Introduction To 5G RADIO NETWORK PLANNING
In the ever-evolving landscape of 5G technology, 5G RADIO NETWORK PLANNING is paramount to ensuring optimal network performance, particularly in uplink transmission and random access procedures. This article delves into the specifics of 5G RADIO NETWORK PLANNING, focusing on PRACH (Physical Random Access Channel) formats, configuration indices, and the zero correlation zone—key components that significantly influence the effectiveness of 5G networks.
PRACH FORMATS
The PRACH Format must be selected from either:
The set of long PRACH Formats (0, 1, 2, 3), or
The set of short PRACH Formats (A1, A2, A3, B1, B4, A1/B1, A2/B2, /A3/83, C0, C2
Formats 'B2' and 'B3' are always used in combination with Formats 'A2' and 'A3' respectively.
Figure below summarises the criteria used to select the PRACH Format. The Frequency Range should be considered as the first criteria - operating bands within Frequency Range 2 only support the short PRACH Formats. In contrast, operating bands within Frequency Range 1 support both the long and short PRACH Formats.
The Slot Format should be considered as the second criteria when selecting a PRACH Format. In using FDD, the Slot Format docs not restrict the PRACH Format because continuous uplink transmission is permitted. In using TDD, the Slot Format can restrict the choice of PRACH Format. For example, if the 30 kHz subcarrier spacing is used in combination with the following Slot Format pattern { D, D, D, F. U, D, D, D, F, U}, the maximum number of contiguous uplink slots is 1. This leads to a maximum uplink transmission period of 0.5 ms (based upon the 30 kHz sub carrier spacing and ignoring uplink symbols within the Flexible slot). Long PRACH Formats have a minimum period of 1 ms so in this case, it would be necessary to configure a Short PRACH Format. Alternatively, if the 30 kHz sub carrier spacing is used in combination with the following Slot Format pattern {D. D, D, D, D. D, D, F, U, U}, the maximum number of contiguous uplink slots is 2. This leads to a maximum uplink transmission period of 1 ms so it becomes possible to select long PRACH Formats 0 and 3.
The cell range requirement is also an important criteria when selecting a PRACH Format. Long PRACH Formats support maximum cell ranges between 14 and 100 km, whereas, short PRACH Formats arc restricted to supporting cell ranges up to 9.3 km. The majority of short PRACH Formats support maximum cell ranges below 5 km.
The coverage requirement can be used as an input when selecting a repetition level for the PRACH sequence. Within this context, coverage requirement refers to deep indoor coverage rather than long range coverage, i.e. improving coverage within the existing cell range rather than extending the maximum cell range.
In the case of the long PRACH Formats, the 'Sequence Duration' column indicates that PRACH Format I benefits from 2 transmissions of the sequence, while PRACH Formats 2 and 3 benefit from 4 transmissions of the sequence. PRACH Format 2 is intended to provide the best coverage performance because it is based upon the transmission of 4 x 800 μs sequences (PRACH Format 3 uses a higher subcarrier spacing and is intended for high mobility scenarios).
In the case of the short PRACH Formats, the 'Sequence Duration' column within Table 173 indicates the repetition level for each PRACH Format. For example, PRACH Format A I benefits from 2 transmissions of the sequence, while Format 84 benefits from 12 transmissions of the sequence.
The UE mobility requirement can be used as a criteria when selecting the subcarrier spacing. Long PRACH Formats 0, I and 2 use a subcarrier spacing of 1.25 kHz while long PRACH Format 3 uses a subcarrier spacing of5 kHz. The larger subcarrier spacing is more robust in the presence of Doppler frequency offsets. Short PRACH Formats can be configured to use subcarrier spacings of 15, 30, 60 or 120 kHz. These larger subcarrier spacings arc more robust in the presence of Doppler frequency offsets. However, the smaller cell ranges associated with the short PRACH Formats will require more frequent handovers if deployed at locations with high UE mobility.
