LTE downlink Physical layer transport mechanism (1)-Antenna port antenna port and cell specific reference signal CRS

In the Last Post, "LTE Physical Transmission Resources (3)-time-frequency resources," the final mention of Pcfich and other downstream physical channels, this blog post originally wanted to write Pcfich channel, but in preparation for writing Pcfich, found the need to use the antenna port related content, and these content is not yet written. So this paper first write the antenna port and downlink reference signal related content.

1. Relationship of antenna port (antenna port) and reference signal (Reference Signal)

The antenna port is a logical concept, and it has no one by one correspondence with the physical antenna. In the downlink, the antenna port is one by one corresponding to the downstream reference signal (Reference signal) : If the same reference signal is transmitted through multiple physical antennas, then these physical antennas correspond to the same antenna port , and if two different reference signals are transmitted from the same physical antenna, the physical antenna corresponds to two separate antenna ports.

The R9 protocol defines four downstream reference signals, and the corresponding relationship between the antenna ports and these reference signals is as follows:

(1) cell-specific reference signal (cell-specific reference Signals,crs), or cell dedicated reference signal. CRS support One, 2, 4 three antenna port configuration, the corresponding port number is: p=0,p={0,1},p={0,1,2,3}.

(2) MBSFN reference signal (MBSFN Reference signals), only transmitted in the antenna port p=4. This kind of signal is not used much, this article does not involve.

(3)UE Specific reference signal (ue-specific Reference signals), or UE dedicated reference signal, or demodulation reference signal (demodulation Reference Signals,dm-rs). can be transmitted in antenna port p=5,p=7,p=8, or p={7,8}. This piece of content is then written in a later post.

(4) Locating the reference signal (positioning Reference signals), transmitted only in the antenna port p=6. This kind of signal is not used much, this article does not involve.

2. Structure of cell-specific reference signals

The purpose of designing a cell-specific reference signal (cell-specific Reference signals) is not to carry user data, but rather to provide a technical means to allow the terminal to perform downlink channel estimation. The terminal can get downlink CQI,PMI,RI and other information by measuring the specific reference signal of the cell.

In each cell, there can be one, 2 or 4 cell-specific reference signals that correspond to 1, 2, or 4 antenna ports respectively. For a cell that supports Pdsch transmission, all of its downstream sub-frames transmit cell-specific reference signals that can be transmitted on port 0 or Port 0, 1, or Port 0, 1, 2, 3. It should be stated that the cell-specific reference signal can only be transmitted in a sub-carrier interval of 15KHz LTE system (there is also a special use for MBSFN transmission, sub-carrier interval is 7.5KHz of the situation remember? Please refer to the post "LTE physical Transmission Resources (1)-frame structure and OFDM symbols").

is a map of the RE mapping of cell-specific reference signals under different antenna ports under the normal CP behavior.

The emphasis on " one " is because of the cell reference signal re location and physical cell ID(remember how to get it?). To read PSS and SSS, please refer to the blog "LTE cell search-physical cell ID and synchronous signal PSS, SSS"), downlink CP type , frequency offset (k=0,1,2,3,4,5), The following specific analysis of why the cell reference signal is such a position.

3. Cell-specific reference signal bearer content

Before describing the location of the re mapping, it is necessary to first describe what is mapped in the cell reference signal. The cell-specific reference signal actually carries a sequence value (called the R sequence), which is mapped to a different RE based on the rules of the first time domain post-frequency domain . the reference signal within each of the OFDM symbols corresponds to the 1- length r sequence , where the sequence values of these 220 points are evenly distributed across the entire bandwidth, and each sequence point is mapped to a single RE . the number of points corresponds to the bandwidth of a series of RB, actually effective sequence points to see the current actual bandwidth, such as the current bandwidth is 10M, that is, the entire bandwidth only 50 RB, then at this time, the 220 sequence points, only the front The six sequence points are valid ( in one OFDM symbol, each RB has 2 re-load sequence points for the reference signal ), at which point the terminal only needs to parse the previous sequence points in each OFDM symbol (of course, The terminal also has no way to parse out the sequence points after 100).

