Question

I would like to ask the question about modulation method.

In wireless communication using radio wave, the various modulation methods are adopted for example AM（Amplitude Modulation）,FM（Frequency Modulation）and QPSK（Quadrature Phase Shift Keying）. I think that the Baseband Digital Modulation which is one of Amplitude Modulation and is also used in optical fiber communication, is the main method in infrared space communication. Are the special Amplitude Modulation for example Pulse Position Modulation and the Analogue Modulation tried for the improvement of throughput?

In radio wave LAN the high speed communication is realized by contriving the modulation method. I am interested to know the trial in infrared LAN.

Answer

In radio wave media including radio wave LAN the various modulation methods are adopted for the effective application of frequency band. In infrared communication the modulation method is adopted based on another reason. Theoretically the same methods as radio wave can be realized, but the following method is the main one based on the simplicity of deployment and the cost. The following explanation is about the case of the outdoor infrared communication.

In almost all cases Baseband Digital Modulation is adopted, which is used in optical fiber communication. The reason is the good S/N ratio and the low price, and the transparent interface is possible between optical fiber and infrared communication system without the additional equipment in the network. The communication speed of 1.25Gbps is already realized and 10Gbps seems to be possible. In this case LD is 1.55μm and the throughput can be maintained sufficiently.

In the special use for example the lump transmission of CATV and the communication between cellular phone base stations, the analogue modulation and the PSK modulation with sub-carrier multiplex are used for the convenience of the interface with another equipment. But the requirement of the S/N ratio and the distortion is severe and the communication distance is reduced.

In the simple image transmission (monitor for parking area, station yard and elevator), FM and AM are used because of the low price.

In infrared communication system, the easy operation (deployment and adjustment) and the elimination of reduction of transmission quality are the important point for the system design.