# LTE OFDMA and Downlink Frame Structure Details

- Downlink OFDMA time-frequency multiplexing
- LTE Spectrum Flexibility and Bandwidth
- FDD downlink frame structure detailed
- TDD frame structure

LTE (any OFDM/OFDMA) band is made up of multiple small spaced channels and we call each of these small channels as “Sub Carrier”. Space between the chhanel and the next channel is always same regardless of the system bandwidth of the LTE band. So if the system bandwidth of LTE channel changes, number of the channels (sub carriers) changes but the space between channels does not change.

Q> What is the space between a subcarrier and the next sub carrier ? A> 15 Khz

Q> What is the number of channels(sub carriers) for 20 Mhz LTE band ? A> 1200 sub carriers.

Q> What is the number of channels(sub carriers) for 10 Mhz LTE band ? A> 600 sub carriers.

Q> What is the number of channels(sub carriers) for 5 Mhz LTE band ? A> 300 sub carriers.

Got any feelings about sub carriers and it’s relation to system bandwidth ?

Now let’s look at the basic units of horizontal axis which is time domain. The minimum unit of the time domain is a Symbol, which amounts to 66.7 us. Regardless of bandwidth, the symbol length does not changes.In case of time domain, we have a couple of other structures as well. The largest unit in time domain is a frame, each of which is 10 ms in length. Each of the frame consists of 10 sub frames, each of which is 1 ms in length. Each of sub frame consists of 2 slots, each of which is 0.5 ms in length.Each of slots consists of 7 symbols, each of which is 66.7 us.With this in mind, let’s think about the scale in reverse direction.

Q> How many symbols are there in a slot ? A> 7 symbols.

Q> How many symbols in a sub frame ? A> 14 symbols.

Q> How many slots are there in a frame ? A> 20 slots.

Now let’s look at the units which is made up of both time domain (horizontal axis) and frequency domain (vertical axis). Let’s call this type of unit a two-dimensional unit.

The minimum two dimensional unit is resource element which is made up of one symbol in time domain and one sub carrier in frequency domain. Another two dimensional unit is resource block(RB) which is made up of one slot in time domain and 12 sub-carrier in frequency domain. Resource Block(RB) is the most important units in LTE both for protocol side and RF measurement side.