PERFORMANCE STUDY OF THE MCCDMA AS PHYSICAL LAYER FOR MOBILE WiMAX TECHNOLOGY

Transcription

1 International Journal of Computer Networking, Wireless and Mobile Communications (IJCNWMC) ISSN Vol. 3, Issue 2, Jun 2013, TJPRC Pvt. Ltd. PERFORMANCE STUDY OF THE MCCDMA AS PHYSICAL LAYER FOR MOBILE WiMAX TECHNOLOGY A. BOUACHA & F. T. BENDIMERAD Telecommunications Laboratory, Faculty of Engineering, Abou-BekrBelkaïd University, Chetouane, Tlemcen, Algeria ABSTRACT Mobile WiMAX (Mobile Worldwide Interoperability for Microwave Acces), is a wireless broadband standard which promised high bandwidth over Long-range transmission.it is OFDM-based technology that supports point to multipoint (PMP) Broadband Wireless Access (BWA). Main application of mobile WiMAX today is for MAN/WAN base stations and link stations with both Line of sight (LOS) communication in the range of 10-66GHz (802.16c) and Non-LOS communication in the range of 2-11GHz (802.16a & d. The major problem in the WiMAX system is that as the bit error rate (BER) increasethe signal to noise ratio (SNR) decrease. So, there is great necessity to optimize the BER to increase the WIMAX performances. In this paper, the performance of mobile WiMAX physical layer using MCCDMA (Multi-Carrier Code Division Multiple Access) as transmission technique is simulate, analyze and compared to the standard mobile WiMAX (IEEE e). The simulation results include the performance analysis based on bit error rate (BER) versus bit energy to noise rate (E b /N o ) plots of different modulation and channel coding schemes according to the standard IEEE e. The results show that MC-CDMA outperforms OFDM in WiMAX system and enhances the performance more when spreading facto (SF) increases. KEYWORDS: WiMAX Technology, MC-CDMA Technique, OFDM-Based Technology INTRODUCTION In recent years a substantial increase in the demand of wireless access technologies for evolving wireless Internet services and improved cellular systems has been observed, but the limitation of RF spectrum presents a big problem [1]. This motivates the need for better techniques to improve spectrum utilization. Research efforts investigating methods of improving wireless systems performance are currently being conducted worldwide, in this way the WiMAX technology based on the IEEE standard is imposed itself as a key technology for broadband wireless metropolitan networks [2]- [3]. It is designed to accommodate both fixed and mobile broadband applications and provides high throughput broadband connections over long distance. WiMAX can be used for a number of applications, including last mile broadband connections, hotspots and high-speed connectivity for business customers. It provides wireless metropolitan area network (MAN) connectivity at speeds up to 70 Mbps and the WiMAX base station on the average can cover between 5 to 10 km [4]. WiMAX is based on an RF technology called Orthogonal Frequency Division Multiplexing (OFDM), which is a very effective means of transferring data when carriers of width of 5MHz or greater can be used. Below 5MHz carrier width, current CDMA based 3G systems are comparable to OFDM in terms of performance. There are several researches of different stages of the WiMAX systems. The aim of this work is to analyze and simulate a MC-CDMA to be used as physical layer in WiMAX instead of OFDM.MC-CDMA combines the advantages offered by both OFDM and CDMA [5-6]. The combinations of these to techniques allow efficient use of existing spectrum. The most advantage of the MC-

2 48 A. Bouacha & F. T. Bendimerad CDMA technique is that it uses longer symbolduration than delay spread. Due to this the subcarriers are affected by the same deep fading at same time, which improves the performances. This paper include the performance analysis based on bit error rate (BER) versus bit energy to noise rate (Eb/No) plots of different modulation and channel coding schemes according to the standard IEEE The rest of this paper is organized as follows. In Section II we present the components of the WiMAX physical layer based on MC-CDMA, in this context we describe the transmitter, the receiver and the used channel model. In section III, the results are presented and discussed. Finally, Section IV concludes the paper. DESCRIPTION OF THE WIMAX PROPOSED PHYSICAL LAYER In this paper the proposed physical layer is based on the MC-CDMA transmission technique. The system model is detailed in this section with different implementation steps, namely the creation and simulation of the transmitter, channel and receiver modules. The block diagram in Figure 1 gives a global overview of the proposed system [3-4]. Figure 1: WiMAX MC-CDMA Simulation Module We can distinguish three important parts: The Transmitter The transmitter consists of a bit random generator and concatenated Reed-Solomon (RS) and Convolutional (CC) encoder. The RS code used in our simulation is a systematic code derived from the RS (255, 239, 8) code, where 255 is the codeword length, 239 is the data length and 8 is the parity length. After RS coding, the code is shortened to achieve a different desired rate and coding by convolutional code of rate R = ½. The next step is an implementation of an interleaver, to avoid long runs of low reliable bits at the decoder input, and mapping the bits to symbols [7][8]. Finely, serial-to-parallel conversion is carried out to form OFDM symbols.pilots and guard carriers are added. All parameters used in our simulation are conform to the IEEE specifications. For MC-CDMA simulation, we used Walsh Hadamard (WH) spreading codes with deferent user snimber [10]. After symbols spreading by WH code, an IFFT is applied to the signal and a cyclic prefix, of length equal to ¼ of totalofdm symbol length, is added.

