International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)

Implementation of 802.11n OFDM Transmitter and Receiver Using FPGA Pratibha Mane1, Varsha Thombare2, Monali Mhaske3, Prof. R. R. Bhambare4 1,2,3,4

Department of Electronics, Pravara Rural Engineering College, Loni In frequency multiplexing technique each carrier is placed on different frequencies. And each carrier is separated by an unused band of frequencies called a “Guard Band” and off course the guard band is waste of the bandwidth resource.[3]

Abstract— The increasing demand on high bit rate and reliable wireless system has led to many new emerging modulation techniques like OFDM (Orthogonal frequency division and multiplexing),which offers reliable high bit rate wireless system with reasonable low complexity. Multiple input multiple output antenna systems provides higher data rates up to 100Mbps.The combination of OFDM with MIMO results in less BER.802.11n which is designed and simulated in this paper. The new standard 802.11n is predicted to be capable of supporting data rates up to 600Mbps by deploying the latest communication method such as MIMO.For this we are using 2.4GHZ frequency. Index Terms— OFDM, FPGA, MIMO, Modelsim6.5, Xilink12.3, MATLAB2008b.

II. OFDM BASIC Orthogonal frequency division and multiplexing (OFDM) Is a combination of multiplexing and modulation technique. In OFDM the available bandwidth is shared by individual modulated data sources. In normal modulation techniques like AM, PM, FM, BPSK, QPSK etc. The incoming information is modulated over a single carrier therefore these are single carrier modulation techniques. OFDM is a multicarrier modulation technique, in which several carriers are transmitted over the allocated bandwidth to carry the information from source to destination. For this, each carrier may use one of the several available digital modulation techniques like BPSK, QPSK, and QAM.

802.11n,

I. INTRODUCTION The need for high speed data transmission has been increased in recent years. The mobile communication industry faces the problem of providing the technology that will be able to support a variety of services ranging from voice communication with a bit rate of few Kbps to wireless multimedia in which bit rate is up to 2Mbps.This method has recently became available with reasonable prices verses performance of hardware implementation. The objective of this paper is to design and implement a baseband OFDM transmitter and receiver on FPGA hardware. Frequency band of 2.4GHZ is used for this. By implementing a MIMO OFDM baseband transmitter and receiver on an FPGA with proper selection of one of the sixteen constellation which vary in terms of the convolution coding rate, parsing method and modulation type (e.g data rate 48Mbps using 16QAM at the code rate of ½ using spatial multiplexing) thereby the project is expected to fulfill the need for high speed data transmission for a wireless communication system with cost effective hardware implementation. OFDM has become popular technique because of it’s spectrum efficiency. OFDM is multiplexing technique.

A. OFDM Spectrum and Symbol:

Fig1: OFDM Spectrum

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014) C. OFDM Specification: Parameters of Specification

Description

N=64

FFT size or total number of subcarriers(used+unused)

Nsp=4

Number of pilot subcarriers are 4

Nsd=48

Number of data subcarriers are 48

Derived Parameters Fig 2: OFDM Symbol

OFDM Bandwidth=20*10^6

B. OFDM Tx And Rx:

Fig 3: OFDM Transmitter

The input data stream is split into N parallel low bandwidth modulated data streams. Due to orthogonality of subcarriers they do not interfere with one another. Each subcarrier has a low symbol rate. But the combination of subcarriers carrying the information in parallel allows for high data rates. Low symbol rate is used to reduce the problem of intersymbol interference (ISI). Before modulation the transmitter stage of an OFDM transceiver takes data, Converts the data and encodes it into a serial stream. The generation of OFDM signal is takes place by using Inverse Fast Fourier Transform (IFFT).Reverse process takes place in receiver stage.

Bandwidth of OFDM

ΔF=OFDM BW/N

Bandwidth for each subcarrierinclude all used and unused subcarriers.

