In FDM, the sub-carriers are separated in the frequency domain to avoid interference between the sub-channels
It results in a loss of spectrum efficiency because the frequency guard band can not be used to send data.The OFDM allows one to remove the frequency guard band.
Benefit: There are more sub-carriers, so more symbols are sent at the same time. The orthogonality brings a better spectrum efficiency. In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitterand the receiver; unlike conventional FDM a separate filter for each sub-channel is not required.
The orthogonality requires that the sub-carrier spacing is Vf = k/(TU) Hertz, where TU seconds is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with N sub-carriers, the total passband bandwidth will be B ? N(delta)Vf (Hz). That leads to the representation of a sub-carrier.
The duration of the symbol depends on the width of the sub-carrier.
- It is inversely proportional. The shorter the symbol, the wider the sub-carrier and vice-versa.
- The frequency center of the sub-carrier is linked to the frequency of the carrier.
- The inter-channel (or inter sub-carrier) interferences are cancelled because they are located in a such way that when there is the peak for a given sub-carrier, the adjacent subcarriers are null.
OFDM allows high density of carriers, without generating Inter-Channel Interference (ICI). BASIC IDEA : The channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.A single wideband signal is transformed into multiple narrow band signals transmitted on orthogonal subcarriers
- One single stream at high rate
- Each symbol occupies the whole bandwidth
- Very short symbol duration to ensure high rate
Notion of Orthogonality in LTE
Suppose you’re trying to understand why LTE can handle so many users efficiently. I’d suggest focusing on the concept of orthogonality. In LTE, orthogonality means that the subcarriers in the frequency domain are spaced in such a way that they don’t interfere with each other. Each subcarrier carries its own signal, but because they are orthogonal, they won’t overlap or cause interference, even when packed closely together.
This is key for LTE’s use of OFDMA. I think of it like this: imagine multiple radio stations broadcasting on the same frequency range, but each one is perfectly spaced so they don’t step on each other’s toes. Because of orthogonality, LTE can make the most efficient use of the available spectrum, maximizing throughput while minimizing interference.
In practical terms, orthogonality allows LTE to handle more users and higher data rates. It’s a big reason why LTE can provide reliable and fast performance, even when the network is crowded.