RICHA ANAND*1,PRASHANT BHATI*2

Transcription

1 Performance Evolution of MIMO-WiMAX System for Text Transmission with Different Modulation Schemes RICHA ANAND*1,PRASHANT BHATI*2 *1(PG student of EC Department, Patel College of science and technology,indore, India) *2 (Ass. professor of of EC Department,Patel College of science and technology, Indore, India) ABSTRACT The telecommunication industry has been developing at a very fast rate. In this era the telecom industry is expected to grow more as demand increases for higher generation wireless network applications as a cost effective solutions. This paper presents the performance enhancement of the WiMAX system (Worldwide Interoperability for Microwave Access) by using MIMO Technique and Zero Forcing Equalizer. In this paper, we investigated the performance of WiMAX system for different modulation technique by using MIMO Technique and Zero Forcing Equalizer. The performance is improved by using multiple antennas at transmitting and receiving side. At last text is transmitted and, successfully received. For simulation MATLAB is used, the results show BER performance of WiMAX system with lower bit error rates. Keywords: WiMAX, OFDM, Equalizer, LMS, RLS Broadband Access, 3G and 4G family of standards is officially called WirelessMAN in IEEE. The IEEE standard is based on the Wireless Metropolitan Area Network (WMAN). It supports multiple frequency allocations from 2-66GHz.WiMAX is one of the various areas of research that is started with on developing its performance in variable environments. Broadband wireless access (BWA) systems,e.g. IEEE standard [1] is WiMAX (Worldwide interoperability for Microwave Access), that defines the physical (PHY) and medium access control (MAC) layers and makes several possible configurations available along with non-mandatory options [2]. WiMAX is one of the hottest broadband wireless technologies around today. WiMAX systems are expected to deliver broadband access services to residential and an enterprise customer in an economical way. It is comprehensive, modern and provides extensible security. IEEE standard gives strong support for authentication, key management, encryption and decryption I.INTRODUCTION Wireless is a spectrum of opportunities. IEEE is a family of Wireless Broadband Network. The 1

2 II. IEEE Protocol Architecture: The second sub-layer is Common Part Sub-layer (CPS), which is tightly integrated with the security sub-layer. This layer defines the rules and mechanisms for system access, bandwidth allocation and connection management. It also defines functions like scheduling, connection control and automatic repeat request. The core MAC layer provides packet fragmentation, ARQ and QOS. Figure1: Protocol Stack The IEEE protocol architecture has two main layers: the Medium Access Control (MAC) layer and the Physical (PHY) layer. The MAC layer is divided into three sublayers: Convergence Sublayer (CS), Common Part Sublayer (CPS) and Security Sublayer 2.1Convergence Sub layer (CS) The first sub-layer is the Service Specific Convergence Sub-layer (CS), which is to converse with higher layers and transform upper level data services to MAC layer service flow and connections. The function of CS sublayer is to receive data from higher layers and to classify them as ATM cell or packet and forward frames to CPS sublayer. 2.2 MAC Common Part Sublayer(MAC CPS) 2.3 Security Sublayer The last sub-layer of MAC layer is the Security Sublayer which also known as privacy sub layer,lies between the MAC CPS and the PHY layer. sublayer provides secure key establishment and encryption. Security sublayer has two main protocols: (a) encapsulation protocol for encrypting packet data across the network. (b) PKM protocol for secure distribution of the key negotiations from the Base Station (BS) to the Subscriber Station (SS). 2.4Physical Layer The PHY layer provides a two-way mapping between MAC protocol data units and the PHY layer frames received and transmitted through coding and modulation of radio frequency signals. The physical layer creates the physical connection between the two communicating entities. As is a digital technology, the PHYsical layer is responsible for transmission of the bit sequences. WiMAX Based MIMO System Multiple-input and multiple-output MIMO is an important part of modern wireless communication 2

