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 occurPRACH 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 preamblesUse different sequences for all neighbour cells

SIB2 value BCCH-DL-SCH-Message ::= message c1 : systemInformation : criticalExtensions systemInformation-r8 : sib-TypeAndInfo sib2 : radioResourceConfigCommon rach-ConfigCommon preambleInfo numberOfRA-Preambles n40, preamblesGroupAConfig sizeOfRA-PreamblesGroupA n32, messageSizeGroupA b144, messagePowerOffsetGroupB dB10 prach-Config rootSequenceIndex 30, prach-ConfigInfo prach-ConfigIndex 4, highSpeedFlag FALSE, zeroCorrelationZoneConfig 8, prach-FreqOffset 3,Lo normal es que sean 64Distrintos grupos distinta cantidad de info

PRACH Configuration Index (1)The PRACH Configuration

Chosen after selecting the Preamble formatThe 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)

PRACH Configuration Index

Preamble FormatCyclic Prefix LengthSequence LengthGuard TimeTotal LengthGuard Time Equiv. Dist.Typical Max. Cell Range00.10 ms0.8 ms0.10 ms1 ms30 km15 km10.68 ms0.8 ms0.52 ms2 ms156 km78 km20.20 ms1.6 ms0.20 ms2 ms60 km30 km30.68 ms1.6 ms0.72 ms3 ms216 km108 km

Frequency Offset (1)2 msPUCCHPUCCHPRACHDefines the position of the PRACH preamble within the channel bandwidthPRACH should be positioned adjacent to the PUCCH

Resource Block signalled in SIB2

PRACH Frequency Offset (2)

Indicates the first PRB available for PRACH in the UL frequency bandPRACH 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 areaParameter 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]

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 IndexThe 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 36.211Each logical rootSeqIndex is associated with a single physical root sequence number. The reuse distance of root sequences should be maximisedThe 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 preamblesThe propagation delay and the cyclic separation are directly related to the cell rangeRecommendation: 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.zeroCorrelationZoneConfig

Zero Correlation Zone (1)SelectingZero Correlation ZoneHigh Speed Flagare prerequisites to planning the Root Sequence Index

The Zero Correlation Zone determines the size of the cyclic shift used to generate the PRACH sequence from the root sequenceLarge cyclic shift (large Zero Correlation Zone) required to support larger cell rangesLarge cyclic shift means that fewer PRACH sequences can be generated from each root sequencePRACH sequences generated from different root sequences are not orthogonalHigh Speed Flag = FALSEHigh Speed Flag = TRUE>250 km/h

Zero Correlation Zone (2)There are 838 root sequences from which to generate the PRACH sequences(64)Each root sequence has a length of 839Each cell requires no more than 64 PRACH sequencesThe number of PRACH sequences which can be generated from each root sequence is given by:PRACH Sequences per Root Sequence = ROUNDDOWN(839 / Number of Cyclic Shifts (NCS))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 sequenceszeroCorrelationZoneConfig

Zero Correlation Zone (3)

PRACH PlanningWrap UpSteps: - Define the prachConfIndexDepends on preamble format (cell range)Depends on the amount of RACH attemptsIt should be different for each cell of a site- Define the prachFreqOffDepends on the PUCCH regionIt can be assumed to be the same for all cells of a network (simplification)- Define the zeroCorrelationZoneConfigDepends on the cell rangeIf for simplicity same cell range is assumed for all network then zeroCorrelationZoneConfig is the same for all cells- Define the rootSeqIndexIt points to the first root sequence It needs to be different for neighbour cellsrootSeqIndex separation between cells depends on how many are necessary per cell (depends on zeroCorrelationZoneConfig)

PRACH Planning example

Assumptions: - prachConfIndex=3 for all cellspreamble format =0One PRACH opportunity per 10ms- prachFreqOff=6 for all cellsPRACH starts at sixth PRB in frequency domain- Define the zeroCorrelationZoneConfig=8 for all cellsMax cell range = 5.5kmEach cell consumes 4 root sequences

