PDCCH - how UEs are scheduled

The book "LTE - from Theory to Practice" provided a very good summary of scheduling process from Control Channel point of view. Here is some of the excerpt:

"A typical seqeuence of steps carried out by eNodeB could be envisaged as follows:

1. Determine which UEs should be granted resources in the uplink, based on information such as channel quality measurements, scheduling requests and buffer status reports. Also decide on which resources should be granted.

2. Determine which UEs should be scheduled for packet transmission in the downlink, based on information such as channel quality indicator reports, and in the case of MIMO, rank indication and perferred precoding matrix

3. Indentify common control channel messages which are requred (e.g. power control commands using DCI format 3)

4. For each message decide on the PDCCH format (i.e. 1,2,4,8 CCEs), and any power offset to be apllied, in order to reach the intended UE(s) which sufficient reliability, while minimizing PDCCH overhead.

5. Determine how much PDCCH resource (in terms of CCEs) will be required, how many OFDM symbols would be needed for these PDCCHs and therefore what should be signalled on PCFICH.

6. Map each PDCCH to a CCE location within the appropriate search space.

7. If any PDCCHs cannot be mapped to a CCE location because all locations in the relevant search space have allready been assigned, either:
- continue to next step (step 9) accepting that one or more PDCCHs will not be transmitted, and now all DL-SCH and/or UL-SCH resources will be used, will a likely loss in throughput.
- allocate one more OFDM symbol to support the required PDCCHs and possibly revisit step 1 and/or 2 and change UE selection and resource allocation (e.g. to fully use uplink and downlink resources)

8) allocation the necessary resources to PCFICH and PHICH

9) Allocate resources to PDCCHs

10) Check that total power level per OFDM symbol does not exceed maximum allowed, and ajust if necessary

11) Transmit downlink control channels"


A few notes to help understanding the above description:

- CCE denotes Control Channel Elements, each CCE is nine REGs, where each REG is four physical resource elements

- LTE has four PDCCH formats (0-3), which has 1,2,4,8 CCEs respectively. The total number of bits can be transmitted over PDCCH for each format is 72, 144, 288, 576 respectively.
note: 1 CCE = 9 REGs = 4REs/REG * 9 REG = 36REs * 2bit/symbol = 72 bits

- the PDCCH can occupy the first 1, 2 or 3 OFDM symbols in a subframe, extending over the entire system bandwidth.

- PCFICH carriers control format indicator (CFI) which indicates the number of OFDM symbols (1,2 or 3) used for control channels. In principle the UE could deduce the value of CFI without PCFICH, for example by multiple attemps to decode the control channels assuming each possible number of symbols, but this would result in significant processing load.

- PHICH carriers HARQ ACK/NACK, it is modulated by BPSK, can be mapped onto 1, 2 or 3 OFDM symbols (and CFI needs to be set correspondingly).