Practical Implementation of PN Scrambler for PAPR Reduction
Practical Implementation of PN Scrambler for PAPR Reduction
Practical Implementation of PN Scrambler for PAPR Reduction
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codes need to be used and as a result, the overall efficiency<br />
<strong>of</strong> transmission becomes reduced.<br />
Finally, the third category is based on OFDM symbol<br />
scrambling and selection <strong>of</strong> the sequence that produces<br />
minimum <strong>PAPR</strong>. The pre-scrambling techniques [7-8]<br />
achieve good <strong>PAPR</strong> reduction but they require multiple<br />
FFT trans<strong>for</strong>ms and somewhat higher processing power.<br />
The method presented in this paper belongs to the third<br />
category <strong>of</strong> the <strong>PAPR</strong> reduction techniques. It uses<br />
conveniently chosen Pseudorandom Noise (<strong>PN</strong>) sequences<br />
applied to the input data bit stream. The method is very<br />
easy to realize in the s<strong>of</strong>tware or hardware environment<br />
which is very important if the <strong>PAPR</strong> needs to be<br />
implemented in Application Specific Integrated Circuits<br />
(ASIC). In such a scenario, the <strong>PN</strong>-<strong>Scrambler</strong> may be<br />
implemented by the addition <strong>of</strong> external FPGA and DSP<br />
hardware to the Commercial Off-The-Shelf (COTS)<br />
ASICs. As a result, one obtains cost efficient and reliable<br />
hardware solutions.<br />
3. OVERVIEW OF THE <strong>PN</strong>-SCRAMBLER<br />
TECHNIQUE FOR <strong>PAPR</strong> REDUCTION<br />
3.1. System overview<br />
Block diagrams <strong>of</strong> the transmitter and receiver<br />
implementing the proposed <strong>PN</strong>-<strong>Scrambler</strong> are presented in<br />
Figs. 1 and 2, respectively.<br />
A/D Real<br />
A/D Imag<br />
Figure 1. <strong>PN</strong>-<strong>Scrambler</strong> Tx Block Diagram<br />
DC<br />
Offset<br />
DC<br />
Offset<br />
Subcarrier<br />
Demapper<br />
I / Q<br />
Imbalance<br />
I / Q<br />
Imbalance<br />
Channel<br />
Estimate<br />
Acquisition / Synchronization<br />
AGC<br />
Receive (Demodulation) Path<br />
Timing<br />
Detect<br />
Carrier<br />
Phase<br />
and<br />
Timing<br />
Drift<br />
Correction<br />
Coarse<br />
Freq<br />
S<strong>of</strong>t<br />
Bit<br />
Decisions<br />
Timing<br />
Fine<br />
Freq<br />
Deinterleaver<br />
and<br />
Convolutional<br />
decoder<br />
Sample<br />
Buffer<br />
Guard<br />
Interval<br />
Removal<br />
Descrambler<br />
FFT<br />
Data Bits<br />
Output<br />
Figure 2. <strong>PN</strong>-<strong>Scrambler</strong> Rx Block Diagram<br />
As seen in Fig. 1, two additional elements are added to<br />
a typical OFDM transmitter. The first element is the<br />
<strong>PAPR</strong> scrambler, and the second one is the <strong>PAPR</strong><br />
threshold compare block. The <strong>PN</strong>-<strong>Scrambler</strong> utilizes a<br />
Maximal-Length Linear Feedback Shift Register (ML-<br />
LFSR) with log2(k) = l taps in order to produce k = 2 l -1<br />
uncorrelated unique sets <strong>of</strong> data from the same input<br />
sequence. The k unique sets <strong>of</strong> data are used to generate k<br />
independent identically distributed (i.i.d.) OFDM symbols.<br />
A block <strong>of</strong> Nb bits comprising one OFDM symbol is<br />
2 <strong>of</strong> 7<br />
scrambled and passed along <strong>for</strong> Forward Error Correction<br />
(FEC) coding, interleaving, modulation, symbol mapping<br />
and IFFT.<br />
In any given OFDM system, Nb is a function <strong>of</strong> the<br />
number <strong>of</strong> subcarriers, the modulation scheme applied to<br />
each subcarrier, and the coding rate. By examining each<br />
individual sample coming out <strong>of</strong> the IFFT, the <strong>PAPR</strong><br />
threshold comparator determines if the scrambler has<br />
achieved a desired <strong>PAPR</strong> on a symbol-by-symbol basis. If<br />
the <strong>PAPR</strong> <strong>of</strong> the symbol is below a desired threshold, then<br />
the data is passed along towards the RF stage <strong>of</strong> the<br />
transmitter. However, if the <strong>PAPR</strong> is still high, the data is<br />
scrambled with a different phase <strong>of</strong> the ML-LFSR’s <strong>PN</strong><br />
sequence. Since this technique operates on the input bit<br />
stream, it is essentially independent <strong>of</strong> the OFDM<br />
modulation and may be adapted to any particular scenario.<br />
The receiver presented in Fig. 2 is a typical OFDM<br />
receiver that needs to per<strong>for</strong>m the tasks <strong>of</strong> down<br />
conversion, channel estimation, and decoding. The only<br />
additional task required by the <strong>PN</strong>-<strong>Scrambler</strong> <strong>PAPR</strong><br />
reduction technique is descrambling <strong>of</strong> the data at the<br />
receiver output. To per<strong>for</strong>m descrambling, the receiver<br />
has to “know” the phase <strong>of</strong> the ML-LFSR used on the<br />
transmission side. This phase is embedded in the data<br />
stream. For example, the first l bits <strong>of</strong> the OFDM symbol<br />
may carry the in<strong>for</strong>mation on the ML-LFSR phase.<br />
3.2. Analytical Per<strong>for</strong>mance Characterization<br />
A practical implementation <strong>of</strong> the <strong>PN</strong>-<strong>Scrambler</strong><br />
<strong>PAPR</strong> reduction technique requires selection <strong>of</strong> several<br />
parameters. These parameters are defined as follows.<br />
1. Number <strong>of</strong> scrambling sequences (k) - defined as the<br />
number <strong>of</strong> <strong>PN</strong> sequences produced by the ML-<br />
LFSR. Each sequence is Nb bits long.<br />
2. <strong>PAPR</strong> threshold (L) – defined as the maximum<br />
<strong>PAPR</strong> <strong>for</strong> the OFDM symbol. This value is used by<br />
the <strong>PAPR</strong> threshold comparator block in order to<br />
discard OFDM symbols with <strong>PAPR</strong> greater than L.<br />
3. IFFT size / number <strong>of</strong> sub-carriers (N) – defined as<br />
the number <strong>of</strong> the non-zero orthogonal subcarriers<br />
per OFDM symbol.<br />
4. Average latency ( k ) – defined as the average<br />
number <strong>of</strong> scrambling attempts per OFDM symbol<br />
in order to pass the threshold level L.<br />
5. Probability <strong>of</strong> clipping (p) – probability that the<br />
<strong>PAPR</strong> exceeds the threshold level L after k<br />
scrambling attempts.<br />
6. <strong>PN</strong> scrambler overhead ( v ) – defined as the ratio <strong>of</strong><br />
the number <strong>of</strong> bits required to represent the phase <strong>of</strong><br />
the ML-LFSR to the number <strong>of</strong> bits per OFDM<br />
symbol Nb.<br />
In any actual design, the above parameters allow different<br />
trade<strong>of</strong>fs. The subsequent section highlights some <strong>of</strong> these<br />
design trades.