LTE System Bandwidth Arrangement
•
Bandwidth for P-SCH/S-SCH and BCH is centered differently compared to
the actual DL bandwidth used for shared channel operation
• UE aligns to center 72 sub-carriers for cell and system acquisition and then re-aligns based on the DL bandwidth.
• Example shown for different transmission bandwidths
• UE aligns to center 72 sub-carriers for cell and system acquisition and then re-aligns based on the DL bandwidth.
• Example shown for different transmission bandwidths
• The different channels are time/frequency multiplexed hence there is no overlap!
System Acquisition
• P-SCH is a time domain sequence, provides slot boundary and time alignment
• P-SCH is a ZC sequence with root 25, 29, 34 and is used to modulate S-SCH
• S-SCH is a pair of m-sequence used for frame alignment and also frequency correction
• P-SCH/S-SCH estimate used for CP estimation
• NCellId detection is used further for corroboration of CP used based on cell specific signal transmission
• All steps prevent redundant information from being sent to improve cell selection and acquisition time
• P-SCH is a ZC sequence with root 25, 29, 34 and is used to modulate S-SCH
• S-SCH is a pair of m-sequence used for frame alignment and also frequency correction
• P-SCH/S-SCH estimate used for CP estimation
• NCellId detection is used further for corroboration of CP used based on cell specific signal transmission
• All steps prevent redundant information from being sent to improve cell selection and acquisition time
• Physical layer cell identities NCellId = 504 (unique values for neighboring/overlapping cells)
• 504 = 168 * 3, 168 groups of 3 cells each as NCellId = 3 * N(1)Id + N(2)Id
N(2)Id = 3 cells identifying the P-SCH are always using a 62 bit ZC Sequence (root =25, 29, 34) transmitted on 72 subcarriers. P-SCH is always transmitted on the last OFDM symbol of the first slot in 0th and 5th subframe.
• N(1)Id comprises of identifiers between 0 to 167 identifying 168 cell id groups. This comprises the S-SCH and is generated from two 31 bit m sequences. The 31 bit sequences are scrambled with an index based on
N(2)Id.
• Cell specific reference signal is generated based on a scrambled code generated from c(i) initialised with OFDM symbol/slot number, NcellId and Ncp (Cyclic Prefix).
N(2)Id = 3 cells identifying the P-SCH are always using a 62 bit ZC Sequence (root =25, 29, 34) transmitted on 72 subcarriers. P-SCH is always transmitted on the last OFDM symbol of the first slot in 0th and 5th subframe.
• N(1)Id comprises of identifiers between 0 to 167 identifying 168 cell id groups. This comprises the S-SCH and is generated from two 31 bit m sequences. The 31 bit sequences are scrambled with an index based on
N(2)Id.
• Cell specific reference signal is generated based on a scrambled code generated from c(i) initialised with OFDM symbol/slot number, NcellId and Ncp (Cyclic Prefix).
• UE has locked on to the cell and has been able to find out NcellId based on the procedure described in earlier slides
• First goal of the UE is to decode PBCH (MIB is always transmitted on BCH mapped to PBCH) to find out DL system bandwidth.
• MIB carries bootstrap System Information (MIB = Master Info Block)
• If DL bandwidth is beyond UE capability, UE will reselect to another frequency as described earlier
• SFN computation requires only 8 bits; MIB is transmitted over 40 msec duration on the PBCH
• SIBType1 and SIBType2+ are found on DL-SCH (mapped to PDSCH) and will be found in specific sub-frames based on UE finding SI-RNTI in UE common search space in PDCCH allocation
SIB Scheduling Rules
• SIBType1 provides the schedule for SIBType2 onward SIBs
• SIBType1 has a fixed schedule always as mentioned in earlier sections
• SIBType2 index in the IE for Scheduling Information is always the first one (n=1)
• SIBs with the same periodicity may be combined in one or transmitted in different SI messages
• SI messages are always transmitted one per SI-Window, there will never be a case when two different SI messages are transmitted in the same SI-Window
• The goal of using the SI-window and related calculations is to ensure that the location of each SI message is clear to the UE (to assist DRX) and also to spread the load of system information evenly across all
radio frames.
• SIBType1 has a fixed schedule always as mentioned in earlier sections
• SIBType2 index in the IE for Scheduling Information is always the first one (n=1)
• SIBs with the same periodicity may be combined in one or transmitted in different SI messages
• SI messages are always transmitted one per SI-Window, there will never be a case when two different SI messages are transmitted in the same SI-Window
• The goal of using the SI-window and related calculations is to ensure that the location of each SI message is clear to the UE (to assist DRX) and also to spread the load of system information evenly across all
radio frames.