*Based on cell range the PRACH format is selected The location of the 6RB of the PRACH is dynamic. Its defined by two parameters:PRACH Configuration index in time domainPRACH Frequency Offset in freq domain

1.PRACH Timing determines the interferences between time synchronised cells 2.PRACH Capacity should match the traffic requirements. Increased PRACH capacity means bigger PRACH overhead which on the hand decreases the number of RBs of PUSCH. So single PRACH per radio frame is a reasonable starting point for a new network3.The selection of the root sequence number is also part of the cell planning process. The number of physical root sequences used per cell depends on the cyclic shift length and preamble formatThe CS is used to ensure sufficient separation between the preambles

Two cells that are assigned the same configuration index, frequency offset and root sequence offset could interfere each other and potentially lead to ghost RACHs. Phantom RACH stands for a non-dedicated PRACH preamle sent by the UE sends and captured by more than one cell. All the cells capturing the preamble send random access response, hence reserving PDCCH and PUSCH capacity

*PRACH configuration index parameter selects one of the 4 possible PRACH durations.It can be sent in any radio frame or only in even numbered ones as shown in the table. PRACH Configuration index specifies also the number of the subframe where PRACH can be. It depends as already said on preamble format. The PRACH capacity is determined by System frame number (SFN). One PRACH is considered per subframe=> PRACH density=1RACH density is based on the expected RACH procedures per second and the maximum collision probability of the RACH preambles it is possible to estimate the RACH density using a formula.

The overhead for random access can be calculated : For example, if the system bandwidth is 10MHz,the number of RB is 50.The PRACH configuration is 9 with random access allowed in all system frame numbers, the overhead is:Overhead = 6/50x3/10=3.6%. configuration 9 allocates 3 out of a total of 10 subframes for random access transmissions.

*To avoid overlapping with PUCCH in frequency domain different frequency offsets are set

**The 64 preamble sequences per cell are grouped in subsets and the set of sequences within each subset is signalled as part of the system information. The terminal randomly selects one sequence in one of the subsets during the random access process. The subset from which the preamble sequence is selected depends on the amount of data the terminal would like to transmit on PUSCH. The set of 64 preamble sequences in a cell is determined by including all the available cyclic shifts of a root ZadoffChu sequence.If the 64 preambles cannot be generated from a single root ZadoffChu sequence, additional preamble sequences are obtained from the root sequences with the consecutive logical indexes until all the 64 sequences are found. Additional ZC root sequences should be used only when the required number of sequences (64) cannot be generated by cyclic shifts of a single root sequence. Furthermore, processing power of eNodeB is increasing with additional root sequences*In order to meet random access coverage requirements only 6-bit information is transmitted using a preamble.The cyclic shift dimensioning is therefore very important in the RACH design.The configuration determines how many cyclic shifts are needed to generate the preamble. The column signatures per root sequence indicate how many signatures are in the zero correlation zone The 4th column shows many root sequence indices are required for that specific PrachCs configuration The 5th column is the respective maximum cell range *PRACH sequence belong to Zadoff-Cho sequence based on CAZAC (Constant Amplitude Zero Autocorrelation)The High Speed Flag should be set to FALSE unless speeds are particularly high, i.e. > 250 km/hSetting the High Speed Flag to TRUE complicates root sequence planning because the number of PRACH sequences per root sequence is no longer constant

In LTE two sets of cyclic shifts unrestricted and restricted cyclic shifts are specified. In the restricted cyclic shifts, only a subset of the total cyclic shifts is allowed to support high mobility scenarios

Zero Correlation Zone should be set to the smallest value which supports the cell range. In this way the size of the root sequence reuse pattern increases.. The sequences generated from a cyclic shift of a single root sequence are orthogonal. They are favoured over non-orthogonal sequences*. Example: Cell Range=15km, PrachCS: 12 cyclic shifts=119, Preamble sequence length Nzc=839Nb of cyclic shifts=ROUNDOWN (839/119) =7To ensure that the cells has 64 preamblesRoot Sequence per cell= ROUNDUP(64/7)= 10