On the Impact of Exponent Multipath and Branch Correlation on MC-CDMA System in Frequency-Selective Fading Environments

The impact of existence with exponent multipath MIP (multipath intensity profile) for an MC-CDMA (multi-carrier coded-division multiple access) system, which is assumed working over the frequency selective fading environments in this paper. We derived the average BER (bit error rate) formulas for MC-CDMA system with MRC (maximal ratio combining) diversity were with an alternative method for the complementary error function. The consideration included not only the correlated subcarriers and independent subcarriers were addressed in the numerical analysis, but the parameter of user capacity is also analyzed. Keywords—MC-CDMA systems, exponent MIP, MRC diversity, Nakagami-m fading I.Introduction In order to reduce ISI (inter-symbol interference) effect and overcoming the channel fading in a transmission channel, multi-carrier modulation scheme has been adopted for high speed transmission applications. A number of multi-carrier modulation techniques have been proposed during the pass decade [1]. To support a wide area of services and high data rate by using a variety of techniques capable of achieving the highest possible spectrum efficiency is the main objective for future generations of wideband wireless communication systems. The CDMA (coded-division multiple-access) scheme has been applied as an attractive multiple access technology in both 2G (second-generation) and 3G (thirdgeneration) wireless radio systems. In general, the multicarrier DS systems have already been proposed and can be categorized into two types: a parallel transmission-scheme of narrowband DS waveforms in the frequency domain, and a combination of OFDM (orthogonal frequency division multiplexing) and CDMA [1]. The available frequency spectrum of carrier wave is divided into M equal band of subcarriers in the former systems. These subcarriers are used to carry a narrowband DS waveform and the number of subcarriers is usually much less than the processing gain. In the latter system, each chip modulates a different carrier conveying a narrowband waveform rather than a DS waveform, and the number of carriers should be equal to the processing gain. On other hand, due to the advantages of spectrum efficient, interference immune, high date rate, and insensitivity to frequency selective channel, etc. Such that multiple access system bases on direct sequence CDMA (coded-division multiple-access) have drawn recent interest in the application of wireless radio systems [1]. Especially, multi-carrier CDMA (MC-CDMA) appears to be a considerable candidate for future mobile radio communication system. The MC-CDMA system based on the spread spectrum techniques. There are a lot of previous researches have been published for investigation about the issues of MC-CDMA system. Besides, the BER (bit error rate) analysis of MC-CDMA based on considering different kinds of assumptions, so far, have been dedicated in numerous previously researches [1, 2, 3]. In [2] the authors analyzed the BER (bit error rate) performance of uplink MC-CDMA system over frequency selective Nakagami-m fading with MRC and EGC receptions. The performance evaluation of MC-CDMA over multipath fading channels was studied in [3]. The results presented in [4] are for uplink channel using MRC (maximal ratio combining) with the assumed frequency offsets condition in correlated fading. The performance of MC-CDMA in non-independent Rayleigh fading was studied in [5]. In [6], which by use of the method of CF (characteristic function) and residue theorem to calculate the performance for downlink MC-CDMA system. Both of the envelopes and phases correlation are considered in [7] to evaluate the performance of a MC-CDMA system operates in Rayleigh fading channel. The literature in [8] illustrated the error probability for MC-CDMA systems assumed that the transmission channel is in Nakagami-m IAENG International Journal of Computer Science, 33:1, IJCS_33_1_ 15 ______________________________________________________________________________________ (Advance online publication: 13 February 2007) fading, and the postdetection of EGC (equal gain combining) is considered. In this paper, some expressions of BER performance for uplink MC-CDMA system working in correlated fading channels is evaluated. The general correlation of channels with Nakagami-m fading distribution is assumed. An average BER formula closed-form is obtained via the sum of Gamma variates to avoid the difficulty of explicitly obtaining the pdf for the SNR (signal-to-noise ratio) at the MRC output. The results analyze and show that how does the channel correlation affects the system performance of a MC-CDMA systems. The rest of this paper is organized as follows: section II gives a description of the MC-CDMA system model. The correlated-Nakagami-m fading channel model is given in section III. In section IV describes the receiver model of MC-CDMA system. The performance of MC-CDMA operating in uncorrelated and correlated fading cannel is carried out in section V. There are numerically results shown in section VI. Finally, section VII draws briefly conclusions. II. System Models We considered an uplink MC-CDMA system model for the study. Assuming that exist K simultaneous users are with N subcarriers within a signal cell. Any effect of correlation among users is going to be ignored by assuming the number of users is uniformed of distribution. As shown in Fig.1, a signal data symbol is replicated into N parallel copies. The signature sequence chip with a spreading code of length L is used to BPSK (binary phase shift keying) modulated each of the N subscriers of the k-th user. Where the subcarrier has frequency b F/T Hz, and where F is an integer number. [1,3]. The technical described above is same as to the performance of OFDM (Orthogonal Frequency Division Multiplexing) on a direct sequence spread-spectrum signal when set 1 F  . The larger values of F , the more transmit bandwidth increase. The transmitted signal the resulting transmitted baseband signal ( ) k S t corresponding to the M data bit size can be expressed as