Sunday, July 29, 2012

Downlink Resource Allocation Type 2 and Uplink Resource Allocation Type 0

This type of resource allocation is mainly used for contiguous RB allocations for uplink (RA Type0) and for compact scheduling of downlink assignments (RA Type 2).

In this type, the resource block assignment information indicates to a scheduled UE a set of contiguously allocated localized VRBs or distributed VRBs.

In case of resource allocation signaled with PDCCH DCI format 1A, 1B or 1D, one bit flag indicates whether localized VRBs or distributed VRBs are assigned (value 0 indicates Localized and value 1 indicates Distributed VRB assignment) . In the case of resource allocation signaled with PDCCH DCI format 1C, only distributed VRBs are assigned.

Localized VRB allocations for a UE vary from a single VRB up to a maximum number of VRBs spanning the system bandwidth.

For indicating contiguous RB assignment, starting position of the RB (RBstart) and the number of RBs is required. Let us consider RBstart = 0th RB, the number of combinations possible = NRB. Similarly when RBstart = 1st RB, then the number of possible combinations are NRB – 1 and so on. There are NRB.(NRB + 1)/2 combinations possible in total.

For downlink, PDCCH DCI format 1A, 1B or 1D, a type 2 resource allocation field consists of a Resource Indication Value (RIV) corresponding to a starting resource block (RBstart) and a length in terms of virtually contiguously allocated resource blocks LCRBs.

For uplink, a resource allocation (type 0) field in the scheduling grant consists of a resource indication value (RIV) corresponding to an RBstart and a length in terms of contiguously allocated physical resource blocks (LCRBs ≥ 1). The RIV value for both uplink and downlink is defined by:

        RIV = NRB (LCRBs — 1) + RBstart                                            if (LCRBs — 1) ≤ NRB /2
                = NRB (NRB — LCRBs + 1) + (NRB — 1 — RBstart)         otherwise

Let us consider NRB = 6 NRB.(NRB + 1)/2 = 21  5-bits are required for indicating any RIV value ranging from 0 to 20. The RIV values for NRB = 6 are illustrated below. Let RBstart = 2 and LCRBs = 3, from the above equations RIV = 14

Reference: 3GPP TS 36.213

Saturday, July 28, 2012

Downlink Resource Allocation Type 0

In Resource Allocation Type 0, Resource Block assignment information includes a bitmap indicating a set of Resource Block Groups (RBGs) to the scheduled UE. An RBG is a set of consecutive PRBs. 

In RA Type 0, the signaling overhead is reduced since the bitmap is defined on a group of RBs. The size of RBG (P) is a function of system bandwidth as shown in the table below:

The total number of RBGs (NRBG) for downlink system bandwidth NRBDL­­ is given by NRBG = NRBDL­­/Pwhere NRBDL­­/Pof the RBGs are of size P and if NRBDL­­ mod P  > 0 then 
one of the RBGs is of  size NRBDL­­ –  P.NRBDL­­/P.  

The bitmap is of size NRBG bits with one bitmap bit per RBG such that each RBG is addressable

An RBG is allocated to the UE if the corresponding bit value in the bitmap is 1, the RBG is not allocated to the UE otherwise

From the above table it could be noted that for smaller bandwidths (NRBDL ≤ 10) the value of P = 1 (RBG size = 1) which means that each RB can be addressed with a bit in the bitmap.

The RBG size (P) is increased with system bandwidth as smaller values of P would require more number of bits to address the entire bandwidth

Let us consider NRBDL = 15. From the above table, P = 2, so total number of RBGs NRBG = 8 out of which 7 RBGs are of size 2 RBs and one RBG is of size 1 RB. The total number of bits required for resource allocation is equal to NRBG which is 8 in this case. The bitmap is done as shown in the below:

In this example if the resource allocation information is 11000001, this means RBGs 1, 2 and 8 (5 RBs in total) are allocated and the remaining RBGs are not allocated to the UE.

Reference: 3PGP TS 36.213

Resource Allocation

The information on which Resource Blocks (RBs) are allocated (Resource Allocation) for both Uplink and Downlink needs to be signalled to the UE.

The Resource Allocation information is carried on Downlink Control Information (DCI) by Physical Downlink Control Channel (PDCCH). Resource Allocation field is one of the major field in the DCI.

In the case of Uplink, the allocated RBs have to be contiguous in order to guarantee single-carrier property (SC-FDMA is used in the Uplink). The contiguous nature of resource allocation requires less number of bits but the scheduler in the eNB will have less flexibility in allocating the resources.
In the case of Downlink, the allocated RBs doesn’t need to be contiguous which will require more bits to signal the Resource Allocation but the eNB’s scheduler will have more flexibility in allocating the resources.

In LTE, ResourceAllocation Type 0, Resource Allocation Type 1 and Resource Allocation Type 2 are defined.

Resource Allocation Type 0 and Resource Allocation Type 1 are used for non-contiguous resource allocation (only for Downlink) which uses bit-map based signaling. 

For allocating contiguous resource blocks in the Uplink, Resource Allocation Type 0 is used where as same for downlink it is called as Resource Allocation Type 2.

The UE shall interpret the Resource Allocation field depending on the PDCCH DCI Format detected. 

For Uplink, Resource Allocation information is conveyed on PDCCH DCI Format 0 using Resource Allocation Type 0.

In the Downlink, PDCCH DCI Formats 1, 2, 2A and 2B uses Resource Allocation Type 0 or Type 1  whereas PDCCH DCI Formats 1A, 1B, 1C and 1D uses Resource Allocation Type 2

Click here for Resource Allocation Type 0 in the Downlink. Check here for Downlink Resource Allocation Type 1. 

Uplink Type 0 and Downlink Type 2 are explained in post 'Downlink Resource Allocation Type2 and Uplink Resource Allocation Type0'.

Reference: 3GPP TS 36.213