INTRODUCTION TO RADIO NETWORK PLANNING
Effective radio network planning is essential for optimizing the performance and coverage of modern wireless networks. This involves a meticulous process of selecting and configuring operating bands and channel frequencies. In 2024, significant updates have been made to the methodologies used in network planning to accommodate the evolving needs of 5G and beyond. This technical blog delves into the key aspects of operating bands, NR-ARFCN, and GSCN in the context of contemporary radio network planning.
OPERATING BAND
In many cases, there will be a one-to-one relationship between the: allocated spectrum and the operating band. In these cases, there is only a single operating band which can be selected.
In some cases, operating bands overlap so multiple operating bands can be associated with a single spectrum allocation. For example, a TDD spectrum allocation from 3300 to 3400 MHz can be associated with operating band n77 (3300 to 4200 MHz) or operating band n78 (3300 to 3800 MHz)
The network implementation may allow multiple operating bands to be configured, e.g. SIB I is capable of broadcasting a list of operating bands rather than just a single operating band.
If the network implementation supports a single operating band per channel then it will be necessary to select one of the candidate operating bands. This selection could be based upon the requirement for specific band combinations. Band combinations are specified between LTE and NR for the Non-Standalone Base Station architecture. Band combinations arc also specified for NR Carrier Aggregation and Dual Connectivity.
All of the required band combinations should be identified and checks should be completed to determine whether or not all combinations have been specified by 3GPP. Checks should also be completed to determine whether or not the network equipment and UE support all band combinations. This may lead to one operating band being more attractive than another operating band. For example, operating band n77 may support all of the required band combinations, while operating band n78 may not support all of the required band combinations.
NR-ARFCN & GSCN
It is necessary to identify centre frequencies for both the channel bandwidth and the set of SS/PBCH Blocks. Initial deployments are likely to use a single set of SS/PBCH Blocks requiring a single centre frequency. More advanced deployments may use multiple sets of SS/PBCH Blocks requiring multiple centre frequencies, e.g. when a channel bandwidth is configured to use multiple Bandwidth Parts. Figure below illustrates a single set of SS/PBCH Blocks within a channel bandwidth.
The New Radio - Absolute Radio Frequency Channel Number (NR-ARFCN) identifies the centre frequency of the channel. The steps associated with selecting a1n appropriate NR-ARFCN include:
Identify the channel raster(s) for the selected operating band. Many operating bands within Frequency Range l are restricted to using a 100 kHz raster to provide coexistence with LTE (which also uses a 100 kHz raster). Other operating bands support two channel rasters. For example, operating bands n77 and n78 support the 15 kHz and 30 kHz channel rasters. When two channel rasters are specified for an operating band, the larger raster is applicable when the channel only uses a subcarrier spacing which equals the larger raster.
Identify the target centre frequency in MHz. In general, the target centre frequency should be positioned towards the centre of the allocated spectrum while occupying a position belonging to the channel raster.
Calculate the NR-ARFCN using the equation.
The Global Synchronisation Channel Number (GSCN) identifies the centre frequency for a set ofSS/PBCH Blocks. The two main requirements associated with selectiing a centre frequency are
The Resource Blocks occupied by the SS/PBCH Blocks must be fully within the channel bandwidth
The SS/PBCH Blocks must be subcarrier aligned with the Resource Blocks belonging to the channel bandwidth. Orthogonality will be lost if the two transmissions are not subcarrier aligned.
In addition, it is necessary to identify a strategy for the general position of the SS/PBCH Blocks, i.e. should the SS/PBCH Blocks be placed within the lower section, middle section or upper section of the channel bandwidth? Placing the SS/PBCH Blocks towards the centre of the channel bandwidth means that SS-RSRP and SS-RSRQ measurements are completed all the centre of the channel. These measurements may be more representative of average channel conditions when compared to measurements completed using the lower or upper sections of the channel . Placing the SS/PBCH Blocks towards the centre of the channel bandwidth also provides a solution analogous to LTE which always uses the centre of the channel for the Synchronisation Signals and PBCH. A counter argument could be based upon the measurement of frequency multiplexed CSI Reference Signals. If the SS/PBCH Blocks occupy the centre of the channel bandwidth, the number of contiguous Resource Blocks available to frequency multiplexed CSI Reference Signals will be reduced (the channel bandwidth will be segmented by the SS/PBCH Blocks). This may have some impact upon measurement accuracy.
As an example, consider a spectrum allocation from 2300 to 2340 MHz within operating band n40. Operating band n40 uses a 100 kHz channel raster so it is possible to place the centre frequency at 2320 MHz. The NR-ARFCN can be calculated as:
NR-/ARFCN = Nref-off + (Center Frequency Fref-offs )/Δ fglobal
NR-ARFCN=0 + (2320 MHz 0)/ 5 kHz= 464 000
The resultant NR-ARFCN is a multiple of 20 which confirms that the centre frequency belongs to the I00kHz channel raster.
Assuming that the SS/PBCH Blocks are to be positioned towards the centre of the channel bandwidth then candidate centre frequencies can be identified using the following equation which is applicable to operating bands below 3 GHz:
Center Frequency= N x 1200 kHz + M x 50 kHz where, N = 1 to 2499 and M= 1, 3 or 5
Setting N=1933, generates candidate centre frequencies of 23I9.65, 2319.75 and 2319.85 MHz for M=1, 3 and 5 respectively. It is then necessary to check which of these candidates provides subcarrier alignment with the channel bandwidth. The first candidate has an offset of 350 kHz relative to the centre of the channel bandwidth. This is not an integer multiple of I5 kHz so is not subcarrier aligned. Similarly, the second candidate has an offset of 250 kHz which is not an integer multiple of 15 kHz so is not subcarrier aligned. The third candidate has an offset of 150 kHz which is an integer multiple of 15 kHz. Thus, in this example, the third candidate should be selected. The GSCN can then be calculated as:
GSCN=3xN+(M 3)/ 2=3x1933+(5 3)/ 2=5800
The appropriate value of 'M' changes as the centre frequency of the channel bandwidth increments in steps of I00kHz.This results from 100 kHz not being an integer multiple of 15 kHz. The preceding example has illustrated that M=5 should be selected, but if the centre frequency of the channel bandwidth is incremented to 2320.1 MHz then the combination of N = 1933 and M= 1 provides subcarrier alignment.
Subcarrier alignment is less complex for operating bands above 3GHz because the 100kHz channel raster is not used. In these cases, the 'M' variable is not required when calculating the SS/PBCH centre frequency and the corresponding GSCN.
CONCLUSION
Understanding the intricacies of operating bands, NR-ARFCN, and GSCN is vital for efficient radio network planning. These elements ensure optimal frequency utilization and robust network performance, essential for the demands of modern wireless communication.
REFERENCES
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