LTE learning path (12) -- pdcch (continued)

12.1 Preface

One of the main functions provided by pdcch is to transmit physical layer Resource Allocation instructions, and pdcch indicates frequency-domain resource allocation on each sub-frame.

The main challenge in designing signal allocation for frequency-domain resources (a group of resource blocks) is to find a good solution between flexibility and scheduling overhead. One feasible method is to send the combined resource allocation message to all ue, but it may be rejected because it requires high power to reach all ue reliably, including the UE at the edge of the cell.

There are three types of LTE resource allocation: type0, type1, and type2. The specific resource allocation type depends on the DCI format selected and the bit configurations in DCI. (For DCI introduction, please read: DCI of pdcch in section 2nd of LTE learning path (11)

Table 1: Relationship between DCI format and downstream Resource Allocation types

12.2 Reg Introduction

The bitmap (Bitmap) is used to indicate the resource block group (rbg) allocated to the scheduled UE. The resource block group is a continuous set of PRB instances. Rbg sizePIs about the system bandwidth function, as shown in the following table.

Table 2: rbg size of type0 Resource Allocation

For example, if the system bandwidth of the following row is 50 RB, the P value is 3, the total number of rbg is 17, and the first 16 rbg each contains 3 pRb, the last rbg contains only two PRBS.

12.3 downstream Resource Allocation Type 0 (Type 0)

In type0, DCI format 1/2/2a/2B/2C uses a bitmap to indicate the rbg allocated to the UE. Bitmap contains bits, each of which corresponds to one rbg. The highest bit indicates rbg 0, and the lowest Bit indicates rbg-1.

This type of push. If an rbg is assigned to a ue, the corresponding bit in bitmap is set to 1; otherwise, it is set to 0.

　　Example:

The formula gives the total number of rbg instances whose downlink system bandwidth is PRB. Figure 1 shows = 25, = 13,P= 2. Therefore, bitmap contains 13 BITs to indicate the scheduling result.

Figure 1

Assume that the bitmap of the Resource Allocated to a ue is: 1001110100010, then the UE is allocated rbg 0, rbg 3, rbg 4, rbg5, rbg 7, rbg 11

Summary:

　　From the above example, we can see that: 1) type0 supports non-continuous RB allocation in the frequency domain; 2) The scheduling granularity is coarse and the smallest unit of scheduling is rbg. For larger bandwidth, resources cannot be allocated according to a single RB.

12.4 downstream Resource Allocation type 1 (type 1)

Type1 is to divide all PRB into P rbg subsets (subset), each rbg subsetP() Include from rbgPStart, all rbg with an interval of P.

Note:The VRB resources allocated to ue must come from the same rbg subset (type1 allocates VRB resources instead of pRb, which is one of the differences with type0 ).

DCI format 1/2/2a/2B/2C indicates the VRB allocated to ue through three fields.

The first field contains a bit, which is used to specify the selected rbg subset, that isPValue;

The second field contains one shift bit, which is used to specify whether the resources in the subset are offset. 1 indicates offset, and 0 indicates no offset;

The third field contains a bitmap. each bit of Bitmap corresponds to a VRB in the selected rbg subset. The highest bit indicates the first VRB In the subset, the lowest Bit indicates the last VRB In the subset, and so on. If a VRB is assigned to a ue, the corresponding bit in bitmap is set to 1; otherwise, it is set to 0. The bitmap size is calculated as follows:

Formula (1)

Note:

The VRB in a selected rbg subset starts with the smallest VRB number + offset in the subset and corresponds to the highest bit in bitmap.

This offset is expressed by the number of VRB instances and is an offset in the selected rbg subset.

If the second field in the resource block allocation information of DCI is 0, then the rbg subsetPIf the second domain in the resource block allocation information of DCI is 1, then the rbg subsetPAnd the bitmap bitwise is adjusted to the last VRB in the corresponding rbg subset.

Is a subset of rbg.PThe number of VRB contained is calculated as follows:

Formula (2)

For rbg SubsetsPEvery bit in its bitmapI(I= 0, 1 ,.....,) The corresponding VRB can be calculated using the following formula:

Formula (3)

Example (the system bandwidth is 25rb ):

1) because the system bandwidth is 25 RB, P = 2 (that is, two subsets): subset 0 (starting from rbg 0) and subset 1 (starting from rbg 1 );

Table 3: subset (= 25rb) in type1)

2), that is, the first domain uses 1 bit to specify the selected rbg subset;

3) The second field uses 1-bit to specify whether the resources in the rbg subset are offset;

4) bitmap contains only 11 VRB values;

5) Each rbg subsetPThe number of VRB contained is:

Therefore, we can see that bitmap is insufficient to indicate all VRB contained in each subset.

6) Next, we will detail the impact of shift bit on the VRB represented by bitmap.

If shift bit = 0, rbg subsetPOffset;

If shift bit = 1, rbg subsetPOffset:

From the previous analysis, we can see which rbg is included in each subset, that is, which VRB is included is also determined. 2:

Figure 2

When shift bit = 0Formula (3) We can see which VRB is corresponding to each Bitmap (for the bandwidth of 25 RB, listen 11bit. The result is as follows:

It can be seen that if shift bit = 0 (no offset occurs), the bitmap of each subset corresponds to VRB, starting from the first VRB In the VRB set given in Figure 8 (corresponding to subset 0, starting VRB is vrb0; corresponding to subset 1, starting VRB is vrb2), 11 VRB are selected in sequence.

When shift bit = 1Formula (3) We can see which VRB is corresponding to each Bitmap (for the bandwidth of 25 RB, listen 11bit. The result is as follows:

It can be seen that if shift bit = 1 (an offset occurs), the bitmap of each subset corresponds to VRB, which is the first VRB In the VRB set given in figure 8, start with offset (corresponding to the subset 0, offset, that is, given in Figure 2PIn a VRB set with 0, the first VRB is vrb4, corresponding to subset 1, offset, that is, given in figure 8.PIn a VRB set with 1, move one forward to the other, and the initial VRB is vrb3.) Select 11 VRB in sequence.

Summary:

From the example above, we can see: 1) Resource allocation type 1 supports non-continuous RB allocation in the frequency domain; 2) compared with resource allocation Type 0, resource allocation type 1 supports allocation of 1 RB; 3) Resource allocation Type 0 and resource allocation type 1 use the same number of BITs to indicate resource allocation; 4) bitmap actually has fewer bits than the number of vrb in the rbg subset. By using shift bit, bitmap can overwrite all VRB.

 12.5 downstream Resource Allocation Type 2 (Type 2)

In type2, the resources allocated to UE are continuous VRB. The VRB can be centralized (localized) or distributed (distributed ).

For DCI format 1A/1B/1d, there is a bit flag to indicate whether centralized VRB is used or distributed VRB. If the bit is set to 0, centralized is used, this bit = 1 indicates that distributed is used, while distributed VRB is always used in DCI format 1C.

For centralized VRB allocation, resources allocated to a UE can be allocated from one VRB to all VRB of the overall system bandwidth.

If DCI format 1A uses a distributed VRB Allocation Method and Its dci crc is disturbed by P-rnti, RA-RNTI, or SI-RNTI, the number of VRB allocated to the corresponding UE can be from 1 to 1. The calculation is as follows: (6.2.3.2 of protocol 36.211)

 

 

Not yet to be continued ....

 

Refer to blog:

Http://blog.sina.com.cn/s/blog_927cff010101a042.html

 

 

LTE learning path (12) -- pdcch (continued)