3 standards, because it offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. Basically the use of multiple antennas at the transmitter and Multiplexing (OFDM) has emerged as a successful air-interface technique. It is the method of Digital Modulation which is based on the principle of transmitting data by splitting the signal into several narrowband channels at different frequencies. The OFDM signal is made up of many orthogonal carriers, and each individual carrier is digitally modulated receiver in wireless system is, popularly known as MIMO technology. MIMO technology has the ability to exploit NLoS channels, and hence they can increase spectral efficiency compared to SISO systems. The merits of MIMO system include diversity gains, multiplexing gains, interference suppression, and array gains. Because it supports a full range of smart antenna technologies, including Space Time Block Codes (STBC), Spatial Multiplexing (SM), and beam forming. MIMO is considered suitable for Mobile WiMAX. MIMO is considered as a critical component in the future developments of mobile WiMAX. The mobile WiMAX system uses the wireless-man-ofdma air interface. the principle of OFDMA consists of different users who share the Fast Fourier Transform (FFT) space. The architecture is based on a scalable sub-channelization structure with variable FFT sizes according to the channel bandwidth. It is expected that Mobile WiMAX with MIMO will be able to carry approximately 4x more traffic than today s 3G networks. MIMO technology constitutes a breakthrough in wireless communication design. with a relatively slow symbol rate. OFDM is one of the applications of a parallel-data transmission scheme, which reduces the effects of multipath fading and makes complex equalizers unnecessary. Orthogonality: The word Orthogonal indicates that there is the precise mathematical relationship between the frequencies of the carrier in the system. Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in the equation 1 and 2 below: Continuous Time T cos(2 nft) cos(2 mft) dt 0; n m 0 (1) Discrete Time N 1 2 kn 2 kn 0 cos( ) cos 0; n m N N Orthogonal Frequency Division Multiplexing(OFDM) In recent years Orthogonal Frequency Division (2) 3

4 The carriers of an OFDM are sinusoids that meet this requirement because each one is a multiple of frequency. Each one has an integer number of cycles in the fundamental period. OFDM signals are made up from a sum of sinusoids, with each corresponding to a subcarrier. The orthogonality requires that the sub-carrier spacing is f=k/tu Hertz, where TU seconds is the useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1. Therefore, with Nsub-carriers, the total pass band bandwidth will be B N f(hz). The orthogonality also allows high spectral efficiency, with a total symbol rate near the Nyquist rate for the equivalent base band signal. It requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers will no longer be orthogonal, causing inter-carrier interference. WiMAX Transceiver Modulated signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 3.2 shows the block diagram of a typical WiMAX transceiver. In the transmitter, the serial to parallel converted data is mapped in the modulator. Also, the fast Fourier transform (IFFT) realizes the time domain WiMax signal by modulating each data symbol onto a unique carrier frequency. The remaining steps are A/D and serial to parallel conversions. Figure 2: WiMAX system diagram [16] The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the modulated signal the calculated time domain signal is then mixed up to the required frequency. The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. [17] The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. III. FORWARD ERROR CORRECTION CODE Forward error correction codes are those codes which detect error at receiver end and correct the data 4

