Do you still remember that picture? It was the description of one PRB
(Physical Resource Block) in 4G LTE. One PRB contains 12 subcarriers
and 7 symbols.
Are we still use that modulation in 5G NR? Based on 3GPP TS38.802
OFDMA is still being used for 5G NR modulation, then the concept of one
PRB also not changed. We might see the new approach of transmitting the
Radio signal in Higher Frequency. 5G NR will be operating in two main
frequency:
- sub-6 GHz for operating frequency below 6 GHz
- mmWave for operating frequency above 6 GHz
In higher frequency, it's hard to guarantee there is no Inter Carrier
Interference (ICI) if we still using default Sub Carrier Spacing (SCS)
with the length 15 kHz between subcarrier. In order to mitigate this
problem, there are several SCS were introduced in 5G NR Release 14.
f(n) = 15 kHz * 2^n ; where (
n is non-negative integer),
One Numerology corresponds to one subcarrier spacing in frequency domain
Maximum channel bandwidth per NR carrier is 400 Mhz in Rel 15. All
details for channel bandwidth up to 100 Mhz per NR Carrier are to be
specified in Rel 15.
At least for single numerology case, candidates of the maximum
numbers of the subcarrier are 3300 or 6600 in Rel 15. Subframe Duration
is fixed to be 1 ms. One TTI duration corresponds to a number of
consecutive symbols for one transmission in the time domain (TR 38.804
5.4.7). The combination of one numerology and one TTI duration
determines how the transmission is made on the physical layer. Frame
Length is fixed to be 10 ms. Scalable numerology should allow the
subcarrier spacing from 15 Khz to 480 Khz. The number of subcarrier per
PRB is 12. The number of OFDM symbols per slot may vary depending on
subcarrier spacing. Slot Aggregation (data transmission scheduling to
span one or multiple slots. All numerologies align on symbol boundaries
every 1 ms in NR carrier (regardless of subcarrier spacing
and CP-overhead).No explicit DC subcarrier is reserved for both Downlink
and Uplink.
5G NR should be able to use more than one numerologies in one NR work frequency
A slot is defined as 7 or 14 OFDM symbols for the same subcarrier
spacing of up to 60kHz with normal CP and as 14 OFDM symbols for the
same subcarrier spacing higher than 60kHz with normal CP. A slot can
contain all downlink, all uplink, or {at least one downlink part and at
least one uplink part}. Slot aggregation is supported, i.e., data
transmission can be scheduled to span one or multiple slots.
Mini-slots having the following lengths are defined. At least above 6
GHz, mini-slot with length 1 symbol supported. Lengths from 2 to slot
length -1. For URLLC, at least 2 is supported
From 3GPP RAN Meeting, there are several discussion from each vendor
about supports SCS and maximum BW for one NR system in two domains
operating frequency which is not fixed in the current study [Rel.14].
For sub 6GHz, 15k, 30k, and 60k are included in all companies
proposals. For above 6GHz (mmWave), 60k, 120k and 240k are included in
most companies proposals. Wider SCS will bring us more symbols can be
transmitted over one TTI duration so higher data rate can be achieved
with this approach.
Source :
Ray KHASTUR
LTE Senior RNO Consultant at P.I.Works, Inc.
