What Is Coupling Loss?

Coupling loss refers to the energy loss that occurs when energy is transmitted from one circuit or one of its components to another circuit. Coupling loss is measured in watts or decibels.

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Coupling loss minimum coupling loss

The minimum coupling loss (Minimum Coupling Loss, MCL) is proposed by the WCDMA equipment manufacturer, and defines the minimum coupling loss between the base station and the transmitting and receiving parts of the mobile phone. The MCL value consists of two parts: the free space loss from the mobile phone to the antenna and the antenna feed system loss from the antenna to the base station receiver.

If the MCL loss is too small, the power from the mobile phone to the base station will be too large, which will increase the noise floor of the entire sector and cause interference. It can be seen that the introduction of the MCL concept is because the CDMA system is an interference-limited system. Although the power from the mobile phone to the receiving side of the base station is determined by the mobile phone's transmit power and intermediate loss, the behavior of the mobile phone is not completely controllable by the network, so the definition of intermediate loss is still Significant.

Assuming the minimum coupling loss is 45dB, the minimum transmit power of the mobile phone -50dbm causes a noise rise of about 9dB, which means that the power required by the base station is increased by 9dB, or the minimum energy-to-noise ratio of the guaranteed service is reduced. When the MCL is higher than 65dB, the increase of the noise level caused by the minimum transmit power of the UE will be ignored.

The MCL value consists of two parts: the free space loss from the mobile phone to the antenna, and the antenna feed system loss from the antenna port to the base station receiver. The minimum space loss from mobile phone to antenna, usually we take 38.5dB of space loss of 1 meter. Antenna system loss mainly includes feeder transmission loss, device distribution loss, etc. In indoor distribution systems, the uplink / downlink antenna system loss is equal. Therefore, we can calculate the equivalent uplink value by calculating the downlink antenna feed system loss, so the antenna feed system loss = base station transmit power-antenna port transmit power.

If MCL65dB meets the system requirements, assuming the base station transmit pilot power is 33dBm, the indoor antenna port transmit power must meet the following requirements:
MCL = 38.5dB + (33-antenna port power) 65dB.

From this calculation, when the antenna port power is 6.5dBm, the MCL meets the system requirements. Because the output power of the base station is basically fixed, as long as the antenna port power is measured, the (downlink) antenna feed loss can be known, so that it is equivalent to know whether the upstream MCL meets the requirements. Inference, the MCL value determines that the indoor distribution system should adopt the design method of low power multiple antennas. ^[1]