WiMAX OFDM SIMULATOR

WiMAX OFDM SMULATOR Wickramasinghe DS *, Perera CJSAH **. Department of Electrical Computer Engineering, The Open University of Sri Lanka. * wickytech35@gmail.com, ** cjper@ou.ac.lk. Abstract WiMAX is a next Generation broadband wireless technology based on EEE 802.16-2004 standard. This paper describes the implementation and verification of WiMAX OFDM physical layer specified in EEE 802.16-2004 standards on MATLAB simulink with GU design. Aim of the WiMAX OFDM simulator is to implement basic features of the WiMAX OFDM physical layer specified in EEE 802.16 to get better understanding of standards and OFDM system. Simulator can be used as an OFDM based research and a teaching tool. Baseband OFDM signal Generation, Channel coding and Error correction coding (RS, CC, nterleaving, viterbi coding), variable Modulations (BPSK, QPSK, 16QAM and 64QAM digital modulation) have been implemented in the Simulator. Random binary source and voice signals have been used for simulation analysis. Simulation analysis has been performed for both AWGN channel and frequency flat fading channels. Channel SNR value, maximum Doppler shift, Cyclic Prefix (CP), Band Width (BW) are the main variables of the simulation model. BER analysis, scatter plots and Spectrum analysis have been performed in the simulation. Key words: WiMAX, OFDM, AWGN, QAM 1.0 ntroduction WiMAX or Worldwide nteroperability for Microwave Access is a standards-based wireless technology for providing high-speed, last-mile broadband connectivity to homes and businesses and for mobile wireless networks.[1] WiMAX is essentially a next-generation wireless technology that enhances broadband wireless access. t is specified in the standards EEE 802.16. EEE Working Group 802.16 on Broadband Wireless Access (BWA) standard is responsible for development of 802.16 and standard contains the specification of, Medium Access Control (MAC) layer for BWA and Physical (PHY) layer. The PHY layers defined in EEE 802.16 are, 1. WirelessMAN SC, a single-carrier PHY layer for frequencies beyond 11GHz requiring a LOS condition. 2. WirelessMAN SCa, a single-carrier PHY for frequencies between 2GHz and 11GHz for point-to-multipoint operations. 3. WirelessMAN OFDM, a 256-point FFT-based OFDM PHY layer for point-to-multipoint operations in non-los conditions at frequencies between 2GHz and 11GHz. 4. WirelessMAN OFDMA, a 2,048-point FFTbased OFDMA PHY for point-to-multipoint operations in NLOS conditions at frequencies between 2GHz and 11GHz.[2] WiMAX OFDM simulator uses WirelessMAN OFDM standards. 2.0 Orthogonal Frequency Division Multiplexing (OFDM) OFDM is a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower-rate subcarriers (SCs)[1]. n a classical parallel-data system, the total signal frequency band is divided into N c non overlapping frequency subchannels. Each subchannel is modulated with a separate symbol, and then the N c subchannels are frequency multiplexed. n OFDM subchannels are frequency overlapped. The spectral overlap results in a waveform that uses the available bandwidth with very high bandwidth efficiency[4]. This is illustrated in Figure 2.1

