This is an old revision of the document!
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 data to send to this user (2048 bits corresponding to one OFDM symbol multiplied by the number of resources blocks allocated to the user). 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 (which is the second OFDM symbol because the first is used for Downlink Control information). 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.
This description is about time scheduling because we consider the resources blocks and ressources blocks per user are already defined. So there is another of scheduling to implement. We have to schedule the attribution of each resources block group (RBG). Following the round robin algorithm, it's quite simple: each RBG is iteratively allocated to a user. Other algorithms as max CQI may include the transmission conditions parameter to allocate . Each RBG is composed of 1 to 4 resources block (in the case of LTE the channel bandwidth is about 20Mhz so we have 100 resources blocks which means 25 to 100 resources blocks groups).