5G NR - Frequency Domain Resource Allocation
- Venkateshu
- Jul 17
- 6 min read
Introduction
At its core, frequency domain resource allocation involves dividing the available radio spectrum into smaller, manageable units and assigning these units to different users or services. Both LTE and 5G NR utilize Orthogonal Frequency Division Multiplexing (OFDM) as their underlying waveform, which naturally lends itself to frequency domain multiplexing.
In OFDM, the total bandwidth is divided into multiple narrowband subcarriers. These subcarriers are orthogonal to each other, meaning their peaks and troughs align in such a way that they do not interfere with one another, even when overlapping in frequency. This allows for high spectral efficiency.
Resource Blocks (RBs)
The fundamental unit of frequency domain resource allocation in both LTE and 5G NR is the Resource Block (RB).
In LTE: An RB consists of 12 consecutive subcarriers, each with a fixed subcarrier spacing of 15 kHz. Therefore, an LTE RB occupies 12 * 15 kHz = 180 kHz of bandwidth.
In 5G NR: While an RB still comprises 12 consecutive subcarriers, the crucial difference lies in the variable subcarrier spacing (SCS). This flexibility is a hallmark of 5G NR, allowing it to support a wider range of services and deployment scenarios. The maximum supported bandwidth is 100MHz in FR1 and 400MHz in FR2.
LTE resource grid
5G Resource grid:
Subcarrier Spacing (SCS):
The subcarrier spacing dictates the width of each individual subcarrier.
LTE: Fixed SCS of 15 kHz. This simplicity made LTE deployments straightforward but also limited its adaptability to diverse use cases.
5G NR: Supports multiple SCS values, typically 15 kHz, 30 kHz, 60 kHz, 120 kHz, and 240 kHz (and up to 480 kHz for specific scenarios like unlicensed spectrum).
Smaller SCS (e.g., 15 kHz, 30 kHz):
More subcarriers within a given bandwidth, leading to finer frequency granularity.
Longer symbol duration, making them more robust to multipath propagation and suitable for wider area coverage.
Higher overhead for cyclic prefix (CP), which reduces spectral efficiency slightly.
Used for services requiring wider coverage and less stringent latency, like enhanced Mobile Broadband (eMBB) in lower frequency bands.
Larger SCS (e.g., 60 kHz, 120 kHz):
Fewer subcarriers within a given bandwidth.
Shorter symbol duration, enabling lower latency and better for high-speed scenarios.
More susceptible to inter-symbol interference (ISI) in highly dispersive channels, requiring shorter cell radii.
Lower overhead for CP.
Ideal for services requiring ultra-low latency and high data rates, such as Ultra-Reliable Low-Latency Communication (URLLC) and millimeter-wave (mmWave) deployments.
Numerology[ μ ] | Nsymslot | Nslotframe,μ | Nslotsubframe,μ | Cyclic Prefix |
0 | 14 | 10 | 1 | Normal |
1 | 14 for Normal CP,12 for Extended CP | 20 | 2 | Normal,Extended |
2 | 14 | 40 | 4 | Normal |
3 | 14 | 80 | 8 | Normal |
4 | 14 | 160 | 16 | Normal |
Resource Allocation in Practice: Downlink and Uplink
In both LTE and 5G NR, the gNB (base station) is responsible for scheduling resources for both downlink (gNB to UE) and uplink (UE to gNB) transmissions. This scheduling information is conveyed to the UEs through Downlink Control Information (DCI) carried on the Physical Downlink Control Channel (PDCCH).
The DCI specifies:
The allocated RBs (starting RB and number of RBs).
Modulation and coding scheme (MCS).
Other relevant parameters for the transmission.
Key Concepts for Frequency Domain Resource Allocation in 5G NR
5G NR introduces several new concepts to provide greater flexibility and control over frequency domain resource allocation, particularly in the context of initial access and synchronization.
Point A (Absolute Frequency Reference)
Point A is a critical reference point in 5G NR that defines the absolute frequency location of the Common Resource Block (CRB) grid. All other frequency-related parameters are defined relative to Point A.
Point A is the lowest subcarrier of CRB 0.
It serves as a common reference for all bandwidth parts (BWPs) and provides a consistent framework for frequency allocation across different numerologies. The gNB broadcasts the frequency location of Point A to the UE.
Offset to SSB (Synchronization Signal Block)
The Synchronization Signal Block (SSB) is a crucial component in 5G NR initial access. It contains the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH). The SSB is periodically transmitted by the gNB to enable UEs to synchronize and acquire essential system information.
offsetToSSB (K_SSB): This parameter, signaled in the Master Information Block (MIB) (specifically, in MIB-PBCH-Config-SIB1), indicates the offset in resource blocks between the lowest subcarrier of the SSB and the lowest subcarrier of CRB 0 (Point A).
Value: It is expressed in terms of the SCS of the SSB. For example, if offsetToSSB is 10 and the SSB SCS is 30 kHz, then the SSB starts 10 12 30 kHz = 3.6 MHz away from Point A.
