What is the use/Influence of the parameter "high speed" in Prach in LTE?

Question

During preamble transmission, how devices choose the frequency band in PRACH for preamble transmission?

If they choose the frequency in PRACH randomly, then there can be the chance of sub-channel(frequency band to send preamble) collision. If above statement is true then how many devices can send the preamble at a random access time slot?

Answer 1

All the preamble sequences should be orthogonal to each other. Otherwise, various preambles from multiple UEs within the same cell can interfere each other. So we have to shift the generated sequence by a specifically designed value and this value is called Cv (Cyclic Shift Value) and it is defined as follows.

First, you would notice that we use different process to calculate Cv depending on whether we use 'unrestricted sets' or 'restricted sets'. This decision is made by 'Highspeedflag' information elements in SIB2. If Highspeedflag is set to be TRUE, we have to use 'restricted sets' and if Highspeedflag is false, we have to use 'unrestricted sets'.

Hope this clarifies the doubt and usage of highspeed flag in prach.

Source: http://www.sharetechnote.com/html/RACH_LTE.html