5 received with error by using error correcting code. This technique used to enhance data reliability. It introduced redundant data, called error correcting code, prior to data transmission or storage. FEC provides the receiver with the ability to correct errors without a reverse channel to request the retransmission of data. Forward error correcting code is further divide in two groups which are as follows. 1) Block code. 2) Convolution code. Block Code: Block codes operate on a block of bits. block codes" are error correcting code that acts on a block of k bits input data to produce n bits of output data (n,k).block codes are referred to as (n, k) codes. A block of k information bits are coded to become a block of n bits. n=k + r where r is the number of parity bits. k is the number of information. Convolution Code Convolution codes were first mentioned by Elias in They can be seen as an attempt to generate the random codes that were successfully used by Shannon. Convolution codes differ from block codes in that the encoder contains memory and the n encoder outputs at any given time unit depend not only on the k inputs at that time unit but also on m previous input blocks. An (n, k, m) convolutional code can be implemented with a k-input, n-output linear sequential circuit with input memory m. typically, n and k are small integers with k < n, but the memory order m must be made large to achieve low error probabilities. The information and code words of convolutional codes are of infinite length, and therefore they are mostly referred to as information and code sequence. Space Time Block Coding Space-time coding is a method used in multiple antenna systems to not only increase the reliability of the communication link, but also increase its throughput. This is accomplished by encoding multiple streams of data across the spatial domain (i.e., antennas) and across the time domain. Space- Time Codes (STCs) have been implemented in cellular communications as well as in wireless local area networks. Space time coding is performed in both spatial and temporal domain introducing redundancy between signals transmitted from various antennas at various time periods. It can achieve transmit diversity and antenna gain over spatially Space Time Block Code (STBC): Space-time block codes operate on block of input symbols, producing a matrix output whose columns represent time and rows represent antennas. The space-time coding scheme, is essentially a joint design of coding, modulation, transmit and receive diversity. A space time block code is defined by a transmission matrix G. The entries of the matrix G are linear combinations of the variables and their conjugates. The number of transmission antennas is n, and we usually use it to separate different codes from each other. For example, 5

6 represents a code which utilizes two transmit antennas and is defined by, Alamouti Encoding: To introduce Space Time Block Codes, The Alamouti code is most commonly used. A simple transmit diversity scheme which improves the signal quality at the receiver on one side of the link by simple processing across two transmit antennas at the opposite end. This is a very special STBC. It is the only orthogonal STBC that achieves rate-1.only STBC can achieve its full diversity gain without needing to sacrifice its data rate. Fading in Wireless Channel Fading is deviation of the attenuation that a carriermodulated telecommunication signal experiences over certain propagation media. The radio waves transmitted from the BS, arrive at the MS after reflection, diffraction and scattering from the natural propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionosphere reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. The effects of multipath include constructive and destructive interference, and phase shifting of the signal. This causes Rayleigh fading. Rician Fading A difference between Rayleigh fading and Rician fading is that there is no dominant component in Rayleigh fading channels. If a channel has a fixed LOS component, the received signal equals the superposition of a complex Gaussian component and a LOS component. The signal envelope in this case is considered to be a Rician distribution P sc = Probability of symbol-error per carrier and man-made objects situated between the BS and the MS. The incoming radio waves arriving from different directions have different propagation delays. These multipath components, having randomly distributed amplitudes, phases and angles of arrival, at the receiver antenna causing the received signal to distort or fade. Thus, fading is the rapid fluctuations in the amplitude phase and the multipath delays of a radio signal over a short period of time. The Flat Rayleigh Fading Channel Model Rayleigh fading is caused by multipath reception. In wireless telecommunications, multipath is the Noise and Interference Bit Error Rate: The bit error rate ratio (BER) is the number of bit errors divided by the total number of transferred bits during a time interval. Signal-to-Noise Ratio (SNR):SNR is defined as the ratio between signal power to noise power and it is normally expressed in decibel (db). The mathematical expression of SNR is; 6

7 IV. SIMULATION MODEL Data Generation Randomization FEC Encoding or Convolution coding Modulation Serial to Parallel Noise + MIMO Encoding Parallel to serial Cyclic Prefix IFFT Zero Padding MIMO Decoding Zero Forcing Equalizer Serial to Parallel Removal of Cyclic Prefix FFT Decoding Demodulation Parallel to serial Removal of Zero Padding Derandomization BER Calculation Figure 3: Basic Simulation Model for WiMAX System using MIMO Technique 7

