RACH Part 3: RACH info in SIB5

All of the RACH related information is broadcasted on SIB5/5bis via BCH. Last time, the PRACH partitioning, which contained the available PRACH preamble signatures and sub-channels, was elaborated in details. But as I mentioned, we need more info to check the exact available signatures and sub-channels.

Once I was confused about SIB5 and SIB5bis and later found they were almost identical. On Agilent website it says:

SIB5bis was introduced by 3GPP as a way to allow new frequency bands to be created that overlap existing frequency bands. For example, when Band IV was introduced it overlapped Band I. To prevent older Band I UEs from camping on Band IV cells (and then transmitting their PRACH messages on the wrong uplink frequency due to a different Tx/Rx frequency separation), Band IV networks transmit SIB5bis instead of SIB5. Band IV UEs expect SIB5bis and can camp to the cell, but older Band I mobiles simply see that SIB5 is missing and thus do not camp to the Band IV cell. SIB5bis is identical to SIB5 except for the SIB type.

In PRACH System Information List of SIB5/5bis, there is one Information Elements block called PRACH RACH Info. Basically, it specifies the available signatures and sub-channels for this PRACH as well as other necessary info like available SF, preamble scrambling code word number and puncturing limit, etc.

With this part of information and PRACH partitioning, we can finnally get the exact available signatures and sub-channels rather than just index or bit number.

SIB 5/5bis

The mapping for available sub-channels is kind of tricky between PRACH Partitioning and PRACH System Information List. Because AICH Transmission Timing should be also taken into account. And you can find the specific mechanism in TSG WG1#5 (99) 650. (I will introduce AICH Transmission Timing in next post and how it impacts Node B process.)

So now we can see the whole mapping is presented as below.