5G: Frames, Subframes, Slots, Minislots, Resource Blocks & Resource Elements
Overview §
Subcarriers §
In order to understand frames (and their component parts), it is helpful to first have a refresher on subcarriers:
- A carrier is formed of multiple smaller subcarriers:
- The subcarrier: the building blocks of a carrier; a smaller frequency channel within the larger carrier.
- The subcarrier spacing (or SCS): the frequency separation between adjacent carriers.
- e.g. 15kHz, 30kHz, 60kHz, 120kHz.
- The frequency band: defines a duplex mode (TDD/FDD), one frequency range (if TDD, or two if FDD), and a range of supported channel bandwidths.
- For example, band n78 (very common in 5G):
- The uplink/downlink frequency is 3300MHz-3800MHz (n78 is TDD, so uplink and downlink share the same frequency range).
- This provides a total available bandwidth of 500MHz…
- …however, of this 500MHz, the supported channel bandwidths are 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100MHz.
- For example, band n78 (very common in 5G):
- The channel bandwidth: the total frequency range occupied by a carrier.
- To calculate the approximate number of subcarriers (ignoring guard bands):
num_subcarriers = bandwidth / subcarrier_spacing- e.g. For a 5MHz bandwidth with 15kHz SCS:
5MHz / 15kHz = 5,000,000 / 15,000 = ~333 subcarriers - Multiple subcarriers are sent/received in parallel using OFDM.
- e.g. For a 5MHz bandwidth with 15kHz SCS:
Frames (and their component parts) §
Now we can dig into frames, their component parts (subframes, slots, resource blocks, resource elements) and how they relate to subcarriers:
- A frame is always 10ms.
- A frame always consists of 10 subframes at 1ms each.
- A subframe consists of between 1 and 64 slots of between 1000μs (1ms) and 15.625μs each.
- The exact value for number of slots (
num_slots) and slot duration (slot_duration) depends upon the numerology (numerology), as do the available options for Cyclic Prefix (cyclic_prefix).+--------------------------+------+-----+-----+-----+------+-------+--------+ | numerology (μ) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | +--------------------------+------+-----+-----+-----+------+-------+--------+ | num_slots | 1 | 2 | 4 | 8 | 16 | 32 | 64 | | subcarrier_spacing (kHz) | 15 | 30 | 60 | 120 | 240 | 480 | 960 | | slot_duration (μs) | 1000 | 500 | 250 | 125 | 62.5 | 31.25 | 15.625 | | cyclic_prefix (normal) | y | y | y | y | y | y | y | | cyclic_prefix (extended) | | | y | | | | | +--------------------------+------+-----+-----+-----+------+-------+--------+ - Bigger numerology = more, shorter slots = narrower Subcarrier Spacing (SCS), e.g.:
- 0μ = 1 × 1000μs slot @ 15kHz (widest SCS).
- 6μ = 64 × 15.625μs slots @ 960kHz (narrowest SCS).
- All numerologies support a normal cyclic prefix.
- Only 2μ (60kHz) supports an extended cyclic prefix.
- The exact value for number of slots (
- A slot consists of 12 or 14 OFDM symbols, depending upon Cyclix Prefix (CP).
- Cyclic Prefix vs num. OFDM symbols:
- Normal Cyclic Prefix = 14 OFDM symbols
- Extended Cyclic Prefix = 12 OFDM symbols
- Cyclic Prefix vs num. OFDM symbols:
- A resource block is 12 consecutive subcarriers.
- It is defined only in the frequency domain; the time is undefined.
- A resource element is one OFDM symbol and one subcarrier.
Frame structure §
- Communication between the UE and gNodeB uses frames that are 10ms long.
- Each frame is divided into ten subframes that are each 1ms long.
10ms
<--------->
+---------+---------+---------+---------+
| Frame 0 | Frame 1 | Frame 2 | Frame 3 |
+---------+---------+---------+---------+
. ' - .
