analog beamforming based distributed antenna system

doc/analog_beamforming.md

@@ -10,7 +10,7 @@ There are three main beamforming techinques: analog, digital and hybrid. The nam
 
 The presence of a limited number of predefined beams at RU poses constraints to the scheduler at gNB. As a matter of fact, the scheduler can serve only a limited number of beams, depending on the RU characteristics (possibly only 1), in a given time scale, that also depends on the RU characteristics (e.g. 1 slot or 1 symbol). This limitation doesn't exist for digital beamforming.
 
-To be noted that in our implementation analog beamforming is only supported in non-split/monolithic mode because we don't support yet SCF P19 interface that would be needed to manage these procedure in a split scenario with SCF FAPI.
+Analog beamforming implementation also allows to enable distributed antenna systems (DAS), where each beam corrisponds to one antenna (or a set of antennas) of the system. In this scenario, the scheduler constaint is alleviated because normally the number of concurrent beams allowed equals the total number of beams.
 
 # Configuration file fields for analog beamforming
 
@@ -22,6 +22,8 @@ In the `MACRLC` section of configuration files, there are three new parameters:
 - `beams_per_period` is the number of concurrent beams the RU can handle in the beam duration (default value is 1)
 - `beam_weights` is a vector field containing the set of beam indices to be provided by the OAI L1 to the RU is also required. In current implementation, the number of beam indices should be equal to the number of SSBs transmitted.
 
+DAS is enabled by setting to 1 the parameter `enable_das` in the L1 section of the configuration file.
+
 # Implementation in OAI scheduler
 
 A new MAC structure `NR_beam_info_t` controls the behavior of the scheduler in presence of analog beamforming. Besides the already mentioned parameters `beam_duration` and `beams_per_period`, the structure also holds a matrix `beam_allocation[i][j]`, whose indices `i` and `j` stands respectively for the number of beams in the period and the slot index (the size of the latter depends on the frame characteristics).
@@ -32,6 +34,8 @@ It is important to note that in current implementation, there are several period
 
 # FAPI implementation
 
+To be noted that in our implementation analog beamforming is only supported in non-split/monolithic mode because we don't support yet SCF P19 interface that would be needed to manage these procedure in a split scenario with SCF FAPI.
+
 In `config_request` structure, a vendor extension (`nfapi_nr_analog_beamforming_ve_t`) configures the lower layers at initialization with the following information:
 - `analog_bf_vendor_ext` which can assume values 1 or 0 for enabling or disabling analog beamforming
 - `num_beams_period_vendor_ext` which corresponds to the configuration parameter `beams_per_period`
@@ -45,6 +49,8 @@ Additionally, L2 provides in each channel FAPI message information about the bea
 To handle multiple concurrent beams, the buffers containing Tx and Rx data in frequency domain (`txdataF` and `rxdataF`) have been extended by one dimension to contain multiple concurrent beams to be transmitted/received.
 The function `beam_index_allocation`, called by every L1 channel, is responsible to match the FAPI analog beam index to the RU beam index and to store the latter `beam_id` structure, which allocates the beams per symbol, despite L2 only supporting beam change at slot level. At the same time, the function returns the concurrent beam index, to be used to store data in frequency domain buffers. While doing so, the function also checks if there is room for current beam in the list of concurrent beams, which should always be the case, if L2 properly allocated the channels.
 
+In case of DAS, since each beam corresponds to a specific antenna port, the `beam_index_allocation` function is simplified in the sense that the beam index corresponds to the antenna port index of the frequency domain buffers.
+
 # RU implementation
 
 The implementation is still work in progress.