An re carries a sequence point of a cell-specific reference signal that consists of a complete sequence R, which can be computed by the following formula:

For the convenience of understanding, it is necessary to explain the above formula:

(1)N_MAX_DLRB is fixed equal to 110, so the range of M is [0,219], so the cell-specific reference signal sequence R includes five points, that is, each OFDM symbol can generate 220 reference signal points .

(2) each R sequence corresponds to 2 c sequences, and these two C sequences correspond to the real and imaginary parts of the R sequence respectively . For example, R (0) corresponds to the two sequences C (0) and C (1), R (219) corresponds to the C (438) and C (439) sequences, so the R sequence of 220 points corresponds to the C sequence of 440 points, each re carries a real part and an imaginary part.

(3) The C sequence consists of sequence x1 and sequence x2. Because of the presence of MOD2, the points of each C, X1, and X2 sequence are 0 or 1, that is, each C, x1, and X2 sequence points correspond to a bit value (0 or 1). Therefore, in each OFDM symbol, 220 points of cell-specific reference signal sequence R is composed of 440 C sequence points, the corresponding bit number is 440bits. In other words, the cell-specific reference signal for each OFDM symbol includes 440 bits of the X1 sequence point and 440 bits of the X2 sequence point.

(4) for the X1 sequence, Bit31 is launched by Bit3 and bit0; for X2 sequences, bit31, Bit3, Bit2, and bit1 are introduced as follows (the other bit values of two sequences can be introduced recursively by the formula).

Through the above analysis, we have obtained the cell specific reference signal sequence R, and then analyze how to map these sequence points to different re.

4. Mapping of cell-specific reference signals

Not all re can carry the sequence Rof a cell-specific reference signal, and the coordinates of the eligible re (k,l) need to satisfy the following formula.

which

(1) k represents the offset position of the sub-carrier , and the range is 0-5. The different carrier offset positions are different, as shown in.

(2)L represents the offset value of the OFDM symbol in each timeslot . This value is related to the antenna port number p, downlink CP type, as shown in the table below. Therefore, the reference signal structure given in the preceding text is only one of the cases.

(3) The same antenna port, within the same OFDM symbol, spaced 6 sub-carriers . antenna ports 0 and 1 are in the 1th and penultimate OFDM symbols per timeslot (l=0,l=4 or l=3), antenna ports 2 and 3 in each timeslot of the 2nd OFDM symbol (l=1). The structure of the four antenna ports, the downstream normal CP, and the frequency offset equals 0 (i.e. n_cell_id=0) is as follows.

Further analysis of the above mapping formula, can be pushed to export the following knowledge points:

(1) The end of the cell-specific reference signal parsing process, occurs after the synchronization of PSS and SSS . The n_cell_id in the formula is the physical cell ID of the current cell, and the terminal can obtain the n_cell_id as well as the sub-frame number and the downlink CP type (reason reference blog "LTE cell search-physical cell ID and synchronous signal PSS, SSS" only after completing the synchronization of PSS and SSS).

(2) the physical community ID of the adjacent cell to ensure that the modulus 6 value is different . As you can see from the formula, the frequency domain offset K value is related to (n_cell_id mod 6). If cell A and cell B are adjacent, and n_cell_id (a) =1,n_cell_id (B) = 7, then the same antenna port p corresponding to the two cells will be the same as the location of the cell-specific reference signal, thus creating interference between each other.

Through the above analysis, we can determine the location of the cell reference signal for each time slot in the entire bandwidth. The following are the cell-specific reference signal distributions in the entire bandwidth (the re position of the red color block labeled) when the TDD format, 1.4MHz bandwidth, downlink NORMALCP, and quad antenna ports.

5. The number of antenna ports corresponding to the cell-specific reference signal from where to get

The previous description can be known that the number of antenna ports are different, will affect the location of the cell-specific reference signal, so the terminal needs to know the current LTE system antenna port number . According to the 36212 Protocol table 5.3.1.1-1,enb in the transmission of PBCH , according to the current antenna port number to select a different CRC mask, so the terminal can decode PBCH, to obtain the current cell specific reference signal corresponding antenna port number . For more information on PBCH, follow-up blog post to write.

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://dhagle.in/LTE

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

(5) 3GPP TS 36.212 V9.4.0 (2011-09) multiplexing and channel coding

LTE downlink Physical layer transport mechanism (1)-Antenna port antenna port and cell-specific reference CRS