A conventional architecture for the physical and link layers has a demodulator and decoder, as shown in Figure 6.1. Packet reception occurs in the DEMOD module that demodulates, samples, and puts out quantized real- or complex-valued soft symbols. These symbols are fed to the DECODER module to produce a data packet. If the packet has a valid CRC, then it passes to the network layer; otherwise it is discarded.
Fig. 6.1 A Conventional Physical/Link Layer Architecture. The thick arrow between demod and decoder denotes a high-rate soft symbol interface.
In the classic multihop approach, the network layer determines the future of each decoded packet. At the final destination, the network layer passes the packet to higher layers. At an intermediate node on a path, the network layer resolves the next node on the path, constructs a packet with the same data but a modified header that indicates the next node as the intended recipient, and passes this modified packet back down to the PHY layer transmitter.
The demodulator and decoder can be tightly coupled since the decoding process handles one packet at a time. This is significant because the interface that passes soft symbols is a high-rate interface. In particular, consider a coded system with M-ary modulation. A sequence of k input bits is coded at rate R into n = k/R code bits. Next, log2 M code bits at a time are passed to the modulator. This leads to the reception of a complex-valued soft symbol at the DEMOD. The real and imaginary components of this complex symbol must each be quantized to b bits. A soft symbol output is thus represented by 2b bits. The number of bits b must be chosen sufficiently large so that quantization noise is negligible.
The above implies that k input message bits result in
bits being passed through the receiver’s soft symbol interfaces. This represents a bandwidth expansion of
which can be substantial. Generally R < 1 and b = 10 bit quantization is typical [55]. For example, under QPSK signaling with M = 4, a rate R = 1/2 code and b = 6 bits quantization, the bandwidth expansion factor is G = 12. Values of G on the order of 10 or 20 are typical. Communication at a message bit rate of 20 Mb/s might thus require soft symbol interfaces that operate at 200-400 Mb/s. Note that it appears that transmitting with a large constellation size M reduces the expansion factor. However increasing M requires finer quantization and thus larger b. Typically, increasing M is a losing proposition in terms of the expansion factor G.
While this discussion of soft symbols may seem overly detailed, minimizing the use of soft-symbol interfaces is an important consideration for hardware designers. We will see that this consideration influences the partitioning of tasks in the network layer.
