PRACH Planning in LTE

PRACH Planning in LTE

PRACH Planning Principle •In LTE it is necessary the radio planner to selects the preamble format for each cell based on maximum estimated cell range. Typical preamble format will be ‘Preamble Format 0’, allowing for cell sizes up to 15km. Other preamble formats allow for larger cell ranges. •PRACH parameters should be planned. PRACH transmission can be separated by: – Time (prachConfIndex) specifies in which subframes the RA can occur PRACH Configuration Index cannot be the same for different cells at the eNB – Frequency (prachFreqOff) avoid overlapping with PUCCH and PUSCH or splitting the PUSCH area.PRACH area is next to PUCCH area either at upper or lower border of frequency band, For simplicity use same configuration for all cells – Sequence (PRACH CS and RootSeqIndex) the cyclic shift ensures sufficient separation between the preambles

Use different sequences for all neighbour cells

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PRACH Configuration Index (1) The PRACH Configuration • Chosen after selecting the Preamble format • The PRACH capacity is determined by the SFN and subframe figures • In the case of FDD, the PRACH cannot be multiplexed in the frequency domain, i.e. only 1 PRACH resource per subframe

Recommendation: Configure different PRACHconfiguration Indexes at cells belonging to the same site. E.g.: 3/4/5 if RACH density=1 or 6/7/8 if RACH density=2 (Preamble Format 0)

Frequency Offset (1)

– Defines the position of the PRACH preamble within the channel bandwidth – PRACH should be positioned adjacent to the PUCCH

PUCCH

PUCCH 2 ms Resource Block signalled in SIB2

PRACH

PRACH Frequency Offset (2) •

Indicates the first PRB available for PRACH in the UL frequency band – PRACH area (6 PRBs) should be next to PUCCH area either at upper or lower border of frequency band in order to maximize the PUSCH area but not overlap with PUCCH area – Parameter is configured based on the PUCCH region i.e. its value depends on how many PUCCH resources are available. – If PRACH area is placed at the lower border of UL frequency band then: PRACH-Frequency Offset= roundup [PUCCH resources/2]

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If PRACH area is placed at the upper border of the UL frequency band then: PRACH-Frequency Offset= NRB -6- roundup [PUCCH resources/2]

NRB: Number of Resource Blocks

Root Sequence Index – The allocated ‘root sequence’ index broadcast in SIB2 is a logical index. The actual physical index is obtained using a look-up table defined within 3GPP TS 36.211 (part of the table is shown below) – Each logical rootSeqIndex is associated with a single physical root sequence number. – The reuse distance of ‘root sequences’ should be maximised – The eNB could be configured with all cells using the same root sequences, provided the cells use different preamble configuration indexes.

PRACH Cyclic Shift for FDD preambles – Cyclic shift is used to assure sufficient separation between the preambles – The propagation delay and the cyclic separation are directly related to the cell range Recommendation: assume all cells have same size=> same PrachCS

The table highlights how the intra-cell interference is optimized with respect to cell size: the smaller the cell size, the larger the number of orthogonal signatures and the better the detection performance.

Zero Correlation Zone (1) – Selecting • Zero Correlation Zone • High Speed Flag are prerequisites to planning the ‘Root Sequence Index’

High Speed Flag = FALSE

High Speed Flag = TRUE

– The Zero Correlation Zone determines the size of the cyclic shift used to generate the PRACH sequence from the ‘root sequence’

– Large cyclic shift (large Zero Correlation Zone) required to support larger cell ranges – Large cyclic shift means that fewer PRACH sequences can be generated from each ‘root sequence’ – PRACH sequences generated from different ‘root sequences’ are not orthogonal

Zero Correlation Zone (2) – – – –

There are 838 ‘root sequences’ from which to generate the PRACH sequences(64) Each ‘root sequence’ has a length of 839 Each cell requires 64 PRACH sequences The number of PRACH sequences which can be generated from each ‘root sequence’ is given by: PRACH Sequences per Root Sequence = ROUNDDOWN(839 / Cyclic Shift)

– The number of root sequences required per cell is then given by: ROUNDUP(64 / PRACH Sequences per Root Sequence) – This determines the size of the reuse pattern when planning ‘root sequences’

PRACH Planning Wrap Up • • • • • • • • • • • • • •

Steps: - Define the prachConfIndex Depends on preamble format (cell range) It should be different for each cell of a site - Define the prachFreqOff Depends on the PUCCH region It can be assumed to be the same for all cells of a network (simplification) - Define the PrachCS Depends on the cell range If for simplicity same cell range is assumed for all network then prachCS is the same for all cells - Define the rootSeqIndex It points to the first root sequence It needs to be different for neighbour cells rootSeqIndex separation between cells depends on how many are necessary per cell (depends on PrachCS)

PRACH Planning example

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Assumptions: - prachConfIndex=3 for all cells preamble format =0 One PRACH opportunity per 10ms - prachFreqOff=6 for all cells PRACH starts at sixth PRB in frequency domain - Define the prachCS=8 for all cells Max cell range = 5.5km Each cell consumes 4 root sequences

Exercise – Plan the PRACH Parameters for the sites attached in the excel – Assumptions: • PUCCH resources =6 • Cell range = 12km (all cells have same range) • BW:10MHz

Exercise Solution