DSP Builder for Intel® FPGAs (Advanced Blockset): Handbook

ID 683337
Date 12/12/2022

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Document Table of Contents Simulink vs Hardware Design Representations

Simulink shows the IP block as a single block, but the data input and output wires from the IP blocks show as a vector with multiple dimension. Multiple wires accommodate all the channels and the Simulink model uses a vector of width 2.
Figure 17. Simulink and Hardware Representations of a Single Rate FIR Filter
Note: To display the ChanWireCount in Simulink, point to Port/Signal Displays in the Format menu and click Signal Dimensions.

In a typical wideband CDMA macro-cell system, the DUC module in the RF card needs to process eight inphase (I) and quadrature (Q) data pairs, resulting in 16 independent channels on the datapath. The input sample rate to a DUC is at sample rate 3.84 MHz as defined in the 3GPP specification. A high-performance FPGA running at 245.76 MHz typically maximizes parallel processing power.

Figure 18. 16-channel WCDMA DUC DesignShows how channel's distribution on wires change in a multirate system.
Table 5.  16-channel WCDMA DUC Design
Signals Clock Rate (MHz) ChanCount Data Sample Rate (MSPS) Period Data Signal Pattern Interpolation Factor
Input to FIR1 245.76 16 3.48 64 I1, I2, ...I8, Q1, ... Q8, zeros(1, 64–16) 2
Input to FIR2 245.76 16 7.68 32 I1, I2, ...I8, Q1, ... Q8, zeros(1, 32–16) 2
Input to CIC 245.75 16 15.36 16 I1, I2, ...I8, Q1, ... Q8 8
Output of CIC 245.75 16 122.88 2 I1, I2, I3, I4, I5, I6, I7, I8, Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8 8

In this example, the input data at low sample rate 3.84 MSPS can accommodate all channels on a single wire. So the ChanWireCount is 1. In fact more time slots are available for processing, since period is 64 and only 16 channels are present to occupy the 64 time slots. Therefore the ChanCycleCount is 16, which is the number of cycles occupied on a wire. As the data travels down the up conversion chain, its sample rate increases and in turn period reduces to a smaller number. At the output of CIC filter, the data sample rate increases to 122.88 Msps, which means only two time slots are available on a wire. As there are 16 channels, spread them out on 8 wires, where each wire supports two channels. At this point, the ChanWireCount becomes 8, and ChanCycleCount becomes 2. The ChanCycleCount does not always equal period, as the input data to FIR1 shows.

For most systems, sample rate is less than clock rate, which gives WirePerChannel=1. In this case, ChanWireCount is the same as WireGroups, and it is the number of wires to accommodate all channels. In a super-sample rate system, a single channel's data needs to be split onto multiple wires. Use parallel signals at a clock rate to give an equivalent sample rate that exceeds the clock rate. In this case, WiresPerChannel is greater than one, and ChanWireCount = WireGroups × WiresPerChannel because one channel requires multiple wires.

When connecting two modules in DSP Builder, the output interface of the upstream module must have the same ChanWireCount and ChanCycleCount parameters as the input interface of the downstream module.