Short PRACH Formats are more suitable for Base Stations configured with large beam sweeping patterns. The short duration of each PRACH occasion allows the Base Station to rapidly sweep through the set of beam positions, i.e. minimising latency. Operating bands within Frequency Range 2 can support up to 64 SS/PBCH beams and in this case it is mandatory to use a short PRACH Format. In the case of Frequency Range 1, PRACH latency can become significant if long PRACH preambles are used in combination with the maximum of8 SS/PBCH beams. In this case, latency can be reduced by allowing multiple beams to share a single PRACH occasion, i.e. the set of64 PR.J\CH preambles can be shared across beams. Alternatively, latency can be reduced by frequency multiplexing PRACH occasions.
PRACH CONFIGURATION INDEX
Frequency Range Considerations:
The PRACH Configuration Index must be selected from one of three tables specified by 3GPP TS 38.211:
Frequency Range 1, Paired Spectrum (FDD) and Supplementary Uplink, or
Frequency Range 1, Unpaired Spectrum (TDD), or
Frequency Range 2, Unpaired Spectrum (TDD)
Each table includes 256 rows which are grouped according to the PRACH Preamble Format. Once the relevant table and PRACH Preamble Format have been identified, the number of rows available for selection is typically reduced to within the range of 15 to 30
Figure below illustrates the criteria used to select the PRACH Configuration Index. The PRACH Preamble Format should be considered as the first criteria because it identifies the set of rows available for selection.
The Slot Format should be considered as the second criteria because it may further restrict the set of rows available for selection. If using FDD, the Slot Format does not restrict the PRACH Configuration Index because all slots arc available for uplink transmission. If using TDD, the Slot Format can restrict the choice of PRACH Configuration Indices because the Configuration Index defines the timing of the PRACH opportunities and these opportunities must coincide with the timing of the uplink slots
Figure below illustrates an example TDD Slot Format which allows uplink transmission during subframes 2 and 7. If using PRACH Format 0, there are only 3 PRACH Configuration Indices available for selection. These are visible within Table 172 as PRACH Configuration Indices 9, 14 and 19.
Capacity and Latency Trade-offs
The PRACH capacity and latency requirements should also be considered when selecting a PRACH Configuration Index. For example, PRACH Configuration Index 0 within Table 172 provides a low capacity and high latency because there is a single PRACH occasion every 160 ms (assuming PRACH occasions arc not frequency multiplexed). The benefit of this configuration is a low PRACH overhead. PRACH Configuration Index 7 provides a moderate capacity and latency because there is a single PRACH occasion every 10 ms. PRACH Configuration Index 27 provides a high capacity and low latency because there arc five PRACH occasions every 10 ms
The PRACH Root Sequence Index planning strategy can also impact the selection of the PRACH Configuration Index.
All neighbouring cells arc allocated a common PRACH Configuration Index, but a different Root Sequence Index.
All cells belonging to a specific Base Station (or Distributed Unit if using the CU/DU Split Base Station architecture) are allocated a common Root Sequence Index, but a different combination of PRACH Configuration Index and PRACH Frequency Offset.
The latter strategy helps to reduce the probability of Root Sequence collisions by increasing the Root Sequence re-use distance, i.e. Root Sequences are allocated 'per Base Station' rather than 'per cell'. The PRACH Configuration Index can be used to provide isolation in the time domain. When using the example shown in Figure above, sector 1 could be allocated Configuration Index 14 to use subframe 2, sector 2 could be allocated Configuration Index 9 to use subframe 7, while sector 3 could also be allocated Configuration Index 9 but with a different PRACH Frequency Offset to provide isolation in the frequency domain.
ZERO CORRELATION ZONE
Selecting a Zero Correlation Zone is a prerequisite to planning the PRACH Root Sequence Indices.
Each PRACH occasion allows the use of 64 preamble sequences. These preamble sequences allow multiple UE to share the same set or time and frequency domain resources when transmitting their PRACH preambles.
