Peak-to-average power ratio (PAPR) reduction

In a system employing OFDM, the high peak-to-average power ratio (PAPR) problem remains a plaguing concern particularly when the number of sub-carriers is large. To remedy this issue, various PAPR reduction strategies have been proposed in the literature. These can be classified into three categories: (a) clipping (b) symbol scrambling and (c) error control coding.

The major drawback with the use of clipping is the effect of non-linear distortion that cannot be corrected at the receiver and significant spectral regrowth. Also, no good coding solutions are available when the number of sub-carriers is large. It is also important to note that for larger block lengths decoding complexity increases manifolds.

From these reasons, a concatenated scheme for PAPR reduction that employs interleaving followed by non-uniform quantization (i.e., companding) of the OFDM waveform was developed at MPRG, which is classified into a symbol scrambling method.

Figure 1: Block Diagram of an OFDM system employing a concatenated scheme for PAPR reduction

Figure 2 : Non-uniform quantization technique

Table 1 : PAPR values for different crest factor reduction techniques

Figure 3: SER performance curves for the concatenated scheme in an AWGN channel, with 10 quantization bits

Figure 3 depicts the SER versus Eb/No curves for the concatenated scheme in an AWGN channel. Observe that the performance loss between the SER curves corresponding to μ = 32 and μ = 0 is relatively small and hence given the extent of PAPR reduction possible with the concatenated scheme (almost 8dB, see Table 1), it is always a viable option to use the proposed concatenated scheme.

Figure 4: SER performance curves for the concatenated scheme in an AWGN channel, with 5 quantization bits and N = 256 subcarriers

Figure 4 shows the SER versus Eb/No curves for the concatenated scheme in an AWGN channel when the number of quantization bits is small (5 bit quantizer). It can be observed an error floor at higher values of Eb/No, and this is because the effect of quantization noise is more pronounced at higher Eb/No values when compared to the noise variance contributed by the AWGN channel.

Figure 5: SER performance curves for the concatenated scheme in slow Rayleigh fading

Now when the channel is slow Rayleigh fading, the SER versus Eb/No curves for the concatenated scheme is shown in figure 4. As seen in the figure, the performance gap between the various curves is relatively small and hence significant reductions in PAPR can be realized with the synergistic use of interleaving and companding (non-uniform quantization) but with only negligible loss in receiver performance.

From these observations, it can be clearly seen that the synergistic use of symbol scrambling and companding provides significant reductions in PAPR with relatively small loss in receiver performance.