Before we understand 5G frame structure, let us look at the below diagram:
From the above image we can see that, each frame will have number of sub frames.
A sub frame can have number of slots.
Each slots can have a number of symbols (7 symbol for normal CP/ 6 for extended CP for LTE and 14 OFDM symbols in 5G).
5G “frame” has a duration of 10ms, that consist of 10 “sub frames” having 1 ms duration, similar to that of 4G/LTE.
In LTE there was fixed sub carrier spacing of 15KHz, but in 5G it is dependent on numerology defined.
The 1 ms sub-frame is then divided into one or more 5G slots. The slots will depend upon the Numerology (μ) used (sub carrier spacing).
In 5G numerology will refer to sub-carrier spacing. In 4G it was 15KHz, but in 5G it depends on the formula “2^μ * 15KHz”
So if μ = 0, then 2^0 * 15 = SCS using will be = 15KHz So if μ = 1, then 2^1 * 15 = SCS using will be = 30KHz So if μ = 2, then 2^2 * 15 = SCS using will be = 60KHz So if μ = 3, then 2^3 * 15 = SCS using will be = 120KHz So if μ = 4, then 2^4 * 15 = SCS using will be = 240KHz
Note:
Irrespective of numerology, each slot will 14 OFDM symbols.
The above sub carrier spacing can be represented as below table:
Calculate the number of sub carriers given below parameters:
Example 1:
If Bandwidth is 50MHz
SCS = 30KHz
Guard Band = 692.5 KHz. So it means, we need to remove from both sides. So we need to remove 692.5 KHz twice.
Effective Bandwidth = 500000Khz – 2(692.5)KHz = 48.615 MHz.
Number of Sub Carriers = Now divide by 30 KHz = 46.615MHz/30KHz = 1620 SC.
Example 2:
If Bandwidth is 50MHz
SCS = 15KHz
Guard Band = 692.5 KHz. So it means, we need to remove from both sides. So we need to remove 692.5 KHz twice.
Effective Bandwidth = 500000Khz – 2(692.5)KHz = 48.615 MHz.
Number of Sub Carriers = Now divide by 15 KHz = 46.615MHz/15KHz = 3241 SC.
Resource Block:
In LTE one resource block is 12 sub carriers * SBS ie 12 *15 = 180KHz.
In 5G also, one resource block will have 12 sub carriers, but will vary according to SBS. i.e 1 RB = 12 * SCS.
Below table gives a brief of resource block bandwidth WRT SCS.
Now let us see how sub-frame will change according to numerology:
Read more at TS 38.101
Below table shows what is the Min and Max channel bandwidth required.
Calculate the number of resource blocks given below parameters:
If Bandwidth is 100MHz
SCS = 60KHz
Guard Band = 1370 KHz. So it means, we need to remove from both sides. So we need to remove 692.5 KHz twice.
Effective Bandwidth = 100000Khz – 2(1370)KHz = 97.26 MHz.
Number of Sub Carriers = Now divide by 60 KHz = 97.26MHz/60KHz = 1621 SC.
Number of Resource Blocks = 1621/12 = 135
5G Frame structure in Time domain:
1 Frame is 10 ms
1 subframe is 1 ms
1 slot is 0.5 ms (depends on numerology)
Each slot will have 14 OFDM symbols in case of Normal CP, 12 OFDM symbols in case of Extended CP.
Symbol Duration will also change depending on Numerology.