3GPP TS 38.211 specifics that preamble sequences are generated from a set of 838 Root Sequences when using a long PRACI I Format and from a set of 138 Root Sequences when using a short PRACH format. Each preamble sequence is generated from its Root Sequence by applying a cyclic shift. The Zero Correlation Zone determines the size of the cyclic shift and the number of preamble sequences which can be generated from each Root Sequence. The key trends associated with selecting large and small Zero Correlation Zones are:
1. Large Zero Correlation Zones:
Large cyclic shifts used to generate preamble sequences from each Root Sequence. TI1is leads to a larger maximum permitted propagation delay and thus, a larger cell range.
Fewer preamble sequences can be generated from each Root Sequence. So each cell requires an increased number of Root Sequences. This reduces the Root Sequence re-use pattern size and increases the potential for Root Sequence collisions.
2. Small Zero Correlation Zones:
Small cyclic shifts used to generate preamble sequences from each Root Sequence.This leads to a smaller maximum permitted propagation delay and thus, a smaller cell range.
More preamble sequences can be generated from each Root Sequence, so each cell requires fewer Root Sequences. This increases the Root Sequence re-use pattern size and decreases the potential for Root Sequence collisions.
It is beneficial to generate as many preamble sequences as possible from the same Root Sequence because these sequences arc orthogonal to one another. Preamble sequences generated from different root sequences arc not orthogonal
Figure below summarises the criteria used to select the Zero Correlation Zone. The combination of PRACH Preamble Format, PRACH subcarrier spacing and UE mobility requirement determines the look-up table used to select the Zero Correlation Zone.
The same relationship when using a long PRACH Format with the 5 kHz subcarrier spacing, i.e. PRACH Format 3. Both of these tables are applicable to low/medium mobility scenarios, i.e. they assume the 'unrestricted' set of cyclic shifts rather than the 'restricted' set of cyclic shifts. The High Speed Flag is used to select between the ·unrestricted' and 'restricted' sets of cyclic shifts. The 'restricted' sets of cyclic shifts are intended for high mobility scenarios.
The appropriate Zero Correlation Zone is identified by selecting the smallest value which satisfies the maximum cell range requirement. This approach ensures that the cell range is sufficient while helping to maximise the Root Sequence re-use pattern size. It is important to ensure that the selected cell range caters for any overshooting of RF transmissions in the live network. !f a cell radiates beyond its maximum cell range then UE which are located outside the maximum cell range will experience failed PRACH procedures
This results from the increased propagation delay appearing similar to an increased cyclic shift, i.e. both increased propagation delay and increased cyclic shift cause the Base Station to receive a signal which appears delayed in time. If the maximum cell range is exceeded, the increased propagation delay causes preamble 'X' to appear similar to preamble 'Y' (because preambles are generated by applying cyclic shifts to the Root Sequence). This means that the: Base Station will provide a Random Access Response for preamble ·y• while the UE is monitoring for a response to preamble 'X', i.e. the MSG 1 to MSG 3 success rate will become poor.
The relationship between Zero Correlation Zone and Root Sequence index re-use pattern size when using a short PRACH Format. In this case, the re-use pattern sims are significantly smaller because there arc only 138 Root Sequences available. The relationship between Zero Con-elation Zone and cell range when using subcarrier spacings of 15, 30, 60 or 120 kHz. These cell ranges are relatively small due to the reduced symbol duration associated with the higher subcarrier spacings. The reduced symbol duration leads to a reduced maximum permitted round trip delay associated with each cyclic shift.
Conclusion
In 5G radio network planning, the careful selection of PRACH formats, configuration indices, and Zero Correlation Zones is essential for optimising network performance, especially in terms of coverage, capacity, and latency. By understanding these elements, network planners can ensure that their 5G deployments meet the demands of both high-mobility environments and deep indoor coverage scenarios, ultimately delivering a robust and reliable service to end users.
This updated guide reflects the latest advancements in 2024, providing a comprehensive overview for professionals involved in 5G network planning.
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