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 N_RBG 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 ⎾N_RB^DL­­/P⏋bits 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 N_RB^TYPE1 and is defined as:

N_RB^TYPE1 = ⎾N_RB^DL­­/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) = N_RB^RBGsubset(p) — N_RB^Type1, where the LSB of the bitmap is justified with the highest VRB number within the selected RBG subset. N_RB^RBGsubset(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, …, N_RB^Type1 — 1 in the bitmap field indicates VRB number

Example:

Let us consider N_RB^DL = 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.  N_RB^TYPE1 = 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