. ' - .
. ' - .
. 1ms ' - .
. <------------> ' - .
+------------+------------+------------+ +------------+
| Subframe 0 | Subframe 1 | Subframe 2 | ... | Subframe 9 |
+------------+------------+------------+ +------------+
Time
------>Slot structure §
- Each subframe is split up into a number of slots, depending upon the SCS (Subcarrier Spacing).
- 15kHz = 1 slot
- 30kHz = 2 slots
- 60kHz = 4 slots
- 120kHz = 8 slots
- All slots within a subframe are not necessarily the same size!
- For timing alignment purposes, some symbols have a longer cyclic prefix than others, which impacts the slot duration.
- Each slot contains:
- 14 OFDM symbols (if using normal cyclic prefix).
- 12 OFDM symbols (if using extended cyclic prefix).
- Each symbol is uplink, downlink or a guard period.
- Comparison of 5G to 4G:
- In 4G, the subframe was the basic unit of transmission, carrying either uplink or downlink data for the whole subframe.
- In 5G, the subframe is further subdivided into slots.
Subframe (1ms)
<--------------------------------------------------------------->
1000μs (1ms)
<--------------------------------------------------------------->
+-------+-------+-------+-------+-------+-------+-------+-------+
15kHz | Slot |
| 14 symbols |
+-------+-------+-------+-------+-------+-------+-------+-------+
500μs
<------------------------------->
+-------+-------+-------+-------+-------+-------+-------+-------+
30kHz | Slot | |
| 14 symbols | |
+-------+-------+-------+-------+-------+-------+-------+-------+
250μs
<--------------->
+-------+-------+-------+-------+-------+-------+-------+-------+
60kHz | Slot | | | |
| 14 symbols | | | |
+-------+-------+-------+-------+-------+-------+-------+-------+
125μs
<------->
+-------+-------+-------+-------+-------+-------+-------+-------+
120kHz | Slot | | | | | | | |
| 14 s. | | | | | | | |
+-------+-------+-------+-------+-------+-------+-------+-------+
Time
------> Subframe (1ms)
<--------------------------------------------------------------->
1000μs (1ms)
<--------------------------------------------------------------->
+-------+-------+-------+-------+-------+-------+-------+-------+
15kHz | Slot |
| 14 symbols |
+-------+-------+-------+-------+-------+-------+-------+-------+
500μs
<------------------------------->
+-------+-------+-------+-------+
30kHz | Slot |
| 14 symbols |
+-------+-------+-------+-------+
250μs
<--------------->
+-------+-------+
60kHz | Slot |
| 14 symbols |
+-------+-------+
125μs
<------->
+-------+
120kHz | Slot |
| 14 s. |
+-------+
Resource grid §
- The slots are carried on the subcarriers using resource blocks.
- A resource block is defined as 12 consecutive subcarriers in the frequency domain.
- It is not defined in the time domain.
- A resource element is the smallest unit of transmission (i.e. the smallest unit that can be used for scheduling purposes).
- It is defined as one subcarrier (in the frequency domain) and one OFDM symbol (in the time domain).
This can be illustrated with a resource grid.
1 slot
(14 symbols)
<------------------------------->
/ \
/ \
/ \
/ \
/ \
/ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . .}
| | | | | | | | | | | | | | | } 1 subcarrier
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . .}
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| . . |
| . . |
| . . |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . . . . . }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
^ | | | | | | | | | | | | | | | } 1 resource block
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
freq. | | | | | | | | | | | | | | | | } (12 subcarriers)
| +--+--+--+--+--+--+--+--+--+--+--+--@@@@--+ }
| | | | | | | | | | | | @ @<---- resource }
+--+--+--+--+--+--+--+--+--+--+--+--@@@@--+ element }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ }
| | | | | | | | | | | | | | | }
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . . . . . }
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| . . |
| . . |
| . . |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Time
------> 1 slot
(14 symbols)
<------------------------------->
/ \
/ \
/ \
/ \
/ \
/ \
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . .}
} 1 subcarrier
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . .}
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . . . . . }
}
}
}
}
}
}
}
}
}
}
} 1 resource block
}
} (12 subcarriers)
}
}
}
}
}
}
}
}
}
}
$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$~~$ . . . . . . }
@@@@
@ @<- -- resource
@@@@ element
In 4G, a resource block was defined as having a duration of one slot (time domain) and a span of 12 consecutive subcarriers (frequency domain). In contrast, 5G defines a resource block only in the frequency domain (12 consecutive subcarriers); the time domain is left unspecified!