=, =0,1 1(2.1 1) Figure 2.1: Concept of the OFDM signals: (a) conventional multicarrier technique, and (b) orthogonal multicarrier modulation technique. t yields the time-domain sequence x {k=1..n}. To mitigate the effects of S caused by channel delay spread, each block of N FFT coefficients is preceded by a cyclic prefix (CP) or a guard interval. 2.1 OFDM Transceiver System Figure 2.5-1 shows the process of a typical FFTbased OFDM transmitter system. The incoming serial data is first converted from serial to parallel and grouped into X bits to form a complex number. The number X determines the signal constellation of the corresponding subcarrier, such as 16QAM or 64QAM. The complex symbols are modulated in a baseband fashion by the nverse FFT (FFT) and converted back to serial data for transmission. The FFT is used to realize the harmonically related and the modulated individual OFDM subcarriers, in order to transform the signal s spectrum to the time domain for transmission over the channel. A guard interval is inserted between symbols to avoid inter symbol interference (S) caused by multipath distortion. The receiver performs the inverse process of the transmitter. Figure 2.1-1: Basic OFDM Transmitter System Let s[n] is a serial stream of binary digits. These are first demultiplexed into N parallel streams, and each one mapped to a complex data symbols ( = + ) chosen from signal constellation of Quadrature amplitude modulation (QAM). Different constellations has different bit-rates. An inverse FFT is computed on each set of symbols, giving a set of complex time-domain samples. Considering QAM mapped data sequence { = 1 N}. DFT of data block is, Figure 2.1-2: Cyclic Prefix addition of an OFDM Symbol After CP addition, these samples are first converted in to parallel to serial symbol stream. Then, it is converted to the analogue domain using digital-to-analogue converters (DACs); the analogue x(t) signals are then used to modulate with the carrier. t produces complex RF multicarrier transmission signal, C(t). OFDM receiver system is shown in figure 2.1-3. Figure 2.1-3: Basic OFDM Receiver System The receiver picks up the signal r(t), which is then quadrature-mixed down to baseband using cosine and sine waves at the carrier frequency. This also creates signals centered on 2, low-pass filters are used to reject these frequencies. The baseband signals are then sampled and digitized using analogue-todigital converters (ADCs), serial data converted to parallel and a forward FFT is used to convert back to the frequency domain. This returns N parallel streams, each of which is converted to a binary stream using an appropriate symbol detector. These streams are then re-combined into a serial stream, [n], which is an estimate of the original binary stream at the transmitter.[6],[4].

3.0 Simulation model WiMAX OFDM Simulator simulation block diagram is shown in figure 3.1. subcarriers and it is modulated by using FFT algorithm to generate time domain signal. Cyclic Prefix has been added to make immunity to S. Then data is transmitted through the channel. AWGN and Frequency flat fading channel models are used for the simulation. Physical layer parameters defined as primitive and derived parameters as shown in table 3.1. Table 3.1: WiMAX Physical layer parameters Figure 3.1: Physical layer Processing First step is to randomize the bit stream produced by the source. Randomization introduces protection through informationtheoretic uncertainty, avoiding long sequences of consecutive ones or consecutive zeros. t is also useful for minimizing the possibility of transmission of unmodulated carriers[5][7]. After the randomization process bits are coded by the Error correction Coding. Error correction is done by the outer Reed Solomon (RS) and inner convolutional coding (CC). Data is interleaved to protect the transmission against long sequences of consecutive errors, which are extremely difficult to correct. These long sequences of error may affect a lot of bits in a row and can then cause many transmitted burst losses [8]. nterleaved data then mapped to QAM symbols which are complex symbols in nature. Next step is to construct frequency domain OFDM symbol as specified in the EEE 806.16 standard[2]. OFDM symbol contain 256 P R M T V E D R V E D Parameter Nominal Channel bandwidth -BW Number of used subcarriers - Sampling Factor - n Ration of guard time to useful symbol time - G (smallest power of 2 greater than Nused) Sampling Frequency - Subcarrier Spacing - Useful Symbol Time - CP Time - OFDM Symbol Time Sampling Time Value 1.75MHz, 3.5MHz,7MHz, 14MHz,1.25MHz, 5MHz,10MHz, 15MHz,8.75MHz 200 BW that are multiple of 1.75MHZ then n = 8/7 BW that are multiple of 1.5 MHZ then n = 86/75 BW that are multiple of 1.25MHZ then n =144/125 BW that are multiple of 2.75MHZ then n =316/275 BW that are multiple of 2 MHZ then n = 57/50 BW not otherwise specified then n = 8/7 1 4, 1 8, 1 16, 1 16 255 ( /8000) 8000 / 1/ G x + /

