Mobile Phone Technology

OFDM is the 'raw' frequency division multiplexing in that it splits splits the spectrum into 15kHz sub-channels, orthogonally. In this form it means that the whole of the sub-channel is allocated to a single user. This can cause issues with efficiency and fairness. What if the sub-channel could be shared between different users over a single timeslots. This is OFDMA.

Time Division Multiplexing (or Time Division Duplex) will split the available frequency space into time-slices, and different users will be allocated a timeslice to send their data, then have to wait their turn for the next timeslice

OFDMA uses OFDM as the base technology and add in TDM meaning the available space to send the data is split by both frequency and time, as shown in the diagram. By using OFDM rather than simple FDM as the base frequency division technology more data cn be sent. Also OFDMA is a fairer and efficient technology to send data. On top of OFDMA will be a modulation scheme such as 64QAM.

In LTE+ the sub-channels are 15kHz wide and allocated in 12 sub-channel resource blocks.The time-slice is 1ms.

In 5G there can be different sub-channel sizes, these sizes and related channel size are determined by the numerology parameter (µ), as shown in the table. Not that only the first two lines (µ = 0 or 1) can be used with FR1 frequencies (sub 6GHz), whereas mmWaves can potentially use all of these combinations. Also the sub-channels are not allocated individually, but in resource blocks of 12 sub-channels

µ	Sub-channel size	Resource block (12 sub-carriers)	FR1 or FR2
0	15kHz	180kHz	FR1	1ms
1	30kHz	360kHz	FR1	0.5ms
2	60kHz	720kHz	FR2	0.25ms
3	120kHz	1440kHz	FR2	0.125ms
4	240kHz	2880kHz	FR2	0.0625ms

In 5G the data is sent within the allocated time slot as 64QAM.

Note OFDMA is used in WiFGi6 as well as LTE+ / 5G