3 Performance Study of the MCCDMA as Physical Layer for Mobile WiMAX Technology 49 Channel Model The channel model used in our simulation for mobile WiMAX in the COST-231model [9], which is an extension to Hata model and it was proposed by European Cooperative for Scientific and Technical (COST) research. Table I give the parameters of the COST-231 model with tree paths [11]. The Receiver Table 1: Cost-231 Channel Model Parameters Tap 1 Tap 2 Tap 3 Delay (μs) Power (db) K factor At the receiver, the reverse operation is performed with a channel estimate by extracting the pilot symbols and the use of least squares (LS) estimation.the estimated symbols are decoded by a Viterbi and Reed-Solomondecoder [12]-[13]. The received bits are compared to the transmitted bits and BER is calculated for different Eb/No values. SIMULATION AND RESULTS DISCUSSIONS In this work we simulated the physical layer as described in the IEEE b standard with a transmission scheme based on the MC-CDMA. Table 2 summarizes the parameters used in the simulation. In our simulation we consider a downlink transmission with a carrier frequency equal to 2.5 GHz and SISO (single input, single output) system for transmission. Simulations are performed accordance to the block diagram presented in figure 1, with different modulation and coding schemes. Table 3 gives these schemes. Table 2: Simulation Parameters for MC-CDMA-WiMAX System Parameter Value for Mobile WiMAX Channel Bandwidth (BW) 5Mhz Number of data Subcarriers 380 Number of pilot Subcarriers 60 Number of guard Subcarriers 92 Ratio of Guard time to useful symbol time, (G) 1/8 Number of used Subcarriers 420 FFT size, 512 SamplingFrequency, 5.76 MHz Subcarrier Spacing, khz Useful Symbol Time, µs Cyclic prefix (CP) Time, µs OFDM Symbol Time, 100 µs Sampling Time, µs Spreading code WALSH-HADAMARD Spreadingfacto (SF) 2, 4, 8 Digital modulation BPSK,QPSK, QAM Table 3: Modulation and Coding Schemes Used in Simulation Modulation RS Code CC Code Rate (R) Overall Code rate (r) QPSK (32,24,4) 2/3 1/2 QPSK (44,36,2) 5/6 3/4 16-QAM (80,72,4) 5/6 3/4 64-QAM (108,96,6) 3/4 2/3 256-QAM (120,108,6) 5/6 3/4 Simulation results are shown in figure 2 to figure 5.

4 50 A. Bouacha & F. T. Bendimerad According to the COST-231 channel model used in our simulation, when the signal passes through the channel it is infected by additive noise AWGN, fading effect and Doppler spread. So at the receiver the signal has three paths (COST-231 modes). In a mobile system, as mobile WiMAX, the Doppler effect depend on the speed movement of the transmitter. Figure 2 show the simulation results for the OFDM-WiMAX standard system and the figure 3 give the same results for the proposed MC-CDMA-WiMAX system for different velocity values. From figures 2 and 3 we observe that the low flow profile provides better performance with small values of E b /N 0. We also note that the large modulation generates more errors bits than small modulation such as QPSK and the MC-CDMA-WiMAX system has better performance than OFDM-WiMAX system in the same conditions. Hence from the simulation result, observation table is given below: Table 4: OFDM, MC-CDMA Performance s Comparison for WiMAX PHY Layer Modulation Code Rate E b /N o (db) BER QPSK ½ 12 OFDM MC-CDMA (SF=2) 16-QAM ½ QAM ¾ 20 QPSK ½ QAM ½ QAM ¾ 20 We can easily say that the MC-CDMA-WiMAX provides better performance than the classical OFDM-WiMAX. Figure 1: BER versus E b /N o for OFDM-WiMAX with Different Modulation and Coding Schemes Figure 2: BER versus E b /N o for MC-CDMA-WiMAX with Different Modulation and Coding Schemes Figure 4 plots BER progress versus E b /N 0 for different transmitter speed (30, 60 and120 km/h). We observe that when the speed increases performances decreases. This is due to the Doppler Effect, which increases with the increase