Figure 3.2: Simulink model of the WiMAX OFDM simulator WiMAX Physical layer simulation model is shown in figure 3.2. Simulation input parameters taken from the Graphical User nterface and other parameters specified in table 3.1 are processed by using MATLAB script. Processed parameters are sent to the Simulink model. Simulink model do the signal processing part as specified in standard. Simulation results are sent back to the GU. 4.0 Simulation Results BER analysis, scatter plot and spectrum plot analysis obtained by using random binary data source. Further, voice signal has been used for get more visualized output of the received signals in the AWGN and flat fading channels. decision boundary for each of the symbol getting lower. Hence, the error probability getting higher. As a result, BER rate increases with the QAM modulation scheme. This is clearly observable in figure 4.2-2. On the other hand, As the QAM modulation scheme is getting higher, the number of bits per symbol is increased. So that, energy that is needed to transmit a symbol is also getting higher. Observing Figure 4.1-1, to achieve nearly zero BER (10-2 ) condition, modulation schemes took following SNR values. BPSK ~ 28dB 16QAM ~ 38dB 10 0 QPSK ~ 32dB, 64QAM ~ 44dB. Bit error rate for different modulation 4.1 BER analysis Simulation performed BER analysis with channel coding and without channel coding. Analysis can be performed for each of modulation scheme with different coding rates. Figure 4.1-1 shows Bit error rate for different modulations without applying channel coding. nput Parameter values; CP = 1/32 and BW = 3.5 MHz. As the Graph shows, Modulation schemes like BPSK, QPSK have low SNR values. t can be concluded that low QAM modulation schemes are less vulnerable to bit errors. Higher the Modulation scheme, higher the errors added to the signal. When we increases QAM modulation scheme from BPSK to 64QAM, Bit error rate 10-1 10-2 BPSK QPSK 16-QAM 64-QAM 10-3 0 5 10 15 20 25 30 35 40 45 50 Signal to noise ratio (db) Figure 4.1-1: BER analysis without channel coding when CP = 1/32, BW = 3.5MHz WiMAX Physical layer support overall seven modulation schemes with variable coding rates. Table 4-1 shows all modulation and coding rates in WiMAX OFDM physical layer. Figure 4.1-2 shows the simulation results of the BER test including channel coding techniques.

t clearly shows that by including channel coding, performance has been increased in the system.to achieve nearly zero BER (10-2 ) condition, SNR values of each modulation schemes are shown below. BPSK ½ ~ 18dB QPSK 1/2 ~ 20dB QPSK ¾ ~ 24dB 16QAM ½~ 27dB 16QAM ¾ ~ 31dB 64QAM 2/3~ 34dB 64QAM ¾ ~ 36dB Symbols of the Received signal scatter plot become cloudier with higher error rates. Figure 4.2-2 shows the transmitted and received scatter plots for 64QAM, 16QAM and 4QAM modulations for AWGN channel with SNR=35dB, CP= ¼ and BW= 3.5MHz. Left hand side shows the transmitted signal while right hand side shows the Received signal. 10 0 Bit error rate for different modulation 10-1 Bit error rate 10-2 BPSK1/2 QPSK1/2 10-3 QPSK3/4 16-QAM1/2 16-QAM3/4 64-QAM2/3 64-QAM3/4 10-4 0 5 10 15 20 25 30 35 40 Signal to noise ratio(db) Figure 4.1-2: BER analysis with Channel Coding with CP = 1/32, BW = 3.5MHz Table 4-1: Modulation schemes with various coding rates Rate D Modulation Overall coding rate 1 BPSK ½ 2 QPSK ½ 3 QPSK ¾ 4 16QAM ½ 5 16QAM ¾ 6 64QAM 2 3 7 64QAM ¾ 4.2 Scatter and Spectrum plots Effect of received signal can be observed by changing channel parameters (SNR, Maximum Doppler shift, K factor etc). Figure 4.1-1 shows the effect of SNR of the AWGN channel on the scatter plot of the received signal for 16QAM modulation scheme. Figure 4.2-2: Transmitted (left side) and Received (Right side) scatter plots for 64QAM, 16QAM & 4QAM respectively. Simulation generates OFDM spectrum of the transmitted signal and the Received signal. Spectrum analysis can be carried out by changing simulation parameters. Transmitted OFDM signal Received OFDM signal SNR = 30dB SNR = 35dB SNR = 50dB BER = 0.00031 BER = 0 BER = 0 Figure 4.1-1: 16QAM received scatter plots for various SNR values Figure 4.1-3: OFDM spectrum, QPSK ¾, SNR = 35dB

