Why 6 PRBs use to send RACH message in LTE?
we know to send RACH message in LTE we require Preamble. we have total 4 preamble format we can refer table Number 36.211.
- total preambles type we have 0,1,2,3.
- Almost all operator uses preamble format 0.
- To send preamble we require 6 PRB in frequency domain and 1 msec in time domain why?
- Preamble is made of an 839 Zadd off-chu sequence.
- 1 preamble = 839 Zadd off-chu sequence [839 frequency sequence]
- 1 PRB=12 Subcarrier
- 6 PRB = 12*6 =72 Subcarrier
- Total occupied frequency
- 1.25Khz X839 = 1.048 Mhz =1.4Mhz
- T=1/f =1/1.25 Khz=0.8 msec =1 msec
- we know 1.4Mhz will have 6 PRB or 72 Subcarrier in frequency domain.
The difference in subcarrier spacing between PRACH preamble and uplink subframe (1.25 kHz vs 15 kHz) is likely due to historical reasons or specific design choices for the RACH process. It allows for simpler implementation of the PRACH signal with narrower subcarriers, potentially improving its robustness in noisy channels.