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The base station permanently but not continuously receives datas (bits stored in a queue). Each bit (fifo) is stored in a buffer when this buffer has free space. This buffer may be either a siple array, in this case each bit has to be “tagged” as belonging to a given user, or several arrays each belonging to one user. Let's take one array per user. Those datas are then available to the “scheduler part” of the program. The scheduler may implement one of the three algorithms mentionned earlier, let's take round robin in this example.
Following the round robin algorithm: the scheduler chooses one queue (one user) and extracts 2048 bits to send to this user (corresponding to one OFDM symbol). This data set is stored (and tagged with the user id) in another queue (fifo) that is destined to the “mapper part” of the program. A counter is then incremented and the scheduler does the same action of the next user. This happens again for each user, when the last user has been choosen, the counter is reseted to prepare the next timeslot.
In the same time the schedulers performs his algorithm, the mapper gets each set of data present on the queue. Those datas are pushed on a map which has time and frequency dimensions. A time slot corresponds to 0.5 ms and 7 OFDM synbols so the first OFDM symbol is then occupied on the first user ressources blocks. The second OFDM symbol on the next user resources blocks is used to store the second user datas and so on. We then have a kind of “stairs topology” on this map (because we use round robin). At the end of the timeslot, as the scheduler resets its counter, the mapper sends the data map to the user devices.