5 Performance Study of the MCCDMA as Physical Layer for Mobile WiMAX Technology 51 ofvelocity, as has already been mentioned. Theperformances of MC-CDMA-WiMAX remain acceptable even for high speeds. Figure 3: BER versus Eb/No of QPSK, RS(32,24,4), CC(2/3) with Different Velocities for MC-CDMA System and SF=2 Figure 4: BER versus Eb/No of 16-QAM, RS(64,48,4), CC(2/3) with Different Spreading Factors for MC-CDMA System at v =30 km/h The last figure shows the performance of the MC-CDMA WiMAX system for 16-QAM with RS (64,48,4), CC (2/3), at a velocity of 30 km/h for different spreading factors (2, 8 and 16). We note that the performances increases as SF increase. It is clear that the MC-CDMA system provides improvements to mobile WiMAX system, but it is limited by the Doppler effects. CONCLUSIONS In this paper, the performance of MC-CDMA for Mobile WiMAX system using different modulation and coding schemas when communicating over Multipath Fading Channel is investigated. The performances of MC-CDMA as physical layer for WiMAX system are evaluated against the OFDM system usually used in WiMAX. In our simulation, the spending code used for MC-CDMA is the Walsh-Hadamard and the fading channel modelis the COST-23. We also use tree different modulation (QPSK, 16-QAM, 256-QAM), with concatenated Reed-Solomon and convolutional coding for channel coding. We investigate the BER versus Eb/N0 parameters for these different modulations schemas. We note chat the higher modulation scheme (16-QAM) is not robust but it carries higher data rate. On the contrary, the lower modulation scheme (QPSK) is more robust but carries lower data rate.the achieved results show that the MC-CDMA-WiMAX

6 52 A. Bouacha & F. T. Bendimerad outperforms OFDM-WiMAX system. However, the MC-CDMA is limited by the effects of multiple access interference (MAI) and Doppler effect how reduce significantly the performances. REFERENCES 1. Wan-Yi Lin; Yu-Cheng Chen; Rui-Yen Chang; Sheng-Hui Chen; Chin-Lung Lee;, "Rapid WiMAX network deployment for emergency services," Wireless and Pervasive Computing (ISWPC), th International Symposium on, vol., no., pp.1-5, 3-5 July IEEE. Standard Part16: Air interface for fixedbroadband wireless access systems, October IEEE. Standard e Part16: Air interface for fixed andmobile broadband wireless access systems Amendment for physicaland medium access control layers for combined fixed and mobileoperation in licensed band, December J.Andrews, A.Ghosh, and R.Muhamed fundamentals of wimax :understanding broadband wireless networking.prentic Hall communication engineering and emerging series February MamounGuenac, Heidi Steendam, Performance Evaluation and Parameter Optimization of MC-CDMA, IEEE Transactions on Vehicular Technology, Vol. 56, No. 3, pp , May H. Atarashi, N. Maeda, S. Abeta, and M. Sawahashi, Broadband packet wireless access based on VSF-OFCDM and MC/DS-CDMA, Proceeding PIMRC-2002, Lisbon, Portugal, pp , September ChanglongXu Soft Decoding Algorithm for RS-CC Concatenated Codes in WIMAX System Vehicular Technology Conference, S Venkatachalam and T Manigandan, Adaptive Modulation Of OFDM And MC-CDMA System, International Journal of Electrical and Electronics Engineering and Telecommunication, Vol. 1, No. 1, October Ashayeri, Mohammad Reza; Taheri, Hassan;, "Mobile WiMAX capacity estimation in Various Conditions," Electrical Engineering (ICEE), th Iranian Conference on, vol., no., pp , May Shang-Ho Tsai; Yuan-Pei Lin; Kuo, C.-C.J.;, "MAI-Free MC-CDMA Systems Based on Hadamard Walsh Codes," Signal Processing, IEEE Transactions on, vol.54, no.8, pp , Aug ManjuKumari, TilotmaYadav, PoojaYadav, Comparative Study of Path Loss Models in Different Environments, International Journal of Engineering Science and Technology (IJEST), Vol. 3 No. 4 Apr 2011, pp F. Sanzi, J. Sven and J. Speidel, A Comparative Study of Iterative Channel Estimation for Mobile OFDM Systems, IEEE Transactions on Wireless Communications, Vol. 2, pp , September I. Cosovic, M. Schnell and A. Springer, Combined equalization for uplink MC-CDMA in Rayleigh fading channels, IEEE Transaction on Communications, Vol. 53, pp , October 2005.