A Process Network Model

We establish a process network we mention in 2.3 to model the OFDM system with parallel computation. In this section, we introduce how to establish the process network model. It should be noticed that we use TLM library which is described in [8]

to model the system because of convenience.

First, we should design how to model a node in a process network by SystemC.

The code in Figure 8 is the pattern of the node of the system written in SystemC. In the example of modeling a node, we model the node A in the process network which is mentioned in 3.1. In the object declaration of SystemC in the code, we see the three parts of it the declarations of ports, the body function “proc”, and the object constructor of SystemC. In the function “proc”, the statement (1) is not executed until the input_1, using the get-interface function of TLM library, has data. The statement (2) has same condition, too. That is, they work with blocking read. Thus, our firing rule of process network model is guaranteed. In other words, we guarantee that the process does not execute until all its input queues has data. The part of the object constructor of SystemC puts the function “proc” to an individual thread. And so forth, we can write the SystemC code of other nodes.

SC_MODULE(NODE_A) {

port<tlm_get_if<packet> > input_1;

port<tlm_get_if<packet> > input_2;

port<tlm_put_if<packet> > output;

void proc() {

Figure 8 SystemC code of a node of the process network

The code in Figure 9 is the pattern of the top-view of the system written in SystemC. It illustrates how the connection between nodes and FIFOs are implemented.

The process network described by the code in Figure 9 is the same as the process network described by the directed graph in Figure 2. There are the declarations of nodes and FIFOs and an object constructor of SystemC in the object declaration of SystemC in the code. The statements in the object constructor of SystemC describe

how to connect the FIFOs and the node. For example, the statement “C.output(p)”

means the connection of the output pin of node C and the FIFO p.

SC_MODULE(platform) {

NODE_A A;NODE_B B;NODE_C C;NODE_D D;

tlm_fifo<packet> p;tlm_fifo<packet> q;tlm_fifo<packet> r;

SC_CTOR(platform):NODE_A("NODE_A"),NODE_B("NODE_B"),

Figure 9 The system top-view written in SystemC

Therefore, we use a process network to model the OFDM system in this work.

We describe our procedure of making process network model here. First, we divide whole system to processes, according to the original data flow of the system. Then, we use FIFOs to connect these processes. Specially, while modeling the OFDM

model of a system for being ease to observe the data flow of the system.

In this work, we use process network to model an OFDM system. Because we want to observe the operation of the receiver, we divide the receiver to small processes. Figure 10 and Figure 11 depicts how the receiver part of the OFDM system blocks are divided into processes. We will use four processes, “Guard”, “Fine signal Detection” “Estimation”, “Coarse Signal Dectection”, and a delay element and some FIFOs connecting these nodes to model the receiver in the OFDM system. The block

“Down Converter” and the block “A/D LPF” are combined to the channel model and the block “Signal Demapper” and the block “P/S” are ignored in the original C code.

Thus we ignore these block in our receiver model. We use the method we mentioned in this section to establish the process network in this work.

Figure 10 Receiver architecture divided into processes

Figure 11 Receiver architecture divided into processes

According to the above-mentioned method, we divide the OFDM system into the process network in Figure 12. We also divide the receiver process into the four smaller processes and one delay element to reflect the detail operation of the receiver.

Figure 12 depicts our process network model of the OFDM system

Figure 12 Process network model of the OFDM system

When using the process network to model the OFDM system, the loop including

estimation value of the last iteration to estimate the channel at this iteration.

Estimation and Coarse-signal detection all need the result of Estimation at last iteration. Thus, “Coarse-signal detection” and “Estimation” can not run at the same time because Coarse-signal detection must wait the result Estimation at the same iteration.

In consequence, we try to add a delay element after the original delay element as shown in Figure 13 shows. That means the OFDM system uses the channel estimation value of the penultimate iteration. Thus, “Coarse-signal detection” and “Estimation”

can run at the same time. It may increase the bit error rate of the OFDM system.

However, it also increases the parallelization efficiency. We use the example to prove that changing architecture in the process network level of our design flow is useful.

Figure 13 Process network model of the OFDM system (with two delay element)