, reduce the downlink data transmission delay. On the other hand, the terminal in the sub-frame at the beginning of a few symbols on the detection of scheduling information, you can know that the terminal is not in the sub-frame is ENB dispatch, if not scheduled, or no information belonging to this terminal, you can not need to receive downstream data in the current sub-frame, or directly shut down the receiving circuit, To reduce the consumption of terminal power.

The location of the Pcfich, PDCCH, Phich channels is shown below. The next few articles will continue to write the relevant content of these channels.

FDD format, 1.4MHz bandwidth:

TDD format, 1.4MHz bandwidth:

Reference documents:

(1) 3GPP TS 36.101 V10.21.0 (2016-1) User equipment (UE) radio transmission and reception

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

(3) Http://dhagle.in/LTE

LTE physical Transmission Resources (3)-time-frequency resources