Analytical Studies of Fragmented-Spectrum Multi-Level OFDM-CDMA Technique in Cognitive Radio Networks

In this paper, we present a multi-user resource allocation framework using fragmented-spectrum synchronous OFDM-CDMA modulation over a frequency-selective fading channel. In particular, given pre-existing communications in the spectrum where the system is operating, a channel sensing and estimation method is used to obtain information of subcarrier availability. Given this information, some real-valued multi-level orthogonal codes, which are orthogonal codes with values of $\{\pm1,\pm2,\pm3,\pm4, ... \}$, are provided for emerging new users, i.e., cognitive radio users. Additionally, we have obtained a closed form expression for bit error rate of cognitive radio receivers in terms of detection probability of primary users, CR users' sensing time and CR users' signal to noise ratio. Moreover, simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the aforementioned system.Comment: 6 pages and 3 figure

Partial transmit sequence based low complexity receiver for multi-user STBC MC-CDMA system

Space Time Block Code Multi Carrier Code Division Multiple Access (STBC MC-CDMA) is a promising technology for 4G wireless communication systems. STBC is a special form of Multiple Input Multiple Output (MIMO) originally employed for 2 transmit antennas (Nt) and 1 receive antenna (Nr) by Alamouti under flat fading conditions. So application of STBC to frequency selective channel is challenging and has attracted attention of many researchers. Hence, STBC is integrated with multicarrier techniques such as Orthogonal Frequency Division Multiplexing (OFDM) and Multi Carrier Code Division Multiple Access (MC-CDMA), which convert frequency selective channel to several flat fading channels thereby eliminating ISI and in turn need of equalization. Like all other multicarrier techniques STBC MC-CDMA also suffers from high Peak-to-Average Power (PAPR) problem. To combat the problem ofhigh PAPR, many techniques have been proposed,among which Partial Transmit Sequence (PTS) is considered to be the best PAPR reduction schemebut at a cost of high computational complexity. This dissertation mainly focusses on implementation of PTS technique to STBC MC-CDMA scheme for downlink scenario. Also, a low complexity receiver is designed for the above scheme where the equalization is carried out in time domain basis. Also the proposedSTBC MC-CDMA with PTS scheme is comparedwith Single Input Single Output (SISO) MC-CDMA with PTS scheme in terms ofComplementary Cumulative Distribution Function (CCDF) andBit Error Rate(BER) performance. The simulation results verify that STBC MC-CDMA outperforms SISO MC-CDMA under fading conditions. Alsoas the no of users increase, CCDF performanceimproves and BER performance degrades

Performance Analyses of OFDM-CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM-CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM-CDMA receivers in multipath fading channels: 1) MC-CDMA and 2) MC-direct sequence (DS)-CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC-CDMA and MC-DS-CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC-DS-CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations