LTE and OFDM

In this post I want to explain what the goal of LTE is, and how frequency utilisation helps to achieve that.

We as consumers want to be able to use our mobile phones as multimedia devices. Listening to music and watching videos on the go, using the mobile network. We ever want it better and faster, not having to wait for downloads. More and more people are getting used to high speed internet access at home, and expect the mobile network to deliver the same services. This means we need to improve the network to handle all that traffic at the speed we want.

One of the main goals of the LTE standard is increasing data throughput rates. LTE networks should bring the worlds of very high speed data access at home and that of high mobility cellular networks closer together.

To achieve this, spectrum will be utilised more efficiently. Key instrument is the modulation technique that is used:  Orthogonal Frequency Division Multiplexing (OFDM). In an attempt to explain what modulation is, I’ll describe it as a method for ‘putting information on the radio waves’. The radio waves can be sent and received using antennas, the modulation and demodulation steps make sure the information is put on the waves, and picked off again at the receiving end.

There are various ways to implement modulation, each with their advantages and disadvantages. Some pose high requirements at the transmitter or receiver, others are extremely difficult to intercept or decode if the interceptor does not have the required key.

Main advantages of OFDM are that it allows for relatively simple transmitters and receivers, and that it can handle bad channel conditions rather well. Interference in the radio spectrum and interference of the signal on itself are dealt with without having to use complex filters.

Disadvantage is that the system is sensitive to frequency synchronisation problems between transmitter and receiver. Doppler shift also raises problems, making OFDM perform worse when users are travelling at high speeds (by car or train for example).

At the end of this post a bit more on the frequency part. One thing that makes the OFDM method a very useful and efficient method is the ‘orthogonality’ part. The modulation puts the information on the radio wave. Orthogonality means that the various ‘waves’ used (real terminology is sub-carriers) will not affect each other. The sub-carriers are chosen in such a way that the frequency bands don’t cause cross-talk. Compare this with multiple persons in a room all talking at the same time; this cross-talk makes it more difficult to understand the one person you are talking with. If all the couples would talk in different languages, is would be easier for them to understand each other. This can be seen as a form of orthogonality, the conversations would not interfere. In ODM this is called ‘Inter-Carrier-Interference’ (ICI), and is at the same time one of the weaknesses of the system. Sub-carriers must be synchronised very accurate to keep this orthogonality principle working. When the frequencies change a bit, the communication channels start talking the same language and will influence each other, degrading performance of the system. 

OFDM sub-carriers do not interfere with eachother. Source: link

The picture above shows that the various carriers (A-E) are 'active' or 'talking' when the others are not, so they do not interfere with each other. Note, this picture is in the frequency domain. As long as the frequencies of the sub-carriers are chosen exactly right, they seem to be invisible to each other.

Given a good synchronised, orthogonal set of sub-carriers, this brings the advantage of multiple channels to send the information. Suppose you have 100 times as much channels to use, you could send each stream of information parallel, 100 times slower per stream, and still send the same amount of information. This then opens up the opportunity to simplify the receiver since working slower is easier.

References:

LTE Encyclopedia