Mobile Phone Technology

FDM - Frequency Division Multiplexing is the basic technology that underlines OFDM. The available bandwidth is split into sub-channels. In the case of 4G these channels are of size 15Khz. A single channel is used by a single client to send / receive their data. The problem is that if these channels are next to each other, and there is be crossover and interference between adjacent channels. In raw FDM there are guard channels between the individual channels to protect against this interference.

In order to reduce the size of these guard channels between the channels alternate channels are send orthogonal to each other. This can be thought of in a similar way to polarising light. If the light is vertically polarised, it can be seen if the polarising filter is aligned with the light and not seen if the filter is 90degrees to the light transmission. OFDM is a FDM technology where the adjacent sub-channels are transmitted, orthogonally (at 90degrees). In 4G the OFDM sub-channels are 15kHz wide.

In addition to splitting the channel into sub-channels, it is incumbent on the mobile operator to use the available space as efficiently as possible and in 4G a modulation scheme such as 64QAM will be used to send 6 bits, 64 values per wave.

OFDM is an efficient way to use the available bandwidth and is used in WiFi, 4G-LTE and 5G