In my experience, some people (incorrectly) consider a 5G resource block to have a duration of one slot. While the slot duration is fixed in 4G, in 5G it varies depending upon factors such as the SCS (subcarrier spacing) and number of OFDM symbols (normal or extended).
I have found some variation in how 5G “resource grid” diagrams (such as the one above) illustrate a resource block: some show it with a duration of one slot, while others show one symbol! Not many make it clear that the time domain is actually unspecified.
One Slot §
The MathWorks website shows a “Resource Block” diagram with 14 OFDM symbols along the x-axis (i.e. one slot) and 12 subcarriers along the y-axis (i.e. one resource block, using the strict frequency domain only definition).
One Symbol §
The ShareTechnote website uses the symbol definition in its “resource grid” diagram, which shows the resource block as having a duration of one resource element (i.e. one symbol). However, the text below the diagram agrees that the definition of a resource block is ambiguous:
Time domain definition of resource block is a little bit ambiguous. Minimum time domain length in a resource block can be one OFDM symbol, but exact time domain length vary depending SLIV.
Usage of frequency domain only definition §
In contrast to the apparent slot duration interpretation by MathWorks, their website also uses a frequency domain only interpretation: this paper describes the SSB (SS Block) specifying the size in the frequency domain in terms of resource blocks, while the time domain is four symbols.
A single SS block spans four OFDM symbols in time and 240 subcarriers in frequency (20 resource blocks).
5G NR in Bullets §
The book 5G NR in Bullets clearly states:
5G Resource Blocks are only defined in the frequency domain. This means that a 5G resource allocation must specify both the number of Resource Blocks and the number of symbols. A 5G Resource Block always occupies 12 consecutive subcarriers in the frequency domain. The bandwidth occupied by a Resource Block depends upon the subcarrier spacing.
Official Definition §
For reference, the official 5G definition of a resource block from 3GPP 38.211 says “A resource block is defined as […] 12 consecutive subcarriers in the frequency domain”.
Conclusion §
Though it is tempting to apply a time domain aspect to the definition of a resource block, it is not strictly correct. While the term “block” evokes the sense of an object which has two or three dimensions, in the context of 5G a resource block has only one dimension: frequency.
Duplex mode (FDD/TDD) §
A frequency band has a duplex mode of either FDD or TDD:
- FDD (Frequency Division Duplex)
- Different frequency used for uplink and downlink.
- TDD (Time Divison Duplex)
- Same frequency used for uplink and downlink.
- Uplink and downlink are allocated time slots.
Supplementary links §
Optionally, a supplementary downlink/uplink may be used:
- SDL (Supplementary Downlink)
- An additional frequency band is assigned for the downlink.
- SUL (Supplementary Uplink)
- An additional frequency band is assigned for the uplink.
- Especially useful at the cell edges because there is more interference from neighbouring cells, and the transmission power of the UE is limited.
- SUL typically uses a lower frequency, because lower frequencies are attenuated less during propagation, so they can travel further.
Slot formats §
The slot format determines whether each symbol in a slot is uplink, downlink or a guard period.
- SFI (Slot Format Indicator)
- Defines allocation of each symbol in a slot
- U = Uplink only.
- D = Downlink only.
- D/U = Flexible (uplink, downlink or guard period).