8 Randomization Data randomization, a technique by which protection against the attacks can be achieved by xoring data with random masks. Data randomization uses static analysis. It assigns a random mask to each class and generates code instrumented to xor data read from or written to memory with the mask of the memory operand s class. Random binary data is generated here for simulation purpose. For Performance analysis of WiMAX system for different modulation scheme by using MIMO technique firstly we generate the stream of binary data, then perform the randomization process, which works on a bit by bit basis. The purpose of the scrambled data which is obtained from the random data generator is to convert long sequences of O's or 1's in a random sequence to improve the coding performance. The main component of the data randomization is a Pseudo Random Binary Sequence generator which is implemented using Linear Feedback Shift Register. Next step is to pass the obtained data through Forward Error Correction block using convolution coding[4]. convolution code introduces redundant bits into the data stream through the use of linear shift register. The convolution encoder shall use the industry standard generator polynomials, g0= 1338 and g1 = 1718, of rate R = 1/2. Higher rates like 2/3 and 3/4, are derived from it by employing puncturing. Modulation is the fundamental components of a digital communication system. Modulation is the process of mapping the digital information to analog form so it can be transmitted over the channel. As it is OFDM based WiMAX system different Modulation technique like BPSK,QPSK and QAM [m]is employed. Modulator BPSK QPSK, 16-QAM, and 64-QAM modulations are used for simulation. Received modulated output that is serial data is to be converted into parallel. An inverse Fourier transform (IFFT) converts the frequency domain data input to time domain which represents OFDM Subcarrier. IFFT is useful for OFDM because it generates samples of a waveform with frequency component satisfying orthogonality condition. CYCLIC PREFIX, In OFDMA to enable the multiple access sub channels are used which are formed by grouping the data subcarriers. Number of different sub channels is allocated to the particular user in order to send & receive data. There are two parts in one OFDMA symbol in time domain. The cyclic prefix (CP) time &useful symbol time. CP is used to remove the effect of ISI & ICI in multipath scenario.cp is the copy of the last part of the OFDM symbol which is appended in the front of the transmitted OFDM symbol. The length of CP or guard time interval must be chosen such that it is longer than the maximum delay spread of the multipath channel[7]. Next process is MIMO encoding in which Alamouti Encoding is deployed.[4]. The Alamouti code is also known as Space Time Block Code (STBC). Fig. Shows the block diagram of the transmitter module in MIMO system based on the Alamouti code. The binary bits enter a modulator and are converted into symbols. These symbols are represented by 8

9 complex numbers and they are fed into the Alamouti encoder. The Alamouti encoder maps the symbols onto the transmitter by using the above mentioned matrix defined by (1). In this matrix, the rows represent the transmit antennas, and columns represent the time. The elements of the matrix define the symbols that to be transmitted from a particular antenna. The Alamouti code works with a pairs of symbols at a time. It takes two time periods to transmit the two symbols.[55] the receiver. In MIMO schemes used in the IEEE (WiMAX)two mandatory profiles has specified, space-time code (STC) and spatial multiplexing (SM). the space time code (STC) is based on Alamouti A algorithm which we have used here for transmit diversity [56]. This code achieves a diversity order that is equal to twice the number of antennas at the receiver. After Demodulation removal of zero padding and derandomization of data take place, the original data received at the receiver on which BER is calculated. This is the last part of the process in which the BER vs SNR graphs for MIMO"OFDM with WiMAX System using Alamouti Algorithm with Space Time Coding are presented for Rayleigh fading channel. Figure4:MIMO Transmitter (using Alamauti code) AWGN noise is added as a channel. AT the receiver inverse operation of transmitter takes place. Zero Forcing Equalizer is used at the receiver to bringing down the intersymbol interference (ISI) The Zero-Forcing Equalizer applies the inverse of the channel frequency response to the received signal, to restore the signal after the channel. The proposed equalizer provides ISI-free communications over the IS I MIMO channels without a long guard period. By Performing a FFT on the received samples after the cyclic prefix is discarded, the periodic convolution is transformed into multiplication. Different demodulation techniques may be applied at V. PERFORMANCE ANALYSIS The behavior of the WiMAX System based on MIMO"OFDM Technique under different environments is studied and the effects of increasing the order of the modulation on the BER performance of the system are presented. Zero Forcing equalization is used decoding the coded data symbols at the receiver side. Results are shown in the form of SNR vs. BER plot for different modulations and different channels. The analysis has been done for wireless fading channels namely Rayleigh Channel. Lastly the performance is evaluated by transmitting thetextdata. Results are presented for different antenna configurations over different fading channels using different modulation levels. This performance is basically analyzed using two criterions namely: 9