Purpose: This offset allows the gNB to place the SSB at different frequency locations within the channel bandwidth, providing flexibility in deployment and enabling efficient multi-beam operations.
offsetToCarrier (Frequency Offset for Initial BWP)
The offsetToCarrier parameter, found in SIB1 (System Information Block 1), defines the offset in resource blocks between the lowest subcarrier of the initial Bandwidth Part (BWP) and Point A.
Value: It's typically expressed in terms of the initial BWP's SCS.
Purpose: The initial BWP is the default BWP that a UE uses after successful initial access. offsetToCarrier specifies where this initial BWP is located relative to the common frequency reference (Point A). This allows the gNB to define a smaller, optimized bandwidth for initial communication, saving power for the UE.
kSSB Parameter (within PDCCH-ConfigSIB1)
The kSSB parameter is part of the PDCCH-ConfigSIB1 in the MIB. It indicates the actual starting position of the SSB in terms of resource blocks relative to the overall carrier bandwidth. It's a more direct indicator of the SSB's position within the frequency domain that the UE will use for initial access.
Significance: kSSB helps the UE precisely locate the SSB and subsequently the associated control channels for initial access. This is especially important for flexible deployment of SSBs across the available bandwidth.
5G NR MIB:
sfn : 652
block_index : 0
half_number : 0
intra_freq_reselection : ALLOWED(0)
cell_barred : NOT_BARRED(1)
pdcch_config_sib1 : 96
controlResourceSetZero : 6
searchSpaceZero : 0
dmrs_typea_position : POS2(2)
ssb_subcarrier_offset : 10 -> This is called kSSB
msb_for_dssb : 0
subcarrier_spacing_common : SCS15(0)
Spare_for_padding : 37
5G NR SIB1:
BCCH_DL_SCH_Message
message
c1
systemInformationBlockType1
cellSelectionInfo
q_RxLevMin = -62
cellAccessRelatedInfo
plmn_IdentityInfoList[0]
plmn_IdentityList[0]
……….
………
………
downlinkConfigCommon
frequencyInfoDL
frequencyBandList[0]
offsetToPointA = 14
scs_SpecificCarrierList[0]
offsetToCarrier = 0
subcarrierSpacing = kHz15
carrierBandwidth = 106
initialDownlinkBWP
genericParameters
locationAndBandwidth = 28875
subcarrierSpacing = kHz15
…………….
…………….
…………….
uplinkConfigCommon
frequencyInfoUL
frequencyBandList[0]
absoluteFrequencyPointA = XXXXXX
scs_SpecificCarrierList[0]
offsetToCarrier = 0
subcarrierSpacing = kHz15
carrierBandwidth = 106
Example Scenario (Illustrative):
Point A: Let's say Point A is defined as 1900 MHz (the absolute frequency of CRB 0's lowest subcarrier).
SSB Configuration:
SSB SCS = 30 kHz
offsetToSSB (K_SSB) = 5 RBs (meaning 5 12 subcarriers 30 kHz/subcarrier=1.8 MHz offset from Point A)
So, the SSB starts at 1900 MHz+1.8 MHz=1901.8 MHz.
Initial BWP Configuration:
Initial BWP SCS = 15 kHz
offsetToCarrier = 10 RBs (meaning 10 12 subcarriers 15 kHz/subcarrier=1.8 MHz offset from Point A)
So, the initial BWP starts at 1900 MHz+1.8 MHz=1901.8 MHz.
This example shows how these parameters collectively define the frequency locations of crucial elements for initial access and subsequent communication.
Frequency Domain Resource Allocation Types in 5G NR
In 5G NR, frequency domain resource allocation primarily refers to how a UE is assigned a set of Resource Blocks (RBs) within a Bandwidth Part (BWP) for uplink or downlink transmission. The scheduling information, conveyed via Downlink Control Information (DCI) on the PDCCH, specifies these allocations.
5G NR defines several types of frequency domain resource allocation, which can be broadly categorized as:
Type 0 (Bitmap Allocation):
This type uses a bitmap to indicate the allocated Resource Block Groups (RBGs). An RBG is a set of contiguous PRBs. The size of an RBG depends on the carrier bandwidth and the BWP's configured SCS. For instance, for smaller bandwidths, an RBG might be 4 PRBs, while for larger bandwidths, it could be 16 PRBs. Each bit in the bitmap corresponds to an RBG; a '1' indicates the RBG is allocated, and a '0' indicates it's not.
Type 1 (Contiguous Allocation):
This type specifies the allocated resources by indicating a starting Physical Resource Block (PRB) and the number of contiguous PRBs from that starting point. This is similar to how allocation is typically done in LTE.
Type 2 (Frequency Hopping):
While not a "resource allocation type" in the same sense as Type 0/1 for a single transmission, 5G NR also supports frequency hopping. This is a mechanism where the allocated frequency resources can change over time within a transmission or across multiple transmissions to achieve frequency diversity. This can involve hopping within a slot or across slots.
References:
1. 5G Radio Access Network Architecture The Dark Side of 5G Edited by Sasha Sirotkin
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