An implementation of IEEE802.11a WLAN system using Subword Parallelism and its Quantization Error EvaluationZaipeng Xie

Muwu HouDaphne J Franklin

Topics Covered

Motivation IEEE 802.11a Standard OFDM System

Transmitter Receiver

Quantization Error Analyses Simulation Results Conclusion Future Work

Motivation

Increasing demand of greater communication capacity High bandwidth efficiency

Effects of multipath fading and delay alleviated Frequency selective fading Narrowband interference

Exploit sub-word parallelism to optimize IFFT/FFT implementations

IEEE 802.11a Standard

Wireless LAN Standard High Data Rates

IEEE 802.11b for 2.4GHz Operation IEEE 802.11a for 5GHz Operation

Offers three time the operating bandwidth Less susceptible to interference

Modulation : BPSK, QPSK, 16-QAM, 64-QAM Coding rate : 1/2, 2/3 and 3/4. Number of subcarriers is 52 OFDM symbol duration : 4 sec

Block Diagram – OFDM System

FFT algorithm

A radix-2 Cooley-Tukery FFT, recursive function, O(Nlog(N))Function Y = fft(N,x)If N==1, Y = x;Else

xeven=[x(0)x(2)… x(N-2)];xodd=[x(1) x(3) … x(N-1)];Yeven=fft(N/2,xeven);Yodd=fft(N/2,xodd);For k=0:N-1,

Y(k)=Yeven(k mod N/2)+Wk*Yodd(k mode N/2);end

end

Quantization Error analysis matlab Fixed point package

6 bit input symbol

0 10 20 30 40 50 60 70-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15Quantization error of fixed point FFT (16bit fixed point)

ideal output

output due to quantization

0 10 20 30 40 50 60 70-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15Quantization error of fixed point FFT (8 bit fixed point)

ideal output

output due to quantization

Subword Parallelism of FFT

C source code: out[k].real = y1.real + ((t.real * y2.real) >> 15) - ((t.imag * y2.imag) >> 15)

out[k].imag = y1.imag + ((t.real * y2.imag) >> 15) + ((t.imag * y2.real) >> 15);

out[k+N/2].real = y1.real - ((t.real * y2.real) >> 15) + ((t.imag * y2.imag) >> 15);

out[k+N/2].imag = y1.imag - ((t.real * y2.imag) >> 15) - ((t.imag * y2.real) >> 15);

PLX instructions: pmulshr , padd, psub, pshift

imag real

imag real

t

y2

Pmushr.15 imag real

real imag

excheckPsub.8

BET Evaluation of WLAN system Established an IEEE802.11a WLAN system AWGN channel model Coding Rate ¾ 64 QAM (Quadrature Amplitude Modulation) SNR 10dB Randomly Generated Packet: -number:100 -width: 1 byte Simulate with Different FFT data width - 8, 16, 24, 32, 40, 48, 56 bits

Simulation Result: Raw data BER vs FFT Data Width

Simulation Result: Data BER vs FFT Data Width

Simulation Result: Data PER vs FFT Data Width

Conclusion

Better BER and PER performance by increasing the FFT Data Width

FFT Data Width represents the size of Complex multiplier, Adder and Subtractor

Tradeoff between FFT Processor size and its Performance and possible Optimization

Future Work

Finish PLX subword parallelism implementation

Evaluate Error Rates vs FFT width in other Modulation mode: BPSK, QPSK, 16-QAM, 64-QAM

Evaluate Error Rates vs FFT width with different Coding rate : 1/2, 2/3 and 3/4.

Thanks

Questions?