Multi-Processor System-on-Chip 2. Liliana Andrade

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СКАЧАТЬ 6G applications span vastly different data rates (10 kb/s – 1 Tb/s, i.e. 10⁸ × span) and latency requirements (2 ms – 2 s i.e. 10³ × span).

We have a broad and dynamic workload space with the total number of applications steadily increasing. These applications are sometimes concurrent in their operation, and in, at other times, they are exclusive, which adds another layer of complexity into consideration. For example, a handset user is either watching a video stream or playing an AR-based game while taking a stroll through the city, not both at the same time.

The challenges presented above, combined with new signal processing tasks associated with radio issues when working in new frequency ranges, make HW design choices particularly hard, efficiently balancing between architecture complexity and size, power consumption and cost. The HW challenge is certainly the “half a trillion dollar question”¹. There is no other way to treat this than as an onion problem, and to peel it layer by layer, going step by step, the first step being standard specifications.

1.2.2. Standard specifications

      High variability is built into modern standards through their many modes of operation. One aspect by which these modes may differ are workloads. Let us analyze the workloads by looking at the most advanced 5G standard.

      1.2.2.1. Processing deadline variability

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

      1.2.2.2. Data throughput variability

      Next, let us investigate throughput requirements of the 5G specifications. At this point, we choose to compute and show requirements per handset modem chip. Note that there are additional device classes that support only a subset of the operating modes shown here. However, an advanced handset of the future should support all of the modes shown here, to use the full potential of different frequency ranges.

Previously, we performed a specification analysis (Damjancevic et al. 2019), although over the past 6 months the 5G specifications have expanded, and here we show the updated information. In Figure 1.3 and Figure 1.4, we have organized the throughput information and presented it in a readable form for FR1 and FR2, respectively. For comparison, we plot also the 4G data throughput requirement, which coexists in FR1, along with other legacy standards. Future FR2 5G systems will coexist with Super High Frequency (SHF) and Extremely High Frequency (EHF) communication and radar systems, which are region-specific, adding an extra layer of flexibility. Throughput is shown in maximum resource blocks (RB)⁶ over time, per channel BW for one spatial MIMO data stream layer⁷,⁸ in compliance with active (3GPP 2019c, d) specifications. From the figures, we can observe that:

      1 1) there are many possible modes of operation;

      2 2) there is a 352× difference between the processing data load corners (LTE, 1.4 MHz) – lower end and (μ = 3, 400 MHz) upper end in terms of RBs.

      Figure 1.3. Processing load in kRB/s for 5G NR FR1 (Damjancevic et al. 2019)

      We see a greater need for flexibility again emerging from the observations, compared to the 4G standard, with many more modes to support on top of the throughput difference. Now that we have identified the throughput corners in RBs,

we can assign and calculate the smallest and highest QAM and code bit rates allowed by the 4G⁹ and 5G¹⁰ specification sets, to the lower and upper ends, respectively.

      Figure 1.4. Processing load in kRB/s for 5G NR FR2

This sets the low end at 200 kb/s per 1.4 MHz BW channel and spatial data layer¹¹. Note that this is a hard bit rate, and the rate at different processing steps may be higher due to oversampling.

The high end, on the other hand, is set at 2.63 Gb/s per 400 MHz BW channel and spatial data layer. Note that we have insisted on emphasizing the “per channel, per layer”, since handsets have many operating modes and some of those modes may require a multitude of channels or layers active simultaneously, for example, CA¹². The 5G standard allows modes that support a multitude of each, further increasing the effective number of RBs communicated. The extra RBs can be used to increase the overall throughput or to increase redundancy by sending the same data on another channel frequency or layer. 5G as of now supports up to 4 × 400 MHz CA (3GPP 2019d) for its upper-end mode and MIMO up to 8 layers of data (3GPP 2019e) extension. Whether or not both extension modes can overlap within 5G is not clear, since the two are often used with opposite goals. Namely, data layer extensions СКАЧАТЬ