Topics: WiMAX, Code division multiple access, Multiplexing Pages: 7 (1847 words) Published: April 6, 2013
PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011


Performance Comparison of OFDM, MC-CDMA and OFCDM for
4G Wireless Broadband Access and Beyond
Syed M. Zafi S. Shah, A. W. Umrani, and Aftab A. Memon
Institute of Information and Communication Technologies
Mehran University of Engineering and Technology
Jamshoro 76062, Pakistan

Abstract— The performance comparison off the three most feasible multiple access techniques proposed for fourth generation wireless communication systems, i.e., Orthogonal Frequency and Code Division Multiplexing (OFCDM), Orthogonal Frequency Division Multiplexing (OFDM), and Multi-Carrier Code Division Multiple Access (MC-CDMA) is evaluated. The objective is to find the most suitable technique for implementation in 4G Communication Systems. The modems of OFDM, MC-CDMA and OFCDM have been redesigned and performance is analyzed in terms of probability of error. The results reveal that OFCDM provides the lowest BER for a given SNR.


The two main candidates for fourth generation (4G) mobile communication systems are WiMAX 802.16e and Long Term Evolution (LTE) Advanced initially endorsed by ITU-R enabling true broadband services with transmission rates up to 100 Mbps with full mobility and 1 Gbps with limited mobility [1–3]. However, none of these systems support such higher transmission rates. The major hindrance is the weak ‘Resource Accessing’ techniques used at the physical layer of the incumbent systems, which either rely on FDMA, TDMA and DS-CDMA or combination of them, to provide access to multiple users. There have been several proposals including Orthogonal Frequency Division Multiplexing (OFDM), Multi-Carrier Code Division Multiple-Access (MC-CDMA) and Orthogonal Frequency & Code Division Multiplexing (OFCDM) for the adoption as multiple access techniques [4–6]. MC-CDMA has been thoroughly explained in [4, 6]. The motivation was to combine the advantages of multicarrier transmission with the ability of CDMA to accommodate a greater number of users. Later, a combination of multicarrier transmission with two-dimensional spreading named Orthogonal Frequency & Code Division Multiplexing (OFCDM) is introduced by [5]. The authors in [5] have advocated the use of OFCDM for future 4G mobile communications by providing its basic structure and main functions over OFDM system. However, the authors do not take into account the consideration that the MC-CDMA is also a potential technique for 4G mobile communications. A comparison of OFCDM with MC-CDMA would have been more appropriate. Motivated by their work we compare, in this paper, the performance of OFCDM, OFDM and MC-CDMA systems, keeping the aim to find the suitable technique for implementation in 4G systems. The modems of OFDM, MC-CDMA and OFCDM have been redesigned and the performance is simulated under a practical 6-tap frequency-selective Rayleigh fading channel. 2. SIGNAL STRUCTURE COMPARISON

The structure for an OFDM symbol is shown in Figure 1, where it is assumed that the multicarrier transmission consists of four subcarriers, i.e., Nc = 4. Multiple access can be provided to users by either allocating each user a particular subcarrier for transmission, i.e., FDMA, illustrated in Figure 1(a), or by allocating all subcarriers to a user for a particular yet a reasonable amount of time, i.e., TDMA, illustrated in Figure 1(b). The signal structure for an MC-CDMA symbol accommodating a single user is illustrated in Figure 2(a). The MC-CDMA symbol is generated as follows: A block of real-valued BPSK modulated symbol, assuming {+1} here from {+1, −1}, is repeated onto the subcarriers. The data is then spread by a frequency-domain spreading sequence, {+1, −1, +1, −1} with the spreading factor SFfreq = 4. The number of subcarriers Nc and SFfreq have been chosen to be equal, however, SFfreq < Nc is also possible. A key observation in the signal structure of MC-CDMA is that spreading in frequency domain does not...
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