Introduction to 5G Radio Network Planning
In the ever-evolving land scape of telecommunications, 5G radio network planning is pivotal for ensuring optimal performance and coverage. This part delves into Physical layer Cell Identity (PCI) allocation, highlighting its significance, methodologies, and best practices.
PCI Allocation
Physical layer Cell Identity(PCI) planning for NR is similar to PCI planning for LTE and scrambling code planning for UMTS
PCI Allocation
NR has 1008 PCI which are organised into 336 groups of 3.
LTE has 504 PCI which are organised into 168 groups of 3.
UMTS has 512 scrambling codes organised into 64 groups of 8.
Key Requirements for PCI Planning
The most important requirements for planning PCI are to remain 'CollisionFree' and 'ConfusionFree':
Collision Free: the physical separation between cells using the same PCI should be sufficiently great to ensure that a UE never simultaneously receives the same PCI from more than a single cell. This is achieved by maximising the re-use distance between cells which are allocated the same PCI. Planning should be simplified for NR relative to LTE because there are twice as many PCI values available to allocate. In the case of Frequency Range 2, site densities may be much higher but propagation losses will help to restrict dominance areas and prevent overshooting into neighbouring coverage areas.
Confusion Free: the physical separation between cells using the same PCI should be sufficiently great to avoid neighbour ambiguity at a Base Station, i.e. the Base Station should be able to link a specific PCI to a specific neighbouring cell without ambiguity. This means that a cell should not have multiple neighbours using the same PCI
Additional PCI Planning Rules
In addition, there arc arguments (described below) for further PC! planning rules. These rules imply that UE should not be able to simultaneously receive multiple PCI with equal:
'PCI mod 3' values
'PCI mod 4' values
'PCI mod 30' values
For example, the 'PCI mod 3' rule means that neighbouring cells should not be allocated PCI of 22 and 28 because they both have 'PCI mod 3' values of 1.
The requirement for a 'PCI mod3' rule is based upon the relationship between the PCI and the sequence transmitted by the Primary Synchronisation Signal (PSS). There are 3 ditlerent sequences which arc re-used across the network and the sequence is determined by the value of 'PCI mod 3'. This means that cells which have the same 'PCI mod 3' result will transmit the same PSS. Simulations have shown that cell acquisition times can increase when a UE receives the same PSS from multiple cells. Receiving the PSS from multiple cells can cause the UE to make misleading channel estimates which arc subsequently applied when attempting to detect the Secondary Synchronisation Signal (SSS).
ln the case of LTE, there is an additional argument for the 'PCI mod 3' rule based upon the subcarriers occupied by the Cell specific Reference Signal (CRS). When cells are configured to transmit the CRS using 2 or more antenna ports, and those cells have equal 'PCI mod 3' results then the CRS transmissions collide (assuming time synchronisation between cells). These collisions can have a negative impact upon reported CQI values and consequently lead to lower allocated throughputs.
The requirement for a 'PCI mod 4' rule is based upon the subcarriers occupied by the Demodulation Reference Signal (DMRS) for the PBCH. Subcarriers are allocated to the DMRS using a 'PCI mod 4' calculation. This leads to the DMRS occupying 25 % of subcarriers and means that there will be DMRS to DMRS interference if neighbouring cells have equal 'PC! mod 4' values. If neighbouring cells have unequal 'PCI mod 4' values then there will be PBCH to DMRS and DMRS to PBCH interference. The latter interference scenario is preferred although the impact is likely to be relatively small
The requirement for a 'PCI mod 30' rule is based upon the allocation of sequences to the PUSCH Demodulation Reference Signal (DMRS) when Transform Precoding is enabled. The full set of sequences is divided into 30 groups. The impact of intercell interference is reduced if different groups arc allocated to neighbouring cells. The allocated group is based upon a 'PCI mod 30' calculation unless a value for PUSCH-ldentity is configured within the DMRS-UplinkConfig. When using a 'PCI mod 30' calculation, ensuring that neighbouring cells do not have equal 'PCI mod 30' values is analogous to ensuring that neighbouring cells use different uplink Reference Signal sequences. If the 'PCI mod 30' rule cannot be satisfied for practical reasons, group hopping can be enabled to help reduce the impact of any group collisions.
If the 'PCI mod 3' rule is satisfied then the 'PCI mod 30' rule will also be satisfied. If a 3-sector Base Station is allocated a PCIGroup, i.e. a set of 3 consecutive PCT values, then both the 'PCI mod 3' and 'PCI mod 4' rules will be satisfied across the cells of that Base Station.
Figure below illustrates an example deployment of 3-sector Base Stations, where each Base Station has been allocated a single group of PCI. An ideal hexagonal cell layout is assumed for the purposes of this example. The 'PCI mod 3' rule is satisfied between all direct neighbours. The 'PCI mod 4• rule is satisfied between neighbours belonging to the same Base Station, but is not satisfied between neighbours belonging to different Base Stations.
Special Considerations
In practice, Base Stations will not have an ideal hexagonal cell layout. The situation is further complicated by heterogeneous network architectures where a single macrocell could be neighboured with many micro, pico or femto cells. In those cases, the 'PCI mod 3' rule may be satisfied intra-BTS but not inter-BTS.
If Base Stations have 6 sectors then PCI planning can be based upon allocating 2 PCI groups to each Base Station. If all sites were 6- sector then the maximum re-use pattern at a Base Station level would reduce from 336 to 168.
In addition to the PCI planning rules described above, arrange of PCI can be excluded from the network plan to allow for future network expansion, or the introduction of Femto cells. Femto cells could be allocated a subset of the total PCI. The size of the subset would depend upon the expected density of the Femto cell deployment. Femto cells typically allocate their own PC! from an allocated range after being switched-on and scanning for the PCI used by neighbouring Base Stations.
Additional rules for PCI planning are required at locations close to international borders where there may be another 5G operator using the same RF carrier. These rules are often specified by regulatory organisations. For example, in Europe the Electronic Communications Committee (ECC) within the European Conference of Postal and Telecommunications Administrations (CEPT) has specified ECC Recommendation (08)02. This document states that coordination can be avoided if signal strengths across the international border are below specific thresholds. Otherwise. it recommends that co-channel PCI should be coordinated between neighbouring 5G systems in border areas.
Conclusion
Effective PCI planning is essential for optimizing 5G networks, minimizing interference, and enhancing overall performance. By adhering to these guidelines and rules, operators can ensure their networks are robust, efficient, and ready for future advancements.
References
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