Читать книгу Multi-Processor System-on-Chip 2 - Liliana Andrade - Страница 19

If we look at the transmission time interval (TTI)² consisting of 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols³ for 5G (3GPP 2019e), extrapolate its duration and compare it (see Figure 1.2) to the duration of 3GPP 4th Generation Long Term Evolution (4G) (3GPP 2018a) TTI of 14 OFDM symbols, we can make the following observations:

1 1) TTI duration is scaled with 2μ, where μ is a parameter⁴;

2 2) TTI duration is not a function of bandwidth (BW) allocated for that TTI;

3 3) for a fixed BW and the same number of OFDM symbols per TTI (i.e. the same amount of data), the duration of the TTI differs (i.e. the deadline to process that data differs);

4 4) there is a 16× difference in processing deadlines between the corner cases.

Depending on the mode of operation, the computational engine may need to process the same amount of data with deadlines shifting 16× during operation.

The processing deadline for baseband physical layer processing is constrained by the hybrid automatic repeat request (HARQ) media access control layer (MAC-L) procedure, which is 3 TTIs long. Let us assume, as a rule of thumb, that we have 1/3 of the 3 TTI budget associated with waveform modulation, while the other 2/3 are reserved for other processing steps⁵. With that in mind, the deadlines are simplified and match the TTI duration for the given subcarrier spacing parameter μ.

Figure 1.2. Comparing 14 OFDM symbols’ TTI duration of 4G and 5G