4.3 Real data Transmission (Voice) Uncoded WiMAX OFDM model is used for voice transmission. Simulation was carried out for both AWGN Channel and Frequency flat fading channel. Transmitted & received audio signals can be viewed as amplitude time graphs and it can also be played back to hear the sound. Source signal is a 22050Hz audio file found on MATLAB. All results were obtained for BW = 3.5MHz and CP = ¼. Other simulated parameters are shown in the figure itself. Left sided plot and right sided plot show the transmitted signal and received signal respectively. Upper figure shows received signal for BPSK modulation while, other shows it for QPSK. Figure 4.3-2: Received signal for BPSK and QPSK for Flat fading channel Figure 4.3-3 shows the received signal through frequency flat fading channel. Here the results were obtained by eliminating Noise effect in the channel by setting SNR to 100dB. Simulation results showed that, BPSK modulated received signal has less effect in the Fading channel than the QPSK modulated received signal. By changing the simulation parameter, more analysis can be carried out to compare the effect to the received signal. 5.0 Conclusion Figure 4.3-1: Received signal for BPSK and QPSK for AWGN channel By Analyzing Figure 4.3-1, it is clearly observable that QPSK modulated received signal has more distortion in amplitude than BPSK modulated received signal for same SNR value in AWGN channel. Noises have been added to the received signal. These results compatible with the simulation results are obtained from random binary source in section 4.1. This paper discusses the mplementation of Physical layer specification specified in EEE 802.16-2004 standard in MATLAB Simulink with GU design. Simulation results verified the validity of the model. Performance analysis can also be carried out by changing parameters and add, removing or altering the blocks. (For example, adding or removing channel coding techniques). OFDM has its own issues in wireless channel application. This Simulation model can be used as OFDM based research tool to find optimal channel coding techniques, to develop

carrier offset algorithms etc. Further, Simulation model can be used as a teaching tool for undergraduate students. Acknowledgments wish to express my gratitude to my supervisor, Eng Mr. Aruna Perara for his great interest in my work and for the guidance that he has given me. would like to specially thank Dr. Sankassa Senevirathne of Lanka Bell LTD for giving me this project idea and providing me resources and guidance to do this project successfully. References [1] Jeffrey G Andrews, Arunabha Ghosh, and Rias Muhamed,"Fundamental of WiMAX:Understanding Broadband Wireless Networking".: Pearson Education, 2007. [2] EEE 802.16-2004, "EEE Standard for Local and Metropolitan Area Networks Part16:Air nterface for Fixed Broadband Wireless Access Systems": EEE-SA Standards Board, June 2004. [3] Surujlal Dasrath Adam Truelove, "Software and Hardware mplementation of an OFDM System," Bradley University, Peoria, Project Report 2001. [4] Mérouane Debbah, "Short introduction to OFDM," France,. [10] Dusan Matic's. (1999) Wireless Communication. [Online]. http://www.wirelesscommunication.nl [11] (2010) Tutorial Point: Self learning center. [Online]. http://www.tutorialspoint.com/wimax/wima x_mac_layer.htm [12] (2010) "MATLAB Help: Communication Blockset".[Online]. http://www.mathworks.com/access/helpdesk /help/toolbox/commblks [13] Chip Fleming. (2006) A Tutorial on Convolutional Coding with Viterbi Decoding. [Online]. http://home.netcom.com/~chip.f/viterbi/tuto rial.html [14] Mohammad Azizul Hasan, "Performance Evaluation of WiMAX/EEE 802.16 OFDM Physical Layer," june 2007. [15] (2010) Wikipedia, the free encyclopedia. [Online]. http://en.wikipedia.org [16] Ramjee Prasad, OFDM for Wireless Communications Systems. London: Artech House, nc, 2004. [17] Gordon L. Stiiber Ye (Geoffrey) Li, Orthogonal Frequency Division Multiplexing For Wireless Communications.: Springer, 2006. [5] Henrik Schulze and Christian Luders, "Theory and Applications of OFDM and CDMA:Wideband Wireless Communications".: John Wiley & Sons, 2005. [18] Yafan Zhang,Pagoti Shirisha Osesina Olukayode saac, "OFDM Carrier Frequency Offset Estimation," Karlstad University Sweden, June 2006. [6] (2010)Wikipedia.[Online]. http://en.wikipedia.org/wiki/orthogonal_fre quency-division_multiplexing [19] (2006) Fading in wireless communications. [Online]. http://www.ylesstech.com [7] Amalia Roca, "mplementation of a WiMAX simulator in Simulink," 2007. [8] Loutfi Nuaymi, "WiMAX: Technology for Broadband Wireless Access".: John Wiley & Sons, 2007. [9] Bernard Sklar, Digital Communications fundamental and Applications, 2nd ed. California: Prentice Hall, 2001.