FDM in LTE
Conventional multi-carrier operation as it is used for FDM works simply by selecting a number of center frequencies – one for each carrier to be used.
The center frequencies must be spaced. In fact there is a trade-off between minimizing interference between different carriers and using the total bandwidth efficiently. In other words fdm in lte having each carrier uses an upper and lower guard band to protect itself from its adjacent carriers.
Nevertheless, there will always be some interference between the adjacent carriers – known as Adjacent Carrier Interference (ACI) Especially for rectangular pulses the guard bands must be quite big in fdm lte, as the rectangular pulse has a huge spectrum. Otherwise we would have to apply a pulse shaping filter in fdm in lte, but this would destroy the rectangular form of our pulse and thus complicate inter-symbol interference handling. For the rectangular pulse there is a better option possible and it is even easier to implement.
OFDM in LTE
The spectrum of a rectangular pulses shows null points exactly at integer multiples of the frequency given by the symbol duration. Orthogonally (OFDM) avoids ACI to some extent. Thus OFDM in LTE simply places the next carrier exactly in the first null point of the previous one. With this we don’t need any pulse-shaping and between OFDM carriers using the same symbol duration TS and the same grid of center frequencies no guard bands are required.
This allows a tight packing of small carrier -called the sub-carriers or tones- into a bigger frequency band for OFDM in LTE. Of course at the edges of this bigger band there might be some guard bands required to protect systems on adjacent bands from out-of spectrum emissions by the OFDM system.
Difference Between OFDM and FDM in LTE
OFDM (Orthogonal Frequency Division Multiplexing) and FDM (Frequency Division Multiplexing) are both techniques used to transmit data over multiple frequencies, but they differ in how the available spectrum is utilized and the efficiency of data transmission.
FDM divides the available spectrum into separate, non-overlapping frequency bands, each used for a different signal. Each signal occupies its own band, which can lead to inefficiencies due to guard bands (unused frequencies between signals) to avoid interference. FDM is simpler but less efficient in utilizing spectrum.
OFDM, used in LTE, divides the spectrum into many smaller, closely spaced subcarriers that are orthogonal to each other. This orthogonality minimizes interference between subcarriers, allowing for a more efficient use of the available spectrum. Additionally, OFDM can handle high data rates and mitigate issues like multi-path interference, making it ideal for high-speed wireless communication like LTE.
In summary, OFDM offers better spectrum efficiency and performance compared to traditional FDM, especially in environments with high data demand and interference, such as those encountered in LTE networks.