Adaptive Quantization Method and Apparatus For An OFDM Receiver

• US 20070258531A1
• Filed: 05/04/2006
• Published: 11/08/2007
• Est. Priority Date: 05/04/2006
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Winbond Electronics Corporation
• Inventors: Chen, Jeng-Hong, Lee, Yumin, Lin, Yuh-Chun
• Granted Patent: US 7,756,222 B2
• Time in Patent Office: Days
• US Class Current: 375/261
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178
• CPC Class Codes: H04L 27/0008 arrangements for allowing a transmitter or receiver to use more than one type of modulation negotiating modulation type for two-way transmission paths H04L5/1453
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

A quadrature frequency division multiplexing (“OFDM”) wireless receiver, including methods and devices for adaptive quantization of OFDM signals according to modulation and coding schemes and sub-carrier frequency responses, is provided. Efficient quantization may be utilized to reduce the large dynamic range of signals to achieve circuit simplification and chip area reduction. In one embodiment, a quantization circuit includes a quantization selector to select quantization thresholds according to modulation and coding schemes and sub-carrier frequency responses, and a non-uniform quantizer to reduce input dynamic range so that an output is represented by fewer bits than an input.

Claims

1. A quantization method for a wireless receiver, comprising:

receiving demapped QAM symbols for each of a plurality of sub-carriers from a QAM demapper;

selecting quantization thresholds according to modulation and coding schemes and sub-carrier frequency responses;

conducting a non-linear quantization to reduce the dynamic range of the demapped QAM symbols; and

providing quantized symbols to a de-interleaver.

2. The quantization method of claim 1, further comprising:

computing the channel power for each of the plurality of sub-carriers;

computing a dynamic range D;

determining a frequency selectivity indicator (FSI);

determining a modulation coding scheme (MCS);

determining a normalized quantization factor K′

from the MCS and FSI;

computing quantization thresholds; and

quantizing an input signal into predetermined output quantization levels.

3. The method of claim 2, wherein computing of channel power further comprises:

computing the channel power for each channel P_CH(i)=|G_CH(i)|² for all of m sub-carriers, where G_ch(i) is the i-th sub-carrier channel frequency response, i=0, . . . , m−

1.

4. The method of claim 2, wherein computing a dynamic range D further comprises:

computing the channel power for each channel P_CH(i)=|G_CH(i)|² where G_ch(i) is the i-th sub-carrier channel frequency response for the m sub-carrier channels; and

computing the dynamic range

$D=\underset{i=0}{\overset{m-1}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>P_{\mathrm{CH}}<br><br>\left(i\right).$

5. The method of claim 2, wherein determining a frequency selectivity indicator (FSI) further comprises:

setting FSI=0 if the channels are frequency non-selective; and

setting FSI=1 if the channels are frequency selective.

6. The method of claim 5, wherein determining a frequency selectivity indicator (FSI) further comprises:

computing

$P_{\max }=\underset{i}{\max }<br><br>\left\{P_{\mathrm{CH}}<br><br>\left(i\right)\right\},$

where the max{ } function computes the maximum value of P_CH(i), i=0, 1, . . . , m−

1;

computing

$P_{\min }=\underset{i}{\min }<br><br>\left\{P_{\mathrm{CH}}<br><br>\left(i\right)\right\},$

where the min{ } function computes the minimum value of P_CH(i), i=0, 1, . . . , m−

1; and

setting FSI=1 if P_max>

T*P_min and FSI=0 if P_max<

T*P_min, wherein T ranges from approximately 5 to 30 dB.

7. The method of claim 2, wherein the modulation coding scheme (MCS) is specified in the signaling OFDM symbol.

8. The quantization method of claim 7, wherein the modulation coding scheme (MCS) includes PBSK, QPSK, 16-QAM, 64-QAM, and 256-QAM.

9. The method of claim 8, wherein determining a normalized quantization factor K′

for a 3-bit quantized output comprises determining K′

based on a look-up table given as follows.

10. The method of claim 9, wherein computing quantization thresholds further comprises:

computing a base quantization threshold θ

₁=K′

×

D; and

computing other quantization thresholds as multiples of θ

₁.

11. The method of claim 10, wherein computing quantization thresholds θ

₀, ±

θ

₁, ±

θ

₂, ±

θ

₃ for a 3-bit quantized output further comprises the following computations;

θ

₀=0
θ

₁=K′

×

D
θ

₂=θ

₁×

S₂
θ

₃=θ

₁×

S₃

where S₂ and S₃ are constants.

12. The method of claim 11, wherein

13. The method of claim 11, wherein S₂=2.5 and S₃=5.

14. The method of claim 11, wherein S₂=2 and S₃=4.

15. The method of claim 11, wherein quantizing an input signal into predetermined output quantization levels further comprising:

computing the output y_j according to a formula given below

$y_j=\{\begin{array}{c}3,\\ i,\\ 2^3-i,\\ 4,\end{array}<br><br>\mathrm{ <br><br> <br><br> <br><br>}<br><br>\begin{array}{c}<br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>x_j\ge <br><br>{\mathrm{\theta <br><br>}}_3\\ <br><br><br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\theta <br><br>}}_i\le <br><br>x_j<<br><br>{\mathrm{\theta <br><br>}}_{i+1}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{for}<br><br>\text{\hspace{1em}<br><br>}<br><br>i\in <br><br>\left\{0,1,2\right\}\\ <br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}-{\mathrm{\theta <br><br>}}_i\le <br><br>x_j<<br><br>-{\mathrm{\theta <br><br>}}_{i-1}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{for}<br><br>\text{\hspace{1em}<br><br>}<br><br>i\in <br><br>\left\{1,2,3\right\}\\ <br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>x_j\le <br><br>-{\mathrm{\theta <br><br>}}_3\end{array},$

wherein x_j is an input signal; and

computing the 3-bit quantized output b_j for y_j according to a formula given below

