PRACH Basic-3

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In this section we will learn about PRACH Preamble Format and its basics as well as interview questions.

Random access with NR supports different PRACH formats and numerologies to cope up with different deployments. The random access sequences themselves are based on the preamble format.

Preamble Format:

(Qus: How many new preamble format are introduce by 3gpp in NSA or SA case?)

In 5G NSA or SA cases, there are 13 types of preamble format for FR1/FR2 and they are known as Format 0/1/2/3, Format A1/A2/A3, Format B1/B2/B3/B4, Format C0/C2. These 13 types of preamble format can be categorized into two categories:

• Sequence length of L = 139, with subcarrier spacing of 15, 30, 60, and 120 kHz. These can be solutions for different coverage situations.

• Sequence length of L = 839, with subcarrier spacing of 1.25 kHz and 5 kHz. This sequence can be used for very large cells (up to 100 km).

(Qus: What is Root sequences for sequence length 839 and 139?)

3GPP has specified sequence lengths of 839 and 139 for the PRACH, so there are 838 and 138 Root Sequences respectively.

The preamble sequence length is indicated by the higher-level parameter prach-RootSequenceIndex . Under FR1, long sequences and short sequences with subcarrier spacing of 15KHz and 30KHz are supported. Under FR2, only short sequences with subcarrier spacing of 60KHz and 120KHz are supported.

preamble	Length	RB	SCS (Khz)
Long sequence	839	6/24	1.25 / 5
short sequence	139	12	15/30/60 (FR1) 60/120/240 (FR2)

If the subcarrier spacing of PRACH preamble is 1.25 or 5 Khz, long sequence (L_RA = 839) is used. This sequence can be used for very large cells (up to 100 km).

Long sequence PRACH preambles can only be used by cells belonging to Frequency Range 1 (450 MHL 10 6 GHz). These preambles use Root Sequences which have a length of 839 (meaning that there are 838 Root Sequences). Long preambles are designed to support relatively large cell ranges (up to 100 km).

Cyclic prefix and guard period duration􀋬 are long to support the round-trip propagation delays associated with large cell ranges. Long preambles use relatively small SCS(subcarrier spacings) ( 1.25 and 5kHz). 

This makes them more vulnerable to frequency offsets. In addition, they may be used to provide coverage for high mobility scenarios with high Doppler frequency offsets. For this reason, the Type A and Type 8 Restricted Sets are specified for use with long PRACH preambles. 

Wherein k the basic time unit between LTE and NR ratio k=Ts/Tc=64.

Similarly if the SCS of PRACH preamble is 15,30,60,120 and 240 Khz, short sequence (L_RA = 139) is used as in the following table. These offer suitable solutions for different coverage situations.

Short sequence PRACH preambles can be used by cells belonging to either Frequency Range 1 (450 MHz to 6 GHz) or Frequency Range 2 (24.25 GHz to 52.6 GHz). These preambles use Root Sequences which have a length of 139 (meaning there are 138 Root Sequences). Short preambles are designed to support relatively small cell ranges (up to about 9 km, but with many configurations having a significantly smaller cell range). 

Short preambles use relatively large subcarrier spacings (15, 30, 60 and 120 kHz). This makes them less vulnerable to frequency offsets generated by either non-ideal oscillators or high mobility (Doppler frequency offsets). This means that it has not been necessary to specify Restricted Sets to protect short preambles from frequency offsets. 

Short preambles are intended for cells which are unable to accommodate relatively long periods of UL transmission. In the case of Frequency Range 2 with analogue beam forming, only a single beam can be active at any point in time so it is not possible to share PRACH occasions across beams nor frequency multiplex the PRACH occasions associated with each beam. 

Short preambles allow the set of PRACH occasions to be time multiplexed with minimal latency, i.e. the time taken to cycle through the set of PRACH occasions belonging to the set of beams is relatively short.

Preamble Format table with 15/30/60/120/240 kHz SCS:

 

Note: later 3gpp remove A0 and C1 formats.

Note: T-GP: GAP length

The preamble length gets shorter As this interval goes higher (e.g, 30, 60, 120, 240 Khz).

Several formats are supported for different coverage situations. Because of the cyclic prefix duration, a short sequence length is suited for cell radio on the order of 4.7 km. After this distance, some degradation can be seen thus, for larger cells, it is better to use a longer sequence.

TDD frame configurations have an impact on the relative capacity split between the UL and DL, on the latency, and also on the maximum cell radius. For instance, if we have 30 kHz subcarrier spacing and just one UL slot in the frame, only the short PRACH format can be utilized but not the long PRACH format. Short PRACH limits the cell radius. 

The maximum cell radius with short PRACH with 30 kHz SCS (subcarrier spacing) is less than 5 km and with 15 kHz SCS less than 10 km. A larger cell radius may be needed, especially in rural and sub-urban cases where heighted base station antennas and open propagation can enable larger cells from the propagation point of view. Long PRACH allows very large cell sizes. Long PRACH is possible with two consecutive UL slots.

PRACH design includes considerations for high-speed mobility. For long sequences and low subcarrier spacing (1.25/5 kHz), restricted sets can be used. For short sequences, large subcarrier spacing (15/30/60/120/240 kHz) provides in-built protection against the Doppler shift. These are based on specific cyclic shifts that can be used for the Zadoff Chu sequence with high Doppler.

In the next section we will learn more about RACH. 

So keep reading……. And Stay tune……………

Pinal Dobariya………..

Reference:

3GPP TS 38.321 
3GPP TS 38.300

3GPP TS 38.213