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 | n | n | n/e | n | n | n | n | +--------------------------+------+-----+-----+-----+------+-------+--------+ - 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 (
n) cyclic prefix. - Only 2μ supports an extended (
e) 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
- 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/LTE, 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/NR defines a resource block only in the frequency domain (12 consecutive subcarriers); the time domain is left unspecified!
In my experience, some people still consider the resource block to have a duration of one slot in 5G. The key difference is that the duration of a slot varies in 5G depending upon factors such as the SCS (subcarrier spacing) and number of OFDM symbols (normal or extended), whereas in 4G the duration of a slot was fixed. However, I do not think this is correct.
From my research online, 5G “resource grid” diagrams (such as the one above) vary: some illustrate a resource block as having a duration of one slot, while others show one symbol!
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.
Slot vs Symbol Definition of Resource Block §
- The choice of slot makes most sense in my opinion, since that matches the pattern from 4G and serves to render a “block” rather than a “line”.
- The choice of symbol seems a little confusing to me, but perhaps such diagrams are attempting to focus on illustrating the “span” in the frequency domain?
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).
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.