$b_j=<br><br>\left[b_{j<br><br>{,}^{2,}}<br><br>b_{j<br><br>{,}^{1,}}<br><br>b_{j<br><br>{,}^0}\right]<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{such}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{that}<br><br>\text{\hspace{1em}<br><br>}<br><br>b_{j,i}\in <br><br>\left\{0,1\right\}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>\underset{i=0}{\overset{2}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>b_{j,i}<br><br>2^i=y_j.$

16. A quantization method for an orthogonal frequency division multiplexing (OFDM) receiver including a plurality of modulation and coding schemes and a plurality of sub-carriers for wireless communication in a multi-path environment, comprising:

selecting quantization thresholds according to the modulation and coding schemes and sub-carrier frequency responses; and

conducting a non-linear quantization to reduce the dynamic range of signals.

17. The quantization method of claim 16, wherein the modulation and coding schemes include PBSK, QPSK, 16-QAM, 64-QAM, and 256-QAM.

18. The quantization method of claim 17, wherein selecting quantization thresholds further comprises:

computing channel power of each one of the plurality of sub-carriers;

computing the dynamic range of the input signals of the plurality of sub-carriers; and

setting quantization thresholds according to the modulation coding scheme, distribution of sub-carrier channel power, and the dynamic range of the signals.

19. The method of claim 18, wherein conducting a non-linear quantization further comprises:

computing quantization output g(x_j) according to the following non-uniform transformation of the input x_j,

$g<br><br>\left(x_j\right)=\mathrm{sgn}<br><br>\left(x_j\right)<br><br>\left[1-\exp <br><br>\left(-\frac{<br><br>x_j<br><br>}{K}\right)\right],$

where K is determined by channel modulation scheme and channel dynamic range, and

$\mathrm{sgn}<br><br>\left(x_j\right)=\{\begin{array}{cc}1,& x_j><br><br>0\\ 0,& x_j=0\\ -1,& x_j<<br><br>0\end{array}$

20. An orthogonal frequency division multiplexing (OFDM) receiver comprising an adaptive quantization circuit, comprising:

a quantization selector wherein quantization thresholds are determined according to modulation and coding scheme and sub-carrier frequency responses; and

a non-uniform quantizer wherein a signal dynamic range is reduced and wherein an output is represented by fewer bits than an input.

21. The OFDM receiver of claim 20, wherein the quantization selector further comprises:

a channel power calculator;

a dynamic range calculator;

a channel frequency selectivity detector;

a look-up table for determining normalized quantization factor K′

; and

a circuit for computing a base quantization threshold.

22. The OFDM receiver of claim 20, wherein the quantizer further comprises:

a quantization threshold calculator;

a comparator; and

a bit mapper.

23. The OFDM receiver of claim 20, wherein the quantization selector computes a base quantization threshold according to the modulation and coding schemes and sub-carrier frequency responses, and then computes other quantization thresholds as multiples of the base quantization threshold.

24. The OFDM receiver of claim 23, wherein the modulation coding schemes include PBSK, QBSK, 16-QAM, 64-QAM, and 256-QAM.

25. The OFDM receiver of claim 24, wherein the quantization thresholds are even-integer multiples of the base quantization threshold such that the quantization thresholds are obtained by shift-and-add digital circuit operation of the base quantization thresholds.

26. The OFDM receiver of claim 25, wherein the quantization thresholds θ

₀, ±

θ

₁, ±

θ

₂, ±

θ

₃ for a 3-bit quantized output are computed as follows.

θ

₀=0
θ

₁=K′

×

D
θ

₂=θ

₁×

2
θ

₃=θ

₁×

4

where K′

is determined by the modulation coding schemes and sub-carrier frequency responses and D is the dynamic range of the input signals.

27. The OFDM receiver of claim 26, wherein K′

is determined by a look-up table given below.

28. The OFDM receiver of claim 27, wherein the frequency selectivity indicator FSI is determined as follows:

computing the channel power for each channel P_CH(i)=|G_CH(i)|² where G_ch(i) is the i^-th sub-carrier channel frequency response for the m sub-carrier channels;

computing

$P_{\max }=\underset{i}{\max }<br><br>\left\{P_{\mathrm{CH}}<br><br>\left(i\right)\right\},$

where the max{ } function computes the maximum value of P_CH(i), i=0, 1, . . . , m−

1;

computing

$P_{\min }=\underset{i}{\min }<br><br>\left\{P_{\mathrm{CH}}<br><br>\left(i\right)\right\},$

where the min{ } function computes the minimum value of P_CH(i), i=0, 1, . . . , m−

1; and

setting FSI=1 if P_max>

T*P_min and FSI=0 if P_max<

T*P_min, wherein T ranges from approximately 5 to 30 dB.