LTE: Downlink Resource Allocation Type 1


Resource Allocation Type 1 uses the same number of RA bits as in Resource Allocation Type 0 but allows Resource Allocation on an individual RB level even for larger bandwidths.

The Resource Allocation Information consists of NRBG bits which will indicate the UE about a set of VRBs (Localized Type) from one of the P RBG subsets. The number of RBG subsets (P) is same as RBG size (P) in RA Type 0 which is as given below:


In an RBG subset number p, where 0 ≤ p < P, it is possible to allocate every Pth RB starting from pth RB and this allocation can be done using a bitmap.

A total of NRBDL­­/Pbits are required for resource allocation as in RA Type 0. Resource Block assignment consists of 3 fields.

The First field with log2 (P) bits used to indicate the selected RBG subset number among P RBG subsets.

The second field with one bit is used to indicate a shift of the resource allocation span within a subset. A bit value of 1 indicates shift is triggered. Shift is not triggered otherwise

The third field indicates a bitmap, where each bit of the bitmap addresses a single VRB in the selected RBG subset. The VRB is allocated to the UE if the corresponding bit value in the bit field is 1, the VRB is not allocated to the UE otherwise. The portion of the bitmap used to address VRBs in a selected RBG subset has size NRBTYPE1 and is defined as:
NRBTYPE1 = NRBDL­­/P⏋— ⎾log2 (P)⏋ — 1


The addressable VRB numbers of a selected RBG subset start from an offset, shift(p)   to the smallest VRB number within the selected RBG subset, which is mapped to the MSB of the bitmap. The offset is in terms of the number of VRBs and is done within the selected RBG subset. If the value of the bit in the second field for shift of the resource allocation span is set to 0, the offset for RBG subset p is given by shift(p) = 0. Otherwise, the offset for RBG subset p is given by shift(p) = NRBRBGsubset(p) — NRBType1, where the LSB of the bitmap is justified with the highest VRB number within the selected RBG subset. NRBRBGsubset(p) is the number of VRBs in RBG subset p and can be calculated by the following equation,



Consequently, when RBG subset p is indicated, bit i for i = 0, 1, …, NRBType1 — 1 in the bitmap field indicates VRB number


Example:
Let us consider NRBDL = 15. From the above table, number of RBG subsets (P) is 2. In this case, 1-bit (log2 (2)) is required to indicate the RBG subset number. One more bit is used to indicate whether a shift is used or not.  NRBTYPE1 = 8-1-1 = 6 bits are used for actual resource allocation bitmap. The resource allocation is illustrated below: 


Let the resource allocation bits are 00110011. MSB indicates RBG subset number, second bit indicates if shift is enabled or not. Remaining 6 bits indicate the bitmap of resource allocation of PRBs. In this case, RBG subset 0 is chosen without a shift. Using the remaining 6 bits 110011, RB numbers 0, 1, 8, and 9 are allocated to the UE. Similarly, if resource allocation bits are 11110011 RB numbers 3, 6, 11, and 14 are allocated to the UE.

Reference: 3GPP TS 36.213

7 comments:

  1. how will you represent subset 2(assuming) in resource allocation bits? will there be an additional bit to represent subset 2?

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  2. how will you represent subset 2(assuming) in resource allocation bits? will there be an additional bit to represent subset 2?

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    Replies
    1. ⎾log2 (P)⏋ number of bits are used to represent the subsets. If there are more than 2 subsets, then more than 1 bit is required. So, in order to represent subset 2, there is an additional bit...

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  3. When should eNodeB use DL resource allocation type 0? When should eNodeB use DL resource allocation type 1?
    How I am going to decide which DL resource allocation type should be used?

    How can I understand which resource allocation type will be more effective according to system bandwidth? (how can you establish a relationship between RGBs and resource allocation types that enodeb should use?)

    Thank you.

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    Replies
    1. RAT policies are mapped according to dci formats. So which RAT should be used depends on which dci format has been used by UE

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  4. RA Type 1 gives flexibility of assigning RBs individually as compared to RA Type 0 which can only allow groups of contiguous RBs perticularly for larger bandwidths.

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  5. I did not actually understood physical significance of shift bit ! What impact it has on RB allocation when it is 1 ? Can anyone explain lucidly with examples

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