Method for transmission of downlink radio signals in a D-MIMO system and a D-MIMO system

A method for transmission of DownLink, DL, radio signals in a Distributed Multiple Input Multiple Output, D-MIMO, system of a Radio Access Network, RAN, the D-MIMO system comprising a Central Unit, CU, with Digital Signal Processing, DSP, for processing UpLink, UL, radio signals for channel estimation and/or for synchronizing the DL radio signals, a first remote radio-head and a second remote radio-head for wireless communication with a wireless communications device, an analogue optical fronthaul link between the CU and the respective remote radio-head for transmitting and receiving the DL and UL radio signals between the CU and the respective remote radio-head, the method comprising:

• receiving, by the CU, a first UL transmission from the wireless communications device via the first remote radio-head over the analogue optical fronthaul link;
• receiving, by the CU, a second UL transmission from the wireless communications device via the second remote radio-head over the analogue optical fronthaul link;
• estimating, by the CU, a first effective UL channel for signals from the wireless communications device via the first remote radio-head based on applying DSP to the first UL transmission, wherein the first effective UL channel includes:
• a wireless UL channel between the wireless communications device and the first remote radio-head, at least one optical component of a first UL part of the analogue optical fronthaul link between the first remote radio-head and the CU, at least one component of a receiver chain of the first remote radio-head and at least one component of a transmitter chain of the wireless communications device;
• estimating, by the CU, a second effective UL channel for signals from the wireless communications device to the second remote radio-head based on applying DSP to the second UL transmission, wherein the second effective UL channel includes:
• a wireless UL channel between the wireless communications device and the second remote radio-head, at least one optical component of a second UL part of the analogue optical fronthaul link between the second remote radio-head and the CU, at least one component of a receiver chain of the second remote radio-head and at least one component of the transmitter chain of the wireless communications device;
• estimating, by the CU, a first calibration channel based on applying DSP to a first reference radio signal transmitted wirelessly from the first remote radio-head and received wirelessly at the second remote radio-head;
• estimating, by the CU, a second calibration channel based on applying DSP to a second reference radio signal transmitted wirelessly from the second remote radio-head and received wirelessly at the first remote radio-head;
• synchronizing, by the CU, a first DL transmission of a first DL radio signal from the first remote radio-head to the wireless communications device with a second DL transmission of a second DL radio signal from the second remote radio-head to the wireless communications device such that at the wireless communications device a respective phase of the received first and second radio signals is phase aligned with each other, wherein synchronizing is performed by
• a) compensating, by the CU, the first DL transmission of the first DL radio signal from the first remote radio-head based on the estimated first effective UL channel;
• and
• b) compensating, by the CU, the second DL transmission of the second DL radio signal from the second remote radio-head based on the estimated second effective UL channel, and further based on the estimated first calibration channel and the estimated second calibration channel;
• and
• transmitting, by the CU, the first DL radio signal to the first remote radio-head and the second DL radio signal to the second remote radio-head over the analogue optical fronthaul link.