Tfft=1/ ΔF

IFFT or FFT period=3.2µs

Tgi=Tfft/4

Guard interval duration – duration of cyclic prefix-1/4th portion of OFDM symbols

Tsignal=Tgi+Tfft

Total duration of OFDM symbol=Guard time+FFT period

Ncp=N*Tgi/Tfft

Number of symbols allocated to cyclic prefix

Nst=Nsd+Nsp

Number of total used subcarriers

nBitsPerSym=Nst

For QPSK number of bits per symbol

D. Multiple Input Multiple Output: Frequency reuse is the main function of the MIMO.To improve efficiency of the spectrum we uses MIMO.Multiple antennas at the transmitter side and multiple antennas at the receiver side are being used in this technique.By using spatial multiplexing and space time block code we can implement MIMO. Fig 4: OFDM Receiver

In receiver stage generation of OFDM signal takes place by using fast Fourier transform (FFT).

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014) •DATA SINK:

i. Space Time Block Code:

It is the output of the OFDM receiver system.

For transmission with two antennas Alamouti has discovered space time block coding scheme. STBC scheme supports maximum likelihood (ML) detection based only on a linear processing at the receiver. The linear processing and the simple structure of the Alamouti construction makes it attractive scheme. It is used in the application where higher order diversity is needed.

A. OFDM system transmitter:

ii. Spatial Multiplexing: A high rate bit stream is decomposed into three independent 1/3 rates bit sequences which are then transmitted simultaneously, using multiple antennas, thus consuming one third of the nominal spectrum.It is similar as a three unknowns resolved from a linear system of three equations. Separation of bit stream is possible only if equations are independent which can be interpreted by each antenna “seeing” a sufficiently different channel in which case the bit streams can be detected and merged together to yield the original high rate signal.

Fig 6: MIMO Transmitter block diagram.

Data bits are given as a inputs to the transmitter. In order to scatter the input sequence to avoid the dependence of input signals power spectrum on the actual transmitted data these bits passed through the scrambler. Scrambler randomizes the bit sequences. Encoding of data bits can be done with the help of convolution encoder. The mapper is used for mapping and puncturing of data bits; Puncturing removes the some of the parity bits. Different operations of MIMO parser on input data bit are specifically based on spatial multiplexing (SM) and space time block coding (STBC).The protection of data from burst errors during transmission is done by using interleaving. This increases the diversity of wireless system. Pilot insertion plays important role to prevent intercarrier interference. The IFFT block transforms frequency domain signal into time domain signal. Guard interval is introduced to preserve orthogonality of subcarriers and the independence of subsequent OFDM symbol.

III. OFDM SYSTEM LEVEL IMPLEMENTATION

B. OFDM system receiver:

Fig 5: OFDM block diagram

It consist of – •DATA SOURCE: Data source is the input to the OFDM transmitter. •IQ MAPPER and OFDM Modulation: These two blocks together forms the Transmitter of the system. •AWGN: It is added in transmitter so as to reduce the error. •OFDM Demodulator and IQ Demapper: These two blocks together forms the OFDM Receiver. Fig 7: Mimo Receiver block diagram

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014) The receiver blocks are depends on method used to code the signal in transmitter. The receiver can be divided into three different parts viz. Synchronization, FFT and MIMO detection unit. After receiving the symbol cyclic prefix should be removed. Then data is transmitted to the FFT block.FFT block converts time domain signal to frequency domain. The signals are decoded using viterbi decoder and finally descrambled. Descrambler can be used to get back original data bits from the scrambled bits.

B. Simulation Results:

IV. HARDWARE OVERVIEW In this paper,we are going to implement OFDM system on FPGA.The implementation on FPGA is better than on a general purpose MPU in terms of speed and ASIC in terms of cost.An FPGA means field programmable gate array which supports implementation of relatively large logic circuits.It consist of three main sources viz.Logic blocks,input output blocks and interconnection wires or switches . For this system design on hardware we are using MATLAB /SIMULINK visual modeling tool set.This tool gives the user a library, which simulates each hardware components behaviour.Then the whole design is implemented on spartan6 FPGA device.

