Proceedings of ICCT2013 Performance Analysis of OFCDM in SUI Channels Rehan Ahmad 1 , Shoab Ahmad Khan 2 1,2 Department of Computer Engineering College of Electrical & Mechanical Engineering, National University of Science and Technology Rawalpindi 46000, Pakistan rehan4ad@gmail.com, shoabak@ceme.nust.edu.pk Abstract: Advancements in the technology of multimedia and mobile communication system demands high data rate transmission especially in the downlink. OFDM provides the high data rate transmission with simple detection scheme but it does not provide frequency diversity. CDMA provides frequency diversity but it is not suitable in multipath interference. Orthogonal frequency and code division multiplexing (OFCDM) is getting popular transmission technique due to some of its advantages over OFDM and CDMA. It provides spreading in time and frequency domain which increases the symbols redundancy, hence increase in reliability. It also provides variable transmission rates by employing variable spreading factor (VSF) in frequency and time domain spreading. In this paper we present the performance analysis of OFCDM for fixed wireless applications. Performance in term of bit error rate (BER) is compared between OFCDM and OFDM in six SUI channels. The analytical BER results show that OFCDM has performed much better than OFDM in all SUI channels. The equalizer at the receiver is characterized by zero forcing technique without multi-code interference (MCI) cancellation. So results will be obtained in worst case scenario in the presence noise and MCI. Keywords: OFDM, Multi carrier CDMA, variable spreading factor (VSF), SUI channels, bit error rate (BER), MCI cancellation, frequency and time domain spreading, zero forcing equalizer. 1. Introduction Current wireless communication systems demands high data rate transmission especially in the downlink. In broadband systems downlink rate is always greater than uplink. The required transmission rate for future wireless communication systems is in multi gigabits per second. Thus future wireless communication system will support online gaming, broadcasting services, video conferencing and 3D Television. Wideband code division multiple access (WCDMA) [1] which is also known as third generation (3G) system supports the data rate of 2 Mb/s in 5 MHz bandwidth. WCDMA was further upgraded with some advanced techniques known as adaptive modulation and coding (AMC) and automatic repeat request (ARQ). Thus improved WCDMA also known as High speed downlink packet access (HSDPA) which supports data rate upto 14 Mb/s. HSDPA is further improved to high speed packet access plus (HSPA +) [2] in which data rate is increased to 28.8 Mb/s in both uplink and downlink within same bandwidth. Data rate requirement for IMT-advanced fourth generation (4G) wireless communication system is 100 Mb/s in high mobility and 1Gb/s in low mobility. Both LTE and mobile WiMAX are in competition to approach for data rate of 1 Gb/s. While LTE advance is expected to be released in 2013 with data rate of 1 Gb/s in downlink and 500 Mb/s in uplink. Various physical layer modulation techniques have been proposed for future fourth generation system. Code division multiple access (CDMA) [3] provides diversity, but it is not suitable in large multipath interference environment. Orthogonal frequency and code division multiplexing (OFDM) [4] has performed well in large multipath interference due to number of parallel subcarriers which increases the symbol duration. In OFDM, Modulation and demodulation of orthogonal subcarriers can be easily implemented by using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). Due to high data rate transmission provided by OFDM, it is chosen to be part of current standards such as IEEE 802.11(WLAN), IEEE 802.16 (WiMAX), LTE and DVB-T. One of the disadvantages of OFDM is that it does not provide frequency diversity. So OFDM is upgraded by combining the advantages of CDMA. Thus two dimensional spreading, in time and frequency domain is introduced in OFDM. This new modulation technique is known as orthogonal frequency and code division multiplexing (OFCDM) [5]. Due to time domain spreading redundancy of symbols can be flexibly changed, hence it controls the transmission rate. While due to spreading in frequency domain frequency diversity is achieved. Orthogonal spreading codes [6] are used for spreading in time and frequency domain. These can be obtained by Walsh Hadamard codes or using orthogonal variable spreading factor (OVSF) code tree shown in fig. 2. Other than these advantages, OFCDM can provide variable spreading factor (VSF) technique to flexibly change the spreading in time and frequency domains. Using VSF, system can work in different cellular environments by providing different transmission rate and frequency diversities. Hybrid detection [7] technique based on minimum mean square error (MMSE) and multi-code interference (MCI) cancellation was proposed at the receiver of OFCDM for equalization process.