How spatial separation can be used to increase the data rate of Li-Fi

Imagine a cocktail party, there are multiple friends and couples who want to talk to each other in a small single room. If they all talk at the same time, the room will be too noisy and no one can hear each other.

How to solve this problem?

Solution 1:  a speaker talks to a listener in turn. The speaker talks for a short while and then stops to let another couple talk. No more than one speaker talks in the room, and so no one has to worry about two conversations mixing. However, the drawback of this solution is obvious that if there are more and more couples in the room, everyone has to wait a long time before they can talk to each other.

Solution 2: speakers are allowed to talk at any time, as each uses a different language. Each listener can only understand the language of their partner. As more and more couples join in the party and talk, the background noise becomes louder, but because of the different languages, conversations do not mix. However, if more and more people join the party, at some point, the listener cannot make out what their partner is talking about without coming closer to them.

Solution 3: speakers are allowed to talk at any time, while a long pipe connects between each speaker and his/her corresponding listener. As long as the leakage of the pipe is considerably small, they can always talk and the conversations do not mix. If more people join in the party, the only thing needed is to increase the number of pipes.

In mobile communication, the requirement of serving multiple users at the same time is a very similar concept to the cocktail party scenario. Solutions 1, 2, 3 correspond to three multiple access schemes: Time-division Multiple Access (TDMA), Code-division Multiple Access (CDMA), and Space-division Multiple Access (SDMA). In SDMA, multiple parallel pipes are created to transmit the signal according to user positions. This makes SDMA superior to other multiple access schemes.

In Radio Frequency, directional narrow-beam signals are generated by changing the amplitude and phase of the signals on an antenna array. However, this beam-forming procedure has a significant computational complexity. Also, the leakage of the spatial pipes is a big issue.

As a complementary alternative to current Radio Frequency techniques, Li-Fi is very suitable for the implementation of SDMA. In a Li-Fi system, LEDs are used as transmitters. LEDs have an inherent feature of a limited field-of-view (FOV). This feature is perfect for generating directional light beams. Therefore, in order to realise optical SDMA, an angle diversity transmitter which consists of multiple directional narrow FOV LED elements is used as the optical transmitter. By turning on different transmitter elements, the angle diversity transmitter can generate narrow light beams of different directions.

Optical SDMA is especially useful in scenarios where the requirement for data transmission is extensive – a boardroom, coffee shop or office would be potential locations for implementing optical SDMA.

Zhe Chen

Li-Fi Postgraduate Researcher