10 1. BER Analysis 2. Spectral Efficiency Analysis BER Analysis In this section BER analysis of WiMAX system is done for different order of Modulation levels. The analysis of the system using M"PSK is presented then Figure6: BER Vs SNR PLOT of 2X2 MIMO same procedure is done for M"QAM. The fading channels used for this purpose is Rayleigh channels. A BER measurement for BPSK, QPSK and M QAM is shown by Figures with code rates 1/2 respectively. Figure7:BER Vs SNR PLOT of 2X4 MIMO VI. CONCLUSION Figure5:BER Vs SNR 2X1 MIMO System In this present work the performance of the WiMAX systems using zero forcing equalizer and MIMO technique for different modulation levels and for different antenna configurations is presented. Performance of the system is analyzed under different fading channels. We have obtained BER versus SNR curves as well as spectral efficiency for different modulation scheme with different FEC techniques for 1/2 code rate. Simulation results show that even with only one selected antenna at the 10

11 receiver, performances in terms of BER still satisfactory. On increasing the no. of transmitting antennas and receiving antennas there is an improvement in, the spectral efficiency, and also better BER values can be obtained. Results show that by using MIMO technique, bit error rate for all modulation techniques are improved. The zero forcing equalize mitigates the effect of the wireless channel and allows subsequent symbol demodulation. Equalization is often used in wireless communication to mitigate the effects of Inter Symbol Interference (ISI) and noise. It increases its applications in this modern era that is highly beneficial according to the demand. [5] Analysis of Mobile WiMAX Security: Vulnerabilities and Solutions by Tao Han, Ning Zhang, Kaiming Liu, Bihua Tang, Yuan'an Liu [6] Performance Evaluation of IEEE STD d Transceiver Mrunal P. Pathak, S. A. Shirsat [7] WimaxTechnology Performance Enhancement Using Phy Layer with Turbo Coding *Shaik Avaes Mohsin,**Kilaru Kalyan, # Suresh Angadi [8] Performance Evaluation of IEEE e (Mobile WiMAX) in OFDM Physical Layer [9] A review on QOS for wireless network using WiMAX IEEE Standard. [10] Signal Processing Requirements for WiMAX (802.16e) Base Station M SHAKEEL BAIG REFERENCES [1] Performance Enhancement of Wimax system equalizer using Adaptive Equalizer by 1Anita Garhwal, 2 Partha Pratim Bhattacharya VOL. 3, NO. 4, April [2] Experimental Evaluation of IEEE WiMAX Performances at 2.5 GHz Band Annalisa Durantini RadioLabs, Consorzio University s Industrial Laboratory di Radio communication Rome. [3] Comparative Study of Various Handover Scenarios in WiMAX Network by Chandan Gupta. [4] WiMAX Security A Formal Analysis using Scyther tool Kahya Noudjoud, Debbah Adel and Nacira Ghoualmi [11] AX_ICStelecom.pdf [12] Gaussian and Flat Rayleigh Fading Channel Influences on PAPR Distribution in MIMO- OFDM Systems [13] BER Comparison of Rayleigh Fading, Rician Fading and AWGN Channel using Chaotic Communication based MIMO-OFDM System [14] Wireless Communications, By Andreas F. Molisch [15] Introduction to Binary Convolutional Codes by Yunghsiang S. Han [16] Convolutional Coding & Viterbi Algorithm, byer Liu