 Modulation scheme used-QPSK and MSK.  Number of subcarriers-Limited by board resources.  Input data rate can be achieved upto 90Mbps and the graph for channel capacity Vs. SNR is as shown above.  Bit Error Rate achieved is 0.000001.the plot is as shown below.

V. SOFTWARE OVERVIEW A.

Simulation Process:  Finalize specifications for OFDM system.  Decide hardware according to the specifications.  Start creating model for MIMO in MATLAB in Simulink generator. a) Develope model for the OFDM transmitter. b) Develope model for the OFDM receiver. c) Check MATLAB simulation.  Generate VHDL code and VHDL test bench.  Verification and functional simulation.  Hardware cosimulation. (burning or dumping bit file into FPGA hardware)  Verify OFDM specifications on hardware.

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014) VI. CONCLUSION

BIOGRAPHY: Mane Pratibha pursuing Bachelor degree in Electronics Engineering from Pravara Rural Engineering College, Loni. University of Pune, Maharashtra.

We conclude that, in the design and implementation of OFDM transmitter and receiver on FPGA. The combination of OFDM with MIMO results in less BER. The bit error rate with QPSK for OFDM using MATLAB simulation is presented. The BER performance varies and the transmitted bits are received at the receiver section. The implementation is done using Xilink spartan6 FPGA board. The good capability of OFDM design and implementation has bright future.

Thombare Varsha pursuing Bachelor degree in Electronics Engineering from Pravara Rural Engineering College, Loni. University of Pune, Maharashtra. Mhaske Monali pursuing Bachelor degree in Electronics Engineering from Pravara Rural Engineering College, Loni. University of Pune, Maharashtra.

REFERENCES

Prof. R. R. Bhambare H.O.D, Department of Electronics at Pravara Rural Engineering College, Loni. University of Pune, Maharashtra.

[1]

http://gaussianwaves.blogspot.com/ 2011/07/simulation -of-ofdmsystem-in-matlab-ber.html [2] Zoha Pajoudi, et. al., “Hardware Implementation of a 802.11n MIMO OFDM Transceiver,” in IEEE Jour. 978-1-4244-2750-5. (2008) [3] Yu Wei Lin Chen Yi Lin, “Design of FFT/IFFT Processor for MIMO OFDM Systems,” IEEE Transaction on circuit and system, Vo1.54, no. 4 pp. 807-815, 2007. [4] Ashok Jhunjhunwala, “Next Generation wireless for rural areas,” in Indian Journal of Radio and Space Physics, Vol.36. pp 165-167, 2007. [5] K. C. Chang, et. al., “FPGA Based Design of a Pulsed-OFDM system," in IEEE Jour. 1-4244-0387-1/06. [6] Z. Y. Ding, et. al. , "Design of a MIMO-OFDM baseband receiver for next generation wireless LAN ," in Proc. of ISCAS 06, vol. 1 , pp. 5650-5654, Island of Kos, Greece, 2006. [7] K.C.Chang G.E. Sobelman, E Saberinia, and A.H. Tewfik, “Transmitter architecture for pulsed OFDM,” IEEE APCCAS, 6-9 Dec. 2004, pp 693-696 [8] K. F. Lee, et. al., "A space-frequency transmitter diversity technique for OFDM systems," in Proc. Globecom 03, San Francisco, USA, vol.3, pp.24-28, 2003. [9] D. Getsberg, et. al. , " From theory to practice: An overview of MIMO space-time coded wireless systems," in IEEE Jour. Sel. Areas in Commun.,vo1.21, no. 38 pp. 281-301, 2003. [10] V. Tarokh, et. al., "Space-time block coding for wireless communications: performance results," in IEEE Jour. On Sel. Areas in Commun., vol 17, no. 3.pp. 451-460, 1999. [11] S. M. Alamouti, “A simple transmit diversity scheme for wireless communications," in IEEE Jour. Sel. Areas in Commun., vo1.16, no. 8 pp. 1451-1458, 1998. [12] Ramjee Prasad “OFDM for wireless communication system” Artech House, Inc. Boston, London.

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