4g LTE: LCP Logical Channel Prioritization

Before we learn about Logical Channel Prioritization (LCP), we need to learn about channel starvation.

Channel Starvation:

Before LCP, every channel is given a priority. So while sending the data, the channels with higher priority are able to send the data.

For example:

If we have 3 channels and their priority and the data each channel has is as given below:

Channel 1 has priority 1 with 60 bytes of data
Channel 2 has priority 2 with 40 bytes of data
Channel 3 has priority 3 with 20 bytes of data

So if MAC layer wants to transmit 100 bytes of data in MAC PDU to PHY layer, then only channel 1 and channel 2 will be able to send the data. Channel 3 will not be able to send the data. This is called as channel starvation.

Channel Starvation definition:
When we send the MAC PDU from MAC layer to PHY layer based on the priority there might be chances that some of the channels will not be able to get chance to send the data. This situation is called as Channel Starvation.

So how to solve this?

This can be solved with the help of Logical Channel Prioritization.

Logical Channel Prioritization (LCP)

In LCP, Prioritize Bit Rate (PBR) is added to each logical channel. Hence while filling the MAC PDU, first it will take and add the minimum data i.e PBR from all the channels into the MAC PDU. After that if there are any space left in MAC PDU, then it will fill according to the priority.
Hence all the channels will get a chance to participate in sending the data.

For example:

Channel 1 has priority 1 with 60 bytes of data PBR is 20 bytes
Channel 2 has priority 2 with 40 bytes of data PBR is 10 bytes
Channel 3 has priority 3 with 20 bytes of data PBR is 10 bytes

So if MAC layer wants to transmit 100 bytes of data in MAC PDU to PHY layer, then first it will take 20 bytes from Channel 1, 10 from channel 2, 10 from channel 3. Total it will fill 40 bytes from all the 3 channels.

Now we have 60bytes of data to be filled. Now it will fill according to the priority. Hence it will take 60bytes from channel 1 and fill total 100 bytes.

How priority will be considered for LCP?

For LCP the priority in decreasing order:
– MAC control element for C-RNTI or data from UL-CCCH;
– MAC control element for BSR, with exception of BSR included for padding;
– MAC control element for PHR;
– data from any Logical Channel, except data from UL-CCCH;
– MAC control element for BSR included for padding.

RRC IEs LogicalChannelConfig
			LogicalChannelConfig
			Priority (1 to 16)
			PrioritisedBitRate (0kbps to 2048kbps)
			BucketSizeDuration (ms50 to ms1000)
			LogicalChannelGroup (0 to 3)

Reference TS 36.321