- Defines allocation of each symbol in a slot
- The gNodeB specifies the SFI to the UE.
As an example, following is an excerpt of the SFI table:
+--------+-----------------------------------------------------------------------------------+
| Slot | Symbol Number in a Slot |
| Format +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Ind. | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 0 | D | D | D | D | D | D | D | D | D | D | D | D | D | D |
| 1 | U | U | U | U | U | U | U | U | U | U | U | U | U | U |
| 2 | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U |
| 3 | D | D | D | D | D | D | D | D | D | D | D | D | D | D/U |
| 4 | D | D | D | D | D | D | D | D | D | D | D | D | D/U | D/U |
| 5 | D | D | D | D | D | D | D | D | D | D | D | D/U | D/U | D/U |
| 6 | D | D | D | D | D | D | D | D | D | D | D/U | D/U | D/U | D/U |
| 7 | D | D | D | D | D | D | D | D | D | D/U | D/U | D/U | D/U | D/U |
| 8 | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | U |
| 9 | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D/U | U | U |
| 10 | D/U | U | U | U | U | U | U | U | U | U | U | U | U | U |
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
| 52 | D | D/U | D/U | D/U | D/U | D/U | U | D | D/U | D/U | D/U | D/U | D/U | U |
| 53 | D | D | D/U | D/U | D/U | D/U | U | D | D | D/U | D/U | D/U | D/U | U |
| 54 | D/U | D/U | D/U | D/U | D/U | D/U | D/U | D | D | D | D | D | D | D |
| 55 | D | D | D/U | D/U | D/U | U | U | U | D | D | D | D | D | D |
+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+Slot types §
Self-contained slots §
5G introduces the concept of a self-contained slot. This was not present in 4G/LTE.
- A self-contained slot is a slot that contains uplink, downlink and guard periods.
- This enables doing acknowledgements within a single slot.
- Useful for URLLC (Ultra-Reliable and Low-Latency Communication).
- The term self-contained slot is not officially recognised by 3GPP, but is commonly used in the industry.
Minislot §
5G introduces the concept of minislots (or mini-slots). This was not present in 4G/LTE.
- A minislot contains 2, 4 or 7 OFDM symbols.
- Essentially, a fraction of a slot.
- This avoids the need for an application to wait until the slot boundary; instead it can begin at any symbol boundary.
- This enables a quick delivery of low latency payloads, even if a small SCS (Subcarrier Spacing) is used.
Modulation §
Modulation determines how many bits can be carried on a single resource element:
- QPSK = 2 bits/symbol
- 16QAM = 4 bits/symbol
- 64QAM = 6 bits/symbol
- 256QAM = 8 bits/symbol
Adaptive Modulation and Coding in 5G §
- Under favourable wireless channel conditions:
- 5G can use up to 256QAM (8 bits/symbol).
- Fewer error correction bits required.
- In bad channel conditions (e.g. further away or in shadow):
- Lower modulation scheme will be used.
- More error correction bits required (i.e. stronger coding schemes).
Deployment considerations §
SCS §
- Large SCS is less sensitive to errors (e.g. 15kHz, 30kHz).
- Small SCS is more efficient (e.g. 60kHz, 120kHz).
SCS vs cell range and latency §
- Smaller SCS (e.g. 15kHz) = longer symbol duration = larger cell range.
- Larger SCS (e.g. 120kHz) = shorter symbol duration = lower latency.
TDD vs FDD §
- FDD requires a separate DL frequency band.
- UE needs to make measurements in the DL band and send them to the gNB.
- gNB uses these measurements for channel quality estimation, and adjusts its transmissions.
- TDD has channel reciprocity.
- The channel conditions are the same in the UL and DL because they use the same frequency.
- The gNB makes the measurements in the UL direction and then estimates for the DL.
- 5G is mostly deployed in the TDD configuration.
- FR1 supports FDD and TDD, but FR2 only supports TDD.