LTE-TDD Random Access Process (5)-How to generate 64 sequence of preamble codes

In LTE, each cell has 64 preamble codes, so how do these preamble sequences preamble sequences generated? This article is intended to illustrate the process of generating a sequence of preamble codes.

The set of preamble sequences consists of the root sequence and the cyclic shift sequence generated by the root sequence, which is divided into two large steps:

(1) Generate a Zc (zadoff-chu) root sequence xu (n), as a datum sequence

(2) Cyclic shift of the reference sequence Xu (n) to generate 63 different cyclic sequences XUV (n)

If there are less than 63 shift sequences in (2) based on the Datum sequence, re-enter (1), generate the next datum sequence, and the corresponding shift sequence of the new datum sequence until 64 preamble sequences are met.

1. Select the reference sequence Xu (n)

The reference sequence Xu (n), which is the ZC sequence with the physical root sequence number u, is calculated by the following formula.

which

Nzc represents the length of the ZC sequence, when the preamble format is 0-3, NZC is fixed equal to 839, the preamble format is 4 o'clock, and Nzc is fixed equal to 139. The format of the preamble is determined by the PRACH configuration Index , specifically referring to the blog "LTE-TDD Random Access Process (2)-The format and time-frequency position of the preamble preamble."

You are the physical root sequence number (physical root sequencenumber), and the logical root sequence (Logical root sequencenumbers) checks the Protocol table 5.7.2-4 (the table is checked in the preamble format 0-3) and table 5.7.2-5 (the table in the preamble Format 4 o'clock) is obtained.

The logical root sequence number (Logical root sequencenumbers) is configured by the rootsequenceindex field of the Prach-config cell in the SIB2 message, the range is 0-837, See below. For example RRC configuration of rootsequenceindex= 3, the preamble format = 2, then the table can get the physical logic root sequence number u=699.

2. Calculating the cyclic shift sequence XUV (n)

The cyclic shift sequence XUV (n) is calculated according to the following formula. Among them, the cyclic shift parameter Cv is calculated using different formulas depending on whether it is a constraint set. Whether the constraint collection is configured by the highspeedflag field of the Prach-config cell in the SIB2 message, a value of false indicates the use of unrestricted, and a value of TRUE indicates the use of the Restricted, see above.

Ncs can be obtained based on the zerocorrelationzoneconfig field of the Prach-config cell in the SIB2 message. The zerocorrelationzoneconfig value range is 0-15, and the actual Ncs value can be obtained by checking the table. For example, prach-configindex=1 (preamble format 0), Highspeedflag=false (unrestricted set),Zerocorrelationzoneconfig = 2, then Ncs= 15.

D_start,n_ra_shift,n_ra_group,_n_ra_shift These four parameters are related to the Du value, the du value depends on the P value, and P is satisfied (p* u) the minimum non-negative number of the modnzc=1. For example, u=1,nzc=839, then p=1,du=p=1, if ncs=0 at this time, then Du>ncs,d_start, N_ra_shift, N_ra_group, _n_ra_shift These four parameters can be calculated in turn according to the above formula.

3. Example Description

Here's how to calculate a datum sequence and a shift sequence, this section provides an example of how to get 64 preamble sequences.

The values of the parameters carried in the SIB2 are as follows:

a) Rootsequenceindex = 22 b) Highspeedflag = False c) Zerocorrelationzoneconfig = 5 d) Prach-configindex = 1

(1) According to Prach-configindex=1, you can know the preamble format =0,nzc=839.

(2) According to rootsequenceindex=22, you can check the table to get u=1.

(3) According to Highspeedflag = False, you can know when calculating CV, select unrestricted set.

(4) According to Zerocorrelationzoneconfig = 5, you can know ncs=26.

(5) Since Ncs is not equal to 0, the range of cv=v*ncs=26*v,v is: v=0,1,...,31.

(7) According to the above results, you can get the preamble as follows (because of the blog editor reasons, Xu's subscript part U is not separately special editing , should not affect understanding):

1th preamble: v=0 shift sequence xu,0 (n) =xu ((n+c0) mod839) =xu ((n) mod839) =xu (n) =X1 (n), which is the datum sequence.

2nd preamble: V=1 shift sequence xu,1 (n) =xu ((N+C1) mod839) =xu ((n+26) mod839)=x1 ((n+26) mod839).

3rd preamble: v=2 shift sequence xu,2 (n) =xu ((N+C2) mod839) =xu ((n+26*2) mod839) =X1 ((n+26*2) mod839).

....

32nd preamble: v=31 shift sequence XU,31 (n) =xu ((N+C31) mod839) =xu ((n+26*31) mod839) =X1 ((n+26*31) mod839).

In this case, the shift sequence with the physical root sequence number U=1 has all been obtained, but the generation of all 64 preamble has not yet been completed, so it is necessary to continue to increment u to obtain a new datum sequence.

U=1 's next is u=838 (table 5.7.2.-4), so the following 32 preamble codes are calculated using u=838:

33rd preamble: v=0 shift sequence xu,0 (n) =xu ((n+c0) mod839) =xu ((n) mod839) =xu (n) =X838 (n), which is the new datum sequence.

34th preamble: V=1 shift sequence xu,1 (n) =xu ((N+C1) mod839) =xu ((n+26) mod839)=x838 ((n+26) mod839).

35th preamble: v=2 shift sequence xu,2 (n) =xu ((N+C2) mod839) =xu ((n+26*2) mod839) =X838 ((n+26*2) mod839).

....

64th preamble: v=31 shift sequence XU,31 (n) =xu ((N+C31) mod839) =xu ((n+26*31) mod839) =X838 ((n+26*31) mod839).

at this point, all 64 nzc-length preamble sequences have been generated, and the random access process selects only one of the NZC-length sequences sent to eNB.

4. Interface mapping

For Pusch RB, each sub-carrier accounts for 15K of bandwidth, so 6 RB (12 sub-carriers per RB) is a total of 1.08M of bandwidth. In particular, the RB for the Prach channel, each sub-carrier is 1.25K, so there is a total of 864 sub-carriers. When the UE chooses a certain preamble, it is mapped to 839 sub-carriers in the middle of the frequency domain (Preamble format 0-3), and 25 sub-carriers on both sides are used to protect the band. As shown in.

5. References

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

(2) 3GPP TS 36.331 V9.18.0 (2014-06) Radio Resource Control (RRC)

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

LTE-TDD Random Access Process (5)-How to generate 64 sequence of preamble codes