Mobile Phone Technology

In OFDM, the spectrum is split into 15 kHz sub-channels orthogonally, which is why it is referred to as 'raw' frequency division multiplexing. This means that the entire sub-channel is allocated to a single user. However, this can c ause issues with efficiency and fairness. What if the sub-channel could be shared between different users in a single timeslot? This is OFDMA.

Time division multiplexing (or time division duplex) splits the available frequency space into time slices, with different users being allocated a timeslice to send their data before having to wait for the next timeslice.

OFDMA uses OFDM as the base technology and adds TDM, meaning the available space for sending data is divided by both frequency and time, as shown in the diagram. Using OFDM rather than simple FDM as the base frequency division technology enables more data to be sent. OFDMA is also a fairer and a more efficient technology for sending data. On top of OFDMA, a modulation scheme such as 64QAM will be used.

In LTE+, the sub-channels are 15 kHz wide and are allocated in 12 sub-channel resource blocks, with a timeslice of 1 ms.

In 5G, different sub-channel sizes are possible; these sizes and the related channel sizes are determined by the numerology parameter µ, as shown in the table. Note that only the first two lines (µ = 0 or 1) can be used with FR1 frequencies (sub-6 GHz), whereas mmWave can potentially use all of these combinations. The sub-channels are also 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 WiFi6 as well as LTE+

deepl.com [AI] has been used to improve the text in this description