LTE : How many types of MAC schedulers are defined and how these schedulers are differ to each other ?

Question

Each carrier in LTE is 15khz so in a calculation i found that useful symbol time = 1/15khz = 66.6 mircosec

wrt to this
Each symbol duration is calculated like this
Normal Cyclic Prefix length = 4.69 microsec --- so each symbol in this = 66.6 + 4.69 = 71.29 microsec
Extended Cyclic Prefix length = 16.67 microsec ----so each symbol in this = 66.6 + 16.67 = 83.27 microsec

So Normal cyclic prefix Slot (7 symbols) duration = 71.29 * 7 = 499.03 microsec(0.49903 millis)
and Extended cyclic prefix Slot (6 symbols) duration = 83.27 * 6 = 499.63 microsec (0.49963 millis)

But slot duration is fixed as 0.5 ms ...

So does this slight difference creates any problem ???
How real time it works ???

Answer 1

Schedulers are Dynamic, persistent and semi persistent.

Dynamic- every TTI, MAC check for UEs to be scheduled, data availability and channel conditions. If data is not available, UEs wont get scheduled. The advantage of Dynamic Scheduling is basically the flexibility to alter the size of data in each subframe but drawback is high PDCCH usage i.e high control overhead

Persistent - To reduce dynamic scheduling drawback(high control overhead), we can use persistent scheduling which is fixed packet size scheduling and doesn't depend on channel conditions. But when you have a retransmissions for any of the data, it contain some extra information, So your retransmissions cant be pushed along with the persistent scheduled Time interval.

Semi persistent - Hybrid way trying to overcome drawbacks from dynamic & persistent scheduling. With SPS, the UE is pre-configured by the eNB with an SPS-RNTI and a periodicity. If the UE were to receive a data using the SPS-RNTI, then once allocated resources would be used by UE instead of allocating the resources periodically

Answer 2

Hi harshitha,

There are so many implementation based schedulers are available.

1. Round Robin:

this scheduler gives equal priority to every UE and schedules one after one.

1. PF scheduler(propositional fair):

This scheduler will consider channel quality and fairness. Like I UE channel quality is poor if you schedules more data to those UE then its of no use because how ever it will be retx. So select a good channel condition UE and schedule more data, but to give fairness to other UE even if channel quality id poor schedule some fairness data to those UE. also based on through put some operator may schedule. They can ensure the same through put to all user. so based on running through put ENb will pick the UE

3 . QOS Scheduler :

This scheduler is purely based on the priority of the user and bearer(GBR,N-GBR,SRB). So always QOS scheduler will pick the UE which has either higher priority bearer or it will pick only the premium user. So in this scheduler there will be a starving of users. Mainly this scheduler will focus quality of service to those user who paid premium.

4 . Delay based Scheduler.

This kind of scheduler will give higher priority to delay sensitive bearers. This will work purely based on the bearer delay. If bearer delay is high then it will schedule only those bearers not UE basis.

Mainly all schedulers are implementation specific. based on operator requirement scheduler will be implemented.

Happy Learning :)